International Workshop on Electronic Correlations in Models and Materials, September 11 - 15, 2011, Augsburg, Germany

(in alphabetical order by participants' last names)

Invited Talk

Nils Blümer (U Mainz)
Double occupancy as a universal probe for antiferromagnetic correlations and entropy in cold fermions on optical lattices   

Ultracold fermionic atoms on optical lattices have been proposed as quantum simulators of correlated solids. A missing link towards this goal is the experimental realization of antiferromagnetic (AF) phases and detection of AF order. This failure is commonly attributed to cooling issues. Indeed, the coldest systems achieved so far have central entropies per particle of s=S/(N kB)≈log(2) while AF long-range order is expected in a cubic system only for s<log(2)/2. In my talk, I will argue that this discrepancy is no obstacle for the experiments currently performed or prepared in this context: both modulation spectroscopy [1] and the superlattice approach [2] address the nearest-neighbor correlation function, similarly to the double occupancy D which we have recently suggested as a signature of AF correlations [3]: within DMFT, the development of (local) AF order at low T and strong coupling is accompanied by a distinct enhancement of D. I will show, by comparisons with direct determinantal QMC calculations, that this DMFT scenario applies even in two dimensions, for which the Néel temperature is zero, and is precise on the percent level, up to rounding effects, for a cubic optical lattice [4]. As a function of entropy s, D is nearly universal with respect to dimensionality; in particular, the minimum in D(s) always occurs at s≈log(2) at strong coupling, as predicted by DMFT. Long-range order appears irrelevant for the current search of AF signatures in cold fermions. Thus, experimentalists need not achieve s<log(2)/2 and should consider lower dimensions, for which the AF effects are larger.

[1] D. Greif, L. Tarruell, T. Uehlinger, R. Jördens, and T. Esslinger, Phys. Rev. Lett. 106, 145302 (2011).
[2] S. Trotzky, Yu-Ao Chen, U. Schnorrberger, P. Cheinet, and I. Bloch, Phys. Rev. Lett. 105, 265303 (2010); K. G. L. Pedersen, B. M. Andersen, G. M. Bruun, O. F. Syljuasen, and A. S. Sorensen, arXiv:1105.4466.
[3] E. V. Gorelik, I. Titvinidze, W. Hofstetter, M. Snoek, and N. Blümer, Phys. Rev. Lett. 105, 065301 (2010).
[4] E. V. Gorelik, T. Paiva, R. Scalettar, A. Klümper, and N. Blümer, arXiv:1105.3356.

Invited Talk

Christoph Bruder (U Basel)
Quantum control   

Quantum mechanics courses discuss how the initial state of a system evolves if the Hamiltonian is time-dependent. In quantum control, the question is reversed: how should the time-dependent Hamiltonian be chosen to generate a prescribed unitary transformation at some final time? After a general introduction, this talk will in particular address the problem of how many quantum bits of a quantum computer one needs to control to run an arbitrary quantum algorithm.

Invited Talk

Krzysztof Byczuk (U Warsaw)
Quantification of correlations in correlated electron systems   

Entanglement and relative local entropies are used to study the amount of correlations in correlated electron systems. We solve the Hubbard model within the dynamical mean-field theory and from this solution we extract the entanglement local entropy as well as the relative local entropy, where different factorized Hartree-Fock like ground states are used as a reference state. The amount of correlations, which is expressed by the relative entropy, with respect to Hartree-Fock solutions are discussed in paramagnetic and antiferromagnetic cases. Transition metal-oxides are also quantified with respect to their correlation.

In collaboration with W. Hofstetter (Frankfurt), J. Kuneš (Prague), and D. Vollhardt (Augsburg).

Invited Poster

Liviu Chioncel (U Augsburg)
Absence of half-metallicity in defect-free digital magnetic heterostructures δ-doped with Cr and Mn   

We present the results of combined density functional and many-body calculations of the electronic and magnetic properties of the defect-free digital ferromagnetic heterostructures obtained by doping GaAs with Cr and Mn. While the local-density approximation +U predicts half-metallicity in these defect-free δ-doped heterostructures, we demonstrate that local many-body correlations captured by dynamical mean-field theory induce within the minority-spin channel nonquasiparticle states just above EF. As a consequence of the existence of these many-body states the half-metallic gap is closed and the carriers' spin polarization is significantly reduced. Below the Fermi level the minority-spin highest valence states are found to localize more on the GaAs layers, being independent of the type of electronic correlations considered. Thus, our results confirm the confinement of carriers in these δ-doped heterostructures, having a spin polarization that follows a different temperature dependence than the magnetization. We suggest that polarized hot-electron photoluminescence experiments might uncover evidence for the existence of many-body states within the minority-spin channel and elucidate their finite-temperature behavior.

Invited Talk

Ralph Claessen (U Würzburg)
From infinite dimensions to just one: Luttinger liquid behavior in atomic gold chains   

The theoretically predicted strange quantum state of interacting electrons in one dimension (1D), commonly referred to as Tomonaga-Luttinger liquid (TLL), has experimentally been rather elusive. Here I present a novel 1D surface reconstruction [1,2], namely self-organized Au atom chains on the Ge(001) surface, which are of single atom width only, thus reaching the ultimate 1D limit. The low-energy behavior of these chains as probed by scanning tunneling spectroscopy and photoemission displays all hallmarks of TLL behavior. The experimental single-particle spectra not only show the expected power-law dependence, but even follow a universal scaling with energy and temperature as predicted for the TLL. The surface character of these atomic Au chains in combination with the local control of scanning tunneling microscopy allows additional studies of such 1D systems previously not possible. As an example we demonstrate the quantitative change in the TLL power law exponent at a chain end, as predicted for bounded Luttinger liquids.

[1] J. Schäfer, C. Blumenstein, S. Meyer, M. Wisniewski, and R. Claessen, Phys. Rev. Lett. 101, 236802 (2008).
[2] S. Meyer, J. Schäfer, C. Blumenstein, P. Höpfner, A. Bostwick, J. L. McChesney, E. Rotenberg, and R. Claessen, Phys. Rev. B 83, 121411(R) (2011).

Invited Poster

Theo A. Costi (FZ Jülich)
Charge Kondo effect in PbTe doped with Tl impurities   

Semiconducting PbTe is one of the most interesting materials for thermoelectric applications. When doped with a small concentration of Tl impurities, acting as acceptors, a number of anomalous properties are found: e.g.,beyond a critical concentration of about 0.3 at. % Tl, the system exhibits superconductivity with remarkably high critical temperatures for such a low carrier system. This and other anomalous phenomena prompted the idea that Tl impurities act as negative U centres leading to a charge Kondo effect and to superconductivity. Here, we combine ab-initio information and numerical renormalization group methods to explore the consequences of this model for the normal state properties, showing that it can explain a number of features in the temperature and doping dependence of the resistivity, and carrier density. Predictions for the photoemission spectrum are also discussed.

