Pressure Effects and Large Polarons in Layered MgB_2 Superconductor

Abstract

We consider the dependence of the MgB_2 superconducting critical temperature on the pressure. Our model exploits the influence of the large polarons on the band structure of the layered MgB_2 superconductor. Namely, the hole Pekar-Froehlich polarons form quasi two-dimensional potential wells in the boron plane which shift the positions of the sigma- and pi-bands. This energy shift depends on the pressure and the Cooper pairing of the correlated sigma-electrons happens inside polaron wells. The results obtained are as follows: dT_c/dp = -\alpha (5.2 \pm 0.9) K/GPa or dT_c/dp = -\alpha (6.9\pm 1.1) K/GPa for a different choice of the Grueneisen parameter. Being compared with known experimental data they give us a resonable interval for the value of the Froehlich electron-phonon coupling constant: \alpha = 0.15 - 0.45.Comment: 6 pages, 1 fig, LaTeX, subm. to Phys. Rev.