Resonant energy transfer from nanocrystal Si to β-FeSi2 in hybrid Si/β-FeSi2 film

The photoluminescence （PL） characteristics of hybrid β-FeSi2/Si and pure β-FeSi2 films fabricated by pulsed laser deposition at 20 K are investigated. The intensity of the 1.54-μm PL from the former is enhanced, but the enhancement vanishes when the excitation wavelength is larger than the widened band gap of Si nanocrystal. Time-resolved PL decay measurements reveal that the lifetime of the photo-excited carriers in the hybrid β-FeSi2/Si film is longer than that in the pure β-FeSi2 film, providing evidence that the PL enhancement results from the resonant charge transfer from nanocrystalline Si to β-FeSi2.     

Puzzle of low temperature phase of α-(ET)2 salts

α-(ET)₂ salts split into two groups: one superconducting and another with mysterious low temperature phase (LTP). The LTP does not exhibit X-ray signiture of conventional CDW or magnetic one for SDW. But the magnetic phase diagram suggests strongly a kind of CDW. We have analysed the threshold electric field of unconventional CDW (UCDW) both for H=0 and H≠0. The threshold electric field for UCDW with H=0 and for 3D weak-pinning limit describes reasonably well the one determined in α-(ET)₂KHg(SCH)₄. The result for H≠0 is also presented. We propose that the LTP in α-(ET)₂ salts is most likely UCDW.     

Low temperature work hardening of Cu-based α-phase solid solutions

Work-hardening of -phase Cu-Al solid solutions of Al concentration up to 14 at% was studied for temperatures 4·2 to 300 K. A substantially non-monotonous concentration dependence has been established for a number of work hardening parameters, that tends to increase at lower temperatures, and correlation has been found between temperature dependences of the yield stress and activation volume. The change in the density of mobile dislocations during strain rate cycling has been estimated and found to increase with temperature reduction.     

Near-zero thermal expansion in β-CuZnV2O7 in a large temperature range

We report a new type of near-zero thermal expansion material β-CuZnV₂O₇ in a large temperature range from 173 K to 673 K. It belongs to a monoclinic structure (C2/c space group) in the whole temperature range. No structural phase transition is observed at atmospheric pressure based on the x-ray diffraction and Raman experiment. The high-pressure Raman experiment demonstrates that two structural phase transitions exist at 0.94 GPa and 6.53 GPa, respectively. The mechanism of negative thermal expansion in β-CuZnV₂O₇ is interpreted by the variations of the angles between atoms intuitively and the phonon anharmonicity intrinsically resorting to the negative Grüneisen parameter.     

Low temperature acoustic properties in YBa2Cu3O7−δ

Temperature dependencies of acoustic lossesQ ^–1 and of relative sound velocity change v/v in YBa₂Cu₃O_7– up to 60 K are calculated by the tunneling model theory. The tunneling systems are related to the off-centered positions of the apical oxygen atoms O(A) and are described through the pseudo-Jahn-Teller effect. Tunneling systems' parameters are distributed in narrow range of values and are in correspondence with the experimentally observed infrared phonon spectra and thermal ellipsoids of O(A). Respective relaxation times are calculated by the adapted reaction rate method. The calculatedQ ^–1(T) and v(T)/v dependencies are in good agreement with the experimental data, which is an additional support to the conclusion about the existence of tunneling systems in YBa₂Cu₃O_7– due to the pseudo Jahn-Teller effect.     

Low temperature vibrational relaxation of OD in the A2Σ, state

The vibrational relaxation of the A ²Σ state of OD has been studied in the low translational temperature environment of an argon free-jet (T_trans near 5 K). Using laser induced fluorescence (LIF), the absolute vibrational relaxation rate coefficients were measured for OD A²Σ (ν′) to be 7.1 ± 2.6 × 10^?11, 5.9 ± 1.4 × 10^?11, and 2.7 ± 1.1 × 10^?11 cm³ s^?1 for the ν = 3, 2 and 1 states, respectively. State-to-state relaxation rate coefficients were also obtained for the ν= 1, ? = 1 level going to ν= 0, ? levels in the A²Σ manifold. The rotational relaxation rate coefficient for ν= 1, ?= 1 in the A state of OD was found to be 9.6 ± 1.0 × 10^?11cm³s^?1. These values are consistent with values measured for OH A²Σ, and the total loss rates are near the capture rate coefficient value. The vibrational relaxation rate coefficients k_ν appear to be governed by the vibrational energy of the molecule rather then by interaction with nearby dissociative states such as the a⁴Σ state. The relative Einstein A factors for the A²σ (ν = 3) state of OD were determined and compared with the available calculated value.     

