We present a theoretical study of the disorder effect due to interface roughness on piezoelectricity in wurtzite group-III-nitride heterostructures, e.g., AlGaN / GaN. We have proved that interface roughness gives rise to random nonuniform fluctuations in the piezoelectric polarization. As a result, besides the uniform density of sheet piezoelectric ͑and spontaneous͒ polarization-induced charges on the interface, reported in the existing literature, there must exist fluctuating densities of bulk piezoelectric charges inside of both the strained and relaxed layers as well as a fluctuating density of sheet piezoelectric charges on the interface. The densities of these charges and their electric field were generally found to be high. The maximal rms density of roughnessinduced bulk charges may be so large as 10 21 e /cm 3 , while the rms density of roughness-induced sheet charges may be of the order of magnitude of the uniform density of sheet piezoelectric charges, up to 10 13 e /cm 2 . Thus, the effects of piezoelectric polarization on the conductivity in actual wurtzite group-III-nitride heterostructures turn out to be counteracting, namely as a source of making up the two-dimensional electron gas, but also as a source of their scattering.

We show that a returning electron wave packet in high-order harmonic generation (HHG) with midinfrared laser pulses converges to a universal limit for a laser wavelength above about 3 μm. The results are consistent among the different methods: a numerical solution of the time-dependent Schrödinger equation, the strong-field approximation, and the quantum orbits theory. We further analyze how the contribution from different electron "trajectories" survives the macroscopic propagation in the medium. Our result thus provides a new framework for investigating the wavelength scaling law for the HHG yields. DOI: 10.1103/PhysRevLett.113.033001 PACS numbers: 33.80.Eh, 42.65.Ky High-order harmonic generation (HHG) has been a very active field of research over the last two decades. With the popular Ti:sapphire laser operating at an 800-nm wavelength, a great deal of experimental and theoretical research has been carried out. This interest is due to two facts. First, HHG provides a tabletop coherent extreme ultraviolet (XUV) light source [1] that can be used for application in science and technology. In particular, it can serve as a source of attosecond pulses [2,3] and attosecond-pulse train [4]. Second, the HHG signal contains information about the target structure [5][6][7]. As few-cycle laser pulses are routinely available, HHG holds great promise to become a spectroscopic tool capable of providing a femtosecond scale temporal resolution.The typical photon energy range available from HHG sources with a Ti:sapphire laser has been limited to about 100 eV or so. The well-known cutoff law Ω c ¼ I p þ 3.17U p , with Ω c , I p , and U p ∝ I 0 λ 2 being the cutoff energy, the ionization potential of the target, and the ponderomotive energy, respectively, suggests that higher energy photons can be produced with longer driving laser wavelengths λ (atomic units are used throughout, unless otherwise indicated). Increasing the peak laser intensity I 0 is not an option because of the ground-state depletion as well as the phase mismatch caused by excessive free electrons in the medium. With recent development in optical parametric amplification techniques, midinfrared (mid-IR) lasers with a wavelength of a few microns are available today, with a sufficient intensity to generate high harmonics, thus pushing the HHG photon energy range beyond the water window and even to the keV region [8][9][10][11].Wavelength scaling of HHG yield has been studied both theoretically [12][13][14][15][16][17][18][19] and experimentally [20,21]. Early theoretical investigation was mostly based on numerical solution of the time-dependent Schrödinger equation (TDSE) [12][13][14]. These studies showed that, apart from small oscillation on a fine λ scale due to the threshold phenomena effect, harmonic yield at constant driving laser intensity drops drastically as λ −x , with x ≈ 5-6. All these studies have been limited to wavelengths below 2 μm, and the analysis mostly for a fixed energy range from 20 to 50 eV. More recent studies within the strong-fiel...

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Organic-inorganic hybrid halide perovskites, in which the A cations of an ABX3 perovskite are replaced by organic cations, may be used for photovoltaic and solar thermoelectric applications. In this contribution, we systematically study three lead-free hybrid perovskites, i.e., methylammonium tin iodide CH3NH3SnI3, ammonium tin iodide NH4SnI3, and formamidnium tin iodide HC(NH2)2SnI3, by first-principles calculations. We find that in addition to the commonly known motif in which the corner-shared SnI6 octahedra form a three-dimensional network, these materials may also favor a two-dimensional (layered) motif formed by alternating layers of the SnI6 octahedra and the organic cations. These two motifs are nearly equal in free energy and are separated by low barriers. These layered structures features many flat electronic bands near the band edges, making their electronic structures significantly different from those of the structural phases composed of three-dimension networks of SnI6 octahedra. Furthermore, because the electronic structures of HC(NH2)2SnI3 are found to be rather similar to those of CH3NH3SnI3, formamidnium tin iodide may also be promising for the applications of methylammonium tin iodide.

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We work out a theory of piezoelectricity in an actual semiconductor heterostructure which is composed of a lattice-mismatched zinc-blende layer grown on a [001]-oriented substrate. In contrast to earlier theories, we predict a large density of fixed bulk piezoelectric charges, which are induced by strain fluctuations connected with interface roughness. The piezoelectric charges create a high electric field. The random piezoelectric field presents a conceptually new important scattering mechanism. The system of charge carriers in such a heterostructure becomes strongly disordered and includes generally both free electron-hole pairs near the interface and excitons far from it.

First principles calculations are presented to resolve the possible pressure-dependent phases of Mg2Si. Although previous reports show that Mg2Si is characterized by the cubic antiflurite F m3m structure at low pressures, the situation at higher P is less clear with many contradicting results. Here we utilize several methods to examine the stability, electron, phonon, and transport properties of this material as a function of pressure and temperature. We find that Mg2Si is thermodynamically stable at low and high pressures. Between 6 and 24 GPa, Mg2Si can transform into Mg9Si5, a defected compound, and vice versa, without energy cost. Perhaps this result is related to the aforementioned inconsistency in the structures reported for Mg2Si within this pressure range. Focusing solely on Mg2Si, we find a new monoclinic C2/m structure of Mg2Si, which is stable at high pressures within thermodynamical considerations. The calculated electrical conductivity and Seebeck coefficient taking into account results from the electronic structure calculations help us understand better how transport can be affected in this material by modulating pressure and temperature.