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PhD projects in the Semiconductor Physics Group

Last updated 16/2/2016.

Professor DA Ritchie, Prof CHW Barnes, Dr HE Beere, Professor CJB Ford and Professor CG Smith.

See here for web pages for some individual projects.

Details of graduate admissions for the whole Physics Department are available at http://www.phy.cam.ac.uk/admissions/graduate/.

Introduction

The Semiconductor Physics Group investigates fundamental quantum transport phenomena using advanced and often unique, semiconductor nanostructures. The fabrication of devices also gives rise to challenging projects in applied physics and technology.

The group has extensive clean-room facilities with all necessary lithographic and deposition facilities for advanced semiconductor device fabrication. These include  state-of-the-art molecular beam epitaxy and electron beam lithography systems. The group possesses a wide range of measurement facilities including several dilution refrigerators for experiments down to 20 mK in high magnetic fields, and a range of optical and microwave equipment.

An Optical Single-Electron Spin Detector

(Prof CJB Ford)

For the use of electron spins in quantum information processing (QIP), we need to detect the spin state (up/down in some basis). This project will develop a spin detection technique for travelling electron wave packets, which would enable the use of electron wave packets as flying qubits interconnecting stationary qubit elements, as well the generation of entangled pairs. Spin measurements will use using a lateral p-n junction LED technology developed jointly by the Semiconductor Physics Group at the Cavendish Laboratory in Cambridge and by NPL. Electrons from a single-electron source will be injected into a p-type region, and the polarisation of emitted photons will be measured. Experiments will be performed both in Cambridge and at NPL.

This project is a collaboration with the National Physical Laboratory and is supported by an industrial CASE award.

Physics of low-dimensional electron systems at ultra-low temperatures (< 1 mK)

(Prof D A Ritchie)

Rapid progress in semiconductor device fabrication capabilities have enabled studies of electron systems confined in reduced dimensions, revealing intriguing phenomena including the integer and fractional quantum Hall effects in 2D, conductance quantisation in 1D, and artificial atom behaviour in 0D quantum dots. To observe these effects, devices must be cooled to suppress thermal noise, typically using dilution refrigerators. Although these fridges can cool the semiconductor lattice to below 10 mK, it is very challenging to thermalize electrons at this temperature. The Semiconductor Physics Group are working with Royal Holloway University of London (RHUL) to develop a platform for cooling electron systems below 1 mK, and use it to perform experiments that will address long-standing questions about the nature of these systems in the zero-temperature limit and perhaps lead to the discovery of new electronic ground states.  This project involves fabrication of semiconductor nano-devices compatible with the refrigeration system and studying them at ultra-low temperature. Physical phenomena to be studied include fragile fractional quantum Hall states and the 2D metal-to-insulator transition.

 

Many-body effects in one-dimensional wires — Measuring spin-charge separation in a Luttinger Liquid

(Prof CJB Ford)

A truly one-dimensional quantum wire allows electrons only to move along the wire, so it is not a Fermi liquid, because the electrons repel each other and cannot "overtake". To calculate this is impossible without making one approximation. This model system is called a Tomonaga-Luttinger (TL) liquid. The calculation makes a very surprising prediction: that there are two ways in which a wave can be excited at a given energy. One wave corresponds to the charge of the electron, and the other is related to the electron's spin. Spin and charge waves are expected to travel at different speeds along the wire.

To test this prediction we place a 1D wire very close to a 2D electron system, so that electrons can tunnel from one to the other. Energy and momentum must be conserved, so the current that flows between the two depends on the energies and speeds of the waves in the two systems. Applying a magnetic field gives electrons extra momentum as they tunnel, and a voltage between the two systems gives them extra energy. Thus one system can be used to map out the excitations of the other. Our experiment showed separate lines corresponding to spin and charge excitations (Jompol et al., Science 325, 597 (2009)). The work is described here.

A new student would study these interaction effects in more detail, exploring new regimes where the Luttinger-liquid approximation breaks down. The project involves semiconductor device fabrication, low-noise electrical measurements at T < 100 mK, and interaction with theorists.

