Showing papers on "Brillouin zone published in 2001"
TL;DR: In this article, a condition for boundary Majorana fermions is expressed as a condition on the bulk electron spectrum, which is satisfied in the presence of an arbitrary small energy gap induced by proximity of a 3D p-wave superconductor.
Abstract: Certain one-dimensional Fermi systems have an energy gap in the bulk spectrum while boundary states are described by one Majorana operator per boundary point. A finite system of length L possesses two ground states with an energy difference proportional to exp(-L/l0) and different fermionic parities. Such systems can be used as qubits since they are intrinsically immune to decoherence. The property of a system to have boundary Majorana fermions is expressed as a condition on the bulk electron spectrum. The condition is satisfied in the presence of an arbitrary small energy gap induced by proximity of a three-dimensional p-wave superconductor, provided that the normal spectrum has an odd number of Fermi points in each half of the Brillouin zone (each spin component counts separately).
3,234 citations
TL;DR: In this article, the authors present a complete Raman spectrum analysis of SnO2 nanoparticles, which comprises modification of the normal vibration modes active in Raman when the spectra are obtained from nanocrystals of Sn O 2 nanoparticles in the region around 475 −775 cm 21, and the appearance of the acoustic modes in the low-frequency region of the spectrum.
Abstract: 14 and space group P4 2 /mnm. The unit cell consists of two metal atoms and four oxygen atoms. Each metal atom is situated amidst six oxygen atoms which approximately form the corners of a regular octahedron. Oxygen atoms are surrounded by three tin atoms which approximate the corners of an equilateral triangle. The lattice parameters are a5b 54.737 A, and c53.186 A. The ionic radii for O 22 and Sn 41 are 1.40 and 0.71 A, respectively. 1 The 6 unit cell atoms give a total of 18 branches for the vibrational modes in the first Brillouin zone. The mechanical representation of the normal vibration modes at the center of the Brillouin zone is given by 2,3 G5G 1 ~ A1g!1G 2 ~ A2g!1G 3 ~ B1g!1G 4 ~ B2g! 1G 5 ~ Eg!12G 1 ~ A2u!12G 4 ~ B1u!14G 5 ~ Eu!, ~1! using the Koster notation with the commonly used symmetry designations listed in parenthesis. The latter will be used throughout this article. Of these 18 modes, 2 are active in infrared ~the single A2u and the triply degenerate Eu), 4 are Raman active ~three nondegenerated modes, A1g , B1g , B2g , and a doubly degenerate Eg), and two are silent ( A2g , and B1u). One A2u and two Eu modes are acoustic. In the Raman active modes oxygen atoms vibrate while Sn atoms are at rest ~see Fig. 1 in Ref. 4!. The nondegenerate mode, A1g , B1g , and B2g , vibrate in the plane perpendicular to the c axis while the doubly degenerated E g mode vibrates in the direction of the c axis. The B 1g mode consists of rotation of the oxygen atoms around the c axis, with all six oxygen atoms of the octahedra participating in the vibration. In the A2g infrared active mode, Sn and oxygen atoms vibrate in the c axis direction, and in the Eu mode both Sn and O atoms vibrate in the plane perpendicular to the c axis. The silent modes correspond to vibrations of the Sn and O atoms in the direction of the c axis (B1u) or in the plane perpendicular to this direction ( A2g). According to the literature, the corresponding calculated or observed frequencies of the optical modes are presented in Table I. When the size of the SnO2 crystal is reduced, the infrared spectrum is modified because the interaction between electromagnetic radiation and the particles depends on the crystal’s size, shape, and state of aggregation. 8‐1 0 Experiments using Raman spectroscopy have also reported spectrum modification, at least partially. Low frequency bands have been observed previously in SnO2, 11 and several authors have reported the existence of bands not observed in single-crystal or polycrystalline SnO 2 which have been found to be closely related to grain size. 12‐15 However, some of these reports do not adequately explain the origin of the abnormal spectrum. The aim of this article is to present a complete Raman spectrum of SnO2 nanoparticles. The analysis comprises ~i! modification of the normal vibration modes active in Raman when the spectra are obtained from nanocrystals of SnO2 ~‘‘classical modes’’ !, ~ii! the disorder activated surface modes in the region around 475‐775 cm 21 , and ~iii! the appearance of the acoustic modes in the low-frequency region of the spectra.
