727 research outputs found
Van der Waals density functional: Self-consistent potential and the nature of the van der Waals bond
We derive the exchange-correlation potential corresponding to the nonlocal
van der Waals density functional [M. Dion, H. Rydberg, E. Schroder, D. C.
Langreth, and B. I. Lundqvist, Phys. Rev. Lett. 92, 246401 (2004)]. We use this
potential for a self-consistent calculation of the ground state properties of a
number of van der Waals complexes as well as crystalline silicon. For the
latter, where little or no van der Waals interaction is expected, we find that
the results are mostly determined by semilocal exchange and correlation as in
standard generalized gradient approximations (GGA), with the fully nonlocal
term giving little effect. On the other hand, our results for the van der Waals
complexes show that the self-consistency has little effect at equilibrium
separations. This finding validates previous calculations with the same
functional that treated the fully nonlocal term as a post GGA perturbation. A
comparison of our results with wave-function calculations demonstrates the
usefulness of our approach. The exchange-correlation potential also allows us
to calculate Hellmann-Feynman forces, hence providing the means for efficient
geometry relaxations as well as unleashing the potential use of other standard
techniques that depend on the self-consistent charge distribution. The nature
of the van der Waals bond is discussed in terms of the self-consistent bonding
charge.Comment: submitted to Phys. Rev.
Repulsive Casimir Force: Sufficient Conditions
In this paper the Casimir energy of two parallel plates made by materials of
different penetration depth and no medium in between is derived. We study the
Casimir force density and derive analytical constraints on the two penetration
depths which are sufficient conditions to ensure repulsion. Compared to other
methods our approach needs no specific model for dielectric or magnetic
material properties and constitutes a complementary analysis.Comment: 11 pages. 3 figures. Misprints corrected in Eq. (4
A possible cooling effect in high temperature superconductors
We show that an adiabatic increase of the supercurrent along a superconductor
with lines of nodes of the order parameter on the Fermi surface can result in a
cooling effect. The maximum cooling occurs if the supercurrent increases up to
its critical value. The effect can also be observed in a mixed state of a bulk
sample. An estimate of the energy dissipation shows that substantial cooling
can be performed during a reasonable time even in the microkelvin regime.Comment: 5 pages, to appear in Phys. Rev.
Orientation-dependent Casimir force arising from highly anisotropic crystals: application to Bi2Sr2CaCu2O8+delta
We calculate the Casimir interaction between parallel planar crystals of Au
and the anisotropic cuprate superconductor Bi2Sr2CaCu2O8+delta (BSCCO), with
BSCCO's optical axis either parallel or perpendicular to the crystal surface,
using suitable generalizations of the Lifshitz theory. We find that the strong
anisotropy of the BSCCO permittivity gives rise to a difference in the Casimir
force between the two orientations of the optical axis, which depends on
distance and is of order 10-20% at the experimentally accessible separations 10
to 5000 nm.Comment: 5 pages, 3 figures. Accepted for publication in Physical Review
pi-Junction behavior and Andreev bound states in Kondo quantum dots with superconducting leads
We investigate the temperature- and coupling-dependent transport through
Kondo dot contacts with symmetric superconducting s-wave leads. For finite
temperature T we use a superconducting extension of a selfconsistent auxiliary
boson scheme, termed SNCA, while at T=0 a perturbative renormalization group
treatment is applied. The finite-temperature phase diagram for the 0--pi
transition of the Josephson current in the junction is established and related
to the phase-dependent position of the subgap Kondo resonance with respect to
the Fermi energy. The conductance of the contact is evaluated in the zero-bias
limit. It approaches zero in the low-temperature regime, however, at finite T
its characteristics are changed through the coupling- and temperature-dependent
0--pi transition.Comment: 12 pages, 12 figure
Dispersion Interactions between Optically Anisotropic Cylinders at all Separations: Retardation Effects for Insulating and Semiconducting Single Wall Carbon Nanotubes
We derive the complete form of the van der Waals dispersion interaction
between two infinitely long anisotropic semiconducting/insulating thin
cylinders at all separations. The derivation is based on the general theory of
dispersion interactions between anisotropic media as formulated in [J. N.
