15,794 research outputs found
Reflection high-energy electron diffraction studies of the growth of lnAs/Ga_(1-x)In_xSb strained-layer superlattices
We have used reflection highâenergy electron diffraction to study the surface periodicity of the growth front of InAs/GaInSb strainedâlayer superlattices (SLSs). We found that the apparent surface lattice spacing reproducibly changed during layers which subsequent xâray measurements indicated were coherently strained. Abrupt changes in the measured streak spacings were found to be correlated to changes in the growth flux. The profile of the dynamic streak spacing was found to be reproducible when comparing consecutive periods of a SLSs or different SLSs employing the same shuttering scheme at the InAs/GaInSb interface. Finally, when the interface shuttering scheme was changed, it was found that the dynamic streak separation profile also changed. Large changes in the shuttering scheme led to dramatic differences in the streak separation profile, and small changes in the shuttering scheme led to minor changes in the profile. In both cases, the differences in the surface periodicity profile occurred during the parts of the growth where the incident fluxes differed
Model-Independent Predictions for Low Energy Isoscalar Heavy Baryon Observables in the Combined Heavy Quark and Large Expansion
Model-independent predictions for excitation energies, semileptonic form
factors and electromagnetic decay rates of isoscalar heavy baryons and their
low energy excited states are discussed in terms of the combined heavy quark
and large expansion. At leading order, the observables are completely
determined in terms of the known excitation energy of the first excited state
of . At next-to-leading order in the combined expansion all heavy
baryon observables can be expressed in a model-independent way in terms of two
experimentally measurable quantities. We list predictions at leading and
next-to-leading order.Comment: 7 pages, LaTe
Excited Heavy Baryons and Their Symmetries III: Phenomenology
Phenomenological applications of an effective theory of low-lying excited
states of charm and bottom isoscalar baryons are discussed at leading and
next-to-leading order in the combined heavy quark and large expansion.
The combined expansion is formulated in terms of the counting parameter
; the combined expansion is in powers of
. We work up to next-to-leading order. We obtain
model-independent predictions for the excitation energies, the semileptonic
form factors and electromagnetic decay rates. The spin-averaged mass of the
doublet of the first orbitally excited sate of is predicted to be
approximately . It is shown that in the combined limit at leading and
next-to-leading order there is only one independent form factor describing
; similarly, and
decays are described by a single independent form factor. These form factors
are calculated at leading and next-to-leading order in the combined expansion.
The electromagnetic decay rates of the first excited states of and
are determined at leading and next-to leading order. The ratio of
radiative decay rates is predicted to be approximately
0.2, greatly different from the heavy quark effective theory value of unity.Comment: 21 pages, 2 figure
Type II superlattices for infrared detectors and devices
Superlattices consisting of combinations of III-V semiconductors with type II band alignments are of interest for infrared applications because their energy gaps can be made smaller than those of any 'natural' III-V compounds. Specifically, it has been demonstrated that both InSb/InAsxSb1-x superlattices and Ga1-xInxSb/InAs superlattices can possess energy gaps in the 8-14 mu m range. The efforts have focused on the Ga1-xInxSb/InAs system because of its extreme broken gap band alignment, which results in narrow energy gaps for very short superlattice periods. The authors report the use of in situ chemical doping of Ga1-xInxSb/InAs superlattices to fabricate p-n photodiodes. These diodes display a clear photovoltaic response with a threshold near 12 mu m. They have also attained outstanding structural quality in Ga1-xInxSb/InAs superlattices grown on radiatively heated GaSb substrates. Cross-sectional transmission electron microscope images of these superlattices display no dislocations, while high resolution X-ray diffraction scans reveal sharp high-order superlattice satellites and strong Pendellosung fringes
Experimental observation of negative differential resistance from an InAs/GaSb interface
We have observed negative differential resistance at room temperature from devices consisting of a single interface between n-type InAs and p-type GaSb. InAs and GaSb have a type II staggered band alignment; hence, the negative differential resistance arises from the same mechanism as in a p+-n+ tunnel diode. Room-temperature peak current densities of 8.2Ă10^4 A/cm^2 and 4.2Ă10^4 A/cm^2 were measured for structures with and without undoped spacer layers at the heterointerface, respectively
Electrical determination of the valence-band discontinuity in HgTe-CdTe heterojunctions
Current-voltage behavior is studied experimentally in a Hg0.78Cd0.22Te-CdTe-Hg0.78Cd0.22Te heterostructure grown by molecular beam epitaxy. At temperatures above 160 K, energy-band diagrams suggest that the dominant low-bias current is thermionic hole emission across the CdTe barrier layer. This interpretation yields a direct determination of 390±75 meV for the HgTe-CdTe valence-band discontinuity at 300 K. Similar analyses of current-voltage data taken at 190â300 K suggest that the valence-band offset decreases at low temperatures in this heterojunction
Calculation of pure dephasing for excitons in quantum dots
Pure dephasing of an exciton in a small quantum dot by optical and acoustic
phonons is calculated using the ``independent boson model''. Considering the
case of zero temperature the dephasing is shown to be only partial which
manifests itself in the polarization decaying to a finite value. Typical
dephasing times can be assigned even though the spectra exhibits strongly
non-Lorentzian line shapes. We show that the dephasing from LO phonon
scattering, occurs on a much larger time scale than that of dephasing due to
acoustic phonons which for low temperatures are also a more efficient dephasing
mechanism. The typical dephasing time is shown to strongly depend on the
quantum dot size whereas the electron phonon ``coupling strength'' and external
electric fields tend mostly to effect the residual coherence. The relevance of
the dephasing times for current quantum information processing implementation
schemes in quantum dots is discussed
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