195 research outputs found
Exotic ground states and impurities in multiband superconductors
We consider the effect of isotropic impurity scattering on the exotic
superconducting states that arise from the usual BCS mechanism in substances of
cubic and hexagonal symmetry where the Fermi surface contains inequivalent but
degenerate pockets (e.g. around several points of high symmetry). As examples
we look at CeCo, CeRu, and LaB; all of which have such Fermi
surface topologies and the former exhibits unconventional superconducting
behavior. We find that while these non s-wave states are suppressed by
non-magnetic impurities, the suppression is much weaker than would be expected
for unconventional superconductors with isotropic non-magnetic impurity
scattering.Comment: 4 pages, no figure
Exact Ground States of the Periodic Anderson Model in D=3 Dimensions
We construct a class of exact ground states of three-dimensional periodic
Anderson models (PAMs) -- including the conventional PAM -- on regular Bravais
lattices at and above 3/4 filling, and discuss their physical properties. In
general, the f electrons can have a (weak) dispersion, and the hopping and the
non-local hybridization of the d and f electrons extend over the unit cell. The
construction is performed in two steps. First the Hamiltonian is cast into
positive semi-definite form using composite operators in combination with
coupled non-linear matching conditions. This may be achieved in several ways,
thus leading to solutions in different regions of the phase diagram. In a
second step, a non-local product wave function in position space is constructed
which allows one to identify various stability regions corresponding to
insulating and conducting states. The compressibility of the insulating state
is shown to diverge at the boundary of its stability regime. The metallic phase
is a non-Fermi liquid with one dispersing and one flat band. This state is also
an exact ground state of the conventional PAM and has the following properties:
(i) it is non-magnetic with spin-spin correlations disappearing in the
thermodynamic limit, (ii) density-density correlations are short-ranged, and
(iii) the momentum distributions of the interacting electrons are analytic
functions, i.e., have no discontinuities even in their derivatives. The
stability regions of the ground states extend through a large region of
parameter space, e.g., from weak to strong on-site interaction U. Exact
itinerant, ferromagnetic ground states are found at and below 1/4 filling.Comment: 47 pages, 10 eps figure
Magnetic state of plutonium ion in metallic Pu and its compounds
By LDA+U method with spin-orbit coupling (LDA+U+SO) the magnetic state and
electronic structure have been investigated for plutonium in \delta and \alpha
phases and for Pu compounds: PuN, PuCoGa5, PuRh2, PuSi2, PuTe, and PuSb. For
metallic plutonium in both phases in agreement with experiment a nonmagnetic
ground state was found with Pu ions in f^6 configuration with zero values of
spin, orbital, and total moments. This result is determined by a strong
spin-orbit coupling in 5f shell that gives in LDA calculation a pronounced
splitting of 5f states on f^{5/2} and f^{7/2} subbands. A Fermi level is in a
pseudogap between them, so that f^{5/2} subshell is already nearly completely
filled with six electrons before Coulomb correlation effects were taken into
account. The competition between spin-orbit coupling and exchange (Hund)
interaction (favoring magnetic ground state) in 5f shell is so delicately
balanced, that a small increase (less than 15%) of exchange interaction
parameter value from J_H=0.48eV obtained in constrain LDA calculation would
result in a magnetic ground state with nonzero spin and orbital moment values.
