The time-reversal odd $\left(\mathcal{T}\text{−}\text{odd}\right)$ transverse spin current induced by the spin-splitter effect (SSE) in $d$-wave altermagnet has gained much attention owing to its high charge-spin conversion efficiency. This study conducted a comprehensive investigation into the unconventional spin Hall effect (SHE) for a series of ${\mathrm{Cr}}_{0.5}{X}_{0.5}{\mathrm{O}}_2$ $(X=\mathrm{Ti},\mathrm{V},\mathrm{Os},\mathrm{Fe})$ compounds through ab initio calculations, and introduced the concept of crystal SHE in our investigated system as the counterpart of SSE in $d$-wave altermagnet. It was found that the crystal SHE-induced transverse spin current exists universally in ferrimagnet and ferromagnet with similar crystal symmetry to $d$-wave altermagnet but different magnetic configuration. The crystal SHE-induced spin Hall conductivity (crystal SHC) $|{\sigma }_{xy}^{z\text{,odd}}|$ of ${\mathrm{Cr}}_{0.5}{\mathrm{Fe}}_{0.5}{\mathrm{O}}_2$ can reach 9081.686 $(\hslash /e)\mathrm{S}/\mathrm{cm}$ at a low scattering rate, surpassing that of ${\mathrm{RuO}}_2$, and the charge-spin conversion ratio $|{\sigma }_{xy}^{z\text{,odd}}/{\sigma }_{xx}|$ of ${\mathrm{Cr}}_{0.5}{\mathrm{Ti}}_{0.5}{\mathrm{O}}_2$ is up to 22%. In the case of low-concentration doping, ${\mathrm{Cr}}_{0.875}{\mathrm{Fe}}_{0.125}{\mathrm{O}}_2$ even exhibits a higher charge-spin conversion ratio $(\sim 29\%)$ than ${\mathrm{RuO}}_2$. Additionally, we investigated the magnetic spin Hall effect, which is also $\mathcal{T}$-odd and only exists in magnetic materials. After replacement, the magnetic spin Hall conductivity (MSHC) of the ${\mathrm{Cr}}_{0.5}{X}_{0.5}{\mathrm{O}}_2$ compounds demonstrate an obvious enhancement compared to ${\mathrm{CrO}}_2$, with ${\mathrm{Cr}}_{0.5}{\mathrm{Os}}_{0.5}{\mathrm{O}}_2$'s MSHC $|{\sigma }_{zx}^{x\text{,odd}}|$ being comparable to the crystal SHC. In comparison, even though a slight shift in the Fermi level can lead to a tenfold increase of the $\mathcal{T}$-even intrinsic spin anomalous Hall conductivity, their magnitude remains substantially inferior to that of their $\mathcal{T}$-odd crystal SHC. Our research provides new paths for high-efficiency charge-spin conversion within an easily controllable magnetic system.

Pair density waves (PDWs) are a inhomogeneous superconducting states whose Cooper pairs possess a finite momentum resulting in a oscillatory gap in space, even in the absence of an external magnetic field. There is growing evidence for the existence of PDW superconducting order in many strongly correlated materials, particularly in the cuprate superconductors and in several other different types of systems. A feature of the PDW state is that inherently it has a CDW as a composite order associated with it. Here we study the structure of the electronic topological defects of the PDW, paying special attention to the half-vortex and its electronic structure that can be detected in STM experiments. We discuss tell-tale signatures of the defects in violations of inversion symmetry, in the excitation spectrum and their spectral functions in the presence of topological defects. We discuss the “Fermi surface” topology of Bogoliubov quasiparticle of the PDW phases, and we briefly discuss the role of quasiparticle interference.

