Low-energy excitations and stripes in superconducting cuprate La1.87Sr0.13CuO4

The conductivity and dielectric permittivity spectra of single-crystalline La1.87Sr0.13CuO4 are directly measured with the electric field polarized perpendicular to the CuO planes ( E ∥ c ) covering the frequency range 10–40 cm−1 and temperatures 5–300 K. We observe in the superconducting state a well pronounced excitation with strongly temperature dependent parameters. We suggest that the excitation is caused by the transverse Josephson plasma mode that appears due to the different strengths of Josephson coupling between the superconducting charge stripes in the neighboring and next-nearest neighboring copper–oxygen planes of La1.87Sr0.13CuO4. A strongly enhanced low-frequency (below 15 cm−1) absorption is seen in the superconducting state that is assigned to delocalized quasiparticles of as yet unknown origin.