886 research outputs found
Zero-variance principle for Monte Carlo algorithms
We present a general approach to greatly increase at little cost the
efficiency of Monte Carlo algorithms. To each observable to be computed we
associate a renormalized observable (improved estimator) having the same
average but a different variance. By writing down the zero-variance condition a
fundamental equation determining the optimal choice for the renormalized
observable is derived (zero-variance principle for each observable separately).
We show, with several examples including classical and quantum Monte Carlo
calculations, that the method can be very powerful.Comment: 9 pages, Latex, to appear in Phys. Rev. Let
Application of the density matrix renormalization group method to finite temperatures and two-dimensional systems
The density matrix renormalization group (DMRG) method and its applications
to finite temperatures and two-dimensional systems are reviewed. The basic idea
of the original DMRG method, which allows precise study of the ground state
properties and low-energy excitations, is presented for models which include
long-range interactions. The DMRG scheme is then applied to the diagonalization
of the quantum transfer matrix for one-dimensional systems, and a reliable
algorithm at finite temperatures is formulated. Dynamic correlation functions
at finite temperatures are calculated from the eigenvectors of the quantum
transfer matrix with analytical continuation to the real frequency axis. An
application of the DMRG method to two-dimensional quantum systems in a magnetic
field is demonstrated and reliable results for quantum Hall systems are
presented.Comment: 33 pages, 18 figures; corrected Eq.(117
Enhancement of Pairing Correlation and Spin Gap through Suppression of Single-Particle Dispersion in One-Dimensional Models
We investigate the effects of suppression of single-particle dispersion near
the Fermi level on the spin gap and the singlet-pairing correlation by using
the exact diagonalization method for finite-size systems. We consider strongly
correlated one-dimensional models, which have flat band dispersions near wave
number k=\pi/2, if the interactions are switched off. Our results for strongly
correlated models show that the spin gap region expands as the single-particle
dispersion becomes flatter. The region where the singlet-pairing correlation is
the most dominant also expands in models with flatter band dispersions. Based
on our numerical results, we propose a pairing mechanism induced by the
flat-band dispersion.Comment: 5 pages, including 5 eps figures, to appear in J.Phys.Soc.Jpn Vol.69
No.
Moduli Spaces of Cold Holographic Matter
We use holography to study (3+1)-dimensional N=4 supersymmetric Yang-Mills
theory with gauge group SU(Nc), in the large-Nc and large-coupling limits,
coupled to a single massless (n+1)-dimensional hypermultiplet in the
fundamental representation of SU(Nc), with n=3,2,1. In particular, we study
zero-temperature states with a nonzero baryon number charge density, which we
call holographic matter. We demonstrate that a moduli space of such states
exists in these theories, specifically a Higgs branch parameterized by the
expectation values of scalar operators bilinear in the hypermultiplet scalars.
At a generic point on the Higgs branch, the R-symmetry and gauge group are
spontaneously broken to subgroups. Our holographic calculation consists of
introducing a single probe Dp-brane into AdS5 times S^5, with p=2n+1=7,5,3,
introducing an electric flux of the Dp-brane worldvolume U(1) gauge field, and
then obtaining explicit solutions for the worldvolume fields dual to the scalar
operators that parameterize the Higgs branch. In all three cases, we can
express these solutions as non-singular self-dual U(1) instantons in a
four-dimensional space with a metric determined by the electric flux. We
speculate on the possibility that the existence of Higgs branches may point the
way to a counting of the microstates producing a nonzero entropy in holographic
matter. Additionally, we speculate on the possible classification of
zero-temperature, nonzero-density states described holographically by probe
D-branes with worldvolume electric flux.Comment: 56 pages, 8 PDF images, 4 figure
Quantification of volumetric morphometry and optical property in the cortex of human cerebellum at micrometer resolution
