Page 22 - ILL Annual Report 2019

MAGNETISM
S.W. Lovesey. British
ISIS & Diamond Light Source Ltd, Harwell Campus, UK
‘I completed one of two apprenticeships. At 17 years I downed tools and took off my work
overalls. Seven years later I was permanent staff
in John Hubbard's group (the eponymous Hamiltonian was published) equipped with an Oxford D.Phil. Shortly
thereafter, I co-authored a monograph on neutron scattering. Aside from work on venture capital projects, and studies at the Harvard Business School, I have worked as a theoretical physicist at various universities and institutes.
Direct observation of anapoles by neutron diffraction
Spin-polarised diffractometer D3
Understanding magnetism at an atomic level of detail remains a great intellectual challenge, alongside parity violation and the homochirality of life, for example [1]. Knowledge about magnetic materials is vital for their use in machines and devices (‘You were not made to live like brute beasts,
but to pursue virtue and knowledge’— Dante’s Inferno, Canto XXVI). Magnetic neutron scattering is a puissant tool for
the experimentalist, and more so with our demonstration that neutrons are scattered by anapoles. They are dipoles in a church of Dirac multipoles; the elementary Dirac monopole—yet to be observed—possesses identical discrete symmetries to those
of the electronic multipoles in question, namely, magnetic and polar ones.
AUTHORS
S.W. Lovesey, D.D. Khalyavin and G. van der Laan (ISIS & Diamond Light Source Ltd, Harwell Campus, UK)
ARTICLE FROM
Phys. Rev. Lett. (2019)—doi: https://doi.org/10.1103/ PhysRevLett.122.047203
REFERENCES
[1] A. Dorta-Urra and P. Bargueño, Symmetry 11 (2019) 661
[2] P. Bourges et al., Phys. Rev. B 98 (2018) 016501
[3] S.W. Lovesey and D.D. Khalyavin, J. Phys. Condens. Matter 29
(2017) 215603
[4] M. Fechner et al., Phys. Rev. B 93 (2016) 174419
An anapole was studied by Zel’dovich in the
course of investigating parity-violating interactions in electromagnetic theory. Parity violation in atomic and molecular systems with the observation of electronic anapoles can be traced back to 1974 [1]. Our
direct observation by neutron diffraction makes a profound statement about the electronic properties
of high-Tc superconductors, on which there is no consensus of opinion. The pseudo-gap phase of these materials probably holds the key to understanding the superconducting state. Does the pseudo-gap phase possess magnetic structure? Many neutron diffraction experiments provide evidence of long-range magnetic order in a variety of ceramic superconductors [2]. However, it is not conventional magnetic order, for this eludes detection by standard laboratory techniques; nor is it the antiferromagnetism predicted by Laughlin. Added to this, the detection of magnetic order by neutron diffraction in one ceramic compound, YBCO, has recently been challenged [2]. Set against this challenge, Dirac multipoles provide a comprehensive interpretation of the entire diffraction pattern obtained for the ceramic superconductor Hg1201, which
is simpler than YBCO in having one Cu-O plane rather than two [3]. This result, that magnetic order in Hg1201 is a motif of Dirac quadrupoles (quantities HKQ in figure 1), occurs in a simulation of the electronic structure and Dirac quadrupoles [4].
ANNUAL REPORT 2019