Altermagnetism, as a new branch of magnetism independent of traditional ferromagnetism and antiferromagnetism, has attracted extensive attention recently. At present, researchers have proved several kinds of three-dimensional altermagnets, but research on two-dimensional (2D) altermagnets remains elusive. Here, we propose a method for designing altermagnetism in 2D lattices: bilayer reversed stacking. This method could enable altermagnetism-type spin splitting to occur intrinsically and the spin splitting could be controlled by crystal chirality. We also demonstrate it through a real material of bilayer $\mathrm{Pt}{\mathrm{Br}}_3$ with AB′ stacking order. Additionally, the combination of stacking order and slidetronics offers opportunities for electrical writing and magnetic reading of electronic devices. In the case of ${\mathrm{AC}}^{\prime }$ stacking, interlayer sliding results in reversible spontaneous polarization. This unique combination of antiferromagnetism and sliding ferroelectricity leads to polarization-controlled spin splitting, thus enabling magnetoelectric coupling, which can be detected by magneto-optical Kerr effect even without net magnetization. Our research highlights that reversed stacking provides a platform to explore rich physical properties of magnetism, ferroelectricity, and spin splitting.

Here, we report on comprehensive structural, magnetic, specific heat, resistivity, and microscopic ${}^{57}\mathrm{Fe}$ Mössbauer spectroscopy investigations of the ${\mathrm{GeFe}}_3\mathrm{N}$ compound. The experimental results indicate that the ground state of ${\mathrm{GeFe}}_3\mathrm{N}$ is short-range ordered ferromagnetic (FM) state, where the FM clusters are embedded in the paramagnetic background. Our density functional theory (DFT) calculations have revealed possible orbital order for the FeII site, and the strong interplay between the local moments for the FeI site and the itinerant moments for the FeII site may be responsible for the observed short-range FM order. We believe that these results are crucial for a complete understanding of the peculiar magnetic properties reported for ${\mathrm{GeFe}}_3\mathrm{N}$ and may also shed light on the understanding of the complex relationships between the structure and magnetism in related compounds.

Below the structural transition occurring at $T_s=90\mathrm{K}$, FeSe exhibits positive transverse magnetoresistance when the current is applied parallel to the $ab$ plane. In this study, we show that, in contrast, when both the magnetic field and the current are aligned along the $c$ axis, the magnetotransport changes significantly. In this configuration, FeSe develops a sizable negative longitudinal magnetoresistance ($\sim 15\%$ at $T=10\mathrm{K}$ and ${\mu }_{0}H=16\mathrm{T}$) in the nematic phase. We attribute this finding to the effect of the applied magnetic field on the scattering from spin fluctuations. Our observations reflect the intricate interplay between spin and orbital degrees of freedom in the nematic phase of FeSe.

Superconductivity in single-layer graphene has attracted considerable interest. Here, using the determinant quantum Monte Carlo method, we study transitions of superconductivity and magnetism in a monolayer graphene with a special periodic strain. Consistent with experiments, the deformation accumulates a series of flat bands, whose robustness under interaction is verified through electron localization in real space. During the reconstruction of the band structure, the superconductivity appears in flat band range with next-nearest neighbor $d+id$ pairing symmetry dominating other modes and is accompanied by ferromagnetism caused by symmetry breaking. We also demonstrate that the strain-induced symmetry breaking would accumulate an energy-gap antiferromagnetic insulating phase at half filling even under the limitation of not strong enough interaction, which shows its potential as a platform that exhibits strongly correlated phenomena.

We study the interplay of attractive electron interactions and topological states in strained kagome lattices with spin-orbit coupling via a Hubbard Hamiltonian in the mean-field approximation. In the unstrained lattice, there is a topological phase transition from a quantum spin Hall state to a charge density wave (CDW) with increasing interaction strength. Upon applying a uniform uniaxial strain to the lattice, we find a new phase with coexisting CDWs and topological states. For increasing interaction strength or strain, the system is driven into a pure CDW, signaling topological phase transitions. The directionality (nematicity) of the CDW is controlled by the direction of the applied strain. When $s$-wave electronic pairing is allowed, the system develops a superconducting order beyond a threshold attraction, which is totally suppressed by the onset of a CDW with increasing interaction. Most interestingly, moderate strain allows the coexistence of superconductivity and CDWs for a range of interaction values. This illustrates how electronic interactions and single-particle topological structures compete to create unusual correlated phases in kagome systems.

