Laser-driven quantum magnonics and terahertz dynamics of the order parameter in antiferromagnets

Abstract

The impulsive generation of two-magnon modes in antiferromagnets by femtosecond optical pulses, so-called femto-nanomagnons, leads to coherent longitudinal oscillations of the antiferromagnetic order parameter that cannot be described by a thermodynamic Landau-Lifshitz approach. We argue that this dynamics is triggered as a result of a laser-induced modification of the exchange interaction. In order to describe the oscillations we have formulated a quantum mechanical description in terms of magnon pair operators and coherent states. Such an approach allowed us to derive an effective macroscopic equation of motion for the temporal evolution of the antiferromagnetic order parameter. An implication of the latter is that the photo-induced spin dynamics represents a macroscopic entanglement of pairs of magnons with femtosecond period and nanometer wavelength. By performing magneto-optical pump-probe experiments with 10 femtosecond resolution in the cubic KNiF 3 and the uniaxial K 2 NiF 4 collinear Heisenberg antiferromagnets, we observed coherent oscillations at the frequency of 22 THz and 16 THz, respectively. The detected frequencies as a function of the temperature fit the two-magnon excitation up to the Néel point. The experimental signals are described as dynamics of magnetic linear dichroism due to longitudinal oscillations of the antiferromagnetic vector.

Investigating ultrafast quantum magnetism with machine learning

“…For the simple cubic lattice we use the following time-dependent perturbation of the spin Hamiltonian (i.e. the Raman scattering operator) [20][21][22][23]…”

“…Our main interest is the study of impulsively stimulated Raman scattering that was recently investigated both experimentally and theoretically on the basis of harmonic magnon theory [4,5,22]. To model this problem, we approximate the time-dependent change of the exchange interaction as a square pulse with height ∆J ex and temporal width τ .…”

Antiferromagnetic magnons as highly squeezed Fock states underlying quantum correlations

“…While investigated theoretically, the latter have been largely forgotten, probably owing to the experimental challenge of generating them. The squeezing concept applies to bosonic modes in general, and squeezed states of magnons [14][15][16][17] and phonons 18 have also been achieved experimentally.…”

Zero-field Optic Mode Beyond 20 GHz in a Synthetic Antiferromagnet

“…The limitations in achieving further increases of H k to realize higher-frequency magnetic devices have led to the exploitation of AF materials that have faster dynamic responses than are available in their ferromagnetic counterparts [1,7,24]. Resonance experiments of single-phase antiferromagnets are typically conducted with terahertzrange optical or electrical field probing techniques [8,[25][26][27][28][29], with limited reports of antiferromagnets with dynamics in the range of conventional microwave electronics [30]. Therefore, there is a particular research interest in synthetic antiferromagnetic (SAF) structures, comprised of an exchange-coupled ferromagnet-nonmagnet-ferromagnet (FM-NM-FM) multilayers.…”