arXiv subjects
Publications and source records attributed to Prabhakar P. Singh.
We report here our study on SiC doped with transition metals using first principle density functional theory calculations. We have considered cubic SiC with 3d transition metals as substitutional impurities for Si and C site separately. Cubic SiC doped with Cr, Mn, show ferromagnetism whereas with Sc, Ti, V and Co show site dependency of magnetic properties. Rests of the impurities are found to be non-magnetic.
In this Comment, using Korringa-Kohn-Rostoker coherent-potential approximation method in the atomic-sphere approximation (KKR-ASA CPA) to describe the effects of disorder due to excess Fe in FeTe alloys, we show that (i) the rigid-band approximation is inadequate to describe the effects of disorder and its application leads to an incorrect description of the underlying physics, (ii) the rigid-band energy shift of ~0.76 eV for going from FeTe to Fe_{1.068}Te (or Fe_{1.063}Te), as obtained by Han and Savrasov (Phys. Rev. Lett. 103, 067001(2009)), is inconsistent with our FP-LMTO as well as the KKR-ASA results, and thus the FS of Fe_{1.063}Te shown in Fig. 3(b) of their paper is not correct.
To understand the pressure-induced changes in the electronic structure and the electron-phonon interaction in yttrium, we have studied hexagonal close-packed (hcp) yttrium, stable at ambient pressure and double hexagonal close-packed (dhcp) yttrium, stable up to around 44 GPa, using density-functional-based methods. Our results show that as one goes from hcp yttrium to dhcp yttrium, there is (i) a substantial charge-transfer from s->d with extensive modifications of the d-band and a sizable reduction in the density of states at the Fermi energy, (ii) a substantial stiffening of phonon modes with the electron-phonon coupling covering the entire frequency range, (iii) an increase in the electron-phonon coupling constant λfrom 0.55 to 1.24, leading to a change in the superconducting transition temperature T_c from 0.3 K to 15.3 K for μ*=0.2.
Using ab initio methods, we have studied the electron-phonon interaction in compressed, body-centered cubic (bcc) yttrium, which is predicted to be stable at 280 GPa [Melsen et al, Phys. Rev. B 48, 15574 (1993)]. We find that compressed, bcc yttrium has a large electron-phonon coupling with lambda=1.8, leading to a superconducting transition temperature T_c ~ 30 K or above. Our results indicate that the large electron-phonon coupling is due to the lattice hardening.
We study the effects of pressure on the electron-phonon interaction in MgB_2 using density-functional-based methods. Our results show that the superconductivity in MgB_2 vanishes by 100 GPa, and then reappears at higher pressures. In particular, we find a superconducting transition temperature T_c \~ 2 K for mu*=0.1 at a pressure of 137 GPa.
Based on first principles calculations, we show that $Mn$ impurities are magnetic in $MgB_{2}$ due to exchange-splitting of $d_{3z^2-1}$ band and they substantially modify $B$ $p_σ$ and $p_π$ bands through hybridization. Thus, $Mn$ impurities could act as strong magnetic scattering centers leading to pair-breaking effects in $MgB_{2}$. In contrast, we find $Fe$ impurities in $MgB_{2}$ to be nearly non-magnetic.
First-principles, density-functional-based electronic structure calculations are employed to study the changes in the electronic properties of ZnC_{y}Ni_{3} and MgC_{y}Ni_{3} using the Korringa-Kohn-Rostoker coherent-potential approximation method in the atomic sphere approximation (KKR-ASA CPA). As a function of decreasing C at%, we find a steady decrease in the lattice constant and bulk modulus in either alloys. However, the pressure derivative of the bulk modulus displays an opposite trend. Following the Debye model, which relates the pressure derivative of the bulk modulus with the average phonon frequency of the crystal, it can thus be argued that ZnCNi_{3} and its disordered alloys posses a different phonon spectra in comparison to its MgCNi_{3} counterparts. This is further justified by the marked similarity we find in the electronic structure properties such as the variation in the density of states and the Hopfield parameters calculated for these alloys. The effects on the equation of state parameters and the density of states at the Fermi energy, for partial replacement of Mg by Zn are also discussed.
