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 quasi-two-dimensional electron gas found at the LaAlO{3}/SrTiO{3} interface offers exciting new functionalities, such as tunable superconductivity, and has been proposed as a new nanoelectronics fabrication platform. Here we lay out a new example of an electronic property arising from the interfacial breaking of inversion symmetry, namely, a large Rashba spin-orbit interaction, whose magnitude can be modulated by the application of an external electric field. By means of magnetotransport experiments we explore the evolution of the spin-orbit coupling across the phase diagram of the system. We uncover a steep rise in Rashba interaction occurring around the doping level where a quantum critical point separates the insulating and superconducting ground states of the system.

Complex transition metal oxides span a wide range of crystalline structures and play host to an incredible variety of physical phenomena. High dielectric permittivities, piezo-, pyro-, and ferroelectricity are just a few of the functionalities offered by this class of materials, while the potential for applications of the more exotic properties like high temperature superconductivity and colossal magnetoresistance is still waiting to be fully exploited. With recent advances in deposition techniques, the structural quality of oxide heterostructures now rivals that of the best conventional semiconductors, taking oxide electronics to a new level. Such heterostructures have enabled the fabrication of artificial multifunctional materials. At the same time they have exposed a wealth of phenomena at the boundaries where compounds with different structural instabilities and electronic properties meet, giving unprecedented access to new physics emerging at oxide interfaces. Here we highlight some of these exciting new interface phenomena.

In materials with strong local Coulomb interactions, simple defects such as atomic substitutions strongly affect both macroscopic and local properties of the system. A nonmagnetic impurity, for instance, is seen to induce magnetism nearby. Even without disorder, models of such correlated systems are generally not soluble in 2 or 3 dimensions, and so few exact results are known for the properties of such impurities. Nevertheless, some simple physical ideas have emerged from experiments and approximate theories. Here, we first review what we can learn about this problem from 1D antiferromagnetically correlated systems. We then discuss experiments on the high Tc cuprate normal state which probe the effect of impurities on local charge and spin degrees of freedom, and compare with theories of single impurities in correlated hosts, as well as phenomenological effective Kondo descriptions. Subsequently, we review theories of impurities in d-wave superconductors including residual quasiparticle interactions, and compare with experiments in the superconducting state. We argue that existing data exhibit a remarkable similarity to impurity-induced magnetism in the 1D case, implying the importance of electronic correlations for the understanding of these phenomena, and suggesting that impurities may provide excellent probes of the still poorly understood ground state of the cuprates.Comment: 66 pages, 48 figures, review articl

The phase diagram of the infinite-range model of spin-glasses exhibits two mixed phases. In these mixed phases, ferromagnetism and spin-glass order coexist, due to freezing of the transverse degrees of freedom or replica symmetry breaking. This may help to interpret a number of recent experimental findings, e.g. , in AuFe.PACS numbers: 75.50.Kj, 05.50.+q Many disordered magnetic materials have been systematically investigated as a function of the concentration of the constituents. In the phase diagrams of these materials, one often finds a spin-glass phase for some range of concentration and a phase with long-range order (ferromagnetic or antiferromagnetic) for other concentrations. ' " Recently, a number of experiments have been performed to study the border region between the spin-glass and ferromagnetic phases, and many interesting data are now available. Unfortunately these data are most often interpreted, qualitatively'"'" and even quantitatively, "'~' with the help of a theory" which, though of great merit in its days, is now obsolete (it has been proven to be incorrect, " at low temperatures, as a solution of its model). Moreover, these theoretical predictions are inadequate because they are concerned with Ising spins, whereas experimental spine are Heisenberg-like (isotropic vector spins). The purpose of this Letter is to draw attention to recent studies, "" which should provide a more satisfactory theoretical basis, and to present new results concerning the nature of two mixed phases, with both spin-glass and ferromagnetic characters. The existence of such intermediate phases may help explain a number of phenomena already observed experimentally, but hitherto ambiguously interpreted.In the spirit of a mean-field theory, we consider the famous infinite-range model" for N classical vector spins S,. Each S; has m components S;" (p. =1, . .. , m) satisfying, for convenience, the following normalization condition: They interact via independent random interactions J;, . distributed according to the following law: P(Z ) = -~e xp --J 2wj 2 (2) so that (J, , ), = J,/N and (J, .~) , =1/N, where ( ),denotes an average over the bond disorder, that is, over P(J,~). In the presence of an external magnetic field H applied along the p, = 1 direction, the Hamiltonian of the model reads where the sum over (ij ) denotes a summation over the N(N -1)/2 distinct pairs of sites.In the limit of large and positive J" the interactions are mainly ferromagnetic and the system exhibits a ferromagnetic phase. For J0=0, the interactions are random in sign and the ordering is of spin-glass type. The border region between ferromagnetic and spin-glass phases occurs for Jo =1. However, it has been shown" how the properties of the model for J, g0 can be simply derived from the case J, = 0, which we therefore first consider.

