Thin oxide films with perovskite or related structures and with transition metal doping show a reproducible switching in the leakage current with a memory effect. Positive or negative voltage pulses can switch the resistance of the oxide films between a low- and a high-impedance state in times shorter than 100 ns. The ratio between these two states is typically about 20 but can exceed six orders of magnitude. Once a low-impedance state has been achieved it persists without a power connection for months, demonstrating the feasibility of nonvolatile memory elements. Even multiple levels can be addressed to store two bits in such a simple capacitor-like structure.

Materials showing reversible resistive switching are attractive for today’s semiconductor technology with its wide interest in nonvolatile random-access memories. In doped SrTiO3 single crystals, we found a dc-current-induced reversible insulator–conductor transition with resistance changes of up to five orders of magnitude. This conducting state allows extremely reproducible switching between different impedance states by current pulses with a performance required for nonvolatile memories. The results indicate a type of charge-induced bulk electronic change as a prerequisite for the memory effect, scaling down to nanometer-range electrode sizes in thin films.

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We have performed a detailed study of the tunneling spectra of bicrystal grain boundary junctions (GBJs) fabricated from the high temperature superconductors (HTS) YBa2Cu3O 7−δ (YBCO), Bi2Sr2CaCu2O 8+δ (BSCCO), La1.85Sr0.15CuO4 (LSCO) and Nd1.85Ce0.15CuO4−y (NCCO). In all experiments the tunneling direction was along the CuO2 planes. With the exception of NCCO, for all materials a pronounced zero bias conductance peak (ZBCP) was observed which decreases with increasing temperature and disappears at the critical temperature. These results can be explained by the presence of a dominating d-wave symmetry of the order parameter resulting in the formation of zero energy Andreev bound states at surfaces and interfaces of HTS. The absence of a ZBCP for NCCO is consistent with a dominating s-wave symmetry of the pair potential in this material. The observed nonlinear shift of spectral weight to finite energies by applying a magnetic field is in qualitative agreement with recent theoretical predictions.To appear in Physical Review B There is strong evidence that the superconducting order parameter (OP) in the HTS has a dominating d-wave symmetry [1,2]. For this pairing symmetry there is a π-phase shift of the OP in orthogonal k-space directions resulting in a positive and negative sign of the pair potential in those directions. This also means that there are directions with nodes of the pair potential, e. g. for a pure d x 2 −y 2 -symmetry, the nodes are along the [110] direction in the CuO 2 plane. For the tunneling spectra of junctions employing HTS electrode materials with a d-wave symmetry of the OP, a pronounced ZBCP has been predicted originating from mid-gap surface (interface) states or zero energy bound states (ZES) at the Fermi level [3][4][5][6][7][8]. The physical reason for these states originates from the fact that quasiparticles incident and reflecting from the surface propagate through different order parameter fields which leads to Andreev reflection. The constructive interference between incident and Andreev reflected quasiparticles results in bound states. Stable ZES are formed if the scattering induces a change in sign of the OP. For a d x 2 −y 2 -wave symmetry such sign change and, hence, the presence of ZES is possible for all surfaces parallel to the c-axis except for those with the lobe directions perpendicular to the surface, whereas for a s-wave symmetry no ZES are possible. The spectral weight of the ZES for a d x 2 −y 2 -wave symmetry depends on the orientation of the surface with respect to the crystal axis. The maximum spectral weigth is expected for a (110) surface and, hence, a maximum ZBCP is expected for tunneling in the direction of the nodal lines, i. e., the [110] direction. This has been observed recently using low temperature scanning tunneling spectroscopy (LTSTS) [9] and planar type junctions [10]. We note that the ZBCP is sensitive to surface roughness making it difficult to distinguish between the directions in the plane [11][12][13].Initially, the ZBCP in the tunneling spectra...

Intrinsic properties of a compound (e.g. electronic structure, crystallographic structure, optical and magnetic properties) define notably its chemical and physical behavior. In the case of nanomaterials, these fundamental properties depend on the occurrence of quantum mechanical size effects and on the considerable increase of the surface to bulk ratio. However, the literature on this size-dependence and on the involved mechanisms is quite elusive and scarce. Here, we explore the size-dependence of both crystal and electronic properties of CeO2 nanoparticles (NPs) with different sizes by state-of-the art spectroscopic techniques. XRD, XPS and HERFD-XANES demonstrate that the as-synthesized NPs crystallize in the fluorite structure and they are predominantly composed of Ce IV ions. The strong dependence of the lattice parameter with the NPs size was attributed to the presence of adsorbed species at the NPs surface thanks to FTIR and TGA measurements. In addition, the size-dependence of the t2g level in the Ce LIII XANES spectra was experimentally observed by HERFD-XANES and confirmed by theoretical calculations.

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We report on a strain-induced martensitic transformation, accompanied by a suppression of magnetic order in epitaxial films of chemically disordered FeRh. X-ray diffraction, transmission electron microscopy and electronic structure calculations reveal that the lowering of symmetry (from cubic to tetragonal) imposed by the epitaxial relation leads to a further, unexpected, tetragonalto-orthorhombic transition, triggered by a band-Jahn-Teller-type lattice instability. The collapse of magnetic order is a direct consequence of this structural change, which upsets the subtle balance between ferromagnetic nearest-neighbor interactions arising from Fe-Rh hybridization and frustrated antiferromagnetic coupling among localized Fe moments at larger distances.