Using a particle-based model, we simulate the behavior of a skyrmion under the influence of asymmetric funnel geometries and ac driving at zero temperature. We specifically investigate possibilities for controlling the skyrmion motion by harnessing a ratchet effect. Our results show that as the amplitude of a unidirectional ac drive is increased, the skyrmion can be set into motion along either the easy or hard direction of the funnel depending on the ac driving direction. When the ac drive is parallel to the funnel axis, the skyrmion flows in the easy direction and its average velocity is quantized. In contrast, when the ac drive is perpendicular to the funnel axis, a Magnus-induced ratchet effect occurs, and the skyrmion moves along the hard direction with a constant average velocity. For biharmonic ac driving of equal amplitude along both the parallel and perpendicular directions, we observe a reentrant pinning phase where the skyrmion ratchet vanishes. For asymmetric biharmonic ac drives, the skyrmion exhibits a combination of effects and can move in either the easy or hard direction depending on the configuration of the ac drives. These results indicate that it is possible to achieve controlled skyrmion motion using funnel geometries, and we discuss ways in which this could be harnessed to perform data transfer operations.

$\mathrm{Eu}{\mathrm{Sn}}_2{\mathrm{As}}_2$ with a layered rhombohedral crystal structure is proposed to be a candidate for an intrinsic antiferromagnetic (AFM) topological insulator. Here, we have investigated systematic magnetoresistance (MR) and magnetization measurements on the high-quality $\mathrm{Eu}{\mathrm{Sn}}_2{\mathrm{As}}_2$ single crystal with the magnetic field both parallel and perpendicular to the (00l) plane. Both the kink of magnetic susceptibility and longitudinal resistivity reveal that $\mathrm{Eu}{\mathrm{Sn}}_2{\mathrm{An}}_2$ undergoes an AFM transition at $T_{\mathrm{N}}=21\mathrm{K}$. At $T=2\mathrm{K}$, the magnetization exhibits two successive plateaus of ∼5.6 and $\sim 6.6{\mu }_{\mathrm{B}}/\mathrm{Eu}$ at the corresponding critical magnetic fields. Combined with the negative longitudinal MR and abnormal Hall resistance, we demonstrate that $\mathrm{Eu}{\mathrm{Sn}}_2{\mathrm{An}}_2$ undergoes complicated magnetic transitions from an AFM state to a canted ferromagnetic (FM) state at $H_c$ and then to a polarized FM state at $H_s$ as the magnetic field increases.

We report a peculiar magnetization reversal observed in magnetron-sputtered Co, Fe, and Ni films. We found that some recoil (minor) magnetization curves lie entirely and way outside the major loop, a phenomenon referred to here as a recoil-curve overshoot. The greatly enhanced recoil curve's remanence and coercivity result in an up to 2.5-fold increase of loop's area. The model of pairs of exchange-coupled grains with misaligned anisotropy axes reproduces, in a very good agreement with the experiment, all key features of the recoil-curve overshoot as well as the kink that some major loops present before saturation. The disclosed features of the ferromagnetic hysteresis provide further insights into this important classical phenomenon.

