Incoherent inelastic neutron scattering study of the low-frequency phonons in superconductingLa1.85Sr0.15CuO4and antiferromagneticLa2CuO4

Abstract

The generalized phonon density of states (DOS) has been measured for the ceramic superconductor ${\mathrm{La}}_{1.85}$ ${\mathrm{Sr}}_{0.15}$ ${\mathrm{CuO}}_4$ ($T_0$=38 K) and antiferromagnetic ($T_N$=250 K) ${\mathrm{La}}_2$ ${\mathrm{CuO}}_4$ from 0.1–100 meV at temperatures between 250 and 30 K. For the superconductor the phonon spectrum, which was temperature independent over the whole range, contained major peaks at 11.5, 18.5, 23.8, 29.5, 38, 55.2, 78 meV while ${\mathrm{La}}_2$ ${\mathrm{CuO}}_4$ had peaks at 12, 18.5 (24), 29.5, 39.8, 57.2, 80 meV. Increased scattering in the range 5–8 meV, the only feature in the DOS of ${\mathrm{La}}_2$ ${\mathrm{CuO}}_4$ which had no analogue in the superconductor was most apparent at 250 K decreasing with temperature to become insignificant at 30 K.