The valley degree of freedom in two-dimensional crystalline materials is another promising candidate for designing dissipationless information devices in addition to the spin degree of freedom. The primary challenge for the application of valley degree of freedom is to achieve a significant and controllable valley polarization. Here, we discover an electrical tunable mechanism for generating valley polarization caused by the backscattering induced by Rashba spin-orbit coupling in monolayer nanoribbon of transition-metal dichalcogenides. Since the Rashba spin-orbit coupling in monolayer transition-metal dichalcogenides can be induced by the external electric field, the valley polarization here can be dynamically controlled by means of the external gating. The results of the valley-dependent transport calculations show that a pronounced valley polarization can be generated when the external electric field-induced Rashba spin-orbit coupling is present in the central scattering region of the nanoribbon. Moreover, the efficiency and direction of the valley polarization can be controlled by tuning the Fermi energy and the strength of Rashba spin-orbit coupling. The calculated results also confirm that the valley polarization remains significant in the presence of atomic vacancy defects. This finding opens up a potential possibility for realizing electrical control of the valleytronics devices.

In two-dimensional higher-order topological insulators, the corner states are separated by a non-negligible distance. The crystalline symmetries protect the robustness of their corner states with long-range entanglement, which are robust against time-reversal-breaking perturbations. Here, we demonstrate the possibility of direct control of the topological corner states by introducing the spin degree of freedom in a rhombus-shaped Kekulé nanostructure with local magnetization and local electric potential. By applying a local magnetization on one corner, the other corner can also be strongly spin polarized. By further applying a local electric potential at the same corner, the sign of the spin polarization can be reversed at both corners. We also prove the robustness of the control of the spin polarization under the disorder. Moreover, we demonstrate the material realization in a $\gamma$-graphyne nanostructure with Mn adsorption and Si replacement at one corner by using first-principles calculations. Other higher-order lattices and the shape of the nanostructure are also discussed. Our studies give a showcase of the remote correlation of quantum states in higher-order topological materials for spintronic and quantum applications.

Resonant structures in modern nanophotonics are non-Hermitian (leaky and lossy), and support quasinormal modes. Moreover, contemporary cavities frequently include two-dimensional (2D) materials to exploit and resonantly enhance their nonlinear properties or provide tunability. Such materials add further modeling complexity due to their infinitesimally thin nature and strong dispersion. Here, a formalism for efficiently analyzing third harmonic generation (THG) in nanoparticles and metasurfaces incorporating 2D materials is proposed. It is based on numerically calculating the quasinormal modes in the nanostructure, it is general, and does not make any prior assumptions regarding the number of resonances involved in the conversion process, in contrast to conventional coupled-mode theory approaches in the literature. The capabilities of the framework are showcased via two selected examples: a single scatterer and a periodic metasurface incorporating graphene for its high third-order nonlinearity. In both cases, excellent agreement with full-wave nonlinear simulations is obtained. The proposed framework may constitute an invaluable tool for gaining physical insight into the frequency generation process in nano-optic structures and providing guidelines for achieving drastically enhanced THG efficiency.

In this work, we develop a systematic approach of constructing and classifying the model Hamiltonians for two-dimensional (2D) higher-order topological phase with corner zero-energy states (CZESs). Our approach is based on the direct construction of an analytical solution of the CZESs in a series of 2D systems stacking the 1D extended Su-Schrieffer-Heeger (SSH) model, two copies of the two-band SSH model, along with two orthogonal directions. Fascinatingly, our approach not only gives the celebrated Benalcazar-Bernevig-Hughes and 2D SSH models but also reveals a novel model and we name it 2D crossed SSH model. Although these three models exhibit completely different bulk topology, we find that the CZESs can be universally characterized by edge winding number for 1D edge states, attributing to their unified Hamiltonian construction form and edge topology. Remarkably, our principle of constructing CZESs can be readily generalized to 3D and superconducting systems. Our work sheds new light on the theoretical understanding of the higher-order topological phases and paves the way to looking for higher-order topological insulators and superconductors.

We explore hydrodynamics of Dirac fermions in neutral graphene in the Corbino geometry. In the absence of a magnetic field, the bulk Ohmic charge flow and the hydrodynamic energy flow are decoupled. However, the energy flow does affect the overall resistance of the system through viscous dissipation and energy relaxation that has to be compensated by the work done by the current source. Solving the hydrodynamic equations, we find that local temperature and electric potential are discontinuous at the interfaces with the leads as well as the device resistance and argue that this makes Corbino geometry a feasible choice for an experimental observation of the Dirac fluid.

