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.     

Phase transition and extended X-ray absorption fine structure of melt-spun amorphous Fe100−xYx alloys

The phase diagram and local structure of melt-spun amorphous (a-) Fe_100−xY_x (22?x?62) alloys were investigated using AC and DC magnetic and extended X-ray absorption fine structure (EXAFS) measurements. The a-Fe–Y system shows reentrant spin glass (RSG) behavior for 42?x?58 and spin glass (SG) behavior for 60?x. Two SG transition temperatures, T_g and T_f, were obtained in the RSG state. The T_g, T_f and Curie temperature T_C decrease with increasing x, and the T_C and T_g vanish at x=60. A new magnetic phase diagram for the melt-spun a-Fe_100−xY_x alloys was obtained from magnetic measurements for higher Y concentration. The magnetic states of the a-Fe_100−xY_x alloys change remarkably around x=60 and an EXAFS study revealed that the average atomic distance between nearest-neighboring Fe atoms changes at approximately x=60.     

Glass transition behavior of binary GaxSe100−x (0?x?10) glass systems

The glass transition behavior of glassy Ga_xSe_100−x (x=0, 2.5, 5, 7.5 and 10) systems were investigated using differential scanning calorimetry (DSC). The variation of glass transition temperature, T_g, with Ga concentration has been studied. The value of activation energy of glass transition, E_g, has been found to increase with increase in Ga content. This increase in E_g has been explained in terms of the average heat of atomization for these glasses.     

Effect of structural relaxation on low energy excitations in amorphous Zr x Cu1−x

We report on an investigation of the liquid-quenched metallic glass Zr _x Cu_1?x (0.6≦x≦0.74) subjected to heat treatments below the glass transition temperatureT _g. Annealing temperatures up to 200°C (<0.8T _g) were chosen as to achieve topological relaxation only. The superconducting transition temperaturesT _c are lowered, as already observed for other metallic glasses. Low temperature measurements of the thermal conductivity (0.5 K≦T≦15 K) and of the specific heat (0.1 K≦T≦3 K) were carried out in order to determine the effect of structural relaxation on the low energy configurational excitations characteristic of the amorphous state. The annealed samples show no detectable (<20%) change in the specific heat forT?T _c, but an increase of the thermal conductivity by a factor of 2 forT?T _c is observed. Within the tunneling model of two level systems (TLS) for the low energy excitations, this behavior can be qualitatively understood in terms of a change of the TLS relaxation time distribution upon annealing. This distribution differs from that of the commonly used standard tunneling model. The change of the phonon scattering by TLS directly observed forT?T_c is largely responsible for the enhancement of the thermal conductivity found also aboveT _c.     

Some results of the differential thermal analysis of TlxAs2−xS3−x glasses

The paper presents the results of the differential thermal analysis of glasses of general composition Tl _x As_2?x S_3?x (0≦x ≦ 1). Character of the composition dependence of the glass-transition temperatureT _g leads to a division of investigated glass-forming interval into two parts. Forx in excess of 0.238 there is a linear decrease ofT _g withx, whereas forx less then 0.238 the decrease ofT _g withx is non-linear. Unlike the former composition range, the latter one is characterized by the fact that the values ofT _Tg and also of the recrystallization temperatureT _r depend on the temperature régime during synthesis and, eventually, on further thermal treatment of these glasses. At the lowestx values are theT _g values influenced by differencies in the starting materials used in synthetizing the glass. Glasses prepared from amorphous TlAsS₂, arsenic and sulphur at given thermal régime were found to form homogeneous materials only with difficulty.     

Magnetic resonance of spin clusters and triplet excitations in a spin-Peierls magnet with impurities

The magnetic resonance spectrum of spin clusters formed in spin-Peierls magnets in the vicinity of impurity ions is investigated. The observed temperature dependences of the effective g-factor and the linewidth of the electron paramagnetic resonance (EPR) in crystals of Cu_1?x Ni_xGeO₃ are described in the model of the exchange narrowing of the two-component spectrum with one component ascribed to spin clusters and exhibiting an anomalous value of the g-factor and the other related to triplet excitations. An estimation of the size of the suppressed dimerization region around the impurity ion is obtained (this region includes about 30 copper ions). The dependence of the effective g-factor and the EPR linewidth on the impurity concentration at low temperatures indicates the interaction of clusters.     

The nature of temperature dependence of energy gaps in layer semiconductors

The temperature dependences of direct and indirect energy gaps in layer semiconductors GaS, GaSe and GaS_xSe_1?x are investigated in the temperature range 5–150 K. The nonmonotonous behaviour of E_g(T) dependences is observed in these crystals. It is shown that the effect of thermal expansion cannot in itself explain the observed anomalies. A new model of electron-phonon interaction explaining the E_g(T) behaviour in layer crystals is proposed.     

