Magnetotransport In (TMTSF)2PF6 and (TMTSF)2C104 Under Pressure

We have shown from the magnetotransport that the Fermi surface of (TMTSF)₂PF₆ is effectively closed two-dimensionally and is compensated in the metallic state at low temperature. These results are inconsistent with a broad regime of one-dimensional superconducting fluctuations.

Independent electron theory is seen to be inadequate to explain either the Fermi surface or the threshold field for oscillations, thereby implying that correlation effects are still important in the metallic state. Finally, an anomaly has been observed in the magnetotransport of (TMTSF)₂C10₄ which is analogous to the threshold field in (TMTSF)₂PF₆.     

Superconducting Transition of (TMTSF)2CLO4 in Magnetic Fields

Abstract

Superconducting transition of (TMTSF)₂ClO₄ was studied by conductivity measurements along the most conductive a-axis in magnetic fields applied along three different principal crystallographic axes. The GL coherence lengths at OK along the a-, b-, and c*-axes, ?_a(0)≈ 600A, ?(0) ≈ 540A, and ?_c*(0) ≈ 60A were obtained from the measurements of the temperature dependence of the upper critical field H_C2 near the transition temperature. The anisotropy is discussed in terms of the dimensionality arising from the crystal structure and of the conductivity anisotropy in the normal state.     

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.     

One Dimensional Organic Superconductivity in (TMTSF)2PF6 and (TMTSF)2C104 Detected Via Tunnel Spectroscopy

Electron tunnelling data in (TMTSF)₂ X-GaSb Schottky barriers support the idea that strong superconducting fluctuations exist in quasi-one-dimensional (TMTSF)₂X up to temperatures which are about one order or magnitude higher than the 3-D ordering critical temperature. Other tunnelling data tend to indicate that stabilization of superconductivity is possible at high temperature via chain cross-linking.     

(TMTSF)2(2,5-Tcnqbr2): Structure and Physical Properties

The 2:1 charge-transfer salt (TMTSF)₂(2,5-TCNQBr2) has been prepared and its physical properties investigated. Its crystal structure consists of segregated stacks of TMTSF donors (ring-over-bond overlap pattern; mean interplanar spacing of 3.6A) and chains of edge-on and disordered 2,5-TCNQBr2 acceptors. Infrared data are suggestive of unit charge on the 2,5-TCNQBr2 molecule and, therefore, half charge on the TMTSF donor. Resistivity data are successfully interpreted on the basis of a percolation construction. Magnetic data are also presented.     

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.     

Some Properties of the (TMTSF)2X Superconductors

Measurements of the anisotropy of the critical field in (TMTSF)₂X superconductors are presented. The results show that the a axis H_c2 is Pauli limited based on a T_c near 1K. The large energy gap obtained from tunneling data is therefore thermodynamically untenable in terms of the present critical field studies and the superconductivity is singlet rather than triplet. Moreover, we find that low levels of irradiation induced defects eliminate the superconductivity in the ClO₄ and PF₆ salts and leave the thermopower of the ClO₄ salt below 100K unchanged. These results are incompatible with the proposal for 1D superconducting fluctuations below 40K. A discussion of the arguments for and against the occurance of superconducting fluctuations in the (TMTSF)₂X salts is given with emphasis on the role of interchain interactions and other possible interpretations of the tunneling data.     

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.     

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.     

稀磁半导体Hg0.89Mn0.11Te中的磁化率和自旋-玻璃转变

利用MPMS-XL型超导量子干涉仪 (superconducting quantum interference device, SQUID)对本实验室生长的Hg0.89Mn0.11Te 晶体的磁化强度和磁化率进行了研究.结果表明,Hg0.89Mn0.11Te在2 K恒温下,-0.5～0.5 kOe磁场范围内出现不可逆磁化. 在温度低于2.8 K时该晶体出现自旋-玻璃转变,以磁化率在冻结温度处出现拐点和低于冻结温度时磁化过程不可逆为主要鉴别特征.这是目前在该材料中发现的最小磁性离子浓度下限(渗透极限).     

Superconductivity in the Organic Charge Transfer Salts: (TMTSF)2X and (TMTTF)2X

Abstract

We report pressure dependent studies of the a-axis resistivity as a function of temperature for several members of the isostructural families of organic charge transfer salts, (TMTSF)₂X and (TMTTF) ₂X. For a typical (TMTSF)₂X material the low temperature metal-insulator transition seen at 1 bar is suppressed above some critical pressure, P_c, where a superconducting transition is observed near 1 K. We find a correlation between P_c and the ambient pressure c lattice parameter which reflects the anion size. The (TMTTF) _cX salts exhibit very different ambient pressure behaviour but we find that with the application of sufficiently high pressures (～30 kbar) their behaviour resembles that seen in the (TMTSF)₂X family but at lower pressures. In particular we find evidence of a possible superconducting transition near 4 K in (TMTTF)₂Br at 25 kbar. At this pressure the conductivity near 4 K is extremely high with a value approaching 10⁶ (Ωcm)^?1 and the resistivity ratio is about 400.     

