Superconductivity at 52 K in hole-doped C60

Isothermal and non-isothermal transport properties of diluted fullerene binary and ternary aromatic solvent mixtures

We present mass transport properties of C60 fullerene in five aromatic solvents, methylnaphthalene, toluene and three xylene isomers. Optical beam deflection and thermogravitational column techniques were used to determine molecular diffusion, thermodiffusion and Soret coefficients. All thermo-optical properties necessary to determine the abovementioned coefficients are also given at a mean working temperature of 298.15 K and an atmospheric pressure of 0.101 MPa. The magnitude of all transport properties is governed by the molecular weight ratio. In the particular case of the isomers, experiments revealed that movement under isothermal conditions (described by molecular diffusion) is dominated by density, while under non-isothermal conditions viscous forces affect the displacement (thermodiffusion depends on the dynamic viscosity). In the case of the Soret coefficients, as a combination of both, density is the dominant parameter and also the moment of inertia.

Light triggered encapsulation and release of C60 with a photoswitchable TPE-based supramolecular tweezers

Stimuli responsive hosts for C₆₀ can control its binding and release on demand. A photoswitchable TPE based supramolecular host can encapsulate C₆₀ in the Z-form with a markedly different visual change in the colour. In addition, the Z-1 bound C₆₀ has been characterized by various spectroscopic methods and mass spectrometry. Upon exposure to visible light (>490 nm), the host switches to the E-form where the structural complementarity with the guest is destroyed as a result of which the C₆₀ is disassembled from the host. The results described herein reveals an actionable roadmap to pursue further advances in component self-assembly particularly light-induced association and dissociation of a guest molecule.

The manipulation of C60 in molecular arrays with an STM tip in regimes below the decomposition threshold

The ability of scanning tunneling microscopy to manipulate selected C(60) molecules within close packed C(60) arrays on a (Au,In)/Si(111) surface has been examined for mild conditions below the decomposition threshold. It has been found that knockout of the chosen C(60) molecule (i.e., vacancy formation) and shifting of the C(60) molecule to the neighboring vacant site (if available) can be conducted for wide ranges of bias voltages (from -1.5 to +0.5 V), characteristic manipulation currents (from 0.02 to 100 nA) and powers (from 2 × 10(-8) to 0.1 μW). This result implies that the manipulation is not associated with the electrical effects but rather has a purely mechanical origin. The main requirement for successful C(60) knockout has been found to be to ensure a proper 'impact parameter' (deviation from central impact on the C(60) sphere by the tip apex), which should be less than ~1.5 Å. A certain difference has been detected for the manipulation of C(60) in extended molecular arrays and molecular islands of a limited size. While it is possible to manipulate a single C(60) molecule in an array, in the case of a C(60) island it appears difficult to manipulate a given fullerene without affecting the other ones constituting the island.

Supramolecular Nano Networks Formed by Molecular-Recognition-Directed Self-Assembly of Ditopic Calix[5]arene and Dumbbell [60]Fullerene

Dumbbell fullerene 1 and ditopic bisdouble-calix[5]arene 2 were synthesized. Their iterative host-guest complexations create the supramolecular nano network. Scanning electron microscopy revealed the formation of the branched fiber, possessing a length of more than 100 mum and widths of 250-500 nm on a glass plate. More detailed information was given by atomic force microscopy. The formed fibers on a mica plate have widths of 60-90 nm and heights of 1.2-1.9 nm. The nanosize assemblies are probably composed of a bundle of 40-60 polymer chains created by entangling the alkyl side chains with van der Waals interaction.

Experimental evidence of a dynamic Jahn-Teller effect in C60(+)

Detailed analysis of the highest occupied molecular orbital band shape in the photoelectron spectrum of gaseous C60 reveals a dynamic Jahn-Teller effect in the ground state of C60(+). The direct observation of three tunneling states asserts a D(3d) geometry for the isolated cation, originating from a strong vibronic coupling. These results show that the ionic motion plays an important role in the electron-phonon interaction.

Staircase in the electron mobility of a ZnO quantum dot assembly due to shell filling

Electron transport in an assembly of ZnO quantum dots has been studied using an electrochemically gated transistor. The electron mobility shows a stepwise increase as a function of the electron occupation per quantum dot. When the occupation number is below two, transport occurs by tunneling between the S orbitals. Transport becomes 3 times faster when the occupation number is between two and eight; tunneling now occurs between the P orbitals. Electron transport is thus critically determined by the quantum properties of the building blocks.

Universal intermediate phases of dilute electronic and molecular glasses

Generic intermediate phases with anomalous properties exist over narrow composition ranges adjacent to connectivity transitions. Analysis of both simple classical and complex quantum percolation shows how topological concepts can be used to understand many mysterious properties of high temperature superconductors, including the remarkably similar phase diagrams of La(2-x)Sr(x)CuO4 and C60(+y). Predictions are made for novel threshold behavior of the impurity band metal-insulator transition in two dimensions.

