Superconducting state

Superconductivity The physical state in which all resistance to the flow of direct-current electricity disappears is defined as superconductivity. The Bardeen-Cooper-Schriefer (BCS) theoiy has been reasonably successful in accounting for most of the basic features observed of the superconducting state for low-temperature superconductors (LTS) operating below 23 K. The advent of the ceramic high-temperature superconductors (HTS) by Bednorz and Miller (Z. Phys. B64, 189, 1989) has called for modifications to existing theories which have not been finahzed to date. The massive interest in the new superconductors that can be cooled with liquid nitrogen is just now beginning to make its way into new applications. 

Three important characteristics of the superconducting state are the critical temperature, the critical magnetic field, and the critical current. These parameters can be varied by using different materials or giving them special metallurgical treatments. 

For all these reasons, the stability of the superconducting state and ways to control it are questions of prime importance. Many studies have addressed the degradation of the properties of HTSC under the influence of a variety of factors. They included more particularly the corrosion resistance of HTSC materials exposed to aqueous and nonaqueous electrolyte solutions as well as to water vapor and the vapors of other solvents. It was seen that the corrosion resistance depends strongly both on the nature (chemical composition, structure, etc.) of the HTSC materials themselves and on the nature of the aggressive medium. 

In some cases the stability of HTSC materials in contact with electrolytes is quite satisfactory, so they can be used for electrochemical measurements. Such measurements are made for various reasons. A number of workers have used cyclic voltam-mograms to characterize the state of HTSC materials. A constant shape of these curves over a certain length of time was evidence for conservation of the superconducting state during this time interval. 

Many papers have been published regarding HTSCs used as inert, nonconsumable electrodes for kinetic and mechanistic studies of various electrode reactions occurring at them. Most of these studies were performed at room temperature when the materials were not in their state of superconductivity. Unfortunately, to date a given reaction has rarely been studied at similar temperatures just above and below r , that is, at temperatures where the same material is once in its normal state and once in its superconducting state. The electronic stracture of materials differs sharply between these two states, and quantitative studies under these conditions might provide valuable information as to the mechanism of the elementary act of charge transfer from the electrode to a reacting species, and vice versa. 

Zn) and Ga( Zn) isotopes, the transition to the superconducting state leads to a change of the electron density on the metal sites, above the transition the center shift is determined by SOD, below by the influence of Bose condensation, a correlation between the change in electron density and the temperature of the transition to the superconducting state is found... 

Gold is the only known dopant to YBa2Cu307, 5, which increases the critical temperature Tc of transition to the superconducting state. In this way, is enhanced from 97 K by 1.5 K. Eibschiitz et al. [401] observed by Mossbauer studies that the... 

In this superconducting state, the electrical resistivity of the material becomes zero, and its thermal properties change. Tc is a function of the material as well as of its purity and of the applied magnetic field. 

In particular, if the magnetic field is strong enough, the material does not enter the superconducting state. The latter property is shown in Fig. 3.4(a) where the specific heat of A1 was measured [17] in no-field and in a moderate field. 

The jump in ce is due to the fact that the superconducting metal has a new degree of freedom, i.e. the possibility of entering the superconducting state. For simple superconductors, such as A1 and Sn, the Bardeen-Cooper-Schrieffer (BCS) theory [18-22] gives ... 

Thermodynamic arguments [23] indicate that the transition from the normal to the superconducting state at zero field does not involve a latent heat and therefore must be a higher-order transition. Experimental evidence indicates that it is second-order transition. 

When the BETS donor replaces the BEDT-TTF electron donor molecule during the electrocrystallization process, crystals of KL-(BETS)2Ag(CF3)4(TCE) have been prepared [29] and structurally characterized. Replacement of the inner sulfur atoms of BEDT-TTF with selenium results in a slight expansion of the unit cell and prevents the stabilization of a superconducting state above 1.2 K. Disorder in one of the BETS ethylene endgroups has been offered as a possible explanation. 

Although the superconducting state of this material has attracted considerable attention, the metallic state is also of interest. For instance, metallic samples are being explored for use as cathode materials in solid oxide fuel cells (Section 6.10). 

Fowler-Nordheim tunneling of, 22 258 in HBTs, 22 167-168 Moore s law and device scaling and, 22 254 in RTDs, 22 170-171 in semiconducting silicon, 22 485-486 in semiconductors, 22 233, 237-239 in SETs, 22 171-172 in single layer OLEDs, 22 215-216 in spinel ferrites, 11 60-61 in the superconducting state, 23 804 Electron spectrometer system, components of, 24 100-101... 

At low temperatures, matter will undergo a transition to a color-superconducting state, with a different quasiparticle structure than presumed in our quasiparticle approach. Nonetheless, pairing affects the thermodynamic bulk properties only at the relative order of 0(A2/fi2), where the estimated gap A < 100 MeV is comfortably smaller than the chemical potential. Therefore, our equation of state is a reasonable approximation even at small temperatures (maybe except for the pressure where it becomes very small). 

---