First-principles method for nonlinear light propagation at oblique incidence

We have developed a computational method to describe the nonlinear light propagation of an intense and ultrashort pulse at oblique incidence on a flat surface. In the method, coupled equations of macroscopic light propagation and microscopic electron dynamics are simultaneously solved using a multiscale modeling. The microscopic electronic motion is described by first-principles time-dependent density functional theory. The macroscopic Maxwell equations that describe oblique light propagation are transformed into one-dimensional wave equations. As an illustration of the method, light propagation at oblique incidence on a silicon thin film is presented.Comment: 14 pages, 8 figure

Nonlinear polarization evolution using time-dependent density functional theory

We propose a theoretical and computational approach to investigate temporal behavior of a nonlinear polarization in perturbative regime induced by an intense and ultrashort pulsed electric field. First-principles time-dependent density functional theory is employed to describe the electron dynamics. Temporal evolution of third-order nonlinear polarization is extracted from a few calculations of electron dynamics induced by pulsed electric fields with the same time profile but different amplitudes. We discuss characteristic features of the nonlinear polarization evolution as well as an extraction of nonlinear susceptibilities and time delays by fitting the polarization. We also carry out a decomposition of temporal and spatial changes of the electron density in power series with respect to the field amplitude. It helps to get insight into the origin of the nonlinear polarization in atomic scale.Comment: 11 pages, 9 figure

Theoretical investigation of vacancy related defects at 4H-SiC(0001ˉ\bar{1})/SiO2_2 interface after wet oxidation

The stability and formation mechanism of the defects relevant to silicon and carbon vacancies at the 4H-SiC(000$\bar{1}$)/SiO$_2$ interface after wet oxidation are investigated by first-principles calculation based on the density functional theory. The difference in the total energy of the defects agrees with the experimental results concerning the dencity of defects. We found that the characteristic behaviors of the generation of defects are explained by the positions of vacancies and antisites in the SiC(000$\bar{1}$) substrate and that the formation of silicon and carbon vacancies is relevant to the generation mechanism of defects. The generation of silicon and carbon vacancies is attributed to the termination of dangling bonds by H atoms introduced by wet oxidation, resulting in generation of carbon-antisite--carbon-vacancy and divacancies defects in wet oxidation.Comment: 12 page

Valley filters using graphene blister defects from first principles

Valleytronics, which makes use of the two valleys in graphenes, attracts considerable attention and a valley filter is expected to be the central component in valleytronics. We propose the application of the graphene valley filter using blister defects to the investigation of the valley-dependent transport properties of the Stone--Wales and blister defects of graphenes by density functional theory calculations. It is found that the intervalley transition from the $\mathbf{K}$ valley to the $\mathbf{K}^\prime$ valleys is completely suppressed in some defects. Using a large bipartite honeycomb cell including several carbon atoms in a cell and replacing atomic orbitals with molecular orbitals in the tight-binding model, we demonstrate analytically and numerically that the symmetry between the A and B sites of the bipartite honeycomb cell contributes to the suppression of the intervalley transition. In addition, the universal rule for the atomic structures of the blisters suppressing the intervalley transition is derived. Furthermore, by introducing additional carbon atoms to graphenes to form blister defects, we can split the energies of the states at which resonant scattering occurs on the $\mathrm{K}$ and $\mathrm{K}^\prime$ channel electrons. Because of this split, the fully valley-polarized current will be achieved by the local application of a gate voltage.Comment: 19 pages, 15 figure

Density functional theory calculations for investigation of atomic structures of 4H-SiC/SiO2_2 interface after NO annealing

We propose the atomic structures of the 4H-SiC/SiO$_2$ interface for the $a$, $m$, C, and Si faces after NO annealing. Our proposed structures preferentially form at the topmost layers of the SiC side of the interface, which agrees with the experimental finding of secondary-ion mass spectrometry, that is, the N atoms accumulate at the interface. In addition, the areal N-atom density is on the order of 10$^{14}$ atom/cm$^2$ for each plane, which is also consistent with the experimental result. Moreover, the electronic structure of the interface after NO annealing, in which the CO bonds are removed and the nitride layer only at the interface is inserted, is free from gap states, although some interface models before NO annealing include the gap states arising from the CO bonds near the valence band edge of the bandgap. Our results imply that NO annealing can contribute to the reduction in the density of interface defects by forming the nitride layer.Comment: 10 pages and 7 figure

