Gertrud Zwicknagl

Calorimetric Evidence for a Fulde-Ferrell-Larkin-Ovchinnikov Superconducting State in the Layered Organic Superconductor κ − ( BEDT − TTF ) 2 Cu ( NCS ) 2

The specific heat of the layered organic superconductor kappa-(BEDT-TTF)(2)Cu(NCS)(2), where BEDT-TTF is bisethylenedithio-tetrathiafulvalene, has been studied in magnetic fields up to 28 T applied perpendicular and parallel to the superconducting layers. In parallel fields above 21 T, the superconducting transition becomes first order, which signals that the Pauli-limiting field is reached. Instead of saturating at this field value, the upper-critical-field increases sharply and a second first-order transition line appears within the superconducting phase. Our results give strong evidence that the phase, which separates the homogeneous superconducting state from the normal state is a realization of a Fulde-Ferrell-Larkin-Ovchinnikov state.

Emerging local Kondo screening and spatial coherence in the heavy-fermion metal YbRh2Si2

The entanglement of quantum states is both a central concept in fundamental physics and a potential tool for realizing advanced materials and applications. The quantum superpositions underlying entanglement are at the heart of the intricate interplay of localized spin states and itinerant electronic states that gives rise to the Kondo effect in certain dilute magnetic alloys. In systems where the density of localized spin states is sufficiently high, they can no longer be treated as non-interacting; if they form a dense periodic array, a Kondo lattice may be established. Such a Kondo lattice gives rise to the emergence of charge carriers with enhanced effective masses, but the precise nature of the coherent Kondo state responsible for the generation of these heavy fermions remains highly debated. Here we use atomic-resolution tunnelling spectroscopy to investigate the low-energy excitations of a generic Kondo lattice system, YbRh2Si2. We find that the hybridization of the conduction electrons with the localized 4f electrons results in a decrease in the tunnelling conductance at the Fermi energy. In addition, we observe unambiguously the crystal-field excitations of the Yb3+ ions. A strongly temperature-dependent peak in the tunnelling conductance is attributed to the Fano resonance resulting from tunnelling into the coherent heavy-fermion states that emerge at low temperature. Taken together, these features reveal how quantum coherence develops in heavy 4f-electron Kondo lattices. Our results demonstrate the efficiency of real-space electronic structure imaging for the investigation of strong electronic correlations, specifically with respect to coherence phenomena, phase coexistence and quantum criticality.

Fermi surface and heavy masses for UPd2Al3

We calculate the Fermi surface and the anisotropic heavy masses of UPd 2 Al 3 by keeping two of the 5f electrons as localized. Good agreement with experiments is found. The theory contains essentially no adjustable parameter except for a small shift of the position of the Fermi energy of the order of a few meV. A discussion is given why localization of two f electrons is justified.

The dual nature of 5f electrons and the origin of heavy fermions in U compounds

We develop a theory for the electronic excitations in UPt3 which is based on the localization of two of the 5f electrons. The remaining f electron is delocalized and acquires a large effective mass by inducing intra-atomic excitations of the localized ones. The measured de Haas–van Alphen frequencies of the heavy quasiparticles are explained as well as their anisotropic heavy mass. A model calculation for a small cluster reveals why only the largest of the different 5f hopping matrix elements is operative, causing the electrons in other orbitals to localize.

Feedback Spin Resonance in Superconducting CeCu2Si2 and CeCoIn5

We show that the recently observed spin resonance modes in heavy-fermion superconductors CeCoIn5 and CeCu2Si2 are magnetic excitons originating from superconducting quasiparticles. The wave vector Q of the resonance state leads to a powerful criterion for the symmetry and node positions of the unconventional gap function. The detailed analysis of the superconducting feedback on magnetic excitations reveals that the symmetry of the superconducting gap corresponds to a singlet d_{x;{2}-y;{2}} state symmetry in both compounds. In particular this resolves the long-standing ambiguity of the gap symmetry in CeCoIn5. We demonstrate that in both superconductors the resonance peak shows a significant dispersion away from Q which can be checked experimentally. Our analysis reveals the similar origin of the resonance peaks in the two heavy-fermion superconductors and in layered cuprates.

