Routes to probe Bismuth induced strong-coupling superconductivity in bimetallic BiIn alloys

Abstract

We report the observation of strong electron-phonon coupling in intergranular linked BiIn superconductors over an infinite range mediated by low-lying phonons. An enhanced superconducting transition temperature was observed from the magnetization, revealing a main diamagnetic Meissner state below T C (0) = 5.86(1) K and a critical field H C (0) = 1355(15) Oe with an In 2 Bi phase of the composite sample. The electron-phonon coupling to low lying phonons is found to be the leading mechanism for observed strong-coupling superconductivity in the BiIn system. Our findings suggest that In 2 Bi is in the strong-coupling region with T C (0) = 5.62(1) K, λ ep = 1.45, ω ln = 45.92 K and α = 2.23. The estimated upper critical field can be well-described by a power law with α value higher than 2, consistent with the strong electron-phonon coupling.Recently a number of electron-phonon coupled bi-and tri-metallic superconductors with enhanced superconducting transition have been discovered, renewing interest in conventional phonon-mediated superconductivity 1-4 . We consider the BiIn bimetallic system, in which Bi is a semimetal and In is a weak-coupled superconductor with transition temperature T C (0) = 3.4 K (at ambient atmosphere). When annealed together they form In 2 Bi, In 5 Bi 3 and InBi bimetallic compounds and a solid solution, α-In 5 . The reported T C (0) for single crystals of In 2 Bi and In 5 Bi 3 are 5.1 K and 4.1 K, respectively, whereas InBi is a non-superconductor down to 0.5 K. Hutcherson et al. 6 reported the two-gap superconductivity in a composite (In 2 Bi + In 5 Bi 3 ) superconductor with T C1 (0) = 5.6 K and T C2 (0) = 4.1 K. A microscopic examination of polished InBi samples revealed a heterogeneous mixture of crystal grains, except for pure crystalline phases indicating granular superconductivity. In such a system, the Josephson tunneling between superconducting grains establishes inter-granular links and results in macroscopic superconductivity 7 . However, a comprehensive study, including electron-phonon coupling strength and the enhanced T C in the InBi system, has not yet been carried out. Conventionally, BCS-Eliashberg theory showed that T C is the combined effect of electron-phonon coupling strength (λ ep ) and phonon energy (ω ln ). In this study, we present a transition of weak to strong electron-coupling superconductivity by tuning the Bi concentration in bimetallic Bi y In 1-y alloys, consisting of two superconducting gaps at y = 0.1, 0.2, 0.6, and 0.7 and a single gap at y = 0.01, 0.2, 0.3, 0.4 and 0.5, respectively. The enhancement of T C (0) = 5.62(1) K was observed at y = 0.3, led by low lying phonons that can be well described by using Allen and Dynes' theory 2 . ResultsCrystal structure analysis of BiIn alloy. Figure 1(a) presents the typical EDS spectra, showing a series of elemental Bi and In constituents that can be assigned to Bi-Ma 1 , In-Lα 1 , In-Lβ 1 , and In-Lβ 2 , respectively. The small peaks of C and Cu were the result of the carbon film on the Cu grid from mounting ...

Phase Diagram and Superconductivity: New Insight into the Fundamentals of InSn Bimetallic Alloys

“…From the above expression, for a known value of the DOS, high-energy phonons lead to weak EP coupling, 9 whereas low-energy phonons lead to strong EP coupling. 10,11 Takayama et al 1213 discovered a family of ternary platinum phosphide APt 3 P (A = Sr, Ca, and La) SC with T C = 8.4, 6.6, and 1.5 K, respectively. A crossover from weak to strong coupling in InPb bimetallic alloys was observed in our previous work 9 using first-principle calculation and by the Eliashberg theory based on the Allen−Dyne formulation.…”

“…According to the Eliashberg theory based on Allen and Dynes, modified from McMillan formulations, the EP coupling constant can be expressed as , where N ( E F ) gives the density of state (DOS) number at the Fermi level, ⟨ I 2 ⟩ represents Fermi surface average of the squared EP coupling interaction, M is atomic mass, and ⟨ω o 2 ⟩ is an average squared phonon frequency. From the above expression, for a known value of the DOS, high-energy phonons lead to weak EP coupling, whereas low-energy phonons lead to strong EP coupling. , Takayama et al discovered a family of ternary platinum phosphide APt 3 P (A = Sr, Ca, and La) SC with T C = 8.4, 6.6, and 1.5 K, respectively.…”

Prediction of Phonon-Mediated Superconductivity with High Critical Temperature in the Two-Dimensional Topological Semimetal W2N3

“…On the other hand, even moderate expansions of the lattice parameter rapidly show dramatic effects on the electron–phonon couplings with λ jumping to a value of 3.5 with a 1% expansion. Such value would represent an absolute record, considerably higher than the highest electron–phonon coupling ever observed in nature (λ = 2.6 in bismuth–lead alloys), and the experimental study of such an extreme regime would be very valuable to understand the limitation of the current theories of superconductivity. Unsurprisingly, for higher strains the system becomes unstable with the emergence of a charge density wave driven by the highly interacting acoustic modes.…”

From Complex Inorganic Oxides to Ag–Bi Nanoalloy: Synthesis by Femtosecond Laser Irradiation

“…In previous works, Ag-rich alloyed NPs, such as those containing Au, Cu, or Pt, demonstrated superior bactericidal properties. 48−50 Although recent studies have also shown that Bi inhibits some parasitic and microbial growth 51−53 and that the addition of Bi could modify the final properties of other alloys, 54,55 to the best of our knowledge, the bactericidal activity of Ag–Bi NAs has not been studied yet.…”