115 research outputs found
Fermi surface and heavy masses for UPdAl
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.Comment: 4 pages, 2 figure
Moment screening in the correlated Kondo lattice model
The magnetic correlations, local moments and the susceptibility in the
correlated 2D Kondo lattice model at half filling are investigated. We
calculate their systematic dependence on the control parameters J_K/t and U/t.
An unbiased and reliable exact diagonalization (ED) approach for ground state
properties as well as the finite temperature Lanczos method (FTLM) for specific
heat and the uniform susceptibility are employed for small tiles on the square
lattice. They lead to two major results: Firstly we show that the screened
local moment exhibits non-monotonic behavior as a function of U for weak Kondo
coupling J_K. Secondly the temperature dependence of the susceptibility
obtained from FTLM allows to extract the dependence of the characteristic Kondo
temperature scale T* on the correlation strength U. A monotonic increase of T*
for small U is found resolving the ambiguity from earlier investigations. In
the large U limit the model is equivalent to the 2D Kondo necklace model with
two types of localized spins. In this limit the numerical results can be
compared to those of the analytical bond operator method in mean field
treatment and excellent agreement for the total paramagnetic moment is found,
supporting the reliability of both methods.Comment: 19 pages, 9 figure
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.Comment: 74 pages, 29 figures. For a version of the manuscript including
higher-resolution figures, see http://www.cpfs.mpg.de/~thalm/SCMaterials.pd
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 electrons in actinide-based strongly correlated metallic compounds,
with some 5 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.Comment: 9 pages, 9 figure
Feedback spin resonance in superconducting CeCuSi and CeCoIn
We show that the recently observed spin resonance modes in heavy-fermion
superconductors CeCoIn and CeCuSi are magnetic excitons originating
from superconducting quasiparticles. The wave vector 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 state symmetry in
both compounds. In particular this resolves the long-standing ambiguity of the
gap symmetry in CeCoIn. We demonstrate that in both superconductors the
resonance peak shows a significant dispersion away from that should
be observed experimentally. Our results suggest a unifying nature of the
resonance peaks in the two heavy-fermion superconductors and in layered
cuprates.Comment: 5 pages, 4 figure
Spectral functions for strongly correlated 5f-electrons
We calculate the spectral functions of model systems describing 5f-compounds
adopting Cluster Perturbation Theory. The method allows for an accurate
treatment of the short-range correlations. The calculated excitation spectra
exhibit coherent 5f bands coexisting with features associated with local
intra-atomic transitions. The findings provide a microscopic basis for partial
localization. Results are presented for linear chains.Comment: 10 Page
Exact Constructions in the (Non-linear) Planar Theory of Elasticity: From Elastic Crystals to Nematic Elastomers
In this article we deduce necessary and sufficient conditions for the presence of “Conti-type”, highly symmetric, exactly stress-free constructions in the geometrically non-linear, planar n-well problem, generalising results of Conti et al. (Proc R Soc A 73(2203):20170235, 2017). Passing to the limit , this allows us to treat solid crystals and nematic elastomer differential inclusions simultaneously. In particular, we recover and generalise (non-linear) planar tripole star type deformations which were experimentally observed in Kitano and Kifune (Ultramicroscopy 39(1–4):279–286, 1991), Manolikas and Amelinckx (Physica Status Solidi (A) 60(2):607–617, 1980; Physica Status Solidi (A) 61(1):179–188, 1980). Furthermore, we discuss the corresponding geometrically linearised problem
Quantum Resistive Transition in Type II Superconductors under Magnetic Field
It is shown that, within a Ginzburg-Landau (GL) formalism, the
superconducting fluctuation is insulating at zero temperature even if the
fluctuation dynamics is metallic (dissipative). Based on this fact, the low
temperature behavior of the -line and the resistivity curves near a
zero temperature transition are discussed. In particular, it is pointed out
that the neglect of quantum fluctuations in data analysis of the dc resistivity
may lead to an under-estimation of the values near zero temperature.Comment: 7 page
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