C. Acha

Mechanism for bipolar resistive switching in transition metal oxides

We introduce a model that accounts for the bipolar resistive switching phenomenon observed in transition-metal oxides. It qualitatively describes the electric-field-enhanced migration of oxygen vacancies at the nanoscale. The numerical study of the model predicts that strong electric fields develop in the highly resistive dielectric-electrode interfaces leading to spatially inhomogeneous oxygen vacancies distribution and a concomitant resistive switching effect. The theoretical results qualitatively reproduce nontrivial resistance hysteresis experiments that we also report providing key validation to our model.

Enhancement of Tc of CyCu1−yBa2Ca2Cu3Ox from 67 K to 120 K by reduction treatments

Abstract The T c of the C y Cu 1− y Ba 2 Ca 2 Cu 3 O 8+δ ( y ≈ 0.5) phase, synthesized under high pressure, has been raised from 67 K to about 120 K by heat-treatments under reducing conditions. An increase of both a and c lattice parameters occurs concomitantly with that of T c (before: a = 3.858 (1), c = 14.766 (9) A , after: a ≈ 3.862(1), c ≈ 14.825 (3) A ). Subsequent heat-treatments under oxygen atmosphere lead, reversibly, to a decrease of T c and of the lattice parameters. Consequently, the increase of T c has been associated with an oxygen loss. Measurements of the resistivity under high pressure indicate that after reduction the overdoped as-prepared samples go towards the optimal doping. A structural model is proposed about the position of the mobile oxygen in the basal plane of the structure.

Electrical resistivity of the Ti

Abstract.We have measured resistivity as a function of temperature and pressure of Ti4O7 twinned crystals using different contact configurations. Pressures over 4 kbar depress the localization of bipolarons and allow the study of the electrical conduction of the bipolaronic phase down to low temperatures. For pressures P > 40 kbar the bipolaron formation transition is suppressed and a nearly pressure independent behavior is obtained for the resistivity. We observed an anisotropic conduction. When current is injected parallel to the principal axis, a metallic conduction with interacting carrier effects is predominant. A superconducting state was not obtained down to 1.2 K, although evidences of the proximity of a quantum critical point were noticed. While when current is injected non-parallel to the crystal’s principal axis, we obtained a logarithmic divergence of the resistivity at low temperatures. For this case, our results for the high pressure regime can be interpreted in the framework of interacting carriers (polarons or bipolarons) scattered by Two Level Systems.

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We make use of a BCS-type approach based on the extended Hubbard Hamiltonian to study the superconductor transition and to give a microscopic interpretation of the pressure effects on Tc in high-temperature superconductors. This method suggests that the applied pressure causes an increase of the superconducting gap and this effect is explored in order to explain the variations of Tc . Our approach is therefore beyond the scope of previous phenomenological models which basically postulate a pressure-induced charge transfer and an intrinsic term linear in pressure. We obtain a microscopic interpretation of this intrinsic term and a general expansion of Tc in terms of the pressure. To demonstrate the efficiency of the method we apply it to the experimental data of the Hg-based superconductors. @S0163-1829~97!07925-3#

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We have measured the pressure sensitivity of Tc in fluorinated HgBa2Ca2Cu3O8 + δ (Hg-1223) ceramic samples with different F contents, applying pressures up to 30 GPa. We obtained that Tc increases with increasing pressure, reaching different maximum values, depending on the F doping level, and decreases for a further increase of pressure. A new high Tc record (166 K±1 K) was achieved by applying pressure (23 GPa) in a fluorinated Hg-1223 sample near the optimum doping level. Our results show that all our samples are at the optimal doping, and that fluorine incorporation decreases the crystallographic a-parameter concomitantly increasing the maximum attainable Tc. This effect reveals that the compression of the a-axis is one of the keys that controls the Tc of high-temperature superconductors.

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The nature of the Mott transition in the absence of any symmetry breaking remains a matter of debate. We study the correlation-driven insulator-to-metal transition in the prototypical 3D Mott system GaTa(4)Se(8), as a function of temperature and applied pressure. We report novel experiments on single crystals, which demonstrate that the transition is of first order and follows from the coexistence of two states, one insulating and one metallic, that we toggle with a small bias current. We provide support for our findings by contrasting the experimental data with calculations that combine local density approximation with dynamical mean-field theory, which are in very good agreement.

Magneli phase under high pressure

We report on DC and pulsed electric field sensitivity of the resistance of mixed valent Mn oxide based La(5/8-y)Pr(y)Ca(3/8)MnO(3) (y = 0.4) single crystals as a function of temperature. The low temperature regime of the resistivity is highly current and voltage dependent. An irreversible transition from high (HR) to a low resistivity (LR) is obtained upon the increase of the electric field up to a temperature dependent critical value (V_c). The current-voltage characteristics in the LR regime as well as the lack of a variation in the magnetization response when V_c is reached indicate the formation of a non-single connected filamentary conducting path. The temperature dependence of V_c indicates the existence of a consolute point where the conducting and insulating phases produce a critical behavior as a consequence of their separation.

Extended Hubbard model applied to study the pressure effects in high-temperature superconductors

Current-voltage (IV) characteristics and the temperature dependence of the contact resistance [R(T)] of Au/YBa2Cu3O7−δ (optimally doped YBCO) interfaces have been studied at different resistance states. These states were produced by resistive switching after accumulating cyclic electrical pulses of increasing number and voltage amplitude. The IV characteristics and the R(T) dependence of the different states are consistent with a Poole-Frenkel (P-F) emission mechanism with trapping-energy levels Et in the 0.06–0.11 eV range. Et remains constant up to a number-of-pulses-dependent critical voltage and increases linearly with a further increase in the voltage amplitude of the pulses. The observation of a P-F mechanism reveals the existence of an oxygen-depleted layer of YBCO near the interface. A simple electrical transport scenario is discussed, where the degree of disorder, the trap energy level, and the temperature range determine an electrical conduction dominated by non-linear effects, either in a P-F e...

High-pressure effects in fluorinated HgBa2Ca2Cu3O8 + δ

We report on the growth and characterization of Ti/La1/3Ca3/2MnO3/SiO2/n-Si memristive devices. We demonstrate that using current as electrical stimulus unveils an intermediate resistance state, in addition to the usual high and low resistance states that are observed in the standard voltage controlled experiments. Based on thorough electrical characterization (impedance spectroscopy, current-voltage curves analysis), we disclose the contribution of three different microscopic regions of the device to the transport properties: an ohmic incomplete metallic filament, a thin manganite layer below the filament tip exhibiting Poole-Frenkel like conduction, and the SiOx layer with an electrical response well characterized by a Child-Langmuir law. Our results suggest that the existence of the SiOx layer plays a key role in the stabilization of the intermediate resistance level, indicating that the combination of two or more active resistive switching oxides adds functionalities in relation to the single-oxide de...

First-order insulator-to-metal Mott transition in the paramagnetic 3D system GaTa4Se8.

Abstract We have measured the resistance as a function of temperature under pressure up to 22GPa for samples of the bi-Hg-layer compound, Hg 2 Ba 2 ( Y 1− x Ca x ) Cu 2 O 8− d . We report here the largest total pressure effect ever observed in the superconducting critical temperature (> 50K over a span of 20GPa, independently of the Ca doping). We analyze our results on the basis of a phenomenological model founded on the Tc dependence with carrier density, generalized to include pressure. The large pressure effect observed in all Hg-based cuprates reveals the existence of a common hidden clue, whose identification is essential to stabilize at ambient pressure the structural arrangement responsible for the higher Tcs. This clue should also explain why the Hg-bearing superconductors are so susceptible to pressure.