First-principles investigations of the high-temperature superconducting system ${\mathrm{Ba}}_2\mathrm{Cu}{\mathrm{O}}_{3+\delta }$, recently discovered at $\delta \approx 0.2$ at $T_c=70$ K, are applied to demonstrate the effects of oxygen ordering on the electronic and magnetic properties. The observed “highly overdoped” superconducting phase displays stretched Cu-planar oxygen ${\mathrm{O}}_{\mathrm{P}}$ distances and anomalously shortened Cu-apical ${\mathrm{O}}_{\mathrm{A}}$ separations compared with other cuprates. The stoichiometric system $\delta =0$, with its strongly one-dimensional (1D) Cu-${\mathrm{O}}_{\mathrm{P}}$ chain structure, when nonmagnetic shows 1D Fermi surfaces that lead, within density functional theory, to antiferromagnetic Cu-${\mathrm{O}}_{\mathrm{P}}$ chains (a spin-Peierls instability). Accounting for 1D fluctuations and small interchain coupling according to the theory of Schulz indicates this system, like ${\mathrm{Sr}}_2\mathrm{Cu}{\mathrm{O}}_3$, is near the 1D Luttinger-liquid quantum critical phase. The unusual Cu-O bond lengths per se have limited effects on other properties for $\delta =0$. We find that a “doubled bilayer” structure of alternating Cu-${\mathrm{O}}_{\mathrm{P}}$ chains and wide rung ${\mathrm{Cu}}_3{\mathrm{O}}_4$ ladders is the energetically preferred one of three possibilities where the additional oxygen ions bridge Cu-${\mathrm{O}}_{\mathrm{P}}$ chains in the superconducting phase $\delta =1/4$. Nominal formal valences of the three Cu sites are discussed. The sixfold (octahedral) site is the most highly oxidized, accepting somewhat more holes in the $d_{z^2}$ orbital than in the $d_{x^2-y^2}$ orbital. The implication is that two-band physics is involved in the pairing mechanism and the superconducting carriers. The Fermi surfaces of this metallic bilayer structure show both 1D and 2D strong (incipient) nesting instabilities, possibly accounting for the lack of clean single-phase samples based on this structure and suggesting importance for the pairing mechanism.

Injection locking can stabilize a source of radiation, leading to an efficient suppression of noise-induced spectral broadening and therefore, to a narrow spectrum. The technique is well established in laser physics, where a phenomenological description due to Adler is usually sufficient. Recently, locking experiments were performed in Josephson photonics devices, where microwave radiation is created by inelastic Cooper pair tunneling across a dc-biased Josephson junction connected in-series with a microwave resonator. An in-depth theory of locking for such devices, accounting for the Josephson nonlinearity and the specific engineered environments, is lacking. Here, we study injection locking in a typical Josephson photonics device where the environment consists of a single mode cavity, operated in the classical regime. We show that an in-series resistance, however small, is an important ingredient in describing self-sustained Josephson oscillations and enables the locking region. We derive a dynamical equation describing locking, similar to an Adler equation, from the specific circuit equations. The effect of noise on the locked Josephson phase is described in terms of phase slips in a modified washboard potential. For weak noise, the spectral broadening is reduced exponentially with the injection signal. When this signal is provided from a second Josephson device, the two devices synchronize. In the linearized limit, we recover the Kuramoto model of synchronized oscillators. The picture of classical phase slips established here suggests a natural extension towards a theory of locking in the quantum regime.

Among the transition-metal oxides, bismuth nickelate is particular in that, under normal conditions, it crystallizes in a triclinic structure with charge ordering in the Bi sites. Upon increasing pressure, a phase transition takes the material to the $\mathrm{Gd}\mathrm{Fe}{\mathrm{O}}_3$-type orthorhombic phase commonly found in perovskite oxides; recent attention to $\mathrm{Bi}\mathrm{Ni}{\mathrm{O}}_3$ arises, in part, because this transition can be engineered to be accompanied by a colossal negative thermal expansion. On the other hand, the closely related rare-earth nickelates also show the orthorhombic phase, but as temperature decreases they undergo a metal-insulator transition to a monoclinic phase where charge ordering appears instead in the Ni sites. Since bismuth is similar in valence and size to some of the rare-earth atoms, it has been conjectured that a $\mathrm{Bi}\mathrm{Ni}{\mathrm{O}}_3$ phase with charge ordering in the Ni sites might also exist—this would make this perovskite unique in that it can harbor charge ordering in either set of cation species, while the other set contains identical nominal ionic charges. In this paper we show that such a phase indeed corresponds to a special point of the energy surface of $\mathrm{Bi}\mathrm{Ni}{\mathrm{O}}_3$ when studied with methods based on density-functional theory. The existence of this phase could explain the somewhat conflicting experimental reports regarding the valence state of the Ni ions in the high-pressure phase of $\mathrm{Bi}\mathrm{Ni}{\mathrm{O}}_3$.

