654 research outputs found
Dynamical Properties of Two Coupled Hubbard Chains at Half-filling
Using grand canonical Quantum Monte Carlo (QMC) simulations combined with
Maximum Entropy analytic continuation, as well as analytical methods, we
examine the one- and two-particle dynamical properties of the Hubbard model on
two coupled chains at half-filling. The one-particle spectral weight function,
, undergoes a qualitative change with interchain hopping
associated with a transition from a four-band insulator to a two-band
insulator. A simple analytical model based on the propagation of exact rung
singlet states gives a good description of the features at large . For
smaller , is similar to that of the
one-dimensional model, with a coherent band of width the effective
antiferromagnetic exchange reasonably well-described by renormalized
spin-wave theory. The coherent band rides on a broad background of width
several times the parallel hopping integral , an incoherent structure
similar to that found in calculations on both the one- and two-dimensional
models. We also present QMC results for the two-particle spin and charge
excitation spectra, and relate their behavior to the rung singlet picture for
large and to the results of spin-wave theory for small .Comment: 9 pages + 10 postscript figures, submitted to Phys.Rev.B, revised
version with isotropic t_perp=t data include
The spectral gap for some spin chains with discrete symmetry breaking
We prove that for any finite set of generalized valence bond solid (GVBS)
states of a quantum spin chain there exists a translation invariant
finite-range Hamiltonian for which this set is the set of ground states. This
result implies that there are GVBS models with arbitrary broken discrete
symmetries that are described as combinations of lattice translations, lattice
reflections, and local unitary or anti-unitary transformations. We also show
that all GVBS models that satisfy some natural conditions have a spectral gap.
The existence of a spectral gap is obtained by applying a simple and quite
general strategy for proving lower bounds on the spectral gap of the generator
of a classical or quantum spin dynamics. This general scheme is interesting in
its own right and therefore, although the basic idea is not new, we present it
in a system-independent setting. The results are illustrated with an number of
examples.Comment: 48 pages, Plain TeX, BN26/Oct/9
Bell-inequality violation with "thermal" radiation
The model of a quantum-optical device for a conditional preparation of
entangled states from input mixed states is presented. It is demonstrated that
even thermal or pseudo-thermal radiation can be entangled in such a way, that
Bell-inequalities are violated
Two-Hole and Four-Hole Bound States in a t-J Ladder at half-filling
The two-hole excitation spectrum of the t-J ladder at half-filling is studied
using linked-cluster series expansion methods. A rich spectrum of bound states
emerges, particularly at small . Their dispersion relations and coherence
lengths are computed, along with the threshold behaviour as the bound states
merge into the continuum. A class of 4-hole bound states is also studied,
leading to the conclusion that phase separation occurs for ,
in agreement with other studies.Comment: revtex
Boundary Conditions and Quasilocal Energy in the Canonical Formulation of All 1+1 Models of Gravity
Within a first-order framework, we comprehensively examine the role played by
boundary conditions in the canonical formulation of a completely general
two-dimensional gravity model. Our analysis particularly elucidates the
perennial themes of mass and energy. The gravity models for which our arguments
are valid include theories with dynamical torsion and so-called generalized
dilaton theories (GDTs). Our analysis of the canonical action principle (i)
provides a rigorous correspondence between the most general first-order
two-dimensional Einstein-Cartan model (ECM) and GDT and (ii) allows us to
extract in a virtually simultaneous manner the ``true degrees of freedom'' for
both ECMs and GDTs. For all such models, the existence of an absolutely
conserved (in vacuo) quantity C is a generic feature, with (minus) C
corresponding to the black-hole mass parameter in the important special cases
of spherically symmetric four-dimensional general relativity and standard
two-dimensional dilaton gravity. The mass C also includes (minimally coupled)
matter into a ``universal mass function.'' We place particular emphasis on the
(quite general) class of models within GDT possessing a Minkowski-like
groundstate solution (allowing comparison between and the
