Co-based shandite ${\mathrm{Co}}_3{\mathrm{Sn}}_2{\mathrm{S}}_2$ is a ferromagnet hosting Weyl fermions in the layered Co kagome structure. The band topology as well as the magnetism is predicted to vary drastically in the atomically thin films depending on the thickness and surface termination, and as an extreme case, the quantum anomalous Hall state is expected in a monolayer of the Co kagome lattice. Given that the bulk Weyl gap depends on the magnetization direction, here we theoretically study how the topological nature and transport properties vary with the magnetization direction in the systems with kagome monolayer, with both Sn and S surface terminations. By using ab initio calculations, we find that in the Sn-end monolayer the anomalous Hall conductivity shows successive discrete changes between different quantized values by rotating the magnetization, indicating several topological transitions between the anomalous quantum Hall insulators with different Chern numbers. Notably, when the magnetization is oriented in-plane and perpendicular to the Co-Co bond, the system exhibits a planar quantum anomalous Hall effect. We clarify that these peculiar behaviors are due to topological changes in the band structures associated with gap closing of the Weyl nodes. In contrast, the S-end monolayer shows rather continuous changes in the transport properties since the system is metallic, although the band structure contains many Weyl nodes. Our results pave the way for controlling Weyl fermions in atomically thin films of Co-based shandite, where the topological nature associated with the Weyl nodes appears more clearly than the bulk.

We propose cooperative two-photon lasing in two quantum dots coupled to a single-mode photonic crystal cavity. We consider both quantum dots are driven either incoherently or coherently using an external pump. We incorporate exciton-phonon coupling using polaron transformed master equation. Using quantum laser theory, single- and multiphoton excess emission rates (difference between emission and absorption) into cavity mode are investigated. The single- and two-photon excess emission rates contribute to cavity photons, predominantly. Varying the pump strength can lead to single-photon excess emission rate change from negative to positive, and thus, by appropriately selecting pump strength, single-photon excess emission rate can be made negligible.

We propose the one-dimensional structure driven by nonreciprocal disorder in the coupling terms of a quantum chain, probing the properties of localization and the skin effect of the system under the competition between disorder and nonreciprocal coupling. The results indicate that such disorder has a significant modulating effect on the energy spectra and localization of the system. To be specific, it induces the pseudo-Hermitian symmetry phase transition in the system. And when the disorder parameters in the two hopping directions are the same, some states have opportunities to localize on the left and right sides. The interesting result is that a localization transition appears at weak disorder strength, and its occurrence is related to the energies of states. Next, when the disorder parameters in the two hopping directions differ, the energy-dependent non-Hermitian skin effect is induced, accompanied by the more intricate localization phenomena. We believe that the findings in this work will enhance the understanding of nonreciprocal disorder effects on one-dimensional quantum systems.

We analytically and numerically study the nonreciprocal surface waves guided by a magnetized graphene tube. When applying the magnetic bias perpendicularly to the cylinder axis, the conductivity profile cross section is nonuniform due to the varying angle between the magnetic bias and the surface-wave guiding tube, which enables us to obtain pronounced nonreciprocal modes with different field distributions for propagation in opposite directions due to the coupling between different values of orbital angular momentum. We show that this property can be leveraged for directional power delivery when changing the source location, and we study the isolation ratio as a function of different parameters. The dependence of the isolation ratio on the magnetic field bias allows simple, yet robust, control over the nonreciprocal wave propagation properties.

Josephson plasmons in layered superconductors represent a natural source of optical nonlinearity, thanks to their intrinsically anharmonic nature. Here we derive the selection rules behind nonlinear plasmonics showing its dependence on plasmonic branches hidden to other spectroscopies, such as RIXS. We benchmark our results for the case of layered cuprates, showing how in a layered system the combined effect of plasmon dispersion and light polarization can move the resonance of the bilayer system away from the plasma edge measured in linear spectroscopy. Our results demonstrate the dependence of the nonlinear THz response on the convoluted plasmon dispersion in a momentum region complementary to RIXS, and offer a possible perspective for the generation of THz pulses by artificially designed Josephson heterostructures.

