216 research outputs found
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 BiSrCaYCuO. Sub-meV
modulations of the electronic band structure are observed at frequencies of
and 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-derived phonons. The Bi- and Sr-derived
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
planes and dictate thermodynamic properties.Comment: 7 pages, 3 figure
Ultrafast Optical Excitation of a Persistent Surface-State Population in the Topological Insulator Bi2Se3
Using femtosecond time- and angle- resolved photoemission spectroscopy, we
investigated the nonequilibrium dynamics of the topological insulator Bi2Se3.
We studied p-type Bi2Se3, in which the metallic Dirac surface state and bulk
conduction bands are unoccupied. Optical excitation leads to a meta-stable
population at the bulk conduction band edge, which feeds a nonequilibrium
population of the surface state persisting for >10ps. This unusually long-lived
population of a metallic Dirac surface state with spin texture may present a
channel in which to drive transient spin-polarized currents
Imaginary-time quantum many-body theory out of equilibrium I: Formal equivalence to Keldysh real-time theory and calculation of static properties
We discuss the formal relationship between the real-time Keldysh and
imaginary-time theory for nonequilibrium in quantum dot systems. The latter can
be reformulated using the recently proposed Matsubara voltage approach. We
establish general conditions for correct analytic continuation procedure on
physical observables, and apply the technique to the calculation of static
quantities in steady-state non-equilibrium for a quantum dot subject to a
finite bias voltage and external magnetic field. Limitations of the Matsubara
voltage approach are also pointed out.Comment: 24 pages, 10 figure
Persistent order due to transiently enhanced nesting in an electronically excited charge density wave
Non-equilibrium conditions may lead to novel properties of materials with
broken symmetry ground states not accessible in equilibrium as vividly
demonstrated by non-linearly driven mid-infrared active phonon excitation.
Potential energy surfaces of electronically excited states also allow to
direct nuclear motion, but relaxation of the excess energy typically excites
fluctuations leading to a reduced or even vanishing order parameter as
characterized by an electronic energy gap. Here, using femtosecond time- and
angle-resolved photoemission spectroscopy, we demonstrate a tendency towards
transient stabilization of a charge density wave after near-infrared
excitation, counteracting the suppression of order in the non-equilibrium
state. Analysis of the dynamic electronic structure reveals a remaining energy
gap in a highly excited transient state. Our observation can be explained by a
competition between fluctuations in the electronically excited state, which
tend to reduce order, and transiently enhanced Fermi surface nesting
stabilizing the order
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