Specific heat of low-$T_c$ ${\mathrm{Tl}}_2{\mathrm{Ba}}_2\mathrm{Cu}{\mathrm{O}}_{6+\delta }$

A sensitive differential ac technique has been used to measure the specific heat of low-$T_c$ single-crystal ${\mathrm{Tl}}_2{\mathrm{Ba}}_2\mathrm{Cu}{\mathrm{O}}_{6+\delta }\left(T_c\approx 15K\right)$ in fields up to 7 T applied parallel to the $c$ axis. In contrast to high-$T_c$ cuprates where a magnetic field substantially broadens the resistive transition, in these low-$T_c$ samples, the field simply shifts the transition to lower temperatures leaving its width unchanged. Our specific-heat results, however, show a quite different behavior. The superconducting anomaly is dramatically reduced in size and broadened by the field, suggesting, in analogy with other cuprates, the presence of strong thermal-fluctuation effects.