By using density functional theory and ab initio quantum-transport simulation, we study the Schottky barrier and the rectifying behavior of diodes consisting of the two-dimensional metal phase $1T\text{−}{\mathrm{MoS}}_2$ and semiconductor phase 2H-${\mathrm{MoS}}_2$. The results show that the Schottky barrier of the out-of-plane (OP) contacted ${\mathrm{MoS}}_2$ heterostructure diode is a little different from that of the in-plane (IP) contacted ${\mathrm{MoS}}_2$ heterostructure diode. The current-voltage characteristics show that the OP diode has the better rectifying behavior compared to the IP diode under the zero gate voltage. The corresponding maximum rectifier ratio of the OP Schottky barrier diode is close to ${10}^7$ at 0.9 V bias voltage. More interestingly, we find that the gate voltage can be used to effectively control the rectifying behavior of the two diodes. The positive gate voltages can increase the current value of two Schottky barrier diodes, but weaken their rectification ratios. The negative gate voltages can reverse the rectifying direction of two Schottky barrier diodes. The above results provide good theoretical guidance for the designing of diode devices based on two-dimensional materials in the future.

Trilayer graphene with a twisted middle layer has recently emerged as a new platform exhibiting correlated phases and superconductivity near its magic angle. A detailed characterization of its electronic structure in the parameter space of twist angle $\theta$, interlayer potential difference $\mathrm{\Delta }$, and top-bottom layer stacking $\mathit{\tau }$ reveals that flat bands with large Coulomb energy versus bandwidth $U/W>1$ are expected within a range of $\pm 0.2^{\circ }$ near $\theta \simeq 1.5^{\circ }$ and $\theta \simeq 1.2^{\circ }$ for ${\mathit{\tau }}_{\mathrm{AA}}$ top-bottom layer stacking, between a wider $1^{\circ }–1.7^{\circ }$ range for ${\mathit{\tau }}_{\mathrm{AB}}$ stacking, whose bands often have finite valley Chern numbers thanks to the opening of primary and secondary band gaps in the presence of a finite $\mathrm{\Delta }$, and below $\theta \lesssim 0.6^{\circ }$ for all $\mathit{\tau }$ considered. The largest $U/W$ ratios are expected at the magic angle $\sim 1.5^{\circ }$ when $\left|\mathrm{\Delta }\right|\sim 0$ meV for AA, and slightly below near $\sim 1.4^{\circ }$ for finite $\left|\mathrm{\Delta }\right|\sim 25$ meV for AB stackings, and near $\theta \sim 0.4^{\circ }$ for both stackings. When $\mathit{\tau }$ is the saddle point stacking vector between AB and BA we observe pronounced anisotropic local density of states (LDOS) strip patterns with broken triangular rotational symmetry. We present optical conductivity calculations that reflect the changes in the electronic structure introduced by the stacking and gate tunable system parameters.

Using Ginzburg-Landau formalism, we theoretically study the isothermal shear modulus anomaly of an unconventional superconductor with a two-component order parameter, where the superconducting transition is split by a symmetry-breaking field. Experimental signatures are proposed for both chiral and nematic superconductors. Particularly striking is the vanishing of $C_{66}$ across the lower transition to a nematic superconducting state. Our findings can guide future experiments and shed new light on materials such as ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ and $M_x{\mathrm{Bi}}_2{\mathrm{Se}}_3$.

Polarization rotation is considered one of the most important intrinsic origins of high piezoelectricity in perovskite ferroelectrics. Such rotation strongly increases the lateral polarization, thus giving rise to the ultrahigh-shear piezoelectric coefficient. The evolution of polarization vector as a function of shear strain in a lead-free perovskite ferroelectric, ${\mathrm{K}}_{0.5}{\mathrm{Na}}_{0.5}{\mathrm{NbO}}_3$ (KNN), is demonstrated using the first-principle calculations. The polarization rotation is found to correlate directly with the shear piezoelectric constant. The larger the rotation angle, the higher the shear piezoelectric constant. The atomic insight reveals the polarization rotation mainly relies on the reorientation of Nb and O atoms along low potential-energy surfaces. We propose the polarization vector is more likely to rotate along the ${\left[111\right]}_{\mathrm{PC}}\to {\left[101\right]}_{\mathrm{PC}}\to {\left[001\right]}_{\mathrm{PC}}$ path instead of direct ${\left[111\right]}_{\mathrm{PC}}\to {\left[001\right]}_{\mathrm{PC}}$ transition in high-performance lead-free piezoelectric materials with broad thermotropic phase boundaries; thus, the critical role of orthorhombic structure should be emphasized. We hope that this work will inspire future experimental studies for further developing lead-free piezoelectric materials.

