306 research outputs found
Fractional charge excitations in fermionic ladders
The system of interacting spinless fermions hopping on a two-leg ladder in
the presence of an external magnetic field is shown to possess a long range
order: the bond density wave or the staggered flux phase. In both cases the
elementary excitations are kinks and carry one half the charge of an
electron.Comment: 4 pages, 3 figure
Lower Bound for the Fermi Level Density of States of a Disordered D-Wave Superconductor in Two Dimensions
We consider a disordered d--wave superconductor in two dimensions. Recently,
we have shown in an exact calculation that for a lattice model with a
Lorentzian distributed random chemical potential the quasiparticle density of
states at the Fermi level is nonzero. As the exact result holds only for the
special choice of the Lorentzian, we employ different methods to show that for
a large class of distributions, including the Gaussian distribution, one can
establish a nonzero lower bound for the Fermi level density of states. The fact
that the tails of the distributions are unimportant in deriving the lower bound
shows that the exact result obtained before is generic.Comment: 15 preprint pages, no figures, submitted to PR
Explicit approximate controllability of the Schr\"odinger equation with a polarizability term
We consider a controlled Schr\"odinger equation with a dipolar and a
polarizability term, used when the dipolar approximation is not valid. The
control is the amplitude of the external electric field, it acts non linearly
on the state. We extend in this infinite dimensional framework previous
techniques used by Coron, Grigoriu, Lefter and Turinici for stabilization in
finite dimension. We consider a highly oscillating control and prove the
semi-global weak stabilization of the averaged system using a Lyapunov
function introduced by Nersesyan. Then it is proved that the solutions of the
Schr\"odinger equation and of the averaged equation stay close on every finite
time horizon provided that the control is oscillating enough. Combining these
two results, we get approximate controllability to the ground state for the
polarizability system
Incommensurate spin correlations in Heisenberg spin-1/2 zig-zag ladders
We develop a low-energy effective theory for spin-1/2 frustrated two-leg
Heisenberg spin ladders. We obtain a new type of interchain coupling that
breaks parity symmetry. In the presence of an XXZ-type anisotropy, this
interaction gives rise to a novel ground state, characterized by incommensurate
correlations. In the case of a single ladder, this state corresponds to a spin
nematic phase. For a frustrated quasi-one-dimensional system of infinitely many
weakly coupled chains, this state develops true three dimensional spiral order.
We apply our theory to recent neutron scattering experiments on .Comment: 4 pages of revtex, 3 figure
New Chiral Universality Class in a Frustrated Three-Leg Spin Ladder
We study a model of three antiferromagnetic Heisenberg spin chains
weakly coupled by on-rung and plaquette-diagonal interchain interactions. It is
shown that the model exhibits a critical phase with central charge C=2 and
belongs to the class of ``chirally stabilized'' liquids recently introduced by
Andrei, Douglas, and Jerez. By allowing anisotropic interactions in spin space,
we find an exact solution at a Toulouse point which captures all universal
properties of the model, including the SU(2) symmetric case. At the new
critical point the massless degrees of freedom are described in terms of an
effective Heisenberg spin chain and two critical Ising models. We
discuss the spectral properties of the model, compute spin-spin correlation
functions and estimate the NMR relaxation rate.Comment: 4 page
Van Hove Singularities in disordered multichannel quantum wires and nanotubes
We present a theory for the van Hove singularity (VHS) in the tunneling
density of states (TDOS) of disordered multichannel quantum wires, in
particular multi-wall carbon nanotubes. We assume close-by gates which screen
off electron-electron interactions. Diagrammatic perturbation theory within a
non-crossing approximation yields analytical expressions governing the
disorder-induced broadening and shift of VHS's as new subbands are opened. This
problem is nontrivial because the (lowest-order) Born approximation breaks down
close to the VHS. Interestingly, compared to the bulk case, the boundary TDOS
shows drastically altered VHS, even in the clean limit.Comment: 4 pages, 2 figures, accepted with revisions in PR
Quasiparticle density of states in dirty high-T_c superconductors
We study the density of quasiparticle states of dirty d-wave superconductors.
We show the existence of singular corrections to the density of states due to
quantum interference effects. We then argue that the density of states actually
vanishes in the localized phase as or depending on whether time
reversal is a good symmetry or not. We verify this result for systems without
time reversal symmetry in one dimension using supersymmetry techniques. This
simple, instructive calculation also provides the exact universal scaling
function for the density of states for the crossover from ballistic to
localized behaviour in one dimension. Above two dimensions, we argue that in
contrast to the conventional Anderson localization transition, the density of
states has critical singularities which we calculate in a
expansion. We discuss consequences of our results for various experiments on
dirty high- materials
Staggered Flux Phase in a Model of Strongly Correlated Electrons
We present numerical evidence for the existence of a staggered flux (SF)
phase in the half-filled two-leg t-U-V-J ladder, with true long-range order in
the counter-circulating currents. The density-matrix renormalization-group
(DMRG) / finite-size scaling approach, generalized to describe complex-valued
Hamiltonians and wavefunctions, is employed. The SF phase exhibits robust
currents at intermediate values of the interaction strength.Comment: Version to appear in Phys. Rev. Let
Phase Diagram of the Half-Filled Extended Hubbard Model in Two Dimensions
We consider an extended Hubbard model of interacting fermions on a lattice.
The fermion kinetic energy corresponds to a tight binding Hamiltonian with
nearest neighbour (-t) and next nearest neighbour (t') hopping matrix elements.
In addition to the onsite Hubbard interaction (U) we also consider a nearest
neighbour repulsion (V). We obtain the zero temperature phase diagram of our
model within the Hartree-Fock approximation. We consider ground states having
charge and spin density wave ordering as well as states with orbital
antiferromagnetism or spin nematic order. The latter two states correspond to
particle-hole binding with symmetry in the charge and spin
channels respectively. For , only the charge density wave and spin
density wave states are energetically stable. For non-zero t', we find that
orbital antiferromagnetism (or spin nematic) order is stable over a finite
portion of the phase diagram at weak coupling. This region of stability is seen
to grow with increasing values of t'.Comment: Latex file, 10 output pages, 3 Figures (available on request to
[email protected]), to appear in Phys. Rev. B (BR
Dynamical Chiral Symmetry Breaking in Unquenched
We investigate dynamical chiral symmetry breaking in unquenched
using the coupled set of Dyson--Schwinger equations for the fermion and photon
propagators. For the fermion-photon interaction we employ an ansatz which
satisfies its Ward--Green--Takahashi identity. We present self-consistent
analytical solutions in the infrared as well as numerical results for all
momenta. In Landau gauge, we find a phase transition at a critical number of
flavours of . In the chirally symmetric phase the
infrared behaviour of the propagators is described by power laws with
interrelated exponents. For and we find small values for the
chiral condensate in accordance with bounds from recent lattice calculations.
We investigate the Dyson--Schwinger equations in other linear covariant gauges
as well. A comparison of their solutions to the accordingly transformed Landau
gauge solutions shows that the quenched solutions are approximately gauge
covariant, but reveals a significant amount of violation of gauge covariance
for the unquenched solutions.Comment: 33 pages, 8 figures, reference added, version to be published in
Phys. Rev.
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