Mott Metal-Insulator Transitions in Pressurized Layered Trichalcogenides

Re-emerging magnetic order in correlated van der Waals antiferromagnet NiPS3

Van der Waals (vdW) gap is a significant feature that distinguishes vdW magnets from traditional magnets. Manipulating the magnetic properties by changing the vdW gap has been hot topic in condensed matter research. Here we report a re-emerging magnetic order induced by pressure in a correlated vdW antiferromagnetic insulator NiPS3. It is found that the interlayer magnetoresistance (MR) nearly vanishes at the critical pressure where the crystal structure transforms fromC2/mphase to the slidingC2/mphase. On further compression within the slidingC2/mphase, a substantially enhanced MR emerges from low temperature associated with an insulator-to-metal transition, indicating a metallic antiferromagnetic phase. The enhanced re-emerging MR in slidingC2/mphase can be ascribed to the increasing magnetic interaction between neighboring layers due to the vdW gap narrowing. Our results provide important experimental clues for understanding the pressure effects on magnetism in correlated layered materials.

First principles investigation of screened Coulomb interaction and electronic structure of low-temperature phase TaS2

By means of ab initio computation schemes, we examine the electronic screening, Coulomb interaction strength, and the electronic structure of a quantum spin liquid candidate monolayer TaS2 in its low-temperature commensurate charge-density-wave phase. Not only local (U) but non-local (V) correlations are estimated within random phase approximation based on two different screening models. Using GW + EDMFT (GW plus extended dynamical mean-field theory) method, we investigate the detailed electronic structure by increasing the level of non-local approximation from DMFT (V=0) to EDMFT and GW + EDMFT.

Two-Dimensional Ultrahigh Unconventional Piezoelectricity Driven by Charge Screening

In the past decade, piezoelectricity has been explored in a series of two-dimensional (2D) materials for nanoelectromechanical applications, while their piezoelectric coefficients are mostly much lower than those of prevalent piezoceramics. In this paper, we propose an unconventional approach of inducing 2D ultrahigh piezoelectricity dominated by charge screening instead of lattice distortion and show the first-principles evidence of such piezoelectricity in a series of 2D van der Waals bilayers, where the bandgap can be remarkably tuned via applying a moderate vertical pressure. Their polarizations can switch between the screened and unscreened state by a pressure-driven metal-insulator transition, which can be realized via tuning interlayer hybridization or inhomogeneous electrostatic potential by substrate layer to change the band splitting or tuning the relative energy shift between bands utilizing the vertical polarization of the substrate layer. Such 2D piezoelectric coefficients can be unprecedented and orders of magnitude higher than those of previously studied monolayer piezoelectrics, and their high efficiency of energy harvesting in nanogenerators can be expected.

Uniaxial compressions induced complementarity and anisotropic behaviors in CuVP2S6

Uniaxial compressions in layered materials can change their electronic structures and properties. In this work, a bimetallic compound CuVP₂S₆is simulated by using Density Functional Theory (DFT) in the presence of uniaxial compressions. Our results clearly show vertical compressions could lead to anisotropic behaviors, which include the compression effect caused by interlayer compression and the anisotropy of intralayer stretching. The vertical compressions change the V-S bonds and the P-S bonds respectively in AA and AB structures. The complementarity between intralayer stretching and interlayer compression could also result in adjustable bandgaps and degeneracy breakdown of V atoms. Results from the electron localization function analysis demonstrate that the free electrons of AA and AB structures tend to delocalize, and ionic features in V-S bonds could be weakened with increasing vertical compressions. Moreover, the two internal binding energies of AA and AB structures and the charge density difference analysis show that the anisotropy in the intralayer stretch and the charge transfer between metal atoms and S atoms increases gradually.

Limited Ferromagnetic Interactions in Monolayers of MPS3 (M = Mn and Ni)

We present a systematic study of the electronic and magnetic properties of two-dimensional ordered alloys, consisting of two representative hosts (MnPS₃ and NiPS₃) of transition metal phosphorus trichalcogenides doped with 3d elements. For both hosts, our DFT + U calculations are able to qualitatively reproduce the ratios and signs of all experimentally observed magnetic couplings. The relative strength of all antiferromagnetic exchange couplings, both in MnPS₃ and in NiPS₃, can successfully be explained using an effective direct exchange model: it reveals that the third-neighbor exchange dominates in NiPS₃ due to the filling of the t_2g subshell, whereas for MnPS₃, the first-neighbor exchange prevails, owing to the presence of the t_2g magnetism. On the other hand, the nearest neighbor ferromagnetic coupling in NiPS₃ can only be explained using a more complex superexchange model and is (also) largely triggered by the absence of the t_2g magnetism. For the doped systems, the DFT + U calculations revealed that magnetic impurities do not affect the magnetic ordering observed in the pure phases, and thus, in general in these systems, ferromagnetism may not be easily induced by such a kind of elemental doping. However, unlike for the hosts, the first and second (dopant-host) exchange couplings are of similar order of magnitude. This leads to frustration in the case of antiferromagnetic coupling and may be one of the reasons of the observed lower magnetic ordering temperature of the doped systems.

Two-dimensional magnetic atomic crystals

Two-dimensional (2D) magnetic crystals show many fascinating physical properties and have potential device applications in many fields. In this paper, the preparation, physical properties and device applications of 2D magnetic atomic crystals are reviewed. First, three preparation methods are presented, including chemical vapor deposition (CVD) molecular beam epitaxy (MBE) and single-crystal exfoliation. Second, physical properties of 2D magnetic atomic crystals, including ferromagnetism, antiferromagnetism, magnetic regulation and anomalous Hall effect are presented. Third, the application of 2D magnetic atomic crystals in heterojunctions reluctance and other aspects are briefly introduced. Finally, the future development direction and possible challenges of 2D magnetic atomic crystals are briefly addressed.

