Elementary excitations of quantum spin liquids (Jan Bruin)

A primary aim of the Department of Quantum Materials is to study new 'electronic phases of matter' that emerge in materials with strong electronic correlations, and a very topical subject is that of quantum spin liquids. A quantum spin liquid has strongly interacting local magnetic moments which nevertheless do not form magnetic order down to zero temperature. Proving that a system is really a spin liquid can be difficult, as it is mainly defined by what it 'is not': No long range magnetic order, no spontaneous time reversal / translational / rotational symmetry breaking, and no spin freezing. However, an important feature of these quantum spin liquids is that they may host charge-neutral excitations (which, depending on the system, may be 'spinons', 'visons' or 'Majorana Fermions'). Detecting the presence of these excitations gives positive proof of the existence of the quantum spin liquid state. These excitations carry entropy but not charge, so they may be detected in thermal transport, but not electrical transport. In addition, their existence may be detected by specific heat and other thermodynamic probes.

In this project, you will be joining our effort to detect elementary excitations in new quantum spin liquid candidates at very low temperature. You will learn to perform sample characterization including magnetic susceptibility and specific heat on single crystals down to 2K on a PPMS (physical property measurement system). In addition, you will learn low-temperature measurement methods, including thermal conductivity measurements on dilution refrigerators down to below 100mK. The task combines a wide variety of skills, such as careful manipulation of small samples under a microscope, handling cryogenic liquids, performing low-noise electrical measurements and programming tasks to analyse and present large data sets. No prior experience is necessary, but a strong motivation engage in all these tasks is essential.

The aim of this project is to do a thorough characterization of low temperature excitations in a novel spin liquid, with the ideal scenario of course being the detection of exotic elementary spin excitations. However, the reality of low-temperature measurements can be quite challenging, so a definite result at the end of this project is not guaranteed. In any case, a dedicated and self-motivated student is likely to learn a large number of valuable practical skills, as well as benefit from the experience in being involved a hot research topic.

1. L. Balents, Spin liquids in frustrated magnets. Nature 464, 199 (2010). (review of quantum spin liquids)

2. M. Li & G. Chen, Thermal transport for probing quantum materials. MRS Bull. 45, 348–356 (2020). (general review of thermal transport in quantum materials)


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