4,198 research outputs found
Space-Time Tradeoffs for Distributed Verification
Verifying that a network configuration satisfies a given boolean predicate is
a fundamental problem in distributed computing. Many variations of this problem
have been studied, for example, in the context of proof labeling schemes (PLS),
locally checkable proofs (LCP), and non-deterministic local decision (NLD). In
all of these contexts, verification time is assumed to be constant. Korman,
Kutten and Masuzawa [PODC 2011] presented a proof-labeling scheme for MST, with
poly-logarithmic verification time, and logarithmic memory at each vertex.
In this paper we introduce the notion of a -PLS, which allows the
verification procedure to run for super-constant time. Our work analyzes the
tradeoffs of -PLS between time, label size, message length, and computation
space. We construct a universal -PLS and prove that it uses the same amount
of total communication as a known one-round universal PLS, and factor
smaller labels. In addition, we provide a general technique to prove lower
bounds for space-time tradeoffs of -PLS. We use this technique to show an
optimal tradeoff for testing that a network is acyclic (cycle free). Our
optimal -PLS for acyclicity uses label size and computation space . We further describe a recursive space verifier for
acyclicity which does not assume previous knowledge of the run-time .Comment: Pre-proceedings version of paper presented at the 24th International
Colloquium on Structural Information and Communication Complexity (SIROCCO
2017
Scattering Mechanism in Modulation-Doped Shallow Two-Dimensional Electron Gases
We report on a systematic investigation of the dominant scattering mechanism
in shallow two-dimensional electron gases (2DEGs) formed in modulation-doped
GaAs/Al_{x}Ga_{1-x}As heterostructures. The power-law exponent of the electron
mobility versus density, mu \propto n^{alpha}, is extracted as a function of
the 2DEG's depth. When shallower than 130 nm from the surface, the power-law
exponent of the 2DEG, as well as the mobility, drops from alpha \simeq 1.65
(130 nm deep) to alpha \simeq 1.3 (60 nm deep). Our results for shallow 2DEGs
are consistent with theoretical expectations for scattering by remote dopants,
in contrast to the mobility-limiting background charged impurities of deeper
heterostructures.Comment: 4 pages, 3 figures, modified version as accepted in AP
The Intrinsic Spin Hall Conductivity in a Generalized Rashba Model
We calculate the intrinsic spin Hall conductivity \sigma^{\mathrm{sH}} of a
two-dimensional electron system within a generalized Rashba model, showing that
it is, in general, finite and model-dependent. Considering arbitrary band
dispersion, we find that \sigma^{\mathrm{sH}} in the presence of the
linear-in-momentum spin-orbit coupling of the Rashba form does not vanish in
the presence of impurities except for the precisely parabolic spectrum. We
show, using the linear response Kubo formalism, how the exact cancellation
happens for the quadratic dispersion, and why it does not occur in general. We
derive a simple quasiclassical formula for \sigma^{\mathrm{sH}} in terms of the
Fermi momenta for the two electron chiralities, and find that
\sigma^{\mathrm{sH}} is in general of the order of the squared strength of the
Rashba term
Spin Bose Glass Phase in Bilayer Quantum Hall Systems at
We develop an effective spin theory to describe magnetic properties of the
Quantum Hall bilayer systems. In the absence of disorder this theory
gives quantitative agreement with the results of microscopic Hartree-Fock
calculations, and for finite disorder it predicts the existence of a novel spin
Bose glass phase. The Bose glass is characterized by the presence of domains of
canted antiferromagnetic phase with zero average antiferromagnetic order and
short range mean antiferromagnetic correlations. It has infinite
antiferromagnetic transverse susceptibility, finite longitudinal spin
susceptibility and specific heat linear in temperature. Transition from the
canted antiferromagnet phase to the spin Bose glass phase is characterized by a
universal value of the longitudinal spin conductance.Comment: 4 pages, 4 eps figure
Carrier relaxation due to electron-electron interaction in coupled double quantum well structures
We calculate the electron-electron interaction induced energy-dependent
inelastic carrier relaxation rate in doped semiconductor coupled double quantum
well nanostructures within the two subband approximation at zero temperature.
In particular, we calculate, using many-body theory, the imaginary part of the
full self-energy matrix by expanding in the dynamically RPA screened Coulomb
interaction, obtaining the intrasubband and intersubband electron relaxation
rates in the ground and excited subbands as a function of electron energy. We
separate out the single particle and the collective excitation contributions,
and comment on the effects of structural asymmetry in the quantum well on the
relaxation rate. Effects of dynamical screening and Fermi statistics are
automatically included in our many body formalism rather than being
incorporated in an ad-hoc manner as one must do in the Boltzman theory.Comment: 26 pages, 5 figure
An extended Hubbard model with ring exchange: a route to a non-Abelian topological phase
We propose an extended Hubbard model on a 2D Kagome lattice with an
additional ring-exchange term. The particles can be either bosons or spinless
fermions . At a special filling fraction of 1/6 the model is analyzed in the
lowest non-vanishing order of perturbation theory. Such ``undoped'' model is
closely related to the Quantum Dimer Model. We show how to arrive at an exactly
soluble point whose ground state manifold is the extensively degenerate
``d-isotopy space'', a necessary precondition for for a certain type of
non-Abelian topological order. Near the ``special'' values, , this space is expected to collapse to a stable topological phase
with anyonic excitations closely related to SU(2) Chern-Simons theory at level
k.Comment: 4 pages, 2 colour figures, submitted to PRL. For an extended
treatment of a more general family of models see cond-mat/030912
Pressure-tuning of the electron-phonon coupling: the insulator to metal transition in manganites
A comprehensive understanding of the physical origin of the unique magnetic
and transport properties of A_(1-x)A'^xMnO_3 manganites (A = trivalent
rare-earth and A' = divalent alkali-earth metal) is still far from being
achieved. The complexity of these systems arises from the interplay among
several competing interactions of comparable strength. Recently the
electron-phonon coupling, triggered by a Jahn-Teller distortion of the MnO_6
octahedra, has been recognised to play an essential role in the insulator to
metal transition and in the closely related colossal magneto-resistance. The
pressure tuning of the octahedral distortion gives a unique possibility to
separate the basic interactions and, at least in principle, to follow the
progressive transformation of a manganite from an intermediate towards a weak
electron-phonon coupling regime. Using a diamond anvil cell, temperature and
pressure-dependent infrared absorption spectra of La_(0.75)Ca_(0.25)MnO_3 have
been collected and, from the spectral weight analysis, the pressure dependence
of the insulator to metal transition temperature T_IM has been determined for
the first time up to 11.2 GPa. The T_IM(P) curve we proposed to model the
present data revealed a universality character in accounting for the whole
class of intermediate coupling compounds. This property can be exploited to
distinguish the intermediate from the weak coupling compounds pointing out the
fundamental differences between the two coupling regimes.Comment: 8 pages, 4 figure
Estimates of electronic interaction parameters for LaO compounds (=Ti-Ni) from ab-initio approaches
We have analyzed the ab-initio local density approximation band structure
calculations for the family of perovskite oxides, LaO with =Ti-Ni
within a parametrized nearest neighbor tight-binding model and extracted
various interaction strengths. We study the systematics in these interaction
parameters across the transition metal series and discuss the relevance of
these in a many-body description of these oxides. The results obtained here
compare well with estimates of these parameters obtained via analysis of
electron spectroscopic results in conjunction with the Anderson impurity model.
The dependence of the hopping interaction strength, t, is found to be
approximately .Comment: 18 pages; 1 tex file+9 postscript files (appeared in Phys Rev B Oct
15,1996
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