67 research outputs found
DMRG and the Two Dimensional t-J Model
We describe in detail the application of the recent non-Abelian Density
Matrix Renormalization Group (DMRG) algorithm to the two dimensional t-J model.
This extension of the DMRG algorithm allows us to keep the equivalent of twice
as many basis states as the conventional DMRG algorithm for the same amount of
computational effort, which permits a deeper understanding of the nature of the
ground state.Comment: 16 pages, 3 figures. Contributed to the 2nd International Summer
School on Strongly Correlated Systems, Debrecen, Hungary, Sept. 200
Quantum phase slips in the presence of finite-range disorder
To study the effect of disorder on quantum phase slips (QPS) in
superconducting wires, we consider the plasmon-only model where disorder can be
incorporated into a first-principles instanton calculation. We consider weak
but general finite-range disorder and compute the formfactor in the QPS rate
associated with momentum transfer. We find that the system maps onto
dissipative quantum mechanics, with the dissipative coefficient controlled by
the wave (plasmon) impedance Z of the wire and with a superconductor-insulator
transition at Z=6.5 kOhm. We speculate that the system will remain in this
universality class after resistive effects at the QPS core are taken into
account.Comment: 4 pages, as accepted at Phys. Rev. Letter
Reflection Symmetry and Quantized Hall Resistivity near Quantum Hall Transition
We present a direct numerical evidence for reflection symmetry of
longitudinal resistivity and quantized Hall resistivity
near the transition between quantum Hall state and insulator, in accord
with the recent experiments. Our results show that a universal scaling behavior
of conductances, and , in the transition regime
decide the reflection symmetry of and quantization of ,
independent of particle-hole symmetry. We also find that in insulating phase
away from the transition region deviates from the quantization and
diverges with .Comment: 3 pages, 4 figures; figure 4 is replace
Hall Resistivity and Dephasing in the Quantum Hall Insulator
The longstanding problem of the Hall resistivity rho(x,y) in the Hall
insulator phase is addressed using four-lead Chalker-Coddington networks.
Electron interaction effects are introduced via a finite dephasing length. In
the quantum coherent regime, we find that rho(x,y) scales with the longitudinal
resistivity rho(x,x), and they both diverge exponentially with dephasing
length. In the Ohmic limit, (dephasing length shorter than Hall puddles' size),
rho(x,y) remains quantized and independent of rho(x,x). This suggests a new
experimental probe for dephasing processes.Comment: RevTeX, 4 pages, 3 figures included with epsf.st
Intersecting Loop Models on Z^D: Rigorous Results
We consider a general class of (intersecting) loop models in D dimensions,
including those related to high-temperature expansions of well-known spin
models. We find that the loop models exhibit some interesting features - often
in the ``unphysical'' region of parameter space where all connection with the
original spin Hamiltonian is apparently lost. For a particular n=2, D=2 model,
we establish the existence of a phase transition, possibly associated with
divergent loops. However, for n >> 1 and arbitrary D there is no phase
transition marked by the appearance of large loops. Furthermore, at least for
D=2 (and n large) we find a phase transition characterised by broken
translational symmetry.Comment: LaTeX+elsart.cls; 30 p., 6 figs; submitted to Nucl. Phys. B; a few
minor typos correcte
Second-order shaped pulses for solid-state quantum computation
We present the constructon and detailed analysis of highly-optimized
self-refocusing pulse shapes for several rotation angles. We characterize the
constructed pulses by the coefficients appearing in the Magnus expansion up to
second order. This allows a semi-analytical analysis of the performance of the
constructed shapes in sequences and composite pulses by computing the
corresponding leading-order error operators. Higher orders can be analyzed with
the numerical technique suggested by us previously. We illustrate the technique
by analysing several composite pulses designed to protect against pulse
amplitude errors, and on decoupling sequences for potentially long chains of
qubits with on-site and nearest-neighbor couplings.Comment: 16 pages, 29 figure
Conductivity tensor of striped quantum Hall phases
We study the transport properties of pinned striped quantum Hall phases. We
show that under quite general assumptions, the macroscopic conductivity tensor
satisfies a semicircle law. In particular, this result is valid for both
smectic and nematic stripe phases, independent of the presence of topological
defects such as dislocations and grain boundaries. As a special case, our
results explain the experimental validity of a product rule for the dissipative
part of the resistivity tensor, which was previously derived by MacDonald and
Fisher for a perfect stripe structure.Comment: 5 pages, 2 figure
Modular Groups, Visibility Diagram and Quantum Hall Effect
We consider the action of the modular group on the set of
positive rational fractions. From this, we derive a model for a classification
of fractional (as well as integer) Hall states which can be visualized on two
``visibility" diagrams, the first one being associated with even denominator
fractions whereas the second one is linked to odd denominator fractions. We use
this model to predict, among some interesting physical quantities, the relative
ratios of the width of the different transversal resistivity plateaus. A
numerical simulation of the tranversal resistivity plot based on this last
prediction fits well with the present experimental data.Comment: 17 pages, plain TeX, 4 eps figures included (macro epsf.tex), 1
figure available from reques
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