Using density-functional-based method, we study the k-resolved sigma- and pi-band holes in Mg_{1-x}Al_{x}B_{2} and Mg(B_{1-y}C_{y})_{2} alloys. We find that the calculated profiles of the loss of sigma- and pi-band holes in these two systems as a function of impurity concentration are in qualitative agreement with experiments, as expected. We also describe its implications vis-a-vis superconductivity in Mg_{1-x}Al_{x}B_{2} and Mg(B_{1-y}C_{y})_{2}.
First-principles, density-functional based electronic structure calculations are carried out for MgC(Ni_{1-x}Co_{x})_{3} alloys over the concentration range 0\leq x\leq1, using Korringa-Kohn-Rostoker coherent-potential approximation (KKR CPA) method in the atomic sphere approximation (ASA). The self-consistent calculations are used to study the changes as a function of x in the equation of state parameters, total and partial densities of states, magnetic moment and the on-site exchange interaction parameter. To study the magnetic properties as well as its volume dependence, fixed-spin moment calculations in conjunction with the phenomenological Landau theory are employed. The salient features that emerge from these calculations are (i) a concentration independent variation in the lattice parameter and bulk modulus at x~0.75 with an anomaly in the variation of the pressure derivative of bulk modulus, (ii) the fixed-spin moment based corrections to the overestimated magnetic ground state for 0.0\leq x\leq0.3 alloys, making the results consistent with the experiments, and (iii) the possibility of multiple magnetic states at x~0.75, which, however, requires further improvements in the calculations.
The influence of vacancies in the $C$ sub-lattice of $MgCNi_{3}$, on its structural, electronic and magnetic properties are studied by means of the density-functional based Korringa-Kohn-Rostoker Green's function method formulated in the atomic sphere approximation. Disorder is taken into account by means of coherent-potential approximation. Characterizations representing the change in the lattice properties include the variation in the equilibrium lattice constants, bulk modulus and pressure derivative of the bulk modulus, and that of electronic structure include the changes in the, total, partial and $\mathbf{k}$-resolved density of states. The incipient magnetic properties are studied by means of fixed-spin moment method of alloy theory, together in conjunction with the phenomenological Ginzburg-Landau equation for magnetic phase transition. The first-principles calculations reveal that due to the breaking of the $C$-$Ni$ bonds, some of the $Ni$ 3d states, which were lowered in energy due to strong hybridization, are transfered back to higher energies thereby increasing the itinerant character in the material. The Bloch spectral densities evaluated at the high symmetry points however reveal that the charge redistribution is not uniform over the cubic Brillouin zone, as new states are seen to be created at the $Γ$ point, while a shift in the states on the energy scale are seen at other high symmetry points.
Using density-functional-based methods, we have studied the effects of incipient magnetism in the cubic perovskites MgCNi_{3} and YBRh_{3} . Our results show that (i) at the equilibrium volume, both MgCNi_{3} and YBRh_{3} alloys remain paramagnetic and (ii) at expanded volumes, only YBRh_{3} shows the possibility of a ferromagnetic phase with a local magnetic moment larger than 0.25 mu_{B} per Rh atom.
The changes in the electronic properties of the substitutionally disordered MgC(Ni_{1-x}T_{x})_{3} (T=Fe, Co or Cu) alloys are studied using the atomic sphere formulation of the Korringa-Kohn-Rostoker coherent-potential approximation method (KKR-ASA CPA), while the effects of incipient magnetism in these alloys are studied phenomenologically using Ginzburg-Landau coefficients in conjunction with fixed-spin moment method. We find that the disordered MgC(Ni_{1-x}T_{x})_{3} alloys have a small magnetic moment localized at Fe and Co sites for low concentrations. The overestimation of the calculated magnetic moment is likely to be due to the limitations of the local-density approximation used in the present study. However, the calculated Ginzburg-Landau coefficients clearly show that the disordered MgC(Ni_{1-x}T_{x})_{3} alloys remain paramagnetic. At expanded volumes, we also find the possibility of a ferromagnetic state for MgC(Ni_{0.95}Fe_{0.05})_{3} and MgC(Ni_{0.90}Co_{0.10})_{3}.
