NMR spectral densities and two dimensional diffusion in TiS2(NH3)1.0 and TaS2(NH3)x

NMR measurements of proton spin-lattice relaxation times T₁ and T_1? in the layered intercalation compounds TiS₂(NH₃)_1.0 and TaS₂(NH₃)_x (x = 0.8, 0.9, 1.0) are reported as functions of frequency and temperature (100 K – 300 K). These observations probe the spectral density of magnetic fluctuations due to motions of the intercalated molecules at frequencies accessible to the T₁ (4–90 MHz) and T_1? (1–100 kHz) measurements. Since the average molecular hopping time (τ) can be changed by varying temperature, different regions of the spectral density can be examined. For T > 200 K, both T^?1₁ and T^?1_1? vary logarithmically with frequency, reflecting the two dimensional character of the molecular diffusion. The temperature dependence of T₁ suggests that a more accurate picture of the short time dynamics is required. No dependence of relaxation rate on vacancy concentration is found.     

Electrical and optical properties of single crystal In2Te3 and Ga2Te3

Electrical conductivity and thermopower measurements are reported for the defect semiconductors p-In₂Te₃ and n-Ga₂Te₃. The hole mobility μ_p in the former varied as Tⁿwheren=+5.98 forT<350 K and n=-4.13 forT>350 K showing a maximum of 210 cm²V^-1 sec^-1 at 350 K. Electron mobility in n-Ga₂Te₃ also went through a maximum at 320 K. The optical band-gaps for both were found to be direct, with values of 1.01 and 1.08 eV for In₂Te₃ and Ga₂Te₃ respectively at 300 K. Pronounced effects of annealing on TEP and optical absorption gave evidence of defect ordering at low temperatures followed by defect creation at T>350 K.     

Magnetostriction, Curie temperature and microstructure of Tb0.29(Dy1−xPrx)0.71Fe1.97 oriented alloys

The Tb_0.29(Dy_1−xPr_x)_0.71Fe_1.97 (x=0, 0.1, 0.2 and 0.3) alloys were prepared by directional solidification method. The orientation, magnetostriction λ, Curie temperature T_c and microstructure of alloys were characterized by XRD, standard resistant strain gauge technique, VSM and SEM-EDS. The results reveal that the alloys have a preferred orientation of 〈1 1 0〉 and 〈1 1 3〉 direction when x>0. With the increase in Pr content, the T_c of alloys decreases gradually and the non-cubic phase appears, resulting in the decline of λ dramatically, from 1935.2×10⁻⁶ for x=0 to 695.9×10⁻⁶ for x=0.3 at a compressive stress of 6 MPa and a magnetic field of H=240 kA m⁻¹.     

Effect of heat treatment on the optical and electrical transport properties of Ge15Sb10Se75 and Ge25Sb10Se65 thin films

The effect of heat treatment on the optical and electrical properties of Ge₁₅Sb₁₀Se₇₅ and Ge₂₅Sb₁₀Se₆₅ thin films in the range of annealing temperature 373-723 K has been investigated. Analysis of the optical absorption data indicates that Tauc's relation for the allowed non-direct transition successfully describes the optical processes in these films. The optical band gap (E_g^opt.) as well as the activation energy for the electrical conduction (ΔE) increase with the increase of annealing temperature (T_a) up to the glass transition temperature (T_g). Then a remarkable decrease in both the E_g^opt. and ΔE values occurred with a further increase of the annealing temperature (T_a>T_g). The obtained results were explained in terms of the Mott and Davis model for amorphous materials and amorphous to crystalline structure transformations. Furthermore, the deduced value of E_g^opt. for the Ge₂₅Sb₁₀Se₆₅ thin film is higher than that observed for the Ge₁₅Sb₁₀Se₇₅ thin film. This behavior was discussed on the basis of the chemical ordered network model (CONM) and the average value for the overall mean bond energy 〈E〉 of the amorphous system Ge_xSb₁₀Se_90−x with x=15 and 25 at%. The annealing process at T_a>T_g results in the formation of some crystalline phases GeSe, GeSe₂ and Sb₂Se₃ as revealed in XRD patterns, which confirms our discussion of the obtained results.     

