Hall effect and magnetoresistivity in the ternary molybdenum sulfides

We have measured the Hall coefficient and magnetoresistance of sputtered films of Cu_XMo₆S₈ and PbMo₆S₈, as well as the magnetoresistance in sintered samples of the same materials. Assuming a single band model, net carrier densities and mean mobilities are determined.     

反应扩散法制备的PbMo6S8超导带的超导特性

本文报道反应扩散法制备Chevrel相PbMo₆S₈超导带材的过程以及带材的超导特性。Mo带与H₂S气体反应生成单相的Mo-S化合物(MoS₂或Mo₂S₃,根据反应温度而定),再与Pb的蒸汽反应生成单相的PbMo₆S₈薄层。PbMo₆S₈超导层的临界温度T_c=13.0 关键词：     

Superconducting tunneling on Chevrel phase thin films

Electron tunneling spectroscopy has been applied for the investigation of Cu_1.8Mo₆S₈, PbMo_6.35S₈ and LaMo₆S₈ d.c. getter-sputtered thin films. Cryogenically deposited Al₂O₃ as the artificial barrier with Al counter-electrode formed the tunnel junctions. The energy gap Δ₀ and the ratio 2Δ₀/k_BT_c were determined from the differential conductance dJ/dV vs. voltage V. Preliminary measurements showed phonon structure in LaMo₆S₈ films.     

Electronic properties of Chevrel phases by positron angular correlation

A comparative study of the Chevrel phases Mo₆S₈, Mo₆Se₈, PbMo₆S₈ and Cu_1.8Mo₆S₈ is made by the positron angular correlation technique at room temperature. Electronic properties like the number of conduction electrons per cluster, the density of states at the Fermi level, the electron effective masses and the Fermi velocities are obtained within the framework of the free electron model. The results are discussed in terms of available theoretical and other experimental data.     

Upper critical field of copper molybdenum sulfide

The upper critical field B_c2(T) of sintered and sputtered copper molybdenum sulfide Cu_xMo₆S₈ was measured. It was found to be higher than the highest prediction for a type II superconductor in the dirty limit with no paramagnetic limiting and with weak isotropic electron phonon coupling. The enhancement of B_c2(T) is probably due either to anisotrophy or to purity or to both. Other ternary molybdenum sulfides appear to show similar anomalies.     

Investigation of electronic structure of ternary molybdenum sulphides by means of x-ray emission and photoelectron spectroscopy

The electronic structure of superconducting Chevrel phases (SnMo₆S₈, PbMo₆S₈, Cu_2.8Mo₆S₈) and nonsuperconducting spinel (GaMo₄S₈) was investigated by X-ray emission (XES) and X-ray photoelectron (XPS) spectroscopy. The comparison of the obtained results with band structure and cluster calculations of given compounds was performed.     

Chevrel相MxMo6S8化合物电荷转移和Mo原子上4d轨道能级的分裂

本文用配位场方法导出了Chevrel相M_xMo₆S₈化合物中M和Mo₆S₈原子簇间电荷转移和Mo原子上4d轨道能级分裂的解析关系。结合PbMo₆S₈的反射光谱,求得Mo₆S₈原子簇至Pb的电子转移数为1.18,和以前一般文献上认为的两个电子从Pb转移至Mo₆S₈不同。Mo₆上的4d电子数为18.8。 关键词：     

Preparation of ternary lead molybdenum sulfide with high superconducting transition temperature

The high field superconducting compound PbMo₆S₈ has been prepared by an isothermal chemical vapour transport reaction at 1273 K. The reaction product was single phase PbMo₆S₈ with lattice parameters of $\text{a}{}_0\text{= 6.545}\text{A}\text{?}$, and α = 89° 12′ and has been found to transform to the superconducting state at a midpoint T_c of 14.7 K.     

