Two-dimensional (2D) covalent organic frameworks (COFs) are a promising class of materials exhibiting significant potential for diverse applications owing to their unique structural characteristics. In particular, some applications crucially rely on mechanoresponsive behaviors of 2D COFs, which thus require a comprehensive understanding. Herein, a machine-learning potential is developed for two representative monolayer 2D COFs (COF-1 and COF-5), which demonstrates both high computational efficiency and accuracy in molecular dynamics (MD) simulations of their mechanical behaviors. Our machine-learning MD simulations reveal distinct planar-to-nonplanar conformation transitions occurring in COF-1 and COF-5 at large tensile strain in the armchair direction. According to density functional theory calculations, the tension-induced conformation transitions in 2D COFs are driven by strong H-H steric repulsions between aryl rings within their deformed linker or knot components. The observed conformation transition is anticipated to provide a new strategy for modulating electronic properties, fracture behaviors, and gas adsorption characteristics of 2D COFs.

Flat-band systems have attracted significant attention as platforms for studying strongly correlated electron physics, where the dominance of electron-electron interactions over kinetic energy gives rise to a variety of emergent phenomena. Quasicrystals are compelling systems for studying these phenomena as they host degenerate strictly localized states at zero energy due to perfect destructive interference patterns. In this study, we use the slave-rotor mean-field approach to investigate the effects of electron interactions within the Hubbard model on the Ammann-Beenker quasicrystal. The phase diagram characterizing metallic and Mott insulator regions indicates a first-order phase transition. Our analysis shows that the local coordination number affects the local quasiparticle weight, displaying varying metallicity across the sites. Furthermore, we focus on the strictly localized states that arise in the noninteracting limit. We find that interactions and deviation from particle-hole symmetry induce spectral splitting, broadening, and partial delocalization of the localized states, depending on the local environment. In particular, certain localized states with higher coordination numbers remain more robust compared to others. Our results highlight the critical role of local geometry in shaping correlation effects in flat-band quasicrystals.

We unveil a peculiar dynamical transition for the propagation of wave packets in non-Hermitian Chern insulators, where the evolution of the topological wave packets at the edge is not determined solely by the topological Chern number. Unlike the Hermitian Chern insulator, where a wave packet initiated at the edge propagates along the system boundary, here, it may instead penetrate into the bulk. This behavior is attributed to the competition between the localization induced by conventional topology and the non-Hermitian skin effect. Specifically, when the former dominates, the wave packet will evolve along the boundary; otherwise, it will spread into the bulk. These features demonstrate that while the generalized-Brillouin-zone framework reliably predicts topological phase transitions, it does not on its own specify the dynamical transitions.

A uranium-based metallic compound ${\mathrm{UNi}}_4\mathrm{B}$ with an orthorhombic structure exhibits antiferromagnetic (AFM) ordering at $T_{\mathrm{N}}=20.4$ K. Although an electric current-induced magnetization below $T_{\mathrm{N}}$ has been observed in this compound [H. Saito et al., J. Phys. Soc. Jpn. 87, 033702 (2018)], its underlying mechanism remains controversial. In this study, we measured thermal expansion and magnetostriction along the $a$ axis under a magnetic field $H$ up to 14 T applied within the $bc$ plane to investigate the rotational symmetry of a magnetic field–temperature $\left(H\text{−}T\right)$ phase diagram in ${\mathrm{UNi}}_4\mathrm{B}$. From the phase diagrams constructed under various $H$ angles between $b$ and $c$, it is suggested that the $H\text{−}T$ phase diagram practically exhibits sixfold rotational symmetry within the $bc$ plane despite an orthorhombic crystal structure, which would be attributed to a quasihexagonal structure formed by U ions with small orthorhombic distortion.

Intriguing phases may emerge when two-dimensional systems are coupled in a bilayer configuration. In particular, a Berezinskii-Kosterlitz-Thouless (BKT) paired superfluid phase was predicted and claimed to be numerically observed in a coupled $XY$ model with ferromagnetic interlayer interactions, as reported in Phys. Rev. Lett. 123, 100601 (2019). However, both our Monte Carlo simulations and analytical analysis show that this model does not exhibit a BKT paired phase. We then propose a new model incorporating paired-phase gradient interlayer interactions to realize the BKT paired phase. Moreover, we observe that the anomalous magnetic dimension varies along the phase transition line between the disordered normal phase and the BKT paired phase. This finding requires an understanding beyond the conventional phase transition theory.

