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1.
By using the Fourier expansion method, we have developed an approach to calculate the effective dielectric index of a two-dimensional photonic crystal. The approach is very general: it can take into account various Bravais lattice structure as well as arbitrary spatial variation of the dielectric index. It has been found that near a nondegenerate frequency ωn(Γ) at Γ point, the transverse magnetic (TM) mode is ordinary, as it is independent of the propagation direction, whereas in general the transverse electric (TE) mode depends on the propagation direction, it is extraordinary. Therefore, a two-dimensional photonic crystal can always be described by an effective dielectric index for TM mode near the nondegenerate frequency ωn(Γ). However, the TE mode is much more complicated unless the lattice structure is highly symmetric. Moreover, a two-dimensional square photonic crystal has been identified as an effective birefringent crystal having two negative refractive indexes from the perspective of Snell's law. 相似文献
2.
We discuss the magnetostatic energy of checkerboard domain structures in ultrathin magnetic films (of a few monolayer thickness) and in an atomic monolayer using simple magnetostatic considerations where the easy direction of magnetization is perpendicular to the film. The checkerboard domain size, D, the domain-wall width, ω, the ratio f of the uniaxial surface anisotropy, Ks, to the dipolar energy and the binding energy, (BE), have been calculated numerically with the variational parameter δ and the number of atomic layers, nl, as parameters. 相似文献
3.
We assume an explicit stochastic dynamics for the microscopic variables in the Heisenberg magnet based on experimental data. We obtain additional terms to the Fokker-Planck Equation and also explicit expressions for the numerical coefficients in the Master Equation, near Tc. 相似文献
4.
Jens O. Andersen 《Annals of Physics》2005,317(2):281-353
There has been significant progress in our understanding of finite-temperature field theory over the past decade. In this paper, we review the progress in perturbative thermal field theory focusing on thermodynamic quantities. We first discuss the breakdown of naive perturbation theory at finite temperature and the need for an effective expansion that resums an infinite class of diagrams in the perturbative expansion. This effective expansion which is due to Braaten and Pisarski, can be used to systematically calculate various static and dynamical quantities as a weak-coupling expansion in powers of g. However, it turns out that the weak-coupling expansion for thermodynamic quantities are useless unless the coupling constant is very small. We critically discuss various ways of reorganizing the perturbative series for thermal field theories in order to improve its convergence. These include screened perturbation theory (SPT), hard-thermal-loop perturbation theory, the Φ-derivable approach, dimensionally reduced (DR) SPT, and the DR Φ-derivable approach. 相似文献
5.
《Solid State Communications》1987,62(8):563-565
Infrared reflectance studies were carried out on Stage 1 and Stage 2 single crystals of silver-intercalated TiS2. The spectra clearly showed the Drude edge and were analyzed using the single carrier Drude model. The Drude parameter values, plasma frequency (ωp), damping constant (Γ), high frequency dielectric constant (ε∞) and carrier concentration were calculated. 相似文献
6.
Padé approximation is superior to Taylor expansion when functions contain poles. This is especially important for response functions in complex frequency domain, where singularities are present and intimately related to resonances and absorption. Therefore, we introduce a rational Padé approximant for the complex medium refractive index n(ω). The approximant is calculated using only local information of medium dispersion properties close to a carrier frequency ω0. In return it typically offers an accurate global representation of medium dispersion and absorption. Moreover, the fulfillment of the causality principle and the Kramers-Kronig relation can be established. In practice, our results are relevant if n(ω) is known only for ω?ω0 whereas optical field is spectrally broad such that (i) the resonance absorption becomes important and (ii) a traditional polynomial dispersion operator diverges and induces huge errors. As an exemplary application we use the approximant to derive a nonlocal envelope model for ultrashort pulses. The model provides a natural bridge between the commonly used local envelope equations and the most general non-envelope models operating directly with the electric field. 相似文献
7.