External hydrostatic pressure ($P$) is another controllable and clean knob to tune the energy position of van Hove singularities in twisted bilayer graphene (TBLG), besides the interlayer rotation angle ($\theta$). Based on total energy density functional theory calculations, we report here the ground-state properties of high-angle TBLG with two families due to their even and odd sublattice-exchange parity, hereafter parity for simplicity, under vertical hydrostatic $P$. Even (odd) parity refers to rotate one layer with respect to the other in a bilayer graphene with stacking AA (AB). We observed a Dirac semimetal (0–20 GPa) semiconductor (20–70 GPa) phase transition at $\theta =21.8^{\circ }$ and $13.4^{\circ }$; however, a reentrant Dirac semimetallic phase is observed for $\theta =9.4^{\circ }$ and $P\ge 70$ GPa whenever TBLG has even parity. Meanwhile, TBLG systems with odd parity and different $\theta$ remain metallic but with an enhanced trigonal warping for $P>50$ GPa. Indeed, the semiconductor phase is only present for high $\theta$. The phase transition in TBLG with even parity and the metallic character of TBLG with odd parity are due to band inversion effect assisted by a band-to-band repulsion mechanism shown by unfolded bands. This work shows the relevance of external $P$ and parity in TBLG electronic structure and their possible implications in experiments.

Electrically controllable quantum coherence in quantum dot clusters and arrays based on graphene stripes with zigzag atomic edges (ZZ stripes) is studied using the Dirac equation and $S$-matrix technique. We find that respective multiqubit circuits promise stable operation up to room temperature when the coherence time is prolonged by a few orders of magnitude through the intrinsic spectral narrowing owing to electron transport between flat bands in adjacent sections. Respectively, the coupling of qubits to a noisy environment is diminished, while the inelastic electron-phonon scattering is suppressed. The Stark splitting technique enables a broad range of operations such as the all-electrical tuning of the energy level positions and width, level splitting, controlling of the interqubit coupling, and the coherence time. At the resonant energies, the phase coherence spreads over thousands of periods. Such phenomena potentially can be utilized in quantum computing and communication applications at room temperature.

The unique electronic and crystal structures driven by external pressure in transition metal chalcogenides (TMCs) can host emergent quantum states. Here we report pressure-induced metallization, nontrivial $Z_2$ band topology, and superconductivity in TMC ${\mathrm{Ta}}_2{\mathrm{Ni}}_3{\mathrm{Te}}_5$. Our electrical transport measurements show that the metallization emerges at 3.3 GPa, followed by appearance of the superconductivity at $P_{\mathrm{c}}=21.3\mathrm{GPa}$ with $T_{\mathrm{c}}\sim 0.4\mathrm{K}$. Room-temperature synchrotron x-ray-diffraction experiments demonstrate the stability of the pristine orthorhombic structure upon compression. Our first-principles calculations further reveal a topological phase transition (from $Z_2=0$ to $Z_2=1$), which occurs after ${\mathrm{Ta}}_2{\mathrm{Ni}}_3{\mathrm{Te}}_5$ is turned into an electron-hole compensated semimetal by pressure. The pressure-induced superconductivity at $P_{\mathrm{c}}$ could be attributed to the abruptly enhanced density of states at the Fermi level. These findings demonstrate that ${\mathrm{Ta}}_2{\mathrm{Ni}}_3{\mathrm{Te}}_5$ is a new platform for realizing exotic quantum phenomena in TMCs as well as exploring the interplay between topological property and superconductivity.

We show that the quasicontinuous gapless spectrum of Andreev bound states in multiterminal semi-classical superconducting nanostructures exhibits a large number of topological singularities. We concentrate on Weyl points in a four-terminal nanostructure and compute their density and correlations in three-dimensional parameter space for a universal random matrix theory model as well as for the concrete nanostructures described by the quantum circuit theory. We mention the opportunities for experimental observation of the effect in a quasicontinuous spectrum.

We study the off-equilibrium critical phenomena across a hysteretic first-order transition in disordered athermal systems. The study focuses on the zero temperature random field Ising model (ZTRFIM) above the critical disorder for spatial dimensions $d=2,3,$ and 4. We use Monte Carlo simulations to show that disorder suppresses critical slowing down in phase ordering time for finite-dimensional systems. The dynamic hysteresis scaling, the measure of explicit finite-time scaling, is used to subsequently quantify the critical slowing down. The scaling exponents in all dimensions increase with disorder strength and finally reach a stable value where the transformation is no longer critical. The associated critical behavior in the mean-field limit is very different, where the exponent values for various disorders in all dimensions are similar. The non-mean-field exponents asymptotically approach the mean-field value ($\mathrm{{\rm Y}}\approx 2/3$) with increase in dimensions. The results suggest that the critical features in the hysteretic metastable phase are controlled by inherent mean-field spinodal instability that gets blurred by disorder in low-dimension athermal systems.

From new neutron powder diffraction experiments on the chiral cubic ($P2{}_{1}3$) magnet manganese germanide (MnGe), we analyze all of the possible crystal symmetry-allowed magnetic superstructures that are determined successfully from the data. The incommensurate propagation vectors $k$ of the magnetic structure are found to be aligned with the [100] cubic axes, and correspond to a magnetic periodicity of about 30 Å at 1.8 K. Several maximal crystallographic symmetry magnetic structures are found to fit the data equally well and are presented. These include topologically nontrivial magnetic hedgehog and “skyrmion” structures in multi-$k$ cubic or orthorhombic 3+3 and orthorhombic 3+2 dimensional magnetic superspace groups respectively, with either potentially responsible for topological Hall effect. The presence of orthorhombic distortions in the space group $P{2}_1{2}_1{2}_1$ caused by the transition to the magnetically ordered state does not favor the cubic magnetic hedgehog structure, and leave both orthorhombic hedgehog and skyrmion models as equal candidates for the magnetic structures. We also report on a combined mechanochemical and solid-state chemical route to synthesize MnGe at ambient pressures and moderate temperatures, and compare with samples obtained by the traditional high pressure synthesis.

