Two-dimensional (2D) antiferromagnets have attracted considerable interest due to their potential applications in spintronic devices, yet manipulating their spin polarization remains difficult. In this study, we utilized density functional theory calculations to predict that various 2D transition metal triiodide ($M{\mathrm{I}}_3$) bilayers can be half-metals. We reveal that $A$-type antiferromagnetic ground states are crucial for half-metallicity when the bilayers are subjected to an external out-of-plane electric field with carrier doping, while additional conditions, such as appropriate filling or suitable competition between crystal field splitting and on-site Coulomb repulsion, have to be fulfilled if without. Our prediction enables the generation of a fully spin-polarized current driven by field effects, opening up new avenues for the use of 2D antiferromagnets in electric-field-controlled spintronics and potentially overcoming significant challenges in the field.

High-quality single-crystal films of $\mathrm{CoTi}{\mathrm{O}}_3$ were fabricated on a $\mathrm{SrTi}{\mathrm{O}}_3$ substrate using pulsed laser deposition methods, allowing for a detailed investigation of the electronic structure and orbital occupancy via x-ray linear dichroism in both in-plane and out-of-plane configurations. The combination of measurements and semiempirical calculations revealed that the trigonal distortion of $\mathrm{Co}{\mathrm{O}}_6$ octahedra deviating from the cubic crystal field gives rise to an additional trigonal crystal field of ${\mathrm{\Delta }}_{\mathrm{trig}}=45\mathrm{meV}$, with a low-lying $e_g^{\pi }$ orbital of the energetic splitting of the $t_{2g}$ band. Additionally, the x-ray magnetic circular dichroism (XMCD) at the Co $L_{2,3}$ edge captured significant magnetic anisotropy and unquenched orbital angular momentum inherent in the film. An in-plane exchange field of −0.65 meV is required in the multiplet simulation to well reproduce the XMCD spectra obtained with field applied in the in- and out-of-plane directions. These findings provide insights into the potential of $\mathrm{CoTi}{\mathrm{O}}_3$ to align with the Kitaev model when it is presented as a thin film, which is crucial for the further realization of quantum spin liquids via strain engineering.

The Arrhenius law predicts transition times between equilibrium states in thermally activated systems, with broad interdisciplinary relevance. In paleomagnetism, it is used to estimate relaxation times and the thermal stability of magnetic nanoparticles. Magnetite, key to both paleomagnetic studies and magnetic hyperthermia, shows large uncertainties in its attempt frequency $f_0$, varying over ${10}^7–{10}^{13}\mathrm{Hz}$ and affecting relaxation rate estimates. We present a dynamical method using atomistic spin dynamics to fully parametrize the Arrhenius-Néel law. For magnetite nanoparticles with cubic anisotropy, we find ${\tau }_0=0.98\pm 0.13\mathrm{ns}$ at room temperature. For particles with enhanced anisotropy, ${\tau }_0$ decreases significantly and shows a different temperature dependence, highlighting the role of anisotropy. Our method applies to a broad range of systems with clearly defined states and enables robust estimates of domain stability, with particular relevance for the micromagnetic analysis of paleomagnetic records and improving our understanding of geomagnetic fidelity over geological time.

Multielectron bubbles are gas-filled cavities in liquid helium containing electrons pinned to their inner surfaces, thus forming a unique curved two-dimensional electron system (2DES). We perform experiments where an externally applied AC electric field is used to induce size and shape oscillations on these bubbles. We have identified the various normal modes and their frequencies, further compared with analytical results. The experimental technique presented here can lead to a quantitative probe of the properties of the curved 2DES.

