We report a detailed inelastic neutron scattering (INS) and muon spin relaxation $\left(\mu \mathrm{SR}\right)$ investigations of a trimer ruthenate ${\mathrm{Ba}}_5{\mathrm{Ru}}_3{\mathrm{O}}_{12}$ system, which undergoes long-range antiferromagnetic ordering at $T_N=60$ K. The INS reveals two distinct spin-wave excitations below $T_N$ : one at $\sim 5.6\mathrm{meV}$ and the other at 10–15 meV. By accompanying the INS spectra based on a linear spin-wave theory using spinw software and machine learning force fields (MLFFs), we show that ${\mathrm{Ba}}_5{\mathrm{Ru}}_3{\mathrm{O}}_{12}$ exhibits spin frustration due to competing exchange interactions between neighboring and next-neighboring Ru moments, exchange anisotropy, and strong spin-orbit coupling, which yields a noncollinear spin structure, in contrast to other ruthenate trimers in this series. Interestingly, these magnetic excitations do not completely vanish even at high temperatures above $T_N$ , evidencing short-range magnetic correlations in this trimer system. This is further supported by $\mu \mathrm{SR}$ spectroscopy, which exhibits a gradual drop in the initial asymmetry around the magnetic phase transition and is further verified through maximum entropy analysis. The results of $\mu \mathrm{SR}$ spectroscopy indicates a dynamic nature of magnetic order, attributed to local magnetic anisotropy within the trimer as a result of local structural distortion and different hybridization, consistent with canted spin structure. We predict the ground state of ${\mathrm{Ru}}_3{\mathrm{O}}_{12}$-isolated trimer through theoretical calculations which agree with the experimentally observed spin excitation.

Ferroelectric control of altermagnetism in momentum space has been studied widely, while the control of magnetism in real space of altermagnets is still rare. We present a design rule to identify multiferroicity in $n=2$ Ruddlesden-Popper halides. Our results show that a Jahn-Teller distortion can cooperate with oxygen octahedral rotations to break inversion symmetry, which we demonstrate in ${\mathrm{K}}_3\mathrm{C}{\mathrm{r}}_2{\mathrm{F}}_7$ and cation-ordered ${\mathrm{KAg}}_2{\mathrm{Cu}}_2{\mathrm{Cl}}_7$, and leads to a ferrielectric-to-ferroelectric phase transition in ${\mathrm{K}}_3\mathrm{C}{\mathrm{r}}_2{\mathrm{F}}_7$. Altermagnetic spin order in the ferrielectric phase of ${\mathrm{K}}_3\mathrm{C}{\mathrm{r}}_2{\mathrm{F}}_7$ transforms into a conventional antiferromagnetic order in the ferroelectric phase, at which strain/pressure engineered sizable changes of weak ferromagnetism can occur. Our study is not only conducive to realize strong magnetoelectric coupling in multiferroics but also reveals more functionalities in altermagnetic materials.

The quantum transduction, or equivalently quantum frequency conversion, between microwave and optical photons is essential for realizing scalable quantum computers with superconducting qubits. Due to the large frequency difference between microwave and optical ranges, the transduction needs to be done via intermediate bosonic modes or nonlinear processes. Regarding the transduction mediated by magnons, previous studies have so far utilized ferromagnetic magnons in ferromagnets. Here, we formulate a theory for the microwave-to-optical quantum transduction mediated by antiferromagnetic magnons in antiferromagnets. We derive analytical expressions for the transduction efficiency in the cases with and without an optical cavity, where a microwave cavity is used in both cases. In contrast to the case of the quantum transduction using ferromagnets, we find that the quantum transduction can occur even in the absence of an external static magnetic field. We also find that, in the case with an optical cavity the transduction efficiency takes a peak structure with respect to the sample thickness, indicating that there exists an optimal thickness, whereas in the case without an optical cavity the transduction efficiency is a monotonically increasing function of the sample thickness. Our study opens up a way for possible applications of antiferromagnetic materials in future quantum interconnects.

