We present results from density-functional theory-based molecular dynamics simulations of fluid mixtures of carbon dioxide ($\mathrm{C}{\mathrm{O}}_2$) and rhenium (Re) metal at pressures up to 45 GPa and a temperature of 4000 K. Covalently bonded carbon chains form as a result of a redox reaction between $\mathrm{C}{\mathrm{O}}_2$ and Re, whereas a pure $\mathrm{C}{\mathrm{O}}_2$ fluid is stable against decomposition at the same conditions. These findings have implications in reducing $\mathrm{C}{\mathrm{O}}_2$ by pressure-induced redox reactions with transition metals, which may play a role in $\mathrm{C}{\mathrm{O}}_2$ destruction and carbon storage as related to possible solutions to the current climate crisis.

We show that slow time-periodic variation of the external magnetic field applied to a polariton topological insulator based on a honeycomb array of microcavity pillars with pronounced TE-TM splitting results in oscillations of the edge states along the boundary of the insulator accompanied by slow transverse drift of the center of mass of the edge state along the boundary. The oscillations and drift are due to time-periodic variation of the sign of the instantaneous group velocity of the edge state with selected central Bloch momentum triggered by the variation of the Zeeman splitting in the external magnetic field. These oscillations are also accompanied by periodic exchange of norm between two polarization components of the edge state. Oscillating edge states survive despite the fact that the magnetic field periodically vanishes resulting in closure of the topological gap in the instantaneous spectrum of the system. The direction of the drift and amplitude of oscillations of the edge state strongly and nonmonotonically depend on its initial central Bloch momentum.

We consider a model of two harmonically driven damped harmonic oscillators that are coupled linearly and with a cross-Kerr coupling. We show how to distinguish this combination of coupling types from the case where a coupling of optomechanical type is present. This can be useful for the characterization of various nonlinear systems, such as mechanical oscillators, qubits, and hybrid systems. We then consider a hybrid system with linear and cross-Kerr interactions and a relatively high damping in one of the modes. We derive a quantum Hamiltonian of a doubly clamped magnetic beam, showing that the cross-Kerr coupling is prominent there. We discuss, in the classical limit, measurements of its linear response as well as the specific higher-harmonic responses. These frequency-domain measurements can allow estimating the magnitude of the cross-Kerr coupling or the magnon population.

Sputtered thin films of binary ${\mathrm{Y}}_x{\mathrm{Co}}_{1–x}$ with $0<x\le 0.54$ and thickness $\approx 15$ nm are investigated to help understand the ferromagnetism of cobalt in amorphous rare-earth cobalt alloys. The magnetic moment per cobalt falls to zero at $x_c\approx 0.50$, where the appearance of magnetism is marked by a paraprocess with a dimensionless susceptibility of up to 0.015. All films are magnetically soft, with densities that fall between those of crystalline Y-Co intermetallic compounds and the density of a relaxed 10 000-atom binary random close-packed model of hard spheres with an Y:Co volume ratio of 3:1, where the packing fractions for all films lie in a narrow range, 0.633 ± 0.004, and Co is coordinated by an average of 3.2 Co and 3.2 Y atoms at $x=0.5$. All films with $x<0.4$ exhibit in-plane shape anisotropy that is about six times as great as an intrinsic perpendicular component. Average cobalt spin and orbital moments obtained by x-ray magnetic circular dichroism were 1.31 and $0.32{\mu }_{\mathrm{B}}$, respectively, for amorphous ${\mathrm{Y}}_{0.25}{\mathrm{Co}}_{0.75}$. Strong local anisotropy is associated with the large cobalt orbital moment, but there is little influence of anisotropy on the ferromagnetic order because of exchange averaging. The films are soft ferromagnets. Hall effect and magnetoresistance are modeled in terms of effective uniform rotation of the magnetization, with spontaneous and band contributions. Amorphous ${\mathrm{Y}}_x{\mathrm{Co}}_{1–x}$ is contrasted with amorphous ${\mathrm{Y}}_x{\mathrm{Fe}}_{1–x}$, which exhibits random noncollinear magnetic order that is very sensitive to the film density.

