Strong electron-phonon coupling in superconductingMgB2: A specific heat study

Abstract

We report on measurements of the specific heat of the recently discovered superconductor MgB2 in the temperature range between 3 and 220 K. Based on a modified Debye-Einstein model, we have achieved a rather accurate account of the lattice contribution to the specific heat, which allows us to separate the electronic contribution from the total measured specific heat. From our result for the electronic specific heat, we estimate the electron-phonon coupling constant λ to be of the order of 2, significantly enhanced compared to common weak-coupling values ≤ 0.4. Our data also indicate that the electronic specific heat in the superconducting state of MgB2 can be accounted for by a conventional, s-wave type 74.25.Kc The recent discovery of superconductivity in MgB 2 below T c ≈ 39 K [1] has caused a remarkable excitement in the solid-state physics community. Critical temperatures of this magnitude inevitably raise the question whether mechanisms other than the common electron-phonon interaction are responsible for the transition. In their very recent work, Bud'ko et al. [2] have investigated the Boron isotope effect in superconducting MgB 2 and found that replacing 11 B by 10 B increases the critical temperature by about 1 K. This was taken as strong evidence that superconductivity in MgB 2 is of conventional nature, i.e., the electron pairing interaction is phonon-mediated. Kotagawa et al. have interpreted their 11 B NMR measurements as indicating strong coupling s-wave superconductivity [3]. Preliminary 11 B NMR measurements at low temperatures yield spectra which are consistent with the expectations for a type-II superconductor in the mixed state [4]. Evidence for sizeable electron-phonon coupling is also provided by recent band-structure calculations [5][6][7]. Various tunneling experiments have provided some evidence for conventional BCS-like superconductivity, but the values of the superconducting energy gap extracted from these measurements vary from 2 to 7 meV [8][9][10].In this letter, we report on measurements of the specific heat C p of MgB 2 and present C p (T ) data in a wide temperature range. Using a consistent model for the contribution of the lattice vibrations to C p (T ) we calculate the electronic specific heat of this material. We show that the electronic specific heat below T c is in good agreement with a conventional BCS-type interpretation.The sample has been prepared from commercially available MgB 2 -powder (Alfa Aesar) by sintering a pressed pellet at 500• C for about 72 h. Electron microprobe investigations of the sample have shown that impurities of heavy elements (Cu, Ni, W) with concentrations of the order of 10 −2 are present in the sample. The specific heat C p (T ) of the sintered MgB 2 sample has been measured using two different experimental techniques in overlapping temperature ranges. A standard relaxation technique was employed in the temperature range between 3 and 45 K. For temperatures between 20 and 220 K an adiabatic continuous heating calorimeter was used. Special car...

Theoretical Study of Specific Heat and Density of States of MgB<sub>2</sub> Superconductor in Two Band Model

“…C es versus T is in satisfactory agreement with experimental results [30]. However, the theoretical values obtained are slightly higher than the experimental ones.…”

“…Now substituting the Green function from Equation (33) in Equation (30) and using the delta function property, …”

Superconducting properties of the misfit-layer compound (SnSe)1.18(TiSe2)2

“…Here, we subtract the normal state specific heat C n from total specific heat C for the superconducting (or electronic) contribution of specific heat C e because we exclude the lattice part of specific heat. The specific heat jump near the transition temperature shows only broad transition like the case of MgB 2 [33]. If we estimate the specific heat jump C e /γT c by linear extrapolation from low temperature to critical midtemperature T c ≈ 2.8 K, the C e /γT c is estimated by 0.88 which is much smaller than the one of the expected weakcoupling BCS superconductors ( C e /γT c = 1.43).…”

Unusual effects of anisotropy on the specific heat of ceramic and single crystal MgB2

“…Shortly after the unexpected discovery of superconductivity at ~ 39 K in MgB 2 [1,2], specific heat (C) measurements were performed on ceramics in order to learn more about the nature of superconductivity in this material [3][4][5][6][7][8][9][10]. After 15 years of investigations of high temperature superconductors (HTS), superconductivity above ~ 30 K was intuitively associated with unconventional pairing, possibly mediated by antiferromagnetic fluctuations, involving short coherence lengths ξ, reduced dimensionality, and thermal fluctuations.…”