We study one-dimensional Dirac fermions in the presence of a spatially varying Dirac velocity $v\left(x\right)$, that can form an approximate laboratory-based Rindler Hamiltonian describing an observer accelerating in Minkowski spacetime. A sudden switch from a spatially homogeneous velocity [$v\left(x\right)$ constant] to a spatially varying velocity [$v\left(x\right)$ inhomogeneous] leads to the phenomenon of particle creation, i.e., an analog Unruh effect. We study the dependence of the analog Unruh effect on the precise form of the velocity profile, finding that while the ideal Unruh effect occurs for $v\left(x\right)\propto \left|x\right|$, a modified Unruh effect still occurs for more realistic velocity profiles that are linear for $\left|x\right|$ smaller than a length scale $\lambda$ and constant for $\left|x\right|\gg \lambda$ [such as $v\left(x\right)\propto tanh\left(\right|x|/\lambda )$]. We show that the associated particle creation is localized to $\left|x\right|\ll \lambda$.

Negative thermal expansion (NTE) is an intriguing phenomenon that is not only scientifically interesting but also technologically significant. Herein, using temperature-dependent high-resolution synchrotron radiation x-ray diffraction, together with dilatometry and specific heat capacity analyses, we systematically study the NTE behaviors of $\mathrm{C}{\mathrm{r}}_2{\mathrm{P}}_2{\mathrm{O}}_7$ and the new physical mechanisms beyond the low-frequency phonon vibration modes within a framework structure. We show that $\mathrm{C}{\mathrm{r}}_2{\mathrm{P}}_2{\mathrm{O}}_7$ can exhibit successively near zero and giant NTE from 100 to 320 K and 320 to 355 K, respectively. In the temperature range of giant NTE, we further clear that giant NTE behavior originates from the intense repulsion between Cr-Cr atoms and result in abrupt contraction along both the a- and c-axes, respectively. However, contrary to the current understanding of NTE in oxides, we confidently confirm that the transverse vibrations of diphosphate bridging oxygen in $\mathrm{C}{\mathrm{r}}_2{\mathrm{P}}_2{\mathrm{O}}_7$ have no contribution to such giant NTE through a detailed analysis of the correlation between temperature dependent P-O-P angles and Cr–Cr distance. Instead, the Jahn-Teller distortion in $\mathrm{Cr}{\mathrm{O}}_6$ octahedra is responsible for the unique thermal expansion behaviors totally. These findings not only extend the NTE family in framework structure materials and new NTE mechanisms, but also propose a new approach to realize NTE materials with the Jahn-Teller effect at the level of electron spin.

Hydrogenated two-dimensional (2D) materials have recently attracted significant attention for their potential to achieve high-temperature superconductivity via the conventional electron-phonon coupling. However, insufficient surface coordination often limits effective hydrogenation in traditional 2D systems. Here, we identify hexagonal MBenes as an ideal platform with the compact atomic stacking for constructing robust hydrogen-based networks. MBenes are boron-based analogs of MXenes, in which boron (B) atoms form the intermediate layer and transition metals (M) make up the outer layers. A comprehensive study of 53 MBene compounds, including pristine MB, monohydride MBH, and dihydride ${\mathrm{MBH}}_2$ phases, identifies 29 dynamically and mechanically stable structures. Hydrogen adsorption significantly enhances the superconducting critical temperature ($T_{\text{c}}$) in Sc-, Ti-, V-, Nb-, and Ta-based MBenes. Among them, ${\mathrm{VBH}}_2$ exhibits a record $T_{\text{c}}$ of 83 K for hydrogenated 2D superconductors. This enhancement is attributed to a Van Hove singularity (VHS) near the Fermi level, induced by strong hybridization between H $s$ and M $d$ orbitals. Biaxial strain calculations further confirm the decisive role of the VHS by showing a suppression of $T_{\text{c}}$ when the VHS shifts away from the Fermi level. Furthermore, except for ${\mathrm{TaBH}}_2, {\mathrm{MBH}}_2$ (M = Sc, Ti, V, Nb) exhibit three-gap superconductivity, with anisotropy stemming from the dispersive nature of M $d$ orbitals. This research constructs a robust theoretical basis to probe high-temperature superconductivity and its fundamental origin in hydrogenated MBenes.

Fractional Chern insulators (FCIs), as a platform for the realization of fractionalization phenomena without external magnetic field, have attracted great interest. To realize FCIs, topological flat bands play a significant role. Most recently, FCIs have been extended into two-dimensional hyperbolic systems and the geometric degree of freedom of hyperbolic FCIs has been revealed. In this work, we construct hyperbolic lattices with a conical singularity and demonstrate the existence of topological flat bands. Subsequently, a bosonic $\nu =1/2$ FCI state is investigated on these hyperbolic topological flat band models with a singularity. The edge excitations for these singular hyperbolic FCIs are investigated. We also construct the optimal trial wave functions for the $\nu =1/2$ FCI state and roughly explore the interplay between the $\nu =1/2$ FCI state and geometric factors originated from the hyperbolic geometry and conical singularity.

