Heat capacity in magnetic and electric fields near the ferroelectric transition in triglycine sulfate

Specific-heat measurements are reported near the Curie temperature (T-C=320 K) on triglycine sulfate. Measurements were made on crystals whose surfaces were either nongrounded or short circuited, and were carried out in magnetic fields up to 9 T and electric fields up to 220 V/cm. In nongrounded crystals the authors find that the shape of the specific-heat anomaly near T-C is thermally broadened. However, the anomaly changes to the characteristic sharp lambda shape expected for a continuous transition with the application of either a magnetic field or an electric field. In crystals whose surfaces were short circuited with gold, the characteristic lambda shape appeared in the absence of an external field. This effect enabled a determination of the critical exponents above and below T-C, and may be understood on the basis that the surface charge originating from the pyroelectric coefficient, dP/dT, behaves as if shorted by external magnetic or electric fields.