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
This project is a collaboration with Prof Simon Clarke in the sub-Department of Inorganic Chemistry at Oxford, and has been funded by EPSRC grants EP/I017844/1, New approaches to iron-based superconductivity. and EP/P018874/1, Soft chemical control to achieve new layered architectures and strongly correlated states.
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
Oxford, OX1 3PU
United Kingdom
phone
+44 (0) 1865 272376
fax
+44 (0) 1865 272400
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 and synchrotron X-ray 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:
Paramagnon dispersion in β-FeSe observed by Fe L-edge resonant inelastic x-ray scattering
M. C. Rahn, K. Kummer, N. B. Brookes, A. A. Haghighirad, K. Gilmore, and A. T. Boothroyd
Phys. Rev. B 99 (2019) 014505 (arXiv:1808.08183)
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)
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 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).
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.
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 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).
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.
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).
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