We conducted a first-principles study of the dielectric properties of liquid propylene glycol (PG) and polypropylene glycol (PPG) using a recently developed chemical-bond-based machine learning (ML) model for dipole moments [T. Amano et al., Phys. Rev. B 110, 165159 (2024)]. The ML dipole models successfully predict the dipole moment of various liquid configurations in close agreement with density functional theory calculations, and they generate $20\mathrm{n}\mathrm{s}$ quantum-accuracy dipole moment trajectories to calculate the dielectric function, when combined with ML potentials. The calculated dielectric function of PG closely matches experimental results. We identified a libration peak at $600{\mathrm{cm}}^{-1}$ and an intermolecular mode at $100{\mathrm{cm}}^{-1}$, previously noted experimentally. Furthermore, the models trained on PG2 training data can apply to longer chain PPGs not included in the training data. The present research marks the first step toward developing a universal bond-based dipole model.

To control magnon transport in magnetic systems is of great interest in magnonics. Due to the feasibility of electric field, the ability to generate and manipulate magnons using purely electrical methods is highly desirable. We propose that the magnon spin current can be generated by applying a time-dependent electric field, with the coupling between magnons and the electric field facilitated by the Aharonov-Casher effect. Furthermore, we propose a magnon Landau-Zener tunneling effect that is the key impetus of the induced magnon spin current. By applying our theory to the one-dimensional ferromagnetic Su-Schrieffer-Heeger model, we demonstrate that the generated magnon spin current is intimately linked to the band geometry. Our findings are expected to broaden the scope of magnonics and advance the understanding of electric control mechanisms for magnons.

The vanadium-based kagome metal $\mathrm{Cs}{\mathrm{V}}_3\mathrm{S}{\mathrm{b}}_5$ has drawn significant interest since it exhibits a wide variety of intriguing topological phases and electronic instabilities. A prominent example includes an unconventional charge-density wave (CDW), which is associated with the Van Hove singularities (vHss). Hole doping of the system via substitution of Ti for V is expected to shift the vHss, providing an effective way to tune the Fermi surface instabilities in $\mathrm{Cs}{\mathrm{V}}_3\mathrm{S}{\mathrm{b}}_5$. We investigate the optical response of $\mathrm{Cs}{\left({\mathrm{V}}_{1-x}\mathrm{T}{\mathrm{i}}_x\right)}_3\mathrm{S}{\mathrm{b}}_5$ ($x=0$, 0.01, 0.02, and 0.06) single crystals by utilizing infrared spectroscopy. The infrared signature of the CDW order, i.e., the decrease in the low-energy spectral weight and the appearance of the absorption peak at about 200 meV, is suppressed upon Ti doping ($x=0.02$) and completely disappears for $x=0.06$. Nevertheless, the ratio of the CDW gap ${\mathrm{\Delta }}_{\mathrm{CDW}}$ to the CDW transition temperature ($T_{\mathrm{CDW}}$), i.e., $2{\mathrm{\Delta }}_{\mathrm{CDW}}/k_{\mathrm{B}}{T}_{\mathrm{CDW}}$ is increased with Ti substitution. In the normal state, the spectral weight of the intraband response is abruptly suppressed with Ti doping ($x\ge 0.02$), suggesting a significant increase in the effective mass of the itinerant carriers due to the shift of the vHs toward the Fermi level. The evolutions of the CDW phase and the normal-state charge dynamics reveal the crucial role of the electronic correlations in the $\mathrm{Cs}{\left({\mathrm{V}}_{1-x}\mathrm{T}{\mathrm{i}}_x\right)}_3\mathrm{S}{\mathrm{b}}_5$ system.

We propose a theory for tunable second-order topological corner states induced by interlayer coupling in bilayer Chern insulators with opposite Chern numbers. We demonstrate that the existence of the topological corner state is determined by the relationship between the twist angle of the bilayer Chern insulators and the normal angles of the two sides of the corner. In addition, the position of these corner states can be sensitively controlled by the twist angle, as confirmed by a rigorous analysis of edge state theory. Our findings serve as a theoretical guide, opening avenues for the design and realization of higher-order topological materials.

Due to its anisotropic crystal structure, the electronic, optical, and thermal properties of two-dimensional black phosphorus (BP) exhibit significant anisotropy. In this study, we investigate the oscillations of the local density of states (LDOS) near a line defect in monolayer BP. Utilizing the two-band effective $\mathbf{k}·\mathbf{p}$ Hamiltonian, we numerically calculate the LDOS using the transfer matrix method and analytically derive the power-law decay indices of the LDOS oscillation patterns based on the stationary phase approximation. We find that the power-law decay indices of the LDOS oscillation patterns are always $-1/2$ irrespective of the orientation of the defects, indicating a remarkable isotropic behavior. While, the magnitude and period of the LDOS patterns are highly sensitive to the orientation of the line defect, which serves as a manifestation of the anisotropy inherent in BP. These results highlight a unique aspect of BP and are also applicable to commensurate multilayer BP, as they share a similar Hamiltonian to that of the monolayer.

