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We present neutron inelastic scattering measurements of the full three-dimensional spin-wave dispersion in the multiferroic material BiFeO3, for temperatures from 5 to 700 K. Despite the presence of strong electromagnetic coupling, the magnetic excitations behave as conventional magnons over all parts of the Brillouin zone. At low temperature the spin waves are well-defined coherent modes, described by a classical model for a G-type antiferromagnet. A softening of the spin-wave velocity and broadening in energy is already present at room temperature, which is well below the Neel temperature TN ~ 640 K, and increases on heating. In addition, a strong hybridization of the Fe 3d and 0 2p states is found to modify significantly the distribution of the spin-wave spectral weight, which implies that the spins are not restricted to the Fe atomic sites as previously believed. [ABSTRACT FROM AUTHOR]

Our inelastic neutron scattering study of spin excitations in iron telluride reveals remarkable thermal evolution of the collective magnetism. In the temperature range relevant for the superconductivity in FeTe1-xSex materials, where the local-moment behavior is dominated by liquidlike correlations of emergent spin plaquettes, we observe unusual, marked increase of magnetic fluctuations upon heating. The effective spin per Fe at T ≈ 10 K, in the phase with weak antiferromagnetic order, corresponds to S ≈ 1, consistent with the recent analyses that emphasize importance of Hund's coupling [K. Haule and G. Kotliar, New J. Phys. 11, 025021 (2009).]. However, it grows to S ≈ 3/2 in the high-T disordered phase, suggestive of the Kondo-type behavior, where local magnetic moments are entangled with the itinerant electrons. [ABSTRACT FROM AUTHOR]

The nature of the magnetic correlations in Fe-based superconductors remains a matter of controversy. To address this issue, we use inelastic neutron scattering to characterize the strength and temperature dependence of low-energy spin fluctuations in FeTe0.35Se0.65 (Tc~14 K). Integrating magnetic spectral weight for energies up to 12 meV, we find a substantial moment (⟨M2⟩LE~0.07μB2/Fe)that shows little change with temperature, from below Tc to 300 K. Such behavior cannot be explained by the response of conduction electrons alone; states much farther from the Fermi energy must have an instantaneous local spin polarization. It raises interesting questions regarding the formation of the spin gap and resonance peak in the superconducting state. [ABSTRACT FROM AUTHOR]

We report inelastic neutron scattering measurements of low-energy (ℏω≲10 meV) magnetic excitations in the "11" system Fe1+yTe1-xSex. The spin correlations are two-dimensional (2D) in the superconducting samples at low temperature, but appear much more three-dimensional (3D) when the temperature rises well above Tc~15 K, with a clear increase of the (dynamic) spin correlation length perpendicular to the Fe planes. This behavior is extremely unusual; typically, the suppression of thermal fluctuations at low temperature would favor the enhancement of 3D correlations, or even ordering, and the reversion to 2D cannot be naturally explained when only the spin degree of freedom is considered. Our results suggest that the low temperature physics in the 11 system, in particular the evolution of low-energy spin excitations towards superconducting pairing, intrinsically involves changes in orbital correlations. [ABSTRACT FROM AUTHOR]

Recently, Co-based honeycomb magnets have been proposed as promising candidate materials to host the Kitaev spin liquid (KSL) state. One of the front-runners is BaCo2(AsO4)2 (BCAO), where it was suggested that the exchange processes between Co2+ ions via the surrounding edge-sharing oxygen octahedra could give rise to bond dependent Kitaev interactions. In this work, we present and analyze a comprehensive inelastic neutron scattering (INS) study of BCAO with fields in the honeycomb plane. Combining the constraints from the magnon excitations in the high-field polarized state and the inelastic spin structure factor measured in zero magnetic field, we examine two leading theoretical models: the Kitaev-type JK 000 model and the XXZ-J1-J3model. We show that the existing experimental data can be consistently accounted for by the XXZ-J1-J3model but not by the JK 000 model, and we discuss the implications of these results for the realization of a spin liquid phase in BCAO and more generally for the realization of the Kitaev model in cobaltates. [ABSTRACT FROM AUTHOR]

The electron-phonon interaction (EPI) is instrumental in a wide variety of phenomena in solid-state physics, such as electrical resistivity in metals, carrier mobility, optical transition, and polaron effects in semiconductors, lifetime of hot carriers, transition temperature in BCS superconductors, and even spin relaxation in diamond nitrogen-vacancy centers for quantum information processing. However, due to the weak EPI strength, most phenomena have focused on electronic properties rather than on phonon properties. One prominent exception is the Kohn anomaly, where phonon softening can emerge when the phonon wave vector nests the Fermi surface of metals. Here we report a new class of Kohn anomaly in a topological Weyl semimetal (WSM), predicted by field-theoretical calculations, and experimentally observed through inelastic x-ray and neutron scattering on WSM tantalum phosphide. Compared to the conventional Kohn anomaly, the Fermi surface in a WSM exhibits multiple topological singularities of Weyl nodes, leading to a distinct nesting condition with chiral selection, a power-law divergence, and non-negligible dynamical effects. Our work brings the concept of the Kohn anomaly into WSMs and sheds light on elucidating the EPI mechanism in emergent topological materials. [ABSTRACT FROM AUTHOR]