Density of states and superconducting critical temperature vs the number of carriers in a triangular lattice under applied pressure

We propose a band structure for a triangular lattice with nearest neighbor hopping, in terms of parameters which depend on applied pressure to the sample. Such a lattice is a representation of the compound, NaxCoO2· yH2O (Sodium Cobalt Oxyhydrate), which is superconducting at a critical temperature of Tc approx. 5 K. We calculate the density of states, N(ω) vs ω, for several values of the parameters which depend on pressure, distorting in this way the geometry of the lattice. We observe the frequency dependence of the density of states for the different parameters. For zero pressure, we have obtained a density of states which is not symmetric around ω = 0 and which is a different result with respect to a square lattice in 2-D. We also study the superconducting critical temperature, Tc, as a function of the chemical potential, μ. Tc and μ are calculated from two self.consistent BCS equations. As a natural result, we have studied the effect of pressure on Tc, relating the different van Hove singularities, which appear in the density of states, with the different Tc,max's. We then prove that ns mv, where ns is the number of logarithmic singularities and mv is the number of Tc,max's. We compare our results with relevant results in the literature. (author)