Structural and vibrational properties of dilute GaNxAs1−x(P1−x)

We report a comprehensive analyzes of the Fourier transform infrared (FTIR) absorption and Raman scattering data on the structural and vibrational properties of dilute ternary GaAs_1−xN_x,[GaP_1−xN_x] (x<0.03) alloys grown on GaAs [GaP] by metal organic chemical vapor deposition (MOCVD) and solid source molecular beam epitaxy (MBE). By using realistic total energy and lattice dynamical calculations, the origin of experimentally observed N-induced vibrational features are characterized. Useful information is obtained about the structural stability, vibrational frequencies, lattice relaxations and compositional disorder in GaNAs (GaNP) alloys. At lower composition (x<0.015) most of the N atoms occupy the As [P] sublattice {N_As[N_P]}—they prefer moving out of their substitutional sites to more energetically favorable locations at higher x. Our results for the N-isotopic shifts of local mode frequencies compare favorably well with the existing FTIR data.     

Crystal structure, electrical and magnetic properties of La1 − xSrxCoO3 − y

The crystal structure and properties of La_{1 − x}Sr_xCoO_{3 − y} with strontium contents ranging from x = 0.1 to x = 0.7 have been studied. The lattice parameters were measured as a function of temperature (4.2–400 K) and the crystal structure was found to change from rhombohedral (at low temperatures and values of x) to cubic. While LaCoO₃ is paramagnetic the oxides in the composition range 0.2 < x < 0.6 are soft ferromagnets. The strontium additions are compensated by the formation of Co⁴⁺ (cobalt ions with one positive effective charge, Co_Co^.) and oxygen vacancies (V_o^..). From the results it is concluded that the relative importance of oxygen vacancies increases with increasing temperature and decreasing oxygen activity. As a result the concentration of electronic charge carriers — and the resultant electrical conductivity — decrease with increasing temperature. The defect structure is discussed and it is concluded that defect associations — probably between oxygen vacancies and strontium ions — and formation of microdomains of perovskite-related phases are important aspects of the overall structure of these perovskite phases.     

The T-contribution to the electrical resistivity at low temperatures in NiAs-type Ni1−xS1−ySey

The y(1−y)-like maxima in Ni_0.98S_1−ySe_y are found beyond y_c in the y-dependences of the coefficient of the T² contribution to the resistivity and of the residual resistivity. Here, the critical concentration of the antiferromagnetic and less-conductive phase is determined to be y_c = 0.095.     

Phonon Raman scattering in Nd1+xBa2−xCu3O7−δ and Pr1+xBa2−xCu3O7−δ single crystals

The polarized Raman spectra of Nd_1+xBa_2−xCu₃O_7−δ (−0.023≤x≤0.107) and Pr_1+xBa_2−xCu₃O_7−δ (0.01≤x≤0.15) single crystals have been investigated. It was found that the Cu(2) A_g mode softens by 6 cm⁻¹ in Nd 1:2:3 and 4 cm⁻¹ in Pr 1:2:3 as x increases. These frequency shifts cannot be explained by the change in the relevant bond lengths due to Nd(Pr)-substitution for Ba. The variations with x of the two low frequency modes may be affected by change of their hybridization and/or change of their force constants. The linewidths of Ba mode in Pr 1:2:3 are broader than those in Y 1:2:3. This result suggests that the Pr substitution on Ba sites occurred even in a very small value of x. In x(yy) geometry the relative intensity of the Ba and O(4) modes in Nd 1:2:3 is greater than those in Pr 1:2:3. The difference between Nd 1:2:3 and Pr 1:2:3 in the relative intensity of the Ba and O(4) modes may be produced by the chains.     

