Structural Phase Transition and the Dielectric Permittivity of the Model Lipid Bilayer [(CH2)12(NH3)2]CuCl4

Structural phase transitions in the lipid-like bilayer material [(CH₂)₁₂(NH₃)₂]CuCl₄ have been observed using differential thermal scanning. The compound shows an irreversible thermochromic transition at ? 465 K and three reversible transitions at T ₁ = 433 ± 4 K and T ₂ = 411 ± 2 K and T ₃ = 358 K. The transition at 350 K is ascribed to chain melting. The other two correspond to crystalline phase transformation.

Phase (IV) T₃ = 358 ± 2K Phase (III) T₂ = 411 ± 2K Phase (II) T₁ = 433 ± 4K Phase (I)

Dielectric permittivity is studied as a function of temperature in the range 300-440 K and frequency, range (60 Hz-100 kHz). It confirms the observed transitions. The dielectric permittivity reflects rotational and conformational transitions for the compound. The variation of the real part of the conductivity with temperature is thermally activated in the temperature range above 350 K, with frequency-dependent activation energy, the values of activation energy lie in the range of ionic hopping. The dependence of the conductivity on frequency follows the universal power law σ = σ₀ + A(T) ω ^{s ( T )} with 0<s<1. Comparison of this material with other members of the series is discussed     

Anomalies of the photo-response and thermal boundary resistance of a YBaCuO/YSZ structure

The photoresponse of a YBaCuO/ZrO₂ bolometric structure was measured under modulated (λ = 630 nm) and pulsed (τ ≈ 7 – 8 ns; λ = 337 nm) laser excitation. The shape of the measured photoresponse was interpreted by a thermal model; nevertheless, the pulse amplitude for vanishing YBaCuO film resistance was 5–6 times greater than predicted; the thermal boundary resistance R_Bd between YBaCuO and YSZ was evaluated ? 10^?2 K × cm²/Watt, which is considerably larger than estimated theoretically for the similar situation of YBaCuO/MgO [5].     

Covalent distribution of spin in V2O3:O17 nuclear resonance

O¹⁷ nuclear magnetic resonance has been observed in metallic V₂O₃ with frequency shifts from (?0.10 ± 0.02)-(?0.05 ± 0.02) per cent between 170 and 460°K respectively, a linewidth of 37 ± 5 oe and spin-lattice relaxation rate 1/T₁ ≈ 60 sec^?1 at 296°K. From these quantities, covalency parameters f_s/2S = ? 0.35 × 10^?3 and ?_π/2S ≈ ? 0.07 are calculated. One of the two vanadium 3d electrons in the antiferromagnetic state below the 170°K metal-insulator transition is inferred to lie in a non-magnetic state, while covalent charge transfer augments the spin moment of the other 3d electron to the observed 1.2 μ_B.     

Characterization of organic–inorganic hybrid layered perovskite and intercalated compound (n-C12H25NH3)2ZnCl4

We report on some electrical properties and solid–solid phase transitions of organic–inorganic hybrid layered halide perovskite and intercalated compound (n-C₁₂H₂₅NH₃)₂ZnCl₄ which is one member of the long-chain compounds of the series (n-C_nH_2n+1NH₃)_2,(n = 8–18). The complex dielectric permittivity ?*(ω,T) and the ac conductivity σ (ω,T) were measured as functions of temperature 100 K < T < 390 K and frequency 5 kHz < f < 100 kHz. Moreover, the differential scanning calorimetery and the differential thermal analysis thermograms were performed. The analysis of our data confirms the existence of a structural phase transition at T ≈ (362?±?2) K, where the compound changes its state from intercalation to non-intercalation with a drastic increase in the c-axis by about 16.4%.

The behavior of the frequency-dependent conductivity follows the Jonscher universal power law: σ (ω, T) α?^s(?,T). The mechanism of electrical conduction in the low-temperature phase (phase II) can be described as quantum mechanical tunneling model.     

Proton magnetic relaxation in ethylene oxide,tetrahydrofuran, and triethylene diamine as clathrate deuterates

Proton spin-lattice relaxation times (T ₁) have been measured for triethylene diamine, ethylene oxide, and tetrahydrofuran as clathrate deuterates. The results are interpreted in terms of anisotropic rotation of the guest molecules. Triethylene diamine is thought to be undergoing rotation about its C ₃ axis with a correlation time given by τ_c/s = 4·87 × 10^-14 exp (1680 K/T) at temperatures between 120 K and the decomposition point (308 K). Between 77 K and 120 K, T ₁ is dominated by conformational distortions of the guest molecule. Ethylene oxide and tetrahydrofuran rotate about at least two axes in the deuterate at rates sufficient to produce some motional narrowing. At high temperatures the relaxation is caused in both cases by rotation about an axis perpendicular to the C ₂ axis, and at lower temperatures by rotation about the C ₂ axis itself. The correlation times are for ethylene oxide τ_c/s = 6·76 × 10^-14 exp (450 K/T), T < 160 K; and for tetrahydrofuran τ_c/s = 4·79 × 10^-14 exp (470 K/T), T < 140 K.

