Electrical conductivity with exactly zero resistance
A high-temperature superconductor levitating above a magnet. A persistent electric current flows on the surface of the superconductor, acting to exclude the magnetic field of the magnet (Meissner effect). This current effectively forms an electromagnet that repels the magnet.
Superconductivity is a set of physical properties observed insuperconductors: materials whereelectrical resistance vanishes andmagnetic fields are expelled from the material. Unlike an ordinary metallicconductor, whose resistance decreases gradually as its temperature is lowered, even down to nearabsolute zero, a superconductor has a characteristiccritical temperature below which the resistance drops abruptly to zero.[1][2] Anelectric current through a loop ofsuperconducting wire can persist indefinitely with no power source.[3][4][5][6]
The superconductivity phenomenon was discovered in 1911 by Dutch physicistHeike Kamerlingh Onnes. Likeferromagnetism andatomic spectral lines, superconductivity is a phenomenon which can only be explained byquantum mechanics. It is characterized by theMeissner effect, the complete cancellation of the magnetic field in the interior of the superconductor during its transitions into the superconducting state. The occurrence of the Meissner effect indicates that superconductivity cannot be understood simply as theidealization ofperfect conductivity inclassical physics.
In 1986, it was discovered that somecuprate-perovskiteceramic materials have a critical temperature above 35 K (−238 °C).[7] It was shortly found (byChing-Wu Chu) that replacing thelanthanum withyttrium, i.e. makingYBCO, raised the critical temperature to 92 K (−181 °C), which was important becauseliquid nitrogen could then be used as a refrigerant. Such a high transition temperature is theoretically impossible for aconventional superconductor, leading the materials to be termedhigh-temperature superconductors. The cheaply available coolantliquid nitrogen boils at 77 K (−196 °C) and thus the existence of superconductivity at higher temperatures than this facilitates many experiments and applications that are less practical at lower temperatures.
Superconductivity was discovered on April 8, 1911, byHeike Kamerlingh Onnes, who was studying the resistance of solid mercury atcryogenic temperatures using the recently producedliquid helium as arefrigerant.[9] At the temperature of 4.2 K, he observed that the resistance abruptly disappeared.[10] In the same experiment, he also observed thesuperfluid transition of helium at 2.2 K, without recognizing its significance. The precise date and circumstances of the discovery were only reconstructed a century later, whenOnnes's notebook was found.[11] In subsequent decades, superconductivity was observed in several other materials. In 1913,lead was found to superconduct at 7 K, and in 1941niobium nitride was found to superconduct at 16 K.
Great efforts have been devoted to finding out how and why superconductivity works; the important step occurred in 1933, whenMeissner andOchsenfeld discovered that superconductors expelled applied magnetic fields, a phenomenon which has come to be known as the Meissner effect.[12] In 1935,Fritz andHeinz London showed that the Meissner effect was a consequence of the minimization of the electromagneticfree energy carried by superconducting current.[13]
The theoretical model that was first conceived for superconductivity was completely classical: it is summarized byLondon constitutive equations. It was put forward by the brothers Fritz and Heinz London in 1935, shortly after the discovery that magnetic fields are expelled from superconductors. A major triumph of the equations of this theory is their ability to explain the Meissner effect,[12] wherein a material exponentially expels all internal magnetic fields as it crosses the superconducting threshold. By using the London equation, one can obtain the dependence of the magnetic field inside the superconductor on the distance to the surface.[14]
The two constitutive equations for a superconductor by London are:
The first equation follows fromNewton's second law for superconducting electrons.
During the 1950s, theoreticalcondensed matter physicists arrived at an understanding of "conventional" superconductivity, through a pair of remarkable and important theories: the phenomenologicalGinzburg–Landau theory (1950) and the microscopic BCS theory (1957).[15][16]
In 1950, thephenomenologicalGinzburg–Landau theory of superconductivity was devised byLandau andGinzburg.[17] This theory, which combined Landau's theory of second-order phase transitions with aSchrödinger-like wave equation, had great success in explaining the macroscopic properties of superconductors. In particular,Abrikosov showed that Ginzburg–Landau theory predicts the division of superconductors into the two categories now referred to as Type I and Type II. Abrikosov and Ginzburg were awarded the 2003 Nobel Prize for their work (Landau had received the 1962 Nobel Prize for other work, and died in 1968). The four-dimensional extension of the Ginzburg–Landau theory, theColeman-Weinberg model, is important inquantum field theory andcosmology.