Superconductivity magnetism and low temperature specific heat in YBa2(Cu1−xFex)3O7−δ with δ0 and δ1

Magnetic and low temperature specific heat measurements have been performed on iron doped YBa₂(Cu_1−xFe_x)₃O_7−δ samples with different oxygen contents (δ0 and δ1). Iron doping induces an orthorhombic to tetragonal transition and a decrease of both T_c and diamagnetic signal. Low temperature specific heat measurements reveal a Schottky type anomaly for δ0 samples with x=0.01 (1.8 K) and x=0.02 (3 K). This anomaly is attributed to magnetic interactions within iron limited chains. A numerical analysis of this effect is proposed.     

Growth and properties of ferromagnetic In1−xMnxSb alloys

We discuss a new narrow-gap ferromagnetic (FM) semiconductor alloy, In_1−xMn_xSb, and its growth by low-temperature molecular-beam epitaxy. The magnetic properties were investigated by direct magnetization measurements, electrical transport, magnetic circular dichroism, and the magneto-optical Kerr effect. These data clearly indicate that In_1−xMn_xSb possesses all the attributes of a system with carrier-mediated FM interactions, including well-defined hysteresis loops, a cusp in the temperature dependence of the resistivity, strong negative magnetoresistance, and a large anomalous Hall effect. The Curie temperatures in samples investigated thus far range up to 8.5 K, which are consistent with a mean-field-theory simulation of the carrier-induced ferromagnetism based on the 8-band effective band-orbital method.     

The structural features of Cu1−xPbx liquid alloys

Structure of Cu_1−xPb_x (x=0, 0.025, 0.05, 0.075, 0.1, 0.125) molten alloys has been studied by means of X-ray diffraction method. Structural parameters obtained from structure factors (SF) and radial distribution functions were analyzed. Partial structure factors were calculated by using Reverse Monte-Carlo method. It is shown that tendency to preferred interaction of atoms of the same kind grows with increasing of lead content.     

Guest-guest interactions in cointercalation compounds of FexCo1/4TiS2 and (FeyCo1−y)1/3TiS2

Lattice spacings, as magnetic susceptibilities, ESR, specific heats, and electrical properties have been measured on the cointercalation compounds of 1T-CdI₂ type host TiS₂, Fe_xCo TiS₂ (0 x ) and (Fe_yCo_1−y) TiS₂(0 y 1).

While the interlayer spacings are expanded rather smoothly with increasing the cointercalated Fe metals, the magnetic, thermal, and transport properties depend sensitively on a small amount of the added Fe metals, showing the presence of strong magnetic interactions, or guest-guest interactions between the cointercalated Fe and Co 3d metals in these systems.     

Preparation and characterization of Y1−xPrxBa2Cu3O7−δ single crystals

We present a reliable method for growing single crystals of Y_1−xPr_xBa₂Cu₃O_7−δ high-T_c superconductors in ZrO₂ crucibles. This method results in crystals with greatly improved superconducting properties compared to crystals grown with the previously reported methods which use Al₂O₃ crucibles. We describe techniques for crystal growth in both Al₂O₃ and ZrO₂ crucibles using an excess of BaCo₃ and CuO as the flux. The crystals were characterized by means of DC magnetic-susceptibility measurements, electrical-resistivity measurements, and electron microprobe analysis. The effects of Al contamination on the conditions for crystal growth and on the superconducting properties of the crystals are found to be quite significant.     

Preparation and characterization of Y1−xPrxBa2Cu3O7−δ single crystals

We present a reliable method for growing single crystals of Y_1−xPr_xBa₂Cu₃O_7−δ high-T_c superconductors in ZrO₂ crucibles. This method results in crystals with greatly improved superconducting properties compared to crystals grown with the previously reported methods which use Al₂O₃ crucibles. We describe techniques for crystal growth in both Al₂O₃ and ZrO₂ crucibles using an excess of BaCo₃ and CuO as the flux. The crystals were characterized by means of DC magnetic-susceptibility measurements, electrical-resistivity measurements, and electron microprobe analysis. The effects of Al contamination on the conditions for crystal growth and on the superconducting properties of the crystals are found to be quite significant.     