An ultrathin two-dimensional (2D) semiconductor, monolayer ${\mathrm{MoS}}_2$, holds stable excitons and charged excitons (trions) even at ambient temperature. Trion formation time is of interest to the physics of 2D electron systems because it relates to strengths of interactions between excitons and charges, exciton size, and exciton diffusion, which depend on dimensionality. In this paper, we investigate the trion formation via exciton-to-trion relaxation using femtosecond time-resolved photoluminescence (PL) measurements for monolayer ${\mathrm{MoS}}_2$ on a sapphire substrate. The number of layers and defect concentration were evaluated by measuring Raman scattering spectra. The electron concentration was estimated from the PL peak-energy difference between A excitons and trions. The time evolutions of exciton and trion PL in the femtosecond and picosecond regions were obtained. The rate-equation analysis provided the time constant of the relaxation from exciton to trion, i.e., the time constant of trion formation. This time constant was compared with that of other 2D electron systems and discussed in terms of the 2D confinement determined by the thickness. In this paper, we advance our understanding of exciton physics in low-dimensional systems.

The novel series of hole-doped quadruple manganite perovskites ${\mathrm{Hg}}_{1-x}{\mathrm{Na}}_x{\mathrm{Mn}}_3{\mathrm{Mn}}_4{\mathrm{O}}_{12}$ (HNMO) has been synthesized and its charge and orbital order behavior investigated through high-resolution synchrotron powder x-ray diffraction techniques. Through careful Rietveld refinements of structural models via symmetry-motivated approaches, we show that the ground state of HNMO compositions adopts a polar $G$-type charge and orbital ordered state, which is rare in manganite perovskites, and is robust as a sole phase up to a critical doping level. Upon this critical doping, coincident with that in which colossal magnetoresistance (CMR) is maximal in canonical manganite perovskites, electronic phase separation occurs between $G$-type and orbital order with charge disorder-type states. The latter state has recently been identified in ${\mathrm{Ca}}_{1-x}{\mathrm{Na}}_x{\mathrm{Mn}}_3{\mathrm{Mn}}_4{\mathrm{O}}_{12}$ perovskites, and proposed to be the competing insulating state from which CMR phenomena emerge. We show that the mechanism for the formation of the $G$-type state is due to charge transfer processes which may occur through a coupling of distortions involving structural and charge and orbital degrees of freedom, ultimately driving the polar ground state through an improper-like ferroelectric polarization mechanism. These results will act as an important recipe for designing novel ferroelectric-active materials, in addition to expanding the richness of charge and orbital ordered states in manganite perovskites.

Weyl points are generic and stable features in the energy spectrum of Hamiltonians that depend on a three-dimensional parameter space. Nongeneric isolated twofold degeneracy points, such as multi-Weyl points, split into Weyl points upon a generic perturbation that removes the fine tuning or protecting symmetry. The number of the resulting Weyl points is at least $\left|Q\right|$, where $Q$ is the topological charge associated to the nongeneric degeneracy point. Here, we show that such a nongeneric degeneracy point also has an birth quota, i.e., a maximum number of Weyl points that can be born from it upon any perturbation. This upper bound is a local multiplicity associated to the nongeneric degeneracy point, an invariant of map germs known from singularity theory. This holds not only for the case of a three-dimensional parameter space with a Hermitian Hamiltonian, but also for the case of a two-dimensional parameter space with a chiral-symmetric Hamiltonian. We illustrate the power of this result for band structures of two- and three-dimensional crystals. Our work establishes a strong and powerful connection between singularity theory and topological band structures and, more broadly, parameter-dependent quantum systems.

