1,112 research outputs found
Single Impurity Anderson Model with Coulomb Repulsion between Conduction Electrons on the Nearest-Neighbour Ligand Orbital
We study how the Kondo effect is affected by the Coulomb interaction between
conduction electrons on the basis of a simplified model. The single impurity
Anderson model is extended to include the Coulomb interaction on the
nearest-neighbour ligand orbital. The excitation spectra are calculated using
the numerical renormalization group method. The effective bandwidth on the
ligand orbital, , is defined to classify the state. This quantity
decreases as the Coulomb interaction increases. In the
region, the low energy properties are described by the Kondo state, where
is the hybridization width. As decreases in this region, the
Kondo temperature is enhanced, and its magnitude becomes comparable to
for . In the region, the local
singlet state between the electrons on the and ligand orbitals is formed.Comment: 5 pages, 3 figures, LaTeX, to be published in J. Phys. Soc. Jpn Vol.
67 No.
Small Energy Scale for Mixed-Valent Uranium Materials
We investigate a two-channel Anderson impurity model with a magnetic
and a quadrupolar ground doublet, and a excited triplet. Using
the numerical renormalization group method, we find a crossover to a non-Fermi
liquid state below a temperature varying as the triplet-doublet
splitting to the 7/2 power. To within numerical accuracy, the non-linear
magnetic susceptibility and the contribution to the linear
susceptibility are given by universal one-parameter scaling functions. These
results may explain UBe as mixed valent with a small crossover scale
.Comment: 4 pages, 3 figures, REVTeX, to appear in Phys. Rev. Let
Exactly solvable toy models of unconventional magnetic alloys: Bethe Ansatz versus Renormalization Group method
We propose toy models of unconventional magnetic alloys, in which the density
of band states, , and hybridization, , are energy
dependent; it is assumed, however, that
, and hence an effective
electron-impurity coupling is
energy independent. In the renormalization group approach, the physics of the
system is assumed to be governed by only rather than by
separate forms of and . However, an exact Bethe
Ansatz solution of the toy Anderson model demonstrates a crucial role of a form
of inverse band dispersion .Comment: A final version. A previous one has been sent to Archive because of
my technical mistake. Sorr
Magnetically Robust Non-Fermi Liquid Behavior in Heavy Fermion Systems with f^2-Configuration: Competition between Crystalline-Electric-Field and Kondo-Yosida Singlets
We study a magnetic field effect on the Non-Fermi Liquid (NFL) which arises
around the quantum critical point (QCP) due to the competition between the
f^2-crystalline-electric-field singlet and the Kondo-Yosida singlet states by
using the numerical renormalization ground method. We show the characteristic
temperature T_F^*, corresponding to a peak of a specific heat, is not affected
by the magnetic field up to H_z^* which is determined by the distance from the
QCP or characteristic energy scales of each singlet states. As a result, in the
vicinity of QCP, there are parameter regions where the NFL is robust against
the magnetic field, at an observable temperature range T > T_F^*, up to H_z^*
which is far larger than T_F^* and less than min(T_{K2}, $Delta).Comment: 8 pages, 9 figur
Zero temperature metal-insulator transition in the infinite-dimensional Hubbard model
The zero temperature transition from a paramagnetic metal to a paramagnetic
insulator is investigated in the Dynamical Mean Field Theory for the Hubbard
model. The self-energy of the effective impurity Anderson model (on which the
Hubbard model is mapped) is calculated using Wilson's Numerical Renormalization
Group method. Results for quasiparticle weight, spectral function and
self-energy are discussed for Bethe and hypercubic lattice. In both cases, the
metal-insulator transition is found to occur via the vanishing of a
quasiparticle resonance which appears to be isolated from the Hubbard bands.Comment: 4 pages, 3 eps-figures include
Criterion for weak spin-orbit coupling in heavy-fermion superconductivity: A numerical renormalization-group study
A criterion for effective irrelevancy of the spin-orbit coupling in the
heavy-fermion superconductivity is discussed on the basis of the impurity
