Superconductor transitions

This work shows the exceptional physics that can be done with a STM operated at cryogenic temperatures and the availability of STMs working down to liquid helium temperature opens broad avenues of research in the coming years. No doubt that among the many future scientific experiments accessible with low temperature STMs, the real-space electronic characterization of the metal-superconductor transition in /c-phases of BEDT-TTF salts, because Tc > 4 K, as well as the study of magnetic ordering in MOMs, will certainly occupy a relevant position. 

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. 

Chapters 13 and 14 use thermodynamics to describe and predict phase equilibria. Chapter 13 limits the discussion to pure substances. Distinctions are made between first-order and continuous phase transitions, and examples are given of different types of continuous transitions, including the (liquid + gas) critical phase transition, order-disorder transitions involving position disorder, rotational disorder, and magnetic effects the helium normal-superfluid transition and conductor-superconductor transitions. Modem theories of phase transitions are described that show the parallel properties of the different types of continuous transitions, and demonstrate how these properties can be described with a general set of critical exponents. This discussion is an attempt to present to chemists the exciting advances made in the area of theories of phase transitions that is often relegated to physics tests. 

An analysis of the IR reflection spectra of organic conductors with activated ag modes makes it possible to determine, among others, the values of e-mv coupling constants, which are instrumental in estimating the electric conductivity of the system or calculating the temperature of a potential metal-superconductor transition. 

On the basis of the electrical properties, it is advisable to divide these transitions into three general classes and to consider these classes in the following order semiconductor-to-semiconductor transitions in the present chapter, and metal-to-semiconductor and metal-to-superconductor transitions in Chapters 9 and 10. The case of polymers is treated separately in Chapters 11, 12, and 13. 

Crucibles of titanium nitride and zirconium nitride are utilized for the melting of lanthanum alloys. ZrN, HfN and TaN are also used as electrodes in electronic valves. Niobium and zirconium nitrides could be used as superconductors due to their relatively high superconductor transition temperatures of 16.8 and 10 K respectively. 

Niobium and zirconium nitrides have relatively high superconductor transition temperatures... 

The problematic nature of the melting transition can be illustrated by comparison with other well-known first-order phase transitions, for instance the normal metal-(low T ) superconductor transition. The normal metal-superconductor and melting transitions have similar symptomatic definitions, the former being a loss of resistance to current flow, and the latter being a loss of resistance to shear. However, superconductivity can also be neatly described as a phonon-mediated (Cooper) pairing of electrons and condensation of Cooper pairs into a coherent ground state wave function. This mechanistic description of the normal metal-super-conductor transition has required considerable theoretical effort for its development, but nevertheless boils down to a simple statement, indicat-... 

In Fig. 1 we show typical R T) behavior in parallel orientation of the magnetic field of 0.5 T for two samples, MLS with N w=5 (S type) and ML6 with Nbii=6 (N type). The resistive characteristics are quite different and depend on the type of the samples symmetry. For the N-type sample (ML6) the usual sharp curve reveals the resistive fall down to zero. For the S-type sample (MLS) the superconductor transition is broadened. At large magnetic fields R T) curves become sharp for both the samples. The behavior shown in Fig. 1 has been found for all our samples with... 

Superconductivity. The insulator-superconductor transition takes place near this composition probably as a result of the percolation of the hole doped clusters. At x = 6.342 small amounts of the material become superconducting (fig. 27b). Superconductivity under pressure A giant ATJAP effect has been measured at RT (sect. 5.4) indicating an increased compressibility at the T-0 transition due to the half-filled chains and strong oxygen-ordering effects. The effect disappears at LT where the mobility of oxygen freezes (fig. 58). 

A very recent study done by Sun et al. showed saniconductor-metal superconductor transitions in CTystalline boron nanowires under high pressure. It is evident from their study that boron nanowires tend to metalize at higher pressure more than bulk P-ihombohedral boron and exhibit super conductivity at 1.5 K at 84 GPa. This tendency can solely be attributed to the size of the material that takes effect at a higher magnitude in nanostructures than in bulk. 

With non-zero expression (Eq. (20)) is very similar to the Landau-Ginzburg functional describing the normal-superconductor transition [1, 22, 23] ... 

Type II (Av5<. Two critieal values of the field are found. Vortiees bearing a quantum flux (j)Q = h/q penetrate the system for H ci-... 

Kf = 0 for 2 Vj > V, the eigendirec-tion is stable. The problem reduces exactly to the normal-superconductor transition [29] which implies Vj =V. There is no such stable fixed point, however, in the smectic case (n = 2, d=3) [30],... 

The SmA liquid crystalline phase results from the development of a one-dimensional density wave in the orientationally ordered nematic phase. The smectic wave vector q is parallel to the nematic director (along the z-axis) and the SmA order parameter i/r= i/r e is introduced by P( ) = Po[1+R6V ]- Thus the order parameter has a magnitude and a phase. This led de Gennes to point out the analogy with superfluid helium and the normal-superconductor transition in metals [7, 59]. This would than place the N-SmA transition in the three-dimensional XY universality class. However, there are two important sources of deviations from isotropic 3D-XY behavior. The first one is crossover from second-order to first-order behavior via a tricritical point due to coupling between the smectic order parameter y/ and the nematic order parameter Q. The second source of deviation from isotropic 3D-XY behavior arises from the coupling between director fluctuations and the smectic order parameter, which is intrinsically anisotropic [60-62]. 

Up to now, we have discussed mostly model intermetallic compounds with simple crystal structures (generally cubic LI2, B2,. . . ) and containing two metal species. We shall now present briefly some properties of point defects in more exotic systems, of considerable interest the A15 superconductors, transition-metal carbides and nitrides, and III-V semiconductors (e.g. GaAs). 

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