3,124 research outputs found
Dynamic structure factor for 3He in two-dimensions
Recent neutron scattering experiments on 3He films have observed a zero-sound
mode, its dispersion relation and its merging with -and possibly emerging from-
the particle-hole continuum. Here we address the study of the excitations in
the system via quantum Monte Carlo methods: we suggest a practical scheme to
calculate imaginary time correlation functions for moderate-size fermionic
systems. Combined with an efficient method for analytic continuation, this
scheme affords an extremely convincing description of the experimental
findings.Comment: 5 pages, 5 figure
Implementation of the Linear Method for the optimization of Jastrow-Feenberg and Backflow Correlations
We present a fully detailed and highly performing implementation of the
Linear Method [J. Toulouse and C. J. Umrigar (2007)] to optimize
Jastrow-Feenberg and Backflow Correlations in many-body wave-functions, which
are widely used in condensed matter physics. We show that it is possible to
implement such optimization scheme performing analytical derivatives of the
wave-function with respect to the variational parameters achieving the best
possible complexity O(N^3) in the number of particles N.Comment: submitted to the Comp. Phys. Com
Equation of state of two--dimensional He at zero temperature
We have performed a Quantum Monte Carlo study of a two-dimensional bulk
sample of interacting 1/2-spin structureless fermions, a model of He
adsorbed on a variety of preplated graphite substrates. We have computed the
equation of state and the polarization energy using both the standard
fixed-node approximate technique and a formally exact methodology, relying on
bosonic imaginary-time correlation functions of operators suitably chosen in
order to extract fermionic energies. As the density increases, the fixed-node
approximation predicts a transition to an itinerant ferromagnetic fluid,
whereas the unbiased methodology indicates that the paramagnetic fluid is the
stable phase until crystallization takes place. We find that two-dimensional
He at zero temperature crystallizes from the paramagnetic fluid at a
density of 0.061 \AA with a narrow coexistence region of about 0.002
\AA. Remarkably, the spin susceptibility turns out in very good
agreement with experiments.Comment: 7 pages, 7 figure
Imaginary Time Correlations and the phaseless Auxiliary Field Quantum Monte Carlo
The phaseless Auxiliary Field Quantum Monte Carlo method provides a well
established approximation scheme for accurate calculations of ground state
energies of many-fermions systems. Here we apply the method to the calculation
of imaginary time correlation functions. We give a detailed description of the
technique and we test the quality of the results for static and dynamic
properties against exact values for small systems.Comment: 13 pages, 6 figures; submitted to J. Chem. Phy
Quantum dislocations: the fate of multiple vacancies in two dimensional solid 4He
Defects are believed to play a fundamental role in the supersolid state of
4He. We have studied solid 4He in two dimensions (2D) as function of the number
of vacancies n_v, up to 30, inserted in the initial configuration at rho =
0.0765 A^-2, close to the melting density, with the exact zero temperature
Shadow Path Integral Ground State method. The crystalline order is found to be
stable also in presence of many vacancies and we observe two completely
different regimes. For small n_v, up to about 6, vacancies form a bound state
and cause a decrease of the crystalline order. At larger n_v, the formation
energy of an extra vacancy at fixed density decreases by one order of magnitude
to about 0.6 K. In the equilibrated state it is no more possible to recognize
vacancies because they mainly transform into quantum dislocations and
crystalline order is found almost independent on how many vacancies have been
inserted in the initial configuration. The one--body density matrix in this
latter regime shows a non decaying large distance tail: dislocations, that in
2D are point defects, turn out to be mobile, their number is fluctuating, and
they are able to induce exchanges of particles across the system mainly
triggered by the dislocation cores. These results indicate that the notion of
incommensurate versus commensurate state loses meaning for solid 4He in 2D,
because the number of lattice sites becomes ill defined when the system is not
commensurate. Crystalline order is found to be stable also in 3D in presence of
up to 100 vacancies
Entangled light pulses from single cold atoms
The coherent interaction between a laser-driven single trapped atom and an
optical high-finesse resonator allows to produce entangled multi-photon light
pulses on demand. The mechanism is based on the mechanical effect of light. The
degree of entanglement can be controlled through the parameters of the laser
excitation. Experimental realization of the scheme is within reach of current
technology. A variation of the technique allows for controlled generation of
entangled subsequent pulses, with the atomic motion serving as intermediate
memory of the quantum state.Comment: 4 pages, 3 figures, revised version (new scheme for generation of
subsequent pairs of entangled pulses included). Accepted for publication in
Phys. Rev. Let
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