Josephson spectroscopy of terahertz losses in [100]-tilt YBa2Cu3O7-x bicrystal junctions

Abstract

Terahertz losses in the [100]-tilt YBa2Cu3O7-x grain-boundary junctions were studied using admittance Josephson spectroscopy. The I-Vcurves of the [100]-tilt YBa2Cu3O7-x junctions, when annealed in atomic oxygen, were described by the resistively shunted junction model (RSJ) with an accuracy of better than 0.5% at the temperature range, where the characteristic voltage IcRn< kT/2e. The losses were shown to increase with the decrease of the oxygen content in the junction and corresponding decrease of junction conductance Rn-1. At low temperatures, where IcRn» kT/2e, the absorption of Josephson radiation by optical phonon modes in YBa2Cu3O7-xwas found to be reflected in the I-V curve of the [100]-tilt junctions. The most prominent structure is situated at the voltages V≌ 9.5 mV, which gives the corresponding Josephson frequency of 4.6 THz in good agreement with the frequency of the strongest IR active optical phonon mode in YBa2Cu3O7-x. Assignment of additional lines in the derived losses is discussed according to available data on lattice dynamic calculations and experimental data for a dynamic conduction Reσ1(f) of YBa2Cu3O7-x. Josephson spectroscopy might be useful for study of low-energy excitations in high-Tc materials.