Spin-polarized STM spectra of Dirac electrons on the surface of a topological insulator

We provide a theory for the tunneling conductance G(V) of Dirac electrons on the surface of a topological insulator as measured by a spin-polarized scanning tunneling microscope tip for low-bias voltages V. We show that if the in-plane rotational symmetry on the surface of the topological insulator is broken by an external field that does not couple to spin directly (such as an in-plane electric field), G(V) exhibits an unconventional dependence on the direction of the magnetization of the tip, i.e., it acquires a dependence on the azimuthal angle of the magnetization of the tip. We also show that G(V) can be used to measure the magnitude of the local out-of-plane spin orientation of the Dirac electrons on the surface. We explain the role of the Dirac electrons in this unconventional behavior and suggest experiments to test our theory.

Effect of pairing fluctuations on low-energy electronic spectra in cuprate superconductors

We describe here a minimal theory of tight-binding electrons moving on the square planar Cu lattice of the hole-doped cuprates and mixed quantum mechanically with their own Cooper pairs. The superconductivity occurring at the transition temperature T(c) is the long-range, d-wave symmetry phase coherence of these Cooper pairs. Fluctuations, necessarily associated with incipient long-range superconducting order, have a generic large-distance behavior near T(c). We calculate the spectral density of electrons coupled to such Cooper-pair fluctuations and show that features observed in angle resolved photoemission spectroscopy (ARPES) experiments on different cuprates above T(c) as a function of doping and temperature emerge naturally in this description. These include ``Fermi arcs'' with temperature-dependent length and an antinodal pseudogap, which fills up linearly as the temperature increases toward the pseudogap temperature. Our results agree quantitatively with experiment. Below T(c), the effects of nonzero superfluid density and thermal fluctuations are calculated and compared successfully with some recent ARPES experiments, especially the observed bending or deviation of the superconducting gap from the canonical d-wave form.

Experimental evidence of Ga-vacancy induced room temperature ferromagnetic behavior in GaN films

We have grown Ga deficient GaN epitaxial films on (0001) sapphire substrate by plasma-assisted molecular beam epitaxy and report the experimental evidence of room temperature ferromagnetic behavior. The observed yellow emission peak in room temperature photoluminescence spectra and the peak positioning at 300 cm(-1) in Raman spectra confirms the existence of Ga vacancies. The x-ray photoelectron spectroscopic measurements further confirmed the formation of Ga vacancies; since the N/Ga is found to be >1. The ferromagnetism is believed to originate from the polarization of the unpaired 2p electrons of N surrounding the Ga vacancy. (C) 2011 American Institute of Physics. [doi:10.1063/1.3654151]

Fracture analysis of concrete using singular fractal functions with lattice beam network and confirmation with acoustic emission study

In the present study singular fractal functions (SFF) were used to generate stress-strain plots for quasibrittle material like concrete and cement mortar and subsequently stress-strain plot of cement mortar obtained using SFF was used for modeling fracture process in concrete. The fracture surface of concrete is rough and irregular. The fracture surface of concrete is affected by the concrete's microstructure that is influenced by water cement ratio, grade of cement and type of aggregate 11-41. Also the macrostructural properties such as the size and shape of the specimen, the initial notch length and the rate of loading contribute to the shape of the fracture surface of concrete. It is known that concrete is a heterogeneous and quasi-brittle material containing micro-defects and its mechanical properties strongly relate to the presence of micro-pores and micro-cracks in concrete 11-41. The damage in concrete is believed to be mainly due to initiation and development of micro-defects with irregularity and fractal characteristics. However, repeated observations at various magnifications also reveal a variety of additional structures that fall between the `micro' and the `macro' and have not yet been described satisfactorily in a systematic manner [1-11,15-17]. The concept of singular fractal functions by Mosolov was used to generate stress-strain plot of cement concrete, cement mortar and subsequently the stress-strain plot of cement mortar was used in two-dimensional lattice model [28]. A two-dimensional lattice model was used to study concrete fracture by considering softening of matrix (cement mortar). The results obtained from simulations with lattice model show softening behavior of concrete and fairly agrees with the experimental results. The number of fractured elements are compared with the acoustic emission (AE) hits. The trend in the cumulative fractured beam elements in the lattice fracture simulation reasonably reflected the trend in the recorded AE measurements. In other words, the pattern in which AE hits were distributed around the notch has the same trend as that of the fractured elements around the notch which is in support of lattice model. (C) 2011 Elsevier Ltd. All rights reserved.

