THz Josephson properties of grain boundary YBaCuO junctions on symmetric, tilted bicrystal sapphire substrates

Superconducting Josephson junctions with high characteristic voltages (I_cR_n larger than 4 mV at 4.2 K) are fabricated by depositing YBa₂Cu₃O_7-x on tilted sapphire bicrystal substrates, where the tilting axis is along the grain boundary. The good junction quality and low microwave losses in sapphire gave high frequency response well into the THz region. High quality YBa₂Cu₃O_7-x epitaxial films were deposited on tilted sapphire substrates with CeO₂ buffer layers by pulsed laser deposition. YBaCuO films have smaller tilt angles, from 1.0° up to 10.3°, compared to inclination angles of the substrates from 1.5° to 13.6°. X-ray diffraction shows only a single orientation of the films in the a-b plane, as well as an absence of a-axis particles and outgrowths. Critical temperatures as high as T_c=88.5-89.0 K and ΔT_c≤1.5 K were obtained in all films. The grain boundary in a common high-T_c superconducting junction is usually much less straight than in the in-plane rotated bicrystal substrate, depressing Josephson currents. The waviness of the artificial grain boundary in a tilted bicrystal film is three times less than in an in-plane (untilted) bicrystal. Tilted Josephson junctions of widths from 1.5 to 6 μm were tested at temperatures from 0.26 to 77 K. I_cR_n products as high as 4.5 mV were observed at T=4.2 K. Shapiro steps were observed at voltages over 3 mV under 300 GHz irradiation. Josephson - radiation from the tilted junction was measured at frequencies up to 1.7 THz by a cryogenic bolometer. Suppressing the critical current with a magnetic field can separate Josephson radiation and thermal radiation. A parabolic dependence of the response on bias voltage for thermal radiation corresponds to an increase of junction temperature from 260 mK at zero bias to 3 K at 1 mV bias.