Reference : UMR8502-FABBER-001
Workplace : ORSAY
Date of publication : Monday, June 20, 2022
Scientific Responsible name : BERT Fabrice
Type of Contract : PhD Student contract / Thesis offer
Contract Period : 36 months
Start date of the thesis : 1 October 2022
Proportion of work : Full time
Remuneration : 2 135,00 € gross monthly
Description of the thesis topic
Spin liquids are novel states of quantum matter where the frustration of the magnetic interactions favors the stabilization of a macroscopically entangled quantum state instead of a conventional long range ordered one. In existing candidate materials, the main goal is to identify the exact nature of their unconventional ground states and their exotic excitations (spinons, magnetic monopoles…) but also their stability under perturbations such as disorder, magnetic anisotropies or even external magnetic fields.
The herbertsmithite mineral compound, with its highly frustrated kagome lattice (made of corner-sharing triangles), is the archetypal candidate for the spin liquid physics. Despite strong antiferromagnetic interactions, it does not order magnetically down to the lowest studied temperatures and signatures of unconventional –fractional- excitations are observed in neutron scattering experiments. The exact nature of its spin liquid ground state remains however a pending and hotly debated question. Beyond this emblematic material, still under study in particular by us at Orsay and Sherbrooke, new compounds are being synthesized with various perturbations to a perfect kagome lattice. In particular, we propose to study the Y-kapellasite compound, closely related to herbertsmithite, but which realizes a still unexplored anisotropic kagome magnetic model. Large single crystals of this material are available (Coll. P. Puphal, Stuttgart). Another facet of the project is dedicated to the study of some molybdenum based oxides which realize a so-called 'breathing' kagome lattice, made of triangles with two different sizes. The ratio of the interactions in both kinds of triangles should determine the nature of the ground states in these systems.
We propose to study these novel states through a combination of two complementary techniques: nuclear magnetic resonance (NMR) to extract the static and dynamic magnetic susceptibilities in such complex systems and ultrasound measurements to reveal subtle phase transitions through magneto-elastic coupling.
This PhD project relies on a well-established collaboration between three members of the newly created International Research Laboratory (IRL “Frontières Quantiques”) gathering CNRS labs and the university of Sherbrooke. NMR studies in the SQM (LPS\Paris-Saclay) group are routinely performed up to 14T and from room temperature down to 40mK. At Sherbrooke, very low temperature (50mK) ultrasound measurements can be performed in the group of Jeffrey Quilliam up to 16T. At the LNCMI, much stronger (pulsed) fields up to 90T can be used to reveal transitions, potentially field-induced, in collaboration with David Le Boeuf, expert of ultrasound measurements in high fields.
-Introduction to Frustrated Magnetism (eds Lacroix, C., Mendels, P. & Mila, F. (Springer, 2010).
-Norman, M. R. Herbertsmithite and the search for the quantum spin liquid. Rev. Mod. Phys. 88, 041002 (2016).
-Mendels, P. & Bert, F. Quantum kagome frustrated antiferromagnets: One route to quantum spin liquids, Comptes Rendus Physique, Académie des Sciences 17, 455 (2016).
-Barthélemy Q. et al Local study of the insulating quantum kagome antiferromagnets YCu3(OH)6OxCl3-x (x = 0, 1/3). Phys. Rev. Materials 3, 074401 (2019).
-Khuntia, P. et al. Gapless ground state in the archetypal quantum kagome antiferromagnet ZnCu3(OH)6Cl2 Nature Physics 16, 469 (2020).
-Tustain, K et al. From magnetic order to quantum disorder in the Zn-barlowite series of S=1/2 kagome antiferromagnets, npj Quantum Mater. 5, 74 (2020).
The candidate will work at the Laboratoire de Physique des Solides, university Paris-Saclay-CNRS, in the team Spectroscopies of Quantum Materials. He/She will benefit from the supervision of 3 permanent researchers, experts in the proposed techniques and subject together with well-established national and international collaborations, in particular at Sherbrooke and LNCMI. He/She will also benefit from the expertise of a research engineer in the group and more generally from the whole technical support from the laboratory (mechanic, cryogenics, ..).
Team Information: https://equipes2.lps.u-psud.fr/sqm/
Laboratory Information: https://www.lps.u-psud.fr/
Constraints and risks
Secondments at Sherbrooke and short missions at LNCMI are expected for ultrasound measurements.
The candidate should show a strong motivation for experiments, extreme conditions (low temperatures, high magnetic fields) and fundamental research. A solid background in quantum mechanics and solid-state physics at the Master 2 level is expected.
Applications must include a detailed CV, at least one recommendation letter, a cover letter of one page and transcript of grades for the Master years 1 and 2.
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