383 research outputs found
Goldstone Mode Relaxation in a Quantum Hall Ferromagnet due to Hyperfine Interaction with Nuclei
Spin relaxation in quantum Hall ferromagnet regimes is studied. As the
initial non-equilibrium state, a coherent deviation of the spin system from the
direction is considered and the breakdown of this Goldstone-mode
state due to hyperfine coupling to nuclei is analyzed. The relaxation occurring
non-exponentially with time is studied in terms of annihilation processes in
the "Goldstone condensate" formed by "zero spin excitons". The relaxation rate
is calculated analytically even if the initial deviation is not small. This
relaxation channel competes with the relaxation mechanisms due to spin-orbit
coupling, and at strong magnetic fields it becomes dominating.Comment: 8 page
Key signal contributions in photothermal deflection spectroscopy
We report on key signal contributions in photothermal deflection spectroscopy
(PDS) of semiconductors at photon energies below the bandgap energy and show
how to extract the actual absorption properties from the measurement data. To
this end, we establish a rigorous computation scheme for the deflection signal
including semi-analytic raytracing to analyze the underlying physical effects.
The computation takes into account linear and nonlinear absorption processes
affecting the refractive index and thus leading to a deflection of the probe
beam. We find that beside the linear mirage effect, nonlinear absorption
mechanisms make a substantial contribution to the signal for strongly focussed
pump beams and sample materials with high two-photon absorption coefficients.
For example, the measured quadratic absorption contribution exceeds 5% at a
pump beam intensity of about in Si and at
in GaAs. In addition, our method also
includes thermal expansion effects as well as spatial gradients of the
attenuation properties. We demonstrate that these effects result in an
additional deflection contribution which substantially depends on the distance
of the photodetector from the readout point. This distance dependent
contribution enhances the surface related PDS signal up to two orders of
magnitude and may be misinterpreted as surface absorption if not corrected in
the analysis of the measurement data. We verify these findings by PDS
measurements on crystalline silicon at a wavelength of 1550 nm and provide
guidelines how to extract the actual attenuation coefficient from the PDS
signal.Comment: 10 pages, 16 figures, submitted to Journal of Applied Physiv
Spin relaxation in a two-electron quantum dot
We discuss the rate of relaxation of the total spin in the two-electron
droplet in the vicinity of the magnetic field driven singlet-triplet
transition. The total spin relaxation is attributed to spin-orbit and
electron-phonon interactions. The relaxation process is found to depend on the
spin of ground and excited states. This asymmetry is used to explain puzzles in
recent high source-drain transport experiments.Comment: 9 pages in the PDF format, 1 figur
Comment on "Spin relaxation in quantum Hall systems"
W. Apel and Yu.A. Bychkov have recently considered the spin relaxation in a
2D quantum Hall system for the filling factor close to unity [PRL v.82, 3324
(1999)]. The authors considered only one spin flip mechanism (direct
spin-phonon coupling) among several possible spin-orbit related ones and came
to the conclusion that the spin relaxation time due to this mechanism is quite
short: around s at B=10 T (for GaAs). This time is much shorter than
the typical time ( s) obtained earlier by D. Frenkel while considering
the spin relaxation of 2D electrons in a quantizing magnetic field without the
Coulomb interaction and for the same spin-phonon coupling. I show that the
authors' conclusion about the value of the spin-flip time is wrong and have
deduced the correct time which is by several orders of magnitude longer. I also
discuss the admixture mechanism of the spin-orbit interaction.Comment: 1 pag
Double-exciton component of the cyclotron spin-flip mode in a quantum Hall ferromagnet
We report on the calculation of the cyclotron spin-flip excitation (CSFE) in
a spin-polarized quantum Hall system at unit filling. This mode has a
double-exciton component which contributes to the CSFE correlation energy but
can not be found by means of a mean field approach. The result is compared with
available experimental data.Comment: 9 pages, 2 figure
Levitating the noise performance of ultra-stable laser cavities assisted by a deep neural network: The non-intuitive role of the mirrors
