2,851 research outputs found
All-optical valley switch and clock of electronic dephasing
2D materials with broken inversion symmetry posses an extra degree of
freedom, the valley pseudospin, that labels in which of the two
energy-degenerate crystal momenta, or , the conducting carriers are
located. It has been shown that shining circularly-polarized light allows to
achieve close to 100% of valley polarization, opening the way to valley-based
transistors. Yet, switching of the valley polarization is still a key challenge
for the practical implementation of such devices due to the short coherence
lifetimes. Recent progress in ultrashort laser technology now allows to produce
trains of attosecond pulses with controlled phase and polarization between the
pulses. Taking advantage of such technology, we introduce a coherent control
protocol to turn on, off and switch the valley polarization at faster
timescales than electronic and valley decoherence, that is, an ultrafast
optical valley switch. We theoretically demonstrate the protocol for hBN and
MoS monolayers calculated from first principles. Additionally, using two
time-delayed linearly-polarized pulses with perpendicular polarization, we show
that we can extract the electronic dephasing time from the valley Hall
conductivity.Comment: 19 pages; 4 figure
Reconstruction of attosecond beating by interference of two-photon transitions in bulk solids
The reconstruction of attosecond beating by interference of two-photon
transitions (RABBIT) is one of the most widely used techniques for resolving
ultrafast electronic dynamics in atomic and molecular systems. As it relies on
the interference of photo-electrons in vacuum, similar interference has never
been contemplated in the bulk of crystals. Here we show that the interference
of two-photon transitions can be recorded directly in the bulk of solids and
read out with standard angle-resolved photo-emission spectroscopy. The phase of
the RABBIT beating in the photoelectron spectra coming from the bulk of solids
is sensitive to the relative phase of the Berry connection between bands and it
experiences a shift of as one of the quantum paths crosses a band. For
resonant interband transitions, the amplitude of the RABBIT oscillation decays
as the pump and probe pulses are separated in time due to electronic
decoherence, providing a simple interferometric method to extract dephasing
times
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Sub-cycle valleytronics: control of valley polarization using few-cycle linearly polarized pulses
So far, it has been assumed that selective excitation of a desired valley in the Brillouin zone of a hexagonal two-dimensional material has to rely on using circularly polarized fields. We theoretically demonstrate a way to control the valley excitation in hexagonal 2D materials on a few-femtosecond timescale using a few-cycle, linearly polarized pulse with controlled carrier–envelope phase. The valley polarization is mapped onto the strength of the perpendicular harmonic signal of a weak, linearly polarized pulse, which allows to read this information all-optically without destroying the valley state and without relying on the Berry curvature, making our approach potentially applicable to inversion-symmetric materials. We show applicability of this method to hexagonal boron nitride and MoS2
Ultrafast dephasing in solid state high harmonic generation: macroscopic origin revealed by real-space dynamics
Using a fully real-space perspective on high harmonic generation (HHG) in
solids, we examine the relationship between microscopic response, macroscopic
propagation of this response to the far field, and the extremely short
dephasing times routinely used in the theoretical simulations of experimentally
measured solid-state HHG spectra. We find that far field propagation naturally
reduces the contribution to the observed HHG emission from electrons that do
not return to the lattice site where they have been injected into the
conduction band. We then show that extremely short dephasing times routinely
used in microscopic simulations suppress many electron trajectories that
contribute to the far-field spectra, leading to significant distortions of the
true high harmonic response. We show that a real-space based dephasing
mechanism, which preferentially suppresses trajectories which veer too far away
from their original lattice site, yield HHG spectra that faithfully retain
those trajectories that contribute to the far-field spectra while filtering out
those which do not, already at the microscopic level. Our findings emphasize
the similarities between atomic and solid-state HHG by highlighting the
importance of the intensity-dependent phase of HHG emission and address the
longstanding issue regarding the origin of extremely short dephasing times in
solid-state HHG.Comment: 8 pages, 3 figure
Molecular diversity and genetic relationships in Secale
