83 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
The fractional quantum Hall effect in infinite layer systems
Stacked two dimensional electron systems in transverse magnetic fields
exhibit three dimensional fractional quantum Hall phases. We analyze the
simplest such phases and find novel bulk properties, e.g., irrational braiding.
These phases host ``one and a half'' dimensional surface phases in which motion
in one direction is chiral. We offer a general analysis of conduction in the
latter by combining sum rule and renormalization group arguments, and find that
when interlayer tunneling is marginal or irrelevant they are chiral semi-metals
that conduct only at T > 0 or with disorder.Comment: RevTeX 3.0, 4p., 2 figs with epsf; reference to the detailed
companion paper cond-mat/0006506 adde
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
Quasi-Fermi Distribution and Resonant Tunneling of Quasiparticles with Fractional Charges
We study the resonant tunneling of quasiparticles through an impurity between
the edges of a Fractional Quantum Hall sample. We show that the one-particle
momentum distribution of fractionally charged edge quasiparticles has a
quasi-Fermi character. The density of states near the quasi-Fermi energy at
zero temperature is singular due to the statistical interaction of
quasiparticles. Another effect of this interaction is a new selection rule for
the resonant tunneling of fractionally charged quasiparticles: the resonance is
suppressed unless an integer number of {\em electrons} occupies the impurity.
It allows a new explanation of the scaling behavior observed in the mesoscopic
fluctuations of the conductivity in the FQHE.Comment: 7 pages, REVTeX 3.0, Preprint SU-ITP-93-1
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
Aging in a Two-Dimensional Ising Model with Dipolar Interactions
Aging in a two-dimensional Ising spin model with both ferromagnetic exchange
and antiferromagnetic dipolar interactions is established and investigated via
Monte Carlo simulations. The behaviour of the autocorrelation function
is analyzed for different values of the temperature, the waiting
time and the quotient , and being the
strength of exchange and dipolar interactions respectively. Different
behaviours are encountered for at low temperatures as is
varied. Our results show that, depending on the value of , the dynamics
of this non-disordered model is consistent either with a slow domain dynamics
characteristic of ferromagnets or with an activated scenario, like that
proposed for spin glasses.Comment: 4 pages, RevTex, 5 postscript figures; acknowledgment added and some
grammatical corrections in caption
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
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