Observations of the stress developed in Si inclusions following plastic flow in the matrix of an Al–Si–Mg alloy

Abstract

Measurements are made of the stress developed in near-spherical elastic inclusions in an elastic plastic matrix under both tension and compression loading. Two experimental conditions are reported. The first case is where no thermal mismatch exists between the inclusions and the matrix, so that the stress in the inclusion is purely a result of the misfit in the elastic moduli and of the distortion of the plastic slip-line field around the inclusion. The observations are believed to be the first such and are in qualitative agreement with finite-element modelling for idealised inclusion distributions. The second case is the more usual one where a thermal misfit stress exists and observations are reported of the stress relief effects caused by the interaction of the plasticity-induced stress with the thermal and elastic misfit stresses.     

Orientation relationship between α-Fe precipitate and α-Al2O3 matrix in iron-implanted sapphire

Fe ions were implanted into α-Al₂O₃ single crystals (sapphire) at room temperature and annealed in a reducing atmosphere. The orientation relationships (ORs) between α-Fe particles and sapphire matrix were investigated using transmission electron microscopy (TEM). All the α-Fe particles have the orientation relationship (OR) of (1 1 1)_α-Fe || (0 0 0 1)_sapphire and ${[1\overline{1}0]}_{\alpha \text{-Fe}}\left|\right|{[11\overline{2}0]}_{\text{sapphire}}$ with sapphire. This OR is predicted precisely by the coincidence of reciprocal lattice points (CRLP) method. The other OR of (1 1 0)_α-Fe || (0 0 0 1)_sapphire and ${[111]}_{\alpha \text{-Fe}}\left|\right|{[5\overline{1}\overline{4}0]}_{\text{sapphire}}$ reported before is confirmed by the same method to be one of the secondary preferred orientation relationships in the α-Fe/sapphire system.     

Low temperature Pmmm and C2/m phases in Sr_2CuO_(3+δ) high temperature superconductor

A new low temperature Pmmm(120 K) phase was found in high temperature superconductor Sr_2 CuO_(3+δ), which was indicated as a pure electronic phase by resonant x-ray diffraction at Cu K-edge. As shown by x-ray absorption fine structure(EXAFS) and x-ray absorption near edge structure(XANES) at Cu K-edge, the strong charge density redistribution and local lattice fluctuations around Cu site at the onset of phase transition were due to the occurrence of superconductive coherence, the redistribution and fluctuation finished at Tc. Finally, the electron–lattice interaction was mainly elaborated to understand the superconductivity of Sr_2 CuO_(3+δ).     

Calculation of the temperature dependence of the thermal emf in amorphous alloys CaxAl1−x and AuxNi1−x

The temperature dependence of the thermal emf of Ca_xAl_1–x and Au_xNi_1–x for four different concentrations of the components of the alloys is calculated on the basis of the concept of dynamic concentrated excitations in amorphous metal systems. It is shown that increasing x from 0.15 to 0.50 in Au_xNi_1–x raises the thermal emf, and a further increase in the Au concentration from 0.50 to 0.80 lowers S(T). For Ca_xAl_1–x the dependence S(T) is calculated in the interval of Ca concentrations from 0.55 to 0.75. In this concentration interval the thermal emf decreases as x is increased. It is shown that for both types of alloys the S(T) curve bends abruptly at a temperature near 10T₀ (where T₀ is the concentration-dependent characteristic temperature of amorphous alloys separating the ranges of strong and weak scattering of electrons by dynamic concentration excitations). The so-called S(T) knee shifts toward lower temperatures when the thermal emf increases with increasing x and toward higher temperatures when S(T) decreases with increasing x. The results agree with experimental data.Institute of Physics of Strength and Materials Science, Siberian Branch, Russian Academy of Sciences. State University, Tomsk. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 43–48, August, 1994.     

