Photoexcited quasiparticle relaxation dynamics are investigated in a YBa2Cu3O 7−δ superconductor as a function of doping δ and temperature T using ultrafast time-resolved optical spectroscopy. A model calculation is presented which describes the temperature dependence of the photoinduced quasiparticle population npe, photoinduced transmission ∆T /T and relaxation time τ for three different superconducting gaps: (i) a temperature-dependent collective gap such that ∆(T ) → 0 as T → Tc, (ii) a temperature-independent gap, which might arise for the case of a superconductor with pre-formed pairs and (iii) an anisotropic (e.g. d-wave) gap with nodes. Comparison of the theory with data of photoinduced transmission |∆T /T |, reflection |∆R/R| and quasiparticle recombination time τ in YBa2Cu3O 7−δ over a very wide range of doping (0.1 < δ < 0.48) is found to give good quantitative agreement with a temperature-dependent BCS-like isotropic gap near optimum doping (δ < 0.1) and a temperature-independent isotropic gap in underdoped YBa2Cu3O 7−δ (0.15 < δ < 0.48). A pure d-wave gap was found to be inconsistent with the data.

Macroscopic quantum phenomena such as high-temperature superconductivity, colossal magnetoresistance, ferrimagnetism and ferromagnetism arise from a delicate balance of different interactions among electrons, phonons and spins on the nanoscale. The study of the interplay among these various degrees of freedom in strongly coupled electron-lattice systems is thus crucial to their understanding and for optimizing their properties. Charge-density-wave (CDW) materials, with their inherent modulation of the electron density and associated periodic lattice distortion, represent ideal model systems for the study of such highly cooperative phenomena. With femtosecond time-resolved techniques, it is possible to observe these interactions directly by abruptly perturbing the electronic distribution while keeping track of energy relaxation pathways and coupling strengths among the different subsystems. Numerous time-resolved experiments have been performed on CDWs, probing the dynamics of the electronic subsystem. However, the dynamics of the periodic lattice distortion have been only indirectly inferred. Here we provide direct atomic-level information on the structural dynamics by using femtosecond electron diffraction to study the quasi two-dimensional CDW system 1T-TaS(2). Effectively, we have directly observed the atomic motions that result from the optically induced change in the electronic spatial distribution. The periodic lattice distortion, which has an amplitude of ∼0.1 Å, is suppressed by about 20% on a timescale (∼250 femtoseconds) comparable to half the period of the corresponding collective mode. These highly cooperative, electronically driven atomic motions are accompanied by a rapid electron-phonon energy transfer (∼350 femtoseconds) and are followed by fast recovery of the CDW (∼4 picoseconds). The degree of cooperativity in the observed structural dynamics is remarkable and illustrates the importance of obtaining atomic-level perspectives of the processes directing the physics of strongly correlated systems.

We report on the synthesis, structure, and self-assembly of single-wall subnanometer-diameter molybdenum disulfide tubes. The nanotubes are up to hundreds of micrometers long and display diverse self-assembly properties on different length scales, ranging from twisted bundles to regularly shaped "furry" forms. The bundles, which contain interstitial iodine, can be readily disassembled into individual molybdenum disulfide nanotubes. The synthesis was performed using a novel type of catalyzed transport reaction including C(60) as a growth promoter.

We present a comparative study of ultrafast photoconversion dynamics in tetracene (Tc) and pentacene (Pc) single crystals and Pc films using optical pump-probe spectroscopy. Photoinduced absorption in Tc and Pc crystals is activated and temperature-independent, respectively, demonstrating dominant singlet-triplet exciton fission. In Pc films (as well as C60-doped films) this decay channel is suppressed by electron trapping. These results demonstrate the central role of crystallinity and purity in photogeneration processes and will constrain the design of future photovoltaic devices.

The superconducting gap ∆ c , the pseudogap ∆ p and pair fluctuations above T c in overdoped Y 1−x Ca x Ba 2 Cu 3 O 7−δ from femtosecond time-domain spectroscopy. 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.

