Linear temperature term of heat capacity in insulating and superconducting La-Ba-Cu-O systems

The heat capacity of ${\mathrm{La}}_{2\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Ba}}_{\mathrm{x}}$${\mathrm{CuO}}_{4}$ was investigated between 0.8 and 10 K. The linear temperature term \ensuremath{\gamma}T of the heat capacity is absent in the antiferromagnetic-ordered region of x\ensuremath{\le}0.02. \ensuremath{\gamma} increases abruptly from x=0.02 to x=0.04 where the system is still insulating at low temperature, and seems to saturate at the order of 5 mJ/mol ${\mathrm{K}}^{2}$ in the superconducting region of x\ensuremath{\ge}0.05. The correlation between magnetic order and vanishing \ensuremath{\gamma}T term and the existence of a finite \ensuremath{\gamma}T term for Ba doping give support to the resonating-valence-bond picture.