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

Genetic Characterization of Fine-Leaved \u3ci\u3eFestuca valesiaca\u3c/i\u3e Germplasm and Evaluation of Their Relationship to the \u3ci\u3eFestuca ovina\u3c/i\u3e Complex

Fine-leaved Festuca valesiaca possesses abiotic stress tolerances. However, their agronomic performances in the western United States and its genetic relationship to species of the Festuca ovina complex have not been investigated. Also, natural hybridization due to open pollination presents difficulties in distinguishing them for closely related taxa using morphological analysis. Given the species’ agronomic potentials, a project was designed to identify Festuca valesiaca accessions possessing high biomass production and seed yield for possible low-maintenance applications and to examine their relatedness to taxa of the Festuca ovina complex by multi-locus AFLP genotyping and chloroplast DNA sequence analysis using primer combinations designed from three intergenic spacers. Plant vigor, height and width, total biomass, and seed weight and seed number of Festuca valesiaca accessions were evaluated from 2009 to 2011 at Blue Creek, Utah in a random complete block design with six replications. The Festuca valesiaca accessions examined produced abundance of small seeds. Seed production was significantly (P = 0.001) correlated (r2 = 0.84) with the total biomass, plant height, and plant vigor rating. The Festuca valesiaca accessions examined possessed lower height than the control ‘Cascade’ but higher biomass, spring green-up, and seed production. Given their morphological attributes, Festuca valesiaca accessions PI 659923, W6 30575, and W6 30588 should be considered for low-maintenance applications and use in plant improvement. The AFLP-based neighbor-joining analysis indicated that Festuca valesiaca is a closely related subcluster of Festuca ovina and should be considered as one species. Festuca trachyphylla is a subcluster under Festuca ovina and Festuca valesiaca. Festuca idahoensis has a close relationship with Festuca roemeri but not with Festuca ovina. Low admixture was detected between the Festuca rubra and Festuca trachyphylla accessions examined, while a comparative high admixture was detected among the commercial cultivars examined. Chloroplast sequences data reconfirmed that the Festuca ovina complex genetically differed from Festuca rubra and the other reference taxa examined. Festuca valesiaca and Festuca ovina possessed the same maternal lineage based on chloroplast DNA sequence analysis. One Festuca valesiaca accession, W6 30537, was genetically similar to the Festuca rubra examined and should be putatively reclassified as Festuca rubra pending further taxonomic analysis

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

Feeling Right at Home: Hometown CEOs and Firm Innovation

Extending the theories of social and place identity, we predict that CEO hometown identity has a positive and significant influence on firm innovation. Our empirical evidence, from publicly traded firms in China during 2002–2016, suggests that a firm whose CEO's hometown is in the same province or city as the firm's headquarters tends to invest more in R&D and generate more patent applications. Our results are robust to the firm fixed effects and we use difference-in-differences analysis and instrument variable regressions to mitigate endogeneity concerns. CEOs' hometown identity still has a strong and positive impact on innovation after we control for measures of social capital of CEOs. We identify the mechanisms behind the positive relation between firm innovation and CEO hometown identity: hometown CEOs enjoy more support from the board of directors, they are more willing to take risks, and they are more likely to have long-term visions

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

Prediction of interesting ferromagnetism in Janus semiconducting Cr2_2AsP 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 Cr$_2$AsP monolayer. The Janus Cr$_2$AsP 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 Cr$_2$As$_{1-x}$Se$_x$P (x = 0.25, 0.50, 0.75) can regulate Cr$_2$AsP 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 Cr$_2$AsP 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 Cr$_2$AsP becomes a half-metallic material. These charming properties render the Janus Cr$_2$AsP 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 Cr2_2TeX (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 Cr$_2$TeX (X= P, As, Sb) monolayers are systematically investigated by the density functional theory. Janus Cr$_2$TeP, Cr$_2$TeAs, and Cr$_2$TeSb 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}$_c$) of these monolayers are about 583, 608, and 597 K, respectively. Additionally, it is found that Cr$_2$TeX (X= P, As, Sb) monolayers still exhibit FM half-metallic properties under biaxial strain from -6% to 6%. At last, the Cr$_2$TeP monolayer has a higher absorption coefficient than the Cr$_2$TeAs and Cr$_2$TeSb monolayers in the visible region. The results predict that Janus Cr$_2$TeX (X= P, As, Sb) monolayers with novel properties have good potential for applications in future nanodevices.Comment: 14 pages, 5 figure