Oral Presentations-Full abstracts

Hilbert v. Löhneysen, Karlsruhe Institute of Technology (KIT), D-76128 Karlsruhe, Germany

In a number of strongly correlated metallic systems, long-range magnetic order can be tuned to zero temperature by an external parameter such as pressure, chemical composition, or magnetic field. Highly anisotropic magnetic fluctuations in the heavy-fermion system CeCu_6-xAu_x are observed by inelastic neutron scattering when approaching the QPT at x = 0.1, despite the fact that long range-incommensurate order for x ³ 0.15 is three-dimensional. The QPT in this system can be tuned not only by Au concentration, but also by hydrostatic pressure or magnetic field, which offers the opportunity to elucidate the role of the tuning parameter. Tuning with Au concentration or hydrostatic pressure leads to surprisingly similar behavior of magnetic ordering wave vector, thermodynamics, and electronic transport close to the QPT. On the other hand, magnetic-field tuning leads to distinctly different behavior of critical fluctuations. We will compare the volume and magnetic Grüneisen parameters obtained from thermal expansion and magnetocaloric effect, respectively.

Bulk LaCoO₃ is in a low-spin S = 0 state for T → 0, but tensile strain on epitaxial LaCoO₃ films exerted by the substrate induces ferromagnetism. Using different substrates to tune the lattice constant <a>, distinctly different dependencies of the Curie temperature and effective magnetic moment on <a> are obtained. The possibility of a QPT obtained by strain tuning in this system will be discussed.

Steffen Wirth, Max Planck Institute for Chemical Physics of Solids, Noethnitzer Str 40, 01187 Dresden, Germany Manuel Brando, Max Planck Institute for Chemical Physics of Solids, Noethnitzer Str 40, 01187 Dresden, Germany Tanja Westerkamp, Max Planck Institute for Chemical Physics of Solids, Noethnitzer Str 40, 01187 Dresden, Germany Philipp Gegenwart, I. Physik. Institut, Georg-August-Universität, Friedrich-Hund-Platz 1, 37077 Göttingen,  Germany Niels Oeschler, Max Planck Institute for Chemical Physics of Solids, Noethnitzer Str 40, 01187 Dresden, Germany Cornelius Krellner, Max Planck Institute for Chemical Physics of Solids, Noethnitzer Str 40, 01187 Dresden, Germany Christoph Geibel, Max Planck Institute for Chemical Physics of Solids, Noethnitzer Str 40, 01187 Dresden, Germany Frank Steglich, Max Planck Institute for Chemical Physics of Solids, Noethnitzer Str 40, 01187 Dresden, Germany Silke Paschen, Vienna University of Technology, Wiedner Hauptstraße 8-10, 1040 Wien, Austria Stefan Kirchner, Max-Planck-Institute for the Physics of Complex Systems, Noethnitzer Str. 38, 01187 Dresden, Germany Qimiao Si, Department of Physics and Astronomy, Rice University, Houston, TX 77005, USA

Conventionally, a quantum critical point (QCP) is described within the quantum generalization of finite temperature phase transitions, the Ginzburg-Landau theory. Discrepancies of experimental observations in heavy-fermion metals with the predictions of this approach stimulated new unconventional scenarios which are based on the breakdown of the Kondo effect. Here, we present results on the prototypical material YbRh₂Si₂ for which the antiferromagnetic QCP is accessed by means of a small magnetic field. On the one hand, we report high-precision Hall-effect measurements establishing a jump of the Hall coefficient at the QCP thus reflecting the Fermi surface reconstruction arising from the breakdown of the Kondo effect. On the other hand, we present results revealing the global phase diagram of YbRh₂Si₂ under positive and negative chemical pressure as realized by Co and Ir substitution on the Rh side [1]. Surprisingly, chemical pressure leads to a detachment of the magnetic instability from the Fermi surface reconstruction. In particular, negative pressure induces a separation of the two with a spin-liquid type ground state emerging in the intermediate field range. These results indicate a new quantum phase arising from the interaction of the Kondo breakdown and the magnetic quantum phase transition.

[1] S. Friedemann et al, Nature Phys. 5, 465 (2009).

J.C. Seamus Davis, Cornell/BNL A.R. Schmidt, Cornell/BNL M. Hamidian, Cornell/BNL G. Luke, McMaster J.D. Garrett, McMaster T.J.  Williams, McMaster P. Wahl, MPI Stuttgart F. Meier, Hamburg A.V.  Balatsky, LANL

Within a Kondo lattice, the strong hybridization between electrons localized in real space (r-space) and those delocalized in momentum-space (k-space) generates exotic electronic states called ‘heavy fermions’. In URu₂Si₂ these effects begin as expected at temperatures T~55K but are suddenly altered by an unidentified electronic phase transition at T_o=17.5 K. Whether this is conventional ordering of the k-space states, or a change in the hybridization of the r-space states at each U atom, is unknown. Here we use spectroscopic imaging scanning tunnelling microscopy (SI-STM) to image the evolution of URu₂Si₂ electronic structure simultaneously in r-space and k-space. Above T_o, the ‘Fano lattice’ electronic structure predicted for Kondo screening of a magnetic lattice is revealed. But below T_o, a partial energy-gap but without any associated density-wave signatures emerges from the Fano lattice. Heavy-quasiparticle interference imaging within this gap reveals its cause as the rapid splitting below T_o of a light k-space band into two new heavy fermion bands. Thus, the URu₂Si₂ ‘hidden order’ state emerges directly from the Fano lattice electronic structure and exhibits characteristics, not of a conventional density wave, but of a sudden alteration in both the hybridization at each U atom its associated heavy fermion states.

The origin of the second-order phase transition observed around 17.5 K in URu₂Si₂, has been investigated intensively for long time. There has not been any clear evidence for time reversal symmetry breaking or lattice distortion in the low-temperature ordered phase, and then the order parameter has not been established so far. Therefore, the order has been called “the hidden order”. Very recently, it is proposed that the space group in the ordered phase is P4₂/mnm (No. 136); one of the subgroup of I4/mmm (No. 139) in the high temperature phase [1]. The transition from No. 139 to No. 136 does not require any kind of lattice distortion and allows the NQR frequency at a Ru site unchanged, therefore it is easily understood that the conventional experimental techniques has been unable to detect the phase transition. The proposed space group is compatible with O_xy-type anti-ferro-quadrupole ordering with Q = (0, 0, 1). Such the anti-ferro-quadrupole coupling should be realized in the system, although good nesting property, as well known in skutterudites [2], could not be found in the Fermi surfaces. It should be emphasized that the coupling can survive any kinds of lattice distortions even in higher temperatures. The characteristic electronic structure of the hidden ordered phase will be discussed based on the local 5f² electron picture.

[1] H. Harima, K. Miyake, J. Flouquet, J. Phys. Soc. Jpn. 79, 033705 (2010).

[2] H. Harima, J. Phys. Soc. Jpn. 77 Supplement A, 114 (2008).

Funding for this work provided by a Grant-in-Aid for Scientific Research on Innovative Areas ‘‘Heavy Electron’’ (No. 20102002) from the Ministry of Education, Culture, Sports, Science and Technology, Japan.

Peter M. Oppeneer, Uppsala University, Sweden Jan Rusz, Uppsala University, Sweden Michi-To Suzuki, Uppsala University, Sweden Saad Elgazzar, Uppsala University, Sweden John A. Mydosh, Leiden University, Netherlands

We have performed extensive electronic structure calculations of the paramagnetic (PM), large moment antiferromagnetic (LMAF), and hidden order (HO) phases of URu₂Si₂, using relativistic, full-potential LSDA, LSDA+U, and DMFT approaches.

A detailed comparison of calculated and known experimental data shows that LSDA calculations, assuming delocalized uranium 5f states, provide an excellent explanation of the low-temperature LMAF and PM phases. This exemplified by calculated results for the equilibrium volume, Fermi surface (FS) gap, spin, orbital magnetic moments, de Haas-van Alphen frequencies, FS nesting vectors, resistivity, infrared optical spectra, compensated metal property and number of carriers.

Temperature-dependent DMFT calculations performed for the PM high-temperature phase show progressive opening of a quasi-particle gap when temperature is reduced.

The influence of long-lived longitudinal AF fluctuations is analyzed in detail. Although an AF mode would normally be only a very soft perturbation, the situation is different for URu₂Si₂. Due to the huge FS gap, coupling strongly to the AF mode a macroscopic gap is induced, which modifies the materials’ bulk properties. This dynamical symmetry breaking explains why the gap in the HO is 70% of that of the LMAF phase and predicts that the dynamical spin-spin correlation should show typical order parameter behavior.