669 citations
TL;DR: The double resonance Raman process provides new assignments for the dispersive and nondispersive features observed in the Raman spectra of disordered graphite and carbon nanotubes, some features having been incorrectly assigned previously, or not assigned at all.
Abstract: The phonon dispersion relations of graphite can be probed over a wide range of the Brillouin zone by double resonance Raman spectroscopy. The double resonance Raman process provides us with new assignments for the dispersive and nondispersive features observed in the Raman spectra of disordered graphite and carbon nanotubes, some features having been incorrectly assigned previously, or not assigned at all.
543 citations
TL;DR: The observed dispersion relation shows evidence for substantial interactions beyond the nearest-neighbor Heisenberg term which can be understood in terms of a cyclic or ring exchange due to the strong hybridization path around the Cu4O4 square plaquettes.
Abstract: The magnetic excitations of the square-lattice spin-1/2 antiferromagnet and high- T(c) parent compound La2CuO4 are determined using high-resolution inelastic neutron scattering. Sharp spin waves with absolute intensities in agreement with theory including quantum corrections are found throughout the Brillouin zone. The observed dispersion relation shows evidence for substantial interactions beyond the nearest-neighbor Heisenberg term which can be understood in terms of a cyclic or ring exchange due to the strong hybridization path around the Cu4O4 square plaquettes.
482 citations
TL;DR: In this article, a review is devoted to both the experimental and theoretical aspects of lateral converse effects observed for spin waves, with the wave vector in the 10-10 cm range, where the magnetic dipole interaction plays the most important role.
452 citations
Abstract: The free energy and other thermodynamic properties of hexagonal-close-packed iron are calculated by direct ab initio methods over a wide range of pressures and temperatures relevant to the Earth’s core. The ab initio calculations are based on density-functional theory in the generalized-gradient approximation, and are performed using the projector augmented wave approach. Thermal excitation of electrons is fully included. The Helmholtz free energy consists of three parts, associated with the rigid perfect lattice, harmonic lattice vibrations, and anharmonic contributions, and the technical problems of calculating these parts to high precision are investigated. The harmonic part is obtained by computing the phonon frequencies over the entire Brillouin zone, and by summation of the free-energy contributions associated with the phonon modes. The anharmonic part is computed by the technique of thermodynamic integration using carefully designed reference systems. Detailed results are presented for the pressure, specific heat, bulk modulus, expansion coefficient and Gruneisen parameter, and comparisons are made with values obtained from diamond-anvil-cell and shock experiments.
262 citations
TL;DR: The anomalous Hall effect in two-dimensional ferromagnets is discussed to be the physical realization of the parity anomaly in (2+1)D, and the band crossing points behave as the topological singularity in the Brillouin zone.
Abstract: The anomalous Hall effect in two-dimensional ferromagnets is discussed to be the physical realization of the parity anomaly in (2+1)D, and the band crossing points behave as the topological singularity in the Brillouin zone. This appears as the sharp peaks and the sign changes of the transverse conductance $\sigma_{xy}$ as a function of the Fermi energy and/or the magnetization. The relevance to the experiments including the three dimensional systems is also discussed.
192 citations
TL;DR: The valence band structure of the layered transition metal dichalcogenide has been determined experimentally by angle resolved photoelectron spectroscopy and theoretically by augmented spherical wave band structure calculations as based on density functional theory as mentioned in this paper.
