Two-dimensional (2D) semiconductors with high mobility comparable to three-dimensional (3D) Si or GaAs are still lacking, hindering the development of high-performance 2D devices. In this work, based on the phonon-energy resolved matching function between the electronic band structure and the phonon spectrum recently proposed in our group, we provide some general rules for screening possible high-mobility 2D semiconductors. In general, a 2D semiconductor with high sound velocity, small effective mass, large atomic mass, high-energy optical phonons, small Born effective charge, large effective in-plane dielectric constant, and large effective layer thickness, is likely to have high phonon-limited mobility. Additionally, if the semiconductor has multiple valleys, the mobility-limiting factor due to intervalley scattering can be weak if the valleys are far apart in reciprocal space and have out-of-plane orbitals. Guided by these rules, we have systematically explored the mobility of traditional semiconductors at the 2D limit (2DTSs), which have been theoretically predicted and experimentally realized recently. Seven 2DTSs are found to have mobilities close to or exceeding 1000 $\mathrm{c}{\mathrm{m}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$. Most notably, BSb reaches record-high values of several thousand $\mathrm{c}{\mathrm{m}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$ at room temperature and the experimentally synthesized 2D AlSb has an electron mobility of 921.9 $\mathrm{c}{\mathrm{m}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$. Using BSb, InN, and AlSb as examples, we reveal that, although the high mobilities of these systems are attributed to different underlying scattering mechanisms, they can all be understood from our proposed general rules. The present work demonstrates that 2DTSs can achieve comparable carrier mobility to 3D, thus opening possible doors to 2D high-performance electronic devices.

We study the interplay between symmetry representations of the physical and virtual space on the class of tensor network states for critical spins systems known as field tensor network states (fTNSs). These are by construction infinite-dimensional tensor networks whose virtual space is described by a conformal field theory (CFT). We can represent a symmetry on the physical index as a commutator with the corresponding CFT current on the virtual space. By then studying this virtual space representation we can learn about the critical symmetry-protected topological properties of the state, akin to the classification of symmetry-protected topological order for matrix product states. We use this to analytically derive the critical symmetry-protected topological properties of the two ground states of the Majumdar-Ghosh point with respect to the previously defined symmetries.

Transition-metal dichalcogenides are good candidates for spintronic technologies owing to their giant intrinsic spin-orbit splitting. Manipulation of energy splitting is highly desirable for further practical devices. In this work, we provide a simple route to modulate spin-orbit splitting in van der Waals ${\mathrm{MoTe}}_2/{\mathrm{WTe}}_2$ heterostructure. By introducing an external electric field pointing from ${\mathrm{MoTe}}_2$ to ${\mathrm{WTe}}_2$, the spin-orbit splitting of individual layers can be switched more than 10 meV. The spin-orbit splitting of ${\mathrm{MoTe}}_2$ switches from 213 meV to $204\sim 180\mathrm{meV}$ under electric field of $0.02\sim 0.24$ V/Å. The energy splitting of ${\mathrm{WTe}}_2$ switches to $490\sim 513\mathrm{meV}$. The underlying physics stems from interlayer charge redistribution and band repulsion, where the parallel spin in the sub-bands plays a role. Our findings provide a clue to actively control the spin-orbit splitting in transition-metal dichalcogenides, which is of great importance for developing innovative nonmagnetic spintronic devices.

Non-Hermitian skin effects (NHSEs) enrich the traditional band theories and give rise to unique non-Hermitian topological properties. Here, we reveal a different type of NHSE, termed the V-shaped NHSE, and explore an on-chip optical ring resonator platform to demonstrate the NHSE in telecommunication wavelength. By tuning the hopping and loss modulation, we work out a phase diagram that reveals the phase transition of the NHSE. Besides the conventional NHSE and the frequency-dependent bipolar NHSE, another type of NHSE is observed featuring an interesting V shape in its special skin states distribution. We further design a silicon ring resonator array with loss-assisted antiresonating coupling as the link resonators to demonstrate the nontrivial NHSE. The simulations show that one can tune the number of states piled up at different ends by switching the NHSE from conventional, bipolar to V-shaped types continuously. Our work enriches the NHSE phase transitions and implies potential in on-chip light modulation by the NHSE.