Also in 1950, Maxwell and Reynolds et al. found that the critical temperature of a superconductor depends on theisotopic mass of the constituent element.[18][19] This important discovery pointed to theelectron–phonon interaction as the microscopic mechanism responsible for superconductivity.
The complete microscopic theory of superconductivity was finally proposed in 1957 byBardeen,Cooper andSchrieffer.[16] This BCS theory explained the superconducting current as a superfluid of Cooper pairs, pairs of electrons interacting through the exchange of phonons. For this work, the authors were awarded the Nobel Prize in 1972.
The BCS theory was set on a firmer footing in 1958, whenN. N. Bogolyubov showed that the BCS wavefunction, which had originally been derived from a variational argument, could be obtained using a canonical transformation of the electronicHamiltonian.[20] In 1959,Lev Gor'kov showed that the BCS theory reduced to the Ginzburg–Landau theory close to the critical temperature.[21][22]
Generalizations of BCS theory for conventional superconductors form the basis for the understanding of the phenomenon ofsuperfluidity, because they fall into thelambda transition universality class. The extent to which such generalizations can be applied tounconventional superconductors is still controversial.
The first practical application of superconductivity was developed in 1954 withDudley Allen Buck's invention of thecryotron.[23] Two superconductors with greatly different values of the critical magnetic field are combined to produce a fast, simple switch for computer elements.
Soon after discovering superconductivity in 1911, Kamerlingh Onnes attempted to make an electromagnet with superconducting windings but found that relatively low magnetic fields destroyed superconductivity in the materials he investigated. Much later, in 1955, G. B. Yntema[24] succeeded in constructing a small 0.7-tesla iron-core electromagnet with superconducting niobium wire windings. Then, in 1961,J. E. Kunzler, E. Buehler, F. S. L. Hsu, and J. H. Wernick[25] made the startling discovery that, at 4.2 kelvin,niobium–tin, a compound consisting of three parts niobium and one part tin, was capable of supporting a current density of more than 100,000 amperes per square centimeter in a magnetic field of 8.8 tesla. The alloy was brittle and difficult to fabricate, but niobium–tin proved useful for generating magnetic fields as high as 20 tesla.
In 1962, T. G. Berlincourt and R. R. Hake[26][27] discovered that more ductile alloys of niobium and titanium are suitable for applications up to 10 tesla. Commercial production ofniobium–titanium supermagnet wire immediately commenced atWestinghouse Electric Corporation and atWah Chang Corporation. Although niobium–titanium boasts less-impressive superconducting properties than those of niobium–tin, niobium–titanium became the most widely used "workhorse" supermagnet material, in large measure a consequence of its very highductility and ease of fabrication. However, both niobium–tin and niobium–titanium found wide application in MRI medical imagers, bending and focusing magnets for enormous high-energy-particle accelerators, and other applications. Conectus, a European superconductivity consortium, estimated that in 2014, global economic activity for which superconductivity was indispensable amounted to about five billion euros, with MRI systems accounting for about 80% of that total.
In 1962,Josephson made the important theoretical prediction that a supercurrent can flow between two pieces of superconductor separated by a thin layer of insulator.[28] This phenomenon, now called theJosephson effect, is exploited by superconducting devices such asSQUIDs. It is used in the most accurate available measurements of themagnetic flux quantumΦ0 = h/(2e), whereh is thePlanck constant. Coupled with thequantum Hall resistivity, this leads to a precise measurement of the Planck constant. Josephson was awarded the Nobel Prize for this work in 1973.[29]
Multiple types of superconductivity are reported in devices made ofsingle-layer materials. Some of these materials can switch between conducting, insulating, and other behaviors.[33]
Twisting materials imbues them with a "moiré" pattern involving tiled hexagonal cells that act like atoms and host electrons. In this environment, the electrons move slowly enough for their collective interactions to guide their behavior. When each cell has a single electron, the electrons take on an antiferromagnetic arrangement; each electron can have a preferred location and magnetic orientation. Their intrinsic magnetic fields tend to alternate between pointing up and down. Adding electrons allows superconductivity by causing Cooper pairs to form. Fu and Schrade argued that electron-on-electron action was allowing both antiferromagnetic and superconducting states.[34]
The first success with 2D materials involved a twisted bilayer graphene sheet (2018, Tc ~1.7 K, 1.1° twist). A twisted three-layer graphene device was later shown to superconduct (2021, Tc ~2.8 K). Then an untwisted trilayer graphene device was reported to superconduct (2022, Tc 1-2 K). The latter was later shown to be tunable, easily reproducing behavior found in millions of other configurations. Directly observing what happens when electrons are added to a material or slightly weakening its electric field enables quick testing of an unprecedented number of recipes to see which lead to superconductivity.[33]
In four and five layer rhombohedral graphene, a form of superconductivity with spontaneously brokentime reversal symmetry known as "chiral superconductivity" was recently observed.[35] These systems were not observed to have any superlattice effects, and they can flip between two possible magnetic states without exiting the superconducting phase.[36] This is in strong contrast to other observations of superconductivity and magnetic fields.