The most precise measurand available to science is the frequency of ultra-stable lasers. With a relative deviation of 4 × 10−17 over a wide range of measuring times between one second and 100 seconds, the smallest effects in nature can thus be made measurable. To enable cutting-edge precision, the laser frequency is stabilized to an external optical cavity. This complex optical device must be manufactured to the highest standards and shielded from environmental influences. Given this assumption, the smallest internal sources of perturbation become dominant, namely the internal noise of the optical components. In this work, we present the optimization of all relevant noise sources from all components of the frequency-stabilized laser. We discuss the correlation between each individual noise source and the different parameters of the system and discover the significance of the mirrors. The optimized laser offers a design stability of 8 × 10−18 for an operation at room temperature for measuring times between one second and 100 seconds
Spin Relaxation in a Quantized Hall Regime in Presence of a Disorder
We study the spin relaxation (SR) of a two-dimensional electron gas (2DEG) in
the quantized Hall regime and discuss the role of spatial inhomogeneity effects
on the relaxation. The results are obtained for small filling factors () or when the filling factor is close to an integer. In either case SR times
are essentially determined by a smooth random potential. For small we
predict a "magneto-confinement" resonance manifested in the enhancement of the
SR rate when the Zeeman energy is close to the spacing of confinement sublevels
in the low-energy wing of the disorder-broadened Landau level. In the resonant
region the -dependence of the SR time has a peculiar non-monotonic shape. If
, the SR is going non-exponentially. Under typical conditions
the calculated SR times range from to s.Comment: 10 pages, 1 figure. To appear in JETP Letter
Effects of Aperture Size on <i>Q</i>Â factor and Shielding Effectiveness of a Cubic Resonator
The EMC properties of a cubic metallic shield are highly affected
by its resonances. At the resonant frequencies, the shielding effectiveness
(SE) collapses, which results in high field strengths inside the cavity. This
can cause failure or even breakdown of electronic devices inside the shield.
The resonant behaviour is mainly determined by the quality or QÂ factor of
the shield. In this paper, the effects of the aperture size on the QÂ factor
and the SE of an electrically large, cubic shield are analysed. At first, a
method is developed in order to determine the QÂ factor based on the
resonance behaviour of the shield in time domain. Only the first resonance of
the shield is considered therefore. The results are evaluated for different
aperture diameters and compared with theory for the QÂ factor. The dominant
coupling mechanism of electromagnetic energy into the shield is thus
identified.
Then the effect of aperture size on the SE is analysed. The excitation of
resonances is very probable if the interfering signal is an ultrawideband
(UWB) pulse, which constitutes a typical intentional electromagnetic
interference (IEMI) scenario. Therefore, the relation between aperture size
and SE is analysed using the theory of the transient SE for a broadband
signal with a constant spectral density distribution. The results show, that
a worst case aperture size exists, where the SE has its minimum
Auger-like Relaxation of Inter-Landau-Level Magneto-Plasmon Excitations in the Quantised Hall Regime
Auger relaxation in 2D strongly correlated electron gas can be represented as
an Auger-like process for neutral magnetoplasmon excitations. The case of
"dielectric" state with lack of free electrons (i.e. at integer filling )
is considered. Really the Auger-like process is a coalescence of two
magnetoplasmons which are converted into a single one of a different plasmon
mode with zero 2D wave-vector. This event turns out to be energetically allowed
for magnetoplasmons near their roton minima where the spectrum has the infinite
density of states. As a result the additional possibility appears for indirect
observation of the magnetorotons by means of anti-Stokes Raman scattering. We
find the rate of this process employing the technique of Excitonic
Representation for the relevant matrix element calculation.Comment: 4 page
Cyclotron spin-flip excitations in a \nu=1/3 quantum Hall ferromagnet
Inelastic light scattering spectroscopy around the \nu=1/3 filling discloses
a novel type of cyclotron spin-flip excitation in a quantum Hall system in
addition to the excitations previously studied. The excitation energy of the
observed mode follows qualitatively the degree of electron spin polarization,
reaching a maximum value at \nu=1/3 and thus characterizing it as a \nu=1/3
ferromagnet eigenmode. Its absolute energy substantially exceeds the
theoretical prediction obtained within the renowned single-mode approximation.
Double-exciton corrections neglected utilizing the single-mode approach are
evaluated within the framework of the excitonic representation and are inferred
to be responsible for the observed effect.Comment: 4 pages,3 figures, submitted to PR
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