Thermoelectric power of Ba(Fe1−x Co x )2As2 (0 ≤ x ≤ 0.05) and Ba(Fe1−x Rh x )2As2 (0 ≤ x ≤ 0.171)

Temperature-dependent, in-plane, thermoelectric power data are presented for single crystals of Ba(Fe_1?x Co _x )₂As₂ (0?≤?x?≤?0.05) and Ba(Fe_1?x Rh _x )₂As₂ (0?≤?x?≤?0.171). Given that previous thermoelectric power and angle resolved photoemission spectroscopy studies of Ba(Fe_1?x Co _x )₂As₂ delineated a rather large Co-concentration range for Lifshitz transitions to occur, and the underdoped side of the phase diagram is poorly explored, new measurements of thermoelectric power on tightly spaced concentrations of Co, 0?≤?x?≤?0.05, were carried out. The data suggest evidence of a Lifshitz transition, but instead of a discontinuous jump in thermoelectric power in the range 0?≤?x?≤?0.05, a more gradual evolution in the S(T) plots as x is increased was observed. The thermoelectric power data of Ba(Fe_1?x Rh _x )₂As₂ show very similar behavior to that of Co substituted BaFe₂As₂. The previously outlined T–x phase diagrams for both systems are further confirmed by these thermoelectric power data.     

Composition-tuned refractive index and oscillator parameters in TlGaxIn1−xS2 layered mixed crystals (0≤x≤1)

The optical properties of TlGa_xIn_1?xS₂ mixed crystals have been studied through transmission and reflection measurements in the wavelength range 400–1100 nm. These measurements allowed determination of the spectral dependence of the refractive index for all compositions of the mixed crystals studied. The dispersion of the refractive index is discussed in terms of the Wemple–DiDomenico single-effective-oscillator model. The compositional dependences of the refractive index dispersion parameters (oscillator energy, dispersion energy and zero-frequency refractive index) were revealed.     

Studying the dielectric, electromechanical, and elastic properties of K1 − x (NH4) x H2PO4 mixed crystals

Temperature dependences of the longitudinal permittivity, piezoelectric coefficient d ₃₆, and elastic constant c ₆₆ ^E of K_{1 ? x} (NH₄) _x H₂PO₄ mixed crystals are studied experimentally. A microscopic model is proposed for crystals of the K_{1 ? x} (NH₄) _x H₂PO₄ type that includes the piezoelectric contribution to the effective pseudospin cluster Hamiltonian. Dielectric elastic and electromechanical properties of these crystals with ammonium concentrations x below 0.40 are calculated in a wide temperature range using the cluster approach. The calculation results are in qualitative agreement with experimental data.     

Optical properties of glasses activated by the compounds CuInSe2 and CuInTe2 and by the solid solutions CuInSe2x Te(1−x)

The spectral properties of amorphous media synthesized on the basis of glasses of the system SiO₂-CaO-R₂O (R=Na, K, Li), the ternary compounds CuInSe₂, and CuInTe₂, and solid solutions based on them have been investigated. It is shown that the absorption limit of the media investigated lies within 1.4–1.6 μm. It is noted that the three alkali ions constituting the system exert a favorable influence on the state and optical properties of the amorphous media activated with the compound CuInTe₂ and with the solid solutions CuInSe_2xTe_(1−x), where x≤0.5. The influence of the concentration of the solid solution CuInSeTe (x=0.5) on the optical properties of the amorphous media is investigated. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 67, No. 1, pp. 132–134, January–February, 2000.     

Influence of silver on the galvanomagnetic properties and energy spectrum of mixed (Bi1− x Sbx)2Te3 crystals

An investigation is made of the temperature dependences of the resistivity in the range 4.2–300 K, the Hall effect, and the Shubnikov-de Haas effect in magnetic fields up to 40 T in (Bi_{1? x} Sb_x)₂Te₃Ag_y single crystals (0 ≤ x ≤ 0.75). Doping (Bi_{1? x} Sb_x)₂Te₃ crystals with silver showed that in Sb₂Te₃ and (Bi_{1? x} Sb_x)₂Te₃ crystals unlike Bi₂Te₃ silver exhibits acceptor properties. The angular and concentration dependences of the Shubnikov-de Haas effect were studied in (Bi_{1? x} Sb_x)₂Te₃Ag_y. It was established that the anisotropy of the ellipsoids of the upper valence band in Bi_0.5Sb_1.5Te₃ remains unchanged as a result of silver doping.     

