2,736 research outputs found
Theory of Kondo lattices and its application to high-temperature superconductivity and pseudo-gaps in cuprate oxides
A theory of Kondo lattices is developed for the t-J model on a square
lattice. The spin susceptibility is described in a form consistent with a
physical picture of Kondo lattices: Local spin fluctuations at different sites
interact with each other by a bare intersite exchange interaction, which is
mainly composed of two terms such as the superexchange interaction, which
arises from the virtual exchange of spin-channel pair excitations of electrons
across the Mott-Hubbard gap, and an exchange interaction arising from that of
Gutzwiller's quasi-particles. The bare exchange interaction is enhanced by
intersite spin fluctuations developed because of itself. The enhanced exchange
interaction is responsible for the development of superconducting fluctuations
as well as the Cooper pairing between Gutzwiller's quasi-particles. On the
basis of the microscopic theory, we develop a phenomenological theory of
low-temperature superconductivity and pseudo-gaps in the under-doped region as
well as high-temperature superconductivity in the optimal-doped region.
Anisotropic pseudo-gaps open mainly because of d\gamma-wave superconducting
low-energy fluctuations: Quasi-particle spectra around (\pm\pi/a,0) and
(0,\pm\pi/a), with a the lattice constant, or X points at the chemical
potential are swept away by strong inelastic scatterings, and quasi-particles
are well defined only around (\pm\pi/2a,\pm\pi/2a) on the Fermi surface or
line. As temperatures decrease in the vicinity of superconducting critical
temperatures, pseudo-gaps become smaller and the well-defined region is
extending toward X points. The condensation of d\gamma-wave Cooper pairs
eventually occurs at low enough temperatures when the pair breaking by
inelastic scatterings becomes small enough.Comment: 15 pages, 14 figure
Frustrated electron liquids in the Hubbard model
The ground state of the Hubbard model is studied within the constrained
Hilbert space where no order parameter exists. The self-energy of electrons is
decomposed into the single-site and multisite self-energies. The calculation of
the single-site self-energy is mapped to a problem of self-consistently
determining and solving the Anderson model. When an electron reservoir is
explicitly considered, it is proved that the single-site self-energy is that of
a normal Fermi liquid even if the multisite self-energy is anomalous. Thus, the
ground state is a normal Fermi liquid in the supreme single-site approximation
(S^3A). In the strong-coupling regime, the Fermi liquid is stabilized by the
Kondo effect in the S^3A and is further stabilized by the Fock-type term of the
superexchange interaction or the resonating-valence-bond (RVB) mechanism beyond
the S^3A. The stabilized Fermi liquid is frustrated as much as an RVB spin
liquid in the Heisenberg model. It is a relevant unperturbed state that can be
used to study a normal or anomalous Fermi liquid and an ordered state in the
whole Hilbert space by Kondo lattice theory. Even if higher-order multisite
terms than the Fock-type term are considered, the ground state cannot be a Mott
insulator. It can be merely a gapless semiconductor even if the multisite
self-energy is so anomalous that it is divergent at the chemical potential. A
Mott insulator is only possible as a high temperature phase.Comment: 11 pages, no figur
Magnetic and charge structures in itinerant-electron magnets: Coexistence of multiple SDW and CDW
A theory of Kondo lattices is applied to studying possible magnetic and
charge structures of itinerant-electron antiferromagnets. Even helical spin
structures can be stabilized when the nesting of the Fermi surface is not sharp
and the superexchange interaction, which arises from the virtual exchange of
pair excitations across the Mott-Hubbard gap, is mainly responsible for
magnetic instability. Sinusoidal spin structures or spin density waves (SDW)
are only stabilized when the nesting of the Fermi surface is sharp enough and a
novel exchange interaction arising from that of pair excitations of
quasi-particles is mainly responsible for magnetic instability. In particular,
multiple SDW are stabilized when their incommensurate ordering wave-numbers
are multiple; magnetizations of different components
are orthogonal to each other in double and triple SDW when magnetic anisotropy
is weak enough. Unless are commensurate, charge density waves
(CDW) with coexist with SDW with . Because the
quenching of magnetic moments by the Kondo effect depends on local numbers of
electrons, the phase of CDW or electron densities is such that magnetic moments
are large where the quenching is weak. It is proposed that the so called stipe
order in cuprate-oxide high-temperature superconductors must be the coexisting
state of double incommensurate SDW and CDW.Comment: 10 pages, no figure
Thermal conductivity of the thermoelectric layered cobalt oxides measured by the Harman method
In-plane thermal conductivity of the thermoelectric layered cobalt oxides has
been measured using the Harman method, in which thermal conductivity is
obtained from temperature gradient induced by applied current. We have found
that the charge reservoir block (the block other than the CoO block)
dominates the thermal conduction, where a nano-block integration concept is
effective for material design. We have further found that the thermal
conductivity shows a small but finite in-plane anisotropy between and
axes, which can be ascribed to the misfit structure.Comment: 4 pages, 4 figures, J. Appl. Phys. (scheduled on July 1, 2004
Opening of a pseudogap in a quasi-two dimensional superconductor due to critical thermal fluctuations
We examine the role of the anisotropy of superconducting critical thermal
fluctuations in the opening of a pseudogap in a quasi-two dimensional
superconductor such as a cuprate-oxide high-temperature superconductor. When