Quantum computation with surface acoustic waves

(Prof CJB Ford and Prof CHW Barnes)

We have proposed a solid-state quantum computer based on the transport of individual electrons in the minima of a surface acoustic wave in a set of parallel narrow channels. These projects aim to construct some of the building blocks of such a computer, namely a number of quantum dots travelling along adjacent channels, together with electrostatic gates and nanoscale magnetic structures to initialise, control, entangle and read out the spins of the electrons. This should reveal a considerable amount of interesting physics as well as establishing the feasibility of such a quantum-computational scheme. A cryogenic scanning optical microscope has been constructed to detect polarised photons emitted from spin-polarised electrons recombining with holes, and this will be used for spin readout and development of a single-photon source. See here for more details of our recent work, or R. P. G. McNeil et al., Nature 477, 439 (2011).

Creating and exploring non-Abelian states of matter

(Prof D A Ritchie)

The fractional quantum Hall effect, where the Hall resistance of a two-dimensional system in a magnetic field becomes quantised at values which are fractional, as well as integer, multiples (called the filling factor, nu) of h/e2, is generally explained by composite Fermion theory. However, a small minority of FQH states have remained stubbornly difficult to explain. The leading theory for one such state, nu = 5/2, predicts a new, not yet observed subset of quasiparticles that obey non-Abelian statistics (i.e. their interchange operator is non-commutative). This exotic property has sparked huge interest, not least as it could be used to produce quantum computer qubits that are particularly resilient to errors (e.g. due to spin decoherence).

The few FQH states possibly obeying non-Abelian statistics have been difficult to observe, due to their sensitivity to disorder caused by the intentional dopants in conventional semiconductors. Using novel techniques, uniquely developed in our group, for fabricating undoped two-dimensional electron systems, these exotic FQH states should be easier to observe. We aim to perform single-electron transistor and/or interference-based measurements to reveal the signature of non-Abelian physics in exotic FQH states. This project involves fabrication of devices and low-temperature electronic measurements.

For a topical review, read Ady Stern, Nature 464, 187 (2010):    Full-text (requires subscription)    Abstract (public access)

Spin and interaction effects in a quantum antidot in the quantum Hall regime

(Prof CJB Ford)

The two-dimensional electron gas formed in a GaAs-AlGaAs heterostructure is a very fruitful system for investigating quantum and interaction effects. At high magnetic fields B, a series of “edge states” forms, with current travelling in opposite directions along the two sides of the sample. If a potential hill is formed in the centre of the sample, edge states may be backscattered to the opposite edge, via circulating edge states that form around this “antidot”. Thus the resistance of the sample can be used to measure the properties of the antidot. The circulating states are quantised, each enclosing one more unit of flux h/e than its neighbour, and so the resistance oscillates with B, with flux period h/e (the Aharonov-Bohm effect). We have shown that the magnetic confinement of electrons around the antidot causes it to behave much like a quantum dot, showing single-electron effects (Coulomb blockade). Very recently, we have injected electrons of up or down spin, and found, surprisingly, that the antidot states are mixed by interactions such that spin and charge excitations have very different energies. These results leave many questions unanswered, and there is now plenty of scope to investigate many-body, spin and charging effects in more detail and in other regimes. With our high-resolution lithography we will also be able to combine antidots, charge detectors and spin filters with ease.

Semiconductor electron-hole bilayers: the search for exciton superfluidity

(Prof DA Ritchie)

Two-dimensional electron-hole (e-h) bilayer systems, where electrons and holes are prevented from recombining by an insulating barrier but are close enough together to interact via Coulomb attraction, are predicted to form a superfluid coherent state of electron-hole pairs (excitons) for sufficiently narrow barriers, low densities and low temperatures. Such predictions have generated intense theoretical studies, but in experiments it has been difficult to fabricate real devices that can reach the exciton regime and can be probed electrically to confirm the existence of superfluidity. Previous work in the Semiconductor Physics Group has demonstrated 2D e-h bilayers in GaAs/AlGaAs devices that are approaching the strongly interacting regime. This project involves further development of these devices and measuring them at low temperature (< 1 K) to study the effects of interactions and look for phase transitions. The project is mostly experimental, involving semiconductor device fabrication, cryogenic techniques and challenging electrical measurements. However, there is also considerable scope for numerical and theoretical work if this is of interested to the student pursuing the project.

