The uniform spin susceptibility, chi(s), is determined from the Gd3+
high frequency ESR Knight shift in the underdoped high temperature
superconductor, Gd:YBa2Cu4O8 (T-c = 82 K) between 8 to 260 K.
Measurements at two fields (5.4 and 8.1 T) permit high precision at low
temperatures. In the range of 0.1T(c) to 0.4T(c), a linear temperature
dependence of chi(s) is observed with a large slope. The corresponding
maximum d-wave superconducting gap is Delta(0) = 190 K, 1.1 times the
weakly coupled d-wave limit. The results support suggestions that the
normal state gap observed below 180 K and the superconducting gap below
T-c have nodes with the same symmetry.
