Superconductivity is one of the most fascinating phenomena in physics, representing a quantum mechanical state where electrical resistance vanishes completely below a critical transition temperature. Materials in the superconducting state conduct electricity without any loss or heating, making superconductors highly desirable for applications ranging from magnets to quantum computing.

Early Exploration of Superconductivity

Superconductivity was first observed in 1911 when Dutch physicist Heike Kamerlingh Onnes discovered that electrical resistance in mercury abruptly vanishes at very low temperatures near absolute zero. This breakthrough earned Onnes the 1913 Nobel Prize and sparked immense interest in understanding zero resistance.

However, progress remained slow for nearly a half century given extreme technical demands of reaching 1-2K temperatures where early superconducting transitions occurred. In 1933 German researchers Walter Meissner and Robert Ochsenfeld made another pivotal discovery that superconductors perfectly expel magnetic fields - now called the Meissner effect - establishing superconductivity as a fundamental new state of matter.

The 1930s brought initial theoretical progress describing this strange behavior. In 1935 brothers Fritz and Heinz London mathematically modeled that superconducting electrons must experience no resistance and demonstrated that external magnetic fields decay exponentially inside superconductors - correctly explaining the Meissner effect. This early model formed the basis of successful microscopic superconductivity theories developed two decades later.

In subsequent years many new superconducting chemical elements and alloys were identified under ultra-cold conditions, but full explanations proved elusive. By the 1960s some 23 elements were found to superconduct including lead, tin, mercury, aluminum and niobium along with many metal alloys. But the lack of an overarching theory left zero resistance largely mysterious over 50 years since its first sighting.

Microscopic Theory of Superconductivity

In 1957 John Bardeen, Leon Cooper and Robert Schrieffer published BCS theory delivering the first complete microscopic explanation for conventional superconductivity. The Nobel Prize winning framework models the quantum mechanical behavior of electrons in a superconductor through interactions causing electrons to form paired “Cooper pairs” that act as stable composite bosons not scattered by lattice vibrations.

Specifically, electron-phonon interactions induce weak attractions between electrons with equal and opposite crystal momentum and opposite spin. Below some critical temperature these attractions overcome Coulomb repulsion leading electrons to form correlated Cooper pairs states commonly referred to as a condensate. Electron scattering is eliminated both within these pairs and between pairs due to quantum effects.

The net result is that many electrons collapse into a single macroscopic quantum state described by a single wavefunction enabling resistanceless current flow. At sufficiently low temperatures where enough electrons become paired, supercurrent flows impeded only by external magnetic fields exceeding some critical strength to break Cooper pairs.

The BCS model accurately predicts key properties experimentally confirmed in conventional low temperature superconductors. By the 1960s physicists attained solid fundamental grasp of superconductivity in these early materials. However, room temperature superconductivity with immense technological potential would remain distant for several more decades.

Conventional Low Temperature Superconductors

As explained by BCS theory, conventional superconductivity arises from electron-phonon interactions leading to paired electrons (Cooper pairs) able to carry current without resistance. Typically these conventional superconductors transition only at very low cryogenic temperatures limiting applications.

The highest temperature threshold for decades was 23K (-250 C) from a niobium-germanium alloy. This family includes elemental superconductors like mercury, lead, aluminum and niobium along with metal alloys and compounds still used in niche applications like medical MRI machines.

Conventional superconductor properties depend strongly on isotopes demonstrating electron-phonon activity underpinning Cooper pair formation. For instance, changing zinc-64 to zinc-68 drops zinc’s critical temperature from 0.85K to 0.3K. Several features constrain maximum temperatures around 20-25K reached in ad-hoc trial and error searches.

For example, increasing phonon frequencies to strengthen electron pairing competes against their tendency to disrupt superconductivity. Electron-phonon coupling exhibits similar counteracting effects. These innate limits stymied hopes for room temperature superconductors until 1986’s game-changing discovery of high temperature cuprate superconductors.

High Temperature Cuprate Superconductors

In 1986 IBM researchers Georg Bednorz and Alex Müller synthesized lanthanum barium copper oxide (LaBaCuO) displaying superconductivity up to 35K, nearly 50% higher than believed possible. This strange copper oxide or cuprate material shattered assumptions guiding prior searches - it showed no isotopic effect and conduction possibly occurred in 2D planes rather than 3D bulk.

Bednorz and Muller’s unexpected findings sparked a gold rush exploring substituted copper oxide compounds boosting the record to 50K by 1987. Within a decade critical temperatures peaked at 138K (-135C) using liquid nitrogen cooling rather than complex liquid helium systems. These high-Tc cuprates exhibited such unusual characteristics that their mechanism remains debated decades later.

Unlike conventional BCS superconductors, cuprates originate from doped Mott insulators where CuO2 planes can conduct electricity without phonon pairing mechanisms. Complex electron correlations derived from copper and oxygen orbitals appear responsible for condensation into a superfluid phase. Doping strips electrons enabling mobility that precipitates superconducting transitions once antiferromagnetic Mott regions sufficiently weaken.

The highest temperature cuprate superconductor is mercury barium calcium copper oxide (HgBaCaCuO) operating above 130K when pressurized. Cuprates show great promise for real world applications given liquid nitrogen temperatures more practical than liquid helium cooling conventional superconductors require. We will revisit modern commercial uses after reviewing additional exotic, still theoretical non-cuprate materials.