Investigation of the magnetic properties of the Gd1−xErx alloy system in the x < 0.62 composition range

The magnetic properties of the Gd---Er system (with erbium concentrations ranging from 0 to 62 at%) were investigated at temperatures between 4.2 and 350 K under external magnetic fields up to 1 T. The results show that, similar to other rare-earth systems, simple ferromagnetism prevails for concentrations with a de Gennes factor G higher than about 11.5 (≈ 30 at % Er) while complex magnetic structures are present for higher Er concentrations (G ≤ 11.5. The magnetic transition temperature decreases with increasing Er content, showing good agreement with phenomenological expectations. In the paramagnetic region, the Curie constant of the alloy corresponds to the linear combination of the constituent elements' contribution, thus proving that both Gd and Er maintain their individual magnetic moments. From magnetization data below the transition, the change in magnetic entropy for a 1 T field was calculated and this showed distinct differences between ferromagnetic-paramagnetic transitions and antiferromagnetic transitions. The Landau theory of second-order phase transitions can be well applied for evaluating the thermodynamic transition temperature and the specific heat anomaly in the ferromagnetic alloys but it fails for the more complex structures if the applied field is not far higher than the critical field. The magnetic entropy change displays well defined peaks at the transition temperature for both ferromagnetic and antiferromagnetic alloys. There is an indication that the spin-reorientation transition occurring in alloys with higher Er concentrations is also coupled with significant magnetic entropy changes.     

Magnetic properties of pseudobinary compounds CrSb1−xAsx

Magnetic susceptibility and crystal structure were studied on CrSb_1−xAs_0.6, structural transformation occurs between MnP-type and NiAs-type at 270 K, the Néel temperature is 300 K. Paramagnetic moments of the system are 2.45−2.7μ_B/Cr, considerably larger than antiferromagnetic moments.     

Electrical properties of perovskite-type La(Cr1−xMnx)O3+δ

Perovskite-type La(Cr_1−xMn_x)O_3+δ (0.0x1.0) was synthesized using a sol–gel process. The crystal structure of La(Cr_1−xMn_x)O_3+δ changes from orthorhombic to rhombohedral at x=0.6. The Mn⁴⁺ ion content increases monotonically in the range 0.2x1.0. The magnetic measurement of La(Cr_1−xMn_x)O_3+δ indicates that a Mn³⁺ ion is a high-spin state with (d)³(dγ)¹. The variation of the average (Cr, Mn)-O distance is explained by ionic radii of the Cr³⁺, the Mn³⁺, the Mn⁴⁺ ions. Since the log σT–1/T curve is linear and the Seebeck coefficient (α) is independent of temperature, it is considered that La(Cr_1−xMn_x)O_3+δ is a p-type semiconductor and exhibits the hopping conductivity.     

Magnetic properties and specific heat of LaMn1−xTixO3+δ (0<x0.2)

We present a magnetic study of the insulating perovskite LaMn_1−xTi_xO_3+δ (0<x0.2) including measurements of magnetization, susceptibility, and magnetic relaxation. The Curie temperature was found to decrease with increasing content of Ti. Two distinct magnetic transitions, irreversibility, non-exponential relaxation and aging effects confirm a reentrant spin–glass state for x=0.2. The time decay of the magnetization has an algebraic functional form for times up to 2 h. The specific heat also displays characteristic features of a spin–glass system by a linear low-temperature dependence and a broadened peak near the temperature of the reentrant transition.     