We present results for properties related to the band structure of a microwave photonic crystal, that is, a flat resonator containing metallic cylinders arranged on a triangular grid, referred to as the Dirac (microwave) billiard, with a threefold-symmetric shape. Such systems have been used to investigate finite-size graphene sheets. It was shown recently that the eigenmodes of rectangular Dirac billiards are well described by the tight-binding model of a finite-size honeycomb-kagome lattice of corresponding shape. We compare properties of the eigenstates of the Dirac billiard with those of the associated graphene and honeycomb-kagome billiard and relativistic quantum billiard. We outline how the eigenstates of threefold-symmetric systems can be separated according to their transformation properties under rotation by $\frac{2\pi }{3}$ into three subspaces, namely singlets, that are rotationally invariant, and doublets that are noninvariant. We reveal for the doublets in graphene and honeycomb-kagome billiards in quasimomentum space a selective excitation of the valley states associated with the two inequivalent Dirac points. For the understanding of symmetry-related features, we extend known results for nonrelativistic quantum billiards and the associated semiclassical approach to relativistic neutrino billiards.

We examine the motion of an individual skyrmion driven through an assembly of other skyrmions in the absence of quenched disorder. The skyrmion behavior is determined by the ratio of the damping and Magnus terms, known as the intrinsic skyrmion Hall angle. For fixed drive in the damping dominated regime, the effective viscosity decreases monotonically with increasing skyrmion density. In contrast, in the Magnus dominated regime, the velocity varies nonmonotonically with density, and there is a regime in which the skyrmion moves faster with increasing density, as well as a pronounced speed-up effect in which a skyrmion traveling through a dense medium moves more rapidly than for low densities or the single-particle limit. The velocity-force curves in the Magnus-dominated regime exhibit an overshoot effect and other differences from the damping-dominated regime. We find a finite threshold force for skyrmion motion which increases with density as well as a drive-dependent skyrmion Hall angle. We map dynamic phase diagrams showing the threshold for motion, nonlinear flow, speed-up, and saturation regimes. In some cases, increasing the density can reduce the skyrmion Hall angle while producing a velocity boost, which could be valuable for applications.

We elucidate the electronic state of a two-dimensional (2D) Rashba square lattice proximitized to a square monolayer $s$-wave superconductor, analyzing the role played by dynamical electron-electron interactions. The $\text{2D}+\text{2D}$ proximity effect induces sharp Bogoliubov and low-energy Andreev-reflected bound states, suppressing the $s$-wave gap globally. Dynamical correlations strongly renormalize the Bogoliubov quasiparticles and the Andreev levels, evolving the spin-resolved Andreev linewidths into a single bound state. We explore the channel- and spin-resolved spectral functions and analyze the reconstructed superconducting state, showing that the Rashba spin-orbit coupling drives strong channel differentiation. The mutual interplay between electron-electron and spin-orbit interactions, with proximity-induced electron pairing, lead us to introduce a generally applicable mechanism for designing Majorana fermions in 2D superconducting structures.

In this paper, we show that the same theoretical tools that successfully explain other hydride systems under pressure seem to be at odds with the recently claimed conventional room-temperature superconductivity of carbonaceous sulfur hydride. We support our conclusions with (i) the absence of a dominant low-enthalpy stoichiometry and crystal structure in the ternary phase diagram. (ii) Only the thermodynamics of C-doping phases appears to be marginally competing in enthalpy against ${\mathrm{H}}_3\mathrm{S}$. (iii) Accurate results of the transition temperature given by ab initio Migdal-Eliashberg calculations differ by more than 110 K from recent theoretical claims explaining the high-temperature superconductivity in carbonaceous hydrogen sulfide. An unconventional mechanism of superconductivity or a breakdown of current theories in this system is possibly behind the disagreement.

We systematically investigated the superconducting properties and the interplay between charge density waves (CDWs) and superconductivity (SC) in lithium-intercalated $2H\text{−}\mathrm{Ta}{\mathrm{S}}_2$. By gradually increasing the lithium content $x$, the CDW formation temperature is continuously suppressed, and the onset temperature of SC is increased with a maximum transition temperature $T_c=3.5\mathrm{K}$ for $x=0.096$. The bulk nature of SC is confirmed by a superconducting shielding fraction of the order of unity for this composition. The electronic contribution to the specific heat and Hall resistivity data demonstrates that the CDW weakens with lithium intercalation, thereby indirectly increasing the carrier density and boosting SC. While the sign of the charge carriers in undoped $2H\text{−}\mathrm{Ta}{\mathrm{S}}_2$ changes from electronlike to hole type near the CDW formation temperature $\sim 75\mathrm{K}$, the lithium-intercalated ${\mathrm{Li}}_x\mathrm{Ta}{\mathrm{S}}_2$ shows predominantly hole-type carriers in the CDW phase even for very low lithium content.

