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α-hexathienyl (α–6T) is a highly promising material for application in thin film transistor devices. Recently, record high mobilities, together with record high current on/off ratios, have been reported.1 Thus far, structural information on this exciting material is sketchy. The crystal structures of several such hexamers have been investigated, but only with powder samples, since the crystal growth has proven exceedingly difficult.2-5 Powder Rietveld refinements on these materials are severely hampered by the large number of overlapping reflections, preferred orientation, ambiguities in symmetry, etc. Here, we present a crystal structure of the high-temperature polymorph of α–6T (α–6T/HT), as determined from a single-crystal structure analysis. In this polymorph, the hexamer crystallizes in the smallest unit cell so far reported for this material, but the molecule is flat. Extended Hückel theory (EHT) band structure calculations show that α–6T/HT is an indirect gap semiconductor, with the conduction band minimum at Y and the valence band maximum at Γ. The conduction and valence bands both show a remarkable degree of dispersion along X and Y for a molecular crystal. The electronic band structure of this material is strikingly similar to that of the two-dimensional organic superconductors based on bis(ethylenedithio)tetrathiafulvalene (ET), such as κ−(ET)2 Cu(NCS)2.

We have investigated the synthesis, structure, and electronic properties of ${(\mathrm{N}{\mathrm{H}}_{3})}_{x}{\mathrm{K}}_{3}{\mathrm{C}}_{60}$ by exposing preformed ${\mathrm{K}}_{3}$${\mathrm{C}}_{60}$ to N${\mathrm{H}}_{3}$ vapor. Measurements were made on bulk powders and films with a variety of techniques including in-situ x-ray diffraction and monitoring of N${\mathrm{H}}_{3}$ pressure, Raman scattering and ac susceptibility. The reaction of N${\mathrm{H}}_{3}$ with ${\mathrm{K}}_{3}$${\mathrm{C}}_{60}$ is completely reversible and leaves the ${({\mathrm{C}}_{60})}^{3\ensuremath{-}}$ charge state intact, while allowing the distance between the ${\mathrm{C}}_{60}$ molecules to vary. We observe two different crystallographic structures at $x=1$ and $x=8\ensuremath{-}10$. The $x=1$ structure is face-centered orthorhombic with a single N${\mathrm{H}}_{3}$ on the octahedral interstitial site. It is single phase at 100\ifmmode^\circ\else\textdegree\fi{}C and N${\mathrm{H}}_{3}$ pressures \ensuremath{\ge}400 torr and remains single phase at 0 torr if the temperature is reduced to 25\ifmmode^\circ\else\textdegree\fi{}C. Although (N${\mathrm{H}}_{3}$)${\mathrm{K}}_{3}$${\mathrm{C}}_{60}$ exhibits no superconductivity at ambient pressure, superconductivity can be induced by hydrostatic pressure with an onset at 28 K. The details of the $x=8\ensuremath{-}10$ structure are uncertain, however it appears that this phase is body-centered tetragonal with N${\mathrm{H}}_{3}$ on both the octahedral and tetrahedral sites. It occurs at room temperature and N${\mathrm{H}}_{3}$ pressures \ensuremath{\ge}500 torr, but is not stable at lower N${\mathrm{H}}_{3}$ pressures. Resistivity measurements of N${\mathrm{H}}_{3}$-doped ${\mathrm{K}}_{3}$${\mathrm{C}}_{60}$ films show that the formation of the new structure results in a rapid increase of the resistivity to values exceeding 1 \ensuremath{\Omega} cm.

We report superconductivity in Cs3C60 at 40K using ac susceptibility measurements under hydrostatic conditions up to 15 kbar. Cs3C60 was prepared by reaction of C60 with Cs in liquid ammonia, followed by heating at 150°C. This route circumvents formation of the energetically more stable Cs1C60 and Cs4C60 phases. We have studied the synthesis and phase formation by measuring the symmetric Ag pentagonal pinch mode of C60 using Raman spectroscopy. Whereas Raman spectroscopy indicates homogeneous charge transfer of three electrons, x-ray diffraction indicates two phases other than the commonly observed fcc structure. This superconducting transition temperature is considerably higher than for known doped C60 compounds and intermetallic compounds.

Crystalline films of C60 with ~1 μm grain size and a preferred [111] texture have been doped with K and Rb to form superconducting compositions that have sharp zero-resistance transitions. The temperature dependence of the resistance above Tc for both K3C60 and Rb3C60 films reveals metallic behavior up to temperatures as high as 520 K without any evidence of saturation. For Rb3C60, electronic mean free paths significantly shorter than nearest-neighbor C60 distances and anomalously high values of the electron-phonon coupling strength are inferred. These results suggest that there is a strong interaction between the conduction electrons and the intramolecular vibrational modes and also bring into question the assumption that all of the donated charge (three electrons per C60) is itinerant.

