We investigate ultrahigh-contrast plasmonic mode conversion in a coupled graphene waveguide structure with manipulated gyration and graphene chemical potential by using both analytic theory and numeric simulations. The ultrahigh contrast is supported by a magnetic resonance at the expense of high sensitivity on the design and material parameters and the operation frequency. The analytical study reveals that the ultrahigh contrast mode conversion can be preserved by adjusting the gyration and the chemical potential in the deviation of structural and material parameter values due to imperfect fabrication and harsh environment and for tuning its operation frequency over a broadband frequency range. We derive explicit analytical results for the resonance gyration and chemical potential that answer to those questions how to manipulate the gyration and the chemical potential according to the design and material parameters and the operation frequency. Numerical simulations demonstrate the robust and broadband tunable plasmonic mode conversion preserves the contrast higher than ${10}^3$ (99.9%) for different design and material parameter values over a one-octave-spanning broadband frequency range. In practice, the chemical potential and the gyration can be manipulated by controlling the gate voltage of the graphene and the external magnetic field, respectively. Our findings can be of great importance in the design of high-integration nanophotonic circuits and for probing of ultrafast magnetization dynamics.

Integrated optical devices that possess bistable characteristics are key elements for optical digital signal processing and all-optical computing. In this paper we report on an optomagnetic bistable memory based on the inverse Faraday effect related nonlinearity in graphene waveguide ring resonators. The effective magnetic field via the plasmon-induced inverse Faraday effect in the graphene resonator has two discrete stable states that represent logic one and logic zero and magnetic switching for memory operation can be achieved by injecting an optical pulse and momentarily blocking the bias input light. Our proposed optomagnetic bistable device based on a graphene magnetoplasmonic configuration has great potential for the development of next generation magnetic memory. It has benefits of ultrafast magnetization switching, simplicity of the considered structure, on-chip compatibility, and a deep subwavelength platform. It also enhances the nonlinear interaction due to strong confinement of the graphene plasmon modes and high-quality factor of traveling modes in the graphene ring resonator, and tunability and robustness by adjusting the gate voltages of the graphene sheets. These advantages will satisfy the demand for fast magnetization switching, low-energy consumption, and high-density recording required in future magnetic memories.

Diffractively coupled nanoparticle arrays are promising candidates for helping to flatten many photonic devices such as lasers, lenses, and metrology instruments. Their performance, however, is directly linked with the size of the metasurfaces, limiting their applicability in nanophotonic applications. Here, we dramatically reduce array sizes of high-$Q$ factor metasurfaces by utilizing strongly dispersive media. The effect is demonstrated by theoretically and numerically studying periodic arrays of plasmonic nanoparticles embedded inside Bragg-reflector waveguides. We demonstrate array dimensions reduction up to two orders of magnitude while still achieving ultrahigh-$Q$ factors in excess of ${10}^4$.

Exceptional points are the most intriguing features of non-Hermitian photonics, providing clues for enhanced sensitivity, asymmetric light reflection, laser-absorber effect, topological switching, and other applications. The materials used in non-Hermitian optical structures are usually supposed to be isotropic. In this paper, we study the archetypal anisotropic non-Hermitian system consisting of the $\mathcal{PT}$-symmetric bilayer (with balanced loss and gain) supplemented with the anisotropic defect (interlayer). Starting from the analysis of isotropic case, we show that anisotropy results in doubling the number of exceptional points and nontrivial splitting of the $\mathcal{PT}$-symmetry-broken regions. We demonstrate that to shift the exceptional points, it is not enough to change the orientation of anisotropic medium optical axis in the transverse plane, but two anisotropic layers with differently oriented axes are needed. In the latter case, an experimentally feasible way is proposed to tune the system to an exceptional point by simply controlling the relative optical axes orientation. Moreover, we show that the exceptional points in anisotropic structures generally lose their association with unity transmission. Our results will be helpful for better understanding exceptional points and other features of non-Hermitian systems and for plenty of applications inspired by the rich physics behind the scene.

We study interaction corrections to the thermoelectric transport coefficient $\alpha$ and the thermopower $S$ in the two-dimensional disordered electron gas with long-range Coulomb interactions. To this end, we analyze the heat density–density correlation function using the recently derived generalized nonlinear sigma model with particle-hole asymmetry. We present a detailed account of the perturbative calculation of the correlation function, the analysis of its structure, and the derivation of the final results for the transport coefficients. We show that both real and virtual processes give rise to logarithmic interaction corrections to $\alpha$ and $S$ with $\delta \alpha /\alpha <0$ and $\delta S/S<0$ in the diffusive transport regime $T\ll 1/\tau$, where $T$ is the temperature and $1/\tau$ is the disorder scattering rate. A short account of our findings is presented in an accompanying letter.

