Nernst effect of stripe ordering La<Subscript>1.8−x</Subscript>Eu<Subscript>0.2</Subscript>Sr<Subscript>x</Subscript>CuO<Subscript>4</Subscript>

We investigate the transport properties of La_1.8−xEu_0.2Sr_xCuO₄ (x = 0.04, 0.08, 0.125, 0.15, 0.2) with a special focus on the Nernst effect in the normal state. Various anomalous features are present in the data. For x = 0.125 and 0.15 a kink-like anomaly is present in the vicinity of the onset of charge stripe order in the LTT phase, suggestive of enhanced positive quasiparticle Nernst response in the stripe ordered phase. At higher temperature, all doping levels except x = 0.2 exhibit a further kink anomaly in the LTO phase which cannot unambiguously be related to stripe order. Moreover, a direct comparison between the Nernst coefficients of stripe ordering La_1.8−xEu_0.2Sr_xCuO₄ and superconducting La_2−xSr_xCuO₄ at the doping levels x = 0.125 and x = 0.15 reveals only weak differences. Our findings make high demands on any scenario interpreting the Nernst response in hole-doped cuprates.     

Magnetocaloric properties of manganites La<Subscript>1−x</Subscript>(Ag,K)<Subscript>x</Subscript>MnO<Subscript>3</Subscript>

The temperature and magnetic-field dependences of the magnetocaloric effect in manganites La_1−x Ag _x MnO₃ (x = 0.1; 0.15; 0.2) and La_1−x K _x MnO₃ (x = 0.1; 0.11; 0.13; 0.15; 0.175) were studied by a direct method. Large changes in the sample temperature were detected as a magnetic field changed by ΔH = 10 kOe. Temperatures of the magnetocaloric effect maxima are near room temperatures. Field dependences of the magnetocaloric effect show no signs of saturation in fields to 30 kOe.     

Microwave characterization of Pb<Subscript>1−x</Subscript>Ca<Subscript>x</Subscript>Fe<Subscript>0.5</Subscript>Nb<Subscript>0.5</Subscript>O<Subscript>3</Subscript> multiferroics at X-band

The microwave characteristics of Pb_1?x Ca _x Fe_0.5Nb_0.5O₃ multiferroics (x = 0.0, 0.4, 0.45, 0.5, 0.55, 0.6), have been investigated as a function of frequency and substitution. The results depict ?13.99 dB reflection loss at 11.65 GHz in composition x = 0.6. Microwave absorption is enhanced with substitution of Ca²⁺ ions and undoped composition 0.0 behaves as electromagnetic shield. The model governing microwave absorption is discussed and different compositions for electromagnetic applications have been suggested.     

Optical activity of γ<Subscript>1</Subscript>-(Ga<Subscript>x</Subscript>In<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>)<Subscript>2</Subscript>Se<Subscript>3</Subscript> crystals

The optical activity of uniaxial γ₁-(Ga_xIn_1?x )₂Se₃ crystals (x=0.1, 0.2, 0.3, 0.4) is studied at T=295 and 77 K in the spectral range 0.5–0.8 πm. It is found that the cationic substitution In → Ga leads to a nonlinear increase in the specific rotation of the plane of polarization ρ and the component g ₃₃ of the gyration tensor. It is shown that the gyrotropy of the crystals studied is determined by high-energy transitions whose energy exceeds the energy of the edge transitions and that the gyrotropy observed has a molecular origin.     

Magnetorefractive effect in (Co<Subscript>50</Subscript>Fe<Subscript>50</Subscript>)<Subscript>x</Subscript>(Al<Subscript>2</Subscript>O<Subscript>3</Subscript>)<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript> granular films

The reflectivity spectra and the magnetorefractive effect (MRE) of (Co₅₀Fe₅₀)_x(Al₂O₃)_1?x metal-dielectric granular films (0.07<x<0.52) are analyzed in the IR spectral range λ=2.5–25 µm. It is revealed that the specific features observed in the spectra at λ≈8.5 and 20 µm are associated with the excitation of phonon modes in the dielectric matrix. The magnetorefractive effect in the films is observed below the percolation thresh-old only in p-polarized light and above the percolation threshold for both the p and s polarizations. It is demonstrated that the optical properties of (Co₅₀Fe₅₀)_x(Al₂O₃)_1?x films in the IR spectral range, to a first approximation, can be interpreted in the framework of the effective-medium theory and the magnetorefractive effect can be explained in terms of the modified Hagen-Rubens relation.     