Abstract: The valence band structure of the layered transition metal dichalcogenide ${\mathrm{WS}}_{2}$ has been determined experimentally by angle resolved photoelectron spectroscopy and theoretically by augmented spherical wave band structure calculations as based on density functional theory. Good agreement between experimental and calculated band structure is observed for single crystal ${\mathrm{WS}}_{2}.$ An experimental band structure of a single layer was determined from an electronically decoupled film prepared on a single crystalline graphite substrate by metal-organic van der Waals epitaxy. The polarization dependent photoemission selection rules of the single layer film are appropriate for a free standing film. The experimental single layer band structure shows some differences compared to band structure calculations using bulk atomic positions within the layer. We conclude that relaxation of the single layer occurs as a consequence of the missing interlayer interactions leading to close agreement between electronic structure of the single layer and single crystal. As a consequence of the missing interlayer interactions the valence band maximum for the single layer is located at the K point of the Brillouin zone.
157 citations
TL;DR: In this article, 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.
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.
156 citations
TL;DR: A high-resolution angle-resolved photoemission spectroscopic study of the electron-doped cuprate superconductor Nd185Ce015CuO4 was performed in this paper.
Abstract: We report a high-resolution angle-resolved photoemission spectroscopic study of the electron-doped ( n-type) cuprate superconductor Nd185Ce015CuO4 We observe regions along the Fermi surface where the near- E(F) intensity is suppressed and the spectral features are broad in a manner reminiscent of the high-energy "pseudogap" in the underdoped p-type (hole doped) cuprates However, instead of occurring near the (pi,0) region, as in the p-type materials, this pseudogap falls near the intersection of the underlying Fermi surface with the antiferromagnetic Brillouin zone boundary
138 citations
TL;DR: In this article, the authors showed that incorporation of small amounts of nitrogen into conventional III-V compounds to form III-N-V alloys leads to splitting of the conduction band into two subbands.
Abstract: Recent high hydrostatic pressure experiments have shown that incorporation of small amounts of nitrogen into conventional III–V compounds to form III–N–V alloys leads to splitting of the conduction band into two subbands. The downward shift of the lower subband edge is responsible for the observed, large reduction of the fundamental band gaps in III–N–V alloys. The observed effects were explained by an anticrossing interaction between the conduction band states close to the center of the Brillouin zone and localized nitrogen states. The interaction leads to a change in the nature of the fundamental from the indirect gap in GaP to a direct gap in GaNP. The predictions of the band anticrossing model of enlarged electron effective mass and enhanced donor activation efficiency were confirmed by experiments in GaInNAs alloys.
TL;DR: In this paper, the phonon frequencies at the center of the Brillouin zone, the Born effective charge tensors, and the dielectric permittivity tensors were obtained for tetragonal zirconia (t-ZrO2).
Abstract: Using the variational density-functional perturbation theory, we investigate the dynamical and dielectric properties of tetragonal zirconia (t-ZrO2). We obtain the phonon frequencies at the center of the Brillouin zone, the Born effective charge tensors, and the dielectric permittivity tensors. For all these quantities, a comparison is made with the related values in the cubic phase. The Born effective charge tensors are found to be quite anisotropic. The calculated phonon frequencies present a better agreement with the infrared and Raman experimental values than previous theoretical calculations. We propose symmetry assignments that solve the contradictions existing in the literature. The electronic and static dielectric permittivity constants are in relatively good agreement with experimental values. We perform a detailed analysis of the contribution of the various infrared-active modes to the static dielectric permittivity and explain its strong anisotropy.
TL;DR: In this article, a method that utilizes a dispersion-shifted fiber having compound compositions with different temperature coefficients in core to simultaneously measure the distributed strain and temperature based on Brillouin frequency shift was proposed and realized.
Abstract: We propose and realize a new method that utilizes a dispersion-shifted fiber having compound compositions with different temperature coefficients in core to simultaneously measure the distributed strain and temperature based on Brillouin frequency shift. In a 3682-m sensing length of large-effective-area nonzero dispersion-shifted fiber, a temperature resolution of 5/spl deg/C, a strain resolution of 60 /spl mu//spl epsi/, and a spatial resolution of 2 m are achieved simultaneously.
TL;DR: In this article, the authors derive simple expressions to connect the linewidths of the waves interacting in the fiber, and show that the magnitude of the narrowing effect depends only on the acoustic damping rate and the cavity loss rate.
