We predict the coexistence of tunneling spin and valley Hall effects when electrons in graphene coherently transmit through a barrier with the broken inversion symmetry and proximity-induced spin-orbit coupling. Due to the rotation of the pseudospin in the tunneling process, the transmitted electrons acquire a finite spin- and valley-dependent backreflection geometric phase when the two interfaces of the barrier are asymmetric. This results in a spin- and valley-dependent skew coherent tunneling, which is responsible for the transverse spin and valley Hall currents. We further demonstrate that the coherent-tunneling assisted charge-spin and charge-valley conversions are highly efficient with large Hall angles. Our work provides a new route for the generation of efficient spin and valley Hall effects, suggesting potential applications for spintronic and valleytronic devices.

Two-dimensional (2D) A-type antiferromagnets, characterized by intralayer ferromagnetic and interlayer antiferromagnetic couplings, exhibit significant tunability in their spin configurations, along with exotic electronic and optical responses. With the recent discovery of various 2D $A$-type antiferromagnets such as ${\mathrm{CrI}}_3$ and ${\mathrm{MnBi}}_2{\mathrm{Ti}}_4$, the diversity in their magnetic parameters—including interlayer coupling strengths and anisotropies—has made their magnetic transitions complex and challenging to fully comprehend. In this work, we employ a linear chain model to systematically investigate the magnetic transitions in 2D $A$-type antiferromagnets. Through numerical simulations, we explore the layer number dependence of spin-flip and spin-flop transitions with different magnetic parameters, under varying magnetic field orientations. Additionally, we derive analytical expressions to describe the layer number dependence of these transitions. Our findings enhance the understanding of magnetic transitions in 2D antiferromagnets by revealing how spin-flip and spin-flop transitions are influenced by layer numbers. Furthermore, our approach provides an effective method to determine crucial parameters, such as interlayer coupling strengths and anisotropic properties, based on experimental observations.

Plasmonic nanocavities are known for their extreme field enhancement and subwavelength light confinement in gaps of just a few nanometers. Pairing this with the ability to host quantum emitters, they form highly promising platforms to control or engineer quantum states at room temperature. Here, we use the lossy nature of plasmonic nanocavities to form subradiant entangled states between two or more quantum emitters, that persist for $\sim 100$ times longer than the plasmonic excitation. We develop a theoretical description that directly links quantum variables to experimentally measurable quantities, such as the extinction cross section, and unlike previous studies includes plasmonic excitations necessary to resonantly form subradiant states. This work paves the way towards engineering quantum entangled states in ambient conditions with plasmonic nanocavities.

In van der Waals bilayer germanene, we show a twist-induced nontrivial topological transition and find it can host the quantum valley Hall effect (QVHE). It is demonstrated that band inversion is induced at each $K$ of the supercell Brillouin zone (SBZ). The topological phase transition comes from the fact that the valley Chern number, $C_v=C_K-C_{K^{\prime }}$, switches from 0 to 2, which characterizes the nontrivial QVHE phase. Taking into account spin-orbit coupling (SOC), this band topology of the moiré superlattice originates from both the low-buckled structure within each layer and the coexistence of distinct stacking domains—namely, AA, AB, and BA regions—in the bilayer, particularly for small twist angles. Our findings advance potential applications for device design in topological valleytronics and twistronics.

We theoretically investigate a thermoelectric heat engine based on a single-level quantum dot, calculating average quantities such as current, heat current, output power, and efficiency, as well as fluctuations (noise). Our theory is based on a diagrammatic expansion of the memory kernel together with counting statistics, and we investigate the effects of strong interactions and next-to-leading order tunneling. Accounting for next-to-leading order tunneling is crucial for a correct description when operating at high power and high efficiency, and in particular affect the qualitative behavior of the Fano factor and efficiency. We compare our results with the so-called thermodynamic uncertainty relations, which provide a lower bound on the fluctuations for a given efficiency. In principle, the conventional thermodynamic uncertainty relations can be violated by the non-Markovian quantum effects originating from next-to-leading order tunneling, providing a type of quantum advantage. However, for the specific heat engine realization we consider here, we find that next-to-leading order tunneling does not lead to such violations, but in fact always pushes the results further away from the bound set by the thermodynamic uncertainty relations.