Temperature-dependent Fe Mössbauer spectroscopy and local structure of the mullite-type Bi2(FexGa1−x)4O9 (0.1≤x≤1) solid solution

The Bi₂(Fe_xGa_1−x)₄O₉ oxide solid solution possessing a mullite-type structure has been investigated by ⁵⁷Fe Mössbauer spectroscopy in dependence of composition (0.1≤x≤1) and temperature (293≤T/K≤1073). The spectra have been fitted with two doublets for tetrahedrally and octahedrally coordinated high-spin Fe³⁺ ions, respectively. The experimental areas of the subspectra were used to determine the distribution of iron on the two inequivalent structural sites. The fraction of iron cations occupying the octahedral site is found to increase with decreasing Fe content and the cation distribution is almost independent of temperature. The unusual temperature dependence of the quadrupolar splitting, QS, observed for the octahedral site with dQS/dT>0 is discussed in connexion with structural data for Bi₂Fe₄O₉. The temperature dependence of Mössbauer isomer shifts and signal intensities is examined in the context of local vibrational properties of iron on the two inequivalent sites of the mullite-type lattice structure.     

Two-dimensional critical behaviour in antiferrodistortive Al ur6(CIO4)3 and Ga ur6(ClO4)3

Critical behaviour with dimensionality d = 2 has been observed for the 300 K antiferrodistortive phase transition in Al ur₆(ClO₄)₃ and Ga ur₆(ClO₄)₃ by means of the temperature dependence of the ESR parameter D. The systems exhibited d = 2 behaviour in the static critical behaviour for T<T_c?40 K for T>T_c + 40 K. From the ESR data including line width measurements the local order parameter relaxation rate ω₁ has been obtained for various temperatures above T_c, with a lowest value of ω₁ = 150 MHz at T_c + 15 K     

Manganese-doped CdGeAs2, ZnGeAs2 and ZnSiAs2 chalcopyrites: A new materials for spintronics

Based on Mn-doped chalcopyrites CdGeAs₂, ZnGeAs₂ and ZnSiAs₂ the new dilute magnetic semiconductors with p-type conductivity were produced. Magnetization, electrical resistivity, magnetoresistance and Hall effect of mentioned compositions were studied. Their curves of temperature dependence of magnetization have the similar form in spite of complicated character, for which the concentration and mobility of the charge carriers are responsible. Thus, for T<15 K, these curves are characteristic for superparamagnetics and for T>15 K for a frustrated ferromagnetics. In compounds with Zn these two states dilute by spinglass-like state. This specific feature is assigned to an attraction of Mn ions occupying neighboring sites and to the competition between the carrier-mediated exchange and superexchange interactions. Curie temperatures of these compounds are above room temperature. These are the highest Curie temperatures in the A^IIB^IVC^V₂:Mn systems.     

Molecular dynamics of [(CH2OH)3CNH3]2SiF6 by phase transition of 178 K

We have investigated the molecular motions of TRIS⁺ ([(CH₂OH)₃CNH₃]⁺) and ions in the [(CH₂OH)₃CNH₃]₂SiF₆ crystal below room temperature from the measurements of the spin-lattice relaxation time T₁ and the NMR absorption line of ¹H and ¹⁹F nuclei, in order to elucidate the changes of the molecular motions by the phase transition of T_c=178 K. The narrowing of the ¹⁹F-NMR line was observed around T_c=178 K and the reorientation of the anion appears above T_c. Moreover, from the analysis of the temperature dependence of T₁, we have observed that the activation energy of the reorientational motion of ions changes from 0.168 eV (T>T_c) to 0.185 eV (T<T_c). Based on these results, we found that the reorientational motion of ions is closely related to the origin of the phase transition at T_c. In addition, from the measurement of the ¹H-NMR line, we also found that the reorientational motion of H₂ in the -CH₂OH group becomes active accompanied by the phase transition.     