Abstract: In Brillouin fiber lasers, the phase fluctuations of the pump laser are transferred to the emitted Stokes field after being strongly reduced. The result is a linewidth narrowing that we study both experimentally and theoretically. We derive simple expressions to connect the linewidths of the waves interacting in the fiber, and we show that the magnitude of the narrowing effect depends only on the acoustic damping rate and the cavity loss rate. We successfully compare these theoretical predictions with experimental results obtained by recording the response of a Brillouin fiber ring laser to frequency modulation of the pump field.
TL;DR: In this article, a simple method of generating evenly spaced multiwavelength Brillouin comb by employing dispersion compensating fiber both for BrillOU Stokes generation and Raman amplification was demonstrated.
Abstract: We demonstrate a simple method of generating evenly spaced multiwavelength Brillouin comb by employing dispersion compensating fiber both for Brillouin Stokes generation and Raman amplification. Multiwavelength output of 798 Brillouin Stokes lines with average channel power of -17 dBm has been obtained with excellent flatness. Channel spacing corresponds to the Brillouin Stokes shift in dispersion compensating fiber and is estimated to be 9.4 GHz with the heterodyne detection method. Coupled interaction of Brillouin, Raman, and Rayleigh scattering explains the unique feature of proposed structure.
TL;DR: In this paper, the authors investigated the spin-wave damping in ferromagnetic (FM)/antiferromagnetic bilayers exhibiting exchange bias and found that the measured linewidths are more than one order of magnitude larger than in similar NiFe films without exchange bias.
Abstract: Ferromagnetic resonance and Brillouin light-scattering techniques have been used to investigate the spin-wave damping in ferromagnetic (FM)/antiferromagnetic bilayers exhibiting exchange bias. The measurements were done in the prototype system NiFe/NiO sputtered on Si(100) as a function of the NiFe film thickness. The linewidths measured with both techniques are more than one order of magnitude larger than in similar NiFe films without exchange bias and increase dramatically with decreasing FM film thickness. The data are consistently explained by a relaxation mechanism based on two-magnon scattering processes due to the local fluctuation of the exchange coupling caused by interface roughness. The local interface energy necessary to account for the measured linewidths is on the same order of the atomic exchange coupling.
TL;DR: Using a complex PT -invariant, periodic potential of Kronig-Penny type, the occurrence of the usual energy band structure was shown in this paper, where the Brillouin zone boundaries (BZBs: kL=0,π) may not be reached and along with band gaps rounded double bands may occur near the BZBs.
TL;DR: In this paper, a detailed experimental and theoretical investigation of the Fermi-surface topology of the layered transition metal dichalcogenide $2H\ensuremath{-}{\mathrm{NbSe}}_{2},$ which undergoes a second-order phase transition into an incommensurate two-dimensional charge-density-wave phase at 33.5 K was performed.
Abstract: We report a detailed experimental and theoretical investigation of the Fermi-surface topology of the layered transition-metal dichalcogenide $2H\ensuremath{-}{\mathrm{NbSe}}_{2},$ which undergoes a second-order phase transition into an incommensurate two-dimensional charge-density-wave phase at 33.5 K. High-resolution angle-resolved photoemission with synchrotron radiation yields two Nb $4d$-related Fermi-surface cylinders and a Se ${4p}_{z}$-derived pocket around the center of the Brillouin zone, in good agreement with the results of fully relativistic ab initio calculations within the local-density approximation of density-functional theory. The measurements were carried out at 50 K to identify characteristic features in the electronic structure of the normal phase that can give important clues as to the origins of the phase transition, and to achieve high resolution, at the same time. The implications of our results on the charge-density-wave mechanism in $2H\ensuremath{-}{\mathrm{NbSe}}_{2}$ are discussed. Our results together with previous data from the literature seem to rule out the saddle-point mechanism, but they reveal at the same time that the driving mechanism for the transition into the charge-density-wave state is not a simple Fermi-surface nesting, as recently suggested by Straub et al. [Phys. Rev. Lett. $82,$ 4504 (1999)].
TL;DR: In this article, structural, electronic, dynamical, and dielectric properties of zircon (ZrSiO4) were investigated using density-functional perturbation theory.
