Nano-magnetism and Spintronics
In the Nanomagnetism and Spintronics research work focuses on experimental studies of magnetic, magneto-optical, and spin-transport phenomena in new functional materials and hybrid nanoscale structures. Current research topics include electric-field control of ferromagnetism, spin transport in ferrimagnetic and ferroelectric tunnel junctions, and magneto-plasmonics of patterned nanostructures.
As consumers of high-tech semiconductor technology, we have become accustomed to faster and smaller products with each new generation of computers, ipods, smart phones, etc. However, the current fabrication methods and device architectures that have enabled this rapid progress are quickly approaching their physical limits. Spintronics, or spin based electronics, attempts to manipulate the spin degree of freedom, instead of or in addition to the charge of electrons to create new functionality and prolong this era of rapid technological advancement. The field of spintronics has already produced several major advances, the most famous being the giant magnetoresistive (GMR) effect which is the primary operating principle behind current hard-drive technology and also the subject of the 2007 Nobel Prize in physics. Future development of spintronics envisions the integration of semiconductor device function with magnetic storage, such as in a spin-transistor a spin-LED, which will revolutionize computing by offering the advantages of nonvolatility, faster data processing speed, low power consumption, and high integration densities. This demands significant advances in magnetic semiconducting materials.
Materials for magentic refrigeration
Magnetocaloric materials utilize magnetocaloric effect, a change in material's temperature upon a change in an applied magnetic field. Such materials heat up when the magnetic field is increased and cool down when magnetic field is reduced. They can be used for magnetic refrigeration (figure on the right) and can offer a greater efficiency than the current vapor-cycle refrigeration.
A conventional magnetocaloric effect is based on ordering of magnetic moments, i.e. on magnetic transition only. The effect can be significantly increased (by a factor of ~2) when a magnetic ordering is coupled to a structural transition. A combined effect is known as a giant magnetocaloric effect.
Our research in this area focuses on discovery of new metal-rich magnetocaloric materials and on manipulating the physical and structural properties of known materials.
Thermoelectric materials can convert heat into electricity (Seebeck effect, figure on the right) or perform cooling/heating when electrical current is passed through them (Peltier effect). Thermoelectric materials are used to generate electricity when other source of electricity are not available (i.e. deep space missions) or to convert waste heat into electricity thus reducing fuel consumption (i.e. in cars). They are also used to perform cooling when other cooling techniques cannot be easily applied (i.e. car seats, spot cooling in electronics).
One of the challenges in the thermoelectric research is to reduce thermal conductivity of materials in order to optimize their performance.
Our group tackles this challenge by utilizing a natural superlattice approach for the material design. We combine two structures with different properties to obaine a material known as "phonon-glass electron-crystal". We also prepare unique suboxide phases, in which a band gap is opened in the original semimetallic material through the incorporation of oxide fragments.
Topological insulators are a new phase of quantum matter exhibiting novel electronic properties. Their ability to maintain a robust quantum state in the presence of scattering by non-magnetic impurities make them excellent candidates for use in spintronic devices and quantum computation.
The origin of the novel properties of these materials traces back to the work by Klaus von Klitzing on the Quantum Hall Effect in 1980, for which he later was awarded the Nobel Prize in Physics. His original paper can be found online with PRL.
The new topological insulators are a realisation of a related theory called the Quantum Spin Hall Effect (QSHE) developed by Kane and Mele in 2005 and published in PRL. One can think of this theory as being like two independent currents each governed by the Quantum Hall Effect, one for all the spin up electrons and the other for the spin down electrons. A good representation of what these spin currents may look like is shown in the figure below.
The Quantum Hall state, while scientifically interesting requires a large external magnetic field to be applied across the material for the state to persist. The requirement of this external field limits the use of such quantum states in a real world device. In the topological insulators however, the equivalent Quantum Spin Hall state is intrinsically invariant under time reversal symmetry and the state will persist without the need for an external field. The robustness of this state opens up interesting experimental opportunities and the possibility of novel future devices.
The first material to show evidence of the Quantum Spin Hall state were HgTe quantum wells published in a 2007 paper by M. König et al. Since then other materials systems have been predicted and researched in this emerging field. Our group focuses on crystal systems of the type , as well as related systems. These are grown by MBE as high purity thin films and then investigated using a variety of characterization tools.
Graphene (Preparation and solar cell device fabrication)
Graphene refers to the single atom sheet of carbon atoms that is obtained by unrolling carbon nanotubes. This material helps to realize a truly two-dimensional electronic system since the carriers are confined to a single atomic layer, unlike in layered semiconductors. In addition, the linear dispersion relation associated with relativistic particles in graphene leads to novel phenomena such as the anomalous quantum Hall effect.