J. E. Schirber

Organic Superconductors—New Benchmarks

Recent advances in the design and synthesis of organic synthetic metals have yielded materials that have the highest superconducting transition temperatures (Tc ≈ 13 kelvin) reported for these systems. These materials have crystal structures consisting of alternating layers of organic donor molecules and inorganic anions. Organic superconductors have various electronic and magnetic properties and crystal structures that are similar to those of the inorganic copper oxide superconductors (which have high Tc values); these similarities include highly anisotropic conductivities, critical fields, and short coherence lengths. The largest number of organic superconductors, including those with the highest Tc values, are charge-transfer salts derived from the electron donor molecule BEDT-TTF or ET [bis(ethylenedithio)-tetrathiafulvalene]. The synthesis and crystal structures of these salts are discussed; their electrical, magnetic, and band electronic structure properties and their many similarities to the copper oxide superconductors are treated as well.

The crystal structure of superconducting La2CuO4.032 by neutron diffraction

Abstract The crystal structure of superconducting (37.5 K) La2CuO4.032 has been refined from single-crystal neutron-diffraction data at room temperature and 15 K. At both temperatures it exhibits the orthorhombic symmetry Cmca. The extra oxygen O4 atoms occupy the special positions ( 1 4 y 1 4 with y=0.243 at 15 K). They are located between two successive LaO layers and are surrounded by distorted cubes built up of two interpenetrated tetrahedra, one comprising four La atoms and the other four O1 atoms (the apical oxygen atoms of the CuO6octahedra). The O4 insertion causes a 0.75 A displacement of 4.8% of the O1 atoms towards new O3 positions (x=0.030(5), y=0.182(2), z=0.100(5)). From the refined values of the occupancy factors at 15 K, it is deduced that for each extra O4 three O1 are displaced to O3 with one short O3–O4 distance of 1.64(3) A. This value indicates the formation of a strong O-O covalent bond of peroxide type with a formal 2-valence. Since the La and Cu sublattices have been found to be fully occupied and the doping does not change the oxygen charge, the La, Cu and O sublattices have the formal valences 6+, 2+, and 8−, respectively. However, the increase in La coordination and the consequent La-O distance readjustment indicate, when compared to the undoped compound structure, that a charge transfer occurs in La2CuO4.032, with the excess positive charge going either to the La or to the O sublattice. In the latter case it would correspond to the formation of holes in the O 2p band.

Stoichiometry of bulk superconducting La2CuO4+δ: A superconducting superoxide?

Abstract A variety of analytical tools have been combined to show that bulk superconducting La2CuO4+δ has a La to Cu ratio of 2.00 (±0.02) and an oxygen stoichiometry of 4.13 (±0.1). The combination of these analytical results and magnetization data with a model leads us to suggest that the excess charged oxygen is introduced as O-2.

Strain index, lattice softness and superconductivity of organic donor-molecule salts: Crystal and electronic structures of three isostructural salts k-(BEDT- TTF)2Cu[N(CN)2]X (X=Cl, Br, I)

Abstract The recently discovered k -phase salts k -(BEDT-TTF) 2 ]Cu[N(CN) 2 ]X (X=Cl,Br, I) are isostructural but differ in their superconducting properties: the Br-salt is an ambient-pressure superconductor with T c =11.6K, the Cl-salt becomes superconducting ( T c =12.8K) under a slight applied pressure of 0.3 kbar, and the I-salt does not exhibit superconductivity under an applied-pressure of up to 5 kbar. We examine the structural and electronic properties of these three salts on the basis of their crystal structures determined at 127 K by single crystal X-ray diffraction measurements. The band electronic structure of the three salts are virtually the same and do not account for the differences in the conduction properties. The differences in the three salts with regard to superconductivity are examined from the viewpoint of lattice softness by calculating strain indices for short intermolecular contacts. The random potential arising from ethylene group conformational disorder prevents superconductivity in the iodide salt.

Intercalation of molecular species into the interstitial sites of fullerene

Molecular species were found to diffuse readily into the octahedral interstitial sites of the fcc lattice of C{sub 60}. The {sup 13}C NMR spectrum of C{sub 60} under magic angle spinning (MAS) conditions consisted of a primary resonance at 143.7 ppm and a minor peak shifted 0.7 ppm downfield. The downfield shift obeys Curies law and is attributed to the Fermi-contact interaction between paramagnetic oxygen molecules and all 60 carbon atoms of rapidly rotating adjacent C{sub 60} molecules. Exposure of C{sub 60} to 1 kbar oxygen for 1.75 h at room temperature resulted in a spectrum of seven evenly spaced resonances corresponding to the filling of 0 to 6 of the adjacent octahedral interstitial sites with oxygen molecules. The distribution of site occupancies about a C{sub 60} molecule provided evidence that the intercalation process is controlled by diffusion kinetics. Exposure to 0.14 kbar hydrogen gas at room temperature for 16 h filled a substantial fraction of the interstitial sites of C{sub 60} and C{sub 70} with hydrogen molecules.