Reizo Kato, RIKEN

Conducting anion radical salts of Pd(dmit)₂ belong to a strongly correlated two-dimensional system with a quasi triangular lattice of [Pd(dmit)₂]₂⁻ dimers. At ambient pressure, most of them are Mott insulators and spin-1/2 Heisenberg antiferromagnets where the spin frustration operates. Among them,  the EtMe₃Sb salt with a nearly regular-triangular lattice has been found in the quantum spin liquid state.  ¹³C-NMR indicates no spin ordering/freezing down to 19.4 mK. Since this temperature is smaller than 0.01 % of J, the absence of spin ordering/freezing is attributed to quantum fluctuations. The nuclear spin-lattice relaxation rate 1/T₁ shows a kink around 1 K, and T²-dependence below 1 K. This suggests a nodal spin gap. On the other hand, each of heat capacity and thermal conductivity shows a T-linear term in the zero-temperature limit, indicating the presence of gapless excitations. Magnetic-field dependence of the thermal conductivity, however, suggests excitation-gap formation at 1 K.

All these results indicate unusual bipartite nature of elementary excitations in this quantum liquid.

This work has been done in collaboration with S. Yamashita, Y. Nakazawa (Osaka Univ.), T. Itou, M. Yamashita, and Y. Matsuda (Kyoto Univ.). This work was supported by Grants-in-Aid for Scientific Research (No. 20110003) from the Ministry of Education, Culture, Sports, Science and Technology, Japan.

Ivar  Martin, LANL Cristian D. Batista, LANL

We study the Kondo Lattice and Hubbard models on a triangular lattice for band filling factor n=3/4. We show that a simple non-coplanar chiral spin ordering (scalar spin chirality) is naturally realized in both models due to perfect nesting of the Fermi surface. The resulting triple-Q magnetic ordering is a natural counterpartof the collinear Neel ordering of the half-filled square lattice Hubbard model. We show thatthe obtained chiral phase exhibits a spontaneous quantum Hall-effect with s_xy= e²/h.

LA-UR-08-04926

Marc Janoschek, University of California, San Diego Florian Bernlochner, Technische Universität München, Germany Sarah Dunsiger , Technische Universität München, Germany Christian Pfleiderer, Technische Universität München, Germany Peter Böni, Technische Universität München, Germany Bertrand Roessli, Paul Scherrer Institut, Switzerland Peter Link , FRM-II, Technische Universitaet München, Germany Achim Rosch, Universität zu Köln, Germany

Recent theoretical studies [1,2] predict that the broken inversion symmetry in the helical phase of MnSi will lead to a rich spectrum of helimagnons for wave vectors that are small compared to the helical propagation vector k. However, for the large wave vector limit, corresponding to locally ferromagnetic magnetic moments, a nearly ferromagnetic dispersion was expected. Our extensive inelastic neutron scattering study in the helical phase shows the existence of broad dispersive excitations that contradicts this expected behavior. Using a parameter free model we quantitatively establish that these excitations represent broad spin wave bands that are purely caused by the tiny magnetic propagation vector [3]. The small magnetic Brillouin zone leads to multiple Umklapp interactions and thus many helimagnon modes. Our study provides a tractable showcase how collective spin excitations may be radically modified even in simple systems by seemingly harmless small magnetic propagation vectors.

[1] Belitz D., Kirkpatrick T. R. and Rosch A., Phys. Rev. B 73 054431 (2006).

[2] Maleyev S. V., Phys. Rev. B 73, 174402 (2006).

[3] Janoschek M., Bernlochner F., Dunsiger S., Pfleiderer C., Böni P., Roessli B., Link P., and Rosch A., arXiv:0907.5576v1 (2009).

This work was supported by the NSF (Grant No. PHY05-51164) and by the SFB 608 of the DFG.

Dmitri N. Basov, University of California, San Diego   http://infrared.ucsd.edu/

Infrared experiments enable an experimental access to the kinetic energy of mobile electrons and thus allow one to quantify the strength of correlations in solids /M.Qazilbash et al. Nature-Physics 5, 647 (2009)/. This analysis uncovers a common aspect of a variety of unconventional superconductors including cuprates, pnictides, and ruthenates. All these systems reveal substantial suppression of the electronic kinetic energy compared to expectations of the band theory. Infrared nano-imaging  experiments show that the electronic correlations in VO₂ are enhanced in the vicinity of the insulator-to-metal transition /Qazilbash et al. Science 318, 1750 (07)/. Electronic phase separation in VO₂ appears to be responsible for persistent changes of resistance and capacitance in this enigmatic compound /Driscoll et al Science  325, 1518 (2009)/.

Jian-Xin Zhu, Theoretical Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA Rong  Yu, Department of Physics & Astronomy, Rice University, Houston, Texas 77005, USA Hangdong Wang, Department of Physics, Zhejiang University, Hangzhou 310027, P. R. China Liang L. Zhao, Department of Physics & Astronomy, Rice University, Houston, Texas 77005, USA M. D.  Jones, University at Buffalo, SUNY, Buffalo, New York 14260, USA Jianhui Dai, Department of Physics, Zhejiang University, Hangzhou 310027, P. R. China Elihu Abrahams, Center for Materials Theory, Rutgers University, Piscataway, New Jersey 08855, USA, and Department of Physics and Astronomy, University of California Los Angeles, Los Angeles, California 90095, USA E. Morosan, Department of Physics & Astronomy, Rice University, Houston, Texas 77005, USA Minghu Fang, Department of Physics, Zhejiang University, Hangzhou 310027, P. R. China Qimiao Si, Department of Physics & Astronomy, Rice University, Houston, Texas 77005, USA

Bad metal properties have motivated a description of the parent iron pnictides as correlated metals on the verge of Mott localization. What has been unclear is whether interactions can push these and related compounds to the Mott insulating side of the phase diagram. Here we consider the iron oxychalcogenides La₂O₂Fe₂O(Se,S)₂, which contain an Fe square lattice with an enlarged unit cell. We show theoretically that they contain enhanced correlation effects through band narrowing compared to LaOFeAs, and we provide experimental evidence that they are Mott insulators with moderate charge gaps. We also discuss the magnetic properties in terms of a Heisenberg model with frustrating J₁-J₂-J₂’ exchange interactions on a “doubled” checkerboard lattice.

Funding for this work provided by the National Nuclear Security Administration of the U.S. DOE  at LANL under Contract No. DE-AC52-06NA25396, the U.S. DOE Office of Science, and the LDRD Program at LANL (J.-X.Z.), the NSF Grant No. DMR-0706625, the Robert A. Welch Foundation Grant No. C-1411, and the W. M. Keck Foundation (R.Y. and Q.S.), the NSFC Grant No.10974175 and 10874147, the National Basic Research Program of China Grant No. 2009CB929104, and the PCSIRT of ChinaContract No. IRT0754 (H.W., J.D. and M.F.), and DoD MURI (L.L.Z. and E.M.).

Chandra Varma, University of California, Riverside.

An effective Hamiltonian for the heavy-fermion problem and its criticality are provided based on the solution of single impurity and  two impurities in a self-consistent media, and variational approximation to the lattice problem both for the paramagnetic and the antiferromagnetic state. A discussion of the energy scales deduced for the problem from the dilute to the concentrated limit and its basis in fundamental theory is also provided.

Moosung Kim, Stony Brook University Meigan Aronson, Stony Brook University

Many of the R₂T₂X (R=rare earth, T=transition metal, X=Pb,Sn,Sb,Bi) form layered compounds where the R atoms lie on triangular units in the geometically frustrated Shastry-Sutherland lattice (SSL)[1]. Depending on the relative strengths of the first and second neighbor exchange interactions, these compounds either order antiferromagnetically or show spin liquid properties[2].  These R₂T₂X compounds are metallic, and thus offer the promise of studying the effects of geometric frustration on quantum criticality.  Yb₂Pt₂Pb and Ce₂Pt₂Pb are of special interest, as they lie very near this antiferromagnetic quantum critical point. Yb₂Pt₂Pb orders antiferromagnetically at 2 K,  with unusually strong fluctuations in the paramagnetic state.  The ordered state is Fermi liquid-like with a Sommerfeld coefficient g=0.05 J/mol-K²[3]. The phase behavior with magnetic field is very complex,[4] terminating in a sequence of magnetization plateaux, as observed previously in insulating SSL systems[5].   In contrast, Ce₂Pt₂Pb appears to be on the spin liquid side of the QCP, and here the ground state is heavy fermion-like, with g-0.6 J/mol-K².  Our results  suggest that heavy-fermion behavior occurs near the quantum critical point in this class of SSL compounds, as for unfrustrated heavy fermion compounds, but is strongly suppressed by magnetic ordering. 

[1]  B. S. Shastry and B. Sutherland, 1981, ``Exact ground state of a quantum mechanical antiferromagnet’’, Physica 108B, 1069.

[2] A. Isacsson and O. F. Syljuasen, Phys. Rev. E 74, 026701 (2006).

[3]  M. S. Kim, M. C. Bennett, and M. C. Aronson, 2008b, ``Yb₂Pt₂Pb: Magnetic frustration in the Shastry-Sutherland lattice'',  Phys. Rev. B77, 144425. 

[4] M. S. Kim,  Y. -J. Jo, and M. C. Aronson, 2009 (unpublished).

[5] H. Kagayama, K. Yoshimura, R. Stern, N. V. Mushikov, K. Onizuka, M. Kato, K. Kosuge, C. P. Slichter, T. Goto, and Y. Ueda, 1999, ``Exact dimer ground state and quantized magnetization plateaus in the two dimensional spin system SrCu₂(BO₃)₂’’, Phys. Rev. Lett. 82, 3168.

