Showing papers on "Brillouin zone published in 2010"

High-frequency homogenization for periodic media

Abstract: An asymptotic procedure based upon a two-scale approach is developed for wave propagation in a doubly periodic inhomogeneous medium with a characteristic length scale of microstructure far less than that of the macrostructure. In periodic media, there are frequencies for which standing waves, periodic with the period or double period of the cell, on the microscale emerge. These frequencies do not belong to the low-frequency range of validity covered by the classical homogenization theory, which motivates our use of the term ‘high-frequency homogenization’ when perturbing about these standing waves. The resulting long-wave equations are deduced only explicitly dependent upon the macroscale, with the microscale represented by integral quantities. These equations accurately reproduce the behaviour of the Bloch mode spectrum near the edges of the Brillouin zone, hence yielding an explicit way for homogenizing periodic media in the vicinity of ‘cell resonances’. The similarity of such model equations to high-frequency long wavelength asymptotics, for homogeneous acoustic and elastic waveguides, valid in the vicinities of thickness resonances is emphasized. Several illustrative examples are considered and show the efficacy of the developed techniques.

On-chip stimulated Brillouin scattering

Abstract: We report the first demonstration of on-chip stimulated Brillouin scattering (SBS) with low average power. The measured Brillouin shift and line width are ∼7.7 GHz and ∼6 MHz in a 7 cm long chalcogenide waveguide.

Topological insulators on the Lieb and perovskite lattices

Abstract: Electrons hopping on the sites of a two-dimensional Lieb lattice and three-dimensional edge-centered cubic (perovskite) lattice are shown to form topologically nontrivial insulating phases when spin-orbit coupling is introduced. These simple models on lattices with cubic symmetry show a Dirac-type structure in the excitation spectrum but with the unusual feature that there is a dispersionless band through the center of the spectrum and only a single Dirac cone per Brillouin zone.

Making a Reconfigurable Artificial Crystal by Ordering Bistable Magnetic Nanowires

Abstract: Spin-wave excitations (magnons) are investigated in a one-dimensional (1D) magnonic crystal fabricated out of Ni80Fe20 nanowires. We find two different magnon band structures depending on the magnetic ordering of neighboring wires, i.e., parallel and antiparallel alignment. At a zero in-plane magnetic field H the modes of the antiparallel case are close to those obtained by zone folding of the spin-wave dispersions of the parallel case. This is no longer true for nonzero H which opens a forbidden frequency gap at the Brillouin zone boundary. The 1D stop band gap scales with the external field, which generates a periodic potential for Bragg reflection of the magnons.

Brillouin light scattering studies of planar metallic magnonic crystals

Abstract: The application of Brillouin light scattering to the study of the spin-wave spectrum of one- and two-dimensional planar magnonic crystals consisting of arrays of interacting stripes, dots and antidots is reviewed. It is shown that the discrete set of allowed frequencies of an isolated nanoelement becomes a finite-width frequency band for an array of identical interacting elements. It is possible to tune the permitted and forbidden frequency bands, modifying the geometrical or the material magnetic parameters, as well as the external magnetic field. From a technological point of view, the accurate fabrication of planar magnonic crystals and a proper understanding of their magnetic excitation spectrum in the gigahertz range is oriented to the design of filters and waveguides for microwave communication systems.

Single Dirac cone topological surface state and unusual thermoelectric property of compounds from a new topological insulator family.

Abstract: Angle resolved photoemission spectroscopy study on TlBiTe2 and TlBiSe2 from a thallium-based ternary chalcogenides family revealed a single surface Dirac cone at the center of the Brillouin zone for both compounds. For TlBiSe2, the large bulk gap (∼200 meV) makes it a topological insulator with better mechanical properties than the previous binary 3D topological insualtor family. For TlBiTe2, the observed negative bulk gap indicates it as a semimetal, instead of a narrow-gap semiconductor as conventionally believed; this semimetality naturally explains its mysteriously small thermoelectric figure of merit comparing to other compounds in the family. Finally, the unique band structures of TlBiTe2 also suggest it as a candidate for topological superconductors.

