Dinesh C. Gupta

Structural, elastic and thermo-electronic properties of paramagnetic perovskite PbTaO3

Self-consistent ab initio calculations with highly precise spin-polarised, density functional theory (DFT) have been performed for the first time, to study the structural stability, mechanical and magneto-electronic properties of cubic perovskite PbTaO3. The DFT as well as the analytically calculated values of tolerance factor, in addition to stable-phase optimization, mechanical and elastic properties show stability of the present material in the cubic phase with a reasonably stiff nature and ductile properties. The symmetric spin-polarised band structure of both the spin (up and down) channels reveals zero spin polarisation at the Fermi level. Moreover, the insignificant total and individual spin magnetic moments of adjacent atoms and magnetic susceptibility calculations via the post-DFT treatment predict the paramagnetic nature of the material. Based on results of the present study, the paramagnetic metal PbTaO3 material is considered a promising candidate in designing new electrode materials.

Robust thermoelectric performance and high spin polarisation in CoMnTiAl and FeMnTiAl compounds

New quaternary Heusler materials, CoMnTiAl and FeMnTiAl, have been investigated. These alloys are found to be stable in a ferromagnetic phase in Y1 type structure; the stability is also confirmed by various conditions of elastic constants. Both alloys show half-metallic (HM) ferromagnetic character with integer magnetic moments and they present a metallic nature. The values of the minority band gap in CoMnTiAl and FeMnTiAl are found to be 0.50 eV and 0.25 eV respectively. These alloys also possess excellent thermoelectric behaviour with Seebeck coefficients for CoMnTiAl (FeMnTiAl) as 31 μV K−1 (20 μV K−1) at room temperature. Here we report that CoMnTiAl achieves a figure of merit of 0.62 in a wide temperature range while FeMnTiAl achieves 0.77. Thus these new quaternary Heusler alloys can prove to be ultimate candidates for spin valves and for the best thermoelectric applicability over an ample temperature range.

Investigation of the transport, structural and mechanical properties of half-metallic REMnO3 (RE = Ce and Pr) ferromagnets

Systematic investigation of the ground state structure, which includes elastic and transport properties, of perovskite oxides REMnO3 (RE = Ce and Pr) has been carried out by first principles calculations. We present the analytical as well as DFT calculated equilibrium lattice constants which show good agreement with experimental data. Three independent elastic constants are emphasised to yield the corresponding mechanical properties, including the elastic moduli (B, G and Y), Poissons ratio (ν), anisotropy factor (A) and Pugh ratio B/G, for these compounds. These calculations predict the brittle PrMnO3 as a less hard material than the ductile CeMnO3 oxide. Post DFT treatment involving Boltzmanns theory is conveniently employed to investigate the thermoelectric properties of these compounds. The analysis of the thermal transport properties specifies the dimensionless figure of merit of 0.24 and 0.19 at room temperature for PrMnO3 and CeMnO3, respectively. Their half-metallic nature with efficient thermoelectric parameters, including electrical conductivity, Seebeck coefficient and thermal conductivity, suggest the likelihood of these materials to have a potential application in the design of shape memory devices and imminent thermoelectric materials.

Pressure induced magnetic, electronic and mechanical properties of SmX (X = Se, Te)

The magnetic, structural, elastic and electronic properties of Sm-chalcogenides in the stable [Formula: see text] and high pressure [Formula: see text] phase have been analyzed using an ab initio pseudo-potential method with a spin-polarized GGA based on exchange-correlation energy optimization, as implemented in SIESTA code. The magnetic phase stability has been determined from the total energy calculations in non-magnetic and magnetic phases, which clearly indicate that at ambient and high pressures, these compounds are ferromagnetically stable. Also, the Sm ion is described in both five and six localized f electrons. Under compression the Sm chalcogenides undergo a first-order transformation from Sm(2+) to a stable valence state (Sm(3+)) with delocalization of the 4f electrons into the 5d states of Sm followed by a structural transition from the B1 to the B2 phase. The structural properties namely, equilibrium lattice constant, bulk modulus, its pressure derivative, transition pressure and volume collapse agree well with the experimental results. We have also computed the electronic structure at different volumes.

Effect of covalency, zero-point energy and charge transfer on the phase-transition, elastic and thermophysical properties of Ca-chalcogenides under compression

The pressure induced phase-transition, elastic and thermophysical properties of Ca-chalcogenides have been investigated by means of many body potential. The modified charge transfer potential consists of long-range Coulomb and charge-transfer interactions modified by covalency and short-range overlap repulsion extended up to second neighbours and zero-point energy effects. Another charge-transfer model excludes covalency and zero-point energy effects. These chalcogenides undergo first-order phase-transition at P T = 39.23, 36.30 and 31.20 GPa and their equation of state show volume collapse of 10.12, 7.61 and 4.55% for CaS, CaSe and CaTe, respectively, which are in good agreement with the experiments. The elastic and thermophysical properties of these compounds have also been computed at normal and high pressures. Both the models are capable of explaining the Cauchy-discrepancy (C12 ≠ C44), elastic, phase-transition and thermophysical properties successfully.