Two-dimensional (2D) layered magnetic materials are generating a great amount of interest for the next generation of electronic devices thanks to their remarkable properties associated with spin dynamics. The recently discovered layered $\mathrm{V}{\mathrm{I}}_3$ ferromagnetic phase belongs to this family, although a full understanding of its properties is limited by the incomplete understanding of its crystallographic structure. The motivation of this work is to address this issue. Here, we investigate the $\mathrm{V}{\mathrm{I}}_3$ crystal structures at low temperature using both synchrotron x-ray and neutron powder diffraction and provide structural models for the two structural transitions occurring at 76 and 32 K. Moreover, we confirm by magnetic measurements that $\mathrm{V}{\mathrm{I}}_3$ becomes ferromagnetic at 50 K and we question the establishment of a long-range magnetic structure by neutron diffraction. We equally determined the magnetic properties of our recently reported $\mathrm{LiV}{\mathrm{I}}_3$ phase, which is like the well-known $\mathrm{Cr}{\mathrm{I}}_3$ ferromagnetic phase in terms of electronic and crystallographic structures and found an antiferromagnetic behavior with a Néel temperature of 12 K. Such a finding provides extra clues for a better understanding of magnetism in these low-dimension compounds. Finally, the easiness of preparing Li-based 2D magnetic materials by chemical/electrochemical means opens wide the opportunity to design materials with exotic properties.

Constraining the melting behavior of magnesium oxide, a major constituent of gaseous and rocky planets, is key to benchmarking their evolutionary models. Using a double-shock technique, we extended the MgO melt curve measurements to 2 TPa; this is twice the pressure achieved by previous melting experiments on any material. A temperature plateau is observed between 1218 and 1950 GPa in the second-shock states, which is attributed to latent heat of melting. At 1950 GPa, the measured melting temperature is 17 600 K, which is 17% lower than recent theoretical predictions. The melting curve is steeper than that of ${\mathrm{MgSiO}}_3$, indicating that MgO is likely solid in the interior of Saturn-sized gas giants and extra-solar super-Earth planets.

A number of recent experiments indicate that the iron-chalcogenide FeSe provides the long-sought possibility to study bulk superconductivity in the cross-over regime between the weakly coupled Bardeen-Cooper-Schrieffer (BCS) pairing and the strongly coupled Bose-Einstein condensation (BEC). We report on ${}^{77}\mathrm{Se}$ nuclear magnetic resonance experiments of FeSe, focused on the superconducting phase for strong magnetic fields applied along the $c$ axis, where a distinct state with large spin polarization was reported. We determine this high-field state as bulk superconducting with high spatial homogeneity of the low-energy spin fluctuations. Further, we find that the static spin susceptibility becomes unusually small at temperatures approaching the superconducting state, despite the presence of pronounced spin fluctuations. Taken together, our results clearly indicate that FeSe indeed features an unusual field-induced superconducting state of a highly spin-polarized Fermi liquid in the BCS-BEC crossover regime.

We analyze the decay of spin waves into Stoner excitations in magnetic Weyl semimetals. The lifetime of a mode is found to have a universal dependence on its frequency and momentum, and on a few parameters that characterize the relativistic Weyl spectrum. At the same time, uniform Gilbert damping by Weyl electrons is absent. The decay rate of spin waves is calculated perturbatively using the s-d model of itinerant Weyl or Dirac electrons coupled to local moments. We show that many details of the Weyl spectrum, such as the momentum-space locations, dispersions, and sizes of the Weyl Fermi pockets, can be deduced indirectly by probing the spin waves of local moments using inelastic neutron scattering.

