Energy Dispersion Curves Research Articles

Rashba spin splitting of Shockley surface states on semi-infinite crystals

We study the Rashba spin splitting of the $L$-gap surface state on the (111) surfaces of Au, Ag, and Cu, and that of the topologically protected surface state on Sb(111) by a first-principles calculation based on the embedded Green's function technique. By taking advantage of semi-infinite surface geometry, we address several issues that are difficult to handle with the finite slab calculation. For Au(111), we investigate the nonlinear dependence on wave vector $\mathbf{k}$ of the spin-splitting energy, and also at what wave number each dispersion curve of the two spin-split bands is merged into a surface-projected bulk band. Then, we discuss why Cu(111) exhibits a larger Rashba effect than Ag(111) despite the fact that the atomic orbitals of Ag have larger spin-orbit matrix elements than those of Cu. We reveal that, while the wave function of the $L$-gap surface state has strong ${p}_{z}$ character (the $z$ axis is surface normal), the Rashba spin splitting occurs through the spin-orbit matrix element between the ${d}_{{z}^{2}}$ and ${d}_{uz}$ orbital components hybridized with ${p}_{z}$, where ${d}_{uz}$ denotes a linear combination of ${d}_{xz}$ and ${d}_{yz}$ with its orbital lobes pointing to the $\mathbf{k}$ direction (defined as the $u$ axis). Cu(111) exhibits a larger spin splitting than Ag(111), since the amount of both the ${d}_{{z}^{2}}$ and ${d}_{uz}$ components mixed in its surface-state wave function is much larger than that of Ag(111). In contrast to the (111) surfaces of noble metals, the spin splitting of the topologically protected surface state on Sb(111) arises from the spin-orbit matrix element between the ${p}_{z}$ and ${p}_{u}$ orbital components. We also discuss under what condition the energy dispersion curve of a surface state intersects the boundary line between a projected bulk band and a projected bulk band gap at a large angle, instead of gradually approaching the boundary line with increasing $|\mathbf{k}|$. Finally, we comment on a recent theoretical work of Krasovskii [Phys. Rev. B 90, 115434 (2014)] on the microscopic origin of the Rashba spin splitting.

Structure of the fluorine states in cadmium molybdate host studied by the electronic band structure calculations of CdMoO4, CdMoO4:F and CdMoO3F2 crystals

The electronic structures of CdMoO4, CdMoO3F2 and the fluorine-doped CdMoO4:F crystals are calculated by the Full-Potential Linear Augmented Plane Wave method. The energy dispersion curves, partial densities of electronic states and spatial distributions of electronic densities are calculated and analyzed in comparison with published experimental data. Relaxation of the crystal structure around the FO impurity in cadmium molybdate is revealed in geometry-optimization calculations carried out for CdMoO4:F. It is found that CdMoO3F2 crystal is as a direct-gap material. The Cd d, F p and O p states electronic states form the top of the Valence band and mainly Mo d states form the bottom of the Conduction band of CdMoO3F2 and all these states are involved into the lowest-energy band-to-band electronic transitions. The low-energy shift of the main bands in intrinsic luminescence emission and excitation spectra of CdMoO3F2 with respect to CdMoO4 may be a consequence of the lower Eg value of the oxyfluoromolybdate. Calculations suggest more complex structure of the luminescence centers in CdMoO3F2 in comparison with CdMoO4.

Electronic Band Calculation of LaTSb2 (T=Cu,Ag,Au)

Abstract The series of compounds La T Sb 2 ( T =Cu,Ag,Au) show various electronic properties. In this paper we have calculated the electronic structure of La T Sb 2 from first-principles. The obtained Fermi surfaces of LaAgSb 2 show strong two-dimensionality, which well agrees with the results of de Haas - van Alphen experiment. On the other hand, one of the Fermi surface of LaCuSb 2 show three-dimensionality. LaAuSb 2 has the intermediate properties. These different Fermi surfaces may cause the occurrence of the charge density wave (CDW) in LaAgSb 2 and LaAuSb 2 , and no occurrence of the CDW in LaCuSb 2 . The conduction electrons are mainly due to Sb-5p electrons, while La-d and T -d electrons also have considerable contribution. The calculated electronic specific heat coefficient well agrees with the experimental results. The energy dispersion curve shows some Dirac points in the case of scalar-relativistic calculation, but these Dirac points are disappeared when we take the spin-orbit interaction into account.

