221 research outputs found
Signatures of four-particle correlations associated with exciton-carrier interactions in coherent spectroscopy on bulk GaAs
Transient four-wave mixing studies of bulk GaAs under conditions of broad
bandwidth excitation of primarily interband transitions have enabled
four-particle correlations tied to degenerate (exciton-exciton) and
nondegenerate (exciton-carrier) interactions to be studied. Real
two-dimensional Fourier-transform spectroscopy (2DFTS) spectra reveal a complex
response at the heavy-hole exciton emission energy that varies with the
absorption energy, ranging from dispersive on the diagonal, through absorptive
for low-energy interband transitions to dispersive with the opposite sign for
interband transitions high above band gap. Simulations using a multilevel model
augmented by many-body effects provide excellent agreement with the 2DFTS
experiments and indicate that excitation-induced dephasing (EID) and
excitation-induced shift (EIS) affect degenerate and nondegenerate interactions
equivalently, with stronger exciton-carrier coupling relative to
exciton-exciton coupling by approximately an order of magnitude. These
simulations also indicate that EID effects are three times stronger than EIS in
contributing to the coherent response of the semiconductor
Optical studies of carrier and phonon dynamics in Ga_{1-x}Mn_{x}As
We present a time-resolved optical study of the dynamics of carriers and
phonons in Ga_{1-x}Mn_{x}As layers for a series of Mn and hole concentrations.
While band filling is the dominant effect in transient optical absorption in
low-temperature-grown (LT) GaAs, band gap renormalization effects become
important with increasing Mn concentration in Ga_{1-x}Mn_{x}As, as inferred
from the sign of the absorption change. We also report direct observation on
lattice vibrations in Ga1-xMnxAs layers via reflective electro-optic sampling
technique. The data show increasingly fast dephasing of LO phonon oscillations
for samples with increasing Mn and hole concentration, which can be understood
in term of phonon scattering by the holes.Comment: 13 pages, 3 figures replaced Fig.1 after finding a mistake in
previous versio
Origin of Magnetic Circular Dichroism in GaMnAs: Giant Zeeman Splitting versus Spin Dependent Density of States
We present a unified interpretation of experimentally observed magnetic
circular dichroism (MCD) in the ferromagnetic semiconductor (Ga,Mn)As, based on
theoretical arguments, which demonstrates that MCD in this material arises
primarily from a difference in the density of spin-up and spin-down states in
the valence band brought about by the presence of the Mn impurity band, rather
than being primarily due to the Zeeman splitting of electronic states.Comment: 4+ pages, 4 figure
Intensive care nurse managers' experiences during the first wave of the Covid-19 pandemic: Implications for future epidemiological crises
BACKGROUND: Nurse managers play an important role in coordinating the multidisciplinary teamwork, which is specifically important in emergency and crises situations like the COVID-19 pandemic. The aim of this qualitative study is twofold: (1) to explore the experiences of the Intensive care units (ICU) nurse managers regarding their work during the first wave of the COVID-19 pandemic, and (2) to analyse what implications might be provided based on experiences of nurse managers for future possible epidemiological crises. METHODS: In-depth phone interviews were conducted to explore the experiences of ward managers-nurses (n = 15) working in different hospitals across Poland. Interviews were taped and transcribed verbatim, and then qualitatively analysed. RESULTS: Three main categories were identified: (1) Challenge of working with the unknown, (2) Nurse managers' expectations, and (3) Methods of coping and received support. The COVID-19 pandemic strongly affected the work of ICU nurse managers and uncovered the malfunctioning of the healthcare system. CONCLUSION: It is important to improve the knowledge and competence of hospital management personnel through exercises and in-service training on how to handle emergencies in order to improve the management of healthcare facilities, increase the safety of patients and employees, and the quality of healthcare
Spin Dynamics and Spin Transport
Spin-orbit (SO) interaction critically influences electron spin dynamics and
spin transport in bulk semiconductors and semiconductor microstructures. This
interaction couples electron spin to dc and ac electric fields. Spin coupling
to ac electric fields allows efficient spin manipulating by the electric
component of electromagnetic field through the electric dipole spin resonance
(EDSR) mechanism. Usually, it is much more efficient than the magnetic
manipulation due to a larger coupling constant and the easier access to spins
at a nanometer scale. The dependence of the EDSR intensity on the magnetic
field direction allows measuring the relative strengths of the competing SO
coupling mechanisms in quantum wells. Spin coupling to an in-plane electric
field is much stronger than to a perpendicular field. Because electron bands in
microstructures are spin split by SO interaction, electron spin is not
conserved and spin transport in them is controlled by a number of competing
parameters, hence, it is rather nontrivial. The relation between spin
transport, spin currents, and spin populations is critically discussed.
