arXiv subjects

C. Y. Mou

Publications and source records attributed to C. Y. Mou.

7 recordsLinked to original sources

arXiv2026

Non-reciprocal circular dichroism of axial phonons coupled to ferro-rotational order

Circular dichroism (CD) in X-ray absorption, defined as the difference in absorption between opposite circular polarizations, is fundamentally enabled by the breaking of time-reversal symmetry or inversion symmetry. It is therefore sensitive to magnetism, chirality, and their interplay. In contrast, the sample symmetry alone is insufficient to determine whether CD in resonant inelastic X-ray scattering (RIXS) is allowed. Rather, RIXS-CD is governed by both the sample symmetry and the scattering geometry. Here, using RIXS, we identify circularly polarized phonons coupled to ferro-rotational order in MnTiO$_3$, which we refer to as ferro-axial phonons. Their excitations provide a direct demonstration of non-reciprocal RIXS-CD, in which the dichroic response changes upon reversing the propagation direction of the incident X-rays, although the system globally preserves both inversion and time-reversal symmetries. We propose that a condensate of these phonons, manifested as standing waves, underlies the ferro-rotational order in MnTiO$_3$. The observed non-reciprocal CD reflects the interplay among photon helicity, phonon polarization, and ferro-rotational order.

cond-mat.str-el↗

arXiv2026

Quantum critical behavior of cuprate superconductors observed by inelastic X-ray scattering

Progress toward a complete understanding of cuprate superconductors has been hindered by their intricate phase diagram, potentially linked to a quantum critical point (QCP). However, conclusive evidence for the QCP is lacking, as the presumed QCP is buried under the superconducting dome, disguising its presence. Here, we use high-resolution resonant inelastic X-ray scattering to examine the dynamical charge-charge correlation in La$_{2-x}$Sr$_x$CuO$_4$ and uncover the quantum critical scaling, a key feature required for a QCP. Specifically, \djh{we observed that the inverse correlation lengths for various dopings and temperatures collapsed onto a universal scaling curve, yielding a critical exponent $ν$ of $0.74 \pm 0.08$. The non-negativity of this exponent confirms the presence of a QCP. Remarkably, the value of $ν$ suggests that while the QCP is manifested through the charge-density wave, other orders also participate, such that the QCP appears to belong to the universality class characterized by the O(4) symmetry, reminiscent of the microscopic SO(4) symmetry in the Hubbard model at half-filling. Further analysis indicates that the QCP is highly dissipative with a short quasi-particle lifetime, reflecting the intertwined quantum fluctuations due to its being buried inside the superconducting state.

cond-mat.str-el↗

arXiv2025

Altermagnetic boosting of chiral phonons

Chirality characterizes the asymmetry between a structure and its mirror image and underlies a wide range of chiral functionalities. In crystallographically chiral materials, phonons with non-zero linear momentum $\textbf{k}$ can acquire a $k$-induced longitudinal magnetization, giving rise to chiral phonons. Helical spin order, with its proper screw-type configuration, breaks all mirror symmetries and therefore carries magnetic chirality. Such helical spins also generate non-relativistic spin splitting for any quasiparticle excitations propagating along the screw axis. To explore the possible connection between chiral phonons and magnetic chirality, we investigated the crystallographically polar and chiral compound (Mn,Ni)$_3$TeO$_6$, which hosts three distinct states: a paramagnetic state, a helical spin state with magnetic chirality, and a collinear spin state without magnetic chirality. We find an approximately tenfold enhancement of chiral-phonon coupling in the helical spin state along the screw axis, compared with both the paramagnetic and collinear spin states. These results identify a new route to amplify chiral phonons through an altermagnetic effect arising from the broken parity-time symmetry in helical spins. %from non-relativistic spin splitting.

cond-mat.str-el↗

arXiv2023

Precise dd excitations and commensurate intersite Coulomb interactions in the dissimilar cuprate YBa_2Cu_3O_(7-x) and La_(2-x)Sr_xCuO_4