Tensile-strain and doping enhanced direct bandgap optical transition of n+ doped Ge/GeSi quantum wells

Band structures of tensile strained and n+ doped Ge/GeSi quantum wells (QWs) are calculated by multiple-band k·p method. The energy dispersion curves of the Γ and L conduction subbands are obtained. The effects of tensile strain and n+ doping in Ge on direct bandgap optical gain and spontaneous radiative recombination rate spectra are investigated including the electron leakage from Γ to L conduction subbands. Our results show that the optical gain and spontaneous radiative recombination rate can be significantly increased with the tensile strain, n-type doping concentration, and injection carrier density in the Ge QW. The free carrier absorption is calculated and cannot be ignored because of the heavily doped Ge. The pure TM mode polarized net optical gain up to 1153 cm−1 can be achieved for the Ge/Ge0.986Si0.014 QW with tensile strain of 1.61% and n-type doping concentration of 30 × 1018 cm−3.

First-Principles Study of the Electronic Structure of Heavy Fermion YbNi2

We investigate the electronic properties of YbNi2 by means of band structure calculations based on the density functional theory within LDA (local density approximation), fully relativistic, and LDA+U schemes. The 4f derived bands are studied within a relativistic framework which yields flat and spin-orbit split bands, and a correlated band method (LDA+U) that includes correlation corrections. In both cases, the 4f bands, which is located roughly 200 meV below the Fermi level (E F ), hybridize weakly with the dispersive Ni-3d bands. When the fully relativistic scheme is applied, the 4f derived bands split into lower and higher bands due to spin-orbit coupling effects. The 4f electrons are delocalized through the hybridization with conduction electrons, and the hybridization between f and conduction d electrons also plays a important role in YbNi2. The on-site Coulomb potential is added to the Yb-derived 4f orbitals, the degeneracy between the 4f orbitals would be lifted partially and they are split into three manifolds bands. The Fermi surface splits into three different sheets which are from main the Yb-4f derived bands and Ni-3d bands. Band structure calculations reveal a saddle points existence at the L point in the energy dispersion curve closed to E F , whereby, we think YbNi2 might have a superconducting properties. In addition, the quasiparticle mass enhancement inferred by comparing γ to the density of states (DOS) at the Fermi level indicates the effective mass of YbNi2 enhanced with the fully relativistic results.

Extremely correlated Fermi liquids: Self-consistent solution of the second-order theory

We present detailed results from a recent microscopic theory of extremely correlated Fermi liquids, applied to the $t$-$J$ model in two dimensions, developed recently by Shastry [Phys. Rev. Lett. 107, 056403 (2011); Phys. Rev. B 87, 125124 (2013)]. The second-order theory in the parameter $\ensuremath{\lambda}$, related to the density, is argued to be quantitatively valid in the overdoped regime for $0\ensuremath{\le}n\ensuremath{\lesssim}0.75$, with $n$ denoting the particle density. The calculation involves the self-consistent solution of equations for an auxiliary Fermi liquid Green's function and an adaptive spectral weight. We present numerical results at low as well as high $T$, at various low to intermediate densities in the normal phase, using a minimal set of band parameters relevant to the cuprate superconductors. We display the momentum space occupation function ${m}_{k}$, energy dispersion curves locating the peaks of spectral functions, the optical conductivity, relaxation rates for quasiparticles, and the electronic spectral functions on an absolute scale. The line shapes have an asymmetric shape and a broad background that is also seen in experiments, and our calculations validate approximate recent versions of the theory. The results also display the experimentally noted high-energy kink and provide an in-depth understanding of its origin and dependence on band parameters.

