Ferromagnetic ordering of monolayer vanadium dichalcogenides (VSe 2 and VS 2 ) has been predicted by density functional theory (DFT), and suggestive experimental evidence for magnetic ordering in VSe 2 monolayers has been reported. However, such ferromagnetic ordering would be in stark contradiction to the known paramagnetic nature of the bulk VSe 2 . Herein, we investigate the electronic structure of VSe 2 monolayers by angle-resolved photoemission spectroscopy (ARPES) and first-principles DFT. The ARPES measurements demonstrate the absence of spin-polarized bands for monolayers in close correspondence to nonmagnetic nature of the bulk VSe 2 . We demonstrate that the stabilization of the nonmagnetic state occurs due to the appearance of a charge density wave (CDW) state in VSe 2 monolayers. In contrast to well-established 4 × 4 × 3 periodicity of the CDW in bulk VSe 2 , we identify a √3 × √7 unit cell for VSe 2 monolayers from both scanning tunneling microscopy imaging and first-principles calculations. Moreover, DFT predicts that the √3 × √7 charge order state is energetically competitive with a ferromagnetic 1 × 1 state. This suggests that the experimentally observed CDW state is the nonmagnetic ground state of a perfect VSe 2 monolayer, consistent with the absence of spin-polarized bands in ARPES measurements. Therefore, monolayer VSe 2 is not an itinerant magnet.

Interlayer interactions in layered transition metal dichalcogenides are known to be important for describing their electronic properties. Here, we demonstrate that the absence of interlayer coupling in monolayer VTe2 also causes their structural modification from a distorted 1T′ structure in bulk and multilayer samples to a hexagonal 1T structure in the monolayer. X-ray photoemission spectroscopy indicates that this structural transition is associated with electron transfer from the vanadium d bands to the tellurium atoms for the monolayer. This charge transfer may reduce the in-plane d orbital hybridization and thus favor the undistorted 1T structure. Phonon-dispersion calculations show that, in contrast to the 1T′ structure, the 1T structure exhibits imaginary phonon modes that lead to a charge density wave (CDW) instability, which is also observed by low-temperature scanning tunneling microscopy as a 4 × 4 periodic lattice distortion. Thus, this work demonstrates a novel CDW material, whose properties are tuned by interlayer interactions.

Tin sulfides, SnxSy, are an important class of materials that are actively investigated as novel photovoltaic and water splitting materials. A first-principles evolutionary crystal structure search is performed with the goal of constructing the complete phase diagram of SnxSy and discovering new phases as well as new compounds of varying stoichiometry at ambient conditions and pressures up to 100 GPa. The ambient phase of SnS2 with P3¯m1 symmetry remains stable up to 28 GPa. Another ambient phase, SnS, experiences a series of phase transformations including α-SnS to β-SnS at 9 GPa, followed by β-SnS to γ-SnS at 40 GPa. γ-SnS is a new high-pressure metallic phase with Pm3¯m space group symmetry stable up to 100 GPa, which becomes a superconductor with a maximum Tc = 9.74 K at 40 GPa. Another new metallic compound, Sn3S4 with I4¯3d space group symmetry, is predicted to be stable at pressures above 15 GPa, which also becomes a superconductor with relatively high Tc = 21.9 K at 30 GPa.

SnxSey crystalline compounds consisting of Sn and Se atoms of varying composition are systematically investigated at pressures from 0 to 100 GPa using the first-principles evolutionary crystal structure search method based on density functional theory (DFT). All known experimental phases of SnSe and SnSe2 are found without any prior input. A second order polymorphic phase transition from SnSe-P nma phase to SnSe-Cmcm phase is predicted at 2.5 GPa. Initially being semiconducting, this phase becomes metallic at 7.3 GPa. Upon further increase of pressure up to 36.6 GPa, SnSe-Cmcm phase is transformed to CsCl-type SnSe-P m 3m phase, which remains stable at even higher pressures. A metallic compound with different stoichiometry, Sn3Se4-I 43d, is found to be thermodynamically stable from 18 GPa to 70 GPa. Known semiconductor tin diselenide SnSe2-P 3m1 phase is found to be thermodynamically stable from ambient pressure up to 18 GPa. Initially being semiconducting, it experiences metalization at pressures above 8 GPa.

Charge mobility of crystalline organic semiconductors (OSC) is limited by local dynamic disorder. Recently, the charge mobility for several high mobility OSCs, including TIPS-pentacene, were accurately predicted from a density functional theory (DFT) simulation constrained by the crystal structure and the inelastic neutron scattering spectrum, which provide direct measures of the structure and the dynamic disorder in the length scale and energy range of interest. However, the computational expense required for calculating all of the atomic and molecular forces is prohibitive. Here we demonstrate the use of density functional tight binding (DFTB), a semiempirical quantum mechanical method that is 2 to 3 orders of magnitude more efficient than DFT. We show that force matching a many-body interaction potential to DFT derived forces yields highly accurate DFTB models capable of reproducing the low-frequency intricacies of experimental inelastic neutron scattering (INS) spectra and accurately predicting charge mobility. We subsequently predicted charge mobilities from our DFTB model of a number of previously unstudied structural analogues to TIPS-pentacene using dynamic disorder from DFTB and transient localization theory. The approach we establish here could provide a truly rapid simulation pathway for accurate materials properties prediction, in our vision applied to new OSCs with tailored properties.

The electronic and magnetic properties of transition metal dichalcogenides are known to be extremely sensitive to their structure. In this paper we study the effect of structure on the electronic and magnetic properties of mono-and bilayer VSe 2 films grown using molecular beam epitaxy. VSe 2 has recently attracted much attention due to reports of emergent ferromagnetism in the two-dimensional (2D) limit. To understand this compound, high-quality 1T and distorted 1T films were grown at temperatures of 200°C and 450°C, respectively, and studied using 4 K scanning tunneling microscopy and spectroscopy. The measured density of states and the charge density wave (CDW) patterns were compared to band structure and phonon dispersion calculations. Films in the 1T phase reveal different CDW patterns in the first layer compared to the second. Interestingly, we find the second layer of the 1T film shows a CDW pattern with 4a × 4a periodicity which is the 2D version of the bulk CDW observed in this compound. Our phonon dispersion calculations confirm the presence of a soft phonon at the correct wave vector that leads to this CDW. In contrast, the first layer of distorted 1T phase films shows a strong stripe feature with varying periodicities, while the second layer displays no observable CDW pattern. Finally, we find that the monolayer 1T VSe 2 film is weakly ferromagnetic, with ∼3.5 μ B per unit similar to previous reports.

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