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A study is performed of different conditions of the synthesis and growth of perfect crystals of the family of Mott insulators for creating memristors (new-generation memory elements). The ceramic synthesis of GaNb4Se8 compound is used in combination with the thermolysis of niobium selenides under nonequilibrium conditions, allowing a material with a high level of chemical uniformity to be created.

Models that describe bipolar resistive switching (BRS) in planar microstructures based on oxide compounds (Bi2Sr2CaCu2O8 + x and Nd2 – xCexCuO4 – y) and bismuth selenide are considered. Metal/insulator/metal planar-type memristor heterostructures in which the microsize is formed by an electrode with a diameter much smaller than the total size of the structure (which can be both Sharvin-type microcontacts and film electric electrodes) are investigated. Another important feature of these heterostructures is the presence of a surface layer several tens of nanometers thick with the specific conductivity significantly reduced relative to the bulk conductivity. The change in the resistive properties of such heterostructures is caused by the formation or destruction of a conducting channel through the mentioned layer. Numerical simulation has shown that the BRS is significantly affected by the topology of the electric field’s distribution. To describe the experimentally observed memristor effects in the investigated heterostructures, a model of a critical field is proposed. In this model it is assumed that the change in the specific conductivity occurs in those parts of the surface layer where the electric field strength exceeds some critical value. The critical field model is based on the numerical calculation of the electrical potential distribution from the distribution of the specific conductivity in the structure. In addition, a model that enables analysis of the influence of the electrodiffusion of oxygen ions on resistive switching in the heterostructures based on Bi2Sr2CaCu2O8 + x is considered. In the numerical realization of the models, a combination of the integrodifferential approximation of differential equations, the multigrid approach for localization of heterogeneities in physical characteristics, the iterative decomposition method, and the composite adaptive meshes are used. This allows tracking the investigated processes with the necessary accuracy. The simulation results are compared with the experimental data.

We have been studying the dynamic effects of bipolar resistive switching (BERS) in memristive planar heterostructures based on c-oriented epitaxial films of the high-temperature superconductor YBa2Cu3O7 (YBCO). We have been observing the suppression of the memristive properties of the structures under sinusoidal and pulse loads and the nucleation and the evolution of the regions of current density and electric field in the structures under study. We found that the limiting frequencies of a sinusoidal type load for observing the switching effect are frequencies of about 104 Hz. It has been shown that the percolation channel is formed by the appearance of electric field domains in which the electric field threshold is reached. The switchings are controlled by two processes, with time t

The study of memristive properties or effect of resistive switchings in four classes of high-temperature superconductors (HTSC), namely Bi2Sr2CaCu2O8+y (BSCCO), YBa2Cu3O7−y (YBCO), Ba0.6K0.4BiO3−y (BKBO), and Nd2−xCexCuO4−y (NCCO), is presented. The purpose of this study is to reveal functional properties of HTSC which become apparent in the effects under discussion, prospects of usage of HTSC-based memristors in applications, and search for new mechanisms of strongly correlated nature to realize new-generation memristors (Tulina 1). The properties are as follows: undergoing the metal-insulator transition at oxygen doping, transport anisotropy, and existence of charge reservoirs through which doping of conductive copper–oxygen layers is carried out. These are the main functional properties of HTSC which allows their usage in memristors. By the example of study of bipolar effect of resistive switching in HTSC-based heterojunctions, it is shown how one can form memristor structures based on HTSC using their functional properties.

Transitions in the resistive switching of diode heterostructures based on bismuth selenide, in which bipolar resistive switching is implemented, are investigated. It is found that the time of transitions from one metastable state to another has a fast component on the order of microseconds and a slow component (a shuttle tail). Results are described using the model of a critical electric field, and the parameters of the investigated structures are calculated numerically.

Reversible and stable bistable resistive switching were observed in planar-type junctions Nd2–x Ce x CuO4–y /Nd2–x Ce x O1.75/Ag. It was shown that current transport in such junctions has a diode character with Schottky-like barriers in highly doped semiconductors. It was revealed that the key factor for switching is the presence of the second phase Nd2–x Ce x O1.75, deficient in oxygen, epitaxially germinated at the Nd2–x Ce x CuO4–y surface.

Reverse and stable bipolar resistive switching effect (BRSE) was observed in planar Nd 2 − x Ce x CuO 4 − y /Nd 2 − x Ce x O x /Ag heterostructure. It was shown that the СVС of the BRSE observed has a diode character. Simulations were used to consider the influence of the nonuniform distribution of an electric field at the interface of a heterojunction on the effect of bipolar resistive switching in investigated structures. The inhomogeneous distribution of the electric field near the contact edge creates regions of higher electric field strength which, in turn, stimulates motion and redistribution of defects, changes of the resistive properties of the whole structure and formation of a percolation channel.

