Search

Two-dimensional (2D) materials have attracted considerable interest due to their remarkable properties and potential applications for nanoelectronics, electrodes, energy storage devices, among others. However, many well-studied 2D materials lack appreciable conductivity and tunable mechanical strength, limiting their applications in flexible devices. Newly developed MXenes open up the opportunity to design novel flexible conductive electronic materials. Here, using density functional theory (DFT), we investigate systematically the effects of several functional groups on the stabilization, mechanical properties, and electronic structures of a representative MXene. It is found that oxygen possesses the largest adsorption energy as compared to other functional groups, indicating its good thermodynamic stabilization. In comparison with bare and other functionalized titanium carbides, the oxygen functionalized one exhibits the most superior ideal strength; however, the premature softening of the long-wave phonon modes might limit the intrinsic strength for Ti3C2O2. Furthermore, the introduction of functional groups can induce a strong anisotropy under tensile loading. By analyzing the deformation paths and the electronic instability under various loadings, we demonstrate that the unique strengthening by oxygen functional groups is attributed to a significant charge transfer from inner bonds to outer surface ones after functionalization. Our results shed a novel view into exploring a variety of MXenes for their potential applications in flexible electronic and energy storage devices. [ABSTRACT FROM AUTHOR]

The mechanical properties and electronic structure of polymorphic tungsten carbides with different stacking sequences have been investigated by means of density functional theory. The stacking sequence strongly influences the stability and mechanical strength of these polymorphs. Using the plastic parameters based on resistance to slip, we found that the newly identified metastable hP4[194] structure (expressed by the Pearson symbol and a space group number) possesses the highest plastic resistance despite its lower elastic moduli as compared to the thermodynamically stable hP2[187]. The cF8[225] structure is thermodynamically and dynamically unstable at 0 K, but it shows anharmonic stabilization at finite temperature, in agreement with the experimental data. Our results provide a way to increase material strength and hardness by varying the crystal lattice stacking sequence. [ABSTRACT FROM AUTHOR]

Taking pure Mg, Mg-Al, and Mg-Zn as prototypes, the effects of strain on the stacking fault energies (SFEs), dislocation core structure, and Peierls stress were systematically investigated by means of density functional theory and the semidiscrete variational Peierls-Nabarro model. Our results suggest that volumetric strain may significantly influence the values of SFEs of both pure Mg and its alloys, which will eventually modify the dislocation core structure, Peierls stress, and preferred slip system, in agreement with recent experimental results. The so-called "strain factor" that was previously proposed for the solute strengthening could be justified as a major contribution to the strain effect on SFEs. Based on multivariate regression analysis, we proposed universal exponential relationships between the dislocation core structure, the Peierls stress, and the stable or unstable SFEs. Electronic structure calculations suggest that the variations of these critical parameters controlling strength and ductility under strain can be attributed to the strain-induced electronic polarization and redistribution of valence charge density at hollow sites. These findings provide a fundamental basis for tuning the strain effect to design novel Mg alloys with both high strength and ductility. [ABSTRACT FROM AUTHOR]

The magnetic ground state of the Eu2+ moments in a series of Eu(Fe1−xCox)2As2 single crystals grown from the Sn flux has been investigated in detail by neutron diffraction measurements. Combined with the results from the macroscopic properties (resistivity, magnetic susceptibility and specific heat) measurements, a phase diagram describing how the Eu magnetic order evolves with Co doping in Eu(Fe1−xCox)2As2 is established. The ground-state magnetic structure of the Eu2+ spins is found to develop from the A-type antiferromagnetic (AFM) order in the parent compound, via the A-type canted AFM structure with some net ferromagnetic (FM) moment component along the crystallographic c direction at intermediate Co doping levels, finally to the pure FM order at relatively high Co doping levels. The ordering temperature of Eu declines linearly at first, reaches the minimum value of 16.5(2) K around x=0.100(4), and then reverses upwards with further Co doping. The doping-induced modification of the indirect Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction between the Eu2+ moments, which is mediated by the conduction d electrons on the (Fe,Co)As layers, as well as the change of the strength of the direct interaction between the Eu2+ and Fe2+ moments, might be responsible for the change of the magnetic ground state and the ordering temperature of the Eu sublattice. In addition, for Eu(Fe1−xCox)2As2 single crystals with 0.10≤x≤0.18, strong ferromagnetism from the Eu sublattice is well developed in the superconducting state, where a spontaneous vortex state is expected to account for the compromise between the two competing phenomena. [ABSTRACT FROM AUTHOR]

High-resolution inelastic neutron scattering reveals that the elementary magnetic excitations in multiferroic MnWO4 consist of low-energy dispersive electromagnons in addition to the well-known spin-wave excitations. The latter can well be modeled by a Heisenberg Hamiltonian with magnetic exchange coupling extending to the 12th nearest neighbor. They exhibit a spin-wave gap of 0.61(1) meV. Two electromagnon branches appear at lower energies of 0.07(1) and 0.45(1) meV at the zone center. They reflect the dynamic magnetoelectric coupling and persist in both the collinear magnetic and paraelectric AF1 phase and the spin spiral ferroelectric AF2 phase. These excitations are associated with the Dzyaloshinskii-Moriya exchange interaction, which is significant due to the rather large spin-orbit coupling. [ABSTRACT FROM AUTHOR]