Altermagnets (AMs) constitute a novel class of spin-compensated materials in which opposite-spin sublattices are connected by a crystal rotation, causing their electronic isoenergy surfaces to be spin split. While cubic and tetragonal crystal symmetries tend to produce AMs in which the splitting of electronic isoenergy surfaces has $d$-wave symmetry, hexagonal AMs, such as CrSb and MnTe, are $g$-wave AMs. Here we investigate the purely magnetic modes and spin textures of $g$-wave AMs and show that they are drastically different for easy-axial (CrSb) and easy-planar (MnTe) materials. We show that in CrSb the splitting of the chiral magnon branches possesses $g$-wave symmetry, with each branch carrying a fixed momentum-independent magnetic moment. The altermagnetic splitting is not affected by the easy-axial anisotropy and is the same as that in the nonrelativistic limit. The magnon splitting of MnTe, however, does not strictly possess $g$-wave symmetry due to its easy-planar anisotropy. Instead, the magnetic moment of each branch becomes momentum dependent, with a distribution that is of $g$-wave symmetry. To generalize the concept of the altermagnetic splitting beyond the nonrelativistic limit, we introduce an alternative, directly observable splitting parameter that encompasses both the magnon eigenenergy and its magnetic moment (a component along the ground-state Néel vector), and exhibits $g$-wave symmetry in both easy-axial and easy-planar cases. The associated altermagnetic domain walls in easy-axial CrSb possess a net magnetization with an amplitude that depends on their orientation.

We demonstrate nonreciprocal macroscopic entanglement between magnon modes in two yttrium-iron-garnet (YIG) spheres induced by the Kerr effect stemming from magnetocrystalline anisotropy. The two YIG spheres with different sizes are placed in a microwave cavity, and the magnon modes couple to the microwave cavity photons by the magnetic dipole interaction. The microwave cavity can be adiabatically eliminated by far detuning from the magnon modes, and thus mediates the indirect interaction between two magnon modes. The magnon Kerr nonlinearity of the small YIG sphere is enhanced by the appropriate drive. Meanwhile, the sign of the Kerr coefficient can be manipulated by changing the direction of the bias magnetic field, in turn causing the nonreciprocity of magnon-magnon entanglement. We numerically analyze the optimal parameter regions for generating the entanglement. The properties of the nonreciprocal entanglement are explored by analyzing the defined bidirectional contrast ratio, which reveals the different strategies for controlling the nonreciprocity of the entanglement. The presented scheme provides an alternative method for the nonreciprocal macroscopic entanglement and may have potential applications in nonreciprocal quantum devices and macroscopic quantum effects.

We investigate the ground state phase diagram of the spin-$1/2$ antiferromagnetic Heisenberg model on the star lattice using infinite projected entangled pair states (iPEPS) and high-order series expansions. The model includes two distinct couplings: $J_d$ on the dimer bonds and $J_t$ on the trimer bonds. While it is established that the system hosts a valence bond solid (VBS) phase for $J_d\ge J_t$, the ground state phase diagram for $J_d<J_t$ has remained unsettled. Our iPEPS simulations uncover a first-order phase transition at $J_d/J_t\approx 0.18$, significantly lower than previously reported estimates. Beyond this transition, we identify a close competition between two valence bond crystal (VBC) states; a columnar VBC and a $\sqrt{3}\times \sqrt{3}$ VBC, with the latter consistently exhibiting lower energy across all finite bond dimensions. The high-order series expansion supports this by finding that the $\sqrt{3}\times \sqrt{3}$ VBC state indeed becomes energetically favorable, but only at sixth order in perturbation theory, revealing the subtle nature of the competition between candidate states.