Metageneralized gradient approximations (meta-GGAs) on the third rung of the functional hierarchy are gaining increasing relevance for electronic structures. Meta-GGAs are constructed from numerous ingredients including the orbital kinetic energy density that make them more flexible than generalized gradient approximations (GGAs) including the heavily used Perdew-Burke-Ernzerhof GGA (PBE-GGA). Still, most meta-GGAs cope with the expected limitations of a semilocal density functional when band gaps or localization of electrons are needed. On the other hand, meta-GGAs are explicit functionals of the orbitals. This feature resembles hybrid density functionals with exact exchange. Efforts in recent years demonstrate that some meta-GGAs can rise beyond the accuracy of semilocal approximations when band gaps are computed. Cerium is an ideal test bed to challenge some recent meta-GGAs. Cerium shows an isostructural $\alpha \text{−}\gamma$ phase transition with delocalized and localized $f$ electrons in each phase, respectively. The correlation of $f$ electron systems is hardly captured by semilocal approximations, but the recent Lebeda-Aschebrock-Kümmel (LAK) meta-GGA with ultranonlocality steps out of the framework of conventional semilocal density functionals and delivers spectacular accuracy for the phase transition of cerium. LAK and further meta-GGAs inspired by the success of LAK can open a forefront of meta-GGAs for quantum materials with localized electrons.

We demonstrate that the $\eta$ symmetry of the pump-induced $\eta$-pairing state in the Hubbard chain can be transiently broken by an additionally applied probe pulse. This leads to characteristic dynamic Higgs oscillations of the $\eta$-pair correlations during the probe pulse and a sharp dynamic negative optical conductivity $\sigma (\omega ;t)$, which appears above the equilibrium Mott gap $\omega >{\mathrm{\Delta }}_{\mathrm{c}}$, long after the pump pulse is finished. This negative peak in the real part of $\sigma (\omega ;t)$ is a distinctive hallmark of the photoinduced $\eta$-pairing state, distinguishing it from parameter regimes dominated by incoherent doublon formation, which do not exhibit this feature. Remarkably, the broadband probe pulse then leads to the emission of photons with the Higgs frequency.

We investigate the indistinguishability of polaritons in optically trapped Bose-Einstein condensates by implementing Hong-Ou-Mandel (HOM) interferometry and test the limitations of two-polariton interference in the coherent, limit-cycle, and thermal statistical regimes. We observe that the HOM dynamics of a circularly polarized condensate follows the condensate coherence time with the characteristic HOM dip approaching the classical limit. Linearly polarized condensates exhibit a combined effect of polariton bunching and two-polariton interference. Under elliptically polarized excitation, the temporal evolution of the spinor condensate results in the revival of the HOM dip at the spinor Larmor precession frequency.

The realization and manipulation of Majorana zero modes have drawn significant attention for their crucial role in enabling topological quantum computation. Conventional approaches to the braiding of Majorana zero modes rely on adiabatic processes. In this work, using a composite 2-Kitaev-chain system accommodating Majorana zero modes as a working example, we propose a nonadiabatic and non-Abelian geometry phase-based protocol to execute operations on these Majorana zero modes. This is possible by locally coupling the edge sites of both quantum chains with an embedded lattice defect, successfully simulating the braiding operation of two Majorana modes in a highly nonadiabatic fashion. To further enhance the robustness against control imperfections, we apply a multiple-pulse composite strategy to our quantum chain setting for second-order protection of the braiding operations. Our proposal can also support the fast and robust realization of the $\pi /8$ gate, an essential ingredient for universal quantum computation. This work hence offers a potential pathway toward the nonadiabatic and fault-tolerant control of Majorana zero modes.

Highly nonequilibrium conditions in femtosecond-laser excited solids cause a variety of ultrafast phenomena that are not accessible by thermal conditions, such as subpicosecond solid-to-liquid or solid-to-solid phase transitions. In recent years, the microscopic pathways of various laser-induced crystal rearrangements could be identified and led to novel applications and/or improvements in optoelectronics, photonics, and nanotechnology. However, it remains unclear what effect a femtosecond-laser excitation has on ionic impurities within an altered crystal environment, in particular on the atomic mobility. Here, we performed ab initio molecular dynamics simulations on laser-excited black silicon, a promising material for high-efficiency solar cells, using the code CHIVES. By computing time-dependent Bragg peak intensities for doping densities of 0.16% and 2.31% we could identify the overall nonthermal weakening of the crystal environment with increasing impurity density. The analysis of Si-S bond angles and lengths after different excitation densities, as well as computing interatomic forces, allowed one to identify a change in ion mobility with increasing impurity density and excitation strength. Our results indicate the importance of impurity concentrations for ionic mobility in laser-excited black silicon and could give significant insight for semiconductor device optimization and materials science advancement.