Sergey Borisenko, IFW-Dresden

We have studied the electronic structure of the Fe-pnictides using angle-resolved photoemission spectroscopy. Among them is a non-magnetic LiFeAs (T_c~18K) superconductor. In LiFeAs we find a notable absence of the Fermi surface nesting, strong renormalization of the conduction bands by a factor of three, high density of states at the Fermi level caused by a Van Hove singularity, strong coupling to phonons and no evidence for either a static or fluctuating order except superconductivity with in-plane isotropic energy gaps. Our observations suggest that these electronic properties capture the majority of ingredients necessary for the superconductivity in iron pnictides [1,2].

[1] S. V. Borisenko et al., arXiv:1001.1147

[2] A. A. Kordyuk et al., arXiv:1002.3149

The project was supported, in part, by the DFG under Grants No. KN393/4, BO 1912/2-1, BE1749/13, 486RUS 113/982/0-1 as well as priority program SPP1458. I.V. Morozov also acknowledges support from the Ministry of Science and Education of the Russian Federation under State Contract P-279.

Suchitra E. Sebastian, U. of Cambridge Neil Harrison, NHMFL, Los Alamos Paul A. Goddard, U. of Oxford Moaz M. Altarawneh, NHMFL, Los Alamos Charles H. Mielke, NHMFL, Los Alamos Ruixing Liang, U. of British Columbia Doug A. Bonn, U. of British Columbia Walter N. Hardy, U. of British Columbia Ole K. Andersen, Max Planck Institute, Stuttgart Gilbert G. Lonzarich, U. of Cambridge

Quantum oscillation measurements as a function of magnetic field and angle are presented on the underdoped cuprate YBa₂Cu₃O_6+x over a wide magnetic field range up to 85T, and a broad angular range in both polar and azimuthal angles, measured between 100 mK and 20K. We show that Fermi Dirac statistics govern the elementary excitations even in this strongly correlated material in close proximity to the Mott insulating phase. The angular dependence of these high resolution measurements enables multiple small sections of the Fermi surface with different topologies to be distinguished and located at different locations in the Brillouin zone [1]. Both electron and hole pockets are indicated, implying reconstruction by a translational-symmetry breaking order parameter. While the precise nature of this order parameter remains elusive, we demonstrate via our measurements that it must involve spin degrees of freedom. We further trace a single small section of Fermi surface toward the Mott insulating regime, and find a dramatic increase in effective mass at a metal-insulator quantum critical point (QCP), located under a local maximum in the YBCO superconducting dome. Possible mechanisms that drive this QCP, and their potential relation to enhanced superconducting temperatures are further investigated.

[1] S. E. Sebastian et al. http://arxiv.org/abs/1001.5015 (2010).

Daniel F. Agterberg, University of Wisconsin - Milwaukee Manfred Sigrist , ETH-Zurich Hirokazu  Tsunetsugu, ISSP, University of Tokyo Zhichao Zheng, University of Wisconsin - Milwaukee

With the groundbreaking work of Fulde, Ferrell, Larkin and Ovchinnikov (FFLO), it was realized that superconducting order can also break translational invariance, leading to a phase in which the Cooper pairs develop a coherent periodic spatially oscillating structure. Such pair density wave (PDW) superconductivity has become relevant in a diverse range of systems, including cuprates, organic superconductors, heavy-fermion superconductors, cold atoms, and high-density quark matter. Here I discuss theories of PDW/FFLO superconductors focusing on two aspects. The first relates to the vortex/dislocation topological defects and the resulting consequences on the superconducting state [1]. The second examines the consequences of co-existing PDW order and d-wave superconductivity at high fields in CeCoIn₅ [2].

D.F. Agterberg and H. Tsunetsugu, Nature Physics 4, 639 (2008).

D.F. Agterberg, M. Sigirst, and H. Tsunetsugu, Phys. Rev. Lett. 102, 207004 (2009).

Funding for this work provided in part by NSF DMR-0906633.

Corinna Kollath, CPHT, CNRS, Ecole Polytechnique

Atomic gases cooled to Nanokelvin temperatures are a new exciting tool to study a broad range of quantum phenomena. In particular, an outstanding degree of control over the fundamental parameters, such as interaction strength, spin composition, or dimensionality, has been achieved. This has facilitated access to strongly correlated quantum many body physics in exceptionally clean samples. The outstanding tunability allows to rapidly change the system parameters, even in real time, and to observe the subsequent quantum evolution.

The cleanliness and the good tunability of these cold quantum gases opens the door to simulate systems from other areas of physics. For example, artificial periodic structures for the atomic gas can be created using laser light to mimic condensed matter systems.

I will report on recent theoretical and experimental progress on the realization of these strongly correlated ultracold gases in optical lattices and their response to perturbations from equilibrium.

Funding for this work has been provided by the Triangle de la Physique, the DARPA-OLE program and ANR (FAMOUS).

Zengwei  Zhu, ESPCI Fauqué Benoît, ESPCI Huan Yang, ESPCI Kamran Behnia, ESPCI

The effect of quantum criticality on thermoelectric response has been the subject of a number of recent experimental studies. Here, we focus on the particular case of graphite in the vicinity of the quantum limit[1]. Each time a Landau tube leaves the Fermi surface there is a van Hove singularity in the density of states. The Nernst coefficient sharply peaks and displays hallmarks of quantum criticality including a temperature-independent critical field.  We argue that a quantum topological phase transition occurs whenever a squeezed Landau level leaves the Fermi surface  and this leads to  a hitherto unexplored case of quantum criticality

[1] Z. Zhu,et al.,  Nature Physics, 6, 26 (2010).

E. Bauer, Vienna University of Technology R.T. Khan, Vienna University of Technology F. Kneidinger, Vienna University of Technology E.  Royanian, Vienna University of Technology G.  Hilscher, Vienna University of Technology H. Michor, Vienna University of Technology G. Rogl, Vienna University of Technology E.-W. Scheidt, University of Augsburg K. Miliyanchuck, University of Vienna P.  Podloucky, University of Vienna P.  Rogl, University of Vienna

Superconductivity in materials without inversion symmetry (NCS) in the respective crystal structures occurs in the presence of an antisymmetric spin-orbit coupling as a consequence of an intrinsic electric field gradient. The superconducting condensate is then a superposition of spin-singlet and spin-triplet Cooper pairs. This scenario accounts for various experimental findings such as nodes in the superconducting gap or extremely large upper critical magnetic fields found in superconductors like CePt₃Si or CeRhSi₃. Spin-triplet pairing can happen in a NCS environment in spite of Anderson's theorem that spin-triplet pairing requires a crystal structure that exhibits inversion symmetry. A central issue that emerges when debating physical properties of non-centrosymmetric superconductors concerns the role of strong correlations among electrons.

The aim of this paper is to compare superconductors without inversion symmetry based either on materials with strong electron correlations (e.g., heavy fermions) or on materials which do not possess significant correlations like recently found 1-1-3 compounds such as BaPtSi₃ or Mo₃Al₂C. This allows disentangling the various states and contributions observed in such unconventional superconductors.

Work supported by the Austrian FWF P22295.

Phillip Phillips, University of Illinois

I focus on two experimental puzzles in the underdoped to optimally doped cuprates: 1) the origin of the abrupt sign change of the thermoelectric power near optimal doping for a wide class of cuprate superconductors and 2) the origin of the temperature dependence of the Hall coefficient.  The latter has been fit[1], without microscopic justification, to a two-fluid model in which one of the components scales with the nominal doping level and the other is thermally activated with an excitation energy proportional to the pseudogap energy scale. Within the context of the Hubbard model, I will show that the sign change of the thermopower in a doped Mott insulator arises from dynamical spectral weight transfer across the Mott gap.  Further, such dynamical mixing plays a central role in the temperature dependence of the Hall coefficient.  Starting from the effective low energy theory of a doped Mott insulator[2] obtained by exactly integrating out the high-energy scale, I show that the effective carrier density in the underdoped regime reproduces the two-fluid model of Gor'kov and Teitel'baum [1]. The doping dependence of the resultant activation energy is in excellent agreement with the experimentally determined pseudogap scale in the cuprates.

1. L.P.Gor'kov and G.B.Teitel'baum Phys. Rev. Lett. 97, 247003 (2006).

2. S. Chakraborty and P. Phillips, Phys. Rev. B 80, 132505 (2009).

Yoshio Kitaoka¹, H. Mukuda¹, S. Shimizu¹, M.Yashima¹, P. Shirage ², K. Miyazawa ², H. Eisaki² and A. Iyo²

¹Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka 560-8531 Japan.

²National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8568, Japan.

