Doping dependence of the thermopower of high-Tc cuprates: Tight-binding band model.

Abstract

Tight-binding calculatoins are performed which include both Cu-O and O-O interactions in the ${\mathrm{CuO}}_{2}$ plane. These calculations reconcile inconsistencies in observed behaviors of the thermopower S and the Hall coefficient ${\mathit{R}}_{\mathit{H}}$: the sign of S of high-${\mathit{T}}_{\mathit{c}}$ cuprates at room temperature becomes negative in the overdoped regime, while ${\mathit{R}}_{\mathit{H}}$ remains positive. A striking feature of the ${\mathrm{CuO}}_{2}$ antibonding band is that a holelike Fermi surface is formed even when the band is less than half-filled. This brings about an unusual electron state in which the Hall (cyclotron) mass parallel to the Fermi surface is holelike (0) but the transport mass perpendicular to it is electronlike (>0). This electronlike transport mass contributes to negative S, while the holelike Hall mass results in positive ${\mathit{R}}_{\mathit{H}}$. In such a state, the electron on the Fermi surface has complete duality: it is holelike in one direction, but electronlike in another. In the overdoped regime, where ${\mathit{R}}_{\mathit{H}}$>0 and S0, the hole doping increases the carrier concentration defined as \ensuremath{\propto} ${\mathit{R}}_{\mathit{H}}^{\mathrm{\ensuremath{-}}1}$, but it decreases the carrier concentration defined as (n/${\mathit{m}}^{\mathrm{*}}$${)}_{\mathit{D}}$ in Drude's formula. This qualitatively explains the recent muon-spin-rotation (\ensuremath{\mu}SR) results that the superconducting carrier concentration ${\mathit{n}}_{\mathit{s}}$/${\mathit{m}}^{\mathrm{*}}$\ensuremath{\sim}(n/${\mathit{m}}^{\mathrm{*}}$${)}_{\mathit{D}}$ decreases with hole doping in the overdoped regime.