3,987 research outputs found
Oscillatory dynamics of a superconductor vortex lattice in high amplitude ac magnetic fields
In this work we study by ac susceptibility measurements the evolution of the
solid vortex lattice mobility under oscillating forces. Previous work had
already shown that in YBCO single crystals, below the melting transition, a
temporarily symmetric magnetic ac field (e.g. sinusoidal, square, triangular)
can heal the vortex lattice (VL) and increase its mobility, but a temporarily
asymmetric one (e.g. sawtooth) of the same amplitude can tear the lattice into
a more pinned disordered state. In this work we present evidence that the
mobility of the VL is reduced for large vortex displacements, in agreement with
predictions of recent simulations. We show that with large symmetric
oscillating fields both an initially ordered or an initially disordered VL
configuration evolve towards a less mobile lattice, supporting the scenario of
plastic flow.Comment: 5 pages, 4 figures. To appear in Phys. Rev.
Enhanced spin accumulation at room temperature in graphene spin valves with amorphous carbon interfacial layers
We demonstrate a large enhancement of the spin accumulation in monolayer
graphene following electron-beam induced deposition of an amorphous carbon
layer at the ferromagnet-graphene interface. The enhancement is 10^4-fold when
graphene is deposited onto poly(methyl metacrylate) (PMMA) and exposed with
sufficient electron-beam dose to cross-link the PMMA, and 10^3-fold when
graphene is deposited directly onto SiO2 and exposed with identical dose. We
attribute the difference to a more efficient carbon deposition in the former
case due to an increase in the presence of compounds containing carbon, which
are released by the PMMA. The amorphous carbon interface can sustain very large
current densities without degrading, which leads to very large spin
accumulations exceeding 500 microeVs at room temperature
Fingerprints of Inelastic Transport at the Surface of the Topological Insulator Bi2Se3: Role of Electron-Phonon Coupling
We report on electric-field and temperature dependent transport measurements
in exfoliated thin crystals of BiSe topological insulator. At low
temperatures ( K) and when the chemical potential lies inside the bulk
gap, the crystal resistivity is strongly temperature dependent, reflecting
inelastic scattering due to the thermal activation of optical phonons. A linear
increase of the current with voltage is obtained up to a threshold value at
which current saturation takes place. We show that the activated behavior, the
voltage threshold and the saturation current can all be quantitatively
explained by considering a single optical phonon mode with energy meV. This phonon mode strongly interacts with the surface states of
the material and represents the dominant source of scattering at the surface at
high electric fields.Comment: Supplementary Material at:
http://journals.aps.org/prl/supplemental/10.1103/PhysRevLett.112.086601/TIPhonon_SM.pd
Spin precession and spin Hall effect in monolayer graphene/Pt nanostructures
Spin Hall effects have surged as promising phenomena for spin logics
operations without ferromagnets. However, the magnitude of the detected
electric signals at room temperature in metallic systems has been so far
underwhelming. Here, we demonstrate a two-order of magnitude enhancement of the
signal in monolayer graphene/Pt devices when compared to their fully metallic
counterparts. The enhancement stems in part from efficient spin injection and
the large resistivity of graphene but we also observe 100% spin absorption in
Pt and find an unusually large effective spin Hall angle of up to 0.15. The
large spin-to-charge conversion allows us to characterise spin precession in
graphene under the presence of a magnetic field. Furthermore, by developing an
analytical model based on the 1D diffusive spin-transport, we demonstrate that
the effective spin-relaxation time in graphene can be accurately determined
using the (inverse) spin Hall effect as a means of detection. This is a
necessary step to gather full understanding of the consequences of spin
absorption in spin Hall devices, which is known to suppress effective spin
lifetimes in both metallic and graphene systems.Comment: 14 pages, 6 figures. Accepted in 2D Materials.
https://doi.org/10.1088/2053-1583/aa882
On the Structure of Dark Matter Halos at the Damping Scale of the Power Spectrum with and without Relict Velocities
We report a series of high-resolution cosmological N-body simulations
designed to explore the formation and properties of dark matter halos with
masses close to the damping scale of the primordial power spectrum of density
fluctuations. We further investigate the effect that the addition of a random
component, v_rms, into the particle velocity field has on the structure of
halos. We adopted as a fiducial model the Lambda Warm Dark Matter cosmology
with a non-thermal sterile neutrino mass of 0.5 keV. The filtering mass
corresponds then to M_f = 2.6x10^12 M_sun/h. Halos of masses close to M_f were
simulated with several million of particles. The results show that, on one
hand, the inner density slope of these halos (at radii <~0.02 the virial radius
Rvir) is systematically steeper than the one corresponding to the NFW fit or to
the CDM counterpart. On the other hand, the overall density profile (radii
larger than 0.02Rvir) is less curved and less concentrated than the NFW fit,
with an outer slope shallower than -3. For simulations with v_rms, the inner
halo density profiles flatten significantly at radii smaller than 2-3 kpc/h
(<~0.010-0.015Rvir). A constant density core is not detected in our
simulations, with the exception of one halo for which the flat core radius is
~1 kpc/h. Nevertheless, if ``cored'' density profiles are used to fit the halo
profiles, the inferred core radii are ~0.1-0.8 kpc/h, in rough agreement with
theoretical predictions based on phase-space constrains, and on dynamical
models of warm gravitational collapse. A reduction of v_rms by a factor of 3
produces a modest decrease in core radii, less than a factor of 1.5. We discuss
the extension of our results into several contexts, for example, to the
structure of the cold DM micro-halos at the damping scale of this model.Comment: 13 pages, 6 figures, accepted for publication in The Astrophysical
Journa
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