H/F CDD 36 months SIMaP ED I-MEP2 UGA New Quantum Spin Liquids
Reference : UMR5266-MATVEL-005 Workplace : ST MARTIN D HERES Date of publication : Thursday, April 28, 2022 Scientific Responsible name : Matias VELAZQUEZ 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
Getting a material the magnetic ground state of which would be a perfect quantum spin liquid constitute one of the most exciting endeavours of current materials science, because of both the scientific advancement and quantum technologies development it would trigger . Indeed, such an unusual magnetic ground state, which challenges Landau's paradigm of symmetry breaking phase transitions, displays long range quantum entanglement of the spins without long range magnetic order. Predicted in the 1970's [2,3], experimental breakthroughs in this direction have been more recent, since the discovery of such a property in the Herbertsmithite, a mineral solid solution of composition ZnxCu4-x(OH)6Cl2. When x=1, due to its peculiar crystal structure, this compound has become an archetype for quantum spin liquids based on geometrical frustration allowed by a Kagome lattice in an antiferromagnetic 2D Heisenberg system. Its properties are still discussed from a theoretical point of view, and experimental investigations have not yet permitted to discriminate between the different possible scenarios. It turns out that to qualify experimentally a quantum spin liquid, single crystals are mandatory. The major current challenge in this system consists in growing such single crystals with a composition as close as possible to the ZnCu3(OH)6Cl2 stoichiometry and with an as low as possible antisite disorder amount. Indeed, Cu2+ cations substituted for Zn2+ cations between the Kagome planes produce a magnetic contribution which impact the quantum spin liquid behaviour in an elusive way. In order to optimize the crystal growth process to get better quality crystals of larger dimensions, it is necessary on the one hand to better understand the equilibrium thermodynamics of their aqueous solution, and on the other hand, to identify their crystal growth mechanisms.  L. Balents, Nature, 464, 199 (2010).  P. W. Anderson, Mater. Res. Bull., 8 (1973) 153.  P. W. Anderson, Science, 235 (1987) 1196.
- To control the Herbertsmithite crystals ZnxCu4-x(OH)6Cl2 composition with x0.9-1.1 ;
- To control the antisite disorder in the 2D Kagome lattice of the single crystalline Herbertsmithite ZnCu3(OH)6Cl2 ;
- To grow new single crystals in this family of compounds, with natural isotopic contents, deuterated or 17O-enriched ;
- To grow new single crystals in this family of compounds, with other diamagnetic bivalent cations than Zn2+ ;
- To optimize the growth process and identify the crystal growth mechanism in order to obtain crystals with controlled points defects and which can be real 2D quantum spin liquids.
- Synthesis and growth in aqueous solution at optimized supersaturation, pH and temperature, optimized by (Phreeqc) thermodynamic calculations validated by means of Raman in situ characterizations of the solution ;
- Surface, magnetic, structural and chemical characterizations, by means of many methods, such as laboratory and synchrotron X-ray diffraction (-Laue et -XRF), atomic force microscopy (AFM), differential or total interferential contrast microscopies (DIC/TIC), LIBS spectroscopic mapping, magnetic susceptibility measurements ;
- Zn-related liquidus and solidus thermodynamic equilibria calculations by means of DFT and CALPHAD methods complemented by screened solution calorimetry measurements, in order to optimize temperature profiles in the three independent heating zones of the crystal growth furnaces ;
- Exploration of new synthetic routes by preliminary synthesis of reactants unvailable on the market and allowing to lower crystal growth temperatures ;
- Development of in situ characterization methods (DIC/TIC, AFM) for the observation of growth interfaces in order to identify growth mechanisms and relate them to Phreeqc calculations.
The SIMaP laboratory (Science et Ingénierie des Matériaux et leurs Procédés, UGA-CNRS-Grenoble INP) hire a PhD student (36 months, I-MEP2 doctoral school of the University Grenoble Alpes). The position, based in Saint Martin d'Hères (Isère, France), will be available from October 2022. SIMaP has internationally reknown expertise in crystal growth and thermodynamics applied to the optimization of processes. The PhD student will be responsible for the activities described above. He/she will benefit from the scientific supervision of CNRS researchers and engineers and also from the SIMaP technical support assistance and of ESRF beamtime access. This research activity is continuously funded since 10 years, and because of the harsh international competition to understand the quantum spin liquid physics, the PhD student will be strongly incentivised to present his results in national and international conferences. She/he will also undertake several experiments described above (solid state NMR and magnetic susceptibility measurements, Raman in situ, LIBS spectroscopy, advanced micro-Laue and micro-XRF mappings at ESRF, inelastic neutron scattering, crystal shaping, AFM characterizations, solution calorimetry) within a broad French collaboration network.
Expected skills and knowledge
Good knowledge of the thermodynamics and physical chemistry of aqueous solutions, and of materials science. Complementary knowledge in crystal growth physics will be appreciated. Marked interest for experimental work is needed.
Constraints and risks
Preparations in glovebox, work with torch. Regulatory constraints in full force and effect. No risk.