Fractal magnetic microstructure in the (Co<Subscript>41</Subscript>Fe<Subscript>39</Subscript>B<Subscript>20</Subscript>)<Subscript>x</Subscript>(SiO<Subscript>2</Subscript>)<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript> nanocomposite films

Magnetostructural methods are applied to determine the exchange bond percolation limit in (Co₄₁Fe₃₉B₂₀)_x(SiO₂)_1?x nanocomposites (x _c = 0.30 ± 0.02), which separates the phase plane along the metal concentration axis into a superparamagnetic region and a ferromagnetic region. It is shown that, with respect to the singularities of the magnetization up to the magnetization saturation curves, the ferromagnetic region is further subdivided into three regions differing in the character of the spatial propagation of the magnetization ripples or in the magnetic correlation function characteristics. The fractal dimension of the nanocomposite magnetic microstructure near the percolation threshold is determined.     

Magnetic ordering in La<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Sr<Subscript>x</Subscript>MnO<Subscript>3−<Emphasis Type="Italic">x</Emphasis>/2</Subscript> anion-deficient manganites

The structural, magnetic, and electrotransport properties of La_1?xSr_xMnO_{3? x/2}(0≤x≤0.30) manganites with perovskite structure are investigated experimentally as a function of oxygen deficiency. In the solid solutions La_1?xSr_xMnO₃, a change in the type of symmetry of the unit cell is observed at x=0.125. Samples with x≤0.125 are characterized by an O′-orthorhombic unit cell, whereas samples with x>0.125 are characterized by a rhombohedral unit cell. The structural properties of the anion-deficient solid solutions La_1?xSr_xMnO_3?x/2 are analogous to those of the stoichiometric system. It is assumed that, as the oxygen content decreases, La_{1? x}Sr_xMnO_3?x/2 anion-deficient solid solutions experience a series of successive magnetic phase transformations in the ground state: from an A-type (x=0) antiferromagnet to a cluster spin-glass-type inhomogeneous magnetic state (0.175>x≤0.30) through a two-phase (antiferromagnetic and ferromagnetic) state (0>x≤0.175). The anion-deficient solid solution with x=0.175 has the maximal value of the ferromagnetic component. As the oxygen deficiency increases, the resistivity of La_{1? x}Sr_xMnO_3?x/2 samples first decreases (up to a value of x=0.175), acquiring an activation character, and then increases (up to a value of x=0.30). In this case, none of the anion-deficient solid solutions exhibits a metal-semiconductor transition in the whole range of concentrations considered. A peak of magnetoresistance at a temperature below the point of magnetic ordering is observed only in the sample with x=0.175. The results of experiments carried out with a series of La_1?xSr_xMnO_3?x/2 anion-deficient solid solutions are summarized in the concentration diagrams of the spontaneous magnetic moment and the critical temperature of magnetic phase transitions. Hypothetical magnetic phase states are pointed out. The experimental results obtained can be interpreted in terms of the phase-separation model and the competition between ferromagnetic and antiferromagnetic indirect superex-change interactions. It is assumed that Mn³⁺-O-Mn³⁺ indirect superexchange interactions in the orbitally disordered phase are positive in the case of octahedral coordination of manganese ions and are negative when the coordination of at least one Mn³⁺ ion is pentahedral.     

Dielectric properties of CuInP<Subscript>2</Subscript>(Se<Subscript>x</Subscript>S<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>)<Subscript>6</Subscript> solid solutions

The dielectric properties of layered crystals of CuInP₂(Se_xS_1?x)₆ solid solutions are studied at x = 0.02, 0.05, 0.20, and 0.40. At a low selenium content (x ≤ 0.05), the solid solutions undergo a transition to the phase with short-range polar order. This transition manifests itself as a diffuse maximum in the temperature dependence of the permittivity ε′(T). Ferroelectric ordering in the solid solutions under investigation is suppressed at x > 1. It is assumed that the structural disordering initiated by the substitution of atoms in the anion sublattice of the solid solutions at 0.1 < x < 0.75 leads to the formation of the state of structured glass. The dielectric relaxation dispersion observed in the radio-frequency range at temperatures of 80–140 K is associated with the freezing of the relaxation dynamics of individual copper atoms.     