In collaboration with V. Zlatić.

Invited Talk

Martin Eckstein (ETH Zurich)
Photo-excited states in correlated electron systems   

Ultrashort laser pulses can switch correlated electron systems between various phases on timescales as short as the inverse bandwidth. A well-known example is the photo- induced transition from a Mott insulator to a metallic state in tantalum disulfide [1]. In many cases, these photo-excited states substantially differ from the equilibrium electronic phases. Recently, nonequilibrium dynamical mean-field theory has been used to compute the dynamics of a Mott insulator during and after a strong laser excitation process [2]. Rapid thermalization to a quasi-equilibrium state at elevated electronic temperature occurs only for initial states in the crossover regime between metal and insulator. The photo-excited Mott insulator, on the other hand, remains in a nonthermal state for times much longer than the inverse bandwidth. The bottleneck for the thermalization process is identified as the slow decay of doublon-hole pairs in the Hubbard model. In this talk these results are discussed, as well as ongoing work to use nonequilibrium DMFT for the microscopic modeling of the time-evolution of photo-excited states along nonthermal paths.

[1] L. Perfetti, P. A. Loukakos, M. Lisowski, U. Bovensiepen, H. Berger, S. Biermann, P. S. Cornaglia, A. Georges, and M. Wolf, Phys. Rev. Lett. 97, 067402 (2006).
[2] M. Eckstein, and P. Werner, Phys. Rev. B 84, 035122 (2011).

Invited Talk

Hidetoshi Fukuyama (Tokyo US)
Effects of magnetic field on Dirac electrons in solids   

It is known for long time that there are cases where electronic band structures are similar to those of Dirac electrons in vacuum. Well-known examples are bismuth and graphite; 4x4 Dirac matrix in the former with strong spin-orbit interaction, while 2x2 massless Dirac described by Weyl equation in the latter. Recently one layer of graphite, graphene, is realized and studied both extensively and intensively. Another recent example is molecular solids, α-(BEDT-TTF)2I3. It has been established theoretically that orbital susceptibility of these Dirac electrons has very particular features resulting from inter-band effects of magnetic field. It is of interest to see such inter-band effects on Hall effects to be compared with orbital susceptibility, which will be introduced in this presentation.

This work is based on the collaborations with Yuki Fuseya, Masao Ogata, Akito Kobayashi, and Yoshikazu Suzumura.

[1] A. Kobayashi, Y. Suzumura, and H. Fukuyama, J. Phys. Soc. Jpn 77, 064718 (2008).
[2] Y. Fuseya, M. Ogata, and H. Fukuyama, Phys. Rev. Lett. 102, 066601 (2009) and in preparation.
[3] H. Fukuyama, Y. Fuseya, and A. Kobayashi, “Perspectives of Mesoscopic Physics – Dedicated to Prof. Yoseph Imry's 70th Birthday”, World Scientific, p.69.

Invited Talk

Antoine Georges (École Polytechnique, Paris)
Strong correlations from Hund's rule coupling   

The proximity of a Mott insulating state (“Mottness”) is responsible for strong correlations in a number of materials, especially oxides of 3d transition metals. In this talk, I will emphasize that the Hund's rule coupling induces strong correlation effects in materials which are not close to a Mott insulator. This is especially relevant to oxides of 4d transition metals such as ruthenates, and also possibly to iron-based superconductors. The Hund's rule coupling will be shown to have antagonistic (“Janus-faced”) effects on the Mott gap and on the quasiparticle coherence scale. For Hund's correlated materials, the self-consistent embedded-atom construction at the heart of Dynamical Mean-Field Theory proves to be especially relevant.

References: J. Mravlje et al., Phys. Rev. Lett. 106, 096401 (2011); L. de'Medici et al., arXiv:1106.0815.

Invited Poster

Markus Greger (U Augsburg)
Kinks in the effective dispersion of the Emery model   

We consider the Emery model (a three-band p-d Hubbard model) in dynamical mean-field theory and study the dependence of kinks in the dispersion on different values of the oxygen-oxygen hopping integral tpp. Furthermore we discuss the role of the hybridization between the oxygen p and copper d bands (which produces the Zhang-Rice singlet) and the marked differences between electron- and hole-doped systems. We study dynamical quantities (Green function, self-energies, spin and charge susceptibilities) using the numerical renormalization group, which yields excellent spectra for the comparably low frequencies under consideration.

In collaboration with Marcus Kollar and Dieter Vollhardt.

Invited Poster

Zsolt Gulácsi (U Debrecen)
Exact multielectronic ground states for hexagon chains   

Using a method based on properties of positive semidefinite operators, exact multi-electronic ground states are deduced for hexagon chains. Polyacene, polyphenanthrene, and polyphenyl type of chains are analyzed by Hubbard type of models deducing ground states for different restricted regions of the parameter space in the interacting case, and physical properties of the deduced phases are analyzed.

In collaboration with R. Trencsényi and E. Kovács.

Invited Talk

Peter Hirschfeld (U Florida)
Gap structure and symmetry of Fe-based superconductors   

I discuss predictions for superconducting gap structure in Fe-pnictide and chalcogenide superconductors based on spin fluctuation pairing theory. While the interactions in all Fe-based systems are similar, there are many material-specific aspects of electronic structure which can strongly influence the order parameter. I focus in particular on the doping evolution of the gap, and on recent experiments and theories related to 3D gap structure.

Invited Talk

Walter Hofstetter (U Frankfurt)
Strong correlations and dynamics in ultracold atoms   

Cold atoms in optical lattices offer a new laboratory for the study of strong correlation phenomena, exotic phases and their quantum dynamics. I will focus on two recent developments:

i) We report the first detection of the Higgs-type amplitude mode using Bragg spectroscopy in a strongly interacting Bose condensate in an optical lattice. By the comparison of our experimental data with a spatially resolved, dynamical Gutzwiller calculation, we obtain good quantitative agreement. This allows for a clear identification of the amplitude mode, showing that it can be detected with full momentum resolution by going beyond the linear response regime.

ii) We study the properties of three-flavor fermions in an optical lattice, where new exotic quantum states such as color superfluids arise in partial analogy to Quantum Chromodynamics. Low-temperature properties of this system are addressed using DMRG and dynamical mean-field theory. We find a strong interplay between magnetization and color superfluidity.

