Brillouin zone

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

One-Dimensional Electronic Structure and Suppression of d-Wave Node State in (La1.28Nd0.6Sr0.12)CuO4

Abstract: Angle-resolved photoemission spectroscopy was carried out on (La1.28Nd0.6Sr0.12)CuO4, a model system of the charge- and spin-ordered state, or stripe phase. The electronic structure contains characteristic features consistent with other cuprates, such as the flat band at low energy near the Brillouin zone face. However, the low-energy excitation near the expected d -wave node region is strongly suppressed. The frequency-integrated spectral weight is confined inside one-dimensional segments in the momentum space (defined by horizontal momenta | kx | = π/4 and vertical momenta | ky | = π/4), deviating strongly from the more rounded Fermi surface expected from band calculations. This departure from the two-dimensional Fermi surface persists to a very high energy scale. These results provide important information for establishing a theory to understand the charge and spin ordering in cuprates and their relation with high-temperature superconductivity.

Fermi surface topology and low-lying quasiparticle dynamics of parent Fe1+xTe/Se superconductor.

Abstract: We report the first photoemission study of Fe_(1+x) Te—the host compound of the newly discovered iron-chalcogenide superconductors (maximum T_c∼27 K). Our results reveal a pair of nearly electron-hole compensated Fermi pockets, strong Fermi velocity renormalization, and an absence of a spin-density-wave gap. A shadow hole pocket is observed at the “X” point of the Brillouin zone which is consistent with a long-range ordered magnetostructural ground state. No signature of Fermi surface nesting instability associated with Q=(π/2,π/2) is observed. Our results collectively reveal that the Fe_(1+x)Te series is different from the undoped phases of the high Tc pnictides and likely harbor an unusual mechanism for superconductivity and magnetic order.

Mobile small polaron

Abstract: small radius of the wave function but a large size of the lattice distortion. We calculate the energy dispersion and the effective mass of the polaron with the 1/� perturbation theory and with the exact Monte Carlo method in the nonadiabatic and adiabatic regimes, respectively. The "small" Frohlich polaron is found to be lighter than the small Holstein polaron by one or more orders of magnitude. A free electron interacting with the dielectric polaris- able continuum was studied by Pekar (1) and Frohlich (2) in the strong and weak coupling limit, respectively. This is the case of carriers interacting with optical phonons in ionic crystals under the condition that the size of the self-trapped state is large compared to the lattice con- stant so the lattice discreteness is irrelevant (3). The most sophisticated treatment of this "large" or "contin- uum" polaron is due to Feynman and co-workers (4) with the path-integral method, substantially extended in the past decade (5). This treatment leads to a mass enhance- ment, but not to a hopping conduction or to a narrow polaron band. When the electron-phonon coupling constantis large, all the states in the Brillouin zone are involved in the for- mation of the polaron wave function, so the polaron ra- dius becomes comparable with the lattice constant a and the continuum approximation is no longer valid. Basic features of the small polaron were well recognised a long time ago by Tjablikov (6), Yamashita and Kurosawa (7), Sewell (8), Holstein (9), Lang and Firsov (10) and oth- ers, and are described in several review papers and text- books (11-15). So far, analytical and numerical studies have been mainly confined to the Holstein model with a short-range electron-phonon interaction. Exact diag- onalization of several vibrating molecules coupled with one electron (16,17), variational (18,19) and Monte Carlo calculations (20) revealed an excellent agreement with an- alytical results of Holstein (9) and Lang and Firsov (10) for the energy of the ground state and first excited states at large �. Polaron mass is very large in the Holstein model, unless phonon frequencies are extremely high. The size of the region, where the small Holstein polaron is localised, is about the same as the size of the lattice distortion, each of the order of the lattice constant. Both sizes are almost identical also for the large Frohlich polaron, but much larger. In this Letter we study a problem of the lattice po- laron with a long-range Frohlich interaction (21). This quasiparticle has a small (atomic) size of the electron lo- calization region but a large size of the lattice distortion. While the large Frohlich polaron is heavier than the large Holstein polaron, the small Frohlich polaron (SFP) turns out to be much lighter than the small Holstein polaron (SHP) with the same binding energy. We argue that SFPs are relevant quasiparticles in the cuprates. A quite general electron-phonon lattice Hamiltonian with one electron and the "density-displacement" type of interaction is given by (9,12,15) H = − X nn' tnnc †'cn + X q� ¯q�(d †�dq� + 1/2)

Fundamental and second-order phonon processes in CdTe and ZnTe

Abstract: Fundamental and higher-order photon-phonon interactions dominate the far infrared absorption and dispersion spectrum of most semiconductors and dielectrics. We present a detailed investigation of the temperature dependence of the dielectric function of the important II-VI semiconductors CdTe and ZnTe in the frequency range below 3 THz, between 10 and 300 K. From the dielectric function we determine the temperature dependence of the fundamental transverse-optical (TO) frequency in CdTe and ZnTe as well as the TO phonon damping rate in CdTe, and the dynamic ionic charge of both crystals is inferred from the measurements. Furthermore, our experimental data enable unambiguous assignment of low-frequency absorption bands to sum and difference combinations of fundamental phonon modes at critical points away from the Brillouin zone center.

Mechanical Weyl Modes in Topological Maxwell Lattices.

Abstract: We show that two-dimensional mechanical lattices can generically display topologically protected bulk zero-energy phonon modes at isolated points in the Brillouin zone, analogs of massless fermion modes of Weyl semimetals. We focus on deformed square lattices as the simplest Maxwell lattices, characterized by equal numbers of constraints and degrees of freedom, with this property. The Weyl points appear at the origin of the Brillouin zone along directions with vanishing sound speed and move away to the zone edge (or return to the origin) where they annihilate. Our results suggest a design strategy for topological metamaterials with bulk low-frequency acoustic modes and elastic instabilities at a particular, tunable finite wave vector.