We develop a first-principles approach to compute infrared (IR) vibrational absorption and Raman scattering spectra with excitonic effects included. Our method is based on a perturbative expansion of electron-phonon and electron-light couplings in the time-dependent adiabatic GW (TD-aGW) theory. We show that excitonic effects in the IR absorption spectrum can be included by replacing the free electron-hole propagators in the perturbative expression for independent particles with their interacting counterparts, which are readily available from standard GW-Bethe-Salpeter-equation calculations. For the Raman spectrum, our derived expression agrees with the single- and double-resonance terms from a diagrammatic approach. We show significant excitonic enhancement in both the IR and resonance Raman scattering intensity for monolayer ${\mathrm{MoS}}_2$, ${\mathrm{WS}}_2$, and ${\mathrm{WSe}}_2$. Moreover, the exciton-phonon coupling strength and exciton energy landscape can be accessed by analyzing resonance Raman spectrum of these two-dimensional materials.

Muon spin relaxation is a powerful technique for probing static and dynamic local magnetic fields. The strong collision model, based on a Gaussian-Markovian process, is commonly used to account for dynamical effects. Yet, it remains limited in describing systems where the local field undergoes discrete state changes. To address this, I introduce a generalized two-state strong collision model that explicitly incorporates transitions between distinct local field environments during fluctuations. This extension allows for a more accurate representation of dynamical effects, particularly in systems where each collision alters the underlying static polarization function. Analytical and numerical solutions are presented, and the model's applicability is demonstrated and discussed across relevant physical systems—including low-dimensional magnets, systems with dynamic disorder, and ion and muon diffusion. These results offer an enhanced framework for interpreting data in complex materials and extend the method's reach to a broader class of dynamical phenomena in condensed matter physics.

Spin-splitting antiferromagnets with spin-polarized band structures in momentum space have garnered intensive research attention due to their zero net magnetic moments, ultrafast spin dynamics as conventional antiferromagnets, and spin-polarized transport properties akin to ferromagnets, making them promising candidates for antiferromagnetic spintronics. However, unlike spin-torque switching of ferromagnets by electric current, efficient electric control of spin-splitting antiferromagnetic order remains challenging. In this work, we identify prototypes of multiferroic spin-splitting antiferromagnets, including $\mathrm{BiFe}{\mathrm{O}}_3, {\mathrm{Fe}}_2{\mathrm{Mo}}_3{\mathrm{O}}_8$, and compensated ferrimagnet $\mathrm{GaFe}{\mathrm{O}}_3$ with ferroelectric polarization as well as spin-polarized electronic structures. We establish design principles for the spin-splitting multiferroic antiferromagnets and compensated ferrimagnet, elucidating the band symmetry features in the Brillouin zone. We demonstrate that the spin polarization in spin-splitting magnets, despite zero net magnetic moment, can be switched by ferroelectric polarization, providing an efficient means of controlling the antiferromagnetic order. Our work may inspire future development of novel multiferroic functional magnets with zero magnetic moments and pave the way for their applications in magnetoelectric spintronic devices.

Motivated by the recent rapid progress in high-$T_c$ nickelate superconductors, we comprehensively study the physical properties of the alternating bilayer trilayer stacking nickelate ${\mathrm{La}}_7{\mathrm{Ni}}_5{\mathrm{O}}_{17}$. The high-symmetry phase of this material, without the tilting of oxygen octahedra, is not stable at ambient conditions but becomes stable under high pressure, where a small hole pocket ${\gamma }_0$, composed of the $d_{3z^2-r^2}$ states in the trilayer sublattice, appears. This pocket was identified in our previous work for trilayer ${\mathrm{La}}_4{\mathrm{Ni}}_3{\mathrm{O}}_{10}$ as important to develop superconductivity. Moreover, using random-phase approximation calculations, we find a leading $s^{\pm }$ pairing state for the high-symmetry phase under pressure with similar pairing strength as that obtained previously for the bilayer ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$ compound, suggesting a similar or higher superconducting transition temperature $T_c$, at the random-phase approximation level. In addition, we find that the dominant magnetic fluctuations in the system driving this pairing state have antiferromagnetic structure both in-plane and between the planes of the top and bottom trilayer and bilayer sublattices, while the middle trilayer is magnetically decoupled.

