Abstract

The van-der-Waals antiferromagnets TMPS3 , where TM = Transition Metal, form an ideal playground for tuning both low-dimensional magnetic and electronic properties . These are layered honeycomb antiferromagnetic Mott insulators, long studied as near-ideal 2D magnetic systems with a rich landscape of competing interactions and a variety of magnetic properties across the family. I will give an overview of my work using high pressure as a continuous tuning parameter to control the dimensionality of these materials. Due to the weak physical inter-planar forces in such van-der-Waals materials, pressure gives us clean and selective control over the interplanar spacing and hence interactions.

I will present magnetic, structural and electrical transport results and compare the behaviour of Fe-, V-, Mn- and NiPS3 as we tune them towards 3D structures – and Mott transitions from insulator to metal. I show multiple enigmatic features in electrical transport in this ‘strange metal’ high pressure regime. I will focus in particular upon our recent ultra-high pressure neutron scattering results, which have unveiled an enigmatic form of short-range magnetic order in metallic FePS3. This phase is particularly important as it most likely forms a precursor to superconductivity, and the direct observation of magnetism here completely overturns the existing wisdom in the literature of a spin-crossover transition as the Fe local environment changes. I will contrast these measurements with our recent equivalent results on sister compound FePSe3, in which superconductivity has in fact recently been observed.

Finally I will describe our recent efforts to engineer magnetic frustration into this lattice, and discuss muon-spin-rotation measurements on some members of the family which give interesting hints towards complex magnetic behaviour.