Modeling of wrinkles and folds formation in single and multilayer graphene sheets lying on flat deformable (polymer) substrates has been carried out. It is shown that the deformability of the substrate leads to the appearance of significant features. In contrast to the flat surfaces of rigid crystals, molecules of soft polymer substrates can penetrate into the wrinkles and folds of the graphene sheet, completely filling the voids beneath the sheet. Moreover, the vertical folds of the sheet can be directed not only upward from the substrate, but also down into the substrate, penetrating it. By modeling the uniaxial compression of the two-component graphene/polymer system, the external hydrostatic pressure and thermal vibrations of the substrate molecules have been taken into account. High pressure $P\ge 0.015$ GPa leads to a noticeable additional stabilization of the initial ground state of the system. At uniaxial compression above the critical value, a system of localized wrinkles, whose interior is filled with molecules of the substrate appears in the graphene sheet. Increasing temperature leads to an increase in the size of wrinkles and to a decrease in their number. The largest wrinkles form before the substrate begins to melt. Melting leads to the complete disappearance of all wrinkles and small folds. Cooling of the melted substrate leads to its crystallization, but the system of wrinkles in the graphene sheet on the surface is not restored. Therefore, melting the polymer substrate and subsequently cooling it can serve as a method for getting rid of localized wrinkles and folds in the graphene sheet.

We propose a source of purely electronic energy-entangled states implemented in a solid-state system with potential applications in quantum information protocols based on electron flying qubits. The proposed device relies on the standard tools of electron quantum optics and exploits entanglement of the Cooper pairs of a BCS superconductor. The latter is coupled via an adjustable quantum point contact to two opposite spin-polarized electron waveguides, which are driven by trains of Lorentzian pulses. This specific choice for the drive is crucial to inject purely electronic entangled states devoid of spurious electron-hole pairs. In the Andreev regime, a perturbative calculation in the tunnel coupling confirms that entangled electron states are generated at the output of the normal side. We introduce a quantity related to charge current cross correlations which allows one to experimentally verify the entangled nature of the emitted state.

Strain engineering stands out as a promising avenue for manipulating the physicochemical characteristics of two-dimensional (2D) van der Waals (vdW) materials. Amidst the flourishing investigations into the modulation of electronic band structures, the influence of elastic strain on the evolution of phonon spectra and, consequently, thermal transport has garnered comparatively less attention. Yet, this facet offers a realm of intricate physics and engineering possibilities. Bending, a process that couples in-plane (IP) and cross-plane (CP) motion, provides a unique opportunity to investigate the influence of elastic anisotropy on the thermal properties of inhomogeneous strain-modulated 2D materials. Here we studied the effects of CP bonding on the IP thermal conductivities of a model layered material using molecular dynamics simulation. Notably, our findings unveiled an intriguing negative correlation between CP bonding strength and IP thermal conductivity while keeping IP bonding constant. This can be attributed to the heightened stress gradient spanning across multiple layers due to the pronounced CP bonding, which in turn, engenders a broadened phonon spectrum, and eventually stronger phonon scattering. Our work sheds light on the intricate interplay between CP bonding and thermal transport in bent vdW materials, offering insights extendable to the broader family of 2D materials.

The polymorphism and mechanism of helium compounds are crucial for understanding the physical and chemical nature of He-bearing materials under pressures. Here, we predict two types of He-bearing compounds, MgHe and $\mathrm{M}{\mathrm{g}}_n\mathrm{He}$ $(n=6, 8, 10, 15, \text{and} 18)$, being formed >750 GPa, by unbiased ab initio structure search. An unexpected band gap is opened in MgHe at as low as ∼200 GPa. This is a case of noble gas–driven metal-nonmetal transition. The same mechanism is demonstrated as also being applicable to other metallic elements and making beryllium transform into a nonmetallic state, an achievement that is impossible otherwise. Furthermore, the stability of the simple cubic phase of Mg (Mg-sc) is greatly enhanced by mixing with He, which lowers the critical pressure of pure Mg-sc from ∼1.1 TPa down to 750 GPa to form ordered substitutional alloying phase of $\mathrm{M}{\mathrm{g}}_n\mathrm{He}$ on a simple cubic lattice of Mg. This is a report on a Mg-based noble gas–substitutional alloy, in sharp contrast with the conventional wisdom that He prefers interstitial sites. The observed striking influences of He demonstrate the rich physics and chemistry of He-bearing compounds under ultrahigh pressures.