The bilayer of ${\mathrm{CrI}}_3$ is a prototypical van der Waals (vdW) 2D antiferromagnetic material with magnetoelectric effect. It is not generally known, however, that for symmetry reasons the flexomagnetic effect, i.e., the strain gradient-induced magnetization, is also possible in this system. In the present paper, based on the first-principle calculations, we estimate the flexomagnetic effect to be $200{\mu }_B·\AA$, which is two orders of magnitude higher than it was predicted for the referent antiperovskite flexomagnetic material ${\mathrm{Mn}}_3\mathrm{GaN}$. The two major factors of flexomagnetic effect enhancement related to the peculiarities of antiferromagnetic structure of vdW magnets are revealed: the strain-dependent ferromagnetic coupling in each layer, and large interlayer distance separating antiferromagnetically coupled ions. Since 2D systems are naturally prone to mechanical deformation, the emerging field of flexomagnetism is of special interest for application in vdW spintronics, and straintronics in particular.

One approach to probe the still controversial superconductivity in ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ is to apply external perturbations that break the underlying tetragonal crystalline symmetry. Chiral $p_x+i{p}_y$ and $d_{xz}+i{d}_{yz}$ states respond to such perturbations in ways that may help to distinguish them from other superconducting pairings. However, past experimental efforts along this line, using uniaxial strains and magnetic fields parallel to the ${\mathrm{RuO}}_2$ plane, have not been able to reach an unambiguous conclusion. In this study, we propose to further examine the possibility of chiral superconducting order in ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ using an alternative tetragonal-symmetry-breaking perturbation—in-plane supercurrent. We study the superconducting phase diagram as a function of both temperature and the applied supercurrent. Supercurrent generically splits the transition of the two chiral order parameter components, and we show that the splitting can give rise to visible specific heat anomalies. Furthermore, supercurrent parallel and antiparallel to the unidirectional propagation of the chiral edge modes impact the edge states in a different manner. This difference manifests in the tunneling spectrum, thereby providing an additional means to probe the chirality even when the related spontaneous edge current is vanishingly small. Finally, we discuss the distinction of supercurrent responses in nonchiral time-reversal-symmetry-breaking superconducting states. Our proposal can be applied to other candidate chiral superconductors.

We use the Gross-Pitaevskii equation to study Josephson tunneling between two weakly coupled Bose-Einstein condensates, which compose spin-1 bosons. We show that a rotating magnetic field on one side can produce a phase difference across the junction, resulting in an oscillatory tunneling spin current. Besides numerical calculation, we derive analytical results in two extreme cases, namely the low- and high-frequency limits: in the low-frequency limit (magnetic field rotates adiabatically), a non-Abelian geometric phase arises and leads to the oscillatory spin current. By sharp contrast, the physics is intrinsically different in the high-frequency limit, where an average Zeeman energy difference leads to an oscillatory spin current. This proposed apparatus should be promising for the future experimental search of spin superfluidity.

Full-shell nanowires formed by a semiconducting core fully wrapped by an epitaxial superconducting shell, have recently been introduced as promising hybrid quantum devices. Despite this, however, their properties when forming a Josephson junction (JJ) have not been elucidated yet. We here fill this void by theoretically studying the physics of JJs based on full-shell nanowires. In the hollow-core limit, where the thickness of the semiconducting layer can be ignored, we demonstrate that the critical supercurrent $I^{\text{c}}$ can be tuned by an external magnetic flux $\mathrm{\Phi }$. Specifically, $I^{\text{c}}\left(\mathrm{\Phi }\right)$ does not follow the Little-Parks modulation of the superconducting pairing $\mathrm{\Delta }\left(\mathrm{\Phi }\right)$ and exhibits steps for realistic values of nanowire radii. The position of the steps can be understood from the underlying symmetries of the orbital transverse channels contributing to the supercurrent.

We report the topological electronic structure and magnetic and magnetotransport properties of a noncentrosymmetric compound GdAlSi. Magnetic susceptibility shows an antiferromagnetic transition at $T_{\mathrm{N}}=32$ K. In-plane isothermal magnetization exhibits an unusual hysteresis behavior at higher magnetic field, rather than near zero field. Moreover, the hysteresis behavior is asymmetric under positive and negative magnetic fields. First-principles calculations were performed on various magnetic configurations, revealing that the antiferromagnetic state is the ground state, and the spiral antiferromagnetic state is a close competing state. The calculations also reveal that GdAlSi hosts multiple Weyl points near the Fermi energy. The band structure measured by angle-resolved photoemission spectroscopy (ARPES) shows relatively good agreement with the theory, with the possibility of Weyl nodes slightly above the Fermi energy. Within the magnetic ordered state, we observe an exceptionally large anomalous Hall conductivity (AHC) of $\sim 1310$ ${\mathrm{\Omega }}^{-1}{\mathrm{cm}}^{-1}$ at 2 K. Interestingly, the anomalous Hall effect persists up to room temperature with a significant value of AHC $(\sim 155 {\mathrm{\Omega }}^{-1}{\mathrm{cm}}^{-1})$. Our analysis indicates that the large AHC originates from the Berry curvature associated with the multiple pairs of Weyl points near Fermi energy.

