Pseudogap of Ortho-III YBa2Cu3O7−x from Cu EPR investigation

• Pseudogap is almost 190 K for YBa2Cu3O7−x (Tc ≈ 86.5 K) based on derivative ∂g∂T.
• Temperature dependent g factors are investigated in a phenomenological and numerical way.
• g factors are linearly dependent of the implicit temperature normalized parameter γ.
• The local structures of Cu2+(1) are reasonably explained in view of pseudo Jahn-Teller effect.

The temperature dependences of g factors of underdoped YBa2Cu3O7−x (Tc ≈ 86.5 K) indicate that the pseudogap onset temperature T∗ (≈190 K) coincides with the phase boundary established by resistivity, Nernst effect, neutron diffraction and ultrasound measurements in YBCO phase diagram. The obtained empirical relationship between g factors and pseudogap is appropriate for the entire charactering temperature range of Ortho-III YBCO. The EPR signals are ascribed to the localized Cu2+(1) sites based on the quantitative calculations of the g factors by involving the local structures of the paramagnetic copper (1) sites. The present work provides another potential spectroscopic probe for the pseudogap in underdoped high Tc superconductors and the origin of EPR and related mechanisms in YBCO which are still controversial.