The objective of this study was to quantify the molecular diversity and to determine the genetic relationships among Secale spp. and among cultivars of Secale cereale using RAPDs, ISSRs and sequence analysis of six exons of ScMATE1 gene. Thirteen ryes (cultivated and wild) were genotyped using 21 RAPD and 16 ISSR primers. A total of 435 markers (242 RAPDs and 193 ISSRs) were obtained, with 293 being polymorphic (146 RAPDs and 147 ISSRs). Two RAPD and nine ISSR primers generated more than 80% of polymorphism. The ISSR markers were more polymorphic and informative than RAPDs. Further, 69% of the ISSR primers selected achieved at least 70% of DNA polymorphism. The study of six exons of the ScMATE1 gene also demonstrated a high genetic variability that subsists in Secale genus. One difference observed in exon 1 sequences from S. vavilovii seems to be correlated with Al sensitivity in this species. The genetic relationships obtained using RAPDs, ISSRs and exons of ScMATE1 gene were similar. S. ancestrale, S. kuprijanovii and S. cereale were grouped in the same cluster and S. segetale was in another cluster. S. vavilovii showed evidences of not being clearly an isolate species and having great intraspecific differences
Leer en igualdad: una aproximación a los requisitos de los textos
We present in this study the need to include among the selection criteria of readings, adequate texts that respect cultural and personal differences, being contrary to violence and away from sexist stereotypes. The educators of the compulsory stage must bear in mind, when working in the classroom, the importance of selecting reading texts that meet the requirements of respect, dignity, and equal rights and opportunities for all.For this, we propose an approach to the requirements of these texts based, on the one hand, on theories that contemplate the performative power of words, based on speech acts and awareness of the effects that messages can produce according to the intentionality of the issuer by choosing certain terms instead of others. The complexity of the communicative process is recognized, emphasizing the fact that language, a tool often used in the transmission of values, is not innocuous. On the other hand, a revision of the current legislation on equality and education in values is carried out.Presentamos en este estudio la necesidad de incluir entre los criterios de selección de lecturas, textos adecuados que respeten las diferencias culturales, personales, contrarios a la violencia y alejados de los estereotipos sexistas. Los educadores de la etapa obligatoria deben tener presente, a la hora de trabajar en el aula, la importancia de realizar una selección de textos de lectura que cumpla con los requisitos de respeto, dignidad e igualdad de derechos y oportunidades para todos. Para ello, proponemos una aproximación a los requisitos de estos textos con base, por un lado, en teorías que contemplan el poder perfomativo de las palabras, partiendo de los actos de habla y la conciencia respecto a los efectos que pueden producir los mensajes según la intencionalidad del emisor al elegir ciertos términos en lugar de otros. Se reconoce la complejidad del proceso comunicativo, incidiendo en el hecho de que el lenguaje, herramienta empleada a menudo en la transmisión de valores, no es inocuo. Por otro lado, se realiza una revisión de la legislación vigente en materia de igualdad y educación en valores
Lightwave-controlled band engineering in quantum materials
Stacking and twisting atom-thin sheets create superlattice structures with
unique emergent properties, while tailored light fields can manipulate coherent
electron transport on ultrafast timescales. The unification of these two
approaches may lead to ultrafast creation and manipulation of band structure
properties, which is a crucial objective for the advancement of quantum
technology. Here, we address this by demonstrating a tailored lightwave-driven
analogue to twisted layer stacking. This results in sub-femtosecond control of
time-reversal symmetry breaking and thereby band structure engineering in a
hexagonal boron nitride monolayer. The results practically demonstrate the
realization of the topological Haldane model in an insulator. Twisting the
lightwave relative to the lattice orientation enables switching between band
configurations, providing unprecedented control over the magnitude and location
of the band gap, and curvature. A resultant asymmetric population at
complementary quantum valleys lead to a measurable valley Hall current,
detected via optical harmonic polarimetry. The universality and robustness of
the demonstrated sub-femtosecond control opens a new way to band structure
engineering on the fly paving a way towards large-scale ultrafast quantum
devices for real-world applications.Comment: 4 pages main text, 4 figure
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