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Register a new userSpectral sensitive phonon wipeout due to a fluctuating spin state in a Fe2+ coordination polymer
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Raman scattering in the spin-crossover system [Fe(pmd)(H2O){Au(CN)2}2]*H2O reveals a complex three-phase spin-state transition in contrast to earlier observations in magnetization measurements. We observe different spin state phases as function of temperature and electromagnetic radiation in the visible spectral range. There exists a fluctuating spin state phase with an unexpected wipeout of the low frequency phonon scattering intensity. Furthermore we observe one phase with reduced symmetry that is attributed to a cooperative Jahn-Teller effect. Pronounced electron-phonon interaction manifests itself as a strong Fano-resonance of phonons related to {FeN6} and {FeN4O2} coordination octahedra. Density functional theory supports this interpretation.
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The narrow gap semiconductor FeSi owes its strong paramagnetism to electron-correlation effects. Partial Co substitution for Fe produces a spin-polarized doped semiconductor. The spin-polarization causes suppression of the metallic reflectivity and increased scattering of charge carriers, in contrast to what happens in other magnetic semiconductors, where magnetic order reduces the scattering. The loss of metallicity continues progressively even into the fully polarized state, and entails as much as a 25% reduction in average mean-free path. We attribute the observed effect to a deepening of the potential wells presented by the randomly distributed Co atoms to the majority spin carriers. This mechanism inverts the sequence of steps for dealing with disorder and interactions from that in the classic Altshuler Aronov approach - where disorder amplifies the Coulomb interaction between carriers - in that here, the Coulomb interaction leads to spin polarization which in turn amplifies the disorder-induced scattering.
Enhanced coupling of material properties offers new fundamental insights and routes to multifunctional devices. In this context 5d oxides provide new paradigms of cooperative interactions driving novel emergent behavior. This is exemplified in 5d osmates that host a metal-insulator transition (MIT) driven by magnetic order. Here we consider the most robust case, the 5d perovskite NaOsO3, and reveal a giant coupling between spin and phonon through a frequency shift of {Delta}{omega}=40 cm-1, the largest measured in any material. We identify the dominant octahedral breathing mode and show isosymmetry with spin ordering which induces dynamic charge disproportionation that sheds new light on the MIT. The occurrence of the dramatic spin-phonon-electronic coupling in NaOsO3 is due to a property common to all 5d materials: the large spatial extent of the 5d ion. This allows magnetism to couple to phonons on an unprecedented scale and consequently offers multiple new routes to enhanced coupled phenomena.
We report a comprehensive investigation of the magnetism of the $S$ = 3/2 triangular-lattice antiferromagnet, $alpha$-CrOOH(D) (delafossites green-grey powder). The nearly Heisenberg antiferromagnetic Hamiltonian ($J_1$ $sim$ 23.5 K) with a weak single-ion anisotropy of $|D|$/$J_1$ $sim$ 4.6% is quantitatively determined by fitting to the electron spin resonance (ESR) linewidth and susceptibility measured at high temperatures. The weak single-ion anisotropy interactions, possibly along with other perturbations, e.g. next-nearest-neighbor interactions, suppress the long-range magnetic order and render the system disordered, as evidenced by both the absence of any clear magnetic reflections in neutron diffraction and the presence of the dominant paramagnetic ESR signal down to 2 K ($sim$ 0.04$J_1$$S^2$), where the magnetic entropy is almost zero. The power-law behavior of specific heat ($C_m$ $sim$ $T^{2.2}$) observed below the freezing temperature of $T_f$ = 25 K in $alpha$-CrOOH or below $T_f$ = 22 K in $alpha$-CrOOD is insensitive to the external magnetic field, and thus is consistent with the theoretical prediction of a gapless U(1) Dirac quantum spin liquid (QSL) ground state. At low temperatures, the spectral weight of the low-energy continuous spin excitations accumulates at the K points of the Brillouin zone, e.g. $|mathbf{Q}|$ = 4$pi$/(3$a$), and the putative Dirac cones are clearly visible. Our work is a first step towards the understanding of the possible Dirac QSL ground state in this triangular-lattice magnet with $S$ = 3/2.
We report on comprehensive results identifying the ground state of a triangular-lattice structured YbZnGaO$_4$ to be spin glass, including no long-range magnetic order, prominent broad excitation continua, and absence of magnetic thermal conductivity. More crucially, from the ultralow-temperature a.c. susceptibility measurements, we unambiguously observe frequency-dependent peaks around 0.1 K, indicating the spin-glass ground state. We suggest this conclusion to hold also for its sister compound YbMgGaO$_4$, which is confirmed by the observation of spin freezing at low temperatures. We consider disorder and frustration to be the main driving force for the spin-glass phase.
The quadruple Calcium manganite (CaMn7O12) is a multiferroic material that exhibits a giant magnetically-induced ferroelectric polarization which makes it very interesting for magnetoelectric applications. Here, we report the Raman spectroscopy study on this compound of both the phonon modes and the low energy excitations from 4 K to room temperature. A detailed study of the Raman active phonon excitations shows that three phonon modes evidence a spin-phonon coupling at TN2 = 50 K. In particular, we show that the mode at 432 cm-1 associated to Mn(B)O6 (B position of the perovskite) rotations around the [111] cubic diagonal is impacted by the magnetic transition at 50 K and its coupling to the new modulation of the Mn spin in the (a,b) plane. At low energies, two large low energy excitations are observed at 25 and 47 cm-1. The first one disappears at 50 K and the second one at 90 K. We have associated these excitations to electro-magneto-active modes.
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