The three-dimensional (3D) topological insulators (TIs), hosting topologically protected helical surface states, can be promoted into second-order TIs when a diagonal Zeeman term, typical of magnetic doping, is introduced. The latter hosts exotic chiral one-dimensional (1D) topological hinge states (THSs). In this paper, we investigate the electronic transport of THSs through a magnetic domain wall (DW) in a 3D TI nanowire. Because of the sign reversal of the out-of-plane magnetization across the DW, four 1D topological boundary states, residing on the edge of the DW, arise and form an enclosed loop mediating the counterpropagating THSs. By applying a uniform magnetic field parallel to the nanowire, we obtain a perfect sinusoidal Aharonov-Bohm oscillation in the two-terminal conductance $G$, formulated by $G=\frac{e^2}{2h}[1-cos(\pi \mathrm{\Phi }/{\mathrm{\Phi }}_0)]$, with $\mathrm{\Phi }$ the magnetic flux through the DW and ${\mathrm{\Phi }}_0=h/2e$ the flux quantum. Applying a phenomenological scattering matrix approach, we explain this Aharonov-Bohm oscillation perfectly, and attribute the constructive (destructive) interference of transmission at $\mathrm{\Phi }={\mathrm{\Phi }}_0$ (0) to the $\pi$-spin rotation of the THSs traveling through the DW. Extending our study to a double-DW junction, where the central region has antiparallel magnetization to the leads, we observe Fabry-Pérot oscillations, in which the conductance minima are tuned by the magnetic flux. Our findings open an avenue for finely controlling the quantum transport of THSs in magnetic systems using magnetic flux, and provide a faithful way for detecting THSs in experiments.

We study 2D spin and orbital systems, in a classical limit, in a regime where their coupling is so strong that orbital fluctuations are able to change the sign of spin exchange. Our aim is to understand how different phases in the orbital sector determine the ordering in the spin sector. The existence of intermediate vortex crystal (VC) phases, beside the canonical Kosterlitz and Thouless (KT) phases is now a well-established fact. Recently, we found [Phys. Rev. Res. 6, 043154 (2024)] that such a phase can explain the phase diagram of FeSe, specific parameters used here come from that ab initio work. Motivated by this, here we develop a renormalization group (RG) method which can capture the formation of the VC phase. We achieve it by incorporating orbital vortex-vortex interactions through space-dependent incommensurability. Simultaneously, we incorporate the coupling with the spin sector for both short- and long-range interactions. We then derive a phase diagram including the effects of long-range interactions and spin-orbit coupling. The presence of the intermediate VC phase in the orbital sector changes the scaling of long-range spin-spin interactions, making them relevant under specific conditions. We thus find a way to bypass the Mermin-Wagner theorem: the regular arrangement of orbital vortices can induce a long-range order in the 2D spin system.

Superconductivity emerges in both $\mathrm{L}{\mathrm{a}}_3\mathrm{N}{\mathrm{i}}_2{\mathrm{O}}_7$ and $\mathrm{L}{\mathrm{a}}_4\mathrm{N}{\mathrm{i}}_3{\mathrm{O}}_{10}$ under high pressure by suppressing their density-wave (DW) transitions, but critical temperature ($T_c$) differs significantly between these two compounds. To gain deeper insights into the distinct superconducting states, it is essential to unravel the nature of the DW states at ambient pressure, a topic that remains largely unexplored. Here, using scanning tunneling microscopy/spectroscopy, we report the direct visualization of an incommensurate unidirectional charge DW (CDW) in $\mathrm{L}{\mathrm{a}}_4\mathrm{N}{\mathrm{i}}_3{\mathrm{O}}_{10}$ in real space. The density of states (DOS) is strongly depleted near $E_{\mathrm{F}}$, indicating the opening of a CDW gap of 2Δ ≈ 71 meV, which is unfavorable for the formation of superconductivity at ambient pressure. We propose that the CDW arises from Fermi surface nesting and is likely a subsidiary phase of a spin-DW. Compared with $\mathrm{L}{\mathrm{a}}_3\mathrm{N}{\mathrm{i}}_2{\mathrm{O}}_7$, the weaker electronic correlation in $\mathrm{L}{\mathrm{a}}_4\mathrm{N}{\mathrm{i}}_3{\mathrm{O}}_{10}$ is likely one reason for the lower $T_c$.

