Paramagnetic resonance study of the high-temperature superconductor YBa2Cu3O7-δand the gap equation

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Abstract
The paramagnetic resonance of Cu2+ in YBa2Cu3O7- delta ( delta approximately=0.1) has been studied. In the normal state at room temperature (RT), the spectrum corresponds to g/sub ///(RT) approximately=2.2167+or-0.0010 and gperpendicular to (RT) approximately=2.0475+or-0.0010 and there is a weak broad line at g approximately=2 which overlaps with the hump describing the gperpendicular to . As the temperature is lowered, it is found that g/sub ///(RT)-g/sub /// varies with temperature as (T-Tc1)-1.1. The onset temperature for the decrease of resistivity is Tc1 approximately=96 K. The intensity of the paramagnetic resonance absorption corresponding to g/sub /// of Cu2+ decreases with decreasing temperature and shows a phase change at 96 K and transitions at 78 K and 30 K and vanishes at about 10 K. For temperatures below Tc1, there is a strong Josephson signal at very low magnetic fields, H approximately=150 G. A numerical solution of the BCS gap equation is performed which shows that the expression kBTc=1.13h(cross) omega exp (-1/N(0)V) requires enhancement of the preexponential factor near 1/N(0)V approximately=1. The spin density suggests that the system is susceptible to the formation of charge-density waves.