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Physics and ISEM Special Lecture

August 19 @ 11:00 am - 12:00 pm


Dr Evan Constable (Technical University of Vienna)


From magnetic monopoles to vibronic processes: Exploring the emergent physics of spin ice with terahertz radiation


The delicate balance between competing spin interactions in magnetically frustrated systems fosters a rich physical environment to explore novel states of matter with interesting collective behaviours. One fascinating consequence of the constrained spin degrees of freedom in a class of frustrated magnets known as “spin ice” is the ability to describe their collective behaviour by emergent gauge fields. Interestingly, this emergent description can be shown to express a novel form of electrodynamics that exists on top of the collective magnetic behaviour and where new quasi-particles such as magnetic monopoles are allowed.
Because this frustrated state is driven by the host lattice geometry, an intimate link between the spin and lattice degrees of freedom prevails. Under certain conditions, vibronic processes emerge where small spin-lattice perturbations may act to melt the spin ice phase forming even more complex ground states such as spin liquids and their quantum variants.

Probing the elementary excitations and dynamics of these frustrated states is a valuable method to characterise and understand the exotic and emergent collective physics at play. While historically, neutron scattering techniques have been the go-to choice in magnetic spectroscopy, state-of-the-art developments in terahertz techniques are shining new light on the subject. In this talk, I will discuss recent terahertz-based spectroscopic investigations into the emergent electrodynamics and spin-lattice coupling of the rare-earth pyrochlore spin ice systems Ho2Ti2O7 and Tb2Ti2O7.


Evan Constable studied physics at the University of Wollongong and completed his PhD on Terahertz spectroscopy of magnetic metal oxides under the supervision of professors R. A. Lewis and J. Horvat at the Institute for Superconducting and Electronic Materials (ISEM) in 2014. His PhD work was awarded the Examiners’ Commendation for Outstanding Thesis as well as the best PhD Thesis Prize from the ISEM in 2015. Following an interest developed during his PhD using complementary neutron and terahertz techniques, in 2015 he took up a postdoc position at the CNRS Neel Institute in Grenoble, France under the supervision of Dr S. DeBrion and Dr V. Simonet. His postdoctoral research in Grenoble focused on complementary neutron scattering and terahertz spectroscopy of novel excitations in magnetically frustrated materials using the large scale neutron and synchrotron facilities at Institut Laue–Langevin (ILL), Laboratoire Leon Brillouin (LLB) and Synchrotron SOLEIL. During his time at Grenoble, he was awarded a Marie Skłodowska-Curie PRESTIGE fellowship and a CMIRA-Accueil Pro-2015 grant funded by the Rhone-Alpes Region. In 2017 he took up a second postdoc position at the Vienna University of Technology in Austria under the supervision of professor A. Pimenov. Since joining the solidstate spectroscopy group in Vienna, he has continued his focus on spectroscopy of novel excitations in frustrated magnets. In 2019 he was granted funding by the Austrian Science Fund (FWF) to pursue a 4-year project as the principal investigator studying Emergent Electrodynamics in Frustrated Magnets.

Selection of recent publications

E. Constable, R. Ballou, J. Robert, C. Decorse, J.-B. Brubach, P. Roy, E. Lhotel, L. Del-Rey, V. Simonet, S. Petit, and S. deBrion. Double vibronic process in the quantum spin ice candidate Tb2Ti2O7 revealed by terahertz spectroscopy. Physical Review B, 95:020415(R) (2017)

E. Constable, S. Raymond, S. Petit, E. Ressouche, F. Bourdarot, J. Debray, M. Josse, O. Fabelo, H. Berger, S. deBrion, and V. Simonet. Magnetic and dielectric order in the kagomelike francisite Cu3Bi(SeO3)2O2Cl. Physical Review B, 96:014413 (2017)

M. Ramakrishnan, E. Constable, A. Cano, M. Mostovoy, J. S. White, N. Gurung, E. Schierle, S. de Brion, C. V. Colin, F. Gay, P. Lejay, E. Ressouche, E. Weschke, V. Scagnoli, R. Ballou, V. Simonet, and U. Staub. Field-induced double spiral spin texture in a frustrated chiral magnet. arXiv:1810.02656 (2018).



August 19
11:00 am - 12:00 pm
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