Anomalous behaviour of the proton spin-lattice relaxation time near the phase transition of the layered antiferroelectric squaric acid

The temperature dependence of the proton spin-lattice relaxation time has been measured at 51, 70 and 300 MHz by different pulse sequences. Besides a strongly frequency dependent background relaxation rate, a frequency independent critical relaxation rate could be resolved particularly in a broad temperature interval above T_c. The temperature dependence of the critical relaxation rate could not be represented by a power law with a single exponent. Rather it most favourably could be fitted by a logarithmic law. The results are discussed with respect to the phase transition mechanism of squaric acid.