ELSEVIER Physiea C 282-287 (1997) 256-259 PHYSICA Evidence for multi-component superconducting order parameter from microwave measurements of YBa2Cu307-5 single crystals * S. Sridhar, H. Srikanth, Z. Zhai, B.A. Willemsen, T. Jacobs a and A. Erb, E. Walker and R. Flukiger b aPhysics Department, Northeastern University, 360 Huntington Avenue, Boston, MA 02115 bDPMC, Universite de Geneve, CH-1211 Geneva, Switzerland The microwave complex conductivity in the superconducting state (~r~ = al -i~r~) of high quality YBa2 Cu3 07-~ single crystals measured at IOGHz using a high-Q Nb cavity reveal two new features - (i) A new peak in crl at ,,, 80K and (ii) enhanced pair conductivity as below ,,- 65K. The results indicate two pairing temperature scales and point to a multi-component superconducting order parameter in YBCO. Electrodynamic measurements of the complex conductivity (~r~ = ~rl - i~2) yield important in- formation regarding the quasiparticle density of states and the nature of the superconducting con- densate [1,2]. Microwave experiments have played an important role in understanding fundamental issues in both low (LTS) and high temperature superconductivity (HTS). In HTS, improvement of materials quality over the years has gradually led to many experimen- tal results which reflect the intrinsic properties untainted by defects. The recent growth of YBa2Cua07_~ single crystals in BaZrOa(BZO) crucibles has ushered in a new generation of high quality samples [3]. This growth method avoids the critical problem of crucible corrosion and leads to single crystals with extremely clean surfaces and purity exceeding 99.995%. We have carried out precision measurements of the surface impedance (Zs = Rs + iXs) of these new high quality YBa2Cu307-a single crystals grown in BZO crucibles. The experiments were done in a superconducting Nb cavity resonator op- erating at IOGHz [4]. Three single crystals from different batches grown in BZO crucible and one crystal of comparable dimensions grown in YSZ crucible were measured to study the systematics in the different samples. The crystals grown in BZO had T~ = 93.4K and the one grown in YSZ had T~ = 92.4K. *Work supported by NSF-DMR-9623720 0921-4534/97/$17.00 © Elsevier Science B.V. All rights reserved. PII S0921-4534(97)00239-6 In Figs.1 and 2, the typical temperature de- pendence of ~2(T) and ch(T) are plotted for YBCO/BZO and YBCO/YSZ samples. Two striking features emerge clearly from a compari- son of the data: (i) ~2 (T) shows two distinct regions of variation for T < 65K and 65K < T < Tc respectively in the YBCO/BZO samples. Since the estimated ,k(0) is lower in these crystals, this feature implies enhanced pair conductivity indicating an onset of additional pairing below 65K. (ii) The normal conductivity crl (T) shows a new peak at around 80K(,-~ 0.9To) in YBCO/BZO crystals which is absent in YBCO/YSZ. Very near To, a third sharp peak is present in ~rl(T) of all four samples and its sharpness at- tests to the quality of the crystals. The sharp- ness of the superconducting transition is evident in the R~ data which indicated a transition width of 0.2K at IOGHz [5], and is confirmed by the sharpness of the ~h peak. A comparison of the in- elastic scattering rates extracted from the normal state conductivity of all the samples shows that the second peak at 80K is prominent in samples with lower inelastic scattering in the normal state. The data indicate that in addition to the pairing at 93K, additional pairing develops below around 65K. This strongly suggests the presence of mul- tiple pairing interactions in YBa:Cu306.95. A peak in O"1 around 30K is seen in all YBCO crys- tals and to account for its location, an argument based on a precipitous drop in quasiparticle scat-