1,691 research outputs found
Zero temperature optical conductivity of ultra-clean Fermi liquids and superconductors
We calculate the low-frequency optical conductivity sigma(w) of clean metals
and superconductors at zero temperature neglecting the effects of impurities
and phonons. In general, the frequency and temperature dependences of sigma
have very little in common. For small Fermi surfaces in three dimensions (but
not in 2D) we find for example that Re sigma(w>0)=const. for low w which
corresponds to a scattering rate Gamma proportional to w^2 even in the absence
of Umklapp scattering when there is no T^2 contribution to Gamma. In the main
part of the paper we discuss in detail the optical conductivity of d-wave
superconductors in 2D where Re sigma(w>0) \propto w^4 for the smallest
frequencies and the Umklapp processes typically set in smoothly above a finite
threshold w_0 smaller than twice the maximal gap Delta. In cases where the
nodes are located at (pi/2, pi/2), such that direct Umklapp scattering among
them is possible, one obtains Re sigma(w) \propto w^2.Comment: 7 pages, 3 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
Attosecond time-resolved photoelectron holography
Ultrafast strong-field physics provides insight into quantum phenomena that evolve on an attosecond time scale, the most fundamental of which is quantum tunneling. The tunneling process initiates a range of strong field phenomena such as high harmonic generation (HHG), laser-induced electron diffraction, double ionization and photoelectron holography—all evolving during a fraction of the optical cycle. Here we apply attosecond photoelectron holography as a method to resolve the temporal properties of the tunneling process. Adding a weak second harmonic (SH) field to a strong fundamental laser field enables us to reconstruct the ionization times of photoelectrons that play a role in the formation of a photoelectron hologram with attosecond precision. We decouple the contributions of the two arms of the hologram and resolve the subtle differences in their ionization times, separated by only a few tens of attoseconds
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
Random walk approach to spin dynamics in a two-dimensional electron gas with spin-orbit coupling
We introduce and solve a semi-classical random walk (RW) model that describes
the dynamics of spin polarization waves in zinc-blende semiconductor quantum
wells. We derive the dispersion relations for these waves, including the
Rashba, linear and cubic Dresselhaus spin-orbit interactions, as well as the
effects of an electric field applied parallel to the spin polarization
wavevector. In agreement with fully quantum mechanical calculations [Kleinert
and Bryksin, Phys. Rev. B \textbf{76}, 205326 (2007)], the RW approach predicts
that spin waves acquire a phase velocity in the presence of the field that
crosses zero at a nonzero wavevector, . In addition, we show that the
spin-wave decay rate is independent of field at but increases as
for . These predictions can be tested experimentally by
suitable transient spin grating experiments
Ion Beam Machining Of Optoelectronic Components
We produce very smooth vertical sidewalls with high anisotropy in III-V semiconductors by ion beam etching. These qualities are used to fabricate mirrors and deflect light in InGaAs and GaAs waveguide structures. Either a maskless technique, using a focussed ion beam (FIB), or a lithographically deposited mask followed by broad-beam etching (CAIBE) are employed to produce such facets. Here, we describe the two examples of application of
high-resolution ion beam etching techniques towards miniaturizing optoelectronic devices. We show the conversion of an edge-emitting laser structure into a surface-emitting structure, by cutting 45° reflection mirrors, and the fabrication of a monolithic InP-based wavelength demultiplexer by etching a diffraction grating
Measurement of electron-hole friction in an n-doped GaAs/AlGaAs quantum well using optical transient grating spectroscopy
We use phase-resolved transient grating spectroscopy to measure the drift and
diffusion of electron-hole density waves in a semiconductor quantum well. The
unique aspects of this optical probe allow us to determine the frictional force
between a two-dimensional Fermi liquid of electrons and a dilute gas of holes.
Knowledge of electron-hole friction enables prediction of ambipolar dynamics in
high-mobility electron systems.Comment: to appear in PR
Nodal Quasiparticle Dispersion in Strongly Correlated d-wave Superconductors
We analyze the effects of a momentum-dependent self-energy on the
photoemission momentum distribution curve (MDC) lineshape, dispersion and
linewidth. We illustrate this general analysis by a detailed examination of
nodal quasiparticles in high Tc cuprates. We use variational results for the
nodal quasiparticle weight Z (which varies rapidly with hole doping x) and the
low energy Fermi velocity (which is independent of x), to show that
the high energy MDC dispersion , so that it is much
larger than the bare (band structure) velocity and also increases strongly with
underdoping. We also present arguments for why the low energy Fermi velocity
and the high energy dispersion are independent of the bare band structure at
small x. All of these results are in good agreement with earlier and recent
photoemission data [Zhou et al, Nature 423, 398 (2003)].Comment: 4 pages, 3 eps fig
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