Ferroelectric nitrides have emerged as promising semiconductor materials for modern electronics. However, their domain structures and associated properties are basically unknown, despite their potential to result in optimized or new phenomena. Density functional theory calculations are performed to investigate the effect of epitaxial strain on multidomains of (Al, Sc)N nitride systems and to compare it with the monodomain case. The multidomain systems are predicted to have five strain-induced regions, to be denoted as Regions I to V, respectively. Each of these regions is associated with rather different values or behaviors of physical properties such as axial ratio, polarizations, internal parameters, bond lengths, etc. Of particular interest is the prediction of bent domains under compressive strain extending beyond $-5.5$%, which indicates that domain walls may play a key role in the mechanical failure properties of these systems. Interestingly, such bending induces the creation of a finite in-plane polarization (in addition to out-of-plane dipoles) due to geometric and symmetry considerations. Strikingly too, the bent domains have lower energy than the wurtzite monodomains and have atomically sharp boundaries. Our findings may pave the way for domain-wall engineering in ferroelectric nitrides.

Ferroelectric solid solutions usually exhibit giant dielectric response and high piezoelectricity in the vicinity of the morphotropic phase boundary (MPB), where the structural phase transitions between the rhombohedral and the tetragonal phases as a result of the composition or strain variation. Here, we propose a compositional ordering driven MPB in the specified compositional solid solutions. By preforming machine-learning potential-based molecular dynamics simulations on lead zirconate titanate, we find a phase transition from the rhombohedral to tetragonal phase with the decrease of compositional ordering, leading to the MPB on temperature-ordering phase diagram. The compositional ordering driven MPB can enhance the piezoelectricity with a magnitude comparable to that at the composition driven MPB. Finally, we demonstrate that the mechanism of high piezoelectricity is polarization rotation driven by external field. This work provides an additional degree of freedom, compositional ordering, to design the high-performance piezoelectric materials.

Strain engineering is a versatile method to boost the carrier mobility of two-dimensional materials-based electronics and optoelectronic devices. In addition, strain is ubiquitous during device fabrication via material deposition on a substrate with a different lattice structure. Here, we show that the polarization properties of the harmonics in graphene under uniaxial strain are strongly, yet differently affected in the lower and higher orders. The polarization plane of the lower-order emitted harmonics is rotated—a manifestation of Faraday rotation due to the broken symmetry planes. In contrast, we observe elliptically polarized higher-order harmonics due to the intricate interplay of the interband and intraband electron dynamics. The implications of these findings are twofold: First, we show how the rotation of the polarization plane of the lower-order harmonics can be used as a probe to characterize the strain's nature, strength, and angle. Second, we demonstrate how strain engineering can be used to alter the polarization properties of higher-order harmonics, relevant for applications in ultrafast chiral-sensitive studies. Our research opens a promising avenue for strain-tailored polarization properties of higher-order harmonics in engineered solids.

Bound states in the continuum (BICs) are embedded states in the radiation spectrum, which have garnered significant interest across various areas, including photonics and acoustics. In this work, by introducing rotational obstacles into acoustic resonators, we report a series of accidental BICs in coupled waveguide-resonator systems. We demonstrate that a general type of accidental BICs would emerge at specific rotating angles, supported by the mode symmetries at the boundary interface between resonators and attached waveguides. We further demonstrate that the presence or absence of accidental BICs is closely related to the geometric parameters of resonators and obstacles, and can be predicted by the mode evolution within closed resonators. Additionally, we show that two BICs of an individual mode could converge, merge, and vanish in a single resonator by changing the geometry parameters. We also explore the topological origins of these phenomena. Our study provides an efficient way to manipulate and engineer BICs through rotating obstacles in acoustic resonator systems.

Phonon anharmonicity plays a crucial role in the lattice dynamics and lattice thermal conductivity ${\kappa }_L$ of solids. Here, by combining first-principles calculations with the linearized Boltzmann transport equation, we unveil that phonon anharmonicity can make an unexpected contribution to ${\kappa }_L$ and thereby lead to anomalous thermal transport behavior in the intermetallic compound ScZn, a recently theoretically predicted topological phonon material. We explicitly consider phonon renormalization, four-phonon (4ph), and phonon-electron scattering to predict the ${\kappa }_L$ of ScZn. The results show that anharmonic phonon renormalization due to quartic anharmonicity gives rise to significant phonon hardening, especially for heat-carrying acoustic phonons. We also reveal that anharmonic effects largely decrease the fourth-order force constants, causing the dramatic suppression of 4ph scattering on ${\kappa }_L$. The incorporation of renormalization-induced phonon hardening and the decrease of quartic force constants makes a consequence of an almost twofold increase in the ${\kappa }_L$ compared to the conventional harmonic treatment, from 3.9 to 7.6 W/mK at room temperature. This is enabled by the suppressed phonon scattering phase space and weakened fourth-order lattice anharmonicity due to the temperature-induced phonon renormalization effect. Moreover, it is found that the electronic contribution ${\kappa }_e$ always plays a dominating role on the thermal conductivity; however, the lattice contribution is still considerable, leading to the substantial deviation of Lorenz number from the Sommerfeld value. This work provides a fundamental understanding of the thermal transport in ScZn and highlights the importance of anharmonic effects on the phonon energies and thermal conductivity of metals.

