Evidence for a phase transition in Bis(tetrathiotetracene) triiodide

The specific heat of bis(tetrathiotetracene) triiodide has been measured from 20 to 65 K using thermal relaxation methods. An anomaly is observed at 50 K which is consistent with a mean field theory for a Peierls transition.     

Low temperature metallic resistivity of the charge-transfer organic conductor (TMTSF)2PF6

The electrical resistivity arising from electron-electron Umklapp scattering is calculated at low temperatures for the anisotropic but two-dimensional charge-transfer complex (TMTSF)₂PF₆. Also, we consider a specular scattering correction to a recent one-phonon scattering mechanism and find a significant change in the temperature dependent behavior of the resistivity. Finally an exact solution is obtained for the resistivity arising from two-libron scattering and is found to give a small contribution at low temperatures. Comparing these results with recent data, we find that the electron-electron mechanism yields the most satisfactory agreement at low temperatures, while contributions from the other two mechanisms may not be negligible at high temperatures.     

On the stoichiometry of the organic metal bis(ethylenedioxy)-tetrathiafulvalene chloride

We report on Shubnikov-de Haas and de Haas-van Alphen measurements of an organic metal based on the molecular donor bis(ethylenedioxy)-tetrathiavulvalene (= BEDO-TTF) and the anion Cl^- with H₂O molecules. The observed single oscillation frequency perfectly follows the two-dimensional dependence with T. The Fermi-surface area of of the first Brillouin zone proves a quarter-filled band, i.e., a 2:1 stoichiometry of the BEDO-TTF donor with respect to the anion. The apparent discrepancy to X-ray data which give a 1:1 ratio between BEDO-TTF and Cl^- is understood by replacement of H₂O molecules with (H₃O)⁺ ions. The proposed stoichiometry therefore is (BEDO-TTF)₂ ⁺(H₅O₂)⁺(Cl^-)₂. The cyclotron effective mass is when deduced from the temperature dependence of the fundamental oscillation amplitude, but strongly reduced when extracted from higher harmonics. This and the strong harmonic content of the oscillations signals an influence of the two-dimensional electronic structure and can be qualitatively understood by applying the concept of magnetic interaction to the effect of the oscillatory chemical potential. Received: 16 April 1998 / Revised: 7 July 1998 / Accepted: 9 July 1998     

Low temperature Hall-effect measurements in some metallic glasses

We report on simultaneous measurements of the Hall-effect and resistivity in the metallic glasses Pd₈₀Si₂₀ and Ni₇₈Si₈B₁₄. Data were taken within a temperature range of 1.7 to 300 K and in magnetic fields up to 1.1 T. Whereas the Hall constant is nearly independent of temperature in case of Pd₈₀Si₂₀, it changes sign at low temperatures in Ni₇₈Si₈B₁₄. The implications of the low temperature results are discused with respect to the origin of a resistivity minimum, present in both alloys.     

Low temperature thermodynamical properties of the organic chain conductor (TMTSF)AsF

We report on specific heat measurements of the quasi-one-dimensional organic salt (TMTSF)₂AsF₆ in its spin density wave state between 75 mK and 7 K. Similarly to (TMTSF)₂PF₆, we find discontinuities in the lattice contribution at 1.9 K an d 3.5 K ascribed to sub-spin density wave phases. Time-dependent effects due to dynamics of low-energy excitations in metastable states occur only below 0.2 K which yields an activation energy for the equilibrium energy relaxation process of 0.34 K, 4-5 times smaller than found for (TMTSF)₂PF₆. Finally the reduction of the low-energy excitations contribution to the specific heat in comparison to PF₆ reveals an intermediate cubic-like regime between 0.25 and 0.5 K that we tentatively describe as the phason contribution of the incommensurate spin density wave modulation. Received: 17 March 1998 / Revised: 27 July 1998 / Accepted: 22 September 1998     

Minimum metallic conductivity in a thin crystal

Electrical conductivities of thin crystals of Bi₂(Te,S)₃ measured from 4.2°K to 300°K fall into four regions: 1) σ < 1.3×10^?5 S with positive temperature coefficient of conductivity; 2) 1.3×10^?5 S < σ < 1.4×10^?5 S with temperature independent conductivity; 3) 1.4×10^?5 S σ < 4×10^?5 S with negative temperature coefficient of conductivity, and 4) σ > 4×10^?5 S with hardly any temperature dependence. A disproportionately high fraction of samples falls into the second range; 1.3×10^?5 S < σ < 1.4×10^?5 S.     

Low temperature elastic properties of a superconducting disordered metal

We have measured the elastic properties of a glass at audiofrequencies (0.2–1.5 kHz) in the temperature interval 0.01 to 10 K. In the superducting glassy metal Cu₆₀Zr₄₀ (T_c=0.31 K) both the sound velocity and attenuation are similar to that of an insulating glass below 0.05 K. Above that temperature (T_c>T>0.05 K) the relaxation process is mainly governed by electrons which are thermally excited above the BCS gap. Above the superconducting transition we observed new features in the sound velocity which were not observed in high frequency measurements.     

