Pseudorotational Averaging of EPR Spectrum of Cu(II)O₅Complex in YBa₂Cu₃O_7-δin Low Temperatures

Abstract

In low temperatures the condensation of oxygen was found to occur at 05 site in an elementary cell of YBa2Cu3O7-δ being the fifth ligand forming the CuO5 complex around Cul copper in chain. This change of coordinantion from CuO4 to CuO5 is the origin of a pseudorotation related to a strong vibronic coupling of two distorted configurations: a tetragonal pyramid C4v and a trigonal bipyramid D3h, which yields a pseudocubic EPR spectrum in low temperatures. The averaged spectroscopic splitting coefficient is related to a superposition of vibronically coupled orbital states |x 2 -y2) and 3z2 -r2 ) . The averaged spectrum was for the first time observed in low temperatures since the oxygen condensation in YBa2Cu3O7-δ at 05 site of the chain occur only when oxygen undergoing fast diffusion among the chains, gets localized with decreasing temperature. The activation energy of oxygen desorption from the 05 site is 36 K.

Magnetic Properties of YBCO-PST Composites

“…For Φw = 0.2, the measured g-factor equal to g 2.13 is the average of g11 and g1. In this case the co-ordination is the superposition of the two orbital s t at es │x2 -y2) and 13z2 -r 2 > a s d i s c u s s e d i n [ 8 ] . The changes in the g-factor and the increase in the EPR line width and intensity are caused by the progress of the reaction between YBCO and PST when Φw < 0.5.…”

“…For Φw = 0.2, the measured g-factor equal to g 2.13 is the average of g ││ and 91. In this case the co-ordination is the superposition of the two orbital states │ x 2 -y 2 ) a n d 1 3 z 2 -r 2 ) a s d i s c u s s e d i n [ 8 ] . The decay of the superconductivity is apparent from the measurements of the superconducting volume ratio Φv.…”

Direct measurement of electron spin-lattice relaxation time of paramagnetic centers and non-linear modulated responses in HTSC

“…This conclusion is applicable only to the "high-temperature" range (T > 60 K), whereas the origin of the "low-temperature" EPR line (T < 40 K) [3,5] can be quite different. To solve this problem an additional investigation is needed.…”

“…The EPR spectrum in these substances is well known (see for example [3][4][5]) but there is no commonly accepted viewpoint conceming its nature. Some authors [4,5] ascribe the observed asymmetric line to various structural defects including oxygen vacancies; others [3] consider this EPR spectrum as originated from impurity phases, primarily from the so-called "green phase" Y2BaCuOs.…”