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).
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.
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.
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.
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.