Alkali metal fullerides (A x C 60 ) are a subject of considerable current interest because of the occurrence of superconductivity for A x C 60 at temperatures surpassed only by the high T c copper oxides. The preparation and characterization of A x C 60 (A = alkali metal, x = 2,3,4,6) by powder X-ray diffraction, NMR ( 13 C, 23 Na and 87 Rb), and d.c. magnetization are reported. The structures are described as intercalation compounds of the FCC structure of pristine c 60 or of hypothetical BCC or BCT structures. The structures and phase diagrams can be rationalized on the basis of ion size and electrostatic considerations. Only the A 3 C 60 compounds are metallic (and superconducting). The superconducting T c increases nearly linearly with unit cell size. EHT (Extended Huckel Theory) calculations and 13 C NMR relaxation measurements indicate higher densities of states for the higher T c compositions.

INTERCALATING solid C60 with dopant atoms yields materials with a remarkable range of properties1. Alkali metal atoms, for example, readily form charge-transfer compounds2,3, AxC60 (where A is an alkali metal), which can be metallic, superconducting or insulating depending on the dopant concentration4–9. In all cases, the superconducting phase has a face-centred cubic (f.c.c.) structure and stoichiometry A3C60, which suggests a common mechanism for superconductivity dependent, at least in part, on the external coordination number of the C60 molecules. More recently, it has been shown10 that the alkaline earth metal calcium can also be intercalated with fulleride to form a superconducting phase, again with a f.c.c.-derived structure, near a Ca:C60 ratio of 5:1. Here we report the intercalation of fulleride with barium, in which a pure body-centred cubic phase with a lattice constant of 11.171 A is realized near a stoichiometry of Ba6C60. This phase is also superconducting (with a transition temperature of 7 K), suggesting that the mechanism of superconductivity is related to an intrinsic property of the C60 molecules, rather than the external coordination number.

Alkali metal fullerides (A x C 60 ) are a subject of considerable current interest because of the occurrence of superconductivity for A x C 60 at temperatures surpassed only by the high T c copper oxides. The preparation and characterization of A x C 60 (A = alkali metal, x = 2,3,4,6) by powder X-ray diffraction, NMR ( 13 C, 23 Na and 87 Rb), and d.c. magnetization are reported. The structures are described as intercalation compounds of the FCC structure of pristine c 60 or of hypothetical BCC or BCT structures. The structures and phase diagrams can be rationalized on the basis of ion size and electrostatic considerations. Only the A 3 C 60 compounds are metallic (and superconducting). The superconducting T c increases nearly linearly with unit cell size. EHT (Extended Huckel Theory) calculations and 13 C NMR relaxation measurements indicate higher densities of states for the higher T c compositions.

Bulk-sensitive, polarization-dependent oxygen K- and copper L-edge absorption spectra of single-domain ${\mathrm{La}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{CuO}}_{4}$ with ${\mathrm{CuO}}_{2}$ planes tilted off the sample surface were measured. A novel experimental configuration was utilized to achieve complete polarization geometry and eliminate optical path variations. Contrary to previous measurements, our data show for both intrinsic and doped holes a significant amount of O2${\mathit{p}}_{\mathit{z}}$ character, likely associated with apical O, but a very weak Cu3${\mathit{d}}_{3\mathit{z}}^{2}$-${\mathit{r}}^{2}$ character. The quantitative results impose strong contraints on superconductivity models invoking out-of-plane orbitals, and suggest that the apical O 2${\mathit{p}}_{\mathit{z}}$ orbital may play an important role in high-${\mathit{T}}_{\mathit{c}}$ phenomena.

We report the results of $^{13}\mathrm{C}$ NMR measurements on ${\mathrm{K}}_{3}$${\mathrm{C}}_{60}$ and ${\mathrm{Rb}}_{3}$${\mathrm{C}}_{60}$ in the normal and superconducting states. Electronic densities of states at the Fermi energy in the normal-state and energy gaps in the superconducting state are estimated from spin-lattice-relaxation data. Implications of the relaxation and spectral data for the electronic properties of these materials are discussed.

IT has been demonstrated1–6 that solid C60 can be readily intercalated with group IA alkali metals to give metallic or insulating compounds, depending on the dopant concentration. The metal atoms diffuse into tetrahedral and octahedral interstitial sites of the C60 lattice with little disturbance to the face-centred cubic (f.c.c.) packing7. The A3C60f.c.c. phases (A is K, Rb, Cs and mixtures of these) exhibit superconductivity with a transition temperature that increases with lattice constant8. At higher dopant concentration a body-centred tetragonal A4C60 phase9 and an insulating, body-centred cubic A4C60 phase10 are found. Here we report that the divalent group IIA intercalant calcium can be intercalated into the f.c.c. sites of C60 to form a solid solution, and that, near a Ca:C60 ratio of 5:1, a phase transformation occurs to a simple cubic phase. Measurements of microwave loss, magnetic susceptibility and Meissner effect show that the simple cubic phase becomes superconducting below 8.4 K.