The discovery of high mobility in quasi-two-dimensional (2D) rare-earth tritellurides has had a transformative impact on the field of quantum materials, particularly in relation to charge-density-wave phenomena. Despite significant progress, the complex electronic properties of these materials remain enigmatic, primarily due to a lack of comprehensive understanding regarding their transport characteristics. In this paper, we present a meticulous analysis of the magnetic-field-induced properties of ${\mathrm{PrTe}}_3$ in both bulk and nanoflake forms. Our investigations unveil a diverse range of distinct frequencies exhibited by the observed quantum oscillation effect. By combining angular-dependent magnetoresistance with Shubnikov–de Haas quantum oscillations, we successfully fit the data to a 1/cos ($\theta$) dependence model, providing compelling evidence for a 2D-like Fermi surface and underscoring the pronounced 2D nature of the Fermi surface pockets. Furthermore, our findings reveal a zero Berry phase, corroborating the Lifshitz-Onsager quantization rule. Notably, we report a remarkable magnetoresistance value of $1800\%$ in the 60 nm device, coupled with an electron mobility of $4.1\times {10}^4\mathrm{c}{\mathrm{m}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$. These exceptional outcomes are poised to stimulate heightened interest in exploring the potential of layered rare-earth tritellurides in the realm of quantum oscillation.

Topological materials with unconventional electronic properties have been investigated intensively for both fundamental and practical interests. Thousands of topological materials have been identified by symmetry-based analysis and ab initio calculations. However, the predicted magnetic topological insulators with genuine full band gaps are rare. Here we employ this database and supervisedly train neural networks to develop a heuristic chemical rule for electronic topology diagnosis. The learned rule is interpretable and diagnoses with a high accuracy whether a material is topological using only its chemical formula and Hubbard $U$ parameter. We next evaluate the model performance in several different regimes of materials. Finally, we integrate machine-learned rules with ab initio calculations to high-throughput screen for magnetic topological insulators in a 2D material database. We discover six new classes (15 materials) of Chern insulators, among which four classes (seven materials) have full band gaps and may motivate for experimental observation. We anticipate the machine-learned rule here can be used as a guiding principle for inverse design and discovery of new topological materials.

The excitonic dynamics in ${\mathrm{MoS}}_2$ monolayer, bilayer, and bulk flakes with different stacking orders, namely $3R$ and $2H$, are investigated through transient absorption spectroscopy at room temperature. Samples are obtained by the mechanical exfoliation of bulk ${\mathrm{MoS}}_2$ crystals with $3R$ and $2H$ stacking orders. Photoluminescence spectroscopy is performed to characterize and compare the samples. Time-resolved differential reflection measurements reveal that monolayer flakes exfoliated from the $3R$ and $2H$ crystals exhibit similar exciton lifetimes of a few picoseconds. However, the exciton lifetime in the $3R$ bilayer flake is approximately 85 ps, about twice as long as that in the $2H$ bilayer. This prolonged exciton lifetime is attributed to the reduction of the electron-hole wavefunction overlap in the $3R$ bilayer due to the built-in polarization. Similarly, in bulk $3R$ ${\mathrm{MoS}}_2$, the exciton lifetime is around 220 ps, approximately twice as long as those in bulk $2H$ ${\mathrm{MoS}}_2$. The extension of the exciton lifetime in $3R$ transition metal dichalcogenides could be advantageous for their applications in optoelectronic devices.