Magnetic coupling of Fe-porphyrin molecules adsorbed on clean andc(2×2) oxygen-reconstructed Co(100) investigated by spin-polarized photoemission spectroscopy

The spin-polarized electronic structure of iron octaethylporphyrin (FeOEP) molecules adsorbed on a pristine and on a $c$($2\ifmmode\times\else\texttimes\fi{}2$) oxygen-reconstructed Co(100) surface has been analyzed by means of spin-polarized photoemission spectroscopy (SPPES) and first-principles density functional theory with the on-site Coulomb repulsion $U$ term ($\mathrm{DFT}+U$) calculations with and without Van der Waals corrections. The aim is to examine the magnetic exchange mechanism between the FeOEP molecules and the Co(100) substrate in the presence or absence of the oxygen mediator. The results demonstrate that the magnetic coupling from the ferromagnetic substrate to the adsorbed FeOEP molecules is ferromagnetic, whereas, the coupling is antiferromagnetic for the FeOEP on the $c$($2\ifmmode\times\else\texttimes\fi{}2$)O/Co(100) system. Spin-resolved partial densities of states extracted from ab initio $\mathrm{DFT}+U$ modeling are in fairly good comparison with the electronic spectral densities seen in angle-integrated SPPES energy dispersion curves for submonolayer coverages of FeOEP. Through combined analysis of these spectra and theoretical results, we determine that hybridization of 2$p$ orbitals of N and O with Co 3$d$ orbitals facilitates indirect magnetic exchange interactions between Fe and Co, whereas, a direct Fe-Co interaction involving the Fe ${d}_{{z}^{2}}$ orbital is also found for FeOEP on Co. It is observed through SPPES that the spin polarization of the photoemission-visible molecular overlayers decreases to zero as coverage is increased beyond the submonolayer regime, indicating that only interfacial magnetic coupling is at work. Microspot low-energy electron diffraction and low-energy electron microscopy were performed to characterize the physical order of the molecular coverage, revealing that FeOEP structural domains are orders of magnitude greater in size on $c$($2\ifmmode\times\else\texttimes\fi{}2$)O/Co(100) than on clean Co(100), which coincides with reduced scattering from the disorder and sharper features seen in SPPES.

Orientation dependence of electronic structure and optical gain of (11N)-oriented III-V-N quantum wells

A ten-band k·p Hamiltonian for III-V-N dilute nitride semiconductor quantum wells (QWs) grown on the (11N)-oriented substrates is presented. The energy dispersion curves, optical transition matrix elements, internal piezoelectric field, and optical gain of InGaAsN/GaAs on the (110), (111), (113), and (11∞)-oriented substrates are investigated including band-anti-crossing, strain, and piezoelectric field effects. The band structures and optical gain are sensitive to the substrate orientation. The fundamental transition energy is the largest for the (111)-oriented QW and the smallest for (11∞)-oriented QW. The absolute values of internal piezoelectric field in the well and barrier layers reach the maximum for the (111)-QW, and zero for the (110) and (11∞)-oriented QWs. There exists an injection current density turning point. When the injection current density is below the turning point, the (111)-oriented QW has the largest peak gain. At the larger injection current density, the (11∞)-oriented QW has the largest peak gain.

Electronic topological transition in Nb3Al under compression: An ab initio study

Very recently Yu et al. studied the structural stability of Nb3Al over a range of pressure up to 39.5GPa. They have noticed an anomaly in the pressure volume plot beyond 19.2GPa. The physical mechanism is not explained by them. The electronic structure is obtained as function of pressure by the all electron full potential linear augmented plane wave method. The energy dispersion curves are plotted. From the plot we observe that the ‘d’ like band of Nb at the zone center moves under pressure which may be the possible reason for the isostructural phase transition reported experimentally. The elastic properties are also calculated which also confirms the transition.