Abstract: We investigate structural, electronic, dynamical, and dielectric properties of zircon (ZrSiO4) within density-functional theory. The atomic structure is fully relaxed and the structural parameters are found to differ by less than 1.5% from the experimental data. The associated electronic band structure and density of states are also presented. Using density-functional perturbation theory, we obtain the phonon frequencies at the center of the Brillouin zone, the Born effective charge tensors, and the dielectric permittivity tensors. The calculated phonon frequencies agree with the infrared and Raman experimental values (rms relative deviations of 2.5%) when available, while the silent modes are predicted to range between 119.6 and 943.3 cm(-1). We compute the Born effective charge tensors, that are found to be quite anisotropic. The electronic and static dielectric permittivity are analyzed in detail. Their difference is mostly due to the lowest infrared-active mode, whose eigenvector corresponds to a distortion of the SiO4 tetrahedra with a displacement of Zr and O atoms in opposite directions.
TL;DR: In this article, an ab initio tight-binding linear muffin-tin orbitals method was used to study the electronic band structure of GaTe single crystals grown by vapor-phase transport.
Abstract: Exciton characteristics of GaTe single crystals grown by vapor-phase transport were studied by optical measurements. A hydrogenlike exciton series up to $n=4$ was clearly observed in the absorption spectra at 2 K. In the $n=1$ exciton energy region three types of exciton lines were found. By analyzing microphotoluminescence and micro-Raman-scattering spectra on the basis of group theory, it was clarified that these exciton lines are not due to different polytypes but to intrinsic exciton states. Furthermore, optical-absorption spectra in a magnetic field at 4.2 K were measured. In the Voigt configuration, one and two components for $E\ensuremath{\parallel}b$ and $E\ensuremath{\perp}b$ polarizations, respectively, were observed in the $n=1$ and 2 exciton lines. These magnetic-field dependencies cannot be interpreted on the basis of the previously proposed L-S coupling regime. The electronic band structure of GaTe was studied by the ab initio tight-binding linear muffin-tin orbitals method. It was found that GaTe is a direct-gap semiconductor and that the band edge is located at an M point of the Brillouin zone. From a comparison of exciton absorption spectra and the calculated band structure, the existence of the three types of excitons was interpreted from the viewpoint of $j\ensuremath{-}j$ coupling. Our model calculation was also able to explain the Zeeman splitting and the diamagnetic shift of the exciton peak energies.
TL;DR: The excitation spectrum of a two-dimensional resonating valence bond (RVB) state is investigated and its characteristic energy scales along different high symmetry directions in the magnetic Brillouin zone are predicted.
Abstract: We investigate the excitation spectrum of a two-dimensional resonating valence bond (RVB) state. Treating the pi-flux phase with antiferromagnetic correlations as a variational ground state, we recover the long wavelength magnon as an "RVB exciton." However, this excitation does not exhaust the entire spectral weight and the high-energy spectrum is dominated by fermionic excitations. The latter can be observed directly by inelastic neutron scattering, and we predict their characteristic energy scales along different high symmetry directions in the magnetic Brillouin zone. We also interpret experimental results on two magnon Raman scattering and midinfrared absorption within this scenario.
TL;DR: It is found that the single mode approximation fails, and that a finite energy width appears in the dynamic correlation function S(q,omega) for q less, similar to that predicted for the two-magnon continuum in the nonlinear sigma-model.
Abstract: The spin-excitation continuum, expected to dominate the low-energy fluctuation spectrum in the Haldane spin chain around the Brillouin zone center, q = 0, is directly observed by inelastic magnetic neutron scattering in the S = 1 quasi-1D antiferromagnet CsNiCl3. We find that the single mode approximation fails, and that a finite energy width appears in the dynamic correlation function S(q,omega) for q less, similar 0.5 pi. The width increases with decreasing q, while S(q,omega) acquires an asymmetric shape qualitatively similar to that predicted for the two-magnon continuum in the nonlinear sigma-model.
TL;DR: In this article, the authors measured the dielectric permittivities, Brillouin backscattering spectra, polarization-electric field hysteresis loops, and domain structures as a function of temperature in relaxor-based ferroelectric single crystals.
