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

Specific-heat measurements are reported near the Curie temperature (TC=320K) 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 9T and electric fields up to 220V∕cm. In nongrounded crystals the authors find that the shape of the specific-heat anomaly near TC is thermally broadened. However, the anomaly changes to the characteristic sharp λ 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 λ shape appeared in the absence of an external field. This effect enabled a determination of the critical exponents above and below TC, 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.