The realization of BCS-BEC crossover in superconductors, which smoothly connects Bardeen-Cooper-Schrieffer (BCS) theory with Bose-Einstein condensation (BEC) in fermion systems, is an intriguing recent topic in strongly correlated electron systems. The organic superconductor $\kappa \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_4{\mathrm{Hg}}_{2.89}{\mathrm{Br}}_8$ ($\kappa$-HgBr) under pressure is one of the leading candidates, owing to its unique metallic spin-liquid nature and tunable electron correlation. We theoretically investigate the extended Hubbard model for $\kappa$-HgBr and discuss the possibility of the BCS-BEC crossover by systematically calculating superconducting correlation function, coherence length, superfluid weight, and chemical potential. Our findings show that the BCS-BEC crossover can be observed when competing phases, such as Mott insulators and charge and/or spin orders, are suppressed by appropriate hole doping. $\kappa$-HgBr is just the case because both the Mott insulating phase and magnetic orders are absent due to its nonstoichiometric Hg composition and geometrical frustration. We further propose that other $\kappa$-type organic superconductors could serve as potential candidates of the BCS-BEC crossover if their band fillings and degree of geometrical frustration are systematically tuned.

We use dynamical mean-field theory to study how electronic transport in multiorbital metals is influenced by correlated (nominally) empty orbitals that are in proximity to the Fermi level. Specifically, we study $2+1$ orbital and $3+2$ orbital (i.e., $t_{2g}+e_g$) models on a Bethe lattice with a crystal field that is set so that the higher lying orbitals are nearly empty at low temperatures but get a non-negligible occupancy at elevated temperature. The high temperature regime is characterized by thermal activation of carriers leading to higher magnetic response (i.e., thermally induced low-spin to high-spin transition) and substantial influence on resistivity, where one can distinguish two counteracting effects: increased scattering due to formation of high spin and increased scattering phase space on one hand and additional parallel conduction channel on the other. The former effect is stronger and one may identify cases where resistivity increases by a factor of 3 at high temperatures even though the occupancy of the unoccupied band remains small ($<10\%$). We discuss implications of our findings for transport properties of correlated materials.

In this study, ultrasonic and magnetic property measurements are performed to investigate whether quadrupolar ordering occurs in the chiral magnet ${\mathrm{DyNi}}_3{\mathrm{Al}}_9$. We discover remarkable elastic softening of the transverse modulus $C_{66}$, accompanied by significant ultrasound attenuation, suggesting quadrupolar ordering, toward the antiferromagnetic ordering temperature $T_{\mathrm{M}}=8.4$ K. Crystal field analysis reveals that this softening arises from quadrupole-mediated interactions of a quasidegenerate quartet ground state. In a magnetic field applied along [100], the phase boundaries overlapping at $T_{\mathrm{M}}$ in zero field separated, confirming simultaneous occurrence of antiferromagnetic ordering and quadrupolar ordering at $T_{\mathrm{M}}$. Below $T_{\mathrm{M}}$, we further detected huge softening and ultrasound attenuation in the $C_{66}$ mode, with a minimum at 3.8 K, revealing a phase transition. These findings indicate successive quadrupolar ordering with different order parameters, facilitated by octupole-mediated interactions, in ${\mathrm{DyNi}}_3{\mathrm{Al}}_9$.

One of the most striking signatures of Weyl fermions in solid-state systems is their surface Fermi arcs. Fermi arcs can also be localized at internal twin boundaries where two Weyl materials of opposite chirality meet. In this work, we derive constraints on the topology and connectivity of these “internal Fermi arcs.” We show that internal Fermi arcs can exhibit transport signatures, and we propose two probes: quantum oscillations and a quantized chiral magnetic current. We propose merohedrally twinned B20 materials as candidates to host internal Fermi arcs, verified through both model and ab initio calculations. Our theoretical investigation sheds light on the topological features and motivates experimental studies on the intriguing physics of internal Fermi arcs.

Terahertz (THz) has broad applications for imaging, spectroscopy, and high-data-rate communication. However, THz detection at room temperature remains a significant challenge. By model Hamiltonian including electron-phonon coupling and photon-electron interaction within dipole approximation, we demonstrate that atomic displacement $\vec{A}\left(t\right)$ driven by THz field $\vec{E}\left(t\right)$ can induce static nonlinear electronic polarization in semiconductors without spatial inversion symmetry by nonlinear optic effect (${\alpha }_{ijk}{E}_j{E}_k$), Raman effect (${\beta }_{ijk}{A}_j{E}_k$), and nonlinear phononic effect (${\gamma }_{ijk}{A}_j{A}_k$). Our numerical calculations also reveals that nonlinear phononic effect dominates the change of static polarization in ferroelectric semiconductor ${\mathrm{BaTiO}}_3$ irradiated by THz field. This work presents a design principle for efficient THz detection at room temperature by monitoring the change of surface polarization induced by THz-driven lattice vibration.

We provide the phase diagram of a two-parameter spin-1 chain that has a symmetry-protected topological (SPT) Haldane phase using computational algorithms along with tensor-network tools. We improve previous results, showing the existence of a new phase and new triple points. New striking features are the triple end of the Haldane phase and the complexity of phases bordering the Haldane phase in proximity—allowing moving to nearby non-SPT phases via small perturbations. These characteristics make the system, which appears in Rydberg excitons, e.g., in ${\mathrm{Cu}}_2\mathrm{O}$, a prime candidate for applications.