the anisotropy between planes and their perpendicular axis is large enough and
its superconducting critical temperature T_c is high enough, the fluctuations
are much developed in its critical region so that lifetime widths of
quasiparticles are large and the energy dependence of the selfenergy deviates
from that of Landau's normal Fermi liquids. A pseudogap opens in such a
critical region because quasiparticle spectra around the chemical potential are
swept away due to the large lifetime widths. The pseudogap never smoothly
evolves into a superconducting gap; it starts to open at a temperature higher
than T_c while the superconducting gap starts to open just at T_c. When T_c is
rather low but the ratio of varepsilon_G(0)/k_BT_c, with varepsilon_G(0) the
superconducting gap at T=0K and k_B the Boltzmann constant, is much larger than
a value about 4 according to the mean-field theory, the pseudogap must be
closing as temperature T approaches to the low T_c because thermal fluctuations
become less developed as T decreases. Critical thermal fluctuations cannot
cause the opening of a prominent pseudogap in an almost isotropic three
dimensional superconductor, even if its T_c is high.Comment: 25 pages, 5 figures (14 subfigures
Rashba spin splitting in biased semiconductor quantum wells
Rashba spin splitting (RSS) in biased semiconductor quantum wells is
investigated theoretically based on the eight-band envelope function model. We
find that at large wave vectors, RSS is both nonmonotonic and anisotropic as a
function of in-plane wave vector, in contrast to the widely used linear and
isotropic model. We derive an analytical expression for RSS, which can
correctly reproduce such nonmonotonic behavior at large wave vectors. We also
investigate numerically the dependence of RSS on the various band parameters
and find that RSS increases with decreasing band gap and subband index,
increasing valence band offset, external electric field, and well width. Our
analytical expression for RSS provides a satisfactory explanation to all these
features.Comment: 5 pages, 4 figures, author names corrected, submitted to Phys. Rev.
Spin-Valley Kondo Effect in Multi-electron Silicon Quantum Dots
We study the spin-valley Kondo effect of a silicon quantum dot occupied by electrons, with up to four. We show that the Kondo
resonance appears in the Coulomb blockade regimes, but not
in the one, in contrast to the spin-1/2 Kondo effect, which
only occurs at odd. Assuming large orbital level spacings, the
energy states of the dot can be simply characterized by fourfold spin-valley
degrees of freedom. The density of states (DOS) is obtained as a function of
temperature and applied magnetic field using a finite-U equation-of-motion
approach. The structure in the DOS can be detected in transport experiments.
The Kondo resonance is split by the Zeeman splitting and valley splitting for
double- and triple-electron Si dots, in a similar fashion to single-electron
ones. The peak structure and splitting patterns are much richer for the
spin-valley Kondo effect than for the pure spin Kondo effect.Comment: 8 pages, 4 figures, in PRB format. This paper is a sequel to the
paper published in Phys. Rev. B 75, 195345 (2007
Theory of itinerant-electron ferromagnetism
A theory of Kondo lattices or a expansion theory, with spatial
dimensionality, is applied to studying itinerant-electron ferromagnetism. Two
relevant multi-band models are examined: a band-edge model where the chemical
potential is at one of band-edges, the top or bottom of bands, and a flat-band
model where one of bands is almost flat or dispersionless and the chemical
potential is at the flat band. In both the models, a novel ferromagnetic
exchange interaction arises from the virtual exchange of pair excitations of
quasiparticles; it has two novel properties such as its strength is in
proportion to the effective Fermi energy of quasiparticles and its temperature
dependence is responsible for the Curie-Weiss law. When the Hund coupling
is strong enough, the superexchange interaction, which arises from the virtual
exchange of pair excitations of electrons across the Mott-Hubbard gap, is
ferromagnetic. In particular, it is definitely ferromagnetic for any nonzero
in the large limit of band multiplicity. Ferromagnetic instability
occurs, when the sum of the two exchange interactions is ferromagnetic and it
overcomes the quenching of magnetic moments by the Kondo effect or local
quantum spin fluctuations and the suppression of magnetic instability by the
mode-mode coupling among intersite spin fluctuations.Comment: 14 pages, 4 figure
Size Dependence In The Disordered Kondo Problem
We study here the role randomly-placed non-magnetic scatterers play on the
Kondo effect. We show that spin relaxation effects (with time )in the
vertex corrections to the Kondo self-energy lead to an exact cancellation of
the singular temperature dependence arising from the diffusion poles. For a
thin film of thickness and a mean-free path , disorder provides a
correction to the Kondo resistivity of the form
that explains both the disorder and sample-size depression of the Kondo effect
observed by Blachly and Giordano (PRB {\bf 51}, 12537 (1995)).Comment: 11 pages, LaTeX, 2 Postscript figure
Kondo effect in two-dimensional disordered electron systems
We investigate the Kondo effect in two-dimensional disordered electron
systems using a finite-temperature quantum Monte Carlo method. Depending on the
position of a magnetic impurity, the local moment is screened or unscreened by
the spin of the conduction electron. On the basis of the results, we show that
the distribution of the Kondo temperature becomes wide and the weight at
becomes large as randomness increases. The average susceptibility shows
a weak power-law or logarithmic divergence at low temperature, indicating a
non-Fermi-liquid behavior.Comment: 2 pages, 2 figures, to be published in supplement of J. Phys. Soc.
Japan, Proceedings of Localisation 2002, (Tokyo, Japan, 2002
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