Resonant plasmon-phonon coupling and its role in magneto-thermoelectricity in bismuth

Using diagrammatic methods we derive an effective interaction between a low energy collective movement of fermionic liquid (acoustic plasmon) and acoustic phonon. We show that the coupling between the plasmon and the lattice has a very non-trivial, resonant structure. When real and imaginary parts of the acoustic plasmon's velocity are of the same order as the phonon's velocity, the resonance qualitatively changes the nature of phonon-drag. In the following we study how magneto-thermoelectric properties are affected. Our result suggests that the novel mechanism of energy transfer between electron liquid and crystal lattice can be behind the huge Nernst effect in bismuth.Comment: accepted in EPJB, to appear with a highligh

Evidence for universality of tunable-barrier electron pumps

We review recent precision measurements on semiconductor tunable-barrier electron pumps operating in a ratchet mode. Seven studies on five different designs of pumps have reported measurements of the pump current with relative total uncertainties around 10-6 or less. Combined with theoretical models of electron capture by the pumps, these experimental data exhibits encouraging evidence that the pumps operate according to a universal mechanism, independent of the details of device design. Evidence for robustness of the pump current against changes in the control parameters is at a more preliminary stage, but also encouraging, with two studies reporting robustness of the pump current against three or more parameters in the range of ∼5 × 10-7 to ∼2 × 10-6. This review highlights the need for an agreed protocol for tuning the electron pump for optimal operation, as well as more rigorous evaluations of the robustness in a wide range of pump designs

Transporting and manipulating single electrons in surface-acoustic-wave minima

A surface acoustic wave (SAW) can produce a moving potential wave that can trap and drag electrons along with it. We review work on using a SAW to create moving quantum dots containing single electrons, with the aims of developing a current standard, emitting single photons, transferring single electrons between static quantum dots, and investigating non-adiabatic effects

Towards Single Electron Interferometry

There have been many studies and suggested technological applications using the two dimensional electron system in the GaAs/AlGaAs heterostructure. These have mostly focused on the behaviour of electrons propagating at or close to the Fermi Energy. More recently, one such application of this two dimensional system is for an electron pump, which isolates electrons from the two dimensional electron gas and pumps them individually at energies typically 100 meV above the Fermi energy, using surface gates to create a dynamic quantum dot. This energy regime had been previously unobtainable. We can utilise the high accuracy output of the pump - consistency that each pumped electron has the same properties, to study fundamental single particle physics, and work towards technological schemes, at this high energy. In this work we set out to continue and extend the previous work in this fi eld. We present new measurements that detail an electron detector barrier that we can use both as a sampling oscilloscope, with a bandwidth approaching 100 GHz, or to measure the wavepacket properties of electrons, including their energy and time of arrival with high resolution. After developing and establishing the electron detector, we detail a series of experiments that utilise it to measure the electron velocity, scattering mechanisms and wavepacket size. We show this work maps consistently to theory, and further, we begin to demonstrate control of the electron wavepacket, with the possibility that this hot electron system could have future technological applications. This is all put together in the construction of an interferometer, which seeks to complete our understanding of electrons in this system by measuring coherence of the wavefunction, a key step to demonstrating construction of a prescribed state

Laser-generated, plane-wave, broadband ultrasound sources for metrology

The accurate quantification of ultrasound fields generated by diagnostic and therapeutic transducers is critical for patient safety. This requires hydrophones calibrated to a traceable national measurement standard over the full range of frequencies used. At present, the upper calibration frequency range available to the user community is limited to a frequency of 60 MHz. However, there is often content at frequencies higher than this, e.g., through nonlinear propagation of high-amplitude pulses or tone-bursts for therapeutic applications, and the increasing use of higher frequencies in diagnostic imaging. To reduce the uncertainties and extend the calibrations to higher frequencies, a source of high-pressure, plane-wave and broadband ultrasound fields is required. This is not possible with current piezoelectric transducer technology, therefore laser-generated ultrasound is investigated as an alternative. This consists of an ultrasound wave generated by the pulsed laser excitation of a thin, planar, layer of light absorbing carbon-polymer nanocomposite materials. The work described in this thesis can be divided into three parts. The first part consisted of the fabrication of various nanocomposites in order to study the effect of different polymer types, composite thickness, laser fluence, and concentration of carbon nanotubes, on the ultrasound generated, as well as their stability. This included an investigation into the nonlinear propagation of MPa range laser-generated ultrasound, and the effect of the bandlimited hydrophone response, using a numerical wave solver (k-Wave). In the second part, the effects on the signal of acoustically reflective and matched backings (the substrates onto which the nanocomposite was coated) were studied. It was found experimentally that the backing material can significantly affect the pressure amplitude when the duration of the laser pulse is longer than the acoustic transit time across the thin nanocomposite layer. An analytical model was developed to describe how the signal generated depends on the backing material, absorbing layer thickness, and laser pulse duration. The model agreed well with measurements performed with a variable pulse duration fibre-laser. Finally, in the third part, a laser-generated, plane-wave, broadband ultrasound source device superficially resembling a standard piezoelectric piston source was designed, fabricated, and tested. The source produced quasi-unipolar pressure-pulse of 9 MPa peak-positive pressure with a bandwidth of 100 MHz, and the ultrasound beam is sufficiently planar to reduce uncertainties due to diffraction to negligible levels for hydrophones up to 0.6 mm in diameter

Aeronautical engineering: A special bibliography, supplement 45, June 1974

This special bibliography lists 430 reports, articles, and other documents introduced into the NASA scientific and technical information system in May 1974

An accurate high-speed single-electron quantum dot pump

Using standard microfabrication techniques, it is now possible to construct devices that appear to reliably manipulate electrons one at a time. These devices have potential use as building blocks in quantum computing devices, or as a standard of electrical current derived only from a frequency and the fundamental charge. To date, the error rate in semiconductor 'tuneable-barrier' pump devices, those which show most promise for high-frequency operation, have not been tested in detail. We present high-accuracy measurements of the current from an etched GaAs quantum dot pump, operated at zero source-drain bias voltage with a single ac-modulated gate at 340 MHz driving the pump cycle. By comparison with a reference current derived from primary standards, we show that the electron transfer accuracy is better than 15 parts per million. High-resolution studies of the dependence of the pump current on the quantum dot tuning parameters also reveal possible deviations from a model used to describe the pumping cycle

Amplitude, temperature, and frequency dependence of quantum pumps in semiconductor heterostructures

In the rapidly growing field of integrated quantum devices, two particular areas of interest are the development of an on-chip cryogenic current comparator (CCC) for completing the metrological triangle and the development of integrated de- vices for fast qubit operations. This thesis aims to significantly further our understanding of a quantum pump, a device integral to the CCC and potentially critical for realising fast qubit operations. A quantum pump is a device that transfers a discrete number of electrons between two electrically isolated regions when a potential barrier is cyclically oscillated. Initially, quantum pumps were single electron turnstile devices, which were limited in operational frequency by the Coulomb potential of the turnstile. Modern quantum pumps, utilising a dynamic quantum dot in a 2-dimensional electron gas (2DEG), are not limited by frequency. The fast operation of these modern pumps makes them very promising devices for accurately measuring the electron charge and performing fast qubit operations. In this study, we address the technical challenges of measuring a Al- GaAs/GaAs quantum pump and detail the processing and measurement setup. One of the challenges is rectified current swamping pump current. We develop a model for the rectified current and investigate ways to suppress it. We then show how the accuracy of a quantum pump changes as a function of amplitude, temperature, and frequency, and develop a model towards explaining the changes