270 research outputs found
Ultrafast photodoping and effective Fermi-Dirac distribution of the Dirac particles in Bi2Se3
We exploit time- and angle- resolved photoemission spectroscopy to determine
the evolution of the out-of-equilibrium electronic structure of the topological
insulator Bi2Se. The response of the Fermi-Dirac distribution to ultrashort IR
laser pulses has been studied by modelling the dynamics of the hot electrons
after optical excitation. We disentangle a large increase of the effective
temperature T* from a shift of the chemical potential mu*, which is consequence
of the ultrafast photodoping of the conduction band. The relaxation dynamics of
T* and mu* are k-independent and these two quantities uniquely define the
evolution of the excited charge population. We observe that the energy
dependence of the non-equilibrium charge population is solely determined by the
analytical form of the effective Fermi-Dirac distribution.Comment: 5 Pages, 3 Figure
Ultrafast Optical Control of the Electronic Properties of
We report on the temperature dependence of the electronic
properties, studied at equilibrium and out of equilibrium, by means of time and
angle resolved photoelectron spectroscopy. Our results unveil the dependence of
the electronic band structure across the Fermi energy on the sample
temperature. This finding is regarded as the dominant mechanism responsible for
the anomalous resistivity observed at T* 160 K along with the change of
the charge carrier character from holelike to electronlike. Having addressed
these long-lasting questions, we prove the possibility to control, at the
ultrashort time scale, both the binding energy and the quasiparticle lifetime
of the valence band. These experimental evidences pave the way for optically
controlling the thermoelectric and magnetoelectric transport properties of
Strong enhancement of d-wave superconducting state in the three-band Hubbard model coupled to an apical oxygen phonon
We study the hole binding energy and pairing correlations in the three-band
Hubbard model coupled to an apical oxygen phonon, by exact diagonalization and
constrained-path Monte Carlo simulations. In the physically relevant
charge-transfer regime, we find that the hole binding energy is strongly
enhanced by the electron-phonon interaction, which is due to a novel
potential-energy-driven pairing mechanism involving reduction of both
electronic potential energy and phonon related energy. The enhancement of hole
binding energy, in combination with a phonon-induced increase of quasiparticle
weight, leads to a dramatic enhancement of the long-range part of d-wave
pairing correlations. Our results indicate that the apical oxygen phonon plays
a significant role in the superconductivity of high- cuprates.Comment: 5 pages, 5 figure
Hubbard exciton revealed by time-domain optical spectroscopy
We use broadband ultra-fast pump-probe spectroscopy in the visible range to
study the lowest excitations across the Mott-Hubbard gap in the orbitally
ordered insulator YVO3. Separating thermal and non-thermal contributions to the
optical transients, we show that the total spectral weight of the two lowest
peaks is conserved, demonstrating that both excitations correspond to the same
multiplet. The pump-induced transfer of spectral weight between the two peaks
reveals that the low-energy one is a Hubbard exciton, i.e. a resonance or bound
state between a doublon and a holon. Finally, we speculate that the pump-driven
spin-disorder can be used to quantify the kinetic energy gain of the excitons
in the ferromagnetic phase.Comment: 5 pages and 6 figures, 9 pages and 12 figures with additional
material
An Integrated Approach To Recruiting And Retaining Appalachian Engineering Students
Recruiting and retaining Appalachian engineering students is difficult for a variety of ecological and cultural reasons. At West Virginia University an NSF STEP grant1* has allowed the development specific interventions to evolve from an ecological model we describe here. The interventions include web-based, realistic engineering design exercises linked to state and federal content standards and objectives; a week-long residential summer camp addressing social and academic challenges for rural and minority students; a full set of retention efforts including "rescue courses" targeting struggling college freshmen in early stages of academic difficulty coupled with required study labs to underscore time management and persistence skills early in a freshman's academic career. Process and impact measures suggest that this package of interventions is effective in building interest in engineering not only in high school teachers but in the high school students themselves. While freshman retention has improved remarkably to an all time high of 84%, we conclude that it may take longer than five years to establish among youth in Appalachia an "engineering identity" as a cultural norm. We discuss the key aspects of our 5 year NSF project along with findings and conclusions
Evidence of reduced surface electron-phonon scattering in the conduction band of Bi_{2}Se_{3} by non-equilibrium ARPES
The nature of the Dirac quasiparticles in topological insulators calls for a
direct investigation of the electron-phonon scattering at the \emph{surface}.
