Superconductivity

From Wikipedia, the free encyclopedia

 

A magnet levitating above a high-temperature superconductor, cooled with liquid nitrogen. Persistent electric current flows on the surface of the superconductor, acting to exclude the magnetic field of the magnet (Faraday's law of induction). This current effectively forms an electromagnet that repels the magnet.

 

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Video of a Meissner effect in a high-temperature superconductor (black pellet) with a NdFeB magnet (metallic)

A high-temperature superconductor levitating above a magnet

Superconductivity is a phenomenon of exactly zero electrical resistance and expulsion of magnetic flux fields occurring in certain materials, called superconductors, when cooled below a characteristic critical temperature. It was discovered by Dutch physicist Heike Kamerlingh Onnes on April 8, 1911, in Leiden. Like ferromagnetism and atomic spectral lines, superconductivity is a quantum mechanical phenomenon. It is characterized by the Meissner effect, the complete ejection of magnetic field lines from 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 the idealization of perfect conductivity in classical physics.

The electrical resistance of a metallic conductor decreases gradually as temperature is lowered. In ordinary conductors, such as copper or silver, this decrease is limited by impurities and other defects. Even near absolute zero, a real sample of a normal conductor shows some resistance. In a superconductor, the resistance drops abruptly to zero when the material is cooled below its critical temperature. An electric current through a loop of superconducting wire can persist indefinitely with no power source.^[1][2][3][4]

In 1986, it was discovered that some cuprate-perovskite ceramic materials have a critical temperature above 90 K (−183 °C).^[5] Such a high transition temperature is theoretically impossible for a conventional superconductor, leading the materials to be termed high-temperature superconductors. The cheaply-available coolant liquid nitrogen boils at 77 K, and thus superconduction at higher temperatures than this facilitates many experiments and applications that are less practical at lower temperatures.

Classification

There are many criteria by which superconductors are classified. The most common are:

Response to a magnetic field

A superconductor can be Type I, meaning it has a single critical field, above which all superconductivity is lost and below which the magnetic field is completely expelled from the superconductor; or Type II, meaning it has two critical fields, between which it allows partial penetration of the magnetic field through isolated points. These points are called vortices. Furthermore, in multicomponent superconductors it is possible to have combination of the two behaviours. In that case the superconductor is of Type-1.5.

By theory of operation

It is conventional if it can be explained by the BCS theory or its derivatives, or unconventional, otherwise.^[6]

By critical temperature

A superconductor is generally considered high-temperature if it reaches a superconducting state when cooled using liquid nitrogen – that is, at only T_c > 77 K) – or low-temperature if more aggressive cooling techniques are required to reach its critical temperature.

By material

Superconductor material classes include chemical elements (e.g. mercury or lead), alloys (such as niobium-titanium, germanium-niobium, and niobium nitride), ceramics (YBCO and magnesium diboride), superconducting pnictides (like fluorine-doped LaOFeAs) or organic superconductors (fullerenes and carbon nanotubes; though perhaps these examples should be included among the chemical elements, as they are composed entirely of carbon).

Elementary properties of superconductors

Most of the physical properties of superconductors vary from material to material, such as the heat capacity and the critical temperature, critical field, and 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. For instance, all superconductors have exactly zero resistivity to low applied currents when there is no magnetic field present or if the applied field does not exceed a critical value. The existence of these "universal" properties implies that superconductivity is a thermodynamic phase, and thus possesses certain distinguishing properties which are largely independent of microscopic details.

Zero electrical DC resistance

Electric cables for accelerators at CERN. Both the massive and slim cables are rated for 12,500 A. Top: regular cables for LEP; bottom: superconductor-based cables for the LHC

 

Cross section of a preform superconductor rod from abandoned Texas Superconducting Super Collider (SSC).

The simplest method to measure the electrical resistance of a sample of some material is to place it in an electrical circuit in series with a current source I and measure the resulting voltage V across the sample. The resistance of the sample is given by Ohm's law as R = V / I. If the voltage 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 in superconducting electromagnets such as those found in MRI machines. Experiments have demonstrated that currents in superconducting coils can persist for years without any measurable degradation. Experimental evidence points to a current 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.^[3] In practice, currents injected in superconducting coils have persisted for more than 22 years in superconducting gravimeters.^[7][8] In such instruments, the measurement principle is based on the monitoring of the levitation of a superconducting niobium sphere of mass 4 grams.

