Preparation and Solid-state Structural, Electronic, and Magnetic Properties of the 5-Cyano-1,3-benzene-Bridged Bis(1,2,3,5-dithiadiazolyl) and Bis(1,2,3,5-diselenadiazolyl) [5-CN-1,3-C6H3(CN2E2)2] (E = S, Se)

The preparation and solid-state characterization of the bifunctional radicals [4,4’-(5-cyanobenzene)-1,3-bis(1,2,3,5-dithiadiazolyl)] and [4,4’-(5-cyanobenzene)-1,3-bis( 1,2,3,5-diselenadiazolyl)] [5-CN-1,3-C6H3(CN2E2)2] (E = S, Se) are described. The crystals of the two title compounds are isomorphous and belong to the monoclinic space group P21/c, with (for E = S) a = 7.00(2), b = 30.050(6), c = 10.713(8) Å, β = 104.80(10)°, V = 2179(6) Å3, Z = 8 and (for E = Se) a = 7.124(4), b = 30.50(2), c = 10.874(2) Å, β = 105.46(3)°, V = 2277(2) Å3, Z = 8. The crystal structures consist of stacks of diradicals running parallel to x; radical dimerization up and down the stack generates a zigzag arrangement, as seen in the related 1,3-phenylene structures. Along the stacking axis the mean intradimer E-E contacts are 3.12 (E = S) and 3.23 Å (E = Se), while the mean interdimer E- - -E distances are 3.89 (E = S) and 3.91 Å (E = Se). Magnetic and conductivity data are presented and discussed in light of extended Hückel band structure calculations

Molecular Conductors from Neutral-Radical Charge-Transfer Salts:Preparation and Characterization of an Iodine-Doped Hexagonal Phase of 1,2,3,5-Dithiadiazolyl ([HCN2S2]∙)

Sublimation of 1,2,3,5-dithiadiazolyl in vacuo affords a triclinic phase of the dimer [HCN2S2]2. The crystals belong to the space group P¯1, a = 6.816(3), b = 13.940(2), c = 14.403(3) Å, α = 116.830(14), β = 98.64(3), γ = 99.18(3)°, FW = 212.4 (for [HCN2S2]2·[N2]0.08) Z = 6. The crystal structure consists of stacked dimers, with three dimers per asymmetric unit. Pairs of asymmetric units, related by an inversion center, generate a pinwheel motif consisting of six dimers. The columnar structure associated with these pinwheels forms close-packed sets of “molecular tubes”. Cosublimation of the radical in the presence of iodine in the mole ratio (HCN2S2:I = 5:1) yields an iodine-doped hexagonal phase of composition [HCN2S2]6[I]1.1. Crystals of this material belong to the space group P61, a = b = 14.132(16), c = 3.352(5) Å, FW = 128.20, Z = 6. The crystal structure consists of sixfold pinwheels in which the now evenly spaced HCN2S2 rings form a spiral about the 61 axis. The iodine atoms lie within the columnar cavity of the pinwheels in a disordered array wrapped tightly about the sixfold screw axis. The single-crystal conductivity of the doped material is 15 S cm-1 at room temperature. Raman spectroscopic and magnetic susceptibility measurements on the doped material are reported

THE EFFECT OF Fe SUBSTITUTION ON THE CHARGE DENSITY WAVE IN VSe2

Les effets de la substitution de fer dans le composé V1-xFexSe2 sont comparés à ceux mesurés dans le composé 1T-Ta1-xFexSe2. Dans le composé pur VSe2 une onde de densité de charge (CDW) apparaît au-dessous de T0 = 112 K, à la différence du composé 1T-TaSe2 ou T0 ≈ 600 K. Plusieurs phénomènes associés à la présence du fer substitutionnel sont observés dans le composé 1T-Ta1-xFexSe2 : (1) la CDW est stable à 300 K pour x ≤ 0,15, (2) le niveau commensurable de la CDW est supprimé à x ≈ 0,06, (3) la conductivité décroît rapidement vers zéro à basses températures quand x ≥ xc ≈ 0,15 (localisation d'Anderson), (4) le fer possède une transition d'un état bas spin à un état de haut spin pour des températures croissantes. On trouve que pour le fer substitutionnel dans le VSe2 : (1) la CDW est éliminée pour x ≥ 0,035, (2) le fer est non magnétique (bas spin), (3) jusqu'à la plus haute concentration étudiée (0 ≤ x ≤ 0,2), on ne peut mettre en évidence la localisation d'Anderson. Les résultats sont cohérents avec une CDW beaucoup moins stable dans VSe2 que dans le 1T-TaSe2.We compare the effect of Fe substitution in V1-xFexSe2 with the previously measured properties of 1T-Ta1-xFexSe2 In pure VSe2 a Charge Density Wave (CDW) appears below T0 = 112 K, while the CDW in 1T-TaSe2 occurs below T0 ≈ 600 K. Several phenomena associated with Fe substitution are observed in 1T -Ta1-xFexSe2 : (1) the CDW is stable at 300 K for x up to at least 0.15, (2) the commensurate CDW state is suppressed at x ≈ 0.06, (3) the conductivity decreases rapidly to zero at low temperatures when x ≥ xc ≈ 0.15 (Anderson localization), (4) the Fe shows a low spin to high spin transition as the temperature is increased. We find for 1T-V1-xFexSe2 x ≤ ½ ; ( 1) the CDW is eliminated for x ≥ 0.035, (2) the Fe is nonmagnetic (low spin), (3) to the concentrations studied (x = 0.2), no evidence for Anderson localization. The results are consistent with the CDW being much less stable in VSe2 than in 1T-TaSe

