178 research outputs found
Excitons at high density in Cuprous Oxide and coupled quantum wells
There is a 40-year long history in the search for Bose-Einstein condensation (BEC) of excitons in semiconductors. This thesis presents research directed toward this goal in bulk crystal Cu2O in three dimensions and in GaAs-based coupled quantum wells (CQW) in two dimensions.The Auger recombination process in Cu2O plays a major role in limiting the density of the excitons. We find that the rate for this process increases with applied stress and lattice temperature. We create paraexcitons in Cu2O through a resonant two-photon excitation in a harmonic potential trap with the Auger recombination process as small as possible (at low temperature and low stress), and find that the exciton creation efficiency in the resonant two-photon excitation is greater for one-beam excitation than for two colliding pulses, but the colliding pulse method may be useful for direct creation of a condensate in the ground state. The paraexciton density in this work is about thirty times less than the required density for BEC of paraexcitons. One promising direction for BEC of excitons in Cu2O is that with higher laser power from stronger IR laser sources, or at lower temperature, the critical density can be approached under one-beam two-photon excitation resonant with the paraexciton state.In two dimensions, the CQW structure has been modified with four design strategies: highest possible barriers, introducing into the barriers a superlattice of 60 angstrom GaAs wells, p-i-n doping, and wider quantum wells, which provides indirect excitons low disorder and high mobility. With a cold near-resonant excitation, we conclude that the excitons act as a free gas, travelling distances of hundreds of microns. We also present observations of a narrow beam of emitted light, when the indirect excitons are confined in a two-dimensional harmonic potential trap, in a way quite similar to the first observations of BEC in alkali atoms. A beam-like emission has been suggested as a telltale for BEC of excitons. This opens the door to a whole range of investigations, including attempts to observe coherence of the emitted light, proof of superfludity of the excitons, and other fascinating effects
Centrosome-associated regulators of the G2/M checkpoint as targets for cancer therapy
In eukaryotic cells, control mechanisms have developed that restrain cell-cycle transitions in response to stress. These regulatory pathways are termed cell-cycle checkpoints. The G2/M checkpoint prevents cells from entering mitosis when DNA is damaged in order to afford these cells an opportunity to repair the damaged DNA before propagating genetic defects to the daughter cells. If the damage is irreparable, checkpoint signaling might activate pathways that lead to apoptosis. Since alteration of cell-cycle control is a hallmark of tumorigenesis, cell-cycle regulators represent potential targets for therapy. The centrosome has recently come into focus as a critical cellular organelle that integrates G2/M checkpoint control and repairs signals in response to DNA damage. A growing number of G2/M checkpoint regulators have been found in the centrosome, suggesting that centrosome has an important role in G2/M checkpoint function. In this review, we discuss centrosome-associated regulators of the G2/M checkpoint, the dysregulation of this checkpoint in cancer, and potential candidate targets for cancer therapy
Identification and control of a Pseudomonas spp (P. fulva and P. putida) bloodstream infection outbreak in a teaching hospital in Beijing, China
SummaryObjectivesAn outbreak of bacteremia caused by Pseudomonas spp (P. fulva and P. putida) was first identified in our hospital in the summer of 2010 and reoccurred in the following year. Based on the epidemiological data collected in these 2 years, we initiated an investigation on the source of the outbreak. The aim of this study was to report the results of the investigation, as well as the intervention strategies that resulted in successful control of the outbreak.MethodsAn infection control team was set up consisting of infectious disease specialists, microbiologists, infection control practitioners, and head nurses. The microbiology and medical records of case-patients with P. fulva or P. putida bloodstream infections were reviewed. Environmental samples and intravenous (IV) solutions from the wards and the pharmacy center were collected for culturing. The molecular characteristics of the bacterial isolates were studied by pulsed-field gel electrophoresis (PFGE). Strict infection control strategies were implemented.ResultsA total of 20 case-patients from five inpatient wards were identified during three summer seasons from 2010 to 2012. Nineteen of them recovered with proper antibiotics. Unfortunately one died from complications of heart failure. A total of 19 isolates of P. fulva and four of P. putida were identified, of which 20 were from blood, two from environmental surface samples from the hospital pharmacy, and one from an in-use compounded solution from a case-patient in the cardiology ward. Molecular analysis revealed that the P. fulva isolated from the in-use compounded solution (5% glucose solution containing insulin, isosorbide dinitrate, and potassium magnesium aspartate) and the environmental samples had the same PFGE type as the clinical isolates.ConclusionsThe investigation identified that contaminated IV solution was the source of the P. fulva bacteremia, which prompted us to implement intensified control measures that resulted in successful control of the outbreak
