Quantum Systems and Nanomaterials

Quantum transport

This theme embraces classical and quantum mechanical aspects of charge transport in nanostructures. This includes quantum interference, the quantum Hall effect and quantum confinement. Novel electronic devices are devised, to study conduction and the interplay between different scattering mechanisms.

Quantum transport, noise and interactions in nanomaterials

• Research lead: Dr David Horsell
• Research Fellows: Liangxu Lin, Alexis Perry, Mark Heath
• PhD Students: Sneha Eashwer-Singhraj, Denis Nikiforov, Lauren Denton, Tobias Bachmann
• MPhys Students: Jack Bartlett, Oliver Pakenham-Walsh
• Alumni: Adam Price, Aleksey Kozikov, Alexey Kaverzin, Sam Hornett, Lily Battershill
• Funding: EPSRC, TSB, Royal Society

Our experimental lab investigates the fundamental physics that lies at the heart of the physical properties of nanomaterials.

The main focus of our research is the scattering processes that control the electrical conduction, which we probe with a number of techniques including weak localisation, mesoscopic fluctuations, flicker noise, non-linear conductance and electrochemistry. Though our work is predominantly fundamental, we also investigate the underlying physics of sensing technologies.

Graphene materials and nanodevices

• Academic lead: Professor Saverio Russo
• Research Fellows: Liping Lu, Laureline Mahe, Iddo Amit, Adolfo De Sanctis
• PhD students: Gareth Jones, Selim Unal, Nicola Townsend, Jake Mhewe, Saad Rhamadan

The physics of future electronics is largely driven by the needs for reducing the size, enhancing the performances and combining novel functionalities in a single electrical device.

The science of systems which are just one or few hundreds of atoms (i.e. nanoscale systems) differs significantly from that of macroscopic devices as it thrives primarily on the fundamental laws of quantum mechanics. This is leading to the discovery of a new realm of physical properties.

Our research group is pioneering the novel science found in nano-systems. In particular, we are currently studying the electrical properties of graphene materials, which are just one or few carbon atom thick with honeycomb structure. In these materials charge carriers have a record high mobility at room temperature and behave as massless Dirac fermions. Here are our main research directions.

Current projects:

• Whole graphene electronics: We are developing graphene-based flexible and transparent electronic devices. This is done by direct writing circuits in an insulating sheet of chemically functionalized graphene with fluorine adatoms with an electron beam.
• Graphene nano-devices: We are exploring novel technologies for fabricating suspended and double gated graphene transistors to access the electric field tuneable low-energy band structure in few-layer graphene and the electro-mechanical properties.
• Search for highly conductive and transparent materials: We have developed GraphExeter, which is the best known transparent material able to conduct electricity and it is a suitable replacement for Indium Tin Oxide (ITO) commonly used by display industries. This material is obtained by introducing molecules which act as dopants between the planes of few-layer graphene.
• Superconducting-graphene hybrid structures: We are exploring the feasibility of solid state entangler devices based on hybrid structures of graphene and superconducting materials.

Thermodynamic properties

This theme investigates the strong electron-electron interactions and collective electronic and phononic excitations that occur in confined systems. These are studied through measurements of magnetisation, Raman scattering and photoluminescence.

Magneto-Ramen of graphene

• Academic lead: Dr Annette Plaut
• Group: Kevin Luong, Paul Medal, Guy Norris, Charles Warren

We are presently in the process of setting up a Raman spectroscopy facility in order to investigate graphene using inelastic light-scattering.

The optical spectroscopy laboratory consists of 5+ lasers of various wavelengths ranging from the UV to the near infra-red. We also have the capability of changing the temperature of our samples from room temperature down to 300 mK, in magnetic fields up to 9 T.

Raman spectroscopy is an optical technique whereby one can measure the energy of excitations, e.g. phonons, in a system. Normally one wishes to map how the energy of these excitations evolve as a function of some parameter, such as the incoming laser photon energy. In this way we can better understand the properties of the material which can lead to its exploitation in devices.

Graphene (a single layer of graphite) is a new 2D system which is now attracting enormous attention because of its intriguing properties and potential applications in carbon-based electronics. It is a zero-gap semiconductor and its charge carriers have zero effective mass and therefore move relativistically. This makes the study of relativistic quantum mechanics, normally a rather exotic activity, potentially accessible using the experimental techniques available in our laboratory.

New states of matter in semiconductors

• Academic lead: Dr Alan Usher
• PhD students: Chris Downs

We explore novel quantum phenomena in nanostructures that occur at low temperatures and high magnetic fields.

Nanostructures, systems having at least one dimension in the range 1 to 100 nm, provide a versatile test-bed for studying quantum phenomena, and also underpin many of the recent technological advanced that enhance our everyday lives. Because of their size, the properties of electrons in such structures are governed by quantum physics: the electrons behave like waves, quantum tunnelling can occur and interference and coherence result in exotic many-body effects.

