Magnetic Field Induced Low-Energy Spin Excitations in YBa2Cu4O8 Measured by High Field Gd3+ ESR

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We have measured the spin susceptibility of the underdoped high temperature superconductor, YBa2Cu4O8 by Gd³⁺ electron spin resonance in single crystals and aligned powders at several magnetic fields between 3 and 15.4 T. At low temperatures and high fields, the spin susceptibility of the CuO2 planes is enhanced slightly in the B∥ c orientation with respect to the B⊥ c orientation. The enhancement in an applied field of 15.4 T (≈ 0.15 Hc₂) at 16 K (0.2 Tc) is approximately 10 percent of the susceptibility measured at Tc. Such a small magnitude suggests that the second critical field of superconductivity, Hc₂≈ 100 T, would not suppress the pseudogap. This work demonstrates the potential of high field ESR in single crystals for studying high Tc superconductors.