Munday, D. Iannuzzi, Yu. S. Barash and F. Capasso, {\sl Phys. Rev. A} {\bf 71},
042102 (2005)]. This formulation is then used to calculate the dispersion
interactions between a pair of single walled carbon nanotubes at all
separations and all angles. Non-retarded and retarded forms of the interactions
are developed separately. The possibility of repulsive dispersion interactions
and non-monotonic dispersion interactions is discussed within the framework of
the new formulation
Casimir torque
We develop a formalism for the calculation of the flow of angular momentum
carried by the fluctuating electromagnetic field within a cavity bounded by two
flat anisotropic materials. By generalizing a procedure employed recently for
the calculation of the Casimir force between arbitrary materials, we obtain an
expression for the torque between anisotropic plates in terms of their
reflection amplitude matrices. We evaluate the torque in 1D for ideal and
realistic model materials.Comment: 8 pages, 4 figs, Submitted to Proc. of QFEXT'05, to appear in J.
Phys.
Van der Waals torque induced by external magnetic fields
We present a method for inducing and controlling van der Waals torques
between two parallel slabs using a constant magnetic field. The torque is
calculated using the Barash theory of dispersive torques. In III-IV
semiconductors such as , the effect of an external magnetic field is to
induce an optical anisotropy, in an otherwise isotropic material, that will in
turn induce a torque.
The calculations of the torque are done in the Voigt configuration, with the
magnetic field parallel to the surface of the slabs. As a case study we
consider a slab made of calcite and a second slab made of . In the
absence of magnetic field there is no torque. As the magnetic field increases,
the optical anisotropy of increases and the torque becomes different
from zero, increasing with the magnetic field. The resulting torque is of the
same order of magnitude as that calculated using permanent anisotropic
materials when the magnetic fields is close to 1 T.Comment: to appear in Journal of Applied Physic
Electronic structure of d-wave superconducting quantum wires
We present analytical and numerical results for the electronic spectra of
wires of a d-wave superconductor on a square lattice. The spectra of Andreev
and other quasiparticle states, as well as the spatial and particle-hole
structures of their wave functions, depend on interference effects caused by
the presence of the surfaces and are qualitatively different for half-filled
wires with even or odd number of chains. For half-filled wires with an odd
number of chains N at (110) orientation, spectra consist of N doubly degenerate
branches. By contrast, for even N wires, these levels are split, and all
quasiparticle states, even the ones lying above the maximal gap, have the
characteristic properties of Andreev bound states. These Andreev states above
the gap can be interpreted as a consequence of an infinite sequence of Andreev
reflections experienced by quasiparticles along their trajectories bounded by
the surfaces of the wire. Our microscopic results for the local density of
states display atomic-scale Friedel oscillations due to the presence of the
surfaces, which should be observable by scanning tunneling microscopy. For
narrow wires the self-consistent treatment of the order parameter is found to
play a crucial role. In particular, we find that for small wire widths the
finite geometry may drive strong fluctuations or even stablilize exotic
quasi-1D pair states with spin triplet character.Comment: 21 pages, 20 figures. Slightly modified version as published in PR
Periodic orbits of the ensemble of Sinai-Arnold cat maps and pseudorandom number generation
We propose methods for constructing high-quality pseudorandom number
generators (RNGs) based on an ensemble of hyperbolic automorphisms of the unit
two-dimensional torus (Sinai-Arnold map or cat map) while keeping a part of the
information hidden. The single cat map provides the random properties expected
from a good RNG and is hence an appropriate building block for an RNG, although
unnecessary correlations are always present in practice. We show that
introducing hidden variables and introducing rotation in the RNG output,
accompanied with the proper initialization, dramatically suppress these
correlations. We analyze the mechanisms of the single-cat-map correlations
analytically and show how to diminish them. We generalize the Percival-Vivaldi
theory in the case of the ensemble of maps, find the period of the proposed RNG
analytically, and also analyze its properties. We present efficient practical
realizations for the RNGs and check our predictions numerically. We also test
our RNGs using the known stringent batteries of statistical tests and find that
the statistical properties of our best generators are not worse than those of
other best modern generators.Comment: 18 pages, 3 figures, 9 table
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