Brillouin zone

About: Brillouin zone is a research topic. Over the lifetime, 13849 publications have been published within this topic receiving 383077 citations.

Band structure, elementary excitations, and stability of a Bose-Einstein condensate in a periodic potential.

Abstract: We investigate the band structure of a Bose-Einstein condensate in a one-dimensional periodic potential by calculating stationary solutions of the Gross-Pitaevskii equation, which have the form of Bloch waves. We demonstrate that loops (``swallow tails'') in the band structure occur both at the Brillouin zone boundary and at the center of the zone, and they are therefore a generic feature. A physical interpretation of the swallow tails in terms of periodic solitons is given. The linear stability of the solutions is investigated as a function of the strength of the mean-field interaction, the magnitude of the periodic potential, and the wave vector of the condensate. The regions of energetic and dynamical stability are identified by considering the behavior of the Gross-Pitaevskii energy functional for small deviations of the condensate wave function from a stationary state. It is also shown how for long-wavelength disturbances the stability criteria may be obtained within a hydrodynamic approach.

Exciton satellites in photoelectron-spectra

Abstract: We propose a new mechanism for satellite peaks in photoelectron spectra. Besides data from the literature new measurements of spectra of early first-row transition-metal compounds are discussed. The observed strong satellites of the metal core peaks of these materials are assigned to the creation of excitons on the anions. In our model, satellite energies can be determined from optical excitation spectra and satellite intensities from anion polarizabilities. It is shown that sharp satellites of this kind are expected if the exciton dispersion is not too large so that exciton states exist over large regions of the Brillouin zone. We also consider competition between charge transfer and exciton satellites. It turns out that only weak charge-transfer satellites are expected for the early $3d$-metal compounds, in agreement with experiment.

Growth, Luminescence, Selection Rules, and Lattice Sums of SiC with Wurtzite Structure

Abstract: Relatively large and pure crystals of the rare $2H$ polytype of SiC (wurtzite structure) have been grown, using the method of Adamsky and Merz, with special attention to purity. Absorption and luminescence measurements (2 to 8\ifmmode^\circ\else\textdegree\fi{}K) show that $2H \mathrm{SiC}$ has an indirect energy gap of 3.330 eV, the largest yet reported for a SiC polytype. The polarized luminescence was analyzed, using group-theoretical selection rules to determine the active phonons, as Lax and Hopfield did for Ge and Si. The observed spectrum is consistent with the selection rules, provided there are conduction-band minima at the $K$ positions of the Brillouin zone (two minima). Certain "forbidden" lines are found to be temperature-dependent, as a similar line is in Ge. It is proposed to extend to all SiC polytypes the lattice sum rule discussed by Brout and by Rosenstock. The $2H$ lattice sum at $K$ is found to be within 2% of the cubic SiC lattice sum at $X$, even though the phonon energies measured in the luminescence spectra are quite different. As far as "trace variable" forces are concerned, SiC resembles C (diamond) more than it does Si.

Ultrafast energy and momentum resolved dynamics of magnetic correlations in photo-doped Mott insulator Sr$_2$IrO$_4$

Abstract: Measuring how the magnetic correlations throughout the Brillouin zone evolve in a Mott insulator as charges are introduced dramatically improved our understanding of the pseudogap, non-Fermi liquids and high $T_C$ superconductivity. Recently, photoexcitation has been used to induce similarly exotic states transiently. However, understanding how these states emerge has been limited because of a lack of available probes of magnetic correlations in the time domain, which hinders further investigation of how light can be used to control the properties of solids. Here we implement magnetic resonant inelastic X-ray scattering at a free electron laser, and directly determine the magnetization dynamics after photo-doping the Mott insulator Sr$_2$IrO$_4$. We find that the non-equilibrium state 2~ps after the excitation has strongly suppressed long-range magnetic order, but hosts photo-carriers that induce strong, non-thermal magnetic correlations. The magnetism recovers its two-dimensional (2D) in-plane Neel correlations on a timescale of a few ps, while the three-dimensional (3D) long-range magnetic order restores over a far longer, fluence-dependent timescale of a few hundred ps. The dramatic difference in these two timescales, implies that characterizing the dimensionality of magnetic correlations will be vital in our efforts to understand ultrafast magnetic dynamics.

Structural Phase Transition and Material Properties of Few-Layer Monochalcogenides.

Abstract: GeSe and SnSe monochalcogenide monolayers and bilayers undergo a two-dimensional phase transition from a rectangular unit cell to a square unit cell at a critical temperature T_{c} well below the melting point. Its consequences on material properties are studied within the framework of Car-Parrinello molecular dynamics and density-functional theory. No in-gap states develop as the structural transition takes place, so that these phase-change materials remain semiconducting below and above T_{c}. As the in-plane lattice transforms from a rectangle into a square at T_{c}, the electronic, spin, optical, and piezoelectric properties dramatically depart from earlier predictions. Indeed, the Y and X points in the Brillouin zone become effectively equivalent at T_{c}, leading to a symmetric electronic structure. The spin polarization at the conduction valley edge vanishes, and the hole conductivity must display an anomalous thermal increase at T_{c}. The linear optical absorption band edge must change its polarization as well, making this structural and electronic evolution verifiable by optical means. Much excitement is drawn by theoretical predictions of giant piezoelectricity and ferroelectricity in these materials, and we estimate a pyroelectric response of about 3×10^{-12} C/K m here. These results uncover the fundamental role of temperature as a control knob for the physical properties of few-layer group-IV monochalcogenides.