Magnetic field induced modification of superfluid density and interplane spectral weight in YBa2 Cu3 Oy

We report on the interlayer infrared response of YBa2 Cu3 Oy in an applied magnetic field. This study explores both the underdoped (y=6.67 and 6.75) and optimally doped (y=6.95) regions of the phase diagram, and includes data for fields applied both parallel to the c axis and to the CuO2 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/c eliminate the high-frequency contribution to the superfluid density, returning the system to a more BCS-like energy scale. For fields H/ CuO2, 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. © 2009 The American Physical Society.