The phase diagrams of AFM and SC in multilayered cuprate systems, particularly in n = 4 and 5 compounds, are remarkably different from those established for LSCO (n=1) and YBCO (n=2), in which the AFM order collapses by doping a very small number of holes having N_h ~0.02 and N_h ~ 0.055, respectively. The QCPs for n = 4 and 5 compounds are located at higher doping levels than those for n=1 and 2 compounds; hence, the AFM uniformly coexists with SC. When n decreases from 5 to 4, the QCP moves to a lower hole-doped region, suggesting that interlayer magnetic coupling [J_cJ_out(n)]^1/2 becomes stronger with increasing n, which stabilizes the AFM order. Here, J_c is magnetic coupling between interunit cells, which is independent of n, but J_out(n) is magnetic coupling between intraunit cells, which increases with n.  We deduce that 1) independent of n, AFM moment decreases with doping and collapses for 0.17< N_h< 0.19; 2) T_N increases as the out-of plane coupling J_out(n) increases; 3) the in-plane superexchange J_in(N_h) for n =4 and 5 is larger than J_in(¥) ~ 1300 K for infinite layered Ca_0.85Sr_0.15CuO₂. Though T_c is maximum close to the QCP, the results presented here strongly suggest that the AFM interaction plays a vital role in T_c by acting as the glue for the Cooper pairs, which will lead us to a genuine understanding of why T_c of cuprate superconductors is very high. We also report NMR/NQR studies of FeAs-based superconductors.

Robert McQueeney, Iowa State University/Ames Laboratory

The AFe₂As₂ (A=Ba,Sr,Ca) based superconductors (SC) are antiferromagnetic (AFM) metals with a layered crystal structure. Electron doping suppresses the AFM transition and leads to the appearance of a SC phase in the presence of AFM spin fluctuations. We have studied the evolution of static magnetic order and spin excitations as a function of doping in Ba(Fe_1-xCo_x)₂As₂ using neutron and x-ray scattering. The spin wave spectra in the AFM parent compounds (A=Ca) reveal large magnetic exchange within the Fe layers and weaker interlayer exchange.  Spin fluctuations in the optimally doped SC compositions (x > 7%), with no long-range AFM order, are more two-dimensional (2D) in character and highlighted by a 2D magnetic resonance feature that develops below T_C. Within a narrow compositional range (3 < x < 6%) at the onset of SC, AFM and SC can actually coexist and compete with each other. This competition is revealed by a strong suppression of the AFM order parameter below T_C.  The spin excitations in the underdoped compositions are notably more 3D than optimally doped compositions, including a magnetic resonance that has strong c-axis dispersion.  Overall, the results suggest that the approach to SC in Ba(Fe_1-xCo_x)₂As₂ coincides with competing weak magnetic order and a crossover in the dimensionality of the system.

Almut Schroeder and Sara Ubaid-Kassis, Kent State University, Thomas Vojta, Missouri University of Science and Technology

We present magnetization (M) measurements of the d-metal alloy Ni1-xVx at vanadium concentrations above xc ≈11.4% where the onset of long-range ferromagnetic order is suppressed to zero temperature. The temperature (T) dependence of the magnetic susceptibility is best described by simple nonuniversal power laws, M/H(T,H→0) ∼ Tγ , rather than Curie Weiss laws. Moreover, the magnetic field (H) dependence of the low-temperature magnetization displays power laws M ∼ Hα  with α = 1-γ. This leads to H/T scaling of the magnetization in a wide temperature (10K < T ≤ 300K) and field (H ≤ 5T) range. The exponent γ is strongly x dependent, decreasing from 1 at x ≈ xc to γ < 0.1 for x=15%. This behavior clearly differs from both classical and quantum critical behavior in a clean 3D ferromagnet. Instead, it closely follows the predictions for a quantum Griffiths phase associated with a quantum phase transition in a disordered metal. 

Funding for this work provided by NSF (DMR-0306766, DMR-0339147, DMR-0906566) and Research Corporation

Archetypical examples of Fermi liquid instability at zero temperature have been found in heavy-fermion intermetallics. So far, quantum critical materials of this kind are known to have an almost integral valence to stabilize local moments considered essential for the criticality. On the contrary, valence fluctuations generally promote the screening of local moments and consequently suppress the critical phenomena.  In this presentation, we show that the mixed valent f-electron superconductor b-YbAlB₄ exhibits quantum criticality at practically zero field [1-4]. In particular, our high precision magnetization measurement has probed the quantum critical scaling properties down to far lower temperatures than the large energy scale of the local physics due to intermediate valence. The systematic evolution of the magnetism found in our recent doping experiments will be also presented [5].

This is the work performed in collaboration with T. Tayama, Y. Shimura, T. Sakakibara, Y. Karaki, Y. Uwatoko, M. Okawa, and S. Shin (ISSP, Univ. of Tokyo), A. Nevidomskyy and P. Coleman (Rutgers Univ.).

[1] S. Nakatsuji et al., Nature Phys. 4, 603 (2008).

[2] K. Kuga et al., Phys. Rev. Lett. 101, 137004 (2008).

[3] M. Okawa et al., arXiv:0906.4899.

[4] Y. Matsumoto et al., preprint (2010).

[5] K. Kuga et al., preprint (2010).

Gertrud Zwicknagl, Techn. Universitaet Braunschweig

We present calculations of the magnetic field-induced changes of the heavy quasiparticles in YbRh2Si2 which are reflected in thermodynamic and transport properties. The quasi­particles are determined by means of the Renormalized Band Method. The progressive de-renormalization of the quasiparticles in the magnetic field is accounted for using field­dependent quasiparticle parameters deduced from Numerical Renormalization Group studies. Consequences for the interpretation of experimental data are discussed.

Oliver Bodensiek, Theoretical Physics, University of Goettingen, Germany/Thomas Pruschke, Theoretical Physics, University of Goettingen, Germany/Rok Zitko, J. Stefan Institute, Ljubljana, Slovenia

We study the Kondo lattice model with an additional Einstein phonon mode coupled via a Holstein term to the electrons within the dynamical mean-field theory. As impurity solver we use the numerical renormalization group. We present results for the paramagnetic case showing the anticipated heavy Fermion physics, including direct evidence for the appearance of a large Fermi surface for antiferromagnetic exchange interaction. Lattice degrees of freedom tend to effectively suppress the Kondo effect, leading to strongly reduced low-energy scale. For too large electron-phonon coupling we observe a complete collaps of the heavy Fermi liquid and the formation of polarons.

By introducing a Nambu notation, we find that increasing electron-phonon coupling favors superconductivity, which however is not BCS like but shows additional structures in the density of states and the gap function. Due to the latter there is a distinction between the mean-field gap and the gap observed in the tunneling density of states. We investigate the dependency of T_c and D on the model parameters and address the question how this superconducting states competes with the antiferromagnetism close to half filling.

Sergio G. Magalhaes, Universidade Federal de Santa Maria Fabio M. Zimmer, Universidade Federal de Santa Maria Bernard Coqblin, L. P. S.,Université Paris-Sud

The interplay between disorder and strong correlations has been observed experimentally in disordered Cerium alloys such as Ce(Ni,Cu) or Ce(Pd,Rh). In the case of Ce(Ni,Cu) alloys with a Cu concentration x between 0.6 and 0.3, the first studies have shown a smooth transition with decreasing temperature from a spin glass phase to ferromagnetism; for x smaller than 0.2, a Kondo phase has been observed. The situation is more complicate now due to the recent observation of magnetic clusters. The competition between the Kondo effect, the spin glass (SG) and the ferromagnetic (FE) ordering has been extensively studied theoretically. The Kondo effect is described by the usual mean field approximation; we have treated the SG behaviour successively by the Sherrington-Kirkpatrick model, then by the Mattis model and finally by the van Hemmen model, which takes both a ferromagnetic part and a site-disorder random part for the inter-site exchange interaction. We present here the results obtained by the van Hemmen-Kondo model: for a large Kondo exchange Jk, a Kondo phase is obtained, while, for smaller Jk, the succession of a spin glass phase, a mixed SG-FE one and finally a FE one has been obtained with decreasing temperature. This model improves the theoretical description of disordered Kondo systems, can account for experimental data and provides a simpler approach for further calculations of magnetic clusters. 

Michael Norman

Recent angle resolved photoemission data of our group addresses the nature of the excitations in the normal state of the cuprate superconductors [1].  This normal state is characterized by two crossover temperature scales:  T* which marks the onset of an energy gap, and T_coh which marks the onset of sharp spectral peaks.  We find that these two crossover scales are strongly doping dependent, and cross each other near optimal doping, as predicted by certain theories of cuprates based on doped Mott insulators.  Moreover, superconductivity only occurs below both temperature scales.  More interestingly, optimal superconductivity emerges from an unusual normal state below these two crossovers that is characterized by gapped, coherent excitations.

[1] U. Chatterjee et al., unpublished.

This work was supported by the US DOE, Office of Science, under Contract No. DE-AC02-06CH11357.

B. J. Kim, Univ. of Michigan

Sr₂IrO₄ is a novel type of Mott insulator induced by relativistic spin-orbit coupling, and a rare realization of the single-band Mott-Hubbard system in a quasi-two-dimensional square lattice [1,2]. Due to its similarities in crystal, electronic, and magnetic structure to high temperature superconducting cuprates, as well as its unique features, such as orbital-dominated magnetic moment and the quantum phase associated with it, it is interesting to investigate metallic phases in proximity to the Mott insulating state. In this talk, I will present spectral evolution across the insulator-metal transition achieved by in-situ doping method and registered by angle-resolved photoemission. I will present clear evidences of quasiparticles emerging in the Mott gap as Kondo-like resonances, consistent with the dynamical mean-field theory (DMFT) description of the Mott transition. Our results provide a first verification on the validity of DMFT based on infinite dimension, in a 2D material.

