●All-in-all-Out magnetic order established in a candidate Weyl semi-metal
An international team from Oxford and UCL has shown that an iridium oxide compound satisfies a necessary requirement for a correlated Weyl semi-metal. The finding points the way to a possible realization in the solid state of an exotic type of particle.
The Weyl semi-metal is a topologically non-trivial electronic state described by a solution to the Dirac equation for massless fermions. It requires the combination of either (i) time-reversal symmetry with broken inversion symmetry, or (ii) inversion symmetry with broken time-reversal symmetry. Examples of the former type of Weyl semi-metal have been found recently, but no example of the latter has yet been confirmed.
The team, which included Marein Rahn, Dr D Prabhakaran and Prof Andrew Boothroyd from Oxford, used a combination of resonant elastic and inelastic X-ray scattering on single crystal samples grown in Oxford. The results showed that the cubic pyrochlore oxide Sm2Ir2O7 transforms at a temperature of 110K into a magnetically ordered state in which the magnetic moments on the Ir atoms point either all-in or all-out of a network of tetrahedra formed by the Ir sites in the crystal. Such a magnetic structure breaks time-reveral symmetry but preserves inversion symmetry, as required for a Weyl semi-metal.
The results support a theoretical prediction that the family of pyrochlore iridates may host Weyl fermions, although electronic correlations also revealed by the study could forestall the Weyl semi-metal state. In that case, it might be necessary to tune the electronic structure by chemical modifications or external perturbations to form the Weyl semi-metal.
●New EPSRC grant:
Title: New correlated electronic states arising from strong spin-orbit coupling
Investigator(s): Prof. A. T. Boothroyd and Dr D. Prabhakaran
Start date: 1 November 2016 (provisional)
Duration: 3 years
●Charge stripes found in a layered cobalt oxide
Stripe phases are a form of complex matter involving coupled spin and charge order. They are observed in certain copper oxide superconductors, as well as in nickelates and manganites. Some theoreticians believe that stripe fluctuations are important to the mechanism of superconductivity in the copper oxide family of high temperature superconductors. A key piece of evidence for this idea is the universal form of the magnetic spectrum of hole-doped copper oxide superconductors as measured by neutron scattering, which is in the shape of an hourglass. This type of spectrum emerges naturally from a stripe-ordered ground state.
In 2011, an insulating, layered cobalt oxide La5/3Sr1/3CoO4 was found to have an hourglass magnetic spectrum [A.T. Boothroyd et al., Nature 471, 341 (2011)]. At the time, there was indirect evidence for stripe order in this material, and the result was significant because it provided an experimental demonstration of how an hourglass spectrum could arise from a stripe-ordered ground state. However, direct evidence for the charge-stripe order has been missing until now. The apparent absence of stripes led to an alternative, stripe-free, model to explain the hourglass spectrum based on two types of short-range magnetic order in coexistence on the nanoscale.
In our new work, published in Nature Communications, we employed polarized neutron diffraction at the Institut Laue–Langevin to detect a signature of charge stripes in La5/3Sr1/3CoO4. The results show that the magnetic ground state is more complicated than initially thought, comprising a nanoscopic coexistence of (i) spin- and charge-stripe order and (ii) charge order in which Co2+ and Co3+ alternative like the black and white squares on a chess board. However, the measurements reinforce the stripe-model as the underlying mechanism for the hourglass magnetic spectrum in the cobaltates. The present results provide an experimental basis for theories that assume a ground state with static or slowly fluctuating stripes in order to explain the hourglass spectrum in cuprates.
Reference (open access): P. Babkevich et al., Nat. Commun. 7:11632 doi: 10.1038/ncomms11632
●When is a ferroelectric not a ferroelectric?
Ferroelectrics are insulating materials with an electrical polarisation that can be switched by an applied voltage. Ferroelectricity cannot occur in metals because it would be screened by the conduction electrons. In an article published in Nature Materials, we report the discovery of a new material called lithium osmate (LiOsO3) which remains a metal down to the lowest temperatures and yet undergoes a structural phase transition that is identical to the ferroelectric transition in the well-known ferroelectrics LiNbO3 and LiTaO3.
Using a variety of techniques, including neutron diffraction at the ISIS Facility, Oxfordshire, Yanfeng Guo, Andrew Princep and Andrew Boothroyd, together with co-workers from ISIS, Japan and China, found that the phase transition in LiOsO3 is characterised by a large shift in the position of the Li ions, a structural effect which has been known for many years to cause ferroelectricity in LiNbO3 and LiTaO3.
The discovery represents the first clear-cut example of a so-called “ferroelectric” metal, a concept first postulated over 50 years ago by Nobel prize-winner Philip Anderson and co-worker Blount. It is scientifically interesting because the mechanisms for structural phase transitions are usually quite distinct in metals and insulators, so it is surprising to find a metal (LiOsO3) that undergoes the same structural transition as occurs in the insulating analogues. The discovery of a “ferroelectric” metal establishes a new class of materials which could have interesting properties, possibility including non-centrosymmetric superconductivity stabilised by the “ferroelectric” structural instability.
Reference: Y. Shi et al. Nature Materials doi:10.1038/nmat3754
News & Views article: doi:10.1038/nmat3774
We are pleased to welcome Yanfeng into the group. Yanfeng was previously a postdoctoral fellow in the National Institute for Materials Science in Tsukuba, Japan, where he worked on synthesis, crystal growth and physical properties investigations of a range of transition metal oxides with exotic properties. Before that he completed his PhD at the Institute of Physics, Chinese Academy of Sciences, Beijing, on pulsed electron deposition of thin films of high temperature superconductors and their properties.
●New EPSRC grant:
Title: Emergence of novel electronic states in 5d transition metal oxides
Investigator(s): Prof. Andrew T. Boothroyd
Start date: 24 September 2012
Duration: 3.5 years
●Andrew Princep joins the group
We welcome Andy into the group. Andy did his undergraduate degree at Curtin University in Western Australia, and recently completed his doctorate at the University of New South Wales, Canberra campus. His thesis work involved the use of synchrotron x-ray and neutron scattering experiments to investigate orbital and magnetic order in rare-earth borocarbides.
●IoP Superconductivity Group Prize 2011
We are proud that the work of the group has been recognised through this award to ATB.
Prof. Andrew Boothroyd
Oxford, OX1 3PU
+44 (0) 1865 272376
+44 (0) 1865 272400