Quantum Fluctuations of Squeezed Magnons in Ferro- and Antiferromagnets

Abstract: 
Magnons are the elementary excitations of spin systems. Here, we discuss the emergence of novel quasiparticles, mediated by magnetic dipolar interactions or anisotropies, that have been hiding in simple spin systems with ordered ground states. These quasiparticles exhibit a spin ranging from zero to above 1ℏ and display a variety of interesting quantum properties.[1,2] The unusual properties originate from quantum mechanical squeezing of the magnon vacuum state and its excitations. In antiferromagnets, the magnons are in highly entangled two-mode-squeezed states that might be a resource for quantum information.[4] We suggest that the quantum properties can be detected by noise correlations of spin transport across a magnet/non-magnetic conductor interface.[3] Finally, we discuss more recent works addressing non-collinear magnetic structures that exhibit unusual quantum correlations not visible in the magnetic structure.[4,5] 

Reference List 

[1]    A. Kamra and W. Belzig, Super-Poissonian shot noise of squeezed-magnon mediated spin transport, Phys. Rev. Lett. 116, 146601 (2016). 
[2]    A. Kamra, U.  Agrawal, and W. Belzig, Noninteger-spin magnonic excitations in untextured magnets, Phys. Rev. B 96, 020411(R) (2017). 
[3]    A. Kamra and W. Belzig, Spin pumping and shot noise in ferrimagnets: bridging ferro- and antiferromagnets, Phys. Rev. Lett. 119, 197201 (2017). 
[4]    A. Kamra, E. Thingstad, G. Rastelli, R. A. Duine, A. Brataas, W. Belzig, and A. Sudbø, Antiferromagnetic Magnons as Highly Squeezed Fock States underlying Quantum Correlations, Phys. Rev. B 100, 174407 (2019). 
[5]    Dennis Wuhrer, Levente Rózsa, Ulrich Nowak, and Wolfgang Belzig, Magnon squeezing in conical spin spirals, Phys. Rev. Research 5, 043124 (2023). 
[6]    Levente Rózsa, Dennis Wuhrer, Sebastián A. Díaz, Ulrich Nowak, Wolfgang Belzig Hidden quantum correlations in the ground states of quasiclassical spin systems Phys. Rev. B 111, 174441 (2025).