We developed a lattice dynamical theory of an atomically thin compressional piezoelectric resonator, based on a two-dimensional (2D) ionic crystal with $D_{3h}$ point group symmetry and finite lateral size. Starting from an anharmonic vibrational Hamiltonian, we derive in-plane acoustic and optical displacement propagators using linear response theory, incorporating symmetry-based selection rules. These propagators satisfy Dyson equations with polarization operators dependent on frequency, temperature, and crystal size. The dynamic piezoelectric susceptibilities governing the direct and converse piezoelectric effects are shown to be equal. Resonant behavior of in-plane longitudinal and transverse acoustic waves is analyzed in both classical and quantum regimes. In the quantum limit near zero temperature, resonance broadenings scale inversely with crystal size. Below a crossover temperature, the quantum zero-point fluctuations become dominant and put an upper limit on the quality factor which is size independent. In the classical regime, the broadenings scale with temperature and the quality factor decreases inversely with crystal size. Scaling relations for temperature, size, and quality factor are established. Anharmonic scattering is shown to be dominated by out-of-plane flexural modes. The theory is showcased numerically on 2D hexagonal boron nitride (h-BN) and $\mathrm{2H}\text{−}{\mathrm{MoS}}_2$. The results suggest that h-BN, despite a smaller static piezoelectric coefficient, is a more robust candidate for resonator applications due to its higher in-plane rigidity. The framework applies broadly to 2D piezoelectric crystals of $D3h$ symmetry, including transition-metal dichalcogenides and dioxides.

Excitons and phonons are the two fundamental types of quasiparticles in solids including atomically layered two-dimensional (2D) transition metal dichalcogenides (TMDs). They themselves and their interactions play a central role in determining the optical properties of 2D-TMDs. Herein, we present a combined study on effects of environmental dielectric screening on the optical properties of excitons and the exciton-phonon scattering in a series of $\mathrm{W}{\mathrm{S}}_2$ monolayers with different environmental dielectric screenings via reflectance spectroscopic measurements and microscopic calculations. As the dielectric screening increases, the linewidth of the A-exciton resonance structure exhibits a significant narrowing and less dependence on temperature. Microscopic theoretical analysis based on the variational Ansatz shows that the noticeable reduction in both radiative recombination rate and phonon-assisted intervalley scattering of excitons result in the observed phenomenon due to the reduction of exciton Bohr radius caused by the increase of the dielectric screening. In this study, we demonstrate that dielectric engineering can offer an effective approach for regulating the excitonic effects and investigating the complicated exciton-phonon interactions in ultimate 2D monolayer systems.

A prerequisite to using the net spin polarization generated by a source-drain bias in three-dimensional topological insulators for spintronic applications is understanding how such a bias alters the transport properties of these materials. At low temperatures, quantum corrections can dominate the temperature dependence of the resistance. Although a dc bias does not break time-reversal symmetry and is therefore not expected to suppress quantum corrections, an increase of the electron temperature due to Joule heating can cause a suppression. This suppression at finite bias can lead to a non-Ohmic differential resistance in the three-dimensional topological insulator (${\mathrm{Bi}}_{1-x}{\mathrm{Sb}}_x{)}_2{\mathrm{Te}}_3$, consisting of a zero-bias resistance peak (from electron-electron interactions) and a high-bias background (from weak antilocalization). We show that the bias voltage dependence of quantum corrections can be mapped to the temperature dependence, while the heating effect on the lattice temperature remains small. When searching for non-Ohmic effects due to novel phenomena in three-dimensional topological insulators, Joule heating should not be overlooked.

