1,738 research outputs found
Cavity-enhanced optical Hall effect in two-dimensional free charge carrier gases detected at terahertz frequencies
The effect of a tunable, externally coupled Fabry-P\'{e}rot cavity to
resonantly enhance the optical Hall effect signatures at terahertz frequencies
produced by a traditional Drude-like two-dimensional electron gas is shown and
discussed in this communication. As a result, the detection of optical Hall
effect signatures at conveniently obtainable magnetic fields, for example by
neodymium permanent magnets, is demonstrated. An AlInN/GaN-based high electron
mobility transistor structure grown on a sapphire substrate is used for the
experiment. The optical Hall effect signatures and their dispersions, which are
governed by the frequency and the reflectance minima and maxima of the
externally coupled Fabry-P\'{e}rot cavity, are presented and discussed. Tuning
the externally coupled Fabry-P\'{e}rot cavity strongly modifies the optical
Hall effect signatures, which provides a new degree of freedom for optical Hall
effect experiments in addition to frequency, angle of incidence and magnetic
field direction and strength
Descriptive Complexity of Deterministic Polylogarithmic Time and Space
We propose logical characterizations of problems solvable in deterministic
polylogarithmic time (PolylogTime) and polylogarithmic space (PolylogSpace). We
introduce a novel two-sorted logic that separates the elements of the input
domain from the bit positions needed to address these elements. We prove that
the inflationary and partial fixed point vartiants of this logic capture
PolylogTime and PolylogSpace, respectively. In the course of proving that our
logic indeed captures PolylogTime on finite ordered structures, we introduce a
variant of random-access Turing machines that can access the relations and
functions of a structure directly. We investigate whether an explicit predicate
for the ordering of the domain is needed in our PolylogTime logic. Finally, we
present the open problem of finding an exact characterization of
order-invariant queries in PolylogTime.Comment: Submitted to the Journal of Computer and System Science
Large K-exciton dynamics in GaN epilayers: the non-thermal and thermal regime
We present a detailed investigation concerning the exciton dynamics in GaN
epilayers grown on c-plane sapphire substrates, focussing on the exciton
formation and the transition from the nonthermal to the thermal regime. The
time-resolved kinetics of LO-phonon replicas is used to address the energy
relaxation in the excitonic band. From ps time-resolved spectra we bring
evidence for a long lasting non-thermal excitonic distribution which accounts
for the rst 50 ps. Such a behavior is con rmed in di erent experimental
conditions, both when non-resonant and resonant excitation are used. At low
excitation power density the exciton formation and their subsequent
thermalization is dominated by impurity scattering rather than by acoustic
phonon scattering. The estimate of the average energy of the excitons as a
function of delay after the excitation pulse provides information on the
relaxation time, which describes the evolution of the exciton population to the
thermal regime.Comment: 9 pages,8 figure
Critical impact of Ehrlich-Schwöbel barrier on GaN surface morphology during homoepitaxial growth
We discuss the impact of kinetics, and in particular the effect of the Ehrlich-Schwöbel barrier (ESB), on the growth and surface morphology of homoepitaxial GaN layers. The presence of an ESB can lead to various self-assembled surface features, which strongly affect the surface roughness. We present an in-depth study of this phenomenon on GaN homoepitaxial layers grown by metal organic vapor phase epitaxy and molecular beam epitaxy. We show how a proper tuning of the growth parameters allows for the control of the surface morphology, independent of the growth technique
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