Optical study of plasmons in ${\mathrm{Tl}}_2$${\mathrm{Ba}}_2$${\mathrm{Ca}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{10}$

The spectral functions of ${\mathrm{Tl}}_2$ ${\mathrm{Ba}}_2$ ${\mathrm{Ca}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{10}$ thin films have been determined accurately from combined reflectance and spectroellipsometric measurements. The plasmon spectrum of ${\mathrm{Tl}}_2$ ${\mathrm{Ba}}_2$ ${\mathrm{Ca}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{10}$ consists of an optic plasmon at about 1.2 eV and a broad band of acoustic plasmons. The latter is revealed by the characteristic quadratic frequency dependence of the loss function, Im(-1/ε)=β${\mathrm{\omega }}^2$, for ħω≤1 eV.