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1. Spin–lattice and electron–phonon coupling in 3d/5d hybrid Sr3NiIrO6

   https://doi.org/10.1038/s41535-019-0184-x  

   O’Neal, Kenneth R. ; Paul, Arpita ; al-Wahish, Amal ; Hughey, Kendall D. ; Blockmon, Avery L. ; Luo, Xuan ; Cheong, Sang-Wook ; Zapf, Vivien S. ; Topping, Craig V. ; Singleton, John ; et al ( August 2019 , npj Quantum Materials)

   While 3d-containing materials display strong electron correlations, narrow band widths, and robust magnetism, 5dsystems are recognized for strong spin–orbit coupling, increased hybridization, and more diffuse orbitals. Combining these properties leads to novel behavior. Sr₃NiIrO₆, for example, displays complex magnetism and ultra-high coercive fields—up to an incredible 55 T. Here, we combine infrared and optical spectroscopies with high-field magnetization and first-principles calculations to explore the fundamental excitations of the lattice and related coupling processes including spin–lattice and electron–phonon mechanisms. Magneto-infrared spectroscopy reveals spin–lattice coupling of three phonons that modulate the Ir environment to reduce the energy required to modify the spin arrangement. While these modes primarily affect exchange within the chains, analysis also uncovers important inter-chain motion. This provides a mechanism by which inter-chain interactions can occur in the developing model for ultra-high coercivity. At the same time, analysis of the on-site Ir⁴⁺excitations reveals vibronic coupling and extremely large crystal field parameters that lead to at_2g-derived low-spin state for Ir. These findings highlight the spin–charge–lattice entanglement in Sr₃NiIrO₆and suggest that similar interactions may take place in other 3d/5dhybrids.
2. Physical origin of giant excitonic and magneto-optical responses in two-dimensional ferromagnetic insulators

   https://doi.org/10.1038/s41467-019-10325-7  

   Wu, Meng ; Li, Zhenglu ; Cao, Ting ; Louie, Steven G. ( May 2019 , Nature Communications)

   The recent discovery of magnetism in atomically thin layers of van der Waals crystals has created great opportunities for exploring light–matter interactions and magneto-optical phenomena in the two-dimensional limit. Optical and magneto-optical experiments have provided insights into these topics, revealing strong magnetic circular dichroism and giant Kerr signals in atomically thin ferromagnetic insulators. However, the nature of the giant magneto-optical responses and their microscopic mechanism remain unclear. Here, by performing first-principlesGWand Bethe-Salpeter equation calculations, we show that excitonic effects dominate the optical and magneto-optical responses in the prototypical two-dimensional ferromagnetic insulator, CrI₃. We simulate the Kerr and Faraday effects in realistic experimental setups, and based on which we predict the sensitive frequency- and substrate-dependence of magneto-optical responses. These findings provide physical understanding of the phenomena as well as potential design principles for engineering magneto-optical and optoelectronic devices using two-dimensional magnets.
3. Exploring few and single layer CrPS ₄ with near-field infrared spectroscopy

   https://doi.org/10.1088/2053-1583/abf251  

   Neal, Sabine N. ; O’Neal, Kenneth R. ; Haglund, Amanda V. ; Mandrus, David G. ; Bechtel, Hans A. ; Carr, G. Lawrence ; Haule, Kristjan ; Vanderbilt, David ; Kim, Heung-Sik ; Musfeldt, Janice L. ( April 2021 , 2D Materials)

   We combine synchrotron-based near-field infrared spectroscopy and first principles lattice dynamics calculations to explore the vibrational response of CrPS₄in bulk, few-, and single-layer form. Analysis of the mode pattern reveals aC2 polar + chiral space group, no symmetry crossover as a function of layer number, and a series of non-monotonic frequency shifts in which modes with significant intralayer character harden on approach to the ultra-thin limit whereas those containing interlayer motion or more complicated displacement patterns soften and show inflection points or steps. This is different from MnPS₃where phonons shift as 1/size²and are sensitive to the three-fold rotation about the metal center that drives the symmetry crossover. We discuss these differences as well as implications for properties such as electric polarization in terms of presence or absence of the P–P dimer and other aspects of local structure, sheet density, and size of the van der Waals gap.
4. Field Angle Tuned Metamagnetism and Lifschitz Transitions in UPt3

   https://doi.org/10.1038/s41598-019-44602-8  

   Shivaram, B. S. ; Holleis, Ludwig ; Ulrich, V. W. ; Singleton, John ; Jaime, Marcelo ( June 2019 , Scientific Reports)

   Strongly correlated electronic systems can harbor a rich variety of quantum spin states. Understanding and controlling such spin states in quantum materials is of great current interest. Focusing on the simple binary system UPt₃with ultrasound (US) as a probe we identify clear signatures in field sweeps demarkating new high field spin phases. Magnetostriction (MS) measurements performed up to 65 T also show signatures at the same fields confirming these phase transitions. At the very lowest temperatures (<200 mK) we also observe magneto-acoustic quantum oscillations which forθ = 90° (B||c-axis) and vicinity abruptly become very strong in the 24.8–30 T range. High resolution magnetization measurements for this same angle reveal a continuous variation of the magnetization implying the subtle nature of the implied transitions. With B rotated away from the c-axis, the US signatures occur at nearly the same field. These transitions merge with the separate sequence of the well known metamagnetic transition which commences at 20 T forθ = 0° but moves to higher fields as 1/cosθ. This merge, suggesting a tricritical behavior, occurs atθ ≈ 51° from the ab-plane. This is an unique off-symmetry angle where the length change along the c-axis is precisely zero due to the anisotropic nature of MS in UPt₃formore »all magnetic field values.

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5. Emergent optical nonreciprocity and chirality-controlled magneto-optical resonance in a hybrid magneto–chiral metamaterial

   https://doi.org/10.1364/OPTICA.480791  

   Peng, Peisong ; Thapa, Grija ; Zhou, Jiangfeng ; Talbayev, Diyar ( January 2023 , Optica)

   Broken spatial and time reversal symmetries in materials often give rise to new emergent phenomena in the interaction between light and matter. The combination of chirality and broken time reversal symmetry in a magnetic field leads to magneto–chiral phenomena, such as the nonreciprocity of transmission. Here, we construct a terahertz hybrid metamaterial that combines the natural optical activity of a chiral metallic gammadion bilayer and the magneto-optical activity of semiconductor indium antimonide in a magnetic field. We report a resonant magneto–chiral effect that leads to polarization-independent nonreciprocal optical transmittance. Furthermore, we discover a magneto-optical Faraday effect that is resonantly controlled by the natural optical activity of the chiral gammadion bilayer. Unlike optical activity due to chirality, the novel Faraday effect is odd under time reversal. Both phenomena are activated by a modest magnetic field, which may open doors for their potential applications in polarization-independent optical isolation and highly efficient polarization control at terahertz frequencies.