arXiv subjects

Prabhakar P. Singh

Publications and source records attributed to Prabhakar P. Singh.

24 records · Page 2Linked to original sources

arXiv2002

Effects of Disorder in Mg_{1-x}Ta_{x}B_2 Alloys using Coherent-Potential Approximation

Using Korringa-Kohn-Rostoker coherent-potential approximation in the atomic-sphere approximation (KKR-ASA CPA) method for taking into account the effects of disorder, Gaspari-Gyorffy formalism for calculating the electron-phonon coupling constant $λ$, and Allen-Dynes equation for calculating T_c, we have studied the variation of T_c in Mg_{1-x}Ta_{x}B_2 alloys as a function of Ta) concentration. Our results show that the T_c decreases with the addition of Ta for upto 40 at% and remains essentially zero from 60 at% to 80 at% of Ta. We also find TaB_2 to be superconducting, albeit at a lower temperature. Our analysis shows that the variation in T_c in Mg_{1-x}Ta_{x}B_2 is mostly dictated by the changes in the B p density of states with the addition of Ta.

cond-mat.supr-con↗

arXiv2002

Theoretical Study of Magnetism and Superconductivity in 3d Transition-Metal- MgB_2 Alloys

We have studied the electronic structure of 3d transition-metal- MgB_{2} alloys, Mg_{0.97}TM_{0.03}B_{2}, (TM\equiv Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn) using KKR-CPA method in the atomic-sphere approximation. For unpolarized calculations, our results for Mg_{0.97}TM_{0.03}B_{2} alloys are similar to that of 3d impurities in other s and s-p metals. In particular, the local densities of states (DOS) associated with the 3d impurities are similar to our earlier work on 3d impurities in bulk Al. For spin-polarized calculations, we find only the alloys of V, Cr, Mn, Fe and Co with MgB_{2} to be magnetic of all the 3d elements. We also find that Cr and Mn in MgB_{2} have a relatively large local magnetic moment of 2.43 μ_{B} and 2.87 μ_{B}, respectively. We have used the unpolarized, self-consistent potentials of Mg_{0.97}TM_{0.03}B_{2} alloys, obtained within the coherent- potential approximation, to calculate the electron-phonon coupling constant λusing the Gaspari-Georffy formalism and the superconducting transition temperature T_{c} using the Allen-Dynes equation. We find that the calculated T_{c} is the lowest for Mg_{0.97}Cr_{0.03}B_{2} and the highest for Mg_{0.97}Zn_{0.03}B_{2}, in qualitative agreement with experiment. The calculated trend in variation of T_{c} from Mn to Zn is also similar to the available experimental data. Our analysis of the variation in T_{c}, in terms of the DOS and the spectral function along Γto A direction, shows the variation to be an interplay between the total DOS at the Fermi energy and the creation/removal of states along Γto A direction [P. P. Singh, cond-mat/0201093].

cond-mat.supr-con↗

arXiv2002

Hole-doped, High-Temperature Superconductors Li_{x}BC, Na_{x}BC and C_{x} : A Coherent-Potential-Based Prediction

Using density-functional-based methods, we show that the hole-doped Li_{x}BC and Na_{x}BC in P6_{3}/mmc crystal structure and C_{x} in graphite structure are capable of showing superconductivity, possibly with a T_{c} much higher than that of MgB_{2}. We use full-potential methods to obtain the optimized lattice constants a and c, coherent-potential approximation to describe the effects of disorder, Gaspari-Gyorffy formalism to obtain the electron-phonon coupling constant $λ$, and Allen-Dynes equation to calculate T_{c} as a function of hole concentration in these alloys.

cond-mat.supr-con↗

arXiv2002

Anisotropic, Fermi-Surface-Induced Variation in T_c in MgB_2 Alloys

Using coherent-potential to describe disorder, Gaspari-Gyorffy approach to evaluate electron-phonon coupling, and Allen-Dynes equation to calculate T_c, we show that in Mg_{1-x}M_{x}B_{2} (M = Al,Li or Zn) alloys (i) the way T_c changes depends on the location of the added/modified k-resolved states on the Fermi surface and (ii) the variation of T_c as a function of concentration is dictated by the B p DOS. In addition, using full-potential calculations for MgMB_4, we show that (i) at x=0.5 a superstructure can form in Mg_{1-x}Al_{x}B_2 but not in Mg_{1-x}Li_{x}B_2 or Mg_{1-x}Zn_{x}B_2, and (ii) B layer shifts towards the impurity layer, more for Al than for Li or Zn.

cond-mat.supr-con↗

arXiv2001

Superconductivity in MgB2 and TaB2: A Full-Potential Electronic Structure Comparison

We present a comparison of electronic structure of MgB2 and TaB2, the two new superconductors, as well as VB2, calculated using full-potential, density- functional-based methods in P6/mmm crystal structure. Our results, described in terms of (i) density of states (DOS), (ii) band-structure, and (iii) the DOS and the electronic charge density in a small energy window around the Fermi energy EF. In particular, the charge density around EF in MgB2 and TaB2 show striking similarity as far as the B plane is concerned. A comparison of their band-structures, coupled with l-character analysis, indicates that TaB2 has substantially more p-character than VB2 along A-L and H-A directions near EF.

cond-mat.supr-con↗

arXiv2001

Role of Boron p-Electrons and Holes in Superconducting MgB2, and other Diborides: A Fully-Relaxed, Full-Potential Electronic Structure Study

We present the results of fully-relaxed, full-potential electronic structure calculations for the new superconductor MgB2, and BeB2, NaB2, and AlB2, using density-functional-based methods. Our results described in terms of (i) density of states (DOS), (ii) band-structure, and (iii) the DOS and the charge density around the Fermi energy EF, clearly show the importance of B p-band for superconductivity. In particular, we show that around EF, the charge density in MgB2, BeB2 and NaB2 is planar and is associated with the B plane. For BeB2 and NaB2, our results indicate qualitative similarities but significant quantitative differences in their electronic structure due to different lattice constants a and c.

cond-mat.supr-con↗