The development and applications of terahertz sources

(Prof DA Ritchie, Dr HE Beere)

We are developing and exploiting high-power sources of coherent terahertz (1 THz = 1012 Hz) radiation. Such sources are potentially of considerable importance for imaging (e.g. in the medical, dental, and pharmaceutical industries) and spectroscopy (e.g. for environmental monitoring, or screening of biological fluids such as blood). Until recently, this significant region of the electromagnetic spectrum has not been exploited owing to the severely limited number of sources previously available, which were bulky, expensive, inefficient, frequently incoherent, and not at all suited to the potential applications. Research topics include: pulsed and continuous (cw) THz generation; mechanisms for visible-to-THz conversion; THz quantum cascade lasers; the fabrication of photonic-band-gap crystals; interactions of THz radiation with organic and inorganic materials. In particular it has been shown that pharmaceutical products can be non-invasively investigated and in the terahertz regime clear differences between healthy and cancerous tissue is observed. CASE awards with TeraView Limited may be available.

The growth of semiconductor nanowires and their incorporation into novel device structures.

(Prof DA Ritchie)

Recent work has established the possibility of growing self-assembled nanowire structures by molecular beam epitaxy, with a core and outer shell of different materials. This project would involve developing growth techniques for structures of this type as well as using electron microscopy, electrical and optical measurements to characterise the wires. Ultimately the aim is to incorporate these wires into novel device structures for a range of applications.

Single and entangled pair photon sources fabricated from InAs quantum dots

(Prof DA Ritchie)

When InAs, is grown on GaAs by molecular beam epitaxy, past a critical thickness, in this case around 1.6 monolayers, the InAs layer self-assembles into quantum dots typically of diameter 25 nm and height 8 nm. InAs dots form near ideal optical emitters with a very narrow linewidth and we are using them to fabricate novel sources emitting over a range of wavelengths. These sources, driven by either optical or electrical excitation have applications in quantum computing and quantum key distribution systems. This work is carried out in a close collaboration with the quantum information group of Toshiba Research Europe Ltd based at the Cambridge Science Park, CASE awards may be available for projects in this area.

Quantum computation using variable g-value systems

(Professor DA Ritchie, Prof CHW Barnes)

As part of a collaborative project with Toshiba Research Europe we have developed devices where by varying the composition the Lande g-value, which determines the spin splitting, will vary with the position of the electron. If coupled dots are formed then the location of the electrons will be determined by the nature of the hybrid state and this can be read off by a “detector” which is a device sensitive to a charge of less than one electron. This opens the way to a number of concepts for quantum computation, as well as “spintronics” in which the transport properties depend on the spin.

Wave-function manipulation and Schrödinger-cat states in moving quantum dots

(Prof CHW Barnes and Prof CJB Ford)

When a quantum box suddenly changes size, an electron in the ground state finds itself in a superposition of excited states of the new system. To cause a fast enough change (in tens of picoseconds) we drag an electron along a narrow channel in a minimum of a surface acoustic wave. Where the channel width changes abruptly, excitation occurs, and the electron’s wave-function starts to evolve in time, oscillating from side to side. This can be detected by measuring oscillations in the small current that tunnels out of a thin barrier forming one side of the channel. This project aims to tailor the channel shape to create particular excited states called coherent states and to detect their evolution. Controlling interactions with a few other electrons in a neighbouring channel will help us try to answer the fundamental question about how the act of measurement causes a wave-function to collapse. Superposition (Schrödinger-cat) states will also be created and measured. Then pairs of electrons will be entangled, separated and measured, giving further insights into the fundamentals of quantum measurement.

Engineering of long quantum coherence times in double quantum dots for possible quantum computation applications

(Prof CG Smith)

This project aims at building on our development of a single electron detector capable of measuring the movement of single electrons in sub-micron semiconductor devices. Recently we have been able to start to look at the time dependent evolution of long lived quantum states within a four quantum dot QCA. In this project we want to continue these measurements at high frequencies where the measurement process is on a time scale comparable to the coherence time for the quantum state. This will allow us to explore the fundamental quantum limits of measurement for both charge and spin dephasing.

Microwave-driven transitions in two coupled semiconductor charge qubits

Petersson K. D., Smith C. G., Anderson D., Atkinson P., Jones G. A. C. and Ritchie D. A., 2009, Phys. Rev. Lett., 103, 016805

Imaging of quantum states in coupled quantum dots defined in agraphene layer using a scanning probe device

(Prof CG Smith)

This project involves using a low temperature scanning force microscope with a conducting tip to modify the transport properties of a sub-micron grapheme device.  Graphene (a single layer of graphite) is a gapless two dimensional semiconductor with a high mobility.  It is predicted that the edges of graphite are spin active.  We will create double quantum dots set up in the spin blockade regime to investigate this. In addition over the last few years we have developed a low temperature scanning probe instrument with 50nm resolution that is able to image the probability distributions of wavefunctions in a 1-D semiconductor device at temperatures below 100mK. We use the conducting tip of the AFM as a movable scatterer, which is able to modify the electron transport in a semiconductor structure. We can use this device to investigate surface states in graphene, and to image individual percolation paths in heavily disordered devices.

