Superconductor/normal metal

From here follow main goals in the SIT problem to find theoretical models which would lead to the negative magnetoresistance to trace how the specific SIT properties appear in the field-induced superconductor-normal metal transition while the normal metal is shifted toward the insulating state to find out whether the low dimensions of the films is crucial or the SIT can happen in 3D materials to find the explanation for the pair localization alternative to the boson-vortex duality. It seems that the first two goals are achieved. 

Superconducting BSCCO films were produced in situ on 3" diameter sapphire substrates by plasma-enhanced halide LPCVD [266, 267]. Films consisting mainly of the Bi-2212 phase were deposited with metal halide precursors at 580°C under 0.1 Torr system pressure in the presence of a rf plasma. These films became superconducting at 70 K with = 2.5 x 10 Acm at 10 K. Plasma-enhanced halide LPCVD was also used to grow Bi-Sr-Ca-Cu-O/Bi-Sr-Cu-O superconductor-normal metal (S-N) heterostructures [259], HRTEM images showed the S/N interface to be atomically abrupt while variable temperature resistivity measurements gave T. = 75 K for the S/N heterostructure. 

In 1962, Hilsch conducted a series of experiments in which the transition temperatures of sandwiches formed from superconductor thin films deposited on top of normal metal thin films were measured [68]. It was noted that the transition temperature of the superconductor/ normal metal sandwich was less than that of the bulk superconductor and that the thinner the superconductor layer, the lower the transition temperature of the sandwich. Hilsch also found that if the superconductor layer was sufficiently thin and held at a constant thickness, the observed transition temperature of the sandwich decreased as the normal metal layer thickness increased. Eventually, the metal layer reached a thickness that exceeded the dimensions of the proximity effect, and the transition temperature of the sandwich arrived at a constant value. 

A number of researchers have investigated the contact resistance phenomena that occur between high temperature superconductors and conventional materials [63,86-90]. Low contact resistance values are necessary for many practical applications. High contact resistance values can lead to local heating and the loss of superconductivity at the contact interface. In addition, future applications may involve the use of superconductor/ normal metal/superconductor (S-N-S) Josephson junctions, and these devices can be fabricated only with materials that exhibit low contact resistance values. 

R. H. Ono, J. A. Beall, M. W. Cromar, T. E. Harvey, M. E. Johansson, C. D. Reintsema, and D. Rudman. High-Tc superconductor-normal metal-superconductor Josephson microbridges with high resistance normal metal links, Appl. Phys. Lett. 59 1126 (1991). 

Tunneling electric current through the normal metal insulator superconductor junction is accompanied with heat flow out of normal metal when property voltage is biased. The phenomenon enables cooling of electrons and phonons (under special conditions) in the region below 1K. At lower bath temperatures, two parasitic heat sources decrease refrigerator performance ... 

FIGURE 10.1 A plot of resistivity, p, vs. temperature, T, illustrating the drop to zero at the critical temperature, Tc, for a superconductor, and the finite resistance of a normal metal at absolute zero. 

Conductor-Superconductor Transition When some metals or compounds are cooled below a certain temperature, their electrical resistance drops abruptly to zero. This temperature is referred to as the superconducting transition temperature. These materials are classified into two categories, type I or type II superconductors, depending upon how a bulk sample behaves in an external magnetic field. In the absence of an external magnetic field, the (superconductor + normal) transition is continuous in both types of superconductors. When a magnetic field is applied, the transition becomes first order in type I superconductors, but remains continuous in the type II superconductors. 

The process of Andreev reflection involves a spin-up electron of energy E coupling with a spin-down electron of energy —E to form a Cooper pair in the superconductor. If the normal metal is replaced by a ferromagnet with a finite polarization P, not all electrons of one spin species will be able to find a corresponding electron of the opposite spin species in order to form Cooper pair. Hence, the probability of Andreev reflection will be reduced by a factor of (l-P), where we define the polarization P by... 

STM Spectroscopy of the Local Density of States in Normal Metal - Superconductor Systems... 

In a normal metal (N) coupled to a superconductor (S), superconducting properties are locally induced by the proximity effect. The characteristic energy scale of this proximity superconductivity is given by the minimum of the bulk superconductor energy gap A and the Thouless energy ec ... 

In conclusion, we mention that the effects of disorder on the kinetics of quasiparticles confined in an insulator/normal-metal/superconductor (INS) hybrid structure due to Andreev reflections was first considered in Ref. [12] within a model where the disorder is provided by irregularities on the I/N boundary through the normal scattering of quasiparticles. 

The quantity er is the relative dielectric constant or permittivity of a dielectric medium, eo = 8.85 x 10 12 As/Vm. The quantity tan 6 represents the loss tangent of a dielectric medium. Metals in the microwave range are usually described by a complex conductivity with dominant real part for normal metals and dominant imaginary part for superconductors. 

A perfect superconductor is a material that, when cooled below a characteristic temperature called the critical temperature, conducts electricity without any losses or any heating, and expels magnetic fields from its interior. The former property is called zero resistance, and the latter is called perfect diamagnetism. At temperatures above T, it is a normal metal, and is ordinarily not a very good conductor. For example, lead and tin become superconductors while copper and silver, which are much better conductors, do not superconduct. 

Binary and ternary alloys and oxides of these elements, as well as pure V, Nb, Gd, and Tc are referred to as Type II or high-field superconductors. In contrast to Type I, these materials exhibit conductive characteristics varying from normal metallic to superconductive, depending on the magnitude of the external magnetic field. It is noteworthy to point out that metals with the highest electrical conductivity (e.g., Cu, Au) do not naturally possess superconductivity. Although this behavior was first discovered in 1911 for supercooled liquid mercury, it was not until 1957 that a theory was developed for this phenomenon. 

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