How superconductivity moved from mercury to modern devices

Who discovered superconductivity? Kamerlingh Onnes in Leiden

Kamerlingh Onnes developed the refrigeration needed to study matter near absolute zero. NobelPrize.org dates his successful liquefaction of helium to 1908 [1]. Liquid helium let his laboratory cool metals into a temperature range that had previously been inaccessible. The discovery came from measuring an ordinary electrical property under extraordinary thermal conditions.

The iScience review places mercury's superconducting transition near 4.2 kelvin [3]. Its resistance did not simply continue a gradual decline as the specimen cooled; it disappeared at a transition. This was a new state of matter, rather than an exceptionally good version of normal metallic conduction. Identifying its magnetic behavior and microscopic origin would require further experiments and theories [2].

The Meissner effect and the London brothers

The iScience review dates Walther Meissner and Robert Ochsenfeld's discovery of magnetic flux expulsion to 1933 [3]. Cooling into the superconducting state could change the magnetic field inside a specimen. A hypothetical perfect conductor would instead preserve its existing internal flux. The Meissner effect therefore established a distinction between zero resistance alone and the equilibrium magnetic response of a superconductor [2].

NobelPrize.org's scientific account dates the theory of brothers Fritz and Heinz London to 1935 [2]. Their equations described superconducting currents and explained why magnetic fields penetrate only a limited distance into the surface. The London penetration depth remains an important material property. The theory captured electromagnetic behavior without yet explaining the microscopic mechanism that creates the superconducting state [2].

Ginzburg-Landau, BCS, and Abrikosov's vortices

NobelPrize.org dates the Ginzburg-Landau theory of Vitaly Ginzburg and Lev Landau to 1950 [2]. It described superconductivity through an order parameter that can vary across a specimen. This made the theory useful for surfaces, magnetic fields, and the boundary between normal and superconducting regions. It was a phenomenological description, built around observable behavior rather than a microscopic account of electron pairing.

NobelPrize.org dates the Bardeen-Cooper-Schrieffer theory, usually called BCS, to 1957 [2]. John Bardeen, Leon Cooper, and Robert Schrieffer explained conventional superconductivity through paired electrons forming a coherent collective state. The theory supplied the microscopic explanation that earlier electromagnetic models lacked. Its original form is not a complete account of every superconducting material discovered since then [2].

NobelPrize.org also dates Alexei Abrikosov's theory of type II superconductors to 1957 [2]. Building on Ginzburg-Landau theory, he showed how magnetic flux could enter as vortices while superconductivity persisted around them. That mixed state helped explain why suitable superconductors could function in strong magnetic fields. In practical conductors, restraining vortex motion is essential because moving flux can produce dissipation [3].

Josephson junctions and practical magnet conductors

NobelPrize.org dates Brian Josephson's prediction of supercurrent through a thin insulating barrier to 1962 [4]. The Josephson effect connected the quantum phases of superconductors across a junction. It became the basis of sensitive magnetic sensors and superconducting circuit elements. A junction is an engineered interface, not a continuous length of superconducting wire [4].

The iScience review identifies the 1960s as a period when niobium-titanium, or NbTi, and niobium-tin, or Nb3Sn, drove the development of practical superconducting magnets [3]. Their material discoveries and their development into useful conductors were separate milestones. NbTi could be drawn into fine filaments within a stabilizing metal matrix; brittle Nb3Sn required different fabrication routes. Turning superconductivity into a magnet technology depended on manufacturing and current capacity as well as transition temperature [3].

Cuprates and the liquid-nitrogen threshold

In their 1986 paper in Zeitschrift fur Physik B, Georg Bednorz and Alex Mueller reported a sharp resistance decrease in a barium-lanthanum-copper-oxide system, with an onset in the 30 kelvin range [5]. The paper cautiously described possible high-temperature superconductivity. It opened the copper-oxide, or cuprate, era and challenged the expectation that substantially higher transition temperatures would be difficult to achieve in known material families.

Physical Review Letters published M. K. Wu and colleagues' Y-Ba-Cu-O result in 1987, reporting a superconducting transition between 80 and 93 kelvin at ambient pressure [6]. This crossed the liquid-nitrogen cooling threshold, which the iScience review places at 77 kelvin [3]. The material family became known as YBCO. Higher transition temperature broadened cooling options, but ceramic brittleness and current transport across grain boundaries still made conductor development demanding [3].

Magnesium diboride and iron-based materials

Nature published Jun Nagamatsu and colleagues' discovery of superconductivity in magnesium diboride, or MgB2, at 39 kelvin in 2001 [7]. This was a comparatively simple compound outside the cuprate family. It showed that important discoveries could still emerge from familiar chemistry. The iScience review describes its development into wires and the need to improve flux pinning for useful operation in magnetic fields [3].

The Journal of the American Chemical Society published Yoichi Kamihara and colleagues' fluorine-doped lanthanum iron arsenide result at 26 kelvin in 2008 [8]. It established a major iron-based high-temperature superconducting family, distinct from the cuprates. New families expanded the scientific questions as well as the candidate materials. A promising transition temperature did not establish that a material could already be manufactured as a long, dependable conductor [3].

High-pressure hydrides and the remaining engineering gap

Nature published Alexander Drozdov and colleagues' sulfur-hydride result at 203 kelvin in 2015 [9]. The experiments combined a resistance drop with magnetic evidence and required extreme pressure. The authors identified a hydrogen-rich sulfur phase as the likely source of the high transition temperature. This made compressed hydrides a major research direction, rather than a material ready for an ambient-pressure cable or magnet.

The history has therefore split into connected scientific and engineering tracks. Discovering a superconducting phase answers whether the state can exist under specified conditions. Making it useful also requires current capacity, field tolerance, mechanical integrity, refrigeration, and reproducible manufacturing. The iScience review treats these constraints as central to practical superconducting materials [3]. A higher transition temperature alone does not complete that work.