Theory Seminar: Spatial structure and non-equilibrium dynamics of magnetic polarons

Kristian Knakkergaard Nielsen (Aarhus University):
In recent years, the quantum simulation [1] of the Fermi-Hubbard Hamiltonian [2] has made it possible to study the underlying physics of a vast class of strongly correlated materials [3].

Kristian Knakkergaard Nielsen (Aarhus University)
Group Seminar via Zoom
Tue 31. August 2021, 11:30 am (MEZ)

Abstract:

In recent years, the quantum simulation [1] of the Fermi-Hubbard Hamiltonian [2] has made it possible to study the underlying physics of a vast class of strongly correlated materials [3]. An important case emerges close to half filling, where each lattice site is occupied by one fermion. Then, strong on-site repulsion leads to the buildup of antiferromagnetic order of the spins of the fermions, which competes with the delocalization of holes that can be present in the lattice [4]. The resulting buildup of magnetic frustrations around such holes leads to the formation of quasiparticles, termed magnetic polarons [5].

Describing the associated spatial correlations recently measured in cold atom experiments [6,7] is inherently highly involved, because of the strongly correlated nature of the system. Nevertheless, in this talk, I will show that it is possible to use a powerful non-perturbative approach for the calculation of the hole Green’s function [8] to also describe hole-spin correlation functions [9], by constructing the full quasiparticle wave function [10].

This enables us to calculate the magnetization cloud in the vicinity of a hole, revealing the spatial structure of magnetic polarons. Crucially, this forces us to extent previous methods to include up to an infinite number of excitations of the underlying spin lattice. Additionally, our calculations reveal a remarkably high spatial symmetry of the polaronic magnetization cloud and a surprising misalignment between its orientation and the polaron crystal momentum.

Finally, I will showcase how this method can be generalized to accurately describe the non-equilibrium quench dynamics of holes in the spin lattice, as recently measured in experiments [11].
[1] A. Mazurenko et al., Nature 545, 462 (2017)
[2] J. Hubbard, Proc. R. Soc. London A, Containing Papers of a Mathematical and Physical Character 276, 238 (1963).
[3] P. A. Lee, N. Nagaosa, and X.-G. Wen, Rev. Mod. Phys. 78, 17 (2006).
[4] Y. A. Izyumov, Physics-Uspekhi 40, 445 (1997).
[5] S. Schmitt-Rink, C. M. Varma, and A. E. Ruckenstein, Phys. Rev. Lett. 60, 2793 (1988).
[6] J. Koepsell et al., Nature 572, 358 (2019).
[7] C. S. Chiu et al., Science 365, 251 (2019).
[8] C. L. Kane, P. A. Lee, and N. Read, Phys. Rev. B 39, 6880 (1989).
[9] A. Ramsak and P. Horsch, Phys. Rev. B 48, 10559 (1993).
[10] G. F. Reiter, Phys. Rev. B 49, 1536 (1994).
[11] G. Ji et al., Phys. Rev. X 11, 021022 (2021).

Paper:
K. K. Nielsen et al., “The spatial structure of magnetic polarons in strongly interacting antiferromagnets” (2021), arXiv:2106.14510 [cond-mat.str-el].

If you'd like to participate in the seminar, please contact us!