At a critically low temperature, certain materials, like metal and alloy, conduct electricity with practically zero resistance. This behaviour allows the particles inside the electric current to maintain its perpetual loop for billions of years without losing any energy. Finding the mechanism behind such capability may pave the way for a large variety of technological applications and globally purposes. This will be further clarified in this article.
The Origin of Superconductivity and BCS Theory
The history of superconductivity began with Dutch physicist Heike Kamerlingh Onnes’s investigation of the resistivity in materials. When Onnes dropped solid mercury into liquid helium at the temperature of 4.19 Kelvins, he observed the resistivity of mercury abruptly disappeared. He unveiled his research in 1911, in a paper titled “On the Sudden Rate at which the Resistance of Mercury Disappears”. Onnes initially stated the phenomenon “supraconductivity”, and it was later adopted by the term “superconductivity”.
In subsequent decades, superconductivity was found in several cold metals at certain temperatures; as lead at 7K, niobium at 10K, and niobium nitride at 16K.
After the breakthrough concept of physics initiated by Heike Kamerlingh Onnes of his 1911’s discovery, a comprehensive theory in 1957s by the American physicists, John Bardeen, Leon Cooper and John Schieffer (their surname initials providing designation BCS) was developed to further explain the behaviours of superconducting materials. Any superconductors, when cooled to cryogenic temperatures, will tend to lose all resistance to the flow of electric current. Knowing that the moving free electrons in the electric current must follow Pauli’s exclusion Principle which forbids particles with asymmetrical wave function (fermions) from sharing the same quantum states within the same region. Nevertheless, throughout the established predictions on the Fermi lattice, Cooper stated that electrons are grouped in pairs when the lattice reached its critical temperature, now known as Cooper pairs. Coupling electrons within a single superconductor constitute a system that can function as a single entity. In other words, when mobile electrons paired together, their properties are no longer fermions, but rather bosons as they behave like single particles. Their overall wave function becomes symmetric and the electron pairs can communally share the same minimum properties within the metal. As the temperature is warmed, electron pairs will separate back into individual, free electrons and the system returns normal, or non-superconducting.
In conventional superconductors, the attraction between electrons in the current is due to the electron-phonon interaction that causes the occurrence of electron pairs. At a critical temperature, Cooper had observed over the behaviours of superconductors that the steady flow of pairing electrons interact with the metal lattice atom via vibrations. The ideas that two moving electrons become coupled seemed contradicting with the common fact that the force between free electrons is repulsive because they carry the same negative electrical charge. Nonetheless, Cooper pairing is a quantum effect, the reason for the paring can be seen from classical explanations.
Although electrons tend to repel each other due to their negative charged, they attract positive ions that create the rigid lattice of the metal. Passing free electrons attract the lattice causing a slight ripple which could be detected and calculated in microscopic levels. The phonon-mediated attraction between mobile electrons occurs in a metal which causes paired state of electrons to have lower energy than Fermi energy. This interaction relates to the quantum of energy, often called phonon, which associated with the vibrational energy of oscillating atoms in the superconductor lattice.
Furthermore, the attraction distorts the ion lattice, moving the ions slightly toward the passing electrons, which increase positive charge density in the vicinity. This positive charge density can attract more electrons at a long distance. As a result, the attraction between electrons due to displaced ions can overcome the electrons’ repulsive force, which causes them to tie up in pairs. The electrons bound in pairs are not necessarily close together; because the interaction is long-range and paired electrons may still be linking over a range of hundreds of nanometres. The energy between the pairing electrons is relatively weak and unstable, at about 10−3 eV, and thermal energy can easily destruct the bond and break them apart. So, a significant amount of the electrons in Cooper pairs can only be found and materialised in metal or other alloys at low temperature.
The Relationship between Superfluidity and Bose-Einstein Condensations
The term ‘superfluidity’ was first discovered in the 1930s, however, superfluid’s inexplicable behaviours are still perplexing researchers to disclose its most subtle mechanism. Among its odd properties, a superfluid is characterised by having zero viscosity, the ability to flow without apparent friction. When stirred, a superfluid can continue to spin indefinitely. At critical points, a superfluid behaves very strangely when placed in a container. When radiation warms the superfluid, it can produce the fountain effect since the temperature gradient is virtually absent. When the container is open, the fluid can creep up the walls of the container and leak out as it flows against gravity. One example was observed to possess the characteristics of a superfluid is supercooled helium-4, which contains bosons that made up of pairs of ultracold fermions. Long after the pioneering observation of these effects in liquid helium, the development of laser cooling technique for atomic gases has offered experimentalists a new framework for investigating the fundamental concepts of superfluidity, and its indeterminate relationship with Bose-Einstein condensation.
