1,373 research outputs found
Variation of the hopping exponent in disordered silicon MOSFETs
We observe a complex change in the hopping exponent value from 1/2 to 1/3 as
a function of disorder strength and electron density in a sodium-doped silicon
MOSFET. The disorder was varied by applying a gate voltage and thermally
drifting the ions to different positions in the oxide. The same gate was then
used at low temperature to modify the carrier concentration.
Magnetoconductivity measurements are compatible with a change in transport
mechanisms when either the disorder or the electron density is modified
suggesting a possible transition from a Mott insulator to an Anderson insulator
in these systems.Comment: 6 pages, 5 figure
A dynamic localization of 2D electrons at mesoscopic length scales
We have investigated the local magneto-transport in high-quality 2D electron
systems at low carrier densities. The positive magneto-resistance in
perpendicular magnetic field in the strongly insulating regime has been
measured to evaluate the spatial concentration of localized states within a
mesoscopic region of the samples. An independent measurement of the electron
density within the same region shows an unexpected correspondence between the
density of electrons in the metallic regime and that of the localized states in
the insulating phase. We have argued that this correspondence manifests a rigid
distribution of electrons at low densities.Comment: 8 pages (incl 4 figures), double colum
Possible zero-magnetic field fractional quantization in In0.75Ga0.25As heterostructures
In this Letter, we report a systematic study of a structure found in zero magnetic field at or near 0.2 ×(e2/h) in In0.75Ga0.25As heterostructures, where e is the fundamental unit of charge and h is Planck's constant. This structure has been observed in many samples and stays at near constant conductance despite a large range of external potential changes, the stability indicating a quantum state. We have also studied the structure in the presence of high in-plane magnetic fields and find an anisotropy which can be related to the Rashba spin–orbit interaction and agrees with a recent theory based on the formation of coherent back-scattering. A possible state with conductance at 0.25 ×(e2/h) has also been found. The quantum states described here will help with the fundamental understanding of low-dimensional electronic systems with strong spin–orbit coupling and may offer new perspectives for future applications in quantum information schemes
Influence of parallel magnetic fields on a single-layer two-dimensional electron system with a hopping mechanism of conductivity
Large positive (P) magnetoresistance (MR) has been observed in parallel
magnetic fields in a single 2D layer in a delta-doped GaAs/AlGaAs
heterostructure with a variable-range-hopping (VRH) mechanism of conductivity.
Effect of large PMR is accompanied in strong magnetic fields by a substantial
change in the character of the temperature dependence of the conductivity. This
implies that spins play an important role in 2D VRH conductivity because the
processes of orbital origin are not relevant to the observed effect. A possible
explanation involves hopping via double occupied states in the upper Hubbard
band, where the intra-state correlation of spins is important.Comment: 10 pages, 4 jpeg figure
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
Spin-incoherent transport in quantum wires
When a quantum wire is weakly confined, a conductance plateau appears at e(2)/h with decreasing carrier density in zero magnetic field accompanied by a gradual suppression of the 2e(2)/h plateau. Applying an in-plane magnetic field B-parallel to does not alter the value of this quantization; however, the e(2)/h plateau weakens with increasing B-parallel to up to 9 T, and then strengthens on further increasing B-parallel to, which also restores the 2e(2)/h plateau. Our results are consistent with spin-incoherent transport in a one-dimensional wire
Coulomb Charging Effects in an Open Quantum Dot
Low-temperature transport properties of a lateral quantum dot formed by
overlaying finger gates in a clean one-dimensional channel are investigated.
Continuous and periodic oscillations superimposed upon ballistic conductance
steps are observed, when the conductance G of the dot changes within a wide
range 0<G<6e^2/h. Calculations of the electrostatics confirm that the measured
periodic conductance oscillations correspond to successive change of the total
charge of the dot by . By modelling the transport it is shown that the
progression of the Coulomb oscillations into the region G>2e^2/h may be due to
suppression of inter-1D-subband scattering. Fully transmitted subbands
contribute to coherent background of conductance, while sequential tunneling
via weakly transmitted subbands leads to Coulomb charging of the dot.Comment: 12 pages, RevTeX, 15 eps figures included, submitted to Phys. Rev.
On the Zero-Bias Anomaly in Quantum Wires
Undoped GaAs/AlGaAs heterostructures have been used to fabricate quantum
wires in which the average impurity separation is greater than the device size.
We compare the behavior of the Zero-Bias Anomaly against predictions from Kondo
and spin polarization models. Both theories display shortcomings, the most
dramatic of which are the linear electron-density dependence of the Zero-Bias
Anomaly spin-splitting at fixed magnetic field B and the suppression of the
Zeeman effect at pinch-off
Experimental Evidence for Coulomb Charging Effects in an Open Quantum Dot at Zero Magnetic Field
We have measured the low-temperature transport properties of an open quantum
dot formed in a clean one-dimensional channel. For the first time, at zero
magnetic field, continuous and periodic oscillations superimposed upon
ballistic conductance steps are observed when the conductance through the dot
exceeds . We ascribe the observed conductance oscillations to
evidence for Coulomb charging effects in an open dot. This is supported by the
evolution of the oscillating features for as a function of both
temperature and barrier transparency. Our results strongly suggest that at zero
magnetic field, current theoretical and experimental understanding of Coulomb
charging effects overlooks charging in the presence of fully transmitted 1D
channels.Comment: To appear in Phys. Rev. Lett. 81 (Oct 19 issue
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