Upper critical fields of superconducting superconducting rare-earth molybdenum selenides

Upper critical fields, H_c2(T), for superconducting LaMo₆Se₈, La_0.8Eu_0.2Mo₆Se₈, La_0.8Ce_0.2Mo₆Se₈ and PrMo₆Se₈ were measured. For the best material, LaMo₆Se₈, T_c = 11.3 K, (dH_c²/dT)_T=Tc=70 kG/K and and H_c² (4.2 K) = 370 kG. These selenide compounds show promise as very high field superconductors.     

Homogeneity and critical current density of Sn-doped PbMo6S8 superconductors

With the overall critical current density,J _c , remaining low for PbMo₆S₈, the addition of 0.3 at. % Sn noticcably improved the performance. The enhancement occurs despite the presence of excess Pb and a slightly suppressedT _c . The increase inJ _c can be attributed to a better homogeneity with respect to the chemical composition of the samples, which may be as a consequence of Sn substitution in the lattice. Detailed calorimetric investigation of undoped PbMo₆S₈ showed aT _c distribution down to 9 K evidenced by a smearing ofT _c . This may be due to a progressive degradation of the sample quality leading to a plausible off-stoichiometry. High-resolution Auger electron spectroscopic experiments suggest a variation in composition indicating excellent correspondence with the specific heat data. However, no such deviation was detected for the Sn-doped samples and therefore the observed raise inJ _c . The results are discussed in conjunction with oxygen, lead and tin incorporated PbMo₆S₈.     

Lack of oxygen substitution in the chevrel compound Mo6S8

We present results from x-ray powder diffraction and lithium intercalation cells on samples of nominal composition “Cu₂Mo₆S₆O₂” and “Mo₆S₆O₂”. Our results show that it is not possible to prepare these materials with any significant fraction of the sulfur substituted by oxygen as claimed by Umarji et al.¹ Instead, the high temperature (1250°C) products from starting materials of CuO, Cu, S, MoO₂ and/or Mo are MoO₂, Mo and Cu_yMo₆S₈ with y>2.     

Metal atom incorporation studies on AxMo6S8 Chevrel phases

Group IIIa metals and Nb, Hg and Pb containing A_xMo₆S₈ Chevrel phases have been synthesized by low temperature reaction (420–430°C) frorn A metal and Mo₆S₈. Some of these phases are hitherto unknown. Electrolysis method of preparation of A_xMo₆S₈ reported by Schöllhorn does not yield the desired phases when A = Pb or Ag. Stoichiometry, thermal stability, electrical and superconducting properties of the compounds have been examined. In the Pb_xMo₆S₈ system, a series of phases with varying x have been prepared and studied. Group IIIa metal and Nb-compounds are found to be semiconductors with low energies of activation exhibiting p-type behavior in the range 77–300 K. Hg-phase behaves as a metal or a degenerate semiconductor and does not become superconducting above 4.2 K. X-Ray, resistivity and T_c studies show that for x ? 0.8 in Pb_xMo₆S₈, the compounds behave more like Mo₆S₈ and only for x ? 1.0 or (PbBi)_1.0, the inherent behavior of the ternary Chevrel phase is exhibited. Seebeck coefficients are small and positive for Mo₆S₈, Cu and Pb-containing phases in the range 77–300 K. The results are discussed in the light of available band structure of these materials.     

Correlations between structure and the superconducting transition temperature in Chevrel phase molybdenum chalcogenides

The superconducting transition temperatures for Chevrel phase materials, M_xMo₆X₈(M=Pb, Sn, rare eart …, X = S, Se, Te) show wide variability even for a single phase, e.g. PbMo₆S₈ with reported T_c's of 9.8–14.7 K. The T_c is shown to be a linear function of the hexagonal crystallographic $\text{c}\text{a}$ ratio when materials are grouped according to the formal charge on the ternary element M. This correlation is discussed in terms of the electron-phonon coupling and electronic density of states.     

