Thermoplastic deformation of silicon surfaces induced by ultrashort pulsed lasers in submelting conditions

A hybrid 2D theoretical model is presented to describe thermoplastic deformation effects on silicon surfaces induced by single and multiple ultrashort pulsed laser irradiation in submelting conditions. An approximation of the Boltzmann transport equation is adopted to describe the laser irradiation process. The evolution of the induced deformation field is described initially by adopting the differential equations of dynamic thermoelasticity while the onset of plastic yielding is described by the von Mise's stress. Details of the resulting picometre sized crater, produced by irradiation with a single pulse, are then discussed as a function of the imposed conditions and thresholds for the onset of plasticity are computed. Irradiation with multiple pulses leads to ripple formation of nanometre size that originates from the interference of the incident and a surface scattered wave. It is suggested that ultrafast laser induced surface modification in semiconductors is feasible in submelting conditions, and it may act as a precursor of the incubation effects observed at multiple pulse irradiation of materials surfaces.Comment: To appear in the Journal of Applied Physic

DETERMINATION OF VICKERS MICROHARDNESS IN β-Ga2O3 SINGLE CRYSTALS GROWN FROM THEIR OWN MELT

The results of microhardness measurements of β-Ga2O3 single crystals for (001) crystallographic face are reported. The crystals were grown by the free crystallization with the "Garnet-2M" equipment. Microhardness values ​​ were determined by the Vickers method at varying loads. A four-sided diamond pyramid was used as an indenter. The average value of gallium oxide microhardness was equal to 8.91 GPa. We have carried out comparison of the values ​​obtained with the microhardness for the other wide bandgap semiconductors - epitaxial GaN layers grown on 6H-SiC and GaP layers grown on GaP:S. The findings are usable for machining process development of β-Ga2O3 single crystal substrates. In particular, silicon carbide and electrocorundum may be recommended for β-Ga2O3 machine processing

Molecular Genetics of T Cell Development

T cell development is guided by a complex set of transcription factors that act recursively, in different combinations, at each of the developmental choice points from T-lineage specification to peripheral T cell specialization. This review describes the modes of action of the major T-lineage-defining transcription factors and the signal pathways that activate them during intrathymic differentiation from pluripotent precursors. Roles of Notch and its effector RBPSuh (CSL), GATA-3, E2A/HEB and Id proteins, c-Myb, TCF-1, and members of the Runx, Ets, and Ikaros families are critical. Less known transcription factors that are newly recognized as being required for T cell development at particular checkpoints are also described. The transcriptional regulation of T cell development is contrasted with that of B cell development, in terms of their different degrees of overlap with the stem-cell program and the different roles of key transcription factors in gene regulatory networks leading to lineage commitment

A new multi-scale dispersive gradient elasticity modelwith micro-inertia: Formulation and C0-finiteelement implementation

Motivated by nano-scale experimental evidence on the dispersion characteristics of materials with a lattice structure, a new multi-scale gradient elasticity model is developed. In the framework of gradient elasticity, the simultaneous presence of acceleration and strain gradients has been denoted as dynamic consistency. This model represents an extension of an earlier dynamically consistent model with an additional micro-inertia contribution to improve the dispersion behaviour. The model can therefore be seen as an enhanced dynamic extension of the Aifantis' 1992 strain-gradient theory for statics obtained by including two acceleration gradients in addition to the strain gradient. Compared with the previous dynamically consistent model, the additional micro-inertia term is found to improve the prediction of wave dispersion significantly and, more importantly, requires no extra computational cost. The fourth-order equations are rewritten in two sets of symmetric second-order equations so that C0-continuity is sufficient in the finite element implementation. Two sets of unknowns are identified as the microstructural and macrostructural displacements, thus highlighting the multi-scale nature of the present formulation. The associated energy functionals and variationally consistent boundary conditions are presented, after which the finite element equations are derived. Considerable improvements over previous gradient models are observed as confirmed by two numerical examples

Lineage Diversion of T Cell Receptor Transgenic Thymocytes Revealed by Lineage Fate Mapping

Background: The binding of the T cell receptor (TCR) to major histocompatibility complex (MHC) molecules in the thymus determines fates of $TCR\alpha\beta$ lymphocytes that subsequently home to secondary lymphoid tissue. TCR transgenic models have been used to study thymic selection and lineage commitment. Most TCR transgenic mice express the rearranged $TCR\alpha\beta$ prematurely at the double negative stage and abnormal TCRαβ populations of T cells that are not easily detected in non-transgenic mice have been found in secondary lymphoid tissue of TCR transgenic mice. Methodology and Principal Findings: To determine developmental pathways of TCR-transgenic thymocytes, we used Cre-LoxP-mediated fate mapping and show here that premature expression of a transgenic $TCR\alpha\beta$ diverts some developing thymocytes to a developmental pathway which resembles that of gamma delta cells. We found that most peripheral T cells with the HY-TCR in male mice have bypassed the RORγt-positive $CD4^{+}8^{+}$ (double positive, DP) stage to accumulate either as $CD4^{-}8^{-}$ (double negative, DN) or as $CD8\alpha^{+}$ T cells in lymph nodes or gut epithelium. Likewise, DN $TCR\alpha\beta$ cells in lymphoid tissue of female mice were not derived from DP thymocytes. Conclusion: The results further support the hypothesis that the premature expression of the $TCR\alpha\beta$ can divert DN thymocytes into gamma delta lineage cells

Identification of CD4−CD8− Double-Negative Natural Killer T Cell Precursors in the Thymus

BACKGROUND: It is well known that CD1d-restricted Valpha14 invariant natural killer T (NKT) cells are derived from cells in the CD4(+)CD8(+) double-positive (DP) population in the thymus. However, the developmental progression of NKT cells in the earlier stages remains unclear, and the possible existence of NKT cell presursors in the earlier stages than DP stage remains to be tested. PRINCIPAL FINDINGS: Here, we demonstrate that NKT cell precursors that express invariant Valpha14-Jalpha18 transcripts but devoid of surface expression of the invariant Valpha14 receptor are present in the late CD4(-)CD8(-) double-negative (DN)4 stage and have the potential to generate mature NKT cells in both in vivo and in vitro experimental conditions. Moreover, the DN4 population in CD1d knock-out (CD1dKO) mice was similar to those with an NKT cell potential in wild-type (WT) C57BL/6 (B6) mice, but failed to develop into NKT cells in vitro. However, these precursors could develop into NKT cells when co-cultured with normal thymocytes or in an in vivo experimental setting, indicating that functional NKT cell precursors are present in CD1dKO mice. CONCLUSIONS: Together, these results demonstrate that thymic DN4 fraction contains NKT cell precursors. Our findings provide new insights into the early development of NKT cells prior to surface expression of the invariant Valpha14 antigen receptor and suggest the possible alternative developmental pathway of NKT cells