Tashi Nautiyal

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.

Effect of spin-orbit coupling on magnetic and orbital order in MgV2O4.

Recent measurements on MgV(2)O(4) single crystals have reignited the debate on the role of spin-orbit (SO) coupling in dictating the orbital order in vanadium spinel systems. Density functional theory calculations were performed using the full-potential linearized augmented-plane-wave method within the local spin density approximation (LSDA), Coulomb correlated LSDA (i.e. LSDA + U), and with SO interaction (LSDA + U + SO) to study the magnetic and orbital ordering in the low temperature phase of MgV(2)O(4). It is observed that, in the experimental antiferromagnetic phase, the spin-orbit coupling affects the orbital order differently in alternate V-atom chains along the c-axis. This observation is consistent with the experimental prediction that the effect of spin-orbit coupling is intermediate between those in the cases of ZnV(2)O(4) and MnV(2)O(4).

The nature of itineracy in CoV2O4: a first-principles study

Inspired by recent experiments, we have theoretically explored the nature of itineracy in CoV2O4 under pressure and investigated, using first-principles density functional theory calculations, whether it has any magnetic and orbital ordering. Our calculations indicate that there could be two possible routes for obtaining the experimentally observed pressure induced metallicity in this system. One is via the spin–orbit interaction coupled with Coulomb correlation, which can take the system from a semiconducting state at ambient pressure to a metallic state under high pressure. The other mechanism, as indicated by our GGA + U calculations, is based on the presence of two kinds of electrons in the system: localized and itinerant. An effective Falicov–Kimball model could then possibly explain the observed insulator to metal transition. Comparison of the two scenarios with existing experimental observations leads us to believe that the second scenario offers a better explanation for the mechanism of the insulator to metal transition in CoV2O4 under pressure.

Electronic and optical properties of free-standing and supported vanadium nanowires

We have investigated theoretically the electronic and optical properties of free-standing and substrate-supported ultrathin nanowires (NWs) of the transition metal vanadium. Ground state of the structures studied, except free-standing zigzag geometry, is found to be magnetic in nature. We show that for some structures, study of the antiferromagnetic state necessitates considering various possible configurations. All the structures, except dimerized, show metallic behavior. Structure with helical geometry possesses decent value of magnetic moment and is exceptionally stable as well as most stiff of all the structures studied. The plasma frequency and dielectric function nicely exhibit the anisotropy due to one-dimensional nature of the nanowires. The latter is structure-dependent and markedly different from that of bulk. More realistic case of linear chains supported on a substrate shows fair impact of the substrate in comparison with free-standing case. There is substantial charge redistribution on relaxi...

Basic nanosystems of early 4d and 5d transition metals: Electronic properties and the effect of spin-orbit interaction

There are various possibilities for the structure as well as for the growth of nanosystems, particularly of nanowires. The ultimate one-dimensional material—linear chains—are difficult to exploit for applications due to their transient nature. Nonetheless these are a good prototype for studying one-dimensional materials and project the kind of behavior one may expect from ultrathin nanowires. Likewise monolayers are the ultimate two-dimensional materials and their study is helpful in understanding the behavior of two-dimensional materials. We present a theoretical study on basic nanosystems—linear chains and monolayers—of the 4d (Y, Zr, Nb, Mo, and Tc) and 5d (Hf, Ta, W, and Re) transition metals of groups 3–7 by means of an all-electron density functional approach. We have explored all kinds of magnetic configurations: nonmagnetic, ferromagnetic, and antiferromagnetic, by (i) inclusion and (ii) omission of spin-orbit interaction. We find that though this interaction has a marginal effect on nanosystems o...

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.

An anomalous interlayer exciton in MoS2

The few layer transition metal dichalcogenides are two dimensional materials that have an intrinsic gap of the order of ≈2 eV. The reduced screening in two dimensions implies a rich excitonic physics and, as a consequence, many potential applications in the field of opto-electronics. Here we report that a layer perpendicular electric field, by which the gap size in these materials can be efficiently controlled, generates an anomalous inter-layer exciton whose binding energy is independent of the gap size. We show this originates from the rich gap control and screening physics of TMDCs in a bilayer geometry: gating the bilayer acts on one hand to increase intra-layer screening by reducing the gap and, on the other hand, to decrease the inter-layer screening by field induced charge depletion. This constancy of binding energy is both a striking exception to the universal reduction in binding energy with gap size that all materials are believed to follow, as well as evidence of a degree of control over inter-layer excitons not found in their well studied intra-layer counterparts.

Theoretical studies on electronic and magnetic properties of ultrathin Mo nanowires

We present a detailed theoretical study on electronic and magnetic properties of Mo nanowires with different structures. The ultrathin nanowires of this 4d transition metal show a unique behavior for the stability. We notice that zigzag structure is stable at the lower values of nearest neighbor distance. On slightly stretching the nanowire, the ladder structure is preferred while the dimerized structure, with the highest value of cohesive energy, is the most stable structure at larger nearest neighbor distances. This work suggests that magnetic ordering of Mo nanowires can be tuned with structure. The linear and ladder structures of Mo nanowires show antiferromagnetic ordering. Equilateral zigzag structure prefers a nonmagnetic state whereas the planar zigzag structure is ferromagnetic. The dimerized structure stands out showing degenerate nonmagnetic and ferromagnetic states. The highest value of magnetic moment (∼1.16 μB/atom) is predicted for linear chains. Relative break force values suggest that the...

Time dependent DFT study of structural and optical properties of bulk CuCl

We have investigated the structural and optical properties of bulk CuCl which is a direct band-gap semiconductor with zinc blende structure, employing the time dependent density functional theory using the boot strap approximation for the exchange-correlation kernel. We have also calculated the optical properties under Random Phase Approximation. Our results show a strong signature of excitons in bulk CuCl.