1,271 research outputs found
Dynamic Phases of Vortices in Superconductors with Periodic Pinning
We present results from extensive simulations of driven vortex lattices
interacting with periodic arrays of pinning sites. Changing an applied driving
force produces a rich variety of novel dynamical plastic flow phases which are
very distinct from those observed in systems with random pinning arrays.
Signatures of the transition between these different dynamical phases include
sudden jumps in the current-voltage curves as well as marked changes in the
vortex trajectories and the vortex lattice order. Several dynamical phase
diagrams are obtained as a function of commensurability, pinning strength, and
spatial order of the pinning sites.Comment: 4 pages, 3 figures. To appear in Physical Review Letters. Movies
available at http://www-personal.engin.umich.edu/~nor
Hall noise and transverse freezing in driven vortex lattices
We study driven vortices lattices in superconducting thin films. Above the
critical force we find two dynamical phase transitions at and
, which could be observed in simultaneous noise measurements of the
longitudinal and the Hall voltage. At there is a transition from plastic
flow to smectic flow where the voltage noise is isotropic (Hall noise =
longitudinal noise) and there is a peak in the differential resistance. At
there is a sharp transition to a frozen transverse solid where the Hall
noise falls down abruptly and vortex motion is localized in the transverse
direction.Comment: 4 pages, 3 figure
Vortex Dynamics and Defects in Simulated Flux Flow
We present the results of molecular dynamic simulations of a two-dimensional
vortex array driven by a uniform current through random pinning centers at zero
temperature. We identify two types of flow of the driven array near the
depinning threshold. For weak disorder the flux array contains few dislocation
and moves via correlated displacements of patches of vortices in a {\it
crinkle} motion. As the disorder strength increases, we observe a crossover to
a spatially inhomogeneous regime of {\it plastic} flow, with a very defective
vortex array and a channel-like structure of the flowing regions. The two
regimes are characterized by qualitatively different spatial distribution of
vortex velocities. In the crinkle regime the distribution of vortex velocities
near threshold has a single maximum that shifts to larger velocities as the
driving force is increased. In the plastic regime the distribution of vortex
velocities near threshold has a clear bimodal structure that persists upon
time-averaging the individual velocities. The bimodal structure of the velocity
distribution reflects the coexistence of pinned and flowing regions and is
proposed as a quantitative signature of plastic flow.Comment: 12 pages, 13 embedded PostScript figure
Dynamical Phase Transition in a Driven Disordered Vortex Lattice
Using Langevin dynamics, we have investigated the dynamics of vortices in a
disordered two dimensional superconductor subjected to a uniform driving
current. The results provide direct numerical evidence for a dynamical phase
transition between a plastic flow regime and a moving ``hexatic glass." The
simulated current-voltage characteristics are in excellent agreement with
recent transport measurements on amorphous thin film
superconductors.Comment: 13 pages, latex, revtex, 4 figures available upon request from
[email protected]
Transverse depinning in strongly driven vortex lattices with disorder
Using numerical simulations we investigate the transverse depinning of moving
vortex lattices interacting with random disorder. We observe a finite
transverse depinning barrier for vortex lattices that are driven with high
longitudinal drives, when the vortex lattice is defect free and moving in
correlated 1D channels. The transverse barrier is reduced as the longitudinal
drive is decreased and defects appear in the vortex lattice, and the barrier
disappears in the plastic flow regime. At the transverse depinning transition,
the vortex lattice moves in a staircase pattern with a clear transverse
narrow-band voltage noise signature.Comment: 4 pages, 4 figure
Dynamic transition in vortex flow in strongly disordered Josephson junction arrays and superconducting thin films
We study the dynamics of vortices in strongly disordered Josephson
junction arrays and superconducting films driven by a current. We find a
dynamic phase transition in vortex flow at a current . Below
there is plastic flow characterized by an average-velocity correlation length
scale in the direction of motion, which diverges when approaching
. Above we find a moving vortex phase with homogeneous flow and
short range smectic order. A finite size analysis shows that this phase becomes
asymptotically a liquid for large length scales.Comment: 4 pages, 4 figure
GTC OSIRIS transiting exoplanet atmospheric survey: detection of sodium in XO-2b from differential long-slit spectroscopy
We present two transits of the hot-Jupiter exoplanet XO-2b using the Gran
Telescopio Canarias (GTC). The time series observations were performed using
long-slit spectroscopy of XO-2 and a nearby reference star with the OSIRIS
instrument, enabling differential specrophotometric transit lightcurves capable
of measuring the exoplanet's transmission spectrum. Two optical low-resolution
grisms were used to cover the optical wavelength range from 3800 to 9300{\AA}.