A theoretical study on the structural, elastic, electronic and lattice dynamic properties of AlxYyB1−x−yN quaternary alloys in zinc-blend phase has been carried out with first-principles methods. Information on the lattice parameter, the lattice matching to available substrates and energy band-gaps is a prerequisite for many practical applications. The dependence of the lattice parameter a, bulk modulus B, elastic constants C11, C12 and C44, band-gaps, optical phonon frequencies (ωTO and ωLO), the static and high-frequency dielectric coefficients ε (0) and ε (∞) and the dynamic effective charge Z? were analyzed for y=0, 0.121, 0.241, 0.362 and 0.483. A significant deviation of the bulk modulus from linear concentration dependence was observed. A set of isotropic elastic parameters and related properties, namely bulk and shear moduli, Young's modulus, Poisson's ratio are numerically estimated in the frame work of the Voigt-Reuss-Hill approximation. The resistance to changes in bond length and lateral expansion in AlxYyB1−x−yN increase with increasing y concentration. We observe that at y concentration about 0.035 and 0.063, AlxYyB1−x−yN changes from brittle to ductile and Γ-X indirect fundamental gap becomes Γ-Γ direct fundamental gap. There is good agreement between our results and the available experimental data for the binary compound AlN, which is a support for those of the quaternary alloys that we report for the first time. 相似文献
8.
A.V. Syromyatnikov 《Journal of magnetism and magnetic materials》2009,321(7):928-930
We discuss the spin-wave interaction in two-dimensional (2D) Heisenberg ferromagnet (FM) with dipolar forces at TC?T?0 using 1/S expansion. A comprehensive analysis is carried out of the first 1/S corrections to the spin-wave spectrum. In particular, we obtain that the spin-wave interaction leads to the gap in the spectrum εk renormalizing greatly the bare gapless spectrum at small momenta k. Expressions for the spin-wave damping Γk are derived self-consistently and it is concluded that magnons are well-defined quasi-particles in both quantum and classical 2D FMs at small T. We observe thermal enhancement of both Γk and Γk/εk at small momenta. In particular, a peak appears in Γk and Γk/εk at small k and at any given direction of k. If S∼1 the height of the peak in Γk/εk is not larger than a value proportional to T/D?1, where D is the spin-wave stiffness. In the case of large spins S?1 the peak in Γk/εk cannot be greater than that of the classical 2D FM found at k=0 whose height is small only numerically: Γ0/ε0≈0.16 for the simple square lattice. 相似文献
9.
We have investigated the current for the system of vibrating quantum dot irradiated with a rotating magnetic field and an oscillating magnetic field by nonequilibrium Green's function. The rotating magnetic field rotates with the angular frequency ωr around the z-axis with the tilt angle ?, and the time-oscillating magnetic field is located in the z-axis with the angular frequency ω. Different behaviors have been shown in the presence of electron-phonon interaction (EPI) which plays a significant role in the transport. The current displays asymmetric behavior as the source-drain bias eV=0, novel side peaks or shoulders can be found due to the phonon absorption and emission procedure, and the negative differential resistance becomes stronger as the parameter g increases. Furthermore, the strong EPI also destroys the quasiperiodic oscillations of current in the region μ0B1>2.5Δ. The electron transport properties are also significantly influenced by the linewidth function Γ. 相似文献
10.