The recent synthesis of macroscopic quantities of spherical molecular carbon compounds, commonly called fullerenes, has stimulated a wide variety of studies of the chemical and physical properties of this novel class of compounds. We discovered that the smallest of the known fullerenes, C60, could be made conducting and superconducting by reaction with alkali metals. In this paper, an overview of the motivation for these discoveries and some recent results are presented.

An insulating polymorph with a composition close to Bi2Sr2CuO6 complicates the synthesis of the single copper layer superconductor Bi2+xSr2−yCuO6±δ. We concentrate on the region where the material is single phase in air synthesis (2.18 < x < 2.29, 1.44 < y < 1.7). The physical properties from insulating to marginally conducting as the composition moves toward x = y = 0. Like to two-layer compound, the material has a superlattice due to a buckling of the planar structure, however the superlattice coherence is degraded by defects at finite values of x and y. Density measurements indicate that the nonstoichiometry of Bi2+xSr2−y CuO6±δ is not accompanied by the formation of a large number of vacancies. The insulating polymorph, Bi2Sr2CuO6, can be suppressed by synthesis in modest oxygen pressures (above 5–10 atm). The result is a bulk superconductor (Tc ≈ 10 K).

The crystal structure of the La 1.6 Sr 0.4 CaCu 2 O 6± δ superconductor has been determined by neutron diffraction powder profile refinement at room temperature on a slightly oxygen deficient sample (δ=-0.06, T c =55 K). The crystal structure is very similar to that of the nonsuperconducting host compound La 1.9 Ca 1.1 Cu 2 O 6−δ . Sr substitutes on the nine coordinate La sites, and there may be some La/Ca mixing on the eight coordinate metal site between the CuO 2 planes. Oxygen deficiency occurs in the apical oxygen sites. The compound has the tetragonal symmetry characteristic of a square CuO 2 plane at room temperature and 10 K, making it differ from the single-layer analog (La, Sr) 2 CuO 4 which has a buckled, distorted CuO 2 plane in the superconducting state.

The structure/property relationships in the Bi-Sr-Cu-O system have been investigated. This system includes Bi2+xSr2-yCuO6±δ, the single Cu layer compound that is one member of the Bi-Sr-Ca-Cu-O family of superconductors. This family includes Bi2Sr2CaCu2O8, a 95K superconductor. We have determined that two structurally related phases exist near the composition Bi2Sr2CuO6. One phase is a solid solution, Bi2+xSr2-yCuO6±5, with a wave modulated structure. The second phase, Bi2Sr2CuO6, has a structure with a step dislocation. The former phase exhibits superconductivity while the latter phase is an insulator. The transition between structures is driven, atleast in part, by a change in cation ratios.

Transport anisotropies of ${\ensuremath{\rho}}_{c}$.${\ensuremath{\rho}}_{\mathrm{ab}}$\ensuremath{\simeq}${10}^{4}$ to ${10}^{5}$ were measured for superconducting and nonsuperconducting ${\mathrm{Bi}}_{2+\mathrm{x}}$${\mathrm{Sr}}_{2\mathrm{\ensuremath{-}}\mathrm{y}}$${\mathrm{CuO}}_{6\ifmmode\pm\else\textpm\fi{}\mathrm{\ensuremath{\delta}}}$ crystals. In superconducting samples ${\ensuremath{\rho}}_{\mathrm{ab}}$ increases linearly with temperature from just above ${T}_{c}$\ensuremath{\simeq}7 to 700 K. The implication of the ${\ensuremath{\rho}}_{\mathrm{ab}}$ results is that classical electron-phonon scattering mechanisms are inadequate. The anisotropy and ${T}_{c}$ for various layered superconductor systems are compared. In all crystals studied ${\ensuremath{\rho}}_{c}$ is nonmetallic, varying as a power law ${T}^{\mathrm{\ensuremath{-}}\ensuremath{\alpha}}$, \ensuremath{\alpha}\ensuremath{\simeq}0.5--1.

A WIDE variety of alkali-metal fullerides (AxC60) have been prepared, with stoichiometries varying between 1⩽=x⩽=11 (refs 1,2). Most of the compounds with x = 3 are superconductors with unusually high transition temperatures (Tc) which increase with the size of the face-centred cubic unit cell3. The increase in Tc on lattice expansion is attributed to a decrease in the conduction band width, and therefore one ultimately expects a saturation or downturn of Tc followed by a metal–insulator transition. We have sought to explore this regime by expanding the A3C60 structure through intercalation of neutral molecules capable of solvating the A+ ions. Here we report that the reaction of ammonia with Na2CsC60 (Tc = 10.5K; lattice constant α = 14.132 A) produces the compound (NH3)4Na2CsC60, which has an expanded unit cell (a = 14.473 A) and an increased Tc of 29.6 K. The structure contains the Na(NH3)+4 cation on the octahedral site.

