2,751 research outputs found
Remark on charge conjugation in the non relativistic limit
We study the non relativistic limit of the charge conjugation operation in the context of the Dirac equation coupled to an electromagnetic field.
The limit is well defined and, as in the relativistic case, ,
(parity) and (time reversal) are the generators of a matrix group
isomorphic to a semidirect sum of the dihedral group of eight elements and
. The existence of the limit is supported by an argument based in quantum
field theory. Also, and most important, the limit exists in the context of
galilean relativity. Finally, if one complexifies the Lorentz group and
therefore the galilean spacetime , then the explicit form of the matrix
for allows to interpret it, in this context, as the complex
conjugation of the spatial coordinates: . This result is
natural in a fiber bundle description.Comment: 8 page
Band Structure Dynamics in Indium Wires
One-dimensional Indium wires grown on Si(111) substrates, which are metallic
at high temperatures, become insulating below K due to the formation
of a Charge Density Wave (CDW). The physics of this transition is not
conventional and involves a multiband Peierls instability with strong interband
coupling. This CDW ground state is readily destroyed with femtosecond laser
pulses resulting in a light-induced insulator-to-metal phase transition. The
current understanding of this transition remains incomplete, requiring
measurements of the transient electronic structure to complement previous
investigations of the lattice dynamics. Time- and angle-resolved
photo\-emission spectroscopy with extreme ultra-violet radiation is applied to
this end. We find that the transition from the insulating to the metallic band
structure occurs within fs that is a fraction of the amplitude mode
period. The long life time of the transient state ( ps) is attributed to
trapping in a metastable state in accordance with previous work.Comment: 14 pages, 7 figure
Direct evidence for efficient ultrafast charge separation in epitaxial WS/graphene heterostructure
We use time- and angle-resolved photoemission spectroscopy (tr-ARPES) to
investigate ultrafast charge transfer in an epitaxial heterostructure made of
monolayer WS and graphene. This heterostructure combines the benefits of a
direct gap semiconductor with strong spin-orbit coupling and strong
light-matter interaction with those of a semimetal hosting massless carriers
with extremely high mobility and long spin lifetimes. We find that, after
photoexcitation at resonance to the A-exciton in WS, the photoexcited holes
rapidly transfer into the graphene layer while the photoexcited electrons
remain in the WS layer. The resulting charge transfer state is found to
have a lifetime of \,ps. We attribute our findings to differences in
scattering phase space caused by the relative alignment of WS and graphene
bands as revealed by high resolution ARPES. In combination with spin-selective
excitation using circularly polarized light the investigated WS/graphene
heterostructure might provide a new platform for efficient optical spin
injection into graphene.Comment: 28 pages, 14 figure
Direct evidence for efficient ultrafast charge separation in epitaxial WS<sub>2</sub>/graphene heterostructures
We use time- and angle-resolved photoemission spectroscopy (tr-ARPES) to investigate ultrafast charge transfer in an epitaxial heterostructure made of monolayer WS2 and graphene. This heterostructure combines the benefits of a direct-gap semiconductor with strong spin-orbit coupling and strong light-matter interaction with those of a semimetal hosting massless carriers with extremely high mobility and long spin lifetimes. We find that, after photoexcitation at resonance to the A-exciton in WS2, the photoexcited holes rapidly transfer into the graphene layer while the photoexcited electrons remain in the WS2 layer. The resulting charge-separated transient state is found to have a lifetime of ∼1 ps. We attribute our findings to differences in scattering phase space caused by the relative alignment of WS2 and graphene bands as revealed by high-resolution ARPES. In combination with spin-selective optical excitation, the investigated WS2/graphene heterostructure might provide a platform for efficient optical spin injection into graphene
Primera valoración genética para la disciplina de raid en el caballo de pura raza árabe español
El esquema de selección del caballo de Pura Raza
Árabe fue aprobado por el Ministerio de Agricullura Pesca
y Alimentación en septiembre de 2005. Dentro de él se
especifica que se realizará una selección para mejorar los
caracteres que potencien el alto rendimiento, que de
forma natural, presenta la raza en la disciplina de raid.
Se ha realizado la primera valoración genética para la
disciplina de raid en el caballo de Pura Raza Árabe para
lo cual se ha contado con datos de 249 caballos con un
total de 547 participaciones en raids de diferentes categarías.
La valoración genética se ha realizado para los
caracteres puesto clasificatorio y tiempo de carrera.
Previamente ha sido preciso realizar un estudio de los factores que afectan al rendimiento de esta disciplina. Los
factores que se han incluido en el modelo de valoración
por resultar estadísticamente significativos han sido el año
de celebración de la prueba de raid, la zona geográfica
donde se realiza la prueba y los kilómetros del recorrido.
Además, se han incluido como covariables el número
total de participantes en la prueba de raid para el carácter
puesto clasificatorio y el tiempo medio de carrera para
el carácter tiempo. las heredabilidades obtenidas presentan un valor bajo-medio (0,18 para el puesto clasificatorio
y 0,13 para el tiempo). La evolución del valor genético
para dichos caracteres nos muestra que el progreso
genético ha sido escaso hasta el momento, pero la elevada
variabilidad del carácter asegura un progreso genético
adecuado si se realiza una apropiada intensidad de selección
para dichos caracteres
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
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
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