Rare-earth (RE) elements exhibit diverse crystal structures and unique physicochemical properties under pressure, such as complex magnetic behavior and intriguing superconducting properties. Among them, lanthanum (La) exhibits enhanced superconductivity under compression. However, pressure-induced structural distortion and symmetry reduction modify its electronic properties, significantly reducing its superconducting temperature ($T_{\mathrm{c}}$) from its maximum value of 13.1 K at pressures exceeding 78 GPa. To optimize its $T_{\mathrm{c}}$, we employ an alloying strategy in this study, partially substituting La with yttrium (Y) to modulate its structural symmetry under high pressure, a key factor for $T_{\mathrm{c}}$. X-ray diffraction measurements reveal a phase transition in the La-Y alloy from an ambient hexagonal structure to a face-centered cubic phase at ∼11 GPa, maintaining high symmetry up to at least 170 GPa. Electrical transport measurements confirm that this cubic phase exhibits robust superconductivity, reaching a maximum $T_{\mathrm{c}}$ of 13.6 K at 65 GPa. These findings provide a platform for modulating structural symmetry and physical properties through alloying, highlighting the potential for discovering superior superconductivity in RE materials under extreme conditions.

Fractional Chern insulators (FCIs) with crystalline symmetry possess topological invariants that fundamentally have no analog in continuum fractional quantum Hall states. Here we demonstrate through numerical calculations on model wave functions that FCIs possess a fractionally quantized electric polarization ${\vec{\mathcal{P}}}_{\text{o}}$, where $\text{o}$ is a high-symmetry point. ${\vec{\mathcal{P}}}_{\text{o}}$ takes fractional values compared to the allowed values for integer Chern insulators because of the possibility that anyons carry fractional quantum numbers under lattice translation symmetries. ${\vec{\mathcal{P}}}_{\text{o}}$, together with the discrete shift ${\mathcal{S}}_{\text{o}}$, determines fractionally quantized universal contributions to electric charge in regions containing lattice disclinations, dislocations, boundaries, and/or corners, which are fractions of the minimal anyon charge. We demonstrate how these invariants can be extracted using Monte Carlo computations on model wave functions with lattice defects for 1/2 Laughlin and 1/3 Laughlin FCIs on the square and honeycomb lattices, respectively, obtained using the parton construction. These results comprise a class of fractionally quantized response properties of topologically ordered states that go beyond the known ones discovered over 30 years ago.

We design an enhanced event-chain Monte Carlo algorithm to study 1D quantum dissipative systems, using their bosonized representation. Expressing the bosonized Hamiltonian as a path integral over a scalar field enables the application of Monte Carlo algorithms developed for classical systems. Specifically, we focus on a dissipative XXZ spin chain, exhibiting critical slowing down, minima degeneracy, and long-range interactions. Addressing all three bottlenecks, we design an algorithm that combines local persistent event-chain Monte Carlo moves with global cluster moves, in an $O\left(1\right)$-complexity implementation. Through systematic performance analysis, we show that such an algorithm outperforms traditional Metropolis algorithms by more than a magnitude factor and is competitive with current state-of-the-art quantum Monte Carlo algorithms. We then use this approach to determine the dissipative spin chain's phase diagram, thereby reinforcing prior analytical predictions.

We measured x-ray linear dichroism (XLD) at the preedge of the x-ray absorption near-edge structure (XANES) of Ti and of one halogen (Cl) entering into the composition of some Ti-based MXene compounds. Density functional theory allowed us to retrieve the experimental spectra with a satisfying degree of accuracy and was used for deciphering the orbital and electronic anisotropies responsible for the XLD signal. XLD, measured at the $K$ edge of the transition metal, depends on the nature of the MXene surface termination. XLD, measured at the $K$ edge of the halogen termination, depends on the structure of the van der Waals (vdW) stacking between two adjacent MXene layers, so that it could be used as a signature of a given vdW stacking.

Band-geometric effects are of broad interest for transport phenomena in quantum materials. The wave-packet transport theory is a well-established framework that intuitively captures these effects, particularly well in the adiabatic regime. Motivated by intriguing possibilities of steering ultrafast electronic processes via band-geometric effects, we aim to extend this theory to the nonadiabatic and transient regime. This extension enables us to investigate macroscopic ways of manifesting microscopic band-geometric effects. Crucially, it highlights the special band-geometric manifestations arising from nonadiabatic driving not available to adiabatic driving. We point out three such manifestations in ultrafast processes. The first is the imprinting of band-geometric properties on the current rate after switching off the laser pulses. The second is the induction of intrinsic macroscopic spin polarization with an orientation not accessible by adiabatic processes. The third relates the microscopic, geometrically rooted intrinsic spin coherence to the spin-mediated parts of the macroscopic photocurrents. Through explicit calculations of an example with Rashba spin-orbit coupling, the spin-mediated part is shown to be discernible from the nonspin-mediated part by the anisotropy of the photocurrents. While our primary goal is to extend a previously established theoretical framework, we also present our own experimental data on SnSe, a material exhibiting apparent anisotropic spin-orbit coupling. We distill the key theoretical principles beyond the Rashba system and find consistent agreement with experiment, supporting the physical distinction between spin-mediated and nonspin-mediated components of the photocurrent.