Strongly correlated electrons

Electron correlations are strong when the on-site electron-electron repulsions U are much larger than the energies associated with the overlap of atomic orbitals belonging to different atoms (resonance energies). The latter are characterized in a solid by the width W of the energy band under consideration. A large ratio U/W is expected in systems involving well-localized 4f or 5f electrons, i.e., rare earth or actinide atoms. Yet systems with d electrons can be strongly correlated, too. A famous example is CoO. If we treated it within the independent electron approximation we would find that this substance is metallic, with an odd number of electrons per unit cell and a partially filled d band. In reality, however, CoO is an insulator. The same holds true for La2CuO4, a prototype for a class of materials with high superconducting transition temperatures (Sect. 14.2). CoO and La2CuO4 are not metallic because the strong electron correlations suppress the charge fluctuations required for a nonvanishing conductivity. Instead they are insulators.

Interplay between Kondo suppression and Lifshitz transitions in YbRh2Si2 at high magnetic fields.

We investigate the magnetic field dependent thermopower, thermal conductivity, resistivity, and Hall effect in the heavy fermion metal YbRh2Si2. In contrast to reports on thermodynamic measurements, we find in total three transitions at high fields, rather than a single one at 10 T. Using the Mott formula together with renormalized band calculations, we identify Lifshitz transitions as their origin. The predictions of the calculations show that all experimental results rely on an interplay of a smooth suppression of the Kondo effect and the spin splitting of the flat hybridized bands.

Hall effect measurements and electronic structure calculations on YbRh2Si2 and its reference compounds LuRh2Si2 and YbIr2Si2

Sven Friedemann, Steffen Wirth, Niels Oeschler, Cornelius Krellner, Christoph Geibel, Frank Steglich, Sam MaQuilon, Zachary Fisk, Silke Paschen, and Gertrud Zwicknagl Max Planck Institute for Chemical Physics of Solids, Nöthnitzer Strasse 40, 01187 Dresden, Germany Department of Physics and Astronomy, University of California, Irvine, CA 92697-4575, USA Institute of Solid State Physics, TU Vienna, Wiedner Hauptstraße 8-10, 1040 Vienna, Austria Institut für Mathematische Physik, TU Braunschweig,

Magnetic-field-induced band-structure change in CeBiPt.

We report on a field-induced change of the electronic band structure of CeBiPt as evidenced by electrical-transport measurements in pulsed magnetic fields. Above approximately 25 T, the charge-carrier concentration increases nearly 30% with a concomitant disappearance of the Shubnikov-de Haas signal. These features are intimately related to the Ce 4f electrons since for the non-4f compound LaBiPt the Fermi surface remains unaffected. Electronic band-structure calculations point to a 4f-polarization-induced change of the Fermi-surface topology.

Similar temperature scale for valence changes in Kondo lattices with different Kondo temperatures

The Kondo model predicts that both the valence at low temperatures and its temperature dependence scale with the characteristic energy TK of the Kondo interaction. Here, we study the evolution of the 4f occupancy with temperature in a series of Yb Kondo lattices using resonant X-ray emission spectroscopy. In agreement with simple theoretical models, we observe a scaling between the valence at low temperature and TK obtained from thermodynamic measurements. In contrast, the temperature scale Tv at which the valence increases with temperature is almost the same in all investigated materials while the Kondo temperatures differ by almost four orders of magnitude. This observation is in remarkable contradiction to both naive expectation and precise theoretical predictions of the Kondo model, asking for further theoretical work in order to explain our findings. Our data exclude the presence of a quantum critical valence transition in YbRh2Si2.The competition between interactions promoting magnetic order and those suppressing magnetism causes unusual electronic behaviour in Kondo lattice materials. Here, the authors show the energy scale for valence fluctuations is not controlled by the Kondo scale, contrary to expectations from single-site models.