Structural,dielectric and Raman scattering studies of (1− x )PbMg1/3Nb2/3O3– x PbTiO3 single crystals (0 ≤ x ≤ 0.38)

The X-ray powder diffraction, dielectric and Raman studies of the (1???x) PbMg_1/3Nb_2/3O₃–xPbTiO₃ (0?≤?x?≤?0.38) single crystals were performed in a wide temperature range. The results of these complementary studies revealed that with the increase in PT content, the relaxor behaviour transforms into the ferroelectric one and a sequence of different structural phase transitions occur. On the base of the structural and dielectric results the phase diagram was determined. In addition, these results were used as a base for the group theory analysis and interpretation of the observed Raman spectra. The Raman spectra were deconvoluted using the independent damped oscillator model. From this model the temperature dependencies of the wavenumbers, reduced intensities and line widths were determined.     

Concentration dependence of EPR spectra in mixed (BaF2)1 − x (CeF3) x crystals

Physics of the Solid State - In the EPR spectra of the (BaF2)1 − x (CeF3) x system, additional lines appear as the concentration x in the mixture increases from 10−3 to 10−2, some...     

Study on the structural properties and relaxation mechanisms in LiRb(1-x)(NH₄)(x)SO₄ (x=0, 0.5, and 1) mixed crystals by ¹H, ⁷Li, and ⁸⁷Rb nuclear magnetic resonance

LiRb_1?x(NH₄)_xSO₄ (x=0, 0.5, and 1) mixed crystals were grown using the slow evaporation method, and their lattice constants were determined by X-ray diffraction. The effects of replacing ammonium ions with rubidium on the phase transitions of and dynamics of LiRb_1?x(NH₄)_xSO₄ mixed crystals were studied. We investigated the structural properties and phase transition temperatures of the LiRb_1?x(NH₄)_xSO₄ mixed crystals by performing ¹H, ⁷Li, and ⁸⁷Rb NMR relaxation and DSC. When a fraction of rubidium ions in the pure LiRbSO₄ crystal is partially replaced by ammonium ions, we compare the structural properties for LiRbSO₄, LiRb_0.5(NH₄)_0.5SO₄, and LiNH₄SO₄ single crystals. The ¹H, ⁷Li, and ⁸⁷Rb NMR relaxation mechanisms of LiRb_0.5(NH₄)_0.5SO₄ are different from those of pure LiRbSO₄ and LiNH₄SO₄; the sequence of the structural phase transitions of LiRb_0.5(NH₄)_0.5SO₄ is different, which might be related to the dynamics and orientation of the sulfate tetrahedral groups in the crystal structure.     

Structural and optical properties of La and Gd substituted Bi4 − x M x V2O11 − δ (0.1 ≤ x ≤ 0.3)

Bi_4???x M _x V₂O_11???δ (M?=?La, Gd; 0.1?≤?x?≤?0.3) is synthesised by a solid state reaction method to study the effect of La³⁺ and Gd³⁺ substitution for Bi on the structural and optical properties. The as-prepared samples are characterised by X-ray diffraction, Fourier transform infrared analysis, UV–visible spectroscopy, scanning electron microscopy and energy-dispersive spectroscopy. The refinement results confirmed that even substituted samples exhibit monoclinic structure with space group C2/m. The parameters like band gap energy; Urbach energy has been calculated from the UV–visible spectra. It has been observed that even substitution at the bismuth site by isovalent cations decreases the energy band gap. The lowest observed band gap is 1.86 eV for Bi_3.9La_0.1V₂O_11???δ . The grain size and defects were observed to increase with increasing substitution along with the amount of secondary phase.     