INTERCALATING solid C60 with dopant atoms yields materials with a remarkable range of properties1. Alkali metal atoms, for example, readily form charge-transfer compounds2,3, AxC60 (where A is an alkali metal), which can be metallic, superconducting or insulating depending on the dopant concentration4–9. In all cases, the superconducting phase has a face-centred cubic (f.c.c.) structure and stoichiometry A3C60, which suggests a common mechanism for superconductivity dependent, at least in part, on the external coordination number of the C60 molecules. More recently, it has been shown10 that the alkaline earth metal calcium can also be intercalated with fulleride to form a superconducting phase, again with a f.c.c.-derived structure, near a Ca:C60 ratio of 5:1. Here we report the intercalation of fulleride with barium, in which a pure body-centred cubic phase with a lattice constant of 11.171 A is realized near a stoichiometry of Ba6C60. This phase is also superconducting (with a transition temperature of 7 K), suggesting that the mechanism of superconductivity is related to an intrinsic property of the C60 molecules, rather than the external coordination number.

SUPERCONDUCTING K3C60 (ref. 1) has been shown2 to have an intercalated face-centred cubic (f.c.c.) structure, and other A3C60 compounds (where A is K, Rb or mixtures of K, Rb and/or Cs) form an isostructural series with superconducting transition temperatures up to 31.3 K (ref. 3). Recently we reported413C NMR studies of KxC60 for x

Despite early recognition that a relatively low‐temperature superconductor occurs in samples of approximate composition Bi2Sr2CuO6, studies of this material have been plagued by difficulties in the preparation of single phase material with a large superconducting fraction. We report the first preparation of ceramic samples in the Bi‐Sr‐Cu‐O system that show bulk superconductivity at 4–15 K. Structural, chemical, and electronic characterizations of superconducting and nonsuperconducting ceramics of composition Bi2.05Sr1.95CuO6 are described.

STARTING with the pioneering work of Bednorz and Muller1, many copper-oxide-based superconductors with high transition temperatures (Tc) have been discovered. All contain layers of copper-oxygen squares, pyramids or octahedra as their electronically active structural components2,3. One structure type, first reported for La2SrCu2O6 and La2CaCu2O6 (ref. 4), has stood as an enigma since the beginning of high-Tc research. This crystal structure4-7 (Fig. 1) is the least complex of all the structures with the double layers of copper oxide pyramids common to the compounds with highest Tc, yet despite considerable effort, both published8,9 and unpublished, it has not until now been made superconducting. Here we report the successful synthesis and preliminary physical characterization of superconducting (La, Sr)2CaCu2O6. The highest transition temperature observed is 60 K at the composition La1.6Sr0.4CaCu2O6. This is a uniquely simple double-layer superconductor, which, like its single-layer analogue (La, Sr)2CuO4, becomes superconducting through the introduction of carriers in an unambiguous manner—by straightforward atomic substitution without the intervention of charge reservoir layers with flexible valence states.

The Na based mixed alkali doped C{sub 60} superconductors show anomalous behavior with respect to the ``universal`` superconducting transition temperature {Tc} vs lattice constant a{sub 0} relation followed by most of the fcc A{sub 3}C{sub 60} superconductors. We have measured dt{sub c}/dP for Na{sub 2}CsC{sub 60} and (NH{sub 3}){sub 4}Na{sub 2}CsC{sub 60} using solid He as the pressure medium to {approximately} 6 kbar finding dT{sub c}/dP equal to {minus}0.8{plus_minus}(0.01) K/kbar and {minus}1.0({plus_minus}0.1)K/kbar for Na{sub 2}CsC{sub 60} and Na{sub 2}(NH{sub 3}){sub 4}C{sub 60} respectively. Our value for Na{sub 2}CsC{sub 60} differs markedly from that obtained by Mizuki et al of about {minus}1.3 K/kbar. However, using N{sub 2} or Ar, we obtain values for dT{sub c}/dP in substantial agreement with Mizuki et al who used fluorinert to generate their pressure. This work emphasizes the need for compressibility measurements with the same pressure medium in the appropriate temperature range so that meaningful comparisons can be made between various pressure measurements and models which are based on lattic spacing.

The Na based mixed alkali doped C60 superconductors show anomalous behavior with respect to the “universal” superconducting transition temperature Tc vs lattice constant ao relation followed by most of the fcc A3C60 superconductors. We have measured dTc/dP for Na2CsC60 and (NH3)4Na2CsC60 using solid He as the pressure medium to ∼ 6 kbar finding dTc/dP equal to −0.88±(0.01) K/kbar and −1.0(±0.1) K/kbar for Na2CsC60 and Na2(NH3)4C60 respectively. Our value for Na2CsC60 differs markedly from that obtained by Mizuki et al of about −1.3 K/kbar. However, using N2 or Ar, we obtain values for dTc/dP in substantial agreement with Mizuki et al who used fluorinert to generate their pressure. This work emphasizes the need for compressibility measurements with the same pressure medium in the appropriate temperature range so that meaningful comparisons can be made between various pressure measurements and models which are based on lattice spacing.