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     

X-Ray Diffuse Scattering Study of Some (TMTSF)2X and (TMTTF)2X Salts

X-ray diffuse scattering experiments on the superconducting salts (TMTSF)₂X with X = PF₆, AsF₆ and ClO₄ reveal no charge density wave instability, in contrast with the poorly conducting (TMTTF)₂PF₆ salt. New structural phase transitions coinciding with conductivity anomalies are found in (TMTSF)₂NO₃, (TMTSF)₂ReO₄, and (TMTTF)₂ClO₄. It is suggested that they correspond to order-disorder transitions involving the non-centrosymmetric counter ions. Relationships between these structural instabilities and superconductivity are also discussed.     

新型配位聚合物[Ni(C6H4NO2)2(H2O)4]n的水热合成和晶体结构

以NiCl2·6H2O和4-氰基吡啶为原料,在中温水热反应条件下,合成了一种新型配位聚合物[Ni(C6H4NO2)2(H2O)4]n单晶体,并对其进行了元素分析、红外光谱表征和X射线单晶衍射测定.该配位聚合物属三斜晶系,P-1空间群,a=0.6298(4)nm,b=0.6907(4)nm,c=0.9243(5)nm,α=96.437(8)°,β=105.184(9)°,γ=113.314(9)°,V=0.3456(3)nm3,Z=2,dc=1.802 g/cm3,μ=1.452mm-1,F(000)=194,R1=0.0309,R2=0.0753.该晶体通过配位键的连接和分子间氢键相互作用形成三维的网状结构.     

Magnetic Susceptibility and Peculiarity of Chemical Bond in the Cd-P(As) System Compounds

Specific features of interatomic interaction of semiconductor compounds of the Cd—P(As) system derived from the analysis of experimental data and theoretical calculations of magnetic susceptibility are discussed. It is shown that the A²B⁵₂ compounds are of predominantly covalent and A²₃B⁵₂ of dominating ionic bond character.     

Magnetic Susceptibility and Peculiarities of Chemical Bond in the AgGa(S1‐xSex)2 Solid Solutions

The measurements of magnetic susceptibility of the AgGa(S_1‐xSe_x)₂solid solutions with the chalcopyrite structure have been performed and the lattice susceptibility separated into the Langevin diamagnetic and the Van Vleck paramagnetic terms. The obtained data were used to evaluate the character of chemical bond in these alloys. The correlations between the parameters of chemical bond and other physical properties are discussed. It is shown that the investigated solid solutions have preferably ionic character of chemical bond. The Ga‐S(Se) (B‐C) bonds are found to be stronger than the Ag‐S(Se) (A‐C) bonds.     

Low-Temperature Electron Spin Resonance in (TMTSF)2PF6 in the High Pressure Metallic Phase

Abstract

The electron spin resonance of (TMTSF)₂PF₆ has been observed at low fields (H_o < 110 Oe) in the high pressure, metallic phase (p > 6.5 kbar) in the temperature range 1-4^?K. The anisotropy in the g value is similar to that observed at ambient pressure above the metal-insulator transition. The linewidth is very narrow and the spin susceptibility strongly decreases as the superconducting transition is approached from above. We interpret this as evidence for singlet-paired superconductivity. Superconductivity is observed at 1.1 K and the critical field has angular dependence in the be plane. These observations lead us to conclude that (TMTSF)₂PF₆ is a singlet paired superconductor.     

新型三维配位聚合物[La(C5H4NCOO)3(H2O)2]n的水热合成和晶体结构

在水热反应条件下,设计合成了一种新型三维配位聚合物[La(C5H4NCOO)3(H2O)2]n的单晶体,对其进行了元素分析、热重分析、红外光谱表征、X射线单晶衍射测定.该配位聚合物属单斜晶系,P2(1)/c空间群,晶胞参数为a=0.9747(4)nm,b=1.9904(7)nm,c=1.1613(4)nm,α=90°,β=111.761(6)°,γ=90°,V=2.0925(13)nm3,Z=4,dc=1.718g/cm3,μ=2.090mm-1,F(000)＝1064,R1=0.0374, wR2=0.0683.X射线衍射结果显示La3+离子之间以4-氰基吡啶的羧基为桥形成一维链状结构,通过4-氰基吡啶的吡啶基与配位的水分子之间的氢键形成三维的网络结构.热重分析表明该配位聚合物在230℃下稳定.     

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.