As an emerging type of two-dimensional (2D) family, transition metal oxyhalides with the chemical formula ${\mathrm{MOX}}_2$ have been studied in recent years. Inspired by the successful synthesis of $\mathrm{ZrO}{\mathrm{I}}_2$ monolayer with excellent photocatalytic and piezoelectric properties, we conducted a systematic and comprehensive investigation of $\mathrm{ZrO}{X}_2 (X=\mathrm{Br},\mathrm{I})$ and its Janus ZrOBrI monolayers using first-principles calculations. The results show that the mechanically, dynamically, and thermally stable $\mathrm{ZrO}{X}_2$ and Janus ZrOBrI are indirect-gap semiconductors with band gaps ranging from 1.89 to 3.48 eV by the hybrid density functional HSE06 method. Besides, their valence-band minimum and conduction-band minimum can straddle the redox potential of water at $p\mathrm{H}=0$, respectively. Interestingly, due to the optimal band alignment mechanism, the band-edge position of Janus ZrOBrI with an intrinsic electric field does not expand. Moreover, biaxial compressive strain (within −6%) effects on the band alignments and band gaps are discussed. What is more, strong anisotropy and high optical absorption in the visible-ultraviolet region ($\sim {10}^5\mathrm{c}{\mathrm{m}}^{-1}$) render these monolayers fantastic polarizers and photoelectronic devices. Besides, the orientation-dependent carrier mobility of these monolayers is much higher than that of many other 2D semiconductors, making them potential electronic and photocatalytic devices. For piezoelectric performance, all of these monolayers exhibit a considerable in-plane transverse piezoelectric coefficient $d_{21}$, reaching about 20 pm/V. Furthermore, the Janus ZrOBrI possesses additional out-of-plane piezoelectric responses due to structural mirror asymmetry. Under AC stacking mode, multilayer Janus ZrOBrI has appreciable vertical piezoelectric coefficients $d_{31}$ and $d_{32}$, reaching 1.17 and 4.61 pm/V, respectively. Our findings highlight that all three monolayers are multifunctional devices, especially in the fields of photocatalysis and piezoelectricity.

Van der Waals magnetic materials are currently of great interest as materials for applications in future ultrathin nanoelectronics and nanospintronics. Due to weak coupling between individual monolayers, these materials can be easily obtained in the monolayer and bilayer forms. The latter are of specific interest as they may be considered as natural two-dimensional spin valves. In this paper, we theoretically study spin waves in bilayers of transition metal dichalcogenides. The considerations are carried within the general spin wave theory based on effective spin Hamiltonian and Holstein-Primakoff-Bogolubov transformation. The spin Hamiltonian includes intralayer as well as interlayer nearest-neighbor exchange interactions, easy-plane anisotropy, and additionally a weak in-plane easy-axis anisotropy. The bilayer systems consist of two ferromagnetic (in-plane magnetization) monolayers that are coupled either ferromagnetically or antiferromagnetically. In the latter case, we analyze the spin-wave spectra in all magnetic phases, i.e., in the antiferromagnetic, spin-flop, and ferromagnetic ones.

A Fermi gas of noninteracting electrons, or ultracold fermionic atoms, has a quantum ground state defined by a region of occupancy in momentum space known as the Fermi sea. The Euler characteristic ${\chi }_F$ of the Fermi sea serves to topologically classify these gapless fermionic states. The topology of a $D$-dimensional Fermi sea is physically encoded in the $D+1$ point equal time density correlation function. In this work, we first present a simple proof of this fact by showing that the evaluation of the correlation function can be formulated in terms of a triangulation of the Fermi sea with a collection of points, links, and triangles and their higher dimensional analogs. We then make use of the topological $D+1$ point density correlation to reveal universal structures of the more general $M$ point density correlation functions in a $D$-dimensional Fermi gas. Two experimental methods are proposed for observing these correlations in $D=2$. In cold atomic gases imaged by quantum gas microscopy, our analysis supports the feasibility of measuring the third-order density correlation, from which ${\chi }_F$ can be reliably extracted in systems with as few as around 100 atoms. For solid-state electron gases, we propose measuring correlations in the speckle pattern of intensity fluctuations in nonlinear x-ray scattering experiments.