For Pu compounds investigated in the present work, predominantly f^6
configuration with nonzero magnetic moments was found in PuCoGa5, PuSi2, and
PuTe, while PuN, PuRh2, and PuSb have f^5 configuration with sizeable magnetic
moment values. Whereas pure jj coupling scheme was found to be valid for
metallic plutonium, intermediate coupling scheme is needed to describe 5f shell
in Pu compounds. The results of our calculations show that both spin-orbit
coupling and exchange interaction terms in the Hamiltonian must be treated in a
general matrix form for Pu and its compounds.Comment: 20 pages, LaTeX; changed discussion on reference pape
Fermi Surface Properties of Low Concentration CeLaB: dHvA
The de Haas-van Alphen effect is used to study angular dependent extremal
areas of the Fermi Surfaces (FS) and effective masses of CeLaB alloys for between 0 and 0.05. The FS of these alloys was previously
observed to be spin polarized at low Ce concentration ( = 0.05). This work
gives the details of the initial development of the topology and spin
polarization of the FS from that of unpolarized metallic LaB to that of
spin polarized heavy Fermion CeB .Comment: 7 pages, 9 figures, submitted to PR
Temperature dependence of the Kondo resonance and its satellites in CeCu_2Si_2
We present high-resolution photoemission spectroscopy studies on the Kondo
resonance of the strongly-correlated Ce system CeCuSi. Exploiting the
thermal broadening of the Fermi edge we analyze position, spectral weight, and
temperature dependence of the low-energy 4f spectral features, whose major
weight lies above the Fermi level . We also present theoretical
predictions based on the single-impurity Anderson model using an extended
non-crossing approximation (NCA), including all spin-orbit and crystal field
splittings of the 4f states. The excellent agreement between theory and
experiment provides strong evidence that the spectral properties of
CeCuSi can be described by single-impurity Kondo physics down to K.Comment: 4 pages, 3 figure
Plaquette operators used in the rigorous study of ground-states of the Periodic Anderson Model in dimensions
The derivation procedure of exact ground-states for the periodic Anderson
model (PAM) in restricted regions of the parameter space and D=2 dimensions
using plaquette operators is presented in detail. Using this procedure, we are
reporting for the first time exact ground-states for PAM in 2D and finite value
of the interaction, whose presence do not require the next to nearest neighbor
extension terms in the Hamiltonian. In order to do this, a completely new type
of plaquette operator is introduced for PAM, based on which a new localized
phase is deduced whose physical properties are analyzed in detail. The obtained
results provide exact theoretical data which can be used for the understanding
of system properties leading to metal-insulator transitions, strongly debated
in recent publications in the frame of PAM. In the described case, the lost of
the localization character is connected to the break-down of the long-range
density-density correlations rather than Kondo physics.Comment: 34 pages, 5 figure
Ba 4d core-level spectroscopy in the YBa2Cu3O6.9 high-Tc superconductor: Existence of a surface-shifted component
Two sets of spin-orbit split Ba 4d core-level photoemission peaks were observed in a crystal of YBa2Cu3O6.9. From constant final-state measurements taken as a function of kinetic energy, the low-binding-energy doublet is identified as a surface component. Possible origins of the surface shift are discussed
The Kondo Resonance in Electron Spectroscopy
The Kondo resonance is the spectral manifestation of the Kondo properties of
the impurity Anderson model, and also plays a central role in the dynamical
mean-field theory (DMFT) for correlated electron lattice systems. This article
presents an overview of electron spectroscopy studies of the resonance for the
4f electrons of cerium compounds, and for the 3d electrons of V_2O_3, including
beginning efforts at using angle resolved photoemission to determine the
k-dependence of the resonance. The overview includes the comparison and
analysis of spectroscopy data with theoretical spectra as calculated for the
impurity model and as obtained by DMFT, and the Kondo volume collapse
calculation of the cerium alpha-gamma phase transition boundary, with its
spectroscopic underpinnings.Comment: 32 pages, 11 figures, 151 references; paper for special issue of J.
Phys. Soc. Jpn. on "Kondo Effect--40 Years after the Discovery
Saturation of electrical resistivity
Resistivity saturation is observed in many metallic systems with a large
resistivity, i.e., when the resistivity has reached a critical value, its
further increase with temperature is substantially reduced. This typically
happens when the apparent mean free path is comparable to the interatomic
separations - the Ioffe-Regel condition. Recently, several exceptions to this
rule have been found. Here, we review experimental results and early theories
of resistivity saturation. We then describe more recent theoretical work,
addressing cases both where the Ioffe-Regel condition is satisfied and where it
is violated. In particular we show how the (semiclassical) Ioffe-Regel
condition can be derived quantum-mechanically under certain assumptions about
the system and why these assumptions are violated for high-Tc cuprates and
alkali-doped fullerides.Comment: 16 pages, RevTeX, 15 eps figures, additional material available at
http://www.mpi-stuttgart.mpg.de/andersen/saturation
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