Structure of haloform intercalated C60 and its influence on superconductive properties

CHCl3 and CHBr3 intercalated C60 have attracted particular interest after a superconductivity transition temperature (Tc) of up to 117 K was discovered. We have determined the structure using synchrotron x-ray powder-diffraction and found that the expansion of the lattice mainly takes place in one dimension (triclinic b axis), leaving planes of C60 molecules on an approximately hexagonal, slightly expanded lattice. We have performed tight-binding band structure calculations for the surface layer. In spite of the slight expansion of the layers, for the range of dopings where a large Tc has been observed, the density of states at the Fermi energy is smaller for C60.2CHCl3 and C60.2CHBr3 than for C60. This suggests that the lattice expansion alone cannot explain the increase of Tc.

Prediction of high T(c) superconductivity in hole-doped LiBC

The layered lithium borocarbide LiBC, isovalent with and structurally similar to the superconductor MgB2, is an insulator due to the modulation within the hexagonal layers (BC vs B2). We show that hole doping of LiBC results in Fermi surfaces of B-C p sigma character that couple very strongly to B-C bond stretching modes, precisely the features that lead to superconductivity at T(c) approximately equal to 40 K in MgB2. Comparison of Li(0.5)BC with MgB2 indicates the former to be a prime candidate for electron-phonon coupled superconductivity at substantially higher temperature than in MgB2.

Cooperative C(60) binding to a porphyrin tetramer arranged around a p-terphenyl axis in 1:2 host-guest stoichiometry

[structure: see text] Porphyrin tetramer 1 was newly designed and synthesized to construct a novel cooperative [60]fullerene (C(60)) binding system. Compound 1 has a p-terphenyl axis, which is expected to act as a scaffold for a guest-binding information transducer. In toluene, 1 can bind 2 equiv of C(60) to produce a 1:2 1/C(60) complex with association constants of 5800 M(-1) (K(1)) and 2000 M(-1) (K(2)). These values are significantly greater than those for control porphyrin dimers such as 2 and 3.

Evidence for insulating behavior in the electric conduction of (NH(3))K(3)C(60) systems

Microwave study using the cavity perturbation technique revealed that the conductivity of the antiferromagnet (NH(3))K(3-x)Rb(x)C(60) at 200 K is already 3-4 orders of magnitude smaller than those of superconductors, K(3)C(60) and (NH(3))(x)NaRb(2)C(60), and that the antiferromagnetic compounds are insulators below 250 K without metal-insulator transitions. The striking difference in the magnitude of the conductivity between these materials strongly suggests that the Mott-Hubbard transition in the ammoniated alkali fullerides is driven by a reduction of lattice symmetry from face-centered-cubic to face-centered-orthorhombic, rather than by the magnetic ordering.

Electrostatic tuning of the hole density in NdBa2Cu3O(7-delta) films and its effect on the Hall response

We have used the ferroelectric field effect in heterostructures based on superconducting NdBa2-Cu(3)O(7-delta) and ferroelectric Pb(Zr0.2Ti0.8)O3 to electrostatically modulate in a reversible and nonvolatile fashion the hole carrier density of the superconducting layer. Reversing the ferroelectric polarization induces a constant relative change in the resistivity and Hall constant of 9% and 6%, respectively, at all temperatures above the superconducting transition. The cotangent of the Hall angle displays a T2 dependence with a slope that increases as the carrier density is reduced.

Structure rationalization and topology prediction of two-distinct-component organic crystals: the role of volume fraction and interface topology

We consider here small-length-scale crystal structures with two clearly different molecular components (e.g., hydrophobic and hydrophilic). Using a perspective developed by studies on large-length-scale block copolymers and liquid crystals, we focus on the crystalline interface between the two components. We examine four types of two-component crystals: aromatic ammonium carboxylates, aromatic oligo(ethylene oxides), cyclohexylammonium carboxylates, and ether-thioether compounds. Of the 111 crystal structures found in the Cambridge Structure Database (CSD), 108 adopt one of the five generic topologies found in diblock copolymers: spheres, columns, perforated layers, layers, and bicontinuous structures. As in diblock copolymers, a key factor controlling the interfacial topology is shown to be the volume ratio of the two components. When the volume fraction of one component is less than 30% of the whole, more than five-sixths of the examined crystal structures are of columnar or spherical type. For volume fractions between 40 and 50% more than three-quarters are of lamellar or bicontinuous type. We use this model to predict the topologies of small-length-scale two-component crystals. We predict the crystal topolgies of six new crystal structures: three are predicted to be columnar, and the other three, lamellar or bicontinuous. The crystal structures of these systems were then determined by single-crystal X-ray methods. Five of the structures form in topologies consistent with the predictions: three in columns and two in layers. The remaining one forms as a perforated layer instead of the predicted columnar structure. Such predictive accuracy is consistent with the statistics of the CSD investigation.