The flow of water through nanochannels has promising applications in desalination, energy conversion, and nanomedicine. We have used molecular dynamics simulations to show that the water molecules passing through a wide single-walled carbon nanotube (CNT) get aligned by the flow to have a net dipole moment along the flow direction. With increasing flow velocity, the net dipole moment first increases and eventually saturates to a constant value. This behavior is similar to that of an Ising chain with the flow velocity acting as an effective aligning field. We show that the microscopic origin of this behavior is the preferential entry of water molecules with their dipole vectors pointing inward along the CNT axis.

We prove that the motion of a single hole induces the nearest-neighbor resonating-valence-bond ground state in the $U=\infty$ Hubbard model on a triangular cactus—a treelike variant of a kagome lattice. The result can be easily generalized to $t-J$ models with antiferromagnetic interactions $J\ge 0$ on the same graphs. This is a weak converse of Nagaoka's theorem of ferromagnetism on a bipartite lattice.

Despite extensive research on magnetically induced ferroelectricity there exist relatively few studies on how a preexisting electric polarization affects magnetic order. Given that well-established magnetoelectric coupling schemes can in principle work in reverse, one might anticipate that primary, polar magnetic structures could be uniquely stabilized in ferroelectric crystals, however, this scenario is apparently rare. Here, we show that in ferroelectric $\mathrm{Bi}{\mathrm{Mn}}_7{\mathrm{O}}_{12}$, a pure, polar $E$-type antiferromagnetic order emerges below $T_1=59$ K, and we present a phenomenological model of trilinear magnetoelectric coupling consistent with ${\mathrm{Bi}}^{3+}$ lone-pair driven polar distortions uniquely stabilizing the polar antiferromagnetism via modulation of Heisenberg exchange pathways, i.e., inverse exchange striction. In addition, below $T_2=55$ K there occurs large commensurate canting of the $E$-type structure due to the onset of ferrimagnetic order on a separate crystallographic sublattice that may be exploited for additional magnetoelectric functionality.

We study the competition between ferromagnetic and spin glass phases using a system of coupled infinite-range Ising and Sherrington-Kirkpatrick models. We obtain the replica-symmetric solution for the free energy of this system in terms of magnetization and Edwards-Anderson order parameters in both subsystems. Using these order parameters we are able to identify different phases in the system and determine which phase is dominant for different strengths of the coupling between two subsystems. We observe that both subsystems are in the same phase although with different order parameters. The phase boundary between paramagnetic-ferromagnetic and paramagnetic–spin glass phases is more or less similar to that in the Sherrington-Kirkpatrick phase diagram. But the boundary between ferromagnetic–spin glass phases becomes qualitatively different as the coupling between the two subsystems changes. Remarkably, for intermediate values of the coupling, this phase boundary is such that there could be a reentrant transition between spin glass and ferromagnetic phases by increasing the temperature. We found that for some range of the coupling, the second-order transition between these phases turns into first order at a tricritical point. Further, we carry out the stability analysis by considering deviations from the replica-symmetric solution.

Experiment has established that spin glasses can support a steady-state spin current ${\vec{j}}_i$. However, the accepted theory of spin-glass dynamics permits oscillations but no steady-state spin current. Onsager's irreversible thermodynamics implies that the spin current is proportional to the gradient of a magnetization. We argue, however, that the magnon distribution function associated with the local equilibrium magnetization $\vec{M}$ cannot diffuse because it represents ${10}^{23}$ variables. We therefore invoke the nonequilibrium magnetization $\vec{m}$, which in spintronics is called the spin accumulation. Applying the theory of irreversible thermodynamics, we indeed find that it predicts spin diffusion, and we consider other experimental consequences of the theory, including a wavelength-dependent coupling between the reactive and the diffusive degrees of freedom.