We have studied several systems:

• Two-dimensional systems in semiconductors: These are atomic-scale sandwiches of different compound semiconductors, for instance gallium arsenide and aluminium gallium arsenide, the two-dimensional sheet of electrons being the "jam" in the sandwich. These so-called heterostructures are typically produced by molecular beam epitaxy, an ultra-high vacuum technique which is therefore rather expensive. They provide the cleanest system for electrons in the solid state, allowing us to observe very fragile new states of matter (such as those responsible for the fractional quantum Hall effect).
• Graphene: This 2D atomic honeycomb of carbon atoms possesses many unique and superlative properties - enormous strength and biocompatibility being among them. Graphene can be suspended to form the only two-dimensional conducting membrane. Graphene also exhibits extraordinary electronic properties due to its novel band-structure, which imparts chirality on the electrons and makes them behave as if they were mass-less. 
• Quantum point-contacts: Fabricated from semiconductor heterostructures, these one-dimensional systems are defined by applying voltages to deplete different regions of the two-dimensional sheet of electrons. The exhibit quantised conductance and are exquisitely sensitive to their electrostatic environment. We probe the properties of these systems by first reducing their temperatures to within a degree or so of absolute zero, and then subjecting them to high magnetic fields. The magnetic fields produce further quantisation of the electrons' motion, effectively transforming a 2D system into a 0D one. The various phenomena that result can collectively be referred to as quantum Hall physics. We have used a variety of measurement techniques to study the properties of electrons in the quantum Hall regime, including electrical measurements and optical probes. Our most novel and powerful technique is that of magnetisation, which provided a direct measurement of a thermodynamic function of state, and also a novel contact-free transport measurement.‌
• ‌Thermodynamic measurements: One of the very few techniques to probe a thermodynamic equilibrium property directly, magnetisation measurements provide important new information about 2D electron systems, and potentially about the exotic new states of matter that occur in the quantum Hall regime. In collaboration with co-workers at Cardiff University, we have developed a unique milli-Kelvin torsion balance magnetometer with the extremely high sensitivity necessary to measure the tiny magnetisations of these sheets of electrons. We have recently been extending these measurements to electrons in graphene and have also been exploring the controversy over whether electrons in graphite share some of the novel properties of those in graphene.
• Contact-free transport: In the quantum Hall regime, electron transport becomes virtually dissipationless, and as a result, large circulating currents can be induced in 2D electron systems by a sweeping magnetic field. Our magnetometers can detect these circulating currents from the magnetic moments they produce. This constitutes a transport measurement (measurement of electrical resistance) in a geometry with no electrical contacts. These measurements provide invaluable information about the breakdown of the quantum Hall effect in 2D systems, and can be used to alter the properties of quantum point-contacts in novel ways.
• Transport measurements of strained graphene: Amongst its many extraordinary properties, graphene can be stretched elastically by up to 18% without tearing. Formally, the effect of such strain is analogous to the application of external magnetic and electric fields, so-called fictitious gauge fields. One of the most exciting prospects of strain engineering (tailoring the electronic properties of graphene via controllable mechanical deformations) is in producing pseudo-magnetic fields; graphene is known to display novel behaviour (e.g. an anomalous quantum Hall effect) in real magnetic fields. We are currently developing methods for straining graphene in order to observe these effects.
  

Theoretical condensed matter

This theme focuses on the electronic, mechanical, thermal and optical properties of systems in which fundamental aspects of quantum mechanics are prominent.

Electromechanical properties of nanomaterials

• Academic lead: Dr Eros Mariani
• Group: Tom Sturges, Claire Woollacott, Alexander Pearce

This group's field of research is theoretical condensed matter physics. We currently focuses mostly on the electromechanical properties of graphene membranes and on nano-electromechanical systems (NEMS).

In particular, we investigate electronic transport in suspended mono- and multi-layer graphene, with a special focus on the fictitious gauge fields induced by elastic mechanical deformations in the electronic Dirac Hamiltonian. Our recent works highlight the importance of flexural phonons in transport through graphene resonators as well as the dramatic effects of strain on the electronic band-structure of bilayer graphene.

In the field of NEMS we analysed the electron-vibron coupling and its effects on electronic transport in different contexts. One of the most remarkable electro-mechanical effects is the Franck-Condon current blockade, recently observed in suspended carbon-nanotubes quantum dots.

A parallel research line recently introduced in our activities deals with the physics of plasmonic metamaterials, with a focus on collective plasmons in lattices of metallic nanoparticles.

Other research activities in the past few years include parametric resonances in quantum many-body systems, cold atoms in optical lattices and the fractional quantum Hall effects (Chern-Simons quantum field theory, composite fermions and non-abelian quantum Hall states).

Optical properties of graphene and carbon nanotubes

• Academic lead: Professor Misha Portnoi
• PhD students: Charles Downing

Our current research is focused on electronic and optical properties of graphene and carbon nanotubes. We have investigated confined zero-energy states in electrostatically-defined graphene waveguides (Figure 1), and vortices in quantum dots and rings; momentum alignment of photo-excited carriers in graphene (optovalleytronics) as well as two-phonon scattering in graphene in the quantum Hall effect regime at elevated temperatures.

Our work on carbon nanotubes is aimed at their prospective applications to terahertz optoelectronics and includes the study of optical transitions and excitons in narrow-gap carbon nanotubes.

Band-gap engineering and transport in quantum systems

• Academic lead: Dr Andrei Shytov
• PhD students: Lachlan Marnham
• MPhys students: Scott Taylor, Giacomo Pope

Our theoretical research group looks at band-gap engineering and transport in quantum systems. Our group has shown that adatoms in graphene-based compounds (fluorinated or hydrogenated graphene) can be ordered by electron-mediated effective interactions.

This opens a route towards engineering a band gap.

Theory of electrons and phonons

• Academic lead: Professor GP Srivastava
• Group Members: Dr Iorwerth Thomas (PDRA), Professor Tome M. Schmidt (visiting academic)
• Collaborators: Professor H. M. Tutuncu (Sakarya University, Turkey); Professor M. Cakmak (Gazi University, Ankara, Turkey); Professor T. M. Schmidt (University of Uberlandia, Brazil); Professor R. Hicken (University of Exeter)

This group conducts theoretical ab inito and semi-ab initio investigations of the physics of electrons and phonons in bulk (3D), surface (2D) and nanocomposite structures. Specific topics include electronic band structure; phonon dispersion relations; electron-electron, electron-phonon and phonon-phonon interactions; thermal transport; thermoelectric properties; BCS superconductivity.