Anderson model with two sets of Kramers doublets. Using Wilson's numerical
renormalization-group method, we demonstrate a formation of the quasiparticle
as well as the renormalization of the rotational symmetry-breaking interaction
in the lower Kramers doublet (quasispin) space. A comparison with the quasispin
conserving interaction exhibits the effective irrelevancy of the
symmetry-breaking interaction for the splitting of two doublets Delta larger
than the characteristic energy of the local spin fluctuation T_K. The formula
for the ratio of two interactions is also determined.Comment: 4 pages, 4 figures (2 color figures
Quasiparticle Interactions for f-Impurity Anderson Model with Crystalline-Electric-Field: Numerical Renormalization Group Study
The aspect of the quasiparticle interaction of a local Fermi liquid, the
impurity version of f-based heavy fermions, is studied by the Wilson
numerical renormalization group method. In particular, the case of the
f-singlet crystalline-electric-field ground state is investigated assuming
the case of UPt with the hexagonal symmetry. It is found that the
interorbital interaction becomes larger than the intraorbital one in contrast
to the case of the bare Coulomb interaction for the parameters relevant to
UPt. This result offers us a basis to construct a microscopic theory of the
superconductivity of UPt where the interorbital interactions are expected
to play important roles.Comment: 9 pages, 5 figure
Room Temperature Kondo effect in atom-surface scattering: dynamical 1/N approach
The Kondo effect may be observable in some atom-surface scattering
experiments, in particular, those involving alkaline-earth atoms. By combining
Keldysh techniques with the NCA approximation to solve the time-dependent
Newns-Anderson Hamiltonian in the infinite-U limit, Shao, Nordlander and
Langreth found an anomalously strong surface-temperature dependence of the
outgoing charge state fractions. Here we employ the dynamical 1/N expansion
with finite Coulomb interaction U to provide a more realistic description of
the scattering process. We test the accuracy of the 1/N expansion in the
spinless N = 1 case against the exact independent-particle solution. We then
compare results obtained in the infinite-U limit with the NCA approximation and
recover qualitative features found previously. Finally, we analyze the
realistic situation of Ca atoms with U = 5.8 eV scattered off Cu(001) surfaces.
Although the presence of the doubly-ionized Ca species can change the absolute
scattered positive Ca yields, the temperature dependence is qualitatively the
same as that found in the infinite-U limit. One of the main difficulties that
experimentalists face in attempting to detect this effect is that the atomic
velocity must be kept small enough to reduce possible kinematic smearing of the
metal's Fermi surface.Comment: 15 pages, 10 Postscript figures; references and typos correcte
Interacting Growth Walk - a model for hyperquenched homopolymer glass?
We show that the compact self avoiding walk configurations, kinetically
generated by the recently introduced Interacting Growth Walk (IGW) model, can
be considered as members of a canonical ensemble if they are assigned random
values of energy. Such a mapping is necessary for studying the thermodynamic
behaviour of this system. We have presented the specific heat data for the IGW,
obtained from extensive simulations on a square lattice; we observe a broad
hump in the specific heat above the -point, contrary to expectation.Comment: 4 figures; Submitted to PR
Gap States in Dilute Magnetic Alloy Superconductors
We study states in the superconducting gap induced by magnetic impurities
using self-consistent quantum Monte Carlo with maximum entropy and formally
exact analytic continuation methods. The magnetic impurity susceptibility has
different characteristics for T_{0} \alt T_{c0} and T_{0} \agt T_{c0}
(: Kondo temperature, : superconducting transition temperature)
due to the crossover between a doublet and a singlet ground state. We
systematically study the location and the weight of the gap states and the gap
parameter as a function of and the concentration of the
impurities.Comment: 4 pages in ReVTeX including 4 encapsulated Postscript figure
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