High-accuracy differential thermal analysis: A tool for calorimetric investigations on small high-temperature-superconductor specimens

We describe a technique for measuring small heat capacities below room temperature, which represents a simple but significant improvement of the conventional differential thermal-analysis (DTA) method by means of high-precision electronic components. It is thereby possible to measure the heat capacity C(T) of milligram samples with a relative accuracy δC/C<0.02%. This simple and fast DTA method is suitable not only to detect the specific-heat discontinuity at ${\mathit{T}}_{\mathit{c}}$ of high-temperature superconductors, but also to study the thermodynamics at the irreversibility boundary ${\mathit{H}}_{\mathrm{irr}}$(T) in the magnetic phase diagram of cuprate superconductors. From respective measurements on a ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ single crystal, we deduce an upper limit for a latent heat L associated with a hypothetical first-order transition at ${\mathit{H}}_{\mathrm{irr}}$(T), namely L<0.05${\mathit{k}}_{\mathit{B}}$T per vortex per layer for an external field ${\mathrm{\mu }}_0$H=7 T parallel to the c axis of the investigated specimen.