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Physical Review B, 2003
The polarized femtosecond spectroscopies obtained from well characterized and YBa 2 Cu 3 O 7-δ thin films are reported. This bulk-sensitive spectroscopy, combining with the well-textured samples, serves as an effective probe to quasiparticle relaxation dynamics in different crystalline orientations. The significant anisotropy in both the magnitude of the photoinduced transient reflectivity change and the characteristic relaxation time indicates that the nature of the relaxation channel is intrinsically different in various axes and planes. By the orientation-dependent analysis, d-wave symmetry of the bulk-superconducting gap in cuprate superconductors emerges naturally. The crucial and yet controversial issues involved in the superconducting gap symmetry of the cuprate superconductors remain unsettled. Numbers of theories and experiments have been proposed and deployed to track down the nature of the order parameter symmetry of the cuprate superconductors. In general, the obtained results can be roughly classified into two sects 1 . Majority of experimental results obtained by surface-sensitive experiments 2-6 , all point to the widely-accepted d-wave symmetry scenario. Nonetheless, recently some bulk-sensitive experiments 7-10 , e.g. the investigation of quasiparticle dynamics by the ultrafast time-resolved experiments, have revealed some disputable features of s-wave symmetry or s-d mixed characters. In particular, very recently, Kabanov et al. 7 compared the calculations on the temperature dependence of the photo-induced transmission amplitude below T c and claimed that the results were more consistent with an isotropic gap in YBa 2 Cu 3 O 7-δ (YBCO). This apparently has revived the extensive interest on this matter. The fact that, in addition to its bulk-sensitive characteristic, there exist intimate correlations between the superconducting gap opening and the amplitude as well as relaxation time of the transient reflectivity change (∆R/R) has made the femtosecond pump-probe technique one of the most powerful tools in studying the ultrafast carrier dynamics relevant to high-T c superconductivities 11-19 . It is, thus, desirable to re-examine this issue with the improved femtosecond laser and more carefully characterized samples. In this study, based on the general consensus that the amplitude and relaxation time of the transient reflectivity in picosecond scale below T c are directly associated with the opening of the superconducting gap, we have measured the (2003). 30 The temperature-dependent ∆R/R along the b-axis and ab-diagonal qualitatively coincides with the temperature-dependent collective gap and the temperature-independent pseudogap, respectively, in the theory of Kabanov et al. 7 .
Physical Review B, 1999
Photoexcited quasiparticle relaxation dynamics are investigated in a YBa 2 Cu 3 O 7Ϫ␦ superconductor as a function of doping ␦ and temperature T using ultrafast time-resolved optical spectroscopy. A model calculation is presented that describes the temperature dependence of the photoinduced quasiparticle population n pe , photoinduced transmission ⌬T/T, and relaxation time for three different superconducting gaps: ͑i͒ a temperature-dependent collective gap such that ⌬(T)→0 as T→T c , ͑ii͒ a temperature-independent gap, which might arise for the case of a superconductor with preformed pairs, and ͑iii͒ an anisotropic ͑e.g., d-wave͒ gap with nodes. Comparison of the theory with data of photoinduced transmission ͉⌬T/T ͉, reflection ͉⌬R/R͉, and quasiparticle recombination time in YBa 2 Cu 3 O 7Ϫ␦ over a very wide range of doping (0.1Ͻ␦Ͻ0.48) is found to give good quantitative agreement with a temperature-dependent BCS-like isotropic gap near optimum doping (␦Ͻ0.1) and a temperature-independent isotropic gap in underdoped YBa 2 Cu 3 O 7Ϫ␦ (0.15Ͻ␦Ͻ0.48). A pure d-wave gap was found to be inconsistent with the data. ͓S0163-1829͑99͒09301-7͔
Physical Review B, 2010
Time-domain spectroscopy, which probes the dynamics of the electronic states near Fermi surface that are associated with superconductivity, has proven to be a powerful method for providing insights into the fundamental nature of both pseudogap and superconducting gap. This study shows that the unique T-x phase diagram with the time-evolving ultrafast dynamics of Y 1−x Pr x Ba 2 Cu 3 O 7−␦ can be used to identify clearly the Nernst, pseudogap, and superconducting regions. All of the orders appear together at a specific delay time, such as 1.2-3 ps, after pumping by an ultrashort pulse. These discoveries suggest that the Nernst effect, the pseudogap and even superconductivity may have the same physical origin.
Phys Rev Lett, 1997
Femtosecond time-resolved spectrocopy has been used to investigate electronic excitations contributing to the superconducting gap function D͑v, T͒ 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 ϳ5 ps, whose amplitude displays a two-fluid-like temperature dependence, and a long-lived component ͑.10 ns͒ 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 ഠ 3.5kT c. [S0031-9007(97)02434-4]
Physical Review B, 2002
Femtosecond pump probe experiments are reported on quasiparticle relaxation and recombination in YSrxBa2−xCu4O8 as a function of temperature and polarization. The data show a 2-component relaxation similar to YBa2Cu3O 7−δ , one component being assocated with the superconducting transition, and the other with the pseudogap below T * . The relaxation time τ p associated with the pseudogap is found to be T -independent, while the relaxation time τ g of the component obseved only below Tc exhibits a clear divergence near Tc. A strong polarisation anisotropy of the picosecond transient is observed below Tc which is attributed to the anisotropy of the probe transition matrix elements.
