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The physics of the superconducting state in two-dimensional (2D) electron systems is relevant to understanding the high-transition-temperature copper oxide superconductors and for the development of future superconductors based on interface electron systems (1). But it is not yet understood how fundamental superconducting parameters, such as the spectral density of states, change when these superconducting electron systems are depleted of charge carriers. Here we use tunnel spectroscopy with planar junctions to measure the behaviour of the electronic spectral density of states as a function of carrier density, clarifying this issue experimentally. We chose the conducting LaAl[O.sub.3]SrTi[O.sub.3]interface (2) as the 2D superconductor, because this electron system can be tuned continuously with an electric gate field (3). We observed an energy gap of the order of 40 microelectronvolts in the density of states, whose shape is well described by the Bardeen-Cooper-Schrieffer superconducting gap function. In contrast to the dome-shaped dependence of the critical temperature, the gap increases with charge carrier depletion in both the underdoped region and the overdoped region. These results are analogous to the pseudogap behaviour of the high-transition-temperature copper oxide superconductors and imply that the smooth continuation of the superconducting gap into pseudogap-like behaviour could be a general property of 2D superconductivity., One of the main challenges in understanding the superconductivity of copper oxide superconductors is to identify the origin and nature of the pseudogap phase in the underdoped regime (4,5). This [...]

The review discusses extensively the phase-sensitive methods supporting d-wave pairing symmetry to explain and clarify the controversies surrounding pairing symmetry in the cuprates. The d-wave theory embodies phase-sensitive methods and half-integer flux-quantum effect studies.

An inhomogeneous grain boundary critical current density was observed in high temperature grain boundary junctions combined with a d(sub x2-y2) symmetry component of the order parameter. The effect is more apparent in grain boundaries with asymmetric 45-degree tilt. A negative critical current density was also observed for a large fraction of the junctions' facets, leading to highly anomalous magnetic field dependences.

A high-resolution scanning superconducting quantum interference device microscopy study of tetragonal single-layer [TI.sub.2][Ba.sub.2][CuO.sub.6+δ] films, deposited on tricrystal [SrTiO.sub.3] substrates, demonstrates the effect of spontaneously generated half flux quanta. This observation shows that in addition to [YBa.sub.2][Cu.sub.3][O.sub.7], the order parameter symmetry in [TI.sub.2][Ba.sub.2][CuO.sub.6+δ] is consistent with that of a [d.sub.[x.sup.2]-[y.sup.2]] pair state. This result also rules out any bilayer or twinning effects and any pairing that is incompatible with the fourfold rotational symmetry as in the [TI.sub.2][Ba.sub.2][CuO.sub.6+δ], superconducting system., An unambiguous determination of the pairing symmetry in copper oxide superconductors is necessary to understand the origin of high-temperature superconductivity. A test of order parameter symmetry requires a definitive measurement [...]

The magnetic properties of thin film pickup loop structures for a superconducting quantum interference device magnetometer are investigated at sub-micrometer dimensions. The structures, which were designed using a three-level-of-metal approach, are analyzed for their magnetic coupling behavior. Results show that the performance of the loop structures is strongly affected by the design of the low-inductance interconnect structure.

Constraints on the pairing symmetry, a superconducting order parameter, is obtained by the use of the half-integer flux quantization in superconducting rings having three grain-boundary Josephson junctions. The constraints reduce the various ways in which the Cooper pairs are formed in the high temperature superconductor YBa2Cu3O(7-delta). This parameter depends on the direction of the crystal phase isotropy.

In the interlayer tunnelling (ILT) model, superconductivity is created by tunnelling of electron pairs between copper oxide planes. It predicts that superconducting condensation energy is almost equal to kinetic energy gain. Both energies can be independently determined to give a quantitative test of the model. Direct imaging of magnetic flux vortices also provides a test, which was performed in the high temperature superconductor TI(sub2)Ba(sub2)Cu0(sub6).

A magnetic imaging technique has been developed using a magnetoresistive spin valve in a dc bias mode as the sensing element. A submicron spatial resolution map of the normal component of the magnetic field is obtained in the temperature range 4.2-300 K. The technique has been applied to image the magnetic domain structure in patterned thin films of La(sub0.67)Sr(sub0.33)MnO(sub3).

Knowledge of the symmentry of the order parameter to describe electron pairing in the superconducting state is important to the development of theoretical models for high temperature superconductivity. An experiment is described studying the single-layer tetragonal superconductor TI sub 2 Ba sub 2 Cu0 sub 6 + delta. The results showed that the material has pure dx squared-y squared pairing symmetry, providing further understanding of the more complicated mixed s + d state appearing in other high temperature superconductors.

The bio-functionalization and detection of 12-nm manganese ferrite nanoparticles (NPs) is reported. A novel method of detecting either protein binding or DNA hybridization at room temperature using the NPs and magnetic tunnel junction (MTJ) biosensor situated in orthogonal magnetic fields is demonstrated.

The dependence on field and temperature of the critical current micro superconducting quantum interference devices (SQUIDs), using Dayem bridges as weak links, is reported. It is seen that phase drops and depression of the order parameter produced by supercurrent flow are often distributed throughout the micro-SQUID structure, rather being localized in the bridge area, for typical micro-SQUID geometries and coherence lengths.

The scanning SQUID microscope has proved to be a valuable tool for the study of fundamental properties of solid state systems. It has the advantages of high magnetic field sensitivity, a linear, easily calibrated response over a huge dynamic range, and uses sensors which (at least for the low-Tc SQUIDs) incorporate a mature technology. It has the disadvantages of the requirement of a cooled sensor and modest spatial resolution. I will briefly describe the current status and potential advances in this instrument, and then discuss some example fundamental applications. These examples include pairing symmetry tests, tests of the interlayer tunneling model for the pairing mechanism, the search for `visons'', and evidence for a precursor to superconductivity well above Tc, all in the high-Tc cuprate superconductors; and measurements of the properties of very small magnetic particles. [Copyright &y& Elsevier]

Examines the limit on spin-charge separation in high-T[subc] superconductors in the absence of a vortex-memory effect. Effect of spin-charge separation on the properties and transition temperatures of T-materials; Analysis of spin-charge separation in T-materials; Benefits of vortex-memory effect for copper oxides.