Resistive and Magnetic Susceptibility Transitions in Superconducting (TMTSF)2C104

We report measurements of the ac magnetic susceptibility and dc resistive superconducting transitions in the organic superconductor (TMTSF)₂C10₄. Inductive measurements show complete diamagnetic shielding below a broad transition and initial flux penetration at very low fields [H_c1(0) < 1 Oe]. The resistive transition is also broad, but occurs at a significantly higher temperature than the inductive transition, T_c = 1.0 K and 0.65 K respectively. Resistance measurements also show evidence of a phase transition in the vicinity of 24 K. Magnetic field induced transitions, measured both inductively and resistively, show marked anistropy both in magnitude and in breadth of the transition. Results suggest that (TMTSF)₂C10₄ is a quasi ID or 2D superconductor at high temperatures and high magnetic fields and an anisotropic bulk superconductor at low temperatures and fields. Associated thermoelectric power measurements suggest that spin density waves coexist with the superconducting state.     

Magnetism of the (TMTSF)2X Family of Organic Conductors

Abstract

The magnetic properties of the (TMTSF)₂X series of conductors have been studied using magnetic susceptibility, electron spin resonance and both CW and pulsed NMR measurements. The results imply the existence of a spin-density-wave ground state, at ambient pressure, in at least the PF₆ ^?, AsF₆ ^? and SbF₆ ^? salts. The amplitude of the SDW, the exchange interaction, the anisotropy energy and the single particle gap are extracted from the experimental data.     

New (TMTSF)2X Derivatives: A Change in the Selenium Network Dimensionality Derived From the Molecular and Crystal Structures of (TMTSF)2(Fso3) [T=298K, 123K] and (TMTSF)2(Bro4) T=298K

Abstract

We report the first crystallographic analysis, as a function of temperature, of a TMTSF derivative. Both (TMTSF)₂(FSO₃) and (TMTSF)₂(BrO₄) are isostructural (triclinic, with space group PI) with superconducting (TMTSF)₂(ClO₄). (TMTSF)₂(FSO₃) undergoes a metal-to-insulator transition at 86-90K as observed by microwave conductivity, D.C. conductivity, and magnetic susceptibility. The crystal structure contains 2-dimensional sheets of short Se-Se contacts in the molecular stacking direction and perpendicular to the stacking direction. The temperature dependent variations in these contact distances appear to be of special importance in determining the conduction properties of these materials, and are observed to change in a surprising manner when (TMTSF)₂(FSO₃) is cooled (298 → 123K). The homoatomic Se separations within each TMTSF molecule appear to increase slightly, but not significantly. At the same time the entire 2-dimensional sheet of intermolecular (intra- and interstack) Se-Se contacts between TMTSF molecules contract quite anisotropically, which results in an increase in “dimensionality” of the Se-Se network. Hence, an increase in electrical conduction, in the absence of insulating phenomena, over the temperature range 298 → 123K is not surprising. The intermolecular Se-Se contact distances in (TMTSF)₂(BrO₄) are significantly longer than in (TMTSF)₂(FSO₃) which suggests that the room temperature electrical conductivity of the (BrO₄)^? salt may be diminished compared to the (FSO₃)^? analogue.     

Pressure Dependence of the Metal-Insulator and Superconducting Phase Transitions in (TMTSF)2 Reo4

Measurements of the pressure and temperature dependence of the a-axis resistivity of (TMTSF) ₂ReO₄, are presented. The metal-insulator transition seen in this material at the remarkably high temperature of ～180 K at ambient pressure and which is associated with an ordering of the ReO₄, anions is suppressed under pressure. For pressures above ～9.5 kbar we observe a superconducting transition near 1.3 K. There is a narrow intermediate pressure regime about 2.5 kbar in width in which both superconductivity and effects of anion ordering are observed. In this regime (i) a superconducting transition is seen near 1.3 K even though ρ just above the transition can be up to 10-100 times greater than ρ(300 K), and (ii) there is an extraordinarily large hysteresis in p below ～ 100 K with the possibility of varying the resistance of the low temperature state by several orders of magnitude by appropriate temperature cycling. These results establish the first order character of the transition. We suggest that at high pressures the anions remain frozen in a metastable disordered state to low temperatures.     

