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Microwave response of single crystal YBa2Cu3O7−δ films as a probe for pairing symmetry

“…Figure 6 demonstrates that these peaks can be also seen in the l ac (T) dependences . For less perfect (e.g., PLD) films, characterized by a higher density of linear defects, stacking faults, not so smooth surface, and much smaller size of the singlecrystalline domains with disordered low-angle tilt boundaries, the R s (T) dependences appear to be monotonous and similar to those obtained in previous works [2,26,27]. The observed two-peak feature of R s (T) is believed to be an intrinsic microscopic feature of the most perfect quasi-single-crystalline YBCO films.…”

“…[23] for this sort of YBCO films. On the other hand, we demonstrate that for the most perfect epitaxially grown single-crystalline YBCO films the nonmonotonous character of R s (T) can be even more complicated than it was observed before: for such films our experiments have revealed two distinct rather narrow peaks on R s (T) curves, positioned at quite different temperatures: T 1 = 27-30 K and T 2 = 48-51 K [24]. This observation clearly indicates that the microscopic scenario of electron properties in HTS cuprates (like YBCO) is more intriguing and sophisticated, than it was assumed before.…”

“…Microwave measurements of YBCO films under study are performed by a use of two different techniques: a) coplanar resonator technique [25] which allows one to measure the surface resistance of films under study at frequencies f = 5.25; 8; 17.75; 21 GHz; b) an end-plate cavity resonator technique [24] which extends the frequency range to higher frequencies: f = = 34, 65, and 134 GHz. Several dc off-axis magnetron Fig.…”

“…We suppose that evidence of the small gap in the R s (T) dependence (and, correspondingly, in the quasi-particle spectrum) is consistent with a scenario of the mixed (s+id-type) electron pairing with the small admixture of s-wave component to the pure d x y 2 2 -pairing, usually assuming for the perfect YBCO single crystals. This admixture [42,43] can arise because of enhanced (compared to that in single crystals) quasi-particle scattering due to the additional static disorder inherent to the films structure, such as: different growth-induced crystal structure defects of films [26][27][28][29][30][31][32][33][34][35][36][37][38] like internal low-angle boundaries, dislocations, stacking faults, twin boundaries, etc. Figure 3 demonstrates the temperature dependence of ac penetration depth l(T), measured at several frequencies by a use of coplanar resonator technique.…”

“…4, 5) [24]. Microwave measurements in this case are performed with a use of cylindrical pure copper cavities 2, 4 and 8 mm in diameter.…”