42 research outputs found
The quasi-periodic Bose-Hubbard model and localization in one-dimensional cold atomic gases
We compute the phase diagram of the one-dimensional Bose-Hubbard model with a
quasi-periodic potential by means of the density-matrix renormalization group
technique. This model describes the physics of cold atoms loaded in an optical
lattice in the presence of a superlattice potential whose wave length is
incommensurate with the main lattice wave length. After discussing the
conditions under which the model can be realized experimentally, the study of
the density vs. the chemical potential curves for a non-trapped system unveils
the existence of gapped phases at incommensurate densities interpreted as
incommensurate charge-density wave phases. Furthermore, a localization
transition is known to occur above a critical value of the potential depth V_2
in the case of free and hard-core bosons. We extend these results to soft-core
bosons for which the phase diagrams at fixed densities display new features
compared with the phase diagrams known for random box distribution disorder. In
particular, a direct transition from the superfluid phase to the Mott
insulating phase is found at finite V_2. Evidence for reentrances of the
superfluid phase upon increasing interactions is presented. We finally comment
on different ways to probe the emergent quantum phases and most importantly,
the existence of a critical value for the localization transition. The later
feature can be investigated by looking at the expansion of the cloud after
releasing the trap.Comment: 19 pages, 20 figure
Haldane-gap chains in a magnetic field
We consider quasi one dimensional spin-1 Heisenberg chains with crystal field
anisotropy in a uniform magnetic field. We determine the dynamical structure
factor in various limits and obtain a fairly complete qualitative picture of
how it changes with the applied field. In particular, we discuss how the width
of the higher energy single magnon modes depends on the field. We consider the
effects of a weak interchain coupling. We discuss the relevance of our results
for recent neutron scattering experiments on the quasi-1D Haldane-gap compound
NDMAP.Comment: 34 pages, 7 figure
Magnetization Plateaus in a Solvable 3-Leg Spin Ladder
We present a solvable ladder model which displays magnetization plateaus at
fractional values of the total magnetization. Plateau signatures are also shown
to exist along special lines. The model has isotropic Heisenberg interactions
with additional many-body terms. The phase diagram can be calculated exactly
for all values of the rung coupling and the magnetic field. We also derive the
anomalous behaviour of the susceptibility near the plateau boundaries. There is
good agreement with the phase diagram obtained recently for the pure Heisenberg
ladders by numerical and perturbative techniques.Comment: 4 pages, revtex, 3 postscript figures, small changes to the text and
references update
A Strong-Coupling Approach to the Magnetization Process of Polymerized Quantum Spin Chains
Polymerized quantum spin chains (i.e. spin chains with a periodic modulation
of the coupling constants) exhibit plateaux in their magnetization curves when
subjected to homogeneous external magnetic fields. We argue that the
strong-coupling limit yields a simple but general explanation for the
appearance of plateaux as well as of the associated quantization condition on
the magnetization. We then proceed to explicitly compute series for the plateau
boundaries of trimerized and quadrumerized spin-1/2 chains. The picture is
completed by a discussion how the universality classes associated to the
transitions at the boundaries of magnetization plateaux arise in many cases
from a first order strong-coupling effective Hamiltonian.Comment: 5 pages REVTeX, three PostScript figures included using psfig.st
Random bond XXZ chains with modulated couplings
The magnetization behavior of q-periodic antiferromagnetic spin 1/2
Heisenberg chains under uniform magnetic fields is investigated in a background
of disorder exchange distributions. By means of both real space decimation
procedures and numerical diagonalizations in XX chains, it is found that for
binary disorder the magnetization exhibits wide plateaux at values of
1+2(p-1)/q, where p is the disorder strength. In contrast, no spin gaps are
observed in the presence of continuous exchange distributions. We also study
the magnetic susceptibility at low magnetic fields. For odd q-modulations the
susceptibility exhibits a universal singularity, whereas for q even it displays
a non-universal power law behavior depending on the parameters of the
distribution.Comment: 4 pages, 3 figures. Final version to appear in PR
Successive opening of the Fermi surface in doped N-leg Hubbard ladders
We study the effect of doping away from half-filling in weakly (but finitely)
interacting N-leg Hubbard ladders using renormalization group and bosonization
techniques. For a small on-site repulsion U, the N-leg Hubbard ladders are
equivalent to a N-band model, where at half-filling the Fermi velocities are
v_{1}=v_{N}<v_{2}=v_{N-1}<... We then obtain a hierarchy of energy-scales,
where the band pairs (j,N+1-j) are successively frozen out. The low-energy
Hamiltonian is then the sum of N/2 (or (N-1)/2 for N odd) two-leg ladder
Hamiltonians without gapless excitations (plus a single chain for N odd with
one gapless spin mode), similar to the N-leg Heisenberg spin-ladders. The
energy-scales lead to a hierarchy of gaps. Upon doping away from half-filling,
the holes enter first the band(s) with the smallest gap: For odd N, the holes
enter first the nonbonding band (N+1)/2 and the phase is a Luttinger liquid,
while for even N, the holes enter first the band pair (N/2,N/2+1) and the phase
is a Luther-Emery liquid, similar to numerical treatments of the t-J model,
i.e., at and close to half-filling, the phases of the Hubbard ladders for small
and large U are the same. For increasing doping, hole-pairs subsequently enter
at critical dopings the other band pairs (j,N+1-j) (accompanied by a diverging
compressibility): The Fermi surface is successively opened by doping, starting
near the wave vector (pi/2,pi/2). Explicit calculations are given for the cases
N=3,4.Comment: 10 pages, 4 figures, to be published in Phys. Rev.
Metal-Kondo insulating transitions and transport in one dimension
We study two different metal-insulating transitions possibly occurring in
one-dimensional Kondo lattices. First, we show how doping the pure Kondo
lattice model in the strong-coupling limit, results in a Pokrovsky-Talapov
transition. This produces a conducting state with a charge susceptibility
diverging as the inverse of the doping, that seems in agreement with numerical
datas. Second, in the weak-coupling region, Kondo insulating transitions arise
due to the consequent renormalization of the backward Kondo scattering. Here,
the interplay between Kondo effect and electron-electron interactions gives
rise to significant phenomena in transport, in the high-temperature delocalized
(ballistic) regime. For repulsive interactions, as a perfect signature of Kondo
localization, the conductivity is found to decrease monotonically with
temperature. When interactions become attractive, spin fluctuations in the
electron (Luttinger-type) liquid are suddenly lowered. The latter is less
localized by magnetic impurities than for the repulsive counterpart, and as a
result a large jump in the Drude weight and a maximum in the conductivity arise
in the entrance of the Kondo insulating phase. These can be viewed as remnants
of s-wave superconductivity arising for attractive enough interactions.
Comparisons with transport in the single impurity model are also performed. We
finally discuss the case of randomly distributed magnetic defects, and the
applications on persistent currents of mesoscopic rings.Comment: 21 pages, two columns, 5 figures and 1 table; Final version: To
appear in Physical Review