40 research outputs found
Momentum noise in a quantum point contact
Ballistic electrons flowing through a constriction can transfer momentum to
the lattice and excite a vibration of a free-standing conductor. We show (both
numerically and analytically) that the electromechanical noise power P does not
vanish on the plateaus of quantized conductance -- in contrast to the current
noise. The dependence of on the constriction width can be oscillatory or
stepwise, depending on the geometry. The stepwise increase amounts to an
approximate quantization of momentum noise.Comment: 4 pages including 4 figure
Nonlinear Transport in a Quantum Point Contact due to Soft Disorder Induced Coherent Mode Mixing
We show that the coherent mixing of different transverse modes, due to
forward scattering of carriers by soft impurity- or boundary potentials leads
to a nonlinear, asymmetric current response of quantum point contacts (QPC).
The oscillating contribution to the current is sensitive both to driving
voltage and to gate voltage in direct analogy to the electrostatic
Aharonov-Bohm effect.
Our calculations are in a good agreement with recent experimental data
showing small-scale conductivity nonlinearities and asymmetry in QPC.Comment: 4 pages, 2 figures (availiable upon request), REVTEX, Applied Physics
Report 93-4
Quantum energy flow in mesoscopic dielectric structures
We investigate the phononic energy transport properties of mesoscopic,
suspended dielectric wires. The Landauer formula for the thermal conductance is
derived and its universal aspects discussed. We then determine the variance of
the energy current in the presence of a steady state current flow. In the final
part, some initial results are presented concerning the nature of the
temperature fluctuations of a mesoscopic electron gas thermometer due to the
absorption and emission of wire phonons.Comment: 20 pages, 2 figures. Submitted to Phys. Rev.
Photoconductance Quantization in a Single-Photon Detector
We have made a single-photon detector that relies on photoconductive gain in
a narrow electron channel in an AlGaAs/GaAs 2-dimensional electron gas. Given
that the electron channel is 1-dimensional, the photo-induced conductance has
plateaus at multiples of the quantum conductance 2e/h. Super-imposed on
these broad conductance plateaus are many sharp, small, conductance steps
associated with single-photon absorption events that produce individual
photo-carriers. This type of photoconductive detector could measure a single
photon, while safely storing and protecting the spin degree of freedom of its
photo-carrier. This function is valuable for a quantum repeater that would
allow very long distance teleportation of quantum information.Comment: 4 pages, 4 figure
Electronic transport through domain walls in ferromagnetic nanowires: Co-existence of adiabatic and non-adiabatic spin dynamics
We study the effect of a domain wall on the electronic transport in
ferromagnetic quantum wires. Due to the transverse confinement, conduction
channels arise. In the presence of a domain wall, spin up and spin down
electrons in these channels become coupled. For very short domain walls or at
high longitudinal kinetic energy, this coupling is weak, leads to very few spin
flips, and a perturbative treatment is possible. For very long domain wall
structures, the spin follows adiabatically the local magnetization orientation,
suppressing the effect of the domain wall on the total transmission, but
reversing the spin of the electrons. In the intermediate regime, we numerically
investigate the spin-dependent transport behavior for different shapes of the
domain wall. We find that the knowledge of the precise shape of the domain wall
is not crucial for determining the qualitative behavior. For parameters
appropriate for experiments, electrons with low longitudinal energy are
transmitted adiabatically while the electrons at high longitudinal energy are
essentially unaffected by the domain wall. Taking this co-existence of
different regimes into account is important for the understanding of recent
experiments.Comment: 10 pages, 6 figure
Conductance of a Quantum Point Contact in the presence of a Scanning Probe Microscope Tip
Using the recursive Green's function technique, we study the coherent
electron conductance of a quantum point contact in the presence of a scanning
probe microscope tip. Images of the coherent fringe inside a quantum point
contact for different widths are obtained. It is found that the conductance of
a specific channel is reduced while other channels are not affected as long as
the tip is located at the positions correspending to that channel. Moreover,
the coherent fringe is smoothed out by increasing the temperature or the
voltage across the device. Our results are consistent with the experiments
reported by Topinka et al.[Science 289, 2323 (2000)].Comment: 5 page
A Current Induced Transition in atomic-sized contacts of metallic Alloys
We have measured conductance histograms of atomic point contacts made from
the noble-transition metal alloys CuNi, AgPd, and AuPt for a concentration
ratio of 1:1. For all alloys these histograms at low bias voltage (below 300
mV) resemble those of the noble metals whereas at high bias (above 300 mV) they
resemble those of the transition metals. We interpret this effect as a change
in the composition of the point contact with bias voltage. We discuss possible
explanations in terms of electromigration and differential diffusion induced by
current heating.Comment: 5 pages, 6 figure
Spin-dependent thermoelectric transport coefficients in near-perfect quantum wires
Thermoelectric transport coefficients are determined for semiconductor
quantum wires with weak thickness fluctuations. Such systems exhibit anomalies
in conductance near 1/4 and 3/4 of 2e^2/h on the rising edge to the first
conductance plateau, explained by singlet and triplet resonances of conducting
electrons with a single weakly bound electron in the wire [T. Rejec, A. Ramsak,
and J.H. Jefferson, Phys. Rev. B 62, 12985 (2000)]. We extend this work to
study the Seebeck thermopower coefficient and linear thermal conductance within
the framework of the Landauer-Buettiker formalism, which also exhibit anomalous
structures. These features are generic and robust, surviving to temperatures of
a few degrees. It is shown quantitatively how at elevated temperatures thermal
conductance progressively deviates from the Wiedemann-Franz law.Comment: To appear in Phys. Rev. B 2002; 3 figure
Bias and temperature dependence of the 0.7 conductance anomaly in Quantum Point Contacts
The 0.7 (2e^2/h) conductance anomaly is studied in strongly confined, etched
GaAs/GaAlAs quantum point contacts, by measuring the differential conductance
as a function of source-drain and gate bias as well as a function of
temperature. We investigate in detail how, for a given gate voltage, the
differential conductance depends on the finite bias voltage and find a
so-called self-gating effect, which we correct for. The 0.7 anomaly at zero
bias is found to evolve smoothly into a conductance plateau at 0.85 (2e^2/h) at
finite bias. Varying the gate voltage the transition between the 1.0 and the
0.85 (2e^2/h) plateaus occurs for definite bias voltages, which defines a gate
voltage dependent energy difference . This energy difference is
compared with the activation temperature T_a extracted from the experimentally
observed activated behavior of the 0.7 anomaly at low bias. We find \Delta =
k_B T_a which lends support to the idea that the conductance anomaly is due to
transmission through two conduction channels, of which the one with its subband
edge \Delta below the chemical potential becomes thermally depopulated as the
temperature is increased.Comment: 9 pages (RevTex) with 9 figures (some in low resolution
Mass Production of Silicon MOS-SETs: Can We Live with Nano-Devices’ Variability?
AbstractIt is very important to study variability of nanodevices because the inability to produce large amounts of identical nanostructures is eventually a bottleneck for any application. In fact variability is already a major concern for CMOS circuits. In this work we report on the variability of dozens of silicon single-electron transistors (SETs). At room temperature their variability is compared with the variability of the most advanced CMOS FET i.e. the ultra thin Silicon-on-Insulator Multiple gate FET (UT SOI MuGFET). We found that dopants diffused from Source –Drain into the edge of the undoped channel are the main source of variability. This emphasizes the role of extrinsic factors like the contact junctions for variability of any nanodevice