Microwave response of single crystal YBa2Cu3O7-d films as a probe for pairing symmetry

Experimental Methods and Applications

Анотація

Temperature dependences of the microwave surface impedance, Z_s(T), are measured in the c-axis oriented single- crystal high-T_c superconducting cuprate YBa₂Cu₃O_7–d (YBCO) thin films deposited by the off-axis dc magnetron sputtering onto CeO₂-buffered single-crystal sapphire substrates (film thickness is d ≈ 150, 300, 480 nm). Measurements are performed by a use of the coplanar resonator as well as the end-plate cylindrical cavity resonator techniques at a number of several discrete frequencies within the range of 5–134 GHz. The measurements have revealed unexpected peculiarities on the Z_s(T)-dependences for the most perfect films under study. The peculiarities appear to be most strongly pronounced on the temperature dependences of the film surface resistance R_s(T) = Re {Z_s(T)}. The most important features of the unusual surface resistance behavior are: (i) the temperature dependence R_s(T) of YBCO films under study at low temperatures obeys the exponential law: R_s(T) = R_res + R₀ exp [–D_s/T] with the small energy gap D_s (D_s ≈ 0.5T_c at f = 5 GHz); (ii) the most perfect films reveal a distinct two-peak structure of the R_s(T) dependence with peaks positioned at 27–30 K and 48–51 K, while such peaks are not observed in less perfect films. The peaks are mostly pronounced at moderate (e.g. 34 GHz) frequencies and gradually disappear both at higher and lower frequencies, while their temperature positions remain unchanged. These features of perfect single-crystalline YBCO films are believed to reveal their intrinsic electron properties. Taking into account the possibility of a mixed (s+id)-type pairing symmetry as well as a significant role of extended out-of-plane crystal defects (such as dislocation lines or twin planes) in Bogolyubov`s quasiparticle scattering within the most perfect YBCO films, one can suggest a consistent explanation for the anomalies observed in the Z_s(T) behavior.