Observation of a Far-Infrared Sphere Resonance in Superconducting ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x\mathrm{Cu}{\mathrm{O}}_{4-y}$ Particles

A far-infrared sphere resonance has been observed in superconducting ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x\mathrm{Cu}{\mathrm{O}}_{4-y}$ isolated small particles at low temperatures. The frequency and strength of this electronic mode increase with increased Sr doping, decrease with applied magnetic field, and decrease as the temperature is raised; the resonance disappears above the superconducting transition temperature. This superconducting sphere resonance occurs at a frequency below the energy gap and is polarized along the $c$ axis of the particle.