VOLUME 78, NUMBER 11 PHYSICAL REVIEW LETTERS 17 MARCH 1997 Evidence for Two-Component High-Temperature Superconductivity in the Femtosecond Optical Response of YBa 2 Cu 3 O 72d C. J. Stevens, D. Smith, C. Chen, and J. F. Ryan Department of Physics, Clarendon Laboratory, Oxford University, Oxford, United Kingdom B. Podobnik and D. Mihailovic Department of Physics, Clarendon Laboratory, Oxford University, Oxford, United Kingdom and Institute J. Stefan, Jamova 39, 61000 Ljubljana, Slovenia G. A. Wagner and J. E. Evetts Superconductivity Materials IRC, Cambridge University, Cambridge, United Kingdom (Received 13 March 1996; revised manuscript received 11 September 1996) Femtosecond time-resolved spectrocopy has been used to investigate electronic excitations contribut- ing to the superconducting gap function Dv, T  in YBa 2 Cu 3 O 72d . The optical response is strongly peaked at 1.5 eV, and contains two distinct components: one with a characteristic relaxation time of 5 ps, whose amplitude displays a two-fluid-like temperature dependence, and a long-lived component .10 ns which is consistent with localized quasiparticle states at the Fermi energy. The latter shows activated behavior below T c with an activation energy 2D 0  3.5kT c . [S0031-9007(97)02434-4] PACS numbers: 74.72.Bk, 73.50.Gr, 78.47.+p The microscopic origin of the boson exchange mecha- nism responsible for electron pairing in high-temperature superconductors remains unresolved, despite numerous investigations during the past ten years or so since their discovery [1]. Existing experimental data support two quite different scenarios in which the charge carriers are either localized polaronic states or extended band- like states, with deviations from canonical Fermi liq- uid behavior arising from strong anisotropy and damping [2]. In the case of conventional low-temperature super- conductors, tunneling spectroscopy in conjunction with Eliashberg theory convincingly demonstrated the funda- mental role of electron-phonon interactions, providing a spectral density that closely matches the phonon fre- quency distribution. With high-temperature supercon- ductors this technique has been limited to energies # 200 meV because it has not been possible to fabricate junctions which can sustain substantially higher voltages. This situation has been significantly changed by the re- cent work of Holcomb et al. [3] who have measured the spectral function up to 5 eV using thermal modulation spectroscopy (TMS). This work found clear evidence of an excitonic contribution to the pairing mechanism with a characteristic energy 1.5 eV in YBa 2 Cu 3 O 72d . A self- consistent Eliashberg analysis gives impressive agreement between the predicted T c , the calculated gap function Dv, T , and the TMS spectra [4]. Guided by these developments, we have investigated the ultrafast dynamical optical response of YBa 2 Cu 3 O 72d . Previous studies of carrier dynamics in high-T c supercon- ductors using degenerate pumpprobe spectroscopy have already established that the optical response in the neigh- borhood of 2 eV is sensitive to the occupancy of quasi- particle states near the Fermi level, and the relaxation rate is reported to be dependent on the magnitude of the en- ergy gap [5–7]. In this Letter we show that the dynami- cal response at 1.5 eV includes a fast (5 ps) component, with a temperature-dependent amplitude which displays two-fluid behavior, and a long-lived (.10 ns) component which shows thermally activated behavior below T c with an activation energy 2D 0  3.5kT c . The dispersion of the fast response over the energy range 0.8 3.0 eV closely follows the behavior of TMS data. The samples used in this investigation included c- oriented films of thickness 1000 Å, i.e., approximately one absorption length, and crystals with well-developed a-c and a-b planes. The optical absorbance measured at 4 K is shown in Fig. 1 for as-grown film, and also for insulating material which had undergone oxygen depletion. The latter shows a sharp charge transfer (CT) transition at 1.8 eV, with a broad continuum extending to higher energies; it is essentially transparent at energies below the CT gap. In addition to a broad high-energy continuum, the superconducting material shows strong absorption at energies below 1 eV, which includes contributions from the extended Drude tail. The 1.8 eV CT exciton of the insulating phase disappears on doping and is not present in the superconducting material. In fact, a distinct new feature appears at 1.5 eV which does not evolve systematically from the 1.8 eV band, and which band structure calculations indicate may arise from chain- to-plane Cu-O transitions [8]. In the present investigation we have measured the dynamical response of the superconducting phase using nondegenerate pump and probe laser beams. The time dependence of the optical reflectance R and transmission 2212 0031-9007 97 78(11) 2212(4)$10.00 © 1997 The American Physical Society