Arnowitt-Deser-Misner mass for such models).Comment: REVTeX, 41 pages, 2 Postscript figures, 10 macro
The fate of spinons in spontaneously dimerised spin-1/2 ladders
We study a weakly coupled, frustrated two-leg spin-1/2 Heisenberg ladder. For
vanishing coupling between the chains, elementary excitations are deconfined,
gapless spin-1/2 objects called spinons. We investigate the fate of spinons for
the case of a weak interchain interaction. We show that despite a drastic
change in ground state, which becomes spontaneously dimerised, spinons survive
as elementary excitations but acquire a spectral gap. We furthermore determine
the exact dynamical structure factor for several values of momentum transfer.Comment: 8 pages of revtex, 7 figures; discussion of physical picture for
ground state and excitations in the "twistless" ladder expanded, version to
appear in Phys Rev
"The fruits of independence": Satyajit Ray, Indian nationhood and the spectre of empire
Challenging the longstanding consensus that Satyajit Ray's work is largely free of ideological concerns and notable only for its humanistic richness, this article shows with reference to representations of British colonialism and Indian nationhood that Ray's films and stories are marked deeply and consistently by a distinctively Bengali variety of liberalism. Drawn from an ongoing biographical project, it commences with an overview of the nationalist milieu in which Ray grew up and emphasizes the preoccupation with colonialism and nationalism that marked his earliest unfilmed scripts. It then shows with case studies of Kanchanjangha (1962), Charulata (1964), First Class Kamra (First-Class Compartment, 1981), Pratidwandi (The Adversary, 1970), Shatranj ke Khilari (The Chess Players, 1977), Agantuk (The Stranger, 1991) and Robertsoner Ruby (Robertson's Ruby, 1992) how Ray's mature work continued to combine a strongly anti-colonial viewpoint with a shifting perspective on Indian nationhood and an unequivocal commitment to cultural cosmopolitanism. Analysing how Ray articulated his ideological positions through the quintessentially liberal device of complexly staged debates that were apparently free, but in fact closed by the scenarist/director on ideologically specific notes, this article concludes that Ray's reputation as an all-forgiving, ‘everybody-has-his-reasons’ humanist is based on simplistic or even tendentious readings of his work
Three-Dimensional Gravity with Conformal Scalar and Asymptotic Virasoro Algebra
Strominger has derived the Bekenstein-Hawking entropy of the BTZ black hole
using asymptotic Virasoro algebra. We apply Strominger's method to a black hole
solution found by Martinez and Zanelli (MZ). This is a solution of
three-dimensional gravity with a conformal scalar field. The solution is not
, but it is asymptotically ; therefore, it has the asymptotic
Virasoro algebra. We compute the central charge for the theory and compares
Cardy's formula with the Bekenstein-Hawking entropy. It turns out that the
functional form does agree, but the overall numerical coefficient does not.
This is because this approach gives the "maximum possible entropy" for the
numerical coefficient.Comment: 26 pages, LaTeX; v2: minor correction
The making of English cricket cultures: Empire, globalization and (post) colonialism
The aim of this article is to understand how English cricket cultures have been made, negotiated and, ultimately, resisted in the context of (post) colonialism. I draw upon research undertaken with white and British Asian cricketers in Yorkshire to identify the place and significance of cricket within the everyday lives of British Asian communities. Over the last decade the number of British Asian cricketers progressing into the upper echelons of the game (mainly the English County Championship) has increased. Many within the game (mainly white people) have used these figures to argue that English cricket is now 'colour blind'. However, I argue that representation is not the equivalent to acceptance and integration, and present evidence to suggest that racial prejudice and discrimination, not to mention inaccurate and essentialized cultural stereotypes of British Asian cricketers, remain firmly and routinely embedded in aspects of the sport at all levels. I argue that the ability of British Asians to resist the hegemonic structures of white 'Englishness', by asserting their own distinctive post-colonial identities in cricket, is paramount to their everyday negotiations of power and racism. © 2011 Taylor & Francis
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