Exponential absorption edges in GeS

The absorption coefficient of GeS at the absorption edge behaves according to Urbach's rule. The electron-phonon interaction involves all LO phonon modes expected in the directions studied. The phonon involved in the b-axis direction has a frequency value equal to 0.034 eV while for the three LO phonons in the a-axis direction the values 0.015, 0.040 and 0.444eV were found respectively. A comparison with results for isomorphic GeSe indicates that the ionic trend obtained using the electron-phonon coupling constant σ₀ agrees with the trend which is established using ionicity values from Philip's scale.     

Electroabsorption in TlInS2

Electroabsorption spectra of single crystals have been studied near the fundamental absorption edge at 77 and 300 K. At 300 K two positive peaks (2.34 and 2.42 eV) and a negative peak (2.38 eV) are observed in the electroabsorption spectrum. At liquid-nitrogen temperature a fine structure corresponding to the formation of a parabolic exciton (2.503 eV) is observed.Values of the width of the forbidden gap E_g, the n = 1 exciton positions, the exciton activation energy ΔE_b, the effective Bohr radius a_exc, the reduced effective mass of an electron-hole pair μ, and the exciton ionization field F(E_g = 2.535 eV, E_exc = 2.503 eV, E_b = 32 meV, $\text{a}{}_{\mathrm{<mtext>exc</mtext>}}\text{=}\text{28}\text{A}\text{A;}\text{;}$;, μ = 0.15 m₀, and F = 1.2 × 10⁵ V cm^-1) have been determined from the electroabsorption spectrum.     

One-dimensional electronic order in underdoped surface of YBa2Cu3Oy studied by STM

We have performed low-temperature scanning tunneling microscopy (STM) experiments on the cold-cleaved surface of YBa₂Cu₃O_y single crystals to study the nanoscale electronic order in high-T_c superconductors. STM images measured at low-bias voltage below ∼50 meV show the one-dimensional (1D) electronic modulation along the Cu-O bonds (parallel to the b-axis). The 1D electronic modulation does not have long-range order and the periodicity along the a-axis varies within the range ∼2a-4a depending on the position on the surface, indicating the glassy electronic order in the underdoped CuO₂ plane.     

Synthesis,magneto-optics and fluorescence of the ferromagnetic semiconductor Eu3(II)SiO5

Small single crystals of the transparent ferromagnet Eu₃SiO₅ have been synthesized by high temperature chemical transport. Magnetic measurements indicate T_c = 9°K and a saturation magnetization very close to 7 μ_B/Eu ion. Crystalline samples show a very low residual optical absorption and an absorption edge near 2 eV which displays a small red shift of 20 meV on cooling below T_c. Samples, containing a small percentage of dissolved EuO clusters, show in addition an absorption band at lower energies with a temperature dependence and magnetic behavior typical for EuO. The photoluminescence of the pure compound has a single emission band near 1·9 eV with a high quantum yield. At low temperatures also the fluorescence displays a red shift similar to that of the absorption edge. The fluorescence is accompanied with photoconductivity.     

Measurement of the barrier height of the reflective W {110} plane boundaries in surface diffusion of single atoms

Plane boundaries are in general reflective to diffusing atoms. From about 2000 heating periods of direct field ion microscope observations we have studied adatom plane edge interaction and determined the barrier heights of the W {110} plane boundaries in single atom diffusion of W, Re, and Ir. Using equations we have derived recently, the barrier heights ΔE_b are found to be about 200 meV for these adatoms.     

Optical properties of thallium indium disulfide (TlInS2) single crystals

Transmission, photoluminescence, and reflectance spectra of TlInS2 single crystals grown by the Bridgman–Stockbarger method were measured at 4.2 K near the fundamental absorption edge. Narrow lines at ~2.5535 and ~2.5694 eV were observed in the transmission spectrum and assigned to ground and excited free-exciton states, respectively. The free-exciton binding energy and band-gap energy Eg were found to be ~21.2 meV and ~2.5747 eV, respectively. A recombination mechanism was proposed for the TlInS2 near-band-edge and deep luminescence.     

Optical anisotropy of orthorhombic InS

The optical anisotropy of InS single crystals in the range of photon energy from 1.8 to 3.5 eV has been studied by absorption, electroreflectance and wavelength-derivative reflectance measurements. These systematic optical measurements for the polarizations, E//a and E//b, have revealed that the transition at the fundamental absorption edge of InS is allowed for E//b, and there exist three distinct doublet transitions having different selection rules in the photon energy region from 2 to 3.5 eV; Bo and B^'₀doublet allowed only for E//b, A₀ and A^'₀ allowed only for E//a, and E₁ and E^'₁ allowed for both polarizations. The observed results are discussed based on the anisotropic nature of two chemical bonds in InS, cation-cation and cation-anion.     