Spin-cluster effects and frustration in the uniaxial spin glass Fe2−xTi1+xO5

Transverse-field (TF) muon-spin-relaxation (μSR) and Mössbauer experiments on the uniaxial insulating spin glass Fe_2?xTi_1+xO₅ (x=0.25) have been performed near and below the spin-glass temperature (T _g). The effect of a transverse field on the spin-freezing process and spin-glass state has been investigated by measuring the field-and temperature-dependencies of the μSR parameters. Spin-cluster effects signaled by anomalous μ-spin relaxation have been observed in a temperature region just aboveT _g. An interpretation supported by recently developed theoretical models addressing non-linear relaxation in an intermediate Griffiths phase is offered.     

Measurement of complex susceptibility on a metallic spin glass with broad relaxation spectrum

AC susceptibility measurements (0.625 H_z?_v?625 H_z) have been performed in a SQUID magnetometer on the amorphous metallic spin glass (Fe_0.06Ni_0.94)₇₅P₁₆B₆Al₃. The in-phase component of the susceptibility (x') shows a sharp peak at the spin glass freezing temperature T_g≈8 K with a frequency dependence of ?T_g/T_g≈0.015 per decade of frequency increase. The out-of-phase component (x″) exhibits a small anomaly at the spin glass transition with the magnitude of the anomaly almost independent of frequency. In particular we find the relation x″=(π;/2)?x'/? lnω to be valid in the vicinity of T_g. This relation and a frequency independence of x″ are fundamental characteristics of a spin glass having a broad spectrum of relaxation times. We argue that similar results from ac susceptibility measurements should be found in many spin glass systems, metallic as well as non-metallic.     

Structural and electrical characterization of SxSe100−x thin films

Thin films of samples of the glassy S_xSe_100−x system with 0 ≤ x ≤ 7.28 have been prepared by thermal evaporation technique at room temperature (300 K). X-ray investigations show that the structure of pure selenium (Se) does change seriously by the addition of small amount of sulphur S ≤7.28%. The lattice parameters were determined as a function of sulphur content. Results of differential thermal analysis (DTA) of the glassy compositions of the system S_xSe_100−x were discussed. The characteristic temperatures (T_g, T_c and T_m) were evaluated. Dark electrical resistivities, ρ, of S_xSe_100−x thin films with different thicknesses from 100 to 500 nm, were measured in the temperature range from 300 to 423 K. Two distinct linear parts with different activation energies were observed. The variation of electrical resistivity of examined compositions has been discussed as a function of the film thickness, temperature and the sulphur content. The application of Mott model for the phonon assisted hopping of small polarons gave the same two activation energies obtained from the resistivity temperature calculations.     

Shifts of g Values in the Excited Triplet States of Metal Complexes Studied by Time-Resolved W-band EPR

A highly time-resolved high-frequency/high-field W-band electron paramagnetic resonance (EPR) (ν ~ 94 GHz) is a powerful technique to determine small g anisotropies of transient paramagnetic species. We applied this method to studies of the lowest excited triplet (T₁)³ ππ* states in metal complexes such as a platinum (Pt) diimine complex and metal (Zn and Mg) porphines in rigid glasses. From the analyses of time-resolved EPR spectra, g anisotropies were obtained as g _z  = 2.0048, g _x  = g _y  = 2.0035 for Pt(b-iq)(CN)₂ (b-iq = 3,3′bi-isoquinoline) and g _z  = 1.9968, g _x  = g _y  = 2.0022 for zinc tetraphenylporphine (ZnTPP). No measurable anisotropies were found for magnesium (Mg) TPP. The g values of the Pt complex are larger than g _e (=2.0023, g value of free electron) and that g _z of ZnTPP is smaller than g _e. These results were interpreted in terms of the nature of the perturbed states: the higher triplet ππ′* state mixes with T₁(ππ*) via spin–orbit coupling in ZnTPP. In contrast, the higher triplet dπ* state is involved in this coupling for the Pt complex. Thus, the nature of the perturbed state can be distinguished from the anisotropic g values of the T₁(ππ*) state.     

