Specific features of the approximation of the dielectric spectra of alkylcyanobiphenyl liquid crystals

The frequency dependences of the longitudinal and transverse permittivities of oriented nematic liquid crystals belonging to the alkylcyanobiphenyl group nCB (n=5–8) are measured in the relaxation region in the meter and decimeter wavelength ranges. It is established that the dispersion of the longitudinal permittivity is well approximated by the sum of two Debye processes with different relaxation times. The frequency dependence of the transverse permittivity is represented by the dispersion relation with a continuous distribution of relaxation times in a specified time range. It is demonstrated that, in the high-frequency range (f>200 MHz), in which the dielectric spectra exhibit a number of weakly pronounced dispersion features, the total dispersion of the permittivity is adequately described by the sum of relaxation and resonance processes.     

Dielectric anisotropy of 5CB liquid crystal in a decimeter wavelength range

Parallel and perpendicular components of the dielectric constant for 5CB nematic liquid crystal are measured in a relaxation region. Two frequencies f ₁≈30 and f ₂≈350 MHz are determined at which the inversion of the dielectric anisotropy sign is observed. It is shown that the observed sizable high-frequency shift of f ₂ with respect to the value calculated according the Debye theory is caused by the dielectric resonance near the relaxation region.     

Impact of resonator on direct-detected rapid-scan EPR at 9.8 GHz

Rapid-scan electron paramagnetic resonance spectra at 9.8 GHz were obtained on a Bruker E580 spectrometer. Spectra of lithium phthalocyanine (LiPc) needles (T ₁=8 μs,T ₂=3.4 μs) and of a 0.2 mM aqueous solution of Nycomed triarylmethyl (trityl-CD₃) radical (T ₁=11.5 μs,T ₂=8 μs) were recorded at scan rates between 3.4·10³ and 7.5·10⁵ G/s at the center of sinusoidal scans. Signals for LiPc were obtained with a split-ring resonator, a rectangular resonator and a dielectric resonator. At faster scan rates the small bandwidth of the high-Q dielectric resonator filters, out high-frequency components of the rapid-scan signals. Field inhomogeneities induced by the rapidly changing magnetic field increase with scan rate and are greater with the dielectric and split-ring resonantors than with the rectangular resonator. Data for trityl-CD₃ were recorded with the rectangular resonator. The extended trityl sample, about 3 mm long, shows larger effects of magnetic field inhomogeneities than the small LiPc crystals.     

Dielectric and ultrasonic investigation of phase transition in cuinp2s6 crystals

Investigation results of dielectric and ultrasonic properties of layered CuInP₂S₆ crystals are presented. At low frequencies, dielectric spectra are highly influenced by the high ionic conductivity with the activation energy of 7357.4?K (0.635?eV). The high-frequency part of the spectra is determined by relaxational soft mode. The critical slowing down and Debye-type dispersion show the order–disorder type of the phase transition. The temperature dependence of the relaxational soft mode and dielectric contribution show a quasi-one-dimensional behaviour. Ultrasonic velocity exhibits critical slowing down which is accompanied by attenuation peaks in the phase transition region. Layered CuInP₂S₆ crystals have extremely large elastic nonlinearity in the direction perpendicular to layers. The nonlinear elastic parameters substantially increases at the PT temperature.     

Influence of exciton decay on the polariton luminescence spectra of CdTe crystal

The polariton luminescence spectra of CdTe crystals have been numerically calculated with allowance for the decay of mechanical excitons and compared with the known experimental low-temperature photoluminescence spectra of these crystals. The mechanism of formation of polariton luminescence in dependence of the optical parameters of crystal for the exciton resonance A _n=1 is discussed.     

Optical and magneto-optical studies of a multiferroic GaFeO<Subscript>3</Subscript> with a high Curie temperature

Single crystals of a noncentrosymmetric orthorhombic pyroelectric ferrimagnet Ga_2?xFe_xO₃ with a Curie temperature within 260–345 K have been grown by the flux method. It has been found that the electrical properties of the single crystals varied over a broad range from 10⁵ to 10¹³ Ω cm depending on the presence of transitionmetal oxide impurities. The dispersion relations for all three principal dielectric functions of orthorhombic GaFeO₃ have been determined in the range 0.7–5.4 eV by spectroscopic ellipsometry. The spectra of the dielectric functions of the orthorhombic Ga_2?xFe_xO₃ crystals are compared with the spectra of the trigonal crystals. The Faraday effect and second-harmonic generation are studied, and the law of the transition to the paramagnetic state has been determined. The crystallographic and magnetic contributions to the second-harmonic generation are analyzed.     

