908 research outputs found
Spin dynamics of SrCuO and the Heisenberg ladder
The Heisenberg antiferromagnet in the ladder geometry is studied as a
model for the spin degrees of freedom of SrCuO. The susceptibility and
the spin echo decay rate are calculated using a quantum Monte Carlo technique,
and the spin-lattice relaxation rate is obtained by maximum entropy analytic
continuation of imaginary time correlation functions. All calculated quantities
are in reasonable agreement with experimental results for SrCuO if the
exchange coupling K, i.e. significantly smaller than in
high-T cuprates.Comment: 11 pages (Revtex) + 3 uuencoded ps files. To appear in Phys. Rev. B,
Rapid Com
Various series expansions for the bilayer S=1/2 Heisenberg antiferromagnet
Various series expansions have been developed for the two-layer, S=1/2,
square lattice Heisenberg antiferromagnet. High temperature expansions are used
to calculate the temperature dependence of the susceptibility and specific
heat. At T=0, Ising expansions are used to study the properties of the
N\'{e}el-ordered phase, while dimer expansions are used to calculate the
ground-state properties and excitation spectra of the magnetically disordered
phase. The antiferromagnetic order-disorder transition point is determined to
be . Quantities computed include the staggered
magnetization, the susceptibility, the triplet spin-wave excitation spectra,
the spin-wave velocity, and the spin-wave stiffness. We also estimates that the
ratio of the intra- and inter-layer exchange constants to be for cuprate superconductor .Comment: RevTeX, 9 figure
NMR relaxation rates for the spin-1/2 Heisenberg chain
The spin-lattice relaxation rate and the spin echo decay rate
for the spin- antiferromagnetic Heisenberg chain are
calculated using quantum Monte Carlo and maximum entropy analytic continuation.
The results are compared with recent analytical calculations by Sachdev. If the
nuclear hyperfine form factor is strongly peaked around the
predicted low-temperature behavior [, ] extends up to temperatures as high as . If has significant weight for there are large
contributions from diffusive long-wavelength processes not taken into account
in the theory, and very low temperatures are needed in order to observe the
asymptotic forms.Comment: 9 pages, Revtex 3.0, 5 uuencoded ps figures To appear in Phys. Rev.
B, Rapid Com
Time of flight observables and the formation of Mott domains of fermions and bosons on optical lattices
We study, using quantum Monte Carlo simulations, the energetics of the
formation of Mott domains of fermions and bosons trapped on one-dimensional
lattices. We show that, in both cases, the sum of kinetic and interaction
energies exhibits minima when Mott domains appear in the trap. In addition, we
examine the derivatives of the kinetic and interaction energies, and of their
sum, which display clear signatures of the Mott transition. We discuss the
relevance of these findings to time-of-flight experiments that could allow the
detection of the metal--Mott-insulator transition in confined fermions on
optical lattices, and support established results on the
superfluid--Mott-insulator transition in confined bosons on optical lattices.Comment: 5 pages, 6 figures, published versio
Universal scaling at field-induced magnetic phase transitions
We study field-induced magnetic order in cubic lattices of dimers with
antiferromagnetic Heisenberg interactions. The thermal critical exponents at
the quantum phase transition from a spin liquid to a magnetically ordered phase
are determined from Stochastic Series Expansion Quantum Monte Carlo
simulations. These exponents are independent of the interdimer coupling ratios,
and converge to the value obtained by considering the transition as a
Bose-Einstein condensation of magnons, alpha_(BEC) = 1.5. The scaling results
are of direct relevance to the spin-dimer systems TlCuCl_3 and KCuCl_3, and
explain the broad range of exponents reported for field-induced ordering
transitions.Comment: 4 pages, 4 eps-figure
A New Approach to Stochastic State selections in Quantum Spin Systems
We propose a new type of Monte Carlo approach in numerical studies of quantum
systems. Introducing a probability function which determines whether a state in
the vector space survives or not, we can evaluate expectation values of powers
of the Hamiltonian from a small portion of the full vector space. This method
is free from the negative sign problem because it is not based on importance
sampling techniques. In this paper we describe our method and, in order to
examine how effective it is, present numerical results on the 4x4, 6x6 and 8x8
Heisenberg spin one-half model. The results indicate that we can perform useful
evaluations with limited computer resources. An attempt to estimate the lowest
energy eigenvalue is also stated.Comment: 10 pages, 2 figures, 8 table
Magnetic Correlations in the Two Dimensional Anderson-Hubbard Model
The two dimensional Hubbard model in the presence of diagonal and
off-diagonal disorder is studied at half filling with a finite temperature
quantum Monte Carlo method. Magnetic correlations as well as the electronic
compressibility are calculated to determine the behavior of local magnetic
moments, the stability of antiferromagnetic long range order (AFLRO), and
properties of the disordered phase. The existence of random potentials
(diagonal or ``site'' disorder) leads to a suppression of local magnetic
moments which eventually destroys AFLRO. Randomness in the hopping elements
(off-diagonal disorder), on the other hand, does not significantly reduce the
density of local magnetic moments. For this type of disorder, at half-filling,
there is no ``sign-problem'' in the simulations as long as the hopping is
restricted between neighbor sites on a bipartite lattice. This allows the study
of sufficiently large lattices and low temperatures to perform a finite-size
scaling analysis. For off-diagonal disorder AFLRO is eventually destroyed when
the fluctuations of antiferromagnetic exchange couplings exceed a critical
value. The disordered phase close to the transition appears to be
incompressible and shows an increase of the uniform susceptibility at low
temperatures.Comment: 10 pages, REVTeX, 14 figures included using psfig.st
The digital challenge for multinational mobile network operators. More marginalization or rejuvenation?
Multinational mobile network operators (MNOs) rapidly emerged in the early 1990s and for a decade and a half were the dominant actors in their industry. We analyze the development and competitiveness of a typical MNO, Telenor. With the introduction of 4G in 2010, we show that Telenor, like other MNOs largely failed to respond to the opportunity that connectivity provided to develop digital services. Instead, these were developed by technology platform companies such as Amazon, Google and Microsoft. Telenor became a marginalized supplier of standardized internet connectivity. We argue that the ‘decade of lost opportunity’ (2010-2020) for Telenor was a product of a lack of ‘recombinant firm-specific advantages’ (FSARs). With the launch of 5G, an emerging global digital infrastructure, this sidelining is set to intensify unless Telenor responds to this new opportunity by developing B2B digital services. We analyze the FSARs that are necessary for a successful transition of capturing the value that 5G provides and the degree to which they are present, or potentially present, in Telenor
Quantum-critical scaling and temperature-dependent logarithmic corrections in the spin-half Heisenberg chain
Low temperature dynamics of the S=1/2 Heisenberg chain is studied via a
simple ansatz generalizing the conformal mapping and analytic continuation
procedures to correlation functions with multiplicative logarithmic factors.
Closed form expressions for the dynamic susceptibility and the NMR relaxation
rates 1/T_1 and 1/T_{2G} are obtained, and are argued to improve the agreement
with recent experiments. Scaling in q/T and \omega/T are violated due to these
logarithmic terms. Numerical results show that the logarithmic corrections are
very robust. While not yet in the asymptotic low temperature regime, they
provide striking qualitative confirmation of the theoretical results.Comment: 4 pages, RevTex, 4 postscript figures in one fil
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