Microwave absorption in YBa2Cu3O x granular thin films at the dc magnetic fields

Localization of microwave absorption in YBa₂Cu₃O _x superconducting thin films in the presence of very low dc magnetic field was studied. The granular YBa₂Cu₃O _x thin films of submicrometer thickness had random orientation of grains, were inserted into a microwave cavity for maximum rf magnetic field. The microwave absorption in low dc magnetic field at temperatures just below the critical temperature shows properties which may be ascribed to superconducting grains. At lower temperatures the hysteretic effects and flux-trapping occur in the intergrain regions of the film.     

Phase transition study of superconducting microstructures

The presented results are part of a feasibility study of superheated superconducting microstructure detectors. The microstructures (dots) were fabricated using thin film patterning techniques with diameters ranging from50µm up to500µm and thickness of1µm. We used arrays and single dots to study the dynamics of the superheating and supercooling phase transitions in a magnetic field parallel to the dot surface. The phase transitions were produced by either varying the applied magnetic field strength at a constant temperature or changing the bath temperature at a constant field. Preliminary results on the dynamics of the phase transitions of arrays and single indium dots will be reported.     

Large Increase of the Critical Field in a Magnet–Superconductor Nanowire Hybrid

We have fabricated two-dimensional periodic arrays of parallel magnetic and superconducting nanowires on a silicon substrate. Parallel magnetic (nickel) nanowires of cross section 90 nm by 300 nm form a periodic array with Pb₈₂Bi₁₈ superconducting nanowires of cross section 200 nm by 100 nm. These nanostructures were characterized with Scanning Electron Microscopy (SEM) and magnetic properties were studied with Magnetic Force Microscopy (MFM). The phase diagram was determined by electrical transport measurements. Depending on the temperature, the second critical field was 2 to 3 times larger than that of a homogeneous Pb₈₂Bi₁₈ superconducting control film. The superconducting phase diagram and transport properties exhibit strong hysteresis in a magnetic field. Results are explained on the basis of the theory of magnet–superconductor hybrids.     

Destruction of the phase coherence by magnetic field in the fluctuation region of thin superconducting films

Reduction of the gradient of current density is observed near T _c in thin superconducting films. This effect is explained by the influence of fluctuation superconductivity on the current density distribution. The observed suppression of this effect by low magnetic field is explained by the destruction of phase coherence by magnetic field. The current density distribution in high magnetic field does not differ from that in the normal state down to low temperature (T << T _c2 ). This is interpreted as evidence that the superconducting state without pinning below H _c2 is not a vortex liquid but a state without phase coherence.     

Coherence Between Superconducting Edge States in Superconducting Periodic Arrays of Artificial Defects

The transition line of superconducting arrays of holes exhibits a rich field structure due to the interference of superconducting states nucleated at the holes edges. We studied by means of resistance measurements their effect on the T* _c (H) line as a function of transverse magnetic field using regular arrays of nanofabricated micron size holes. The arrays transition fields are higher than for the bulk. Moreover we found a nontrivial field modulation of the T* _c (H) line with an inversion, with increasing field, of the modulation concavity which we assigned to a crossover from a collective to an isolated edge state regime. The high field regime is well described by the nucleation at a single hole in an infinite film. The modulation at low fields was found to be dominated by the interference of neighbor edge states when the inter-hole distance w becomes comparable to the coherence length (T* _c ). A comparison between arrays of different hole shape shows the influence of geometry on the type of interaction established, which can described either as a superconducting wire network or as a weak link array.     

Superconducting Transition of Two- and Three-Dimensional Networks in a Magnetic Field

We investigate the superconducting transition temperature T _C of two- and three-dimensional superconducting networks in a magnetic field. Making use of the de Gennes–Alexander equation, magnetic field dependence of the T _C and quantized fluxoid distributions are obtained. Especially, we calculate phase winding numbers of the superconducting order parameter around each closed path of the networks, and then deduce the fluxoid distributions. Two dimensional square lattice networks with edges have higher T _C than periodic square networks. For a three dimensional tetragonal network, T _C in the magnetic field perpendicular to the one of its face is much reduced in contrast to the case of a cubic network.     

Influence of the Trapped Field on the Levitation Performance of the Magnetized Bulk High-<Emphasis Type="Italic">T</Emphasis><Subscript><Emphasis Type="Italic">c</Emphasis></Subscript> Superconductor

A magnetized bulk high-T _c superconductor (HTSC) magnet is a good candidate to improve the levitation performance of the high-T _c superconducting (HTS) maglev system. Compared with the unmagnetized bulk HTSC, the magnetized bulk HTSC magnet can supply stronger levitation or guidance force above a permanent magnet guideway (PMG). Different from the permanent magnet, the magnetic field of a magnetized bulk HTSC magnet is sustained by the induced superconducting current produced during the magnetizing process. Given that the induced superconducting current within the magnetized bulk HTSC magnet is very sensitive to the magnetic field, the levitation performance of the magnetized bulk HTSC magnet is directly related to its own trapped field and the magnetic field of the PMG. This article discusses the influence of trapped and external magnetic fields on the levitation performance of a magnetized bulk HTSC magnet by experiments, and the Critical State Model is used to analyze the test results. The analyses and conclusions of this article are useful for the application of magnetized bulk HTSC magnet in practical HTS maglev systems.     

