The Ruddlesden-Popper series of iridates (Srn+1IrnO3n+1) have been the subject of much recent attention due to the anticipation of emergent phenomena arising from the cooperative action of spin-orbit-driven band splitting and Coulomb interactions. However, an ongoing debate over the role of correlations in the formation of the charge gap and a lack of understanding of the effects of doping on the low-energy electronic structure have hindered experimental progress in realizing many of the predicted states. Using scanning tunnelling spectroscopy we map out the spatially resolved density of states in Sr3Ir2O7 (Ir327). We show that its parent compound, argued to exist only as a weakly correlated band insulator, in fact possesses a substantial ~ 130 meV charge excitation gap driven by an interplay between structure, spin-orbit coupling and correlations. We find that single-atom defects are associated with a strong electronic inhomogeneity, creating an important distinction between the intrinsic and spatially averaged electronic structure. Combined with first-principles calculations, our measurements reveal how defects at specific atomic sites transfer spectral weight from higher energies to the gap energies, providing a possible route to obtaining metallic electronic states from the parent insulating states in the iridates.

Scanning tunneling spectroscopy measurements were performed on an epitaxial thin film of La 0.7 Ca 0.3 MnO 3 grown on a NdGaO 3 substrate. The temperature variation of the density of states ͑DOS͒ close to the ferromagnetic transition temperature ͑T c ͒ was investigated. The strain-free film exhibiting a sharp metal-insulator transition at T p Ϸ T c Ϸ 268 K shows no phase separation as seen by the conductivity map, allowing unambiguous determination of the tunneling spectra as a function of T. The temperature dependence of the conductance ͑dI / dV͒ curves and the normalized DOS clearly indicate a depletion in DOS near T c , which fills up as the sample is cooled below T c. The metal-insulator transition at T c also shows up in the bias dependence of the tunneling curve as the temperature is changed across the transition. In the metallic phase we find that the DOS is similar to what is expected in a correlated metal.

We have used scanning tunneling microscopy and potentiometry to study the microstructure and nanoscopic current transport in thin films of La 0.7 Ca 0.3 MnO 3 grown on SrTiO 3 and NdGaO 3 substrates. Thin ͑50 nm͒ films, which are strained, show step-terrace growth, whereas relatively thicker film ͑200 nm͒, which is strain relaxed, shows well-connected grains. Charge transport in these films is inhomogeneous on the scale of nanometers. There are large variations in the local potential at grain boundaries as well as step edges. The value of the average field distribution at grain boundaries and step edges is found to be dependent on the strain in the film. Within the terraces local variations in potentials correlate with the nature of the strain in the film and this gives rise to inhomogeneous current flow in them.Electronic transport in rare-earth manganites which show colossal magnetoresistance ͑CMR͒ has been a topic of much current interest. 1,2 In these materials the resistivity () shows a peak at a temperature T p which is close to the ferromagnetic transition temperature T c . For a given chemical composition and oxygen stoichiometry the values of T p , T c , and depend on the physical state of the sample. This is particularly true for thin films. For instance, films of the same CMR material grown epitaxially on different substrates can have very different , T p , and T c and, consequently, magnetoresistance ͑MR͒. It has been proposed that the presence of biaxial strain ⑀ bi stabilizes the Jahn-Teller distortion around the Mn 3ϩ ions and reduces T p and increases . 3 The effect of ⑀ bi is expected to be important in thin films of manganites. Recent experiments have established this. 4 However, strain is not the only factor. In thin films of CMR materials it is likely that the strain and microstructure together determine the nature of the charge transport. While techniques like x-ray diffraction ͑XRD͒, which measure lattice constants averaged over a large length scale and can characterize the strain as ''uniform,'' however, in mesoscopic or submicron length scales the strain can be inhomogeneous. The film will also contain internal boundaries, discontinuities, and steps. It is therfore expected that the electrical conduction in these materials will be inhomogeneous on submicron length scales. As a result an average parameter like ⑀ bi is not enough to describe the current transport in these films. In this paper we address specifically these issues. By using a combination of macroscopic and nanoscopic techniques, we can identify that the inhomogeneities seen in the current path depend on the strain in the film.͑001͒-oriented La 0.7 Ca 0.3 MnO 3 ͑LCMO͒ films were grown using pulsed laser deposition ͑PLD͒ on SrTiO 3 ͑STO͒ and NdGaO 3 ͑NGO͒ substrates. 5 Three types of films were studied: namely, 50-nm-thick film grown on STO ͓LCMO/ STO͑50͔͒, 200-nm-thick film grown on STO ͓LCMO/ STO͑200͔͒ and 50-nm-thick film grown on NGO ͓LCMO/ NGO͑50͔͒. In Table I, we give some of the relevant details of the samples used. We ha...

We report investigation of non-linear electronic transport through artificial grain-boundary junctions made on epitaxial films of La 0.7 Ca 0.3 MnO 3 on bicrystal SrTiO 3 substrates. The experiments carried out over the temperature range 4.2 K-300 K in magnetic field up to 3 T allow us to identify some of the conduction mechanisms that may give rise to nonlinear transport in these grain boundary junctions. The nonlinear transport is associated with multistep inelastic processes in the grain-boundary region, which is moderately affected by the applied magnetic field. However the primary effect of the magnetic field is to enhance the zero-bias conductance ͓G 0 ϭ(dI/dV) Vϭ0 ͔. The dominant voltage dependent contribution to the dynamic conductance (GϭdI/dV) comes from a term of the type V 4/3 at lower temperatures. Other voltage dependent contributions to G, which are of higher order in V, appear only for Tу75 K. In addition we found a contribution to G arising from a V 0.5 term, which is likely to arise from the disordered region around the grain boundary ͑GB͒. The magnetoresistance in the GB depends on the bias used and it decreases at higher bias. The bias dependence is found to be reduced as temperature is increased. We discuss the physical origins of the various contributions to the nonlinear conduction.

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