In bilayers consisting of a normal metal (N) with spin-orbit coupling and a ferromagnet (F), the combination of the spin-Hall effect, the spin-transfer torque, and the inverse spin-Hall effect gives a small correction to the in-plane conductivity of N, which is referred to as spin-Hall magnetoresistance (SMR). We here present a theory of the SMR and the associated off-diagonal conductivity corrections for frequencies up to the terahertz regime. We show that the SMR signal has pronounced singularities at the spin-wave frequencies of F, which identifies it as a potential tool for all-electric spectroscopy of magnon modes. A systematic change of the magnitude of the SMR at lower frequencies is associated with the onset of a longitudinal magnonic contribution to spin transport across the F-N interface.

A Kohn-Sham density functional approach has recently been developed for the fractional quantum Hall effect, which maps the strongly interacting electrons into a system of weakly interacting composite fermions subject to an exchange-correlation potential as well as a density-dependent gauge field that mimics the “flux quanta” bound to composite fermions. To get a feel for the role of various terms, we study the behavior of the self-consistent solution as a function of the strength of the exchange-correlation potential, which is varied through an ad hoc multiplicative factor. We find that a crystal phase is stabilized when the exchange-correlation interaction is sufficiently strong relative to the composite-fermion cyclotron energy. Various properties of this crystal are examined.

Rare-earth nickelates $R^{3+}{\mathrm{Ni}}^{3+}{\mathrm{O}}_3$ ($R=\mathrm{Lu}\text{−}\mathrm{Pr}$, Y) show a striking metal-insulator transition in their bulk phase whose temperature can be tuned by the rare-earth radius. These compounds are also the parent phases of the newly identified infinite layer $R\mathrm{Ni}{\mathrm{O}}_2$ superconductors. Although intensive theoretical works have been devoted to understand the origin of the metal-insulator transition in the bulk, there have only been a few studies on the role of hole and electron doping by rare-earth substitutions in $R\mathrm{Ni}{\mathrm{O}}_3$ materials. Using first-principles calculations based on density functional theory (DFT) we study the effect of hole and electron doping in a prototypical nickelate $\mathrm{SmNi}{\mathrm{O}}_3$. We perform calculations without Hubbard-like $U$ potential on Ni $3d$ levels but with a meta–generalized gradient approximation better amending self-interaction errors. We find that at low doping, polarons form with intermediate localized states in the band gap resulting in a semiconducting behavior. At larger doping, the intermediate states spread more and more in the band gap until they merge either with the valence (hole doping) or the conduction (electron doping) band, ultimately resulting in a metallic state at 25% of $R$ cation substitution. These results are reminiscent of experimental data available in the literature and demonstrate that DFT simulations without any empirical parameter are qualified for studying doping effects in correlated oxides and exploring the mechanisms underlying the superconducting phase of rare-earth nickelates.

The ultrafast hole dynamics triggered by the photoexcitation of molecular targets is a highly correlated process even for those systems, such as organic molecules, having a weakly correlated ground state. We provide a unifying framework and a numerically efficient matrix formulation of state-of-the-art nonequilibrium Green's function (NEGF) methods such as second-Born as well as $GW$ and $T$-matrix without and with exchange diagrams. Numerical simulations are presented for a paradigmatic, exactly solvable molecular system, and the shortcomings of the established NEGF methods are highlighted. We then develop a NEGF scheme based on the Faddeev treatment of three-particle correlations; the exceptional improvement over established methods is explained and demonstrated. The Faddeev NEGF scheme scales linearly with the maximum propagation time, thereby opening up prospects for femtosecond simulations of large molecules.

Black phosphorus is a quasi-two-dimensional layered semiconductor with a narrow direct band gap of 0.3 eV. A giant surface Stark effect can be produced by the potassium doping of black phosphorus, leading to a semiconductor to semimetal phase transition originating from the creation of a strong surface dipole and associated band bending. By using time- and angle-resolved photoemission spectroscopy, we report the partial photoinduced screening of this band bending by the creation of a compensating surface photovoltage. We further resolve the detailed dynamics of this effect at the pertinent timescales and the related evolution of the band structure near the Fermi level. We demonstrate that after a fast rise time, the surface photovoltage exhibits a plateau over a few tens of picoseconds before decaying on the nanosecond timescale. We support our experimental results with simulations based on drift-diffusion equations.