Current distribution measurement in YBCO thin film for a superconducting fault current limiter

Current distribution in the superconducting film for a resistive fault current limiter is important, because it influences AC loss and a uniformity of S/N transition. The lateral current distribution of the film was reconstructed from the magnetic field distribution which is measured by multiple Hall probes. The following results were obtained. (1) Non-uniform current distribution in the superconducting film was observed when the current was less than 1.3 times of critical current (I_c). (2) The current in a superconducting film was uniform when the current was much higher than I_c. The current can be considered uniform when the film works as a fault current limiter, because the S/N transition starts about twice of I_c. (3) The validity of the measurement was verified by the comparison with the electric circuit simulation.     

Superconductivity and Magnetism in R 2CaBa2Cu5O z (R=La, Pr, Nd and Eu)

A series of compounds R ₂CaBa₂Cu₅O _z (R=La, Pr, Nd and Eu) have been synthesized by conventional solid state reaction and characterized for their structural, superconducting and magnetic properties. All compounds crystallize with the tetragonal LaBa₂Cu₃O _z type structure, space group P4/mmm. Among the four compounds studied here, R=La, Nd and Eu are superconductors with superconducting transition temperatures (T _c ^R=0) of 72, 40 and 55 K, respectively. On the other hand, neither superconductivity nor magnetic ordering is seen for R=Pr down to 3 K. The effect of the magnetic field on the susceptibility and (magneto) resistance for the superconducting samples has been investigated. The superconductivity of compounds with magnetic rare-earth ions Nd and Eu exhibit a profound influence of the magnetic field, whereas the application of the magnetic field has a limited effect on the La compound. The Pr compound is paramagnetic and does not exhibit magnetic ordering either.     

Study and Application of Controlled Vortex Dynamics in?Patterned YBCO Films

Quantitative imaging of the local magnetic field and of current density distribution in superconductors (with microscopic resolution over macroscopic length scales) is achieved by means of the Magneto-Optical Imaging technique with an indicator film. We exploit this technique to study the vortex arrangement and the corresponding supercurrent distribution in high temperature superconducting YBa₂Cu₃O_7?x films. Several patterned superconducting films were studied, either non-simply connected structures, which imply macroscopic flux quantization, and superconductors whose local properties were tailored by means of confined heavy-ion irradiation. Moreover, by means of electrical transport measurements coupled with the real-time imaging of the magnetic pattern, it is directly shown how the local current distribution in patterned superconductors is affected by the electrical transport both in the Meissner and in the vortex regimes. The relevance of a controlled and localized dissipation induced by the confined vortex motion in tailored superconducting films is demonstrated for direct applications of this phenomenology to superconducting devices, such as magnetic field and photon detectors.     

Transport properties in a superconducting proximity sandwich with magnetic impurities

The thermal conductivity and ultrasonic attenuation rate are calculated numerically in a superconducting proximity sandwich containing magnetic impurities on the normal-metal side. The calculations are carried out on the basis of the theory given by Machida, in which the McMillan tunneling model is adopted to describe the proximity effect. In the limit of a thick superconducting film, analytical expressions are obtained near the transition temperature T = T _c and near T = 0. As in the case of a bulk superconductor with magnetic impurities, the existence of a bound state within an effective energy gap gives a significant effect on the transport properties. The calculation is compared with experiment on the thermal conductivity for AgMn/Pb sandwiches, and reasonable agreement is obtained between them.     

High resolution magneto-optical studies of the intermediate state in thin film superconductors

The structure of the intermediate state in superconducting lead films has been investigated as a function of magnetic field and film thickness. The detection system utilized the high specific Faraday rotation in thin films of a mixture of EuS and EuF₂ in combination with a polarizing microscope, yielding a resolution of about 1 μm. The thickness of the Pb films ranged between 0.7 and 9 μm, thus including the critical film thickness at which the transition from the intermediate state to the vortex state occurs. At low fields a liquid-like mixed state of multi-quanta flux tubes was observed which appeared to be stable up to increasing magnetic fields with decreasing film thickness. The diameter of these flux tubes varied approximately with the square root of the film thickness. At intermediate fields the intermediate state pattern was found to persist down to a film thickness of 0.7 μm, the smallest thickness investigated. The periodicity length of the intermediate state structure was in reasonable agreement with the non-branching model of Landau. Just below H_c, small superconducting domains were observed in increasing field, whereas long threads of superconducting material were formed abruptly in decreasing field. These superconducting threads were absent in the specimentsthinner than 1–2μ, being replaced by a liquid-like mixed state of superconducting tubes. After the passage of a sufficiently high electrical current through the specimen, the flux structure was found to be rearranged into long domains oriented predominantly perpendicular to the current, leading to current hysteresis effects. Finally, some dynamic observations were made during current induced flux flow.     

