We report on the interlayer infrared response of ${\text{YBa}}_2{\text{Cu}}_3{\text{O}}_y$ in an applied magnetic field. This study explores both the underdoped ($y=6.67$ and 6.75) and optimally doped $\left(y=6.95\right)$ regions of the phase diagram, and includes data for fields applied both parallel to the $c$ axis and to the ${\text{CuO}}_2$ planes in this anisotropic superconductor. We focus on the transfer of optical conductivity spectral weight from high-frequency regions of the spectrum to the nondissipative superconducting condensate, and examine the effect of magnetic field upon this process. A sum-rule analysis reveals that magnetic fields $H\parallel c$ eliminate the high-frequency contribution to the superfluid density, returning the system to a more BCS-like energy scale. For fields $H\parallel {\text{CuO}}_2$, however, the high-energy contribution scales with the superfluid density, and the anomalous scheme of condensate formation is maintained, at least in underdoped $y=6.67$ and 6.75 samples. This behavior is discussed in relation to the change in electronic kinetic energy and shown to be closely related to the suppression of interplane phase coherence.

• Received 16 October 2008

DOI:https://doi.org/10.1103/PhysRevB.79.104516