Acoustic Debye temperature and the role of phonons in cuprates and related superconductors

Elastic Properties of MgB₂ and SiC-Doped MgB₂Superconductors

We investigated the elastic properties of MgB2 and 5 wt.% SiC-doped MgB2 superconductors utilizing the pulse-echo overlap technique. Longitudinal and shear ultrasound velocities were measured for each sample at 80 K and 300 K. The measured velocities at 80 K were used to calculate the various elastic moduli, i.e. longitudinal (CL), shear (G), bulk (B) and Young’s (Y) modulus, and the Debye temperature D, for both samples. The high D at 694 K and 706 K obtained for MgB2 and SiC-doped MgB2, respectively, provide strong evidences via direct acoustic measurements to support numerous theoretical and non-acoustic data-based calculations. At 300 K, the higher longitudinal velocity of the pure MgB2 compared to the doped MgB2 but converging towards the same value of longitudinal modulus at 80 K, seems to suggest that the SiC-doped MgB2 undergoes greater elastic stiffening through temperature range of 300 K down to 80 K. Using the D values, the electron-phonon coupling constant  was calculated within the BCS framework and the two dimensional van Hove scenario. The results led us to conclude that MgB2 is a moderately-strong coupled superconductor.

Strain and high temperature superconductivity: unexpected results from direct electronic structure measurements in thin films

Angle-resolved photoemission spectroscopy reveals very surprising strain-induced effects on the electronic band dispersion of epitaxial La(2-x)Sr(x)CuO(4-delta) thin films. In strained films we measure a band that crosses the Fermi level (E(F)) well before the Brillouin zone boundary. This is in contrast to the flat band reported in unstrained single crystals and in our unstrained films, as well as in contrast to the band flattening predicted by band structure calculations for in-plane compressive strain. In spite of the density of states reduction near E(F), the critical temperature increases in strained films with respect to unstrained samples. These results require a radical departure from commonly accepted notions about strain effects on high temperature superconductors, with possible general repercussions on superconductivity theory.