Spectroscopic probes for doped quantum magnets -- new directions

Single-particle spectral functions, which are usually measured using photoemission experiments in electron systems, contain direct information about fractionalization and the quasiparticle excitation spectrum. In this talk, I will present recent developments that enable angle-resolved photo-emission spectroscopy (ARPES) of the Fermi-Hubbard model using ultra cold atoms in optical lattices. 

I will discuss numerical results for the one-dimensional t-J model, where a sharp asymmetry in the distribution of spectral weight appears, that can be explained by a slave-fermion mean-field theory of the spin excitations (spinons). While in one dimension the spin (spinon) and charge (chargon) excitations are deconfined, several theories suggest that in two dimensions, dopants can be understood as bound states of these partons. 

Recent progress in the microscopic description of mobile dopants allows us to conjecture a one-to-one relation of the one-dopant spectral function and the properties of the constituting spinons in the undoped parent antiferromagnet (AFM). Using time-dependent matrix product state calculations of the spectral function of a single hole doped into a two-dimensional Heisenberg AFM, we thoroughly test this hypothesis and obtain excellent agreement with our semi-analytical predictions. 

We directly probe the microscopic nature of the spinon-chargon bound states through a new extension of ARPES, which uncovers long-lived rotational resonances. Similar to Regge trajectories in high-energy physics, which reflect the quark structure of mesons, we establish a linear dependence of the rotational energy on the super-exchange coupling. 

Our findings suggest that the rich physics of lightly doped cuprates may originate from an emergent parton structure.