Superconductor-to-metal transition in overdoped ${\mathrm{La}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{CuO}}_4$

The low-temperature transition from superconductor to metal of overdoped ${\mathrm{La}}_{2\mathrm{-}\mathit{x}}$ ${\mathrm{Sr}}_{\mathit{x}}$ ${\mathrm{CuO}}_4$ has been monitored by measurements of resistivity versus temperature as a function of pressure over the compositional range 0.15≤x≤0.30. Polycrystalline samples were annealed under oxygen for periods up to 1 month to obtain homogeneous, stoichiometric samples. The pressure dependence of the critical temperature is ${\mathit{dT}}_{\mathit{c}}$/dP>0 in the orthorhombic phase found for x≤0.22; it vanishes in the tetragonal phase occurring with x>0.22. Pressure favors the tetragonal phase, lowering the orthorhombic-tetragonal transition temperature ${\mathit{T}}_{\mathit{t}}$ and shifting the transitional composition ${\mathit{x}}_{\mathit{t}}$ to ${\mathit{x}}_{\mathit{t}}$${\mathit{x}}_{\mathit{t}}$ is not due to orthorhombic-phase fluctuations, as has been speculated. Whereas the superconductive transition temperature ${\mathit{T}}_{\mathit{c}}$ decreases with increasing x, an elastic transition at ${\mathit{T}}_{\mathit{d}}$≊37 K persists over the entire compositional range, including the metallic phase at x=0.30. Careful measurements of resistance and Seebeck coefficient across ${\mathit{T}}_{\mathit{d}}$ reveal a ${\mathit{T}}_{\mathit{d}}$≥${\mathit{T}}_{\mathit{c}}$; but superconductive-pair fluctuations may occur below a ${\mathit{T}}_{\mathit{f}}$>${\mathit{T}}_{\mathit{d}}$ where ${\mathit{T}}_{\mathit{c}}$≊${\mathit{T}}_{\mathit{d}}$, which is consistent with a transition from two- to three-dimensional metallic conduction. The data also suggest that the superconductor-metal transition is not smooth.