Superconductivity: A Developing Yet Highly Practical Field

Sometimes, the most important scientific discoveries are serendipitous. This was almost certainly the case with Kamerlingh Onnes’s 1911 discovery of the phenomenon known as superconductivity [1]. As he investigated the resistance of certain metals at extremely low, sub-10K temperatures, he stumbled upon a startling fact: resistivity of electrical conductors drops to zero below a certain temperature, known as the critical temperature [1].

Scientists further refined Onnes’ observations and, in the process, developed many theories as to why superconductivity occurs. The prevalent theory as to why this phenomenon occurs is known as BCS theory, which posits that superconductivity is caused by the pairing of electrons in a specific manner. Cooper pairs, as these pairs as known, are two electrons that are brought together by phonons, which are basic forms of different quanta, or levels of energy [2, 3]. A simplified explanation is that in a normal metal, the electrons are free to “roam,” and they are attracted to the positive metal ions. Thus, around them, there is a net positively charged field. When another electron is attracted to this field, the attractive force becomes much stronger than the two electrons’ mutually repulsive force. These pairs are the charge carriers that travel through the superconductor’s lattice [2, 4].

The scientists who postulated the BCS theory won the Nobel Prize for Physics in 1972, and this led to a renewed wave of interest in the theory of superconductivity, especially in its applications to magnetism and what magnetic effect, if any, would surround the superconducting wire. The Meissner effect, scientists discovered, followed directly from BCS theory [4].

The Meissner effect, in which magnetic fields are excluded from the area around the superconducting wire, seems to be perhaps the most useful and feasible application of superconductivity [2]. Teachers commonly demonstrate this to their classes by showing the “levitation” of a piece of metal above a superconductor; this is merely the repulsive effect of the magnetic field around the superconductor [2].

Significant effort has been put into discovering superconductors that can work at the relatively high temperatures of 90 to 110 degrees Kelvin [4]. This minimizes the cost of the coolant required to operate it. For many superconductors, liquid helium is necessary, providing for temperatures up to 5 K, its boiling point [4]. Naturally, this is a very limiting range, so liquid nitrogen has been proposed as an alternative. Usage of liquid nitrogen provides for temperatures up to 77 K, its boiling point, and this has become the coolant of choice [5]. Added to the fact that liquid nitrogen is universally available and inexpensive to synthesize, physicists the world over have seen this as an excellent aid to their research.

Superconductors whose critical temperature is above the boiling point of liquid nitrogen are some of the most coveted discoveries in condensed matter physics today [1,6]. These are referred to as high-temperature superconductors, and all known high-temperature superconductors are Type II superconductors, which have different properties and composition than Type I superconductors that lie in the bonding and the phenomena exhibited [1].

Type II superconductors are ceramics, some of which are insulators at higher temperatures [2]. Almost all of them contain some amount of copper, hence they are they referred to as cuprates. The amount of most elements varies, shown by formulas such as Y(Ba1-xSrx)2Cu4O8, and depending upon how metals such as yttrium and lanthanum are combined with other nonmetal atoms such as oxygen, the critical temperature for the resulting superconductor can be very high or very low [7]. Type II superconductors, such as YBCO (yttrium-barium-copper-oxide), are the preferred type of superconductors in commercial and practical usage.

The Meissner effect, coupled with the use of high-temperature Type II superconductors, is the basis of technology ranging from maglev trains in Japan to MRI machines in Silicon Valley hospitals. In fact, the train that set the world speed record ran on superconducting magnets [8]. The repulsion between the magnets on the bottom of the train and the superconductors in the “track” helps move the maglev train forward while keeping it centimeters off the track [6]. MRIs use superconducting magnets to create an extremely potent magnetic field that affects the atoms in the human body [8]. The reactions of these atoms are picked up by specialized equipment. Even though many people do not even know the role superconductors play in our current day-to-day lives, let alone that such a technology exists, novel uses of these devices are quickly being invented and refined in order to benefit society as a whole.

Superconductors have also been used to accelerate subatomic particles in the Large Hadron Collider at CERN in Switzerland up to 99.9% of the speed of light [9]. The efficacious use of superconductors, like at CERN, can help yield dramatic results, such as the discovery of the Higgs boson this past summer [10]. Experiments such as these may help shed light on the origins of the universe. Other experiments at CERN have made use of SQUIDs, or superconducting quantum interference devices. These extremely sensitive devices are able to measure magnetic fields on the order of 5 attoTesla (5*10-18 Tesla) [10]. Magnetic fields this small can help in such diverse activities as ghost hunting to controlling MRI scanners more precisely; magnetic fields this small can also help in precise factory automation [11].

Clearly, superconductivity has many applications from fields ranging from transportation to particle physics to even medicine. So now, many people may ask “When can I get zero-resistance current delivered to my house?”

These people automatically assume that superconducting wires can easily be used to transmit power from power plants to residential neighborhoods. Currently, this is a rather infeasible option because of the extremely low temperatures needed for most superconductors– perhaps making YBCO power lines useful in Antarctica [6]. In other more habitable places where the average temperature ranges from 10-20 degrees Celsius (283-293 K), superconducting power lines are impossible as of now [12]. However, that may soon change.

Researchers of the University of Leipzig stumbled upon a startling fact that may soon change the impossibility of superconducting power lines. They doped flakes of graphite with water and tested the flakes for superconductivity at temperatures well above 373 K [13]. It appeared to demonstrate the Meissner effect, an “identity test” for a superconductor [6, 13].

Naturally, it will take years for superconducting graphite power lines to catch on, as the technology and physics behind these must be refined and commercialized. As the allure of discovering high-temperature superconductors becomes greater, consideration and implementation of the plethora of superconductors’ potential applications hinges on the successful discovery of many more of these novel Type II superconductors.

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Rahul Jayaraman is a student at The Harker School. Follow The Triple Helix Online on Twitter and join us on Facebook.