1,109 research outputs found
Confinement-induced Berry phase and helicity-dependent photocurrents
The photocurrent in an optically active metal is known to contain a component
that switches sign with the helicity of the incident radiation. At low
frequencies, this current depends on the orbital Berry phase of the Bloch
electrons via the "anomalous velocity" of Karplus and Luttinger. We consider
quantum wells in which the parent material, such as GaAs, is not optically
active and the relevant Berry phase only arises as a result of quantum
confinement. Using an envelope approximation that is supported by numerical
tight-binding results, it is shown that the Berry phase contribution is
determined for realistic wells by a cubic Berry phase intrinsic to the bulk
material, the well width, and the well direction. These results for the
magnitude of the Berry-phase effect suggest that it may already have been
observed in quantum well experiments.Comment: 4 pages, 2 figure
Diffusion of Nonequilibrium Quasiparticles in a Cuprate Superconductor
We report a transport study of nonequilibrium quasiparticles in a high-Tc
cuprate superconductor using the transient grating technique. Low-intensity
laser excitation (at photon energy 1.5 eV) was used to introduce a spatially
periodic density of quasiparticles into a high-quality untwinned single crystal
of YBa2Cu3O6.5. Probing the evolution of the initial density through space and
time yielded the quasiparticle diffusion coefficient, and both inelastic and
elastic scattering rates. The technique reported here is potentially applicable
to precision measurement of quasiparticle dynamics, not only in cuprate
superconductors, but in other electronic systems as well.Comment: 5 pages, 4 figure
Photoinduced Changes of Reflectivity in Single Crystals of YBa2Cu3O6.5 (Ortho II)
We report measurements of the photoinduced change in reflectivity of an
untwinned single crystal of YBa2Cu3O6.5 in the ortho II structure. The decay
rate of the transient change in reflectivity is found to decrease rapidly with
decreasing temperature and, below Tc, with decreasing laser intensity. We
interpret the decay as a process of thermalization of antinodal quasiparticles,
whose rate is determined by an inelastic scattering rate of quasiparticle
pairs.Comment: 4 pages, 4 figure
TBC experience in land based gas turbines
Prior and on-going machine evaluations of TBC coatings for power generation applications are summarized. Rainbow testing of various TBC's on turbine nozzles, shrouds and buckets are described along with one test on combustor liners. GEPG has conducted over 15 machine tests with TBC coated turbine nozzles of various coatings. Rainbow test times generally range between 10,000 to 24,000 hours. TBC performance has been quite good and additional testing, including TBC's on shrouds and buckets is continuing. The results show that TBC's have the capability of surviving in power generation machines for the times required. The earlier rainbow tests which evaluated various top coat compositions resulted in confirmation of the superiority of YSZ and especially the 6-8 YSZ composition. On-going tests are more focused on TBC process and property variations. The prevalent failure modes seen thus far in the various rainbow tests are erosion, foreign object damage and buildup of deposits. Additional post test analysis is required to investigate bond coat oxidation and other time/temperature dependent changes to the system
Observation of ferromagnetic resonance in strontium ruthenate (SrRuO3)
We report the observation of ferromagnetic resonance (FMR) in SrRuO3 using
the time-resolved magneto-optical Kerr effect. The FMR oscillations in the
time-domain appear in response to a sudden, optically induced change in the
direction of easy-axis anistropy. The high FMR frequency, 250 GHz, and large
Gilbert damping parameter, alpha ~ 1, are consistent with strong spin-orbit
coupling. We find that the parameters associated with the magnetization
dynamics, including alpha, have a non-monotonic temperature dependence,
suggestive of a link to the anomalous Hall effect.Comment: submitted to Phys. Rev. Let
Determination of the spin-flip time in ferromagnetic SrRuO3 from time-resolved Kerr measurements
We report time-resolved Kerr effect measurements of magnetization dynamics in
ferromagnetic SrRuO3. We observe that the demagnetization time slows
substantially at temperatures within 15K of the Curie temperature, which is ~
150K. We analyze the data with a phenomenological model that relates the
demagnetization time to the spin flip time. In agreement with our observations
the model yields a demagnetization time that is inversely proportional to T-Tc.
We also make a direct comparison of the spin flip rate and the Gilbert damping
coefficient showing that their ratio very close to kBTc, indicating a common
origin for these phenomena
High-Rate Entanglement Source via Two-Photon Emission from Semiconductor Quantum Wells
We propose a compact high-intensity room-temperature source of entangled
photons based on the efficient second-order process of two-photon spontaneous
emission from electrically-pumped semiconductor quantum wells in a photonic
microcavity. Two-photon emission rate in room-temperature semiconductor devices
is determined solely by the carrier density, regardless of the residual
one-photon emission. The microcavity selects two-photon emission for a specific
signal and idler wavelengths and at a preferred direction without modifying the
overall rate. Pair-generation rate in GaAs/AlGaAs quantum well structure is
estimated using a 14-band model to be 3 orders of magnitude higher than for
traditional broadband parametric down-conversion sources
Observation of Coherent Helimagnons and Gilbert damping in an Itinerant Magnet
We study the magnetic excitations of itinerant helimagnets by applying
time-resolved optical spectroscopy to Fe0.8Co0.2Si. Optically excited
oscillations of the magnetization in the helical state are found to disperse to
lower frequency as the applied magnetic field is increased; the fingerprint of
collective modes unique to helimagnets, known as helimagnons. The use of
time-resolved spectroscopy allows us to address the fundamental magnetic
relaxation processes by directly measuring the Gilbert damping, revealing the
versatility of spin dynamics in chiral magnets. (*These authors contributed
equally to this work
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