RESUMO
The interface formed between two wide band-gap insulators, NdGaO3 and SrTiO3 renders metallic behavior, similar to the LaAlO3/SrTiO3 interface. The interface conductivity depends strongly upon oxygen pressure during growth of the NdGaO3 film and subsequent annealing in oxygen. The conductivity of a (10 uc) NdGaO3/SrTiO3 film, pulsed laser deposited at low (pO2 = 10(-4) mbar) oxygen pressure, vanishes after annealing at 600 °C in oxygen atmosphere. For a similar interface formed at high oxygen pressure (pO2 = 0.3 mbar), the metallic conductivity remains also after post annealing. Medium energy ion spectroscopy (MEIS) in random (non-channeling) direction showed that a substantial part of Ga is missing in films deposited at low pressure, while optimal stoichiometry is approached in films deposited at high pressure. Aligned (channeling) MEIS likewise show that the Ga/Nd ratio approaches the stoichiometric value as the pressure is increased from 10(-4) to 0.3 mbar. This is interpreted as due to gallium desorption from a growing film at high temperature and low oxygen pressure while the re-evaporation of gallium is considerably suppressed at higher pressure. We discuss the possible role of stoichiometry on electrical transport properties.
RESUMO
The symmetry of Cooper pairs is central to constructing a superconducting state. The demonstration of a d(x²-y²)-wave order parameter with nodes represented a breakthrough for high critical temperature superconductors (HTSs). However, despite this fundamental discovery, the origin of superconductivity remains elusive, raising the question of whether something is missing from the global picture. Deviations from d(x²-y²)-wave symmetry, such as an imaginary admixture d(x²-y²)+ is (or id(xy)), predict a ground state with unconventional properties exhibiting a full superconducting gap and time reversal symmetry breaking. The existence of such a state, until now highly controversial, can be proved by highly sensitive measurements of the excitation spectrum. Here, we present a spectroscopic technique based on an HTS nanoscale device that allows an unprecedented energy resolution thanks to Coulomb blockade effects, a regime practically inaccessible in these materials previously. We find that the energy required to add an extra electron depends on the parity (odd/even) of the excess electrons on the island and increases with magnetic field. This is inconsistent with a pure d(x²-y²)-wave symmetry and demonstrates a complex order parameter component that needs to be incorporated into any theoretical model of HTS.
RESUMO
We show the influence of pulsed laser deposition fluence on the transport properties of the LaAlO(3)/SrTiO(3) (LAO/STO) heterointerface. Structural characterization by x-ray diffraction and medium energy ion spectrometry enables us to deduce that the electronic behaviour is extremely sensitive to the stoichiometry of the LAO layer as well as the structural quality of the STO surface. An optimum balance of these two quantities is demonstrated for an intermediate laser fluence.
RESUMO
Medium-energy ion spectroscopy (MEIS) has been used to study the depth profile and deduce the distribution of possible cationic substitutions in LaAlO3/SrTiO3 (LAO/STO) heterointerfaces. Analysis of La and Sr peaks in aligned and random MEIS spectra indicates that the surface layers of LAO on an STO substrate are not homogeneous and stoichiometric if the film thickness is less than 4 unit cell layers. This is possibly caused by a redistribution of La and Sr at the interface. Kelvin probe force microscopy reveals an inhomogeneous distribution of the surface potential in a 4 unit cell LAO film, indicating micrometer-sized regions of different compositions. Our findings provide a novel view on the microstructural origin of the electrically conductive interfaces.
RESUMO
The escape rate from the zero voltage state in a superconducting Josephson junction (JJ) is determined by the temperature, but it saturates at low temperature due to macroscopic quantum tunneling (MQT). Complications due to d-wave symmetry in a high temperature superconductor, like low energy quasiparticles and an unconventional current-phase relation, may influence the escape rate. We report, for the first time to our knowledge, the observation of MQT in a YBa(2)Cu(3)O(7-delta) grain boundary biepitaxial JJ. This proves that dissipation can be significantly reduced by a proper junction configuration, which is of significance for quantum coherence.
RESUMO
We have measured intrinsic-tunneling spectra of a single CuO-layer La-doped Bi2(Sr(2-x)Lax)CuO(6+delta) (Bi2201-Lax). Despite a difference of a factor of 3 in the optimal superconducting critical temperatures for Bi2201-La0.4 and Bi2212 (32 and 95 K, respectively) and different spectral energy scales, we find that the pseudogap vanishes at a similar characteristic temperature T* approximately 230-300 K for both compounds. We find also that, in Bi2201-Lax, pseudogap humps are seen as sharp peaks and, in fact, even dominate the intrinsic spectra.
RESUMO
We study single-electron transport in quench-condensed bismuth films. By lateral confinement, we select a specific cluster of about 10(3) atoms and use tunneling barriers that appear naturally during thin-film formation. A remarkable reversible increase of the sample conductance up to 5 times was found as the temperature was lowered from 11 to 4 K. We attribute this effect to a spontaneous distortion of the cluster shape and discuss its relation to a phase transition predicted for free metallic clusters.
RESUMO
We present intrinsic tunneling spectroscopy measurements on small Bi2Sr2CaCu2O8+x mesas. The tunnel conductance curves show both sharp peaks at the superconducting gap voltage and broad humps representing the c-axis pseudogap. The superconducting gap vanishes at Tc, while the pseudogap exists both above and below Tc. Our observation implies that the superconducting and pseudogaps represent different coexisting phenomena.
