634 research outputs found
Quantum Monte Carlo treatment of elastic exciton-exciton scattering
We calculate cross sections for low energy elastic exciton-exciton scattering
within the effective mass approximation. Unlike previous theoretical
approaches, we give a complete, non-perturbative treatment of the four-particle
scattering problem. Diffusion Monte Carlo is used to calculate the essentially
exact energies of scattering states, from which phase shifts are determined.
For the case of equal-mass electrons and holes, which is equivalent to
positronium-positronium scattering, we find a_s = 2.1 a_x for scattering of
singlet-excitons and a_s= 1.5 a_x for triplet-excitons, where a_x is the
excitonic radius. The spin dependence of the cross sections arises from the
spatial exchange symmetry of the scattering wavefunctions. A significant
triplet-triplet to singlet-singlet scattering process is found, which is
similar to reported effects in recent experiments and theory for excitons in
quantum wells. We also show that the scattering length can change sign and
diverge for some values of the mass ratio m_h/m_e, an effect not seen in
previous perturbative treatments.Comment: 6 pages, 6 figures. Revision has updated figures, improved paper
structure, some minor correction
Unitary Fermi gas, epsilon expansion, and nonrelativistic conformal field theories
We review theoretical aspects of unitary Fermi gas (UFG), which has been
realized in ultracold atom experiments. We first introduce the epsilon
expansion technique based on a systematic expansion in terms of the
dimensionality of space. We apply this technique to compute the thermodynamic
quantities, the quasiparticle spectrum, and the critical temperature of UFG. We
then discuss consequences of the scale and conformal invariance of UFG. We
prove a correspondence between primary operators in nonrelativistic conformal
field theories and energy eigenstates in a harmonic potential. We use this
correspondence to compute energies of fermions at unitarity in a harmonic
potential. The scale and conformal invariance together with the general
coordinate invariance constrains the properties of UFG. We show the vanishing
bulk viscosities of UFG and derive the low-energy effective Lagrangian for the
superfluid UFG. Finally we propose other systems exhibiting the nonrelativistic
scaling and conformal symmetries that can be in principle realized in ultracold
atom experiments.Comment: 44 pages, 15 figures, contribution to Lecture Notes in Physics
"BCS-BEC crossover and the Unitary Fermi Gas" edited by W. Zwerge
Comparison of exact and approximate cross-sections in relativistic Coulomb excitation
We present a new method of obtaining time-dependent matrix elements of the
electromagnetic pulse produced by a highly-relativistic projectile. These
matrix elements are used in a coupled-channel calculation to predict the
cross-sections for population of 1- and 2-phonon states of the giant dipole
resonance. Comparisons are made with the predictions of the long-wavelength and
Born approximations.Comment: 26 pages, LaTex2
The extraction of nuclear sea quark distribution and energy loss effect in Drell-Yan experiment
The next-to-leading order and leading order analysis are performed on the
differential cross section ratio from Drell-Yan process. It is found that the
effect of next-to-leading order corrections can be negligible on the
differential cross section ratios as a function of the quark momentum fraction
in the beam proton and the target nuclei for the current Fermilab and future
lower beam proton energy. The nuclear Drell-Yan reaction is an ideal tool to
study the energy loss of the fast quark moving through cold nuclei. In the
leading order analysis, the theoretical results with quark energy loss are in
good agreement with the Fermilab E866 experimental data on the Drell-Yan
differential cross section ratios as a function of the momentum fraction of the
target parton. It is shown that the quark energy loss effect has significant
impact on the Drell-Yan differential cross section ratios. The nuclear
Drell-Yan experiment at current Fermilab and future lower energy proton beam
can not provide us with more information on the nuclear sea quark distribution.Comment: 17 pages, 4 figure
Signatures of Superfluidity in Dilute Fermi Gases near a Feshbach Resonance
We present a brief account of the most salient properties of vortices in
dilute atomic Fermi superfluids near a Feshbach resonance.Comment: 6 pages, 1 figure, and jltp.cls. Several typos and a couple of
inaccuracies have been correcte
An Improved Upper Bound for the Ground State Energy of Fermion Lattice Models
We present an improved upper bound for the ground state energy of lattice
fermion models with sign problem. The bound can be computed by numerical
simulation of a recently proposed family of deformed Hamiltonians with no sign
problem. For one dimensional models, we expect the bound to be particularly
effective and practical extrapolation procedures are discussed. In particular,
in a model of spinless interacting fermions and in the Hubbard model at various
filling and Coulomb repulsion we show how such techniques can estimate ground
state energies and correlation function with great accuracy.Comment: 5 pages, 5 figures; to appear in Physical Review
Adaptive Sampling Approach to the Negative Sign Problem in the Auxiliary Field Quantum Monte Carlo Method
We propose a new sampling method to calculate the ground state of interacting
quantum systems. This method, which we call the adaptive sampling quantum monte
carlo (ASQMC) method utilises information from the high temperature density
matrix derived from the monte carlo steps. With the ASQMC method, the negative
sign ratio is greatly reduced and it becomes zero in the limit
goes to zero even without imposing any constraint such like the constraint path
(CP) condition. Comparisons with numerical results obtained by using other
methods are made and we find the ASQMC method gives accurate results over wide
regions of physical parameters values.Comment: 8 pages, 7 figure
Triton photodisintegration in three-dimensional approach
Two- and three- particles photodisintegration of the triton is investigated
in a three-dimensional (3D) Faddeev approach. For this purpose the Jacobi
momentum vectors for three particles system and spin-isospin quantum numbers of
the individual nucleons are considered. Based on this picture the three-nucleon
Faddeev integral equations with the two-nucleon interaction are formulated
without employing the partial wave decomposition. The single nucleon current as
well as and like exchange currents are used in an appropriate
form to be employed in 3D approach. The exchange currents are derived from AV18
NN force. The two-body t-matrix, Deuteron and Triton wave functions are
calculated in the 3D approach by using AV18 potential. Benchmarks are presented
to compare the total cross section for the two- and three- particles
photodisintegration in the range of . The 3D Faddeev
approach shows promising results
Engineering Entanglement between two cavity modes
We present scheme for generation of entanglement between different modes of
radiation field inside high-Q superconducting cavities. Our scheme is based on
the interaction of a three-level atom with the cavity field for pre-calculated
interaction times with each mode. This work enables us to generate complete set
of Bell basis states and GHZ state
Two-Boson Exchange Physics: A Brief Review
Current status of the two-boson exchange contributions to elastic
electron-proton scattering, both for parity conserving and parity-violating, is
briefly reviewed. How the discrepancy in the extraction of elastic nucleon form
factors between unpolarized Rosenbluth and polarization transfer experiments
can be understood, in large part, by the two-photon exchange corrections is
discussed. We also illustrate how the measurement of the ratio between
positron-proton and electron-proton scattering can be used to differentiate
different models of two-photon exchange. For the parity-violating
electron-proton scattering, the interest is on how the two-boson exchange
(TBE), \gamma Z-exchange in particular, could affect the extraction of the
long-sought strangeness form factors. Various calculations all indicate that
the magnitudes of effect of TBE on the extraction of strangeness form factors
is small, though can be large percentage-wise in certain kinematics.Comment: 6 pages, 5 figures, prepared for Proceedings of the fifth
Asia-Pacific Conference on Few-Body Problems in Physics (APFB2011), Seoul,
Korea, August 22-26, 2011, to appear in Few-Body Systems, November 201
- …