Using full-potential, density-functional-based methods we have studied electron-phonon interaction in ZrB2 and TaB2 in P6/mmm crystal structure. Our results for phonon density of states and Eliashberg function show that the electron-phonon coupling in ZrB2 is much weaker than in TaB2. In particular, we find that the average electron-phonon coupling constant λis equal to 0.14 for ZrB2 and 0.72 for TaB2. The solutions of the isotropic Eliashberg gap equation indicate no superconductivity for ZrB2 but a superconducting transition temperature Tc of around 12 K for TaB2 with μ* ~0.16.
We have studied the changes in the electronic structure and the superconducting transition temperature T_c of Mg(B_{1-x}C_{x})_{2} alloys as a function of x with 0\leq x\leq 0.3. Our density-functional-based approach uses coherent-potential approximation to describe the effects of disorder, Gaspari-Gyorffy formalism to estimate the electron-phonon matrix elements and Allen-Dynes equation to calculate T_c in these alloys. We find that the changes in the electronic structure of Mg(B_{1-x}C_{x})_{2} alloys, especially near the Fermi energy E_F, come mainly from the outward movement of E_F with increasing x, and the effects of disorder in the B plane are small. In particular, our results show a sharp decline in both B and C p_{x(y)} states for 0.2\leq x\leq 0.3. Our calculated variation in T_{c} of Mg(B_{1-x}C_{x})_{2} alloys is in qualitative agreement with the experiments.
Using the Korringa-Kohn-Rostoker coherent-potential approximation in the atomic-sphere approximation (KKR-ASA CPA) we have studied the changes in the electronic structure and the superconducting transition temperature T_{c} in Nb_{1-x}B_{2} alloys as a function of x. We find that the variation in the electronic structure of Nb_{1-x}B_{2} alloys as a function of x is consistent with the rigid-band model. However, the variation of T_{c}, obtained using the Allen-Dynes equation within the Gaspari-Gyorffy formalism to estimate the electron-phonon matrix elements, does not follow the expected trend. We associate this disagreement to the use of a constant ω_{rms} in the Allen-Dynes equation over the whole range of vacancy concentration, thereby indicating the importance of lattice dynamical effects in these systems.
We show that the electron-phonon coupling in the transition-metal diborides NbB2 and TaB2 is dominated by the longitudinal acoustical (LA) mode, in contrast to the optical E_{2g} mode dominated coupling in MgB2. Our ab initio results, described in terms of phonon dispersion, linewidth, and partial electron-phonon coupling along Gamma to A, also show that (i) NbB2 and TaB2 have a relatively weak electron-phonon coupling, (ii) the E_{2g} linewidth is an order of magnitude larger in MgB2 than in NbB2 or TaB2, (iii) the E_{2g} frequency in NbB2 and TaB2 is considerably higher than in MgB2, and (iv) the LA frequency at A for TaB2 is almost half of that of MgB2 or NbB2.
We show that the differences in the magnetic properties of Ni-Pd and Ni-Pt alloys arise mainly due to relativity. In particular, we find that the local magnetic moment of Ni increases with the addition of Pd in Ni-Pd while it decreases with the addition of Pt in Ni-Pt, as found experimentally, only if relativity is present. Our analysis is based on the effects of relativity on (i) the spin-polarized densities of states of Ni, (ii) the splitting of majority and minority spin d-band centers of Ni, and (iii) the separation between s-d band centers of Pd and Pt in Ni-Pd and Ni-Pt alloys.
We present a comparison of electron-phonon interaction in NbB_2 and MgB_2, calculated using full-potential, density-functional-based methods in P6/mmm crystal structure. Our results, described in terms of (i) electronic structure, (ii) phonon density of states F(ω), (iii) Eliashberg function α^2F(ω), and (iv) the solutions of the isotropic Eliashberg gap equation, clearly show significant differences in the electron-phonon interaction in NbB_2 and MgB_2. We find that the average electron-phonon coupling constant λis equal to 0.59 for MgB_2 and 0.43 for NbB_2, leading to superconducting transition temperature T_c of around 22 K for MgB_2 and 3 K for NbB_2.