Pressure-induced phase transitions and electronic structure of GaAs

The structural and electronic properties of GaAs under high pressure have been investigated using an ab initio pseudo-potential approach within the framework of density functional theory. We use the local density approximation based on exchange–correlation energy optimization for calculating the total energy. The phase transition in bulk GaAs is investigated. Results of the first-principles calculation of the structural phase transition and the electronic properties of GaAs in the three different crystallographic structures are reported. GaAs is found to undergo structural transition from B3 to B1 phase, demonstrating a reasonably good agreement with experimental data. The equation of state for these transformations also shows good agreement with experimental results. The calculated value of volume collapse is close to the observed data. The energy of the B2 phase is found to be slightly higher than that of the B1 phase. A possible mechanism for the transition characterized by the space group is discussed. The calculated values show that there is a relatively small value of enthalpy for the transition as compared to the transition, which provides the transition behaviour of GaAs from the kinetic viewpoint. The electronic structures have also been computed at different volumes.

A Correlative Study of Geomagnetic Storms Associated with Solar Wind and IMF Features During Solar Cycle 23

A geomagnetic storm is a global disturbance in Earths magnetic field usually occurred due to abnormal conditions in the interplanetary magnetic field (IMF) and solar wind plasma emissions caused by various solar phenomenon. Furthermore the magnitude of these geomagnetic effects largely depend upon the configuration and strength of potentially geo-effective solar/interplanetary features. In the present study the identification of 220 geomagnetic storms associated with disturbance storm time (Dst) decrease of more than -50 nT to -300 nT, have been made, which are observed during 1996-2007, the time period spanning over solar cycle 23. The study is made statistically between the Dst strength (used as an indicator of the geomagnetic activity) and the peak value obtained by solar wind plasma parameters and IMF B as well as its components. We have used the hourly values of Dst index and the wind measurements taken by various satellites. Our results inferred that yearly occurrences of geomagnetic storms are strongly correlated with 11-year sunspot cycle. We observed that IMF B is highly geo-effective during the main phase of magnetic storms, while it more significant at the time of storm peak, which is further contributed by southward component of IMF Bz, substantiating earlier findings. The correlation between Dst and wind velocity is higher, as compared with IMF Bz and ion density. It has been verified that geomagnetic storm intensity is correlated well with the total magnetic field strength of IMF better than with its southward component.

Effect of high pressure on polymorphic phase transition and electronic structure of XAs (X=Al, Ga, In)

The structural and electronic properties of XAs (X = Al, Ga, In) under pressure have been investigated using ab-initio pseudo-potential approach within local density approximation in B3→B1→B2 phases. The values of phase transition pressures show reasonably good agreement with the experimental data and better than others. The B1→B2 phase transition in InAs is not seen. The volume collapse computed from equation of state (EOS) is found to be in good agreement with the experimental values. Under ambient conditions, the energy of B3 phase is lowest as compared to other phases, while at high pressures beyond B1→B2 phase transition, the energy of B2 phase is found to be lower than that of B1 phase showing correct stability of the phases. There is relatively smaller enthalpy associated with B3→B1 transition as compared to B3→B2 transition. The electronic structures have also been computed at different pressures. We have also reported the effect of pressure on energy gap and valence band width.

Magnetic, Electronic, and Mechanical Properties of Strongly Correlated Samarium Mono-chalcogenides under High Pressure

The magnetic, structural, electronic and optical properties of Sm-chalcogenides in the stable Fm3m and high-pressure Pm3m phase have been analyzed by means of ab-initio pseudo-potential method within the framework of density functional theory. The spin polarized GGA based on exchange-correlation energy optimization has been used for calculating the total energy as implemented in SIESTA code. The magnetic phase stability was determined from the total energy calculations for both the non-magnetic and magnetic phases, which clearly indicate that at ambient and high pressures, these compounds are ferromagnetically stable. Also, the Sm ion is considered with five and six localized f electrons. Under compression the Sm chalcogenides undergo first-order transition from Sm 2+ to mixed valent Sm 3+ due to the delocalization of 4f electrons into 5d states of Sm followed by a structural transition from B1 to B2 phase. The structural properties viz., equilibrium lattice constant, bulk modulus, its pressure derivative, transition pressure and volume collapse agree well with the experimental results. Electronic structures at different volumes have also computed.

Structural properties of silver iodide and copper iodide

The structural changes within the Silver iodide (AgI) and Copper iodide (CuI) induced by pressure have been investigated using an effective interaction potential. CuI and AgI in their parent zinc blende (ZnS) to rock salt (NaCl) through an intermediate structure have been reported. The calculated values for the phase transition pressures and associate volume collapses are generally in good agreement with measured data.