In a normal conductor, an electric current may be visualized as a fluid of electrons 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 into heat, which is essentially the vibrational kinetic 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 and Joule 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 bound pairs of electrons known as Cooper pairs. This pairing is caused by an attractive force between electrons from the exchange of phonons. Due to quantum mechanics, the energy spectrum of this Cooper pair fluid possesses an energy 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 the thermal energy of the lattice, given by kT, where k is Boltzmann's constant and T is the temperature, the fluid will not be scattered by the lattice. The Cooper pair fluid is thus a superfluid, meaning it can flow without energy dissipation.

In a class of superconductors known as type II superconductors, including all known high-temperature superconductors, an extremely low but nonzero 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 of magnetic 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 tiny 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.

Superconducting phase transition

Behavior of heat capacity (c_v, blue) and resistivity (ρ, green) at the superconducting phase transition

In superconducting materials, the characteristics of superconductivity appear when the temperature T is lowered below a critical temperature T_c. 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. Solid mercury, for example, has a critical temperature of 4.2 K. As of 2009, the highest critical temperature found for a conventional superconductor is 39 K for magnesium diboride (MgB₂),^[9][10] although this material displays enough exotic properties that there is some doubt about classifying it as a "conventional" superconductor.^[11] Cuprate superconductors can have much higher critical temperatures: YBa₂Cu₃O₇, one of the first cuprate superconductors to be discovered, has a critical temperature of 92 K, and mercury-based cuprates have been found with critical temperatures in excess of 130 K. The explanation for these high critical temperatures remains unknown. Electron pairing due to phonon exchanges explains superconductivity in conventional superconductors, but it does not explain superconductivity in the newer superconductors that have a very high critical temperature.

Similarly, at a fixed temperature below the critical temperature, superconducting materials cease to superconduct when an external magnetic field is applied which is greater than the critical magnetic field. This is because the Gibbs 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 longer London 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 a phase transition. For example, the electronic heat 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 as e^−α/T for some constant, α. This exponential behavior is one of the pieces of evidence for the existence of the energy gap.

The order of the superconducting phase transition was long a matter of debate. Experiments indicate that the transition is second-order, meaning there is no latent 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^[12] 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 a disorder field theory, in which the vortex lines of the superconductor play a major role, that the transition is of second order within the type II regime and of first order (i.e., latent heat) within the type I regime, and that the two regions are separated by a tricritical point.^[13] The results were strongly supported by Monte Carlo computer simulations.^[14]

Meissner effect

When a superconductor is placed in a weak external magnetic field H, 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 the London 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 of diamagnetism one would expect in a perfect electrical conductor: according to Lenz's law, when a changing 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 which 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 brothers Fritz and Heinz London, who showed that the electromagnetic free energy in a superconductor is minimized provided

${\displaystyle \nabla ^{2}\mathbf {H} =\lambda ^{-2}\mathbf {H} \,}$

where H is the magnetic field and λ is the London penetration depth.

This equation, which is known as the London equation, predicts that the magnetic field in a superconductor decays 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 value H_c. Depending on the geometry of the sample, one may obtain an intermediate state^[15] consisting of a baroque pattern^[16] 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 value H_c1 leads to a mixed state (also known as the vortex state) in which an increasing amount of magnetic 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 strength H_c2, superconductivity is destroyed. The mixed state is actually caused by vortices in the electronic superfluid, sometimes called fluxons because the flux carried by these vortices is quantized. Most pure elemental superconductors, except niobium and carbon nanotubes, are Type I, while almost all impure and compound superconductors are Type II.

London moment

Conversely, a spinning superconductor generates a magnetic field, precisely aligned with the spin axis. The effect, the London moment, was put to good use in Gravity 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.

History of superconductivity

Heike Kamerlingh Onnes (right), the discoverer of superconductivity. Paul Ehrenfest, Hendrik Lorentz, Niels Bohr stand to his left.