Alkali metal fullerides (A x C 60 ) are a subject of considerable current interest because of the occurrence of superconductivity for A x C 60 at temperatures surpassed only by the high T c copper oxides. The preparation and characterization of A x C 60 (A = alkali metal, x = 2,3,4,6) by powder X-ray diffraction, NMR ( 13 C, 23 Na and 87 Rb), and d.c. magnetization are reported. The structures are described as intercalation compounds of the FCC structure of pristine c 60 or of hypothetical BCC or BCT structures. The structures and phase diagrams can be rationalized on the basis of ion size and electrostatic considerations. Only the A 3 C 60 compounds are metallic (and superconducting). The superconducting T c increases nearly linearly with unit cell size. EHT (Extended Huckel Theory) calculations and 13 C NMR relaxation measurements indicate higher densities of states for the higher T c compositions.

THE discovery of conductivity1 in AxC60 (where A represents an alkali metal) and superconductivity2 in KxC60 has been followed by reports of superconductivity in other alkali-metal-doped full-erides with transition temperatures as high as 33 K (ref. 3). Elucidation of phase diagrams and understanding the relationship between structure and superconducting properties is essential to a detailed understanding of superconductivity in these systems. So far, structural data have been reported only for the non-conducting, intercalated body-centred cubic (b.c.c.) structures A6C60 (where A is K or Cs; ref. 4), a body-centred tetragonal structure for A6C60 (where A is K, Rb, Cs; ref. 5) and the superconducting, intercalated face-centred cubic (f.c.c.) material K3C60 (ref. 6). Here we report the preparation of a series of single-phase, isostructural f.c.c. superconductors with composition A3C60 (where A is K, Rb, Cs or a mixture of these), and show that Tcc increases monotonically with the size of the unit cell. Extended Hiickel band-structure calcula-tions also show a monotonic increase in the density of states at the Fermi level, N(EF), with lattice parameter. The primary impli-cation of these results is that all A3C60 superconductors have the same structure and that changes in Tc can be accounted for by changes in N(EF).

We report extended Huckel theory (EHT) band structure calculations for the high-temperature polymorph of α-6T (α-sexithiophene) and C60, two organic materials that have shown promising device characteristics, as the active element in thin-film transistors (TFTs). We also report calculations for α-3T and κ-(ET)2Cu(NCS)2[where ET = bis(ethylenedithio)tetrathiafulvalene] for comparison purposes. Both α-6T/HT and C60 show well developed band dispersions. Whereas C60 is an isotropic three-dimensional (3D) molecular solid, α-6T/HT shows a strongly 2D electronic structure, and the band structure is quite similar to that of the ET organic superconductors such as κ-(ET)2Cu(NCS)2. Factors important in determining the mobilities of organic semiconductors are reviewed.

We have obtained anisotropic thermal parameters for ${\mathrm{Ba}}_{2}$Y${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ from a dual-wavelength x-ray crystal structure determination on a single crystal with a small amount of twinning. Anharmonic thermal motion was found for both Ba and Cu atoms with the largest anharmonic feature attributed to a Ba-Ba breathing mode. The anisotropic and anharmonic thermal parameters are consistent with the coordination polyhedra as well as the bonding distances involved.

Ambient temperature measurements of the crystallographic cell parameters for oxygen deficient Ba 2 YCu 3 O 7−δ prepared by gettered annealing indicate the presence of microscopic differences in the oxygen configuration at fixed δ dependent on annealing temperature. The loss of superconductivity with increasing oxygen deficiency is shown to be due to a step-like increase in length of the plane copper-apical oxygen bond, and not to the orthorhombic to tetragonal transition. The crystal structure of the 84K superconductor Bi 2 Sr 2 CaCu 2 O 8+δ is described, as is the stabilization of 110K superconductivity via partial Pb/Bi substitution with long time annealing.

We have observed evidence of finite-range order in quenched crystals of nominal composition ${\mathrm{Ba}}_{2}$${\mathrm{YCu}}_{3}$${\mathrm{O}}_{6.7}$. X-ray scattering shows diffuse peaks with a reduced wave vector of (1/2,0,0) in the oxygen-deficient compound but not in fully oxidized ${\mathrm{Ba}}_{2}$${\mathrm{YCu}}_{3}$${\mathrm{O}}_{7}$. Although the crystals are twinned, we find that the diffuse satellites occur symmetrically around (h,0,0) peaks rather than (0,k,0) peaks indicating a doubling of the unit cell along the a axis (transverse to the Cu-O chains). The width of the diffuse peaks yields coherence lengths of 21, 16, and 9 A\r{} for the ${a}_{0}$, ${b}_{0}$, and ${c}_{0}$ directions, respectively.