Pressure-induced isostructural phase transition in α-Ni(OH)_2 nanowires

High pressure structural phase transition of monoclinic paraotwayite type α-Ni(OH)_2 nanowires with a diameter of15 nm–20 nm and a length of several micrometers were studied by synchrotron x-ray diffraction(XRD) and Raman spectra.It is found that the α-Ni(OH)_2 nanowires experience an isostructural phase transition associated with the amorphization of the H-sublattice of hydroxide in the interlayer spaces of the two-dimensional crystal structure at 6.3 GPa–9.3 GPa. We suggest that the isostructural phase transition can be attributed to the amorphization of the H-sublattice. The bulk moduli for the low pressure phase and the high pressure phase are 41.2(4.2) GPa and 94.4(5.6) GPa, respectively. Both the pressure-induced isostructural phase transition and the amorphization of the H-sublattice in the α-Ni(OH)_2 nanowires are reversible upon decompression. Our results show that the foreign anions intercalated between the α-Ni(OH)_2 layers play important roles in their structural phase transition.     

Effect of calcination temperature on phase transformation,structural and optical properties of sol–gel derived ZrO2 nanostructures

Zirconia (ZrO₂) nanostructures of various sizes have been synthesized using sol–gel method followed by calcination of the samples from 500 to 700 °C. The calcined ZrO₂ powder samples were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier-transform infra-red spectroscopy (FT-IR), UV–visible spectroscopy (UV–vis.), Raman spectroscopy (RS) and thermogravimetric analysis (TGA). The phase transformation from tetragonal (t) to monoclinic (m) was observed. The average diameter of the ZrO₂ nanostructures calcined at 500, 600 and 700 °C was calculated to be 8, 17 and 10 nm, respectively. The ZrO₂ sample calcined at 500 °C with tetragonal phase shows a direct optical band gap of 5.1 eV. The value of optical band gap is decreased to 4.3 eV for the ZrO₂ calcined at 600 °C, which contains both tetragonal (73%) and monoclinic (27%) phases. On further calcination at 700 °C, where the ZrO₂ nanostructures have 36% tetragonal and 64% monoclinic phases, the optical band gap is calculated to be 4.8 eV. The enhancement in optical band gap for ZrO₂ calcined at 700 °C may be due to the rod like shape of ZrO₂ nanostructures. The tetragonal to monoclinic phase transformation was also confirmed by analyzing Raman spectroscopic data. The TG analysis revealed that the ZrO₂ nanostructure with dominance of monoclinic phase is found to be more stable over the tetragonal phase. In order to confirm the phase stability of the two phases of ZrO₂, single point energy is calculated corresponding to its monoclinic and tetragonal structures using density functional theory (DFT) calculations. The results obtained by theoretical calculations are in good agreement with the experimental findings.     

X-ray and luminescent analysis of finely dispersed β-FeSi2 films formed in Si by pulsed ion-beam treatment

Finely dispersed β-FeSi₂ films were formed by implanting Fe⁺ ions with an energy of 40 keV and a dose of 1×10¹⁶ cm⁻² in Si single crystals, followed by nanosecond pulsed ion-beam treatment. The results of glancing incidence x-ray diffraction indicate the formation of a highly grain-oriented film consisting of inclusions of the iron disilicide phase (β-FeSi₂) with a grain size of approximately 40 nm surrounded by a polycrystalline Si matrix. The photoluminescence spectroscopy data reveal that the photoluminescence signal with a peak around 1.56 μm, which is observed up to 210 K, is associated with direct interband transitions in β-FeSi₂ and not with the contribution from the dislocation-induced line D1. __________ Translated from Fizika Tverdogo Tela, Vol. 43, No. 9, 2001, pp. 1569–1572. Original Russian Text Copyright ? 2001 by Bayazitov, Batalov, Terukov, Kudoyarova.     

Interface trap density in amorphous La2Hf2O7/SiO2 high-κ gate stacks on Si

The present paper investigates the interface trap density of a new high- gate dielectric stack, La₂Hf₂O₇/SiO₂ on Silicon. Amorphous La₂Hf₂O₇ thin films are deposited by metal evaporation in the presence of atomic oxygen beams on an ultra-thin SiO₂ layer (1.5 nm) grown by rapid thermal oxidation on a p-type Si substrate. A combination of electrical (C–V) and cross sectional TEM measurements indicates a value of the dielectric constant of about 19±2.2. The interface states density (D_it) was measured using the conductance method for different La₂Hf₂O₇ thicknesses ranging from 3 nm to 14 nm. D_it ranges from 3.4×10¹⁰ eV^-1 cm^-2 to 4.8×10¹² eV^-1 cm^-2 and shows a quasi-linear dependence on the La₂Hf₂O₇ layer thickness. The interface state density increase with film thickness could have different explanations, such as oxygen de-passivation and/or defect generation at the Si-SiO₂ interface, formation of a new Si-SiO₂ interface by Si oxidation or alternatively, the SiO₂ interfacial layer reduction by oxygen vacancies. PACS 73.40.Qv; 77.22.Ch; 77.55.+f; 77.84.Bw     

