Nature of low-energy excitations in $\mathrm{La_{1.87}Sr_{0.13}CuO_{4}}$ superconducting cuprate

The spectra of the conductivity and dielectric constant of $\mathrm{La_{1.87}Sr_{0.13}CuO_{4}}$ cuprate have been directly measured in the frequency range of $0.3$ to $1.2$ THz ($10$–$40$ cm$^{-1}$) and the temperature range of $5$ to $300$ K in the $E \| c$ polarization (the electric field vector of radiation is perpendicular to the copper-oxygen planes). Excitation has been observed in the superconducting phase, and its nature has been attributed to the transverse optical excitation of the condensate of Cooper pairs, which appears because Josephson junctions between $\mathrm{CuO}$ planes are modulated due to in-plane magnetic and charge stripes. Additional quasiparticle absorption of unknown origin has been detected at frequencies below ${\approx}\,15$ cm$^{-1}$ at liquid helium temperatures.