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[en] Highlights: • Unfolding supercells with vacancies is discussed. • Fully vacant supercells reduce all PC band probabilities uniformly. • Net fully vacant supercells also reduce all PC band probabilities uniformly. - Abstract: We study how vacancies alter the effective primitive cell bands projected out of supercell eigenstates via Brillouin zone unfolding. Two types of vacant primitive cells are of particular interest: Fully vacant, in which all atoms in a single cell are missing; and net fully vacant, in which the atoms comprising a full set for a single cell are missing from more than one cell. We find that a fully vacant primitive cell and a net fully vacant primitive cell have the same effect on the primitive cell bands. We show that the probability reduction for any primitive cell band is the same, regardless of band or wavevector in the primitive cell Brillouin zone, for both fully and net fully vacant primitive cells. We illustrate these results with a two-band model.
[en] The mechanism of opening a band gap in the free phononic crystal (PC) thin plate with or without a mirror plane is investigated. It is found that, in a PC plate with a mirror plane, the permitted modes can be separated into symmetric and antisymmetric modes, and the band gap in such a system can be opened by the interaction between the modes of the same kind and/or the breaking of the degeneracy of the mode at the edge of the Brillouin zone. However, for a PC plate without a mirror plane, mode separation can no longer be performed, and interaction can occur between any two permitted modes. As a result, a new kind of band gap can be opened.
[en] Inelastic and quasielastic neutron scattering studies were carried out on a K-Hollandite sample composed of 25 single well oriented crystals. An anomalous behaviour of a TA-Λ3/4-dispersion branch along (00ζ) was observed. Above ζ about 0.21 the branch becomes unobservable in the Brillouin zones investigated so far and deviated from a 'normal' behaviour in the range 0.15 < ζ < 0.21 with respect to energy width and integrated intensity. Weak but distinct scattering occurs within the Q,ω-plane around/above ζ about 0.21. It seems to be plausible that this is due to a framework distortion around empty cavities along the K+-chains. No clear evidence for an interaction between a bottle-neck mode and a 1-d motion of the K+-ions was found. Remarkable quasielastic scattering - as predicted by theoretical work based on a FK-model - was observed and gross features may be reproduced. A better agreement, however, is achieved with a simple damped harmonic oscillator model. (author) 5 refs., 7 figs
[en] We present a comprehensive study of the temperature and doping dependence of the 500 meV peak observed at q = (π,0) in resonant inelastic x-ray scattering (RIXS) experiments on La2CuO4. The intensity of this peak persists above the Neel temperature (TN = 320 K), but decreases gradually with increasing temperature, reaching zero at around T = 500 K. The peak energy decreases with temperature in close quantitative accord with the behavior of the two-magnon B1g Raman peak in La2CuO4 and, with suitable rescaling, agrees with the Raman peak shifts in EuBa2Cu3O6 and K2NiF4. The overall dispersion of this excitation in the Brillouin zone is found to be in agreement with theoretical calculations for a two-magnon excitation. Upon doping, the peak intensity decreases analogous to the Raman mode intensity and appears to track the doping dependence of the spin-correlation length. Taken together, these observations strongly suggest that the 500 meV mode is magnetic in character and is likely a two-magnon excitation.
[en] Femtosecond time-resolved pump-probe experiments were carried out to study ultrafast lattice dynamics of ferroelectric lithium tantalate. Both the fully symmetric (A1 mode) and doubly degenerate (E mode) coherent phonons at the center of the Brillouin zone were excited via impulsive stimulated Raman scattering, as confirmed by the excitation intensity dependence.
[en] We theoretically investigate the optical transition of an electron in graphene that is excited by near-field around a conical Au tip. The interaction between the near-field and the electron is calculated by tight-binding method. In the case of near-field, the wavevector of the electron changes by the optical absorption from the valence band to the conduction band. We show that the change of the wavevector is inversely proportional to the localization width of the near-field, which is given as a function of the distance between the tip and graphene. We calculate the near-field absorption probability as a function of k in the Brillouin zone. (paper)
[en] Electron momentum spectroscopy (EMS) is a relatively new and novel technique that can be used to study the electronic structure of a variety of solids. The technique is of great significance as it allows direct comparison with calculations of the ground state electronic structure of solids. The measured intensity, referred to as the spectral momentum density (SMD) is proportional to the probability of finding an electron with a particular energy-momentum combination and is the modulus square of the wave function in momentum space. The EMS technique differs from angle resolved photoemission spectroscopy (ARPES), the standard technique for measuring band structure, in that we measure the real momentum of the electrons and do not rely on the sample being crystalline nor complicated reconstruction techniques. In fact the EMS method also works exceedingly well for polycrystalline and amorphous targets. The EMS method has a number of different characteristics to ARPES. EMS measurements penetrate deeper into the bulk of the material as it is a transmission experiment and thus less reliant on the surface condition. EMS also allows analysis of line shapes and intensities; ideas that are rarely discussed when describing ARPES measurements. Most importantly, the technique is able to selectively probe the momentum density in other Brillouin zones allowing more stringent comparison with theory. (author). 5 refs., 3 figs