Topological Berry phase and semiclassical quantization of cyclotron orbits for two dimensional electrons in coupled band models

Abstract: The semiclassical quantization of cyclotron orbits for two-dimensional Bloch electrons in a coupled two band model with a particle-hole symmetric spectrum is considered. As concrete examples, we study graphene (both mono and bilayer) and boron nitride. The main focus is on wave effects – such as Berry phase and Maslov index – occurring at order \(\hbar\) in the semiclassical quantization and producing non-trivial shifts in the resulting Landau levels. Specifically, we show that the index shift appearing in the Landau levels is related to a topological part of the Berry phase – which is basically a winding number of the direction of the pseudo-spin 1/2 associated to the coupled bands – acquired by an electron during a cyclotron orbit and not to the complete Berry phase, as commonly stated. As a consequence, the Landau levels of a coupled band insulator are shifted as compared to a usual band insulator. We also study in detail the Berry curvature in the whole Brillouin zone on a specific example (boron nitride) and show that its computation requires care in defining the “k-dependent Hamiltonian” H(k), where k is the Bloch wavevector.

Electronic and defect structure of CuSCN

Abstract: Copper thiocyanate (CuSCN) is a candidate as a transparent solid p-type conductor for optoelectronic and photovoltaic applications, such as solar cells. We calculate the band structure, bonding characteristics, and basic native defect configurations of hexagonal β-CuSCN. β-CuSCN is predicted to be an indirect-gap semiconductor with an unusual orbital character: although the highest valence bands have the expected character of Cu 3d levels hybridized with S 3p states, the conduction band minimum (at the K point of the hexagonal Brillouin zone) has mostly cyanide antibonding character. This quasi-molecular character results in some unusual properties, including that the electron effective masses are comparable to or even larger than the hole effective masses. Calculated results match well with the valence band spectrum of thin film CuSCN, although optical absorption measurements do not conclusively confirm the predicted indirect nature of the lowest transitions. The dominant p-type character of this materia...

Experimental study of Brillouin scattering in perfluorinated polymer optical fiber at telecommunication wavelength

Abstract: Brillouin scattering properties in a perfluorinated graded-index polymer optical fiber (POF) with 120 μm core diameter were experimentally investigated using a laser with an operating wavelength of 1.55 μm. The Brillouin frequency shift and the Brillouin bandwidth were 2.83 GHz and 105 MHz, respectively. The calculated Brillouin gain coefficient of 3.09×10−11 m/W was comparable to that of fused silica fibers. The Brillouin threshold power of the 100 m POF was estimated to be 24 W, which we believe can be reduced by employing POFs with smaller cores.

High Spatial and Spectral Resolution Long-Range Sensing Using Brillouin Echoes

Abstract: High spatial ( cm) and spectral ( MHz) resolution Brillouin sensing is realized with enhanced signal to noise ratio using a pre-activated acoustic field and an optical phase control over the interrogating pulse. Pre-activation of the acoustic field preserves the Brillouin natural linewidth and a differential gain technique extends the method to long ranges. Experimentally, fully resolved measurements of the Brillouin frequency shift of a 5 cm spot perturbation at the far end of a 5 km fiber have been performed with a frequency resolution of 3 MHz (2) , using a 500 ps (5 cm) phase shift pulse.

Distributed Brillouin Fiber Sensor Assisted by First-Order Raman Amplification

Abstract: Distributed optical fiber Brillouin sensors provide innovative solutions for the monitoring of temperature and strain in large structures. The effective range of these sensors is typically of the order of 20-30 km, which limits their use in certain applications in which the distance to monitor is larger. In this work, we have developed a new technique to significantly extend the measurement distance of a distributed Brillouin Optical Time-Domain Analysis (BOTDA) sensor. Distributed Raman Amplification in the sensing fiber provides the means to enhance the operating range of the setup. Three Raman pumping configurations are theoretically and experimentally investigated: co-propagating, counter-propagating and bidirectional propagation with respect to the Brillouin pump pulse. We show that some of the amplification schemes tested can extend the measurement range and improve the measurement quality over long distances.