Synthesis,structural, electrical and magnetic studies of La0.5Ca0.45−xSrxBa0.05MnO3

Half doped mixed valence manganite system La_0.5Ca_0.45?xSr_xBa_0.05MnO₃ (with x=0.1, 0.2, and 0.3) synthesized through a low temperature nitrate route is systematically investigated in this paper. The electronic transport and magnetic properties are analyzed and compared apart from the study of unit cell structure and composition. The system is found to crystallize only in orthorhombic structure (Pnma) and the electronic phase transitions are observed to be of second order. The charge and orbital ordering have been observed to coexist with ferromagnetism in x=0.1 compound. Application of small polaron and variable range hopping models to resistivity data of the system corresponding to high temperature range shows increasing mobility of e_g electrons with x, with the later model describing the electronic transport very closely than the former. The temperature dependent magnetization of the compounds shows monotonic increase of paramagnetic to ferromagnetic transition (T_C) with x. Ferromagnetism is exhibited for the complete temperature range down from respective T_C in contrast to antiferromagnetism usually exhibited by half-doped compounds in the low temperature range. The plots of magnetization versus magnetic field reveal a transition from soft to hard magnetic character for all the compounds as the temperature is lowered.     

Unusual EPR Spectra with Inverse Axial g values of Chlorosalicylate–Cu(II)–2,6-Pyridinedimethanol Complex in Frozen Water–Methanol Solution

Copper(II) complex systems containing 3,5-di, 4-, or 5-chlorosalicylic acids (X-ClsalH) and different copper(II) salts (copper acetate (Cu(ac)₂) or copper sulphate (CuSO₄)), with varying 2,6-pyridinedimethanol (pydime) concentration, [Cu(ac)₂(aq) or CuSO₄(aq) + 2 (X-ClsalH(solv)) + x pydime(solv)], where X = 3,5-di, 4-, or 5- and x = 0, 1, 2, 4, or 8, were prepared. The effects of two copper(II) salts (containing anions of different basicity) and N-donor ligand (pydime) with varying ligand-to-metal ratio (x) on the formation of resulting complexes were studied by electron paramagnetic resonance (EPR) spectroscopy in frozen water/methanol (1:3 v/v) solutions. For x ≥ 2, unusual Cu(II) EPR spectra with "inverse" axial g values of (g _⊥ > g _∥ > 2.0023) were observed, which can indicate the compressed octahedral geometry of the central copper atom with the unpaired electron/hole localized on the $ d_{{z^{2} }} $ orbital. However, for x = 1, composite Cu(II) EPR spectra with both "usual" and "inverse" axial g values were detected. Finally, for x = 0 (ligand not present) Cu(II) EPR spectra only with the ‘usual’ axial g values of g _∥ > g _⊥ > 2.0023 were collected, which can indicate the elongated octahedral geometry of the central copper atom with the unpaired electron/hole localized on $ d_{{x^{2} - y^{2} }} $ orbital. The above described observations are independent of the usage of different copper(II) salts and X-chlorosalicylic acids.     

Cu-Zr-Ti系Cu基块体非晶合金的形成和成分优化

利用与团簇相关的变电子浓度判据研究了过渡金属Cu-Zr-Ti系中Cu基块体非晶合金的形成区域和成分特征.据此判据在Cu-Zr-Ti系相图中确定出三条特殊的成分线，(Cu_9／13Zr_4／13)_100-xTi_x，(Cu_0.618Zr_0.382)_100-xTi_x和(Cu_0.56Zr_0.44)关键词： 块体非晶合金 Cu-Zr-Ti合金 原子团簇 电子浓度     

Kinetic study of non-isothermal crystallization of BixSe100−x chalcogenide glasses

Results of differential thermal analysis (DTA) under non-isothermal conditions on four glasses of Bi_xSe_100−x (x=5, 10, 15 and 25 at%) are reported and discussed. The glass transition temperatures (T_g), the onset crystallization temperatures (T_c) and the peak temperatures of crystallization (T_p) were found to be dependent on the compositions and the heating rates. From the dependence on the heating rates of (T_g) and (T_p), the activation energy for glass transition, E_g, and the activation energy for crystallization, E_c, are calculated and their composition dependence is discussed. The crystalline phases resulting from DTA and scanning electron microscopy (SEM) have been identified using X-ray diffraction. According to the Avrami exponent (n), the results show a one-dimensional growth for the composition Bi₅Se₉₅ and two-dimensional growth for the compositions Bi₁₀Se₉₀, Bi₁₅Se₈₅ and Bi₂₅Se₇₅. The kinetic parameters determined have made it possible to discuss the glass-forming ability.     

Muon spin relaxation measurements in kagomé lattice system SrCr8Ga4O19

Dynamical spin fluctuations in SrCr_8–xGa_4+xO₁₉ a frustrated spin system on a kagomé lattice, is examined by the longitudinal field muon spin relaxation technique. This system shows a spin-glass (SG)-like cusp in the susceptibility atT _g=3.5(2) K. The slowing down of Cr spin fluctuations is found to occur over a very wide temperature rangeT _g<T<30T _g. AsT/T _g 0 these fluctuations remain without static polarization (order parameter). Such strong fluctuations belowT _g have not been observed before in a conventional SG system.     

