13,086 research outputs found
Efficient first-principles calculation of the quantum kinetic energy and momentum distribution of nuclei
Light nuclei at room temperature and below exhibit a kinetic energy which
significantly deviates from the predictions of classical statistical mechanics.
This quantum kinetic energy is responsible for a wide variety of isotope
effects of interest in fields ranging from chemistry to climatology. It also
furnishes the second moment of the nuclear momentum distribution, which
contains subtle information about the chemical environment and has recently
become accessible to deep inelastic neutron scattering experiments. Here we
show how, by combining imaginary time path integral dynamics with a carefully
designed generalized Langevin equation, it is possible to dramatically reduce
the expense of computing the quantum kinetic energy. We also introduce a
transient anisotropic Gaussian approximation to the nuclear momentum
distribution which can be calculated with negligible additional effort. As an
example, we evaluate the structural properties, the quantum kinetic energy, and
the nuclear momentum distribution for a first-principles simulation of liquid
water
Transition to ballistic regime for heat transport in helium II
The size-dependent and flux-dependent effective thermal conductivity of
narrow capillaries filled with superfluid helium is analyzed from a
thermodynamic continuum perspective. The classical Landau evaluation of the
effective thermal conductivity of quiescent superfluid, or the Gorter-Mellinck
regime of turbulent superfluids, are extended to describe the transition to
ballistic regime in narrow channels wherein the radius is comparable to (or
smaller than) the phonon mean-free path in superfluid helium. To do so
we start from an extended equation for the heat flux incorporating non-local
terms, and take into consideration a heat slip flow along the walls of the
tube. This leads from an effective thermal conductivity proportional to
(Landau regime) to another one proportional to (ballistic regime). We
consider two kinds of flows: along cylindrical pipes and along two infinite
parallel plates.Comment: 16 page
Generation of pure spin currents by superconducting proximity effect in quantum dots
We investigate electronic transport in a three-terminal hybrid system,
composed by an interacting quantum dot tunnel coupled to one superconducting,
one ferromagnetic, and one normal lead. Despite the tendency of the charging
energy to suppress the superconducting proximity effect when the quantum dot is
in equilibrium, the non-equilibrium proximity effect can give rise to a large
Andreev current. The presence of the ferromagnet can lead to a finite spin
accumulation on the dot. We find that the interplay of the Andreev current and
spin accumulation can generate a pure spin current, with no associated charge
transport, in the normal lead. This situation is realised by tuning the
quantum-dot spectrum by means of a gate voltage.Comment: 6 pages, 5 figure
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