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1. Cavity magnonics

   https://doi.org/10.1016/j.physrep.2022.06.001  

   Zare Rameshti, Babak; Viola Kusminskiy, Silvia; Haigh, James A.; Usami, Koji; Lachance-Quirion, Dany; Nakamura, Yasunobu; Hu, Can-Ming; Tang, Hong X.; Bauer, Gerrit E.W.; Blanter, Yaroslav M. (September 2022, Physics Reports)

   Fulvio Parmigiani (Ed.)

   Cavity magnonics deals with the interaction of magnons — elementary excitations in magnetic materials — and confined electromagnetic fields. We introduce the basic physics and review the experimental and theoretical progress of this young field that is gearing up for integration in future quantum technologies. Much of its appeal is derived from the strong magnon–photon coupling and the easily-reached nonlinear regime in microwave cavities. The interaction of magnons with light as detected by Brillouin light scattering is enhanced in magnetic optical resonators, which can be employed to cool and heat magnons. The microwave cavity photon-mediated coupling of a magnon mode to a superconducting qubit enables measurements in the single magnon limit. 

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2. Shot noise in a strange metal

   https://doi.org/10.1126/science.abq6100  

   Chen, Liyang; Lowder, Dale T; Bakali, Emine; Andrews, Aaron Maxwell; Schrenk, Werner; Waas, Monika; Svagera, Robert; Eguchi, Gaku; Prochaska, Lukas; Wang, Yiming; et al (November 2023, Science)

   S trange-metal behavior has been observed in materials ranging from high-temperature superconductors to heavy fermion metals. In conventional metals, current is carried by quasiparticles; although it has been suggested that quasiparticles are absent in strange metals, direct experimental evidence is lacking. We measured shot noise to probe the granularity of the current-carrying excitations in nanowires of the heavy fermion strange metal YbRh₂Si₂. When compared with conventional metals, shot noise in these nanowires is strongly suppressed. This suppression cannot be attributed to either electron-phonon or electron-electron interactions in a Fermi liquid, which suggests that the current is not carried by well-defined quasiparticles in the strange-metal regime that we probed. Our work sets the stage for similar studies of other strange metals. 

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3. Quadrupolar magnetic excitations in an isotropic spin-1 antiferromagnet

   https://doi.org/10.1038/s41467-022-30065-5  

   Nag, A.; Nocera, A.; Agrestini, S.; Garcia-Fernandez, M.; Walters, A. C.; Cheong, Sang-Wook; Johnston, S.; Zhou, Ke-Jin (December 2022, Nature Communications)

   Abstract The microscopic origins of emergent behaviours in condensed matter systems are encoded in their excitations. In ordinary magnetic materials, single spin-flips give rise to collective dipolar magnetic excitations called magnons. Likewise, multiple spin-flips can give rise to multipolar magnetic excitations in magnetic materials with spin S ≥ 1. Unfortunately, since most experimental probes are governed by dipolar selection rules, collective multipolar excitations have generally remained elusive. For instance, only dipolar magnetic excitations have been observed in isotropic S = 1 Haldane spin systems. Here, we unveil a hidden quadrupolar constituent of the spin dynamics in antiferromagnetic S = 1 Haldane chain material Y 2 BaNiO 5 using Ni L 3 -edge resonant inelastic x-ray scattering. Our results demonstrate that pure quadrupolar magnetic excitations can be probed without direct interactions with dipolar excitations or anisotropic perturbations. Originating from on-site double spin-flip processes, the quadrupolar magnetic excitations in Y 2 BaNiO 5 show a remarkable dual nature of collective dispersion. While one component propagates as non-interacting entities, the other behaves as a bound quadrupolar magnetic wave. This result highlights the rich and largely unexplored physics of higher-order magnetic excitations. 

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4. Spin-correlated exciton–polaritons in a van der Waals magnet

   https://doi.org/10.1038/s41565-022-01204-2  

   Dirnberger, Florian; Bushati, Rezlind; Datta, Biswajit; Kumar, Ajesh; MacDonald, Allan H.; Baldini, Edoardo; Menon, Vinod M. (September 2022, Nature Nanotechnology)

   Strong coupling between light and elementary excitations is emerging as a powerful tool to engineer the properties of solid-state systems. Spin-correlated excitations that couple strongly to optical cavities promise control over collective quantum phenomena such as magnetic phase transitions, but their suitable electronic resonances are yet to be found. Here, we report strong light–matter coupling in NiPS3, a van der Waals antiferromagnet with highly correlated electronic degrees of freedom. A previously unobserved class of polaritonic quasiparticles emerges from the strong coupling between its spin-correlated excitons and the photons inside a microcavity. Detailed spectroscopic analysis in conjunction with a microscopic theory provides unique insights into the origin and interactions of these exotic magnetically coupled excitations. Our work introduces van der Waals magnets to the field of strong light–matter physics and provides a path towards the design and control of correlated electron systems via cavity quantum electrodynamics. 

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5. Polariton Photonics Using Structured Metals and 2D Materials

   https://doi.org/10.1002/adom.201901090  

   Cai, Ziqiang; Xu, Yihao; Wang, Chuangtang; Liu, Yongmin (October 2019, Advanced Optical Materials)

   Abstract Polaritons are quasiparticles originating from strong interactions between photons and elementary excitations that could enable high tunability, tight electromagnetic field confinement, and large density of photonic states, making it possible to achieve novel and otherwise inaccessible functionalities. For these reasons, polaritons spawn great interest in the fields of physics, materials science, and optics for both fundamental studies as well as potential applications (e.g., modulators, photodetectors, photoluminescence, etc.). In recent years, the explosive growth of research in graphene and other 2D van der Waals materials is witnessed because they provide a new platform that substantially complements conventional metals, dielectrics, and semiconductors to investigate different polariton modes. This review highlights the works published in recent years on the topic of polariton photonics based on structured metals, graphene, and transition‐metal dichalcogenides (TMDs). The exotic optical properties of the polaritons in metallic structures and 2D van der Waals materials offer bright prospects for the development of high‐performance photonic and optoelectronic devices. 

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