Anisotropic Propagation and Damping of Spin Waves in a Nanopatterned Antidot Lattice

Abstract: All-electrical spin-wave spectroscopy, Brillouin light scattering, as well as the magneto-optical Kerr effect are combined to study spin-wave propagation through a magnetic antidot lattice nanopatterned into a Ni80Fe20 thin film. The propagation velocities and, in particular, the relaxation are found to depend characteristically on the applied in-plane magnetic field. We explain the observed anisotropies by magnetic field-controlled spin-wave guiding in a network of interconnected nanowires which takes place over distances of up to 20 μm.

Anderson localization in tight-binding models with flat bands

Abstract: We consider the effect of weak disorder on eigenstates in a special class of tight-binding models. Models in this class have short-range hopping on periodic lattices; their defining feature is that the clean systems have some energy bands that are dispersionless throughout the Brillouin zone. We show that states derived from these flat bands are generically critical in the presence of weak disorder, being neither Anderson localized nor spatially extended. Further, we establish a mapping between this localization problem and the one of resonances in random impedance networks, which previous work has suggested are also critical. Our conclusions are illustrated using numerical results for a two-dimensional lattice, known as the square lattice with crossings or the planar pyrochlore lattice.

Intermediate-band solar cells: Influence of band formation on dynamical processes in InAs/GaAs quantum dot arrays

Abstract: We present a theoretical model for design and analysis of semiconductor quantum dot (QD) array-based intermediate-band solar cell (IBSC). The plane-wave method with periodic boundary conditions is used in expansion of the $\mathbf{k}\ensuremath{\cdot}\mathbf{p}$ Hamiltonian for calculation of the electronic and optical structures of InAs/GaAs QD array. Taking into account realistic QD shape, QD periodicity in the array, as well as effects such as band mixing between states in the conduction and valence band, strain and piezoelectric field, the model reveals the origin of the intermediate-band formation inside forbidden energy gap of the barrier material. Having established the interrelation between QD periodicity and the electronic structure across the QD array Brillouin zone, conditions are identified for the appearance of pure zero density-of-states regions, that separate intermediate band from the rest of the conduction band. For one realistic QD array we have estimated all important absorption coefficients in IBSC, and most important, radiative and nonradiative scattering times. Under radiative-limit approximation we have estimated efficiency of such IBSC to be 39%.

Adiabatic and nonadiabatic phonon dispersion in a Wannier function approach

Abstract: We develop a first-principles scheme to calculate adiabatic and nonadiabatic phonon frequencies in the full Brillouin zone. The method relies on the stationary properties of a force-constant functional with respect to the first-order perturbation of the electronic charge density and on the localization of the deformation potential in the Wannier function basis. This allows for calculation of phonon-dispersion curves free from convergence issues related to Brillouin-zone sampling. In addition our approach justifies the use of the static screened potential in the calculation of the phonon linewidth due to decay in electron-hole pairs. We apply the method to the calculation of the phonon dispersion and electron-phonon coupling in ${\text{MgB}}_{2}$ and ${\text{CaC}}_{6}$. In both compounds we demonstrate the occurrence of several Kohn anomalies, absent in previous calculations, that are manifest only after careful electron- and phonon-momentum integration. In ${\text{MgB}}_{2}$, the presence of Kohn anomalies on the ${\text{E}}_{2g}$ branches improves the agreement with measured phonon spectra and affects the position of the main peak in the Eliashberg function. In ${\text{CaC}}_{6}$ we show that the nonadiabatic effects on in-plane carbon vibrations are not localized at zone center but are sizable throughout the full Brillouin zone. Our method opens perspectives in large-scale first-principles calculations of dynamical properties and electron-phonon interaction.

Demonstration of Brillouin Distributed Discrimination of Strain and Temperature Using a Polarization-Maintaining Optical Fiber

Abstract: Distributed discrimination of strain and temperature is demonstrated by localizing both the stimulated Brillouin scattering (SBS) and the dynamic acoustic grating generated in the SBS process in a polarization-maintaining fiber with a correlation-domain continuous-wave technique. A 12- ?? strain resolution and 0.3°C temperature resolution together with a 10-cm spatial resolution are experimentally validated.