We investigate the decoherence of magnonic qubits in small ferromagnetic insulators and compute the relaxation and dephasing rates due to magnon-phonon and magnon-magnon interactions. We combine a Bloch-Redfield description with Keldysh nonequilibrium field theory to find explicit expressions for the rates. For a quadratic dispersion and assuming a uniform mode defines the qubit, we find that decay into two phonons is the only allowed relaxation process at zero temperature. The low resonance frequency and heavy unit cell strongly suppress this process in yttrium iron garnet. We also find that the dephasing rate scales with the inverse of size and damping of the magnet and could become large for small and clean magnets. Our calculation thus provides additional insight into the viability of magnon-based quantum devices.

The compounds of the $\mathrm{B}{\mathrm{a}}_2\mathrm{LnFe}{\mathrm{O}}_5$ series (Ln being a lanthanide) undergo an antiferromagnetic transition involving both the $\mathrm{F}{\mathrm{e}}^{3+}$ and $\mathrm{L}{\mathrm{n}}^{3+}$ spins. The $\mathrm{L}{\mathrm{n}}^{3+}$ cations sit in distorted octahedra and the associated crystalline electric-field (CEF) is expected to have a drastic impact on the magnetism. In this study, the nature of the spin ordering was investigated by Powder Neutron Diffraction (PND), while the $\mathrm{L}{\mathrm{n}}^{3+}$ spin anisotropy was derived from calculations in the point charge approximation. Different propagation vectors were observed for the various Ln, and the magnetic structures were solved in all cases, except for Ln = Er. A couple of easy axis (EA) consistent with the space group symmetry was observed for each Ln (except Er), and the orientation of these EAs exhibits a huge variation along the series. These features are discussed in connection with the CEF results and the literature on related families of materials.

The study of entanglement and magic properties in topologically frustrated systems suggests that, in the thermodynamic limit, these quantities decompose into two distinct contributions. One is determined by the specific nature of the model and its Hamiltonian, and another arises from topological frustration itself, resulting in being independent of the Hamiltonian's parameters. In this work, we test the generality of this picture by investigating an additional quantum resource, namely, quantum coherence, in two different models where topological frustration is induced through an appropriate choice of boundary conditions. Our findings reveal a perfect analogy between the behavior of quantum coherence and that of other quantum resources, particularly magic, providing further evidence in support of the universality of this picture and the topological nature of this source of frustration.

Stimulated by recent interest in altermagnets, a novel class of antiferromagnets with macroscopic time-reversal symmetry breaking, we investigate the coexistence of altermagnetism and superconductivity. By developing a Ginzburg-Landau theory based on microscopic models, we show that a phase-modulated Fulde-Ferrell superconducting state is stabilized via altermagnetic spin splitting, in contrast to the typical amplitude-modulated states that occur under the uniform Zeeman field. We apply our framework to different models to compare the resulting phase diagrams: a two-sublattice model with altermagnetic order, a continuum model with an anisotropic Zeeman field mimicking altermagnetic spin splitting, and a conventional square-lattice model with two kinds of anisotropic Zeeman fields. We show that the multisublattice structure is crucial for realizing the phase-modulated superconductivity, and highlight spin-split altermagnets as a promising platform for exploring this exotic superconductivity without external magnetic fields.

We visualize the topological ladder and band inversions in ${\mathrm{PtTe}}_2$ using spin-polarized photoemission spectroscopy augmented by three-dimensional momentum imaging. This approach enables the detection of spin polarization in dispersive bands and provides access to topological properties beyond the reach of conventional methods. Extensive mapping of spin-momentum space reveals distinct topological surface states, including a surface Dirac cone at a binding energy $E_B\sim 2.3\mathrm{eV}$ and additional states at $E_B\sim 1.6\mathrm{eV}, E_B\sim 1.0\mathrm{eV}$, and near the Fermi level. The electronic structure analysis demonstrates strong hybridization between Pt and Te atomic orbitals, confirming the nontrivial topology of these surface states. Furthermore, by comparison to one-step model photoemission calculations, we identify a robust correlation between the initial-state and measured spin polarizations while revealing asymmetries in specific experimental spin textures. These asymmetries, absent in the initial states due to symmetry constraints, arise from the breaking of time-reversal symmetry during the photoemission process, emphasizing the crucial influence of symmetries on experimental signatures of topology. A phase analysis of ladder states in real space suggests a route to probe them using photoemission, paving the way towards spectroscopic access to the quantum geometric tensor.