[1] B. J. Kim et al., Phys. Rev. Lett. 101, 076402 (2008)

[2] B. J. Kim et al., Science 323,1329 (2009)

Jon Lawrence, University of California, Irvine CuiHuan Wang, Oak Ridge National Laboratory Eric Bauer, Los Alamos National Laboratory

This talk [1] explores differences between rare-earth-based and uranium-based heavy Fermion (HF) compounds that reflect the underlying difference between local 4f moments and itinerant 5f moments. The focus is on the scaling laws relating the low temperature neutron spectra to the specific heat and susceptibility. The scaling laws are seen to work very well for the rare earth compounds. For a number of key uranium compounds, however, the scaling laws fail badly. There are two main reasons for this failure. First, the scaling laws require knowledge of the high temperature entropy and moment, which are often undetermined for itinerant 5f electrons. Second, the scaling laws neglect the presence of antiferromagnetic (AF) fluctuations, which contribute significantly to the specific heat. By making plausible corrections for both effects, good agreement with the scaling laws is obtained for the uranium compounds. We point out that while both the uranium HF compounds and the rare earth intermediate valence (IV) compounds have spin fluctuation characteristic energies greater than ~ 10 meV, they differ in that the AF fluctuations that are usually seen in the U compounds are never seen in the rare earth IV compounds. This suggests that the 5f itineracy increases the f-f exchange relative to the rare earth case.

[1] www.physics.uci.edu/~jmlawren/UvsREpptTalk.pdf

Research at UC Irvine supported by the U.S. Department of Energy, Office of

Basic Energy Sciences, Division of Materials Sciences and Engineering under Award DE-FG02-03ER46036.

Peter Woelfle, ITKM, Karlsruhe Institute of Technology, Germany

We consider the modifications of the Fermi liquid behavior in the quantum disordered phase on approaching the critical point. At low energies, the quasiparticle properties effective mass, Landau parameters and the q.p. scattering amplitude develop in a characteristic way as a function of the control parameter measuring the closeness to the transition. At higher energies, non-Fermi liquid behavior arises. In the regime of high magnetic field the latter may be calculated within a slave boson theory of the Kondo lattice model. Applications to ESR and transport properties will be discussed.

C.  Stock, NIST C. Broholm, Johns Hopkins/C. Petrovic, Brookhaven

The CeXIn₅ (with X=Rh,Ir and Co) compounds are metallic heavy fermion systems which display a delicate balance between antiferromagnetism and superconductivity [1]. While CeCoIn₅ has the highest superconducting transition (T_c=2.3 K) of any known heavy fermion compound, CeRhIn₅ is an antiferromagnet (T_N=3.8 K) with bulk superconductivity only below 0.1 K [2]. The presence of unusually high temperature superconductivity and two-dimensional planes of magnetic Ce³⁺ ions links these systems to other unconventional superconductors such as the cuprates [3]. The quasi two-dimensional nature of these compounds is reflected in the electronic properties measured by quantum oscillation experiments. I will present a series of experiments investigating the magnetic fluctuations in CeCoIn₅ and CeRhIn₅ measured with the use of neutron inelastic scattering. I will show that the magnetic fluctuations in the superconducting state of CeCoIn₅ are dominated by a 'resonance' peak located in momentum at Q=(1/2,1/2,1/2) [4]. I will present studies investigating the properties of the spin fluctuations as a function of both temperature and magnetic field. Comparisons to antiferromagnetic CeRhIn₅ will be made and various theories describing the resonance peak will be discussed.

[1] C. Petrovic et al., J. Phys. Condens. Matter 13, L337 (2001).

[2] J. Paglione et al., Phys. Rev. B 77, 100505(R) (2008).

[3] R.J. Birgeneau et al., J. Phys. Soc. Jpn., 75, 111003 (2006).

[4] C. Stock et al., Phys. Rev. Lett. 100, 087001 (2008).

Erik  Slooten, University of Amsterdam/Alessia  Gasparini, University of Amsterdam/Ying Kai Huang, University of Amsterdam/Takashi Naka , NIMS, Tsukuba/Anne de Visser, University of Amsterdam

The unusual coexistence of ferromagnetism and superconductivity, reported for the intermetallics UGe₂ (under pressure), UIr (under pressure), URhGe and UCoGe, attracts much attention [1]. In these metallic ferromagnets superconductivity is realized well below the Curie temperature without expelling magnetic order, and, even more peculiar, superconductivity and ferromagnetism are carried by the same 5f electrons. This is at odds with the standard BCS theory for phonon-mediated s-wave superconductivity, since the ferromagnetic exchange field is expected to inhibit spin-singlet Cooper pairing. Consequently, it has been proposed – and ample evidence has been put forward – that superconducting ferromagnets are p-wave superconductors, and that superconductivity is mediated by magnetic interactions. Here we use UCoGe as a test case system to study spin fluctuation mediated spin-triplet superconductivity. We report on the response to pressure and magnetic field, which reveals that superconductivity is enhanced near the pressure and field-induced magnetic quantum critical points.

[1] A. de Visser, Superconducting ferromagnets, in: Encyclopedia of Materials: Science and Technology, Eds K.H.J. Buschow et al. (Elsevier, Oxford, 2010), pp. 1-6.

Rebecca Flint, Rutgers University Piers Coleman, Rutgers University

In the highest Tc heavy fermion superconductors, the heavy electrons are forming as they pair, and the internal structure of the pair becomes as important as the forces holding it together.  The 115 family of superconductors [CeMIn₅ (M={Co,Ir,Rh}) and  PuMGa₅ (M={Co,Rh}) and NpPd₅Al₂] provide an extreme example of this phenomena, as free local moments are present down to the superconducting transition temperature.   We examine the internal structure of the heavy fermion condensate to show that it necessarily consists of two types of pairs: a d-wave pair of quasiparticles on neighboring sites condensed in tandem with a d-wave composite pair of electrons bound to a local moment.  We demonstrate this tandem pairing within a symplectic-N solution of the two-channel Kondo-Heisenberg model, showing that the two mechanisms couple linearly to enhance the transition temperature.  Tuning the relative strengths of these interactions naturally explains the two dome structure observed in Ce(Rh,Ir)In₅.  We also predict that the composite component will be observable as a superconducting valence shift.

Funding for this work provided by NSF DMR-0907179

Yuji Matsuda, Department of Physics, Kyoto University/Hiroaki Shishido, Department of Physics, Kyoto University/Takasada Shibauchi, Department of Physics, Kyoto University/Yuta Mizukami, Department of Physics, Kyoto University/Hiroshi Kontani, Department of Physics, Nagoya University/Takahito Terashima, Research Center for Low Temperature and Materials Sciences, Kyoto University

Electronic structure in heavy fermion compounds is essentially three-dimensional. We realized experimentally a two-dimensional heavy fermion system, adjusting the dimensionality in a controllable fashion. Artificial superlattices of the antiferromagnetic heavy fermion compound CeIn₃ and the conventional metal LaIn₃ were grown epitaxially. By reducing the thickness of the CeIn₃ layers, the magnetic order was suppressed and the effective electron mass was further enhanced. Heavy fermions confined to two dimensions display striking deviations from the standard Fermi liquid low-temperature electronic properties, and these are associated with the dimensional tuning of quantum criticality.  We also report the superconducting properties of epitaxially thin films of CeCoIn₅.

[1] H. Shishido, T. Shibauchi, K. Yasu, T. Kato, H. Kontani, T. Terashima and  Y. Matsuda, Science 327, 980 (2010)

E.V.  Sampathkumaran, Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai-400005, India.  K. Mukherjee, ata Institute of Fundamental Research, Homi Bhabha Road, Mumbai-400005, India.  Kartik K Iyer, ata Institute of Fundamental Research, Homi Bhabha Road, Mumbai-400005, India.  Niharika Mohapatra, ata Institute of Fundamental Research, Homi Bhabha Road, Mumbai-400005, India.  Sitikantha  D Das, ata Institute of Fundamental Research, Homi Bhabha Road, Mumbai-400005, India.

Despite intense research in the field of strongly correlated electron behavior for the past few decades, there has been very little effort to understand this phenomenon in nano particles (with sizes less than 1 μm)   of the Kondo lattices.  Recently, we have initiated some work  in this direction to understand strong correlation effects [1] in d- and f-electron intermetallics synthesized by high-energy ball-milling. After briefly summarizing our recent efforts on new Kondo lattices which have not been paid much attention (e.g., Ce₂RhSi₃, CeCuAs₂ etc), we will focus on our findings on the magnetic behavior of fine particles of some Kondo lattices, based on magnetization and heat-capacity studies. For instance, CeRu₂Si₂, a non-magnetic heavy-fermion in the bulk form, exhibits features attributable to magnetic ordering below 8 K. However, in the composition obtained by substitution of Rh for Ru, viz., CeRh_1.2Ru_0.8Si₂, lying at the quantum critical point in the bulk form, magnetic ordering is not induced down to 0.5 K. Interestingly, in isostructural CeRh₂Si₂, magnetic ordering (seen below 36 K in bulk form) is apparently suppressed in fine particles. Results on a few other Kondo lattices also will be summarized. We hope these findings trigger intense research in this direction.