Unconventional Superconductivity Frontiers

High Tc cuprates dispelled old limits predicting low temperature superconductivity barely above absolute zero. These strange copper oxides hinted at deeper undiscovered principles and spurred searches for even higher functioning exotic superconductors.

Candidates share common traits like layered structures, magnetic properties, broken symmetries and complex electron pairing interactions potentially generalizable into new models. Let’s survey unusual recent examples demonstrating active exploratory phases without definitive explanations.

Iron pnictides superconduct above 50K exhibiting nanoscale stoichiometry tuned by doping and pressure. Some theorists propose electron pairing through magnetic fluctuations rather than phonons similar to cuprates. Structured boron-doped diamond hits 11K showing potential for room temperature goals. Controllable manufacture remains challenging.

Carbon nanotubes and graphene intrigue researchers for ultra low density superfluidity. Lattice bending theoretically enables phonon couplings suppressed in flat graphene. Proof of concept awaits experimental confirmation. Yttrium hydrogen material briefly touched room temperature superconduction at high pressures around 200 GPa according to 2022 reports. Reproducibility issues linger but optimism runs high.

Unified explanations remain elusive with many exotic superconductors displaying magnetism and spin effects hinting at new paradigms related to spin fluctuations or orbitally selective pairing mechanisms. Outside cuprates which dominate commercial applications, unconventional materials largely occupy research domains chasing ever-higher performance.

Commercial Applications of Superconductors

Despite functioning only well below freezing, costly refrigeration systems have enabled superconductor technologies improving medicine, science, aviation and many aspects of modern civilization over several decades.

Contemporary applications take advantage of two chief superconductor properties - perfect diamagnetism generating strong magnetic fields due to the Meissner effect and zero resistance enabling lossless power transmission over long distances.

Magnetic resonance imaging (MRI) machines rely on niobium-titanium coils leveraging Meissner effects which perfectly expel internal magnetic fields. Field strengths 100,000 greater focus signals producing detailed patient scan images. MRI now constitutes a multi-billion dollar healthcare industry dependent on superconductivity.

The Large Hadron Collider particle accelerator also capitalizes on high field magnets circling 27 km tunnels to accelerate proton beams to nearly light speed. Millions of niobium-titanium filaments make this immense project feasible through immense magnetic forces.

paired electrons (Cooper pairs) able to carry current without resistance. Typically these conventional superconductors transition only at very low cryogenic temperatures limiting applications.

The highest temperature threshold for decades was 23K (-250 C) from a niobium-germanium alloy. This family includes elemental superconductors like mercury, lead, aluminum and niobium along with metal alloys and compounds still used in niche applications like medical MRI machines.

Conventional superconductor properties depend strongly on isotopes demonstrating electron-phonon activity underpinning Cooper pair formation. For instance, changing zinc-64 to zinc-68 drops zinc’s critical temperature from 0.85K to 0.3K. Several features constrain maximum temperatures around 20-25K reached in ad-hoc trial and error searches.

Levitated Superconducting Trains

Japanese railways lead the way on levitated superconducting maglev trains eliminating mechanical friction for incredible speeds over 400 mph. Persistent current flows within superconducting coils induce stabilizing magnetic fields suspending entire vehicles through repulsive forces just above guideways.

Zero resistance transmission lines similarly carry current thousands of miles with nearly 100% efficiency compared to 6-8% losses over equivalent traditional copper cables. Unfortunately ultra-low temperatures require limiting use to short bursts synchronizing regional power grids.

As manufacturing costs gradually fall, applied usage should continue expanding to leverage these unique advantages. More ubiquitous commercial manifestation may ultimately rest on the most tantalizing but elusive goal - room temperature superconductors described next.

Pursuit of Room Temperature Superconductors

Ever improving cryogenics expands adoption of low temperature superconductors but the ultimate prize remains materials capable of near-zero resistance under easily maintained conditions at or above room temperature. This represents the supreme grand challenge at the highest echelon of condensed matter physics.

Some cuprates have come close but no verified examples reliably function warmer than 138K under high pressure. Compelling leads follow from less understood unconventional candidates though - diamond doping, carbon nanotubes, metallic hydrogen and still hypothetical particles like bose metals.

If achieved, civilization transforming technologies become feasible across energy, transport, computing and more. Perfectly efficient power handling could catalyze renewable energy advances and usher in ultra-fast levitated air and land vehicles. Electrical transmission reaches global ranges solving messy geospatial supply and demand mismatches. Quantum computation can run on desktops rather than complex refrigeration since low temperatures boost coherence.

Essentially world economies and societies stand to radically transform through room temperature zero resistance flow. While mere speculation today, incremental milestones towards these dreams quicken pulse rates across physics. We conclude by pondering what such a future may hold if this allure becomes reality.

Concluding Thoughts on Superconducting Progress

The wondrous state of zero resistance confounded science for nearly half a century since its 1911 discovery until the theories of Bardeen, Cooper and Schrieffer explained conventional superconductivity. Their insightful model unlocked several decades of incremental gains in critical temperatures and magnetic field strengths along with the monumental applied breakthrough of MRI imaging.

Cuprate high temperature superconductors then overturned perceived limits by demonstrating potentials beyond 123K. Now exotic room temperature candidates beckon within reach promising unprecedented technological leaps should initial reports reliably repeat at industrial scales.

Where the next revolutions emerge remains unknowable - yet persistent curiosity lifts veils, piecemeal at first, eventually disclosing landscapes of transformer technologies like superconductors...tumbling backward assumptions as the endless strange comes into brilliant focus.