Ellipsometry study of interband transitions in TlGaS2xSe2(1−x) mixed crystals (0≤x≤1)

In this paper, the spectroscopic ellipsometry measurements on TlGaS_2xSe_2(1?x) mixed crystals (0≤x≤1) were carried out on the layer-plane (001) surfaces with light polarization E⊥c^? in the 1.2–6.2 eV spectral range at room temperature. The real and imaginary parts of the dielectric function, refractive index and extinction coefficient were calculated from ellipsometric data using the ambient-substrate optical model. The critical point energies in the above-band gap energy range have been obtained from the second derivative spectra of the dielectric function. Particularly for TlGaSe₂ crystals, the determined critical point energies were assigned tentatively to interband transitions using the available electronic energy band structure. The effect of the isomorphic anion substitution (sulfur for selenium) on critical point energies in TlGaS_2xSe_2(1?x) mixed crystals was established.     

Influence of Ba addition on the dielectric and optic properties of (1 – x) Na0.5Bi0.5TiO3–xBaTiO3 (x = 0, 0.025, 0.0325 and 0.05) single crystals

High-quality, large-size lead-free (1 – x)Na_0.5Bi_0.5TiO₃–xBaTiO₃ ((1 – x)NBT–xBT) single crystals (x = 0, 0.025, 0.0325 and 0.05) were grown using the Czochralski method. Dielectric and transmitted light intensity properties were measured for these crystals. The broad anomalies exhibited in the temperature dependence of the transmitted light corresponded to structural and dielectric anomalies and were related to the temperature dependence of polar regions and the appearance of a long-range ferroelectric state. We explain our results based on local electromechanical fields, by inhomogeneity of the ion distribution and the mismatch in ion size. We suggested that the NBT–BT system can be a promising lead-free piezoelectric material for ultrasonic delay-line applications, broadband transducers and sensors.     

Features of the structure and dynamics of Me1−x (NH4) x SCN (Me = K, Rb) mixed crystals

Neutron scattering is used to study the structure and dynamics of Me_{1 − x} (NH₄) _x SCN (Me = K, Rb) mixed crystals along the concentration section of 0.0 < x < 1.0 at room temperature 10 and 290 K. Phase transitions in Me_{1 − x} (NH₄) _x SCN mixed crystals are analyzed by neutron powder diffraction. The measured spectra of inelastic incoherent neutron scattering from mixed crystals in a concentration range of 0.0 < x < 1.0 at 10 are transformed into the generalized phonon density of states G(E) in the one-phonon incoherent approximation. Using G(E), we determine the changes in ammonium ion dynamics during phase transitions. Low energy resonance and local translational (two bands) and librational (two bands) modes are observed in the disordered rhombic phase at 10 K. The low energy resonance mode is not found in the ordered monoclinic phase at 10 K, though the local translational mode in the form of two bands and the local librational mode in the form of four bands are present there. The low energy resonance mode appears due to hybridization of the phonon spectrum of the host crystal with rotational tunneling modes of the split librational ground state of the impurity’s molecular ammonium ion.     

Optical spectroscopy and electronic structure of compounds HoNi5 − x Al x (x = 0, 1, 2)

The optical properties of the compounds HoNi_{5 ? x} Al _x (x = 0, 1, 2) have been investigated using the ellipsometric method in the wavelength range from 0.22 to 16 μm. The electronic structure of these intermetallic compounds has been calculated in the local electron-spin density approximation with the correction for strong electronic interactions in the 4f shell of the holmium ions. The experimental dispersion dependences of optical conductivity in the region of interband light absorption have been interpreted based on the results of the calculation of the electron density of states. The plasma and relaxation frequencies of electrons have been determined.     