High-frequency dielectric spectra from liquid crystals of series nCB and nOCB

The design of a resonant frequency-tunable high-sensitivity microstrip sensor is suggested. The permittivity dispersion of liquid crystals of two homologic series, alkylcyanobiphenyls (7CB and 8CB) and alkyloxycyanobiphenyls (7OCB and 8OCB), is studied at frequencies of 100–900 MHz. The dielectric spectra are shown to be the sum of the Debye relaxation and dielectric resonances observed at f≈160, 280, 360, 450, 550, and 650 MHz. The dielectric resonances are present in the spectra of all the samples in both the nematic and isotropic phase. The substitution of an oxygen atom (series nOCB) for a carbon atom (series nCB) in liquid crystal molecules has a minor effect on the dielectric resonance frequencies but changes the resonance intensities and splits some of the resonance lines.     

The parameter of the optical indicatrix of guanidinium aluminum-sulfate hexahydrate crystals

The absorption coefficient (220–1200 nm) and refractive index (800–450 nm) spectra of a strongly anisotropic guanidinium aluminum-sulfate hexahydrate (GASH) crystals are measured. It is shown that the optical anisotropy of GASH crystals in the visible spectral region is significant, while the crystal in the polar direction becomes isotropic. The contribution of IR oscillators to the electronic polarizability of GASH is sharply anisotropic. It is found that the constants of the Sellmeier dispersion formula qualitatively agree with the vacuum ultraviolet spectra calculated from first principles. The experimental refractive indices coincide with the calculated values with an accuracy of 0.10 (n _e ) and 0.1% (n _o ). It is proposed to use the GASH crystal for development of optical phase difference compensators.     

EPR and dielectric properties of Pb5Ge3O11 crystals with off-center Cu2+ ions

The electron paramagnetic resonance (EPR) and dielectric properties of Pb₅Ge₃O₁₁ crystals activated by copper ion are investigated. It is shown that Cu²⁺ ions replace Pb²⁺ in trigonal symmetry positions and occupy three off-center positions displaced from a crystal lattice site in a plane perpendicular to the polar axis C. The temperature variation of EPR spectra and dielectric properties indicates the presence of thermally activated jumps of Cu²⁺ ions between off-center positions. The EPR and dielectric data are used to determine the activation energy W=0.24 eV and the eigenfrequency τ ₀ ^?1 ～ 10¹² Hz of local dynamics of Cu²⁺ ions.     

The dielectric tensor components of anthracene derived from energy loss experiments

Energy loss spectra of anthracene single crystals corresponding to the dielectric tensor elements ? _xx , ? _yy and ? _zz were measured with 60 keV electrons, after having fixed the main axes of the dielectric tensor corresponding to ? _xx and ? _zz . The real and imaginary parts of the dielectric tensor elements are derived from the loss functions by a Kramers-Kronig-analysis. The results are compared with those obtained by other autors and with measurements on graphite.     

Inter-relationship between optical dielectric constant and interatomic separation of mixed crystals in ionic and covalent binary compound families

An ion dependent dielectric model is developed for mixed binary crystals. The interatomic separation (R) and optical dielectric constant (ε_∞) of the mixed crystals are computed from the measured values of R and ε_∞ of pure crystals. An empirical relation between ε_∞ and R is therefore found for mixed crystals by using the computed values. It is found that the dielectric behaviour of ionic mixed crystals is cation dependent while that of covalent mixed crystals is anion dependent. The prescribed theory can therefore be used to form different mixed crystals with particular values of ε_∞ required in any specific opto-electronic devices.     

Interference of ordinary and extraordinary waves in AgAsS2 crystals

The anisotropy of the near-bandgap absorption is investigated in AgAsS₂ crystals. The refraction indices, n^‖ and n^⊥ respectively for the E‖c and E⊥c polarizations as well as the spectral dependence of the refraction indices difference, Δn=n‖−n^⊥ are determined from the interference spectra of AgAsS₂ crystals. A transmission band with four maxima is observed in the transmission spectra of crystals placed between crossed polarizers. The optical parameters n, k, ε₁, and ε₂ for the E‖c and E⊥c polarizations are calculated from the reflection spectra by using the Kramers–Kronig relations.     

Multiplasmon replicas of the emission lines of bound excitons in ZnSe crystals

This paper reports on the results of investigations into the cathodoluminescence spectra of ZnSe crystals grown from a vapor phase and annealed in a bismuth melt [ZnSe(Bi) crystals], as well as in a bismuth melt containing aluminum [ZnSe(Bi,Al) crystals], at a temperature of 1200 K for 120 h. The longitudinal optical (LO) phonon replicas of the emission lines of free excitons are dominant in the cathodoluminescence spectra of the samples subjected to annealing in the bismuth melt with subsequent quenching. The emission spectra of all the samples studied, including the initial crystals, contain series of I ₁ ^s,d -nLO lines of bound excitons. It is revealed that plasmon replicas are characteristic not only of the I ₁ ^s -nLO lines. A weak exciton-plasmon interaction manifests itself in the form of single-plasmon Stokes satellites of the I ₁ ^d -nLO spectral lines. A theory describing the multiphoton optical transitions of bound excitons with the participation of mixed plasmonphonon vibrational modes is proposed. This theory makes it possible to calculate the shape function of an emission spectrum without recourse to model concepts. The results of the theoretical calculations are in agreement with the experimental data.     