Uniaxial Strain Orientation Dependence of Superconducting Transition Temperature (Tc) and Critical Superconducting Pressure (Pc) in β-(BDA-TTP)2I3

Dependence of the superconducting transition temperature (T(c)) and critial superconducting pressure (P(c)) of the pressure-induced superconductor β-(BDA-TTP)(2)I(3) [BDA-TTP = 2,5-bis(1,3-dithian-2-ylidene)-1,3,4,6-tetrathiapentalene] on the orientation of uniaxial strain has been investigated. On the basis of the overlap between the upper and lower bands in the energy dispersion curve, the pressure orientation is thought to change the half-filled band to the quarter-filled one. The observed variations in T(c) and P(c) are explained by considering the degree of application of the pressure and the degree of contribution of the effective electronic correlation at uniaxial strains with different orientations parallel to the conducting donor layer.

Variational study of a mobile hole in a two-dimensional quantum antiferromagnet using entangled-plaquette states

We study the properties of a mobile hole in the $t\ensuremath{-}J$ model on the square lattice by means of variational Monte Carlo simulations based on the entangled-plaquette ansatz. Our energy estimates for small lattices reproduce available exact results. We obtain values for the hole energy dispersion curve on large lattices in quantitative agreement with earlier findings based on the most reliable numerical techniques. Accurate estimates of the hole spectral weight are provided.

Quantitative Interpretation of IR Spectra and Heat Capacity Calculations for Environment friendly Polymer

Biodegradable polymers are the need of the day. Due to their natural spontaneous degradation such materials are becoming the subject of intensive research. Poly(5-hydroxylevulinic acid) (PHLA) is a biodegradable polymer obtained from biomass resources. In the present study, one dimensional isolated chain model is used for quantitative characterization of FTIR spectra by vibrational dynamics giving potential energy distribution and dispersion curves for PHLA. The predicted values of heat capacity calculated in the range 10–500 K, via density-of-states are being reported.

Ab-initio study of electronic and elastic properties of B 2-type ductile YM (M=Cu, Zn and Ag) intermetallics

A theoretical study of structural, electronic, elastic, thermal and mechanical properties of nonmagnetic intermetallics YM (M=Cu, Zn and Ag), which crystallize in CsCl-type structure, is performed using first principles density functional theory based on full potential linearized augmented plane wave (FP-LAPW) method. The calculations are carried out within the generalized gradient approximation (GGA) for the exchange correlation potential. The calculated ground state properties such as lattice constants, bulk modulus and elastic constants agree well with the experiment. From energy dispersion curves, it is found that these compounds are metallic in nature. The ductility of these intermetallics is determined by calculating the bulk to shear ratio B/ G H . The calculated results indicate that YAg is the most ductile amongst the present YM compounds. The results obtained are compared with the available experimental and theoretical results. The mechanical and thermal properties are predicted from the calculated values of elastic constants.

Vibrational properties of graphene and graphene layers

AbstractThe phonon dispersions of graphene and graphene layers are theoretically investigated within fifth‐nearest‐neighbor force‐constant approach. Based on their symmetry groups, the number of Raman‐ and infrared‐active modes at the Γ point is given. Interatomic force constants are recalculated by fitting them to experimental phonon energy dispersion curves. Wavenumbers of optically active modes are presented as a function of number of layers (n). Our calculated results reproduce well the experimental data of G peak for graphene (1587 cm−1) and graphite (1581.6 cm−1) and clearly give the relation that ωG = 1581.6 + 11/(1 + n1.6). Copyright © 2009 John Wiley & Sons, Ltd.

Effect of some preparative parameters on optical properties of spray deposited iridium oxide thin films

Abstract Optical properties of iridium oxide films fabricated by the spray pyrolysis technique (SPT) have been investigated. The transmission and reflection spectra of the sprayed films were measured by using a double-beam spectrophotometer in the wavelength range from 200 to 2500 nm. Influences of the preparative parameters; namely, substrate temperature (350–500 °C) and solution molarity (0.005–0.03 M), on the optical characteristics were examined. The solution molarity of the iridium chloride solution was varied so as to prepare iridium oxide thin films with thicknesses ranging from 160 to 325 nm. Some important characteristics of optical absorption, such as optical dispersion energies, the dielectric constant, the ratio of the number of charge carriers to the effective mass, the single oscillator wavelength, and the average value of the oscillator strength, were evaluated. The value of the refractive index was found to depend on the chemical composition as well as the degree of stoichiometry of IrO2. The values obtained for the high frequency dielectric constant through two procedures are in the range of 2.8–3.9 and 3.3–4.6 over the relevant ranges of the substrate temperature and solution molarity, respectively. Analysis of the energy dispersion curve of the absorption coefficient indicated a direct optical transition with the bandgap energy ranging between 2.61 and 2.51 eV when the substrate temperature increases from 350 to 500 °C.

