Nirpendra Singh

Electronic structure and optical properties of rare earth sesquioxides (R2O3, R=La, Pr, and Nd)

The electronic structure and optical properties of the rare earth sesquioxides (R2O3, R=La, Pr, and Nd) have been studied using the full potential linearized augmented plane wave method within local spin density approximation (LSDA) and Coulomb corrected local density approximation (LSDA+U) as implemented in the WIEN2K code. Our calculations show that the LSDA+U results give a better representation of the band structure compared to LSDA. Our calculated reflectivity and optical conductivity spectra are compared with the experimental data. The peaks in the optical conductivity can be explained as arising out of the transitions from R-p and O-p to R-d states. We have also calculated the dielectric function and the coefficient of refraction for these rare earth sesquioxides. We do not find any significant differences in the optical properties calculated using LSDA and LSDA+U.

Electronic structure and optical properties of rare earth hexaborides RB6 (R = La, Ce, Pr, Nd, Sm, Eu, Gd)

The optical and electronic properties of the rare earth hexaborides RB6 (R = La, Ce, Pr, Nd, Sm, Eu, Gd) are studied using the full potential linearized augmented plane wave method. To account better for the on-site f-electron correlation, we adopted the Coulomb corrected local spin density approximation (LSDA+U) to the exchange correlation functional in the calculations. Our electronic structure calculation shows the overlapping of R 5d states and B 2p states at the X symmetry point. The magnetic moment of the ferromagnetic rare earth hexaborides increases with increasing 4f occupation. The calculated reflectivity and optical conductivity spectra are in agreement with the experimental data, although the structures in the calculated optical spectra are sharper.

S-nitrosylation of UCHL1 induces its structural instability and promotes α-synuclein aggregation

Ubiquitin C-terminal Hydrolase-1 (UCHL1) is a deubiquitinating enzyme, which plays a key role in Parkinson’s disease (PD). It is one of the most important proteins, which constitute Lewy body in PD patient. However, how this well folded highly soluble protein presents in this proteinaceous aggregate is still unclear. We report here that UCHL1 undergoes S-nitrosylation in vitro and rotenone induced PD mouse model. The preferential nitrosylation in the Cys 90, Cys 152 and Cys 220 has been observed which alters the catalytic activity and structural stability. We show here that nitrosylation induces structural instability and produces amorphous aggregate, which provides a nucleation to the native α-synuclein for faster aggregation. Our findings provide a new link between UCHL1-nitrosylation and PD pathology.

Thermoelectric performance of electron and hole doped PtSb2

We investigate the thermoelectric properties of electron and hole doped PtSb2. Our results show that for doping of 0.04 holes per unit cell (1.5×1020u2009cm−3) PtSb2 shows a high Seebeck coefficient at room temperature, which can also be achieved at other temperatures by controlling the carrier concentration (both electron and hole). The electrical conductivity becomes temperature independent when the doping exceeds some 0.2 electrons/holes per unit cell. The figure of merit at 800 K in electron and hole doped PtSb2 is comparatively low at 0.13 and 0.21, respectively, but may increase significantly with As alloying due to the likely opening of a band gap and reduction of the lattice thermal conductivity.

ZnIr2O4: An efficient photocatalyst with Rashba splitting

Semiconductor-based photocatalysts nowadays are of central interest for the splitting of water into hydrogen and oxygen. However, the efficiency of the known materials is small for direct utilization of the solar energy. Using first-principles calculations, we show that ZnIr2O4 can overcome this shortage. Modified Becke-Johnson calculations give an indirect band of 2.25 eV, which can be reduced to the visible energy range by S doping. For 25% S doping we find a direct band gap of 1.25 eV and a Rashba spin splitting of 220 meV ?. The valence band edge potential is 2.89 V against the standard hydrogen electrode, which is sufficient for photocatalytic water oxidation and pollutant degradation. The optical absorption of S-doped ZnIr2O4 is strongly enhanced, making the material an efficient photocatalyst for visible light.

