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Register a new userResonant two-magnon Raman scattering in two-dimensional and ladder-type Mott insulators
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We investigate the resonant two-magnon Raman scattering in the two-dimensional (2D) and ladder-type Mott insulators by using a half-filled Hubbard model in the strong coupling limit. By performing numerical diagonalization calculations for small clusters, we find that the model can reproduce the experimental features in the 2D that the Raman intensity is enhanced when the incoming photon energy is not near the absorption edge but well above it. In the ladder-type Mott insulators, the Raman intensity is found to resonate with absorption spectrum in contrast to the 2D system. The difference between 2D and the ladder systems is explained by taking into account the fact that the ground state in 2D is a spin-ordered state while that in ladder is a spin-gapped one.
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We present results of low-temperature two-magnon resonance Raman excitation profile measurements for single layer Sr_2CuO_2Cl_2 and bilayer YBa_2Cu_3O_{6 + delta} antiferromagnets over the excitation region from 1.65 to 3.05 eV. These data reveal composite structure of the two-magnon line shape and strong nonmonotic dependence of the scattering intensity on excitation energy. We analyze these data using the triple resonance theory of Chubukov and Frenkel (Phys. Rev. Lett., 74, 3057 (1995)) and deduce information about magnetic interaction and band parameters in these materials.
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Although the parent iron-based pnictides and chalcogenides are itinerant antiferromagnets, the use of local moment picture to understand their magnetic properties is still widespread. We study magnetic Raman scattering from a local moment perspective for various quantum spin models proposed for this new class of superconductors. These models vary greatly in the level of magnetic frustration and show a vastly different two-magnon Raman response. Light scattering by two-magnon excitations thus provides a robust and independent measure of the underlying spin interactions. In accord with other recent experiments, our results indicate that the amount of magnetic frustration in these systems may be small.
We have used Raman scattering to investigate the magnetic excitations and lattice dynamics in the prototypical spin-orbit Mott insulators Sr2IrO4 and Sr3Ir2O7. Both compounds exhibit pronounced two-magnon Raman scattering features with different energies, lineshapes, and temperature dependencies, which in part reflect the different influence of long-range frustrating exchange interactions. Additionally, we find strong Fano asymmetries in the lineshapes of low-energy phonon modes in both compounds, which disappear upon cooling below the antiferromagnetic ordering temperatures. These unusual phonon anomalies indicate that the spin-orbit coupling in Mott-insulating iridates is not sufficiently strong to quench the orbital dynamics in the paramagnetic state.
(Sr$_{2}$,Ba$_{2}$)Cu$_{3}$O$_{4}$Cl$_{2}$ are antiferromagnetic insulators which are akin to the parent compounds of the cuprate superconductors but with two distinct magnetic ordering temperatures related to two magnetic Cu$_{I}$ and Cu$_{II}$ spin sublattices. Here we present a study of these materials by means of Raman spectroscopy. Following the temperature and polarization dependence of the data we readily identify two distinct features at around 3000 cm$^{-1}$ and 300 cm$^{-1}$ that are related to two-magnon scattering from the two sublattices. The estimated spin-exchange coupling constants for the Cu$_{I}$ and Cu$_{II}$ sublattices are found to be J$_{I}sim$139-143(132-136) meV and J$_{II}sim$14(11) meV for Sr(Ba) compounds. Moreover, we observe modes at around 480 and 445 cm$^{-1}$ for the Sr and Ba containing samples respectively, that disappears at the ordering temperature of the Cu$_{II}$. We argue that this modes may also be of magnetic origin and possibly related to interband transitions between the Cu$_{I}$-Cu$_{II}$ sublattices.
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