73 research outputs found
First-principles Study of the RKKY Interaction and the Quadrupole Order in the Pr 1-2-20 systems PrT2Al20 (T=Ti, V)
Electronic states and quadrupole orders in the Pr 1-2-20 systems PrT2Al20
(T=Ti, V) are investigated on the basis of the first-principles calculations.
The effective 196 orbital model is derived to reproduce the first-principles
electronic structures of LaT2Al20 (T=Ti, V) without contribution from the Pr 4f
electrons which are considered to be well localized and is employed to
calculate the Ruderman-Kittel-Kasuya-Yosida (RKKY) interactions between
quadrupole and octupole moments of the Pr ions. Within the random phase
approximation for the RKKY Hamiltonian, the most divergent susceptibility is
found to be the quadrupole one for the wave vector Q = (0,0,0) in the case of
PrTi2Al20 while that for Q = (pi/a,0,pi/a) in the case of PrV2Al20 as
consistent with experimental observations in the both cases which exhibit the
ferro-quadrupole (FQ) and the antiferro-quadrupole (AFQ) orders, respectively.
We also discuss the ordered states using the mean-field approximation and find
that, in the case of PrTi2Al20, the 1st-order phase transition to the O20 FQ
order with a tiny discontinuity takes place as predicted by the Landau theory.
In the case of PrV2Al20, the system exhibits two distinct O22 AFQ orders, AFQ-I
and AFQ-II, and shows subsequent two phase transitions, the 2nd-order one from
normal to AFQ-I and the 1st-order one from AFQ-I to AFQ-II, that may be
responsible for the double transitions observed by specific heat measurements.Comment: 6 pages, 6 figure
Effect of the spin-orbit interaction and the electron phonon coupling on the electronic state in a silicon vacancy
The electronic state around a single vacancy in silicon crystal is
investigated by using the Green's function approach. The triply degenerate
charge states are found to be widely extended and account for extremely large
elastic softening at low temperature as observed in recent ultrasonic
experiments. When we include the LS coupling on each Si
atom, the 6-fold spin-orbital degeneracy for the state with the valence
+1 and spin 1/2 splits into doublet groundstates and
quartet excited states with a reduced excited energy of . We also consider the effect of couplings between electrons and
Jahn-Teller phonons in the dangling bonds within the second order perturbation
and find that the groundstate becomes quartet which is responsible
for the magnetic-field suppression of the softening in B-doped silicon.Comment: 4 pages, 2 figure
Crystalline-Electric-Field Effect on the Resistivity of Ce-based Heavy Fermion Systems
The behavior of the resistivity of Ce-based heavy fermion systems is studied
using a 1/-expansion method a la Nagoya, where is the spin-orbital
degeneracy of f-electrons. The 1/-expansion is performed in terms of the
auxiliary particles, and a strict requirement of the local constraints is
fulfilled for each order of 1/N. The physical quantities can be calculated over
the entire temperature range by solving the coupled Dyson equations for the
Green functions self-consistently at each temperature. This 1/N-expansion
method is known to provide asymptotically exact results for the behavior of
physical quantities in both low- and high-energy regions when it is applied to
a single orbital periodic Anderson model (PAM). On the basis of a generalized
PAM including crystalline-electric-field splitting with a single conduction
band, the pressure dependence of the resistivity is calculated by
parameterizing the effect of pressure as the variation of the hybridization
parameter between the conduction electrons and f-electrons. The main result of
the present study is that the double-peak structure of the -dependence of
the resistivity is shown to merge into a single-peak structure with increasing
pressure.Comment: 37 pages, 22 figure
Field dependent effective masses in YbAl
We show for the intermediate valence compound YbAl that the high field
(40 60T) effective masses measured by the de Haas-van
Alphen experiment for field along the direction are smaller by
approximately a factor of two than the low field masses. The field
40T for this reduction is much smaller than the Kondo field ( 670K) but is comparable to the field
where 40K is the temperature for the onset
of Fermi liquid coherence. This suggests that the field scale does not
arise from 4 polarization but is connected with the removal of the anomalies
that are known to occur in the Fermi liquid state of this compound.Comment: 7 pages plus 3 figures Submitted to PRL 9/12/0
Periodic Anderson model with degenerate orbitals: linearized dynamical mean field theory approach
We investigate a multi-orbital extension of the periodic Anderson model with
particular emphasis on electron correlations including orbital fluctuations. By
means of a linearized version of the dynamical mean-field theory, we compute
the renormalization factor, the density of states, the spectral gap and the
local correlation functions for a given set of the intra- and inter-orbital
Coulomb interactions as well as the Hund coupling. It is found that when a
certain condition is met for the intra- and inter-orbital interactions for
electrons, orbital fluctuations are enhanced, thereby enlarging the Kondo
insulating gap. This effect is suppressed in the presence of the Hund coupling.