We theoretically study the nonlinear electron dynamics of Haldane model quantum dots placed in the field of an ultrashort optical pulse. One of the tuning parameters of the Haldane model is the phase accumulated by an electron during its transfer between the next-nearest-neighbor sites. We study how this parameter affects the nonlinear electron dynamics and the generation of high optical harmonics. With increasing the phase from its zero value to ${90}^{\circ }$, the low-energy electron states in the conduction and valence bands become more localized near the edges of the quantum dot, resulting in suppression of the band gap, enhancement of the dipole interband coupling, and strong suppression of the average low-energy electron density of the states. As a result, the nonlinear response of the electron system of a Haldane model quantum dot is the strongest at intermediate values of the phase, $\approx 30\text{–}{40}^{\circ }$. At these values of the phase, the electron dynamics is irreversible and a few first high-order harmonics have the largest intensities. When the phase approaches a ${90}^{\circ }$ value, the generation of high harmonics is strongly suppressed.

We study the boundary criticality enriched by boundary fermions, which ubiquitously emerge in topological phases of matter, with a focus on topological insulators and topological superconductors. By employing dimensional regularization and bosonization techniques, we uncover several alternative boundary universality classes. These include the boundary Gross-Neveu-Yukawa critical point and the special Berezinskii-Kosterlitz-Thouless (BKT) transition, both resulting from the interplay between edge modes and bulk bosons. We present a comprehensive sketch of the phase diagram that accommodates these boundary criticalities and delineate their critical exponents. Additionally, we explore a ($1+1$)-dimensional conformal defect decorated with fermions, where a defect BKT transition is highlighted. We conclude with a discussion on potential experimental realizations of these phenomena.

We present an algorithm that integrates pseudosymmetry search with first-principles calculations to systematically identify achiral parent structures and establish potential chiral displacive transitions linking them to their corresponding chiral phases within the 22 enantiomorphic space groups. This approach enables a robust exploration of structural relationships, offering new insights into symmetry-driven properties. Our workflow streamlines the discovery of displacive chiral phase transitions driven by soft phonon modes, providing insights into the mechanisms of structural chirality in inorganic materials. We apply this methodology on the chiral phases of ${\mathrm{TeO}}_2$, ${\mathrm{Na}}_2{\mathrm{SeO}}_9$, ${\mathrm{Sr}}_2{\mathrm{As}}_2{\mathrm{O}}_7$, ${\mathrm{As}}_2{\mathrm{O}}_5$, ${\mathrm{Rb}}_2{\mathrm{Be}}_2{\mathrm{O}}_3$, and ${\mathrm{CaTe}}_2{\mathrm{O}}_3$. Demonstrating that some do not have a minimal supergroup that allows for an achiral phase; some can have a minimal supergroup, still, no unstable phonon mode exists in the achiral phase; and somewhere the minimal supergroup exists with a soft phonon mode connecting the identified achiral phase and the chiral phase through small continuous displacements.

Classical techniques for the simulation of complex many-body systems are often constrained by the exponential growth of the dimensions of quantum systems, requiring prior assumptions to efficiently extract nonlocal quantum properties. To address this limitation, we propose efficient short-depth quantum algorithms for estimating nonlocal quantum properties, such as quantum coherence and discrete Berry phases, to probe quantum phase transitions in many-body systems. The gate complexities of the quantum circuits scale linearly while the depth complexity scales logarithmically with system size. We demonstrate the applicability of these algorithms by investigating the well-known Haldane-to-quantum-paramagnetic phase transitions in spin-1 Heisenberg antiferromagnetic chains. These transitions, being topological in nature, cannot be identified using local order parameters, necessitating the measurement of nonlocal quantum properties. Our results show that the algorithms successfully extract nonlocal information and provide accurate signatures of topological phase transitions near critical points.

