571 research outputs found
Single-particle vs. pair superfluidity in a bilayer system of dipolar bosons
We consider the ground state of a bilayer system of dipolar bosons, where
dipoles are oriented by an external field in the direction perpendicular to the
parallel planes. Quantum Monte Carlo methods are used to calculate the
ground-state energy, the one-body and two-body density matrix, and the
superfluid response as a function of the separation between layers. We find
that by decreasing the interlayer distance for fixed value of the strength of
the dipolar interaction, the system undergoes a quantum phase transition from a
single-particle to a pair superfluid. The single-particle superfluid is
characterized by a finite value of both the atomic condensate and the
super-counterfluid density. The pair superfluid phase is found to be stable
against formation of many-body cluster states and features a gap in the
spectrum of elementary excitations.Comment: 4 figure
Theoretical Analysis of the No-Slip Boundary Condition Enforcement in SPH Methods
The aim of the present work is to provide an in-depth analysis of the most representative mirroring techniques used in SPH to enforce boundary conditions (BC) along solid profiles. We specifically refer to dummy particles, ghost particles, and Takeda et al. [Prog. Theor. Phys. 92 (1994), 939] boundary integrals. The analysis has been carried out by studying the convergence of the first- and second-order differential operators as the smoothing length (that is, the characteristic length on which relies the SPH interpolation) decreases. These differential operators are of fundamental importance for the computation of the viscous drag and the viscous/diffusive terms in the momentum and energy equations. It has been proved that close to the boundaries some of the mirroring techniques leads to intrinsic inaccuracies in the convergence of the differential operators. A consistent formulation has been derived starting from Takeda et al. boundary integrals (see the above reference). This original formulation allows implementing no-slip boundary conditions consistently in many practical applications as viscous flows and diffusion problems
Spin reversal in Fe8 under fast pulsed magnetic fields
We report measurements on magnetization reversal in the Fe8 molecular magnet using fast pulsed magnetic fields of 1.5 kT s−1 and in the temperature range of 0.6–4.1 K. We observe and analyze the temperature dependence of the reversal process, which involves in some cases several resonances. Our experiments allow observation of resonant quantum tunneling of magnetization up to a temperature of ~4 K. We also observe shifts in the maxima of the relaxation within each resonance field with temperature that suggest the emergence of a thermal instability—a combination of spin reversal and self-heating that may result in a magnetic deflagration process. The results are mainly understood in the framework of thermally-activated quantum tunneling transitions in combination with emergence of a thermal instability
FIBONACCI SUPERLATTICES OF NARROW-GAP III-V SEMICONDUCTORS
We report theoretical electronic structure of Fibonacci superlattices of
narrow-gap III-V semiconductors. Electron dynamics is accurately described
within the envelope-function approximation in a two-band model.
Quasiperiodicity is introduced by considering two different III-V semiconductor
layers and arranging them according to the Fibonacci series along the growth
direction. The resulting energy spectrum is then found by solving exactly the
corresponding effective-mass (Dirac-like) wave equation using tranfer-matrix
techniques. We find that a self-similar electronic spectrum can be seen in the
band structure. Electronic transport properties of samples are also studied and
related to the degree of spatial localization of electronic envelope-functions
via Landauer resistance and Lyapunov coefficient. As a working example, we
consider type II InAs/GaSb superlattices and discuss in detail our results in
this system.Comment: REVTeX 3.0, 16 pages, 8 figures available upon request. To appear in
Semiconductor Science and Technolog
Spin reversal in Fe8 under fast pulsed magnetic fields
We report measurements on magnetization reversal in the Fe8 molecular magnet using fast pulsed magnetic fields of 1.5 kT s−1 and in the temperature range of 0.6–4.1 K. We observe and analyze the temperature dependence of the reversal process, which involves in some cases several resonances. Our experiments allow observation of resonant quantum tunneling of magnetization up to a temperature of ~4 K. We also observe shifts in the maxima of the relaxation within each resonance field with temperature that suggest the emergence of a thermal instability—a combination of spin reversal and self-heating that may result in a magnetic deflagration process. The results are mainly understood in the framework of thermally-activated quantum tunneling transitions in combination with emergence of a thermal instability
Hidden dimers and the matrix maps: Fibonacci chains re-visited
The existence of cycles of the matrix maps in Fibonacci class of lattices is
well established. We show that such cycles are intimately connected with the
presence of interesting positional correlations among the constituent `atoms'
in a one dimensional quasiperiodic lattice. We particularly address the
transfer model of the classic golden mean Fibonacci chain where a six cycle of
the full matrix map exists at the centre of the spectrum [Kohmoto et al, Phys.
Rev. B 35, 1020 (1987)], and for which no simple physical picture has so far
been provided, to the best of our knowledge. In addition, we show that our
prescription leads to a determination of other energy values for a mixed model
of the Fibonacci chain, for which the full matrix map may have similar cyclic
behaviour. Apart from the standard transfer-model of a golden mean Fibonacci
chain, we address a variant of it and the silver mean lattice, where the
existence of four cycles of the matrix map is already known to exist. The
underlying positional correlations for all such cases are discussed in details.Comment: 14 pages, 2 figures. Submitted to Physical Review
Microscopic description of anisotropic low-density dipolar Bose gases in two dimensions
A microscopic description of the zero energy two-body ground state and
many-body static properties of anisotropic homogeneous gases of bosonic dipoles
in two dimensions at low densities is presented and discussed. By changing the
polarization angle with respect to the plane, we study the impact of the
anisotropy, present in the dipole--dipole interaction, on the energy per
particle, comparing the results with mean field predictions. We restrict the
analysis to the regime where the interaction is always repulsive, although the
strength of the repulsion depends on the orientation with respect to the
polarization field. We present a series expansion of the solution of the zero
energy two-body problem which allows us to find the scattering length of the
interaction and to build a suitable Jastrow factor that we use as a trial wave
function for both a variational and diffusion Monte Carlo simulation of the
infinite system. We find that the anisotropy has an almost negligible impact on
the ground state properties of the many-body system in the universal regime
where the scattering length governs the physics of the system. We also show
that scaling in the gas parameter persists in the dipolar case up to values
where other isotropic interactions with the same scattering length yield
different predictions.Comment: 9 figures, 1 tabl
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