We analyze two simple model planar molecules: an ionic molecule with $D_3$ symmetry and a covalent molecule with $D_6$ symmetry. Both symmetries allow the existence of chiral molecular orbitals and normal modes that are coupled to each other in a Jahn-Teller manner, invariant under U(1) symmetry with the generator a pseudo-angular momentum. In the ionic molecule, the chiral mode possesses an electric dipole but lacks physical angular momentum, whereas in the covalent molecule, the situation is reversed. In spite of that, we show that in both cases, the chiral modes can be excited by a circularly polarized light and are subsequently able to induce rotational motion of the entire molecule.

We study the effects of magnon-magnon interactions in the two-dimensional van der Waals ferromagnet ${\mathrm{CrBr}}_3$ focusing on its honeycomb lattice structure. Motivated by earlier theoretical predictions of temperature-induced spectral shifts and van Hove singularities in the magnon dispersion [S. S. Pershoguba et al., Phys. Rev. X 8, 011010 (2018)], we go beyond the commonly used thermal magnon approximation by applying second-order perturbation theory in a fully numerical framework. Our analysis uncovers significant deviations from previous analysis: in particular, the predicted singularities are absent, consistent with recent inelastic neutron scattering measurements [S. E. Nikitin et al., Phys. Rev. Lett. 129, 127201 (2022)]. Moreover, we find that the temperature dependence of the renormalized magnon spectrum exhibits a distinct $T^3$ behavior for the optical magnon branch, while retaining $T^2$ behavior for the acoustic or down magnon band. This feature sheds light on the collective dynamics of Dirac magnons and their interactions. We further compare the honeycomb case with a triangular Bravais lattice, relevant for ferromagnetic monolayer ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$, and show that both systems lack singular features while displaying quite distinct thermal trends.

Among the important phenomena that arise from the interfacial exchange interaction between different magnetic materials are exchange bias (EB) and exchange spring (XS). The exchange bias occurs between ferromagnetic (FM) and antiferromagnetic (AF) materials, and its main manifestation consists of a shift of the hysteresis loop of the FM layer along the field axis. The exchange spring is observed when two FM materials are in contact, producing a single hysteresis loop combining the properties of the two. Experiments to study the EB effect are usually carried out with AF/FM bilayers in which both films are very thin. Here we report the observation of the reciprocal exchange bias effect in a thick single-crystal slab of the Heisenberg AF $\mathrm{RbMn}{\mathrm{F}}_3$ interfaced with a very thin film of the soft FM permalloy ($\mathrm{N}{\mathrm{i}}_{91}\mathrm{F}{\mathrm{e}}_{19}$, Py). Measurements of the magnetic moment versus applied magnetic field in a wide temperature $T$ range (5–300 K) reveal a hysteresis loop of the AF material shifted along the field axis in the positive direction. For thicker Py films that have a magnetic moment comparable to that of the AF slab, we observe XS effects. Interestingly, both effects manifest in the AF phase of $\mathrm{RbMn}{\mathrm{F}}_3 (T<83\mathrm{K})$ as well as in the paramagnetic phase up to room temperature.

Heterostructures comprised of a magnetic topological insulator (MTI) placed in the proximity of an $s$-wave superconductor have emerged as a platform for the practical realization of Majorana bound states (MBSs). More specifically, it has been theoretically predicted that MBS can appear in proximitized MTI nanoribbons (PNRs) in the quantum anomalous Hall regime. As with all MBS platforms, disorder and device imperfections can be detrimental to the formation of robust and well-separated MBSs that are suitable for fusion and braiding experiments. Here, we identify the optimal conditions for obtaining a topological superconducting gap that is robust against disorder, with spatially separated stable MBSs in PNRs, and introduce a figure of merit that encompasses these conditions. Particular attention is given to the thin-film limit of magnetic topological insulators (MTIs), where the hybridization of the surface states cannot be neglected, and to the role of electron-hole asymmetry in the low-energy physics of the system. Based on our numerical results, we find that (1) MTI thin films that are normal (rather than quantum spin Hall) insulators for zero magnetization are favorable, (2) strong electron-hole asymmetry causes the stability and robustness of MBS to be very different for chemical potentials above or below the Dirac point, and (3) the magnetization strength should preferably be comparable to the hybridization or confinement energy of the surface states, whichever is largest.

