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1. Magnetic dynamics of strained non-collinear antiferromagnet

   https://doi.org/10.1063/5.0192467  

   He, Zhiping; Liu, Luqiao (March 2024, Journal of Applied Physics)

   In this work, we theoretically study the switching and oscillation dynamics in strained non-collinear antiferromagnet (AFM) Mn3X (X = Sn, Ge, etc.). Using the perturbation theory, we identify three separable dynamic modes—one uniform and two optical modes, for which we analytically derive the oscillation frequencies and effective damping. We also establish a compact, vector equation for describing the dynamics of the uniform mode, which is in analogy to the conventional Landau–Lifshitz–Gilbert (LLG) equation for ferromagnet but captures the unique features of the cluster octuple moment. Extending our model to include spatial inhomogeneity, we are able to describe the excitations of dissipative spin wave and spin superfluidity state in the non-collinear AFM. Furthermore, we carry out numerical simulations based on coupled LLG equations to verify the analytical results, where good agreements are reached. Our treatment with the perturbative approach provides a systematic tool for studying the dynamics of non-collinear AFM and is generalizable to other magnetic systems in which the Hamiltonian can be expressed in a hierarchy of energy scales. 

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2. Linear response theory of open systems with exceptional points

   https://doi.org/10.1038/s41467-022-30715-8  

   Hashemi, A.; Busch, K.; Christodoulides, D. N.; Ozdemir, S. K.; El-Ganainy, R. (December 2022, Nature Communications)

   Abstract Understanding the linear response of any system is the first step towards analyzing its linear and nonlinear dynamics, stability properties, as well as its behavior in the presence of noise. In non-Hermitian Hamiltonian systems, calculating the linear response is complicated due to the non-orthogonality of their eigenmodes, and the presence of exceptional points (EPs). Here, we derive a closed form series expansion of the resolvent associated with an arbitrary non-Hermitian system in terms of the ordinary and generalized eigenfunctions of the underlying Hamiltonian. This in turn reveals an interesting and previously overlooked feature of non-Hermitian systems, namely that their lineshape scaling is dictated by how the input (excitation) and output (collection) profiles are chosen. In particular, we demonstrate that a configuration with an EP of order M can exhibit a Lorentzian response or a super-Lorentzian response of order M s with M s  = 2, 3, …,  M , depending on the choice of input and output channels. 

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3. Signatures of Mottness and Hundness in archetypal correlated metals

   https://doi.org/10.1038/s41467-019-10257-2  

   Deng, Xiaoyu; Stadler, Katharina M.; Haule, Kristjan; Weichselbaum, Andreas; von Delft, Jan; Kotliar, Gabriel (June 2019, Nature Communications)

   Abstract Physical properties of multi-orbital materials depend not only on the strength of the effective interactions among the valence electrons but also on their type. Strong correlations are caused by either Mott physics that captures the Coulomb repulsion among charges, or Hund physics that aligns the spins in different orbitals. We identify four energy scales marking the onset and the completion of screening in orbital and spin channels. The differences in these scales, which are manifest in the temperature dependence of the local spectrum and of the charge, spin and orbital susceptibilities, provide clear signatures distinguishing Mott and Hund physics. We illustrate these concepts with realistic studies of two archetypal strongly correlated materials, and corroborate the generality of our conclusions with a model Hamiltonian study. 

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4. Dynamics and thermodynamics of air-driven active spinners

   https://doi.org/10.1039/c8sm00403j  

   Farhadi, Somayeh; Machaca, Sergio; Aird, Justin; Torres Maldonado, Bryan O.; Davis, Stanley; Arratia, Paulo E.; Durian, Douglas J. (January 2018, Soft Matter)

   We report on the collective behavior of active particles in which energy is continuously supplied to rotational degrees of freedom. The active spinners are 3D-printed disks, 1 cm in diameter, that have an embedded fan-like structure, such that a sub-levitating up-flow of air forces them to spin. Single spinners exhibit Brownian motion with a narrow Gaussian velocity distribution function, P ( v ), for translational motion. We study the evolution of P ( v ) as the packing fraction and the average single particle spin speeds are varied. The interparticle hydrodynamic interaction is negligible, and the dynamics is dominated by hyperelastic collisions and dissipative forces. As expected for nonequilibrium systems, P ( v ) for a collection of many spinners deviates from Gaussian behavior. However, unlike translationally active systems, phase separation is not observed, and the system remains spatially homogeneous. We then search for a near-equilibrium counterpart for our active spinners by measuring the equation of state. Interestingly, it agrees well with a hard-sphere model, despite the dissipative nature of the single particle dynamics. 

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5. A phase-space semiclassical approach for modeling nonadiabatic nuclear dynamics with electronic spin

   https://doi.org/10.1063/5.0093345  

   Wu, Yanze; Bian, Xuezhi; Rawlinson, Jonathan I.; Littlejohn, Robert G.; Subotnik, Joseph E. (July 2022, The Journal of Chemical Physics)

   Chemical relaxation phenomena, including photochemistry and electron transfer processes, form a vigorous area of research in which nonadiabatic dynamics plays a fundamental role. However, for electronic systems with spin degrees of freedom, there are few if any applicable and practical quasiclassical methods. Here, we show that for nonadiabatic dynamics with two electronic states and a complex-valued Hamiltonian that does not obey time-reversal symmetry (as relevant to many coupled nuclear-electronic-spin systems), the optimal semiclassical approach is to generalize Tully’s surface hopping dynamics from coordinate space to phase space. In order to generate the relevant phase-space adiabatic surfaces, one isolates a proper set of diabats, applies a phase gauge transformation, and then diagonalizes the total Hamiltonian (which is now parameterized by both R and P). The resulting algorithm is simple and valid in both the adiabatic and nonadiabatic limits, incorporating all Berry curvature effects. Most importantly, the resulting algorithm allows for the study of semiclassical nonadiabatic dynamics in the presence of spin–orbit coupling and/or external magnetic fields. One expects many simulations to follow as far as modeling cutting-edge experiments with entangled nuclear, electronic, and spin degrees of freedom, e.g., experiments displaying chiral-induced spin selectivity. 

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