Laminated composites show great promise for achieving substantial magnetoelectric (ME) coupling through the manipulation of magnetostriction and piezoelectric engineering. However, the ME coupling mechanism in composites is less understood due to complex extrinsic effects at the interfaces, unlike the well-understood ME mechanism in chemically single-phase compounds. Our study reveals unexplored ferroelectric (FE) orders along $c$-axis and magnetostriction-driven ME couplings, indicating type-II multiferroic order in ${\mathrm{TbFeO}}_3$ and providing insight into the controversy surrounding ferroelectricity in $R{\mathrm{FeO}}_3$ orthoferrites. Neutron and synchrotron x-ray diffraction (SXD) results indicate exchange striction-driven FE order below 200 K, with Tb order leading to ferroelectricity below 3 K. SXD results also reveal significant magnetostriction along the $c$-axis, correlating with ME coupling at both FE orders. These findings directly demonstrate magnetostriction-driven ME coupling, and they suggest it as one of the potential mechanisms underlying ME coupling in multiferroics.

Multiferroic materials, characterized by the existence of coupled magnetic and electric orders, offer a rich platform for studying magnetoelectric excitations such as electromagnons. In this study, we have investigated the electromagnon excitations in ${\mathrm{GdMn}}_2{\mathrm{O}}_5$, a rare-earth manganite known for its strong magnetoelectric effects. Using terahertz time-domain spectroscopy performed under an external magnetic field up to 7 T, we identified an electromagnon at $36\mathrm{c}{\mathrm{m}}^{-1}$ (4.5 meV) that emerges below the commensurate magnetic ordering temperature $(T_{\mathrm{N}2}\approx 33\mathrm{K})$. Our results show that the electromagnon in ${\mathrm{GdMn}}_2{\mathrm{O}}_5$ is highly sensitive to the spin configurations of Gd and Mn, which are strongly influenced by the Gd-Mn exchange interaction. The observed hysteresis in the electromagnon spectrum, corresponding to the magnetic hysteresis, further indicates a strong coupling between magnetic and electric orders in this material.

The Chern number is a crucial topological invariant for distinguishing the phases of Chern insulators. Here, we find that for Chern insulators with inversion symmetry, the Chern number alone is insufficient to fully characterize their topology. Specifically, distinct topological phases can be differentiated based on skyrmions at different high-symmetry points. Interfaces between these topological phases exhibit gapless helical states, which provide counterpropagating transport channels and robust quantized transport. Additionally, we identify topological transitions that do not involve changes in the Chern number but can be characterized by transitions of skyrmions between high-symmetry points. These transitions arise due to the toroidal structure of the two-dimensional Brillouin zone, which is generally applicable to two-dimensional periodic lattice systems. Our research introduces different degrees of freedom for controlling topological optical transport and deepens the understanding of Chern insulators with inversion symmetry.

Predicting the thermodynamically stable arrangements of molecular building blocks is crucial for understanding the properties and behaviors of crystalline molecular compounds. Several molecular structure prediction methods have been proposed, leading to successful applications. However, some evolutionary approaches that involve direct sampling within the infinite atomic coordinate space, without symmetry-preserving constraints, tend to suffer from biased sampling, where structures with $P1$ symmetry become disproportionately dominant. To overcome this limitation, we propose a method that utilizes a finite set of symmetrically inequivalent Wyckoff encodes to sparsely represent the search space of crystalline molecular compounds, thereby improving the coverage of the search space. This method can be used to predict the molecular crystal structures with varying numbers of molecules in the asymmetric unit, from fractional to multiple integers. The applications of this method to three-dimensional molecular crystals of ammonia dihydrate, the ${\mathrm{NH}}_3\text{−}{\mathrm{H}}_2\mathrm{O}\text{−}{\mathrm{CH}}_4$ mixture, and two-dimensional layered ices revealed a series of energetically quasidegenerate structures with diverse crystal packing motifs. This method expands the scope and utility of molecular building blocks for designing stable molecular crystals, providing a robust framework for achieving extensive tunability in molecular materials design.

Non-Abelian multiband topology, characterized by frame charges, has garnered significant attention in the past few years. The non-Abelian frame charge evolution can be deduced from the rotation of the multiband in momentum space, which can promise potential applications based on the dynamic non-Abelian topological effects. In this work, based on two-dimensional phononic crystals, we directly observe the eigenstate frame rotation to verify non-Abelian band topology, which further predicts the location of the corner modes in this multigap system. These one-dimensional edge states of band gaps or bulk band continuum that induce zero-dimensional corner states are also confirmed experimentally. The corresponding higher-order topology is examined by detecting the acoustic responses in a finite phononic crystal sample. Our work provides experimental evidence for the unique non-Abelian band topology and a convenient platform for studying novel higher-order topological matter phases.

