The $c$-axis reflectivity spectrum of underdoped ${\mathrm{Y}\mathrm{B}\mathrm{a}}_2{\mathrm{C}\mathrm{u}}_3{\mathrm{O}}_{6.6}$ (YBCO) is measured below $T_c=59\mathrm{K}$ in parallel magnetic fields $H\parallel {\mathrm{C}\mathrm{u}\mathrm{O}}_2$ up to 7 T. Upon application of a parallel field, a new peak appears at finite frequency in the optical conductivity at the expense of suppression of $c$-axis condensate weight. We conclude that the dramatic change originates from different Josephson coupling strengths between bilayers with and without Josephson vortices. We find that the $400{\mathrm{c}\mathrm{m}}^{-1}$ broad conductivity peak in YBCO gains the spectral weight under parallel magnetic field; this indicates that the condensate weight at $\omega =0$ is distributed to this peak as well as to the new optical Josephson mode.

• Received 29 April 2002

DOI:https://doi.org/10.1103/PhysRevLett.89.247001