2,543 research outputs found
Exchange interaction effects in inter-Landau level Auger scattering in a two-dimensional electron gas
We consider the influence of spin effects on the inter-Landau level
electron-electron scattering rate in a two-dimensional electron gas. Due to the
exchange spin splitting, the Landau levels are not equidistant. This leads to
the suppresion of Auger processes and a nonlinear dependence of the lifetime on
the concentration of the excited electrons even at very low excitation levels.Comment: 10 pages, 3 figure
Quantum Decoherence of Two Qubits
It is commonly stated that decoherence in open quantum systems is due to
growing entanglement with an environment. In practice, however, surprisingly
often decoherence may equally well be described by random unitary dynamics
without invoking a quantum environment at all. For a single qubit, for
instance, pure decoherence (or phase damping) is always of random unitary type.
Here, we construct a simple example of true quantum decoherence of two qubits:
we present a feasible phase damping channel of which we show that it cannot be
understood in terms of random unitary dynamics. We give a very intuitive
geometrical measure for the positive distance of our channel to the convex set
of random unitary channels and find remarkable agreement with the so-called
Birkhoff defect based on the norm of complete boundedness.Comment: 5 pages, 4 figure
Measurement of miniband parameters of a doped superlattice by photoluminescence in high magnetic fields
We have studied a 50/50\AA superlattice of GaAs/AlGaAs
composition, modulation-doped with Si, to produce
cm electrons per superlattice period. The modulation-doping was tailored
to avoid the formation of Tamm states, and photoluminescence due to interband
transitions from extended superlattice states was detected. By studying the
effects of a quantizing magnetic field on the superlattice photoluminescence,
the miniband energy width, the reduced effective mass of the electron-hole
pair, and the band gap renormalization could be deduced.Comment: minor typing errors (minus sign in eq. (5)
Optical study of superconducting Ga-rich layers in silicon
We performed phase-sensitive terahertz (0.12 - 1.2 THz) transmission
measurements of Ga-enriched layers in silicon. Below the superconducting
transition, T_{c} = 6.7 K, we find clear signatures of the formation of a
superconducting condensate and of the opening of an energy gap in the optical
spectra. The London penetration depth, \lambda(T), and the condensate density,
n_{s} = \lambda^{2} 0)/\lambda^{2}(T), as functions of temperature demonstrate
behavior, typical for conventional superconductors with \lambda(0) = 1.8 \mu m.
The terahertz spectra can be well described within the framework of Eliashberg
theory with strong electron-phonon coupling: the zero-temperature energy gap is
2\Delta(0) = 2.64 meV and 2\Delta(0)/k_{B}T_{c} = 4.6 \pm 0.1, consistent with
the amorphous state of Ga. At temperatures just above T_{c}, the optical
spectra demonstrate Drude behavior.Comment: 5 pages, 4 figure
Disparity of superconducting and pseudogap scales in low-Tc Bi-2201 cuprates
We experimentally study transport and intrinsic tunneling characteristics of
a single-layer cuprate Bi(2+x)Sr(2-y)CuO(6+delta) with a low superconducting
critical temperature Tc < 4 K. It is observed that the superconducting energy,
critical field and fluctuation temperature range are scaling down with Tc,
while the corresponding pseudogap characteristics have the same order of
magnitude as for high-Tc cuprates with 20 to 30 times higher Tc. The observed
disparity of the superconducting and pseudogap scales clearly reveals their
different origins.Comment: 5 page
Correlation between Fermi surface transformations and superconductivity in the electron-doped high- superconductor NdCeCuO
Two critical points have been revealed in the normal-state phase diagram of
the electron-doped cuprate superconductor NdCeCuO by exploring
the Fermi surface properties of high quality single crystals by high-field
magnetotransport. First, the quantitative analysis of the Shubnikov-de Haas
effect shows that the weak superlattice potential responsible for the Fermi
surface reconstruction in the overdoped regime extrapolates to zero at the
doping level corresponding to the onset of superconductivity.
Second, the high-field Hall coefficient exhibits a sharp drop right below
optimal doping where the superconducting transition
temperature is maximum. This drop is most likely caused by the onset of
long-range antiferromagnetic ordering. Thus, the superconducting dome appears
to be pinned by two critical points to the normal state phase diagram.Comment: 9 pages; 7 figures; 1 tabl
Approximately optimal domain adaptation with Fisher's Linear Discriminant
We propose a class of models based on Fisher's Linear Discriminant (FLD) in
the context of domain adaptation. The class is the convex combination of two
hypotheses: i) an average hypothesis representing previously seen source tasks
and ii) a hypothesis trained on a new target task. For a particular generative
setting we derive the optimal convex combination of the two models under 0-1
loss, propose a computable approximation, and study the effect of various
parameter settings on the relative risks between the optimal hypothesis,
hypothesis i), and hypothesis ii). We demonstrate the effectiveness of the
proposed optimal classifier in the context of EEG- and ECG-based classification
settings and argue that the optimal classifier can be computed without access
to direct information from any of the individual source tasks. We conclude by
discussing further applications, limitations, and possible future directions
Parameterizing anisotropic reflectance of snow surfaces from airborne digital camera observations in Antarctica
The surface reflection of solar radiation comprises an important boundary condition for solar radiative transfer simulations. In polar regions above snow surfaces, the surface reflection is particularly anisotropic due to low Sun elevations and the highly anisotropic scattering phase function of the snow crystals. The characterization of this surface reflection anisotropy is essential for satellite remote sensing over both the Arctic and Antarctica. To quantify the angular snow reflection properties, the hemispherical-directional reflectance factor (HDRF) of snow surfaces was derived from airborne measurements in Antarctica during austral summer in 2013/14. For this purpose, a digital 180∘ fish-eye camera (green channel, 490–585 nm wavelength band) was used. The HDRF was measured for different surface roughness conditions, optical-equivalent snow grain sizes, and solar zenith angles. The airborne observations covered an area of around 1000 km × 1000 km in the vicinity of Kohnen Station (75∘0′ S, 0∘4′ E) at the outer part of the East Antarctic Plateau. The observations include regions with higher (coastal areas) and lower (inner Antarctica) precipitation amounts and frequencies. The digital camera provided upward, angular-dependent radiance measurements from the lower hemisphere. The comparison of the measured HDRF derived for smooth and rough snow surfaces (sastrugi) showed significant differences, which are superimposed on the diurnal cycle. By inverting a semi-empirical kernel-driven bidirectional reflectance distribution function (BRDF) model, the measured HDRF of snow surfaces was parameterized as a function of solar zenith angle, surface roughness, and optical-equivalent snow grain size. This allows a direct comparison of the HDRF measurements with the BRDF derived from the Moderate Resolution Imaging Spectroradiometer (MODIS) satellite product MCD43. For the analyzed cases, MODIS observations (545–565 nm wavelength band) generally underestimated the anisotropy of the surface reflection. The largest deviations were found for the volumetric model weight fvol (average underestimation by a factor of 10). These deviations are likely linked to short-term changes in snow properties
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