Pauli Spin Blockade in Carbon Nanotube Double Quantum Dots

Buitelaar M. R., Fransson J., Cantone A. L., Smith C. G., Anderson D., , Ardavan A., Khlobystov A. N., Morley G. W., Porfyrakis K. and Briggs G. A. D.,
2008, Phys. Rev.B., 77, 245439.

"Conductance quantization at a half-integer plateau in a symmetric GaAs quantum wire"

Crook R., Prance J., Thomas K. J., Chorley S. J., Farrer I., Ritchie D. A., Pepper M. and Smith C. G., 2006, Science, 312, 1359–1362.

A scanning capacitance probe to measure polarisation of charge in molecules

(Prof CG Smith)

We have recently been able to measure the change in capacitance that occurs when two states in two quantum dots come into alignment.  At this point electrons can tunnel from one dot to the other.  This measurement was performed using a resonant circuit connected to one end of the double dot pair resonating at 400 MHz.  This measurement can be made when no electron transport is possible through the pair of dots.  We will extend this measurement to our low temperature scanning probe.  This will allow us to measure capacitance changes with nm resolution and MHz band width, which will allow us to investigate polarization in dimmer molecule and thus identify molecules that could be used for quantum computing applications. 

"Gated-charge force microscopy for imaging a surface-acoustic-wave-induced charge in a depleted one-dimensional channel"

Crook R., Schneble R. J., Kataoka M., Beere H. E., Ritchie D. A., Anderson D., Jones G. A. C., Smith C. G., Ford C. J. B. and Barnes C. H. W., 2008, Phys. Rev. B, 78, 125330.

Reducing Charge dephasing in coupled quantum dots for quantum computing applications.

(Prof CG Smith)

Coupled quantum dots can be used as qubits for quantum computing applications.  Both the spin and charge of the electron can be used for this purpose, and the spin life time has been shown to be longer in many systems.  However spin quantum computing requires coupling of spin states in different dots via the exchange interaction.  During this coupling the spin state is sensitive to charge dephasing.  The dominant charge dephasing mechanism is through electron phonon interactions.  This project will look at ways of controlling this by using free-standing membranes where the phonon states are quantuized.  This allows the dephasing to be controlled.

"Single electron transport in a free-standing quantum dot"

Chorley S. J., Smith C. G., Perez-Martinez F., Prance J. R., Atkinson P., Ritchie D. A. and Jones G. A. C.,
2008, Microelectronics Journal, 39 No 3-4, 314–317

Cooling electrons below the lattice temperature.

(Prof CG Smith)

We have recently demonstrated that two quantum dots can be used as energy filters at the entrance and exit of a ten microns square region of two dimensional electron gas grown in a semiconductor hetersostructure.  By setting up the energy filters appropriately we can drive a current through the central regions that injects cold electrons and remove hot electrons.  The resulting cooling will be studied using a third quantum dot, the occupancy of which can be monitored by a near by 1-D channel.   We will look at investigating cooling the electrons from 100mK to 10mK  using this technique.  Cooling quantum systems is a very important way of reducing dephasing and enhancing interaction phenomena. 

"Electronic refrigeration of a two-dimensional electron gas"

Prance J. R., Smith C. G., Griffiths J. P., Chorley S. J., Anderson D., Jones G. A. C., Farrer I. and Ritchie D. A.,2008, Phys. Rev. Lett., 102, 146602.

Pumping particles using microfluidic techniques for display applications

(Prof CG Smith)

Recently we have developed a new technique for pumping fluids using patterned electrodes and low voltage ac fields. The technique allows fluids to be pumped along  the surface and so enables plug flow to be demonstrated.  This technique has applications in the growing area of microanalysis systems where small quantities of chemical reagents are brought together in a small reaction chamber.  The project will involve investigating the use of this pumping technique to move reflective particles at each pixel to construct a reflective display.

Low-voltage plug-flow pumping using anisotropic electrode arrays

Mpholo M., Smith C. G. and Brown A. B. D., 2003, Sensors and Actuators B: Chemical, 92, 262–268.