We have measured the spin susceptibility, ${\chi }_{\mathrm{s}}$, of the ${\mathrm{CuO}}_2$ planes in the underdoped high $T_c$ superconductor, ${\mathrm{YBa}}_2{\mathrm{Cu}}_4{\mathrm{O}}_8$ by ${\mathrm{Gd}}^{3+}$ electron spin resonance (ESR) in single crystals and aligned powders in fields up to 15.4 T. At low temperatures and high fields, ${\chi }_{\mathrm{s}}$ is enhanced slightly in the $B\parallel c$ orientation with respect to the $B\perp c$ orientation. The enhancement at $15.4\mathrm{T}$ ( $\approx {0.15H}_{c2\mathrm{}}$) at $16\mathrm{K}$ ( $0.2T_c$) is small: approximately $10\%$ of ${\chi }_{\mathrm{s}}\left(T_c\right)$, suggesting that the second critical field of superconductivity, $H_{c2\mathrm{}}\approx 100\mathrm{T}$, would not suppress the pseudogap. This work demonstrates the potential of high field ESR in single crystals for studying high $T_c$ superconductors.

• Received 15 February 2000

DOI:https://doi.org/10.1103/PhysRevLett.85.5627