  We present a detailed study on the temperature-dependent THz spectra of the
polycrystalline amino acids L-serine and L-cysteine for wave numbers from 20 to
120 cm-1 and temperatures from 4 to 300 K. Even though the structure of these
two amino acids is very similar, with a sulfur atom in the side chain of
cysteine instead of an oxygen atom in serine, the excitation spectra are
drastically different. Obviously, the vibrational dynamics strongly depend on
the ability of cysteine to form sulfur-hydrogen bonds. In addition, cysteine
undergoes an order-disorder type phase transition close to 80 K, with
accompanying anomalies in our THz results. On increasing temperatures,
well-defined vibrational excitations, exhibit significant shifts in
eigenfrequencies with concomitant line-broadening yielding partly overlapping
modes. Interestingly, several modes completely lose all their dipolar strength
and are unobservable at ambient conditions. Comparing the recent results with
published work utilizing THz, Raman, and neutron-scattering techniques, as well
as with ab-initio simulations, we aim at a consistent analysis of the results
ascribing certain eigenfrequencies to distinct collective lattice modes. We
document that THz spectra can be used to fine-tune parameters of model
calculations and as a fingerprint property of certain amino acids. In addition,
we analyzed the temperature-dependent heat capacity of both compounds and
detected strong excess heat capacities at low temperatures compared to the
canonical Debye behavior of crystalline solids, indicating soft excitations and
a strongly enhanced phonon-density of states at low frequencies.