Prediction of interesting ferromagnetism in Janus semiconducting CrAsP monolayer
Two-dimensional (2D) half-metallic materials that have sparked intense
interest in advanced spintronic applications are essential to the developing
next-generation nanospintronic devices. Here we have adopted a first-principles
calculation method to predict the magnetic properties of intrinsic, Se-doped,
and biaxial strain tuning CrAsP monolayer. The Janus CrAsP monolayer is
proved to be an intrinsic ferromagnetic (FM) semiconductor with a exchange
splitting bandgap of 0.15 eV at the PBE+U level. Concentration-dependent Se
doping such as CrAsSeP (x = 0.25, 0.50, 0.75) can regulate
CrAsP from FM semiconductor to FM half-metallicity. Specifically, the
spin-up channel crosses the Fermi level, while the spin-down channel has a
bandgap. More interestingly, the wide half-metallic bandgaps and spin bandgaps
make them have important implications for the preparation of spintronic
devices. At last, we also explore the effect of biaxial strain from -14% to 10%
on the magnetism of the CrAsP monolayer. There appears a transition from FM
to antiferromagnetic (AFM) at a compressive strain of -10.7%, originating from
the competition between the indirect FM superexchange interaction and the
direct AFM interaction between the nearest-neighbor Cr atoms. Additionally,
when the compressive strain to -2% or the tensile strain to 6%, the
semiconducting CrAsP becomes a half-metallic material. These charming
properties render the Janus CrAsP monolayer with great potential for
applications in spintronic devices.Comment: 14 pages, 4 figure
First-principles calculations on the mechanical, electronic, magnetic and optical properties of two-dimensional Janus CrTeX (X= P, As, Sb) monolayers
Janus materials possess extraordinary physical, chemical, and mechanical
properties caused by symmetry breaking. Here, the mechanic properties,
electronic structure, magnetic properties, and optical properties of Janus
CrTeX (X= P, As, Sb) monolayers are systematically investigated by the
density functional theory. Janus CrTeP, CrTeAs, and CrTeSb are
intrinsic ferromagnetic (FM) half-metals with wide spin gaps and half-metallic
gaps. Monte Carlo simulations based on the Heisenberg model estimate the Curie
temperature (\emph{T}) of these monolayers are about 583, 608, and 597 K,
respectively. Additionally, it is found that CrTeX (X= P, As, Sb)
monolayers still exhibit FM half-metallic properties under biaxial strain from
-6% to 6%. At last, the CrTeP monolayer has a higher absorption coefficient
than the CrTeAs and CrTeSb monolayers in the visible region. The
results predict that Janus CrTeX (X= P, As, Sb) monolayers with novel
properties have good potential for applications in future nanodevices.Comment: 14 pages, 5 figure
Resonant two-photon excitation of 1s paraexcitons in Cuprous Oxide
We have created paraexcitons in Cuprous Oxide via resonant two-photon
generation, and examined their population dynamics by means of time-correlated
single photon detection. Confining the excitons to a constant volume in a
harmonic potential trap made with inhomogeneous applied stress along the [001]
axis, we find that paraexcitons are created directly, and orthoexcitons appear
primarily through the well-known excitonic Auger process. Hot excitons are also
created via a three-photon process when the IR laser is non-resonant. Also we
generate excitons with two colliding pulses, and the luminescence is weaker
than that from one beam excitation with same total laser power. These results
show that resonant one-beam two-photon generation of paraexcitons is a
promising way to pursue Bose-Einstein condensation of paraexcitons.Comment: 22 pages, 9 figure
Detecting topological phase transitions in a double kicked quantum rotor
We present a concrete theoretical proposal for detecting topological phase transitions in double kicked atom-optics kicked rotors with internal spin-1/2 degree of freedom. The implementation utilizes a kicked Bose-Einstein condensate evolving in one-dimensional momentum space. To reduce the influence of atom loss and phase decoherence, we aim to keep experimental durations short while maintaining a resonant experimental protocol. Experimental limitations induced by phase noise, quasimomentum distributions, symmetries, and the ac-Stark shift are considered. Our results thus suggest a feasible and optimized procedure for observing topological phase transitions in quantum kicked rotor
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