CdGd2(WO4)4单晶的结构与磁性研究

采用提拉法生长尺寸为φ12mm×50mm的钨酸钆镉单晶,名义组分为CdGd2(WO4)4,研究表明该晶体具有白钨矿结构,属于四方晶系,晶格常数为a=b=0.5203nm,c=1.1359nm.样品的磁性测量质量比磁化强度与温度σ-T曲线,以及室温下磁化曲线σ-H,都表明晶体具有朗之万顺磁性,但又具有磁各向异性,室温下的x//=3.5018×10-3,χ⊥=3.4403×10-2;电子自旋共振实验也显示出各向异性的特征.a-b平面的两个最近邻Gd3+离子间电子自旋耦合作用较强,沿晶体c轴方向Gd3+离子间电子自旋耦合作用较弱,晶体的磁各向异性体现了晶体的结构特征.     

Magnetotransport and Nonlinear Effects In (TMTSF)2PF6

We have measured the magnetoresistance, Hall effect and the nonlinear conductivity characteristics in pure and radiation damaged (TMTSF>₂PF₆. We find that the material below the metal-insulator transition temperature is best described as quasi-two-dimensional with a very high mobility (10⁵ cm²/volt-sec). The nonlinear conductivity is suppressed by the application of a magnetic field and by radiation damage. The magnetoresistance, metal-insulator transition temperature and the superconducting transition temperature are also reduced by radiation damage of order 1000 ppm for the latter and 100 ppm for the former.     

Meissner Anisotropy in Deuterated (TMTSF)2Clo4

Both the Meissner-signal (flux expulsion on cooling through T_c) and the shielding signal (obtained by turning on a field in the superconducting state) have been observed in deuterated single crystals for fields oriented both along and normal to the chain axis. A possible isotope effect on T_c is discussed. We find complete diamagnetism in normal fields and incomplete (～ 40 %) diamagnetism in parallel fields. A dramatic drop of the Meissner-effect in parallel fields well below H_c1 is observed and not yet well explained.     

Crystal Growth in Magnetic Fields (II) Crystallization of Co(NH4)2(SO4)2 · 6 H2O from Aqueous Solutions in High Magnetic Fields

The displacement rate of (110) single crystal faces of Co(NH₄)₂(SO₄)₂ · 6 H₂O has been measured in dependence on the supersaturation of aqueous solutions in the zero field and at magnetic flux densities of 7 T. The results show that in a given external field measurable effects near the equilibrium don't occur (thermostatic field effect). On the contrary a marked influence at extreme relative supersaturations can be observed (kinetic field effect).     

Superconductivity in (TMTSF)2Clo4 at Zero Pressure

The organic charge transfer salt di-tetramethyl-tetraselenafulvalenium perchlorate becomes superconducting with transition temperatures from 0.9-1.4 kelvin. We compare our results to those of other groups and discuss possible reasons for the observed differences. Preliminary results are given for an alloy with the corresponding perrhenate.     

Thermal Expansion in TMTSF-Dmtcnq & (TMTSF)2PF6

We have measured the anisotropic thermal expansion of TMTSF-DMTCNQ and (TMTSF)₂PF₆ in the temperature range 10-300 K using an X-ray diffraction technique with a relative accuracy of 3 parts in 10⁴. Between 10 K and 300 K we find expansion in the a*, b* and c* directions to be respectively 2.5%, 0.2% and 1.7% for TMTSF-DMTCNQ and 3.7%, 1.6% and 1.3% for (TMTSF)₂PF₆     

DC conductivity studies in K3Na(SeO4)2 single crystals

DC electrical conductivity studies were carried out along the three crystallographic axes for Tripotassium sodium diselenate (K₃Na(SeO₄)₂ or KNSe). Earlier studies of phase transition in this crystal show successive phase transitions at 334 K, 346 K, 730 K, and 758 K. In this paper we report the dc electrical conductivity measurements in the temperature region 303 K – 430 K along a, b and c – axes. An anomaly in conductivity was obtained around 341 K and another one around 333 K. These can be attributed as due to phase transitions in this crystal. A strong anomaly also has been observed along the c‐axis and comparatively week one along a and b axes around 395 K for the first time. This can be due to newly observed phase transition in the crystal. DSC taken for the sample also shows endothermic peak supporting the occurrence of newly observed phase transition. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Crystal structure,thermal analysis and IR spectrometric investigation of bis (2‐amino‐6‐methyl) pyridinium sulfate

