449 research outputs found
Charge transport through weakly open one dimensional quantum wires
We consider resonant transmission through a finite-length quantum wire
connected to leads via finite transparency junctions. The coherent electron
transport is strongly modified by the Coulomb interaction. The low-temperature
current-voltage () curves show step-like dependence on the bias voltage
determined by the distance between the quantum levels inside the conductor, the
pattern being dependent on the ratio between the charging energy and level
spacing. If the system is tuned close to the resonance condition by the gate
voltage, the low-voltage curve is Ohmic. At large Coulomb energy and low
temperatures, the conductance is temperature-independent for any relationship
between temperature, level spacing, and coupling between the wire and the
leads
Current and noise expressions for radio-frequency single-electron transistors
We derive self-consistent expressions of current and noise for
single-electron transistors driven by time-dependent perturbations. We take
into account effects of the electrical environment, higher-order co-tunneling,
and time-dependent perturbations under the two-charged state approximation
using the Schwinger-Kedysh approach combined with the generating functional
technique. For a given generating functional, we derive exact expressions for
tunneling currents and noises and present the forms in terms of transport
coefficients. It is also shown that in the adiabatic limit our results
encompass previous formulas. In order to reveal effects missing in static
cases, we apply the derived results to simulate realized radio-frequency
single-electron transistor. It is found that photon-assisted tunneling affects
largely the performance of the single-electron transistor by enhancing both
responses to gate charges and current noises. On various tunneling resistances
and frequencies of microwaves, the dependence of the charge sensitivity is also
discussed.Comment: 18 pages, 9 figure
Ballistic Composite Fermions in Semiconductor Nanostructures
We report the results of two fundamental transport measurements at a Landau
level filling factor of 1/2. The well known ballistic electron transport
phenomena of quenching of the Hall effect in a mesoscopic cross-junction and
negative magnetoresistance of a constriction are observed close to B~=~0 and
. The experimental results demonstrate semi-classical charge
transport by composite fermions, which consist of electrons bound to an even
number of flux quanta.Comment: 9 pages TeX 3.1415 C version 6.1, 3 PostScript figure
Analysis of the temperature-dependent quantum point contact conductance in view of the metal-insulator transition in two dimensions
The temperature dependence of the conductance of a quantum point contact has
been measured. The conductance as a function of the Fermi energy shows
temperature-independent fixed points, located at roughly multiple integers of
. Around the first fixed point at e/h, the experimental data for
different temperatures can been scaled onto a single curve. For pure thermal
smearing of the conductance steps, a scaling parameter of one is expected. The
measured scaling parameter, however, is significantly larger than 1. The
deviations are interpreted as a signature of the potential landscape of the
quantum point contact, and of the source-drain bias voltage. We relate our
results phenomenologically to the metal-insulator transition in two dimensions.Comment: 5 pages, 3 figure
Sensitivity and back-action in charge qubit measurements by a strongly coupled single-electron transistor
We consider charge-qubit monitoring (continuous-in-time weak measurement) by
a single-electron transistor (SET) operating in the sequential-tunneling
regime. We show that commonly used master equations for this regime are not of
the Lindblad form that is necessary and sufficient for guaranteeing valid
physical states. In this paper we derive a Lindblad-form master equation and a
corresponding quantum trajectory model for continuous measurement of the charge
qubit by a SET. Our approach requires that the SET-qubit coupling be strong
compared to the SET tunnelling rates. We present an analysis of the quality of
the qubit measurement in this model (sensitivity versus back-action).