Dynamic magnetic susceptibility of compounds in Gd<Subscript>5</Subscript>Si<Subscript>2−x</Subscript>Ge<Subscript>2−x</Subscript>Sn<Subscript>2x</Subscript> (2x = 0 − 0.1) system

The dynamic magnetic susceptibility (χ_ac) of magnetically ordered Gd₅Si_2?xGe_2?xSn_2x compounds with the partial substitution for silicon and germanium atoms by isovalent tin atoms (2x = 0 ? 0.1) has been investigated experimentally. From the temperature dependence of χ_ac the Curie temperatures of these alloys are determined. It is established that tin-doped alloys have higher Curie temperatures as compared to Gd₅Si₂Ge₂ (ΔT _C ≈ 15 K).     

Orbital phase transition in Pr<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Ca<Subscript>x</Subscript>MnO<Subscript>3</Subscript>

The phenomenological theory of phase transition in Pr_0.6 Ca_0.4 MnO₃ manganite is developed. It is shown that this is the orbital phase transition and that the two electronic states of the manganese ion, which are discussed in the literature, result from two different types of condensation of the same orbital order parameter. Thus, the manganese ions in Pr_1?xCa_x MnO₃ manganites with 0.3≤_x≤0.5 may be in either of the two electronic states, depending on the thermodynamic parameters.     

Resonant Raman scattering in superconducting Ba<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>K<Subscript>x</Subscript>BiO<Subscript>3</Subscript>

The effect of the photon energy of the exciting laser radiation on the Raman spectra of Ba_1?xK_xBiO₃ with x=0.25, 0.40, and 0.50 is studied. An increase in the laser wavelength from 488 to 750 nm scarcely affects the amplitudes and frequencies of the spectral lines in the Raman spectra of the nonsuperconducting compound with x=0.25. For the optimally doped (x=0.40) and overdoped (x=0.50) superconducting compounds, a substantial increase in the line intensity and a considerable shift of the characteristic frequencies are observed. This result suggests that, in the whole range of superconducting compositions 0.37≤x≤0.50, the local symmetry of the Ba_1?xK_xBiO₃ crystal lattice differs from the perfect cubic symmetry, which should take place according to the literature data. The fact that resonance phenomena are observed when the laser photon energy is shifted toward the optical gap testifies to the presence of local electron pairs in the whole range of superconducting compositions 0.37≤x≤0.50 and is evidence in favor of the superconductivity mechanism proposed for Ba_1?xK_xBiO₃ on the basis of the X-ray absorption studies in our previous paper.     

Hall effect in La<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Sr<Subscript>x</Subscript>MnO<Subscript>3</Subscript> crystals

A study of the temperature dependences of the ordinary and anomalous Hall coefficients and of the Hall mobility of carriers in single crystals of the ferromagnetic manganites La_1?xSr_xMnO₃ (x=0.15, 0.20, 0.25) was carried out in the temperature interval from 85 to 400 K. The nature of the carriers and the conduction mechanisms in these compounds are discussed.     

Thermal expansion of (Sr<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>La<Subscript>x</Subscript>)<Subscript>3</Subscript>Ru<Subscript>2</Subscript>O<Subscript>7</Subscript> single crystals at low temperatures

For single-crystal samples of the (Sr_1?xLa_x)₃Ru₂O₇ ruthenates, the temperature dependence of the thermal expansion coefficient α(T)) is measured in the range 4.2–80 K. The effect of magnetic fields H ≤ 3.5 T on thermal expansion is analyzed. It is found that the (Sr_1?xLa_x)₃Ru₂O₇ ruthenates exhibit an anomalous (negative) thermal expansion coefficient in the temperature range T ≤ 18 K. The position and width of the anomaly revealed in the temperature dependence of the thermal expansion coefficient α(T)) depend substantially on the magnetic field. The origin of the thermal expansion anomaly in ruthenates, the correlation of this anomaly with the stability of the crystal lattice, and the common nature of the anomalies in the thermal properties of ruthenates and high-temperature superconductors are discussed.     

Spectroscopic study of magnetic phase transitions in Nd<Subscript>x</Subscript>Gd<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Fe<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript>

Magnetic ordering and spin reorientation in iron borates Nd_xGd_1?xFe₃(BO₃)₄ (x = 0.01, 0.04, 0.25, 1.0) are studied using a rare-earth spectroscopic probe.     

