Gapless spin excitations in superconducting La2−xCa1+xCu2O6 with Tc up to 55 K

Abstract

We report inelastic neutron scattering on single crystals of the bilayer cuprate family La2−xCa1+xCu2O6+δ, including two crystals made superconducting (transitions at 45 and 55 K) by high-pressure annealing in an oxygen-containing atmosphere. The magnetic excitations in the nonsuperconducting crystal have a similar temperature dependence as those in weakly hole-doped cuprates. In the superconducting crystals, there is a near-uniform suppression of the magnetic spectral weight with increasing temperature; in particular, there are no signs of a spin gap or “resonance” peak. This is different from the temperature dependence seen in many optimally doped cuprates but similar to the behavior seen in certain underdoped cuprates. We discuss the possible connection with pair-density-wave superconductivity.