We have mapped the temperature and composition dependence of the tetragonal-orthorhombic phase-transition temperature ${T}_{d}$ in the high-temperature superconductor ${\mathrm{La}}_{2\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Sr}}_{\mathrm{x}}$${\mathrm{CuO}}_{4\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ using high-resolution powder x-ray diffraction and measurements of the thermal expansion. The maximum volume fraction of ideal diamagnetism occurs in the composition range where ${T}_{d}$ is approaching the superconducting transition. In the composition range xg0.20 where no orthorhombic distortion is observed above 12 K, no bulk superconductivity is observed in ${\mathrm{La}}_{2\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Sr}}_{\mathrm{x}}$${\mathrm{CuO}}_{4\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$.

We report the Raman spectrum of single crystals of Bi/sub 2.2/Ca/sub 0.8/Sr/sub 2/Cu/sub 2/O/sub 8+//sub delta/. The most prominent features are at 120, 295, 462, and 625 cm/sup -1/. The 462-cm/sup -1/ mode is tentatively assigned to axial O(3) symmetric stretching. The 625-cm/sup -1/ mode is assigned to planar Cu-O(1) bond stretching. The 120-cm/sup -1/ mode shows a strong a-b anisotropy. The effect of the b-axis superstructure upon the Raman spectrum is emphasized.

Superconducting Y/sub 1/Ba/sub 2/Cu/sub 3/O/sub 7-//sub x/ films were produced by metal molecular-beam epitaxy with one electron beam and two thermal cells and a molecular oxygen-gas source. The films were epitaxially grown on SrTiO/sub 3/(100) with an a-axis orientation perpendicular to the plane, and b and c axes ordered in the plane. A sharp resistive superconducting transition with T/sub c/(R = 0) at 82 K was obtained. A lower limit of the average J/sub c/ in the plane is 10/sup 3/ A/cm/sup 2/ at 77 K 5 x 10/sup 4/ A/cm/sup 2/ at 70 K, and 10/sup 6/ A/cm/sup 2/ at 4.2 K.

The dissipation observed at the superconducting transition of the high-${T}_{c}$ superconductor ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$Ca${\mathrm{Cu}}_{2}$${\mathrm{O}}_{8}$ is explained quantitatively by the Kosterlitz-Thouless theory of vortex-antivortex pair excitations within the Cu${\mathrm{O}}_{2}$ planes. This conclusion is drawn from the observation of an exponential square-root singularity in the resistivity and a power-law dependence of the resistivity on magnetic field. The Kosterlitz-Thouless phase transition (${T}_{c}=84.7$ K) is at the $\ensuremath{\rho}=0$ point, and the mean-field Ginzburg-Landau transition (${T}_{c0}=86.8$ K) at \ensuremath{\sim}25% of the normal-state resistivity.

We have observed a periodic superlattice modulation in Nd 2 CuO 4 and in the “electron-doped” Nd 2 CuO 4−x F x and Nd 2−x Ce x− CuO 4 by electron diffraction and microscopy. The superlattice modulation characterized by a wave vector q=( 1 4 1 4 ,0) parallel to the Cu-O plane was found in both superconducting and non-superconducting samples, but most of the samples examined were not homogeneous with respect to the superlattice. The orthorhombic superlattice phase is found to exist in four possible variants with its c-axis parallel to the four equivalent [111] zone-axes in the underlying tetragonal lattice. Samples quenched from high temperature invariably showed no superlattice or superconductivity; bulk superconductivity and regions of superlattice were observed on annealing in the range 250–750°C. We believe that oxygen migration is responsible for this disproportionation into superlattice and nonsuperlattice regions.

A strongly anisotropic resistivity tensor was measured from ${T}_{c}=81 \mathrm{to} 600$ K in crystals of the new high-${T}_{c}$ superconductor ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2.2}$${\mathrm{Ca}}_{0.8}$ ${\mathrm{Cu}}_{2}$${\mathrm{O}}_{8}$. The $a\ensuremath{-}b$ plane anisotropy is $\frac{{\ensuremath{\rho}}_{a}}{{\ensuremath{\rho}}_{b}}\ensuremath{\approx}2$, with ${\ensuremath{\rho}}_{a}$ and ${\ensuremath{\rho}}_{b}$ linear in $T$. The perpendicular component ${\ensuremath{\rho}}_{c}\ensuremath{\sim}10$ \ensuremath{\Omega} cm is ${10}^{5}$ greater than ${\ensuremath{\rho}}_{a}$ or ${\ensuremath{\rho}}_{b}$. Evidence for nonmetallic temperature dependence is found for the $\frac{{\ensuremath{\rho}}_{c}}{{\ensuremath{\rho}}_{\mathrm{ab}}}$ resistivity ratio. The data are consistent with the notion of quasiparticle tunneling between planes, but an alternative explanation in terms of weak interplanar coupling by topological defects is also presented.