Absorption spectra and optical gap energies of WxMo1−xO3

Optical absorption spectra of single crystals and powder specimens of W_xMo_1-xO₃ reveal fundamental absorption edges with gap energies of 2.77 eV (WO₃) and 3.05 eV (MoO₃). A continuous shift of the absorption edge and a simultanous appearance of absorption tails was found for ternary oxides. A theoretical model is proposed, based on the Anderson-localisation model with potential fluctuations due to tungsten-molybdenum disorder.     

The absorption edges of GeS and Urbach's rule

The low-energy absorption tail of the E∥a exciton in GeS obeys Urbach's rule over at least three orders of magnitude in absorption coefficient for 4.2 ? T ? 240 K. The E∥b polarization (for which no exciton is observed) does not obey Urbach's rule. The E∥a Urbach edge is caused by interactions between the excitons and a 8.7 meV rigid-layer vibrational mode. Our results are consistent with Sumi and Toyazawa's theory of the Urbach edge, and inconsistent with the models of Dow, Redfield and Skettrup. The absorption edge of GeS is confirmed as a direct edge.     

Optical absorption and electrical conductivity of flash evaporated CuGaTe2 thin films

The optical absorption edge of CuGaTe₂ thin films in the energy range 1 to 2·3 eV was studied. The characteristic band gaps were found to be 1·23 eV and 1·28 eV whereas the acceptor ionization energy was 170 meV. Electrical conductivity measurements were carried out in the temperature range 300–550 K and two acceptor states with ionization energies 400 meV and 140 meV were found. The origin of acceptor states is explained based on covalent model.     

Optical absorption edge in SnS

Optical absorption in single crystals of tin sulfide has been studied at many temperatures between 100 and 300 °K, in the wavelength range 2·2–0·8 μ. From the interference fringe patterns the absorption coefficient, reflection coefficient and index of refraction as a function of wavelength were determined for two light polarizations (ε∥a and ε∥b). From an analysis of the data, indirect band gaps of 1·142 and 1·095 eV were found for the two directions of polarization. Also it was found that the phonon assisted transitions required the participation of two phonons at different energy thresholds with energies 0·033 or 0·038 eV and 0·082 or 0·113 eV, with reference to the two axis. The temperature dependence of the indirect band gap for each direction of light polarization is linear with a slope ?4·05 × 10^?3eV and ?4·37 × 10^?3 eV respectively.     

Fundamental optical properties of Cd1-xMnxSe single crystals

The absorption near the fundamental edge of Cd_1-xMn_xSe was measured in the composition range 0<x<0.3 at room and liquid nitrogen temperature with the electric field of the radiation parallel and perpendicular to the hexagonal axis. An exponential dependence of the absorption coefficient versus photon energy was found, and a linear dependence of energy gap E₀ on composition was obtained. The room temperature reflectivity measurements in the energy range 2.5–5.2eV, for two polarization of light were performed, and a linear dependence of the interband transitions energies vs. alloy composition was found.     

Electronic excitations and luminescence of SrMgF4 single crystals

The electronic and crystal structures of SrMgF₄ single crystals grown by the Bridgman method have been investigated. The undoped SrMgF₄ single crystals have been studied using low-temperature (T = 10 K) time-resolved fluorescence optical and vacuum ultraviolet spectroscopy under selective excitation by synchrotron radiation (3.7–36.0 eV). Based on the measured reflectivity spectra and calculated spectra of the optical constants, the following parameters of the electronic structure have been determined for the first time: the minimum energy of interband transitions E _g = 12.55 eV, the position of the first exciton peak E _n = 1 = 11.37 eV, the position of the maximum of the “exciton” luminescence excitation band at 10.7 eV, and the position of the fundamental absorption edge at 10.3 eV. It has been found that photoluminescence excitation occurs predominantly in the region of the low-energy fundamental absorption edge of the crystal and that, at energies above E _g , the energy transfer from the matrix to luminescence centers is inefficient. The exciton migration is the main excitation channel of photoluminescence bands at 2.6–3.3 and 3.3–4.2 eV. The direct photoexcitation is characteristic of photoluminescence from defects at 1.8–2.6 and 4.2–5.5 eV.     

Reflection spectra of orthorhombic indium iodide

The reflection spectra of InI single crystal have been measured using linearly polarized light. Pronounced anisotropy is observed between two directions of polarization E ∥ c and E ∥ a. The first peak at 2.018 eV for E ∥ c is direct allowed exciton transition with binding energy 4.3 meV. The E ∥ a spectrum suggests that the transition is forbidden.     

Anomalies in the optical properties of nanocrystalline copper oxides CuO and Cu2O near the fundamental absorption edge

A 0.1–0.15-eV displacement of the fundamental absorption edge in the optical absorption spectra of nanocrystalline oxide n-CuO (relative to the position of the fundamental absorption edge in the spectra of CuO single crystals) towards lower energies (red shift) is observed against the background of strong blurring. Nanocrystalline n-Cu₂O exhibits a displacement of the fundamental absorption edge towards higher energies (blue shift) by approximately 0.35 eV. The size of crystallites in n-CuO and n-Cu₂O ranges from 10 to 90 nm. The blue shift of the fundamental absorption edge of n-Cu₂O is typical of classical wide-gap semiconductors and can be explained by size quantization upon a change in the particle size. The anomalous red shift of the fundamental absorption edge of the strongly correlated nanocrystalline oxide n-CuO can be attributed to the highly defective structure of n-CuO, anomalies in the electronic structure of strongly correlated compounds based on 3d metals, and their tendency to electronic phase separation with the formation of metal-like inclusions.     

