Field-Induced Commensurate Phase in Thiourea

An experimental observation of anomalies in the dielectric susceptibility of thiourea under field is reported. Four anomaly lines T_± are determined; they follow the laws (T_±-198.1)²=αE and (T_±-198.8)²=αE, with α=0.11±0.01 K² V^-1 mm (for $E<100$ V/mm). A theoretical discussion, based on a Landau-Ginzburg formulation of the free energy, suggests that a pinned commensurate phase, of commensurability order $p=8$, exists under field at temperatures T_-<T<T₊. This clarifies the nature of the phase diagram of SC(ND₂)₂.