${\mathrm{SrCr}}_2{\mathrm{As}}_2$ is an itinerant antiferromagnet in the same structural family as the ${\mathrm{SrFe}}_2{\mathrm{As}}_2$ high-temperature superconductors. We report our calculations of exchange-coupling parameters $J_{ij}$ for ${\mathrm{SrCr}}_2{\mathrm{As}}_2$ using a static linear-response method based on first-principles electronic-structure calculations. We find that the dominant nearest-neighbor exchange coupling $J_1>0$ is antiferromagnetic whereas the next-nearest-neighbor exchange coupling $J_2<0$ is ferromagnetic with $J_2$/$J_1$ = $-0.68$, reinforcing the checkerboard in-plane magnetic structure. Thus, unlike other transition-metal arsenides based on Mn, Fe, or Co, we find no competing magnetic interactions in ${\mathrm{SrCr}}_2{\mathrm{As}}_2$, which aligns with experimental findings. Moreover, the orbital resolution of exchange interactions shows that $J_1$ and $J_2$ are dominated by direct exchange mediated by the Cr $d$ orbitals. To validate the calculations we conduct inelastic neutron-scattering measurements on powder samples that show steeply dispersive magnetic excitations arising from the magnetic $\mathrm{\Gamma }$ points and persisting up to energies of at least 175 meV. The spin-wave spectra are then modeled using the Heisenberg Hamiltonian with the theoretically calculated exchange couplings. The calculated neutron-scattering spectra are in good agreement with the experimental data.

The process by which magnetic moments switch directions is crucial for understanding the performance of magnetic storage materials and in biomedical applications such as magnetic particle imaging and magnetic hyperthermia. Here, we utilize the geodesic nudged elastic band method to identify the atomic-level minimum energy path for a field-free reorientation of magnetic moments in bulk magnetite and single-domain cuboidal magnetite nanoparticles terminated by $\left\{100\right\}$ surfaces. We show that this magnetization reversal involves three successive elementary rotations of magnetic moments in distinct $\left\{110\right\}$ planes. For iron-rich terminated nanoparticles, this energy barrier depends on the degree of surface spin anisotropy. However, no such effect was observed for oxygen-rich terminations. The former effect is particularly pronounced in nanoparticles with large surface-to-volume ratio and diminishes as particle size increases.

First-principles based molecular-dynamics simulations have been performed for binary ${\mathrm{Cu}}_x{\mathrm{Ti}}_{1-x}$ ($x$ = 0.31, 0.50, and 0.76) alloys to investigate the relationship between local structure and dynamical properties in the liquid and undercooled melt. The undercooled melts show a pronounced short-range order, majorly a fivefold symmetry (FFS) around the Cu atoms, which competes with bcc ordering. This complex SRO is also reflected in the partial coordination numbers, where mainly a Z12 coordination is present around Cu, which corresponds to an icosahedral ordering. Higher coordination numbers were obtained for Ti compatible with Frank-Kasper polyhedra. The increasing Frank-Kasper polyhedra coordination scenario around Ti impacts the interatomic distances of Ti atoms, which increase with increasing Ti content. The ${\mathrm{Cu}}_{50}{\mathrm{Ti}}_{50}$ composition exhibits the highest FFS ordering and amount of Frank-Kasper polyhedra, which explains the slowest melt dynamics, found experimentally and in simulations for this composition. Thus, our results suggest that the high undercooling degree originates from the high complexity of the local structure rather than due to the preferred formation of Cu-Ti pairs, as Cu-Ti interactions were found to be weak.

Solids are often assumed to behave as viscous fluids under high-strain rates. This behavior has been studied experimentally in metals but largely unexplored in brittle ceramic materials. In this study, we present a technique for measuring the viscosity of MgO using time-resolved velocimetry to track the growth rate of a perturbation caused by the Richtmyer-Meshkov instability at the OMEGA EP laser facility. To interpret the results, we use an in-house Eulerian hydrocode to simulate our experiments and model the plastic deformation of solid MgO as a viscous fluid. Results indicate that MgO has a surprisingly low upper bound to its effective viscosity of $\sim {10}^2\mathrm{Pa}\mathrm{s}$ at $175\pm 15\mathrm{GPa}, \sim 3500\mathrm{K}$, and ${10}^6-{10}^7{\mathrm{s}}^{-1}$ strain rate.

