Raman-active phonons and mode softening in superconducting HgBa[sub 2]CuO[sub 4+[delta]]

Polarized micro-Raman spectra of a single crystallite of HgBa[sub 2]CuO[sub 4+[delta]] have been measured and used to assign phonon modes of [ital A][sub 1[ital g]] and [ital E][sub [ital g]] symmetry intrinsic to the superconducting phase. Predictions of the peak positions, linewidths, and Raman cross sections from compounds with similar bonding, i.e., HgO, Y-124, and Y-123, are closely matched and suggest assignments of the high-frequency [ital A][sub 1[ital g]] modes at 592 and 568 cm[sup [minus]1] to apical oxygen stretch vibrations of the ideal structure and of a Cu or O defect (in the Hg plane). The low-frequency 168, 161, and 75 cm[sup [minus]1] modes are assigned to O and Ba vibrations of [ital E][sub [ital g]],[ital A][sub 1[ital g]] and [ital E][sub [ital g]] symmetry, respectively. An anomaly in the phonon frequency and linewidth of the 568-cm[sup [minus]1] [ital A][sub 1[ital g]] mode is observed below [ital T][sub [ital c]]. If interpreted as a change in the phonon self-energy due to the onset of superconductivity, this behavior is consistent with a gap energy near 568 cm[sup [minus]1], i.e., 8.7[ital kT][sub [ital c]] in HgBa[sub 2]CuO[sub 4+[delta]] and constitutes a first estimate of the superconducting gap in this material. Themore » Raman tensor elements for the assigned [ital A][sub 1[ital g]] and [ital E][sub [ital g]] modes are given in absolute units.« less