C. G. Smith

High-frequency single-electron transport in a quasi-one-dimensional GaAs channel induced by surface acoustic waves

We report on an experimental investigation of the direct current induced by transmitting a surface acoustic wave (SAW) with frequency 2.7 GHz through a quasi-one-dimensional (1D) channel defined in a GaAs - AlGaAs heterostructure by a split gate, when the SAW wavelength was approximately equal to the channel length. At low SAW power levels the current reveals oscillatory behaviour as a function of the gate voltage with maxima between the plateaux of quantized 1D conductance. At high SAW power levels, an acoustoelectric current was observed at gate voltages beyond pinch-off. In this region the current displays a step-like behaviour as a function of the gate voltage (or of the SAW power) with the magnitude corresponding to the transfer of one electron per SAW cycle. We interpret this as due to trapping of electrons in the moving SAW-induced potential minima with the number of electrons in each minimum being controlled by the electron - electron interactions. As the number of electrons is reduced, the classical Coulomb charging energy becomes the Mott - Hubbard gap between two electrons and finally the system becomes a sliding Mott insulator with one electron in each well.

SPIN-VALVE EFFECTS IN A SEMICONDUCTOR FIELD-EFFECT TRANSISTOR : A SPINTRONIC DEVICE

We present a spintronic semiconductor field-effect transistor. The injector and collector contacts of this device were made from magnetic permalloy thin films with different coercive fields so that they could be magnetized either parallel or antiparallel to each other in different applied magnetic fields. The conducting medium was a two-dimensional electron gas (2DEG) formed in an AlSb/InAs quantum well. Data from this device suggest that its resistance is controlled by two different types of spin-valve effect: the first occurring at the ferromagnet-2DEG interfaces; and the second occurring in direct propagation between contacts.

Effect of low numerical-aperture femtosecond two-photon absorption on (SU-8) resist for ultrahigh-aspect-ratio microstereolithography

We report the quantitative characterization and analysis on the solidification of SU-8, a chemically amplified near-ultraviolet ultrathick resist, based on two-photon-absorbed (TPA) near-infrared photopolymerization. The resolution of TPA photopolymerized SU-8 voxels and lines is studied as a function of laser-pulse energy, single-shot exposure time, and scanning speed. Two-photon microstereolithography using SU-8 as the matrix material was verified by the fabrication of SU-8 photoplastic structures with subdiffraction-limit resolution. We show that the nonlinear velocity dependence of TPA photopolymerization can be used as the shutter mechanism for disruptive three-dimensional (3D) lithography. This mechanism, when combined with low numerical-aperture optics is exploited for the rapid 3D microfabrication of ultrahigh-aspect-ratio (up to 50:1) photoplastic pillars, planes, and cage structures.

SU-8 for real three-dimensional subdiffraction-limit two-photon microfabrication

We report the inherent utility of two-photon-absorption (TPA) in the fabrication of real three-dimensional (3D) structures with subdiffraction-limit resolution, based on SU-8 as the threshold polymer media. We exploit the nonlinear velocity dependence of TPA photopolymerization as the shutter mechanism for disruptive 3D lithography. We show that low numerical aperture optics can be used for the rapid microfabrication of ultrahigh-aspect ratio photoplastic pillars, planes, and cage structures.

Magnetization reversal and magnetoresistance in a lateral spin-injection device

We have investigated the magnetization reversal and magnetoresistance (MR) behavior of a lateral spin-injection device. The device consists of a two-dimensional electron gas (2DEG) system in an InAs quantum well and two ferromagnetic (Ni80Fe20) contacts: an injector (source) and a detector (drain). Spin-polarized electrons are injected from the first contact and propagating through InAs are collected by the second contact. By engineering the shape of the permalloy film distinct switching fields (Hc) from the injector and the collector have been observed by scanning Kerr microscopy and MR measurements. Magneto-optic Kerr effect (MOKE) hysteresis loops demonstrate that there is a range of magnetic field (20–60 Oe), at room temperature, over which magnetization in one contact is aligned antiparallel to that in the other. The MOKE results are consistent with the variation of the magnetoresistance in the spin-injection device.

Simultaneous fabrication of nanogap gold electrodes by electroless gold plating using a common medical liquid

We report a simple and high yield method for fabricating multiple nanogaps simultaneously by an electroless gold plating technique using electroless gold plating solution which consists of common medical liquid of iodine tincture and L(+)-ascorbic acid (vitamin C). The distance between the gold electrodes (33nm in average) on the SiO2∕Si substrate was decreased by selective deposition of gold onto the surface of the gold electrodes. By electroless gold plating, we fabricated nanogaps below 5nm in width with a 41% process yield. We also demonstrated the Coulomb blockade effect in octanethiol(C8)-protected Au nanoparticles by using such a fabricated nanogap.

Imaging fractal conductance fluctuations and scarred wave functions in a quantum billiard.

We present scanning-probe images and magnetic-field plots which reveal fractal conductance fluctuations in a quantum billiard. The quantum billiard is drawn and tuned using erasable electrostatic lithography, where the scanning probe draws patterns of surface charge in the same environment used for measurements. A periodicity in magnetic field, which is observed in both the images and plots, suggests the presence of classical orbits. Subsequent high-pass filtered high-resolution images resemble the predicted probability density of scarred wave functions, which describe the classical orbits.

Single shot charge detection using a radio-frequency quantum point contact

We report on charge sensing measurements of a GaAs semiconductor quantum dot device using a radio frequency quantum point contact (rf-QPC). The rf-QPC is fully characterized at 4K and millikelvin temperatures and found to have a bandwidth exceeding 20MHz. For single shot charge sensing, we achieve a charge sensitivity of ∼2×10−4e∕Hz referred to the neighboring dot’s charge. The rf-QPC compares favorably with rf single electron transistor electrometers and promises to be an extremely useful tool for characterizing and measuring semiconductor quantum systems on fast time scales.

Gigahertz quantized charge pumping in graphene quantum dots

Single-electron pumps are set to revolutionize electrical metrology by enabling the ampere to be redefined in terms of the elementary charge of an electron. Pumps based on lithographically fixed tunnel barriers in mesoscopic metallic systems and normal/superconducting hybrid turnstiles can reach very small error rates, but only at megahertz pumping speeds that correspond to small currents of the order of picoamperes. Tunable barrier pumps in semiconductor structures are operated at gigahertz frequencies, but the theoretical treatment of the error rate is more complex and only approximate predictions are available. Here, we present a monolithic, fixed-barrier single-electron pump made entirely from graphene that performs at frequencies up to several gigahertz. Combined with the record-high accuracy of the quantum Hall effect and proximity-induced Josephson junctions, quantized-current generation brings an all-graphene closure of the quantum metrological triangle within reach. Envisaged applications for graphene charge pumps outside quantum metrology include single-photon generation via electron-hole recombination in electrostatically doped bilayer graphene reservoirs, single Dirac fermion emission in relativistic electron quantum optics and read-out of spin-based graphene qubits in quantum information processing.

Charge and Spin State Readout of a Double Quantum Dot Coupled to a Resonator

State readout is a key requirement for a quantum computer. For semiconductor-based qubit devices it is usually accomplished using a separate mesoscopic electrometer. Here we demonstrate a simple detection scheme in which a radio frequency resonant circuit coupled to a semiconductor double quantum dot is used to probe its charge and spin states. These results demonstrate a new noninvasive technique for measuring charge and spin states in quantum dot systems without requiring a separate mesoscopic detector.