Invited Talk

Masatoshi Imada (U Tokyo)
Electron-correlation physics of iron-based superconductors   

A la Multi-energy-scale Ab initio scheme for Correlated Electrons (MACE), effective low-energy models for iron-based superconductors have been derived. By solving and analyzing them by the dynamical mean-field theory as well as by the multi-variable variational Monte Carlo method, we show that unique interplay of orbital and spin crucially controlled by electron correlations determines the variety and systematic change in properties of the iron-based families.

Invited Talk

Mark Jarrell (Lousiana SU)
Quantum criticality and superconductivity in the Hubbard model   

In large scale dynamical cluster quantum Monte Carlo simulations of the two-dimensional (2D) Hubbard model with only nearest neighbor hopping, we find a quantum critical point (QCP) at finite doping separating a Fermi liquid region at low filling from a non-Fermi liquid pseudogap region near half-filling. Marginal Fermi liquid behavior is seen in the thermodynamics and single-particle properties for a wide range of doping and temperatures above the QCP. The QCP is due to the second-order terminus of a line of first order phase separation transitions that is driven to zero temperature as the next near-neighbor hopping t' vanishes. The superconducting dome surrounds the QCP. The proximity the QCP and the dome is due to an algebraic divergence, replacing the BCS log divergence, of the bare pairing polarization. This behavior is captured with a simple variation of the quantum critical BCS formalism, and can be traced to a van Hove singularity concomitant with the QCP.

Invited Poster

Anna Kauch (U Augsburg)
Mott-insulator and superfluid phases of correlated bosons in the bosonic dynamical mean-field theory with the strong coupling impurity solver   

We investigate the phase diagram of correlated lattice bosons using the bosonic dynamical mean field theory (BDMFT). The BDMFT, formulated by Byczuk and Vollhardt [Phys. Rev. B 77, 235106 (2008)], is a comprehensive and thermodynamically consistent approximation in which the normal and condensed bosons are treated on equal footing. Within BDMFT the lattice bosonic problem is replaced by a single impurity coupled to two bosonic baths (corresponding to normal and condensed bosons, respectively). The resulting set of equations, the so-called “impurity problem”, has to be solved self-consistently. Our approach is the strong coupling expansion within which the phase transition between Mott-insulating and superfluid phases can be described. Different thermodynamical quantities as well as the bosonic density of states are investigated across the transition lines.

Invited Talk

Stefan Kehrein (LMU Munich)
Weak interaction quenches and thermalization in quantum many-body systems   

I will review recent work on quantum quenches in the Hubbard model and in quantum impurity models. The interaction quench in the Hubbard model permits to explore the opposite limit of the adiabatic switching procedure in the Landau Fermi liquid paradigm. The real time evolution shows the phenomenon of prethermalization, which is related to a universal relation between nonequilibrium and equilibrium expectation values for weak interaction quenches. Based on the flow equation method we will also find universal relations for the work distribution function for quenches in quantum impurity systems.

Invited Talk

Gabriel Kotliar (Rutgers)
Iron pnictides and chalcogenides: A new class of strongly correlated metals   

The recently discovered iron-based superconductors provide a new challenge to electronic structure methods. In this talk I will highlight some LDA+DMFT based predictions, and how they are faring vis-à-vis experiments.

Invited Talk

Jan Kuneš (Academy of Sciences, Prague)
Ordering and spatial inhomogeneities in the vicinity of high-spin low-spin transitions   

The central theme of correlated electron physics is the competition between atomic like Mott state and the Fermi liquid state. In real materials with multiple orbitals it is possible to tune the system so that atomic states of different symmetry are almost degenerate. A typical example is a spin state transition driven by pressure. These additional degrees of freedom lead under certain circumstances to formation of long-range order or nanoscale inhomogeneities. We will present results of model calculations using the dynamical mean-field theory and discuss them in the context of LaCoO3 and its doped analogs.

Invited Poster

Christine A. Kuntscher (U Augsburg)
Pressure-induced electronic and structural phase transitions in TiOCl and TiOBr   

The titanium oxyhalides TiOCl and TiOBr are low-dimensional materials which undergo an unconventional transition to a spin-Peierls state via a structural incommensurate state. With the electronic configuration 3d1 these compounds are Mott-Hubbard insulators with a charge gap of approximately 2 eV. They have been discussed to exhibit a resonating valence bond state and high-temperature superconductivity upon doping. However, despite several attempts, up to now a metallization of TiOX (X = Cl, Br) upon doping was not successful.

Our recent pressure-dependent infrared spectroscopic investigations on TiOX [1] suggest that the application of external pressure is an alternative way to induce an insulator-to-metal transition in TiOX. By infrared microspectroscopy we could show the filling of the Mott-Hubbard gap at the critical pressure 16 GPa (14 GPa) for TiOCl (TiOBr) at room temperature. The possibility of a pressure-induced insulator-metal transition is, however, heavily debated [2,3]. Furthermore, there are inconsistencies regarding the experimental findings for the high-pressure crystal structure of TiOCl [3]. According to our pressure-dependent x-ray powder-diffraction data, the gap closure coincides with a structural phase transition, where the crystal structure changes from orthorhombic Pmmn to monoclinic P21/m with a 2a x 2b x c superstructure [4]. At pc2 ≈ 22 GPa a pressure-induced isostructural phase transition takes place for the monoclinic phase with anomalies in the lattice parameters [4].

[1] C. A. Kuntscher et al., Phys. Rev. B 74, 184402 (2006); C. A. Kuntscher et al., Phys. Rev. B 76, 241101 (R) (2007); C. A. Kuntscher et al., Phys. Rev. B 78, 035106 (2008).
[2] M. K. Forthaus et al., Phys. Rev. B 77, 165121 (2008).
[3] S. Blanco-Canosa et al., Phys. Rev. Lett. 102, 056406 (2009).
[4] J. Ebad-Allah et al., Phys. Rev. B 82, 134117 (2010).

In collaboration with J. Ebad-Allah1, A. Schönleber2, S. van Smaalen2, M. Hanfland3, M. Klemm1, S. Horn1, S. Glawion4, M. Sing4, R. Claessen4.
1Experimentalphysik 2, Universität Augsburg, D-86135 Augsburg, Germany
2Laboratory of Crystallography, Universität Bayreuth, D-95440 Bayreuth, Germany
3European Synchrotron Radiation Facility, BP 220, F-38043 Grenoble, France
4Experimentelle Physik 4, Universität Würzburg, D-97074 Würzburg, Germany

Financial support by the Deutsche Forschungsgemeinschaft is gratefully acknowledged.