Bipolar effect of resistive switchings (BERS) in epitaxial film-based heterostructures Nd 2−x Ce x CuO 4−y and YBa 2 Cu 3 O 7−y is investigated in the paper using the fundamental properties of the HTSC parent compounds – antiferromagnetic Mott insulators, which exhibiting a transition between a metal and an insulator owing to oxygen doping. The studies of electronic structure of the NCCO and YBCO epitaxial films surfaces by XPS and AFM have shown that the surface layer (∼30 nm) doped with oxygen is changing from metal (in a film bulk) to insulate state on the surface. The current-voltage characteristics of BERS devices obey a diode-like model upon the approach based on the double-diode equation .

Reversible and reliable bistable resistive switchings are observed in planar-type Nd2–xCexCuO4–y/Nd2–xCexO1.75/Ag junctions based on the Nd2–xCexCuO4–y superconducting films. Investigation of electronic structure of surface of the Nd0.5Ce0.5O1.75/Nd2–xCexCuO4–y via high-resolution X-ray photoelectron spectroscopy and atomic force microscopy (AFM) show that the key factor for the switchings is the presence of an oxygen-deficient Nd2-xCexO1.75 phase, epitaxially intergrown on surfaces of Nd2–xCexCuO4–y.

The bipolar effects of resistive switching (BRS) in mesoscopic heterostrucures based on amorphous and crystalline niobium oxide films are considered. The BRS effect is more pronounced in the structures based on multiphase nanocrystalline niobium oxide films. It is shown that the threshold frequency for observing the BRS effect is of the order of 104 Hz. Switching is controlled via oxygen electrodiffusion to vacancies as the doping level of the contact area and the resistive properties of the heterocontact change.

The effect applied voltage frequency has on bipolar resistive switching in microcontact type heterostructures based on thin films and single crystals of bismuth selenide is studied both experimentally and via numerical simulation.

Numerical simulation is used to study the effect anisotropic electrical conductivity and anisotropic oxygen diffusion have on the bipolar resistive switching effect in oxide compounds.

Investigations of bipolar resistive switchings in Bi 2 Sr 2 CaCu 2 O 8+ δ single crystal heterostructures are reported. The current–voltage characteristics of heterocontacts with switching effects exhibit a diode nature with Schottky-like barriers on high doped semiconductors. In this case the carrier tunneling processes are responsible for the current transfer in the structure. The voltage of switching into the metastable state is determined by barrier-controlled carrier tunneling. Spatial carrier inhomogeneity produces a field influence on the potential barrier in the interface of heterostructures with the BRS effect, redistribution of carriers and first order phase transition in the phase separation system. The numerical simulation method was used to consider the influence of inhomogeneous electrical field distribution in the heterocontact interface on the BRS effect in the oxide compounds.

Bi 2 Se 3 /Ag, Cu x Bi 2 Se 3 /Ag heterojunctions have been obtained in which the bipolar resistive switching effect has been observed and studied for the first time. The carrier type was determined by the switching voltage polarity. During resistive switching the conductivity of the metastable “On” and “Off” states changes from metallic to non-metallic (activation).

Thin-film heterojunctions Nd2 − x Ce x CuO4 − y /Ag were obtained. The bipolar effect of resistive switching in these heterostructures was detected and investigated. X-ray diffraction data indicate the presence of a second phase in thin films; along with the basic phase Nd2 − x Ce x CuO4 − y , it affects the behavior of the interface of investigated heterojunctions and leads to an alteration of the type of conductivity. The threshold frequency of alternating voltage at which the resistive switching effect is observed in heterojunctions was detected.

The transport properties of the percolation channels of memristive structures based on YBa2Cu3O7−δ epitaxial films were studied. Molecular electronics and Andreev reflection spectroscopy were utilised, and the influence of the superconductive transition of electrodes on resistive switching effects in these structures was examined. Based on the analysis of the conductivity mechanisms in the obtained heterostructures, it is assumed that percolation channels form through a chain of domains disordered by oxygen about 10 nm in diameter, with a maximum T c of 60 K. Zero-bias anomalies of the dynamic resistance of the studied structures display temperature dependence of the critical current of typical superconductor—normal—superconductor weak links within the framework of Kulik–Omel'yanchuk theory in the dirty limit. A simulation was used to determine the physical parameters of the studied heterostructures.

