Random Field Effects and Quantum Annealing in the Disordered Transverse Field Dipolar Ising Model

by Dr Steffen Säubert (Colorado State University)

Monday 7 Dec 2020, 14:30 → 15:30 Europe/Berlin

Zoom Webinar

Quantum annealing refers to a method for solving optimization problems through quantum fluctuations rather than the more traditional thermal fluctuations and has been argued to produce a speed-up for some complex optimization problems [1]. A model system to study quantum annealing is the transverse field Ising model (TFIM) that is experimentally realized in the tetragonal material LiHoF4 and its disordered variant LiHoxY1-xF4 [2]. The disordered TFIM provides a complex optimization problem, where the ground state of the system can be reached more efficiently by quantum fluctuations that induce tunneling under an applied transverse field. This has already been observed, through ac magnetic susceptometry, in the magnetically diluted form LiHoxY1-xF4 (x=0.44) [3], however, without gaining direct information on the microscopic spin correlations that are involved, i.e. the quantity that is actually being optimized. Moreover, in the diluted LiHoxY1-xF4 a transverse field induces random fields [4] that are hypothesized to change the energy landscape of the system and thus the optimization problem itself. I discuss the influence of these random fields on the microscopic spin correlations via a measurement of elastic diffuse magnetic neutron scattering in three dimensions. Further, I report the evolution of these spin correlations for quantum vs thermal annealing.

 

[1] A. Dutta, G. Aeppli, B.K. Chakrabarti, U. Divakaran, T.F. Rosenbaum,

and D. Sen. Quantum Phase Transitions in Transverse Field Spin Models:

From Statistical Physics to Quantum Information. Cambridge University

Press (2015).

[2] M.J.P. Gingras, P. Henelius, Collective Phenomena in the LiHoxY1-xF4 Quantum Ising Magnet: Recent Progress and Open Questions. J. Phys.:

Conf. Ser. 320 012001 (2011).

[3] J. Brooke, D. Bitko, T.F. Rosenbaum, and G. Aeppli. Quantum Annealing

of a Disordered Magnet. Science, 284(5415):779–781 (1999).

[4] S.M.A. Tabei, M.J.P. Gingras, Y.-J. Kao, P. Stasiak, and J.-Y. Fortin.

Induced Random Fields in the LiHoxY1-xF4 Quantum Ising Magnet in a

Transverse Magnetic Field. Phys. Rev. Lett. 97, 237203 (2006).