We focus on the infinite-temperature dynamical spin structure factor of the asymmetric Hatsugai-Kohmoto model, a relative of the asymmetric Hubbard model. It is characterized by distinct single-particle energies for the two spin species, which interact with each other through a contact interaction in momentum space. We evaluate its spin structure factor exactly and follow the evolution of its excitation spectrum for all fillings and interactions, identifying signatures of the Mott transition and fingerprints of the asymmetric hoppings. The longitudinal spin structure factor exhibits soundlike and interaction-induced gapped excitations, whose number gets doubled in the presence of hopping asymmetry. The transverse response displays the competition of interaction- and asymmetry-induced gaps and results in a quadratic excitation branch at their transition. The complete asymmetric case features a momentum-independent dynamical structure factor, characteristic of transitions involving a flat band.

Local integrals of motion (LIOMs) play a key role in understanding the long-time properties of closed macroscopic systems. They were found for selected integrable systems via complex analytical calculations. The existence of LIOMs and their structure can also be studied via numerical methods, which, however, involve exact diagonalization of Hamiltonians, posing a bottleneck for such studies. We show that finding LIOMs in translationally invariant lattice models or unitary quantum circuits can be reduced to a problem for which one may numerically find an exact solution in the thermodynamic limit. We develop a simple algorithm and demonstrate its efficiency by calculating LIOMs and bounds on correlations (the Mazur bounds) for infinite integrable spin chains and unitary circuits. Finally, we demonstrate that this approach identifies slow modes in nearly integrable spin models and estimates their relaxation times.

We investigate the energy per particle, static structure factor, and momentum distribution of the uniform electron gas for different conditions defined by the dimensionless temperature $\mathrm{\Theta }=0.25-1.0$ and average interparticle distance $r_s=0.5-80.0$ using path-integral Monte Carlo (PIMC) simulations. For small $r_{\text{s}}$ ($r_{\text{s}}\le 10$) where the sign problem is particularly challenging, we employ a recent approach based on an analytic continuation of the partition function using a real parameter $\xi$, which allows a generalization from bosons ($\xi =1$) to fermions ($\xi =-1$). We show that the results are in good agreement with other state-of-the-art methods while requiring low computational resources. For large $r_{\text{s}}$ ($r_{\text{s}}=80$), we use direct PIMC exploiting the good behavior of the thermodynamic properties for negative $\xi$. In this framework we demonstrate that, for large $r_s$, the small negative region of $\xi$ can be utilized to extract information about the true fermionic limit, where $\xi =-1$.

Doping carriers into a correlated quantum ground state offers a promising route to generate new quantum states. The recent advent of moiré superlattices provided a versatile platform with great tunability to explore doping physics in systems with strong interplay between strong correlation and nontrivial topology. Here we study the effect of electron doping in the quantum anomalous Hall insulator realized in a transition-metal dichalcogenide moiré superlattice at filling $\nu =1$, which can be described by the canonical Kane-Mele-Hubbard model. By solving the Kane-Mele-Hubbard model using an unrestricted real-space Hartree-Fock method, we find that doping generates quantum anomalous Hall crystals (QAHCs) and topological domain walls. In the QAHC, doping induces skyrmion spin textures, which host one or two electrons in each skyrmion as in-gap states. The skyrmions crystallize into a lattice, with the lattice parameter being tunable by the density of doped electrons. Remarkably, we find that the QAHC can survive even in the limit of vanishing Kane-Mele topological gap for a significant range of fillings. Furthermore, doping can also induce domain walls separating topologically distinct domains with different electron densities, hosting chiral localized modes.

${\mathrm{LiGa}}_5{\mathrm{O}}_8$ with a cubic spinel-type structure was recently reported to be an ultrawide-band-gap semiconductor with unintentional $p$-type conduction. While the origin of $p$-type doping is still unclear, the fundamental properties of this material are of interest. Here, we present a first-principles study of the phonons using density functional perturbation theory. The phonon band structures show no unstable modes, verifying the stability of the structure. We focus mainly on the phonons at the Brillouin zone center for which a full symmetry analysis is presented. We present the dielectric function contributions from the infrared active modes as well as the Raman spectra and their polarization dependence. The phonon density of states integrated over the Brillouin zone is also presented as this may be related to disordered Raman spectra.