1,467 research outputs found
Improved Torsion Pendulum for Ground Testing of LISA Displacement Sensors
We discuss a new torsion pendulum design for ground testing of prototype LISA
(Laser Interferometer Space Antenna) displacement sensors. This new design is
directly sensitive to net forces and therefore provides a more representative
test of the noisy forces and parasitic stiffnesses acting on the test mass as
compared to previous ground-based experiments. We also discuss a specific
application to the measurement of thermal gradient effects.Comment: 4 pages 1 figure, to appear in the Proceedings of the 10th Marcel
Grossmann Meeting on General Relativit
Tracking primary thermalization events in graphene with photoemission at extreme timescales
Direct and inverse Auger scattering are amongst the primary processes that
mediate the thermalization of hot carriers in semiconductors. These two
processes involve the annihilation or generation of an electron-hole pair by
exchanging energy with a third carrier, which is either accelerated or
decelerated. Inverse Auger scattering is generally suppressed, as the
decelerated carriers must have excess energies higher than the band gap itself.
In graphene, which is gapless, inverse Auger scattering is instead predicted to
be dominant at the earliest time delays. Here, femtosecond
extreme-ultraviolet pulses are used to detect this imbalance, tracking both the
number of excited electrons and their kinetic energy with time- and
angle-resolved photoemission spectroscopy. Over a time window of approximately
25 fs after absorption of the pump pulse, we observe an increase in conduction
band carrier density and a simultaneous decrease of the average carrier kinetic
energy, revealing that relaxation is in fact dominated by inverse Auger
scattering. Measurements of carrier scattering at extreme timescales by
photoemission will serve as a guide to ultrafast control of electronic
properties in solids for PetaHertz electronics.Comment: 16 pages, 8 figure
Achieving geodetic motion for LISA test masses: ground testing result
The low-frequency resolution of space-based gravitational wave observatories
such as LISA (Laser Interferometry Space Antenna) hinges on the orbital purity
of a free-falling reference test mass inside a satellite shield. We present
here a torsion pendulum study of the forces that will disturb an orbiting test
mass inside a LISA capacitive position sensor. The pendulum, with a measured
torque noise floor below 10 fNm/sqrt{Hz} from 0.6 to 10 mHz, has allowed
placement of an upper limit on sensor force noise contributions, measurement of
the sensor electrostatic stiffness at the 5% level, and detection and
compensation of stray DC electrostatic biases at the mV level.Comment: 4 pages (revtex4) with 4 figure
Enhanced electron-phonon coupling in graphene with periodically distorted lattice
Electron-phonon coupling directly determines the stability of cooperative
order in solids, including superconductivity, charge and spin density waves.
Therefore, the ability to enhance or reduce electron-phonon coupling by optical
driving may open up new possibilities to steer materials' functionalities,
potentially at high speeds. Here we explore the response of bilayer graphene to
dynamical modulation of the lattice, achieved by driving optically-active
in-plane bond stretching vibrations with femtosecond mid-infrared pulses. The
driven state is studied by two different ultrafast spectroscopic techniques.
Firstly, TeraHertz time-domain spectroscopy reveals that the Drude scattering
rate decreases upon driving. Secondly, the relaxation rate of hot
quasi-particles, as measured by time- and angle-resolved photoemission
spectroscopy, increases. These two independent observations are quantitatively
consistent with one another and can be explained by a transient three-fold
enhancement of the electron-phonon coupling constant. The findings reported
here provide useful perspective for related experiments, which reported the
enhancement of superconductivity in alkali-doped fullerites when a similar
phonon mode was driven.Comment: 12 pages, 4 figure
Ultrafast Momentum Imaging of Pseudospin-Flip Excitations in Graphene
The pseudospin of Dirac electrons in graphene manifests itself in a peculiar
momentum anisotropy for photo-excited electron-hole pairs. These interband
excitations are in fact forbidden along the direction of the light
polarization, and are maximum perpendicular to it. Here, we use time- and
angle-resolved photoemission spectroscopy to investigate the resulting
unconventional hot carrier dynamics, sampling carrier distributions as a
function of energy and in-plane momentum. We first show that the
rapidly-established quasi-thermal electron distribution initially exhibits an
azimuth-dependent temperature, consistent with relaxation through collinear
electron-electron scattering. Azimuthal thermalization is found to occur only
at longer time delays, at a rate that depends on the substrate and the static
doping level. Further, we observe pronounced differences in the electron and
hole dynamics in n-doped samples. By simulating the Coulomb- and
phonon-mediated carrier dynamics we are able to disentangle the influence of
excitation fluence, screening, and doping, and develop a microscopic picture of
the carrier dynamics in photo-excited graphene. Our results clarify new aspects
of hot carrier dynamics that are unique to Dirac materials, with relevance for
photo-control experiments and optoelectronic device applications.Comment: 23 pages, 12 figure
Tripes (Thysanoptera: Thripidae) em plantios de bananeiras Cv. Prata Anã.
