Magnetic-field-induced low-energy spin excitations in YBa2Cu4O8 measured by high field Gd3+ electron spin resonance

We have measured the spin susceptibility, chi (s), of the CuO2 planes in th e underdoped high T-c superconductor, YBa2Cu4O8 by Gd3+ electron spin reson ance (ESR) in single crystals and aligned powders in fields up to 15.4 T. A t low temperatures and high fields, chi (s) is enhanced slightly in the B p arallel to c orientation with respect to the B perpendicular to c orientati on. The enhancement at 15.4 T (approximate to0.15H(c2)) at 16 K (0.2 T-c) i s small: approximately 10% of chi (s)(T-c), suggesting that the second crit ical field of superconductivity, H-c2 approximate to 100 T, would not suppr ess the pseudogap. This work demonstrates the potential of high field ESR i n single crystals for studying high T-c superconductors.