The Science of Existence (64-6) Superconductivity


Electrons lose their individual identities in superconductors, in which electrons pair up to form a pervasive sea. Thus, electrons become their own antiparticles. ~ Frank Wilczek

Helium was first liquefied in 1908 by Dutch physicist Heike Kamerlingh Onnes, who studied the resistance of solid mercury when supercooled. Mercury is the only metal that is liquid at room temperature; hence its nom de plume: quicksilver. In 1911, Onnes discovered that at 4.2 K, quicksilver got quicker: shedding electric resistance entirely.

Normally, the pull of one passing electron is drowned out by ion vibrations. Cooling a metal down can lessen ionic wiggles enough that an electron’s gentle tug can be felt by another, and so the 2 electrons form a Cooper pair:  entranced to dance together via a pied-piper phonon. The temperature below which a material becomes superconducting is called its critical temperature.

A Cooper pair passing through are attractive to the ions in a superconductive metal. The metal ions stray as far toward the electron’s wake as their lattice structure allows. This distorts the crystal for a short time, creating a concentrated positive charge that encourages free electrons in the vicinity to pair up and join the flow. (Most inorganic solids, including metals, are polycrystals: a plenitude of microscopic crystals (crystallites) fused together.)

Once paired, electrons stop behaving as ordinary matter particles and enter a collective hd quantum state, where they become entangled; oblivious to the ions, and so lose no energy bumping into them. Current passes through the crystal without resistance.

Superconductivity is zero electrical resistance, resulting from electrons overcoming their mutual repulsion and pairing up, creating a coherent flow. Electrons act oddly on their way to flowing freely. In crystals that become superconductive when cooled, electrons become 1,000 times more massive than normal as they become entangled. These hot-to-trot heavy electrons are actually composite objects, mixing opposite behaviors together: localized bonding to individual atoms and freely flitting between atoms in the lattice.

Entangled electrons can become so obese that they refuse to budge: freezing into a magnetized state while stuck at an atom, spinning in unison. Tweaking the crystal composition can charm the fat electrons to dance in superconducting entanglement as the crystal is cooled.

Superconductivity is a full-blown quantum field party, characterized by the Meissner effect: the complete expulsion of magnetic field lines from the inside of a superconductor as it transitions to a superconducting state. Owing to the Meissner effect, a magnet put over a superconducting substance levitates. The electromagnetic free energy of the superconductor approaches zero, creating a repulsion of objects with intense magnetic field lines, like the repulsion between a magnet and a diamagnetic object.

Absence of resistance does not make a magnet grow fonder; quite the opposite. Magnetism surrenders to superconductivity.

As antiferromagnetism breaks down, superconductivity appears, encouraging electrons to pair up and flow freely. Long-range magnetism – where atoms align their magnetic moments – ceases in giving way to superconductivity. This abdication belies a deeper connection.

Magnetism is the quantum glue underlying the emergence of superconductivity. ~ Dirk Morr

The electromagnetism in the electrons involved in superconductive substances bifurcates into an electric charge and a magnet. American physicist Peter Anderson suggested the fractionalization of electron properties (“fractional particles”) as facilitating superconductivity in 1987. Altering the interactivity of magnetism is key to superconductivity.

Whereas atomic magnetism vanishes with superconductivity, local magnetic moments become more powerful, as the electrons in individual atoms synchronize. Superconductivity is a transformation of the turbulent flow of an electron sea into coherence by ordering quantum spin.

Relatively high-temperature superconductivity has been shown in insulators. Intuitive expectation would be that conductors would become more conductive when cooled. One would not expect that insulators would reverse themselves, flipping from resisting to superconducting.

The search for high-temperature superconductivity in hydrogen-rich compounds hinges on a theory that, under certain circumstances, elements that have low atomic masses can contribute to high critical temperatures. Hydrogen, being the lightest element, is optimal for high critical temperatures.  ~ American physicist James Hamlin

Temperature is not the only variable affecting potential superconductive materials. Applied pressure matters. Under pressure (specifically, 16,000 times atmospheric pressure (1.6 Gpa)), iron selenides superconduct at 30–32 K. Pressed further, superconductivity disappears at 9 Gpa, only to reappear at 12.4 Gpa. Yet the basic structure of the compound is retained throughout the pressuring.

More recent research has focused on the internal structure of Cooper pairs. Cooper pairs in “conventional” superconductors spin in opposite directions, resulting in the pair having zero spin.

In more exotic “triplet” superconductors, spins line up, such that a Cooper pair carries some spin of its own. Lanthanum-nickel-carbon (LaNiC2) and lanthanum-nickel-gallium (LaNiGa2) are exemplary triplet superconductors.

Unlike conventional superconductors, triplet superconductors are not ferromagnetic. The magnetic moments of the Cooper pairs themselves create the magnetism favorable to superconductivity.

Non-unitary triplet pairing bootstraps itself into superconductivity via quantum spin liquidity generated by emergent superconductivity. This is a superconducting analogue of how magnetism develops in ferromagnetic metals.

Superconductivity appears in divergent materials at different temperatures owing to their magnetic properties and tendency to become a better conductor of electricity in certain directions. Subatomic alignment matters.

Superconductivity shows how the electromagnetic force is highly variable, depending upon material and ambient conditions. Superconductivity remains a mysterious demonstration of 4d effects from hd interactions.

Emergence, the coming into being through evolution, is an important concept in modern condensed-matter physics. Superconductivity is a classic example of emergence in the realm of quantum matter: as the energy scale decreases, the effective electron–electron interactions responsible for Cooper pairing, and thus superconductivity, evolves from the elementary microscopic Hamiltonian through unanticipated modifications.

This evolution is why it is so difficult to derive superconductivity from first principles. Finding the microscopic mechanism of Cooper pairing means discovering the nature of the ultimate effective electron–electron interaction at the lowest energy scales. ~ Scottish Irish American physicist Séamus Davis & Taiwanese American physicist Dung-Hai Lee