Using high-resolution extreme ultraviolet resonant inelastic X-ray scattering (EUVRIXS) spectroscopy at Cu M-edge, we observed the doping dependent spectral shifts of inter-orbital (dd) excitations of YBa_2Cu_3O_(7-x) and La_(2-x)Sr_xCuO_4. With increasing hole doping level from undoped to optimally doped superconducting compositions, the leading edge of dd excitations is found to shift towards lower energy loss in a roughly linear trend that is irrespective to the cuprate species. The magnitude of energy shift can be explained by including a 0.15 eV Coulomb attraction between Cu 3d_(x^2-y^2) electrons and the doped holes on the surrounding oxygens in the atomic multiplet calculations. The consistent energy shift between distinct cuprate families suggests that this inter-site Coulomb interaction energy scale is relatively material-independent, and provides an important reference point for understanding charge density wave phenomena in the cuprate phase diagram.

cond-mat.supr-con↗

arXiv2021

Quantum fluctuations of charge order induce phonon softening in a superconducting cuprate

Quantum phase transitions play an important role in shaping the phase diagram of high-temperature cuprate superconductors. These cuprates possess intertwined orders which interact strongly with superconductivity. However, the evidence for the quantum critical point associated with the charge order in the superconducting phase remains elusive. Here we show the short-range charge orders and the spectral signature of the quantum fluctuations in La$_{2-x}$Sr$_x$CuO$_4$ (LSCO) near the optimal doping using high-resolution resonant inelastic X-ray scattering. On performing calculations through a diagrammatic framework, we discovered that the charge correlations significantly soften several branches of phonons. These results elucidate the role of charge order in the LSCO compound, providing evidence for quantum critical scaling and discommensurations associated with charge order.

cond-mat.supr-con↗

arXiv2018

Strange superconductivity near an antiferromagnetic heavy fermion quantum critical point

The heavy fermion CeMIn5 family with M = Co, Rh, Ir provide a prototypical example of strange superconductors with unconventional d-wave pairing and strange metal normal state, emerged near an antiferromagnetic quantum critical point. The microscopic origin of strange superconductor and its link to antiferromagnetic quantum criticality and strange metal state are still open issues. We propose a microscopic mechanism for strange superconductor, based on the coexistence and competition between the Kondo correlation and the quasi-2d short-ranged antiferromagnetic resonating-valence-bond spin-liquid near the antiferromagnetic quantum critical point via a large-N Kondo-Heisenberg model and renormalization group analysis beyond the mean-field level. We find the coexistence (competition) between the two types of correlations well explains the overall features of superconducting and strange metal state. The interplay of these two effects provides a qualitative understanding on how superconductivity emerges from the SM state and the observed superconducting phase diagrams for CeMIn5 near the anti-ferromagnetic quantum critical point.

cond-mat.supr-con↗

arXiv2015

Raman and fluorescence characteristics of resonant inelastic X-ray scattering from doped superconducting cuprates

Measurements of spin excitations are essential for an understanding of spin-mediated pairing for superconductivity; and resonant inelastic X-ray scattering (RIXS) provides a considerable opportunity to probe high-energy spin excitations. However, whether RIXS correctly measures the collective spin excitations of doped superconducting cuprates remains under debate. Here we demonstrate distinct Raman- and fluorescence-like RIXS excitations of Bi$_{1.5}$Pb$_{0.6}$Sr$_{1.54}$CaCu$_{2}$O$_{8+δ}$ in the mid-infrared energy region. Combining photon-energy and momentum dependent RIXS measurements with theoretical calculations using exact diagonalization provides conclusive evidence that the Raman-like RIXS excitations correspond to collective spin excitations, which are magnons in the undoped Mott insulators and evolve into paramagnons in doped superconducting compounds. In contrast, the fluorescence-like shifts are due primarily to the continuum of particle-hole excitations in the charge channel. Our results show that under the proper experimental conditions RIXS indeed can be used to probe paramagnons in doped high-$T_c$ cuprate superconductors.

cond-mat.supr-con↗