Mode-Selective Coupling of Coherent Phonons to the Bi2212 Electronic Band Structure

Cuprate superconductors host a multitude of low-energy optical phonons. Using time- and angle-resolved photoemission spectroscopy, we study coherent phonons in Bi_{2}Sr_{2}Ca_{0.92}Y_{0.08}Cu_{2}O_{8+δ}. Sub-meV modulations of the electronic band structure are observed at frequencies of 3.94±0.01 and 5.59±0.06  THz. For the dominant mode at 3.94 THz, the amplitude of the band energy oscillation weakly increases as a function of momentum away from the node. Theoretical calculations allow identifying the observed modes as CuO_{2}-derived A_{1g} phonons. The Bi- and Sr-derived A_{1g} modes which dominate Raman spectra in the relevant frequency range are absent in our measurements. This highlights the mode selectivity for phonons coupled to the near-Fermi-level electrons, which originate from CuO_{2} planes and dictate thermodynamic properties.

Mode selective excitation of terahertz vibrations in single crystalline rubrene

Organic molecular crystals have a variety of low frequency vibrational modes composed of intra- and inter-molecular oscillations. They are mixed intricately in the terahertz (THz) region. We are interested in the controllability of the vibrational energy distribution among such THz vibrational modes based on the femtosecond double-pulse excitation scheme. Single crystalline rubrene is prepared by physical vapor transport. The optical response of vibrational modes in the electric ground state of rubrene is detected by the ultrafast pump-probe reflectivity measurement at 90 K. Three oscillation modes at 3.20, 3.67, and 4.18 THz are detected, and we demonstrate selective enhancement and depletion of each mode by properly tuning the double-pulse delay. The amplitude of the selected vibrational mode is modulated between 0.149 and 1.87, where 1.0 corresponds to the amplitude excited with a single pump pulse. The double-pulse delay dependence of the observed vibrational amplitude is simulated based on the classical driven harmonic oscillator model, and the results reasonably reproduce our experimental signals. Such selective manipulation of the vibrational amplitude can be a potential tool to investigate the vibronic and electron-phonon couplings which plays an important role for the charge transport characteristics and various optoelectronic properties in organic molecular crystals.

Local Control Theory in Trajectory Surface Hopping Dynamics Applied to the Excited-State Proton Transfer of 4-Hydroxyacridine

The application of local control theory combined with nonadiabatic ab initio molecular dynamics to study the photoinduced intramolecular proton transfer reaction in 4-hydroxyacridine was investigated. All calculations were performed within the framework of linear-response time-dependent density functional theory. The computed pulses revealed important information about the underlying excited-state nuclear dynamics highlighting the involvement of collective vibrational modes that would normally be neglected in a study performed on model systems constrained to a subset of the full configuration space. This study emphasizes the strengths of local control theory for the design of pulses that can trigger chemical reactions associated with the population of a given molecular excited state. In addition, analysis of the generated pulses can help to shed new light on the photophysics and photochemistry of complex molecular systems.

Coherent dynamics of the charge density wave gap in tritellurides

The dynamics of the transient electronic structure in the charge density wave (CDW) system RTe3 (R = rare-earth element) is studied using time- and angle-resolved photoemission spectroscopy (trARPES). Employing a three-pulse pump-probe scheme we investigate the effect of the amplitude mode oscillations on the electronic band structure and, in particular, on the CDW energy gap. We observe coherent oscillations in both lower and upper CDW band with opposite phases, whereby two dominating frequencies are modulating the CDW order parameter. This demonstrates the existence of more than one collective amplitude mode, in contrast to a simple Peierls model. Coherent control experiments of the two amplitude modes, which are strongly coupled in equilibrium, demonstrate independent control of the modes suggesting a decoupling of both modes in the transient photoexcited state.