Nearly Room-Temperature Ferromagnetism in a Pressure-Induced Correlated Metallic State of the van der Waals Insulator CrGeTe_{3}

A complex interplay of different energy scales involving Coulomb repulsion, spin-orbit coupling, and Hund's coupling energy in 2D van der Waals (vdW) material produces a novel emerging physical state. For instance, ferromagnetism in vdW charge transfer insulator CrGeTe_{3} provides a promising platform to simultaneously manipulate the magnetic and electrical properties for potential device implementation using few nanometers thick materials. Here, we show a continuous tuning of magnetic and electrical properties of a CrGeTe_{3} single crystal using pressure. With application of pressure, CrGeTe_{3} transforms from a ferromagnetic insulator with Curie temperature T_{C}∼66  K at ambient condition to a correlated 2D Fermi metal with T_{C} exceeding ∼250  K. Notably, absence of an accompanying structural distortion across the insulator-metal transition (IMT) suggests that the pressure induced modification of electronic ground states is driven by electronic correlation furnishing a rare example of bandwidth-controlled IMT in a vdW material.

Effects of pressure and strain on physical properties of VI3

The van der Waals ferromagnetic material VI₃is a magnetic Mott insulator. In this work, we investigate the effects of isotropic and anisotropic pressure on the atomic structure and the electronic structure of VI₃using the first-principles method. The in-plane strain induces structural distortion and breaks the three-fold rotational symmetry of the lattice. Both the in-plane and out-of-plane strain widen the conduction and the valence bands, reduce the energy band gap and drive VI₃from a semiconductor to a three-dimensional metal. The structural distortion is not the cause of insulator-to-metal transition. Calculations of the magnetocrystalline anisotropy energy indicate an easy-axis to easy-plane transition when the pressure is higher than 2 GPa. The ferromagnetic Curie temperature falls from 63 K at 0 GPa to 25 K at 6 GPa.

Phase Transitions and Water Splitting Applications of 2D Transition Metal Dichalcogenides and Metal Phosphorous Trichalcogenides

2D layered materials turn out to be the most attractive hotspot in materials for their unique physical and chemical properties. A special class of 2D layered material refers to materials exhibiting phase transition based on environment variables. Among these materials, transition metal dichalcogenides (TMDs) act as a promising alternative for their unique combination of atomic-scale thickness, direct bandgap, significant spin-orbit coupling and prominent electronic and mechanical properties, enabling them to be applied for fundamental studies as catalyst materials. Metal phosphorous trichalcogenides (MPTs), as another potential catalytic 2D phase transition material, have been employed for their unusual intercalation behavior and electrochemical properties, which act as a secondary electrode in lithium batteries. The preparation of 2D TMD and MPT materials has been extensively conducted by engineering their intrinsic structures at the atomic scale. In this study, advanced synthesis methods of preparing 2D TMD and MPT materials are tested, and their properties are investigated, with stress placed on their phase transition. The surge of this type of report is associated with water-splitting catalysis and other catalytic purposes. This study aims to be a guideline to explore the mentioned 2D TMD and MPT materials for their catalytic applications.

Spin-crossover induced ferromagnetism and layer stacking-order change in pressurized 2D antiferromagnet MnPS3

Spin-crossover combined with metal-insulator transition and superconductivity has been found in 2D transition-metal phosphorous trichalcogenides when tuning them by high pressure. Simulation of such intriguing spin-crossover behaviors is crucial to understanding the mechanism. The Hubbard U correction is widely used to describe the strong on-site Coulomb interaction in the d electrons of transition-metal compounds, while the U values are sensitive to the crystal field and spin state varying greatly with pressure. In this work, we show that taking MnPS3 as an example and based on a uniform parameter set, the hybrid functional calculations give a spin-crossover pressure of 35 GPa consistent with experimental observation (30 GPa), which is less than half of the existing reported value (63 GPa) using the Hubbard U correction. Notably, we find a spin-crossover induced transition from an antiferromagnetic semiconductor with monoclinic stacking-order to a ferromagnetic semiconductor with rhombohedral stacking-order, and the ferromagnetism originates from the partially occupied t2g orbitals. Different from previous understanding, the Mott metal-insulator transition of MnPS3 does not occur simultaneously with the spin-crossover but in a pressurized low-spin phase.

P2S5 Reactive Flux Method for the Rapid Synthesis of Mono- and Bimetallic 2D Thiophosphates M2-xM'xP2S6

We report a reactive flux technique using the common reagent P₂S₅ and metal precursors developed to circumvent the synthetic bottleneck for producing high-quality single- and mixed-metal two-dimensional (2D) thiophosphate materials. For the monometallic compound, M₂P₂S₆ (M = Ni, Fe, and Mn), phase-pure materials were quickly synthesized and annealed at 650 °C for 1 h. Crystals of dimensions of several millimeters were grown for some of the metal thiophosphates using optimized heating profiles. The homogeneity of the bimetallic thiophosphates MM'P₂S₆ (M, M' = Ni, Fe, and Mn) was elucidated using energy-dispersive X-ray spectroscopy and Rietveld refinement. The quality of the selected materials was characterized by transmission electron microscopy and atomic force microscopy measurements. We report two novel bimetallic thiophosphates, MnCoP₂S₆ and FeCoP₂S₆. The Ni₂P₂S₆ and MnNiP₂S₆ flux reactions were monitored in situ using variable-temperature powder X-ray diffraction to understand the formation reaction pathways. The phases were directly formed in a single step at approximately 375 °C. The work functions of the semiconducting materials were determined and ranged from 5.28 to 5.72 eV.