Inelastic electron tunneling across magnetically active interfaces in cuprate and manganite heterostructures modified by electromigration processes

We report a study of the electron tunneling transport in point-contact junctions formed by a sharp Ag tip and two different highly correlated oxides, namely, a magnetoresistive manganite La₀.₆₆Ca₀.₃₄MnO₃ and a superconducting cuprate LaBa₂Cu₃O₇₋x. Strong chemical modifications of the oxide surface (supposedly, oxygen ion displacements) caused by applying high voltages to the junctions have been observed. This effect is believed to be responsible for an enormous growth of inelastic tunneling processes across a transition region that reveals itself in an overall "V"-shaped conductance background, with a strong temperature impact. The mechanism of the inelastic scattering is ascribed to charge transmission across magnetically active interfaces between two electrodes forming the junction. To support the latter statement, we have fabricated planar junctions between Cr and Ag films with an antiferromagnetic chromium oxide Cr₂O₃ as a potential barrier and at high-bias voltages have found an identical conductance trend with a similar temperature effect

Can one extract the electron-phonon-interaction from tunneling data in case of the multigap superconductor MgB2_2?

In the present work we calculate the tunneling density of states (DOS) of MgB% $_{2}$ for different tunneling directions by directly solving the two-band Eliashberg equations (EE) in the real-axis formulation. This procedure reveals the fine structures of the DOS due to the optical phonons. Then we show that the numeric inversion of the standard \emph{single-band} EE (the only available method), when applied to the \emph{two-band} DOS of MgB$_{2}$, may lead to wrong estimates of the strength of certain phonon branches (e.g. the $E_{2g}$) in the extracted electron-phonon spectral function $\alpha^{2}F(\omega)$. The fine structures produced by the two-band interaction at energies between 20 and 100 meV turn out to be clearly observable only for tunneling along the $ab$ planes, when the extracted $\alpha ^{2}F(\omega)$ contains the combination $\alpha ^{2}F_{\sigma \sigma}(\omega)$\textbf{+}$\alpha ^{2}F_{\sigma \pi }(\omega)$, together with a minor $\alpha ^{2}F_{\pi \pi}(\omega )$\textbf{+}$\alpha ^{2}F_{\pi \sigma} (\omega)$ component. Only in this case it is possible to extract information on the $\sigma$-band contribution to the spectral functions. For any other tunneling direction, the $\pi$-band contribution (which does not determine the superconducting properties of MgB$_{2}$) is dominant and almost coincides with the whole $\alpha^2F(\omega)$ for tunneling along the c axis. Our results are compared with recent experimental tunneling and point-contact data.Comment: 5 pages, 3 figures. Submitted to Phys. Rev. B (Brief Reports

Normal-superconducting transition induced by high current densities in YBa2Cu3O7-d melt-textured samples and thin films: Similarities and differences

Current-voltage characteristics of top seeded melt-textured YBa2Cu3O7-d are presented. The samples were cut out of centimetric monoliths. Films characteristics were also measured on microbridges patterned on thin films grown by dc sputtering. For both types of samples, a quasi-discontinuity or quenching was observed for a current density J* several times the critical current density Jc. Though films and bulks much differ in their magnitude of both Jc and J*, a proposal is made as to a common intrinsic origin of the quenching phenomenon. The unique temperature dependence observed for the ratio J*/Jc, as well as the explanation of the pre-quenching regime in terms of a single dissipation model lend support to our proposal.Comment: 10 pages, 10 figures, submitted to Physical Review

Manifestation of multiphonon structure and orbitons in the tunnel spectra of manganites

The study results of the tunnel conductance of heterostructures formed by a silver tip and a doped Ag/La₀.₅₇Ca₀.₄₃MnO₃ manganite in the voltage region above manganite phonon and magnon frequencies are presented. Three maxima have been observed at energies corresponding to the peaks in the Raman spectra of LaMnO₃. It is supposed that the maxima are due to the interaction of tunneling electrons with excitations in the orbital subsystem of a degraded region nearby the metal-manganite interface.Представлено результати досліджень тунельної провідності гетероструктур, утворених срібним вістрям і легованим манганітом Ag/La₀.₅₇Ca₀.₄₃MnO₃, в області напруг вище фононних і магнонних частот манганітів. Знайдено три максимуми при енергіях, відповідmrх пікам у спектрах комбінаційного розсіяння LaMnO₃. Передбачається, що вказані максимуми обумовлені взаємодією тунелюючих електронів із збудженнями в орбітальній підсистемі деградованої області, що примикає до інтерфейсу метал манганіт.Представлены результаты исследований туннельной проводимости гетероструктур, образованных серебряным острием и легированным манганитом Ag/La₀.₅₇Ca₀.₄₃MnO₃, в области напряжений выше фононных и магнонных частот манганитов. Обнаружены три максимума при энергиях, соответствующих пикам в спектрах комбинационного рассеяния LaMnO₃. Предполагается, что указанные максимумы обусловлены взаимодействием туннелирующих электронов с возбуждениями в орбитальной подсистеме деградированной области, примыкающей к интерфейсу металл манганит

Manifestation of multiphonon structure and orbitons in the tunnel spectra of manganites

The study results of the tunnel conductance of heterostructures formed by a silver tip and a doped Ag/La₀.₅₇Ca₀.₄₃MnO₃ manganite in the voltage region above manganite phonon and magnon frequencies are presented. Three maxima have been observed at energies corresponding to the peaks in the Raman spectra of LaMnO₃. It is supposed that the maxima are due to the interaction of tunneling electrons with excitations in the orbital subsystem of a degraded region nearby the metal-manganite interface.Представлено результати досліджень тунельної провідності гетероструктур, утворених срібним вістрям і легованим манганітом Ag/La₀.₅₇Ca₀.₄₃MnO₃, в області напруг вище фононних і магнонних частот манганітів. Знайдено три максимуми при енергіях, відповідmrх пікам у спектрах комбінаційного розсіяння LaMnO₃. Передбачається, що вказані максимуми обумовлені взаємодією тунелюючих електронів із збудженнями в орбітальній підсистемі деградованої області, що примикає до інтерфейсу метал манганіт.Представлены результаты исследований туннельной проводимости гетероструктур, образованных серебряным острием и легированным манганитом Ag/La₀.₅₇Ca₀.₄₃MnO₃, в области напряжений выше фононных и магнонных частот манганитов. Обнаружены три максимума при энергиях, соответствующих пикам в спектрах комбинационного рассеяния LaMnO₃. Предполагается, что указанные максимумы обусловлены взаимодействием туннелирующих электронов с возбуждениями в орбитальной подсистеме деградированной области, примыкающей к интерфейсу металл манганит