4,446 research outputs found
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
Are We Seeing Magnetic Axis Reorientation in the Crab and Vela Pulsars?
Variation in the angle between a pulsar's rotational and magnetic
axes would change the torque and spin-down rate. We show that sudden increases
in , coincident with glitches, could be responsible for the persistent
increases in spin-down rate that follow glitches in the Crab pulsar. Moreover,
changes in at a rate similar to that inferred for the Crab pulsar
account naturally for the very low braking index of the Vela pulsar. If
increases with time, all pulsar ages obtained from the conventional
braking model are underestimates. Decoupling of the neutron star liquid
interior from the external torque cannot account for Vela's low braking index.
Variations in the Crab's pulse profile due to changes in might be
measurable.Comment: 14 pages and one figure, Latex, uses aasms4.sty. Accepted to ApJ
Letter
New Physics and CP Violation in Singly Cabibbo Suppressed D Decays
We analyze various theoretical aspects of CP violation in singly Cabibbo
suppressed (SCS) D-meson decays, such as . In particular, we
explore the possibility that CP asymmetries will be measured close to the
present level of experimental sensitivity of . Such measurements
would signal new physics. We make the following points: (i) The mechanism at
work in neutral D decays could be indirect or direct CP violation (or both).
(ii) One can experimentally distinguish between these possibilities. (iii) If
the dominant CP violation is indirect, then there are clear predictions for
other modes. (iv) Tree-level direct CP violation in various known models is
constrained to be much smaller than . (v) SCS decays, unlike Cabibbo
favored or doubly Cabibbo suppressed decays, are sensitive to new contributions
from QCD penguin operators and especially from chromomagnetic dipole operators.
This point is illustrated with supersymmetric gluino-squark loops, which can
yield direct CP violating effects of .Comment: 36 pages, 5 figure
Search for T Violation in Charm Meson Decays
Using data from the FOCUS (E831) experiment, we have searched for T violation
in charm meson decays using the four-body decay channels , , and . The T violation asymmetry is obtained using triple-product
correlations and assuming the validity of the CPT theorem. We find the
asymmetry values to be
,
, and
.
Each measurement is consistent with no T violation. New measurements of the
CP asymmetries for some of these decay modes are also presented.Comment: 17 pages,6 figures,submitted to Phys.Lett.
Structures and orientational transitions in thin films of tilted hexatic smectics
We present detailed systematic studies of structural transformations in thin
liquid crystal films with the smectic-C to hexatic phase transition. For the
first time all possible structures reported in the literature are observed for
one material (5 O.6) at the variation of temperature and thickness. In unusual
modulated structures the equilibrium period of stripes is twice with respect to
the domain size. We interpret these patterns in the frame work of
phenomenological Landau type theory, as equilibrium phenomena produced by a
natural geometric frustration in a system having spontaneous splay distortion.Comment: 7 pages, 6 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
- âŠ