2D materials other than graphene have also been made to superconduct.Transition metal dichalcogenide (TMD) sheets twisted at 5 degrees intermittently achieved superconduction by creating a Josephson junction. The device used used thin layers ofpalladium to connect to the sides of atungsten telluride layer surrounded and protected byboron nitride.[37] Another group demonstrated superconduction inmolybdenum telluride (MoTe₂) in 2Dvan der Waals materials using ferroelectric domain walls. The Tc was implied to be higher than typical TMDs (~5–10 K).[38]
A Cornell group added a 3.5-degree twist to an insulator that allowed electrons to slow down and interact strongly, leaving one electron per cell, exhibiting superconduction. Existing theories do not explain this behavior.
Fu and collaborators proposed that electrons arrange to form a repeating crystal that allows the electron grid to float independently of the background atomic nuclei and the electron grid to relax. Its ripples pair electrons the way phonons do, although this is unconfirmed.
A superconductor can beType I, meaning it has a singlecritical field, above which superconductivity is lost and below which the magnetic field is completely expelled from the superconductor; orType II, meaning it has two critical fields, between which it allows partial penetration of the magnetic field through isolated points[39] calledvortices.[40] Furthermore, in multicomponent superconductors it is possible to combine the two behaviours. In that case the superconductor is ofType-1.5.[41]
A superconductor is generally consideredhigh-temperature if it reaches a superconducting state above a temperature of 30 K (−243.15 °C);[45] as in the initial discovery byGeorg Bednorz andK. Alex Müller.[7] It may also reference materials that transition to superconductivity when cooled usingliquid nitrogen – that is, at onlyTc > 77 K, although this is generally used only to emphasize that liquid nitrogen coolant is sufficient. Low temperature superconductors refer to materials with a critical temperature below 30 K, and are cooled mainly byliquid helium (Tc > 4.2 K). One exception to this rule is theiron pnictide group of superconductors that display behaviour and properties typical of high-temperature superconductors, yet some of the group have critical temperatures below 30 K.
Top: Periodic table of superconducting elemental solids and their experimental critical temperature (T) Bottom: Periodic table of superconducting binary hydrides (0–300 GPa). Theoretical predictions indicated in blue and experimental results in red[46]
Several physical properties of superconductors vary from material to material, such as the critical temperature, the value of thesuperconducting gap, the critical magnetic field, and the critical current density at which superconductivity is destroyed. On the other hand, there is a class of properties that are independent of the underlying material. The Meissner effect, the quantization of themagnetic flux or permanent currents, i.e. the state of zero resistance are the most important examples. The existence of these "universal" properties is rooted in the nature of thebroken symmetry of the superconductor and the emergence ofoff-diagonal long range order. Superconductivity is athermodynamic phase, and thus possesses certain distinguishing properties which are largely independent of microscopic details. Off diagonal long range order is closely connected to the formation ofCooper pairs.
Electric cables for accelerators atCERN. Both the massive and slim cables are rated for 12,500A.Top: regular cables forLEP;bottom: superconductor-based cables for theLHCCross section of a preformed superconductor rod from the abandonedTexas Superconducting Super Collider (SSC)
The simplest method to measure theelectrical resistance of a sample of some material is to place it in anelectrical circuit in series with acurrent sourceI and measure the resultingvoltageV across the sample. The resistance of the sample is given byOhm's law asR = V / I. If the voltage drop across the sample is zero, this means that the resistance is zero.