Thin filmed heterojunctions Ag/Bi/BiOx/Ag were fabricated and the effect of frequency of applied ac voltage on bipolar resistive switching in the obtained heterojunctions was observed for the first time. It has been found that the frequency dependence of the effect has a universal character for some oxide compounds. This observation confirms the significant role of oxygen-ion migration in the resistive switching phenomena in structures based on oxide compounds.

The transitional processes in heterocontacts based on strongly correlated electron systems (SCES) are studied for analyzing of the effect of resistive switching (ERS). It has been shown that the process is asymmetric with respect to switching into “on” and “off” states, the switching time is controlled by a voltage level, this time can be less than microseconds, on the other hand, relaxation processes can reach tens seconds. The switching is controlled by two processes: a change in the resistance state of the normal metal/SCES interface under effect of electric current field and by electrodiffusion of oxygen to vacancies, at that the doping level of the contact area and resistive properties of the heterocontact change. In particular, electrodiffusion of mobile oxygen induced by the electric field makes it possible to use a device with ERS as a memristor. On the other hand, a possibility to control the switching time and ON and OFF parameters show the possibilities to use these devices as memory elements “RAM”.

In this work we have performed the relaxation studies “in situ” of the electron instability effect (EIE) in the heterostructures based on BSCCO single crystals. The new effect of suppression of EIE or colossal electroresistance via application of an alternating low frequency electric field to the heterojunctions in the BSCCO-based single crystals has been found. It has been shown that the top possible frequencies for observation of the effect are of the order of 103 Hz. This fact is interpreted as accumulation of the oxygen ions driven by the electric field to the interface. On the other hand, it has been shown that the switching events are limited by two time processes: t ≈ 1 ms and about ten seconds. The first ones are caused by rearrangement of a charge net in the degraded surface at the electric field switching. The latter are caused by oxygen diffusion to vacancies under electric field above some threshold value. The considered experimental data confirm the correlation character of the HTSC properties as Mott systems, which appears in extreme sensitivity to the doping level, in the tendency to phase separation under external actions, in the hysteresis character of the metal–insulator transition.

Studies of the electron instability effects (EIEs) in structures based in strongly correlated electron systems (SCESs), such as high-temperature superconductors (HTSCs) and doped manganites (compounds with colossal magnetoresistance), are reviewed. These effects manifest themselves in a change of several orders of magnitude in the resistive state of the normal metal–HTSC or normal metal–DM (doped manganite) interface in an electric field under significant current injection conditions. The results of studying HTSC- and doped-manganite-based heterojunctions are considered. EIEs in heterostructures are compared with the electric field effect on the properties of an SCES in thin fllms and gate-containing devices. The general features and distinctions in the physics of these ðhenomena are analyzed.

The electronic structure of degraded (as grown and chemically polished) and ion-sputtered La 0.8 Ca 0.2 MnO 3 single crystal surfaces has been studied by X-ray photoemission and Auger electron spectroscopy techniques. XPS spectra of deep core levels (Mn2p, O1s, La3d, La4d) were taken from as grown, chemically polished and cleaned in UHV by Ar + ion-bombardment surfaces. XPS data show different oxidation state of Mn and La ions on the degraded and ion-sputtered surfaces of the manganite crystal. Mn2p 3/2 line width and energy position suggest mixed valence state of the manganese ions at the degraded and ion-sputtered crystal surfaces.

Simulations are used to study the influence of nonuniform electric field distribution at a heterojunction interface on the effect of bipolar resistive switching in oxide compounds. Computer models show that a conducting channel forms at the edge of a heterojunction area characterized by strong local increase in the electric field intensity. The computations are confirmed by the low temperature properties of metastable phases in heterojunctions based on high temperature superconductors.

Electrodynamic and thermal properties of doped-manganites-based heterojunctions are numerically calculated for a model description of electron instability effects revealed in them under a strong current injection. The calculations are performed by jointly solving the thermal conduction and Poisson equations. Comparison of experimental and calculated data suggests that the effect of transport current on resistive properties of manganites-based heterojunctions is indicative of impurity-assisted phase separation in manganites. The motion of charged oxygen ions in the surface layer with high density of vacancies under the action of an electric field catalyzes phase separation.

The current–voltage characteristics (IVC) and transport properties of point contacts in normal metal–manganite heterojunctions have been investigated on the base of La 1− x Sr x MnO 3 , La 1− x Ca x MnO 3 , La 1− x Ba x MnO 3 ( x =0.15–0.3) single crystals. At considerable current levels the IVC of point contacts show reproducible switching with memory effect. Negative or positive voltage switch the heterocontact resistance between high- and low-resistive states. It has been assumed that the observed current switching effects are due to phase separation of manganites under the electric current.

