111 research outputs found
Electron density effects in the modulation spectroscopy of strained and lattice-matched InGaAs/InAlAs/InP high-electron-mobility transistor structures
The effects of the channel electron density on the interband optical transitions of strained (x=0.6 and 0.65) and lattice-matched (x=0.53) InxGa1âxAs/In0.52Al0.48As/InP high-electron-mobility transistor structures have been investigated by phototransmittance at room temperature. Analysis of the ground and first excited transitions for low and high densities, respectively, enabled a separate estimation of the electron densities occupying each one of the first two subbands. It was found necessary to include the modulation of the phase-space filling in the analysis of the spectra, especially for the samples with a high electron density, in which case this modulation mechanism becomes dominant
Electricâfield dependence of interband transitions in In_(0.53)Ga_(0.47)As/In_(0.52)Al_(0.48)As single quantum wells by roomâtemperature electrotransmittance
Roomâtemperature electrotransmittance has been used in order to investigate the interband excitonic transitions in a 250âĂ
âthick In_(0.53)Ga_(0.47)As/In_(0.52)Al_(0.48)As singleâquantumâwell system as a function of an externally applied electric field. Parity forbidden transitions, involving conductionâband states with quantum numbers up to n=5, which become more pronounced at high electric fields were observed. The groundâstate and the forbidden transitions showed a significant red shift due to the quantum confined Stark effect. A comparison with previously reported results on thinner InGaAs/InAlAs quantum wells indicated that the wideâwell sample exhibits the largest shift, as expected from theory. Despite the appreciable Stark shift, the rather large, fieldâinduced linewidth broadening and the relatively low electric field at which the groundâstate exciton is ionized poses limitations on using this wideâquantumâwell system for electroâoptic applications
Exploiting the close-to-dirac point shift of the Fermi level in the Sb2Te3/Bi2Te3 topological insulator heterostructure for spin-charge conversion
Exploiting the close-to-Dirac point shift of Fermi level in Sb2Te3/Bi2Te3 topological insulator heterostructure for spin-charge conversion
Properly tuning the Fermi level position in topological insulators is of
vital importance to tailor their spin-polarized electronic transport and to
improve the efficiency of any functional device based on them. Here we report
the full in situ Metal Organic Chemical Vapor Deposition (MOCVD) and study of a
highly crystalline Bi2Te3/Sb2Te3 topological insulator heterostructure on top
of large area (4'') Si(111) substrates. The bottom Sb2Te3 layer serves as an
ideal seed layer for the growth of highly crystalline Bi2Te3 on top, also
inducing a remarkable shift of the Fermi level to place it very close to the
Dirac point, as visualized by angle-resolved photoemission spectroscopy. In
order to exploit such ideal topologically-protected surface states, we
fabricate the simple spin-charge converter Si(111)/Sb2Te3/Bi2Te3/Au/Co/Au and
spin-charge conversion (SCC) is probed by spin pumping ferromagnetic resonance.
A large SCC is measured at room temperature, which is interpreted within the
inverse Edelstein effect (IEE), thus resulting in a conversion efficiency
lambda_IEE of 0.44 nm. Our results demonstrate the successful tuning of the
surface Fermi level of Bi2Te3 when grown on top of Sb2Te3 with a full in situ
MOCVD process, which is highly interesting in view of its future technology
transfer.Comment: Main text: 19 pages, 6 figures. Supplementary information are also
included in the file with additional 4 page
Electron density effects in the modulation spectroscopy of strained and latticeâmatched InGaAs/InAlAs/InP highâelectronâmobility transistor structures
Re-entrance of the metallic conductance in a mesoscopic proximity superconductor
We present an experimental study of the diffusive transport in a normal metal
near a superconducting interface, showing the re-entrance of the metallic
conductance at very low temperature. This new mesoscopic regime comes in when
the thermal coherence length of the electron pairs exceeds the sample size.
This re-entrance is suppressed by a bias voltage given by the Thouless energy
and can be strongly enhanced by an Aharonov Bohm flux. Experimental results are
well described by the linearized quasiclassical theory.Comment: improved version submitted to Phys. Rev. lett., 4 pages, 5 included
epsf figure
Superconducting proximity effect in a mesoscopic ferromagnetic wire
We present an experimental study of the transport properties of a
ferromagnetic metallic wire (Co) in metallic contact with a superconductor
(Al). As the temperature is decreased below the Al superconducting transition,
the Co resistance exhibits a significant dependence on both temperature and
voltage. The differential resistance data show that the decay length for the
proximity effect is much larger than we would simply expect from the exchange
field of the ferromagnet.Comment: 4 pages, 6 included epsf figures, published version with small
change
Resistive transport in a mesoscopic proximity superconductor
We review transport measurements in a normal metal (N) in contact with one or
two superconducting (S) islands. From the experiment, we distinguish the
Josephson coupling, the mesoscopic fluctuations and the proximity effect. In a
loop-shaped N conductor, we observe large h/2e-periodic magnetoresistance
oscillations that decay with temperature T with a 1/T power-law. This behaviour
is the signature of the long-range coherence of the low-energy electron pairs
induced by the Andreev reflection at the S interface. At temperature and
voltage below the Thouless energy , we observe the re-entrance
of the metallic resistance. Experimental results agree with the linearized
quasiclassical theory.Comment: 8 pages, 6 included epsf figures, Invited paper at the LT21
Conference, Praha, August 1996. To appear in Czech. J. of Phys. 46, Part S6
(1996
Phase Dependent Thermopower in Andreev Interferometers
We report measurements of the thermopower S of mesoscopic Andreev
interferometers, which are hybrid loops with one arm fabricated from a
superconductor (Al), and one arm from a normal metal (Au). S depends on the
phase of electrons in the interferometer, oscillating as a function of magnetic
flux with a period of one flux quantum (= h/2e). The magnitude of S increases
as the temperature T is lowered, reaching a maximum around T = 0.14 K, and
decreases at lower temperatures. The symmetry of S oscillations with respect to
magnetic flux depends on the topology of the sample.Comment: 4 pages, 4 figure
Interband transitions in InxGa1-xAs/In0.52Al0.48As single quantum wells studied by room-temperature modulation spectroscopy
Room-temperature phototransmittance and electrotransmittance of single quantum wells of InxGa1-xAs with In0.52Al0.48As barriers have been used to study the excitonic interband transitions between the confined conduction- and valence-band states. Peak assignment has been confirmed by photocurrent spectroscopy. The lattice-matched (x=0.53) and strained (x=0.6) structures were considered for two different well widths of 50 and 250 Ă
. Transition energies and broadening parameters were measured from the spectra of the wide well samples and studied as a function of the principal quantum number. Reasonably good agreement between theory and experiment has been achieved by using published values of the electronic band-structure parameters. An observed monotonic increase of the linewidth with the quantum number has been associated with the presence of well-width fluctuations due to rough interfaces
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