Superconducting state dynamics following excitation of a superconductor with a femtosecond optical pulse is studied in terms of a phenomenological Rothwarf and Taylor model. Analytical solutions for various limiting cases are obtained. The model is found to account for the intensity and temperature dependence of both photoinduced quasiparticle density, as well as pair-breaking and superconducting state recovery dynamics in conventional as well as cuprate superconductors.In recent years numerous studies of non-equilibrium carrier dynamics in superconductors (SC) have been performed utilizing femtosecond real-time techniques [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18]. Research focused on the identification of relaxation processes and direct measurements of the relaxation times.One of the open issues at the moment is whether cuprates are in the so called phonon bottleneck regime as conventional SCs [19], or in the weak bottleneck regime, where relaxation is governed by the biparticle recombination kinetics [9,17,18]. The theoretical model that has been most commonly used to interpret the dynamics is a phenomenological Rothwarf-Taylor (RT) model which describes the evolution of quasiparticle (QP) and high frequency phonon (HFP) populations via a set of two non-linear differential equations [20], which were shown recently[21] to follow from the general set of kinetic equations for a SC [22]. While the RT model has been known for almost 40 years, no rigorous attempt to solve it has been made thus far, and neither has a comparison to the experimental data been made.In this Letter we present a detailed study of the evolution of the SC state following excitation by ultrashort laser pulse using the RT model. We have obtained analytical solutions of the model in the limit of a strong and a weak bottleneck, which are in excellent agreement with numerical simulations. The solutions enable comparison of the model to the experimental results. We show that RT model can account for most of the experimental observations in conventional as well as cuprate SC, and that both conventional and cuprates SCs are in the strong bottleneck regime, where SC state recovery is governed by the HFP decay dynamics.Rothwarf and Taylor have pointed out that the phonon channel should be considered when studying the SC relaxation [20]. When two QPs with energies ≥ ∆, where ∆ is the SC gap, recombine a HFP (ω > 2∆) is created. Since HFP can subsequently break a Cooper pair creating two QPs the SC recovery is governed by the decay of the HFP population. The dynamics of QP and HFP populations is determined by [20]:Here n and N are concentrations of QPs and HFPs, respectively, η is the probability for pair-breaking by HFP absorption, and R the bare QP recombination rate with the creation of a HFP. N T is the concentration of HFP in thermal equilibrium at temperature T , and γ their decay rate. I 0 and J 0 represent the external sources of QPs and HFPs, respectively [12]. Physically γ is governed by the fastest of the two processes: ...

3Spontaneous symmetry breaking gives rise to a new quantum ground state featuring characteristic low-energy elementary excitations 3,11,14,[18][19][20][21][22] Ultrashort pulses in the terahertz (1 THz = 10 12 Hz) range have been used to trace electronic order via direct coupling to such excitations 22,23 . We demonstrate that THz pulses may simultaneously also track the crystalline order during an ultrafast phase transition.This idea is tested in a prominent reference system, 1T-TiSe 2 . Within the family of layered transition-metal dichalcogenides, this material has attracted special attention: Upon cooling below T c ≈ 200 K, it undergoes a transition into a commensurate CDW accompanied by the formation of a structural (2×2×2) superlattice 21 (Fig. 1a). In its high-temperature phase, TiSe 2 is a semimetal 20 with electron and hole pockets at the L and  points of the Brillouin zone, respectively 15,24 (Fig. 1b). The spatial reconstruction due to the CDW maps these two points on top of each other and leads to the partial opening of an electronic energy gap as well as a dramatic reduction of the density of free charge carriers 20 (Fig. 1b). Superconductivity emerges when the CDW is suppressed, e.g. by Cu intercalation 7 or pressure 25 . This discovery as well as novel chiral properties 26 have intensified the interest in the nature of the CDW in 1T-TiSe 2 . Yet, the microscopic mechanisms remain elusive 24,[27][28][29] . A first hypothesis assumes electron-phonon coupling based on a Jahn-Teller effect as the driving force 27 . A competing model suggests that the transition is purely electronically driven 24,28 . Coulomb attraction may render the system unstable against the formation of excitons between the electron-and hole-like Fermi pockets, leading to lattice deformation with the corresponding wave vector. Combinations of the two scenarios have also been proposed 29 . Time-resolved x-ray diffraction 16 and photoemission 10,15 experiments have separately tracked the dynamics of either structural or electronic orders. 4Evidence for both excitonic and phononic contributions was obtained in this way, leaving a controversial picture.Here we disentangle the two coupled components of the CDW order parameter by simultaneously tracing the ultrafast THz response of PLD-related phonons and electronic conductivity while a femtosecond pulse selectively melts the excitonic order. Our data reveal a transient phase in which the PLD persists in the absence of excitonic correlations. A quantummechanical theory 29 corroborates our conclusions.In TiSe 2 , the transition to the CDW ordered phase modifies the low-frequency optical response in three distinct ways: (i) The CDW-induced energy gap introduces a broad single- (Fig. 1d). Above T c , we observe a single TO phonon resonance at 17 meV. Below T c , back-folding of the uppermost acoustic branch from the L to the  point 21 yields an additional IR-active in-plane mode at 19 meV. The weaker peak at 22 meV likely originates from a folded optical branch at the M point 21 . 5W...

Single particle and collective excitations in the one-dimensional charge density wave solid K 0.3 MoO 3 probed in real time by femtosecond spectroscopy.