Normal-state transport properties of Ba1−xKxBiO3 crystals

The normal-state transport properties of Ba_1−xK_xBiO₃ crystals with a wide range of potassium compositions (0≤x≤0.62) were studied. Although the host material BaBiO₃ has a monoclinic structure, the system changes from a monoclinic to an orthorhombic structure with a small doping of potassium (0≤x<0.35) and behaves similar to a doped semiconductor, without exhibiting superconductivity. In the composition range, holes are majority carriers in the transport phenomena. When x exceeds a critical value (0.35), the system goes into a cubic superconducting phase with a single metallic band. The vicinity of the critical composition transport phenomena is easy to understand assuming the existence of two conducting channels that are made up of metallic and semiconducting phases. Maximum T_c exceeding 30 K was observed at x0.4, where carrier density was at its maximum. Overdoping with potassium suppresses superconductivity. In the metallic composition of x>0.45, transport seems to correlate with the phonon mode with an energy distribution of 15–43 meV.     

Quantum critical point, scaling and universality in high Tc(CaxLa1−x)(Ba2−c−xLac+x)Cu3Oy

Using charge transport in sintered ceramic samples it is observed that at all doping, including non superconducting overdoped samples, there exists a temperature in which below it dR/dT < 0. This suggests that either the quantum critical point is not necessarily inside the superconducting dome or that the CuO₂ plane is never overdoped. Data relating experimental Cooper pair density, conductivity and T_c suggest that Homes’ relation might need a more specific definition of the conductivity σ.     

Nax/2Bi1−x/2MoxV1−xO4 and Bi1−x/3MoxV1−xO4 New Scheelite-related phases

New Scheelite-related solid solutions of the compositions Na_x/2Bi_1−x/2Mo_xV_1−xO₄ (0≤x≤1) and Bi_1−x/3 Mo_xV_1−xO₄(0≤x≤0.2) have been synthesised by the substitution of Na and Mo at the A and B sites respectively of the ABO₄ type ferroelastic BiVO₄. The phases were characterised using chemical analysis, powder X-ray diffraction, scanning electron microscopy, EDAX, and Raman spectroscopy. While almost a continuous solid solution is obtained for the series Na_x/2Bi_1−x/2Mo_xV_1−xO₄, the absence of Na at the A-site results only in a narrow stability region for the other series, Bi_1−x/3 Mo_xV_1−xO₄ where 0≤x≤0.2. Raman spectra of selected samples at room temperature also suggest that vanadium and molybdenum atoms are disordered at the tetrahedral sites.     

Synthesis and dielectric properties of Bi1−xNdxFeO3 perovskites

Single-phase polycrystalline samples of Bi_1−xNd_xFeO₃ were prepared by standard solid state reaction method. X-ray diffraction (XRD) patterns of the powder samples were recorded and analyzed for the confirmation of crystal structure lattice parameters. Further, these samples were characterized by IR technique to identify and understand the aspect of bonding in the present samples. The dielectric measurements were carried out on the samples as a function of frequency in the range 100 Hz to 1 MHz at room temperature and also as a function of temperature in the range 300–750 K at certain fixed frequencies. The DC and AC electrical resistivity studies were carried out in order to understand conduction mechanism in the samples.     

Magnetic properties of the Cr3S4−xTex

It was found that a transition from the antiferromagnetic state to the ferromagnetic state took place at about x = 1.6 and that a monoclinic phase exists between x = 3.0 and 4.0 and a monoclinic structure became a hexagonal structure at about x = 3.5.     

Field induced flux pinning in Nd1+xBa2−xCu3O7−δ single crystal

The flux pinning behavior of a Nd_1+xBa_2−xCu₃O_7−δ (Nd123) single crystal, which exhibited a peak effect, has been studied by monitoring the time decay of the magnetic moment. The apparent pinning energy (U₀^*) was deduced from flux creep data on the basis of the Anderson-Kim model. The magnetic field dependence of U₀^* showed maxima at peak fields which depended on the temperatures, in a similar manner to those of critical current densities. In addition, the temperature dependence of U₀^* showed several features. To explain the increase in U₀^* with the field as well as its temperature and field dependence, we made a numerical calculation by introducing an additional pinning energy which increased with increasing field. The results are in good agreement with the experimental data, especially at temperatures above 60 K, where the contribution of field induced pinning centers is believed to be dominant.