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1. Dipolar-octupolar correlations and hierarchy of exchange interactions in ${\mathrm{Ce}}_2{\mathrm{Hf}}_2{\mathrm{O}}_7$

   https://doi.org/10.1103/j451-ztvr  

   Porée, Victor; Bhardwaj, Anish; Lhotel, Elsa; Petit, Sylvain; Gauthier, Nicolas; Yan, Han; Pomjakushin, Vladimir; Ollivier, Jacques; Quilliam, Jeffrey A; Nevidomskyy, Andriy H; et al (November 2025, Physical Review B)

   High-resolution neutron spectroscopy on ${\mathrm{Ce}}_2{\mathrm{Hf}}_2{\mathrm{O}}_7$reveals a correlated state characterized by distinct dipolar scattering signals—quasielastic and inelastic contributions consistent with ‘photon’ and ‘spinon’ excitations in quantum spin ice. These signals coexist with weak octupolar scattering. Fits of thermodynamic data using numerical methods indicate a dominant octupolar exchange, $J_x$or $J_y$, with substantial dipolar $J_z$and minute dipole-octupole $J_{xz}$couplings. The $J_{xz}$value is corroborated by an independent fit of the neutron scattering amplitude balance between dipolar and octupolar ‘photon’ contributions, highlighting its importance to understand neutron scattering results in this family. ${\mathrm{Ce}}_2{\mathrm{Hf}}_2{\mathrm{O}}_7$enriches the landscape of dipole-octupole pyrochlore physics, and reveals a ‘quantum multipolar liquid’ where hybrid correlations involve multiple terms in the moment series expansion, opening questions regarding their intertwining and hierarchy in quantum phases. 

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2. Correlated libration in liquid water

   https://doi.org/10.1063/5.0200094  

   Shelton, David P. (March 2024, The Journal of Chemical Physics)

   The libration spectrum of liquid H2O is resolved into an octupolar twisting libration band at 485 cm−1 and dipolar rocking–wagging libration bands at 707 and 743 cm−1 using polarization analysis of the hyper-Raman scattering (HRS) spectrum. Dipole interactions and orientation correlation over distances less than 2 nm account for the 36 cm−1 splitting of the longitudinal and transverse polarized bands of the dipolar rocking–wagging libration mode, while the intensity difference observed for the bands is the result of libration correlation over distances larger than 200 nm. The coupled rock and wag libration in water is similar to libration modes in ice. The libration relaxation time determined from the width of the spectrum is 36–54 fs. Polarization analysis of the HRS spectrum also shows long range correlation for molecular orientation and hindered translation, bending and stretching vibrations in water. 

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3. Hydrogen bond network modes in liquid water

   https://doi.org/10.1103/PhysRevb.108.174203  

   Shelton, David P. (November 2023, Physical Review B)

   Collective modes and dynamics of dense disordered materials such as water are not well understood, but nonlinear optics provides a sensitive probe to study polar modes in these materials. Here we report the hyper- Raman (HRS) light scattering spectrum measured for liquid D2O decomposed into contributions from transverse and longitudinal dipolar modes and octupolar modes, using the polarization dependence of HRS. Transverse HRS is observed for orientation and stretching modes, while longitudinal HRS is observed for translation, libration, and bending modes. The HRS observations indicate molecular correlation at distances >200 nm for all modes of orientation, libration, and vibration. The rocking/wagging, twisting, and translation modes for D2O molecules in the hydrogen bonded network are distinguished. The LO-TO splitting is 28 cm−1 for the libration mode and 16 cm−1 for the translation mode, and the relaxation time for libration modes is about 80 fs. The long-range correlation of the orientation and stretching modes is explained as the result of the dipole-dipole orientation correlation in a dipolar fluid. The long-range correlation of the longitudinal polarized libration and bending modes needs further study. 

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4. Spin Waves and Magnetic Exchange Hamiltonian in CrSBr

   https://doi.org/10.1002/advs.202202467  

   Scheie, Allen; Ziebel, Michael; Chica, Daniel_G; Bae, Youn_June; Wang, Xiaoping; Kolesnikov, Alexander_I; Zhu, Xiaoyang; Roy, Xavier (July 2022, Advanced Science)

   Abstract CrSBr is an air‐stable two‐dimensional (2D) van der Waals semiconducting magnet with great technological promise, but its atomic‐scale magnetic interactions—crucial information for high‐frequency switching—are poorly understood. An experimental study is presented to determine the CrSBr magnetic exchange Hamiltonian and bulk magnon spectrum. TheA‐type antiferromagnetic order using single crystal neutron diffraction is confirmed here. The magnon dispersions are also measured using inelastic neutron scattering and rigorously fit the excitation modes to a spin wave model. The magnon spectrum is well described by an intra‐plane ferromagnetic Heisenberg exchange model with seven nearest in‐plane exchanges. This fitted exchange Hamiltonian enables theoretical predictions of CrSBr behavior: as one example, the fitted Hamiltonian is used to predict the presence of chiral magnon edge modes with a spin‐orbit enhanced CrSBr heterostructure. 

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5. Sleuthing out exotic quantum spin liquidity in the pyrochlore magnet Ce2Zr2O7

   https://doi.org/10.1038/s41535-022-00458-2  

   Bhardwaj, Anish; Zhang, Shu; Yan, Han; Moessner, Roderich; Nevidomskyy, Andriy H.; Changlani, Hitesh J. (May 2022, npj Quantum Materials)

   Abstract The search for quantum spin liquids—topological magnets with fractionalized excitations—has been a central theme in condensed matter and materials physics. Despite numerous theoretical proposals, connecting experiment with detailed theory exhibiting a robust quantum spin liquid has remained a central challenge. Here, focusing on the strongly spin-orbit coupled effectiveS = 1/2 pyrochlore magnet Ce₂Zr₂O₇, we analyze recent thermodynamic and neutron-scattering experiments, to identify a microscopic effective Hamiltonian through a combination of finite temperature Lanczos, Monte Carlo, and analytical spin dynamics calculations. Its parameter values suggest the existence of an exotic phase, aπ-flux U(1) quantum spin liquid. Intriguingly, the octupolar nature of the moments makes them less prone to be affected by magnetic disorder, while also hiding some otherwise characteristic signatures from neutrons, making this spin liquid arguably more stable than its more conventional counterparts. 

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