The fluorescence of Nd3+ in lead borate and bismuth borate glasses with large stimulated emission cross section

The stimulated emission cross section σp for the 1060 nm transition of Nd3+ in lead borate and bismuth borate glasses has been determined from fluorescence measurements. The compositional dependence of σp, which has been evaluated using radiative transition probability, refractive index of the host glass, effective fluorescence linewidth, and position of the band, with PbO/Bi2O3 content is investigated. The σp values of the 1060 nm band of Nd3+ for lead borate and bismuth borate glasses are found to be in the range 2.6–5.7×10−20 cm2 at 298 K and 3.0–6.3×10−20 cm2 at 4.2 K. The σp values are comparatively large suggesting the possible utilization of these materials in laser applications.

Simple theoretical analysis of the Fowler-Nordheim field emission from microstructures and quantum wires of optoelectronic materials

We present a simplified theoretical formulation of the Fowler-Nordheim field emission (FNFE) under magnetic quantization and also in quantum wires of optoelectronic materials on the basis of a newly formulated electron dispersion law in the presence of strong electric field within the framework of k.p formalism taking InAs, InSb, GaAs, Hg(1-x)Cd(x)Te and In(1-x)Ga(x) As(y)P(1-y) lattice matched to InP as examples. The FNFE exhibits oscillations with inverse quantizing magnetic field and electron concentration due to SdH effect and increases with increasing electric field. For quantum wires the FNFE increases with increasing film thickness due to the existence van-Hove singularity and the magnitude of the quantum jumps are not of same height indicating the signature of the band structure of the material concerned. The appearance of the humps of the respective curves is due to the redistribution of the electrons among the quantized energy levels when the quantum numbers corresponding to the highest occupied level changes from one fixed value to the others. Although the field current varies in various manners with all the variables in all the limiting cases as evident from all the curves, the rates of variations are totally band-structure dependent. Under certain limiting conditions, all the results as derived in this paper get transformed in to well known Fowler-Nordheim formula. (C) 2011 Elsevier Ltd. All rights reserved.

Development of novel multiple quantummethodologies for the analyses of complex protonNMR spectra of scalar coupled spins

One of the significant advancements in Nuclear Magnetic Resonance spectroscopy (NMR) in combating the problem of spectral complexity for deriving the structure and conformational information is the incorporation of additional dimension and to spread the information content in a two dimensional space. This approach together with the manipulation of the dynamics of nuclear spins permitted the designing of appropriate pulse sequences leading to the evolution of diverse multidimensional NMR experiments. The desired spectral information can now be extracted in a simplified and an orchestrated manner. The indirect detection of multiple quantum (MQ) NMR frequencies is a step in this direction. The MQ technique has been extensively used in the study of molecules aligned in liquid crystalline media to reduce spectral complexity and to determine molecular geometries. Unlike in dipolar coupled systems, the size of the network of scalar coupled spins is not big in isotropic solutions and the MQ 1H detection is not routinely employed,although there are specific examples of spin topology filtering. In this brief review, we discuss our recent studies on the development and application of multiple quantum correlation and resolved techniques for the analyses of proton NMR spectra of scalar coupled spins.

Synthesis and characterization of self-assembled nanofiber-bundles of V(2)O(5): their electrochemical and field emission properties

High-quality self-assembled V(2)O(5) nanofiber-bundles (NBs) are synthesized by a simple and direct hydrothermal method using a vanadium(V) hydroxylamido complex as a vanadium source in the presence of HNO(3). The possible reaction pathway for the formation of V(2)O(5) NBs is discussed and demonstrated that HNO(3) functions both as an oxidizing and as an acidification agent. V(2)O(5) NBs are single-crystals of an orthorhombic phase that have grown along the [010] direction. A bundle is made of indefinite numbers of homogeneous V(2)O(5) nanofibers where nanofibers have lengths up to several micrometres and widths ranging between 20 and 50 nm. As-prepared V(2)O(5) NBs display a high electrochemical performance in a non-aqueous electrolyte as a cathode material for lithium ion batteries. Field emission properties are also investigated which shows that a low turn-on field of similar to 1.84 V mu m(-1) is required to draw the emission current density of 10 mu Lambda cm(-2).