The free induction decay shapes indicate that, in each case, low frequency motion is occurring about all axes throughout the temperature range studied (77 K to the decomposition temperature in each case). From the lack of an observable signal from the clathrate deuterates of hexamethylene tetramine and dioxan, it is deduced that there is no reorientational motion of these guests at frequencies greater than their rigid-lattice linewidths.     

Elektronen-Spin-Gitter- und Spin-Spin-Relaxation von Cu(II) in Ni(II)-bis(1,1-dicyanoäthylen-2,2-dithiolat)-Einkristallen

The temperature-dependence of the electron-spin-lattice- and -spin-spin-relaxation times T₁ and T₂ of Cu(II) in (Tetrabutyl-ammonium)-Ni(II)bis(1,1-dicyanoethylene-2,2-dithiolate) single crystals are reported. In the range of the direct processes (T = 2…7°K) a dependence of the spin-lattice relaxation rate on the orientation of the Cu-nuclear spin was observed. From the temperature-dependence of the RAMAN -process the DEBYE -temperature and the averaged soundvelocity of the molecular crystal have been derived being θ = (100 ± 10)°K and 〈v〉 ≈ 0.7. 10⁵ cm/sec. In order to explain the unusual small line-width of the ^63,65Cu-hyperfine structure lines of the EP R-spectrum in the investigated large temperature-range of T = 2.0…300°K the mechanisms contributing to the line width were analyzed. The possible influence of the strong covalency of the Cu-ligand bonds to T₁ is considered.     

Grain size of MnBi films

MnBi films were prepared by annealing Bi-Mn composite films. The grain size of the MnBi films was strongly influenced by the temperatureT _s of the film substrate during evaporation of Bi and Mn. For 55-nm-thick films, the numberN of MnBi grains per mm² decreased from 4×10⁶ per mm² forT _s =+90°C to 1 per mm² forT _s =−40°C. The diameters of the largest grains observed were about 5 mm. For interpretation of this result, the formation process of MnBi films was observed magneto-optically and electronmicroscopically. It was found thatN was determined by the structure of Bi in the Bi-Mn composite films and that this strongly depended onT _s .     

Femtosecond Optical and Magneto-Optical Spectroscopy Study of Magnetic and Electronic Inhomogeneities in a Pd0.94Fe0.06 Thin Film

The results of a femtosecond optical and magneto-optical spectroscopy study of a thin epitaxial film of a low-temperature magnetically soft Pd_0.94Fe_0.06 alloy on a ({dy001})-MgO substrate are reported. The photoinduced demagnetization and magnetization recovery times are determined. The latter increases critically at approaching the ferromagnetic ordering temperature T_C = 190 K from below. It is shown that the reflectivity dynamics after a photoexcitation pulse evolves from a two-exponential in the paramagnetic phase to a four-component at 80 K < T < T_C, simplifying to a three-component at T < 50 K. According to our interpretation, such an evolution, along with the manifestation of an additional increasing component in the magnetic response at 80 K < T < T_C, indicates a magnetic and electronic inhomogeneity of the film associated with the distribution of local iron concentrations. The fraction of small-scale inclusions of the paramagnetic phase is estimated as ≈10 vol %.

     

Relaxation Times and Population Inversion of Excited States of Arsenic Donors in Germanium

The relaxation times of excited states of arsenic dopant in germanium at cryogenic temperatures T < 15 K have been experimentally studied by the optical pump-probe method using radiation of a free-electron laser. Two variants of the excitation of impurity centers have been used in the experiment: (i) from the 1s(A₁) ground state of the dopant and (ii) from the 1s(T₂) first excited state having a finite thermal population. In the former variant, it has been shown that the decay times of the 2p₀ and 3p_± states are about 0.8 and 0.6 ns, respectively. In the latter variant, a single measurement can simultaneously provide the relaxation times of two 2p_± and 1s(T₂) states about 0.6 and no more than 0.16 ns, respectively. The data obtained have indicated the possibility of forming population inversion and the gain of terahertz radiation at the 2p_± → 1s(T₂) and 2p₀ → 1s(T₂) transitions at the optical excitation of the mentioned impurity centers.