We also clarify how the Kondo insulator is continuously changed to the Mott
insulator when electron correlations among conduction electrons are increased.Comment: 7 pages, 10 figure
Cooperative Effect of Coulomb Interaction and Electron-Phonon Coupling on the Heavy Fermion State in the Two-Orbital Periodic Anderson Model
We investigate the two-orbital periodic Anderson model, where the local
orbital fluctuations of f-electrons couple with a two-fold degenerate
Jahn-Teller phonon, by using the dynamical mean-field theory. It is found that
the heavy fermion state caused by the Coulomb interaction between f-electrons U
is largely enhanced due to the electron-phonon coupling g, in contrast to the
case with the single-orbital periodic Anderson model where the effects of U and
g compete to each other. In the heavy fermion state for large and g, both
the orbital and lattice fluctuations are enhanced, while the charge (valence)
and spin fluctuations are suppressed. In the strong coupling regime, a sharp
soft phonon mode with a large spectral weight is observed for small U, while a
broad soft phonon mode with a small spectral weight is observed for large U.
The cooperative effect of U and g for half-filling with two f-electrons per
atom is more pronounced than that for quarter-filling with .Comment: 8 pages, 11 figures, accepted for publication in JPS
Phase Diagram of the Electron-Doped Cuprate Superconductors
We investigate the phase diagram of the electron-doped systems in high-Tc
cuprates. We calculate the superconducting transition temperature Tc, the
antiferromagnetic transition temperature TN, the NMR relaxation rate 1/T1 with
the antiferromagnetic fluctuations in the fluctuation-exchange (FLEX)
approximation and with the superconducting fluctuations in the self-consistent
t-matrix approximation. Obtained phase diagram has common features as those in
the hole-doped systems, including the antiferromagnetic state, the
superconducting state and the spin gap phenomenon. Doping-dependences of TN, Tc
and Tsg (spin gap temperature) are, however, different with those in the
hole-doped systems. These differences are due to the intrinsic nature of the
ingap states which are intimately related with the Zhang-Rice singlets in the
hole-doped systems and are correlated d-electrons in the electron-doped
systems, respectively, which has been shown in the d-p model.Comment: 4 pages, 3 figure
Renormalization group approaches to strongly correlated electron systems
In recent years the numerical renormalization group (NRG) method has been extended to the calculation of dynamic response functions and transport properties of magnetic impurity models. The approach can now be applied more widely to lattice models of strongly correlated electron systems by the use of dynamical mean field theory (DMFT), in which the lattice problem is transformed into one for an e ective impurity with an additional self-consistency constraint. We review these developments and assess the potential for further applications of this approach. We also discuss an alternative approach to renormalization, renormalized perturbation theory, in which the leading asymptotically exact results for the low temperature regime for a number of magnetic impurity models can be obtained within nite order perturbation theory
Magnetization Process in the One-Dimensional Doped Kondo Lattice Model
The magnetization process in the one-dimensional Kondo lattice model for the
doped (n_{c}<1) case is studied by the density matrix renormalization group
(DMRG) method. A rapid increase of the magnetization is caused by the collapse
of the intersite incommensurate correlation of f spins. On the contrary, the
intrasite f-c singlet correlation survives in the larger magnetic field. The
crossover from large to small Fermi surfaces for majority and minority spins is
observed, whereas the Fermi surfaces are always contributed by f spins. A
magnetization plateau appears with the magnitude of 1-n_{c}. Both ends of the
plateau are related to the coherence temperature and the Kondo temperature
which are characteristic energies essential in heavy electron systems.Comment: 4 pages, 3 eps figure
The numerical renormalization group method for quantum impurity systems
In the beginning of the 1970's, Wilson developed the concept of a fully
non-perturbative renormalization group transformation. Applied to the Kondo
problem, this numerical renormalization group method (NRG) gave for the first
time the full crossover from the high-temperature phase of a free spin to the
low-temperature phase of a completely screened spin. The NRG has been later
generalized to a variety of quantum impurity problems. The purpose of this
review is to give a brief introduction to the NRG method including some
guidelines of how to calculate physical quantities, and to survey the
development of the NRG method and its various applications over the last 30
years. These applications include variants of the original Kondo problem such
as the non-Fermi liquid behavior in the two-channel Kondo model, dissipative
quantum systems such as the spin-boson model, and lattice systems in the
framework of the dynamical mean field theory.Comment: 55 pages, 27 figures, submitted to Rev. Mod. Phy
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