S. Fujita J.P. Riley C.C. Chen Ching-Lung Ko C.S. Raju T. Yamaguchi 《Journal of Physics and Chemistry of Solids》1973,34(12):2075-2089
The correlation function formula for the dynamic conductivity of a system of non-interacting electrons in the field of impurities is analyzed in terms of proper connected diagrams. By selecting those diagrams appropriate in the region of weak coupling and low impurity concentration, a set of coupled equations for the energy broadening γ (ω, ε, ns) and the energy shift Δ(ω, ε, ns) is derived, where both γ and Δ depend on the frequency ω of a probing field, the energy ε of the electron, and the concentration, ng, of impurities. With the assumption of a finite range potential, these equations are solved. It is found that γ (ω, ns) is smaller than that extrapolated value which the conventional expression γ0 for the low-concentration collision frequency would predict, in the entire region studied, that the difference γ0-γ becomes appreciable when the ratio of the average time between scatterings, τc, to the average duration of a scattering, τd, is 100 or less, that γ (ω, ns) decreases monotonically from its static value γ (0, ns), and becomes vanishingly small in the region ω≈1/τd, and that in the static limit (ω=0), γ=γ0[1?(2/π) (γ0τd)+…], that the energy shift Δ is positive, and increases from 0 and reach a peak of magnitude γ0 as ω is raised from 0. By using the γ and Δ obtained, the dynamic conductivity σ(ω, ns) for degenerated electrons is calculated. The deviation, σ-σ0, from the conventional expression σ0=(?i) (nee2/M) [ω-iΓ0]?1, (ne]=number density of electrons), for 0°K, is appreciable when the ratio τc/τd is 100 or less. The field-term correction, which arises from the modification of the scattering due to the probing field, is found to be negligible in the entire region studied. 相似文献
11.
Jian-Jun Jiang Yong-Jun Liu Cui-Hong Yang 《Journal of magnetism and magnetic materials》2009,321(19):3300-3306
We investigate the effect of frustration on spin-wave excitation spectra and the properties of the quasi-one-dimensional Heisenberg chain using a spin-wave-wave analysis, the exact diagonalization method and the density matrix renormalization group method. The results show that frustration can cause the softening of the acoustic excitation spectrum ω3, as well as the hardening of the optical excitation spectrum ω1. As a function of the frustration parameter α, the phase diagram exhibits a ferromagnetic phase, a narrow canted phase and a singlet phase. The results obtained from numerical methods show that the spin gap obviously opens and the tetramer-dimer state dominates the properties of the ground state in the singlet phase. 相似文献
12.
Mervat H. Simbel 《Zeitschrift für Physik A Hadrons and Nuclei》1983,313(4):311-317
Using a simple model which assumes that neutrons evaporated from an excited nucleus follow a Poisson probability law, and that the ratios of neutron emission to fission reduced widths,Γ n /Γ f remain essentially constant during the evaporation cascade, we evaluateΓ n /Γ f for a large number of reactions between heavy ions where fission appreciably competes with neutron evaporation. We discuss the systematics ofΓ n /Γ f in terms of the mass number of the compound nucleus and the fissility parameter. The results obtained agree well with the previous systematics and with the prediction of fission-spallation reactions. 相似文献
13.
The optical properties of the SrFX (X=Cl, Br, I) compound have been reported using the full potential linearized augmented plane wave (FP-LAPW) method as implemented in the WIEN2K code. We employed the generalized gradient approximation (GGA), which is based on exchange-correlation energy optimization to calculate the total energy. Also we have used the Engel-Vosko GGA formalism, which optimizes the corresponding potential for band structure calculations. Our calculations show that the valence band maximum (VBM) and conduction band minimum (CBM) are located at Γ resulting in a direct energy gap. We present calculations of the frequency-dependent complex dielectric function ε(ω) and its zero-frequency limit ε1(0). We find that the value of ε1(0) increases on decreasing the energy gap. The reflectivity spectra and absorption coefficient have been calculated and compared with the available experimental data. 相似文献
14.
Fabio Caruso Feliciano Giustino 《The European Physical Journal B - Condensed Matter and Complex Systems》2016,89(11):238
We study the spectral function of the homogeneous electron gas using many-body perturbation theory and the cumulant expansion. We compute the angle-resolved spectral function based on the GW approximation and the “GW plus cumulant” approach. In agreement with previous studies, the GW spectral function exhibits a spurious plasmaron peak at energies 1.5ωpl below the quasiparticle peak, ωpl being the plasma energy. The GW plus cumulant approach, on the other hand, reduces significantly the intensity of the plasmon-induced spectral features and renormalizes their energy relative to the quasiparticle energy to ωpl. Consistently with previous work on semiconductors, our results show that the HEG is characterized by the emergence of plasmonic polaron bands, that is, broadened replica of the quasiparticle bands, red-shifted by the plasmon energy. 相似文献
15.
