SrTiO 3. Whereas in oxygen-deficient SrTiO 3−x and Nb-doped SrTiO 3 films the Hall constant increases markedly below 100 K (29), it is less temperature dependent in LaAlO 3 /SrTiO 3 hetero-structures. In addition, the upper critical field of the heterostructures is an order of magnitude smaller than that of Nb-SrTiO 3 with the same T c. Finally, our observation of both superconducting and insulating behavior on the same sample, depending on the precise LaAlO 3 layer thickness, is very hard to reconcile with a pure oxygen vacancy scenario. Magnetic anisotropy allows magnets to maintain their direction of magnetization over time. Using a scanning tunneling microscope to observe spin excitations, we determined the orientation and strength of the anisotropies of individual iron and manganese atoms on a thin layer of copper nitride. The relative intensities of the inelastic tunneling processes are consistent with dipolar interactions, as seen for inelastic neutron scattering. First-principles calculations indicate that the magnetic atoms become incorporated into a polar covalent surface molecular network in the copper nitride. These structures, which provide atom-by-atom accessibility via local probes, have the potential for engineering anisotropies large enough to produce stable magnetization at low temperatures for a single atomic spin.

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The Seebeck coefficient of a metal is expected to display a linear temperature-dependence in the zero-temperature limit. To attain this regime, it is often necessary to cool the system well below 1K. We put under scrutiny the magnitude of this term in different families of strongly-interacting electronic systems. For a wide range of compounds (including heavy-fermion, organic and various oxide families) a remarkable correlation between this term and the electronic specific heat is found. We argue that a dimensionless ratio relating these two signatures of mass renormalisation contains interesting information about the ground state of each system. The absolute value of this ratio remains close to unity in a wide range of strongly-correlated electron systems.

Simultaneous resistivity and ac specific heat measurements have been performed under pressure on singlecrystalline CeCu 2 Si 2 to over 6 GPa in a hydrostatic helium pressure medium. A series of anomalies was observed around the pressure coinciding with a maximum in the superconducting critical temperature, T c max. These anomalies can be linked with an abrupt change of the Ce valence and suggest a second quantum critical point at a pressure P v Ӎ4.5 GPa, where critical valence fluctuations provide the superconducting pairing mechanism, as opposed to spin fluctuations at ambient pressure. Such a valence instability-and associated superconductivity-is predicted by an extended Anderson lattice model with Coulomb repulsion between the conduction and f electrons. We explain the T-linear resistivity found at P v in this picture, while other anomalies found around P v can be qualitatively understood using the same model.

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The superconducting transport properties of the conducting LaAlO3/SrTiO3 interface have been investigated in perpendicular and parallel magnetic fields. A large anisotropy in the transport properties is measured and the two-dimensional nature of the superconducting gas is confirmed. Analyses of the resistance versus temperature and magnetic field, as well as of the correlation length as a function of the magnetic field close to the superconducting critical temperature (about 200 mK), yield an estimate of ∼10 nm for the superconducting layer thickness.