Femtosecond spectroscopy of relaxation processes in metals and high-T c superconductors

Spectral dependences of charge carrier relaxation rates, γ _e-e (?ω) and γ _e-ph (?ω), were observed in Au and Cu films and YBa₂Cu₃O_7-δ high-T _c superconductor films. The relaxation rates decreased substantially in the spectral region corresponding to interband transitions to the Fermi level region (?ω_Au = 2.45 eV, ?ω_Cu = 2.15 eV, and ? ω₁ = 1.89 eV and ?ω₂ = 2.08 eV for YBa₂Cu₃O_7-δ). This relaxation deceleration opens up possibilities for developing a new method, based on the spectral dependences of relaxation rates, for the determination of the Fermi level position and the parameters of electron-electron and electron-phonon interactions on the one hand and for studying deviations from the Fermi-liquid behavior in strongly correlated electronic systems. The linear γ _e-e (?ω) ∝ |?ω? E _F| spectral dependence was observed for a YBa₂Cu₃O_7-δ film near ?ω₂ = 2.08 eV, which may be evidence of a non-Fermi-liquid behavior of the electronic subsystem.     

Electrical properties of novel Li4.08Zn0.04Si0.96O4 ceramic electrolyte at high temperatures

Novel Li_4.08Zn_0.04Si_0.96O₄ electrolyte was synthesized by citric acid-assisted sol–gel method. The compound was studied by X-ray diffraction and complex impedance spectroscopy in the frequency range from 10 Hz to 10 MHz and temperature range from 573 to 773 K. The conductivity–frequency spectra exhibited two regions of conductivity dispersion related to Li⁺ ion transport in the bulk and grain boundaries. The activation energy of the bulk conductivity was found to be equal to the activation energy of relaxation frequency in the bulk. This indicated that the increase in conductivity with temperature was due to the increase in ion mobility while the number of charge carrier concentration was found to be constant with selected temperature range. The observation was in agreement with the calculated charge carrier concentration and ion mobility derived from conductance spectra, σ _ac(ω)?=?σ _o ?+?Aω ^α .     

Magnetic ordering in a [Fe/Co]<Subscript>35</Subscript> BCC film studied by nuclear resonant reflectometry

An analysis of the angular dependences of nuclear resonant reflectivity time spectra for different models of magnetic ordering in films reveals an ambiguity in the magnetization direction determined from spectra measured at one orientation of the sample. This analysis explains features in the spectra of the nuclear resonant reflectivity from a MgO/[Fe(6 ML)/Co(3 ML)]₃₅/V (1 nm) film measured before and after sample rotation by 90° about the normal to the surface. It is shown that the spectrum measured only at one orientation of the sample determines only the effective azimuth angle of magnetization γ^eff. This does not exclude the occurrence of a domain structure, while the angle γ^eff does not correspond to the true direction of the preferred orientation of magnetization. The results of measurements at two orientations of the sample can be satisfactorily matched using a model that considers a coherent mixture of states with magnetization directed along the 〈110〉 axis (77%) and with a chaotic orientation of the magnetic hyperfine field B _hf in the film plane for the other nuclei.     

Amplitude modulation and demodulation of an electromagnetic wave in magnetised acousto-optic diffusive semiconductor plasmas: Hot carrier effects

In communication processes, amplitude modulation is very helpful to save power using a single band transmission. Using the hydrodynamical model of semiconductor plasma analytical investigations are made for the amplitude modulation as well as demodulation of an electromagnetic wave incorporating carrier heating (CH) effects in acousto-optic magnetised semiconductor plasma. The CH effects add new dimensions in the present analysis. Analysis are made under different wave number regions over a wide range of cyclotron frequencies. It is found that incorporation of CH effects modifies the amplitude modulation and demodulation processes effectively. Numerical estimations are made for III–V semiconductor crystal irradiated by pump wave of frequency 1.6×10¹³ s^?1. Complete absorption of the waves takes place in all the possible wavelength regimes when the cyclotron frequency ω_c becomes nearly equal to ω₀, the pump frequency on neglecting the collision term in modulation/ demodulation indices.     

Quantized longitudinal exciton-polaritons in periodic metal–semiconductor nanostructures

We theoretically investigate the optical properties of one-dimensional photonic crystals composed of two alternating layers, namely a semiconductor film and a metallic one. The nonlocal optical response of the semiconductor is here described by using a resonant excitonic dielectric function, whereas the local response function of the metal film is modeled with Drude formula. We calculate optical spectra of the metal–semiconductor 1D photonic crystal for both s- and p-polarization geometries. In both cases the spectra exhibit a rich resonance structure due to the coupling of size-quantized excitons inside the semiconductor film with light. We show the difference between s- and p-polarization reflectivity as the angle of incidence is increased. In the p-polarization geometry, besides transverse exciton-polariton modes, longitudinal polarization waves are excited producing additional spectral resonances. The spectra become radically different when the frequency corresponding to the minimum of the first photonic pass-band is close to the exciton resonance, since such a frequency is distinct for s- and p-polarized modes. We also show how reflectivity spectra for both polarizations are modified with varying the metal filling fraction which controls the width of the gap below the lowest frequency band.     