Spectroscopy of charge states of a superconducting single-electron transistor in an engineered electromagnetic environment

We study charge states of a superconducting single-electron transistor (SSET) fabricated on a heterostructure substrate that can be switched from insulating to conductive. We probe the charge states by microwave irradiation and subsequent observation of photon-assisted Josephson quasiparticle current, which allow us to obtain the energy dispersion relation in the quasicharge space. When the substrate is insulating, the charge states show an energy-dispersion curve with a gap, manifesting that coherent superposition of the two charge states is realized. When the substrate becomes conductive, its two-dimensional electrons capacitively couple to the SSET and modify the degree of dissipation to its electromagnetic environment. The dispersion relation then becomes linear, and its slope agrees with the value estimated from the charging energy including the capacitance of the two-dimensional electrons. This indicates that quantum coherence between the two charge states is lost due to the coupling with the environment.

Vibrational properties of single-wall BC 3 nanotubes

The phonon dispersions of single-wall BC 3 nanotubes with any chirality are calculated within a symmetry-based force constant model of the lattice dynamics. Based on the non-symmorphic symmetry group of the BC 3 tubes, the symmetries and number of the Raman- and infrared-active modes at Γ point of the one-dimensional Brillouin zone are given. The neighbor atom–atomic interaction force constants are recalculated by fitting them to the experimental phonon energy-dispersion curves of honeycomb BC 3 sheet. The frequencies of the optically active modes are presented as the function of diameters and chiralities for BC 3 tubes. The obtained phonon density-of-states spectra, phonon dispersion relations, and vibrational patterns of the zone-center phonons are presented and discussed in detail. The calculated frequencies of infrared-active modes are compared with the experimental values reported in the literature. The results provide comprehensive information about the vibrational properties of the BC 3 tubes and shed light on the interpretation of Raman scattering and infrared spectroscopies.

Fermi arcs in cuprate superconductors: Tracking the pseudogap belowTcand aboveT*

Using an energy-momentum dispersion for ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}\mathrm{Ca}{\mathrm{Cu}}_{2}{\mathrm{O}}_{8}$ obtained from angle-resolved photoelectron spectroscopy, we show that the shrinking Fermi arc model of the pseudogap is inconsistent with the Raman scattering below ${T}_{c}$ and specific heat near ${T}^{*}$. By simulating the quasiparticle energy dispersion curves, we show that Fermi arcs are an artifact of a $T$-dependent scattering rate.

Surface quantum well state at the striped Cu(1 1 0)(2 × 1)O surface studied by angle resolved photoemission spectroscopy

We have performed an angle resolved photoemission spectroscopy with high energy and high momentum resolutions and have observed the k dependent energy dispersion curves of the striped Cu(1 1 0)(2 × 1)O surface. It is found that the Shockley surface state electron is confined in the clean surface along the perpendicular direction to the stripes and forms a quantum well state (QWS). It has also been clearly observed that an electron of Cu–O antibonding state is confined within the oxygen covered surface.

Spin Polarons in Antiferromagnetic Planes

In the lecture the theory of spin polarons is reviewed on the basis of its analogy with the theory of lattice polarons. The energy dispersion curve for a single polaron in a two-dimensional antiferromagnetic square lattice is calculated in the self-consistent Born approximation. Also in self-consistent Born approximation the energy of a pair of interacting spin polarons is calculated. The results of calculations for realistic values of parameters of the model lead to the conclusion that pairing of spin polarons is not a likely mechanism of superconductivity in cuprates.