Fundamental absorption edge and urbach rule in lithium hydride single crystals

The fundamental absorption edge of optically transparent lithium hydride single crystals grown from the melt by Bridgman—Stockbarger method has been investigated in the temperature range 77–594K. The absorption edge behaviour proved to obey the Urbach rule at temperatures above 200 K. The values of σ₀, ħω₀ and K₀ were found to be 0.96, 5.05 eV and 2.5 × 10⁶ cm⁻¹, respectively. It was shown that the absorption edge was originated mainly by the acoustic phonons. Using the obtained value of σ₀, the ionicity and the exciton-phonon interaction constant in LiH single crystals have been estimated.     

STM spectroscopy of an organic superconductor

Electron tunneling spectroscopy of the organic superconductor κ-(BEDT-TTF)₂Cu(NCS)₂using low temperature scanning tunneling microscope (STM) is reported. The tunneling differential conductance in the superconducting phase was obtained in theb–cplane of a single crystal, by varying the tip position on the sample surface. The differential conductance is reduced near zero bias voltage and enhanced at the gap edge, associated with the superconducting gap structure below[formula] K. The gap width differs slightly from sample to sample, while the overall functional shape of the conductance is sample-independent. The tunneling conductance is reduced to almost zero near zero bias voltage, while it is finite inside the gap edge. The curve obtained cannot be fit to the BCS density of states withs-wave pairing symmetry, even if the life-time broadening of one-electron levels is taken into account. Finite conductance inside the gap edge suggests anisotropy of the gap. However, the conductance curve obtained is not explained by a simpled-wave symmetry for Δ(k). The reduced conductance near zero bias voltage suggests a finite gap. An anisotropic model with a finite gap, in which Δ(k) varies depending on the direction ink-space, is examined. The tunneling conductance in the low-energy region is almost fit by the model with Δ_min = 2 meV and Δ_max = 6 meV. The finite conductance is explained by introducing a small effect of life time broadening. We conclude that the gap is anisotropic and is finite (at least Δ_min = 2 meV) on the entire Fermi surface.     

Correlations of particle orientation in monolayer films

Two-dimensional and three-dimensional models of an extended layer that consists of orientation-ally ordered particles are used to describe the ordering in monolayer films. In both models, when the distance between particles of the layer increases, the correlation function decreases to zero (for the parameter of adhesion to the substrate a = 0) or remains constant (for a ≠ 0, i.e., when the energy of interaction between the particles and the substrate is taken into account). In the latter case, this means that the layers have a long-range orientation order. The proposed models of an extended monolayer can be used to interpret data obtained by the light scattering and molecular dynamics methods for Langmuir-Blodgett films with the adhesion parameter a = 0.05 and the particle-particle interaction parameter b = 0.6 in the three-dimensional model or b = 1 in the two-dimensional model. The smaller value of b in the three-dimensional layer model can be explained by a stronger effect of cooperation of particle-particle interactions as compared to the two-dimensional layer model.     

Magnetic structure of KFeS2 — A linear chain antiferromagnet and a spin analog of active sites of two iron ferredoxins — By neutron diffraction

Magnetic structure of KFeS₂ — a spin analog of the oxidized two iron ferredoxins — was determined by the neutron diffraction method. Spins are antiferromagnetically coupled along the c-axis chain, and they make ferromagnetic sheets in the planes containing a and b axes below the transition temperature of 250.0 ± 0.5 K. The iron moment of 2.43 ± 0.03 μ_B lies in the a?c plane and is canted by 125.3 ± 0.7 degrees from a-axis at 4.2 K.     

Investigation of the optoelectronic properties of columbite ZnNb2O6 crystal

A high-quality ZnNb₂O₆ single-crystal grown by optical floating zone method has been used as a research prototype to analyze the optoelectronic parameters by measuring the absorption coefficient and transmittance spectra along the b-axis from 200 nm to 1000 nm at room temperature. The optical interband transitions of ZnNb₂O₆ have been determined as a direct transition with a band gap of 3.84 eV. The refractive index, extinction coefficient, and real and imaginary parts of the complex dielectric constants as functions of the wavelength for ZnNb₂O₆ crystal are obtained from the measured absorption coefficients and transmittance spectra. In the Urbach tail of 3.16–3.60 eV, the validity of the Cauchy–Sellmeier equation has also been evaluated. Using the single effective oscillator model, the oscillator energy E_o is found to be 4.77 eV. The dispersion energy E_d is 26.88 eV and ZnNb₂O₆ crystal takes an ionic value.