Transition-metal dichalcogenides are rich in their structural phases, e.g., $1T-\mathrm{Ta}{\mathrm{S}}_2$ and $1T-\mathrm{Ta}{\mathrm{Se}}_2$ form charge density wave (CDW) under low temperature with interesting and exotic properties. Here, we present a systematic study of different structures in two-dimensional $\mathrm{Ta}{X}_2$ ($X=\mathrm{S}$, Se, Te) using density-functional theory calculations with consideration of van der Waals interaction. All the normal phases present metal characteristics with various ground state and magnetic properties. The lattice reconstruction of CDW drastically affects the electronic and structural characteristics of $1T-\mathrm{Ta}{\mathrm{S}}_2$ and $1T-\mathrm{Ta}{\mathrm{Se}}_2$, leading to a transition from metal to insulator and an emergence of magnetic moment within periodic atomic clusters called the Star of David. The evaluated Heisenberg couplings indicate the weak ferromagnetic coupling between the clusters in monolayer. Furthermore, in bilayer commensurate CDW cases, we find the intriguing phenomenon of the varying magnetic properties with different stacking orders. The magnetic moment in each layer disappears when two layers are coupled, but may sustain in certain stackings of interlayer antiferromagnetic configurations.

We present a theoretical calculation of optical functions for ${\mathrm{Cu}}_2\mathrm{O}$ quantum well (QW) with Rydberg excitons, in an external homogeneous electric field of an arbitrary strength. Two configurations of an external electric field perpendicular and parallel to the QW plane are considered in the energetic region for discrete excitonic states. With the help of the real density matrix approach, which enables the derivation of the analytical expressions for the QW electro-optical functions, absorption spectra are calculated for the case of the excitation energy below the effective band gap.

We present a theoretical calculation of optical functions for a ${\mathrm{Cu}}_2\mathrm{O}$ quantum well (QW) with Rydberg excitons. In particular, an external homogeneous electric field parallel to the QW plane is considered and the spectra are obtained in the energy region above the effective band gap, which is a suitable condition to observe the Franz-Keldysh (FK) oscillations. We quantitatively describe the amplitudes and periodicity of FK modulations and the influence of both Rydberg excitons and confinement effect on this phenomenon.

How do the many-particle interactions evolve in semiconductors is crucial for understanding light-matter interactions. We observe Coulomb-correlated electron-hole plasma formation via its interaction with excitons in a transition metal dichalcogenide semiconductor. We observe that under intense photoexcitation $\sim {10}^{19}$ per ${\mathrm{cm}}^3$, huge damping destroys the Coulomb correlation and hinders the plasma formation until a majority of the free carriers recombine and the plasma oscillation period becomes sufficiently smaller than the damping time constant. Moreover, only 1%–3% of the injected free carriers form Coulomb-correlated plasma. Our study sheds light on exciton-plasma interactions and quasistatic Coulomb screening, which play pivotal roles in device engineering.

When hexagonal boron nitride (hBN) and graphene are aligned at zero or a small twist angle, a moiré structure is formed due to the small lattice constant mismatch between the two structures. In this paper, we analyze magnetic ordering tendencies, driven by on-site Coulomb interactions, of encapsulated bilayer graphene (BG) forming a moiré structure with one (hBN-BG) or both hBN layers (hBN-BG-hBN), using the random phase approximation. The calculations are performed in a fully atomistic Hubbard model that takes into account all $\pi$ electrons of the carbon atoms in one moiré unit cell. We analyze the charge neutral case and find that the dominant magnetic ordering instability is uniformly antiferromagnetic. Furthermore, at low temperatures, the critical Hubbard interaction $U_c$ required to induce magnetic order is slightly larger in those systems where the moiré structure has caused a band gap opening in the noninteracting picture, although the difference is less than 6%. Mean-field calculations are employed to estimate how such an interaction-induced magnetic order may change the observable single-particle gap sizes.

