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Strong electron–phonon coupling and multiband effects in the superconducting β-phase Mo1−x Rex alloys

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Strong electron–phonon coupling and multiband effects in the superconducting β-phase Mo1−x Rex alloys
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Superconducting transition temperature TC of some of the cubic β phase Rex alloys with is an order of magnitude higher than that in the elements Mo and Re. We investigate this rather enigmatic issue of the enhanced superconductivity with the help of experimental studies of the temperature dependent electrical resistivity (ρ(T)) and heat capacity (CP(T)), as well as the theoretical estimation of electronic density of states (DOS) using band structure calculations. The ρ(T) in the normal state of the Rex alloys with is distinctly different from that of Mo and the alloys with We have also observed that the Sommerfeld coefficient of electronic heat capacity γ, superconducting transition temperature TC and the DOS at the Fermi level show an abrupt change above . The analysis of these results indicates that the value of electron–phonon coupling constant λep required to explain the TC of the alloys with is much higher than that estimated from γ. On the other hand the analysis of the results of the ρ(T) reveals the presence of phonon assisted inter-band s–d scattering in this composition range. We argue that a strong electron–phonon coupling arising due to the multiband effects is responsible for the enhanced TC in the β phase Rex alloys with .
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Transcript: English(auto-generated)
Greetings from Raja Ramana Centre for Advanced Technology and Alibaba National Institute, Indore, India. I am Sindhu Nilroy and I am Shyam Sindhar and the work which we are going to present is the part of my doctoral thesis. Discovered in 1960s, binary molybdenum rhenium alloys are interesting superconductors,
with the transition temperature about an order of magnitude higher than that in the constituent elements molybdenum and rhenium. Experimental studies done in mid-1980s and early 1990s suggested rhenium influence softening of the phonon spectrum and change in the electron density upstairs in the molybdenum
rhenium alloys. But this phenomena could not explain completely the reason behind the enhanced superconductivity in molybdenum rhenium alloys. In a parallel development, the existence of Fermi pockets and associated electronic topological transition ETT have been established in the molybdenum rhenium alloys above the critical rhenium concentration of about 6 to 11 percent.
However, any correlation between the ETT and the enhanced superconducting properties of these alloys is yet to be established. In a recent study, we have shown the existence of the multiband effects in the superconducting state of the molybdenum rhenium alloys with 25 and 40 percent rhenium concentration. It is to be noted here that the multiband superconductivity is drawing considerable
attention from early 2000, especially after the discovery of this phenomenon in magnesium diboride and iron-based superconductors. In the present paper, we build on this recent work and address to this unresolved issue of the enhanced superconductivity in molybdenum rhenium alloys.
First of all, we find that in these alloys, superconducting transition temperature Tc and the coefficient of the electronic contribution to the heat capacity γ show an abrupt change in the concentration regime with more than 10 percent rhenium. Theoretically estimated electronic density of states at the Fermi level does not follow the compositional dependence of Tc and γ.
This rules out the dominant role of density of states in enhancing the Tc of the molybdenum rhenium alloys. The analysis of specific heat results supports the lattice softening picture in the molybdenum rhenium alloys. However, the enhancement of Tc above 10 percent rhenium concentration could not be accounted for by the lattice softening alone.
We then carefully study the temperature dependence of electrical resistivity. The resistivity predominantly varies with temperature as T to the power 5 in molybdenum and molybdenum rhenium alloys with low rhenium concentration. With increasing rhenium concentration, these behavior changes gradually to a Tq variation. These indicates a changeover from an intraband SS electron scattering in molybdenum
The analysis of all these results suggests that such phonon-assisted intraband SD electron scattering in the presence of the multiple bands at the Fermi level can possibly explain the announcement of Tc in molybdenum rhenium alloys above 10 percent rhenium concentration.