Research
 

Iron-based superconductors

The iron-based superconductors are a new family of unconventional superconductors with superconducting transition temperatures up to 56 K. The first member of this family to be found was fluorine-doped LaFeOAs, discovered by Hosono and co-workers in 2008. Although there are several different structural variants, all the Fe-based superconducting compounds contain layers of tetrahedrally-coordinated Fe atoms arranged on a square lattice.

Like the copper oxide high temperature superconductors, the superconducting phase in Fe-based superconductors forms in close proximity to an antiferromagnetic parent phase. Superconductivity is induced by doping or application of pressure. The key scientific challenges are: (i) understanding the mechanism that causes superconductivity and (ii) finding new Fe-based superconductors.

 

Current projects

  • Synthesis & properties of new Fe-based superconductors

    This project is a collaboration with Prof Simon Clarke in the sub-Department of Inorganic Chemistry at Oxford, and has been funded by EPSRC grant EP/I017844/1, New approaches to iron-based superconductivity.

  • Investigation of magnetic correlations in Fe-based superconductors

    This project is undertaken in collaboration with Prof Toby Perring and Dr Russell Ewings at the ISIS Facility, and scientists at the Paul Scherrer Institut. Funding was provided by EPSRC grant EP/G067457/1, Magnetic correlations in superconducting iron aresenides.

     

  • Prof. Andrew Boothroyd

    Clarendon Laboratory

    Department of Physics

    Oxford University

    Oxford, OX1 3PU

    United Kingdom

    phone

     +44 (0) 1865 272376

    fax

    +44 (0) 1865 272400

    a.boothroyd@physics.ox.ac.uk

     

     

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    Synthesis & properties of superconductors

    An early success was the discovery of superconductivity with Tc = 17 K in stoichimetric LiFeAs. Recently, it has been found possible to intercalate Li+ ions and small molecules such as ammonia/amide and hydroxide between the iron layers in FeSe, and this raises Tc from 8 K to above 30 K.

    For more information, see

    Structure and Superconductivity of LiFeAs

    M.J. Pitcher, D.R. Parker, P. Adamson, S.J.C. Herkelrath, A.T. Boothroyd, R.M. Ibberson, M. Brunelli and S.J. Clarke

    Chem. Commun. (2008) 5918–5920.

     

    Coexistence of incommensurate magnetism and superconductivity in Fe1+ySexTe1–x

    R. Khasanov, M. Bendele, A. Amato, P. Babkevich, A. T. Boothroyd, A. Cervellino, K. Conder, S. N. Gvasaliya, H. Keller, H.-H. Klauss, H. Luetkens, V. Pomjakushin, E. Pomjakushina, and B. Roessli

    Phys. Rev. B 80 (2009) 140511(R).

     

    Tuning the superconducting and magnetic properties of FeySe0.25Te0.75 by varying the iron content

    M. Bendele, P. Babkevich, S. Katrych, S. N. Gvasaliya, E. Pomjakushina, K. Conder, B. Roessli, A. T. Boothroyd, R. Khasonov, and H. Keller

    Phys. Rev. B 82 (2010) 212504.

     

    Neutron scattering studies

    We have concentrated on exploring the magnetic excitation spectrum, which comprises magnetic fluctuations characteristic of the parent antiferromagnets together with a superconductivity-induced spin resonance at an energy comparable with the superconducting gap. Early on, we discovered that dispersive excitations in the parent phase BaFe2As2 extend up above 200 meV in energy, showing that spin fluctuations have sufficient energy to mediate high-Tc superconductivity. We discovered antiferromagnetic fluctuations and a spin resonance in superconducting LiFeAs, and we have also investigated the spin resonance in FeTe0.5Se0.5, CsxFe2–ySe2 and molecular-intercalated FeSe. We found that there are strong spin fluctutaions in the simplest iron-based superconductor FeSe.

    For more details of this work see:

    High energy spin excitations in BaFe2As2 observed by inelastic neutron scattering

    R.A. Ewings, T.G. Perring, R.I. Bewley, T. Guidi, M.J. Pitcher, D.R. Parker, S.J. Clarke, and A.T. Boothroyd

    Phys. Rev. B 78 (2008) 220501(R).

     

    Magnetic excitations of Fe1+ySexTe1−x in magnetic and superconductive phases

    P. Babkevich, M. Bendele, A. T. Boothroyd, K. Conder, S. N. Gvasaliya, R. Khasanov, E. Pomjakushina,

    and B. Roessli

    J. Phys.: Condens. Matter 22 (2010) 142202.

     

    Spin anisotropy of the resonance peak in superconducting FeSe0.5Te0.5

    P. Babkevich, B. Roessli, S. N. Gvasaliya, L.-P. Regnault, P. G. Freeman, E. Pomjakushina, K. Conder, and A. T. Boothroyd

    Phys. Rev. B 83 (2011) 180506(R).

     

    Antiferromagnetic spin fluctuations in LiFeAs observed by neutron scattering

    A. E. Taylor, M. J. Pitcher, R. A. Ewings, T. G. Perring, S. J. Clarke, and A. T. Boothroyd

    Phys. Rev. B 83 (2011) 220514(R).

     

    Spin-wave excitations and superconducting resonant mode in CsxFe2–ySe2

    A. E. Taylor, R. A. Ewings, T. G. Perring, J. S. White, P. Babkevich, A. Krzton-Maziopa, E. Pomjakushina, K. Conder, and A. T. Boothroyd

    Phys. Rev. B 86 (2012) 094528.

     

    Spin fluctuations away from (pi,0) in the superconducting phase of molecular-intercalated FeSe

    A. E. Taylor, S. J. Sedlmaier, S. J. Cassidy, E. A. Goremychkin, R. A. Ewings, T. G. Perring, S. J. Clarke, and A. T. Boothroyd

    Phys. Rev. B 87 (2013) 220508(R).

     

    Strong (π,0) spin fluctuations in β−FeSe observed by neutron spectroscopy

    M. C. Rahn, R. A. Ewings, S. J. Sedlmaier, S. J. Clarke, and A. T. Boothroyd

    Phys. Rev. B 91 (2015) 180501(R).

     

    Spin resonance in the superconducting state of Li1-xFexODFe1-ySe observed by neutron spectroscopy

    N. R. Davies, M. C. Rahn, H. C. Walker, R. A. Ewings, D. N. Woodruff, S. J. Clarke, and A. T. Boothroyd

    Phys. Rev. B 94 (2016) 144503 (arXiv:1607.05588)

     

    Facilities and Equipment in the Group

    The group has a range of state-of-the art equipment for sample preparation, characterisation and fundamental measurements.

     

     

     

     

    Antiferromagnetic fluctuations and spin resonance in LiFeAs