6,813 research outputs found
Non-equilibrium dynamics of Andreev states in the Kondo regime
The transport properties of a quantum dot coupled to superconducting leads
are analyzed. It is shown that the quasiparticle current in the Kondo regime is
determined by the non-equilibrium dynamics of subgap states (Andreev states)
under an applied voltage. The current at low bias is suppressed exponentially
for decreasing Kondo temperature in agreement with recent experiments. We also
predict novel interference effects due to multiple Landau-Zener transitions
between Andreev states.Comment: Revtex4, 4 pages, 4 figure
The entangling side of the Unruh-Hawking effect
We show that the Unruh effect can create net quantum entanglement between
inertial and accelerated observers depending on the choice of the inertial
state. This striking result banishes the extended belief that the Unruh effect
can only destroy entanglement and furthermore provides a new and unexpected
source for finding experimental evidence of the Unruh and Hawking effects.Comment: 4 pages, 4 figures. Added Journal referenc
On The spectrum of a Noncommutative Formulation of the D=11 Supermembrane with Winding
A regularized model of the double compactified D=11 supermembrane with
nontrivial winding in terms of SU(N) valued maps is obtained. The condition of
nontrivial winding is described in terms of a nontrivial line bundle introduced
in the formulation of the compactified supermembrane. The multivalued
geometrical objects of the model related to the nontrivial wrapping are
described in terms of a SU(N) geometrical object which in the
limit, converges to the symplectic connection related to the area preserving
diffeomorphisms of the recently obtained non-commutative description of the
compactified D=11 supermembrane.(I. Martin, J.Ovalle, A. Restuccia. 2000,2001)
The SU(N) regularized canonical lagrangian is explicitly obtained. In the limit it converges to the lagrangian in (I.Martin, J.Ovalle,
A.Restuccia. 2000,2001) subject to the nontrivial winding condition. The
spectrum of the hamiltonian of the double compactified D=11 supermembrane is
discussed.
Generically, it contains local string like spikes with zero energy.
However the sector of the theory corresponding to a principle bundle
characterized by the winding number , described by the SU(N) model we
propose, is shown to have no local string-like spikes and hence the spectrum of
this sector should be discrete.Comment: 16 pages.Latex2
Order in driven vortex lattices in superconducting Nb films with nanostructured pinning potentials
Driven vortex lattices have been studied in a material with strong pinning,
such as Nb films. Samples in which natural random pinning coexists with
artificial ordered arrays of defects (submicrometric Ni dots) have been
fabricated with different geometries (square, triangular and rectangular).
Three different dynamic regimes are found: for low vortex velocities, there is
a plastic regime in which random defects frustrate the effect of the ordered
array; then, for vortex velocities in the range 1-100 m/s, there is a sudden
increase in the interaction between the vortex lattice and the ordered dot
array, independent on the geometry. This effect is associated to the onset of
quasi long range order in the vortex lattice leading to an increase in the
overlap between the vortex lattice and the magnetic dots array. Finally, at
larger velocities the ordered array-vortex lattice interaction is suppresed
again, in agreement with the behavior found in numerical simulations.Comment: 8 text pages + 4 figure
Design strategies for optimizing holographic optical tweezers setups
We provide a detailed account of the construction of a system of holographic
optical tweezers. While much information is available on the design, alignment
and calibration of other optical trapping configurations, those based on
holography are relatively poorly described. Inclusion of a spatial light
modulator in the setup gives rise to particular design trade-offs and
constraints, and the system benefits from specific optimization strategies,
which we discuss.Comment: 16 pages, 15 figure
Using binary statistics in Taurus-Auriga to distinguish between brown dwarf formation processes
Whether BDs form as stars through gravitational collapse ("star-like") or BDs
and some very low-mass stars constitute a separate population which form
alongside stars comparable to the population of planets, e.g. through
circumstellar disk ("peripheral") fragmentation, is one of the key questions of
the star-formation problem. For young stars in Taurus-Auriga the binary
fraction is large with little dependence on primary mass above ~0.2Msun, while
for BDs it is <10%. We investigate a case in which BDs in Taurus formed
dominantly through peripheral fragmentation. The decline of the binary
frequency in the transition region between star-like and peripheral formation
is modelled. A dynamical population synthesis model is employed in which
stellar binary formation is universal. Peripheral objects form separately in
circumstellar disks with a distinctive initial mass function (IMF), own orbital
parameter distributions for binaries and a low binary fraction. A small amount
of dynamical processing of the stellar component is accounted for as
appropriate for the low-density Taurus-Auriga embedded clusters. The binary
fraction declines strongly between the mass-limits for star-like and peripheral
formation. The location of characteristic features and the steepness depend on
these mass-limits. Such a trend might be unique to low density regions hosting
dynamically unprocessed binary populations. The existence of a strong decline
in the binary fraction -- primary mass diagram will become verifiable in future
surveys on BD and VLMS binarity in the Taurus-Auriga star forming region. It is
a test of the (non-)continuity of star formation along the mass-scale, the
separateness of the stellar and BD populations and the dominant formation
channel for BDs and BD binaries in regions of low stellar density hosting
dynamically unprocessed populations.Comment: accepted for publication in A&A, 11 pages, 4 figures, 1 tabl
Hysteresis and Fractional Matching in Thin Nb Films with Rectangular Arrays of Nanoscaled Magnetic Dots
We have investigated the periodic pinning of magnetic flux quanta in thin Nb
films with rectangular arrays of magnetic dots. In this type of pinning
geometry, a change in the periodicity and shape of the minima in the
magnetoresistance occurs for magnetic fields exceeding a certain threshold
value. This has been explained recently in terms of a reconfiguration
transition of the vortex lattice due to an increasing vortex-vortex interaction
with increasing magnetic field. In this picture the dominating elastic energy
at high fields forces the vortex lattice to form a square symmetry rather than
being commensurate to the rectangular geometry of the pinning array. In this
paper we present a comparative study of rectangular arrays with Ni-dots,
Co-dots and holes. In the magnetic dot arrays, we found a strong fractional
matching effect up to the second order matching field. In contrast, no clear
fractional matching is seen after the reconfiguration. Additionally, we
discovered the existence of hysteresis in the magnetoresistance in the
crossover between the low and the high field regime. We found evidence that
this effect is correlated to the reconfiguration phenomenon rather than to the
magnetic state of the dots. The temperature and angular dependences of the
effect have been measured and possible models are discussed to explain this
behavior.Comment: 1 Table, 5 Figure
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