Electronic transport and magnetic studies of La1−xCax−0.08Sr0.04Ba0.04MnO3

Lanthanum based mixed valence manganite system La_1−xCa_x−0.08Sr_0.04Ba_0.04MnO₃ (LCSBMO; x=0.15, 0.24 and 0.33) synthesized through the sol-gel route is systematically investigated in this paper. The electronic transport and magnetic susceptibility properties are analyzed and compared, apart from the study of unit cell structure, microstructure and composition. Second order phase transition is observed in all the samples and significant difference is observed between the insulator to metal transition temperature (T_MI) and paramagnetic (PM) to ferromagnetic (FM) transition temperature (T_C). In contrast to the insulating FM behaviour usually observed in La_1−xCa_xMnO₃ (LCMO) for x=0.15, a clear insulator to metal transition is observed for LCSBMO for the same percentage of lanthanum. The temperature dependent resistivity of polycrystalline pellets, when obeying the well studied law ρ=ρ_o+ρ₂T² for T<T_MI, is observed to differ significantly in the values of ρ_o and ρ₂, with the electrical conductivity increasing with x. The variable range hopping model has been found to fit resistivity data better than the small polaron model for T>T_MI. AC magnetic susceptibility study of the polycrystalline powders of the manganite system shows the highest PM to FM transition of 285 K for x=0.33.     

Magnetoconductance of Au-SiO2 films

Measurements of the magnetoconductance in Au-SiO₂ thin films show that spin-orbit coupling is dominant at low temperatures. By fitting the data with the theory of Maekawa and Fukuyama the temperature dependence of the inelastic scattering time has been determined to be approximately T>^?1.5. The spin-orbit scattering times turn out to be significantly smaller than those used by Bergmann.     

Electron hopping mechanism in hematite (α-Fe2O3)

The frequency dependence of the real (?′) and imaginary (?″) parts of the dielectric constant of polycrystalline hematite (α-Fe₂O₃) has been investigated in the frequency range 0-100 kHz and the temperature range 190-350 K, in order to reveal experimentally the electron hopping mechanism that takes place during the Morin transition of spin-flip process. The dielectric behaviour is described well by the Debye-type relaxation (α-dispersion) in the temperature regions T<233 K and T>338 K. In the intermediate temperature range 233 K<T<338 K a charge carrier mechanism takes place (electron jump from the O²⁻ ion into one of the magnetic ions Fe³⁺) which gives rise to the low frequency conductivity and to the Ω-dispersion. The temperature dependence of relaxation time (τ) in the −ln τ vs 10³/T plot shows two linear regions. In the first, T<238 K, τ increases with increasing T implying a negative activation energy −0.01 eV, and in the second region T>318 K τ decreases as the temperature increases implying a positive activation energy 0.12 eV. The total reorganization energy (0.12-0.01) 0.11 eV is in agreement with the adiabatic activation energy 0.11 eV given by an ab initio model in the literature. The temperature dependence of the phase shift in the frequencies 1, 5, 10 kHz applied shows clearly an average Morin temperature T_Mo=284±1 K that is higher than the value of 263 K corresponding to a single crystal due to the size and shape of material grains.     

Excitation of the T> components of the giant dipole states in N > Z nuclei by muon capture

Muon capture on N > Z nuclei is proposed as a means of studying the T_> isospin component of giant dipole states. Calculations for nickel isotopes (except ⁵⁶Ni) indicate that these states are strongly excited by muon capture. Possible experiments to detect these states are discussed.     

Structural, magnetotransport and morphological studies of Sb-doped La2/3Ba1/3MnO3 ceramic perovskites