     

Magnetotransport parameters of La0.67Ca0.33MnO3 films grown on neodymium gallate substrates

Weakly mechanically stressed 40-nm-thick La_0.67Ca_0.33MnO₃ films have been grown coherently on a (001)NdGaO₃ substrate by laser evaporation. The electrical resistivity ρ of the La_0.67Ca_0.33MnO₃ film reaches a maximum at a temperature T _C ≈ 255 K. At temperatures below 0.6T _C, the temperature dependences of ρ are well approximated by the relation ρ = ρ_def + C ₁ T ² + C ₂ T ^4.5, in which the first term on the right-hand side accounts for the contribution of structural defects to electrical resistivity, and the second and third terms stand for those of the electron-electron and electron-magnon interactions, respectively. The parameters ρ_def ≈ 1 x 10^?4 Ω cm and C ₁ ≈ 7.7 × 10^?9 Ω cm K^?2 do not depend on temperature and magnetic field H. The coefficient C ₂ decreases with increasing H to reach about 4.9 × 10^?15 Ω cm K^?4.5 at μ₀ H = 14 T.     

Generation of coherent phonons in ruby

In conditions of inversion of spin-level populations at T = 1.7° K the resonant stationary coherent phonon emission has been revealed in ruby at frequency ν = 9.12 GHz. The intensity of emission is I_S ≈ 10^?6 W·cm^?2. The frequency spectrum consists of very narrow lines generated simultaneously on several modes of hypersonic ruby cavity. Monochromatism of emission in each line is Δν/ν ? 2 × 10^?6. The spectrum nature permits to suppose the levelwidth of elementary spin excitation to be Δν_s < 3 × 10⁵ Hz, i.e. at least 2–3 orders it is narrower than the inhomogeneously broadened resonant line in ruby. The lifetime of resonant phonons in ruby was defined: τ = 7 × 10^?6 sec.     

钛离子辐照对MgB2超导薄膜的载流能力和磁通钉扎能力的影响

利用混合物理化学气相沉积法(hybrid physical-chemical vapor deposition, HPCVD)可以制备出高性能的MgB₂超导薄膜, 再对薄膜进行钛(Ti)离子辐照处理.经过辐照处理后的样品被掺入了Ti元素, 与未处理的干净MgB₂样品相比,其超导转变温度没有出现大幅度的下降, 而在外加磁场下的临界电流密度得到了明显的提高,同时样品的上临界磁场也得到了提高. 在温度5 K, 外加垂直磁场为4 T的情况下, Ti离子辐照剂量为1× 10¹³/cm²的样品的临界电流密度达到了1.72× 10⁵ A/cm², 比干净的MgB₂要高出许多,而其超导转变温度仍能维持在39.9 K的较高水平.     

Amorphous germanium III. Optical properties

The optical properties of thick sputtered films (～30μ) of amorphous Ge, grown with different substrate temperatures (0ˇ-T _sˇ-350°C), were obtained between 0·05 and 4·5 eV by a combination of reflectance, transmittance and ellipsometric measurements. The refractive index at 0·15 eV decreases monotonically with increasing T _s, or equivalently, with increasing density, and is 4·13±0·05 eV in the highest density films. The absorption edge is approximately exponential (10²?α?10⁴ cm^?1) but shifts monotonically to higher energy and increases in slope with increasing T _s. Similarly, the peak in ε₂ grows by about 10% and shifts by about 0·15 eV to higher energies, reaching a maximum of about 23 at 2·90±0·05 eV in the high density films. The peak in the transition strength ω²ε₂ occurs at 4·2±0·2 eV in all films, but increases in magnitude with increasing T _s. The sum rules for n _eff(ω) and ε_0,eff(ω) are evaluated for ▄ω?5 eV and vary monotonically with T _s. These trends are neither compatible with Galeener's void resonance theory nor with changes in the oxygen content of the films, determined by the examination of absorption peaks at 0·053 eV and 0·09 eV. An explanation, suggested here and expanded in I, is based on the observed changes in the structure of the network and voids.     

NMR of 89Y in normal and superconducting YBa2 Cu3 O9-δ

Pulsed NMR data for ⁸⁹Y in the high-T_c superconductor YBa₂Cu₃O_9-δ (δ ≈ 2.1) in both the normal and mixed states are presented. Spin-lattice relaxation exhibits a Korringa temperature dependence in the normal state, with T₁T = (4.6 ± 0.1) × 10³ s-K. Below T_c, T₁ increases much more quickly than for a conventional superconductor. From linewidth measurements, the zero-field penetration depth is estimated to be λ ≈ 3800 Å. The Knight shift in the normal state is small, K ≈ 0.02%. Spin-spin relaxation times, determined by Y-Cu dipolar interactions, decrease below T_c due to the detuning of Cu-Cu interactions by fluxoid field gradients.     