邮票上的物理学史(71)--低温物理学(续)

再看超流．液氦的超流动性是苏联物理学家卡皮察于1938年发现的(图13,俄国2000年,俄国20世纪科学．上面的俄文是“卡皮察在1938年发现液氦的超流动性”)．他在用毛细管测量液氦的黏性时发现,液氦的黏性在低于2．2K时趋于消失,可以完全畅通无阻地流过极细的管道或狭缝而不损耗其动能;同时热导率异常高．     

Vibrational spectroscopic and DFT calculation studies of a new organic–inorganic compound of bis (4-acetylanilinium) tetrachlorocadmiate (II)

The FT-IR and Raman vibrational spectra of bis (4-acetylanilinium) tetrachlorocadmiate (II) compound have been measured at room temperature by FT-infrared spectroscopy (4000–400 cm⁻¹) on polycrystalline samples, and by Raman spectroscopy (3600–30 cm⁻¹) on monocrystals. The structure of the [C₈H₁₀NO] ₂CdCl₄ formed by two cations [C₈H₁₀NO]⁺ of same type and one type of anion [CdCl₄]²⁻ was optimized by density functional theory (DFT) using the B3LYP method. The theoretical wavenumbers spectra were scaled by multiple scaling factors, yielding a good agreement between the experimentally recorded and the theoretically calculated values. Root mean square (rms) value was calculated and the small difference between experimental and calculated modes has been interpreted by intermolecular interactions in the crystal. The comparison between the [C₈H₉NO] ligand and the [C₈H₁₀NO]₂[CdCl₄] compound of the Raman spectra showed a decrease in the wavenumber of the bands assigned to the stretching vibration of (NH₃) group in the compound due to the effect of the protonation of the nitrogen.     

Universal temperature dependence of the conductivity of a strongly disordered granular metal

A disordered array of metal grains with large and random intergrain conductances is studied within the one-loop accuracy renormalization group approach. While, at low level of disorder, the dependence of conductivity on logT is nonuniversal (it depends on the details of the array’s geometry), for strong disorder, this dependence is described by a universal nonlinear function, which depends only on the array’s dimensionality. This function is found numerically in two dimensions. The dimensional crossover in granular films is discussed. The text was submitted by the authors in English.     

Reflectivity spectra of new organic metal (BEDO-TTF)5[CsHg(SCN)4]2

The polarized reflectivity and optical conductivity spectra of microcrystals of the new organic conductor (BEDO-TTF)₅[CsHg(SCN)₄]₂ based on the donor molecule bis(ethylenedioxy)tetrathiafulvalene (BEDO-TTF) have been studied in the spectral ranges 600–6500 and 9000–40000 cm^?1 at 300 K for three principal lattice directions. The optical evidence for the quasi-two-dimensional character of the conducting electronic system is obtained. The conclusion is made that the studied crystal is the quasi-two-dimensional semimetal with overlapping electron energy bands. The basic parameters of the electronic system of the crystal are determined in the framework of the Drude model. It is found that the allowed electron energy bands of the crystal are somewhat narrower than those of the previously studied structurally allied superconductor based on the same molecule. The features of vibrational structure are identified in the σ(ω) spectra for the specified three polarizations.     

Shubnikov-de Haas oscillations in a new dual-layered quasi-two-dimensional organic metal (BETS)4CoBr4(C6H4Cl2)

The interlayer and intralayer resistances and Shubnikov-de Haas oscillations in a new dual-layered quasi-two-dimensional organic metal (BETS)₄CoBr₄(C₆H₄Cl₂) with a periodically varying structure of cation layers have been studied. It has been shown that the interlayer resistivity corresponds to an incoherent or weakly incoherent transport regime. The oscillations of the magnetoresistance have been described by a model of a chain of coherent magnetic breakdown orbits taking into account the quantum interference effect. The behavior of the interlayer transport, as well as quantum oscillations, is in good agreement with the theoretical calculations of the band structure.     

Low temperature saturation of the channel conductivity in silicon inversion layers

The conductivity of silicon inversion layers in MOS structures has been measured in the temperature range 0.03 ? T ? 4.2 K at low carrier densities. The conductivity is activated but saturates at the lowest temperatures for both n- and p-channels. The saturation conductivity is found to increase with reverse substrate bias.     

