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Van der Waals (vdW) heterostructures composed of different two-dimensional (2D) materials offer an easily accessible way to combine properties of individual materials for applications. Owing to the discovery of a set of unanticipated physical phenomena, the twisted 2D vdW heterostructures have gained considerable attention recently. Here, we report enhanced valley splitting in twisted 2D vdW WSe2/CrI3 heterostructures. In particular, the splitting can be 1200% (or 5.18 meV) of the value for a non-twisted heterostructure. According to the k·p model, this value is equivalent to a ~20 T external magnetic field applied perpendicular to the 2D sheet. The thermodynamic stability of 2D vdW WSe2/CrI3 heterostructures, on the other hand, depends linearly on the interlayer twisting angle.

A real-space method using generating integers is used to determine possible commensurate lattice Moiré patterns for a bilayer of two equal hexagonal lattices, which can in principle be extended to lattice mismatched bilayers. These Moiré patterns can be classified by a pair of relatively prime integers (n , m) , wherein a rotation θ (n , m) of the top hexagonal lattice maps its lattice vector (n , m) to (m , n) of the bottom lattice. Within this formulation, the area of the commensurate supercell is proportional to (n 2 + m 2 + n m) and the number of coincident lattice sites per supercell is given by (n − m) 2. Taking bilayer boron nitride (BN) as an example, we present how to systematically generate Moiré patterns and explore the differences in local chemistry in the interstitial region by impurity intercalation. Systematic calculations of the properties of intercalated 3d transition metals were performed in an h-BN (4 , 3) bilayer, corresponding to a rotation of 9.43 degrees. These calculations reveal that local symmetry in the intercalated region significantly affect the energetics and magnetization of the intercalated species. These results highlight that Moiré pattern physics is not limited to optoelectronic/electronic phenomena, such as interfacial exciton formation or magic angle superconductivity, but it also produces chemical and magnetic atomic site selectivity, which may play important roles in adsorption, catalysis, or quantum information. [ABSTRACT FROM AUTHOR]

Despite the explosion of interest in topological materials over the last decades, their applications remain limited due to challenges in growth and incorporation with today's microelectronics. As a potential bridge to close this gap, we investigate the group-IV alloy Ge1–xPbx, in the Ge-rich condition using density functional theory and show that relatively low concentrations of Pb (~9.4%) can lead to a topological phase transition. Furthermore, the calculation of the Z2 invariant for both the random alloy and the alloy with short-range order (SRO) indicate that the topological phase of the material can be directly modified by the degree of SRO. These findings are understood in terms of local structural relaxation, which decreases the bandgap in the random alloy. However, in the SRO case, the mutual avoidance of Pb leads to minimal structural relaxation, alleviating strain. Our findings not only highlight the emerging importance of SRO in alloy properties but also indicate the possibility of constructing topological interfaces between materials of identical composition (and nominally identical structure). Moreover, they uncover a viable avenue toward the monolithic integration of quantum materials with today's semiconductor industry. [ABSTRACT FROM AUTHOR]

We investigate a strain-induced topological phase transition in the ferromagnetic Janus monolayer MnSbBiS2Te2 using first-principles calculations. The electronic, magnetic, and topological properties are studied under biaxial strain within the range of −8 to +8%. The ground state of monolayer MnSbBiS2Te2 is metallic with an out-of-plane magnetic easy axis. A band gap is opened when a compressive strain between −4% and −7% is applied. We observe a topological phase transition at a biaxial strain of −5%, where the material becomes a Chern insulator exhibiting a quantum anomalous hall (QAH) effect. We find that biaxial strain and spin–orbit coupling (SOC) are responsible for the topological phase transition in MnSbBiS2Te2. In addition, we find that biaxial strain can alter the direction of the magnetic easy axis of MnSbBiS2Te2. The Curie temperature is calculated using the Heisenberg model and is found to be 24 K. This study could pave the way to the design of topological materials with potential applications in spintronics, quantum computing, and dissipationless electronics. [ABSTRACT FROM AUTHOR]