Superconductors are also able to maintain a current with no applied voltage whatsoever, a property exploited insuperconducting electromagnets such as those found inMRI machines. Experiments have demonstrated that currents in superconducting coils can persist for years without any measurable degradation. Experimental evidence points to a lifetime of at least 100,000 years. Theoretical estimates for the lifetime of a persistent current can exceed the estimated lifetime of the universe, depending on the wire geometry and the temperature.[5] In practice, currents injected in superconducting coils persisted for 28 years, 7 months, 27 days in a superconducting gravimeter in Belgium, from August 4, 1995 until March 31, 2024.[50][51] In such instruments, the measurement is based on the monitoring of the levitation of a superconducting niobium sphere with a mass of four grams.
In a normal conductor, an electric current may be visualized as a fluid ofelectrons moving across a heavy ionic lattice. The electrons are constantly colliding with the ions in the lattice, and during each collision some of the energy carried by the current is absorbed by the lattice and converted intoheat, which is essentially the vibrationalkinetic energy of the lattice ions. As a result, the energy carried by the current is constantly being dissipated. This is the phenomenon of electrical resistance andJoule heating.
The situation is different in a superconductor. In a conventional superconductor, the electronic fluid cannot be resolved into individual electrons. Instead, it consists of boundpairs of electrons known asCooper pairs. This pairing is caused by an attractive force between electrons from the exchange ofphonons. This pairing is very weak, and small thermal vibrations can fracture the bond. Due toquantum mechanics, theenergy spectrum of this Cooper pair fluid possesses anenergy gap, meaning there is a minimum amount of energy ΔE that must be supplied in order to excite the fluid. Therefore, if ΔE is larger than thethermal energy of the lattice, given bykT, wherek is theBoltzmann constant andT is thetemperature, the fluid will not be scattered by the lattice.[52] The Cooper pair fluid is thus asuperfluid, meaning it can flow without energy dissipation.
In the class of superconductors known astype II superconductors, including all knownhigh-temperature superconductors, an extremely low but non-zero resistivity appears at temperatures not too far below the nominal superconducting transition when an electric current is applied in conjunction with a strong magnetic field, which may be caused by the electric current. This is due to the motion ofmagnetic vortices in the electronic superfluid, which dissipates some of the energy carried by the current. If the current is sufficiently small, the vortices are stationary, and the resistivity vanishes. The resistance due to this effect is minuscule compared with that of non-superconducting materials, but must be taken into account in sensitive experiments. However, as the temperature decreases far enough below the nominal superconducting transition, these vortices can become frozen into a disordered but stationary phase known as a "vortex glass". Below this vortex glass transition temperature, the resistance of the material becomes truly zero.
Behavior of heat capacity (cv, blue) and resistivity (ρ, green) at the superconducting phase transition
In superconducting materials, the characteristics of superconductivity appear when the temperatureT is lowered below a critical temperatureTc. The value of this critical temperature varies from material to material. Conventional superconductors usually have critical temperatures ranging from around 20 K to less than 1 K. Solidmercury, for example, has a critical temperature of 4.2 K. As of 2015, the highest critical temperature found for a conventional superconductor is 203 K for H2S, although high pressures of approximately 90 gigapascals were required.[53]Cuprate superconductors can have much higher critical temperatures:YBa2Cu3O7, one of the first cuprate superconductors to be discovered, has a critical temperature above 90 K, and mercury-based cuprates have been found with critical temperatures in excess of 130 K. The basic physical mechanism responsible for the high critical temperature is not yet clear. However, it is clear that a two-electron pairing is involved, although the nature of the pairing ( wave vs. wave) remains controversial.[54]
Similarly, at a fixed temperature below the critical temperature, superconducting materials cease to superconduct when an externalmagnetic field is applied which is greater than thecritical magnetic field. This is because theGibbs free energy of the superconducting phase increases quadratically with the magnetic field while the free energy of the normal phase is roughly independent of the magnetic field. If the material superconducts in the absence of a field, then the superconducting phase free energy is lower than that of the normal phase and so for some finite value of the magnetic field (proportional to the square root of the difference of the free energies at zero magnetic field) the two free energies will be equal and a phase transition to the normal phase will occur. More generally, a higher temperature and a stronger magnetic field lead to a smaller fraction of electrons that are superconducting and consequently to a longerLondon penetration depth of external magnetic fields and currents. The penetration depth becomes infinite at the phase transition.