Typically, the strong coupling when the SET island is occupied causes
back-action on the qubit beyond the quantum back-action necessary for its
sensitivity, and hence the conditioned qubit state is mixed. However, in one
strongly coupled, asymmetric regime, the SET can approach the limit of an ideal
detector with an almost pure conditioned state. We also quantify the quality of
the SET using more traditional concepts such as the measurement time and
decoherence time, which we have generalized so as to treat the strongly
responding regime.Comment: About 11 pages, 6 figures. Changes in v2: we made general
improvements to the manuscript including, but not limited to(!), the removal
of one reference, and modification of the footnote
Time Dependent Current Oscillations Through a Quantum Dot
Time dependent phenomena associated to charge transport along a quantum dot
in the charge quantization regime is studied. Superimposed to the Coulomb
blockade behaviour the current has novel non-linear properties. Together with
static multistabilities in the negative resistance region of the I-V
characteristic curve, strong correlations at the dot give rise to
self-sustained current and charge oscillations. Their properties depend upon
the parameters of the quantum dot and the external applied voltages.Comment: 4 pages, 3 figures; to appear in PR
Multiple Projection Optical Diffusion Tomography with Plane Wave Illumination
We describe a new data collection scheme for optical diffusion tomography in
which plane wave illumination is combined with multiple projections in the slab
imaging geometry. Multiple projection measurements are performed by rotating
the slab around the sample. The advantage of the proposed method is that the
measured data can be much more easily fitted into the dynamic range of most
commonly used detectors. At the same time, multiple projections improve image
quality by mutually interchanging the depth and transverse directions, and the
scanned (detection) and integrated (illumination) surfaces. Inversion methods
are derived for image reconstructions with extremely large data sets. Numerical
simulations are performed for fixed and rotated slabs
Localization fom conductance in few-channel disordered wires
We study localization in two- and three channel quasi-1D systems using
multichain tight-binding Anderson models with nearest-neighbour interchain
hopping. In the three chain case we discuss both the case of free- and that of
periodic boundary conditions between the chains. The finite disordered wires
are connected to ideal leads and the localization length is defined from the
Landauer conductance in terms of the transmission coefficients matrix. The
transmission- and reflection amplitudes in properly defined quantum channels
are obtained from S-matrices constructed from transfer matrices in Bloch wave
bases for the various quasi-1D systems. Our exact analytic expressions for
localization lengths for weak disorder reduce to the Thouless expression for 1D
systems in the limit of vanishing interchain hopping. For weak interchain
hopping the localization length decreases with respect to the 1D value in all
three cases. In the three-channel cases it increases with interchain hopping
over restricted domains of large hopping
Periodic and Aperiodic Bunching in the Addition Spectra of Quantum Dot
We study electron addition spectra of quantum dots in a broad range of
electron occupancies starting from the first electron. Spectra for dots
containing <200 electrons reveal a surprising feature. Electron additions are
not evenly spaced in gate voltage. Rather, they group into bunches. With
increasing electron number the bunching evolves from occurring randomly to
periodically at about every fifth electron. The periodicity of the bunching and
features in electron tunneling rates suggest that the bunching is associated
with electron additions into spatially distinct regions within the dots.Comment: 4 pages, 2 figures. Submitted to PR
The antiangiogenic agent ZD4190 prevents tumour outgrowth in a model of minimal residual carcinoma in deep tissues
BACKGROUND: Tumour cells may persist at the operative site after seemingly adequate surgery. Radiotherapy is often given in an attempt to prevent repopulation, but this modality cannot be relied upon to prevent locoregional recurrence. An alternative strategy is to take advantage of the requirement of tumour cells to develop an independent blood supply and block this process to prevent recurrence. METHODS: In this study, we evaluate the effect of the angiogenesis inhibitor, ZD4190, using a rodent model of residual carcinoma in deep tissues, mimicking the clinical scenario where low numbers of malignant cells persist at the operative site. RESULTS: The tumour burden that could be eliminated was dependent on the site where the cells were implanted. Immediate treatment with ZD4190 prevented outgrowth of up to 2.5 x 10(5) cells in the rectus muscle and 1 x 10(5) in the gastrocnemius, whereas control animals developed large tumours. When more than 2.5 x 10(6) cells were implanted into the rectus or 1 x 10(6) into the gastrocnemius and treatment was maintained for 3 weeks, the carcinomas that developed in ZD4190-treated animals showed a reduced microvessel density and increased necrosis when compared with the vehicle-treated controls, but an infiltrative growth pattern was common. CONCLUSION: These findings suggest that antiangiogenic agents have a role to play in preventing outgrowth of residual carcinoma and are likely to be most effective when the tumour burden is minimal
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