We investigate the absorption spectra of the $F$ centers in MgO and CaO using material-specific hybrid functionals combined with time-dependent hybrid-functional theory calculations. Our study accounts for electron-hole interaction, zero-phonon renormalization, and finite-size effects, which are all found to contribute to the achieved accuracy. The spectra of the neutral states solely present absorption lines stemming from internal defect-to-defect transitions without involving conduction band states of the host. At variance, the spectra of the singly positively charged states show two lines, one analogous to those seen in the neutral states and an extra line corresponding to electron excitations from the valence band to the unoccupied defect level in the band gap. In MgO, the three transition energies pertaining to these two charged states are found to coincide, but are spread out in the case of CaO. The agreement with the energies of experimental lines is excellent, indicating that our scheme reaches an accuracy of 0.2 eV or better.

In moiré materials, excitons serve as optical probes because of the sensitivity to both valley-dependent electronic structure and many-body excitonic interactions. For angle-tuned twisted bilayers, lattice reconstruction becomes significant at marginal twist angles, raising interesting questions about the optical characteristics of excitons both in the steady-state and nonequilibrium regimes. In this work, we investigate both the steady-state and the ultrafast transient exciton behaviors in a twisted ${\mathrm{WSe}}_2$ homobilayer (t-${\mathrm{WSe}}_2$) using reflection contrast, polarization-resolved photoluminescence, and ultrafast pump–probe spectroscopy. We report the emergence of two distinct intralayer excitons in the lattice reconstructed t-${\mathrm{WSe}}_2$, which are used to probe local electronic asymmetries and interlayer coupling in moiré domains. These excitons possess distinct temperature- and doping-dependent valley coherence and population dynamics, arising from the asymmetric interlayer coupling. Theoretical modeling via the Lindblad master equation highlights that the pure dephasing rate increases with hole doping, attributed to the enhanced electron-hole interactions. Ultrafast degenerate pump-probe spectroscopy reveals distinct fast decaying dynamics ($<1$ ps) for the two intralayer excitonic absorption species, $X_1$ and $X_2$, where the asymmetric interlayer coupling contributes more to the faster $X_1$ decay than $X_2$. Optical-pump and white-light probe spectroscopy further unveils a biexponential decay, where the fast component (${\tau }_{\mathrm{fast}}\approx 0.7–0.9\mathrm{ps}$) signifies rapid radiative recombination with repopulation effects in the optical light cone. The slow component (${\tau }_{\mathrm{slow}}\approx 100–300\mathrm{ps}$) is linked to exciton-phonon scattering and population relaxation via interlayer breathing phonons. We also present that intervalley scattering pathways, i.e., mediated by the $KQ$ transitions, are distinct from the monolayer counterpart. This work provides detailed insights into the exciton population dynamics in twisted homobilayers, highlighting the role of intervalley and exciton-phonon interactions in the transient multiple exciton complex behaviors.

Insulators containing chains of magnetic transition metal cations provide platforms for probing spin-$\frac{1}{2}$ dynamics and quantum critical behavior. ${\mathrm{Li}}_2{\mathrm{CoCl}}_4$ contains edge-sharing ${\mathrm{CoCl}}_6$ octahedra that form chains along the crystallographic $c$ axis and orders antiferromagnetically at zero field, but questions remain about its applied-field magnetic structure and the ${\mathrm{Co}}^{2+}$ spin state. Here, we show with neutron diffraction on a polycrystalline sample how the antialigned chains of cobalt moments begin to transition to a ferromagnetic state above 1.6 T. Further, using magnetic resonance absorption measurements and noninteracting spin models, we reveal the strongly anisotropic nature of the ${\mathrm{Co}}^{2+}$ ion's $XY$-like magnetic behavior ($g_{\parallel }=2.77$ and $g_{\perp }=5.23$) and its $J=\frac{1}{2}$ ground state. We therefore supply the magnetic structures and anisotropic description needed to explore the dynamics of the field-driven magnetic phases, laying the foundation for further experimental and theoretical studies.