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We carry out an experimental feasibility study of a magnetic field sensor based on the kinetic inductance of the high-$T_\mathrm{c}$ superconductor yttrium barium copper oxide. We pattern thin superconducting films into radio-frequency resonators that feature a magnetic field pick-up loop. At 77 K and for film thicknesses down to 75 nm, we observe the persistence of screening currents that modulate the loop kinetic inductance. According to the experimental results the device concept appears attractive for sensing applications in ambient magnetic field environments. We report on a device with a magnetic field sensitivity of 4 pT/Hz${}^{1/2}$, an instantaneous dynamic range of 11 $\mu$T, and operability in magnetic fields up to 28 $\mu$T., Comment: 12 pages, 12 figures

Heterostructures consisting of a cuprate superconductor YBa2Cu3O7x and a ruthenate/manganite (SrRuO3/La0.7Sr0.3MnO3) spin valve have been studied by SQUID magnetometry, ferromagnetic resonances and neutron reflectometry. It was shown that due to the influence of magnetic proximity effect a magnetic moment is induced in the superconducting part of heterostructure and at the same time the magnetic moment is suppressed in the ferromagnetic spin valve. The experimental value of magnetization induced in the superconductor has the same order of magnitude with the calculations based on the induced magnetic moment of Cu atoms due to orbital reconstruction at the superconductor-ferromagnetic interface. It corresponds also to the model that takes into account the change in the density of states at a distance of order of the coherence length in the superconductor. The experimentally obtained characteristic length of penetration of the magnetic moment into superconductor exceeds the coherence length for cuprate superconductor. This fact points on the dominance of the mechanism of the induced magnetic moment of Cu atoms due to orbital reconstruction., Comment: 20 pages, 9 figures

The interplay between the quasi 1-dimensional CuO-chains and the 2-dimensional CuO2 planes of YBa2Cu3O6+x (YBCO) has been in focus for a long time. Although the CuO-chains are known to be important as charge reservoirs that enable superconductivity for a range of oxygen doping levels in YBCO, the understanding of the dynamics of its temperature-driven metal-superconductor transition (MST) remains a challenge. We present a combined study using x-ray absorption spectroscopy and resonant inelastic x-ray scattering (RIXS) revealing how a reconstruction of the apical O(4)-derived interplanar orbitals during the MST of optimally doped YBCO leads to substantial hole-transfer from the chains into the planes, i.e. self-doping. Our ionic model calculations show that localized divalent charge-transfer configurations are expected to be abundant in the chains of YBCO. While these indeed appear in the RIXS spectra from YBCO in the normal, metallic, state, they are largely suppressed in the superconducting state and, instead, signatures of Cu trivalent charge-transfer configurations in the planes become enhanced. In the quest for understanding the fundamental mechanism for high-Tc-superconductivity (HTSC) in perovskite cuprate materials, the observation of such an interplanar self-doping process in YBCO opens a unique novel channel for studying the dynamics of HTSC., Comment: 9 pages, 4 Figures

Heterostructures composed of ferromagnetic La0.7Sr0.3MnO3, ferromagnetic SrRuO3, and superconducting YBa2Cu3Ox were studied experimentally. Structures of composition Au/La0.7Sr0.3MnO3/SrRuO3/YBa2Cu3Ox were prepared by pulsed laser deposition, and their high quality was confirmed by X-ray diffraction and reflectometry. A non-collinear magnetic state of the heterostructures was revealed by means of SQUID 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., Comment: 10 pages, 6 figures

Magnetoencephalography (MEG) enables the study of brain activity by recording the magnetic fields generated by neural currents and has become an important technique for neuroscientists in research and clinical settings. Unlike the liquid-helium cooled low-Tc superconducting quantum interference devices (SQUIDs) that have been at the heart of modern MEG systems since their invention, high-Tc SQUIDs can operate with liquid nitrogen cooling. The relaxation of thermal insulation requirements allows for a reduction in the stand-off distance between the sensor and the room-temperature environment from a few centimeters to less than a millimeter, where MEG signal strength is significantly higher. Despite this advantage, high-Tc SQUIDs have only been used for proof-of-principle MEG recordings of well-understood evoked activity. Here we show high-Tc SQUID-based MEG may be capable of providing novel information about brain activity due to the close proximity of the sensor to the head. We have performed single- and two-channel high-Tc SQUID MEG recordings of spontaneous brain activity in two healthy human subjects. We demonstrate modulation of the occipital alpha rhythm and the mu rhythm found in the motor cortex. Furthermore, we have discovered uncharacteristically high-amplitude activity in the theta-band from the occipital region of the brain. Our results suggest high-Tc SQUIDs can provide a closer look into the brain.

We report on the study of electron transport in the oxide heterostructures with superconductor/magnetic matter (S/M) interfaces where anomaly large penetration of superconducting correlations in magnetic matter (proximity effect) is realized. The developed theoretical model based on multilayer magnetic structure of M-interlayer and experiment show presence of the long-range proximity effect at S/M-interface with antiferromagnetic (AF) ordering of the interlayer magnetization. The investigated hybrid heterostructures include cuprate superconductor, the AF-cuprate interlayer and conventional superconductor, Nb. The superconducting critical current with density 1-10A/cm2 and the characteristic voltage, 0.1-0.2 mV, are observed at liquid helium temperature for 15-50 nm thick M-interlayer made of AF film CaSrCuO. These heterostructures demonstrate deviation from sin-type superconducting current-phase relation and had the critical current of the second harmonic of 10-20% of the first harmonic one. The hybrid heterostructures with S/AF interface show high sensitivity to the external magnetic field that is possibly caused by the influence of an external magnetic field on the canting of magnetic moments of individual layers of AF -interlayer. Substituting the AF cuprate by a manganite film, no critical current was observed, although the M- interlayer was made very thin, down to 5 nm.

Electrical transport of a polar heterointerface between two insulating perovskites, KTaO3 and SrTiO3, is studied. It is formed between a thin KTaO3 film deposited on a top of TiO2- terminated (100) SrTiO3 substrate. The resulting (KO)1-(TiO2)0 heterointerface is expected to be hole-doped according to formal valences of K (1+) and Ti (4+). We observed electrical conductivity and mobility in the KTaO3/SrTiO3 similar to values measured earlier in electron-doped LaAlO3/SrTiO3 heterointerfaces. However, the sign of the charge carriers in KTaO3/SrTiO3 obtained from the Hall measurements is negative. The result is an important clue to the true origin of the doping at perovskite oxide hetero-interfaces., Comment: 6 pages, 3 figures

Strontium titanate, SrTiO3, a widely used substrate material for electronic oxide thin film devices, has provided many interesting features. In a combination with a similar oxide material, LaAlO3, it has recently received great interest. It was suggested that two-dimensional electron gas is formed at the interface between SrTiO3 and LaAlO3, resulting in high electrical conductivity and mobility. In this report we demonstrate that the transport properties in those heterostructures are very sensitive to the deposition parameters during thin film growth. Using cathode- and photoluminescence studies in conjunction with measurements of electrical transport properties and microstructure we show that the electronic properties observed at a LaAlO3/SrTiO3 interface can be explained by oxygen reduced SrTiO3. In addition, we demonstrate that oxygen can be pushed in and out of the sample, but that re-oxygenation of an initially oxygen depleted LaAlO3/SrTiO3 heterostructure is partly prevented by the presence of the film., Comment: 19 pages, 5 figures