[1]        S. Narayana Jammalamadaka et al, Appl. Phys. Lett. 92, 192506 (2008); Sitikantha D Das et al, Phys. Rev. B 80, 024401 (2009).

L. Degiorgi, ETH Zurich/M. Lavagnini, ETH Zurich/F. Pfuner, ETH Zurich/R. Hackl, WMI Garching/I.R. Fisher, Stanford University

The rare-earth (R) tri-tellurides RTe₃ host an unidirectional, incommensurate CDW already well above room temperature for all R elements lighter than Dy, while in the heavy rare-earth tri-tellurides (i.e., R=Tm, Er, Ho, Dy) the corresponding transition temperature, T_CDW1, lies below 300 K and decreases with increasing R mass. In the latter systems, a further transition to a bidirectional CDW state occurs at T_CDW2, ranging from 180 K for TmTe₃ to 50 K for DyTe₃. We will present a large wealth of data collected with different spectroscopic methods, as x-ray diffraction, and infrared and Raman spectroscopy as a function of both temperature and externally applied pressure. First of all, our x-ray investigations allow us to extract the lattice constants and the CDW modulation wave-vector. We observe that the intensity of the CDW satellite peaks tend to zero with increasing pressure, thus providing direct evidence for a pressure-induced quenching of the CDW phase. With optical reflectivity method, we consistently discover that the CDW gap of RTe₃ progressively collapses when the lattice constant is reduced. Finally, we will present novel Raman scattering experiments as a function of temperature on DyTe₃ and on LaTe₃ at 6 GPa, which clearly display the emergence of the collective CDW amplitude excitations for both the uni- and bidirectional states.

L. Shan, Institute of Physics, CAS/Y. L. Wang, Institute of Physics, CAS/B. Zeng, Institute of Physics, CAS/B. Shen, Institute of Physics, CAS/G.  Mu, Institute of Physics, CAS/C. Ren, Institute of Physics, CAS/H. Wen, Institute of Physics, CAS

Low temperature specific heat has been measured on iron pnictide superconductors with various structures: 1111, 122, 11 and 42622. In the K-doped BaFe2As2 system, we found a huge specific heat jump near Tc, being much larger than the predicted value of BCS theory. This indicates a moderate electron correlation effect in this sample. Also in K-doped BaFe2As2 single crystals, we performed the low-T STM measurements, we found that the very clean tunneling spectrum shows clear evidence of a nodal superconducting gap.  The angle resolved specific heat measurements are extended to the samples FeSe0.5Te0.5, a four fold oscillations were clearly shown, indicating also the evidence for a nodal gap.

[1] Gang Mu, Bin Zeng, Peng Cheng, Zhaosheng Wang, Lei Fang, Bing Shen, Lei Shan, Cong Ren, Hai-Hu Wen, arXiv:0906.4513.

[2] Gang Mu, Huiqian Luo, Zhaosheng Wang, Zhian Ren, Lei Shan, Cong Ren, Hai-Hu Wen, Phys. Rev. B 79, 174501 (2009).

Oliver Stockert

J.  Arndt, Max-Planck-Institut CPfS, Dresden, Germany/E. Fauhaber, Technische Universität Dresden, Dresden, Germany/C. Geibel, Max-Planck-Institut CPfS, Dresden, Germany/H.S. Jeevan, Universität Göttingen, Göttingen, Germany/S. Kirchner, Max-Planck-Institut PKS, Dresden, Germany/M. Loewenhaupt, Technische Universität Dresden, Dresden, Germany/K. Schmalzl, Jülich Centre for Neutron Science at Institut Laue-Langevin, Grenoble, France/W. Schmidt, Jülich Centre for Neutron Science at Institut Laue-Langevin, Grenoble, France/Q. Si, Rice University, Houston, USA/F. Steglich, Max-Planck-Institut CPfS, Dresden, Germany

The origin of unconventional superconductivity is still discussed controversially and of general interest. Even in the first discovered heavy-fermion superconductor CeCu₂Si₂ the unconventional pairing state remains puzzling. Spin excitations instead of phonons are thought to be responsible for the formation of Cooper pairs. Using high-resolution inelastic neutron scattering we observed for the first time a clear spin excitation gap of the heavy quasiparticles in the superconducting state of CeCu₂Si₂, at an incommensurate wave vector determined by the nesting properties of the Fermi surface. We analyze the neutron scattering data both in the superconducting and normal states, and show that there is a saving of the magnetic exchange energy as the system condenses into a superconducting state. Moreover, this magnetic energy gain is considerably larger than the superconducting condensation energy, which we determine from the specific heat data. Our calculation reveals that magnetic excitations are the primary driving force for superconductivity in CeCu₂Si₂. In contrast to other unconventional superconductors CeCu₂Si₂ has experimentally been shown to be located near a quantum critical point. This is further supported by the considerable slowing down of the magnetic normal state response seen in our data. We will compare our results to other correlated electron superconductors.

Michel Kenzelmann, Laboratory for Developments and Methods, Paul Scherrer Institute, CH-5232 Villigen

We have studied the magnetic order inside the superconducting phase of CeCoIn₅ for fields along the [1 0 0] crystallographic direction using neutron diffraction [1]. We find a spin-density wave order with an incommensurate modulation Q = (q,q,1/2) and q = 0.45(1), which within our experimental uncertainty is indistinguishable from the spin-density wave found for fields applied along the [1 -1 0] reciprocal direction [2]. The magnetic order is thus modulated along the lines of nodes of the d_x2-y2 superconducting order parameter, suggesting that it is driven by the electron nesting along the superconducting line nodes. We postulate that the onset of magnetic order leads to reconstruction of the superconducting gap function and a magnetically-induced pair density wave.

[1] M. Kenzelmann, S. Gerber, N. Egetenmeyer, J. Gavilano, Th. Strässle, A.D. Bianchi, E. Ressouche, R. Movshovich, E.D. Bauer, J.L. Sarrao, J.D. Thompson, to be published, Phys. Rev. Lett.

[2] M. Kenzelmann, et al, Science 321, 1652 (2008).

Youichi Yanase, Niigata University Manfred Sigrist, ETH-Honggerberg

The Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state  near the antiferromagnetic (AFM) quantum critical point (QCP) is discussed [1,2].

The AFM order in the d-wave FFLO state is studied on the basis of Bogoliubov-de Gennes equations [1]. We show that the incommensurate AFM order is stabilized in the FFLO state by the appearance of the Andreev bound state localized around the spatial nodes of the FFLO order parameter. The AFM-FFLO state is further enhanced by the induced pair density wave (PDW). The AFM order occurs in the FFLO state even when it is neither stable in the normal state nor in the BCS state. Roles of the AFM fluctuations beyond the BdG equations are discussed on the basis of the fluctuation-exchange (FLEX) approximation [2]. The relevance of the AFM-FFLO state to the field-induced magnetism in the heavy fermion superconductor CeCoIn5 [3,4] is discussed on the basis of the recent experimental results. We propose experiments to examine the possibility of the FFLO superconducting state in CeCoIn5.

[1] Y. Yanase and M. Sigrist, J. Phys. Soc. Jpn. 78 (2009) 114715. 

[2] Y. Yanase, J. Phys. Soc. Jpn. 77 (2008) 063705.

[3] B.-L. Young et al., Phys. Rev. Lett. 98 (2007) 036402.

[4] M. Kenzelmann et  al., Science 321 (2008) 1652.

This study has been supported by Grant-in-Aid for Scientific Research on Priority Areas "Superclean" (No.20029008), Grant-in-Aid for for Scientific Research on Innovative Areas "Heavy Electrons" (No.21102506), and Grant-in-Aid for Young Scientists (B) (No.20740187) from the MEXT, Japan.

Andrew M Berridge, University of Birmingham Santiago A Grigera, UNLP and University of St Andrews Andrew G Green, University of St Andrews Ben D Simons, University of Cambridge

The phase diagram of Sr₃Ru₂O₇ contains a metamagnetic transition that bifurcates to enclose an anomalous phase with intriguing properties - a large resistivity with anisotropy that breaks the crystal-lattice symmetry [1]. We propose that this is a magnetic analogue of the spatially inhomogeneous superconducting Fulde-Ferrell-Larkin-Ovchinnikov

state. Based on a microscopic theory of Stoner magnetism we derive a Ginzburg-Landau expansion where the magnetisation transverse to the applied field can become spatially inhomogeneous. We show that this reproduces the observed phase diagram of Sr₃Ru₂O₇ [2]. We consider the thermodynamic signatures of such transitions and the effect of the complex bandstructure of Sr₃Ru₂O₇.