Invited Talk (Industry Session)

Markus Laukamp (DONG Energy, Copenhagen)
Against the wind: What Europe's renewable targets mean for the offshore wind industry   

Offshore wind farms are an indispensable component of Europe's plans to meet its 2020 renewable energy targets. The expected contribution translates roughly to a tenfold expansion of the current installed base. In addition to the implied steep volume and construction rate increases the reserved locations are generally further offshore, which on one hand means better wind conditions but also makes installation and access to the sites harder. Finally the importance of offshore wind farms for the implementation of key political targets exposes the technology and its costs to stronger scrutiny by governments and the public.

As a consequence, the offshore wind industry needs to fundamentally change the way offshore wind farms are planned, financed, constructed and operated. Planning and developing of new sites will have to be drastically accelerated. In financing even the large energy companies' balance sheets are too small to take it all, so that the financial markets will have to be tapped. Installation will have to change character from project to production, and finally operations and maintenance will have to guarantee higher availability despite more difficult access to the sites. Success is not a given, the needed concepts are still under development, and the timeline is tight. Based on the status quo the largest challenges for expansion are identified along with approaches to address them. Finally the most crucial problems and preconditions for success will be spelled out.


Invited Poster

Ivan Leonov (U Augsburg)
Electronic correlations at the α-γ structural phase transition in paramagnetic iron   

We present an application of a novel ab initio approach to calculate the total energy of materials with correlated electrons [1]. It combines band structure and dynamical mean-field theory, and is implemented in terms of plane-wave pseudopotentials. Here we employ this computational scheme to study the equilibrium crystal structure and phase stability of paramagnetic iron at the α(bcc)-γ(fcc) phase transition as a function of temperature [2]. For this purpose we analyzed the energetics of the bcc-fcc lattice transformation in Fe using the Bain transformation path. We find that at ambient pressure the temperature of the bcc-fcc structural phase transition occurs at ≈200 K above the calculated Curie temperature. The structural optimization performed for paramagnetic Fe yields the correct lattice constants and predicts a 2% shrinking of the volume at the bcc-fcc phase transition. The magnetic correlation energy is found to be an essential driving force behind the bcc-fcc structural phase transition in paramagnetic iron. The phonon dispersion curves calculated for paramagnetic iron at the bcc-fcc structural phase transition show good agreement with experimental data.

[1] I. Leonov et al., Phys. Rev. Lett. 101, 096405 (2008); I. Leonov et al., Phys. Rev. B 81, 075109 (2010).
[2] I. Leonov et al., Phys. Rev. Lett. 106, 106405 (2011).

Invited Talk

Alexander I. Lichtenstein (U Hamburg)
Non-local correlation effects in crystals   

Dynamical mean field theory (DMFT) in combination with the first-principle LDA-scheme (LDA+DMFT) is an optimal starting point to go beyond static density functional approximation and include effects of local spin and orbital fluctuations. In order to go beyond the DMFT approximation we formulate a general multi-orbital dual-fermions scheme that include a full vertex of impurity problem as a effective interactions in non-local action. Different possibilities to calculate the interaction vertex as well as simple non-local scheme beyond the DMFT will be discussed.

Invited Talk

Alois Loidl (U Augsburg)

During the last decade the phenomenon of coexistence of magnetic and polar order gained considerable attention after reports of spin-driven ferroelectricity in perovskite manganites. In these systems ferroelectricity appears in magnetic phases with spiral or helical order where the spin structure breaks inversion symmetry. An inverse Dzyaloshinskii-Moriya interaction or spin currents inducing electric polarization have been identified as possible driving forces for this unusual ground state. Meanwhile different scenarios to establish coexisting magnetic and ferroelectric order have been established, ranging from systems where both phenomena seem to rather independent like in BiFeO3, spin-driven multiferroics where the spin structure constitutes a S=1/2 quantum spin chain, e.g. like in LiCu2O2 or in LiCuVO4 below 2.5 K, or systems where polar order is established via charge ordering like in Fe3O4. Recently a number of further systems with exotic new ground states have been identified, including the quantum spin-tube compound Sul-Cu2Cl4, where ferroelectricity appears at the magnetic field driven transition to an incommensurate chiral high-field phase which also constitutes a quantum critical point, but also in LiCuVO4, where a high-temperature, high-field polar phase exists where the spins obviously constitute a spiral spin liquid which gets partly ordered via an external field and probably can be described as nematic-like spiral spin liquid phase. In the last part of the talk we will focus on recent developments concerning the dynamics of multiferroics focusing mainly on electromagnons, excitations which can be described as magnons gaining polar weight via a strong interaction with phonons. The Lyddane-Sachs Teller relation will be discussed with respect to these new class of excitations in multiferroics.

Invited Talk

Jochen Mannhart (MPI-FKF Stuttgart)
Two-dimensional electron systems at oxide interfaces   

Two-dimensional electron gases based on conventional semiconductors such as Si or GaAs have played a pivotal role in fundamental science and technology. The high mobilities achieved enabled the discovery of the integer and fractional quantum Hall effects and are exploited in high electron mobility transistors. Recent work has shown that two-dimensional electron systems can also exist at oxide interfaces [1]. These electron systems are characterized by properties that fundamentally differ from those of semiconductor interfaces – they may, for example, form electron liquids and be superconducting as well as magnetic. In the presentation I will provide an overview of these exciting interfaces.

[1] A. Ohtomo et al., Nature 419, 378 (2002).

Invited Poster

Erwin Müller-Hartmann (U Cologne)
Hybridizing Wannier states for transition metal oxide chains and ladders   

The application to one-dimensional transition metal oxide systems of the concept of hybridizing Wannier states which was introduced by Zhang and Rice for CuO2-planes is considered. It is shown that realistic modeling of 1D systems tends to imply exponential localization of the Wannier states. This demonstrates that hybridizing Wannier states are useful for one-dimensional systems as well.