Structural and compositional characterization of polycrystals and single crystals in the Bi- and Tl-superconductor systems: Crystal structure of TlCaBa2Cu2O7

Abstract The growth of mm-sized crystals in the Tl-Ca-Ba-Cu-O and Bi-Ca-Sr-Cu-O superconductor systems shows the common presence of polycrystals, which can be described as syntactic intergrowth crystals, as well as normal single crystals. Three distinct phases with superconducting transitions at 103 K, 106–108 K and 112–114 K have been grown in the Tl-Ca-Ba-Ca-O system. The newly identified 103 K phase is TlCaBa 2 Cu 2 O 7 which is structurally similar to YBa 2 Cu 3 O 7 but without the presence of chains. Transmission electron diffraction and electron microprobe analysis on several polycrystals show that they are composed of interlayers of these distinct phases grown epitaxially along the c -axis. Crystals have been grown for two phases in the Bi-Ca-Sr-Cu-O system that exhibit transition temperatures at 80 K and at 92 K. Oxygen annealing substantially improves the superconducting properties of most of the crystals.

Two-phase structural refinement of La2CuO4.032 at 15 K

Abstract A two-phase refinement of a crystal for the compound La2CuO4+δ (δ=0.032), based on the neutron diffraction data collected at 15 K with the D9 diffractometer at ILL, using ǂ=0.48 A , has been carried out. One phase (30%) consists of stoichiometric La2CuO4 domains, while the other (70%) of oxygen-rich La2CuO4.048 domains. The percentages of each phase, which have been refined together with the other parameters (scale factor, positional parameters, thermal factors and occupancy factors of the oxygen atoms), agree very well with the value determined from χAC measurements. The La2CuO4.048 structure is essentially the same as the average structure reported in ref. [1], the only difference being the oxygen content which is found to be δ= δ 0.70 . The extra oxygen, O(4), is found to be in between two LaO layers in a similar position as the oxygen atoms located between two Nd layers in the N2CuO4 structure. The insertion of extra oxygen causes the displacement of some of the oxygen O(1) towards the O(3) positions. Different models are proposed for the distortion induced by this insertion according to the experimental value, 3.3 [6], found for the ratio of the amount of O(3) to that of O(4). If this ratio is assumed to be 3 and the O(3) atoms are localized about the insertion, then the formation of a short O(4)-O(3) bond would occur. The models not requiring the formation of the short bond correspond to ratios of 2 or 4. In the latter case there would be four displaced O(1), but due to the rigidity of the oxygen octahedra only two O(3) would be bonded to O(4), the other two being the apically opposite oxygen atoms of the same octahedra.

Light-hole conduction in InGaAs/GaAs strained-layer superlattices

We report the first observation of light‐hole band carriers in In0.2Ga0.8As/GaAs strained‐layer superlattices by direct measurements of their effective mass (m*mo=0.14) using oscillatory magnetoresistance data. Preferential population of light‐hole states, due to splitting of the degenerate bulk valence bands by built‐in strain, allows this direct observation.

p‐channel, strained quantum well, field‐effect transistor

A p‐channel field‐effect transistor with a 3.5 μm Cr/Au gate was fabricated from a modulation‐doped GaAs/In0.2Ga0.8As/GaAs quantum well structure. Well‐behaved transistor action was observed at both 300 and 77 K with extrinsic transconductances of 6.2 and 11.3 mS/mm, respectively. Shubnikov–deHaas measurements prove the existence of a two‐dimensional hole gas with a strain‐shifted light‐hole ground state associated with a light‐hole mass of 0.154m0.

GaAs/(In,Ga)As, p‐channel, multiple strained quantum well field‐effect transistors with high transconductance and high peak saturated drain current

GaAs/In0.2 Ga0.8 As structures with two paralleled 10 nm quantum wells, modulation doped from the top, bottom, and middle with Be, have been fabricated into multiple strained quantum well field‐effect transistors (MQWFET’s) with 1×150 μm2 Ti/Au gates and examined both illuminated and in the dark at 300 and 77 K. Measurements on van der Pauw structures fabricated simultaneously with the transistors showed hole mobilities and sheet carrier densities to be 200, 3100, and 8040 cm2/V s, and 5.7×1012, 1.8×1012, and 1.5×1012 cm−2 , at 300, 77, and 4 K, respectively. Shubnikov–de Haas measurements made below 4 K verified the existence of a double‐channel two‐dimensional hole gas with a strain‐shifted light‐hole ground state in the quantum wells with an effective hole mass of 0.15 me . A representative p‐channel MQWFET showed well‐saturated common‐source output characteristics, both illuminated and unilluminated, at all measurement temperatures. Measured peak extrinsic transconductances and peak saturated drain cu...