The surface of the human cerebellar cortex is much more tightly folded than the cerebral cortex. Volumetric analysis of cerebellar morphometry in magnetic resonance imaging studies suffers from insufficient resolution, and therefore has had limited impact on disease assessment. Automatic serial polarization-sensitive optical coherence tomography (as-PSOCT) is an emerging technique that offers the advantages of microscopic resolution and volumetric reconstruction of large-scale samples. In this study, we reconstructed multiple cubic centimeters of ex vivo human cerebellum tissue using as-PSOCT. The morphometric and optical properties of the cerebellar cortex across five subjects were quantified. While the molecular and granular layers exhibited similar mean thickness in the five subjects, the thickness varied greatly in the granular layer within subjects. Layer-specific optical property remained homogenous within individual subjects but showed higher cross-subject variability than layer thickness. High-resolution volumetric morphometry and optical property maps of human cerebellar cortex revealed by as-PSOCT have great potential to advance our understanding of cerebellar function and diseases
Control of Superconducting Correlations in High-Tc Cuprates
A strategy to enhance d-wave superconducting correlations is proposed based
on our numerical study for correlated electron models for high-Tc cuprates. We
observe that the pairing is enhanced when the single-electron level around
(pi,0) is close to the Fermi level E_F, while the d-wave pairing interaction
itself contains elements to disfavor the pairing due to shift of the
(pi,0)-level. Angle-resolved photoemission results in the cuprates are
consistently explained in the presence of the d-wave pairing interaction. Our
proposal is the tuning of the (pi,0)-level under the many-body effects to E_F
by optimal design of band structure.Comment: 4 pages, 6 eps figure
Superconductivity from correlated hopping
We consider a chain described by a next-nearest-neighbor hopping combined
with a nearest-neighbor spin flip. In two dimensions this three-body term
arises from a mapping of the three-band Hubbard model for CuO planes to a
generalized model and for large O-O hopping favors resonance-valence-bond
superconductivity of predominantly -wave symmetry. Solving the ground state
and low-energy excitations by analytical and numerical methods we find that the
chain is a Luther-Emery liquid with correlation exponent , where is the particle density.Comment: 10 pages, RevTeX 3.0 + 2 PostScript figs. Accepted for publication in
Phys.Rev.
Thermodynamic and diamagnetic properties of weakly doped antiferromagnets
Finite-temperature properties of weakly doped antiferromagnets as modeled by
the two-dimensional t-J model and relevant to underdoped cuprates are
investigated by numerical studies of small model systems at low doping. Two
numerical methods are used: the worldline quantum Monte Carlo method with a
loop cluster algorithm and the finite-temperature Lanczos method, yielding
consistent results. Thermodynamic quantities: specific heat, entropy and spin
susceptibility reveal a sizeable perturbation induced by holes introduced into
a magnetic insulator, as well as a pronounced temperature dependence. The
diamagnetic susceptibility introduced by coupling of the magnetic field to the
orbital current reveals an anomalous temperature dependence, changing character
from diamagnetic to paramagnetic at intermediate temperatures.Comment: LaTeX, 10 pages, 10 figures, submitted to Phys. Rev.
The Operator Product Expansion of the Lowest Higher Spin Current at Finite N
For the N=2 Kazama-Suzuki(KS) model on CP^3, the lowest higher spin current
with spins (2, 5/2, 5/2,3) is obtained from the generalized GKO coset
construction. By computing the operator product expansion of this current and
itself, the next higher spin current with spins (3, 7/2, 7/2, 4) is also
derived. This is a realization of the N=2 W_{N+1} algebra with N=3 in the
supersymmetric WZW model. By incorporating the self-coupling constant of lowest
higher spin current which is known for the general (N,k), we present the
complete nonlinear operator product expansion of the lowest higher spin current
with spins (2, 5/2, 5/2, 3) in the N=2 KS model on CP^N space. This should
coincide with the asymptotic symmetry of the higher spin AdS_3 supergravity at
the quantum level. The large (N,k) 't Hooft limit and the corresponding
classical nonlinear algebra are also discussed.Comment: 62 pages; the footnotes added, some redundant appendices removed, the
presentations in the whole paper improved and to appear in JHE
Anomalous Zero Sound
We show that the anomalous term in the current, recently suggested by Son and
Yamamoto, modifies the structure of the zero sound mode in the Fermi liquid in
a magnetic field.Comment: 14 pages, 2 figure
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