Electron characteristic loss spectra of molybdenum dichalcogenides

The complete sets of the fundamental optical functions of molybdenum dichalcogenides have been considered for the first time. The energies of their bulk and surface plasmons of two types are determined. It is found that the energies of long-wavelength plasmons correlate with the energies of the deep minima in the reflectivity and ?₂ E ² spectra and the maxima in the reflectivity phase spectra.     

Quantum size effect in the photoemission intensity emitted from thin metal films

Photoemission spectra are calculated for thin films of Ag on Au(111) using the method of Green's function matching. For very low photon energies (ω ≲ 10 eV) the peak intensities of the thin film states oscillate with the film thickness. These oscillations are caused by the relaxation of the k_⊥-conservation of the optical excitation in photoemission.     

Photon echoes in single- and trilayer semiconductor films of different nanosized thicknesses and their properties

A femtosecond stimulated photon echo is detected for the first time in thin semiconductor films with thicknesses of 100, 800, and 2400 nm. It is established that relaxation time T ₁ varies with ZnO film thickness, taking values of 0.96, 2.96, and 4.312 ps at thicknesses of 2400, 800, and 100 nm, respectively. For ZnO/Si⁺/Si^? trilayer films, the relaxation time is T ₁ = 12.73 ps.     

掺杂半导体/金属膜系的光谱透射反射特性

提出了以改变半导体薄膜的掺杂浓度来调节它的等离子体频率ω_p,使它的高透射区移至可见光带.选择本征吸收频率在近紫外区的金属与之构成最佳的D/M光谱透射-反射膜系,同时结合掺杂半导体膜与金属膜的最佳厚度组合以形成较理想的透明隔热复合膜. 关键词：     

Optical properties of cubic and tetragonal KTa0.5Nb0.5O3 by density functional theory

Using density functional methods, optical properties of KTa_0.5Nb_0.5O₃ (KTN) are investigated. The imaginary part of dielectric function ε₂(ω), the optical absorption coefficient I(ω) and the reflectivity R(ω) of KTN in the cubic (paraelectric) and tetragonal (ferroelectric) phases are calculated. The origin of the different behavior of the optical spectra between the two phases is discussed.     

Excitation by light ofω + andω − surface plasma waves in thin metal layers

Calculations with the extended Fresnel formulae show, thatω ₊ andω _? surface plasma waves can be excited by light, using the following optical arrangement: The metal foil is embedded between two dielectric layers of low index of refraction and equal thickness. Both sides of this 3-layer packet are in optical contact with a high index prism. Total reflection is broken byω ₊ andω _? resonances in reflectivity, transmission and absorption. Theω ₊ resonance is mainly transmissive, (transmission through a 800 Å thick silver foil greater than 50% at a wavelength of 5,461 Å) theω _? resonance is mainly absorptive. The polarization of the transmitted light exceeds 99%-theω ₊ resonance is proposed as a principle for a new optical polarizer.     

Superlattice space-charge-wave analyses in a modified drift-diffusion model and in a simplified balance-equation formulation

Unstable space-charge-waves in superlattice miniband transport are investigated using a modified drift-diffusion model and using a simplified one-dimensional hydrodynamic balance-equation formulation with a relaxation-time approximation. We point out that the earlier widely accepted notion, that in the small wavevector limit the space-chargewave (SCW) propagates at a phase velocity equal to the carrier drift velocity and with an amplitude-growth rate equal to the negative value of the mobility frequency, -ω _c, was crucially based on the implicit assumption that the system momentum relaxation time τ_m is extremely small. Taking account of a finite momentum relaxation time, we find that even the drift-diffusion model would yield results significantly different from the above predictions: the phase velocity of a long-wavelength traveling SCW can be much slower than the carrier drift velocity and its amplitude-growth rate much smaller than -ω_c. A hydrodynamic balance-equation formulation, which properly treats energy dissipation and further reduces the amplitude-growth rate, provides a convenient tool for improved qualitative analyses in SCW-related problem in semiconductor superlattices.     

Composition depth profiles of Bi3.15Nd0.85Ti3O12 thin films studied by X-ray photoelectron spectroscopy

In the present work, X-ray photoelectron spectroscopy (XPS) was used to investigate the composition depth profiles of Bi_3.15Nd_0.85Ti₃O₁₂ (BNT) ferroelectric thin film, which was prepared on Pt(1 1 1)/Ti/SiO₂/Si(1 0 0) substrates by chemical solution deposition (CSD). It is shown that there are three distinct regions formed in BNT film, which are surface layer, bulk film and interface layer. The surface of film is found to consist of one outermost Bi-rich region. High resolution spectra of the O 1s peak in the surface can be decomposed into two components of metallic oxide oxygen and surface adsorbed oxygen. The distribution of component elements is nearly uniform within the bulk film. In the bulk film, high resolution XPS spectra of O 1s, Bi 4f, Nd 3d, Ti 2p are in agreement with the element chemical states of the BNT system. The interfacial layer is formed through the interdiffusion between the BNT film and Pt electrode. In addition, the Ar⁺-ion sputtering changes lots of Bi³⁺ ions into Bi⁰ due to weak Bi-O bond and high etching energy.     