We report on a study of magnetotransport in LaAlO3/SrTiO3 interfaces characterized by mobilities of the order of several thousands cm 2 /Vs. We observe Shubnikov-de Haas oscillations that indicate a two-dimensional character of the Fermi surface. The frequency of the oscillations signals a multiple sub-bands occupation in the quantum well or a multiple valley configuration. From the temperature dependence of the oscillation amplitude we extract an effective carrier mass m * 1.45 me.An electric field applied in the back-gate geometry increases the mobility, the carrier density and the oscillation frequency.Several experimental and theoretical studies have uncovered a number of remarkable electronic properties of the interface between the complex oxides LaAlO 3 and SrTiO 3 [1][2][3][4][5]. One of the main unresolved issues pertains to the dimensionality of the conducting layer [6]. While it is now clear that, using appropriate growth and annealing conditions, a confined metallic and superconducting electron gas can be formed at such interfaces [7,8], no conclusive demonstration of two-dimensional character in the normal state has been obtained so far.X-Ray absorption spectroscopy experiments [9], as well as density functional theory calculations [10], indicate that an orbital reconstruction occurs at this interface. These studies reveal that, even at room temperature, the degeneracy of the Ti 3d t 2g levels is lifted and the first available states for the conducting electrons are generated from 3d xy orbitals, which give rise to strongly twodimensional bands and present a negligible inter-plane coupling. Moreover, the momentum quantization in the quantum well brings about a sub-bands fine structure which was calculated [10,11] but not yet confirmed experimentally. The presence of a very strong spin-orbit coupling adds further complexity to the low-energy electronic structure [12,13].The Fermi surface of two-dimensional electronic states generates clear experimental signatures in the Shubnikovde Haas (SdH) effect: for a two-dimensional electron gas (2DEG) the quantum oscillations depend only on the perpendicular component of the magnetic field. Previous studies reported quantum oscillations in LaAlO 3 /SrTiO 3−δ heterostructures characterized by a large carrier density (of the order of 10 16 cm −2 ) delocalised in the SrTiO 3 substrate [1,14]. The lack of dependence of the oscillations on the field orientation points to a three-dimensional Fermi surface consistently with previous studies in Nb-doped SrTiO 3 single crystals [15]. Very recently, quantum oscillations with two-dimensional character have been reported for Nb-doped thin films of SrTiO 3 [16]. However, in LaAlO 3 /SrTiO 3 heterostructures where the electrons are confined at the interface [6], magnetotransport studies have been carried out so far in the diffusive regime, where the scattering time is not sufficiently long to give rise to well defined Landau levels [12,[17][18][19].The requirements for observing quantum conductance oscillations are [20] ω c ...

Transport in ultrathin films of LaNiO3 evolves from a metallic to a strongly localized character as the film's thickness is reduced and the sheet resistance reaches a value close to h/e 2 , the quantum of resistance in two dimensions. In the intermediate regime, quantum corrections to the Drude lowtemperature conductivity are observed; they are accurately described by weak localization theory. Remarkably, the negative magnetoresistance in this regime is isotropic, which points to magnetic scattering associated with the proximity of the system to either a spin glass state or the charge ordered antiferromagnetic state observed in other rare earth nickelates.Complex oxides are exciting materials in which the interplay between charge, spin, orbital and lattice degrees of freedom leads to a wealth of novel and exotic phenomena. Among these materials, LaNiO 3 (LNO), a metal and paramagnet lacking any ordering phenomena in bulk [1][2][3], has recently become the subject of intense research. This was mainly triggered by theoretical work [4] which suggested the possibility of orbital ordering and high-T c superconductivity in superlattices where very thin LNO layers are separated by insulating layers, confining the conduction to two dimensions (2D). These predictions have yet to be confirmed experimentally. LNO bulk and thin films have been extensively studied in the past, showing a large effective mass and enhanced Pauli paramagnetism, which has been attributed to strong correlations [1,2,5]. This material has also been reported to be sensitive to the degree of disorder. For instance, increasing the amount of oxygen vacancies can lead to weak localization and metal-insulator (MI) transitions, as well as antiferromagnetic ordering and spin glass behaviour [6][7][8]. LNO based heterostructures have been grown and transport measurements have revealed a MI transition as the LNO thickness is reduced to only a few unit cells (u.c.) [9,10]. This behaviour is in agreement with the MI transition observed in single ultrathin LNO films [11]. Understanding the nature of transport for this compound in the ultrathin limit appears essential for future investigation of LNO-based heterostructures and serves as the motivation for this work. Upon reduction of the LNO thickness, transport evolves from a metallic to a strongly localized behavior as the room temperature sheet resistance approaches h/e 2 ∼ 25 kΩ, the quantum of resistance in 2D. We find a transition region between the metallic and the strongly localized states where the films show an upturn in the resistivity at low temperatures which can be explained by the theory of weak localization. Surprisingly, the magnetoresistance (MR) is found to be isotropic in this regime. We attribute this to the presence of magnetic scattering and discuss its possible origins. Films were grown on (001) SrTiO 3 substrates by off-axis rf magnetron sputtering, leading to fully strained epi-