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Abstract: Quasiparticles describe collective excitations in many-body systems, and their symmetry classification is of fundamental importance for physical processes such as excited states, transport phenomena, and phase transitions. Recent studies have introduced chirality as an additional degree of freedom in condensed matter physics, leading to a range of novel phenomena. Among these, chiral phonons are of special interest because they carry angular momentum and therefore intrinsically break time reversal symmetry, which non-trivially bridges the spin system with the lattice. Here, we directly prove the presence of chiral phonons in a prototypical polar LiNbO3 crystal. Our demonstration of chiral phonons in a ferroelectric enables in-situ electrical control of momentum-dependent “magnetic” polarization with the reversible phonon handedness. This ferroic control of phonon chirality has substantial potential in the emerging field of chiral phononics, particularly along the associated control of its phonon angular momentum.

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Abstract: X-ray magnetic circular dichroism provides a means to identify ferromagnetic, chiral, and altermagnetic orders via their time-reversal-symmetry (𝒯) breaking. However, the symmetry properties that govern circular dichroism (CD) in resonant inelastic x-ray scattering (RIXS) remain poorly understood. We show that, due to the inherent irreversibility of the RIXS process, RIXS-CD does not require 𝒯 breaking to be present, but reflects the change in unitary symmetries associated with magnetic ordering. Using the altermagnetic MnTe as a model system, we observe an azimuthal-angle dependent RIXS-CD signal in the magnon excitations. Our findings highlight the sensitivity of RIXS-CD to the relativistic symmetry in magnetic systems and its potential application as a probe of magnetic domains.

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Abstract: Mott-insulating 5d double perovskites (DPs) have recently emerged as strongly correlated electron systems of interest due to the novel multipolar physics they display. Key to understanding these properties is proper identification of the energy scales in the electronic Hamiltonian, for which resonant inelastic X-ray scattering (RIXS) is an ideal tool. In the A2MgReO6 (A = Ca, Sr, Ba) 5d1 DPs, Re L3 edge RIXS reveals that the 5d shell is split at a high level by the crystalline electric field (CEF) and at a low level by strong spin-orbit coupling (SOC). Unexpectedly, the SOC excitations are dressed by Jahn-Teller (JT) active phonon modes, implying the presence of a dynamic JT effect. This dynamic JT effect couples the electronic and lattice degrees of freedom, resulting in a spin-orbit-lattice entangled ground state. Vibronic RIXScalculations support these findings and estimate the dynamic JT effect to be one order of magnitude weaker than the SOC. Higher resolution O K edge RIXS measurements of the A2MgReO6 DPs are also presented, wherein the 5d1 ground state is probed via Re 5d - O 2p orbital hybridization. This higher resolution resolves the individual vibronic modes dressing the SOC excitations, and expanded temperature dependence measurements reveal systematic evolution of the dynamic JT effect. Significant enhancement of low energy phononic features is observed at the O K edge as predicted theoretically.Intermediate state effects are believed to imprint the spectra in O K edge RIXS due to the longer core-hole lifetime. In the 5d2 DPs Ba2YReO6 and Sr2CrReO6, Re L2 and L3 edge RIXS reveals that the 5d levels are split at a high level by the CEF as in the 5d1 case. The 5d2 and 5d1 cases diverge at lower energy where the 5d2 levels are split into several multiplets by the interplay of SOC and Hund’s coupling introduced by the presence of a second electron. Computational methods are used to successfully untangle the individual energy scales of the Hund’s and spin-orbit couplings. Differences between the L2 and L3 spectra reflective of dipole selection rules prove key to being able to unambiguously separate these energy scales.

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Abstract: We use uniaxial strain in combination with ultra-high-resolution resonant inelastic x-ray scattering (RIXS) at the oxygen-𝐾 and copper-𝐿3 edges to study the excitations stemming from the charge ordering wave vector in La1.875⁢Sr0.125⁢CuO4. By detwinning stripe ordering, we demonstrate that the optical phonon anomalies do not show any stripe anisotropy. The low-energy charge excitations also retain an in-plane fourfold symmetry. As such, we find that both phonon and charge excitations are decoupled entirely from the strength of static charge ordering. The almost isotropic character of charge excitations is indicative of a quantum critical behavior and remains a possible source for the strange metal properties found in the normal state of cuprate superconductors.

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Abstract: Multi-band Mott insulators with moderate spin-orbit and Hund’s coupling are key reference points for theoretical concept developments of correlated electron systems. The ruthenate Mott insulator Ca2RuO4 has therefore been intensively studied by spectroscopic probes. However, it has been challenging to resolve the fundamental excitations emerging from the hierarchy of electronic energy scales. Here we apply high resolution resonant inelastic x-ray scattering to probe deeper into the low-energy electronic excitations found in Ca2RuO4. In this fashion, we probe a series of spin-orbital excitations. By taking advantage of enhanced energy resolution, we probe a 40 meV mode through the oxygen K-edge. The polarization dependence of this low-energy excitations exposes a distinct orbital nature, originating from the interplay of spin-orbit coupling and octahedral rotations. Additionally, we discuss the role of magnetic correlations to describe the occurrence of excitations with amplitudes which are multiple of a given energy. Such direct determination of relevant electronic energy scales sharpens the target for theory developments of Mott insulators’ orbital degree of freedom.