We study how the supercurrent flow pattern is altered by inhomogeneities in superconducting films. Working in the vicinity of the critical temperature and assuming a model of short-range disorder in the quadratic term of the Ginzburg-Landau functional, we develop a perturbation theory in the inhomogeneity strength. Absorbing the ultraviolet divergences into the renormalization of the critical temperature, we arrive at a well-defined theory governed by large-scale physics. In the presence of inhomogeneities, the correlation functions of the order parameter and supercurrent exhibit a long-range power-law behavior, which can be attributed to the mixing of the amplitude and phase modes. The fluctuation magnitude grows with increasing the average current, and the system becomes strongly inhomogeneous near the critical current. An inhomogeneous superflow will generate a random magnetic field, whose magnitude in thin NbN films can be comparable to Earth's magnetic field. We discuss the feasibility of detecting it using the SQUID-on-tip technique.

Two-dimensional tungsten-based transition metal dichalcogenides (TMDCs), $M{X}_2$ ($M$ = W, Mo; $X$ = S, Se, Te) monolayers (MLs) with a $1T^{\prime }$ structure, serve as significant-gap quantum spin Hall insulators. However, due to the centrosymmetric nature of these crystals, spin degeneracy persists throughout their electronic band structures, limiting their potential for spintronic applications. By modifying the chalcogen ($X$) atoms in the TMDCs ML surface to create a highly stable Janus $MX{X}^{\prime }$ MLs structure, we demonstrate through density-functional theory calculations that substantial spin splitting of the electronic states can be achieved. Taking the Janus $1T^{\prime }$ WSTe ML as a representative case, we identify pronounced anisotropic spin-splitting bands, with maximum spin splittings of 0.14 eV and 0.10 eV occurring at the highest occupied states and lowest unoccupied states, respectively. These significant band splittings give rise to canted persistent spin textures (PST) in the spin polarization, which differ significantly from those in commonly studied PST materials. We demonstrate that this intricate spin splitting and unique spin textures stem from strong in-plane $p-d$ orbital interactions between tungsten (W) and the chalcogen atoms (Te and Se), driven by the reduced symmetry of the crystal's point group. Further analysis using a $\vec{k}·\vec{p}$ model derived from symmetry considerations corroborates the origins of the observed anisotropic spin splitting and canted PST. The spin-split states are highly sensitive to surface imperfections caused by surface alloying effects, such as variations in the chalcogen composition on the monolayer surface. These findings underscore the potential of Janus $1T^{\prime }MX{X}^{\prime }$ MLs as promising candidates for next-generation spintronic devices.

We employ the “learning by confusion” technique, an unsupervised machine learning approach for detecting phase transitions, to analyze quantum Monte Carlo simulations of the two-dimensional Holstein model—a fundamental model for electron-phonon interactions on a lattice. Utilizing a convolutional neural network, we conduct a series of binary classification tasks to identify Holstein critical points based on the neural network's learning accuracy. We further evaluate the effectiveness of various training datasets, including snapshots of phonon fields and other measurements resolved in imaginary time, for predicting distinct phase transitions and crossovers. Our results culminate in the construction of the finite-temperature phase diagram of the Holstein model.

Quantum spin liquids (QSLs) represent a unique quantum disordered state of matter that hosts long-range quantum entanglement and fractional excitations. However, structural disorder resulting from site mixing between different types of ions usually arises in real QSL candidates, which is considered to be an obstacle to gaining insight into the intrinsic physics. Here, we have synthesized two new rare-earth compounds, ${\mathrm{Rb}}_3\mathrm{Yb}{\left({\mathrm{VO}}_4\right)}_2$ and ${\mathrm{Cs}}_3\mathrm{Yb}{\left({\mathrm{VO}}_4\right)}_2$. X-ray diffractions reveal a perfect triangular-lattice structure with no detectable disorder. Magnetic susceptibility measurements do not capture any phase transition or spin freezing down to 1.8 K. A fit to low-temperature data indicates dominant antiferromagnetic interactions with a Curie-Weiss temperature of $-1.40$ and $-0.43$ K for ${\mathrm{Rb}}_3\mathrm{Yb}{\left({\mathrm{VO}}_4\right)}_2$ and ${\mathrm{Cs}}_3\mathrm{Yb}{\left({\mathrm{VO}}_4\right)}_2$, respectively. Specific-heat results show no sign of long-range magnetic order down to $\sim 0.1\mathrm{K}$ either, but only a Schottky anomaly that is continuously mediated by the external magnetic fields. Additionally, inelastic neutron scattering is employed to detect low-energy spin excitations in ${\mathrm{Rb}}_3\mathrm{Yb}{\left({\mathrm{VO}}_4\right)}_2$. The absence of magnetic excitation signals as well as static magnetic order down to 97 mK aligns with the results from magnetic susceptibility and specific heat. Collectively, these findings point to a quantum disordered ground state with persistent spin dynamics, reminiscent of QSL behaviors. Our work provides a promising platform for further exploration of quantum magnetism in this new disorder-free system.