We report here the structural, magnetotransport and morphological studies of Sb-doped La_2/3Ba_1/3Mn_1−xSb_xO₃ perovskite manganites. Pristine material La_2/3Ba_1/3MnO₃ (LBMO) shows two insulator-metal (I-M) transitions in the electrical resistivity-temperature (ρ-T) behavior. While the higher temperature transition (T_P1) at ∼340 K is reminiscent of the usual I-M transition in manganites, the lower temperature transition (T_P2) at ∼250 K has been ascribed to the grain boundary (GB) effects arising out of the ionic size mismatch between the ions present at the rare-earth site (La³⁺ and Ba²⁺). With Sb-doping T_P1 shifts to lower temperatures while T_P2 remains invariant up to 3% and shifts to lower temperature for 5%. Room temperature electrical resistivity and the peak values also increase successively with Sb-doping. Scanning electron micrographs of the samples exhibit a gradual increase in their grain sizes with Sb indicating a gradual decrease in the GB density. Shift of T_P1 with doping is explained on the basis of a competition between double-exchange and super-exchange mechanisms. The overall electrical resistivity increases and the shift in the electrical resistivity hump (T_P2) with Sb-doping is found related to be gradually decreasing GB density and the ensuing lattice strain increase at the GBs. The intrinsic magnetoresistance (MR) gets suppressed and extrinsic MR gets enhanced with Sb-doping. At T>T_P1, the electrical resistivity is found to follow the adiabatic polaron hopping model whereas the electron-magnon scattering is found to dominate in the metallic regime (T<T_P1).     

Room temperature magnetic entropy change and magnetoresistance in La0.70(Ca0.30−xSrx)MnO3:Ag 10% (x=0.0−0.10)

The magnetic and magnetocaloric properties of polycrystalline La_0.70(Ca_0.30−xSr_x)MnO₃:Ag 10% manganite have been investigated. All compositions are crystallized in single phase orthorhombic Pbnm space group. Both, the insulator–metal transition temperature (T^IM) and Curie temperature (T_c) are observed at 298 K for x=0.10 composition. Though both T^IM and T_c are nearly unchanged with Ag addition, the MR is increased. The MR at 300 K is found to be as large as 31% with magnetic field change of 1 T, whereas it reaches up to 49% at magnetic field of 3 T for the La_0.70Ca_0.20Sr_0.10MnO₃:Ag_0.10 sample. The maximum entropy change (ΔS_Mmax) at near its T_c (300.5 K) is 7.6 J kg⁻¹ K⁻¹ upon the magnetic field change of 5 T. The La_0.70Ca_0.20Sr_0.10MnO₃:Ag_0.10 sample having good MR (31%^1 T, 49%^3 T) and reasonable change in magnetic entropy (7.6 J kg⁻¹.K⁻¹, 5 T) at 300 K can be a potential magnetic refrigerant material at ambient temperatures.     

A Theoretical Study of ãA2 CH2

The potential energy surface and dipole moment surfaces of the ã⁴A₂ electronic state of CH₂⁺ are calculated ab initio using an augmented correlation-consistent polarized valence quadruple-ζ (aug-cc-pVQZ) basis set, with the incorporation of dynamical correlation using the coupled cluster method with single and double excitations and perturbatively connected triple excitations [CCSD(T)]. We use these surfaces in the MORBID program system to calculate rotation and rotation-vibration term values for ã-state CH₂⁺, CD⁺₂, and CHD⁺ and to simulate the rotation and rotation-vibration absorption spectrum of CH₂⁺ in the ã⁴A₂ electronic state. Our work is motivated by studies of CH₂⁺ that use the Coulomb explosion imaging technique and by the goal of predicting spectra that may be obtained from discharge sources. Although the ã state is the lowest-lying excited state above the X?/Ã ground state pair, it turns out to be relatively high-lying, and we determine that T_e(ã)=30447.5 cm⁻¹. The equilibrium bond angle for ã-state CH₂⁺ is only 77.1°; as a result the asymmetric top κ value is close to 0, and the molecule is equally far from the oblate and prolate symmetric top limits in this electronic state.     

Magnetoelectric and Mössbauer studies of Fe2 TeO6

The magnetic symmetry of antiferromagnetic Fe₂TeO₆ indicates that this material should exhibit magnetoelectricity. This prediction has been confirmed by the observation of the electrically induced magnetoelectric (ME) effect in powder samples. Both parallel and perpendicular ME susceptibilities were measured as a function of increasing temperature. The ME effect vanishes at 209°K which is identified as the Néel point of the compound. The single crystal ME susceptibilities are derived from the powder results. The maximum value of the axial ME susceptibility α₃₃ = M₃/E₃ is (in Gaussian units) 3 × 10⁻⁵ at T = 175°K. In addition to magnetoelectricity the magnetic symmetry of Fe₂ TeO₆ indicates that no hyperfine field should exist at the Te⁶⁺ sites. This prediction was confirmed by Mössbauer studies on I¹²⁹ produced in Te⁶⁺ sites by irradiating samples of Fe₂ TeO₆ in a reactor.     

