As discovered by Ohtomo and Hwang, a large sheet charge density with high mobility exists at the interface between SrTiO 3 and LaAlO 3 . Based on transport, spectroscopic, and oxygen-annealing experiments, we conclude that extrinsic defects in the form of oxygen vacancies introduced by the pulsed laser deposition process used by all researchers to date to make these samples is the source of the large carrier densities. Annealing experiments show a limiting carrier density. We also present a model that explains the high mobility based on carrier redistribution due to an increased dielectric constant.

The recent discovery of superconductivity in the iron oxypnictide family of compounds has generated intense interest. The layered crystal structure with transition-metal ions in planar square-lattice form and the discovery of spin-density-wave order near 130 K (refs 10, 11) seem to hint at a strong similarity with the copper oxide superconductors. An important current issue is the nature of the ground state of the parent compounds. Two distinct classes of theories, distinguished by the underlying band structure, have been put forward: a local-moment antiferromagnetic ground state in the strong-coupling approach, and an itinerant ground state in the weak-coupling approach. The first approach stresses on-site correlations, proximity to a Mott-insulating state and, thus, the resemblance to the high-transition-temperature copper oxides, whereas the second approach emphasizes the itinerant-electron physics and the interplay between the competing ferromagnetic and antiferromagnetic fluctuations. The debate over the two approaches is partly due to the lack of conclusive experimental information on the electronic structures. Here we report angle-resolved photoemission spectroscopy (ARPES) of LaOFeP (superconducting transition temperature, T(c) = 5.9 K), the first-reported iron-based superconductor. Our results favour the itinerant ground state, albeit with band renormalization. In addition, our data reveal important differences between these and copper-based superconductors.

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We report results of low-temperature thermodynamic and transport measurements of Pb1−xTlxTe single crystals for Tl concentrations up to the solubility limit of approximately x = 1.5%. For all doped samples, we observe a low-temperature resistivity upturn that scales in magnitude with the Tl concentration. The temperature and field dependence of this upturn are consistent with a charge Kondo effect involving degenerate Tl valence states differing by two electrons, with a characteristic Kondo temperature TK ∼ 6 K. The observation of such an effect supports an electronic pairing mechanism for superconductivity in this material and may account for the anomalously high Tc values. The Kondo effect arises from the interaction of conduction electrons with degenerate degrees of freedom in a material and is usually associated with dilute magnetic impurities in a nonmagnetic host. In such cases, the two degenerate states correspond to the impurity spins oriented up or down. Second order scattering processes involving virtual intermediate states lead to the well-known logarithmic increase in resistivity at low temperatures, which saturates in the unitary scattering limit below a characteristic Kondo temperature [1]. However, other systems comprising two degenerate degrees of freedom can also lead to Kondo-like phenomena [2]. In particular, a "charge Kondo effect," corresponding to dilute impurities with two degenerate charge states, has been proposed in the negative-U Anderson model [3], though to date there has not been an experimental realization of such an effect. Significantly, the quantum valence fluctuations implicit in such a model, which involve pairs of electrons that tunnel on and off impurity sites, also provide an electronic pairing mechanism for superconductivity [4,5,6,7].Thallium is one of several elements that is known to skip valences, such that only Tl 1+ and Tl 3+ are observed in ionic compounds, corresponding to electron configurations 6s 2 and 6s 0 respectively. Compounds that one would otherwise expect to contain divalent Tl are found to disproportionate. For example, TlBr 2 is more specifically Tl I Tl III Br 4 , and TlS is likewise Tl I Tl III S 2 [8]. This effect is driven by the stability of a filled shell in conjunction with the polarizability of the material. In this case, Tl 2+ can be characterized by a negative effective U , where U n = (E n+1 − E n ) − (E n − E n−1 ) < 0 and n labels the valence state [9,10,11]. For this reason, valence-skipping elements provide experimental access to negative-U behavior and are therefore suitable candidate impurities for realizing a charge Kondo effect in a bulk material.

SrRuO 3 is an itinerant ferromagnet with T c ∼ 160 K and a 'bad metal' in the limit of k F l = O(1). While the magnetic properties of SrRuO 3 in the paramagnetic phase, near the ferromagnetic phase transition and at low temperatures are normal and consistent with its being a strong itinerant ferromagnet, the transport properties (resistivity and magnetoresistance) sharply deviate from that of good metallic ferromagnets. We conjecture that the distinct transport behaviour of SrRuO 3 is related to its being a 'bad metal' in the k F l = O(1) limit, and discuss the possible relevance of our results to the unusual transport properties of other 'bad metals' such as high-temperature superconductors.