Magnetoencephalography (MEG) is a noninvasive technique for investigating neuronal activity in the living human brain. The time resolution of the method is better than 1 ms and the spatial discrimination is, under favorable circumstances, 2-3 mm for sources in the cerebral cortex. In MEG studies, the weak 10 fT-1 pT magnetic fields produced by electric currents flowing in neurons are measured with multichannel SQUID (superconducting quantum interference device) gradiometers. The sites in the cerebral cortex that are activated by a stimulus can be found from the detected magnetic-field distribution, provided that appropriate assumptions about the source render the solution of the inverse problem unique. Many interesting properties of the working human brain can be studied, including spontaneous activity and signal processing following external stimuli. For clinical purposes, determination of the locations of epileptic foci is of interest. The authors begin with a general introduction and a short discussion of the neural basis of MEG. The mathematical theory of the method is then explained in detail, followed by a thorough description of MEG instrumentation, data analysis, and practical construction of multi-SQUID devices. Finally, several MEG experiments performed in the authors' laboratory are described, covering studies of evoked responses and of spontaneous activity in both healthy and diseased brains. Many MEG studies by other groups are discussed briefly as well.

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Magnetoencephalography—theory, instrumentation, and applications to noninvasive studies of the working human brain

Magnetic flux can penetrate a type-II superconductor in the form of Abrikosov vortices (also called flux lines, flux tubes, or fluxons) each carrying a quantum of magnetic flux phi 0=h/2e. These tiny vortices of supercurrent tend to arrange themselves in a triangular flux-line lattice (FLL), which is more or less perturbed by material inhomogeneities that pin the flux lines, and in high-Tc superconductors (HTSCs) also by thermal fluctuations. Many properties of the FLL are well described by the phenomenological Ginzburg-Landau theory or by the electromagnetic London theory, which treats the vortex core as a singularity. In Nb alloys and HTSCs the FLL is very soft mainly because of the large magnetic penetration depth lambda . The shear modulus of the FLL is c66~1/ lambda 2, and the tilt modulus c44(k)~(1+k2 lambda 2)-1 is dispersive and becomes very small for short distortion wavelengths 2 pi /k<< lambda . This softness is enhanced further by the pronounced anisotropy and layered structure of HTSCs, which strongly increases the penetration depth for currents along the c axis of these (nearly uniaxial) crystals and may even cause a decoupling of two-dimensional vortex lattices in the Cu-O layers. Thermal fluctuations and softening may `melt` the FLL and cause thermally activated depinning of the flux lines or ofthe two-dimensional `pancake vortices` in the layers. Various phase transitions are predicted for the FLL in layered HTSCs. Although large pinning forces and high critical currents have been achieved, the small depinning energy so far prevents the application of HTSCs as conductors at high temperatures except in cases when the applied current and the surrounding magnetic field are small.

https://iopscience.iop.org/article/10.1088/0034-4885/58/11/003/pdf

The flux-line lattice in superconductors

Magnetic flux can penetrate a type-II superconductor in form of Abrikosov vortices. These tend to arrange in a triangular flux-line lattice (FLL) which is more or less perturbed by material inhomogeneities that pin the flux lines, and in high-$T_c$ supercon- ductors (HTSC's) also by thermal fluctuations. Many properties of the FLL are well described by the phenomenological Ginzburg-Landau theory or by the electromagnetic London theory, which treats the vortex core as a singularity. In Nb alloys and HTSC's the FLL is very soft mainly because of the large magnetic penetration depth: The shear modulus of the FLL is thus small and the tilt modulus is dispersive and becomes very small for short distortion wavelength. This softness of the FLL is enhanced further by the pronounced anisotropy and layered structure of HTSC's, which strongly increases the penetration depth for currents along the c-axis of these uniaxial crystals and may even cause a decoupling of two-dimensional vortex lattices in the Cu-O layers. Thermal fluctuations and softening may melt the FLL and cause thermally activated depinning of the flux lines or of the 2D pancake vortices in the layers. Various phase transitions are predicted for the FLL in layered HTSC's. The linear and nonlinear magnetic response of HTSC's gives rise to interesting effects which strongly depend on the geometry of the experiment.

The Flux-Line Lattice in Superconductors

The properties of the Y1−xPrxBa2Cu3O7 (YPrBCO) system are reviewed. These include superconducting, normal state, structural, chemical, optical, magnetic, and thermal properties. The destruction of superconductivity with Pr doping is discussed in view of possible models such as hole filling, localization, magnetic pair-breaking, and the role of hybridization. Applications to electronic devices using YBCO/PrBCO/YBCO multilayers are also reviewed.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0884291400017489

A review of the superconducting and normal state properties of Y1−xPrxBa2Cu3O7

Crystalline defects on the nano-scale, which are called artificial pinning centers (APCs), were successfully introduced into high-temperature superconductors (HTS) by nanotechnology, in order to strongly pin the quantized vortices. The critical current densities, Jc, of the HTS films were dramatically improved by APCs. It is possible to form APCs in high-quality epitaxial films, keeping the desired dimensionality, volume fraction, spatial distribution and so on. The in-field Jc of HTS films at 77 K was improved by one order of magnitude compared with previous values using APCs. This technology can be applied to the coated conductor technology in progress, and a high Jc has already been reported. A current outline of the research is described in this review.

https://iopscience.iop.org/article/10.1088/0953-2048/23/1/014001/pdf

Artificial pinning center technology to enhance vortex pinning in YBCO coated conductors

Niobium nitride‐niobium nitride Josephson tunnel junctions have been fabricated using amorphous magnesium oxide (a‐MgO) films as barriers. These junctions have excellent tunneling characteristics. For example, a large gap voltage (Vg=5.1 mV), a large product of the maximum critical current and the normal tunneling resistance (IcRn=3.25 mV), and a small subgap leakage current (Vm=45 mV, measured at 3 mV) have been obtained for a NbN/a‐MgO/NbN junction. The critical current of this junction remains finite up to 14.5 K.