The magnetic structure of the doped perovskite manganites La0.7Sr0.3MnO3 and La0.8Ca0.2MnO3 at a temperature of 10 K has been investigated by using the decoration technique in a wide region of external magnetic fields (up to 1000 Oe). We observe the multidomain ferromagnetic state in La0.7Sr0.3MnO3 and the coexistence of the ferro- and nonmagnetic phases in La0.8Ca0.2MnO3. The dependence of the structure on an external magnetic field has been investigated in La0.8Ca0.2MnO3.

We have studied the electronic instability of Bi 2 Sr 2 CaCu 2 O 8+ y -based heterojunctions effected by an electric field. The instability has been shown to be dictated by a change in the number of carriers in the surface depleted layer under influence of an electrical field to about 10 5 V/cm.

Resistive properties of point contacts in normal-metal-manganite heterojunctions have been investigated on the base of La0.7Sr0.3MnO3, La0.8Sr0.2MnO3, La0.8Ca0.2MnO3 single crystals. At considerable current levels the current-voltage characteristics of point contacts exhibit features in the form of resistance peaks at lower temperatures. Colossal magnetoresistance also considerably increases under transport current in the investigated heterojunctions. It has been assumed that the observed current switching effects are due to phase separation of manganites under the electric current.

The normal metal-degraded Bi 2 Sr 2 CaCu 2 O 8+ x surface boundary has been shown to demonstrate the diode properties under specific conditions. The current transport across such a boundary proceeds in hysteresis manner, which indicates phase separation of the degraded surface to conducting and dielectric phases under the action of an electric field.

It is shown that resistive switchings in heterocontacts formed by a doped manganite single crystal and a normal metal lead to the formation of a phase separated state in the manganite surface layer; the resistive and magnetic properties of this state can be significantly varied by changing the resistive switching parameters.

The electrophysical properties of heterojunctions several microns in size, obtained by successive deposition of the metal-oxide high-temperature superconductor YBa2Cu3Ox, a normal metal Au, and the low-temperature superconductor Nb, were studied experimentally. Current flows in the [001] direction of the epitaxial YBa2Cu3Ox film. It is shown, by comparing the experimental data with existing theoretical calculations, that for the experimentally realizable transmittances (D=10−5–10−6) of the YBa2Cu3Ox—normal metal boundary the critical current of the entire heterostructure is low (of the order of the fluctuation current) because of a sharp change in the amplitude of the potential of the superconducting carriers at this boundary. The current-voltage characteristics of the heterostructure studied correspond to tunnel junctions consisting of a superconductor with \(d_{x^2 - y^2 } \) type symmetry of the superconducting wave function and a normal metal.

We have studied the effect of transparency of the normal metal–high temperature superconductor boundary on the voltage–current characteristics, temperature dependence of the dynamic resistance of point-contact on the base of the fresh cleavages and, also, degraded surfaces of Bi2Sr2CaCu2O8+Y single crystals. A normal metal-to-fresh single crystal cleavage contact may be a direct conduction channel free of tunnel interlayers. The temperature dependence of the dynamic resistance of the point-contact is a sensitive and informative characteristic reflecting the superconducting state near-surface layers of a single crystal and their degradation as the oxygen is being lost. It is shown that the current, flowing across the boundary, governs the relationship between the `ab' and `c' components, each of which is dominant in its own temperature and `voltaic' interval. Surface layers with degraded superconducting properties control the I–V characteristic features near zero voltage even at high boundary transparency as these features are related with resistivity arising at the Andreev reflection and with current redistribution between the `c' and `ab' directions in the boundary region. At large biases the boundary regions are normal state, the current flows in the λab layer i parallel with the surface. VAC exhibits a hysteresis in this voltaic region. Once a homogeneous degraded interlayer is formed at the boundary, the VAC form becomes quasitunnel with a zero-bias anomaly feature that weakly depends on a magnetic field and invariant in form (insplit). Such zero-bias anomaly may be regarded as the Andreev reflection at the diffuse normal metal–high temperature superconductor boundary.

The effect of electron instability of heterojunctions based on Ba0.6K0.4BiO3−x single crystals has been found experimentally. This effect is shown to have the opposite sign along the electric field of the current compared to a similar effect observed in structures based on hole-doped systems.

The current transport across the normal metal–Bi 2 Sr 2 CaCu 2 O 8+ x interface in the c -direction has been studied using the current–voltage characteristics and the temperature dependence of the resistance of point contacts on the base of Bi 2 Sr 2 CaCu 2 O 8+ x single crystals. At temperatures below the critical temperature the resistive contribution of the interface to the total point-contact resistance dominates This is due to the effect of a normal metal on a HTSC and to the suppression of superconductivity in the point-contact region. In the normal state two current contributions are observed, namely, the linear contribution with respect to the temperature, R a b and the thermal activation one R c .

