58 research outputs found
Spectroscopy of the Potential Profile in a Ballistic Quantum Constriction
We present a theory for the nonlinear current-voltage characteristics of a
ballistic quantum constriction. Nonlinear features first develop because of
above-barrier reflection from the potential profile, created by impurities in
the vicinity of the constriction. The nonlinearity appears on a small voltage
scale and makes it possible to determine distances between impurities as well
as the magnitude of the impurity potentials.Comment: 3 pages, 4 figures (availiable upon request), REVTEX, Applied Physics
Report 93-5
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
Observation of supercurrent enhancement in SNS junctions by non-equilibrium injection into supercurrent carrying bound Andreev states
We report for the first time enhancement of the supercurrent by means of
injection in a mesoscopic three terminal planar SNSNS device made of Al on
GaAs. When a current is injected from one of the superconducting Al electrodes
at an injection bias , the DC Josephson current between the
other two superconducting electrodes has a maximum, giving evidence for an
enhancement due to a non-equilibrium injection into bound Andreev states of the
underlying semiconductor. The effect persists to temperatures where the
equilibrium supercurrent has vanished.Comment: 7 pages + 3 figures. Resubmitted to Phys. Rev. Lett. Contents change
Nanotextured Si Surfaces Derived From Block-copolymer Self-assembly With Superhydrophobic, Superhydrophilic, or Superamphiphobic Properties
We demonstrate the use of wafer-scale nanolithography based on block-copolymer (BCP) self-assembly for the fabrication of surfaces with enhanced wetting properties. All classes of wetting behaviour derived from the same BCP nanolithography step are demonstrated. An in situ etch mask is defined by self-assembly of polystyrene (PS) and dimethylsiloxane (PDMS) domains to form a predominantly hexagonal array with pitch size (72 ± 3) nm. The subsequent branched processing scheme, exclusively employing dry chemistry and reactive ion etching (RIE), allows the fabrication of nanoholes, nanopillars, or high aspect ratio nano-hoodoo features (overhang profile structures) with a diameter below 100 nm. The surfaces are finally functionalized with either hydrophobic surface chemistry by self-assembly from the precursor perfluorodecyltrichlorosilane (FDTS), or hydrophilic surface chemistry obtained by oxygen plasma treatment. The different texture and surface chemistry configurations are characterized with respect to their wetting properties with water, alkanes and organic oils. While, both nano-pillar and nano-hole surfaces feature excellent superhydrophobic properties with water contact angles (WCAs) exceeding 170° and roll-off angles below 5°, only the nano-pillar surfaces exhibit convincing superhydrophilicity with WCAs below 5°. The repellency of low surface tension liquids known as amphiphobicity is demonstrated for the nano-hoodoo surfaces
Magnetoresistance of a 2-dimensional electron gas in a random magnetic field
We report magnetoresistance measurements on a two-dimensional electron gas
(2DEG) made from a high mobility GaAs/AlGaAs heterostructure, where the
externally applied magnetic field was expelled from regions of the
semiconductor by means of superconducting lead grains randomly distributed on
the surface of the sample. A theoretical explanation in excellent agreement
with the experiment is given within the framework of the semiclassical
Boltzmann equation.Comment: REVTEX 3.0, 11 pages, 3 Postscript figures appended. The manuscript
can also be obtained from our World Wide Web server:
http://roemer.fys.ku.dk/randmag.ht
Weakly nonlinear quantum transport: an exactly solvable model
We have studied the weakly non-linear quantum transport properties of a
two-dimensional quantum wire which can be solved exactly. The non-linear
transport coefficients have been calculated and interesting physical properties
revealed. In particular we found that as the incoming electron energy
approaches a resonant point given by energy , where the transport is
characterized by a complete reflection, the second order non-linear conductance
changes its sign. This has interesting implications to the current-voltage
characteristics. We have also investigated the establishment of the gauge
invariance condition. We found that for systems with a finite scattering
region, correction terms to the theoretical formalism are needed to preserve
the gauge invariance. These corrections were derived analytically for this
model.Comment: 15 pages, LaTeX, submitted to Phys. Rev.
Fiber-Based, Injection-Molded Optofluidic Systems: Improvements in Assembly and Applications
We present a method to fabricate polymer optofluidic systems by means of injection molding that allow the insertion of standard optical fibers. The chip fabrication and assembly methods produce large numbers of robust optofluidic systems that can be easily assembled and disposed of, yet allow precise optical alignment and improve delivery of optical power. Using a multi-level chip fabrication process, complex channel designs with extremely vertical sidewalls, and dimensions that range from few tens of nanometers to hundreds of microns can be obtained. The technology has been used to align optical fibers in a quick and precise manner, with a lateral alignment accuracy of 2.7 ± 1.8 μm. We report the production, assembly methods, and the characterization of the resulting injection-molded chips for Lab-on-Chip (LoC) applications. We demonstrate the versatility of this technology by carrying out two types of experiments that benefit from the improved optical system: optical stretching of red blood cells (RBCs) and Raman spectroscopy of a solution loaded into a hollow core fiber. The advantages offered by the presented technology are intended to encourage the use of LoC technology for commercialization and educational purposes
Wigner Function Description of the A.C.-Transport Through a Two-Dimensional Quantum Point Contact
We have calculated the admittance of a two-dimensional quantum point contact
(QPC) using a novel variant of the Wigner distribution function (WDF)
formalism. In the semiclassical approximation, a Boltzman-like equation is
derived for the partial WDF describing both propagating and nonpropagating
electron modes in an effective potential generated by the adiabatic QPC. We
show that this quantum kinetic approach leads to the well-known stepwise
behavior of the real part of the admittance (the conductance), and of the
imaginary part of the admittance (the emittance), in agreement with the latest
results, which is determined by the number of propagating electron modes. It is
shown, that the emittance is sensitive to the geometry of the QPC, and can be
controlled by the gate voltage. We established that the emittance has
contributions corresponding to both quantum inductance and quantum capacitance.
Stepwise oscillations in the quantum inductance are determined by the harmonic
mean of the velocities for the propagating modes, whereas the quantum
capacitance is a significant mesoscopic manifestation of the non-propagating
(reflecting) modes.Comment: 23 pages (latex), 3 figure
Noise in a Quantum Point Contact due to a Fluctuating Impurity Configuration
We propose a theoretical model for the low-frequency noise observed in a
quantum point contact (QPC) electrostatically defined in the 2D electron gas at
a GaAs-AlGaAs interface. In such contacts electron scattering by soft impurity-
or boundary potentials coherently splits an incoming wave function between
different transverse modes. Interference between these modes have been
suggested to explain observed non-linearities in the QPC-conductance. In this
study we invoke the same mechanism and the time-dependent current due to soft
dynamical impurity scattering in order to analyze the low-frequency
(telegraph-like) noise which has been observed along with a nonlinear
conductance. For the simplified case of a channel with two extended (current
carrying) modes, a simple analytical formula for the noise intensity is
derived. Generally we have found qualitative similarities between the noise and
the square of the transconductance. Nevertheless, incidentally there may be
situations when noise is suppressed but transconductance enhanced.Comment: 9 revte
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