16,053 research outputs found
Pairing and Excitation Spectrum in doped t-J Ladders
Exact diagonalization studies for a doped t-J ladder (or double chain) show
hole pairing in the ground state. The excitation spectrum separates into a
limited number of quasiparticles which carry charge and spin and a triplet mode. At half-filling the former vanish but the latter
evolves continuously into the triplet band of the spin liquid. At low doping
the quasiparticles form a dilute Fermi gas with a strong attraction but
simultaneously the Fermi wavevector, as would be measured in photoemission, is
large.Comment: 10 pages and 4 PostScript figures, RevTeX 3.0, ETH-TH/94-0
Competing states in the t-J model: uniform d-wave state versus stripe state
Variational studies of the t-J model on the square lattice based on infinite
projected-entangled pair states (iPEPS) confirm an extremely close competition
between a uniform d-wave superconducting state and different stripe states. The
site-centered stripe with an in-phase d-wave order has an equal or only
slightly lower energy than the stripe with anti-phase d-wave order. The optimal
stripe filling is not constant but increases with J/t. A nematic anisotropy
reduces the pairing amplitude and the energies of stripe phases are lowered
relative to the uniform state with increasing nematicity.Comment: 6 pages, 4 figures, 4 pages of supplemental materia
Spin transport and spin dephasing in zinc oxide
The wide bandgap semiconductor ZnO is interesting for spintronic applications
because of its small spin-orbit coupling implying a large spin coherence
length. Utilizing vertical spin valve devices with ferromagnetic electrodes
(TiN/Co/ZnO/Ni/Au), we study the spin-polarized transport across ZnO in
all-electrical experiments. The measured magnetoresistance agrees well with the
prediction of a two spin channel model with spin-dependent interface
resistance. Fitting the data yields spin diffusion lengths of 10.8nm (2K),
10.7nm (10K), and 6.2nm (200K) in ZnO, corresponding to spin lifetimes of 2.6ns
(2K), 2.0ns (10K), and 31ps (200K).Comment: 7 pages, 5 figures; supplemental material adde
Micrometre-scale refrigerators
A superconductor with a gap in the density of states or a quantum dot with
discrete energy levels is a central building block in realizing an electronic
on-chip cooler. They can work as energy filters, allowing only hot
quasiparticles to tunnel out from the electrode to be cooled. This principle
has been employed experimentally since the early 1990s in investigations and
demonstrations of micrometre-scale coolers at sub-kelvin temperatures. In this
paper, we review the basic experimental conditions in realizing the coolers and
the main practical issues that are known to limit their performance. We give an
update of experiments performed on cryogenic micrometre-scale coolers in the
past five years
The Impact of Increased Import Competition from the People’s Republic of China on Income Inequality and Household Welfare in Viet Nam
This paper examines the surge of imports from the PRC to Viet Nam from 2000 to 2014 in order to evaluate the effects of increased exposure to trade with the PRC on income inequality and household welfare in Viet Nam. Using household level data from the Viet Nam Household Living Standard Survey and combining it with measures of trade exposure, we find that increased imports led to a fall in inequality at the provincial and district level. We distinguish between intermediate and final goods and find similar results. In order to better understand the relative gains and losses across income groups, we apply a quantile regression approach. Our results indicate that increased imports were more often positively correlated with household income for households located in the lower quantiles. In contrast, for households in the upper quantiles the correlation is either negative or less pronounced
Localization for MCMC: sampling high-dimensional posterior distributions with local structure
We investigate how ideas from covariance localization in numerical weather
prediction can be used in Markov chain Monte Carlo (MCMC) sampling of
high-dimensional posterior distributions arising in Bayesian inverse problems.
To localize an inverse problem is to enforce an anticipated "local" structure
by (i) neglecting small off-diagonal elements of the prior precision and
covariance matrices; and (ii) restricting the influence of observations to
their neighborhood. For linear problems we can specify the conditions under
which posterior moments of the localized problem are close to those of the
original problem. We explain physical interpretations of our assumptions about
local structure and discuss the notion of high dimensionality in local
problems, which is different from the usual notion of high dimensionality in
function space MCMC. The Gibbs sampler is a natural choice of MCMC algorithm
for localized inverse problems and we demonstrate that its convergence rate is
independent of dimension for localized linear problems. Nonlinear problems can
also be tackled efficiently by localization and, as a simple illustration of
these ideas, we present a localized Metropolis-within-Gibbs sampler. Several
linear and nonlinear numerical examples illustrate localization in the context
of MCMC samplers for inverse problems.Comment: 33 pages, 5 figure
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