The reduced total isotope effect and its implications on the nature of superconductivity in MgB2

TL;DR

This paper investigates the isotope effect in MgB2, revealing a reduced total isotope effect coefficient of 0.32, which challenges conventional BCS theory and constrains models explaining its high superconducting transition temperature.

Contribution

It provides the first measurement of the isotope effect coefficient in MgB2, showing a significant deviation from the expected BCS value, thus impacting theoretical understanding.

Findings

Total isotope effect coefficient is 0.32, lower than 0.5.

Results suggest strong electron-phonon coupling and electron-electron interactions.

Constraints on theoretical models of superconductivity in MgB2.

Abstract

The recent discovery of superconductivity at ~39 K in MgB2, by Nagamatsu et al. establishes this simple binary compound as having the highest bulk superconducting transition temperature, Tc, of any non-copper-oxide material. Much of the initial research has focused on whether MgB2 is a conventional BCS, electron-phonon mediated superconductor, and, if it is, why Tc is so high. Isotope effect measurements, in which the atom masses are manipulated to systematically change the phonon frequencies, are one of the fundamental experimental tests of electron-phonon mediated superconductivity. One would expect the total isotope effect coefficient, i.e. the sum of both the Mg and B coefficients, to be 1/2 for a high- Tc, phonon mediated, simple sp orbital superconductor like MgB2. We find a value of 0.32(1), much reduced from the canonical BCS value of 0.5. This requires large values of l (electron-phonon coupling constant) and m* (repulsive electron-electron pseudopotential) to account for the high Tc and, thus, constrains any theoretical model for superconductivity in MgB2.