High-resolution DPP-BOTDA over 50 km LEAF using return-to-zero coded pulses.

Abstract: Coded optical probe pulses in return-to-zero/non-return-to-zero (RZ/NRZ) formats are used for long-range distance sensing based on a differential pulse-width pair Brillouin optical time-domain analysis (DPP-BOTDA) in order to enhance the spatial resolution and measurement accuracy. It is found that using the RZ format maintains the Brillouin spectral shape, enhances the sensing range and leads to a higher signal-to-noise ratio compared to a single-pulse Brillouin optical time-domain analysis. With 512 bit RZ-coded pulse pairs of 60/55 ns for the DPP-BOTDA, a spatial resolution of approximately 0.5 m and a strain resolution of 12 microepsilon (which is equivalent to a 0.7 MHz Brillouin frequency shift) have been achieved over a 50 km large effective area fiber.

Potential of Brillouin scattering in polymer optical fiber for strain-insensitive high-accuracy temperature sensing.

Abstract: We investigated the dependences of Brillouin frequency shift (BFS) on strain and temperature in a perfluorinated graded-index polymer optical fiber (PFGI-POF) at 1.55μm wavelength. They showed negative dependences with coefficients of −121.8MHz/% and −4.09MHz/K, respectively, which are −0.2 and −3.5 times as large as those in silica fibers. These unique BFS dependences indicate that the Brillouin scattering in PFGI-POFs has a big potential for strain-insensitive high-accuracy temperature sensing.

Sensitivity of the superconducting state and magnetic susceptibility to key aspects of electronic structure in ferropnictides

Abstract: Experiments on the iron–pnictide superconductors appear to show some materials where the ground state is fully gapped, and others where low-energy excitations dominate, possibly indicative of gap nodes. Within the framework of a five-orbital spin fluctuation theory for these systems, we discuss how changes in the doping, the electronic structure or interaction parameters can tune the system from a fully gapped to a nodal sign-changing gap with s-wave (A1g) symmetry (s±). In particular, we focus on the role of the hole pocket at the (π, π) point of the unfolded Brillouin zone, identified as crucial to the pairing by Kuroki et al (2009 Phys. Rev. B 79 224511), and show that its presence leads to additional nesting of hole and electron pockets, which stabilizes the isotropic s± state. The pocket's contribution to the pairing can be tuned by doping, surface effects and by changes in interaction parameters, which we examine. Analytic expressions for orbital pairing vertices calculated within the random phase approximation (RPA) fluctuation exchange approximation allow us to draw connections between aspects of the electronic structure, interaction parameters and the form of the superconducting gap.

High-Spatial-Resolution Time-Domain Simultaneous Strain and Temperature Sensor Using Brillouin Scattering and Birefringence in a Polarization-Maintaining Fiber

Abstract: We report on a high-spatial-resolution simultaneous strain and temperature measurement in time domain through measuring Brillouin frequency shift (BFS) and birefringence-induced frequency shift (BireFS) in a polarization-maintaining fiber. High-spatial-resolution BFS and BireFS measurement are obtained through differential pulsewidth pair Brillouin optical time-domain analysis and a local Brillouin grating, respectively. Simultaneous strain and temperature measurement with a spatial resolution of 20 cm is demonstrated in a 6-m Panda fiber. The temperature and strain accuracy is 0.4°C and 9 μe, and their range can be up to 700°C and 14 me, respectively.

High-Frequency Asymptotics, Homogenisation and Localisation for Lattices

Abstract: A two-scale approach, for discrete lattice structures, is developed that uses microscale information to find asymptotic homogenised continuum equations valid on the macroscale. The development recognises the importance of standing waves across an elementary cell of the lattice, on the microscale, and perturbs around the, potentially high frequency, standing wave solutions. For examples of infinite perfect periodic and doubly periodic lattices, the resulting asymptotic equations accurately reproduce the behaviour of all branches of the Bloch spectrum near each of the edges of the Brillouin zone. Lattices in which properties vary slowly upon the macroscale are also considered and the asymptotic technique identifies localised modes that are then compared with numerical simulations.