Graphene as a Nanocarrier for Tamoxifen Induces Apoptosis in Transformed Cancer Cell Lines of Different Origins

A cationic amphiphile, cholest-5en-3 beta-oxyethyl pyridinium bromide (PY(+)-Chol), is able to efficiently disperse exfoliated graphene (GR) in water by the physical adsorption of PY(+)-Chol on the surface of GR to form stable, dark aqueous suspensions at room temperature. The GRPY(+)-Chol suspension can then be used to solubilize Tamoxifen Citrate (TmC), a breast cancer drug, in water. The resulting TmCGRPY(+)-Chol is stable for a long time without any precipitation. Fluorescence emission and UV absorption spectra indicate the existence of noncovalent interactions between TmC, GR, and PY(+)-Chol in these suspensions. Electron microscopy shows the existence of segregated GR sheets and TmC ribbons in the composite suspensions. Atomic force microscopy indicates the presence of extended structures of GRPY(+)-Chol, which grows wider in the presence of TmC. The slow time-dependent release of TmC is noticed in a reconstituted cell culture medium, a property useful as a drug carrier. TmCGRPY(+)-Chol selectively enhanced the cell death (apoptosis) of the transformed cancer cells compared to normal cells. This potency is found to be true for a wide range of transformed cancer cells viz. HeLa, A549, ras oncogene-transformed NIH3T3, HepG2, MDA-MB231, MCF-7, and HEK293T compared to the normal cell HEK293 in vitro. Confocal microscopy confirmed the high efficiency of TmCGRPY(+)-Chol in delivering the drug to the cells, compared to the suspensions devoid of GR.

Evidence of aggregation induced emission enhancement and keto-enol-tautomerism in a gallic acid derived salicylideneaniline gel

A novel salicylideneaniline type fluorescent organogelator based on a 3,4,5-(tri-dodecyloxy)benzoyl group immobilizes aromatic solvents. The resulting gels show enhancement in emission and thermochromic/non-photochromic behaviour during sol-to-gel transition.

Study on the structure and vibrational spectra of efavirenz conformers using DFT: Comparison to experimental data

Efavirenz, (S)-6-chloro-4-(cyclopropylethynyl)-1,4-dihydro-4-(trifluoromethyl)-2H-3 ,1-benzoxazin-2-one, is an anti HIV agent belonging to the class of the non-nucleoside inhibitors of the HIV-1 virus reverse transcriptase. A systematic quantum chemical study of the possible conformations, their relative stabilities and vibrational spectra of efavirenz has been reported. Structural and spectral characteristics of efavirenz have been studied by vibrational spectroscopy and quantum chemical methods. Density functional theory (DFT) calculations for potential energy curve, optimized geometries and vibrational spectra have been carried out using 6-311++G(d,p) basis sets and B3LYP functionals. Based on these results, we have discussed the correlation between the vibrational modes and the crystalline structure of the most stable form of efavirenz. A complete analysis of the experimental infrared and Raman spectra has been reported on the basis of wavenumber of the vibrational bands and potential energy distribution. The infrared and the Raman spectra of the molecule based on OFT calculations show reasonable agreement with the experimental results. The calculated HOMO and LUMO energies shows that charge transfer occur within the molecule. (C) 2011 Elsevier B.V. All rights reserved.

Gallium and indium co-doped ZnO thin films for white light emitting diodes

Ga and In co-doped ZnO (GIZO) thin films together with ZnO, In-doped ZnO (IZO), Ga-doped ZnO (GZO), and IZO/GZO multilayer for comparison, were grown on corning glass and boron doped Si substrates by PLD. The photoluminescence spectra of GIZO showed a strong white light emission and the current-voltage characteristics showed relatively lower turn-on voltage and larger forward current. The CIE coordinates for GIZO were observed to be (0.31, 0.33) with a correlated colour temperature of 6650 K, indicating a cool white light, and establishing a possibility of white light emitting diodes. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim