PQI Collaborative Seminar - Allen Scheie

About this event

Title: Ground states and Hamiltonians of triangular KYbSe2 and NaYbSe2

Allen Scheie

Grove City College

The 2D triangular lattice has sparked over fifty years of theoretical discovery. If the interactions are tuned just right, this model produces spin liquids and fractionalization. However, until recently, there were few experimental examples with which to compare theory. This changed with the emergence of Yb delafossites as a clean materials realization. I will discuss our experiments on the 2D triangular lattice magnets KYbSe2 and NaYbSe2. Neutron scattering reveals a diffuse continuum in both compounds with a well-defined lower boundary. The KYbSe2 Hamiltonian fits to a very isotropic J1/J2 model that has magnetic order but is extremely close to a quantum spin liquid phase [1], and close enough to harbor signatures of fractionalization. The NaYbSe2 compound shows no magnetic order, but also substantial sample-dependence in bulk properties [2,3]. Follow-up work on these materials has revealed two surprises which theory is struggling to explain: a sub-ballistic collective magnetic transport [4], and what appears to be a quantum deconfinement transition under magnetic field [5]. Both are beyond what semiclassical models can explain. Thus the 2D triangular lattice continues to yield fresh insight in quantum magnetism: both what exotic behavior exists in nature, and a target for many-body quantum theory.

[1] A. Scheie et al, Nature Physics 20 , 1 (2024)

[2] A. Scheie et al, Phys. Rev. B , 109 , 014425 (2024)

[3] A. Scheie et al, ArXiv:2406.17773 (2026)

[4] A. Scheie et al, ArXiv:2602.02433 (2026)

[5] Hosoi et al, Science Advances 12 , 43 (2026)