High-frequency electron paramagnetic resonance investigation of Mn3+ centers in SrTiO3

A high-frequency (49 and 70 GHz) electron paramagnetic resonance (EPR) study is reported on SrTiO₃ single crystals doped with 0.5-at% of manganese. EPR spectra originating from the S=2 ground state of Mn³⁺ ions are shown to belong to the different types of Jahn–Teller centers that arise as a result of varied ordering of the oxygen vacancies due to the reducing process. The present results indicate the strong covalent reduction of the density of unpaired spins (Fermi term) at the manganese nucleus for the various types of centers.     

Influence of the composition of TlGa1 − x Er x Se2 crystals on their dielectric characteristics and parameters of localized states

Frequency dependences of the real (?′) and imaginary (?″) parts of the complex permittivity, the dielectric loss tangent (tanδ), and the ac conductivity (σ_ac) in frequency range f = 5 × 10⁴?3.5 × 10⁷ Hz have been investigated for TlGa_{1 ? x} Er _x Se₂ crystals of various compositions. It has been established that the relaxation dispersion of ?′ and ?″ takes place for the studied crystals. The influence of the erbium content in the crystals on their dielectric coefficients has been studied. The ac conductivity of the TlGa_{1 ? x} Er _x Se₂ single crystals in the high-frequency range obeys the law σ_ac ～ f ^0.8, which is characteristic of the hopping mechanism of charge transfer over the states localized in the vicinity of the Fermi level. Parameters of the states localized in the band gap of TlGa_{1 ? x} Er _x Se₂ and the influence of the composition of the crystals on these parameters have been evaluated.     

Electron paramagnetic resonance of Cr<Superscript>3+</Superscript> ions in ABO<Subscript>3</Subscript> (A = Sc,Lu, In) diamagnetic crystals

A magnetic resonance method is applied to the investigation of a number of isostructural diamagnetic compounds ABO₃ (A = Sc, Lu, In) with small additions of Cr³⁺ ions (S = 3/2) sufficient to observe single-ion spectra. It is shown that the resonance spectra for isolated Cr³⁺ ions can be described to a good accuracy by the ordinary axial spin Hamiltonian for 3d ions in octahedral oxygen environment. The parameters of the spin Hamiltonian are determined. It is established that Cr³⁺ ions in these crystals are characterized by easy-axis-type anisotropy.     

Effect of dopant Cr ions on the dielectric properties of melt-grown ZnSe crystals

The frequency and temperature dependences of the real and imaginary parts of the permittivity of Cr-doped (n _Cr ? 10¹⁸ cm^?3) melt-grown ZnSe crystals have been measured in the low-frequency region. It has been established that such doping reduces the heterogeneity of the dielectric properties and the level of energy losses of ac electric field in a crystalline ingot. The effect of dopant atoms on the dielectric properties has been explained as being caused by formation of defect associates with participation of these atoms and intrinsic crystal defects.     

Far-infrared spectroscopy of CdTe1−xSex(In): Phonon properties

The far-infrared reflectivity spectra of CdTe_0.97Se_0.03 and CdTe_0.97Se_0.03(In) single crystals were measured at different temperatures. The analysis of the far-infrared spectra was carried out by a fitting procedure based on the dielectric function which includes spatial distribution of free carriers as well as their influence on the plasmon–phonon interaction. We found that the long wavelength optical phonon modes of CdTe_1−xSe_x mixed crystals exhibit a two-mode behavior. The local In mode at about 160 cm⁻¹ is observed. In both sample, a surface layer with a low concentration of free carriers (depleted region) are formed.     

Dependence of the refractive index on crystal thickness due to a Pekar extra light wave

Transmission spectra of CdS crystals of different thickness are calculated in the region of A _n =1 exciton using the Pekar theory taking into account an extra light wave. It is shown that the dependence of the refractive index variance on the thickness of superthin crystals at low temperatures, which has been observed earlier, is associated with interference of the extra light wave, to which the crystal is opaque, with the fundamental wave transmitted by the crystal.     

Excitons in monoclinic zinc diphosphide: Orthoexciton and polariton effects at n=1 resonance

Quantitative investigations of the hydrogen-like exciton B series in the absorption spectra of the β-ZnP₂ crystal for various wave vector directions and polarization states of radiation are conducted. It is shown that the B spectrum constitutes a single orthoexciton series with S-type envelope functions, and low-energy components in doublet lines belong to the S-type for lines in the series with n≥3. Polariton effects are clearly manifested at the B _n=1 exciton resonance, and Bouguer’s law is violated. The oscillator strength tensor components are determined for transitions to the exciton states of the B series, and the polariton parameters at the B _n=1 exciton resonance are calculated.