A sensitive method to investigate the superconducting properties of a HTSC single crystals surface at a depth of the order of 10 −6 –10 −5 cm is demonstrated. The method is based on the measurements of the temperature dependence of the point-contact resistance. The cleavage surface degradation of a Bi 2 Sr 2 CaCu 2 O 8 + δ single crystal at a depth of the order of 10 −5 cm is studied as a function of time from the change of the single crystal surface layer resistance. The characteristic times are of the order of several hours. The dependence of the critical temperature of the near-barrier layer of a Bi 2 Sr 2 CaCu 2 O 8 + δ single crystal on the resistance in this layer is obtained.

We have observed polarity dependent reversible and irreversible effects of applied electric fields on the tunnel characteristics of Bi 2 Sr 2 CaCu 2 O 8+δ single crystal junctions. The characteristic times of the electric field-induced effects were of the order of several minutes whereas the equilibrium state recovery took several hours.

We have investigated high-temperature superconductor (HTSC) tunnel junctions with various barriers. We show that an anomalous linear contribution to the conductance at high voltage (linear-conductance background) is related to the surface degraded layer, not to the intrinsic properties of HTSCs. The tunnel junctions were obtained on fresh cleavages of Bi 2 Sr 2 CaCu 2 O 8+δ single crystals without an anomalous linear-conductance background. The tunneling characteristic of such a junction is described by the BCS dependence with the gap parameter Δ = 20 meV (2 Δ / kT c ≈ 5.8).

Andreev reflection has been studied in a normal metal-cleavage of a HTSC single crystal heterocontact. The effect of a considerable increase of a point-contact resistance d V /d I ( T ) at T = T c has been observed as compared with d V /d I ( V ) at V = Δ / e . This fact made it possible to estimate the real position of the normal metal-superconductor interface and to define the superconducting energy gap in a YBa 2 Cu 3 O 8− x single crystal in the “ c ”-direction of Δ (4.2 K)=4∓1 meV.

The point-contact of tunnel type transition has been realized on fresh cleavages of the Bi2Sr2CaCu2O8 single crystal. The deviation from the BCS dependence of tunneling characteristics may rather be due to nonideal barrier properties than to the nature of HTSC. Also, we have obtained the maximum ratio 2Δ/kTc=5.6∓0.5 for Bi2Sr2CaCu2O8.

The effect of resistive switching in doped manganites being in the ferromagnetic state has been studied using resistive and magneto-optic methods. The visualization of magnetic structure of La 0.75 Sr 0.25 MnO 3− x single crystals, and its transformation under electric current proposed local superheating of the material above the Curie temperature, which was supported by numerical calculation. The obtained results suggest a significant role of micrometer-scale inhomogeneity of manganites in phase separation, magnetic and transport properties of the material.

Point-contact investigations have been made on the Mo1-xRex system in the b.c.c. single phase region with 0

Tunneling junctions of the point-contact type of Bi 2 Sr 2 CaCu 2 O 8 single crystals have been studied, a natural oxide of aluminium electrode serving as a tunneling barrier. The ratio 2Δ/ kT c =5–6 was obtained in this investigation. A phenomenological model was applied for describing the behaviour of tunneling characteristics. From the numerical calculations and on the basis of this model, the phonon singularities in tunneling characteristics were treated by the standard program for the solutions of the Eliashberg equations and the electron-phonon interaction function, α 2 ( ω ) F ( ω ), was found.

The effect of tensional plastic deformation and of defects produced during this process on the change in temperature (Tc) of the superconducting transition of Re single crystals has been considered. The Tc dependence on the degree of plastic deformation has been obtained for different deformation temperatures and crystal orientations. Electron microscopy show that the defect structure of deformed Re is characterized by the presence of planar defects, i.e. a combination of slip bands and twin layers, between which single dislocations, dislocation arrays, loops, and dipoles are observed. Annealing and quenching experiments on deformed single crystals show that point defects and dislocations have a marked influence on the change in Tc while planar defects have an insignificant effect. [Russian Text Ignored]

Temperature dependence of critical magnetic field of Pb-ternary molybdenum sulphides is studied. The derivative ∂H c 2 ∂T increases anomalously near Tc. Possible physical mechanisms of this effect are discussed.

The V 3 Si point-contact has been studied. The low-frequency mode of electron-phonon interaction has been observed by eV ∼ 16 meV unknown previously. The “dirty” point-contacts exhibited the effects of the negative contribution to the dynamic electric resistance which might be the manifestation of the electron-electron correlations.