Speaker: Blair Lebert
Abstract: Iron’s alpha-epsilon transition, induced by hydrostatic compression around 13 GPa, marks the passage from the well-known bcc, ferromagnetic alpha phase, to the more elusive hcp epsilon phase, whose magnetic properties have been object of study and debate for several decades, their interest spanning from geophysics to the unconventional superconducting state arising at low temperatures. The experimental evidence provided so far has proven to be difficult to reconcile within a single picture and has left room to different theoretical interpretations, ranging from a perfect paramagnet to a long-range ordered antiferromagnetic state.
I will discuss some new experimental results of x-ray emission spectroscopy and neutron powder diffraction, which find appreciable local magnetic moments, but no magnetic long-range order down to very low temperatures (1.8 K) in epsilon iron. I propose an interpretation based on first-principles calculations and classic Monte Carlo calculations, which reveal a ‘spin-smectic’ state lower in energy than previous results. This state forms antiferromagnetic bi-layers separated by null spin bilayers, which allows a complete relaxation of the inherent frustration of antiferromagnetism on a hexagonal close-packed lattice. The magnetic bilayers are likely orientationally disordered, owing to the soft interlayer excitations and the near-degeneracy with other smectic phases. Th e spin-smectic phase may shed some light on the aforementioned contradictions in high-pressure iron and could be integral to explaining its puzzling superconductivity.
I will speculate about the role of the spin-smectic state in epsilon iron's superconductivity and also discuss the challenges of experimentally confirming this spin-smectic state.
Speaker: Adarsh Patri
Abstract: Symmetry broken phases involving higher order multipolar degrees of freedom are historically referred to as so-called “hidden orders”, due to the formidable task of detecting them with conventional probes. In this talk, we theoretically propose a novel and powerful means to directly probe higher-order symmetry breaking: magnetostriction. To that end, we focus on the family of Pr-based cage compounds with strongly correlated f-electrons, Pr(Ti,V,Ir) 2 (Al,Zn) 20 , whose low energy degrees of freedom are composed of purely higher-order multipoles. Employing a symmetry-constructed Landau theory of multipolar moments, we provide key scaling behaviours of the magnetostriction in a range of temperature regimes. These findings provide a way to have clear access to higher order multipolar moments.
Reference: Nature Communications volume 10, Article number: 4092 (2019).