Operation of Brillouin Optical Correlation-Domain Reflectometry: Theoretical Analysis and Experimental Validation

Abstract: We theoretically and experimentally analyze the operation of Brillouin optical correlation-domain reflectometry (BOCDR). First, we experimentally confirm that BOCDR is not based on stimulated Brillouin scattering but on spontaneous Brillouin scattering. Then, we theoretically prove that the spatial resolution of BOCDR is given well by the same expression as that of Brillouin optical correlation-domain analysis (BOCDA). Finally, we demonstrate that the modulation amplitude of the laser frequency, which has been conventionally limited to a half of the Brillouin frequency shift, can be enhanced further by employing a sensing fiber shorter than a half of the measurement range.

First-principles elastic and thermal properties of TiO2: a phonon approach

Abstract: Elastic and thermal properties of the TiO2 lattice in anatase and rutile phases were studied in the framework of density functional perturbation theory within the local density approximation (LDA) and generalized-gradient approximation (GGA) The full elastic constant tensors of the polymorphs were calculated by linear fits to the acoustic branches of the phonon band structure near the center of the first Brillouin zone in symmetry directions of the crystals It was observed that the elastic constants within the GGA are in better agreement with experiment In addition, the Born effective charges, dielectric tensor, heat capacity, mean sound velocity and Debye temperature were calculated The heat capacity at room temperature and the Debye temperature within the LDA are in better agreement with the experimental results Therefore, using the phonon band structure and the density of states, one can obtain the important mechanical and thermal properties of materials

Dynamical polarizability of graphene beyond the Dirac cone approximation

Abstract: We compute the dynamical polarizability of graphene beyond the usual Dirac cone approximation, integrating over the full Brillouin zone. We find deviations at $\ensuremath{\hbar}\ensuremath{\omega}=2t$ (where $t$ is the hopping parameter) which amount to a logarithmic singularity due to the Van Hove singularity and derive an approximate analytical expression. Also at low energies, we find deviations from the results obtained from the Dirac cone approximation which manifest themselves in a peak spitting at arbitrary direction of the incoming wave vector $\mathbf{q}$. Consequences for the plasmon spectrum are discussed.

Sensitivity of the superconducting state and magnetic susceptibility to key aspects of electronic structure in ferropnictides

Abstract: Experiments on the iron-pnictide superconductors appear to show some materials where the ground state is fully gapped, and others where low-energy excitations dominate, possibly indicative of gap nodes. Within the framework of a 5-orbital spin fluctuation theory for these systems, we discuss how changes in the doping, the electronic structure or interaction parameters can tune the system from a fully gapped to nodal sign-changing gap with s-wave ($A_{1g}$) symmetry ($s^\pm$). In particular we focus on the role of the hole pocket at the $(\pi,\pi)$ point of the unfolded Brillouin zone identified as crucial to the pairing by Kuroki {\it et al.}, and show that its presence leads to additional nesting of hole and electron pockets which stabilizes the isotropic $s^\pm$ state. The pocket's contribution to the pairing can be tuned by doping, surface effects, and by changes in interaction parameters, which we examine. Analytic expressions for orbital pairing vertices calculated within the RPA fluctuation exchange approximation allow us to draw connections between aspects of electronic structure, interaction parameters, and the form of the superconducting gap.