The onset of superconductivity is accompanied by abrupt changes in various physical properties, which is the hallmark of aphase transition. For example, the electronicheat capacity is proportional to the temperature in the normal (non-superconducting) regime. At the superconducting transition, it suffers a discontinuous jump and thereafter ceases to be linear. At low temperatures, it varies instead ase−α/T for some constant,α. This exponential behavior is one of the pieces of evidence for the existence of theenergy gap.
Theorder of the superconductingphase transition was long a matter of debate. Experiments indicate that the transition is second-order, meaning there is nolatent heat. However, in the presence of an external magnetic field there is latent heat, because the superconducting phase has a lower entropy below the critical temperature than the normal phase. It has been experimentally demonstrated[55] that, as a consequence, when the magnetic field is increased beyond the critical field, the resulting phase transition leads to a decrease in the temperature of the superconducting material.
Calculations in the 1970s suggested that it may actually be weakly first-order due to the effect of long-range fluctuations in the electromagnetic field. In the 1980s it was shown theoretically with the help of adisorder field theory, in which thevortex lines of the superconductor play a major role, that the transition is of second order within thetype II regime and of first order (i.e.,latent heat) within thetype I regime, and that the two regions are separated by atricritical point.[56] The results were strongly supported by Monte Carlo computer simulations.[57]
Meissner effect in a high-temperature superconductor (black pellet) with a NdFeB magnet (metallic)
When a superconductor is placed in a weak external magnetic fieldBa=Ha, and cooled below its transition temperature, the magnetic field is ejected. The Meissner effect does not cause the field to be completely ejected but instead, the field penetrates the superconductor but only to a very small distance, characterized by a parameter λ, called theLondon penetration depth, decaying exponentially to zero within the bulk of the material. The Meissner effect is a defining characteristic of superconductivity. For most superconductors, the London penetration depth is on the order of 100 nm.
The Meissner effect is sometimes confused with the kind ofdiamagnetism one would expect in a perfect electrical conductor: according toLenz's law, when achanging magnetic field is applied to a conductor, it will induce an electric current in the conductor that creates an opposing magnetic field. In a perfect conductor, an arbitrarily large current can be induced, and the resulting magnetic field exactly cancels the applied field.
The Meissner effect is distinct from this – it is the spontaneous expulsion that occurs during transition to superconductivity. Suppose we have a material in its normal state, containing a constant internal magnetic field. When the material is cooled below the critical temperature, we would observe the abrupt expulsion of the internal magnetic field, which we would not expect based on Lenz's law.
The Meissner effect was given a phenomenological explanation by the brothersFritz andHeinz London, who showed that the electromagneticfree energy in a superconductor is minimized provided whereB is the magnetic field andλ is the London penetration depth.
This equation, which is known as theLondon equation, predicts that the magnetic field in a superconductordecays exponentially from whatever value it possesses at the surface.
A superconductor with little or no magnetic field within it is said to be in the Meissner state. The Meissner state breaks down when the applied magnetic field is too large. Superconductors can be divided into two classes according to how this breakdown occurs. In Type I superconductors, superconductivity is abruptly destroyed when the strength of the applied field rises above a critical valueHc. Depending on the geometry of the sample, one may obtain an intermediate state[58] consisting of a baroque pattern[59] of regions of normal material carrying a magnetic field mixed with regions of superconducting material containing no field. In Type II superconductors, raising the applied field past a critical valueHc1 leads to a mixed state (also known as the vortex state) in which an increasing amount ofmagnetic flux penetrates the material, but there remains no resistance to the flow of electric current as long as the current is not too large. At a second critical field strengthHc2, superconductivity is destroyed. The mixed state is actually caused by vortices in the electronic superfluid, sometimes calledfluxons because the flux carried by thesevortices isquantized. Most pureelemental superconductors, exceptniobium andcarbon nanotubes, are Type I, while almost all impure and compound superconductors are Type II.
Conversely, a spinning superconductor generates a magnetic field, precisely aligned with the spin axis. The effect, the London moment, was put to good use inGravity Probe B. This experiment measured the magnetic fields of four superconducting gyroscopes to determine their spin axes. This was critical to the experiment since it is one of the few ways to accurately determine the spin axis of an otherwise featureless sphere.