我国低温等离子体研究进展(Ⅱ)

5低温等离子体应用 随着我国国民经济的发展，低温等离子体的应用获得了越来越多的重视，这从国内越来越多的等离子体实验室的成立，和一些小的实验室扩大成地方的重点实验室可以得到证明．事实上，大多数涉及到材料或物质的理工科实验室都会有一些与低温等离子体有关的装置或研究，无论该实验室是研究物理的、化学的、生物的或各类工程的，毕竟宇宙中99％以上的物质处于等离子体态．下面就我国低温等离子体的应用方面分类举一些例子．     

我国低温等离子体研究进展(Ⅰ)

低温等离子体物理与技术的研究在国内受到了越来越多的重视．在等离子体中发现的一些有趣的物理现象，如磁场重联、尘埃等离子体等，使人们对等离子体物理的研究掀起了新的热潮．在应用方面，几乎所有理工类实验室都有涉及低温等离子体技术的实验装置，这使得在我国低温等离子体应用方面的研究非常普及，包括微电子工业中的等离子体工艺，各种坚硬、耐腐蚀、耐摩擦材料的制备，纳米材料的制备，聚合物以及生物材料的表面改性，等等．随着低温等离子体技术的发展，低温等离子体的诊断技术也随之发展起来．文章简要地介绍了近几年来低温等离子体研究在我国的发展，介绍了一些有关低温等离子体的热点研究课题．     

A new stable organic metal: -(BETS)C(CN). The first -type radical cation salt with a planar-triangular discrete organic anion

The -(BETS)₂C(CN)₃ radical cation salt was prepared by electrocrystallization, and its crystal structure was determined by single crystal X-ray diffraction. The electronic structure of -(BETS)₂C(CN)₃ was studied by means of the extended Hückel tight binding method. The electrical conductivity of this salt as a function of temperature shows a metallic behaviour down to 1.3 K. Shubnikov-de Haas oscillations reveal both the classical and magnetic breakdown orbits on the Fermi surface typical of the -type organic conductors. In addition, a low frequency oscillation (250 T) which is not predicted by the band structure calculations has been found in the oscillation spectrum. Received: 11 March 1998 / Revised: 9 June 1998 / Accepted: 11 June 1998     

Low temperature specific heat of (TTF) (TCNQ)

Low temperature specific heat measurements on (TTF) (TCNQ) are reported. The Debye temperature is 89.5 K. No term linear in temperature is observed, and comparison with other data indicates the low temperature phase is that of a small band gap non-magnetic semiconductor.     

Effect of irradiation-induced disorder on the conductivity and critical temperature of the organic superconductor kappa-(BEDT-TTF)2Cu(SCN)2

We have introduced defects into clean samples of the organic superconductor kappa-(BEDT-TTF)(2)Cu(SCN)(2) in order to determine their effect on the temperature dependence of the interlayer conductivity and the critical temperature T(c). We find a violation of Matthiessen's rule that can be explained by a model of involving a defect-assisted interlayer channel which acts in parallel with the bandlike conductivity. We observe an unusual dependence of T(c) on residual resistivity, inconsistent with the generalized Abrikosov-Gor'kov theory for an order parameter with a single component, providing an important constraint on models of the superconductivity in this material.     

Low temperature specific heat and thermal conductivity of bulk metallic glass (Cu50Zr50)94Al6

The low temperature specific heat and thermal conductivity of (Cu₅₀Zr₅₀)₉₄Al₆ bulk metallic glass have been studied experimentally. A low temperature anomaly in the specific heat is observed in this alloy. It is also found that in addition to Debye oscillators, the localized vibration modes whose vibration density of state has a Gaussian distribution should be considered to explain the low temperature phonon specific heat anomaly. The phonon thermal conductivity dependence on temperature for the sample does not show apparent plateau characteristics as other glass materials do; however, the influence of the resonant scattering from the localized modes on the lattice thermal conductivity is prominent in the bulk metallic glass at low temperatures.     

The electric conductivity of a laminated metal system (alternating magnetic and nonmagnetic layers)

A set of kinetic equations for the distribution functions of carriers differing both by the energy spectrum and by the spin projection is used to investigate the conductivity of a multilayer sample (alternating layers of magnetic (m) and nonmagnetic (_n) metals). The boundary conditions on the interlayer surfaces are derived in an approximation in which the surface scattering is divided into “specular” and “diffuse” scattering and is characterized by scattering parameters (reflection and transmission) which are related to each other by relations dependent on spin projections and on the type of spectrum. The problem on the longitudinal (with respect to the layers) current is treated; situations are analyzed in which the variation in conductivity due to the change of mutual orientation of magnetization in successive m layers from antiparallel to parallel may be of the order of the values of the conductivity proper (the so-called giant magnetoresistance effect). This is possible only in the case of thin (compared with the free path) n layers (in m layers, the ratios of the characteristic dimensions may be arbitrary) and in the mandatory presence of specular surface scattering. Results are given for different possible ratios of Fermi momenta of electron groups and for different fractions of specular and diffuse scattering. The possibility of realizing the effects of both signs is demonstrated.