The states of two electrons in tunnel-coupled semiconductor quantum dots can be effectively described in terms of a two-spin Hamiltonian with an isotropic Heisenberg interaction. A similar description needs to be generalized in the case of holes due to their multiband character and spin-orbit coupling, which mixes orbital and spin degrees of freedom and splits $j=3/2$ and $j=1/2$ multiplets. Here we investigate two-hole states in prototypical coupled Si and Ge quantum dots via different theoretical approaches. Multiband $\mathit{k}·\mathit{p}$ and configuration-interaction calculations are combined with entanglement measures in order to thoroughly characterize the two-hole states in terms of band mixing and justify the introduction of an effective spin representation, which we analytically derive a from generalized Hubbard model. We find that, in the weak interdot regime, the ground state and first excited multiplet of the two-hole system display—unlike their electronic counterparts—a high degree of $J$ mixing, even in the limit of purely heavy-hole states. The light-hole component additionally induces $M$ mixing and a weak coupling between spinors characterized by different permutational symmetries.

Spectroscopic interrogation of materials in the midinfrared with nanometer spatial resolution is inherently difficult due to the long wavelengths involved, reduced detector efficiencies, and limited availability of spectrally bright, coherent light sources. Technological advances are driving techniques that overcome these challenges, enabling material characterization in this relatively unexplored spectral regime. Synchrotron infrared nanospectroscopy (SINS) is an imaging technique that provides local sample information of nanoscale target specimens in an experimental energy window between 330 and 5000 ${\mathrm{cm}}^{-1}$. Using SINS, we analyzed a series of individual gold nanorods patterned on a ${\mathrm{SiO}}_2$ substrate and on a flake of hexagonal boron nitride. The SINS spectra reveal interactions between the nanorod photonic Fabry-Pérot resonances and the surface phonon polaritons of each substrate, which are characterized as avoided crossings. A coupled oscillator model of the hybrid system provides a deeper understanding of the coupling and provides a theoretical framework for future exploration.

We demonstrate the occurrence of nodal non-Hermitian (NH) phases featuring exceptional degeneracies in chiral-symmetric disordered quantum wires, where NH physics naturally arises from the self-energy in a disorder-averaged Green's function description. Notably, we find that at least two nodal points in the clean Hermitian system are required for exceptional points to be effectively stabilized upon adding disorder. We identify and study experimental signatures of our theoretical findings both in the spectral functions and in mesoscopic quantum transport properties of the considered systems. Our results are quantitatively corroborated and exemplified by numerically exact simulations on a microscopic lattice model. The proposed setting provides a conceptually minimal framework for the realization and study of topological NH phases in quantum many-body systems.

Optically inactive or dark excitons play an important role in exciton and polariton devices. On one hand, they supply excitons to the light cone and feed the photoluminescence signal. On the other hand, they repel radiatively active excitons due to the exchange interaction and contribute to the formation of lateral potentials for exciton and polariton condensates. On top of this, they play an important role in scattering and energy relaxation dynamics of quasiparticles in semiconductors. So far, because of optical inaccessibility, studies were focused typically on one experimental technique, giving information about one quantity of dark excitons. Here we present a comprehensive study of the dark exciton reservoir in a high-quality 14-nm GaAs/AlGaAs quantum well using several experimental techniques. We develop a new method of nonradiative broadening spectroscopy of exciton resonances and combine it with nondegenerate pump-probe spectroscopy. The exciton and carrier dynamics in the reservoir is monitored via dynamic broadening of exciton resonances induced by exciton-exciton and exciton-carrier scattering. The dynamics is found to be strongly dependent on the optical excitation conditions. Based on the experimental results, we develop a model of dynamics in a reservoir of excitons and free carriers. The model allows us to describe the experimentally measured photoluminescence kinetics with no fitting parameters. We also demonstrate the optical control of the dark exciton density by means of an additional excitation that creates imbalance of free carriers depleting the reservoir. These results shed light onto the dynamics of the excitonic “dark matter” and pave the way to the high-precision engineering of optically induced potentials in exciton-polariton and integrated photonic devices. We expect that the observed results can be transferred also to other semiconductors so that the current quantum well serves as a high-quality model system.