[1]  S. A. Grigera et al., Science 306, 1154 (2004);  R. A. Borzi et al., Science 315, 214 (2007);  A. W. Rost et al., Science 325, 1360 (2009)

[2]  A.M. Berridge, A.G. Green, S.A. Grigera and B.D. Simons, Phys. Rev. Lett. 102, 136404 (2009);   A.M. Berridge, S.A. Grigera, B.D. Simons and A.G. Green, Physical Review B 81, 054429 (2010).

Anton Vorontsov, Montana State University

Questions of the pairing glue, symmetry and structure of the condensed state in the Fe-based superconducting class is still open despite intense experimental and theoretical efforts. Contributors to this uncertainty are:  (a) the multi-band nature of electronic structure; (b) complex phase diagram where superconductivity appears close and sometimes together with magnetism; (c) several different families of compounds with different physical properties.

In this presentation I will discuss various proposed superconducting pairing states, - conventional and unconventional, including the most `popular' extended s-wave state, - their  experimental signatures in transport and thermodynamic properties. In particular, I will talk about one of the most characteristic features of these materials – proximity of superconducting (SC) and magnetically ordered spin-density-wave (SDW) states. I will discuss how the interplay between these two states depends on the Fermi surface shape, the order parameter structure and the strength of SC and SDW interactions. Taking into account all theoretical predictions and comparison them with experimental findings leads to strong limitations on the pairing states possible in these materials.

M A McGuire, ORNL B C  Sales, ORNL D Mandrus, ORNL; UT A S Sefat, Oak Ridge National Laboratory

This talk is an overview of the various synthesis and doping techniques used in making the 122 families, and the more complex 42622, and the 32522s. An overview of some of the basic properties and the phase diagrams will be discussed.

Lev P. Gor'kov, NHMFL, Florida State University, 1800 E Paul Dirac Dr., Tallahassee FL 32310, USA Gregory B. Teitel'baum, E. K. Zavoiskii Institute for Technical Physics of the RAS, Kazan 420029, Russia

Spatial coexistence of antiferromagnetic (SDW) and superconducting (SC) phases observed in iron pnictides by means of NMR, μSR and magnetic force microscopy presents the new basic feature which cannot be ascribed to the mere presence of defects in the system pinning an incomplete first order transition. We argue that inherent to iron pnictides is the tendency to formation of the new magnetic phase with deep, periodic in space, modulations of the staggered magnetization resulting in a finite density of states. Depending on the value of tuning parameters (doping or pressure), the new phase may acquire the shape of domain walls - the solitons. We derive the equations that describe the new phase. The equations not permitting an analytical approach, we establish the correspondence between this new phase in pnictides and the FFLO-state for superconductors in an exchange field where the soliton phase was studied numerically. Our analysis also shows how the domain structure grows out of the commensurate SDW thus superseding the putative 1-st order transition into paramagnetic state. We discuss experiments that prove that SC indeed emerges on the background of the soliton state. The findings bring the new insight to understanding this new class of HTSC materials where competition between magnetism and SC is currently the subject of numerous studies.

The work of L.P.G. was supported by the NHMFL through NSF cooperative agreement DMR-0654118 and the State of Florida, that of G.B.T. through the RFBR Grant N 07-02-01184.

Ali Yazdani, Joseph Henry Laboratories and Department of Physics, Princeton University, Princeton, NJ 08544, USA

Heavy electronic states originating from the overlap of f orbitals underlie a rich variety of quantum phases of matter. We use atomic scale imaging and spectroscopy with the scanning tunneling microscope (STM) to examine the novel electronic states that emerge from the uranium f states in URu₂Si₂. We find that as the temperature is lowered, partial screening of the f electrons’ spins gives rise to a Kondo-Fano resonance with the same periodicity as the atomic lattice. At T=17.5 K, URu₂Si₂ is known to undergo a 2^nd order phase transition from the Kondo lattice state into a phase with a hidden order parameter. Using spectroscopic mapping, we identify a spatially modulated, bias-asymmetric energy gap with a mean-field temperature dependence that serves as an order parameter for the hidden order state.

Work done in collaboration with P. Aynajian, E. da Silva Neto, C. Parker, Y. Huang, A. Pasupathy, J. Mydosh and supported by DOE-BES

Tien-Ming Chuang, Cornell & National High Magnetic Field Lab/Milan Allan, Cornell & University of St. Andrews/Jinho Lee, Cornell & Brookhaven National Lab/Yang Xie, Cornell/Ni Ni, Ames Lab & Iowa State University/Sergey Bud'ko, Ames Lab & Iowa State University/Gregory Boebinger, National High Magnetic Field Lab/Paul Canfield, Ames Lab & Iowa State University/J. C. Seamus Davis, Cornell, Brookhaven National Lab & University of St. Andrews

The mechanism of high-temperature superconductivity in the newly discovered iron-based superconductors is unresolved. We use spectroscopic imaging–scanning tunneling microscopy to study the electronic structure of a representative compound CaFe_1.94Co_0.06As₂ in the “parent” state from which this superconductivity emerges. Static, unidirectional electronic nanostructures of dimension eight times the inter–iron-atom distance a_Fe-Fe and aligned along the crystal a-axis are observed. In contrast, the delocalized electronic states detectable by quasiparticle interference imaging are dispersive along the b-axis only and are consistent with a nematic a₂ band with an apparent band folding having wave vector along the a-axis. All these effects rotate through 90 degrees at orthorhombic twin boundaries, indicating that they are bulk properties. As none of these phenomena are expected merely due to crystal symmetry, underdoped ferropnictides may exhibit a more complex electronic nematic state than originally expected [1].

Shinji Watanabe, Osaka University/Kazumasa Miyake, Osaka University

Roles of critical valence fluctuations (CVF) in Ce- and Yb-based heavy fermion metals are discussed from a theoretical point of view. It has been recognized gradually [1] that CVF  are origin of not only enhanced superconducting transition temperature but  also the anomalous properties beyond those of canonical quantum critical point (QCP) on the antiferromagnetic transition. Recently, it turned out that the critical end point of valence transition (CEP-VT) is controlled  effectively by applying magnetic field [2], and that the critical valence fluctuations associated with CEP-VT appear more ubiquitously than expected in general [3]. The applied magnetic field induces rather easily the CEP-VT giving rise to a new universality class of  QCP around which a metamagnetic behavior is expected.  Physical properties around  this new QCP enable us to understand anomalous properties of some Ce- and Yb-based compounds [4,5], especially old and new puzzling behaviors of Ce(Rh,Ir)In₅ and  YbRh₂Si₂, and b-YbAlB₄.

[1] K. Miyake, J. Phys.: Condens. Matter 19, 125201 (2007).

[2] S. Watanabe, A. Tsuruta, K. Miyake, J. Flouquet, Phys. Rev. Lett. 100, 236401 (2008).

[3] S. Watanabe, A. Tsuruta, K. Miyake, J. Flouquet, J. Phys. Soc. Jpn. 78, 104706 (2009).

[4] S. Watanabe, K. Miyake, J. Phys. Soc. Jpn. 79, 033707 (2010).

[5] S. Watanabe, K. Miyake, preprint.

A. Severing, University of Cologne, Germany T. Willers, University of Cologne, Germany Z. Hu, Max Planck Institute CPfS, Dresden, Germany L.H.  Tjeng, Max Planck Institute CPfS, Dresden, Germany A.  Tanaka, Hiroshima Univeristy, Japan E.D. Bauer, Los Alamos National Laboratory, Los Alamos, N.M., USA J.L. Sarrao, Los Alamos National Laboratory, Los Alamos, N.M., USA

Linear polarized soft-x ray absorption (XAS) and inelastic neutron scattering (INS) experiments have been performed on CeMIn₅ with M = Rh, Ir, and Co to determine the crystal-field scheme and characteristic Kondo temperature T^* for the hybridization between 4f and conduction electrons [1]. Soft-XAS at the Ce M_4,5 edges can be used as a complementary technique to neutron scattering since polarization dependent XAS reflects the initial state symmetry and gives thus gives direct information concerning the J_z admixtures of the ground state. In the present work we have determined the ground state wave functions from the polarization dependent soft-XAS data and the crystal-field energies with INS. The characteristic temperature T^* has been determined from the line widths of high resolution INS data and our findings are qualitatively in accordance with the 4f ⁰ spectral weights in our XAS data. We further find that the quasielastic line widths of the superconducting compounds CeCoIn₅ and CeIrIn₅ are comparable with the low energy crystal-field splitting.

