62 research outputs found
Polarization dependence of coherent phonon generation and detection in the 3D topological insulator Bi2Te3
We have studied the polarization dependence of coherent phonons in the
topological insulator Bi2Te3. Using polarization-dependent femtosecond
pump-probe spectroscopy, we measured coherent phonons as a function of angle
when the pump and probe polarizations were fixed, and the crystal orientation
was rotated. For isotropic detection, depending on the spot position,
oscillations either from only low- and high-frequency phonons of A1g symmetry,
or in addition from the mode at 3.6 THz were observed. All the modes were found
to be independent of the orientation of electric field vector with respect to
the crystal axes testifying to their full symmetry while no modes of lower
symmetry appeared in any polarization geometry. For anisotropic detection both
modes of Eg symmetry could be detected, but their amplitudes were considerably
smaller than those of A1g symmetry. To clarify the coherent phonon assignment
and the process of coherent phonon generation in Bi2Te3, the time-domain
measurements were complemented by spontaneous Raman scattering. The comparison
of frequency- and time-domain results and the polarization dependence suggest
that the 3.6 THz mode belongs to crystalline Te arising due to tellurium
segregation. A discrepancy between the time- and frequency domain data is
discussed.Comment: Accepted to Pysical Review RB, 27 pages, 11 figure
Optical excitation of a coherent transverse optical phonon in a polycrystalline Zr metal film
Copyright © 2007 The American Physical SocietyOptical pump-probe measurements of transient reflectivity were made on a sputtered polycrystalline thin film of hcp Zr. On top of a slow multiexponential decay, an oscillatory signal was observed with frequency varying from the initial value of f(0)=2.24 +/- 0.06 THz at zero time delay to a value of 2.56 +/- 0.12 THz after 2 ps, which is similar to that of a transverse optical (TO) phonon near the Gamma point of the Brillouin zone. From the data, the TO phonon relaxation time was determined to be 0.67 ps. The excitation of the coherent phonon was identified as being mediated by resonant coherent Raman scattering
Superfluidity of bosons on a deformable lattice
We study the superfluid properties of a system of interacting bosons on a
lattice which, moreover, are coupled to the vibrational modes of this lattice,
treated here in terms of Einstein phonon model. The ground state corresponds to
two correlated condensates: that of the bosons and that of the phonons. Two
competing effects determine the common collective soundwave-like mode with
sound velocity , arising from gauge symmetry breaking: i) The sound velocity
(corresponding to a weakly interacting Bose system on a rigid lattice) in
the lowest order approximation is reduced due to reduction of the repulsive
boson-boson interaction, arising from the attractive part of phonon mediated
interaction in the static limit. ii) the second order correction to the sound
velocity is enhanced as compared to the one of bosons on a rigid lattice when
the the boson-phonon interaction is switched on due to the retarded nature of
phonon mediated interaction. The overall effect is that the sound velocity is
practically unaffected by the coupling with phonons, indicating the robustness
of the superfluid state. The induction of a coherent state in the phonon
system, driven by the condensation of the bosons could be of experimental
significance, permitting spectroscopic detections of superfluid properties of
the bosons. Our results are based on an extension of the Beliaev - Popov
formalism for a weakly interacting Bose gas on a rigid lattice to that on a
deformable lattice with which it interacts.Comment: 12 pages, 14 figures, to appear in Phys. Rev.
Pairing interactions and pairing mechanism in high temperature copper oxide superconductors
The polaron binding energy E_{p} in undoped parent cuprates has been
determined to be about 1.0 eV from the unconventional oxygen-isotope effect on
the antiferromagnetic ordering temperature. The deduced value of E_{p} is in
quantitative agreement with that estimated from independent optical data and
that estimated theoretically from the measured dielectric constants. The
substantial oxygen-isotope effect on the in-plane supercarrier mass observed in
optimally doped cuprates suggests that polarons are bound into the Cooper
pairs. We also identify the phonon modes that are strongly coupled to
conduction electrons from the angle-resolved photoemission spectroscopy,
tunneling spectra, and optical data. We consistently show that there is a very
strong electron-phonon coupling feature at a phonon energy of about 20 meV
along the antinodal direction and that this coupling becomes weaker towards the
diagonal direction. We further show that high-temperature superconductivity in
cuprates is caused by strong electron-phonon coupling, polaronic effect, and
significant coupling with 2 eV Cu-O charge transfer fluctuation.Comment: 11 pages, 7 figure
C-axis electronic Raman scattering in Bi_2Sr_2CaCu_2O_{8+\delta}
We report a c-axis-polarized electronic Raman scattering study of
Bi_2Sr_2CaCu_2O_{8+\delta} single crystals. In the normal state, a resonant
electronic continuum extends to 1.5 eV and gains significant intensity as the
incoming photon energy increases. In the superconducting state, a coherence
2\Delta peak appears around 50 meV, with a suppression of the scattering
intensity at frequencies below the peak position. The peak energy, which is
higher than that seen with in-plane polarizations, signifies distinctly
different dynamics of quasiparticle excitations created with out-of-plane
polarization.Comment: 12 pages, REVTEX, 3 postscript figure
Optical studies of gap, hopping energies and the Anderson-Hubbard parameter in the zigzag-chain compound SrCuO2
We have investigated the electronic structure of the zigzag ladder (chain)
compound SrCuO2 combining polarized optical absorption, reflection,
photoreflectance and pseudo-dielectric function measurements with the model
calculations. These measurements yield an energy gap of 1.42 eV (1.77 eV) at
300 K along (perpendicular) to the Cu-O chains. We have found that the lowest
energy gap, the correlation gap, is temperature independent. The electronic
structure of this oxide is calculated using both the
local-spin-density-approximation with gradient correction method, and the
tight-binding theory for the correlated electrons. The calculated density of
electronic states for non-correlated and correlated electrons shows
quasi-one-dimensional character. The correlation gap values of 1.42 eV
(indirect transition) and 1.88 eV (direct transition) have been calculated with
the electron hopping parameters t = 0.30 eV (along a chain), t_yz = 0.12 eV
(between chains) and the Anderson-Hubbard repulsion on copper sites U= 2.0 eV.
We concluded that SrCuO_2 belongs to the correlated-gap insulators.Comment: 24 pages, 8 figures, to be published in Phys.Rev.
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