The discovery of superconductivity in Ruddlesden-Popper (RP) nickelates has drawn great attention. Many works have been done to study the superconductivity as well as to find more superconducting RP nickelates. However, there is a lack of general understanding regarding the key factors that contribute to the superconductivity of RP nickelates. Here, we systematically study the series of RP nickelates under doping or high-pressure conditions by means of density functional theory plus dynamical mean-field theory. We find that enhanced quasiparticle weights and local spin fluctuation of the $\mathrm{Ni}\text{−}{e}_g$ orbitals are commonly realized by hole doping or high pressure in the known superconducting RP nickelates, suggesting that they are crucial to the superconductivity. We also summarize experimentally synthesized RP nickelates into phase diagrams with local spin moment and local entanglement entropy as parameters, where phases of spin density wave/antiferromagnetism, superconductivity, and Fermi liquid are distinguished. At last, we predict a promising candidate for superconducting RP nickelates, which is constructed in a “bilayer-trilayer” stacking sequence.

A recent experiment has examined ultracold, fermionic, spin-1/2 ${}^6\mathrm{Li}$ atoms in the Lieb lattice at different Hubbard repulsion $U$ and filling fractions $\nu$ [Lebrat et al., arXiv:2404.17555]. At $\nu =1/2$ and small $U$, they observe an enhanced compressibility on the $p_{x,y}$ sites, pointing to a flat band near the Fermi energy. At $\nu =1/2$ and large $U$ they observe an insulating ferrimagnet. Both small and large $U$ observations at $\nu =1/2$ are consistent with theoretical expectations. Surprisingly, near $\nu =1/4$ and large $U$, they again observe a large $p_{x,y}$ compressibility, pointing to a flat $p_{x,y}$ band of fermions across the Fermi energy. Our Hartree-Fock computations near $\nu =1/4$ find states with canted magnetism (and related spiral states) at large $U$, which possess nearly flat $p_{x,y}$ bands near the Fermi level. We employ parton theories to describe quantum fluctuations of the magnetic order found in Hartree-Fock. We find a metallic state with ${\mathbb{Z}}_2$ fractionalization possessing gapless, fermionic, spinless “chargons” carrying ${\mathbb{Z}}_2$ gauge charges which have a nearly flat $p_{x,y}$ band near their Fermi level: this fractionalized metal is also consistent with observations. Our DMRG study does not indicate the presence of magnetic order, and so is in agreement with a fractionalized ground state. Given the conventional ferrimagnetic insulator at $\nu =1/2$, the ${\mathbb{Z}}_2$ fractionalized metal at $\nu =1/4$ represents a remarkable realization of doping-induced fractionalization.

Polyacene polymers are promising organic one-dimensional materials for optoelectronic and energy-conversion applications that can host nontrivial topologically insulating phases, without the need of spin-orbit coupling. In this work, by using ab initio methods, we show that in its multibridged configuration, it is possible to tune their topological phase by varying the bridges' number and relative position. Moreover, we show that these multibridged polymers can be described by a stacked Su-Schrieffer-Heeger (SSH) model, where interchain hoppings and next-nearest-neighbor interactions play a crucial role in stabilizing a multitude of nontrivial topologically insulating phases. The generality of this model is such that it can be used to describe any couple of interacting SSH chains.

The interplay between strong electronic correlations and the inherent frustration of certain lattice geometries is a common mechanism for the formation of nontrivial states of matter. In this work, we theoretically explore the collective electronic effects in the monolayer ${\mathrm{Nb}}_3{\mathrm{Cl}}_8$, a recently discovered triangular lattice Mott insulator. Our advanced many-body numerical simulations predict the emergence of a phase separation region upon doping this material. Notably, in close proximity to the phase separation, the static charge susceptibility undergoes a drastic change and reveals a distinctive bow-tie structure in momentum space. The appearance of such a fingerprint in the context of spin degrees of freedom would indicate the formation of a spin ice state. This finding allows us to associate the observed phase separation to a charge ice state, a state with a remarkable power law dependence of both the effective exchange interaction and correlations between electronic densities in real space.

