## Calculation of the Field and Temperature Dependence of the
c-axis Plasmon in BiSr_2Ca_2Cu_2O_{8+x}

#### Ing-Jye Hwang and D. Stroud,
Department of Physics,
The Ohio State University, Columbus, Ohio 43210

We calculate the c-axis Josephson plasmon frequency \omega_{pl}(B, T)
versus temperature T and magnetic induction B in BSCCO, treating the
order parameter fluctuations in the lowest Landau
level (LLL)~approximation. The calculation agrees qualitatively with
recent field-cooled measurements of Matsuda {\it et al}, assuming a
$c$-axis dielectric constant $\epsilon_0 ~~20--50.
For any given B, \omega_{pl} falls monotonically with increasing T,
remains finite in the flux liquid state, and varies continuously across
the solid-liquid transition when disorder is included.
We also obtain an expression for \omega_{pl} which is valid everywhere in the
flux liquid state and is not restricted to the LLL approximation. This
expression leads to \omega_{pl} \sim 1/\sqrt{B} in the flux liquid state,
in agreement with experiment.