Niobium nitride Josephson tunnel junctions with magnesium oxide barriers

The current-voltage (I-V) characteristics are investigated in epitaxial ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{Ca}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{\mathit{x}}$ films in magnetic fields parallel to the c axis. Scaling analysis of the nonlinear I-V data demonstrates the existence of a quasi-two-dimensional vortex-glass--liquid transition in which the correlation length in the a-b plane only scales with \ensuremath{\Vert}T-${\mathit{T}}_{\mathit{g}}$\ensuremath{\Vert}, where ${\mathit{T}}_{\mathit{g}}$ is the vortex-glass transition temperature. The critical scaling region is very wide: from 4.2 to 50--80 K in fields of 0.5--4.0 T. In the vortex-liquid regime a finite linear resistivity ${\mathrm{\ensuremath{\rho}}}_{\mathrm{lin}}$=(E/J${)}_{\mathit{J}\ensuremath{\rightarrow}0}$ is observed, while it appears to vanish in the vortex-glass regime.

Quasi-two-dimensional vortex-glass transition observed in epitaxial bi2sr2ca2cu3ox thin films

Flux pinning centres having correlation along the c-axis in epitaxial YBCO films were investigated by measuring the magnetic-field angle ? dependence of the critical current density JC and by observing the microstructure. First, c-axis oriented YBCO films were prepared on four different substrates at three different target-to-substrate distances D by using a pulsed-laser-deposition method. Films deposited on LaAlO3 (100) and CeO2/Al2O3 with large D (112 or 142 mm) showed narrow peaks in the JC versus ? curves when the magnetic field was applied parallel to the c-axis (B//c). In these films, a high density (up to 5.5 ?m?1) of planar defects, most probably stacking faults parallel to the c-axis, were evident. The ratio of JC (B//c) to JC (B//a?b) significantly increased as the density of the planar defects increased. In contrast, films deposited on SrTiO3 (100), LaAlO3 (100) and NdGaO3 (110) with small D (50?60 mm) showed high, broad JC peaks around B//c, and had a high density (up to 430 ?m?2) of nanometre-sized precipitates that were elongated along the c-axis. The ratio of JC (B//c) to JC (B//a?b) increased as the density of these small elongated precipitates increased. In conclusion, planar defects such as stacking faults parallel to the c-axis, and the small elongated precipitates are effective as c-axis correlated flux pinning centres.

https://iopscience.iop.org/article/10.1088/0953-2048/17/1/010/pdf

Flux pinning centres correlated along the c-axis in PLD-YBCO films

Flux pinning properties of pinning centres having correlation along the c-axis in epitaxial YBCO films were investigated by measuring the magnetic-field angle ψ-dependence of the critical current density JC and the E–J-characteristics. YBCO films were prepared by using the pulsed-laser-deposition method on four different substrates at three different target-to-substrate distances D. The ψ-dependence of JC showed large peaks when magnetic field B was applied parallel to the c-axis (), and we observed two types of JC-peak: that is, a broad peak for the films deposited at small D (50–60 mm), and a narrow peak for the films deposited at large D (112 and 142 mm). The E–J-characteristics followed the power law, , and the ψ-dependence of the n-value also showed broad peaks around for the films deposited at D = 50–60 mm, and narrow peaks for the film deposited at D = 112 mm. Based on these results and our previous microstructural observations by AFM and TEM, we confirm that the broad-angle flux pinning effect around may be attributed to a high density of elongated precipitates, and the narrow-angle pinning effect around may be attributed to dense planar defects parallel to the c-axis.

https://iopscience.iop.org/article/10.1088/0953-2048/17/2/056/pdf

Flux pinning properties of c-axis correlated pinning centres in PLD-YBCO films

Series arrays of 1440 Josephson tunnel junctions were fabricated out of Nb/AN-oxide/Nb- and Nb/NbN/MgO/NbN/Nb-sandwiches The arrays provide microwave-induced constant voltage steps of at least one volt. The good stability of hard superconductor tunnel junctions against room temperature storage and thermal cyling suggests that these arrays might be very useful for the constructions of practical lasting Josephson voltage standards of high precision.

Shin Kosaka

Papers

Niobium nitride Josephson tunnel junctions with magnesium oxide barriers

Quasi-two-dimensional vortex-glass transition observed in epitaxial bi2sr2ca2cu3ox thin films

Flux pinning centres correlated along the c-axis in PLD-YBCO films

Flux pinning properties of c-axis correlated pinning centres in PLD-YBCO films

Nb/Al-oxide/Nb and NbN/MgO/NbN Tunnel Junctions in Large Series Arrays for Voltage Standards