Importance of transients and sharp gradients for generating spin magnetization
by electric fields and for ballistic spin transport is clarified.Comment: Invited talk at the 3rd Intern. Conf. on Physics and Applications of
Spin-Related Phenomena in Semiconductors, Santa Barbara (CA), July 21 - 23.
To be published in the Journal of Superconductivity. 7 pages, 2 figure
Controlling Curie temperature in (Ga,Ms)As through location of the Fermi level within the impurity band
The ferromagnetic semiconductor (Ga,Mn)As has emerged as the most studied
material for prototype applications in semiconductor spintronics. Because
ferromagnetism in (Ga,Mn)As is hole-mediated, the nature of the hole states has
direct and crucial bearing on its Curie temperature TC. It is vigorously
debated, however, whether holes in (Ga,Mn)As reside in the valence band or in
an impurity band. In this paper we combine results of channeling experiments,
which measure the concentrations both of Mn ions and of holes relevant to the
ferromagnetic order, with magnetization, transport, and magneto-optical data to
address this issue. Taken together, these measurements provide strong evidence
that it is the location of the Fermi level within the impurity band that
determines TC through determining the degree of hole localization. This finding
differs drastically from the often accepted view that TC is controlled by
valence band holes, thus opening new avenues for achieving higher values of TC.Comment: 5 figures, supplementary material include
Resonant Spectroscopy of II-VI Self-Assembled Quantum Dots: Excited States and Exciton-LO Phonon Coupling
Using resonantly excited photoluminescence along with photoluminescence
excitation spectroscopies, we study the carrier excitation processes in
CdTe/ZnTe and CdSe/ZnSe self-assembled quantum dots. Photoluminescence
excitation spectra of single CdTe quantum dots reflect two major mechanisms for
carrier excitation: The first, associated with the presence of sharp and
intense lines in the spectrum, is a direct excited state ? ground state
transition. The second, associated with the appearance of up to four much
broader excitation lines, is a LO phonon-assisted absorption directly into the
quantum dot ground states. LO phonons with energies of both quantum dots and
ZnTe barrier material are identified in the photoluminescence excitation
spectra. Resonantly excited PL measurements for the dot ensemble as a function
of excitation energy makes it possible to separate the contributions of these
two mechanisms. We find that for CdTe quantum dots the distribution of excited
states coupled to the ground states reflects the energy distribution of the
quantum dot emission, but shifted up in energy by 100 meV. This large splitting
between excited and ground states in CdTe quantum dots suggests strong spatial
confinement. In contrast, the LO phonon-assisted absorption shows significant
size selectivity. In the case of CdTe dots the exciton-LO phonon coupling is
strongly enhanced for smaller-sized dots which have higher emission energies.
In contrast, for CdSe quantum dots the exciton-LO phonon coupling is uniform
over the ensemble ? that is, the energy distribution determines the intensities
of LO phonon replicas. We show that for CdTe quantum dots after annealing, that
is after an increase in the average dot size, the exciton-LO phonon interaction
reflects the dot energy distribution, as observed for CdSe quantum dots.Comment: 28 pages, 5 figure
Academic student satisfaction and perceived performance in the e-learning environment during the COVID-19 pandemic: Evidence across ten countries
The outbreak of the COVID-19 pandemic has dramatically shaped higher education and seen the distinct rise of e-learning as a compulsory element of the modern educational landscape. Accordingly, this study highlights the factors which have influenced how students perceive their academic performance during this emergency changeover to e-learning. The empirical analysis is performed on a sample of 10,092 higher education students from 10 countries across 4 continents during the pandemic’s first wave through an online survey. A structural equation model revealed the quality of e-learning was mainly derived from service quality, the teacher’s active role in the process of online education, and the overall system quality, while the students’ digital competencies and online interactions with their colleagues and teachers were considered to be slightly less important factors. The impact of e-learning quality on the students’ performance was strongly mediated by their satisfaction with e-learning. In general, the model gave quite consistent results across countries, gender, study fields, and levels of study. The findings provide a basis for policy recommendations to support decision-makers incorporate e-learning issues in the current and any new similar circumstances.info:eu-repo/semantics/publishedVersio
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