Ramesh Sharma

Electron-phonon coupling in superconducting β-PdBi2

We have studied the electronic, transport and vibrational properties of low temperature superconductor β-PdBi2. The band manifold clearly demonstrates the 2D-layered structure with multiple gaps. The intersection of bands at EF in the Γ-P, Γ-N directions gives rise to complicated Fermi surface topology, which contains quite complicated multiple connected sheets, as well as hole and electron-like pockets. From the low temperature specific heat, we have estimated the electron-phonon coupling constant λel-ph which has a very high value of 3.66. The vibrational properties clearly illustrates that the strong coupling makes the lattice unstable. The calculated properties confirm that β-PdBi2 is an intermediate coupling superconductor.

Ferromagnetism in Fe-doped transition metal nitrides

Early transition metal mononitrides ScN and YN are refractory compounds with high hardness and melting points as well semiconducting properties. The presence of nitrogen vacancies in ScN/YN introduces asymmetric peaks in the density of states close to Fermi level, the same effects can be achieved by doping by Mn or Fe-atoms. Due to the substitution of TM atoms at Sc/Y sites, it was found that the p–d hybridization induces small magnetic moments at both Sc/Y and N sites giving rise to magnetic semiconductors (MS). From the calculated temperature dependent transport properties, the power factor and ZT is found to be lowered for doped ScN whereas it increases for doped YN. It is proposed that these materials have promising applications as spintronics and thermoelectric materials.Early transition metal mononitrides ScN and YN are refractory compounds with high hardness and melting points as well semiconducting properties. The presence of nitrogen vacancies in ScN/YN introduces asymmetric peaks in the density of states close to Fermi level, the same effects can be achieved by doping by Mn or Fe-atoms. Due to the substitution of TM atoms at Sc/Y sites, it was found that the p–d hybridization induces small magnetic moments at both Sc/Y and N sites giving rise to magnetic semiconductors (MS). From the calculated temperature dependent transport properties, the power factor and ZT is found to be lowered for doped ScN whereas it increases for doped YN. It is proposed that these materials have promising applications as spintronics and thermoelectric materials.

Dynamical and electronic properties of rare-earth aluminides

Rare-earth dialuminides belong to a large family of compounds that stabilize in cubic MgCu2 structure. A large number of these compounds are superconducting, amongst these YAl2, LaAl2 and LuAl2 have been chosen as reference materials for studying 4f-electron systems. In order to understand the role of the RE atoms, we have applied the FPLAPW and PAW methods within the density functional theory (DFT). Our results show that the contribution of RE atoms is dominant in both electronic structure and phonon dispersion. The anomalous behavior of superconducting LaAl2 is well explained from an analysis of the electron localization function (ELF), Bader charge analysis, density of electronic states as well as the dynamical phonon vibrational modes. The interaction of phonon modes contributed by low frequency vibrations of La atoms with the high density La 5d-states at EF in LaAl2 lead to strong electron-phonon coupling.

Thermoelectric properties of defect chalcopyrites

The electronic structure and transport properties of the XIn2Te4 (X=Zn, Cd) compounds using the full potential linearized augmented plane-wave (FP-LAPW) method and the semi-classical Boltzmann theory are presented in this paper. The ternary chalcogenides ZnIn2Te4 and CdIn2Te4 are direct band gap semiconductors with a band gap of 1.08 and 1.03 eV respectively. Although the optical properties are very similar, the transport properties of these chalcopyrites differ. From the calculated Hall coefficient, it is found that the tellurides are p-type materials with electron concentration of 8.09×1019 and 4.80×1019 cm-3 respectively. The figure of merit shows variation with temperature in CdIn2Te4, whereas it is nearly constant over the entire temperature range (10-800 K) in ZnIn2Te4.The electronic structure and transport properties of the XIn2Te4 (X=Zn, Cd) compounds using the full potential linearized augmented plane-wave (FP-LAPW) method and the semi-classical Boltzmann theory are presented in this paper. The ternary chalcogenides ZnIn2Te4 and CdIn2Te4 are direct band gap semiconductors with a band gap of 1.08 and 1.03 eV respectively. Although the optical properties are very similar, the transport properties of these chalcopyrites differ. From the calculated Hall coefficient, it is found that the tellurides are p-type materials with electron concentration of 8.09×1019 and 4.80×1019 cm-3 respectively. The figure of merit shows variation with temperature in CdIn2Te4, whereas it is nearly constant over the entire temperature range (10-800 K) in ZnIn2Te4.

Photoelectric properties of defect chalcogenide HgGa2X4 (x=S, Se, Te)

We present results of ab initio study of ordered vacancy compounds of mercury. The electronic structure, charge density, optical and transport properties of the semiconductor family HgGa2X4 (X=S, Se, Te) are calculated using the full potential linearized augmented plane wave method which is based on the density functional theory. A direct bandgap is observed in these compounds, which reduces in the order S>Se>Te. From the density of states it is observed that there is strong hybridization of Hg-d, Ga-d and X-p states. The optical properties show a red shift with increasing size and atomic no. of the chalcogenide atoms. We have also reported the transport properties of mercury thiogallates for the first time. The selenide compound exhibits n-type nature whereas HgGa2S4 and HgGa2Te4 show p-type behavior. The power factor and ZT for the HGS increases at low temperatures, the figure of merit is highest for HgGa2Se4 (1.17) at 19 K.

Thermodynamical study of boron doped CeX3 (X=Pd, Rh)

The structural, electronic, thermal, and optical properties of cubic non magnetic CeX3(X=Pd, Rh) compounds which crystallize in the Au3Cu structure have been studied using the projected augmented wave (PAW) method within the density functional theory (DFT) with generalized gradient approximation (GGA) for exchange correlation potential. In this paper we have calculated the band structure which are interpreted using the density of states. The optical properties such as extinction coefficients clearly illustrate the changes in CeX3 due to intercalation of boron. Lattice instability is observed in CePd3B from the calculated dynamical properties.

Thermodynamical and thermoelectric properties of boron doped YPd3 and YRh3

The structural, electronic, thermal, and optical properties of borides of cubic non-magnetic YX3 (X=Rh, Pd) compounds and their borides which crystallize in the AuCu3 structure have been studied using the density functional theory (DFT). The flat bands in the vicinity of EF which are associated with superconductivity appear in YPd3 and YRh3 band structures. However, the B s-states enhance the flat band only in YRh3B. The optical properties clearly show that boron insertion modifies the absorption and transmittance. The YX3 alloys and their borides exhibit valuable changes in the thermopower and ZT. It is observed that the properties of the Y-X intermetallics change significantly for the Y-Rh and Y-Pd alloys and the presence of single boron atom modifies the properties to a great extent.

Magnetism in dilute iron doped YN semiconductors

The full potential linearized augmented plane-wave (FP-LAPW) scheme of computation is used to explore the electronic and magnetic properties of Fe doped into YN. Band structure calculations show that YN is a semicon ductor with a narrow indirect band gap of 0.08 eV along Г-X direction. Optical properties such as reflectivity, absorption coefficient are reported and are discussed on the basis of corresponding electronic structure. Spin polarized results indicate that the ground state of Y1-xFexN (x=0.06, 0.12, 0.25) is ferromagnetic with a high moment on Fe-atom and zero moment on Y and N atoms, except in the case of 25 % doping. A discussion of the transport properties of YN and Y1-xFexN is given in order to get insights of the Fe substitution effects.