Chalcogenide perovskites have emerged as promising semiconductor materials due to their appealing properties, including tunable bandgaps, high absorption coefficients, reasonable carrier lifetimes and mobilities, excellent chemical stability, and environmentally benign nature. However, beyond the well‐studied BaZrS3, reports on chalcogenide perovskite thin films with diverse compositions are scarce. In this study, the realization of four different types of chalcogenide perovskite thin films with controlled phases, through CS2 annealing of amorphous chalcogenide precursor films deposited by pulsed laser deposition (PLD), is reported. This achievement is guided by a thorough theoretical investigation of the phase stability of chalcogenide perovskites. Upon crystallization in the distorted perovskite phase, all materials exhibit photoluminescence (PL) with peak positions in the visible range, consistent with their expected bandgap values. However, the full‐width‐at‐half‐maximum (FWHM) of the PL spectra varies significantly across these materials, ranging from 99 meV for SrHfS3 to 231 meV for BaHfS3. The difference is attributed to the difference in kinetic barriers between local structural motifs for the Sr and Ba compounds. The findings underscore the promise of chalcogenide perovskite thin films as an alternative to traditional halide perovskites for optoelectronic applications, while highlighting the challenges in optimizing their synthesis and performance. [ABSTRACT FROM AUTHOR]

Ferroelectric memory is a promising candidate for next‐generation nonvolatile memory owing to its outstanding performance such as low power consumption, fast speed, and high endurance. However, the ferroelectricity of conventional ferroelectric materials will be eliminated by the depolarization field when the size drops to the nanometer scale. As a result, the miniaturization of ferroelectric devices was hindered, which makes ferroelectric memory unable to keep up with the development of integrated‐circuit (IC) miniaturization. Recently, a two‐dimensional (2D) In2Se3 was reported to maintain stable ferroelectricity at the ultrathin scale, which is expected to break through the bottleneck of miniaturization. Soon, devices based on 2D In2Se3, including the ferroelectric field‐effect transistor, ferroelectric channel transistor, synaptic ferroelectric semiconductor junction, and ferroelectric memristor were demonstrated. However, a comprehensive understanding of the structures and the ferroelectric‐switching mechanism of 2D In2Se3 is still lacking. Here, the atomic structures of different phases, the dynamic mechanism of ferroelectric switching, and the performance/functions of the latest devices of 2D In2Se3 are reviewed. Furthermore, the correlations among the structures, the properties, and the device performance are analyzed. Finally, several crucial problems or challenges and possible research directions are put forward. We hope that this review paper can provide timely knowledge and help for the research community to develop 2D In2Se3 based ferroelectric memory and computing technology for practical industrial applications. [ABSTRACT FROM AUTHOR]

Lead halide perovskites have emerged as promising photovoltaic (PV) materials owing to their superior optoelectronic properties. However, they suffer from poor stability and potential toxicity. Here, computational screening with experimental synthesis is combined to explore stable, lead‐free, and defect‐tolerant PV materials. Heavy cations with lone‐pair electrons and mixed anions of chalcogens and halogens as a descriptor for simultaneous realization of defect tolerance and high stability are adopted. Together with the criteria of possessing direct band gap and optimal gap value, the inorganic material database is screened and CuBiSCl2 in the post‐perovskite structure is identified with an ideal band gap of 1.37 eV. The electronic structure and defect calculations suggest its defect‐tolerant characteristics. By optical absorption measurement, its band gap is confirmed to be ≈1.44 eV, with strong absorption near the band edge. The material is stable against thermal decomposition up to 300 °C and can survive from 25 days of storage at ambient conditions with 60% relative humidity. Prototype solar cells are fabricated and demonstrate an open circuit voltage of 1.09 V and a power conversion efficiency of 1.00%. With the excellent properties above, CuBiSCl2 is proposed to be a promising candidate for PV application. [ABSTRACT FROM AUTHOR]

