Yu. V. Kislinskii

Triplet superconducting correlations in oxide heterostructures with a composite ferromagnetic interlayer

The superconducting current induced by the penetration of the long-range triplet component of superconducting correlations into a composite ferromagnetic interlayer has been detected in mesa-heterostructures based on oxide cuprate superconductors YBa2Cu3O7 − δ and Au/Nb bilayer films with the composite oxide interlayer that is made of ferromagnetic films of manganite La0.7Sr0.3MnO3 and ruthenate SrRuO3 and has a thickness much larger than the length of correlations determined by the exchange field. The deviation of the superconducting current in the mesa-heterostructure with the fraction of the second harmonic of 13% from a sinusoidal current-phase relation has been detected; this deviation can also be due to the generation of the triplet component of superconducting correlations in the ferromagnet.

Evidence for spin-triplet superconducting correlations in metal-oxide heterostructures with noncollinear magnetization

Heterostructures composed of ferromagnetic La0.7Sr0.3MnO3, ferromagnetic SrRuO3, and superconducting YBa2Cu3O6+x were studied experimentally. Structures of composition Au/La0.7Sr0.3MnO3/SrRuO3/YBa2Cu3O6+x were prepared by pulsed laser deposition, and their high quality was confirmed by x-ray diffraction and reflectometry. Anoncollinear magnetic state of the heterostructureswas revealed by means of superconducting quantum interference device magnetometry and polarized neutron reflectometry. We have further observed superconducting currents in mesa structures fabricated by deposition of a second superconducting Nb layer on top of the heterostructure, followed by patterning with photolithography and ion-beam etching. Josephson effects observed in these mesa structures can be explained by the penetration of a triplet component of the superconducting order parameter into the magnetic layers.

Superconducting current in hybrid heterojunctions of metal-oxide superconductors : Size and frequency dependences

We have detected experimentally considerable deviations of the frequency dependences of the Shapiro step amplitudes and the critical current of Nb/Au/YBa2Cu3Ox thin-film hybrid Josephson heterojunctions prepared on YBa2Cu3Ox metal-oxide superconductor films with a tilted c axis from the regularities inherent in Josephson junctions of traditional superconductors with an s-symmetry of the order parameter. It is shown that possible formation of “splintered” fluxons with a size λs<λJ due to faceting of the interface and formation of a chain of nanosize 0 and π junctions must be taken into account in describing processes in lumped heterojunctions (whose size L is smaller than the Josephson penetration depth λJ determined from the averaged value of the critical current density). For heterojunctions with a size λs < L < λJ, a substantial decrease in the maximal amplitude of the first Shapiro step with increasing voltage (Josephson oscillation frequency) is observed at voltages much smaller than the energy gap in niobium (V « ΔNb/e); this effect is manifested most strongly when the size L is greater than λs. A fractional Shapiro step and a subharmonic detector response have been observed in the current-voltage characteristics of heterojunctions; the dynamic processes responsible for their emergence and indicating the presence of the second harmonic in the current-phase relation are studied. It is shown that the effect of interface faceting on the current-phase relation increases with a heterojunction size L>λs.

Submillimeter wave signal detection by bicrystal YBCO Josephson junctions at liquid nitrogen temperatures

The submillimeter wave signal detection has been experimentally studied for a high-Tc superconducting device, consisting of a bicrystal YBa2Cu3Ox Josephson junction, incorporated with a log-periodic antenna. Measurements were carried out in wide range of temperatures T ¼ 20–80 K. At relatively high temperatures T > 60 K, the thermally activated phase slippage began to predominate. At the same time, a set of clearly resolved detector response functions have been registered up to the temperatures close to the critical one. At the highest temperature T ’ 80 K, where detector response has been registered, the ratio of the frequency of applied signal fs ’ 500 GHz to the critical frequency of Josephson junction fc has been estimated as large as fs=fc ’ 40. 2002 Elsevier Science B.V. All rights reserved.

Spin-filter tunneling in superconducting mesa structures with a ferromagnetic manganite interlayer

We have studied the current transport and magnetism in epitaxial hybrid superconducting mesa structures consisting of a cuprate superconductor and superconducting niobium with a manganite LaMnO3 (LMO) interlayer. We have shown experimentally using magnetic resonance that the magnetization, magnetic anisotropy parameters, and transition temperature to the ferromagnetic state of the interlayer of the structures are analogous to those of an autonomous LMO film grown on a neodymium gallate substrate. The estimate of the barrier height obtained from the dependence of the characteristic resistance of mesa structures on the interlayer thickness has shown the barrier height variation with the thickness in the range of 5–30 mV. The temperature dependences of the conductivity of the mesa structure in the range between superconducting transition temperatures of the superconductors can be described in the theory taking into account the d-wave nature of the superconductivity for one of the electrodes and the spin-filtering of carriers passing through the tunnel interlayer. Spin-filtering is confirmed by the tunnel magnetoresistance and the high sensitivity of mesa structures to a weak external magnetic field in a voltage interval smaller than the gap of niobium.

