Crystal Symmetry Breaking in Few-Quintuple Bismuth Telluride Films: Applications in Nanometrology of Topological Insulators

We report results of micro-Raman spectroscopy investigation of the "graphene-like" mechanically exfoliated single-crystal bismuth telluride films with the thickness ranging from a few-nm-range to bulk limit. It is found that the optical phonon mode A1u, which is not-Raman active in bulk bismuth telluride crystals, appears in the atomically-thin films due to crystal-symmetry breaking. The intensity ratios of the out-of-plane A1u and A1g modes to the in-plane Eg mode grow with decreasing film thickness. The evolution of Raman signatures with the film thickness can be used for identification of bismuth telluride crystals with the thickness of few-quintuple layers, which are important for topological insulator and thermoelectric applications.Comment: 13 pages, 2 tables, 3 figures; to be presented at MRS Spring Meeting, 201

Strong enhancement of Jc in binary and alloyed in-situ MgB2 wires by a new approach: Cold high pressure densification

Cold high pressure densification (CHPD) is presented as a new way to substantially enhance the critical current density of in situ MgB2 wires at 4.2 and 20 K at fields between 5 and 14 T. The results on two binary MgB2 wires and an alloyed wire with 10 wt.% B4C are presented The strongest enhancement was measured at 20K, where cold densification at 1.85 GPa on a binary Fe/MgB2 wire raised both Jcpara and Jcperp by more than 300% at 5T, while Birr was enhanced by 0.7 T. At 4.2K, the enhancement of Jc was smaller, but still reached 53% at 10 T. After applying pressures up to 6.5 GPa, the mass density dm of the unreacted (B+Mg) mixture inside the filaments reached 96% of the theoretical density. After reaction under atmospheric pressure, this corresponds to a highest mass density df in the MgB2 filaments of 73%. After reaction, the electrical resistance of wires submitted to cold densification was found to decrease, reflecting an improved connectivity. A quantitative correlation between filament mass density and the physical properties was established. Monofilamentary rectangular wires with aspect ratios a/b < 1.25 based on low energy ball milled powders exhibited very low anisotropy ratios, Gamma = Jcpara/Jcperp being < 1.4 at 4.2 K and 10T. The present results can be generalized to alloyed MgB2 wires, as demonstrated on a wire with B4C additives. Based on the present data, it follows that cold densification has the potential of further improving the highest Jcpara and Jcperp values reported so far for in situ MgB2 tapes and wires with SiC and C additives. Investigations are under work in our laboratory to determine whether the densification method CHPD can be applied to longer wire or tape lengths.Comment: Submitted to Superconductors Science and Technolog

A hybrid model for studying the size effects on flow stress in micro - forming with the consideration of grain hardening

© 2019 Trans Tech Publications Ltd, Switzerland. Size effects extremely exist in the metal micro-forming process. When a deformation process scales down to micro scale, the appearances of geometry size and single grain size starts to play a major role in deformation. Generally, the size effects are unavoidable in the experimental work and cannot be neglect in the optimization of micro-forming processes. In this paper, size effect on flow stress is investigated in the form of the coupled effect of workpiece geometry (sample thickness) and grain size, (T/D) by the micro tensile test of pure copper foil. Following the previous approaches, a new hybrid material model is projected to describe the hardening behavior of grains in polycrystalline material. Tensile tests performed on the copper foil with constant thickness and width, while to get dissimilar grain sizes, the foil annealed for different times. The ratio of thickness to grain size (T/D) is limited to larger than 1 (T/D>1). A hybrid material model is proposed and established based on grain heterogeneity and sample thickness. The hybrid material model builds a relationship between the surface layer and sheet interior. The hybrid material model developed by the strain gradient theory in which the dislocation cell structure, cell densities (interior and wall) engaged to define the polycrystalline aggregate and calculated the dislocations in a grain (grain interior and grain wall). The results show that flow stress varies with the different values of T/D, but with an increase of the share of the grains flow stress start to decreases. After applying the hybrid material model of flow stress, the micro-tensile test of copper foil is simulated by finite element method. The simulation outcomes well matched with experimental results

