Nonlinear Absorption Response of Zirconium Carbide Films.

Zirconium carbide (ZrC), a novel representative of the MXene family, has attracted considerable interest because of its outstanding physicochemical properties and potential applications in optoelectronic devices. For improving its performance as an optical modulator for ultrashort lasers, there is a call to continue studying the nonlinear optical behavior of MXene ZrC. Herein, for the first time, MXene ZrC films were fabricated on fused silica by magnetron sputtering deposition technology and used as a saturable absorber (SA) optical modulator in a passive Q-switched Nd:YAG laser. The saturation absorption behaviors of the prepared ZrC films were characterized by the Z-scan method. Their morphology, band structure, damage threshold, carrier recovery time, and saturation absorption properties were analyzed. The experimental results show that the MXene ZrC SA films exhibit excellent nonlinear optical characteristics, with a saturation intensity of 48.4 MW/cm2, a large modulation depth of 6.9%, and an ultrashort recovery time of 2.72 ps. In addition, the damage threshold of MXene ZrC SA films was estimated to be greater than 0.2516 J/cm2. By integrating the ZrC SA film optical modulator into the oscillator of the Nd:YAG laser, we achieved stable operation of the Q-switched laser with a central wavelength at 1.06 µm, with the shortest pulse width of 78 ns. The results of this study demonstrate the potential use of MXene ZrC SA films as optical modulators in ultrashort lasers.

Structural, magnetic, and electronic properties of EuSi2 thin films on the Si(111) surface.

Searching for magnetic silicide thin films has long been a hot topic in condensed matter physics and materials science based on their fundamental physics and promising device applications. Here we report a systematic study on the structural, magnetic, and electronic properties of EuSi2 thin films on the Si(111) surface by ab initio calculations. Total energy calculations show that the EuSi2 thin film in AA stacking is more favorable than that in AB or ABC stacking. The Eu2 + ions are coupled ferromagnetically within each layer and antiferromagnetically across the adjacent silicene layers with a large local spin moment of 6.96-7.00µB derived from the Eu-4f orbital electrons. Electronic band structure calculations indicate that the monolayer EuSi2 thin film is a semiconductor with an indirect surface band gap of 0.45 eV, while the multilayer EuSi2 thin films exhibit metallic behavior. These findings provide a systematic understanding of rare-earth metal silicides on the Si surface and will provide guidance for Si-based nanoelectronics and spintronics.

La induced Si3 trimer bilayer on the Si(111) surface.

Using first-principles calculations, we identify a robust R30° reconstruction of a Si3 trimer bilayer on the Si(111) surface with a La coverage of 2/3 monolayer. Each surface unit cell contains two Si3 trimers and two La atoms, where the upper Si3 trimer is located just above the lower one with a rotation of about 60°, while two La atoms with different heights are distributed between the Si3 trimers and located on the T4 top site of the Si(111) surface, forming a honeycomb-like network structure. We find that the two La atoms have different valence states, La2+ and La3+, respectively. The high structural stability is attributed to the lower La atom saturating all the three dangling bonds of the upper Si3 trimer, while the higher La atom compensates two electrons to the lower Si3 trimer. The electronic band structure and band-decomposed charge density distribution show a semiconducting characteristic with a small surface band gap of 42 meV. Moreover, simulated STM images show a good structural match with the recent experimental observations.

Novel electronic properties of monoclinic MP4 (M = Cr, Mo, W) compounds with or without topological nodal line.

Transition metal phosphides hold novel metallic, semimetallic, and semiconducting behaviors. Here we report by ab initio calculations a systematical study on the structural and electronic properties of [Formula: see text] (M = Cr, Mo, W) phosphides in monoclinic C2/c ([Formula: see text]) symmetry. Their dynamical stabilities have been confirmed by phonon modes calculations. Detailed analysis of the electronic band structures and density of states reveal that [Formula: see text] is a semiconductor with an indirect band gap of 0.47 eV in association with the p orbital of P atoms, while [Formula: see text] is a Dirac semimetal with an isolated nodal point at the [Formula: see text] point and [Formula: see text] is a topological nodal line semimetal with a closed nodal ring inside the first Brillouin zone relative to the d orbital of Mo and W atoms, respectively. Comparison of the phosphides with group VB, VIB and VIIB transition metals shows a trend of change from metallic to semiconducting behavior from [Formula: see text] to VIIB-[Formula: see text] compounds. These results provide a systematical understandings on the distinct electronic properties of these compounds.

Mn-Doped Sr/Si(111)-(3 × 2) HCC Surfaces: Antiferromagnetic Semiconductors for Spintronic Applications.

Manganese and manganese silicide as promising candidates for spintronic applications have attracted great interest in recent years. Here, we adopt Sr-induced Si(111)-(3 × 2) honeycomb-chain channel (HCC) surface as a template and perform a systematical study on the structural stability and magnetic and electronic properties of Mn-doped Sr/Si(111)-(3 × 2) HCC surfaces by ab initio calculations. Our energetic and kinetic results show two robust inserting structures M6 and H4, where Mn atoms are located below the honeycomb Si chain and on the top or hollow site of the Si(111) surface. Their high structural stabilities are attributed to the doped Mn atoms that saturate all the dangling bonds of Si(111) surface. In these two structures, Mn atoms prefer antiferromagnetic coupling with the same local magnetic moment of 3 µB. Electronic band structures and band-decomposed charge density distributions reveal that these two stable surface structures have a semiconducting characteristic with a surface band gap of 0.21-0.28 eV. This work provides an antiferromagnetic system for the possible application in spintronics.

Structural stability and electronic properties of alkaline-earth metal induced Si(111)-(3 × 2) surfaces.

Alkaline-earth metal (Ca, Sr, and Ba) induced Si(111)-(3 × 2) honeycomb chain-channel (HCC) surfaces have been systematically studied by means of ab initio calculations. The large adsorption energy and anisotropic diffusion energy barriers ensure the high structural stability of the one-dimensional HCC structure. Electronic band structures and band-decomposed charge density distributions reveal that the first conduction band and the third valence band level are contributed by the surface Si and metal atoms, while the top first and second valence bands are caused by the bulk silicon atoms. These results identify a larger surface band gap of 1.65-1.68 eV and provide an excellent explanation for the recent experimental observations of a band gap of 1.7 eV for the Sr/Si(111)-(3 × 2) HCC surface.