Marcelo Rozenberg

Resistance Switching and Formation of a Conductive Bridge in Metal/Binary Oxide/Metal Structure for Memory Devices

The resistance switching mechanism of a metal/CuO/metal sandwich with a planar device structure has been studied. We report the direct observation of a conducting bridge within the CuO channel of the device, which is formed upon the initial voltage application (forming process). It is found that the resistance switching phenomenon only occurs when just a single bridge is formed during a soft dielectric breakdown. We argue that the reduction and oxidation of this conducting bridge by the action of an applied field and/or current gives rise to a novel nonvolatile memory effect.

Universal Electric-Field-Driven Resistive Transition in Narrow-Gap Mott Insulators

A striking universality in the electric-field-driven resistive switching is shown in three prototypical narrow-gap Mott systems. This model, based on key theoretical features of the Mott phenomenon, reproduces the general behavior of this resistive switching and demonstrates that it can be associated with a dynamically directed avalanche. This model predicts non-trivial accumulation and relaxation times that are verified experimentally.

Orientational Tuning of the Fermi Sea of Confined Electrons at theSrTiO3(110) and (111) Surfaces

We report the existence of confined electronic states at the (110) and (111) surfaces of SrTiO3. Using angle-resolved photoemission spectroscopy, we find that the corresponding Fermi surfaces, subband masses, and orbital ordering are different from the ones at the (001) surface of SrTiO3. This occurs because the crystallographic symmetries of the surface and sub-surface planes, and the electron effective masses along the confinement direction, influence the symmetry of the electronic structure and the orbital ordering of the t2g manifold. Remarkably, our analysis of the data also reveals that the carrier concentration and thickness are similar for all three surface orientations, despite their different polarities. The orientational tuning of the microscopic properties of two-dimensional electron states at the surface of SrTiO3 echoes the tailoring of macroscopic (e.g. transport) properties reported recently in LaAlO3/SrTiO3 (110) and (111) interfaces, and is promising for searching new types of 2D electronic states in correlated-electron oxides.

Engineering two-dimensional electron gases at the (001) and (101) surfaces ofTiO2anatase using light

We report the existence of metallic two-dimensional electron gases (2DEGs) at the (001) and (101) surfaces of bulk-insulating TiO2 anatase due to local chemical doping by oxygen vacancies in the near-surface region. Using angle-resolved photoemission spectroscopy, we find that the electronic structure at both surfaces is composed of two occupied subbands of d_xy orbital character. While the Fermi surface observed at the (001) termination is isotropic, the 2DEG at the (101) termination is anisotropic and shows a charge carrier density three times larger than at the (001) surface. Moreover, we demonstrate that intense UV synchrotron radiation can alter the electronic structure and stoichiometry of the surface up to the complete disappearance of the 2DEG. These results open a route for the nano-engineering of confined electronic states, the control of their metallic or insulating nature, and the tailoring of their microscopic symmetry, using UV illumination at different surfaces of anatase.

Two resistive switching regimes in thin film manganite memory devices on silicon

Bipolar resistive switching in low cost n-Si/La2/3Ca1/3MnO3/M (M = Ti + Cu) devices was investigated. For low SET compliance currents (CC), an interfacial-related resistive switching mechanism, associated to the migration of oxygen vacancies close to the manganite/metal interface, is operative. Simulations using the voltage enhanced oxygen vacancies drift model validate our experimental results. When further increasing the CC, we have observed the onset of a second, filamentary, resistive switching regime with a concomitant collapse of the ON/OFF ratio. We finally demonstrate that it is possible to delay the onset of the filamentary regime by controlling the film thickness.

Two-dimensional electron gas with six-fold symmetry at the (111) surface of KTaO3

Two-dimensional electron gases (2DEGs) at transition-metal oxide (TMO) interfaces, and boundary states in topological insulators, are being intensively investigated. The former system harbors superconductivity, large magneto-resistance, and ferromagnetism. In the latter, honeycomb-lattice geometry plus bulk spin-orbit interactions lead to topologically protected spin-polarized bands. 2DEGs in TMOs with a honeycomb-like structure could yield new states of matter, but they had not been experimentally realized, yet. We successfully created a 2DEG at the (111) surface of KTaO3, a strong insulator with large spin-orbit coupling. Its confined states form a network of weakly-dispersing electronic gutters with 6-fold symmetry, a topology novel to all known oxide-based 2DEGs. If those pertain to just one Ta-(111) bilayer, model calculations predict that it can be a topological metal. Our findings demonstrate that completely new electronic states, with symmetries not realized in the bulk, can be tailored in oxide surfaces, promising for TMO-based devices.

Avalanche breakdown in GaTa4Se8−xTex narrow-gap Mott insulators

Mott transitions induced by strong electric fields are receiving growing interest. Recent theoretical proposals have focused on the Zener dielectric breakdown in Mott insulators. However, experimental studies are still too scarce to conclude about the mechanism. Here we report a study of the dielectric breakdown in the narrow-gap Mott insulators GaTa4Se(8-x)Te(x). We find that the I-V characteristics and the magnitude of the threshold electric field (Eth) do not correspond to a Zener breakdown, but rather to an avalanche breakdown. Eth increases as a power law of the Mott-Hubbard gap (Eg), in surprising agreement with the universal law Eth is proportional to Eg(2.5) reported for avalanche breakdown in semiconductors. However, the delay time for the avalanche that we observe in Mott insulators is over three orders of magnitude greater than in conventional semiconductors. Our results suggest that the electric field induces local insulator-to-metal Mott transitions that create conductive domains that grow to form filamentary paths across the sample.Mott transitions induced by strong electric fields are receiving a growing interest. Recent theoretical proposals have focused on the Zener dielectric breakdown in Mott insulators, however experimental studies are still too scarce to conclude about the mechanism. Here we report a study of the dielectric breakdown in the narrow gap Mott insulators GaTa

Memristive and neuromorphic behavior in a Li x CoO 2 nanobattery

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Tailoring conductive filaments by electroforming polarity in memristive based TiO2 junctions

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Electrode-Geometry Control of the Formation of a Conductive Bridge in Oxide Resistance Switching Devices

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