Resistance switching effects in metal/perovskite contacts based on epitaxial c-axis oriented YBa 2 Cu 3 O 6+c (YBCO) thin films with different crystallographic orientations have been studied. Three types of Ag/YBCO junctions with the contact restricted to (i) c-axis direction, (ii) ab-plane direction, and (iii) both were designed and fabricated, and their current-voltage characteristics have been measured. The type (i) junctions exhibited conventional bipolar resistance switching behavior, whereas in other two types the low-resistance state was unsteady and their resistance quickly relaxed to the initial high-resistance state. Physical mechanism based on the oxygen diffusion scenario, explaining such behavior, is discussed.The resistive switching (RS) effect observed in capacitor-like metal/insulator/metal junctions belongs to the most promising candidates for the next generation of the memory cell technology based on a sudden change of the junction resistance caused by an electric field applied to the metal electrodes. Despite a burst of activities triggered by increasing technological interest for this phenomenon 1,2 , details of its physical mechanism are still debated. The most studied are highly insulating binary transition metal oxides, where a key ingredient of the RS effect is believed to be a redistribution of oxygen ion vacancies which results in the formation (low-resistive state -LRS) and rupture (high-resistive state -HRS) of conductive filaments within the insulating media. 1,2 For more complex oxides like perovskites, the process of the RS effect seems to be more complicated. In this case the switching was usually bipolar whereas in simple binary oxides bi-and unipolar behavior has been observed. [3][4][5] In the following we are studying the RS effect in contacts based on conducting YBa 2 Cu 3 O 6+c (YBCO) films, compounds with a crystal structure which promotes electronic transport primarily within a twodimensional layer of atoms. One of the ways to shed light on their basic properties is to study an electricfield effect in the normal state. Up to now, two mutually exclusive field-effect mechanisms have been proposed. The first one, fundamentally electronic, is based on a conventional approach which takes into account the Coulomb interaction of an applied electric field with mobile current carriers. 6 It is fast, symmetric with respect to the bias polarity and results in the enhancement or depletion of the number of charge carriers within a few near-surface atomic layers. 6 The second mechanism 7 is related to direct interaction of oxygen ions with an applied electric field which causes significant charge carrier rearrangement due to a comparatively small oxygen migration energy and high density of vacancies in the oxygen sub-lattice in an optimal doping state. Such a process is characterized by a slower time constant and can be unequal in magnitude at positive and negative voltage biases of identical absolute value. 7 To choose between the two mechanisms of field-induced changes in normal-state...
The present paper examines the influence of a nonlinear relationship between the local oxygen‐vacancy concentration and the local resistivity on resistive switching effects in complex oxides. The continuity equation has been used as a model for the motion of oxygen vacancies when a periodic time‐dependent electrical current is applied. The question of endurance of the switching cycles is discussed. It is found that nonlinearity of the resistivity–concentration dependence enhances the endurance.
Because local concentration of vacancies in any material is bounded, their motion must be accompanied by nonlinear effects. Here, we look for such effects in a simple model for electric field-driven vacancy motion in a memristor, solving the corresponding nonlinear Burgers’ equation with impermeable nonlinear boundary conditions exactly. We find non-monotonous relaxation of the resistance while switching between the stable (‘on’ and ‘off’) states, and qualitatively different dependencies of switching time (under applied current) and relaxation time (under no current) on the memristor length. Our solution can serve as a useful benchmark for simulations of more complex memristor models.
Two possible mechanisms for the partial or complete loss of information contained in the quantum-mechanical phase of an electron moving in a stochastic solid-state structure are examined. The first involves phase randomization of the electron characteristics (for example, by elastic scattering of electrons on defects in thin metallic layers) and the second arises from inelastic interactions of current carriers with external degrees of freedom. With a double-barrier heterostructure as an example, it is shown that in the first case the quantum-mechanical approach reduces to a semiclassical method, in which the probabilities of individual events appear, rather than the quantum-mechanical probability amplitudes. The second case corresponds to a transition to the classical theory of charge transport. The effect of decoherence on the differential conductivity and shot noise in double-barrier tunnelling systems with a superconducting electrode is evaluated and the changes in these owing to the transition from quantum to incoherent classical electron transport are analyzed.
The present stage of development in microelec tronics poses increased requirements on the quality of thin films. The task of maintaining thorough control over their characteristics has become even more important in the case of thin complex conducting materials, the surface characteristics of which may strongly differ from their bulk properties. These differ ences, while insignificant for macroscopic samples, are of fundamental value for layers with thicknesses on the nanometer scale. For example, an analysis of the properties of complex oxides of transition metals (including cuprates [1], manganites [2], ferrites [3], etc.) in the conducting state showed that, even for the bulk material composition corresponding to the nom inal stoichiometry, the composition and structure of near surface layers can significantly vary, which leads to radical changes in their electrical and magnetic characteristics. The most frequently encountered defects in near surface layers of these materials are oxygen vacancies [4], the presence of which leads to the formation of a natural dielectric layer at the surface of conducting cuprates and manganites. This layer is a potential barrier for electrons tunneling from a metal electrode to complex oxides [5]. Since these oxides usually contain nanodimensional metal inclusions [6], electron transport in such systems frequently proceeds by charge carrier hopping between grains [2].The present investigation was aimed at developing a simple and effective approach to characterization of the microstructure of inhomogeneous near surface layers of conducting films with hopping conductivity. The proposed method is based on the creation of tun neling contacts on the film surface and an analysis of the influence of inelastic processes with phonon emis sion on the shape of the current-voltage characteristic I(V) of this heterostructure. Naturally, this approach is most applicable to materials with strong electronphonon interactions, such as high temperature cuprate superconductors in the normal state and man ganites [7]. In an interval of relatively low voltages V, where the surface barrier transparency weakly depends on the voltage sign and the distribution of defects is uniform, the inelastic contribution to the differential conductivity G(V) = dI(V)/dV is also independent of the sign of V and, hence, determines the even (with respect to voltage) contribution G (+) (V) = [G(V) + G(⎯V)]/2. The behavior of this even function will be considered below.The electron states in a dielectric barrier with cha otically distributed defects are usually localized and their wave functions exponentially decay at distances on the order of the localization length κ -1 . If the bar rier layer thickness d is greater than κ -1 , the main mechanism of conduction at sufficiently low temper atures is the hopping transport of charge carriers, whereby an electron jumps from one localized state to another (e.g., from state 1 to 2), which occur inside the barrier and are spaced by distance l (Fig. 1). Since the probability D of...
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