Metal-insulator transition and electrically-driven memristive characteristics of SmNiO3 thin films

The correlated oxide SmNiO3 (SNO) exhibits an insulator to metal transition (MIT) at 130 {\deg}C in bulk form. We report on synthesis and electron transport in SNO films deposited on LaAlO3 (LAO) and Si single crystals. X-ray diffraction studies show that compressively strained single-phase SNO grows epitaxially on LAO while on Si, mixed oxide phases are observed. MIT is observed in resistance-temperature measurements in films grown on both substrates, with charge transport in-plane for LAO/SNO films and out-of-plane for Si/SNO films. Electrically-driven memristive behavior is realized in LAO/SNO films, suggesting that SNO may be relevant for neuromorphic devices

Conductivity noise study of the insulator-metal transition and phase co-existence in epitaxial samarium nickelate thin films

Interaction between the lattice and the orbital degrees of freedom not only makes rare-earth nickelates unusually "bad metal", but also introduces a temperature driven insulator-metal phase transition. Here we investigate this insulator-metal phase transition in thin films of $\mathrm{SmNiO_3}$ using the slow time dependent fluctuations (noise) in resistivity. The normalized magnitude of noise is found to be extremely large, being nearly eight orders of magnitude higher than thin films of common disordered metallic systems, and indicates electrical conduction via classical percolation in a spatially inhomogeneous medium. The higher order statistics of the fluctuations indicate a strong non-Gaussian component of noise close to the transition, attributing the inhomogeneity to co-existence of the metallic and insulating phases. Our experiment offers a new insight on the impact of lattice-orbital coupling on the microscopic mechanism of electron transport in the rare-earth nickelates.Comment: 5 pages, 4 figure

Universal logic with encoded spin qubits in silicon

Qubits encoded in a decoherence-free subsystem and realized in exchange-coupled silicon quantum dots are promising candidates for fault-tolerant quantum computing. Benefits of this approach include excellent coherence, low control crosstalk, and configurable insensitivity to certain error sources. Key difficulties are that encoded entangling gates require a large number of control pulses and high-yielding quantum dot arrays. Here we show a device made using the single-layer etch-defined gate electrode architecture that achieves both the required functional yield needed for full control and the coherence necessary for thousands of calibrated exchange pulses to be applied. We measure an average two-qubit Clifford fidelity of $97.1 \pm 0.2\%$ with randomized benchmarking. We also use interleaved randomized benchmarking to demonstrate the controlled-NOT gate with $96.3 \pm 0.7\%$ fidelity, SWAP with $99.3 \pm 0.5\%$ fidelity, and a specialized entangling gate that limits spreading of leakage with $93.8 \pm 0.7\%$ fidelity

Origins of bad-metal conductivity and the insulator–metal transition in the rare-earth nickelates

For most metals, increasing temperature (T) or disorder hastens electron scattering. The electronic conductivity (σ) decreases as T rises because electrons are more rapidly scattered by lattice vibrations. The value of σ decreases as disorder increases because electrons are more rapidly scattered by imperfections in the material. This is the scattering rate hypothesis, which has guided our understanding of metal conductivity for over a century. However, for so-called bad metals with very low σ this hypothesis predicts scattering rates so high as to conflict with Heisenberga's uncertainty principle. Bad-metal conductivity has remained a puzzle since its initial discovery in the 1980s in high-temperature superconductors. Here we introduce the rare-earth nickelates (RNiO₃, R = rare-earth) as a class of bad metals. We study SmNiO₃ thin films using infrared spectroscopy while varying T and disorder. We show that the interaction between lattice distortions and Ni-O covalence explains bad-metal conductivity and the insulator-metal transition. This interaction shifts spectral weight over the large energy scale established by the Ni-O orbital interaction, thus enabling very low σ without violating the uncertainty principle.United States. Army Research Office (Grant W911-NF-09-1-0398)National Science Foundation (U.S.) (Grant DMR-0952794)National Science Foundation (U.S.) (Grant DMR-1206519

Commensurate growth and diminishing substrate influence in a multilayer film of a tris(thieno)hexaazatriphenylene derivative on Au(111) studied by scanning tunneling microscopy

© 2008 The American Physical SocietyThe electronic version of this article is the complete one and can be found online at: http://link.aps.org/doi/10.1103/PhysRevB.77.085433DOI: 10.1103/PhysRevB.77.085433Layer-by-layer growth of the electron-transport material tris 2,5-bis 3,5-bis-trifluoromethyl-phenyl -thieno 3,4-b,h,n -1,4,5,8,9,12-hexaazatriphenylene THAP on Au 111 is probed by scanning tunneling microscopy STM . A relative of discotic liquid crystalline molecules, THAP is shown to grow in commensurate ordered planes from the first to fourth monolayers. The four monolayers all show a concordant ordered structure in which the molecules arrange parallel to the substrate in a hexagonal close-packed lattice with a herringbone pattern defined by alternating rows of molecules with antiparallel orientation. The unit cell is rectangular with two molecules per cell and is nearly equivalent for each layer. The spatial broadening of the local density of states due to the metallic substrate is appreciably diminished in upper layers, as expected and as evidenced by the localization of states seen in STM. There is good agreement between the highest occupied molecular orbital obtained in density functional theory calculations for a single molecule and STM images of the upper layers, in accord with the localized nature of electronic states on molecules under minimal substrate influence