Bose-Einstein condensations first predicted by Einstein in 1925s and based on ideas by Indian physicist Satyendranath Bose. BECs is a state of matter, which is typically formed when a gas of bosons at low density is cooled to a temperature near absolute zero. Einstein speculated that, at such ultra-cold condition, bosons would condensate and fall into their lowest energy quantum state, resulting in a new form of matter. Accordingly, electrons with the lowest quantum state would be able to surf through the resistance atom in the electric current without dissipating energy. This mechanism is theoretically a key contributor to the understanding of puzzling condensed matter phenomena, such as high-energy superconductivity.
Superfluidity is sometimes coincidental with the existence of Bose-Einstein condensation. BECs sometime behave as superfluids and their postulation are somewhat similar, but neither phenomenon is directly related to each other. As not all superfluids are regarded as Bose-Einstein condensates and vice versa. However, later until 1938s, a German physicist Fritz Wolfgang London proposed that the BECs could perform as a mechanism for superfluidity in Helium-4, Knowing that there is no limitation for boson to share the same quantum state in the same place. Below a transition temperature, bosons accumulate a single one-particle quantum state that might be responsible for superfluid of liquid helium. Therefore, it was quickly believed that superfluidity was due to partial Bose-Einstein condensation of the liquid. The properties of a superfluid – a liquid that flows without viscosity and transfers heat without temperature gradient – are intimately related to BEC that occur in this strongly interacting fluid.
Warm Superconductors
In the 1980s, superconductor technology took off. Late until 1986s, Swish researchers discovered a pioneering new class of ceramic materials that became a superconductor at unpredictably warm temperature – so-called ‘high-temperature superconductor’. The first well-known compound was discovered as having the ability to superconduct at high temperatures, the combination of lanthanum, transitioned to superconducting behaviour at 38 Kelvins. A year later, there had been surprising progress in technology, Yttrium compound was the material that became a superconductor at 92 Kelvins, warmer than the widely used coolant liquid hydrogen. Using perovskite-based ceramic and mercuric-cuprates, superconducting temperatures have now reached around 140 Kelvins. The temperatures keep marching on to even higher critical temperatures with advance hydrogen-rich compounds that are attainable at particularly high pressure.
Conventional superconductors are typically made of metal, which usually works below -200°C – so-called low-temperature superconductors. On the other hand, the majority of high-temperature superconductors are ceramics materials. Such ceramics materials are generally brittle and non-metallic. They are notoriously excellent thermal insulators and can withstand high temperatures. Especially, ceramics possess an intrinsic property – atomic magnetic moments – that exhibit a certain type of permanent magnetism. The major advantage of a high-temperature ceramic superconductor is that they can be cooled by using liquid nitrogen. Whereas, metallic superconductors at low temperature require more difficult coolant – mostly liquid helium.
Although holding an adequate understanding of the characteristics and properties of ceramic materials, physicists are still searching for a new theory to explain the accurate operation behind a warm superconductor.
Uses of Superconductors
Superconductors are used to make extremely powerful electromagnets to accelerate charged particles and electricity. It is predicted that they are capable of operating Maglev (magnetic levitation) trains. This works efficiently on superconductors because, theoretically, they tend to repel the magnetic field. So, a magnet will float above the superconductor, resulting in the virtual elimination of friction between the train and the track. All in all, there is an effort on the search for high-temperature superconductors that might lead to dramatic technological implementations in the future.
Bibliography
Baker, J. (2014). Superconductivity. In J. Baker, 50 Physics Ideas You Really Need To Know (pp. 124-127). London: Quercus .
Carusotto, I. (2010, January 19). Sorting superfluidity from Bose-Einstein condensation in atomic gases. Retrieved from Physics.: https://physics.aps.org/articles/v3/5
Wikipedia. (2014, December 1). Cooper pair. Retrieved from https://en.wikipedia.org/wiki/Cooper_pair
Wikipedia. (2017, July 2). Superfluidity. Retrieved from https://en.wikipedia.org/w/index.php?title=Superfluidity&action=history
Wikipedia. (2019, December 9). High-temperature superconductivity. Retrieved from https://en.wikipedia.org/wiki/High-temperature_superconductivity
Wikipedia. (2020, May 28). Bose-Einstein condensate. Retrieved from https://en.wikipedia.org/wiki/Bose–Einstein_condensate
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