Analysis of collective spin-wave modes at different points within the hysteresis loop of a one-dimensional magnonic crystal comprising alternative-width nanostripes

Abstract: The Brillouin light-scattering technique has been applied to study collective spin waves in a dense array of dipolarly coupled ${\text{Ni}}_{80}{\text{Fe}}_{20}$ stripes of alternating widths, during the magnetization reversal process. Both the saturated ``ferromagnetic'' state, where the magnetizations of wide and narrow stripes are parallel, and the ``antiferromagnetic'' state, characterized by an antiparallel alignment of the static magnetization in adjacent stripes, have been analyzed. The experimental data provide strong evidence of sustained collective excitations in the form of Bloch waves with permitted and forbidden magnonic energy bands. The measured frequencies as a function of the exchanged wave vector have been satisfactorily reproduced by numerical simulations which enabled us to calculate the spatial profiles of the Bloch waves, showing that some of the modes are preferentially localized in either the wide or the narrow stripes. We estimated the expected light-scattering cross section for each mode at different magnetic ground states, achieving a good agreement with the measured intensities. The alternating-width stripes system studied here represents a one-dimensional artificial magnonic crystal with a complex base and can be considered as a model system for reprogrammable dynamical response, where the band structure of collective spin waves can be tailored by changing the applied magnetic field.

Anisotropic dynamical coupling for propagating collective modes in a two-dimensional magnonic crystal consisting of interacting squared nanodots

Abstract: Collective spin excitations in a planar array of interacting submicron magnetic squared dots have been studied by the Brillouin light-scattering technique. The dispersion curves of collective spin modes are characterized by periodical oscillations determined by the width of the artificial Brillouin zone. Because of the uniaxial symmetry introduced by the application of an external magnetic field ${H}_{0}$, the dynamical coupling and the frequency dispersion of collective modes are different when the wave vector is perpendicular or parallel to ${H}_{0}$. An analytical model has been exploited to calculate the dispersion of collective spin modes by numerically solving eigenvalue/eigenfunction problem for a band matrix which originates from linearized Landau-Lifshitz magnetic torque equation. A very good agreement between calculation and experiment was found. In addition, a micromagnetic approach has been exploited to achieve an independent evaluation of the collective modes frequency and a visualization of their spatial profile on a limited $(3\ifmmode\times\else\texttimes\fi{}3)$ array of dots.

Systematic variations in structural and electronic properties of BiFeO3 by A-site substitution

Abstract: Y4 s 2 4p 6 4d 1 5s 2 ,L a 5s 2 5p 6 5d 1 6s 2 , and Sb 5s 2 5p 3 were considered as valence electrons. All calculations were performed with an energy cutoff of 500 eV for the plane wave expansion of the PAW, a 222 Monkhorst Pack grid of k points, and the Fermi-smearing for the Brillouin zone integrations. We used the semiempirical LSDA+U method, where the strong Coulomb repulsion between localized d states is treated by adding a Hubbard-type term U to the effective potential, leading to a better description of the correlation effect in transition-metal oxides. 16 In this study, we used a value of Ueff=6 eVU=6 eV and J=0 eV in the framework of Dudarev’s approach. 17,18

Effects of filling in CoSb3: Local structure, band gap, and phonons from first principles

Abstract: We use ab initio computations to investigate the effect of filler ions on the properties of CoSb3 skutterudites. We analyze global and local structural effects of filling, using the Ba-filled system as an example. We show that the deformation of Sb network induced by the filler affects primarily nearest neighboring Sb sites around the filler site as the soft Sb rings accommodate the distortion. Rearrangement of Sb atoms affects the electronic band structure and we clarify the effect of this local strain on the band gap. We compute the phonon dispersions and identify the filler-dominated modes from the lowest-frequency optical modes at Gamma. Their weak dispersion across the Brillouin zone indicates that they are localized and a force-constant analysis shows that the filler vibration is strongly coupled with nearby Sb atoms.

Atomic Bloch-Zener oscillations and Stückelberg interferometry in optical lattices.

Abstract: We report on experiments investigating quantum transport and band interferometry of an atomic Bose-Einstein condensate in an optical lattice with a two-band miniband structure, realized with a Fourier-synthesized optical lattice potential. Bloch-Zener oscillations, the coherent superposition of Bloch oscillations and Landau-Zener tunneling between the two bands, are observed. When the relative phase between paths in different bands is varied, an interference signal is observed, demonstrating the coherence of the dynamics in the miniband system. Measured fringe patterns of this Stuckelberg interferometer allow us to interferometrically map out the band structure of the optical lattice over the full Brillouin zone.