A DC SQUID consists of a superconducting loop with two Josephson junctions or weak links. Its operation is based on the fact that as a result of quantum interference the maximum dissipationless current I c that can flow through the SQUID is periodic in the magnetic flux Φ through the loop 7
General arguments suggest that first-order phase transitions become less sharp in the presence of weak disorder, while extensive disorder can transform them into second-order transitions; but the atomic level details of this process are not clear. The vortex lattice in superconductors provides a unique system in which to study the first-order transition on an inter-particle scale, as well as over a wide range of particle densities. Here we use a differential magneto-optical technique to obtain direct experimental visualization of the melting process in a disordered superconductor. The images reveal complex behaviour in nucleation, pattern formation, and solid-liquid interface coarsening and pinning. Although the local melting is found to be first-order, a global rounding of the transition is observed; this results from a disorder-induced broad distribution of local melting temperatures, at scales down to the mesoscopic level. We also resolve local hysteretic supercooling of microscopic liquid domains, a non-equilibrium process that occurs only at selected sites where the disorder-modified melting temperature has a local maximum. By revealing the nucleation process, we are able to experimentally evaluate the solid-liquid surface tension, which we find to be extremely small.
Transport studies in a Corbino disk geometry suggest that the Bragg glass phase undergoes a first-order transition into a disordered solid. This transition shows a sharp reentrant behavior at low fields. In contrast, in the conventional strip configuration, the phase transition is obscured by the injection of the disordered vortices through the sample edges, which results in the commonly observed vortex instabilities and smearing of the peak effect in NbSe 2 crystals.These features are found to be absent in the Corbino geometry, in which the circulating vortices do not cross the sample edges.
A nanometer-size superconducting quantum interference device (nanoSQUID) is fabricated on the apex of a sharp quartz tip and integrated into a scanning SQUID microscope.A simple self-aligned fabrication method results in nanoSQUIDs with diameters down to 100 nm with no lithographic processing. An aluminum nanoSQUID with an effective area of 0.034 µm 2 displays flux sensitivity of 1.8 × 10 −6 Φ 0 / √ Hz and operates in fields as high as 0.6 T.With projected spin sensitivity of 65 µ B / √ Hz and high bandwidth, the SQUID on a tip is a highly promising probe for nanoscale magnetic imaging and spectroscopy.Imaging magnetic fields on a nanoscale is a major challenge in nanotechnology, physics, chemistry, and biology. One of the milestones in this endeavor will be the achievement of sensitivity sufficient for detection of the magnetic moment of a single electron. patterning methods; 3-11 the large in-plane size of the devices precludes bringing the SQUID loop into sufficiently close proximity to the sample (due to alignment issues) to scan it with optimal sensitivity. Recently, a terraced SQUID susceptometer was developed that is based on a multilayered lithographic process combined with FIB etching. This device includes a 600 nm pickup loop which can be scanned 300 nm above the sample surface. 12 Here we present a simple method for the self-aligned fabrication of a DC nanoSQUID on a tip with effective diameter as small as 100 nm that can be scanned just a few nm above the sample.We have fabricated several SQUID-on-tip (SOT) devices of various sizes. A quartz tube of 1 mm outside diameter is pulled to a sharp tip with apex diameter that can be controllably varied between 100 and 400 nm. The fabrication of the SOT consists of three "self-aligned" steps of thermal evaporation of Al, as shown schematically in Fig. 1a. In the first step, 25 nm of Al are deposited on the tip tilted at an angle of -100 • with respect to the line to the source. Then the tip is rotated to an angle of 100 • , followed by a second deposition of 25 nm. As a result, two leads on opposite sides of the quartz tube are formed, as shown in Fig. 1b. In the last step 17 nm of Al are evaporated at an angle of 0 • , coating the apex ring of the tip. The two areas where the leads contact the ring form "strong" superconducting regions, whereas the two parts of the ring in the gap between the leads, indicated by arrows in Fig. 1c, constitute two weak links, thus forming the SQUID. The resulting nanoSQUID requires no lithographic processing, its size is controlled by a conventional pulling procedure of a quartz tube, and it is located at the apex of a sharp tip that is ideal for scanning probe microscopy.The studies were carried out at 300 mK, well below the critical temperature T c ≈ 1.6 K of granular thin films of aluminum in our deposition system. Instead of the commonly used current 2 bias, the SOT was operated in a voltage bias mode, as shown schematically in the inset to Fig. 2. We use a low bias resistance R b of about 2 Ω and the SOT current...
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