Ferroelectric materials hold significant potential for high-density nonvolatile memory applications. Understanding the dynamic mechanism of ferroelectricity is crucial for maximizing the utilization of these materials. In this study, we investigate the ferroelectric properties of monolayer ${\mathrm{AgCr}}_2{\mathrm{S}}_4$ by combining density functional theory (DFT) calculations with molecular dynamics (MD) simulations that utilize machine learning potentials. Our investigation reveals that the off-center displacement of Ag ions primarily drives the ferroelectricity in monolayer ${\mathrm{AgCr}}_2{\mathrm{S}}_4$. Remarkably, the system exhibits a high ferroelectric-paraelectric phase transition temperature of 600 K. Under low temperatures, a distinctive domain structure emerges, characterized by domain walls that induce silver vacancy defects and interstitial atoms. Moreover, ferroelectric hysteresis loops indicate that the out-of-plane and in-plane polarizations of the ${\mathrm{AgCr}}_2{\mathrm{S}}_4$ monolayer are switchable by applying an external electric field. Thus, our findings suggest that monolayer ${\mathrm{AgCr}}_2{\mathrm{S}}_4$ retains ferroelectricity at room temperature, highlighting its promising potential for applications in ferroelectric devices.

Noble metals are currently considered ideal materials for applications in the field of plasmonics. However, the poor confinement of surface plasmon polaritons to thin surface layers significantly limits the performance of plasmon-based devices utilizing noble metals, despite their plasmon frequencies effectively covering the visible light spectrum. Atomically thin two-dimensional (2D) materials appear to offer an alternative to overcome this limitation. Nevertheless, most accessible 2D materials host plasmon in the terahertz and infrared regions, where the development of light sources and photodetectors remains less advanced compared to the visible and ultraviolet ranges. Consequently, identifying 2D materials with high plasmon frequency comparable to those of bulk noble metals is of significant interest. In this study, we employ first-principles calculations to investigate the plasmon dispersion relations of two recently synthesized 2D MBenes, ${\mathrm{Ti}}_2{\mathrm{B}}_2$ and ${\mathrm{Mo}}_2{\mathrm{B}}_2$. Our results reveal that in the long-wavelength region, the plasmon frequencies of these MBenes span from the visible to the ultraviolet range, exceeding those of typical 2D materials and rivaling those of noble metals. Additionally, the plasmon dispersion of these MBenes lies well outside the single-particle excitation region, indicating that the coupling between plasmons and electron-hole pairs is suppressed, thereby leading to longer plasmon lifetimes. Furthermore, applying a perpendicular electric field or gate doping induces an acoustic plasmon mode in the infrared frequency range in these MBenes, while maintaining their intrinsic plasmonic characteristics. These properties make ${\mathrm{Ti}}_2{\mathrm{B}}_2$ and ${\mathrm{Mo}}_2{\mathrm{B}}_2$ promising candidates for versatile plasmonic applications.

Phonon thermal transport properties of two-dimensional carbon monochalcogenides are computed using a first-principles approach and the Boltzmann transport theory. At room temperature (300 K), the three-phonon thermal conductivity (${\kappa }_{3\mathrm{ph}})$ of CS (34.98 W/m K) is the highest, followed by CTe (26.40 W/m K) and CSe (10.26 W/m K), which contradicts the typical behavior of lattice thermal conductivity with a mass of the chalcogen. Notably, the inclusion of four-phonon scattering drastically reduces the thermal conductivity $({\kappa }_{3+4\mathrm{ph}})$ for CTe by 40%, while CS and CSe remains largely unaffected, yet the anomalous thermal conductivity trend persists. Computed phonon mean free paths for CTe are significantly larger than the CSe and CS, which may restrict the scattering events in the CTe monolayer. Furthermore, the electron localization function implies that the buckling structure restricts the delocalization of electrons and thus a nonbonding lone pair of electrons in the CSe induces strong phonon anharmonicity and reduces lattice thermal conductivity than the CS and CTe monolayers. The present findings provide insights into the origin of the unusual behavior of lattice thermal conductivity in monolayer carbon monochalcogenides.