We report an infrared ellipsometry study of the charge carrier dynamics in polycrystalline ${\mathrm{K}}_{x}p$-terphenyl samples with nominal $x=3$, for which signatures of high-temperature superconductivity were previously reported. The infrared spectra are dominated by two Lorentzian bands with maxima around 4000 and $12 000 {\mathrm{cm}}^{-1}$ which, from a comparison with calculations based on a Hückel model, are assigned to intramolecular excitations of $\pi$ electrons of the anionic $p$-terphenyl molecules. The intermolecular electronic excitations are much weaker and give rise to a Drude peak and a similarly weak Lorentzian band around $220 {\mathrm{cm}}^{-1}$. A dc resistivity of about $0.3 \mathrm{\Omega }\mathrm{cm}$ at $300\mathrm{K}$ is deduced from the IR data, comparable to values measured by electrical resistivity on a twin sample. The analysis of the temperature dependence of the low-frequency response reveals a gradual decrease of the plasma frequency and the scattering rate of the Drude peak below $300\mathrm{K}$ that gets anomalously enhanced below $90\mathrm{K}$. The corresponding missing spectral weight of the Drude peak appears blueshifted towards the Lorentz band at $220 {\mathrm{cm}}^{-1}$. This characteristic blueshift signifies an enhanced localization of the charge carriers at low temperatures and contrasts the behavior expected for a bulk superconducting state for which the missing spectral weight would be redshifted to a delta function at zero frequency that accounts for the loss-free response of the superconducting condensate. Our data might still be compatible with a filamentary superconducting state with a volume fraction well below the percolation limit for which the spatial confinement of the condensate can result in a plasmonic resonance at finite frequency.

We study the properties of the upper critical field of superconductivity in nodal-line semimetals in a continuous model, which has a nodal line on the $k_z=0$ plane. Using the semiclassical Green's function method, we calculate the upper critical field for the two limiting cases: the dirty limit with many impurities and the clean limit with few impurities. The results show the large anisotropy of the magnitude of the upper critical field and the unusual temperature dependence. The obtained results are compared with recent experimental data of ${\mathrm{PbTaSe}}_2$.

The superconducting critical current oscillations as a function of magnetic field were previously observed in heterostructure Josephson junctions fabricated by twisting and stacking superconducting van der Waals materials. The interference patterns, however, are often distorted. Here, we report a model for understanding the shape of oscillation patterns in heterostructure Josephson junctions. To this end, we fabricated Josephson junctions by stacking two bulk superconducting $2H\text{−}{\mathrm{NbSe}}_2$ flakes on top of each other. We were able to control the junction fabricated to be either diffusive or clean by varying the temperature during the stacking process. The measured magnetic field dependence of the critical current exhibits either a Gaussian decay or a clear oscillatory pattern with maxima and minima. We identify the shape of the interference patterns not only depends on the type of junction; they are also strongly dictated by field orientation and interface geometry. This is a consequences of the fact that heterostructure junctions often have inhomogeneous widths that extend along the field direction. We calculate the expected interference pattern from the junction geometry by applying our model and find excellent agreement to our experimental data.

We investigate the density of electrostatic force in a continuous medium modeled as an ionic crystal similar to that of NaCl. The theoretical model is three-dimensional and consists of a lattice of ions of alternating signs. The ions can interact with each other and the externally applied electric field and pressure via elastic springs, which model short-range chemical bonds, and long-range Coulomb interactions. The condition of equilibrium can be solved numerically to yield the basic physical parameters of the structure including Young's modulus, the dielectric permittivity, and the electrically induced strain. We then compare these results to the theoretical predictions obtained according to various formulas for the electrostatic force density that can be found in the literature. It is shown that Helmholtz force density predicts the numerically computed strain almost perfectly while other expressions result in large errors. The numerical simulations are limited to statics.

Recently, it has been predicted that the Berry curvature of electrons can produce an angular momentum for phonons. In systems with time-reversal symmetry, the direction of the phonon angular momentum is locked to the phonon wave vector. Accordingly, this phenomenon has received the name of “phonon helicity.” Here, we present a theory to unveil the signatures of such phonon helicity using Raman scattering. We show that the intensity of Raman scattering for circularly polarized light in ${\mathrm{BaMnSb}}_2$ (a prototypical nonchiral Dirac insulator) changes under a reversal of the phonon wave vector, and that the phonon helicity can be inferred from that change. We compare our results to recent reports of Raman-based detection of phonon angular momentum in chiral crystals.