Genericness of inflation in isotropic loop quantum cosmology

Non-perturbative corrections from loop quantum cosmology (LQC) to the scalar matter sector is already known to imply inflation. We prove that the LQC modified scalar field generates exponential inflation in the small scale factor regime, for all positive definite potentials, independent of initial conditions and independent of ambiguity parameters. For positive semi-definite potentials it is always possible to choose, without fine tuning, a value of one of the ambiguity parameters such that exponential inflation results, provided zeros of the potential are approached at most as a power law in the scale factor. In conjunction with generic occurrence of bounce at small volumes, particle horizon is absent thus eliminating the horizon problem of the standard Big Bang model.Comment: 4 pages, revtex4, one figure. Only e-print archive numbers correctedi in the second version. Reference added in the 3rd version. Final version to appear in Phys. Rev. Lett. Explanations improve

A highly efficient two level diamond based single photon source

An unexplored diamond defect centre which is found to emit stable single photons at a measured rate of 1.6 MHz at room temperature is reported. The novel centre, identified in chemical vapour deposition grown diamond crystals, exhibits a sharp zero phonon line at 734 nm with a full width at half maximum of ~ 4 nm. The photon statistics confirm the center is a single emitter and provides direct evidence of the first true two-level single quantum system in diamond.Comment: 3 pages, 4 figure

Numerical solutions of the three-dimensional magnetohydrodynamic alpha-model

We present direct numerical simulations and alpha-model simulations of four familiar three-dimensional magnetohydrodynamic (MHD) turbulence effects: selective decay, dynamic alignment, inverse cascade of magnetic helicity, and the helical dynamo effect. The MHD alpha-model is shown to capture the long-wavelength spectra in all these problems, allowing for a significant reduction of computer time and memory at the same kinetic and magnetic Reynolds numbers. In the helical dynamo, not only does the alpha-model correctly reproduce the growth rate of magnetic energy during the kinematic regime, but it also captures the nonlinear saturation level and the late generation of a large scale magnetic field by the helical turbulence.Comment: 12 pages, 19 figure

A perfect storm : polycystic ovary syndrome masking underlying yype 1 Von Willebrand disease

Von Willebrand Disease (vWD) is the most common inherited bleeding diathesis worldwide and results in defects in von Willebrand Factor (vWF), inducing a hypocoagulable state. Polycystic Ovary Syndrome (PCOS) is characterized by chronic inflammation and hyperestrogenism, both of which induce acute phase reactions and increase serum levels of vWF and Factor VIII, yielding a prothrombotic state. These laboratory elevations may obscure the diagnosis of underlying vWD in patients with both conditions. We report a case of a 23 year-old female with PCOS and menorrhagia who presented prior to a surgical procedure for evaluation of bleeding risk

Online Garments Inventory Management System

Management Systems are usually designed to enhance the efficiency of handling the information of any system, which is running through an inefficient procedure and is expensive, time consuming, insecure and it needs more manpower. The Online Garments Inventory Management System is a system which makes the working procedure of present manual system of Garments easier and increases its efficiency to a high degree. The main objective of this application is to automate the existing system which is manually maintaining. The whole system is accessed from different terminals of the network. There are three major sectors in this system- employee management, product inventory and raw materials inventory. In the employee management system all information of the employees are stored. In the product inventory system, the invoice and the delivery information is maintained. Raw materials inventory controls the purchase information, shipping information, total materials of stock in the garments. The Entity Relationship model is used to design the database that will store and organize the garments data. The data is stored in MySQL Database Server and populated it with some sample data. Using HTML, PHP, Java Script and CSS, an internet based graphical user interface has been created that allows garments authority and administrator to access the system remotely

Effective State Metamorphosis in Semi-Classical Loop Quantum Cosmology

Modification to the behavior of geometrical density at short scales is a key result of loop quantum cosmology, responsible for an interesting phenomenology in the very early universe. We demonstrate the way matter with arbitrary scale factor dependence in Hamiltonian incorporates this change in its effective dynamics in the loop modified phase. For generic matter, the equation of state starts varying near a critical scale factor, becomes negative below it and violates strong energy condition. This opens a new avenue to generalize various phenomenological applications in loop quantum cosmology. We show that different ways to define energy density may yield radically different results, especially for the case corresponding to classical dust. We also discuss implications for frequency dispersion induced by modification to geometric density at small scales.Comment: Revised version; includes expanded discussion of natural trans-Planckian modifications to frequency dispersion and robustness to quantization ambiguities. To appear in Class. Quant. Gra