1,721,053 research outputs found
Optical and transport experiments in nano-structures with semiconductor superlattices
No full textI presented the recent results on the RRS, PLE, and PL in ordered and intentionally disordered semiconductor SL’s in the presence of an external magnetic field. From the RRS and PLE
spectra, we calculated and compared the behaviours of the energy dispersions of the dephasing times, relating them to the ordered and disordered nature of the SL’s. We also studied
the behaviour of the emission energy and linewidth with the magnetic field, comparing the results with the available results obtained on QW systems. As regards the dependence of the PL linewidth on the field, we found distinct difference in the evolution of the PL width between the ordered and
disordered SL’s. While in the ordered SL the PL linewidth increases monotonously with the field, in the disordered SL the evolution is not monotonous and this behaviour can be correlated to the interplay between localization length and exciton extension in the z direction along which the intentional disorder is present
Unusual behaviour of the conductance in Gaussian superlattices
No full textWe study the conductance at finite temperature of pass-band GaAs-Al(x)Ga(1−xAs) superlattices with Gaussian modulated Al mole fraction. Such structures present bands of almost unscattered electronic states and high peak-to-valley ratio in the j–V characteristic. We found a critical point, indicating the onset for the transition from the conducting to the nonconducting regime, by tuning the chemical potential in the vicinity of the band of unscattered states. Remarkably, the conductance of the Gaussian superlattice remains finite around the critical point even at zero temperature
Optical study of the strain effect in pseudomorphic In1-xGaxAs-InP heterostructures
No full texthotoreflectance (PR) and optical absorption (OA) have been used to study the energy-gap dependence on temperature in In1-xGaxAs thin layers, metalorganic vapor-phase epitaxy grown on InP substrates under nearly lattice-matched conditions (‖Δa/a0‖≤10-3). Both PR and OA spectra show a split structure in the band-gap energy region. Moreover, the energy-gap values at all temperatures, obtained by Franz-Keldysh-oscillations analysis of PR line shapes, are blueshifted with respect to the values of the perfectly lattice-matched alloy (x=0.472). The results are related to the valence-band splitting at k=0 generated by the biaxial strain in In1-xGaxAs, due to the small lattice mismatch. We calculated the splitting and shift values using the orbital-strain Hamiltonian and accounting for compositional effects. The values are in agreement with the experimental results giving optical evidence of coherent growth of the alloy film
Tunneling escape time of electrons from a quantum well confined by delta-like barriers
No full textAn analytic approach is presented for calculations of the electron tunneling escape time from a quantum well confined by ultrathin δ-like barriers. It is shown that the quasibound state decays exponentially and its lifetime depends strongly on well width and cladding barrier opacity
Modification of Fano resonances by resonant polaron coupling in bulk GaAs.
No full textStrong modifications of the magneto-excitonic Fano lineshapes have been observed in the photoluminescence-excitation spectra of bulk GaAs when the energies of the transitions are tuned by an external magnetic field. We identify the origin of these changes as resonant polaronic coupling between the electrons or holes, forming the excitons, with longitudinal-optical phonons, in conduction and valence bands respectively. The resonant polaron effect alters the magneto-excitonic discrete states, destroying the quantum interferences responsible for the Fano asymmetry
HRTEM, raman and optical study of CdS(1-x)Se(x) nanocrystals embebbed in silicate glass
No full textWe studied CdS(1-x)Se(x) nanocrystals embedded in a silicate glass by means of complementary techniques like high resolution transmission electron microscopy (HRTEM), micro-Raman spectroscopy and optical transmission and reflectivity. Transmission Electron Microscopy gives complete information on crystallization and size distribution of the nanocrystals wile Raman scattering is particularly useful in the determination of the composition of the nanocrystals for low-concentration or small-crystallite size composite. Having the size distribution and composition of the nanocrystals we have explained the transmission spectra of the studied samples. Optical transmission spectra evidence the quantization of the electronic states of the nanoparticles system with a size distribution described by a Gaussian function
Thermoreflectance study of the direct optical gap in epitaxial AlxGa1-xSb (x<=0.5)
No full textThermoreflectance spectroscopy was used to precisely determine the direct optical gap EgΓ, as a function of composition and temperature of a series of AlxGa1-xSb layers (0.0<~x<~0.5) epitaxially grown on GaSb. The experimental line shapes were fitted with a critical-point functional form including excitonic effects, to derive the direct gap and broadening parameter values. The relation between EgΓ and x shows a x-dependent bowing, which was compared with previous results and theoretical models, leading to the conclusion that EgΓ(x) curves in AlxGa1-xSb alloys have a cubic polynomial form
Percolation transitions in bilayer graphene encapsulated by hexagonal boron nitride
No full textWe studied the plateau-plateau transitions that characterize the electrical transport in the quantum Hall regime
in a high mobility bilayer graphene flake encapsulated by hexagonal boron nitride at magnetic fields up to 9 T and
temperatures above 300 mK. We measured independently the exponent κ of the temperature-induced transition
broadening, the critical exponent γ of the localization length, and the exponent p ruling the temperature scaling of
the coherence length, finding consistencywith the relation γ = p/2κ. The observed value of κ = 0.30(0.28,0.32)
deviates from that of the quantum Hall critical point. The obtained γ = 1.25(0.96,1.54) questions the validity of
a pure Anderson transition, and reveals percolation as the underlying driving mechanism
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