• Quasiparticle dynamics across the full Brillouin zone of Bi<inf>2</inf>Sr<inf>2</inf>CaCu<inf>2</inf>O<inf>8+δ</inf> traced with ultrafast time and angle-resolved photoemission spectroscopy

      Dakovski, GL; Durakiewicz, T; Zhu, JX; Riseborough, PS; Gu, G; Gilbertson, SM; Taylor, A; Rodriguez, G (2015-09-01)
      © 2015 Author(s). A hallmark in the cuprate family of high-temperature superconductors is the nodal-antinodal dichotomy. In this regard, angle-resolved photoemission spectroscopy (ARPES) has proven especially powerful, providing band structure information directly in energy-momentum space. Time-resolved ARPES (trARPES) holds great promise of adding ultrafast temporal information, in an attempt to identify different interaction channels in the time domain. Previous studies of the cuprates using trARPES were handicapped by the low probing energy, which significantly limits the accessible momentum space. Using 20.15 eV, 12 fs pulses, we show for the first time the evolution of quasiparticles in the antinodal region of Bi2Sr2CaCu2O8+δ and demonstrate that non-monotonic relaxation dynamics dominates above a certain fluence threshold. The dynamics is heavily influenced by transient modification of the electron-phonon interaction and phase space restrictions, in stark contrast to the monotonic relaxation in the nodal and off-nodal regions.
    • Universality and chaoticity in ultracold K+KRb chemical reactions

      Croft, JFE; Makrides, C; Li, M; Petrov, A; Kendrick, BK; Balakrishnan, N; Kotochigova, S; Li, Ming|0000-0003-0827-5976 (2017-07-19)
      © The Author(s) 2017. A fundamental question in the study of chemical reactions is how reactions proceed at a collision energy close to absolute zero. This question is no longer hypothetical: quantum degenerate gases of atoms and molecules can now be created at temperatures lower than a few tens of nanokelvin. Here we consider the benchmark ultracold reaction between, the most-celebrated ultracold molecule, KRb and K. We map out an accurate ab initio ground-state potential energy surface of the K2Rb complex in full dimensionality and report numerically-exact quantum-mechanical reaction dynamics. The distribution of rotationally resolved rates is shown to be Poissonian. An analysis of the hyperspherical adiabatic potential curves explains this statistical character revealing a chaotic distribution for the short-range collision complex that plays a key role in governing the reaction outcome.