Research Highlights for Quantum Materials

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.

Now published in Physical Review Letters!

Candidate quantum spin ice materials include Pr-based pyrochlores, but if you study them with muons you are in for a surprise. The ground state of the Pr ion is a non-Kramers doublet, meaning that it is not protected from non-magnetic perturbations (as a Kramers doublet would be) and we have been able to show that the anisotropic distortion field produced by the presence of the muon is enough to split the doublet. This leads to an enhancement of the nuclear coupling to the muon and this dominates the response.

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.

We report a form of incommensurate charge density wave localisation in the insulating ternary oxide YbFe2O4. This state has a unique signature in the form of tightly wound ‘helices’ of scattering in reciprocal space. Below T ≈ 320 K the helices break up into Bragg peaks with intensities modelled quantitatively by a helical incommensurate strain pattern.

Understanding the complex electronic behaviour of multiband superconductors relies on separating the contribution of different components that contribute to its conductivity. By measuring these components of the magnetoresitivity tensor both at very low temperatures using ultra-high magnetic fields up to 90 T and at high temperatures using fields up to 14 T we were able to disentangle to origin of different electronic bands.

A nematic state is a form of electronic order which breaks the rotational symmetries without changing the translational symmetry of the lattice, and this state may play an important role in understanding high temperature superconductivity. We have revealed the existence of a novel electronic state in an highly unconventional multiband superconductor, FeSe, from the evolution of its electronic structure from the high-temperature tetragonal phase into the electronic nematic phase using angle resolved photoemission spectroscopy (ARPES).

Cd3As2 is a candidate three-dimensional Dirac semimetal which has exceedingly high mobility and non-saturating linear magnetoresistance that may be relevant for future practical applications. Using ultra-high magnetic fields and a large range of temperatures we find that the nonsaturating linear magnetoresistance persists up to 65 T and it is likely caused by disorder effects, as it scales with the high mobility.