Density functional theory study on effect of NO annealing for SiC(0001) surface with atomic-scale steps

Density functional theory calculations for the electronic structures of the 4H-SiC(0001)/SiO$_2$ interface with atomic-scale steps are carried out to investigate the effect of NO annealing. The characteristic behavior of the conduction band edge states of SiC is strongly affected over a wide area of the interface by the Coulomb interaction of the O atoms in the SiO$_2$ region as well as the step structure of the interface, resulting in the discontinuity of the inversion layers at the step edges under the gate bias. The spatially discontinued band only allows the very limited conduction paths in the inversion layer, leading to the significantly decreased mobile carrier density. It is found that the Coulomb interaction of the O atoms is screened and the inversion layers become continuous when the nitrided layers are inserted at the interface by NO annealing. This result is in good agreement with experimental findings that the improvement of the performance of SiC metal-oxide-semiconductor field-effect-transistors by NO annealing is attributed to an increase in the mobile electron density rather than an increase in the mobility of electrons in the inversion layer.Comment: 12 page

First-principle study of spin transport property in L10L1_0-FePd(001)/graphene heterojunction

In our previous work, we synthesized a metal/2D material heterointerface consisting of $L1_0$-ordered iron-palladium (FePd) and graphene (Gr) called FePd(001)/Gr. This system has been explored by both experimental measurements and theoretical calculations. In this study, we focus on a heterojunction composed of FePd and multilayer graphene referred to as FePd(001)/$m$-Gr/FePd(001), where $m$ represents the number of graphene layers. We perform first-principles calculations to predict their spin-dependent transport properties. The quantitative calculations of spin-resolved conductance and magnetoresistance (MR) ratio (150-200%) suggest that the proposed structure can function as a magnetic tunnel junction in spintronics applications. We also find that an increase in $m$ not only reduces conductance but also changes transport properties from the tunneling behavior to the graphite $\pi$-band-like behavior. Furthermore, we examine the impact of lateral displacements (sliding) at the interface and find that the spin transport properties remain robust despite these changes; this is the advantage of two-dimensional material hetero-interfaces over traditional insulating barrier layers such as MgO.Comment: 18 pages, 8 figure

Attosecond state-resolved carrier motion in quantum materials probed by soft x-ray XANES

Recent developments in attosecond technology led to table-top x-ray spectroscopy in the soft x-ray range, thus uniting the element- and state-specificity of core-level x-ray absorption spectroscopy with the time resolution to follow electronic dynamics in real-time. We describe recent work in attosecond technology and investigations into materials such as Si, SiO2, GaN, Al2O3, Ti, and TiO2, enabled by the convergence of these two capabilities. We showcase the state-of-the-art on isolated attosecond soft x-ray pulses for x-ray absorption near-edge spectroscopy to observe the 3d-state dynamics of the semi-metal TiS2 with attosecond resolution at the Ti L-edge (460 eV). We describe how the element- and state-specificity at the transition metal L-edge of the quantum material allows us to unambiguously identify how and where the optical field influences charge carriers. This precision elucidates that the Ti:3d conduction band states are efficiently photo-doped to a density of 1.9 x 1021 cm 3. The light-field induces coherent motion of intra-band carriers across 38% of the first Brillouin zone. Lastly, we describe the prospects with such unambiguous real-time observation of carrier dynamics in specific bonding or anti-bonding states and speculate that such capability will bring unprecedented opportunities toward an engineered approach for designer materials with pre-defined properties and efficiency. Examples are composites of semiconductors and insulators like Si, Ge, SiO2, GaN, BN, and quantum materials like graphene, transition metal dichalcogens, or high-Tc superconductors like NbN or LaBaCuO. Exiting are prospects to scrutinize canonical questions in multi-body physics, such as whether the electrons or lattice trigger phase transitions