We find that sub-mmag level slit losses between the target and reference star
prevent full optical transmission spectra from being constructed, limiting our
analysis to differential absorption depths over ~1000{\AA} regions. Wider long
slits or multi-object grism spectroscopy with wide masks will likely prove
effective in minimising the observed slit-loss trends. During both transits, we
detect significant absorption in the planetary atmosphere of XO-2b using a
50{\AA} bandpass centred on the Na I doublet, with absorption depths of
Delta(R_pl/R_star)^2=0.049+/-0.017 % using the R500R grism and 0.047+/-0.011 %
using the R500B grism (combined 5.2-sigma significance from both transits). The
sodium feature is unresolved in our low-resolution spectra, with detailed
modelling also likely ruling out significant line-wing absorption over an
~800{\AA} region surrounding the doublet. Combined with narrowband photometric
measurements, XO-2b is the first hot Jupiter with evidence for both sodium and
potassium present in the planet's atmosphere.Comment: 9 pages, 10 figures, 1 table, accepted for publication in MNRA
Moving glass theory of driven lattices with disorder
We study periodic structures, such as vortex lattices, moving in a random
potential. As predicted in [T. Giamarchi, P. Le Doussal Phys. Rev. Lett. 76
3408 (1996)] the periodicity in the direction transverse to motion leads to a
new class of driven systems: the Moving Glasses. We analyse using several RG
techniques the properties at T=0 and : (i) decay of translational long
range order (ii) particles flow along static channels (iii) the channel pattern
is highly correlated (iv) barriers to transverse motion. We demonstrate the
existence of the ``transverse critical force'' at T=0. A ``static random
force'' is shown to be generated by motion. Displacements grow logarithmically
in and algebraically in . The persistence of quasi long range
translational order in at weak disorder, or large velocity leads to
predict a topologically ordered ``Moving Bragg Glass''. This state continues
the static Bragg glass and is stable at , with non linear transverse
response and linear asymptotic behavior. In , or in at intermediate
disorder, another moving glass exist (the Moving Transverse Glass) with smectic
quasi order in the transverse direction. A phase diagram in force and
disorder for static and moving structures is proposed. For correlated disorder
we predict a ``moving Bose glass'' state with anisotropic transverse Meissner
effect and transverse pinning. We discuss experimental consequences such as
anomalous Hall effect in Wigner crystal and transverse critical current in
vortex lattice.Comment: 74 pages, 27 figures, RevTe
Directly Imaging Rocky Planets from the Ground
Over the past three decades instruments on the ground and in space have
discovered thousands of planets outside the solar system. These observations
have given rise to an astonishingly detailed picture of the demographics of
short-period planets, but are incomplete at longer periods where both the
sensitivity of transit surveys and radial velocity signals plummet. Even more
glaring is that the spectra of planets discovered with these indirect methods
are either inaccessible (radial velocity detections) or only available for a
small subclass of transiting planets with thick, clear atmospheres. Direct
detection can be used to discover and characterize the atmospheres of planets
at intermediate and wide separations, including non-transiting exoplanets.
Today, a small number of exoplanets have been directly imaged, but they
represent only a rare class of young, self-luminous super-Jovian-mass objects
orbiting tens to hundreds of AU from their host stars. Atmospheric
characterization of planets in the <5 AU regime, where radial velocity (RV)
surveys have revealed an abundance of other worlds, is technically feasible
with 30-m class apertures in combination with an advanced AO system,
coronagraph, and suite of spectrometers and imagers. There is a vast range of
unexplored science accessible through astrometry, photometry, and spectroscopy
of rocky planets, ice giants, and gas giants. In this whitepaper we will focus
on one of the most ambitious science goals --- detecting for the first time
habitable-zone rocky (<1.6 R_Earth) exoplanets in reflected light around nearby
M-dwarfsComment: 8 pages, 1 figure, Astro2020 Science White Pape
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