Josephson Plasma Wave propagation in Bi₂Sr₂CaCu₂O₈ crystals over macroscopic length scales

Extremely anisotropic cuprate superconductors such as Bi₂Sr₂CaCu₂O₈ contain 'intrinsic' Josephson junctions, stacked along the crystalline c-axis. Consequently, it has been theoretically predicted that these compounds should support the propagation of Josephson plasmons at frequencies of hundreds of GHz or more. These plasmons are expected to transport energy in the direction of the CuO₂ planes via oscillating Josephson tunneling currents. In Bi₂Sr₂CaCu₂O₈, it is predicted that Josephson plasmons could propagate over distances of centimeters, due to the highly underdamped nature of the Josephson oscillations in this material. This phenomenon could have future applications such as highly efficient mixers and detectors for terahertz frequencies.



We have observed the propagation of Josephson plasma waves at 0.45 THz through an optimally-doped Bi₂Sr₂CaCu₂O₈ slab with diameter 5 mm and thickness 0.21 mm, cut from a TSFZ growth rod and consisting of multiple single crystals. Our results imply a plasmon decay length that varies from 0.3 mm at 85 Kelvin (slightly below T_c) to 3 mm at 4.4 Kelvin. This length scale is about an order of magnitude shorter than what is predicted for such plasmons theoretically. We provisionally attribute this discrepancy to non-ideal sources of THz energy dissipation in our Bi₂Sr₂CaCu₂O₈ sample, such as plasmon scattering off stacking faults and/or crystalline grain boundaries. Nonetheless, this propagation length is sufficient to permit technological applications of cuprate Josephson plasmons.