This work presents the evolution of the electronic properties of kagome superconductor ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$ under pressure. The magnetoresistance under high fields of 43 T showed clear Shubnikov–de Haas (SdH) oscillations with multiple frequencies up to 2000 T. With the application of pressure, we observed a sudden change in SdH oscillations with the disappearance of the high-frequency signal near the critical pressure $P_{c1}\sim$ 0.7 GPa. We argue that this change could be due to a reconstruction of the Fermi surface (FS) in ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$. To interpret our experimental data, we computed the electronic band structures and FS of ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$ using ab initio density functional theory. Our results indicate that both the electronic bands and FS of ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$ are highly sensitive to external pressure. The deformation of FS pockets with increasing pressure qualitatively explains our experimental observations. The pressure-driven FS instability in ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$ may induce changes in its electronic states, such as superconductivity, charge density wave, nontrivial topology, and more. Therefore, these results are invaluable for gaining insights into these electronic states in ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$, as well as in other kagome materials.

In this paper, we analyze the effects of electron-electron correlations in the high-order harmonic (HH) spectrum of perovskite ${\mathrm{BaTiO}}_3$ excited by an intense, few-cycle midinfrared laser pulse by using time-dependent density-functional theory with the exchange-correlation (XC) kernel obtained from the dynamical mean-field theory solution of the Hubbard model. As we show, the correlation effects considerably alter the HH spectrum, shifting the spectral weight to higher frequencies and leading to the generation of “superharmonics”—an energy-periodic enhancement and suppression of certain harmonic orders—when the correlation strength exceeds a critical value. The results are stable for variation of the peak field strength, duration, and central frequency of the driving laser pulse. Using a correlated two-level model, we find that the superharmonic emission results from transitions between correlation-induced Hubbard subband states and has frequency proportional to the effective electron-electron correlation energy.

We extend the single mode approximation (SMA) into quantum Monte Carlo simulations to provides an efficient and fast method to obtain the dynamical dispersion of quantum many-body systems. Based on stochastic series expansion (SSE) and its projector algorithms, the $\mathrm{SMA}+\mathrm{SSE}$ method can simply extract the dispersion of the dynamical dispersion in the long-wavelength limit and the upper bound of the dispersion elsewhere, without external calculations and high technique barriers. Meanwhile, numerical analytic continuation methods require the fine data of imaginary time correlations and complex programming. Therefore, our method can approach the excitation dispersion of large systems, e.g., we take the two-dimensional Heisenberg model on a $512\times 512$ square lattice. We demonstrate the effectiveness and efficiency of our method with high precision via additional examples. We also demonstrate that SMA combined with SSE goes beyond spin-wave theory with numerical results. We further illustrate that SMA is able to extract useful information in strongly correlated systems with competing states.

We report the Ni-doping effect on the magnetic and electronic properties of thiospinel ${\mathrm{Co}}_{1-x}{\mathrm{Ni}}_x\left[{\mathrm{Co}}_{0.3}{\mathrm{Ir}}_{1.7}\right]{\mathrm{S}}_4$ ($0\le x\le 1$). The parent compound $\mathrm{Co}\left[{\mathrm{Co}}_{0.3}{\mathrm{Ir}}_{1.7}\right]{\mathrm{S}}_4$ exhibits antiferromagnetic order below $T_{\mathrm{N}}\sim 292\mathrm{K}$ within the $A$-site diamond sublattice, along with a narrow charge-transfer gap. Upon Ni doping, an insulator-to-metal crossover occurs at $x\sim 0.35$, and the antiferromagnetism is gradually suppressed, with $T_{\mathrm{N}}$ decreasing to 23 K at $x=0.7$. In the metallic state, a spin-glass-like transition emerges at low temperatures. The antiferromagnetic transition is completely suppressed at $x_{\mathrm{c}}\sim 0.95$, around which a non-Fermi-liquid behavior emerges, evident from the $T^{\alpha }$ temperature dependence with $\alpha \approx 1.2$–1.3 in resistivity and divergent behavior of $C/T$ in specific heat at low temperatures. Meanwhile, the electronic specific-heat coefficient $\gamma$ increases substantially, signifying an enhancement of the quasiparticle effective mass. The magnetic phase diagram has been established, in which an antiferromagnetic quantum critical point is avoided at $x_{\mathrm{c}}$. Conversely, the observed glasslike tail above the critical concentration aligns more closely with theoretical predictions for an extended region of the quantum Griffiths phase in the presence of strong disorder.