The effective manipulation magnetic properties of two-dimensional magnets through electric field presents profound implications for spintronics. In this study, we investigate the ferroelectric-controlled magnetoelectric coupling within the $\mathrm{Cr}{\mathrm{I}}_3/\mathrm{Hf}{\mathrm{O}}_2$(111)/$\mathrm{Cr}{\mathrm{I}}_3$ heterostructure by using first-principles calculations. It is found that the magnetic moments of $\mathrm{Cr}{\mathrm{I}}_3$ are notably responsive to the polarization orientation—either directed towards ($P_{\mathrm{in}}$) or away ($P_{\mathrm{out}}$) from the interface. The polarization of $\mathrm{Hf}{\mathrm{O}}_2$ triggers the accumulation (depletion) of screening electrons at the $P_{\mathrm{in}}$ ($P_{\mathrm{out}}$) interface, which affects the relative population of the exchange-split spin bands at the interface and thus results in the enhanced (reduced) total magnetic moments of the monolayer $\mathrm{Cr}{\mathrm{I}}_3$. The easy magnetization axis of $\mathrm{Cr}{\mathrm{I}}_3$ switches from an out-of-plane ($P_{\mathrm{out}}$ state) to an in-plane ($P_{\mathrm{in}}$ state) direction by the reversal of ferroelectric polarization of $\mathrm{Hf}{\mathrm{O}}_2$. Additionally, the Curie temperature ($T_C$) of monolayer $\mathrm{Cr}{\mathrm{I}}_3$ is enhanced by 78% in the $P_{\mathrm{out}}$ state. This work provides useful guidelines for the study of interfacial magnetoelectric coupling and design of multiferroic devices.

The copper metaborate ${\mathrm{CuB}}_2{\mathrm{O}}_4$ with a unique noncentrosymmetric crystal structure and the two structurally nonequivalent $4b$ and $8d$ magnetic subsystems of ${\mathrm{Cu}}^{2+}$ ions are characterized by several commensurate and incommensurate magnetic phases below the Néel temperature of $T_{\mathrm{N}}=20$ K. Transitions between them can be induced by varying the sample temperature or the applied magnetic field. High-resolution optical spectroscopy of the Frenkel exciton states is used to probe the magnetic phase transitions. The absorption spectra are measured in the temperature range from 4.3 K up to $T_{\mathrm{N}}$ and in magnetic fields up to 2 T. The exciton transitions are observed in the range of $1.405–1.407$ eV and are related to the lowest-energy electronic transition in the $4b$ magnetic subsystem of ${\mathrm{Cu}}^{2+}$ ions, thus allowing us to monitor the behavior of this particular subsystem. The exciton lines show abrupt changes of intensity and energy splitting, which can be unambiguously attributed to the magnetic phase transitions in the $4b$ subsystem. The magnetic phase diagram reconstructed from the spectroscopic experiments on the $4b$ subsystem is compared with those from magnetic measurements, which reflect the magnetic behavior of both the $4b$ and $8d$ subsystems. The suggested spectroscopic approach enables separate monitoring of the temperature and magnetic field behavior of different magnetic subsystems in structurally and magnetically complex compounds.

The tunneling diodes are commonly used to control the direction of current in semiconductor devices and are an essential circuit element in modern electronics. Here, we propose a multifunctional and reconfigurable spin-dependent tunneling diode (spin TD) based on a symmetric van der Waals (vdW) double-barrier magnetic tunnel junction (DBMTJ), where if the magnetic moments of two ferromagnetic electrodes are antiparallel, a significant spin TD effect is produced in the symmetric vdW DBMTJ due to the spin splitting of the middle ferromagnetic metal layer. Using first-principles calculations, we find that the proposed spin TD effect and three significantly different resistance states with giant tunneling magnetoresistance are achieved in the vdW DBMTJs based on the ${\mathrm{Fe}}_3{\mathrm{GaTe}}_2$/${\mathrm{GeI}}_2/{\mathrm{Fe}}_3{\mathrm{GaTe}}_2/{\mathrm{GeI}}_2/{\mathrm{Fe}}_3{\mathrm{GaTe}}_2$ and ${\mathrm{Fe}}_3{\mathrm{GaTe}}_2/{\mathrm{GeI}}_2/{\mathrm{Fe}}_3{\mathrm{GeTe}}_2/{\mathrm{GeI}}_2/{\mathrm{Fe}}_3{\mathrm{GaTe}}_2$ vdW heterostructures. Moreover, the spin TD effect can be modulated by changing the magnetic configuration of the symmetric vdW DBMTJ through the magnetic field. Our work holds valuable implications for developing multifunctional and reconfigurable nondoped diode devices.