The synthesis method, crystal structure determination, phase transitions studied by thermal analysis and IR spectrometric investigation of 2C₆H₉N₂⁺.SO₄^2– are reported. The compound crystallizes in the monoclinic space group C2/c (no. 15) with a = 10.5068(4) Å, b = 10.2225(5) Å, c = 14.0422(7) Å, and β = 104.489(3)°. The atomic arrangement can be described by layers built by all the components of the structure and centered by planes z = 1/4 and 3/4. The organic molecules form channels parallel to the c direction with dimensions of 4.163(1)Å and 5.148(4)Å. Thermal analysis shows that the anhydrous compound presents an irreversible weak phase transition. The IR study, based on theoretical analyses and on the literature data allows the interpretation of the IR spectrum. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Structural Phase Transition of an Intercalation Compound Mn1/4NbS2

Abstract

A structural phase transition of an intercalation compound Mn_1/4NbS₂ has been investigated by X-ray diffraction at high temperatures. The lattice parameter c exhibited a discontinuous change at 640K. The superlattice reflections observed below 640K disappeared suddenly above 640K. The phase transition at 640K took an aspect of the first-order phase transition. The precise structure analyses were performed at various temperatures above and below the phase-transition temperature. It was revealed that Mn atoms were arranged in disorder in the high-temperature phase, while the Mn atoms were ordered forming the 2a ₀ × 2a ₀ × c ₀ superlattice in the low-temperature phase. The Nb and S atoms around the ordered Mn atoms slightly shifted from the high-symmetry position in the low-temperature phase. The order parameters were the degree of order of the Mn atoms and the degree of displacement of the Nb and S atoms.     

Transport Properties of (NH4)2 CuCl4.2H2O Low Dimensional Single Crystals

The X‐ray diffraction and Infrared (IR) spectral studies of (NH₄)₂ CuCl₄.2H₂O single crystals reveals that these crystals contains tetragonal crystal structure with the unit cell dimensions of a = 7.58Å, c = 7.95Å, z= 2, β =90° and two water molecules in the unit cell. The temperature dependence of thermally stimulated depolarization current (TSDC) and dc electrical conductivity (σ) studies of this two‐dimensional (NH₄)₂ CuCl₄.2H₂O single crystal have been carried out in 77K–300K temperature region. The TSDC thermograph shows only one sharp peak at 248K with a peak current of 130nA, which is attributed to the Maxwell‐Wagner peak. The activation energy (U), relaxation time (τ) are calculated as 0.78eV and 3.44×10^‐15 s respectively. Dc electrical conductivity studies of these crystals show a first order phase transition at about 248K.     

Possibility of Coexistence of Spin Density Wave and Superconductivity in Organic Conductor (TMTSF)2PF6

A theory for the coexistence problem of the spin density wave (SDW) and superconductivity (SC) in highly anisotropic materials is presented. On the basis of a simplified electronic band model a Hartree-Fock approximation is applied. It is concluded that SDW is precluded when SC developes at a higher temperature. When the SDW onset temperature is higher than that of SC, these long range orders generally coexist unless two orders interchange by a first order phase transition. Discussions on possible phase diagrams for (TMTSF)₂PF₆ under pressure are given.     

