Our research

The interactions between electrons in solids are responsible for a large number of exciting physical phenomena, including ferromagnetism, antiferromagnetism and superconductivity. In these materials, we cannot treat the electrons as independent entities but have to consider their correlated behaviour. Understanding electron-electron interactions in a variety of systems, including transition metal oxides and organic molecular solids, is a central part of our research.

For enquiries about the group, please contact: Professor Stephen Blundell (group leader), s.blundell@physics.ox.ac.uk Dr Arzhang Ardavan (group leader), a.ardavan@physics.ox.ac.uk

Latest news

The new high-field muon instrument at ISIS, called HiFi, provides applied longitudinal fields up to 5 T. It is now fully operational! Funds to build HiFi have come from a grant made to the ISIS muon group and Prof. Steve Blundell at Oxford University for £2.1M from the STFC Facility Development Board. It represents the first significant investment in the ISIS muon facility for over ten years, and will open up new science areas for the muon technique at ISIS.

Recent results

Quantum mapmakers complete first voyage through spin liquid

Scientists from Oxford University and the Rutherford Appleton Laboratory have completed a painstaking mapping of one of the most exotic places in the universe found close to absolute zero. The quantum spin liquid is a strange state of matter whose existence was proposed in the 1970s but which has only been observed recently. Until now, there has been extremely limited information available describing its physical characteristics. The new research, published on 31 March 2011 in the science journal Nature, demonstrates the effect of temperature and magnetic field on this delicate state of matter. The measurements were made by implanting muons into the spin liquid in order to measure the microscopic magnetism. The quantum spin liquid state is found in 70 milligrams of tiny black crystals of an organic material cooled to just a couple of hundredths of a degree above absolute zero. Inside the material, magnetic atoms are arranged on triangular grids and behave as quantum spins. The interactions between these spins make them liquid-like, never freezing into one configuration, even at temperatures approaching absolute zero. This behaviour is completely different to the more familiar magnets found in everyday life in which, at some particular temperature, the quantum spins become locked into a particular configuration.

Magnetic and non-magnetic phases of a quantum spin liquid

F. L. Pratt, P. J. Baker, S. J. Blundell, T. Lancaster, S. Ohira-Kawamura, C. Baines, Y. Shimizu, K. Kanoda, I. Watanabe and G. Saito
Nature 471, 612 (2011) Link

Good chemistry between magnetism and superconductivity

Scientists from Spain and the United Kingdom have recently used ISIS muons to demonstrate the success of a new method for combining materials with different properties. Both magnets and superconductors have separate technological applications, but making materials that display both properties is challenging. Professor Stephen Blundell, who carried out the ISIS muon experiments, explained that: "Superconductivity and magnetism are usually sworn enemies and refuse to cohabit in the same compound." The most common method of making materials with both these properties has been to deposit alternating layers of compounds with each of these properties. The success of this approach is limited if the compounds have different sizes at the atomic level, leading to distortions that change the properties of each layer. The new method forms each layer in solution and the layers are brought together by electrostatic attraction. Professor Blundell said: "Rather than build up the material atom by atom, molecule by molecule, nanosheets with different functions are self-assembled." Professor Eugenio Coronado, whose group at the University of Valencia developed the chemical technique, described this as "like constructing a building by adding entire pre-assembled floors rather than brick by brick, and is the secret behind combining these two inimical properties." Bringing magnetic and superconducting layers together in such close proximity offers the possibility of using one property to affect the other. To examine the effect of this coupling in these new materials, ISIS muons provided a way to probe each property at a microscopic level. This allowed the volume of the sample that becomes magnetically ordered and the strength of the superconducting state to be determined.

Coexistence of superconductivity and magnetism by chemical design

E. Coronado, C. Mart\'i-Gastaldo, E. Navarro-Moratalla, A. Ribera, S. J. Blundell and P. J. Baker
Nature Chemistry 2, 1031 (2010) Link

Iron-based superconductors tuned using chemical subsitution

The response of the superconducting state and crystal structure of the layered materials LiFeAs and NaFeAs to chemical substitutions has been probed using high-resolution X-ray and neutron diffraction measurements, magnetometry, and muon-spin rotation spectroscopy. The superconductivity is extremely sensitive to composition: Li-deficient materials (Li1-yFe1+yAs with Fe substituting for Li) show a very rapid suppression of the superconducting state, which is destroyed when y exceeds 0.02, echoing the behavior of the Fe1+ySe system. Substitution of Fe by small amounts of Co or Ni results in monotonic lowering of the superconducting transition temperature, Tc, and the superfluid stiffness as the electron count increases. Tc is lowered monotonically at a rate of 10 K per 0.1 electrons added per formula unit irrespective of whether the dopant is Co and Ni, and at higher doping levels superconductivity is completely suppressed. These results and the demonstration that the superfluid stiffness in these LiFeAs-derived compounds is higher than in all of the iron pnictide materials underlines the unique position that LiFeAs occupies in this class. We also show how the magnetic state in NaFeAs can be tuned into superconductivity by replacing Fe by either Co or Ni. The electron count is the dominant factor, since Ni doping has double the effect of Co doping for the same doping level. We follow the structural, magnetic, and superconducting properties as a function of doping to show how the superconducting state evolves, concluding that the addition of 0.1 electrons per Fe atom is sufficient to traverse the superconducting domain, and that magnetic order coexists with superconductivity at doping levels less than 0.025 electrons per Fe atom.

Compositional control of the superconducting properties of LiFeAs,

M. J. Pitcher, T. Lancaster, J. D. Wright, I. Franke, A. J. Steele, P. J. Baker, F. L. Pratt, W. Trevelyan-Thomas, D. R. Parker, S. J. Blundell and S. J. Clarke
J. Am. Chem. Soc. 132, 10467 (2010) Link

Control of the Competition between a Magnetic Phase and a Superconducting Phase in Cobalt-Doped and Nickel-Doped NaFeAs Using Electron Count,

D. R. Parker, M. J. P. Smith, T. Lancaster, A. J. Steele, I. Franke, P. J. Baker, F. L. Pratt, M. J. Pitcher, S. J. Blundell and S. J. Clarke,
Phys. Rev. Lett. 104, 057007 (2010) Link