Model for the a.c. hall effect of high-Tcsuperconductors

The sign of the Cooper pairs in the superconducting state of the currently found high-T _c superconductors can be best measured by utilizing the so-called a.c. Hall effect. In this experiment a d.c. magnetic field is applied normal to the surface of the superconductor and a planar electromagnetic wave perpendicularly polarized incident upon the sample surface. By measuring the reflected electric field polarized in the incident plane, one could determine the sign as well as the magnitude of the a.c. Hall coefficient. The measurement provides a direct means to determine the sign and density of the Cooper pairs in the superconducting state of the high-T _c superconducting sample.     

Possibility of higher order critical fields in superconducting rings

The behavior of a small superconducting ring or hollow cylinder in an axial magnetic field is studied using the nonlinear Ginzburg-Landau equations. We find that, if the radius of the cylinder is smaller than half the superconducting coherence length (T), a periodic alternation between the superconducting and the normal state occurs as the magnetic field is increased. This defines higher order critical magnetic fields for vanishing and reentrant superconductivity.     

Hall probe Cdx Hg1-x Te for use in magnetic investigations of Nb3Sn superconducting layers

The critical temperature of a superconducting Nb₃Sn layer has been measured by repulsion of magnetic flux as a function of temperature, using a Hall probe. Hall probes of an active semiconductor film, Cd_xHg_1-xTe (x = 0.175), have been made by thermal evaporation in a vacuum. The chemical composition of the Cd_xHg_1-xTe thin films have been determined by spectrophotometric analysis. The Hall probes have been characterized by electric measurements over a temperature range of 4.2–18.5 K in a magnetic field. The probes are particulary suitable for magnetic measurements of superconducting Nb₃Sn layers at low temperatures.     

The Angular Dependence of the Critical Current in UPt3 and the Symmetry of the Order Parameter

The superconducting critical current along the c-axis of the unconventional superconductor UPt₃ has been determined as a function of magnetic field direction in the basal (a-a*) plane in the three (A, B and C) phases of the mixed state. The critical current has maxima for field directions parallel to both the a- and a*-axis in the superconducting C phase, but has maxima only parallel to the a-axis in the A and B phases. Our experiments thus show that the anisotropy of the superconducting critical current in UPt₃ changes at the same field at which the symmetry of the order parameter changes. This demonstrates for the first time that the critical current is sensitive to the symmetry of the superconducting order parameter.     

Fabrication of Metallic Magnetic Calorimeter X-ray Detector Arrays

Microcalorimeters with metallic magnetic sensors show great promise for use in astronomical X-ray spectroscopy. We describe the design and fabrication of a lithographically patterned magnetic microcalorimeter. A paramagnetic AuEr film is sputter-deposited as the sensor, which is coupled to a low noise SQUID via a meander superconducting pickup loop used as an inductor. This inductor also provides the magnetic field bias to the sensor. The AuEr film is deposited over this meander such that the field created by a large current flowing in the loop magnetizes the sensor material. The use of thin film techniques in the fabrication of these magnetic sensors not only allows strong magnetic coupling between the sensor and the inductor, it also is scalable for array fabrication.      

A Compact and Versatile Scanning Tunnelling Microscope with High Energy Resolution for Use in a 3He Cryostat

We present a simple design for a very-low temperature STM for the investigation of mesoscopic superconductors. The nonmagnetic microscope operates in a conventional ³He cryostat at T = 240 mK and in magnetic fields up to B = 1 T. Efficient filtering of all electrical lines at base temperature results in a voltage resolution ≈ 20μ V, which has been tested by measuring the differential conductance of superconducting aluminium. The successful operation in magnetic field is demonstrated by spectroscopy on superconducting aluminium as well as by demonstrating atomic resolution of HOPG.     

Vortex flow in granular aluminum films

The vortex flow and the resistive transition of granular Al films have been measured as a function of the magnetic field perpendicular to the film surface. The effects of fluctuations of the superconducting order parameter on the vortex flow resistanceR _f are discussed and compared with the theory of Maki and Takayama. The depression ofR _f belowH _c2 has been observed together with an electric field dependence ofR _f. The depression ofR _f is shown to agree qualitatively with the theoretical prediction. The field dependence ofR _f appears to be related to that of the excess conductivity aboveH _c2.     

The Little-Parks Effect in an Inhomogeneous Superconducting Ring

An inhomogeneous superconducting ring (hollow cylinder) placed in a magnetic field is considered. It is shown that the superconducting transition of the section with the lowest critical temperature may be a first order phase transition if the magnetic flux contained within the ring is not divisible by the flux quantum. In the vicinity of this transition, thermal fluctuations can induce the voltage in the ring with rather small sizes.