The observation of stripelike charge density wave (CDW) order in pressurized $\mathrm{Cs}{\mathrm{V}}_3\mathrm{S}{\mathrm{b}}_5$ implies unique electronic correlation effects in this kagome superconductor. However, it remains unknown how the stripe order affects the band structure of $\mathrm{Cs}{\mathrm{V}}_3\mathrm{S}{\mathrm{b}}_5$ at high pressure. Here, we perform anisotropic resistivity ratio and thermopower measurements under pressure and find that their behavior significantly changes when the system enters the stripelike CDW state. More intriguingly, the anisotropic resistivity ratio suddenly decreases and at the same time the thermopower shows a humplike behavior below $T_{\mathrm{CDW}}$ in the stripelike CDW phase. Our results indicate the enhancement of the interlayer coupling in the stripelike CDW phase, which couples to the band structure below $T_{\mathrm{CDW}}$. These findings provide an alternative experimental framework for understanding the underlying physics behind the pressure-induced stripelike CDW order in kagome superconductors.

Trigonal ${\mathrm{PtBi}}_2$ is a layered semimetal without inversion symmetry, featuring 12 Weyl points in the vicinity of the Fermi energy. Its topological Fermi arcs were recently shown to superconduct at low temperatures where bulk superconductivity is absent. Here, we perform first-principles calculations to investigate in detail the bulk and surface electronic structure of ${\mathrm{PtBi}}_2$, and obtain the spin texture as well as the momentum-dependent localization of the arcs. Motivated by the experimentally observed recovery of inversion symmetry under pressure or upon doping, we interpolate between the two structures and determine the energy and momentum dependence of the Weyl nodes. For deeper insights into the surface superconductivity of ${\mathrm{PtBi}}_2$, we construct a symmetry-adapted effective four-band model that accurately reproduces the Weyl points of ${\mathrm{PtBi}}_2$. We supplement this model with an analysis of the symmetry-allowed pairings between the Fermi arcs, which naturally mix spin-singlet and spin-triplet channels. Moreover, the presence of surface-only superconductivity facilitates an intrinsic superconductor-semimetal-superconductor Josephson junction, with the semimetallic phase sandwiched between the two superconducting surfaces. For a phase difference of $\pi$, zero-energy Andreev bound states develop between the two terminations.

We introduce a direct (Seebeck) and inverse (Peltier) thermoelectric effect induced by electron tunneling between closely separated conducting films. When a transverse temperature gradient is applied along one of two films, a bias voltage is induced in the second thanks to the heat transfer mediated by electrons tunneling through the separation gap. We highlight a non trivial behavior for this Seebeck effect with respect to geometric characteristics of interacting films. Conversely, when an electric current passes through one of two films a strong thermal power can be removed from or inserted in the second film through an induced Peltier effect. In particular we highlight conditions where the induced Seebeck and Peltier coefficients are larger than in the bulk. These induced thermoelectric effects could find broad applications in the fields of energy conversion and cooling at nanoscale.

Lead-free antiferroelectric perovskite ${\mathrm{AgNbO}}_3$ is nowadays attracting extensive research interests due to its promising applications in energy storage. Although great progress has been made in optimizing the material performance, fundamental questions remain regarding the mechanism stabilizing the antiferroelectric $Pbcm$ phase. Here, combining structural symmetry analysis and first-principles calculations, we identified crucial anharmonic couplings of oxygen octahedra rotations and cation antipolar motions that contribute significantly to lowering the energy of the $Pbcm$ phase. The stabilization of this phase shows close similarities with the stabilization of the $Pbam$ phase in ${\mathrm{PbZrO}}_3$ except that in ${\mathrm{AgNbO}}_3$ the octahedra rotations are the primary distortions while the antipolar cation motions appear to be secondary. The appearance and significant amplitude of the latter are explained from the combination of hybrid-improper and triggered mechanisms.