Spin-triplet electron transport in hybrid superconductor heterostructures with a composite ferromagnetic interlayer

Hybrid YBa2Cu3O7 − x/SrRuO3/La0.7Sr0.3MnO3/Au-Nb superconductor mesastructures with a composite manganite-ruthenate ferromagnetic interlayer are studied using electrophysical, magnetic, and microwave methods. The supercurrent in the mesastructure is observed when the interlayer thickness is much larger than the coherence length of ferromagnetic materials. The peak on the dependence of the critical current density on the interlayer material thickness corresponds to the coherence length, which is in qualitative agreement with theoretical predictions for a system with spit-triplet superconducting correlations. The magnetic-field dependence of the critical current is determined by penetration of magnetic flux quanta and by the magnetic domain structure, as well as by the field dependence of disorientation of the magnetization vectors of the layers in the composite magnetic interlayer. It is found that the supercurrent exists in magnetic fields two orders of magnitude stronger than the field corresponding to entry of a magnetic flux quantum into the mesastructure. The current-phase relation (CPR) of the supercurrent of mesastructures is investigated upon a change in the magnetic field from zero to 30 Oe; the ratio of the second CPR harmonic to the first, determined from the dependence of the Shapiro steps on the microwave radiation amplitude, does not exceed 50%.

Electron transport in metal oxide superconducting heterojunctions

Research results on electron transport in Au/YBa2Cu3Ox and Nb/Au/YBa2Cu3Ox thin-film heterojunctions are reviewed. The experimental current–phase relations of Nb/Au/YBa2Cu3Ox heterojunctions on c-oriented YBCO films exhibit a second harmonic, temperature dependence, and a phase shift that is explained in terms of a combined symmetry dx2−y2+s of the superconducting order parameter of YBa2Cu3Ox. The current–voltage characteristics of Au/YBa2Cu3Ox and Nb/Au/YBa2Cu3Ox heterojunctions on (1 1 20)YBa2Cu3Ox thin films with an inclined crystallographic c axis display an anomaly of the conductance at low voltages, the behavior of which is studied at various temperatures and magnetic fields. The experimental results are analyzed in the framework of a model for the appearance of bound states caused by multiple Andreev reflection in junctions containing a superconductor with dx2−y2 symmetry of the superconducting order parameter. Studies of the noise characteristics of Nb/Au/(1 1 20)YBa2Cu3Ox heterojunctions at T=4.2...

Andreev states and the Josephson effect in superconducting heterojunctions on thin YBa2Cu3Ox films

Conductance anomalies at low bias voltages and superconducting currents in Au/YBa2Cu3Ox and Nb/Au/YBa2Cu3Ox heterojunctions in which the c axis of the YBa2Cu3Ox (YBCO) epitaxial film is rotated in the (110) YBCO plane through 11° with respect to the normal to the substrate plane were studied experimentally. The films were prepared by laser deposition onto (7 2 10)-oriented NdGaO3 substrates. The current-voltage characteristics of the heterojunctions exhibit conductance anomalies at low voltages. The behavior of these anomalies is studied at various temperatures and in various magnetic fields. The critical current and Shapiro steps observed in the current-voltage characteristics of Nb/Au/YBa2Cu3Ox were evidence of the Josephson effect in these heterojunctions. The experimental results are analyzed in terms of the model of the arising of bound states caused by Andreev reflection in superconductors with d-type symmetry of the superconducting order parameter.

Generation of microwave oscillations in a superconducting tunnel mesa-structure with a ferromagnetic insulator interlayer

Spin-polarized current in thin-film tunnel mesa-structures formed by epitaxial cuprate superconducting (YBa2Cu3O7–δ) and manganite (LaMnO3) films and an upper superconducting Au–Nb bilayer is studied experimentally. Intrinsic narrow-band generation in the microwave range is reported. Its frequency is tuned by the bias voltage and an external magnetic field.

Growth technology and characteristics of thin strontium iridate films and iridate–cuprate superconductor heterostructures

A technology for epitaxial growth of thin films of strontium iridate (Sr2IrO4) and related heterostructures with cuprate superconductor (Sr2IrO4/YBa2Cu3O7 − δ) has been proposed and developed. It is established that the two-layer structure grows epitaxially and the cuprate superconductor layer has the same critical temperature as that (~91 K) of an autonomous film. Crystallographic parameters of the obtained iridate films are close to tabulated values and the temperature dependence of their electric resistivity is consistent with published data.