We have experimentally and theoretically investigated the spin injection/detection polarization in a Si-based ferromagnetic tunnel junction with an amorphous MgO layer and demonstrated that the experimental features of the spin polarization in a wide bias range can be well explained using our theoretical model based on the band diagram of the junction and the direct tunneling mechanism. It is shown that the spin polarization originates from the band diagrams of the ferromagnetic Fe layer and $n^{+}\text{−}\mathrm{Si}$ channel in the junction, while the spin selectivity of the MgO tunnel barrier is not necessary. Besides, we clarified the mechanism of the reduction in spin polarization when the bias is high and nonlinear properties are prominent, where the widely used spin injection/detection model proposed by Valet and Fert is not applicable. The dominant mechanism of such reduction is found to be “spin accumulation saturation (SAS)” at the $n^{+}\text{−}\mathrm{Si}$ interface in contact with the MgO layer as the bias is increased in the spin extraction geometry, which is inevitable in semiconductor-based ferromagnetic tunnel junctions. Our experiments and theoretical model provide a deep understanding of the spin injection/detection phenomena in semiconductor-based spin transport devices toward the realization of high performance under reasonably high bias conditions for practical use.

In this paper, we investigate the emergence of non-Hermitian phase transitions on quantum torus surfaces. We consider a single fermion whose dynamics are governed by the Dirac equation confined to move on a quantum torus surface. The effects of the geometry are taken into account using the tetrad formalism and the spin connection. The Dirac equation gives rise to two coupled first-order differential equations for each spinor component. The eigenvalues and eigenfunctions for each spinor component are computed numerically, and the non-Hermitian phase transitions are investigated in terms of the geometric features of the torus and the magnitude of the imaginary component of the mass.

Ferroelectricity (FE), antiferromagnetism (AFM), and the quantum anomalous Hall (QAH) effect are three fundamental phenomena in condensed-matter physics that could enable the realization of novel devices, thereby attracting substantial attention. Here, we show theoretical evidence that two-dimensional (2D) even-layer $\mathrm{MnB}{\mathrm{i}}_2{\mathrm{Te}}_4$ allows for the simultaneous presence of FE, AFM, and the QAH effect. Importantly, through rational van der Waals sliding, these exotic properties are strongly coupled. Such coupling could demonstrate distinctive physics, such as the ferroelectric control of the magnetic spin distribution in the AFM phase and the sign of the quantized anomalous Hall plateau. The explored phenomena and mechanisms would not only enrich research in 2D ferroelectricity and topological magnets but also guide the design of low-consumption, high-speed quantum devices.

Chirality-induced current-in-plane magnetoresistance (CCIP-MR) has been recently observed in a structure where the chiral molecules do not serve as the drain current channels [J. Am. Chem. Soc. 144, 7302 (2022)], which is different from the conventional chiral-induced spin selectivity devices. In this theoretical work, we propose the concept of chiral virtual leads (CVLs) to characterize the role of the chiral molecules and put forward a phenomenological model to interpret the origin of CCIP-MR. We find that the spin preference for charge exchange at the interface between the CVLs and the scattering region can lead to a significant CCIP-MR. The theoretical results align well with the experimental observations. Our study provides insights into manipulating device properties using chiral materials.

We investigate nonequilibrium transport of charge and heat through an interacting quantum dot coupled to a finite electron reservoir. Both the quantum dot and the finite reservoir are coupled to conventional electric contacts, i.e., infinite electron reservoirs, between which a bias voltage can be applied. We develop a phenomenological description of the system, combining a rate equation for transport through the quantum dot with standard expressions for bulk transport between the finite and infinite reservoirs. The finite reservoir is assumed to be in a quasiequilibrium state with a time-dependent chemical potential and temperature which we solve for self-consistently. We show that the finite reservoir can have a large impact on the stationary state transport properties, including a shift and broadening of the Coulomb diamond edges. We also demonstrate that there is a region around the conductance lines where a heat current flows out of the finite reservoir. Our results reveal the dependence of the temperature that can be reached by this thermoelectric cooling on the system parameters, in particular the coupling between the finite and infinite reservoirs and additional heat currents induced by electron-phonon couplings, and can thus serve as a guide for experiments on quantum-dot-enabled thermoelectric cooling of finite electron reservoirs. Finally, we study the full dynamics of the system, with a particular focus on the timescales involved in the thermoelectric cooling.

We employ tensor-network renormalization to explore the dynamical spectra of the easy-axis triangular-lattice antiferromagnet (TLAF) in a magnetic field. Our analysis identifies two distinct low-energy magnon excitations: a gapless Goldstone mode and a gapped mode. At zero field, the spectra display two nearly degenerate roton modes near the $M$ point. With the increase of the magnetic field within the Y-shaped superfluid phase, these modes diverge, with the roton excitation vanishing from the Goldstone mode branch, suggesting that the roton dip in this mode may just result from the energy-level repulsion imposed by the roton excitation in the gapped mode. Moreover, the in-plane spectral function shows substantial weight in high energies in the same spin excitation channel where the low-energy roton excitation appears. However, these roton excitations are absent in the V-shaped supersolid phase.