Invited Poster

Andrzej M. Oleś (MPI-FKF Stuttgart, Jagellonian U)
Entangled spin-orbital phases in the d9 model   

The phase diagram of the Kugel-Khomskii (d9) spin-orbital model for a bilayer depends on Hund's exchange JH and the eg orbital splitting Ez. In the (classical) mean-field approach with on-site spin ⟨ Szi⟩ and orbital ⟨ τzi⟩ order parameters and factorized spin-and-orbital operators we demonstrate a competition between the phases with either G-type or A-type antiferromagnetic (AF) or ferromagnetic (FM) long-range order. A more complete phase diagram is obtained using a Bethe-Peierls-Weiss method with a Lanczos exact diagonalization of a cube coupled to its neighbors in ab planes by the mean-field terms. We show that the long-range spin order is unstable in a large part of the phase diagram which contains then six phases, including also the valence-bond phase with interlayer spin singlets stabilized by holes in 3z2-r2 orbitals (VBz phase), a disordered plaquette valence-bond (PVB) phase and a crossover phase between the VBz and the A-type AF phase. When on-site spin-orbital coupling is also included by the ⟨ Sziτzi⟩ order parameter, we discover in addition two spin-disordered phases with spin-orbital entanglement [1]. They compete with A-type AF phase and another crossover phase in between the G-AF phase with occupied x2-y2 orbitals and the PVB phase. We analyze the order parameters in all phases and identify situations where spin-orbital entanglement is crucial and mean-field factorization of the spin and orbital degrees of freedom leads to qualitatively incorrect results [2]. We point out that spin-orbital entanglement may play a role in a bilayer fluoride K3Cu2F7 which is an experimental realization of the VBz phase.

[1] A. M. Oleś, P. Horsch, L. F. Feiner, and G. Khaliullin, Phys. Rev. Lett. 96, 147205 (2006).
[2] W. Brzezicki and A. M. Oleś, Phys. Rev. B 83, 214408 (2011).

In collaboration with Wojciech Brzezicki (Jagellonian U).

Invited Talk

Thomas Pruschke (U Göttingen)
Magnetism and superconductivity in the Kondo lattice model with phonons   

I present results for the Kondo lattice model within dynamical mean-field theory (DMFT). This model is one of the paradigms for understanding the physics of Heavy-Fermion (HF) materials. A particular interest will be laid on the identification of the low-energy scale in photoemission quantities and the influence of lattice degrees of freedom on it. HF compounds frequently exhibit magnetic order at low temperature, and quite often a quantum phase transition between the ordered state and the HF phase. While a characterization of quantum criticality within DMFT is surely not possible, one can at least discuss more principle questions like the actual nature of the ordered state (HF or local moment driven) and identify possible regimes in parameter space for quantum criticality if one allowed for additional fluctuations. I will discuss some of these aspects and in particular present evidence for a transition between a local-moment like antiferromagnet and an ordered state emerging from the HF. Adding phonons will open the possibility that the system undergoes a superconducting transition. The physical aspects of this transition, in particular whether it stays in direct competition with the Kondo effect will be discussed.

Invited Poster

Xinguo Ren (FHI Berlin)
Towards a general-purpose first principles method: A critical assessment of the random phase approximation and beyond   

Owing to its broad applicability and promise to overcome several intrinsic deficiencies of popular approximations to density-functional theory (DFT) (including local-density approximation, generalized gradient approximation, and hybrid functionals), the random phase approximation for the ground-state correlation energy (cRPA) in combination with exact exchange (EX) has brought DFT one step further towards a “general-purpose first principles method”. This is largely due to three attractive features: The exact-exchange energy (EX) cancels the spurious self-interaction error present in the Hartree energy exactly. The RPA correlation (cRPA) energy is fully non-local and includes long-range van der Waals (vdW) interactions automatically and highly accurately. Moreover, dynamic electronic screening is taken into account by summing up a sequence of “ring diagrams” to infinite order, which makes EX+cRPA applicable to small-gap or metallic systems. However, the standard RPA practice, i.e., evaluating both the EX and cRPA terms using single-particle orbitals from local, semi-local, or hybrid functionals, systematically underestimates bond strengths across a variety of systems. Two recent additions to cRPA have successfully ameliorated the underbinding problem: second-order screened exchange (SOSEX) [1,2] and renormalized single excitation corrections (RSE) [3]. From a diagrammatic point of view, SOSEX and RSE correspond to different types of many-body correlation terms that are, however, compatible with each other. In this work, we systematically benchmark the influence of SOSEX, RSE, and their combination on the atomization energies of the covalently bound G2 molecular set, the binding energies of the weakly bound S22 molecular set, and hydrogen-transfer and non-hydrogen-transfer reaction barrier heights (HTBH38/04 and NHTBH38/04 sets). We found that both SOSEX and RSE corrections to cRPA improve upon the notorious tendency of EX+cRPA to underbind. Surprisingly, reaction barrier heights obtained using EX+cRPA based on a KS reference alone are already remarkably accurate. EX+cRPA+SOSEX+RSE lives up to the challenge of providing a comparable level of accuracy for reaction barrier heights and overall gives the most balanced performance, which holds great promise for widespread application in the future.

[1] A. Grüneis et al., J. Chem. Phys.131, 154115 (2009).
[2] J. Paier et al., J. Chem. Phys. 132, 094103 (2010).
[2] X. Ren et al., Phys. Rev. Lett. 106, 153003 (2011).

In collaboration with Joachim Paier2, Patrick Rinke1, and Matthias Scheffler1.
1Fritz Haber Institute of the Max Planck Society, Berlin
2Institute for Chemistry, Humboldt University, Berlin

Invited Talk

T. Maurice Rice (ETH Zurich, Hong Kong U, Brookhaven NL)
Superconductivity in the pseudogap phase of underdoped cuprates   

Underdoped cuprates in the pseudogap phase are characterized by a variety of very anomalous properties. The proposals to explain these anomalies range from strong superconducting phase fluctuations due to the reduced phase stiffness, through exotic electronic liquid crystal phases to precursors to the Mott phase at stoichiometry. A phenomenological theory [1] for the latter was proposed several years ago has proved able to consistently describe many of these anomalies [2]. The standard superconductivity of the overdoped cuprates is modified in the pseudogap phase by a charge gap that opens in the antinodal regions of reciprocal space. As this charge gap opens in the superconductor a bound hole pair or cooperon can appear as a finite energy excitation. Coupling to the cooperons contributes to the superconductivity on the Fermi pockets in a similar way as in a recently analyzed model with lightly doped Hubbard ladders [3].

[1] K.-Y. Yang, T. M. Rice, and F. C. Zhang, Phys. Rev. B 73, 174501 (2006).
[2] T. M. Rice, K.-Y. Yang, and F. C. Zhang, Rep. Prog. Physics (in press).
[3] R. M. Konik, T. M. Rice, and A. M. Tsvelik, Phys. Rev. B 82, 054501 (2010).

Invited Talk

Achim Rosch (U Cologne)
Topological phases and spintorques in chiral magnets   

In chiral magnets, for example in MnSi, spins can form a lattice of magnetic vortices, a skyrmion lattice, similar to the vortex lattice of type II superconductors. The topology of the skyrmions leads to a very efficient coupling of magnetism and electric currents associated with Berry phases picked up by the electrons when their spins follows the magnetic texture. The magnetic structure is affected by current densities almost six orders of magnitude smaller than typically used in spin-torque experiments. We discuss the theoretical concepts underlying the formation of skyrmions and their coupling to currents.

