Measurements of the relaxation time, of electron systems to a disturbance, by two different spectroscopic methods are examined in detail, with the purpose to establish how the presence of fluctuations near a solid state phase transition are made evident in insulators, conductors and superconductors. The absolute temperature and the relaxation time determine the thermodynamic stability of the electronic system near a phase transition by the Uncertainty Principle. At a given temperature T, Landau and Lifshitz obtain the stability from the lower limit of the uncertainty in entropy in units of the Boltzmann constant, S/kB << 1 when T >> 3.82 K ps. Magnetic resonance can measure >> 10-10 s, when v = 9 GHz. X-ray spectroscopy can measure < 10-16s for hv > 5 keV. The results extract information about phenomena that occur at the phase transition by following the evolution of spectral features versus T and crystal orientation. Electron spin resonance identifies the phase transition by the evolution of doublet, triplet and antiferromagnetic resonance, and energy loss. Analysis of the x-ray absorption near an element edge determines one, the relative valence: V(Cu in chains)–V(Cu in planes) 1 in YBa2Cu3O7-, two, the appearance of allowed Cu K pre-edge quadrupole transitions at Tc, three, the enhancement of Ba L3,2 edge transitions by an order of magnitude, just above Tc, at a crystal orientation of the c-axis to the x-ray polarization of 8 /18, and four, difference x-ray absorption spectra, relative to the transition temperature, identify the bonds as well as the atoms involved in the transition. The figure abstract shows the changes in electron density obtained by temperature difference x-ray absorption near the Y K-edge in YBa2Cu3O7- below Tc.
Juana Vivó Acrivos. "Solid state phase transitions characterized by esr and xas" Solid State Sciences (2000): 807-820. doi:10.1016/S1293-2558(00)01089-X