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 tri-glycine sulfate. Measurements were made on crystals whosesurfaces were either non-grounded or short-circuited, and were carried out inmagnetic fields up to 9 T and electric fields up to 220 V/cm. In non-groundedcrystals we find that the shape of the specific-heat anomaly near $T_C$ isthermally broadened. However, the anomaly changes to the characteristic sharp$\lambda$-shape expected for a continuous transition with the application ofeither a magnetic field or an electric field. In crystals whose surfaces wereshort-circuited with gold, the characteristic $\lambda$-shape appeared in theabsence of an external field. This effect enabled a determination of thecritical exponents above and below $T_C$, and may be understood on the basisthat the surface charge originating from the pyroelectric coefficient, $dP/dT$,behaves as if shorted by external magnetic or electric fields.Comment: 4 Pages, 4 Figures. To Appear in Applied Physics Letters_ January 200