Kristian Knakkergaard Nielsen

Max Plank Institute of Quantum Optics

Non-equilibrium dynamics of dopants in quantum magnets

The physics of mobile dopants in magnets is key to our understanding of strongly correlated materials, including high-temperature superconductors. Despite decades of research many open questions remain about their properties and possible quasiparticle formation, especially at nonzero temperatures. However, recent breakthroughs in using ultra cold atoms as powerful quantum simulators have now made it possible to get detailed and microscopic insights into the propagation of such dopants. In my talk, I will showcase two main contributions from my side to the understanding of such dynamics. 

First, I will show you how a quench experiment in an ultra cold quantum simulator [1], in which a dopant is suddenly released from a fixed position, can be accurately described [2] by an extension of a well-known technique in condensed matter -- the self-consistent Born approximation. This makes it possible to discern three dynamical evolution stages: (1) initial free ballistic motion via (2) a crossover stage with so-called string excitations to (3) a final ballistic propagation of quasiparticles. 

Second, I will describe a novel variant of Anderson localisation [3]. Namely that the motion of a dopant in an Ising spin environment can be rendered localised at elevated temperatures, leading to a reversed metal to insulator crossover. This emergent phenomenon is traced back to thermally induced spin fluctuations, and is in stark contrast to the origin of localisation for quenched disorder in "regular" Anderson localisation. Finally, I provide explicit evidence that the predicted localisation can be realistically probed in current experimental setups using ultra cold Rydberg-dressed atoms in optical lattices [4].

[1] G. Ji et al., PRX 11, 021022 (2021) 

[2] K. Knakkergaard Nielsen et al., PRL 129, 246601 (2022)

[3] K. Knakkergaard Nielsen, arXiv:2310.11193.

[4] J. Zeiher et al., Nature Phys. 12, 1095-1099 (2016)