The magnetic transition in Fe-substituted hexagonal β-Al9SiMn3 phase

Recently we have studies β-Al₉Si(Mn_1?x Fe _x )₃ using Mössbauer effect spectroscopy withx=0.28 and discovered a new transition at about 80 K. Neutron diffraction studies down to low temperatures reveal no structural changes, so that this transition at 80 K is indeed of magnetic origin. The magnetic susceptibility shows a small maximum near this temperature superimposed on a large Curie-like background. In addition to this transition, the Curie-like background shows a cusp near 4 K. We thus infer a magnetic double transition in this disordered alloy. These magnetic transitions have been followed to lower Fe concentrations (x=0.10 and 0.20) by both Mössbauer spectroscopy (T _c ) and by magnetic susceptibility (T _g). It is found thatT _C decreases rapidly with decreasing Fe concentration, whileT _g remains relatively constant. The saturation hyperfine fieldB _hf(0) decreases rapidly with decreasingx, indicating that the Fe atoms loose their magnetic moments in the limit of infinite dilution, and the important role played by Fe?Fe nearest neighbors in determining the magnetic properties.     

Ferromagnetic spin fluctuations in antiferromagnetic Pr1−xCaxMnO3: An ESR study

Electron spin resonance (ESR) measurements have been performed on polycrystalline samples of Pr_1−xCa_xMnO₃ (x=0.4, 0.5) in the temperature range of 100-300 K. The temperature dependence of ESR intensity, g value and linewidth shows the existence of ferromagnetic spin correlations in the paramagnetic state. With decreasing temperature, the ferromagnetic spin correlations switch to antiferromagnetic spin correlations in the charge ordering state and vanish at the antiferromagnetic ordering temperature T_N.     

Thermal properties and structures of chlorocadmate, bromocadmate, and bromochlorocadmate glasses

Glass transition temperatures (T_g), crystallization temperatures (T_x), and ¹¹³Cd magic-angle spinning (MAS) NMR spectra of the chlorocadmate, bromocadmate, and bromochlorocadmate glasses and the CdCl₂, CdBrCl, and CdBr₂ crystals have been measured. The T_g and T_x, and thermal stabilities (T_x−T_g) decreased with increasing Br-content. No enhancement of (T_x−T_g) of bromochlorocadmate glasses by anion-mixing was observed. The CdCl₂ and CdBr₂ crystals gave the isotropic chemical shifts of 183 and −24 ppm, which are assigned to the CdCl₆ and CdBr₆ octahedra, respectively. The isotropic chemical shifts of CdBrCl crystal (186, 166, 135, 101, 63, 25 and −21 ppm) were assigned to CdBr_nCl_(6−n) (n=0-6) octahedra, respectively. It was found that the Br and Cl atoms in CdBrCl randomly occupy chlorine sites in the CdCl₂ structure. The ¹¹³Cd MAS NMR spectra revealed that the chlorocadmate, bromocadmate, and bromochlorocadmate glasses are composed of the CdCl₆, CdBr₆ and CdBr_nCl_(6−n) octahedra (n=0-6), respectively, and that the Cd²⁺ in all the bromochlorocadmate glasses was more preferentially bonded to Br⁻ than Cl⁻. The relationship between the thermal properties and the anion coordination environments around Cd²⁺ in these glasses is discussed.     

Infrared contribution to the double-logarithmic small-x asymptotics of structure functions

The role playing by the “soft” (κ _T < 1 GeV/c) region in the small-x behaviour ofg ₁(x,Q ²) and the non-singlet structure functionf _{1, NS}(x, Q²) has been studied with the help of the effective QCD Lagrangian which takes into account the lightest degrees of freedom — the constituent quarks and the π-mesons (Goldstone bosons). It has been shown that the quark-quark interaction due to the pion exchange has a negative couplingg for the isovector component (I = 1 in thet-channel) off ₁(x,Q ²) and isosinglet component ofg ₁(x, Q²). Here the pion induced interaction changes mainly the normalization of the quark distribution (it decreasesf _1,NS ^{I = 1} (x,Q ²) two times atx < 3·10^?3) and changes slightly the effective exponents λ (?_1,NS ,g ₁ ～x ^?λ atx → 0). On the other hand due to a positive value of couplingg the value of λ increases by 15% for the isovector part ofg ₁ (x,Q ²) and up to λ ≈ 0.5 (instead of λ ≈ 0.2 without the pion contribution) for the isoscalar non-singlet structure functionf _1,NS ^{I = 0} (x,Q ²).