●Discovery of an ideal Weyl semimetal
Weyl semimetals exhibit exceptional electronic transport due to the presence of topological band crossings called Weyl nodes. The nodes come in pairs with opposite chirality, but their number and location in momentum space is otherwise material-specific.
Together with colleagues in the Rudolf Peierls Centre for Theoretical Physics, Oxford, and a team of international collaborators, we have found that the layered intermetallic EuCd2As2 in a magnetic field is what Bernevig has termed the hydrogen atom of a Weyl semimetal, i.e. one with a single pair of Weyl nodes at the Fermi level and without overlapping electron bands.
The discovery opens the door to exploration of a wide range of exotic physics predicted for Weyl fermions in the solid state.
Reference:J.-R. Soh et al., Phys. Rev. B 100 (2019) 201102(R)
●New EPSRC grant:
Title: A state-of-the-art floating-zone furnace for crystal growth at high pressures
Investigator(s): A. T. Boothroyd, D. Prabhakaran, P. G. Radaelli, I. A. Walmsley
Start date: 1 March 2018
Duration: 2.5 years
●Understanding the magnetic properties of YIG
Researchers from the University of Oxford have for the first time mapped the complete spectrum of magnetic oscillation modes in yttrium iron garnet (YIG). The results are important for understanding magnon transport phenomena in YIG which are being exploited in a variety of potential magnonic applications.
YIG is already used in a broad range of technological devices, from microphones to lasers. However, it is also key to the emerging field of magnonics, which seeks to create devices that replace conventional electrical circuitry with magnetization currents. If YIG is to fulfil this potential, its magnetic properties, particularly at room temperature, need to be well understood.
Andrew Princep, D. Prabhakaran and Andrew Boothroyd, together with colleagues from the ISIS Facility, the Paul Scherrer Institut and the German technology company Innovent, used inelastic neutron scattering to measure the full magnon spectrum of a large crystal of YIG, and used advanced modelling software to determine values for the exchange interactions between the spins.
The results have been used to develop a model which can be applied to understand and predict the magnetic behaviour of YIG over a wide range of temperature, including the operational conditions for YIG-based magnonics devices.
Reference:A. J. Princep et al., npj Quantum Materials 2, 63 (2017)
●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
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
Prof. Andrew Boothroyd
Oxford, OX1 3PU
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