3,832 research outputs found
Spin and energy correlations in the one dimensional spin 1/2 Heisenberg model
In this paper, we study the spin and energy dynamic correlations of the one
dimensional spin 1/2 Heisenberg model, using mostly exact diagonalization
numerical techniques. In particular, observing that the uniform spin and energy
currents decay to finite values at long times, we argue for the absence of spin
and energy diffusion in the easy plane anisotropic Heisenberg model.Comment: 10 pages, 3 figures, gzipped postscrip
Exact renormalization of the random transverse-field Ising spin chain in the strongly ordered and strongly disordered Griffiths phases
The real-space renormalization group (RG) treatment of random
transverse-field Ising spin chains by Fisher ({\it Phys. Rev. B{\bf 51}, 6411
(1995)}) has been extended into the strongly ordered and strongly disordered
Griffiths phases and asymptotically exact results are obtained. In the
non-critical region the asymmetry of the renormalization of the couplings and
the transverse fields is related to a non-linear quantum control parameter,
, which is a natural measure of the distance from the quantum critical
point. , which is found to stay invariant along the RG trajectories and
has been expressed by the initial disorder distributions, stands in the
singularity exponents of different physical quantities (magnetization,
susceptibility, specific heat, etc), which are exactly calculated. In this way
we have observed a weak-universality scenario: the Griffiths-McCoy
singularities does not depend on the form of the disorder, provided the
non-linear quantum control parameter has the same value. The exact scaling
function of the magnetization with a small applied magnetic field is calculated
and the critical point magnetization singularity is determined in a simple,
direct way.Comment: 11 page
Are acceptance, support, and the factors that affect them, different? Examining perceptions of U.S. fuel economy standards
Understanding the acceptance of and support for transportation policies focused on the environment, such fuel economy standards, is important because of the positive impact policies can have on the environment and overall sustainability goals. This study investigates the acceptance of and support for fuel economy standards through an online survey of Maine residents. Specifically, we assess the Corporate Average Fuel Economy (CAFE) standards, which aim to increase fuel economy of vehicles, while decreasing greenhouses gas emissions and foreign fuel dependence in the United States. We assess how perceptions of the policy and economic views of the market affect acceptance and support. We differentiate acceptance and support on two dimensions, a temporal and attitudinal–behavioral dimension. In doing so, we improve upon traditional measures of these variables and provide evidence that acceptance and support are distinct constructs. We find that perceived fairness, perceived effectiveness, and a subscription to a free-market ideology play a role in acceptance and support. The implications of the findings are discussed in relation to survey methods, policy communications, and an interdisciplinary understanding of environmental policy
Griffiths-McCoy singularities in random quantum spin chains: Exact results through renormalization
The Ma-Dasgupta-Hu renormalization group (RG) scheme is used to study
singular quantities in the Griffiths phase of random quantum spin chains. For
the random transverse-field Ising spin chain we have extended Fisher's
analytical solution to the off-critical region and calculated the dynamical
exponent exactly. Concerning other random chains we argue by scaling
considerations that the RG method generally becomes asymptotically exact for
large times, both at the critical point and in the whole Griffiths phase. This
statement is checked via numerical calculations on the random Heisenberg and
quantum Potts models by the density matrix renormalization group method.Comment: 4 pages RevTeX, 2 figures include
Finite-size scaling properties of random transverse-field Ising chains : Comparison between canonical and microcanonical ensembles for the disorder
The Random Transverse Field Ising Chain is the simplest disordered model
presenting a quantum phase transition at T=0. We compare analytically its
finite-size scaling properties in two different ensembles for the disorder (i)
the canonical ensemble, where the disorder variables are independent (ii) the
microcanonical ensemble, where there exists a global constraint on the disorder
variables. The observables under study are the surface magnetization, the
correlation of the two surface magnetizations, the gap and the end-to-end
spin-spin correlation for a chain of length . At criticality, each
observable decays typically as in both ensembles, but the
probability distributions of the rescaled variable are different in the two
ensembles, in particular in their asymptotic behaviors. As a consequence, the
dependence in of averaged observables differ in the two ensembles. For
instance, the correlation decays algebraically as 1/L in the canonical
ensemble, but sub-exponentially as in the microcanonical
ensemble. Off criticality, probability distributions of rescaled variables are
governed by the critical exponent in both ensembles, but the following
observables are governed by the exponent in the microcanonical
ensemble, instead of the exponent in the canonical ensemble (a) in the
disordered phase : the averaged surface magnetization, the averaged correlation
of the two surface magnetizations and the averaged end-to-end spin-spin
correlation (b) in the ordered phase : the averaged gap. In conclusion, the
measure of the rare events that dominate various averaged observables can be
very sensitive to the microcanonical constraint.Comment: 24 page
Lifespan theorem for constrained surface diffusion flows
We consider closed immersed hypersurfaces in and evolving by
a class of constrained surface diffusion flows. Our result, similar to earlier
results for the Willmore flow, gives both a positive lower bound on the time
for which a smooth solution exists, and a small upper bound on a power of the
total curvature during this time. By phrasing the theorem in terms of the
concentration of curvature in the initial surface, our result holds for very
general initial data and has applications to further development in asymptotic
analysis for these flows.Comment: 29 pages. arXiv admin note: substantial text overlap with
arXiv:1201.657
The open XXZ-chain: Bosonisation, Bethe ansatz and logarithmic corrections
We calculate the bulk and boundary parts of the free energy for an open
spin-1/2 XXZ-chain in the critical regime by bosonisation. We identify the
cutoff independent contributions and determine their amplitudes by comparing
with Bethe ansatz calculations at zero temperature T. For the bulk part of the
free energy we find agreement with Lukyanov's result [Nucl.Phys.B 522, 533
(1998)]. In the boundary part we obtain a cutoff independent term which is
linear in T and determines the temperature dependence of the boundary
susceptibility in the attractive regime for . We further show that at
particular anisotropies where contributions from irrelevant operators with
different scaling dimensions cross, logarithmic corrections appear. We give
explicit formulas for these terms at those anisotropies where they are most
important. We verify our results by comparing with extensive numerical
calculations based on a numerical solution of the T=0 Bethe ansatz equations,
the finite temperature Bethe ansatz equations in the quantum-transfer matrix
formalism, and the density-matrix renormalisation group applied to transfer
matrices.Comment: 35 pages, 8 figure
Dimer and N\'eel order-parameter fluctuations in the spin-fluid phase of the s=1/2 spin chain with first and second neighbor couplings
The dynamical properties at T=0 of the one-dimensional (1D) s=1/2
nearest-neighbor (nn) XXZ model with an additional isotropic
next-nearest-neighbor (nnn) coupling are investigated by means of the recursion
method in combination with techniques of continued-fraction analysis. The focus
is on the dynamic structure factors S_{zz}(q,\omega) and S_{DD}(q,\omega),
which describe (for q=\pi) the fluctuations of the N\'eel and dimer order
parameters, respectively. We calculate (via weak-coupling continued-fraction
analysis) the dependence on the exchange constants of the infrared exponent,
the renormalized bandwidth of spinon excitations, and the spectral-weight
distribution in S_{zz}(\pi,\omega) and S_{DD}(\pi,\omega), all in the
spin-fluid phase, which is realized for planar anisotropy and sufficiently
weak nnn coupling. For some parameter values we find a discrete branch of
excitations above the spinon continuum. They contribute to S_{zz}(q,\omega) but
not to S_{DD}(q,\omega).Comment: RevTex file (7 pages), 8 figures (uuencoded ps file) available from
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