Synthesis and physical characterization of superconductivity-magnetism crossover compound RuSr2EuCeCu2O10−δ

We carefully studied the nonsuperconducting sample of the magneto-superconducting RuSr₂(Eu_1−xCe_x)Cu₂O_10−δ series with composition RuSr₂EuCeCu₂O_10−δ. This compound seems to exhibit a complex magnetic state as revealed by host of techniques like resistivity, thermopower, magnetic susceptibility, and MR measurements. The studied compound exhibited ferromagnetic like M(H) loops at 5, 20, and 50 K, and semiconductor like electrical conduction down to 5 K, with −MR^7 T of up to 4% at low temperatures. The −MR^7 T decreases fast above 150 K and monotonically becomes close to zero above say 230 K. Below, 150 K −MR^7 T decreases to around 3% monotonically down to 75 K, with further increase to 4% at around 30 K and lastly having a slight decrease below this temperature. The thermopower S(T) behavior closely followed the −MR^7 T steps in terms of d(S/T)/dT slopes. Further, both MR^7 T steps and d(S/T)/dT slopes are found in close vicinity to various magnetic ordering temperatures (T_mag) of this compound.     

The Phosphorescence Excitation Spectrum of Jet-Cooled Xanthione in the Region of the T1←S0 and S1←S0 Absorption Transitions

The ³A₂(nπ*)→¹A₁X (T₁→S₀) phosphorescence excitation spectrum of jet-cooled xanthione was investigated in the region 14 920-17 600 cm⁻¹. The structure observed is shown to be due to the T₁←S₀ absorption and an assignment in terms of the vibronic structure of that band is proposed. A previous assignment of the S₁←S₀ origin is considered in detail and the transition involved is shown to be most probably due to absorption of a vibronic triplet state T_1z,ν7. An alternative but tentative assignment of the S_1,0←S_0,0 transition is suggested. Comparison is made with previous spectroscopic and theoretical work on the molecule and its congeners, 4H-pyran-4-thione and 4H-1-benzopyran-4-thione.     

Magnetic and electrical behavior of La1−xAxMnO3 (A=Li, Na, K and Rb) manganites

With a view to understand the magnetic and electrical behavior of monovalent substituted lanthanum manganites, a series of materials were prepared by sol-gel route by sintering at 1200 °C. The X-ray diffraction data were analyzed using Rietveld refinement technique and it has been found that all the samples were found to crystallize into rhombohedral structure with R3¯c space group. The values of ferro to paramagnetic (T_C) and metal-insulator transition (T_P) temperatures were obtained using ac susceptibility and electrical resistivity data, respectively. It has been found that sodium-, potassium- and rubidium-doped samples exhibit two peaks in the electrical resistivity vs. temperature plots. The observed behavior has been explained on the basis of oxygen deficiency present in the samples. The electrical resistivity data were analyzed using various theoretical models and it has been concluded that the electrical resistivity data in the low-temperature regime (T<T_P) can be explained using the equation ρ_(T)=ρ₀+ρ₂T²+ρ_4.5T^4.5, signifying the importance of the grain/domain boundary, electron-electron and two magnon scattering processes. On the other hand, the high-temperature resisitivity data (T>T_P) were explained using variable range and small polaron hopping mechanisms.     

Dielectric dispersion in the incommensurate phases of [N(CH3)4]2CoCl4 and [N(CH3)4]2ZnCl4

The complex dielectric permittivity ?(ω) of [N(CH₃)₄]₂CoCl₄ and [N(CH₃)₄]₂ZnCl₄ along the a-axis was measured between 0.35 MHz and 100 MHz. It has been found that for both substances the relaxation frequencies are about 5 MHz at T_c. The dielectric relaxation of both substances could be described by a polydispersive process β = 0.74 in the vicinity of T_c. However, for the temperature region of (T?T_c) > 0.6 for [N(CH₃)₄]₂CoCl₄ the dielectric absorption seems to be rather monodispersive.