168 research outputs found

### Soft Hubbard gaps in disordered itinerant models with short-range interaction

We study the Anderson-Hubbard model in the Hartree-Fock approximation and the
exact diagonalization under the coexistence of short-range interaction and
diagonal disorder. We show that there exist unconventional soft gaps, where the
single-particle (SP) density of states (DOS) $A$ follows a scaling in energy
$E$ as $A(E)\propto \exp[-(-\gamma\log |E-E_F|)^d]$ irrespective of electron
filling and long-range order. Here, $d$ is the spatial dimension, $E_F$ the
Fermi energy and $\gamma$ a non-universal constant. We propose a multi-valley
energy landscape as their origin. Possible experiments to verify the present
theory are proposed.Comment: 4 pages, 4 figure

### Numerical Study for the Ground State of Multi-Orbital Hubbard Models

Ground state properties of multi-orbital Hubbard models are investigated by
the auxiliary field quantum Monte Carlo method. A Monte Carlo technique
generalized to the multi-orbital systems is introduced and examined in detail.
The algorithm contains non-trivial cases where the negative sign problem does
not exist. We investigate one-dimensional systems with doubly degenerate
orbitals by this new technique. Properties of the Mott insulating state are
quantitatively clarified as the strongly correlated insulator, where the charge
gap amplitude is much larger than the spin gap. The insulator-metal transitions
driven by the chemical potential shows a universality class with the
correlation length exponent $\nu=1/2$, which is consistent with the scaling
arguments. Increasing level split between two orbitals drives crossover from
the Mott insulator with high spin state to the band insulator with low spin
state, where the spin gap amplitude increases and becomes closer to the charge
gap. Experimental relevance of our results especially to Haldane materials is
discussed.Comment: 36 pages LaTeX including 17 PS figures, to be published in
J.Phys.Soc.Jpn. 67 (1998) vol.

### Theory of Electron Transport near Anderson-Mott Transitions

We present a theory of the DC electron transport in insulators near
Anderson-Mott transitions under the influence of coexisting electron
correlation and randomness. At sufficiently low temperatures, the DC electron
transport in Anderson-Mott insulators is determined by the single-particle
density of states (DOS) near the Fermi energy. Anderson insulators, caused by
randomness, are characterized by a nonzero DOS at the Fermi energy. However,
recently, the authors proposed that coexisting randomness and short-ranged
interaction in insulators open a soft Hubbard gap in the DOS, and the DOS
vanishes only at the Fermi energy. Based on the picture of the soft Hubbard
gap, we derive a formula for the critical behavior for the temperature
dependence of the DC resistivity. Comparisons of the present theory with
experimental results of electrostatic carrier doping into an organic conductor
kappa-(BEDT-TTF)_2Cu[N(CN)_2]Br demonstrate the evidence for the present
soft-Hubbard scaling.Comment: 4 pages, 4 figures, 1 tabl

### Origin of high-Tc superconductivity in doped Hubbard models and their extensions: Roles of uniform charge fluctuations

Doped Hubbard model is a simple model for the high-Tc cuprate
superconductors, while its ground state remains a challenge. Here, by
performing state-of-the-art variational Monte Carlo calculations for the
strong-coupling Hubbard model, we find evidences that the d-wave
superconducting phase emerges always near the phase separation region and the
superconducting order has one-to-one correspondence with the enhancement of
charge compressibility. The order as well as the phase separation are
vulnerable to realistic intersite Coulomb interaction while the superexchange
interaction enhances both. An appropriate combination of these two widens the
stable superconducting phase.Comment: 17 pages, 20 figure

### Superconductivity Emerging from Excitonic Mott insulator - Theory of Alkaline Doped Fullerene

A three-orbital model derived from the two-dimensional projection of the $ab$
$initio$ Hamiltonian for alkaline doped fullerene A$_3$C$_{60}$ with A=Cs,Rb,K
is studied by a variational Monte Carlo method. We correctly reproduce the
experimental isotropic s-wave superconductivity around the $ab$ $initio$
parameters. With narrowing the bandwidth, the transition to an insulator is
also reproduced, where orbital symmetry is found to be spontaneously broken
with emergence of an excitonic Mott insulator for two orbitals and an
antiferromagnetic insulator nearly degenerate with a spin liquid for the third
orbital. The superconductivity is a consequence of exciton melting.Comment: 10 pages, 10 figure

### Thermodynamics and Optical Conductivity of a Dissipative Carrier in a Tight Binding Model

Thermodynamics and transport properties of a dissipative particle in a
tight-binding model are studied through specific heat and optical conductivity.
A weak coupling theory is constituted to study the crossover behavior between
the low-temperature region and the high-temperature region analytically. We
found that coherent part around zero frequency in the optical conductivity
disappears for 0<s<2, where s is an exponent of a spectral function of the
environment. Detailed calculation is performed for ohmic damping (s=1). In this
case, the specific heat shows an unusual $T$-linear behavior at low
temperatures, which indicates that the environment strongly influences the
particle motion, and changes the low-energy states of the dissipative particle.
The optical conductivity \sigma(\omega) takes a non-Drude form even at zero
temperature, and the high-frequency side behaves as \omega^(2K-2), where K is a
dimensionless damping strength. The high frequency side of the optical
conductivity is independent of temperatures, while the low frequency side
depends on the temperature, and behaves as T^(2K-2) at high temperatures. We
also comment on the application of this model to the description of incoherent
motion in correlated electron systems.Comment: 30 pages, 11 figs, to be published on J. Phys. Soc. Jpn., improved
version by adding one new figure and removing some mistake