Abstract: Dielectric permittivities, Brillouin backscattering spectra, polarization-electric field hysteresis loops, and domain structures have been measured as a function of temperature in relaxor-based ferroelectric single crystals (PbMg1/3Nb2/3O3)1−x(PbTiO3)x(PMN-xPT) for x=0.24 and 0.34. For PMN-24%PT, a diffuse phase transition which is associated with a broad frequency-dependent dielectric maximum was observed near 380 K. As the temperature increases, the PMN-24%PT crystal gradually develops cubic regions and is fully converted into the cubic state near 375 K. An extra dielectric anomaly appears at 370 K, possibly due to the percolating polar cluster induced by an external electric field. PMN-34%PT exhibits a nearly normal ferroelectric phase transition near 445 K from the tetragonal to the cubic phase. In addition, a weak diffuse phase transition observed near 280 K may result from partial conversion of rhombohedral phase to tetragonal phase. The dielectric thermal hysteresis confirms that the transitions ne...
TL;DR: In this article, a Brillouin optical time domain reflectometer-based distributed temperature sensor utilizing a fiber Bragg grating notch filter to suppress the Rayleigh backscatter was presented.
Abstract: We report on a Brillouin optical time domain reflectometer-based distributed temperature sensor utilizing a fiber Bragg grating notch filter to suppress the Rayleigh backscatter in order to separate the Brillouin signal. The Brillouin light path is thus subject to minimum attenuation and is frequency independent. A 2-m spatial resolution is achieved over a range of 25 km with the temperature resolution rising from 1/spl deg/C at the near end to 7/spl deg/C at the far end in a measurement time of 10 min. This is reduced to <1/spl deg/C over /spl sim/20 km if the measurement time is increased to 180 min.
TL;DR: In this paper, a twenty-band Hamiltonian taking into account the spin-orbit coupling is used to describe the valence band and the first two conduction bands all over the Brillouin zone.
Abstract: A twenty-band $\mathbf{k}\ensuremath{\cdot}\mathbf{p}$ Hamiltonian taking into account the spin-orbit coupling is used to describe the valence band and the first two conduction bands all over the Brillouin zone. The basis functions are ${\mathrm{sp}}^{3}{s}^{*}$-like functions used in linear combination of atomic orbitals. To get the right dispersion up to the Brillouin zone edge, the influence of other bands $(d$ bands) is mimicked via Luttinger-like parameters in the valence band and in the conduction band. The method is applied to GaAs and Si. A satisfying agreement is obtained near the band extrema as well in the direct gap semiconductor (GaAs) as in the indirect gap semiconductor (Si). In particular, while the $\mathbf{k}\ensuremath{\cdot}\mathbf{p}$ Hamiltonian parameters are adjusted to get the longitudinal mass 0.92 of the silicon conduction band, the transverse mass, which results from the calculation without further adjustment, is equal to 0.19 which is the experimental value.
TL;DR: In this paper, the elastic properties of CuxMo1−x thin films with thicknesses varying from 200 up to 430 nm, elaborated by ion beam sputtering, have been investigated and two independent effective elastic constants of these films have been determined, allowing the calculation of their elastic moduli.
Abstract: Brillouin scattering has been used to investigate the elastic properties of CuxMo1−x thin films with thicknesses varying from 200 up to 430 nm, elaborated by ion beam sputtering. Two compositions around Cu30Mo70 and Cu70Mo30 and pure Cu and Mo films have been studied. The samples are essentially randomly oriented polycrystals, leading to an isotropic effective symmetry. By fitting all the Brillouin spectral lines, two independent effective elastic constants of these films have been determined, allowing the calculation of their elastic moduli. Results indicate that pure copper thin films’ elastic properties are similar to the bulk copper ones whereas pure molybdenum thin films' Young's modulus is weaker (about 10%) than the bulk Mo one. Concerning Cu–Mo solid solutions, elastic constants values lie between pure copper and molybdenum ones. Finally, the effect of annealing at 760 K on the mechanical properties of the solid solutions in relation with the microstructural evolution is discussed. The annealed so...