Low temperature structural distortion in the high Tc Chevrel-phase superconductors PbMo6S8 and SnMo6S8

High-resolution powder neutron diffraction data reveal that, at temperatures below 100 K, oxygen-free, high-T_c samples of PbMo₆S₈ and SnMo₆S₈ exhibit a small structural distortion from the R$\text{3}$ rhombohedral space group. The data cannot be refined in a simple P$\text{1}$ triclinic space group as previously reported for EuMo₆S₈ and BaMo₆S₈. Thus, a supercell ordering is suggested.     

Anisotropy of Hc2 in the Chevrel phases

The anisotropy of the upper critical field of PbMo₆S₈, PbMo₆Se₈ and SnMo₆Se₈ has been measured. All three compounds show an anisotropy of about 20 %, with the maximum H_c2 value when H is perpendicular to the ternary axis. The effective mass model with the mass ratio ε₂≈ 0.69 fits well our results.     

NMR study of Chevrel phase superconductor

The nuclear spin lattice relaxation time, T₁, the Knight shift and the spin echo decay time, T₂, of ²⁰⁷Pb and ⁷⁷Se were measured in Pb_1.125Mo₆Se_7.5 and Pb_1.125Mo₆S_7.5 at He temperatures. The temperature dependence of T₁ shows the existence of the energy gap of 3.5kT_c at both sites of Pb and Se. The Knight shift dose not change with temperature even below T_c. A remarkable decrease of T₂ below T_c is observed, which is opposite to the case in usual type II superconductor.     

Magnetic ordering in the superconducting state of rare earth molybdenum sulphides, (RE)1.2Mo6S8 (RE = Tb, DyandEr)

The resistance and ac susceptibility measurements on Tb_1.2Mo₆S₈, Dy_1.2Mo₆S₈ and Er_1.2Mo₆S₈ revealed a magnetic ordering in their superconducting state at 0.9, 0.4 and 0.15 K, respectively. Some of the important features of the ordering in each compound are discussed with reference to their upper critical field curves.     

EPR sites of molybdenum ions in single crystals of Cs2HfCl6 and Cs2ZrCl6

Five different spectra of Molybdenum ions in single crystals of Cs₂ZrCl₆ and Cs₂HfCl₆ are reported. Two of them have a Γ₈ ground state and were assigned to the Mo³⁺ ion. The third is fitted to an axial spin Hamiltonian with S = 1. The two others have a Kramers' doublet ground state of S = 1/2 and due to Mo⁵⁺.     

Surface and bulk properties of hot-pressed PbMo6S8 superconductor studied by Auger electron spectroscopy and calorimetry

A direct observation of the grain-boundary phases for several well-characterized hot-pressed PbMo₆S₈ samples were made by Auger electron spectroscopy. The surface elemental concentrations are completely different when compared to its bulk composition. The thickness of this altered composition is in the range 100–200 Å gradually increasing with increase in hotpressing temperature. Also, evidence of segregation of impurities, such as carbon and SiO_x, to the grain-boundaries were noticed for some of the samples. Calorimetric experiments show a continuous broadening and a reduction in amplitude of the specific heat anomaly atT _c . This in terms of superconducting volume fraction indicates aT _c distribution in the range 9–15 K. Such an observation can be related to the local inhomogeneities with respect to the ternary composition, i.e., a deviation from ideal stoichiometry, PbMo₆S₈. The results are discussed in conjunction with X-ray diffraction, scanning electron microscope, and energy dispersive X-ray analysis. By considering the grain-boundary phases and the calorimetric observation of inhomogeneities, a plausible explanation is given for the low critical current densities in these materials.     

Fast neutron irradiation of YBa2Cu3O7

The influence of fast neutron irradiation on the superconducting transition temperature,T _c , the transition width, upper critical field,B _c2 , and critical current density,j _c , has been investigated in YBa₂Cu₃O₇ up to a fluence of 10¹⁹ cm^–2 (E>1 MeV). TheT _c degradation with fluence is slightly less than in PbMo₆S₈, but larger than in A 15 compounds. The irradiation induced increase of the normal state resistivity is accompanied by a remarkable decrease of both the intergrainj _c and the superconducting volume fraction.