Invited Talk

George A. Sawatzky (U British Columbia)
Importance of explicit inclusion of O 2p states in model Hamiltonians of the high-Tc cuprates   

We report on a recent study of the importance of explicit inclusion of the O 2p orbitals in model Hamiltonians for the High-Tc Cuprate superconductors. In this exact diagonalization study of large clusters containing up to 32 Cu and 64 O sites and zero one or two holes we find that the so called 3 spin polaron model Hamiltonian which contains spins on Cu and mobile holes on O 2p orbitals contains new physics not represented by a single band Hubbard or a t,t',J model. We derive an appropriate Hamiltonian for this model which contains all the ingredients for differentiating between the Zhang-Rice singlet model and the 3 spin polaron models. We find that there is a strong ferromagnetic correlation between Cu spins sandwiching an O 2p hole and longer range ferromagnetic spin correlations not present in the single band or t,t',J models. These correlations lead to strong reduction of the quasi particle spectral weight with zero weight for momenta (π,0), (π,π), and (0,0). In fact the lowest energy quasiparticle in certain regions of momentum space have a spin of 3/2 and are not reachable by single electron removal from a spin 0 quantum ground state. We also report on the two hole dispersion and binding energies and speculate on the importance for the higher doping range and pairing.

In collaboration with Bayo Lau and Mona Berciu.

Invited Poster

Kurt Scharnberg (U Hamburg)
d-wave superconductors with finite range impurities of arbitrary strength   

Disorder is ubiquitous in solid state physics and is often invoked to explain observations that one does not otherwise understand. But do we understand disorder? The frequently used zero range potentials (δ-functions) of arbitrary strength do not lead to scattering in two or more dimensions! Only when one ignores the divergent real part of the Green function, δ-function potentials give not unreasonable results. However, unitary scattering is only obtained for infinitely strong potentials. When the momentum dependence of the scattering potential is taken seriously, a large self energy at zero frequency – leading to midgap states – can be found for reasonable values of matching ranges and potential strengths. For given impurity concentration, the size of the self energy can exceed the s-wave result in the normal state obtained from the δ-function approach by factors 2 - 5, depending on the strength of the potential, when several scattering channels are included. When the potential is so weak, that the Born approximation is applicable, a large impurity concentration is required to produce measurable effects. But then the overlap of scattering potentials cannot be ignored, which requires a different theoretical approach. We shall discuss the dependencies of the three self energy parts proportional to the Pauli matrices σl, l = 0,1,3, on the various parameters. The focus is on fairly small concentrations of reasonably strong scattering potentials with small but finite ranges. All three self energy parts need to be included in the calculation of the momentum and energy dependence of the spectral function as measured in ARPES. Provided these measurements are sensitive enough and are performed at sufficiently low temperatures to freeze out inelastic scattering, observation of the pronounced frequency dependence of the quasi-particle scattering rates in the superconducting state would give valuable information on characteristics of the disorder present. The effect of range and strength of the scattering potential on the transition temperature of a d-wave superconductor will also be discussed.

In collaboration with Carsten T. Rieck.

Invited Talk

Jörg Schmalian (KIT, Karlsruhe)
Impurity induced superconductivity and charge Kondo effect in PbTe   

We present a theory of superconductivity in systems with impurities undergoing resonant quantum valence fluctuations. We find superconductivity induced by charge Kondo impurities, study how pairing of a superconducting host is enhanced due to charge Kondo centers and investigate the interplay between Kondo-scattering and inter-impurity Josephson coupling. We discuss the implications of our theory for Tl-doped PbTe.

Invited Poster

Michael Sekania (U Augsburg)
Real-time dynamics of charge and spin densities in one-dimensional strongly correlated systems   

We investigate the space-time evolution of the charge and spin densities induced by adding a single electron to the ground state of the Hubbard model in the Mott-insulating phase or in the metallic phase close to it. In the first case, where the Mott-insulating ground state serves as a ``host'' system, we observe ballistic spreading of the induced spin and charge densities. The speed of the charge density propagation is only slightly enhanced in comparison to the non-interacting case. In contrast, the speed of the spin density spreading is heavily influenced by the interaction strength. Particularly interesting is the second case (close to half-filling), where the spreading of the induced charge perturbation changes form the ballistic into the diffusive and then into the subdiffusive regime. Numerical simulations are performed using the adaptive time-dependent Density-Matrix Renormalization Group (t-DMRG).

In collaboration with A. P. Kampf.

Invited Talk

B. Sriram Shastry (UC Santa Cruz)
Extreme correlations view of angle-resolved photoemission in high-Tc systems   

A recent theoretical approach to the t-J model gives a remarkably good account of the unusual line shapes encountered in ARPES of optimally doped high-Tc systems, both with conventional light sources and laser sources. I will present the essentials of the new approach highlighting the “Extremely Correlated Fermi Liquid.”

Invited Talk

Qimiao Si (Rice U)
Electron correlations and superconductivity in iron pnictides and selenides   

I will summarize the description of iron pnictides as bad metals close to a Mott transition [1]. As a particular consequence, isoelectronic substitution that increases the kinetic energy will suppress antiferromagnetic order and give rise to a quantum critical point; this is by now verified by experiments in P doped compounds of parent iron arsenides. Within the same framework, I will also consider the implications of the recently discovered 122 iron selenides for Mott transition and local-moment magnetism [2]. Finally, I will discuss how this description makes it natural that the pairing strength of the 122 iron selenides is similar to that of their pnictides counterparts in spite of very different Fermi surfaces [3].

[1] J. Dai et al, Proc. Nat. Acad. Sci. 106, 4118 (2009); Q. Si and E. Abrahams, Phys. Rev. Lett. 101, 076401 (2008).
[2] R. Yu, J.-X. Zhu, and Q. Si, Phys. Rev. Lett. 106, 186401 (2011); R. Yu, P. Goswami, and Q. Si, arXiv:1104.1445.
[3] R. Yu et al, arXiv:1103.3259; P. Goswami, P. Nikolic, and Q. Si, EPL 91, 37006 (2010).