The $d$-wave pseudospin current order $\left(d\mathrm{PSCO}\right)$ with staggered circulating pseudospin current has been proposed as the hidden electronic order to describe the unexpected breaking of spatial symmetries in stoichiometric ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ and the unconventional pseudogap phenomena in electron doped ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$. However, a microscopic model for the emergence of $d\mathrm{PSCO}$ is still lacking. The nearest-neighbor Coulomb repulsion $V$, which is expected to be significant in ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ due to the large spatial extension of the Ir $5d$ orbitals, is capable of driving $d\mathrm{PSCO}$ on the mean-field level, albeit the latter is energetically degenerate to the staggered flux phase with circulating charge current. We find the in-plane anisotropy ${\mathrm{\Gamma }}_2$ in the effective superexchange interaction between $J_{\text{eff}}=\frac{1}{2}$ pseudospins, originating from the cooperative interplay between Hund's rule coupling and spin-orbit coupling of Ir $5d$ electrons, is able to lift the degeneracy and stabilize the pseudospin currents. The effective single-orbital model of $J_{\text{eff}}=\frac{1}{2}$ electrons, including onsite Coulomb repulsion $U$, nearest-neighbor Coulomb repulsion $V$, and the in-plane anisotropy ${\mathrm{\Gamma }}_2$, is then studied. We obtain the mean-field ground states, analyze their properties, and determine the phase diagram of stoichiometric ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ in the plane spanned by $U$ and $V$ at a fixed ${\mathrm{\Gamma }}_2$. We demonstrate the realization of $d\mathrm{PSCO}$, as well as its competition and coexistence with antiferromagnetism. Remarkably, we find the coexistence of $d\mathrm{PSCO}$ and antiferromagnetism naturally leads to spin bond nematicity, with the spin directions of these three orders forming nontrivial chirality. Furthermore, we show that the emergence of the coexistent state and its chirality can be tuned by carrier doping.

The Hatano-Nelson and the non-Hermitian Su-Schrieffer-Heeger models are paradigmatic examples of non-Hermitian systems that host nontrivial boundary phenomena. In this work, we use recently developed graph-theoretical tools to design systems whose isospectral reduction, akin to an effective Hamiltonian, has the form of either of these two models. In the reduced version, the couplings and onsite potentials become energy dependent. We show that this leads to interesting phenomena such as an energy-dependent non-Hermitian skin effect, where eigenstates can simultaneously localize on either ends of the systems, with different localization lengths. Moreover, we predict the existence of various topological edge states, pinned at nonzero energies, with different exponential envelopes, depending on their energy. Overall, our work sheds light on the nature of topological phases and the non-Hermitian skin effect in one-dimensional systems.

We investigate the twisted topology of the complex eigenspectrum of a one-dimensional non-Hermitian system under the influence of long-range unidirectional coupling. Unlike the complex energy spectrum of the conventional Hatano-Nelson chain, which takes the form of a single loop with a topological winding index of a definite sign, the introduction of long-range unidirectional hopping results in the creation of multiple twisted loops. These twisted loops exhibit opposite signs of the topological winding index, which correlate to alternating clockwise and anticlockwise energy windings. The simultaneous presence of both signs of the winding index translates into a bipolar non-Hermitian skin effect (NHSE), which challenges the conventional wisdom that the NHSE localization is dependent on the direction of the dominant nearest-neighbor interactions. In this bipolar NHSE, the exponents of the complex energy eigenvectors corresponding to clockwise and anticlockwise windings lie inside and outside of the complex unit circle, respectively. Interestingly, at the intersections of oppositely oriented energy loops where the sign of the topological winding index flips, the energy becomes real valued, leading to a suppression of the NHSE. This marks the emergence of Bloch-like contact points, where both the bipolar and conventional NHSE vanish. Based on the non-Hermitian model, we provide analytical insights into the effects of long-range unidirectional coupling to the winding topology of its complex energy spectra and their broader implications for the field of condensed matter physics.

We explore a solid-state qubit defined on valley isospin of an electron confined in a gate-defined quantum dot created in an area of monolayer ${\mathrm{MoS}}_2/{\mathrm{WS}}_2$ lateral junction where a steep dipolar potential emerges. We show that the junction oriented along an armchair direction can induce intervalley transitions of the electron confined in the neighboring quantum dot when the (gate-controllable) overlapping with the junction is significant and pumping frequency tuned. The pumping scheme that induces transitions is all-electrical: Obtained by applying oscillating voltages to control gates and thus enabled for scalable qubit architectures. We also report another possibility of valley qubit manipulation by accumulating non-Abelian valley Berry phase. To model the nanodevice we solve the time-dependent Schrödinger-Poisson equations in a tight-binding approach and obtain exact time evolution of the valley qubit system.

We theoretically show how structural modifications and controlling quantum coherency can enhance linear and nonlinear thermoelectric performance in graphene nanostructure heat engines. Although graphene has emerged as a promising material for a nanoscale heat engine due to its high coherency and tunable electronic properties, its large lattice thermal transport often limits its thermal efficiency. Using the density-functional tight-binding method, we demonstrate that one can suppress lattice thermal transport, degrading the thermal efficiency by deliberately manipulating the junction's bending angle at low temperatures. We further argue that applying an optimal local gate voltage unleashes its great potential in achieving excellent efficiency and reasonably high output power that persist in the fully nonlinear regime.