arXiv subjects

G. Zwicknagl

Publications and source records attributed to G. Zwicknagl.

27 records · Page 2Linked to original sources

arXiv2005

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 ~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.

cond-mat.str-el↗

arXiv2004

Superconductivity in heavy fermion compounds

We review the current state of experimental and theoretical investigations of heavy fermion superconductors. We discuss most of the Ce-based compounds like Ce122, Ce115, Ce218 and Ce131 classes and U-based superconductors like UBe_13 and UPd_2Al_3. In the former the emphasis is on the connection to quantum critical phenomena and non-Fermi liquid behaviour. Recent neutron scattering and hydrostatic pressure results on SDW/SC competition in the Ce122 system are included. For the U-compounds we discuss the significance of dual models with both localised and itinerant 5f electrons for mass enhancement and superconducting pair formation. Itinerant spin fluctuation theories for unconventional superconductivity are also reviewed.

cond-mat.str-el↗

arXiv2004

Approximative treatment of 5f-systems with partial localization due to intra-atomic correlations

Increasing experimental and theoretical evidence points towards a dual nature of the 5$f$ electrons in actinide-based strongly correlated metallic compounds, with some 5$f$ electrons being localized and others delocalized. In a recent paper (PRB xxx, 2004), we suggested the interplay of intra-atomic correlations as described by Hund's rules and a weakly anisotropic hopping (hybridization) as a possible mechanism. The purpose of the present work is to provide a first step towards a microscopic description of partial localization in solids by analyzing how well various approximation schemes perform when applied to small clusters. It is found that many aspects of partial localization are described appropriately both by a variational wavefunction of Gutzwiller type and by a treatment which keeps only those interactions which are present in LDA+U calculations. In contrast, the energies and phase diagram calculated within the Hartree Fock approximation show little resemblence with the exact results. Enhancement of hopping anisotropy by Hund's rule correlations are found in all approximations.

cond-mat.str-el↗

arXiv2003

On a dual model for 5f electrons

We study the effect of intra-atomic correlations on anisotropies of hopping of 5f electrons. It is shown that they may considerably enhance these anisotropies to the extend that electrons in some of the 5f orbitals remain localized. This provide a microscopic basis for a previously made assumption that some of the 5f electrons must be partially localized when Fermi surfaces and effective masses of the component are calculated. Calculations are performed on two- and three-sites clusters. They include phase diagrams in the presence and absence of an external magnetic field as function of anisotropic hopping.

cond-mat↗

arXiv2003

Fermi surface and heavy masses for UPd$_2$Al$_3$

We calculate the Fermi surface and the anisotropic heavy masses of UPd2Al3 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.

cond-mat.str-el↗

arXiv2003

Kondo Effect in a Metal with Correlated Conduction Electrons: Diagrammatic Approach

We study the low-temperature behavior of a magnetic impurity which is weakly coupled to correlated conduction electrons. To account for conduction electron interactions a diagrammatic approach in the frame of the 1/N expansion is developed. The method allows us to study various consequences of the conduction electron correlations for the ground state and the low-energy excitations. We analyse the characteristic energy scale in the limit of weak conduction electron interactions. Results are reported for static properties (impurity valence, charge susceptibility, magnetic susceptibility, and specific heat) in the low-temperature limit.

cond-mat.str-el↗

arXiv2001

Structural and electronic properties of an azamacrocycle, C26H18N6

We compute the structure of an azamacrocycle, C26H18N6. Two approximatively planar elliptical structures with C2 or CI symmetry are found to be nearly degenerate. The roughly circular conformation observed in metal complexes turns out to be ~ 0.6 eV higher in energy. We suggest that this difference is mainly due to electrostatic interactions. We discuss the results on various levels of theory (Hartree-Fock, local density and gradient corrected density functional calculations).

cond-mat↗

arXiv2001

Origin of heavy quasiparticles in UPt_3

We propose a microscopic description of heavy fermions in UPt_3. It is based on the assumption that two of the three 5f electrons of the U ions are localized. Band-structure calculations based on this supposition reproduce the observed de Haas-van Alphen frequencies very well. The observed enhancement of the quasiparticle mass as compared with the LDA band mass results from the local Coulomb interaction of the delocalized f electrons with the localized ones. Diagonalization of the Coulomb interaction matrix yields the level scheme of the localized f^2 states. Assuming a splitting of the ground-state doublet by the crystal field of similar size as in UPd_2Al_3 results in a mass enhancement factor of order 10 in agreement with experiments.

cond-mat↗

arXiv1997

Kondo Effect in a Metal with Correlated Conduction Electrons

The large-degeneracy expansion for dilute magnetic alloys is extended to account for conduction electrons interactions. Particular attention is paid to the renormalization of the hybridization vertex which affects the low-energy excitations. As a first example, we calculate the enhanced characteristic energy $k_BT_0$ in the limit of weakly correlated conduction electrons. The metallic regime with strongly correlated electrons is discussed.

cond-mat.str-el↗