Description du sujet de thèse

Quantum spin liquids are fascinating new states of matter. Unlike conventional ferro- or antiferromagnetic ground states consisting of long-range ordered spins, spin liquids are highly entangled disordered states, which breaks the paradigm of the Landau-Ginzburg-Wilson theory of phase transitions. Quantum fluctuations are so strong that the semi-classical picture of individual spins, relevant for conventional states, become irrelevant. Instead, the spins combine to form singlet states. Spin liquid states result from the quantum superposition of these individual singlets to form a macroscopically entangled state. There are many ways to achieve this superposition and therefore many different types of quantum spin liquids possible. Which ones can actually be made in real materials and how to identify them are central questions. A common imprint of these states is the emergence of unconventional excitations, fractional spinons, emerging photon modes, majorana fermions... which can be detected experimentally.

Magnetic frustration has long been recognized and used as an effective mechanism to promote these exotic states for quantum antiferromagnets (spin-1/2): all the richness of this concept is illustrated by the award of the Nobel Prize to G. Parisi in 2021. Several materials such as herbertsmithite or kapellasite – initially natural minerals – are now synthesized and studied around the world and in our group for their unique magnetic properties. Rare earth pyrochlores, Kitaev magnets or quantum materials with strong spin-orbit coupling are also promising avenues for achieving such exotic states.

We propose to study such new spin liquid materials thanks to our well-established international collaborations, with an original approach combining very complementary high-resolution spectroscopic techniques (NMR, muon spin relaxation, inelastic neutron scattering) and thermodynamic (heat capacity) and acoustic measurements at low temperature.

Role of the PhD student:
The hired student will be responsible for experimental spectroscopy and thermodynamic measurements and data analysis under the supervision of Edwin Kermarrec. His research project will revolve around the study of new spin liquid candidates for which he/she will have to develop expertise (bibliographic work). His work must be submitted for publication and presented at conferences.
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Contexte de travail

The Laboratoire de Physique des Solides (LPS) is a joint research unit (UMR 8502) of the Université Paris-Saclay and the CNRS. It is affiliated with the CNRS Institute of Physics and the 28th section of the National Council of Universities. The LPS is a member of the Friedel-Jacquinot Federation, the physics coordination structure on the Moulon plateau in Orsay (IdF). It brings together around a hundred researchers and professors, experimentalists and theorists, and the research activity is supported by nearly sixty engineers, technicians and administrative staff. The laboratory welcomes a large number of undergraduate and graduate students each year, including many doctoral students, as well as postdoctoral researchers and visiting scientists. The lab covers a wider variety of topics, and aims to address the full diversity of condensed matter physics. Research activity is organized around three main axes, each of which involves approximately the same number of scientists:
• New electronic states of matter
• Physical phenomena with reduced dimensions
• Soft matter and the physics-biology interface
In the first axis are brought together both experimental and theoretical studies relating to the properties of systems in which electronic correlations are generally strong and which are the seats of remarkable properties and unconventional electronic states such as superconductivity, magnetism, metal-insulator transitions etc. In the second are activities relating to “nanosciences” in the broad sense. They are approached here from the point of view of fundamental properties, when the dimensions of an object become as small as certain characteristic distances (coherence length, mean free path, etc.). The third axis extends the concept of “soft matter” to biological systems. The themes therefore range from complex systems to living tissues, from liquid crystals to foams, including polymers or granular systems. These physical studies are at the interface with physico-chemistry and biology.
The research work will be carried out within the SQM team at the Solid State Physics Laboratory (CNRS-UMR 8502).