Invited Talk

Manfred Sigrist (ETH Zurich)
Topological and symmetry aspects of chiral p-wave pairing in Sr2RuO4-Ru eutectics   

There is good evidence for the realization of chiral p-wave pairing in Sr2RuO4, a pairing state analogous to the A-phase of superfluid He-3. In Sr2RuO4-Ru eutectic samples, including micrometer-size Ru-metal inclusions, it was found that inhomogeneous superconductivity appears at a temperature roughly twice the bulk transition temperature. The nature of this superconducting phase – called 3-Kelvin phase – is filamentary, most likely due to the nucleation at the interface between Sr2RuO4 and the Ru-inclusions. We show here that the path to bulk superconductivity here requires a further symmetry breaking transition which at the same time involves a change of the topology of the filamentary phase. The presence of such a transition is consistent with experiments on the critical current and quasiparticle tunneling in the 3-Kelvin phase. Very recent experiments on the Josephson effect between a Pb-film connected through a Ru-inclusion to Sr2RuO4 shows an anomalous temperature dependence of the Josephson critical current, with a rather sharp drop at the bulk Tc of Sr2RuO4. We show that the Josephson coupling of s-wave superconductivity (Pb-Ru) to the chiral p-wave state indeed leads to phase frustration which limits the critical current through the appearance of spontaneous flux pattern. We argue that these properties may be responsible for the observed features.

Invited Poster

Michael Stark (U Augsburg)
Two-stage thermalization in weakly correlated electrons   

We investigate the time evolution of quantum many-body systems after a sudden switch of a two-body interaction, starting from a noninteracting ground state. Such systems do not thermalize directly, but rather first attain an intermediate quasi stationary state. After being trapped in this “prethermalized” state for some time, the system relaxes further to a thermal state. Here we analyze this thermalization process by deriving a quantum Boltzmann equation for the momentum distribution. We evaluate this equation for the Hubbard Model in infinite dimensions and compare with exact numerical results. We find that the quantum Boltzmann equation provides a quantitative description of the transient behavior and the thermalization on long time scales.

In collaboration with Marcus Kollar.

Invited Talk

Frank Steglich (MPI-CPfS Dresden)
Interplay of superconductivity, quantum criticality and f-electron localization in rare-earth based 122 systems   

Both CeCu2Si2 and YbRh2Si2 crystallize in the tetragonal ThCr2Si2 structure. Recent neutron-scattering results on normal-state CeCu2Si2 reveal a slowing down of the quasielastic response which conforms to the scaling expected for a quantum critical point (QCP) of itinerant, i.e., three-dimensional spin-density-wave (SDW) type. This interpretation is in full agreement with the non-Fermi-liquid behavior observed in transport and thermodynamic measurements. The momentum dependence of the magnetic excitation spectrum reveals to branches of an overdamped dispersive mode whose coupling to the heavy charge carriers is strongly retarded. These overdamped spin fluctuations are considered the driving force for superconductivity in CeCu2Si2 (Tc = 600 mK).

The weak antiferromagnet YbRh2Si2 (TN = 70 mK) exhibits a magnetic-field induced QCP at BN = 0.06 T (B ⊥ c) but no superconductivity above T = 10 mK. The magnetic QCP appears to concur with a breakdown of the Kondo effect as concluded from, e.g., the field dependencies of both Hall coefficient and thermoelectric power. Doping-induced variations of the average unit-cell volume result in a detachment of the magnetic and electronic instabilities.

Work done in collaboration with J. Arndt, M. Brando, S. Friedemann, P. Gegenwart, C. Geibel, S. Hartmann, H. S. Jeevan, S. Kirchner, C. Krellner, M. Loewenhaupt, Q. Si, O. Stockert, T. Westerkamp, S. Wirth, and G. Zwicknagl.

Invited Talk (Industry Session)

Rainer Strack (BCG)
From strongly correlated electron systems to strategy consulting: 17 years as a physicist at The Boston Consulting Group   

An overview of a life at BCG will be given with project highlights at large international companies: From my first assignment for a mechanical engineering company in Karlsruhe in 1994, over new shareholder value management theories to my presentation about Global Talent Risk at the World Economic Forum in Davos in 2011.

Invited Talk

Liu Hao Tjeng (MPI-CPfS Dresden)
Surprising cobaltites: A spectroscopic perspective   

The class of cobalt-oxide based materials has attracted increasing interest in the last decade. A key aspect of the cobaltites that distinguishes them clearly from the Cu, Ni, and Mn oxides is the spin state degree of freedom of the Co3+ and Co4+ ions: the ions can be low spin, high spin, and perhaps even intermediate spin. This aspect comes on top of the orbital and charge degrees of freedom that already make the Cu, Ni, Mn systems so exciting. It is, however, also precisely this aspect that causes considerable debate in the literature. In this presentation we would like to show how synchrotron based soft-x-ray spectroscopies can successfully resolve the local electronic structure of the Co ions and thus contribute to a better understanding of the physical properties of the cobaltites. In particular, we will address the issue of spin state transitions, metal insulator transitions and the newly proposed spin-blockade phenomenon in several layered cobalt materials. The results also call for the application of LDA+DMFT type of approaches in order to provide a more accurate description of the electronic structure of these materials than what is possible so far on the basis of standard band structure calculations.

Invited Talk

Kazuo Ueda (U Tokyo)
Kondo effects of a magnetic ion vibrating in a metal   

Recently various materials which involve cage structures have been synthesized. Typical examples of this class of materials include skutterudites, clathrates and β-pyrochlore compounds. When some ions are contained in the cage structures, we may expect new interesting physics associated with the vibrational modes of ions. In this presentation I will discuss effects of electron correlations when the ions are magnetic. A generalized Anderson model is formulated, which takes account of the vibrational degrees of freedom. Accompanied by creation or absorption of the vibrational modes new hybridization channels open. Interplay between the conventional hybridization and phonon-assisted one produces interesting physics which span among the conventional s-wave Kondo effect, the phonon-assisted p-wave Kondo effect and the Yu-Anderson type Kondo effect.

Invited Talk

Götz Uhrig (TU Dortmund)
Kinks in the electronic dispersion of strongly correlated models: Physical origin of a generic feature   

We study kinks in the electronic dispersion of a generic strongly correlated system by dynamic mean-field theory (DMFT) using three different algorithms to clearly distinguish physical features from numerical artefacts. The focus is on doped systems where no particle-hole symmetry holds and valence fluctuations matter potentially.

Our findings extend the picture that the kinks reflect the coupling of the fermionic quasiparticles to emergent collective modes, namely the spin fluctuations, beyond half-filling to finite doping. The energies of the kinks and their doping dependence fit well to the kinks in the cuprates, which is surprising in view of the spatial correlations neglected by DMFT.