Single crystal sof the 84-K superconductor ${\mathrm{Bi}}_{2.2}{\mathrm{Sr}}_{2}{\mathrm{Ca}}_{0.8}{\mathrm{Cu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$ were characterized by x-ray diffraction, dc magnetic susceptibility, electrical resistivity, and microwave absorption. The structure has ${[\mathrm{Cu}{\mathrm{O}}_{2}]}_{\ensuremath{\infty}}$ planes separated by calcium atoms, edge-shared bismuth oxide double layers, and an incommensurate superlattice along $b$ with a period of 4.76. The in-plane resistivity above ${T}_{c}$ is linear in $T$, with ${\ensuremath{\rho}}_{\mathrm{RT}}=130$ \ensuremath{\mu}\ensuremath{\Omega} cm. Initial results on Pb substitution yielding ${T}_{c}'\mathrm{s}$ of 107 K are reported.

Single crystals of dilute iron alloys of the layer structures NbSe/sub 2/, TaSe/sub 2/, and TaS/sub 2/ have been studied using measurements of magnetoresistance and Hall effect in magnetic fields up to 220 kG. Iron concentrations in the range 0--10 at.% can induce spin-exchange scattering, metal-insulator transitions, or enhanced superconducting anisotropy depending on the phase of the crystal and the type of iron doping. Kondo-like resistance minima accompanied by negative magnetoresistance and anomalous Hall effects are observed in the 2H phase of Fe/sub x/TaSe/sub 2/ and Fe/sub x/NbSe/sub 2/. A ln H behavior at high fields and a ln T behavior at zero field or low fields have been fit to theoretical expressions and antiferromagnetic exchange constants on the order of -0.1 eV are obtained. Excess iron doping of 2H-TaS/sub 2/ produces enhanced superconductivity characterized by parameters similar to those observed for organic intercalation. The interlayer coupling strength is consistent with a tunneling model describing both the iron-doped material and the organic intercalates. Substitutional iron doping of TaS/sub 2/ and TaSe/sub 2/ stabilizes the 1T phase and produces a dramatic rise in resistivity at low temperature following at T/sup -..cap alpha../ power-law behavior.

High-purity (97%) samples of 27-A phase of Y-Ba-Cu-O have been prepared in superconducting thin films on (100) SrTiO/sub 3/. The possibility that the observed superconductivity might arise from a residual 3% Ba/sub 2/YCu/sub 3/O/sub 7/ impurity in this sample is ruled out using transmission electron microscopy which shows that it is present only as isolated particles. This allows for an unambiguous determination that the 27-A phase is superconducting with an onset T/sub c/ of 82 K and T/sub c/(R = 0) of 79 K.

The intercalation of oxygen in twinned single crystals of ${\mathrm{Ba}}_{2}\mathrm{Y}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ occurs by diffusion along basal $a\ensuremath{-}b$ planes, as shown by time-dependent resistivity measurements at elevated temperatures. Separation of the oxygen-deficient 60-K phase in crystals annealed in ${\mathrm{O}}_{2}$ is revealed by an anomalous temperature dependence of the resistance along the $c$ direction. Susceptibility analysis shows that oxygenation and the sharpness of the superconducting transition can be enhanced significantly, with no change in ${T}_{c}$, by $F$-ion implantation followed by rediffusion of oxygen.

Critical current densities and resistivities were measured in a single crystal of the high Tc superconductor Bi2Sr2CaCu2O8 within the ab basal plane (∥) and along the c direction (⊥). A large anisotropy in critical current density is found, Jc∥/Jc⊥≊103, in quantitative agreement with the large normal‐state resistivity anisotropy near Tc. The data provide strong evidence for a two‐dimensional layered structure of metallic planes separated by semi‐insulating barriers.

The discovery of superconductivity at temperatures above the boiling point of nitrogen has led to intense interest in the physics and chemistry of materials exhibiting this behaviour. Although some experiments can be performed on sintered polycrystalline samples, large single crystals are important for many measurements of physical properties, including investigations of the relationship between structural parameters and superconducting properties. Like its predecessors YB2Cu3O7 (refs 1–4) and La2–xSrxCuO4 (ref. 5), the recently discovered 84-K bismuth–strontium–calcium cuprate superconductor6–9 melts incon-gruently, and crystals have been grown from eutectic melts10. This technique yields crystals large enough for some experiments, but is generally difficult to control, placing limits on the size of crystals that can be grown. In addition, the crystals must be mechanically separated from the melt. Here we report the growth of single crystals of Bi2.2Sr2Ca0.8Cu2O8+δ from alkali chloride fluxes. The crystals are superconductors, showing large Meissner effects and zero-resistance transitions above 80 K. Flux growth is a standard high-temperature solution growth technique, and should allow crystal size and quality to be controlled by parameters such as equilibration temperature, cooling rate and melt composition. The fluxes reported here are easily washed from the crystals.