The compound $\mathrm{Sr}{\mathrm{Co}}_2{\mathrm{P}}_2$ is a Pauli paramagnet very close to ferromagnetic order. To study its electronic structure in close vicinity to the Fermi level, we report measurements of the de Haas–van Alphen effect in magnetic fields up to 35 T in combination with density-functional-theory band-structure calculations in different approximations. The resulting electronic band structure not only depends significantly on the choice of the functional, but also crucially on the exact values of the structural parameters that have been determined at low temperatures by synchrotron x-ray diffraction. We find the best correspondence between the measured and the calculated de Haas–van Alphen frequencies for the general gradient approximation functional and the structural parameters that were determined at 10 K. Although $\mathrm{Sr}{\mathrm{Co}}_2{\mathrm{P}}_2$ crystallizes in the uncollapsed tetragonal structure with a large P-P distance between the ${\mathrm{Co}}_2{\mathrm{P}}_2$ layers, we observe a rather three-dimensional Fermi-surface topology. We obtain a mass-enhancement factor of about 2 deduced from the ratio between experimental and calculated quasiparticle masses. The temperature dependence of the structural parameters leads to a significant reduction of the electronic density of states at the Fermi level and in comparison with the measured Sommerfeld coefficient to an overall mass renormalization in line with our experiment.

One-dimensional hinge states are the hallmark of three-dimensional (3D) higher-order topological insulators (HOTIs), which may lead to interesting transport properties. Here, we study the Aharonov-Bohm (AB) effect in an interferometer constructed by the hinge states in normal metal-HOTI junctions with a transverse magnetic field. We show that the AB oscillation of the conductance can clearly manifest the spatial configurations of such hinge states. The magnetic fluxes encircled by various interfering loops are composed of two basic ones, so that the oscillation of the conductance by varying the magnetic field contains different frequency components universally related to each other. Specifically, the four dominant frequencies ${\omega }_{x,y}$ and ${\omega }_{x\pm y}$ satisfy the relations ${\omega }_{x\pm y}={\omega }_x\pm {\omega }_y$, which generally hold for different magnetic fields, sample sizes, bias voltages, and weak disorders. Our results provide a unique and robust signature of the hinge states and pave the way for exploring the AB effect in 3D HOTIs.

In recent years, low-dimensional ferromagnetic semiconductors have attracted tremendous interest due to their great significance for both fundamental physics and device applications. In this work, a one dimensional (1D) ferromagnetic semiconductor $\mathrm{CrSb}{\mathrm{Se}}_3$ with high Curie temperature $\left(T_{\mathrm{C}}\right)$ and sizable magnetic anisotropy is proposed based on the first-principle calculations and theoretical model. Quasi-1D chain structure with a moderate cleavage energy of $0.49\mathrm{J}/{\mathrm{m}}^2$ suggests the possibility to exfoliate a 1D $\mathrm{CrSb}{\mathrm{Se}}_3$ ladder (a ladder structure formed by two rows of 1D monoatomic Cr chain) from the bulk. Monte Carlo simulation based on anisotropic Heisenberg model predicts $T_{\mathrm{C}}$ to be up to 170 K, which is much higher than that of the bulk. The high $T_{\mathrm{C}}$ of 1D $\mathrm{CrSb}{\mathrm{Se}}_3$ ladder originates from the combined effect of the strong intraladder ferromagnetic exchange interaction and large magnetic anisotropy, while the lower $T_{\mathrm{C}}$ of the bulk results from the weak interladder indirect exchange interaction. In addition, we find that the splitting of top valence bands depends on the direction of magnetization due to the reduced structural symmetry and spin-orbit coupling, indicating a strong magnetoband structure effect. Our theoretical prediction not only provides a material platform for understanding the fundamental physics of 1D magnetism, but also provides a strategy for the search for the 1D ferromagnetic semiconductor with high Curie temperature.