Heat capacity of the Pb<Subscript>5</Subscript>(Ge<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Si<Subscript>x</Subscript>)<Subscript>3</Subscript>O<Subscript>11</Subscript> ferroelectric system

The temperature dependences of the molar heat capacity at constant pressure, C_p, of Pb₅(Ge_1?xSi_x)₃O₁₁ crystals with x=0, 0.39, and 0.45 in the range 5–300 K, as well as of their permittivity, dielectric losses, and the pyroelectric effect, have been measured. Experimental data on the temperature behavior of the heat capacity are presented in the form of a sum of two Debye and one Einstein terms, C_p(T)=0.405C_D1(Θ_D1=160 K, T)+0.53C_D2(Θ_D2=750 K, T)+0.046C_E(Θ_E=47 K, T). Besides a peak in the region of the ferroelectric Curie point T_c=450 K for crystals with x=0, the temperature dependences of the heat capacity did not reveal any other pronounced anomalies.     

Photoluminescence of TlCu<Subscript>1−x</Subscript>GaSe<Subscript>2</Subscript> single crystals

The results of the study of photoluminescence and its excitation spectra in Tl _x Cu_1−x GaSe₂ single crystals are presented. The crystals under study are layered and characterized by anisotropic optical properties. In this respect, it is important to investigate optical properties of the crystals under study.     

Correlation between raman spectra and structural properties of Zn<Subscript>2 − 2<Emphasis Type="Italic">x</Emphasis></Subscript>Cu<Subscript>x</Subscript>In<Subscript>x</Subscript>Se<Subscript>2</Subscript> films

The Raman spectra of Zn_{2 ? 2x} Cu_xIn_xSe₂ (ZCIS) semiconductor films designed for use as optically active layers in thin-film solar cells have been investigated. The Raman spectra of ZCIS films are characterized by the presence of the dominant mode A ₁, which is observed in A^IB^IIIC ₂ ^VI compounds with chalcopyrite structure. The spectra of CuInSe₂ films (x = 1) obtained at low temperatures (T ≤ 400°C) contain and additional mode at 258 cm^?1, which is due to the presence of the impurity Cu_xSe phase. All modes observed in the spectra of ZCIS films with a Zn concentration ≤20 at % obtained under optimal conditions (520–540°C) correspond to the symmetry of vibrations in the chalcopyrite structure. The broadening and blue shift of the A ₁ mode occurring with an increase in the Zn concentration are indicative of degradation of the chalcopyrite crystal structure and the chalcopyrite → sphalerite phase transition at Zn concentrations exceeding 20 at %.     

Spectral-luminescent properties of AgLa<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Eu<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>(MoO<Subscript>4</Subscript>)<Subscript>2</Subscript> solid solutions

The luminescent properties of AgLa_{1 − x} Eu _x (MoO₄)₂ (x = 0.1, 0.2, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0) under laser excitation (λ = 337.1 nm) are studied. It is shown that, upon substitution of Eu³⁺ for La³⁺, the symmetry of luminescence centers does not vary. According to the X-ray diffraction data, all samples have scheelite-like structure; the pattern of variation in volumes of their unit cells counts in favor of the presence of a continuous series of solid solutions. It is found that an increase in the europium concentration in AgLa_{1 − x} Eu _x (MoO₄)₂ leads to an increase in the luminescence intensity with a maximum at x = 0.9.     

Field-induced valence transition in EuPtP<Subscript>1−x</Subscript>As<Subscript>x</Subscript>

The ternary compound EuPtP exhibits two valence transitions at T ₁ = 235 K and T ₂ = 190 K. In order to examine a field-induced valence transition of Eu, we synthesized EuPtP_1−x As _x compounds with 0.05 ≤ x ≤ 0.5 and studied the magnetic and valence behavior. The substitution of As for P increases the lattice volume linearly and decreases both valence transition temperatures, T ₁ and T ₂, in contrast to the behavior under external pressures. The magnetization process in a pulsed magnetic field revealed that EuPtP_0.5As_0.5 exhibits an onset of metamagnetic transition above 50 T with a large hysteresis, which evidences a first-order field-induced valence transition. The analysis of the magnetization curves of x = 0.5 at various temperatures has demonstrated that the field-induced transition is essentially the same as the transition induced by temperature at T ₁.     

Spectral-luminescent properties of LiRbLa<Subscript>2 − <Emphasis Type="Italic">x</Emphasis></Subscript>Eu<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>(MoO<Subscript>4</Subscript>)<Subscript>4</Subscript> solid solutions

The luminescent properties of LiRbLa_{2 − x} Eu _x (MoO₄)₄ (x = 0.002, 0.02, 0.1, 0.2, 0.5, 1.0, 1.5, 2.0) solid solutions are investigated under laser excitation {λ_ex = 337.1 nm). It is established that the composition containing 50 at % Eu³⁺ is the brightest of the considered set of phosphors and has the highest quantum yield.