Role of anisotropy in the dissipative behavior of high-temperature superconductors

In a comparative study of the dissipative behavior of various classes of high-Tc superconductors in large magnetic fields, we demonstrate that materials with a large electronic anisotropy, like the Bi and Tl compounds, have intrinsically smaller pinning energies than more isotropic materials. Consequently, the highly anisotropic materials exhibit thermally assisted dissipation down to temperatures far below Tc. In spite of small pinning energies, large critical current densities Jc can be observed in the critical-state regime at T < Tcr. In this regime irradiation damage produces large changes in Jc, but only modest changes in the magnitude of the pinning energies as evidenced in the thermally activated regime. We introduce a general criterion, which denotes the temperature and magnetic-field regime in which superconductors can be applied.

Electronic excitations of the YBa\u3csub\u3e2\u3c/sub\u3eCu\u3csub\u3e3\u3c/sub\u3eO\u3csub\u3e7-x\u3c/sub\u3e superconductor: A study by transmission electron-energy-loss spectroscopy with an electron microprobe

Interband and core-level transitions and plasmon resonance in the high-temperature oxide superconductor YBa2Cu3O7-x were studied by transmission electron-energy-loss spectroscopy using an electron microprobe ∼5-10 Å in size. Both single-crystal and polycrystalline ceramic samples have been examined. No significant change in electronic structure was observed at the grain boundaries. A 13.5-eV interband-transition peak was found to be sensitive to damages created by ion-beam bombardment

Transport Entropy of Vortex Motion in YBa2Cu3O7

We report measurements of entropy transport due to vortex motion in single-crystal YBa2Cu3O7. From these magnetothermal measurements we are able to determine thermodynamic properties, such as the transport line energy and superconducting phase boundary Tc(H), as well as transport parameters associated with flux motion. Below Tc, the data are compared with the microscopic theory of Caroli and Maki. Interestingly, we also find large entropy transport in the fluctuation regime above Tc, which grows with applied field and for which no theory exists.

Thermally Activated Dissipation in Bi2.2Sr2Ca0.8Cu2O8+δ

A new dissipation behavior is reported in superconducting Bi2.2Sr2Ca0.8Cu2O8+δ for all temperatures below Tc and all magnetic fields exceeding Hc1. The current-independent electrical resistivity is thermally activated and can be described by an Arrhenius law with a single prefactor and a magnetic-field- and orientation-dependent activation energy U0(H,φ). This behavior is markedly different from past observations and will be discussed in terms of flux creep and flux flow. This thermally activated behavior implies a finite resistance at all temperatures and all fields exceeding Hc1 determined by the activation energy as the only parameter.

Structure and Properties of Reduced Early Transition Metal Oxide Single Crystals Grown from Borate Fluxes

Reduced ternary oxides of the early transition metals exhibit a variety of interesting physical and structural properties. Superconductivity (LiTi2O4 [1], tetragonal and hexagonal Mo and W bronzes [2]) and charge density wave behavior (Mo ’blue’ bronzes [3]) have been observed in these systems. Structures range from simple perovskites like SrxNbO3 [4] to complicated structures involving metal-metal bound clusters [5]. Single crystals are highly desirable for a better understanding of structure-property relations. In systems having low-melting high oxidation state ternary precursors (like A2MO4 and A2M2O7; A = alkali metal, M = Mo, W) molten salt electrolysis provides a convenient route to single crystals. For other systems (particularly niobates) other methods have to be employed. Here we describe the use of borate fluxes to produce single crystals of various reduced ternary niobates and titanates in inert atmosphere. Borate fluxes have been regularly employed in the crystal growth of ferroelectrics and garnets [6], but rarely used for reduced compounds. An interesting aspect of the borates AnOm ·xB2O3 is that they can act both as a source and as a buffer for AmOn , allowing the product formation to be influenced by the flux composition when the A-cation is also part of the reduced ternary system. An advantage of the Ba, Sr, and La-borates is their low volatility, which allows their use under high-vacuum conditions