The forces induced by electromagnetic field fluctuations have been thoroughly studied at distances around 100 nm. However, only a few measurements have been conducted at distances below 50 nm. Meanwhile, the distance range below 50 nm has practical significance for microelectromechanical systems (MEMS), as in this range, soft elements of MEMS are susceptible to spontaneous stiction. We use the method of adhered cantilever, which maintains stability at small distances, to measure the adhesion energy due to dispersion forces. Silicon minicantilevers with a length of 12 mm, width of $200\mu \mathrm{m}$, and thickness of $10\mu \mathrm{m}$ were microfabricated. One end of the cantilever is firmly fixed at a height of about $5\mu \mathrm{m}$, while the other end adheres to a gold-coated substrate. The shape of the cantilever is measured using an interferometric technique with a precision of 2 nm over the entire length. The adhesion energy is determined using parameters extracted from the shape measurements. A careful analysis of the surface roughness shows that the effects of short-range forces and capillary bridges can be neglected, while the main contribution to the adhesion energy comes from dispersion forces with a smaller part from electrostatic interaction. Using the Lifshitz theory, we determined from the adhesion energy the distance on contact with a precision of 1–2 nm. These distances, different for each cantilever, are varied in a range of 30–41 nm due to different realization of the roughness. The results of this study not only provide a better understanding of dispersion forces but also have practical implications for MEMS applications. Ways to significantly improve measurement precision are discussed.

We analyze a class of bound defect states in the continuum electronic spectrum of bilayer materials, which emerge independent of symmetry protection or additional degrees of freedom. Taking graphene as a prototypical example, our comparative analysis of $AA$- and $AB$-stacked bilayer graphene demonstrates that these states originate from the intrinsic algebraic structure of the tight-binding Hamiltonian when trigonal warping is neglected rather than any underlying symmetry. The inclusion of trigonal warping and higher-order hoppings broadens the bound states into long-lived resonances. This discovery provides a distinct pathway to defect and band-structure engineering. We conclude with a proposed protocol for observing these states in scanning tunneling microscopy experiments.

The competition between non-Hermitian skin effect and Anderson localization leads to various intriguing phenomena concerning spectrums and wave functions. Here, we study the linear response of disordered non-Hermitian systems, which is precisely described by the Green's function. We show that the average maximum value of matrix elements of Green's functions, which quantifies the maximal response against an external perturbation, exhibits different phases characterized by different scaling behaviors with respect to the system size. Whereas the exponential-growth phase is also seen in the translation-invariant systems, the algebraic-growth phase is unique to disordered non-Hermitian systems. We explain the numerical findings using the large deviation theory, which provides analytical insights into the algebraic scaling factors of non-Hermitian disordered Green's functions. Furthermore, we show that these scaling behaviors can be observed in the steady states of disordered open quantum systems, offering a quantum-mechanical avenue for their experimental detection. Our work highlights an unexpected interplay between non-Hermitian skin effect and Anderson localization.

Zintl phase ${\mathrm{EuIn}}_2{\mathrm{As}}_2$ has garnered growing attention as an axion insulator candidate, triggered by the identification of a commensurate double-$\mathbf{Q}$ broken-helix state in previous studies, however, its periodicity and symmetry remain subjects of debate. Here, we perform resonant x-ray scattering experiments on ${\mathrm{EuIn}}_2{\mathrm{As}}_2$, revealing an incommensurate nature of the broken-helix state, where both the wave number and the amplitude of the helical modulation exhibit systematic sample dependence. Furthermore, the application of an in-plane magnetic field brings about a fanlike state that appears to preserve the double-$\mathbf{Q}$ nature, which might be attributed to multiple-spin interactions in momentum space. We propose that the itinerant character of ${\mathrm{EuIn}}_2{\mathrm{As}}_2$, most likely induced by Eu deficiency, gives rise to the helical modulation and impedes the realization of a theoretically predicted axion state with the collinear antiferromagnetic order.

To clarify spin-related phenomena within a single conduction valley of multivalley semiconductors, we experimentally study the effect of carrier concentration $\left(n\right)$ on spin lifetime $\left({\tau }_{\mathrm{S}}\right)$ in strained ${\mathrm{Si}}_{0.1}{\mathrm{Ge}}_{0.9}$ at various temperatures. The spin diffusion length $\left(\lambda \right)$ of strained ${\mathrm{Si}}_{0.1}{\mathrm{Ge}}_{0.9}$ is estimated from the contact distance dependence of the nonlocal spin signals for spin transport measurements in lateral spin-valve devices with varying $n$. The obtained $\lambda$ is found to depend on $n$ at temperatures below $\sim 100\mathrm{K}$, with the estimated ${\tau }_{\mathrm{S}}$ value at 8 K varying significantly with $n$, ranging from 0.2 to 6.2 ns. These features suggest the presence of intravalley spin scattering at low temperatures even in doped multivalley semiconductors. Thus, strained $n\text{−}{\mathrm{Si}}_{0.1}{\mathrm{Ge}}_{0.9}$ with $L$-valley energy splitting serves as a powerful tool for investigating the spin relaxation mechanisms in a single conduction valley of doped multivalley semiconductors.