Preparation of Pb2CrO5 thin films by an electron-beam evaporation technique

Pb₂CrO₅ thin films have been prepared by an electron-beam evaporation deposition technique on glass substrates using ceramic disks. The thin film fabrication conditions are studied by x-ray diffraction, replica electron micrography and scanning electron microscopy as parameters of substrate temperature, annealing temperature and annealing time. As-deposited Pb₂CrO₅ thin films in the amorphous state are crystallized by heat treatment. Annealed thin films are structurally classified into three types according to the preferred orientations which depend on substrate temperatures (T _s:

1. (i)

   (020) atT _s=room temperature

2. (ii)

   mainly (310) atT _s=100 ‡C, and

3. (iii)

   (200) atT _s=350 ‡C.

The substrate temperature contributes to film orientations; annealing temperature and time-enhance film crystallinity. The annealing temperature is fixed between 400 and 500 ‡C in making appropriate Pb₂CrO₅ thin films. These thin films, ranging between 0.3 and 2.0 Μm in thickness, are prepared at a deposition rate of 1500 å/m.

     

多层膜外退火方法制备MgB2超导薄膜

报道了利用电子束蒸发的Mg/B多层膜作为前驱体，然后退火制备MgB₂薄膜的工作. 实验中发现，采用翻转膜面的退火处理方式可以有效地避免降温过程中Mg蒸气在薄膜表面形成的颗粒凝结，由此稳定地实现了面积为10 mm×10 mm，均匀、平整的超导薄膜的制备，T_c达35 K，转变宽度为0.8 K，在5 μm×5 μm的区域内薄膜的平均粗糙度小于10 nm. 为了便于后续器件制作过程中的微加工工艺，研究了膜厚小于1000 ?时薄膜的成相规律，发现当样品厚度减薄后，T_c会有明显降低. 通过调整前驱薄膜中的不同分层厚度，仍可实现转变温度达30 K以上、厚度约600 ?的MgB₂薄膜，在20 K时的临界电流密度为2.4×10⁶ A/cm².     

Study of an Underexpanded Plasma Jet

Supersonic jets (Mach number M = 1.4 – 2.0) of dense plasma were produced by a plasmatron in a pulsed regime. The shock structure was investigated by fast mirror cameras, by the schlieren technique and by probe and spectroscopic methods. It was found that during jet expansion the bow shock ahead the gas-plasma interface and the shock behind it are superimposed with the quasistationary shock structure which includes the Mach disk. Whereas the plasma in the discharge chamber is in LTE (n_E ≈ 5 × 10²³ m^?3, T ≈ 14000 K), the electron density of the expanding jet plasma falls to a plateau value of n_E ≈ 5 × 10¹⁹ m^?3 below the LTE limit. At this phase, the jet plasma formed a ball-like turbulent plasmoid which moves at a subsonic velocity through the air up to some milliseconds.     

Mechanisms of low-temperature high-frequency conductivity in systems with a dense array of Ge<Subscript>0.7</Subscript>Si<Subscript>0.3</Subscript> quantum dots in silicon

High-frequency (HF) conductivity in systems with a dense (with a density of n = 3 × 10¹¹ cm^?2) array of self-organized Ge_0.7Si_0.3 quantum dots in silicon with different boron concentrations n_B is determined by acoustic methods. The measurements of the absorption coefficient and the velocity of surface acoustic waves (SAWs) with frequencies of 30–300 MHz that interact with holes localized in quantum dots are carried out in magnetic fields of up to 18 T in the temperature interval from 1 to 20 K. Using one of the samples (n_B = 8.2 × 10¹¹ cm^?2), it is shown that, at temperatures T ≤ 4 K, the HF conductivity is realized by the hopping of holes between the states localized in different quantum dots and can be explained within a two-site model in the case of

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, where ω is the SAW frequency and τ₀ is the relaxation time of the populations of the sites (quantum dots). For T > 7 K, the HF conductivity has an activation character associated with the diffusion over the states at the mobility threshold. In the interval 4 K < T < 7 K, the HF conductivity is determined by a combination of the hopping and activation mechanisms. The contributions of these mechanisms are distinguished; it is found that the temperature dependence of the hopping HF conductivity approaches saturation at T* ≈ 4.5 K, which points to a τ₀ ≤ 1. A value of τ₀(T*) ≈ 5 × 10^?9 s is determined from the condition ωτ₀(T*) ≈ 1.