1,353 research outputs found
Distinguishing impurity concentrations in GaAs and AlGaAs, using very shallow undoped heterostructures
We demonstrate a method of making a very shallow, gateable, undoped
2-dimensional electron gas. We have developed a method of making very low
resistivity contacts to these structures and systematically studied the
evolution of the mobility as a function of the depth of the 2DEG (from 300nm to
30nm). We demonstrate a way of extracting quantitative information about the
background impurity concentration in GaAs and AlGaAs, the interface roughness
and the charge in the surface states from the data. This information is very
useful from the perspective of molecular beam epitaxy (MBE) growth. It is
difficult to fabricate such shallow high-mobility 2DEGs using modulation doping
due to the need to have a large enough spacer layer to reduce scattering and
switching noise from remote ionsied dopants.Comment: 4 pages, 5 eps figure
Fano effect and Kondo effect in quantum dots formed in strongly coupled quantum wells
We present lateral transport measurements on strongly, vertically coupled
quantum dots formed in separate quantum wells in a GaAs/AlGaAs heterostructure.
Coulomb oscillations are observed forming a honeycomb lattice consistent with
two strongly coupled dots. When the tunnel barriers in the upper well are
reduced we observe the Fano effect due to the interfering paths through a
resonant state in the lower well and a continuum state in the upper well. In
both regimes an in plane magnetic field reduces the coupling between the wells
when the magnetic length is comparable to the center to center separation of
the wells. We also observe the Kondo effect which allows the spin states of the
double dot system to be probed.Comment: 4 pages, 5 figure
Probing spin-charge separation in a Tomonaga-Luttinger liquid
In a one-dimensional (1D) system of interacting electrons, excitations of
spin and charge travel at different speeds, according to the theory of a
Tomonaga-Luttinger Liquid (TLL) at low energies. However, the clear observation
of this spin-charge separation is an ongoing challenge experimentally. We have
fabricated an electrostatically-gated 1D system in which we observe spin-charge
separation and also the predicted power-law suppression of tunnelling into the
1D system. The spin-charge separation persists even beyond the low-energy
regime where the TLL approximation should hold. TLL effects should therefore
also be important in similar, but shorter, electrostatically gated wires, where
interaction effects are being studied extensively worldwide.Comment: 11 pages, 4 PDF figures, uses scicite.sty, Science.bs
Effects of Zeeman spin splitting on the modular symmetry in the quantum Hall effect
Magnetic-field-induced phase transitions in the integer quantum Hall effect
are studied under the formation of paired Landau bands arising from Zeeman spin
splitting. By investigating features of modular symmetry, we showed that
modifications to the particle-hole transformation should be considered under
the coupling between the paired Landau bands. Our study indicates that such a
transformation should be modified either when the Zeeman gap is much smaller
than the cyclotron gap, or when these two gaps are comparable.Comment: 8 pages, 4 figure
Structural, electrical, and optical characterization of as grown and oxidized zinc nitride thin films
Zinc Nitride (Zn3N2) films were grown by DC sputtering of a Zn target in a N2 plasma under a
variety of different growth conditions, which resulted in the deposition of films with variable
compositions. The as deposited films exhibited a polycrystalline Zn3N2 structure, which was
converted to a ZnO-based structure after several weeks of ambient exposure. Zn3N2 films that were
N-poor exhibited electrical properties indicative of a natively doped semiconductor and reached a
minimum carrier concentration in the order of 1018 cm3 at compositions, which approached the stoichiometric
ratio of Zn3N2. A maximum carrier mobility of 33 cm2 V1 s
1 was obtained in N-rich
films due to an improved microstructure. The Zn3N2 films had an optical band gap of 1.31–1.48 eV
and a refractive index of 2.3–2.7. Despite a wide range of Zn3N2 samples examined, little variation of
its optical properties was observed, which suggests that they are closely related to the band structure
of this material. In contrast to the as grown films, the oxidized film had a band gap of 3.44 eV and the
refractive index was 1.6–1.8, similar to ZnO and Zn(OH)2
Chromosomal copy number variation, selection and uneven rates of recombination reveal cryptic genome diversity linked to pathogenicity
This project was funded by the UK Natural Environmental Research Council (NERC) grant NE/E006701/1, the European Research Council (ERC) grant 260801-BIG_IDEA, the Swiss National Science Foundation grant 31-125099 and the Biodiversa project RACE: Risk Assessment of Chytridiomycosis to European Amphibian Biodiversity (http://www.bd-maps.eu). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.Peer reviewedPublisher PD
Incipient Formation of an Electron Lattice in a Weakly Confined Quantum Wire
We study the low-temperature transport properties of 1D quantum wires as the confinement strength V-conf and the carrier density n(1D) are varied using a combination of split gates and a top gate in GaAs/AlGaAs heterostructures. At intermediate V-conf and n(1D), we observe a jump in conductance to 4e(2)/h, suggesting a double wire. On further reducing n(1D), plateau at 2e(2)/h returns. Our results show beginnings of the formation of an electron lattice in an interacting quasi-1D quantum wire. In the presence of an in-plane magnetic field, mixing of spin-aligned levels of the two wires gives rise to more complex states
Unusual conductance collapse in one-dimensional quantum structures
We report an unusual insulating state in one-dimensional quantum wires with a
non-uniform confinement potential. The wires consist of a series of closely
spaced split gates in high mobility GaAs/AlGaAs heterostructures. At certain
combinations of wire widths, the conductance abruptly drops over three orders
of magnitude, to zero on a linear scale. Two types of collapse are observed,
one occurring in multi-subband wires in zero magnetic field and one in single
subband wires in an in-plane field. The conductance of the wire in the collapse
region is thermally activated with an energy of the order of 1 K. At low
temperatures, the conductance shows a steep rise beyond a threshold DC
source-drain voltage of order 1 mV, indicative of a gap in the density of
states. Magnetic depopulation measurements show a decrease in the carrier
density with lowering temperature. We discuss these results in the context of
many-body effects such as charge density waves and Wigner crystallization in
quantum wires.Comment: 5 pages, 5 eps figures, revte
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