Ferromagnetic (FM) films with perpendicular magnetic anisotropy (PMA) are crucial building blocks of state-of-the-art perpendicular magnetic devices. This study investigated the effects of triggering PMA in FM thin films by applying ${\mathrm{Mn}}_{1-x}{\mathrm{Pd}}_x$-based antiferromagnetic (AFM) thin films with strong AFM exchange coupling from Mn and high spin-orbit coupling from Pd atoms. Our results reveal that ${\mathrm{Mn}}_{1-x}{\mathrm{Pd}}_x$ films can trigger PMA in Co/Ni films with weak in-plane magnetic anisotropy at room temperature through either ${\mathrm{Mn}}_{1-x}{\mathrm{Pd}}_x$-facilitated interfacial perpendicular crystalline anisotropy (spin-orbit coupling, $x \ge 10\%$) or incorporation of them with antiferromagnet-induced exchange coupling ($x \le 10\%$). In Co/Fe films with strong in-plane magnetic anisotropy, Mn-rich ${\mathrm{Mn}}_{1-x}{\mathrm{Pd}}_x$ films ($x \le 10\%$) can induce stable PMA at low temperatures through concurrently enhanced spin-orbit coupling and antiferromagnet-induced exchange coupling across the AFM–FM interface. Our research clarifies the dual effects of exchange and spin-orbit coupling in ${\mathrm{Mn}}_{1-x}{\mathrm{Pd}}_x$ films, which trigger the PMA of adjacent FM films. This indicates that ${\mathrm{Mn}}_{1-x}{\mathrm{Pd}}_x$ films are promising candidates for achieving controllable PMA.

We study the effects of symmetry-breaking perturbations in the out-of-equilibrium quantum dynamics of many-body systems slowly driven by a time-dependent symmetry-preserving parameter, across the quantum critical regime associated with a continuous quantum transition (CQT). For this purpose, we analyze the out-of-equilibrium dynamics arising from generalized Kibble-Zurek (KZ) protocols, within a dynamic renormalization-group framework allowing for finite-size systems. We show that the time dependence of generic observables develops an out-of-equilibrium finite-size scaling (FSS) behavior, arising from the interplay between the timescale $t_s$ of the parameter variations in the KZ protocol, the size $L$ of the system, and the strength $h$ of the symmetry-breaking perturbation, in the limit of large $t_s$ and $L$. Moreover, scaling arguments based on the first-order adiabatic approximation of slow variations in quantum systems allow us to characterize the approach to the adiabatic regimes for some limits of the model parameters (for example, when we take $t_s\to \infty$ before $L\to \infty$), predicting asymptotic power-law suppressions of the nonadiabatic behaviors in the adiabatic limits. This out-of-equilibrium FSS is supported by numerical analyses for the paradigmatic quantum Ising chain along generalized KZ protocols, with a time-dependent transverse field crossing its CQT, in the presence of a static longitudinal field breaking the ${\mathbb{Z}}_2$ symmetry.

Recently, the interplay between spin and lattice degrees of freedom has gained a lot of attention due to its importance for various fundamental phenomena as well as for spintronic and magnonic applications. Examples are ultrafast angular momentum transfer between the spin and lattice subsystems during ultrafast demagnetization, frustration driven by structural distortions in transition-metal oxides, or in acoustically driven spin-wave resonances. In this work, we provide a systematic analysis of spin-lattice interactions for ferro- and antiferromagnetic materials and focus on the role of lattice symmetries and dimensions, magnetic order, and the relevance of spin-lattice interactions for angular momentum transfer as well as magnetic frustration. For this purpose, we use a recently developed scheme, which allows an efficient calculation of spin-lattice interaction tensors from first principles. In addition to that, we provide a more accurate and self-consistent scheme to calculate ab initio spin-lattice interactions by using embedded clusters, which allows us to benchmark the performance of the scheme introduced previously.

We theoretically study the coherent nonlinear response of electrons confined in semiconductor quantum wells under the effect of an electromagnetic radiation close to resonance with an intersubband transition. Our approach is based on the time-dependent Schrödinger-Poisson equation stemming from a Hartree description of Coulomb-interacting electrons. This equation is solved by standard numerical tools and the results are interpreted in terms of approximated analytical formulas. For growing intensity, we observe a redshift of the effective resonance frequency due to the reduction of the electric dipole moment and the corresponding suppression of the depolarization shift. The competition between coherent nonlinearities and incoherent saturation effects is discussed. The strength of the resulting optical nonlinearity is estimated across different frequency ranges from mid-IR to THz with an eye to ongoing experiments on Bose-Einstein condensation of intersubband polaritons and to the speculative exploration of quantum optical phenomena such as single-photon emission in the mid-IR and THz windows.