Josephson Plasma Waves in Bi₂Sr₂CaCu₂O_8+δ crystals and enhancement of plasmon propagation via magnetic vortices

Extremely anisotropic cuprate superconductors contain 'intrinsic' Josephson junctions, stacked along the crystalline c-axis. Consequently, these compounds can support the propagation of Josephson plasmons at frequencies of hundreds of GHz or more. In Bi₂Sr₂CaCu₂O_8+δ, it has been predicted that these plasmons could propagate for 10 cm or more, due to the highly underdamped Josephson oscillations in this material. This phenomenon could have technological applications, such as for highly efficient terahertz mixers or detectors.

 

We have measured propagation of Josephson plasma waves at 0.45 THz through an optimally doped Bi₂Sr₂CaCu₂O_8+δ slab with diameter 5 mm and thickness 0.21 mm, cut from a TSFZ growth rod and consisting of multiple single crystals. We obtain a zero-Gauss plasmon decay length of 0.4 mm at 85 K, which increases to 0.8 mm at 25 K. We find that the decay length can be either strongly enhanced or suppressed - depending on the precise magnitude of the field - by applying magnetic fields of the order of a few tens of Gauss. Applying 27.5 G increases the decay length to as much as 1.3 mm at 13 K. We attribute this behavior to transmission of THz energy via lattices of pinned vortices.