In the celebrated Stern-Gerlach experiment, an inhomogeneous static magnetic field separates a beam of charge-neutral atoms with opposite spins, thereby driving a “spin current” normal to the propagation direction. Here, we generalize it to the dynamic scenario by demonstrating a spin transfer from an ac inhomogeneous magnetic field to electrons or charge-neutral excitons and phonons. We predict that parametric pumping can efficiently radiate the angular momentum of local ac magnetic sources to their dc spin currents with van der Waals semiconductors as prototypes. This mechanism brings a unified and efficient paradigm in the spin transport of distinct mobile carriers.

In this work, we highlight an overlooked mechanism that couples superconducting order parameters to orders in the charge and spin sectors such that the latter emerge as improper orders. At the core of this mechanism are nonsymmorphic symmetries and new families of modulated order parameters with two components of opposite parity. While nonsymmorphic systems have been extensively discussed in the context of topological band theory, our work demonstrates the profound effects of such symmetries on the interplay of ordered phases of matter. As a working example, we focus on the space group P4/nmm and provide a real-space perspective on the classification of order parameters. Our findings can account for the unusual phenomenology of ${\mathrm{CeRh}}_2{\mathrm{As}}_2$, a recently discovered heavy fermion superconductor, and open the door for the exploration of nonsymmorphic symmetries in the broader context of improper orders with potential applications to functional topological materials.

The pinch point is a spectral discontinuity found in the neutron diffraction image of spin ice. A similar spectral singularity is commonly observed in a broad range of systems that have a close connection with flat bands. We focus on the electron flat band and its two topologically distinct classes of wave functions: the compact localized state (CLS) and the noncontractible loop state (NLS). We establish their simple mathematical relationship, showing that different Bloch NLSs can be derived as momentum derivatives of a Bloch CLS, depending on the approaching direction toward the singular point. This CLS-NLS correspondence helps visualize the pinch point as an interference pattern among NLSs through a “polarizer,” which encodes the information about the location of singular momentum and the experimental techniques like spin-polarized photoemission spectroscopy. It helps extract topological information knitted to microscopic electronic and magnetic structures.

In this study, we identify ${\mathrm{EuCd}}_2{X}_2$ (for $X$ = P, As, Sb) as a series of magnetic semiconductors. We examine how the band gap of the series responds to $X$ changing from phosphorus (P), to arsenic (As), and finally antimony (Sb). We characterize the samples using electronic transport and magnetization measurements. Based on infrared spectroscopy, we find that the band gap reduces progressively from 1.23 eV in ${\mathrm{EuCd}}_2{\mathrm{P}}_2$, to 0.77 eV in ${\mathrm{EuCd}}_2{\mathrm{As}}_2$, and finally 0.52 eV in ${\mathrm{EuCd}}_2{\mathrm{Sb}}_2$. In a magnetic field, all three systems show a strong response and their band gaps decrease at 4 K. This decrease is nonmonotonic as we change $X$. It is strongest in the phosphorous compound and weakest in the antimony compound. For all three compositions, ${\mathrm{EuCd}}_2{X}_2$ remains a semiconductor up to the highest magnetic field applied (16 T).

In materials with spin-momentum locked spin textures, such as Rashba states and topological surface states, the current-induced shift of the Fermi contour in the $k$ space leads to spin polarization, known as the Edelstein effect, which depends linearly on the applied current. However, its nonlinear counterpart has not yet been discovered. Here, we report the observation of the nonlinear Edelstein effect in few-layer $\mathrm{WT}{\mathrm{e}}_2$. Under a current bias, an out-of-plane magnetization is induced in $\mathrm{WT}{\mathrm{e}}_2$, which is electrically probed using an $\mathrm{F}{\mathrm{e}}_3\mathrm{GeT}{\mathrm{e}}_2$ electrode, a van der Waals ferromagnet with perpendicular magnetic anisotropy. Notably, with an applied ac at frequency ω, an induced magnetization with second-harmonic response at frequency 2ω is observed, and its magnitude demonstrates a quadratic dependence on the applied current, characteristic of the nonlinear Edelstein effect. This phenomenon is well explained by the current-induced orbital magnetization via the Berry connection polarizability tensors in $\mathrm{WT}{\mathrm{e}}_2$. The orbital degree of freedom plays the primary role in the observed nonlinear Edelstein effect, that is, the nonlinear orbital Edelstein effect. This can, in turn, give rise to a nonlinear spin Edelstein effect through spin-orbit coupling.