Formation of nanostructured ω-Al 7 Cu 2 Fe crystalline phase by the ball milling technique

Crystalline ω-Al₇Cu₂Fe bulk samples were prepared by arc furnace and then by means of milling, the average grain size of these samples is reduced to the nanometer scale. The structural and magnetic properties of the nanostructured ω-Al₇Cu₂Fe phase have been studied by X-ray diffraction employing Rietveld method, Mössbauer spectroscopy and vibrating sample magnetometry. The results indicate that the average grain size of the synthesized sample (ω-phase) rapidly decreases from 79 to 12 nm after 5 h of milling. Furthermore, the hyperfine parameters of the nanostructured samples are higher than the values for the bulk ω-phase. Magnetic measurements show a weak ferromagnetic behavior with M _s ?=?0.46 emu.g^???1 for the bulk ω-phase. After the milling process this value increases to M _s ?=?1.50 emu.g^???1 due to the formation and growth of a magnetic interstitial region after reducing the average grain size of the sample.     

Single-Ion Mechanism of Weak Ferromagnetism and a Spin-Flop Transition in One- and Two-Position Antiferromagnets

Nuclear resonance reflectivity from a [Dy₁₉Gd₁₉] × 20 superlattice has been measured utilizing the 25.652 keV nuclear level of ¹⁶¹Dy. The measured time spectra of nuclear resonance reflectivity make it possible to reveal the variation of the hyperfine magnetic field B_hf on dysprosium nuclei in the temperature range of 4–110 K and to determine the relaxation time of the hyperfine field, using the decay speed-up of the excited state of ¹⁶¹Dy nuclei.     

On the dynamic conductivity for an electron-impurity system

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, n_s) and the energy shift Δ(ω, ε, n_s) 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, n_g, of impurities. With the assumption of a finite range potential, these equations are solved. It is found that γ (ω, n_s) 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, τ_c, to the average duration of a scattering, τ_d, is 100 or less, that γ (ω, n_s) decreases monotonically from its static value γ (0, n_s), and becomes vanishingly small in the region ω≈1/τ_d, and that in the static limit (ω=0), γ=γ₀[1?(2/π) (γ₀τ_d)+…], 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, n_s) for degenerated electrons is calculated. The deviation, σ-σ₀, from the conventional expression σ₀=(?i) (n_ee²/M) [ω-iΓ₀]^?1, (n_e]=number density of electrons), for 0°K, is appreciable when the ratio τ_c/τ_d 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.     

First-principles study of the electronic and optical properties of rutile TiO2

The electronic, structural properties and optical properties of the rutile TiO₂ 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 results including lattice parameter, bulk modulus, density of states, the reflectivity spectra, the refractive index and band gap are compared with the experimental data. We present calculations of the frequency-dependent complex dielectric function ε(ω) and its zero-frequency limit ε₁(0).     

Manifestation of multiband optical properties of MgB2

The optical conductivity of MgB₂ has been determined on a dense polycrystalline sample in the spectral range 6 meV–4.6 eV using a combination of ellipsometric and normal incidence reflectivity measurements. σ₁(ω) features a narrow Drude peak with anomalously small plasma frequency (1.4 eV) and a very broad ‘dome’ structure, which comprises the bulk of the low-energy spectral weight. This fact can be reconciled with the results of band structure calculations by assuming that charge carriers from the 2D σ-bands and the 3D π-bands have principally different impurity scattering rates and negligible interband scattering. This also explains a surprisingly small correlation between the defect concentration and T_c, expected for a two-gap superconductor. The large 3D carrier scattering rate suggests their proximity to the localization limit.     

Infrared optical modulators for missile testing

The continued proliferation of imaging infrared (I²R) missile systems has created the need for fully representative infrared scene generators. In order to test these I²R systems correctly a very large dynamic range is required of the scene generator. This cannot easily be produced by the standard approach of an array of suspended resistor heater elements. This paper therefore describes the fabrication of infrared optical modulators for use in high dynamic range and high frame rate scene generation. Modulation is produced by the heating of a variable reflectivity coating. This is achieved by coating a suitable substrate with a thin film of vanadium dioxide (VO₂) via a reactive sputtering process. VO₂ undergoes a semiconductor to metal phase transition at approximately 68°C and the associated change in reflectivity is exploited to create the modulator. The correct stoichiometry of the thin film of VO₂ is critical in producing high frame rate devices. Both transmissive and reflective modulators are described.     

“Negative” absorption and “negative” emission in nonlinear optics

The nonlinear susceptibilities X³ (ω₂ = ω₁ ? ω₁ + ω₂) calculated for stimulated Raman scattering (SRS) and two- photon absorption (TPA) under the condition of one-photon resonance in the case of a multilevel system with broad overlapping absorption bands (corresponding to polyatomic molecules) are discussed. Spectra of Im X³ with changed sign in some spectral regions are shown. The result is connected with a disbalance of the system, due to the large relaxation constants.