Nearly fractionalized excitations in 2D quantum antiferromagnets

          @misc{ pirsa_PIRSA:17050086,
            doi = {10.48660/17050086},
            url = {https://pirsa.org/17050086},
            author = {Sandvik, Anders},
            keywords = {Condensed Matter},
            language = {en},
            title = {Nearly fractionalized excitations in 2D quantum antiferromagnets},
            publisher = {Perimeter Institute},
            year = {2017},
            month = {may},
            note = {PIRSA:17050086 see, \url{https://pirsa.org}}
          }
          

Abstract

The 2D S = 1/2 square-lattice Heisenberg model is a keystone of theoretical studies of quantum magnetism. It also has very good realizations in several classes of layered insulators with localized electronic spins. While spin-wave theory provides a good understanding of the antiferromagnetic ground state and low-lying excitations of the Heisenberg model, an anomaly in the excitations at higher energy around wave-number q = (\pi, 0) has been diffi_cult to explain. At first sight, the anomaly is just a suppression of the excitation energy by a few percent, but it also represents a more dramatic shift of spectral weight in the dynamic spin structure factor from the single- magnon (spin wave) pole to a continuum. Recent neutron scattering experiments on the quasi-2D material Cu(DCOO)2_.4D2O (the best realization so far of the 2D Heisenberg model) were even interpreted as a complete lack of magnon pole at the anomaly; instead it was suggested that the excitations there are fractional (spinons) [1]. I will discuss recent quantum Monte Carlo and stochastic analytic continuation results pointing to the existence of fragile q~(\pi,0) magnon excitations in the Heisenberg model [2], which can be fractionalized by interactions competing with the nearest-neighbor exchange coupling. This phenomenon can be understood phenomenologically within a simple theory of magnon-spinon mixing.