《Nuclear Physics B》1996,472(3):529-590
We analyze the large-order behaviour in perturbation theory of classes of diagrams with an arbitrary number of chains (i.e. photon lines, dressed by vacuum polarization insertions). We derive explicit formulae for the leading and subleading divergence as n, the order in perturbation theory, tends to infinity, and a complete result for the vacuum polarization at the next-to-leading order in an expansion in l / N f, where N f is the number of fermion species. In general, diagrams with more chains yield stronger divergence. We define an analogue of the familiar diagrammatic R-operation, which extracts ultraviolet renormalon counterterms as insertions of higher-dimension operators. We then use renormalization group equations to sum the leading (in n/ N f ) k corrections to all orders in l INf and find the asymptotic behaviour in n up to a constant that must be calculated explicitly order by order in 1/Nf. 相似文献
16.
In the derivation of Holographic Dark Energy (HDE), the area law of the black hole entropy assumes a crucial role. However, the entropy-area relation can be modified including some quantum effects, motivated from the Loop Quantum Gravity (LQG), string theory and black hole physics. In this paper, we study the cosmological implications of the interacting logarithmic entropy-corrected HDE (LECHDE) model in the framework of Brans-Dicke (BD) cosmology. As system’s infrared (IR) cut-off, we choose the average radius of Ricci scalar curvature, i.e. R ?1/2. We obtain the Equation of State (EoS) parameter ω D , the deceleration parameter q and the evolution of energy density parameter $\varOmega'_{D}$ of our model in a non-flat universe. Moreover, we study the limiting cases corresponding to our model without corrections and to the Einstein’s gravity. 相似文献
17.
W. Wonneberger 《Zeitschrift für Physik B Condensed Matter》1983,53(3):167-173
The third order response properties of the classical model for charge transport by charge density waves above the conductivity threshold are investigated analytically. The model is isomorphic to that of a resistively shunted Josephson circuit (RSJ model) and thus exhibits an oscillating solution above a threshold electric driving field with a characteristic internal frequencyω ph . By a systematic perturbation expansion in powers of the perturbing field strengths thedc response of the time varying state to twoac fields of frequencies ω and 2ω is evaluated. This determines the harmonic mixing signal (HMS) as a third order response. The general formula is explicitly evaluated far above threshold giving a result similar to the Kanter-Vernon formula for the rectified signal in second order. In addition to the resonance atω ph =ω a second resonance is found atω ph =2ω. A characteristic feature for the HMS above threshold is the absence of an out of phase component, i.e. the internal phase shift is zero. 相似文献
18.
We analyze a fermionic Ising spin glass model in the presence of a transverse magnetic field Γ within a cluster mean field theory. The model considers a Sherrington-Kirkpatrick type interaction between magnetic moments of clusters with a ferromagnetic intra-cluster coupling J0. The spin site operators are written as a bilinear combination of fermionic operators. In these quantum spin glass model, the inter-cluster disorder is treated by using a framework of one-step replica symmetry breaking within the static approximation. The effective intra-cluster interaction is then computed by means of an exact diagonalization method. Results for several cluster size ns, values of Γ and J0 are presented. For instance, the specific heat shows a broad maximum (for ns>1) at a temperature above the freezing temperature Tf, which is characterized by the inter-cluster replica symmetry breaking. Phase diagrams T versus Γ show that the critical temperature Tf(Γ) decreases for any value of ns when Γ increases until it reaches a quantum critical point at some value of Γc. 相似文献
19.
20.
We calculate all direct π-exchange ring diagrams to arbitrary orders in nuclear matter. Our model incorporates intermediate Δ(1236) resonances, correlations, ρ- as well as π-meson exchange and mesonic form factors. We find that the convergence of the diagram summation is governed by two quantities which have an immediate physical interpretation: the Landau parameter G'0 and the threshold for pion condensation. The contribution to the energy of nuclear matter from terms of third and higher order is 2 MeV/particle (repulsion) at normal density, with a strong density dependence. 相似文献