TL;DR: In this article, a lead zinc niobate [Pb(Zn1/3Nb2/3)O3 or PZN] single crystal oriented along (111) under zero-field cooling, and zero field heating after field cooling conditions.
Abstract: Raman and optical experiments have been performed on a lead zinc niobate [Pb(Zn1/3Nb2/3)O3 or PZN] single crystal oriented along (111) under zero-field cooling, and zero-field heating after field cooling conditions. At high temperature, the Raman spectra originate from a phonon–polarization coupling whose selection rules depend upon the wave vector of the quasistatic polarization, and that activates modes in the entire Brillouin zone. At room temperature, ferroelectric macrodomains were induced and stabilized under field cooling conditions. Their optically uniaxial behavior along [111] reflects a rhombohedral orientational order. However, Raman analysis shows that a zone center line assignment on the basis of the C3v point group is not appropriate, because of translational symmetry breaking due to chemical and structural disorder. Raman spectroscopy was also performed on a single crystalline sample of PZN oriented along (001). Raman measurements evidence the similarity of the spectra for both crystals in ...
TL;DR: In this article, a high-resolution helium-atom scattering study on the surface structure and dynamics of in situ cleaved MgO(001) single crystals has been performed under ultrahigh vacuum conditions, in both high symmetry directions.
Abstract: A high-resolution helium-atom scattering study on the surface structure and dynamics of in situ cleaved MgO(001) single crystals has been performed under ultrahigh vacuum conditions, in both high symmetry directions. Refined bound state resonance information is given for the energy levels of the He-MgO(001) surface potential, including confirmation of the lowest level at $\ensuremath{\sim}\ensuremath{-}10 \mathrm{meV}.$ A weak half-order reconstruction has been observed in diffraction scans in the $〈100〉$ direction which is interpreted as due to a defect-induced release of the compressional surface stress. The Rayleigh modes were observed over most of the reduced surface Brillouin zone and found to agree with previous measurements and theory. At the zone boundaries the Rayleigh wave is no longer observed being replaced in the inelastic HAS spectrum by the first observations of the longitudinal Lucas mode and the crossing mode, both in agreement with given theoretical arguments.
TL;DR: In this article, it was shown that the electronic kinetic energy of high-Tc superconductors is reduced below Tc, primarily in those compounds in which the superconducting transition at Tc is preceded by the formation of the partial gap in the density of states ~pseudogap! at T *Tc.
Abstract: Insight into the nature of the pairing of charge carriers in high- Tc superconductors may be provided by a systematic investigation of the condensation energy. In this work we report on studies of the electronic kinetic energy across the complex phase diagram of these materials. The c-axis component of the electronic kinetic energy ~determined from an analysis of the optical constants! is shown to be reduced below Tc , primarily in those compounds in which the superconducting transition at Tc is preceded by the formation of the partial gap in the density of states ~pseudogap! at T*.Tc . An examination of the doping dependence of the infrared conductivity in conjunction with the results of photoemission spectroscopy suggests that the lowering of the kinetic energy is a property of the electronic states close to the intersection of the two-dimensional Fermi surface with the boundary of the Brillouin zone. We contrast the c-axis results with the energetics associated with the nodal quasiparticles probed through the in-plane conductivity.
TL;DR: In this paper, an angle-resolved photoemission study of overdoped La1.78Sr0.22CuO4 was performed, and the authors observed sharp nodal quasiparticle peaks in the second Brillouin zone that are comparable to data from Bi2Sr2CaCu2O8+d.
Abstract: We have performed an angle-resolved photoemission study of overdoped La1.78Sr0.22CuO4, and have observed sharp nodal quasiparticle peaks in the second Brillouin zone that are comparable to data from Bi2Sr2CaCu2O8+d. The data analysis using energy distribution curves, momentum distribution curves and intensity maps all show evidence of an electron-like Fermi surface, which is well explained by band structure calculations. Evidence for many-body effects are also found in the substantial spectral weight remaining below the Fermi level around (pi,0), where the band is predicted to lie above EF.