Postgraduate Positions in the Correlated Electron Systems Group

In some materials the electrons form a solid, rather like the water molecules in ice. Some become magnetic, and some become superconducting. Such systems are said to exhibit different forms of order which is to say that the electrons they contain organize themselves in different ways when they are cooled down. One of the most important themes in 21st century materials science is the search for materials whose electrons exhibit completely new forms of order, that one day may be used to build entirely new forms of electronic device. This project explores how new forms of quantum order can arise in systems where no simple form of order wins outright, and the electrons are frustrated (much like people) by the bewildering array of choices which they face. The experimental systems we consider are at the forefront of modern materials science, and unique in their behavioural aspects. In LiV₂O₄, vanadium d-electrons become as massive as muons. In Na₄Ir₃O₈, a hyperkagome structure with strong spin frustration (see figure above), magnetic Ir ions fail to order magnetically at any temperature. PrBa₂Cu₄O₈ is the most one-dimensional metal known to exist. And the organic compounds kappa-(ET)₂Cu₂(CN)₃ and kappa-(ET)₂Cu[N(CN)₂]Cl, provide a long-sought realization of quantum spins on a highly-frustrated triangular lattice. Our main experimental approach will be to measure how these systems transport heat and electricity. The ratio of thermal and electrical conductivities, measured over a range of temperatures, is a very sensitive test of the way in which the electrons in a given material have organized themselves. Measuring this ratio will enable us to establish that these materials are completely unlike conventional metals and insulators. This project will also benefit from very strong theoretical support from Dr. Nic Shannon, also based here in the Physics Department. In particular Dr. Shannon will try to understand how electrons work together to form new types of ``elementary excitation'', for example particles with half the charge of an electron ! If successful, this work has the potential to change how we think about metals forever...

Contact Prof. Hussey for more details.

Materials whose electronic properties (such as the electrical resistance) become anisotropic at low temperatures without a change of crystal symmetry are currently very topical in condensed matter physics. Such materials have become known as "electronic nematic phases" (ENP's) and include high-temperature (copper-oxide) superconductors and two-dimensional electronic systems (2DES) at high magnetic fields. This project is to investigate electronic nematic behaviour in the recently discovered iron-based superconductors (e.g. Ba(Fe_1-xCo_x)₂As₂). We will induce electronic nematic behaviour, measure the collective magnetic correlations using neutron and x-ray scattering and use our measurements to understand the electronic properties. The project offers the possibility to work at international facilities in France, Switzerland, the USA and the UK, with complementary measurements in Bristol. There is scope for developing theoretical modelling and data handling software.

Contact Prof. S. Hayden for more details.

A quantum critical point (QCP) is the point where a continuous phase transition (e.g. ferromagnetism) occurs at zero temperature. "Quantum critically" is currently a hot topic because the occurrence a number of interesting phases, such as high-temperature superconductivity, appears to be connected to QCP's. This project concerns the layered ruthenate Sr₃Ru₂O₇ which has a magnetic-field induced QCP (see figure). The unique feature of Sr₃Ru₂O₇ is the occurrence of a new phase with strong electronic anisotropies at the QCP. This phase is not understood, but appears to be due to a new form of electronic order. In this project, we will use neutron scattering to investigate the magnetic excitations and correlations in the new phase. We have some evidence that the electronic order is driven by the magnetic degrees of freedom of the system. The project offers the possibility to work at international neutron scattering facilities in France and the UK, and involves experiments at high magnetic fields (10T) and low temperatures (50 mK) at the facilities and in Bristol.

Contact Prof. S. Hayden for more details.