Chirality exists universally in nature and is closely related to various exotic physical phenomena. In nanoscale functional materials, various chiral topological structures exist fruitfully together, forming an exotic chiral topological zoo. Chirality has always been challenging to control. Ferroelectric vortices, as well-known in-plane chiral topological domain structures, have been extensively studied experimentally and theoretically in recent years. Compared with ferromagnetic vortices, ferroelectric vortices are two-dimensional, and their chirality only includes circulation but no polarity, which has led to the longstanding misconception that polar vortices, like Bloch-type merons, do not exist in ferroelectric materials. In this paper, we conduct a series of phase-field simulations using ferroelectric epitaxial nanodisks to propose an effective scheme for constructing a three-dimensional polar vortex domain structure with central polarity and a series of strategies for manipulating the chirality of polar vortices (including circulation and polarity), providing inspiration for the design of multistate information memories. The results show that misfit strain, the aspect ratio of a nanodisk, and the mole fraction of Zr and Ti atoms in $\mathrm{PbZ}{\mathrm{r}}_{1-x}\mathrm{T}{\mathrm{i}}_x{\mathrm{O}}_3$ are decisive factors to polar vortex generation. Furthermore, applying either an axial electric field or in-plane torque, the polarity of a polar vortex can be changed while keeping its circulation unchanged; the combined application of the axial electric field and in-plane torque realizes alternating changes in the polarity and circulation of a polar vortex. In this paper, we enrich the understanding of ferroelectric topological physics, which is conducive to facilitating the development of ferroelectric nanoelectronic devices.

In this study, we probe the coupling between magnetism and lattice dynamics in ${\mathrm{UO}}_2$, a 3k antiferromagnet that undergoes magnetic ordering below its Néel temperature ($T_N$) of 30.8 K. Ultrasound measurements provide insights into the interplay between the material's magnetic properties and lattice vibrations in response to the applied high magnetic field. A model analysis based on ab initio calculated superexchange interactions predicts a change in the magnetic structure from 3k to 2k at around 50 T. Although this change is not evident in the magnetization curve, we observe a crossover of ultrasound velocity $\mathrm{\Delta }v/v$ using a phase-sensitive pulse-echo technique in pulsed magnetic fields of up to 65 T. We show that at ${\mathrm{H}}_z>$ 50 T, the structure remains antiferromagnetic in the $xy$ plane and becomes ferromagnetic in the $z$ direction. A further transition into the 1k structure is theoretically predicted to take place at a magnetic field of $\sim 104\mathrm{T}$.

Hydrogen-based compounds demonstrate high-temperature superconductivity under pressure, offering promising avenues for exploring metal hydrides with elevated critical temperatures ($T_{\mathrm{c}}$), while hydrides comprising ${\mathrm{H}}_2$ molecular units were previously deemed unfavorable. Searching for new forms of hydrogen within metal hydrides promised a route for developing novel high-temperature superconductors. In this study, we utilized the evolutionary algorithm alongside first-principles calculations to investigate the high-pressure crystal structure of the $\mathrm{Bi}{\mathrm{H}}_{\mathrm{n}}$ ($n=7-18$) system. Notably, we discovered stable cyclo-${\mathrm{H}}_{12}\mathrm{Bi}/\mathrm{Pb}$ compounds in which the ${\mathrm{H}}_{12}$ resembles cyclohexanelike cyclo-${\mathrm{H}}_{12}$ rings. These compounds are stable above 180 GPa and exhibit a higher $T_{\mathrm{c}}$ than $\mathrm{Bi}{\mathrm{H}}_8$ composed of ${\mathrm{H}}_2$ units. In cyclo-${\mathrm{H}}_{12}$, the hydrogen has a slightly elongated intramolecular H-H length compared to $\mathrm{Bi}{\mathrm{H}}_8$, while the intermolecular H-H distance is much shorter than in H cages, forming a resonant structure. The resonance led to in-/out-of-plane vibration modes for the six ${\mathrm{H}}_2$ units in cyclo-${\mathrm{H}}_{12}$, forming broad midfrequency vibrational bands and boosting the electron-phonon coupling. These findings highlight the cycloform of hydrogen-based hydrides as promising candidates for high $T_{\mathrm{c}}$ superconductors, propelling further exploration of superconductivity in binary superhydrides with different forms of hydrogen.