Heat capacity and electron spin echo evidence for low frequency vibrational modes and lattice disorder in L-alanine at cryogenic temperatures

With the view of understanding the low frequency (40-50 cm(-1)) motional processes in L-alanine around 4 K, we have carried out heat capacity (CP) and electron spin echo (ESE) measurements on L-alanine and L-alanine-d7. The obtained CP data show the so-called boson peak (seen as a maximum in CP/T3 versus T plots) in the low temperature region (1.8-20 K). The phase memory time, T(M), and spin lattice relaxation time, T1, of the spin probe, the so-called first stable alanine radical (SAR1), *CHCH3COOH, have been measured between 4 and 105 K. The obtained relaxation rate 1/T1 shows an anomalous increase which coincides with the emergence of a boson peak in the low temperature region (4-20 K). Together, the ESE and the CP data confirm the existence of a thermally activated dynamic orientational disorder in the lattices of both compounds below 20 K. The results help explain the discrepancy between the CP data from powders and single crystals of alanine, as well as the proanomalous relaxation mechanisms for SAR1 in these lattices, and they also provide a mechanism for the spin-lattice relaxation process for SAR1 at cryogenic temperatures.