Research Highlights


Topological Protection and Non-Equilibrium States in Strongly Correlated Electron Systems

The ambitious goal of this Programme is to lay the experimental and theoretical foundations for fully quantum mechanical electronics.  We investigate the potential for performing coherent operations using exotic ‘topologically protected’ states of interacting particles, and the quantum non-equilibrium physics that must be understood if we are to have smaller and faster devices.

Recent highlights of our research are: 

Figure 1

Hidden spin polarisations uncovered

Engineering spin-polarised electronic states in solids is an essential prerequisite for realising a number of emerging schemes of electronics that would exploit the electron’s spin (a.k.a. spintronics) to deliver fast and energy-efficient devices. Creating large enough energy splittings between spin-polarised states for practical applications, however, has proved challenging to date. Now, a study led by Dr. Phil King and his research group has observed a giant spin splitting of electronic states in the semiconductor WSe2, despite this material possessing structural symmetries that conventional wisdom would dictate precludes it from hosting spin-polarised states at all.

Dr. King’s group, working in collaboration with researchers from the Norwegian University of Science and Technology, the Universities of Tokyo (Japan), Aarhus (Denmark), Suranaree (Thailand), the Max-Planck Institute for Solid State Research in Stuttgart (Germany), MAX-lab (Sweden) and Diamond Light Source (UK), made their discovery using spin- and angle-resolved photoemission spectroscopy. Via surface-sensitive measurements, they showed how the spin texture reverses on successive layers of the crystal, allowing strong local spin polarisations to develop but without violating any fundamental symmetry requirements of the bulk structure. The observation of such hidden spin polarisations reveals that a whole class of materials which we previously thought must have only spin-degenerate energy bands can in fact locally host spin-polarised states, opening new potential for exploiting a wide array of new compounds in spintronics.

The results of this study are published in Nature Physics, 10 (2014) 835.

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Figure 1

Quantum correlations in the one-dimensional driven dissipative XY model

C. Joshi, F. Nissen, J. Keeling. Phys. Rev. A 88 063835 (2013).

We are routinely used to the idea that we can use light to look at the nature of materials, and that various forms of advanced microscopy or spectroscopy (measuring the distribution of wavelengths emitted or scattered) can tell us about the structure and composition of materials. In the last decade however, we have started to view light and materials in a new way --- making synthetic materials using light.  The first example of this is that laser beams can be used to trap and move cold atoms.  This is however still using atoms as the matter, and using light to control where it exists.  Even more recently, there has been a lot of interest in trying to explore a kind of matter made out of light.   One example of this involves arrays of optical or microwave cavities, where light can leak from one cavity to the next, and can be forced to interact in such a way that one is driven towards states with definite numbers of photons in each cavity.

One big question in this field is whether the ideas that apply to normal matter --- statistical mechanics, thermodynamics etc. --- really apply to something like light, which can leak out, and needs pumping to replace it.  Using state-of-the-art numerical approaches, we have explored the behaviour in one example of such a system, and found that the behaviour can be very different indeed.  Most normal matter is governed by finding low energy states --- i.e. atoms and electrons arrange themselves to lower their energy.  Not so for the driven arrays of cavities:  sometimes they arrange themselves to maximise their energy.  This idea is not new -- a famous example is an analogue of 'Indian rope trick', where it is possible to balance a rod upside down by vibrating the bottom at the right frequency.  What we have shown is that this kind of 'inverted' state can occur in the collective behaviour of a driven system.

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