A new, ordered perovskite structure was stabilized as a majority phase in superconducting Y2Ba5Cu7O20−δ’(F/Cl)δ‘ films epitaxially grown on SrTiO3(100). The new phase was formed via fluorination or chlorination introduced during the post‐deposition O2 furnace anneal for recrystallization. The structure is determined by x‐ray diffraction to be an orthorhombic unit cell [a∼b=3.86(1) A, c=27.24(6) A] with reflection conditions consistent with a fully twinned A‐centered cell. Films containing primarily the new phase show a lower resistivity ρ(300 K)∼177 μΩ cm, a comparable ratio of ρ(300 K)/ρ(100 K)∼3.0, and a lower superconducting temperature of 78–74 K when compared to Y1Ba2Cu3O7.

Using powder x-ray diffraction, we have measured the temperature dependence of the lattice parameters of the 30-K superconductor, Ba/sub 0.6/K/sub 0.4/BiO/sub 3/, in the range 14--300 K. We find that within the resolution of our measurement, Ba/sub 0.6/K/sub 0.4/BiO/sub 3/ remains cubic over the entire range of temperature with no peak splitting or additional peaks observed. This means that the compound maintains an undistorted, three-dimensional Bi-O array at the superconducting transition temperature.

Superconducting films of Ca‐Sr‐Bi‐Cu oxides have been prepared by coevaporation of CaF2, SrF2, Bi, and Cu, followed by post‐oxidation in wet O2. The films were characterized by four‐probe resistivity measurements, Rutherford backscattering, transmission electron microscopy, x‐ray diffraction, and Hall measurements. Zero resistance was achieved at ∼80 K, although evidence of traces of superconductivity at higher temperatures was seen in resistivity and Hall data. The critical current at 4.2 K was 1.0×106 A cm−2. The films were epitaxial on 〈100〉 and 〈110〉 SrTiO3 substrates. The electrical and structural properties of the films were insensitive to film composition over a wide range of stoichiometries.

The simple cubic perovskite crystal structure of superconducting (Rb,Ba)BiO3 makes it an ideal system for the study of perovskite heteroepitaxy. We report novel growth phenomena which have been observed in (Rb,Ba)BiO3 films grown by molecular beam epitaxy (MBE). For growth on MgO, the orientation of the film is observed to depend on the substrate temperature during growth, changing from (1 1 0) to (1 0 0) with increasing temperature. For films on SrTiO3, the large (10%) lattice mismatch appears to cause small grain rotations, resulting in a film with many low angle grain boundaries. In at least one growth regime, (2 2 1) oriented grains nucleate and grow on Ʃ3 coincidence boundaries on a faceted (1 0 0) oriented film.

In reply to the comment on vortex/minus/pair excitation near the superconducting transition of BiSrCaCuO crystals that it is premature to invoke a new mechanism. (AIP)

Orientation ordered high-Tc oxide films of Y1Ba2Cu3O7 were prepared by molecular beam epitaxy on SrTiO3 substrates. A combination of effusion cells and e-beam evaporators was used with the addition of an oxygen jet. The superconducting phase is formed by further O2 anneal at temperatures ≥800° C. For SrTiO3 (100), the Y1Ba2Cu3O7 films grow in the orientation of either the c-axis or the a-axis perpendicular to the film plane. Films with predominant “c⊥” orientation exhibit a high degree of crystalline order with a rocking curve 0.3° wide. The Jc is about two times higher than those with the “a⊥” orientation of a comparable Tc (R=0). For a typical film of 9000Å thickness, a Tc (R=O) of 87K, and a Jc of 7 × 104 A/cm2 at 77K were reproducibly achieved. For SrTiO3 (110), X-ray showed twinning of (110) and (103) orientations in Y1Ba2Cu3O7, thus precluding the observation of anisotropy expected for the in-plane axes of [001] and [110].

The remarkable discovery of high Tc superconducting ceramics[1] has been followed by intense efforts to realize their potential in the form of devices, films, and wires. A number of techniques have been shown to be effective for growing thin films of these superconductors including electron beam/thermal co-evaporation,[2-41 RF and DC sputtering, [5-8] laser [11-12] evaporation,[9-10] and spin-on/pyrolysis. Of all of these techniques, the e-beam/thermal co-evaporation methods have been shown to give the best electrical characteristics in Ba2YCu3O7 films, particularly with respect to the critical currents which have reached 106 A/cm2 at 77K. [6,13]© (1988) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Recent breakthrough of superconductivity over 90 K in the Cu oxide based perovskite-type materials is of immense importance in both science and technology.1,2 The superconducting oxide crystals prepared in thin film form are vital for fundamental physical studies as well as for device applications.3,4 We report here the structural and superconducting properties of single crystal Y1Ba2Cu3O7−x films5 prepared by molecular beam epitaxy (MBE) developed for thin film semiconductor technology. The films are epitaxial crystals grown on SrTiO3 (100) face with an orientation of a axis perpendicular to the film plane. Sharp resistive superconducting transitions with Tc (R=0) above 77 K have been reproducibly achieved. The critical current densities are 103 A/cm2 at 77 K, 5 × 104 A/cm2 at 70 K, and ~ 106 A/cm2 at 4.2 K.