Crystal Growth in Magnetic Fields (I) Crystallization of Me (NH4)2(SO4)2 · 6 H2O (Me = Zn,Cu, Ni,Fe) from Aqueous Solutions in Moderate Magnetic Fields

There are several reports in the literature concerning the study of the influence of magnetic fields on nucleation and crystal growth. Because of the partially opposite findings, the question whether the application of magnetic fields especially for mass crystallization processes may be of advantage has not been cleared yet. Therefore, the displacement rates of [110] and [001] single crystal faces of four Tutton's salts Me(NH₄)₂(SO₄)₂ · 6 H₂O (Me = Zn, Cu, Ni, Fe) from supersaturated aqueous solutions in a transversal direct magnetic field (B ≦ 1.4 T) has been measured and compared. Within the detection limit no reproducible effect was observed with Me = Zn, Cu, and Fe. A small growth acceleration occured in case of Ni(NH₄)₂(SO₄)₂ · 6 H₂O. The results which will be completed by measurements at higher magnetic flux densities, support the viewpoint that the application of magnetic fields on crystallization of diamagnetic substances from solutions offers no real advantage.     

Synthesis and characterization of (NH4)2CuBr2Cl2·2H2O crystals

Single crystals of a new compound, (NH₄)₂CuBr₂Cl₂.2H₂O, were grown from saturated aqueous solution at room temperature by slow evaporation method. The grown crystals were characterized through elemental, powder XRD, thermal and DSC analyses and FTIR and far IR spectra. The elemental analysis and the decomposition pattern formulated using the TG‐DTG studies confirm the stoichiometry of the compound. The crystallinity of the compound is confirmed from the powder XRD pattern. A preliminary single crystal X‐ray diffraction structural analysis reveals that the title compound belongs to the orthorhombic system with a = 7.7466 Å, b = 7.783 Å and c = 8.1211 Å. The low temperature DSC shows thermal anomalies at –161.1, –156.5, –152.4, –145.2, –134, –18.5, and 1.4°C during the heating run and at –4.3, –54.8, –66.1, –90.6, –109.7 and –147.2 °C during the cooling run. The thermal hysterses indicate first order phase transitions in the title compound at these temperatures. The FTIR spectra were used to assign the characteristic vibrational frequencies due to NH₄⁺, CuX₄^2– ions and other chemical bonds. The effect of substitution of two bromine atoms on the phase transitions of a closely related crystal, diammonium tetrachloro cuprate dihydrate is also discussed. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Cao.4Bao.6Nb2O6晶体的生长和低温拉曼光谱研究

采用提拉发生长了Ca0.4Ba06Nb2O6晶体,利用偏振拉曼光谱研究了Ca0.4Ba06Nb2O6晶体在低温25 ～260 K 范围内的结构稳定性.实验发现,随温度降低,NbO6六面体的内振动(v5)模的非对称性增加,由Gauss+ Lor Amp 拟合得出的相对低频(v5)模波数和相对高频(v5)模峰宽在70 ～100 K时发生突变,表明该晶体在此温度范围内可能存在相变.同时分别讨论了在x(口zz)x和x(zy)x不同偏振配置下,外振动模的相对强度比随温度的变化趋势,同样发现其在该温度范围内发生明显异常.最后简单讨论了布居因子对拉曼散射强度和实验结果的影响.     

Growth and spectroscopic investigation of Yb/Nd co‐doped KLu(WO4)2 laser crystals

Single crystals of Yb, Nd: KLu(WO₄)₂ (Yb, Nd: KLW) of dimensions up to 40mm× 40mm×5mm have been grown by top‐seeded solution growth (TSSG) method. X‐ray powder diffraction pattern was measured and compared with that of Nd: KLuW and Yb: KLuW. Absorption and fluorescence spectra were measured at room temperature. The Judd‐Ofelt theory was applied to analyze the spectrum of Nd, Yb: KLuW crystal. The intensity parameters Ω_t (t=2, 4 and 6) were calculated as Ω₂=20.68×10^‐20cm², Ω₄=11.04×10^‐20cm², Ω₆=6.74×10^‐20cm² respectively, with a root mean square deviation of 0.58×10^‐20 cm². (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

石英谐振器在强磁场中的磁-敏特性

研究了石英晶体谐振器在强磁场影响下频率变化的原因.结果表明石英晶体谐振器的磁性支架在受到磁场的影响下产生相应的应力,该应力沿石英晶片的边缘作用于石英晶体引起谐振器频率的变化.该实验结果对如何减小磁场对石英晶体谐振器的影响有一定的参考价值.