High-temperature phases of solids are often dynamically stable only. First-principles study of point defects in such solids at 0 K is prohibited by their static instability, which results in random structures of the defect-containing supercell so that the total energy of the supercell is randomly affected by structural distortions far away from the defect. Taking cubic perovskite α -CsPbI3 as an example, we first present the problem incurred by the static instability and then propose an approach based on molecular dynamics to carry out ensemble average for tackling the problem. Within affordable simulation time, we obtain converged defect ionization energies, which are unattainable by a standard approach and allow us to evaluate its defect tolerance property. Our work paves the way for studying defects in statically unstable solids. [ABSTRACT FROM AUTHOR]

Van der Waals (vdW) heterostructures composed of different two-dimensional (2D) materials offer an easily accessible way to combine properties of individual materials for applications. Owing to the discovery of a set of unanticipated physical phenomena, the twisted 2D vdW heterostructures have gained considerable attention recently. Here, we report enhanced valley splitting in twisted 2D vdW WSe2/CrI3 heterostructures. In particular, the splitting can be 1200% (or 5.18 meV) of the value for a non-twisted heterostructure. According to the k·p model, this value is equivalent to a ~20 T external magnetic field applied perpendicular to the 2D sheet. The thermodynamic stability of 2D vdW WSe2/CrI3 heterostructures, on the other hand, depends linearly on the interlayer twisting angle. [ABSTRACT FROM AUTHOR]

Lead halide perovskites have been intensively developed to be high-performance photovoltaic and optoelectronic materials, where the unique defect tolerance properties are believed to contribute to their excellence. However, defect tolerance, especially for the donors, has not been conclusively understood yet. Using hybrid functional calculations including spin–orbit coupling (SOC), we study the intrinsic defects of γ-CsPbI3. With a sufficiently large supercell and full structural relaxation, we show that most of the donor levels are deep in hybrid functional calculations, but become shallow due to the strong SOC effect, which reconstructs the potential energy surfaces of the donors by eliminating the large structural distortions. By quantifying the variation of electronic and elastic energies associated with defects, we show how the SOC effect changes the energy balance and reconstructs the defect structures. We reveal the decisive role of degeneracy of electronic states in the SOC effect, which significantly downshifts the conduction band minimum due to the quasi-three-fold degeneracy of Pb 6p orbitals, but has little effect on the non-degenerate defect states. [ABSTRACT FROM AUTHOR]

Disorder‐induced electronic localization is responsible for the OFF state of the Ovonic threshold switching (OTS) device, which is an indispensable component in the present 3D‐crossbar‐architecture phase‐change memory circuit. However, the atomic mechanism of the OTS device, especially the role of thermal effect, remains a long‐term open question. Recent researches suggest that the working current of the OTS ON‐state is often large enough to melt the adjacent phase‐change memory material in the OTS+PCM devices. Thus, Joule heating‐induced atomic/electronic structure transition in OTS materials must be seriously considered. Taking the typical OTS material GeSe as an example, first‐principles calculations reveal an unexpected electronic delocalization induced by the enhanced structural disorder upon solid‐liquid transition. Meanwhile, as the temperature rises, the band gap decreases or even closes, leading to an increase in carrier concentration. Therefore, the melting filament with high conductivity could be responsible for the holding‐ON state of OTS materials. The results in this study may provide possible explanations for some puzzles of OTS devices, such as the reasons for holding‐ON state and limited endurance. [ABSTRACT FROM AUTHOR]

High-temperature phases of solids are often dynamically stable only. First-principles study of point defects in such solids at 0 K is prohibited by their static instability, which results in random structures of the defect-containing supercell so that the total energy of the supercell is randomly affected by structural distortions far away from the defect. Taking cubic perovskite α -CsPbI3 as an example, we first present the problem incurred by the static instability and then propose an approach based on molecular dynamics to carry out ensemble average for tackling the problem. Within affordable simulation time, we obtain converged defect ionization energies, which are unattainable by a standard approach and allow us to evaluate its defect tolerance property. Our work paves the way for studying defects in statically unstable solids. [ABSTRACT FROM AUTHOR]