Hydrostaticity and hidden order: effects of experimental conditions on the temperature–pressure phase diagram of URu2Si2

The pressure dependence of the hidden order phase transition of URu₂Si₂ is shown to depend sensitively upon the quality of hydrostatic pressure conditions during electrical resistivity measurements. Hysteresis in pressure is demonstrated for two choices of pressure medium: the commonly used mixture of 1:1 Fluorinert FC70/FC77 and pure FC75. In contrast, no hysteresis is observed when the pressure medium is a 1:1 mixture of n-pentane/isoamyl alcohol, as it remains hydrostatic over the entire studied pressure range. Possible ramifications for the interpretation of the temperature–pressure phase diagram of URu₂Si₂ are discussed.     

Low temperature electrical transport in La1−xNdxNiO3−δ

We have investigated low temperature electrical transport in La_1−xNd_xNiO_3−δ perovskite oxide samples. Samples were prepared by a sol–gel method and were characterized by X-ray diffraction and chemical methods. High precision electrical resistivity, magnetoresistance (MR) and electron tunneling conductance measurements were performed. Crystal structure investigations showed a phase transition from rhombohedral to orthorhombic phase at x=0.4. In the orthorhombic phase Ni–O–Ni bond angle was found to depend very sensitively on the value of x; as the value of x increases Ni–O–Ni bond angle decreases resulting in the tilting of NiO₆ octahedra. A Correlation between the Ni–O–Ni bond angle and electrical transport has been observed. The analysis of the electrical resistivity data showed the presence of disorder driven quantum correction effects, namely e–e interactions and weak localization, in the system. A dip in the tunneling density of states and negative MR also suggest the presence of e–e interaction and weak localization effects in the system.     

Observation of a structural phase transition in a CoSi2 layer buried in 〈111〉 Si

Single crystalline multilayered structures of Si/CoSi₂/Si111 made by high dose implantation of Co in a Si wafer were investigated with⁵⁷Co source Mössbauer spectroscopy, channeling measurements and X-ray diffraction. The results point to a structural phase transition in the CoSi₂ buried layer between 180 and 220 K.     

Molecular mechanism of α-β phase transformation in poly-p-xylylene

The molecular mechanism of the α→β phase transformation of poly-p-xylylene was studied by transmission electron microscopy using solution-grown single crystals. The observations suggest that the phase transformation from the α to β-phase occurs by a coordinated lateral movement of the molecules from the lattice sites of the α-phase to the sites of the β-phase.     

Low temperature hole mobility in strained p-Si/Si1 − xGex/p-Si selectively doped double heterojunctions

We present a systematic theoretical study of the low-temperature (T =  0 K) quasi-two-dimensional-hole gas mobility in strained p-Si/Si_0.8Ge_0.2/p-Si selectively doped double heterojunctions. Ionized-impurity (remote and background), interface-roughness and alloy-scattering mechanisms are taken into account. In our calculations we use self-consistently calculated wavefunctions and a multi-subband transport model. We investigate the mobility dependence on the structural parameters, such as the spacer thickness and the well width. We comment on the significance of every scattering mechanism looking for the maximum hole mobility in these systems. Alloy scattering seems to be the main mobility-limiting mechanism resulting in a hole mobility which increases with the spacer thickness and the well width. Our theoretical results are consistent with experiment.     

Structure of the magnetic-field-induced modulated phase of an α-Fe2O3: Ga single crystal

The field and orientational dependences of magnetic linear birefringence in α-Fe₂O₃: Ga single crystals are investigated. The experimental results make it possible to refine and specify the structure and reveal the features of technical magnetization of the modulated magnetic phase of this weak ferromagnet. It is shown that the model of the modulated magnetic structure used in this study adequately describes the experimental situation.