Ultrafast demagnetization in bulk versus thin films: an ab initio study

We report ab initio simulations of the quantum dynamics of electronic charge and spins when subjected to intense laser pulses. By performing these purely electron-dynamics calculations for a thin film and for the bulk of Ni, we conclude that formation of surfaces has a dramatic influence of amplifying the laser induced demagnetization. The reason for this amplification is enhanced spin-currents on the surface of the thin films. We show that the underlying physics of demagnetization for bulk is dominated by spin-flips induced by spin-orbit coupling. In the case of thin films, the dominant cause of demagnetization is a combination of the flow of spin-currents and spin-flips. Furthermore, a comparison of our results with experimental data shows that belowu2009u2009∼120 fs processes of demagnetization are entirely dominated by purely electronic processes followed by which dissipative effects like the Elliott-Yafet mechanism start to contribute significantly.

Comparative study of optical and magneto-optical properties of GdFe2 and GdCo2

We report calculations of the optical and magneto-optical properties of GdFe(2) and GdCo(2) using the full-potential linear-augmented-plane-wave method. Calculations with the Coulomb corrected local spin density approximation (LSDA+U) give a better representation of the band structure, density of states and magnetic moments compared to LSDA alone. However, both LSDA and LSDA+U approximations give fairly good agreement with experiment for the diagonal optical conductivity. The calculated results for GdCo(2) are in better agreement with the oxide corrected data. Our results suggest that the data for GdFe(2) are most likely influenced by surface oxidation, due to the high reactivity of these compounds. For the much smaller off-diagonal components and Kerr rotation, LSDA results are better than the LSDA+U results, particularly in the energy range 0-3xa0eV. We show that the unphysical negative ellipticity values are taken care of by the use of stronger relaxation, which also improves the qualitative agreement with experimental data. Overall we have obtained a fair agreement with the experimental data for both optical and magneto-optical properties. We feel that measurements over a larger energy range are required for facilitating an exhaustive and decisive comparison and also to strengthen the bond between theory and experiments.

Optical and magneto-optical properties of gadolinium

We report calculations of the optical and magneto-optical properties of elemental rare earth gadolinium using the full potential linear augmented plane wave method and including spin-orbit coupling. Calculations are performed within the local spin density approximation (LSDA) and Coulomb corrected local spin density approximation (LSDA+U). While LSDA+U gives better agreement for density of states and magnetic moments, a comparison with the experimental data shows that both LSDA and LSDA+U are at par in representing the diagonal components of the optical conductivity. For the much smaller off-diagonal components, interestingly, the LSDA results have an edge over the LSDA+U results.

Superior thermoelectric response in the 3R phases of hydrated NaxRhO2

Density functional theory is used to investigate the thermoelectric properties of the 3R phases of NaxRhO2 for different Na vacancy configurations and concentrations. As compared to the analogous 2H phases, the modified stacking of the atomic layers in the 3R phases reduces the interlayer coupling. As a consequence, the 3R phases are found to be superior in the technologically relevant temperature range. The Rh orbitals still govern the valence band maxima and therefore determine the transport properties. A high figure of merit of 0.35 is achieved in hydrated Na0.83RhO2 at 580u2005K by water intercalation, which is 34% higher than in the non-hydrated phase.

The preprotein translocase YidC controls respiratory metabolism in Mycobacterium tuberculosis

The YidC–Oxa1–Alb3 preprotein translocases play a vital role in membrane insertion of proteins in eukaryotes and bacteria. In a recent study we observed that Rv3921c, which encodes putative YidC translocase in Mycobacterium tuberculosis (Mtb), is essential for in vitro growth of bacteria. However, the exact function of this particular protein remains to identify in mycobacterial pathogens. By performing a systematic study here we show that YidC of Mtb is an envelope protein, which is required for production of ATP and maintenance of cellular redox balance. Drastic effects of depletion of Rv3921c on the expression of hypoxic genes, ATP synthases, and many proteins of central metabolic and respiratory pathways shed a significant light on the function of YidC towards controlling respiratory metabolism in Mtb. Association of YidC with proteins such as succinate dehydrogenases and ubiquinol-cytochrome C reductase further confirms its role in respiration. Finally we demonstrate that YidC is required for the intracellular survival of Mtb in human macrophages.