0.23 Chapter 8. superconductors.  (Page 3/3)

    The first company to capitalize on high-temperature superconductors was Illinois Superconductor (today known as ISCO International ), formed in 1989. This amalgam of government, private-industry and academic interests introduced a depth sensor for medical equipment that was able to operate at liquid nitrogen temperatures (~ 77K).

   In recent years, many discoveries regarding the novel nature of superconductivity have been made. In 1997 researchers found that at a temperature very near absolute zero an alloy of gold and indium was both a superconductor and a natural magnet. Conventional wisdom held that a material with such properties could not exist!  Since then, over a half-dozen such compounds have been found. Recent years have also seen the discovery of the first high-temperature superconductor that does NOT contain any copper (2000), and the first all-metal perovskite superconductor (2001).

     Also in 2001 a material that had been sitting on laboratory shelves for decades was found to be an extraordinary new superconductor. Japanese researchers measured the transition temperature of magnesium diboride at 39 Kelvin - far above the highest Tc of any of the elemental or binary alloy superconductors. While 39 K is still well below the Tc's of the "warm" ceramic superconductors , subsequent refinements in the way MgB ₂ is fabricated have paved the way for its use in industrial applications. Laboratory testing has found MgB ₂ will outperform NbTi and Nb ₃ Sn wires in high magnetic field applications like MRI .

     Though a theory to explain high-temperature superconductivity still eludes modern science, clues occasionally appear that contribute to our understanding of the exotic nature of this phenomenon. In 2005, for example, Superconductors.ORG discovered that increasing the weight ratios of alternating planes within the layered perovskites can often increase Tc significantly . This has led to the discovery of more than 70 new high-temperature superconductors , including a candidate for a new world record .

     The most recent "family" of superconductors to be discovered is the "pnictides". These iron-based superconductors were first observed by a group of Japanese researchers in 2006. Like the high-Tc copper-oxides, the exact mechanism that facilitates superconductivity in them is a mystery. However, with Tc's over 50K , a great deal of excitement has resulted from their discovery.

     Researchers do agree on one thing: discovery in the field of superconductivity is as much serendipity as it is science.

Section 8.1.Meisnner Effect in Superconductors.

As the temperature falls below a critical temperature there is phase change and the metal under consideration becomes super-conductor as shown in Figure 8.1. In normal metal no such sudden drop in resistance is observed or measured.

In Figure 8.2. a long cylindrical YBCO bar is shown kept in an external magnetic field. Below critical temperature, as the metal becomes superconductor it becomes strongly diamagnetic. In presence of an external magnetic field, strong eddy currents are set up within the body which in accordance with Lenz’s law completely excludes the external magnetic field from the interior of the body as shown in Figure 8.2.b. As the metal returns to normal condition above the critical temperature, external magnetic field passes through the body but causes no induced magnetization as shown in Figure 8.2.a.

Table 8.1. tabulates some well known superconductors and some recently discovered high temperature ceramics of cuparate variety. The record high critical temperature to date is 153K. It also tabulates the critical magnetic field above which superconductivity gets killed. For ceramic YBCO type superconductors also there is a critical Magnetic Field H _C2 and H _C1 but it has not been shown in the Table.

Section 8.2. BCS theory of Superconductivity.

In 1957, John Bardeen, Leon Cooper and John Schriffer proposed the cooper pair theory. According to this theory electrons are Fermions and real particles with J _S (Spin Angular Momentum)= ±(1/2)ħ hence susceptible to photon and defect scattering resulting in resistive metals.

But as temperature falls, electrons couple to form virtual particles with J _S = 0ħ. These are Bosons which are virtual particles and hence not susceptible to photon and defect scattering. This leads to sudden drop in resistance as shown in Figure 8.1.

This theory came to be known as BCS Theory.

BCS Theory predicts isotope effect in the following manner:

This equation predicts that for lighter isotopes super-conductivity can be maintained till higher temperatures.

BCS Theory predicts a critical Magnetic Field also. External Magnetic Fields greater than B _C (the critical magnetic flux density) kills the superconductivity phase of the given metal.

BCS theory is strictly for metals. It completely fails to explain the superconductivity in Cuparate Ceramics.