Understanding and simulating the thermodynamic and dynamical properties of materials affected by strong ionic anharmonicity is a central challenge in material science. Much interest is in material displaying critical displacive behavior, such as near a ferroelectric transition, charge-density waves, or in general displacive second-order transitions. In these cases, molecular dynamics suffers from a critical slowdown and emergent long-range fluctuations of the order parameter. Two prominent methods have emerged to solve this issue: self-consistent renormalization of the phonons like the self-consistent harmonic approximation and self-consistent phonons, and methods that fit the potential energy landscape from short molecular dynamics trajectories, like the temperature-dependent effective potential (TDEP). Despite their widespread use, the limitations of these methods are often overlooked in the proximity of critical points. Here, we establish a guiding rule set for the accuracy of each method on critical quantities: free energy for computing the phase diagrams, static correlation functions for inferring phase stability and critical behaviors, and dynamic correlation functions for vibrational spectra and thermal transport. Also, a new TDEP implementation is introduced to fix the calculation of dynamical spectra, restoring the correct perturbative limit violated by the standard TDEP approach. Results are benchmarked both against an exact one-dimensional anharmonic potential and two prototypical anharmonic crystals: the ferroelectric PbTe and the metal-halide perovskite ${\mathrm{CsSnI}}_3$.

We propose a method based on the discrete truncated Wigner approximation (DTWA) for computing out-of-time-order correlators. This method is applied to long-range interacting quantum spin systems where the interactions decay as a power law with distance. As a demonstration, we use a squared commutator of local operators and its higher-order extensions that describe quantum information scrambling under Hamilton dynamics. Our results reveal that the DTWA method accurately reproduces the exact dynamics of the average spreading of quantum information (i.e., the squared commutator) across all time regimes in strongly long-range interacting systems. We also identify limitations in the DTWA method when capturing dynamics in weakly long-range interacting systems and the fastest spreading of quantum information. Then we apply the DTWA method to investigate the system-size dependence of the scrambling time in strongly long-range interacting systems. We reveal that the scaling behavior of the scrambling time for large system sizes qualitatively changes depending on the interaction range. This work provides and demonstrates a technique to study scrambling dynamics in long-range interacting quantum spin systems.

We present a formalism based on color symmetry to analyze the momentum-space spin textures of noncollinear antiferromagnets. We show that, out of the spin textures allowed by the magnetic point group, one can extract a component that is invariant by general rotations in spin space, and can exist in the absence of spin-orbit coupling, in complete analogy to spin textures in altermagnets. We demonstrate this approach in the case of three complex, noncollinear magnets, ${\mathrm{Mn}}_3\mathrm{Ir}(\mathrm{Ge},\mathrm{Si})$, ${\mathrm{Pb}}_2{\mathrm{MnO}}_4$, and ${\mathrm{Mn}}_3\mathrm{GaN}$. For ${\mathrm{Mn}}_3\mathrm{GaN}$, we also show that the predictions of color-symmetry analysis are consistent with density functional theory calculations performed on the same system both with and without spin-orbit coupling.

Flexomagnetism is a prominent example of flexoeffects and shows great potential in micro/nanodevices where strain gradient could be extremely large. Herein, we demonstrate intrinsic flexomagnetic response in antiferromagnetic monolayer FeSe by applying a constant strain gradient and decipher the associated mechanism via first-principles calculations. We find that the flexomagnetic response of monolayer FeSe is linear under longitudinal, in-plane shear, out-of-plane shear, and transverse strain gradients, with an intrinsic flexomagnetic coefficient up to $3.1{\mu }_{\text{B}}\AA$. By introducing a geometrical bond-angle parameter $\mathrm{\Delta }\theta$, we identify the crystal environment difference of magnetic atoms as the origin of the strain-gradient-induced net magnetic moment, enabling a potentially unified framework to decipher the flexomagnetic mechanism in antiferromagnets. The $d_{yz}$ and $d_{z^2}$ orbitals are further found as the main contributors to flexomagnetism, serving as the direct electronic basis for the flexomagnetic effect. We also reveal that strain gradients could manipulate the spin-orbit coupling strength of $d_{xz}\text{−}{d}_{yz}$ and $d_{z^2}\text{−}{d}_{yz}$ orbitals and thus tune the magnetic anisotropy. Our findings provide a comprehensive discussion on the magnetic responses to strain gradients and their underlying mechanisms in monolayers and could inspire the future exploration of flexible magnetic applications by leveraging flexomagnetism.