Muons and magnets

Abstract:

${\text{CoTi}}_2{\mathrm{O}}_5$ has the paradox that low temperature static magnetic order is incompatible with the crystal structure owing to a mirror plane that exactly frustrates magnetic interactions. Despite no observable structural distortion with diffraction, ${\text{CoTi}}_2{\mathrm{O}}_5$ does magnetically order below $T_N\sim 25\mathrm{K}$ with the breaking of spin ground state degeneracy proposed to be a realization of the spin Jahn-Teller effect in analogy to the celebrated orbital Jahn-Teller transition. We apply neutron and Raman spectroscopy to study the dynamics of this transition in ${\text{CoTi}}_2{\mathrm{O}}_5$ . We find anomalous acoustics associated with a symmetry breaking strain that characterizes the spin Jahn-Teller transition. Crucially, the energy of this phonon coincides with the energy scale of the magnetic excitations, and has the same symmetry of an optic mode, observed with Raman spectroscopy, which atypically softens in energy with decreasing temperature. Taken together, we propose that the energetics of the spin Jahn-Teller effect in ${\text{CoTi}}_2{\mathrm{O}}_5$ are related to cooperative magnetoelastic fluctuations as opposed to conventional soft critical dynamics which typically drive large measurable static displacements. Published by the American Physical Society 2025

Abstract:

Muon spectroscopy has become a leading tool for the investigation of local magnetic fields in condensed matter physics, finding applications in the study of superconductivity, magnetism, ionic diffusion in battery materials, and numerous other fields. Though the muon yields quantitative information about the material, this can only be fully interpreted if the nature of the muon site and its stability is fully understood. Electronic structure calculations are of paramount importance for providing this understanding, particularly through a group of techniques that has become known as DFT +μ, density functional theory including the presence of the implanted muon. We describe how these electronic structure calculations can be used to underpin muon spectroscopy, and some examples of the science that follows from this, as well as some of the available software tools that are currently being developed.