The scaling of the proximity exchange field, which ferromagnetic Fe exerts on the spin accumulation at the MgO/Si interface of Fe/MgO/Si tunnel contacts, was investigated using the inverted Hanle effect. Data as a function of the applied bias voltage shows that the exchange field varies with bias in the same way as the square of the tunnel spin polarization (TSP). The decay of the exchange field with increasing MgO thickness $t_{\mathrm{MgO}}$ is rather weak and nonexponential, which we argue to be related to the improved epitaxial quality of the MgO, and the, consequently, improved spin-filtering properties at larger MgO thickness. Using the TSP data to correct for this yields an exponential decay of the exchange field with $t_{\mathrm{MgO}}$ with a decay length of $\lambda =-0.42$ nm. Comparing this to the decay length for the tunnel conductance ($\lambda =-0.21$ nm) suggests that the exchange interaction is dominated by two-step tunneling processes.

Spin-spiral texture can substantially change charge transport properties in helimagnets. We find the anomalous Hall effect (AHE) exhibiting the dramatic behavior with respect to chemical potential $\mu$ in conical magnetic structures. The direct conductivity demonstrates kinks, and the anomalous Hall current exhibits minima and maxima changing the sign. We analytically derive the expression for energy bands and eigenstates in the most general case. Because of the conical potential, the energy bands are split into two nonparabolic bands where the lower band can have one- or two-minima shapes in the $k_z$ direction ($\widehat{\mathit{z}}$ is a direction of the spiral axis). We prove that the origin of the anomalous Hall effect is not topological and is due to the interplay between the asymmetry of energy bands in the $x$ and $z$ directions and spin restrictions in the phase space due to the conical potential. We also investigate the dependence of transport properties on cone half- angle $\theta$, and find that the effects are most pronounced at $\theta =\pi /2$ (a helical state). Electric current is calculated using the Boltzmann equation where the relaxation is caused by electron-acoustic phonon interaction. The transition probability is found to be a $2\times 2$ matrix with nonvanishing off-diagonal elements indicating the strong interband transitions. The origin of interband transitions is because of the nature of the conical potential where conduction electron spins interact with localized magnetic moments. To verify the proposed theory, we calculate the temperature dependence of resistivity for $\mathrm{MnSi}$ crystals and find the discontinuity at the phase transition between conical and paramagnetic phases. The calculations are in the excellent agreement with the experimental data. In addition, we predict the discontinuity behavior for the anomalous Hall resistivity at the phase transition where the resistivity exhibits the abrupt change at $T=T_C$, (a) to zero if the relativistic effects for the conduction electrons are small or (b) to a nonzero value if Rashba/Dresselhaus effects are taken into account.

Differential conductance spectroscopy performed in the high bias regime—in which the applied voltage exceeds the sample work function—is a suboptimal measure of the local density of states due to the effects of the changing tunnel barrier. Additionally, the large applied voltage oftentimes makes constant-height measurement experimentally impractical, lending constant-current spectroscopy an advantageous edge; but the differential conductance in that case is even further removed from the local density of states due to the changing tip height. Here, we present a normalization scheme for extracting the local density of states from high bias scanning tunneling spectroscopy, obtained in either constant-current or constant-height mode. We extend this model to account for the effects of the in-plane momentum of the probed states to the overall current. We demonstrate the validity of the proposed scheme by applying it to laterally confined field-emission resonances, which appear as peak-shaped spectroscopic features with a well-defined in-plane momentum.

High-temperature properties of ceria surfaces are important for many applications. Here, we report the temperature dependencies of surface energy for (111) and (110) ${\mathrm{CeO}}_2$ obtained in the framework of the extended two-stage up-sampled thermodynamic integration using Langevin dynamics. The method was used together with machine-learning potentials called moment tensor potentials (MTPs), which were fitted to the results of the ab initio molecular dynamics calculations for (111) and (110) ${\mathrm{CeO}}_2$ at different temperatures. The parameters of MTP training and fitting were tested, and the optimal algorithm for the ceria systems was proposed. We found that the temperature increases from 0 to 2100 K led to the decrease of the Helmholtz free energy of (111) ${\mathrm{CeO}}_2$ from 0.78 to 0.64 $\mathrm{J}/{\mathrm{m}}^2$. The energy of (110) ${\mathrm{CeO}}_2$ dropped from 1.19 $\mathrm{J}/{\mathrm{m}}^2$ at 0 K to 0.92 $\mathrm{J}/{\mathrm{m}}^2$ at 1800 K. We show that it is important to consider anharmonicity, as simple consideration of volume expansion gives the wrong temperature dependencies of the surface energies.