High-temperature superconductivity (high-Tc or HTS) is superconductivity inmaterials with a critical temperature (the temperature below which the material behaves as a superconductor) above 77 K (−196.2 °C; −321.1 °F), theboiling point ofliquid nitrogen.[60] They are "high-temperature" only relative to previously known superconductors, which function only closer toabsolute zero. The first high-temperature superconductor was discovered in 1986 byIBM researchersGeorg Bednorz andK. Alex Müller.[61][62] Although the critical temperature is around 35.1 K (−238.1 °C; −396.5 °F), this material was modified byChing-Wu Chu to make the first high-temperature superconductor with critical temperature 93 K (−180.2 °C; −292.3 °F).[63] Bednorz and Müller were awarded theNobel Prize in Physics in 1987 "for their important break-through in the discovery of superconductivity in ceramic materials".[64] Most high-Tc materials aretype-II superconductors.
The major advantage of high-temperature superconductors is that they can be cooled using liquid nitrogen,[61] in contrast to previously known superconductors, which require expensive and hard-to-handle coolants, primarilyliquid helium. A second advantage of high-Tc materials is they retain their superconductivity in higher magnetic fields than previous materials. This is important when constructingsuperconducting magnets, a primary application of high-Tc materials.
The majority of high-temperature superconductors areceramics, rather than the previously known metallic materials. Ceramic superconductors are suitable for some practical uses but encounter manufacturing issues. For example, most ceramics arebrittle, which complicates wire fabrication.[65]
The main class of high-temperature superconductors iscopper oxides combined with other metals, especially therare-earth barium copper oxides (REBCOs) such asyttrium barium copper oxide (YBCO). The second class of high-temperature superconductors in the practical classification is theiron-based compounds.[66][67]Magnesium diboride is sometimes included in high-temperature superconductors: It is relatively simple to manufacture, but it superconducts only below 39 K (−234.2 °C), which makes it unsuitable for liquid nitrogen cooling.
Superconductors are promising candidate materials for devising fundamental circuit elements of electronic, spintronic, and quantum technologies. One such example is a superconducting diode,[68] in which supercurrent flows along one direction only, that promise dissipationless superconducting and semiconducting-superconducting hybrid technologies.
Superconducting magnets are some of the most powerfulelectromagnets known. They are used inMRI/NMR machines,mass spectrometers, the beam-steering magnets used inparticle accelerators and plasma confining magnets in sometokamaks. They can also be used for magnetic separation, where weakly magnetic particles are extracted from a background of less or non-magnetic particles, as in thepigment industries. They can also be used in large wind turbines to overcome the restrictions imposed by high electrical currents, with an industrial grade 3.6 megawatt superconducting windmill generator having been tested successfully in Denmark.[69]
Other early markets are arising where the relative efficiency, size and weight advantages of devices based onhigh-temperature superconductivity outweigh the additional costs involved. For example, inwind turbines the lower weight and volume of superconducting generators could lead to savings in construction and tower costs, offsetting the higher costs for the generator and lowering the totallevelized cost of electricity (LCOE).[73]
Promising future applications include high-performancesmart grid,electric power transmission,transformers,power storage devices,compact fusion power devices,electric motors (e.g. for vehicle propulsion, as invactrains ormaglev trains),magnetic levitation devices,fault current limiters, enhancing spintronic devices with superconducting materials,[74] and superconductingmagnetic refrigeration. However, superconductivity is sensitive to moving magnetic fields, so applications that usealternating current (e.g. transformers) will be more difficult to develop than those that rely upondirect current. Compared to traditional power lines,superconducting transmission lines are more efficient and require only a fraction of the space, which would not only lead to a better environmental performance but could also improve public acceptance for expansion of the electric grid.[75] Another attractive industrial aspect is the ability for high power transmission at lower voltages.[76] Advancements in the efficiency of cooling systems and use of cheap coolants such as liquid nitrogen have also significantly decreased cooling costs needed for superconductivity.
As of 2022, there have been fiveNobel Prizes in Physics for superconductivity related subjects:
Heike Kamerlingh Onnes (1913), "for his investigations on the properties of matter at low temperatures which led, inter alia, to the production of liquid helium".
Leo Esaki,Ivar Giaever, andBrian D. Josephson (1973), "for their experimental discoveries regarding tunneling phenomena in semiconductors and superconductors, respectively" and "for his theoretical predictions of the properties of a supercurrent through a tunnel barrier, in particular those phenomena which are generally known as the Josephson effects".
Georg Bednorz andK. Alex Müller (1987), "for their important break-through in the discovery of superconductivity in ceramic materials".
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