32 research outputs found
Disorder Induced Phase Transition in a Random Quantum Antiferromagnet
A two-dimensional Heisenberg model with random antiferromagnetic
nearest-neighbor exchange is studied using quantum Monte Carlo techniques. As
the strength of the randomness is increased, the system undergoes a transition
from an antiferromagnetically ordered ground state to a gapless disordered
state. The finite-size scaling of the staggered structure factor and
susceptibility is consistent with a dynamic exponent .Comment: Revtex 3.0, 10 pages + 5 postscript figures available upon request,
UCSBTH-94-1
A mathematical analysis of the evolution of perturbations in a modified Chaplygin gas model
One approach in modern cosmology consists in supposing that dark matter and
dark energy are different manifestations of a single `quartessential' fluid.
Following such idea, this work presents a study of the evolution of
perturbations of density in a flat cosmological model with a modified Chaplygin
gas acting as a single component. Our goal is to obtain properties of the model
which can be used to distinguish it from another cosmological models which have
the same solutions for the general evolution of the scale factor of the
universe, without the construction of the power spectrum. Our analytical
results, which alone can be used to uniquely characterize the specific model
studied in our work, show that the evolution of the density contrast can be
seen, at least in one particular case, as composed by a spheroidal wave
function. We also present a numerical analysis which clearly indicates as one
interesting feature of the model the appearence of peaks in the evolution of
the density constrast.Comment: 21 pages, accepted for publication in General Relativity and
Gravitatio
Phase separation and stripe formation in the 2D t-J model: a comparison of numerical results
We make a critical analysis of numerical results for and against phase
separation and stripe formation in the t-J model. We argue that the frustrated
phase separation mechanism for stripe formation requires phase separation at
too high a doping for it to be consistent with existing numerical studies of
the t-J model. We compare variational energies for various methods, and
conclude that the most accurate calculations for large systems appear to be
from the density matrix renormalization group. These calculations imply that
the ground state of the doped t-J model is striped, not phase separated.Comment: This version includes a revised, more careful comparison of numerical
results between DMRG and Green's function Monte Carlo. In particular, for the
original posted version we were accidentally sent obsolete data by Hellberg
and Manousakis; their new results, which are what were used in their Physical
Review Letter, are more accurate because a better trial wavefunction was use
Charge kinks as Raman scatterers in quarter-filled ladders
Charge kinks are considered as fundamental excitations in quarter-filled
charge-ordered ladders. The strength of the coupling of the kinks to the
three-dimensional lattice depends on their energy. The integrated intensity of
Raman scattering by kink-antikink pairs is proportional to or where is the order parameter. The exponent is determined by the
system parameters and by the strength of the electron-phonon coupling.Comment: To be published in Phys. Rev.B (june 2001
A switchable controlled-NOT gate in a spin-chain NMR quantum computer
A method of switching a controlled-NOT gate in a solid-stae NMR quantum
computer is presented. Qubits of I=1/2 nuclear spins are placed periodically
along a quantum spin chain (1-D antiferromagnet) having a singlet ground state
with a finite spin gap to the lowest excited state caused by some quantum
effect. Irradiation of a microwave tuned to the spin gap energy excites a
packet of triplet magnons at a specific part of the chain where control and
target qubits are involved. The packet switches on the Suhl-Nakamura
interaction between the qubits, which serves as a controlled NOT gate. The
qubit initialization is achieved by a qubit initializer consisting of
semiconducting sheets attached to the spin chain, where spin polarizations
created by the optical pumping method in the semiconductors are transferred to
the spin chain. The scheme allows us to separate the initialization process
from the computation, so that one can optimize the computation part without
being restricted by the initialization scheme, which provides us with a wide
selection of materials for a quantum computer.Comment: 8 pages, 5 figure
Frustration and the Kondo effect in heavy fermion materials
The observation of a separation between the antiferromagnetic phase boundary
and the small-large Fermi surface transition in recent experiments has led to
the proposal that frustration is an important additional tuning parameter in
the Kondo lattice model of heavy fermion materials. The introduction of a Kondo
(K) and a frustration (Q) axis into the phase diagram permits us to discuss the
physics of heavy fermion materials in a broader perspective. The current
experimental situation is analysed in the context of this combined "QK" phase
diagram. We discuss various theoretical models for the frustrated Kondo
lattice, using general arguments to characterize the nature of the -electron
localization transition that occurs between the spin liquid and heavy Fermi
liquid ground-states. We concentrate in particular on the Shastry--Sutherland
Kondo lattice model, for which we establish the qualitative phase diagram using
strong coupling arguments and the large- expansion. The paper closes with
some brief remarks on promising future theoretical directions.Comment: To appear in a special issue of JLT
Cosmological parameters from SDSS and WMAP
We measure cosmological parameters using the three-dimensional power spectrum
P(k) from over 200,000 galaxies in the Sloan Digital Sky Survey (SDSS) in
combination with WMAP and other data. Our results are consistent with a
``vanilla'' flat adiabatic Lambda-CDM model without tilt (n=1), running tilt,
tensor modes or massive neutrinos. Adding SDSS information more than halves the
WMAP-only error bars on some parameters, tightening 1 sigma constraints on the
Hubble parameter from h~0.74+0.18-0.07 to h~0.70+0.04-0.03, on the matter
density from Omega_m~0.25+/-0.10 to Omega_m~0.30+/-0.04 (1 sigma) and on
neutrino masses from <11 eV to <0.6 eV (95%). SDSS helps even more when
dropping prior assumptions about curvature, neutrinos, tensor modes and the
equation of state. Our results are in substantial agreement with the joint
analysis of WMAP and the 2dF Galaxy Redshift Survey, which is an impressive
consistency check with independent redshift survey data and analysis
techniques. In this paper, we place particular emphasis on clarifying the
physical origin of the constraints, i.e., what we do and do not know when using
different data sets and prior assumptions. For instance, dropping the
assumption that space is perfectly flat, the WMAP-only constraint on the
measured age of the Universe tightens from t0~16.3+2.3-1.8 Gyr to
t0~14.1+1.0-0.9 Gyr by adding SDSS and SN Ia data. Including tensors, running
tilt, neutrino mass and equation of state in the list of free parameters, many
constraints are still quite weak, but future cosmological measurements from
SDSS and other sources should allow these to be substantially tightened.Comment: Minor revisions to match accepted PRD version. SDSS data and ppt
figures available at http://www.hep.upenn.edu/~max/sdsspars.htm
Photodynamic Therapy – In Vitro Investigation Using an Electrochemical Microsensor System
Inclined lidar observations of boundary layer aerosol particles above the Kongsfjord, Svalbard
An inclined lidar with vertical resolution of 0.4 m was used for detailed boundary layer studies and to link observations at Zeppelin Mountain (474 m) and Ny-Ålesund, Svalbard. We report on the observation of aerosol layers directly above the Kongsfjord. On 29 April 2007, a layer of enhanced backscatter was observed in the lowest 25 m above the open water surface. The low depolarization ratio indicated spherical particles. In the afternoon, this layer disappeared. The ultrafine particle concentration at Zeppelin and Corbel station (close to the Kongsfjord) was low. On 1 May 2007, a drying process in the boundary layer was observed. In the morning, the atmosphere up to Zeppelin Mountain showed enhanced values of the backscatter coefficient. Around noon, the top of the highly reflecting boundary layer decreased from 350 to 250 m. The top of the boundary layer observed by lidar was confirmed by radiosonde data