### Pseudogap and Kinetic Pairing Under Critical Differentiation of Electrons in Cuprate Superconductors

Superconducting mechanism of cuprates is discussed in the light of the
proximity of the Mott insulator. The proximity accompanied by suppression of
coherence takes place in an inhomogeneous way in the momentum space in
finite-dimensional systems. Studies on instabilities of metals consisted of
such differentiated electrons in the momentum space are reviewed from a general
point of view. A typical example of the differentiation is found in the
flattening of the quasiparticle dispersion discovered around momenta $(\pi,0)$
and $(0,\pi)$ on 2D square lattices. This flattening even controls the
criticality of the metal-insulator transition. Such differentiation and
suppressed coherence subsequently cause an instability to the superconducting
state in the second order of the strong coupling expansion. The d-wave pairing
interaction is generated from such local but kinetic processes in the absence
of disturbance from the coherent single-particle excitations. The
superconducting mechanism emerges from a direct kinetic origin which is
conceptually different from the pairing mechanism mediated by bosonic
excitations as in magnetic, excitonic, and BCS mechanisms. Pseudogap phenomena
widely observed in the underdoped cuprates are then naturally understood from
the mode-mode coupling of d-wave superconducting (dSC) fluctuations repulsively
coupled with antiferromagnetic (AFM) ones. When we assume the existence of a
strong d-wave channel repulsively competing with AFM fluctuations under the
formation of flat and damped single-particle dispersion, we reproduce basic
properties of the pseudogap seen in the magnetic resonance, neutron scattering,
angle resolved photoemission and tunneling measurements in the cuprates.Comment: 12pages including 1 figure, Proceedings of Advanced Research Workshop
on Open Problems in Strongly Correlated Electron Systems in Ble

### Nonequilibrium Pump-Probe Photoexcitation as a Tool for Analyzing Unoccupied Equilibrium States of Correlated Electrons

Relaxation of electrons in a Hubbard model coupled to a dissipative bosonic
bath is studied to simulate the pump-probe photoemission measurement. From this
insight, we propose an experimental method of eliciting unoccupied part of the
single-particle spectra at the equilibrium of doped-Mott insulators. We reveal
first that effective temperatures of distribution functions and electronic
spectra are different during the relaxation, which makes the frequently
employed thermalization picture inappropriate. Contrary to the conventional
analysis, we show that the unoccupied spectra at equilibrium can be detected as
the states that relax faster.Comment: 9 pages, 7 figure

### Superfluid-Insulator Transition of Interacting Multi-Component Bosons

Various types of superfluid-insulator transitions are investigated for
two-component lattice boson systems in two dimensions with on-site hard-core
repulsion and the component-dependent intersite interaction. The mean-field
phase diagram is obtained by the Gutzwiller-type variational technique in the
plane of filling and interaction parameters. Various ground-state properties
are also studied by the quantum Monte Carlo method. Our model exhibits two
types of diagonal long-range orders; the density order around the density
$n=1/2$ and the Ising-type component order near $n=1$. The quantum Monte Carlo
results for the transitions from the superfluid state to these two ordered
states show marked contrast with the Gutzwiller results. Namely, although they
are both accompanied by phase separation into commensurate ($n=1/2$ or $n=1$)
and incommensurate density phases, these transitions are both continuous. The
continuous growth of the component correlation severely suppresses the
superfluidity as well as the inverse of the effective mass in the critical
region of the component order transition in contrast to the persistence of the
superfluidity in the density-ordered state. We propose a mechanism of the mass
enhancement observed even far from the Mott insulating filling $n=1$, when the
Ising-type component order persists into $n \neq 1$. Possible relevance of this
type of mass enhancement in other systems is also discussed.Comment: 23 pages LaTeX including 7 PS figure

### Competition among Superconducting, Antiferromagnetic, and Charge Orders with Intervention by Phase Separation in the 2D Holstein-Hubbard Model

Using a variational Monte Carlo method, we study competitions of strong
electron-electron and electron-phonon interactions in the ground state of
Holstein-Hubbard model on a square lattice. At half filling, an extended
intermediate metallic or weakly superconducting (SC) phase emerges, sandwiched
by antiferromagnetic and charge order (CO) insulating phases. By the carrier
doping into the CO insulator, the SC order dramatically increases for strong
electron-phonon couplings, but largely hampered by wide phase separation (PS)
regions. Superconductivity is optimized at the border to the PS

- β¦