Physical Review Letters, 1999
The low-energy electronic excitation spectrum and gap structure in optimally doped and overdoped Y1−xCaxBa2Cu3O 7−δ single crystals are investigated by real-time measurements of the quasiparticle relaxation dynamics with femtosecond optical spectroscopy. From the amplitude of the photoinduced reflectivity as a function of time, temperature and doping x we find clear evidence for the coexistence of two distinct gaps in the entire overdoped phase. One is a temperature-independent "pseudogap" ∆p and the other is a T-dependent collective gap ∆c(T) which has a BCS-like Tdependence closing at Tc. From quasiparticle relaxation time measurements above Tc we ascertain that fluctuations associated with the collective gap ∆c(T) are limited to a few K, consistent with time-dependent Ginzburg-Landau theory and are distinct from the pseudogap whose presence is apparent well above Tc for all x.
Physical Review Letters, 2010
We measure the anisotropic mid-infrared response of electrons and phonons in bulk YBa 2Cu3O7−δ after femtosecond photoexcitation. A line shape analysis of specific lattice modes reveals their transient occupation and coupling to the superconducting condensate. The apex oxygen vibration is strongly excited within 150 fs demonstrating that the lattice absorbs a major portion of the pump energy before the quasiparticles are thermalized. Our results attest to substantial electron-phonon scattering and introduce a powerful concept probing electron-lattice interactions in a variety of complex materials. 74.25.Kc, 78.47.jg The interaction of electrons with the crystal lattice represents one of the most elusive, yet pivotal aspects of high-temperature superconductors (HTSCs). Although purely phonon-mediated BCS-type pairing fails to explain essential properties of superconducting (SC) cuprates, convincing evidence of significant electronphonon contributions have been provided by angleresolved photoemission [1-3], inelastic neutron scattering [4], tunneling [5] and Raman [6] spectroscopies. For time-integrated techniques it is difficult, however, to disentangle the interplay between elementary excitations.
Characteristic features of the pseudogap and superconducting states of YBa2Cu3O7−x
Journal of Experimental and Theoretical Physics Letters, 2002
For the past few years, one of the major problems in high-temperature superconductivity (HTSc) has been the study of the physics of the pseudogap state, which exists in the metallic phase at temperatures T * > T c . Today, this problem seems to be among the most topical ones in the physics of high-temperature superconductivity, and its solution will undoubtedly contribute to the elucidation of the microscopic mechanism of HTSc. The width of the pseudogap state region T * -T c in the phase diagram depends on the carrier concentration. It is maximal for underdoped compounds and decreases to zero at a certain critical carrier concentration; the latter is somewhat higher than the concentration at which the critical temperature T c is maximal. Recent theoretical papers suggest an inhomogeneity of the pseudogap phase and the existence of the temperature crossover, which separates the regions of the pseudogap states with different dynamic properties of quasiparticles . Progress in laser technology has made it possible to decrease the laser pulse duration down to a few femtoseconds, and this has opened up new research prospects by making studies of quasiparticle dynamics and lattice vibrations accessible in real time. Such studies, whose typical example is the pumping technique with subsequent probing by two laser pulses separated in time, are actively performed on HTSc systems . The understanding of the mechanism underlying the establishment of equilibrium after its perturbation by a laser pulse gives an insight into the characteristic features of the dynamics of charge carriers and crystal lattice excitations, as well as the dynamics of their interaction. Using the pumping-probing technique, we performed detailed studies of the relaxation dynamics of the lattice and charge carriers in the temperature range covering both superconducting and pseudogap states. The aim of our studies was to determine the degree of homogeneity of the phase diagram region for T > T c and to reveal the characteristic features of the superconducting state.
Anisotropic Ultrafast Dynamics in Doped Y 1−x Ca x Ba 2 Cu 3 O 7−δ Superconducting Thin Films
The anisotropic dynamics of photoinduced quasiparticle in theY 1−x Ca x Ba 2 Cu 3 O 7−δ (YCBCO) is revealed by using the orientation-resolved femtosecond reflection spectroscopy. This bulk-sensitive spectroscopy, combining with the well-textured (110)-and (100)-YCBCO thin films, serves as an effective probe to quasiparticle relaxation dynamics along different crystallographic orientations. Since the temperature at which the peak of temperature-dependent relaxation time (τ ) manifested in the transient reflectance (∆R/R) measurements is believed to intimately relate to the opening of superconducting gap, the results reported here should shed some light on the issue of the competing order scenarios for over-doped high-T c superconductors.
ACS Symposium Series
Ultrafast time-resolved optical spectroscopy in high-temperature superconductors enables the direct real-time measurement of nonequilibrium quasiparticle recombination dynamics. In addition, it also gives detailed information about the symmetry of the superconducting gap and the "pseudogap", their doping dependence and their temperature dependence. Experimental data, together with theoretical models on the photoinduced transmission amplitude and relaxation time as a function of temperature and doping in YBa 2 Cu 3 O 7-δ gives a consistent picture of the evolution of low-energy structure, where a temperature-independent gap is shown to exist in the underdoped state which evolves with doping into a two-component state near optimum doping, where the dominant response is from a T-dependent BCS-like superconducting gap.