Invited Talk

Roser Valentí (U Frankfurt)
Two-orbital Hubbard model: Single-site versus cluster dynamical mean-field theory and applications   

We shall discuss the anisotropic two-orbital Hubbard model with different bandwidths and degrees of frustration in each orbital in the framework of both single-site dynamical mean-field theory (DMFT) as well as its cluster extension (DCA) for clusters up to four sites combined with a continuous-time quantum Monte Carlo algorithm. This model shows a rich phase diagram which includes the appearance of orbital selective phase transitions, non-Fermi liquid behavior as well as antiferromagnetic metallic states [1,2,3]. We will argue that such a minimal model may be of relevance to understand the nature of the antiferromagnetic metallic state in the iron-pnictide superconductors as well as the origin of the small staggered magnetization observed in these systems [2,3].

[1] H. Lee, Y.-Z. Zhang, H. O. Jeschke, R. Valentí, and H. Monien, Phys. Rev. Lett 104, 026402 (2010).
[2] H. Lee, Y.-Z. Zhang, H. O. Jeschke, and R. Valentí, Phys. Rev. B 81, 220506(R) (2010).
[3] H. Lee, Y.-Z. Zhang, H. O. Jeschke, and R. Valentí, Phys. Rev. B (R) in press (2011).

Invited Talk

Matthias Vojta (TU Dresden)
Cubic interactions and quantum criticality in dimerized antiferromagnets   

In certain Mott-insulating dimerized antiferromagnets, triplet excitations of the paramagnetic phase can decay into the two-particle continuum. When such a magnet undergoes a quantum phase transition into a magnetically ordered state, this coupling becomes part of the critical theory provided that the lattice ordering wavevector is zero. One microscopic example is the staggered-dimer antiferromagnet on the square lattice, for which deviations from O(3) universality have been reported in numerical studies. Using both symmetry arguments and microscopic calculations, we show that a non-trivial cubic term arises in the relevant order-parameter quantum field theory, and assess its consequences using a combination of analytical and numerical methods. All data can be consistently interpreted in terms of critical exponents identical to that of the standard O(3) universality class, but with anomalously large corrections to scaling. We argue that the two-particle decay of critical triplons, although irrelevant in two spatial dimensions, is responsible for the leading corrections to scaling due to its small scaling dimension. Thus we conclude that two distinct universality classes exist for the zero-field ordering transition of coupled-dimer magnets.

Invited Talk

Hilbert v. Löhneysen (KIT, Karlsruhe)
Experimental studies of systems with weak and strong electronic correlations in the vicinity of metal-insulator transitions: Doped semiconductors and manganates   

The metal-insulator (MI) transition of weakly correlated Si:P can be tuned by varying the P concentration or – for barely insulating samples – by application of uniaxial stress S. On-site Coulomb interactions lead to the formation of localized magnetic moments and the Kondo effect on the metallic side, and to a Hubbard splitting of the donor band on the insulating side. Continuous stress tuning allows the observation of dynamic scaling of the conductivity σ(T,S). The issue of half-filling vs. away from half-filling of the donor band (i.e., uncompensated vs. compensated semiconductors) is discussed in detail.

Transition-metal oxides are paradigms of strongly interacting systems. The MI transition in doped manganates, e.g., in La1-xSrxMnO3 gives rise to the colossal magnetoresistance (CMR). In contrast, Pr1-xCaxMnO3 does not exhibit a MI transition for any x at zero magnetic field. However, a MI transition can be induced, e.g., at x = 0.3, via negative chemical pressure by replacing Pr3+ by the larger La3+ ions. We have performed a detailed study of Pr1-xCaxMnO3 and (Pr1-yLay)0.7Sr0.3MnO3 of the near-edge x-ray absorption in order to determine the electronic and magnetic structures of these systems.

Invited Poster

Werner Weber (TU Dortmund)
Multi-band Gutzwiller theory for iron pnictides   

For the iron pnictide LaOFeAs we investigate multi-band Hubbard models which are assumed to capture the relevant physics [1,2]. In our calculations, we employ the Gutzwiller variational theory which is a genuine many particle approach [3]. We will present results both on the paramagnetic and antiferromagnetic phases of our model systems. These results show that a five band-model is not adequate to capture the relevant physics in LaOFeAs [4]. However, our results for the eight band-model which includes the arsenic 4p bands reproduce the experimental data, especially the magnetic moment, for a broad parameter regime.

[1] S. Graser, T. A. Maier, P. J. Hirschfeld, and D. J. Scalapino, New J. Phys. 11, 025016 (2009).
[2] O. K. Andersen and L. Boeri, Annalen der Physik 523, 8 (2011).
[3] J. Bünemann, F. Gebhard, and W. Weber, in: A. Narlikar (ed.), Frontiers in Magnetic Materials, Springer, Berlin, 2005.
[4] T. Schickling, F. Gebhard, and J. Bünemann, Phys. Rev. Lett. 106, 146402 (2011).

In collaboration with Tobias Schickling1, Lilia Boeri2, Jörg Bünemann3, Florian Gebhard1, Ole Krogh Andersen2.
1Fachbereich Physik, Philipps Universität, D-35037 Marburg, Germany
2Max Planck Institute for Solid State Research, D-70569 Stuttgart, Germany
3Institut für Physik, BTU Cottbus, D-03013 Cottbus, Germany

Invited Talk

Peter Wölfle (KIT, Karlsruhe)
Quasiparticles beyond the Fermi liquid and heavy fermion criticality   

The concept of Landau quasiparticles in a Fermi liquid is extended into the non-Fermi liquid regime of heavy Fermion compounds near a magnetic quantum phase transition. It is shown how the critical behavior of the quasiparticles (diverging effective mass) induced by antiferromagnetic fluctuations acts back on the spin fluctuation spectrum. A self-consistent equation for the effective mass has, in the case of 3d AFM fluctuations, a new strong coupling solution characterized by fractional power laws and scaling behavior [1]. The theory accounts well for many of the observed properties of YbRh2Si2.

[1] P. Wölfle and E. Abrahams, arXiv:1102.3391.

Invited Poster

Unjong Yu (Gwangju IST)
Nonlocal effects on ferromagnetism in a diluted magnetic semiconductor system   

Nonlocal effects play a crucial role in some magnetic materials such as the diluted magnetic semiconductor (DMS) systems. We studied the magnetic properties of the diluted magnetic semiconductor Ga1-xMnxAs within the dynamical cluster approximation (DCA), which is a cluster extension of the dynamical mean-field theory (DMFT). We show that nonlocal effects are essential for explaining the experimentally observed transition temperature, saturation magnetization, and magnetic anisotropy of this system. In addition, we propose a new kind of magnetic anisotropy (cluster anisotropy) to explain the temperature-driven spin reorientation in this system.