ABSTRACT
The temperature dependence (10-290 K) of the low-frequency (20-150 cm(-1)) Raman-active phonon modes of deeply supercooled confined water in L,L-diphenylalanine micro/nanotubes was analyzed. The isolated dynamics of a specific geometry of a water cluster (pentamer) in a supercooled confined regime was studied in detail. A fragile-to-strong transition at 204 K was observed and related to the crossing of the Widom line. Analysis of peptide vibrational modes coupled to water hydrogen bonds indicated that hydrogen bond fluctuations play an irrelevant role in this system. Our results are in agreement with the second critical point of water existence hypothesis.
Subject(s)
Nanotubes , Phenylalanine/analogs & derivatives , Water/chemistry , Dipeptides , Phenylalanine/chemistry , Spectrum Analysis, Raman , TemperatureABSTRACT
A scaling behavior between heat capacity C*(P) and thermal expansion coefficient times temperature ΩTλ, where λ is a scale factor, is obtained for ferromagnetic La(1 - x)Ca(x)MnO(3) with x = 0.20, 0.25, 0.30, 0.34, 0.40, and 0.45 compounds. The pressure derivative of the magnetic phase transition temperature obtained through a scaling method is in good agreement with experimental results for all samples. The critical exponents associated with the specific heat (α) for x = 0.25, 0.30, and 0.34 are very close to the phase boundary where continuous phase transitions become discontinuous. This is attributed to strong coupling among the spin, charge, and lattice degrees of freedom, which indicates that the magnetization alone would be a poor choice for the order parameter in these systems. Based on thermodynamic arguments, a phase diagram with diverging, cusp-like, near first order, and first order phase transitions is presented.