Cathodoluminescence spectroscopy has recently emerged as a novel platform for nanoscale control of nonclassical features of light. Here, we propose a theoretical model for cathodoluminescence from a multilevel quantum emitter. Employing a master equation approach and treating the electron-beam excitation as an incoherent broadband field source, we show that quantum interference can arise between the different relaxation pathways. The induced interference can significantly modify the time-dependent spectra resulting in the enhancement or suppression of cathodoluminescence. We find that the excitation rate, initial state of the emitter, and excited level spacing play a crucial role in determining the influence of interference. Our findings shed light on electron-beam-induced quantum interference in cathodoluminescence and provides a theoretical basis for exploring quantum optical phenomena in electron-driven multilevel systems.

Using a recently established band representation analysis, we discover two distinct types of topological nodal magnons in the real-space antiferroic ordering of whirling spin arrangements in the Tb-based icosahedral quasicrystal 1/1 approximants, both of which originate from a composite band (co)representation $A\uparrow P_{I}n\overline{3}\left(24\right)$ and its constituent elementary band representations. The first type is doubly degenerate nodal line network and nodal planes associated with two-dimensional irreducible band representation, while the second type is a nodal line network due to accidental band inversions. Since our analysis, which relies solely on magnetocrystalline symmetry, is valid for a wide range of materials and spin textures belonging to the same magnetic space group irrespective of composition, these findings offer universal insights into the research of Tb-based quasicrystal approximants as well as a contribution to broadening the range of topological magnon-hosting materials.

We explore the superconducting properties of the bilayer Hubbard model, which exhibits a high transition temperature $\left(T_c\right)$ for an $s_{\pm }$ pairing, using a cluster extension of the dynamical mean-field theory. Unlike the single-layer Hubbard model, where the $d$-wave superconductivity emerges by doping the Mott insulator, the parent state of the bilayer system is a correlated band insulator. Above $T_c$, slight hole (electron) doping introduces a striking dichotomy between electron and hole pockets: The electron (hole) pocket develops a pseudogap while the other becomes a nearly incipient band. We reveal that the superconductivity is driven by kinetic (potential) energy gain in the underdoped (overdoped) region. We also find a very short coherence length, for which we argue the relevance to multiorbital physics. Our Letter offers crucial insights into the superconductivity in the bilayer Hubbard model potentially relevant to ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$.

The Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state is a superconducting phase characterized by broken translational symmetry, where Cooper pairs form with nonzero momentum between Zeeman-split Fermi surfaces. This state is highly sensitive to band structure and pairing symmetry. In multiband superconductors, the FFLO state can significantly deviate from its standard form, but experimental verification has remained challenging. Here, we present ${}^{75}\mathrm{As}$ nuclear magnetic resonance (NMR) measurements on the multiband superconductor ${\mathrm{KFe}}_2{\mathrm{As}}_2$. In the low-temperature, high-magnetic-field region above the upper critical field $B_{c2}$, we observe a clear increase in the second moment of the NMR spectrum, along with a strong enhancement in the spin-lattice relaxation rate divided by temperature $1/T_{1}T$. These results indicate an emergence of superconducting spin smecticity and Andreev bound states from the spatial modulation of the superconducting gap, providing microscopic evidence for the FFLO state. The obtained phase diagram reveals a distinct boundary line between the FFLO and homogenous superconducting states with a low critical temperature of the FFLO state $T^{*}\approx 0.2T_c$, which can be attributed to the multiband effects in ${\mathrm{KFe}}_2{\mathrm{As}}_2$. Our results show that the iron-based superconductors are a good material platform for studying the FFLO state and highlight the importance of the multiband effects on this exotic phase.

We present the realization of a spin-$\frac{1}{2}$ hexagonal-plaquette chain with Ising anisotropy, an unexplored quantum spin model that serves as a platform for investigating anisotropic quantum magnetism. Specific heat at zero field reveals a sharp peak at $T_{\mathrm{N}}=1.0$ K, indicating a phase transition to a $\mathrm{N}\acute{\mathrm{e}}\mathrm{el}$ order stabilized by interchain couplings. A perturbative analysis maps the system onto an effective spin-$\frac{1}{2}$ Ising-like chain, supporting the presence of an anisotropy-induced excitation gap. Furthermore, the interchain interactions may induce discrete excitations in the spinon continuum, reminiscent of Zeeman ladder physics observed in related one-dimensional Ising-like systems. These results establish a well-defined model system for correlated spin phenomena in anisotropic magnets and highlight a route for engineering Ising-like quantum states in molecular-based frameworks.