By comparing time-resolved ARPES measurements of the TI Bi_{2}Se_{3} with
different probing depths we show that the relaxation dynamics of the electronic
temperature of the conduction band is much slower at the surface than in the
bulk. This observation suggests that surface phonons are less effective in
cooling the electron gas in the conduction band.Comment: 5 pages, 3 figure
Structural Performance-Based Design Optimisation of a Secondary Mirror for a Concentrated Solar Power (CSP) Plant
Concentrated Solar Power (CSP) plants use mirrors to reflect and concentrate sunlight onto a receiver, to heat a fluid and store thermal energy, at high temperature and energy density, to produce dispatchable heat and/or electricity. The secondary mirror is a critical component in the optical system of certain Solar Power Tower plants (SPT), as it redirects the concentrated sunlight from the primary mirror onto the receiver, which can be arranged at ground level. In this study, we propose a design optimisation for the secondary mirror of a CSP plant. The design optimisation method consists of two steps. The first step involves the use of the finite element simulation software Abaqus 2022 to analyse the structural performance of the secondary mirror under thermal loads and wind. The second step consists of the use of simulation results to identify the combination of design parameters and best performances, with respect to both design constraints and structural safety. This is carried out by developing an algorithm that selects those configurations which satisfy the constraints by using safety coefficients. The proposed optimisation method is applied to the design of a potential configuration of a secondary mirror for the beam-down of the CSP Magaldi STEMÂź technology, although the methodology can be extended to other components of CSP plants, such as primary mirrors and receivers, to further enhance the structural performance of these systems
The momentum and photon energy dependence of the circular dichroic photoemission in the bulk Rashba semiconductors BiTeX (X = I, Br, Cl)
Bulk Rashba systems BiTeX (X = I, Br, Cl) are emerging as important
candidates for developing spintronics devices, because of the coexistence of
spin-split bulk and surface states, along with the ambipolar character of the
surface charge carriers. The need of studying the spin texture of strongly
spin-orbit coupled materials has recently promoted circular dichroic Angular
Resolved Photoelectron Spectroscopy (cd-ARPES) as an indirect tool to measure
the spin and the angular degrees of freedom. Here we report a detailed photon
energy dependent study of the cd-ARPES spectra in BiTeX (X = I, Br and Cl). Our
work reveals a large variation of the magnitude and sign of the dichroism.
Interestingly, we find that the dichroic signal modulates differently for the
three compounds and for the different spin-split states. These findings show a
momentum and photon energy dependence for the cd-ARPES signals in the bulk
Rashba semiconductor BiTeX (X = I, Br, Cl). Finally, the outcome of our
experiment indicates the important relation between the modulation of the
dichroism and the phase differences between the wave-functions involved in the
photoemission process. This phase difference can be due to initial or final
state effects. In the former case the phase difference results in possible
interference effects among the photo-electrons emitted from different atomic
layers and characterized by entangled spin-orbital polarized bands. In the
latter case the phase difference results from the relative phases of the
expansion of the final state in different outgoing partial waves.Comment: 6 pages, 4 figure
Momentum resolved spin dynamics of bulk and surface excited states in the topological insulator
The prospective of optically inducing a spin polarized current for spintronic
devices has generated a vast interest in the out-of-equilibrium electronic and
spin structure of topological insulators (TIs). In this Letter we prove that
only by measuring the spin intensity signal over several order of magnitude in
spin, time and angle resolved photoemission spectroscopy (STAR-PES) experiments
is it possible to comprehensively describe the optically excited electronic
states in TIs materials. The experiments performed on
reveal the existence of a Surface-Resonance-State in the 2nd bulk band gap
interpreted on the basis of fully relativistic ab-initio spin resolved
photoemission calculations. Remarkably, the spin dependent relaxation of the
hot carriers is well reproduced by a spin dynamics model considering two
non-interacting electronic systems, derived from the excited surface and bulk
states, with different electronic temperatures.Comment: 5 pages and 4 figure
A Noninvasive Test for Vesico-Ureteric Reflux in Children
Objective To report the development and testing of a device for the noninvasive diagnosis of vesicoâureteric reflux (VUR) which avoids the need for urethral catheterization (currently required to reliably determine the presence of VUR), and which thus avoids the anxiety of parents and patients that causes many families to refuse such evaluation.
Patients and methods Fiftyâfour children (49 girls and five boys, mean age 7.2âyears, range 4â14) previously evaluated as having VUR volunteered to participate; no child was symptomatic at the time of the study. Refluxing units were known to be present by voiding cystoâurethrography (within 1 year, mean 7 months) in 45 and absent in 16. The device developed acquires electronically processed acoustic signals from the child during an observed urination. The signals are then analysed âoffâlineâ to determine the presence or absence of VUR. The initial preparation for the test included: (i) a full bladder [at least 0.80 Ă {(2+age) Ă30 mL}] measured by ultrasonography; and (ii) localization of the pelviâureteric junction by ultrasonography to accurately place the device\u27s sensors on the child\u27s back. The children were then positioned at a commode after placing the sensors; the recording was started and continued until voiding occurred. The children were tested with the recording and analysis team unaware of the presence and/or degree of VUR. The first 47 studies were singleâkidney examinations and the remaining seven included simultaneous monitoring of both kidneys.
Results Sixtyâone renal units were assessed and interpretable signals were obtained from 54 (89%). There were seven episodes of âsystem failureâ when no interpretable data were obtained. One unit with no VUR had a ârefluxâ signal; in four kidneys, spontaneous (two) and postsurgical (two) resolution of reflux was predicted by the testing and subsequently verified by cyclic radionuclide cystography.
Conclusions This noninvasive diagnostic technique detected VUR in 35 of 37 refluxing units and verified no reflux in 16 of 17 units without VUR. Further refinements may allow this technology to be used in all children with suspected VUR
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