Projet Porteur - Excitations émergentes dans les matériaux quantiques / Harnessing emergent excitations in quantum materials

Cookies will be available in the IQ kitchenette starting at 10:30, at which time coffee will be free until 11:00. Talk starts at 11:00. A nice occasion to discuss!

Zoom : https://umontreal.zoom.us/j/88647167648?pwd=RjQxTFIzcDA5bWQ1YkdvYkwvOXVYdz09

 Premier conférencier : Q. Barthélemy
Institut Quantique, Département de physique & RQMP, Université de Sherbrooke, Sherbrooke, Québec J1K 2R1, Canada

Heat conduction in herbertsmithite:
field dependence at the onset of the quantum spin liquid regime

Résumé: Herbertsmithite ZnCu3(OH)6Cl2 is an emblematic quantum spin liquid candidate because it is the closest realization of the nearest-neighbor S = 1/2 kagome Heisenberg antiferromagnet with a dynamical ground state. After recalling some selected developments of our 17O NMR and high-field specific studies [1, 2], I will present our recent thermal transport study [3], where we report thermal conductivity measurements on high quality single crystals over a wide range of temperatures (0.05-120 K) in magnetic fields up to 15 T. We also report measurements of the thermal Hall effect, found to be vanishingly small. At high temperatures, in the paramagnetic regime, the thermal conductivity has a negligible field dependence. Upon cooling and the development of correlations, the onset of a clear monotonic field dependence below about 20 K signals a new characteristic temperature scale that may reflect the subtle crossover towards the quantum spin liquid regime. Deconfined spinons, if present, are not detected and phonons, as the main carriers of heat, are strongly scattered by the intrinsic spin excitations and the magnetic defects. In view of the colossal fields required to affect the intrinsic spins, most of the field-induced evolution is attributed to the progressive polarization of some magnetic defects. By elaborating a phenomenological model, we extract the magnetization of these main scattering centers which does not resemble the Brillouin function for free spins 1/2, requiring to go beyond the paradigm of isolated paramagnetic spins. Besides, the onset of a nonmonotonic field dependence below about 2 K underlines the existence of another characteristic temperature scale, previously highlighted with other measurements, and sheds new light on the phase diagram of herbertsmithite down to the lowest temperatures.
[1] P. Khuntia et al., Nature Physics 16, 469 (2020)
[2] Q. Barthélemy et al., Physical Review X 12, 011014 (2022)
[3] Q. Barthélemy et al., arXiv:2211.08546 (2022) 

Deuxième conférencier: N. Gauthier

The quest for quantum spin liquids in Ce-based pyrochlores

 Rare-earth based pyrochlore magnets form alarge family of geometrically frustrated magnets that exhibit analogous effects to the proton disorder in water ice. The disordered magnetic ground state in pyrochlore materials was named spin ice due to this similarity with water ice. Classical spin ice states have been extensively studied in Dy₂Ti₂O₇and Ho₂Ti₂O₇. Such states are driven by thermal fluctuations and exhibit slow dynamics at low temperatures. In contrast, quantum fluctuations can generate long-range entanglement of the magnetic moments, a state of matter called quantum spin liquid, and more specifically, a quantum spin ice for pyrochlore materials. The fundamentally quantum nature of this state attracts great interest, in particular because it is a playground to study emergent gauge theories with fractionalized excitations. The quantum spin ice state is predicted to host visons and photons, emergent excitations without any classical analogue. Demonstrating the existence of these excitations would be a milestone to establish experimentally the existence of quantum spin liquids.

Ce-based pyrochlore magnets are promising candidates to host a quantum spin ice state and I will discuss our past, present and future works on these materials. Using a combination of experimental techniques, we previously evidenced that Ce₂Sn₂O₇exhibits many key features expected from a quantum spin ice state. More recently, we investigated the low energy excitations of Ce₂Hf₂O₇using specific heat measurements. Our results showcase the relevant energy scales of the emergent excitations. In this context, I will describe the predicted experimental signatures associated with the emergent photon. I will specifically discuss the feasibility of a recent proposal to measure the photon velocity using ultrasound velocity measurements [Simon, et al., PRB 106, 064427(2022)].