543 research outputs found
Ground State and Excitations of Quantum Dots with "Magnetic Impurities"
We consider an "impurity" with a spin degree of freedom coupled to a finite
reservoir of non-interacting electrons, a system which may be realized by
either a true impurity in a metallic nano-particle or a small quantum dot
coupled to a large one. We show how the physics of such a spin impurity is
revealed in the many-body spectrum of the entire finite-size system; in
particular, the evolution of the spectrum with the strength of the
impurity-reservoir coupling reflects the fundamental many-body correlations
present. Explicit calculation in the strong and weak coupling limits shows that
the spectrum and its evolution are sensitive to the nature of the impurity and
the parity of electrons in the reservoir. The effect of the finite size
spectrum on two experimental observables is considered. First, we propose an
experimental setup in which the spectrum may be conveniently measured using
tunneling spectroscopy. A rate equation calculation of the differential
conductance suggests how the many-body spectral features may be observed.
Second, the finite-temperature magnetic susceptibility is presented, both the
impurity susceptibility and the local susceptibility. Extensive quantum
Monte-Carlo calculations show that the local susceptibility deviates from its
bulk scaling form. Nevertheless, for special assumptions about the reservoir --
the "clean Kondo box" model -- we demonstrate that finite-size scaling is
recovered. Explicit numerical evaluations of these scaling functions are given,
both for even and odd parity and for the canonical and grand-canonical
ensembles.Comment: 16 pages; published version, corrections to figure and equation,
clarification
Kosterlitz-Thouless Universality in a Fermionic System
A new extension of the attractive Hubbard model is constructed to study the
critical behavior near a finite temperature superconducting phase transition in
two dimensions using the recently developed meron-cluster algorithm. Unlike
previous calculations in the attractive Hubbard model which were limited to
small lattices, the new algorithm is used to study the critical behavior on
lattices as large as . These precise results for the first time
show that a fermionic system can undergo a finite temperature phase transition
whose critical behavior is well described by the predictions of Kosterlitz and
Thouless almost three decades ago. In particular it is confirmed that the
spatial winding number susceptibility obeys the well known predictions of
finite size scaling for and up to logarithmic corrections the pair
susceptibility scales as at large volumes with for .Comment: Revtex format; 4 pages, 2 figure
Constraints and Period Relations in Bosonic Strings at Genus-g
We examine some of the implications of implementing the usual boundary
conditions on the closed bosonic string in the hamiltonian framework. Using the
KN formalism, it is shown that at the quantum level, the resulting constraints
lead to relations among the periods of the basis 1-forms. These are compared
with those of Riemanns' which arise from a different consideration.Comment: 16 pages, (Plain Tex), NUS/HEP/9320
The Loop Algorithm
A review of the Loop Algorithm, its generalizations, and its relation to some
other Monte Carlo techniques is given. The loop algorithm is a Quantum Monte
Carlo procedure which employs nonlocal changes of worldline configurations,
determined by local stochastic decisions. It is based on a formulation of
quantum models of any dimension in an extended ensemble of worldlines and
graphs, and is related to Swendsen-Wang algorithms. It can be represented
directly on an operator level, both with a continuous imaginary time path
integral and with the stochastic series expansion (SSE). It overcomes many of
the difficulties of traditional worldline simulations. Autocorrelations are
reduced by orders of magnitude. Grand-canonical ensembles, off-diagonal
operators, and variance reduced estimators are accessible. In some cases,
infinite systems can be simulated. For a restricted class of models, the
fermion sign problem can be overcome. Transverse magnetic fields are handled
efficiently, in contrast to strong diagonal ones. The method has been applied
successfully to a variety of models for spin and charge degrees of freedom,
including Heisenberg and XYZ spin models, hard-core bosons, Hubbard, and
tJ-models. Due to the improved efficiency, precise calculations of asymptotic
behavior and of quantum critical exponents have been possible.Comment: Third Edition, July 2002. (78 pages, 11 figures). To appear in
Adv.Phys. Updated. New chapter on Operator Formulation, with continuous time
and with SS
On the Sign Problem in the Hirsch-Fye Algorithm for Impurity Problems
We show that there is no fermion sign problem in the Hirsch and Fye algorithm
for the single-impurity Anderson model. Beyond the particle-hole symmetric case
for which a simple proof exists, this has been known only empirically. Here we
prove the nonexistence of a sign problem for the general case by showing that
each spin trace for a given Ising configuration is separately positive. We
further use this insight to analyze under what conditions orbitally degenerate
Anderson models or the two-impurity Anderson model develop a sign.Comment: 2 pages, no figure; published versio
General bounds on the Wilson-Dirac operator
Lower bounds on the magnitude of the spectrum of the Hermitian Wilson-Dirac
operator H(m) have previously been derived for 0<m<2 when the lattice gauge
field satisfies a certain smoothness condition. In this paper lower bounds are
derived for 2p-2<m<2p for general p=1,2,...,d where d is the spacetime
dimension. The bounds can alternatively be viewed as localisation bounds on the
real spectrum of the usual Wilson-Dirac operator. They are needed for the
rigorous evaluation of the classical continuum limit of the axial anomaly and
index of the overlap Dirac operator at general values of m, and provide
information on the topological phase structure of overlap fermions. They are
also useful for understanding the instanton size-dependence of the real
spectrum of the Wilson-Dirac operator in an instanton background.Comment: 26 pages, 2 figures. v3: Completely rewritten with new material and
new title; to appear in Phys.Rev.
Availability and Accessibility of Research Outputs in NARS: A case study with IARI
This article focuses on the trends in publication, authorship pattern, availability, and accessibility of articles during 2008–2010 from the Indian Agricultural Research Institute (IARI), a constituent of the National Agricultural Research System in India. The data reveal that during the period of study, researchers from IARI produced 1,833 publications, most of which were jointly authored, and that the most preferred journal for publication by researchers is the Indian Journal of Agricultural Sciences, which is now an Open Access journal. While publications from IARI are available to subscribers of the Consortium for e-Resources in Agriculture (CeRA), public availability to IARI publications is very meager. Hence, in order to make their research output more accessible and available to a wider audience, IARI researchers should deposit their work in IARI’s Open Access repository Eprints@IARI. However, making such a deposit requires an Open Access policy, which IARI is yet to adopt
Superposition in nonlinear wave and evolution equations
Real and bounded elliptic solutions suitable for applying the Khare-Sukhatme
superposition procedure are presented and used to generate superposition
solutions of the generalized modified Kadomtsev-Petviashvili equation (gmKPE)
and the nonlinear cubic-quintic Schroedinger equation (NLCQSE).Comment: submitted to International Journal of Theoretical Physics, 23 pages,
2 figures, style change
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