Os danos causados por tripes no fruto da bananeira, puncturas de cor marrom e ásperas ao tato, são bastante significativos, pois reduzem o valor comercial do produto (1).
Kinematics and hydrodynamics of spinning particles
In the first part (Sections 1 and 2) of this paper --starting from the Pauli
current, in the ordinary tensorial language-- we obtain the decomposition of
the non-relativistic field velocity into two orthogonal parts: (i) the
"classical part, that is, the 3-velocity w = p/m OF the center-of-mass (CM),
and (ii) the so-called "quantum" part, that is, the 3-velocity V of the motion
IN the CM frame (namely, the internal "spin motion" or zitterbewegung). By
inserting such a complete, composite expression of the velocity into the
kinetic energy term of the non-relativistic classical (i.e., newtonian)
lagrangian, we straightforwardly get the appearance of the so-called "quantum
potential" associated, as it is known, with the Madelung fluid. This result
carries further evidence that the quantum behaviour of micro-systems can be
adirect consequence of the fundamental existence of spin. In the second part
(Sections 3 and 4), we fix our attention on the total 3-velocity v = w + V, it
being now necessary to pass to relativistic (classical) physics; and we show
that the proper time entering the definition of the four-velocity v^mu for
spinning particles has to be the proper time tau of the CM frame. Inserting the
correct Lorentz factor into the definition of v^mu leads to completely new
kinematical properties for v_mu v^mu. The important constraint p_mu v^mu = m,
identically true for scalar particles, but just assumed a priori in all
previous spinning particle theories, is herein derived in a self-consistent
way.Comment: LaTeX file; needs kapproc.st
Softening of the insulating phase near Tc for the photo-induced insulator-to-metal phase transition in vanadium dioxide
We use optical-pump terahertz-probe spectroscopy to investigate the
near-threshold behavior of the photoinduced insulator-to-metal (IM) transition
in vanadium dioxide thin films. Upon approaching Tc a reduction in the fluence
required to drive the IM transition is observed, consistent with a softening of
the insulating state due to an increasing metallic volume fraction (below the
percolation limit). This phase coexistence facilitates the growth of a
homogeneous metallic conducting phase following superheating via
photoexcitation. A simple dynamic model using Bruggeman effective medium theory
describes the observed initial condition sensitivity.Comment: accepted for publication in Physical Review Letter
Measuring the LISA test mass magnetic proprieties with a torsion pendulum
Achieving the low frequency LISA sensitivity requires that the test masses
acting as the interferometer end mirrors are free-falling with an unprecedented
small degree of deviation. Magnetic disturbances, originating in the
interaction of the test mass with the environmental magnetic field, can
significantly deteriorate the LISA performance and can be parameterized through
the test mass remnant dipole moment and the magnetic susceptibility
. While the LISA test flight precursor LTP will investigate these effects
during the preliminary phases of the mission, the very stringent requirements
on the test mass magnetic cleanliness make ground-based characterization of its
magnetic proprieties paramount. We propose a torsion pendulum technique to
accurately measure on ground the magnetic proprieties of the LISA/LTP test
masses.Comment: 6 pages, 3 figure
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