Optical properties of GaAs1−x N x alloys grown by molecular beam epitaxy

Optical properties of GaAs_1?x N _x alloys grown by molecular beam epitaxy using GaAs (001) as the substrate have been studied. These include photoluminescence (PL), cathodoluminescence (CL), photocurrent and photomemory effects. The low-temperature (77?K) PL characteristics were measured on samples with 0–0.105% N content. The wide emission band indicates the defective nature of the materials. The widening of the band for materials with increasing nitrogen concentration also suggested that the concentration of defect states in these materials dramatically increased with increasing nitrogen content. The PL and CL spectra for GaAs_1?x N _x layer 1854 did not show identical characteristics. Some layers showed a very sharp fall in photocurrent at low temperatures, indicating a very sharp photoquenching and an interaction between antisite, interstitial and vacancy defects. The photomemory effect, which causes photoquenching and the transition from the EL2 to the EL2? metastable state, was strongly influenced by the optical exposure and thermal history of the sample.     

Optical and structural properties of Zn1−x Mg x O ceramic materials

This paper reports structural, optical and cathodoluminescence characterizations of sintered Zn_1?x Mg _x O composite materials. The effects of MgO composition on these film properties have been analyzed. X-ray diffraction (XRD) confirms that all composites are polycrystalline with prominent hexagonal wurtzite structure along two preferred orientations (002) and (101) for the crystallite growth. Above doping content x = 10 %, the formation of the hexagonal ZnMgO alloy phase and the segregation of the cubic MgO phase start. From reflectance and absorption measurements, we determined the band gap energy which tends to increase from 3.287 to 3.827 eV as the doping content increases. This widening of the optical band gap is explained by the Burstein–Moss effect which causes a significant increase of electron concentration (2.89 × 10¹⁸?5.19 × 10²⁰ cm^?3). The luminescent properties of the Zn_1?x Mg _x O pellets are studied by cathodoluminescence (CL) at room and liquid nitrogen temperatures under different electron beam excitations. At room temperature, the CL spectra of the Zn_1?x Mg _x O composites exhibit a dominant broad yellow-green light band at 2.38 eV and two ultraviolet emission peaks at 3.24 and 3.45 eV corresponding to the luminescence of the hexagonal ZnO and ZnMgO structures, respectively. For the doped ZnO samples, it reveals also new red peaks at 1.72 and 1.77 eV assigned to impurities’ emissions. However, the CL spectra recorded at 77 K show the presence of excitonic emission peaks related to recombination of free exciton (X _A), neutral donor-bound excitons (D⁰X) and their phonon replicas. The CL intensity and energy position of the green, red and ultraviolet emission peaks are found to depend strongly on the MgO doping content. The CL intensity of the UV and red emissions is more enhanced than the green light when the MgO content increases. CL imaging analysis shows that the repartition of the emitting centers in Zn_1?x Mg _x O composites is intimately connected to the film composition and surface morphology.     

Tunneling measurements on (La1−x Ce x )Al2 single crystals with wax barriers

In the investigation of the Kondo superconductor (La_1–x Ce _x )Al₂ a new technique is developed to fabricate tunnel junctions of the type: bulk sample — wax barrier — evaporated counterelectrode. Pb, Al, Ag, Au and Mg are used as counterelectrodes. Measurements employing Pb and Al yield the order parameter of nominally pure LaAl₂: ₀=(0.47±0.03) meV=1.63kT _c. The large influence of localized states within the gap is confirmed.     

Growth and transport properties of CaFeAsF_(1-x) single crystals

Using self-flux method,we have successfully grown the parent phase of the single crystals of CaFeAsF1-x.The X-ray di?raction indicates good crystallinity.In-plane resistivity shows a bad metallic behavior with a sharp drop of resistivity at about T SDW=119K.This anomaly is associated with the possible spin density wave(SDW)order.Interestingly near T SDW,the resistivity exhibits a cusp-like feature,which may be understood as the strong coupling effect between the electrons and the antiferromagnetic(AF)spin fluctuations.A reduction of fluorine or application of a high pressure will suppress the SDW feature and induce superconductivity.Hall effect measurements reveal a positive Hall coefficient below T SDW indicating a dominant role of the hole-like charge carriers in the parent phase.Strong magnetoresistance has been observed below T SDW suggesting multiple conduction channels of the charge carriers.