Magnetic-field-induced low-energy spin excitations in YBa(2)Cu(4)O(8) measured by high field Gd(3+) electron spin resonance.

We have measured the spin susceptibility, chi(s), of the CuO(2) planes in the underdoped high T(c) superconductor, YBa(2)Cu(4)O(8) by Gd(3+) electron spin resonance (ESR) in single crystals and aligned powders in fields up to 15.4 T. At low temperatures and high fields, chi(s) is enhanced slightly in the B parallel c orientation with respect to the B perpendicular c orientation. The enhancement at 15.4 T (approximately equal to 0.15H(c2)) at 16 K (0.2 T(c)) is small: approximately 10% of chi(s)(T(c)), suggesting that the second critical field of superconductivity, H(c2) approximately equal to 100 T, would not suppress the pseudogap. This work demonstrates the potential of high field ESR in single crystals for studying high T(c) superconductors.