Saturation of electrical resistivity in metals at large temperatures

We present a microscopic model for systems showing resistivity saturation. An essentially exact quantum Monte Carlo calculation demonstrates that the model describes saturation. We give a simple explanation for saturation, using charge conservation and considering the limit where thermally excited phonons have destroyed the periodicity. Crucial model features are phonons coupling to the hopping matrix elements and a unit cell with several atoms. We demonstrate the difference to a model of alkali-doped C60 with coupling to the level positions, for which there is no saturation.

Superconductivity in CaCuO2 as a result of field-effect doping

Understanding the doping mechanisms in the simplest superconducting copper oxide-the infinite-layer compound ACuO2 (where A is an alkaline earth metal)-is an excellent way of investigating the pairing mechanism in high-transition-temperature (high-Tc) superconductors more generally. Gate-induced modulation of the carrier concentration to obtain superconductivity is a powerful means of achieving such understanding: it minimizes the effects of potential scattering by impurities, and of structural modifications arising from chemical dopants. Here we report the transport properties of thin films of the infinite-layer compound CaCuO2 using field-effect doping. At high hole- and electron-doping levels, superconductivity is induced in the nominally insulating material. Maximum values of Tc of 89 K and 34 K are observed respectively for hole- and electron-type doping of around 0.15 charge carriers per CuO2. We can explore the whole doping diagram of the CuO2 plane while changing only a single electric parameter, the gate voltage.

Superconductivity in single crystals of the fullerene C70

The observation of superconductivity in doped C60 has attracted much attention, as these materials represent an entirely new class of superconductors. A maximum transition temperature (Tc) of 40 K has been reported for electron-doped C60 crystals, while a Tc of 52 K has been seen in hole-doped crystals; only the copper oxide superconductors have higher transition temperatures. The results for C60 raise the intriguing questions of whether conventional electron-phonon coupling alone can produce such high transition temperatures, and whether even higher transition temperatures might be observed in other fullerenes. There have, however, been no confirmed reports of superconductivity in other fullerenes, though it has recently been observed in carbon nanotubes. Here we report the observation of superconductivity in single crystals of electric-field-doped C70. The maximum transition temperature of about 7 K is achieved when the sample is doped to approximately four electrons per C70 molecule, which corresponds to a half-filled conduction band. We anticipate superconductivity in smaller fullerenes at temperatures even higher than in C60 if the right charge density can be induced.

Magnetic carbon

The discovery of nanostructured forms of molecular carbon has led to renewed interest in the varied properties of this element. Both graphite and C60 can be electron-doped by alkali metals to become superconducting; transition temperatures of up to 52 K have been attained by field-induced hole-doping of C60 (ref. 2). Recent experiments and theoretical studies have suggested that electronic instabilities in pure graphite may give rise to superconducting and ferromagnetic properties, even at room temperature. Here we report the serendipitous discovery of strong magnetic signals in rhombohedral C60. Our intention was to search for superconductivity in polymerized C60; however, it appears that our high-pressure, high-temperature polymerization process results in a magnetically ordered state. The material exhibits features typical of ferromagnets: saturation magnetization, large hysteresis and attachment to a magnet at room temperature. The temperature dependences of the saturation and remanent magnetization indicate a Curie temperature near 500 K.

Magnetic ordering in hexagonally bonded sheets with first-row elements

We report first-principles total-energy electronic-structure calculations in the density-functional theory performed for hexagonally bonded honeycomb sheets consisting of B, N, and C atoms. We find that the ground state of BNC sheets with particular stoichiometry is ferromagnetic. Detailed analyses of energy bands and spin densities unequivocally reveal the nature of the ferromagnetic ordering, leading to an argument that the BNC sheet is a manifestation of the flat-band ferromagnetism.

RETRACTED: Field-induced superconductivity in a spin-ladder cuprate

We report on the modulation of the transport properties of thin films, grown by molecular beam epitaxy, of the spin-ladder compound [CaCu2O3]4, using the field effect in a gated structure. At high hole-doping levels, superconductivity is induced in the nominally insulating ladder material without the use of high-pressure or chemical substitution. The observation of superconductivity is in agreement with the theoretical prediction that holes doped into spin ladders could pair and possibly superconduct.

High-temperature superconductivity in lattice-expanded C60

C60 single crystals have been intercalated with CHCl3 and CHBr3 in order to expand the lattice. High densities of electrons and holes have been induced by gate doping in a field-effect transistor geometry. At low temperatures, the material turns superconducting with a maximum transition temperature of 117 K in hole-doped C60/CHBr3. The increasing spacing between the C60 molecules follows the general trend of alkali metal-doped C60 and suggests routes to even higher transition temperatures.