B. Batlogg

Superconductivity near 70 K in a new family of layered copper oxides

A new family of high-temperature superconductors is described, with the general formula Pb2Sr2ACu3O8+δ. Although they have the planes of CuO5 square pyramids characteristic of the other copper-oxide superconductors, the new compounds belong to a distinct structural series, with wide scope for elemental substitution. Their unusual electronic configuration also gives new insight into the role of charge distribution among the structural building blocks in controlling superconductivity.

ELECTRONIC STATES IN LA2-XSRXCUO4+GAMMA PROBED BY SOFT-X-RAY ABSORPTION

Oxygen {ital K}-edge absorption spectra of carefully characterized La{sub 2{minus}{ital x}}Sr{sub {ital x}}CuO{sub 4+{delta}} samples were measured using a bulk-sensitive fluoresence-yield-detection method. They reveal two distinct pre-edge peaks which evolve systematically as a function of Sr concentration. The measured spectra are quantitatively described by calculations based on the Hubbard model, including local Coulomb interactions and core-hole excitonic correlations. The absorption data are consistent with a description of electronic states based on a doped charge-transfer insulator.

Superconductivity in molecular crystals induced by charge injection

Progress in the field of superconductivity is often linked to the discovery of new classes of materials, with the layered copper oxides being a particularly impressive example. The superconductors known today include a wide spectrum of materials, ranging in complexity from simple elemental metals, to alloys and binary compounds of metals, to multi-component compounds of metals and chalcogens or metalloids, doped fullerenes and organic charge-transfer salts. Here we present a new class of superconductors: insulating organic molecular crystals that are made metallic through charge injection. The first examples are pentacene, tetracene and anthracene, the last having the highest transition temperature, at 4 K. We anticipate that many other organic molecular crystals can also be made superconducting by this method, which will lead to surprising findings in the vast composition space of molecular crystals.

Gate-induced superconductivity in a solution-processed organic polymer film

The electrical and optical properties of conjugated polymers have received considerable attention in the context of potentially low-cost replacements for conventional metals and inorganic semiconductors. Charge transport in these organic materials has been characterized in both the doped-metallic and the semiconducting state, but superconductivity has not hitherto been observed in these polymers. Here we report a distinct metal–insulator transition and metallic levels of conductivity in a polymer field-effect transistor. The active material is solution-cast regioregular poly(3-hexylthiophene), which forms relatively well ordered films owing to self-organization, and which yields a high charge carrier mobility (0.05–0.1 cm2 V-1 s-1) at room temperature. At temperatures below ∼2.35 K with sheet carrier densities exceeding 2.5 × 1014 cm-2, the polythiophene film becomes superconducting. The appearance of superconductivity seems to be closely related to the self-assembly properties of the polymer, as the introduction of additional disorder is found to suppress superconductivity. Our findings therefore demonstrate the feasibility of tuning the electrical properties of conjugated polymers over the largest range possible—from insulating to superconducting.

Superconductivity at 52 K in hole-doped C 60

Superconductivity in electron-doped C60 was first observed almost ten years ago. The metallic state and superconductivity result from the transfer of electrons from alkaline or alkaline-earth ions to the C60 molecule, which is known to be a strong electron acceptor. For this reason, it is very difficult to remove electrons from C60—yet one might expect to see superconductivity at higher temperatures in hole-doped than in electron-doped C60, because of the higher density of electronic states in the valence band than in the conduction band. We have used the technique of gate-induced doping in a field-effect transistor configuration to introduce significant densities of holes into C60. We observe superconductivity over an extended range of hole density, with a smoothly varying transition temperature Tc that peaks at 52 K. By comparison with the well established dependence of Tc on the lattice parameter in electron-doped C60, we anticipate that Tc values significantly in excess of 100 K should be achievable in a suitably expanded, hole-doped C60 lattice.

Trapping in organic field-effect transistors

Current–voltage characteristics of single- and polycrystalline organic field-effect transistors are analyzed. The effect of bulk, interface, and grain boundary traps is investigated. The frequently observed dependence of the field-effect mobility on the gate voltage is ascribed to trapping processes rather than to an intrinsic charge transport mechanism in these organic semiconductors. Furthermore, the thermally activated mobility in polycrystalline devices, frequently observed, is ascribed to the formation of a potential barrier at the grain boundaries of the polycrystalline semiconductor. The barrier height depends significantly on the trap density and the position of the Fermi energy and therefore on the gate voltage.

Modeling of the temperature dependence of the field-effect mobility in thin film devices of conjugated oligomers

A simple model is proposed for charge carrier transport across grain boundaries with an acceptor-like trap level. Potential wells between the grains are formed due to negatively charged grain boundaries. Based on this model, a variety of temperature dependencies of the charge carrier mobility can be described. Using realistic parameters, this model reproduces very well the measured temperature dependencies of the field-effect mobility in polycrystalline pentacene and oligothiophene thin film devices. Therefore, it seems to be difficult to investigate the intrinsic material properties of organic semiconductors using only polycrystalline field-effect devices, since they may be masked by the effects of traps and grain boundaries.

Perylene: A promising organic field-effect transistor material

Field-effect transistors based on single crystalline perylene have been prepared and analyzed in the temperature range from 50 to 300 K. Room temperature electron mobilities as high as 5.5 cm2/V s have been achieved. In addition, ambipolar device operation, i.e., n- and p-channel activity, is observed. The temperature dependence of the electron and hole mobilities is discussed in the limits of hopping and band-like transport mechanisms.

Enhancing the low field magnetoresistive response in perovskite manganites

We report significant improvement in the magnetoresistive (MR) response of perovskite manganites for small applied fields by the use of heterostructures with soft ferromagnets. At room temperature, a 5900‐fold enhancement of the MR response at 10 Oe in La2/3Ca1/3MnO3 has been achieved using (Mn,Zn)Fe2O4 as the soft ferromagnet. By utilizing soft ferromagnets with various magnetic properties, this technique allows for enormous flexibility in ‘‘designing’’ a desired MR response for the manganites, particularly in low fields.

Cuprate superconductors: Science beyond high Tc

The development of the field of high temperature superconductivity is presented as a new chapter in modern condensed matter science. Beyond the early quest to find an answer to the most obvious questions about the microscopic mechanism that leads to pairing at unprecedentedly high temperatures, we have now come to recognize that the occurrence of superconductivity in cuprates requires our revisiting basic tenets.