[1] T. Willers, Z. Hu, P.O. Körner, J. Gegner, T. Burnus, H. Fujiwara, A. Tanaka, D. Schmitz, H.H. Hsieh, H.-J. Lin, C.T. Chen, E.D. Bauer, J.L. Sarrao, E. Goremychkin, M. Koza, L.H. Tjeng, and A. Severing, to be published

Yuri Grin, Max-Planck-Institut für Chemische Physik fester Stoffe, Dresden, Germany

Features of chemical bonding in intermetallic phases play a crucial role for appearance of distinct physical behaviors. Recently this interplay is, in particular, recognized and discussed widely for thermoelectric materials. For design of new strongly correlated electronic systems among intermetallic compounds the understanding of the atomic interactions may open also new opportunities. Especially the covalent interactions between the metal atoms seem to have an effect on physical behavior. New quantum chemical tools based on the electron localizability approach [1,2] allow analysis of the chemical bonding using the electron localizability indicator (ELI-D representation) especially in metallic systems. Decomposition of ELI-D into contributions of the states belonging to certain energy ranges in the electronic density of states [2] allows visualizing and investigation of the role of d and f electrons in the direct bonding in EuRh₂Ga₈ [3] and La₇Os₄C₉ [4].    

[1] M. Kohout, Int. J. Quantum Chem. 97, 651 (2004).

[2] F. R. Wagner et al. Chem. Eur. J. 13, 5724 (2007).

[3] O. Sichevich et al, Inorg. Chem. 48, 6261 (2009).

[4] E. Dashjav et al. J. Solid State Chem. 181, 3121 (2008).

S. Paschen, Institute of Solid State Physics, Vienna University of Technology, Wiedner Hauptstr. 8-10, 1040 Vienna, Austria H. Winkler, Institute of Solid State Physics, Vienna University of Technology, Wiedner Hauptstr. 8-10, 1040 Vienna, Austria K.-A. Lorenzer, Institute of Solid State Physics, Vienna University of Technology, Wiedner Hauptstr. 8-10, 1040 Vienna, Austria J. Custer, Institute of Solid State Physics, Vienna University of Technology, Wiedner Hauptstr. 8-10, 1040 Vienna, Austria A. Strydom, Department of Physics, University of Johannesburg, Auckland Park 2006, South Africa A. Prokofiev, Institute of Solid State Physics, Vienna University of Technology, Wiedner Hauptstr. 8-10, 1040 Vienna, Austria

In Kondo insulators the hybridization of conduction with f electrons is held responsible for the opening of a narrow energy gap. Of particular interest are compounds where the hybridization vanishes along a symmetry axis of the crystal to produce nodes in the gap [1,2]. First investigations on single crystalline CeRu₄Sn₆ have revealed that the magnetization and the specific heat in applied fields are highly anisotropic [3]. Here we present electrical transport measurements and discuss them in the context of previous findings [3-6].

[1] H. Ikeda and K. Miyake, J. Phys. Soc. Jpn. 65, 1769 (1996).

[2] J. Moreno and P. Coleman, Phys. Rev. Lett. 84, 342 (2000).

[3] S. Paschen et al., J. Phys.: Conf. Series 200, 012156 (2010).

[4] I. Das and E.V. Sampathkumaran, Phys. Rev. B 46, 4250 (1992).

[5] E.M. Bruening et al., J. Magn. Magn. Mater. 310, 397 (2007).

[6] A.M. Strydom et al., J. Magn. Magn. Mater. 310, 377 (2007).

We acknowledge financial support by the Austrian Science Fund (FWF project P19458) and the European Research Council (ERC Advanced Grant No 227378).

Unconventional magnetism and superconductivity in the ternary actinide compounds AnPd₅Al₂

 ^1,2Y. Haga, ³Y. Homma, ⁴D. Aoki, ¹T. D. Matsuda, ¹N. Tateiwa, ¹H. Sakai, ^1,2S. Yoshio, ^1,2T. Sugai, ¹A. Nakamura, ¹E. Yamamoto, ⁵R. Settai, ^1,5Y. Onuki, ⁶K. Nakajima, ⁶Y. Arai

¹Advanced Science Research Center, JAEA, Tokai 319-1195, Japan

²Graduate School of Science, Tohoku Univ., Sendai, 319-1313, Japan

³INAC/SPSMS, CEA-Grenoble, Grenoble 38054, France

²Institute for Materials Research, Tohoku Univ., Oarai, 319-1313, Japan

⁵Graduate school of Science, Osaka Univ., Toyonaka 560-0043, Japan

⁶Nuclear Science and Engineering Directorate, JAEA, Tokai 319-1195, Japan

AnPd₅Al₂ compounds (An = rare earths and actinides) crystallize in the tetragonal ZrNi₂Al₅-type structure [1-4].  Among them, NpPd₅Al₂ is the first Np compound that shows heavy fermion superconductivity [1].  The strongly Pauli-limited upper critical fields (H_c2) reflecting the magnetic anisotropy and the first-order phase transition at H_c2 are very similar characteristics as observed in other heavy fermion supercoductors CeCoIn₅ and URu₂Si₂.  On the other hand, other isostructural compounds show antiferromagnetic or paramagnetic ground states with a uniaxial magnetic anisotropy along the tetragonal [001] direction.  It is remarkable that only Np analogue with a superconducting ground state shows XY-type magnetic anisotropy.  Detailed magnetic and superconducting study for these compounds including new experimental developments particularly for the transuranium compounds will be discussed.

[1] D. Aoki et al., J. Phys. Soc. Jpn. 76 (2007) 063701.

[2] Y. Haga et al., J. Alloys Compds. 464 (2008) 47.

[3] K. Gofryk et al., Phys. Rev. B 77 (2008) 092405.

[4] R. A. Ribeiro et al., J. Phys. Soc. Jpn. 76 (2007) 123710.

Steffen Wirth, MPI for Chemical Physics of Solids, Dresden, Germany Sunil Nair, MPI for Chemical Physics of Solids, Dresden, Germany Oliver Stockert, MPI for Chemical Physics of Solids, Dresden, Germany Stefan Ernst, MPI for Chemical Physics of Solids, Dresden, Germany Michael Nicklas, MPI for Chemical Physics of Solids, Dresden, Germany Frank Steglich, MPI for Chemical Physics of Solids, Dresden, Germany John L Sarrao, Los Alamos National Laboratory, Los Alamos, USA Joe D Thompson, Los Alamos National Laboratory, Los Alamos, USA Andrea D Bianchi, University of California, Irvine, USA Zachary Fisk, University of California, Irvine, USA Andy J Schofield, University of Birmingham, United Kingdom

Heavy fermion metals have advanced to suitable model systems by means of which electronic interactions can be studied in detail.  Here we focus on magnetotransport and tunneling investigations of CeMIn₅ (M = Co, Ir) and CeCo(In_1‑xCd_x)₅.  Pressure-dependent Hall effect measurements on CeCoIn₅ exhibit a well developed feature that can unambiguously be related to spin fluctuations associated with the departure from Landau Fermi liquid behavior.  We infer related, yet separate quantum and superconducting critical fields.  Magnetotransport measurements on CeIrIn₅ indicate a precursor state to superconductivity.  A model-independent, single parameter scaling of the Hall angle governed solely by this precursor state is observed.  A detailed comparative scaling analysis indicates a weak scattering of the quasiparticles by magnetic excitations.  These findings are corroborated by recent measurements on CeCo(In_0.925Cd_0.075)₅, which exhibits local coexistence of antiferromagnetic order and superconductivity.

We also report on low temperature Scanning Tunneling Microscopy/Spectroscopy. A gap detected in CeCoIn₅ is compatible with d_x²_−y² symmetry of the superconducting order parameter and is, again, consistent with a precursor state to superconductivity.

This work was supported by DFG Research Unit 960, through NSF-DMR-071042 and by the U.S. Department of Energy/Office of Science.

Pinaki  Sengupta, Cristian Batista

While the mechanism for the formation of any possible supersolid phase in solid ⁴He remains unresolved, theoretical studies have shown that such  a phase with simultaneous diagonal and off-diagonal long range order  can be stabilized in interacting bosons on a lattice. Motivated by the realization of novel bosonic phases in quantum magnets (e.g., Bose Einstein condensation of magnons), we have extended the idea of the supersolid phase to spin systems. In this talk I describe the  charateristic features of the spin supersolid phase and discuss the  minimal spin model that can support such a phase. I shall elaborate the  mechanism of its formation using analytic results in one dimension and  present results for experimentally observable signatures of  the phase (e.g., magnetization, static structure factors, and specific heat) from numerical simulations in higher dimensions. Finally, I shall discuss  models relevant to real quantum magnets that have a spin supersolid ground state.

Faulkner Thomas, KITP/Nabil Iqbal, MIT/Hong Liu, MIT/John McGreevy, MIT/David Vegh, Stony Brook

Since the mid-eighties there has been an accumulation of metallic materials whose thermodynamic and transport properties differ significantly from those predicted by Fermi liquid theory. Examples of these so-called non-Fermi liquids include the strange metal phase of high transition temperature cuprates, and heavy fermion systems near a quantum phase transition. We report on a class of non-Fermi liquids discovered using gauge/gravity duality. The low energy behavior of these non-Fermi liquids is shown to be governed by a nontrivial infrared (IR) fixed point which exhibits nonanalytic scaling behavior only in the temporal direction. Within this class we find examples whose single-particle spectral function and transport behavior resemble those of strange metals. In particular, the contribution from the Fermi surface to the conductivity is inversely proportional to the temperature. In our treatment these properties can be understood as being controlled by the scaling dimension of the fermion operator in the emergent IR fixed point.