Superconductivity was discovered on April 8, 1911 by Heike Kamerlingh Onnes, who was studying the resistance of solid mercury at cryogenic temperatures using the recently produced liquid helium as a refrigerant. At the temperature of 4.2 K, he observed that the resistance abruptly disappeared.^[17] In the same experiment, he also observed the superfluid 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, when Onnes's notebook was found.^[18] In subsequent decades, superconductivity was observed in several other materials. In 1913, lead was found to superconduct at 7 K, and in 1941 niobium 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, when Meissner and Ochsenfeld discovered that superconductors expelled applied magnetic fields, a phenomenon which has come to be known as the Meissner effect.^[19] In 1935, Fritz and Heinz London showed that the Meissner effect was a consequence of the minimization of the electromagnetic free energy carried by superconducting current.^[20]

London theory

The first phenomenological theory of superconductivity was London theory. 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,^[19] 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.^[21]

There are two London equations:

${\displaystyle {\frac {\partial \mathbf {j} _{s}}{\partial t}}={\frac {n_{s}e^{2}}{m}}\mathbf {E} ,\qquad \mathbf {\nabla } \times \mathbf {j} _{s}=-{\frac {n_{s}e^{2}}{m}}\mathbf {B} .}$

The first equation follows from Newton's second law for superconducting electrons.

Conventional theories (1950s)

During the 1950s, theoretical condensed matter physicists arrived at an understanding of "conventional" superconductivity, through a pair of remarkable and important theories: the phenomenological Ginzburg-Landau theory (1950) and the microscopic BCS theory (1957).^[22][23]
In 1950, the phenomenological Ginzburg-Landau theory of superconductivity was devised by Landau and Ginzburg.^[24] This theory, which combined Landau's theory of second-order phase transitions with a Schrödinger-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, the Coleman-Weinberg model, is important in quantum field theory and cosmology.

Also in 1950, Maxwell and Reynolds et al. found that the critical temperature of a superconductor depends on the isotopic mass of the constituent element.^[25][26] This important discovery pointed to the electron-phonon interaction as the microscopic mechanism responsible for superconductivity.

The complete microscopic theory of superconductivity was finally proposed in 1957 by Bardeen, Cooper and Schrieffer.^[23] 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, when N. 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 electronic Hamiltonian.^[27] In 1959, Lev Gor'kov showed that the BCS theory reduced to the Ginzburg-Landau theory close to the critical temperature.^[28][29]

Generalizations of BCS theory for conventional superconductors form the basis for understanding of the phenomenon of superfluidity, because they fall into the lambda transition universality class. The extent to which such generalizations can be applied to unconventional superconductors is still controversial.

Further history

The first practical application of superconductivity was developed in 1954 with Dudley Allen Buck's invention of the cryotron.^[30] Two superconductors with greatly different values of 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 ^[31] 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 ^[32] made the startling discovery that, at 4.2 kelvin, 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. Despite being brittle and difficult to fabricate, niobium-tin has since proved extremely useful in supermagnets generating magnetic fields as high as 20 tesla. In 1962 T.G. Berlincourt and R.R. Hake ^[33][34] discovered that alloys of niobium and titanium are suitable for applications up to 10 tesla. Promptly thereafter, commercial production of niobium-titanium supermagnet wire commenced at Westinghouse Electric Corporation and at Wah Chang Corporation. Although niobium-titanium boasts less-impressive superconducting properties than those of niobium-tin, niobium-titanium has, nevertheless, become the most widely used “workhorse” supermagnet material, in large measure a consequence of its very-high ductility and ease of fabrication. However, both niobium-tin and niobium-titanium find wide application in MRI medical imagers, bending and focusing magnets for enormous high-energy-particle accelerators, and a host of 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.^[35] This phenomenon, now called the Josephson effect, is exploited by superconducting devices such as SQUIDs. It is used in the most accurate available measurements of the magnetic flux quantum Φ₀ = h/(2e), where h is the Planck constant. Coupled with the quantum Hall resistivity, this leads to a precise measurement of the Planck constant. Josephson was awarded the Nobel Prize for this work in 1973.

In 2008, it was proposed that the same mechanism that produces superconductivity could produce a superinsulator state in some materials, with almost infinite electrical resistance.^[36]

High-temperature superconductivity

Timeline of superconducting materials

Until 1986, physicists had believed that BCS theory forbade superconductivity at temperatures above about 30 K. In that year, Bednorz and Müller discovered superconductivity in a lanthanum-based cuprate perovskite material, which had a transition temperature of 35 K (Nobel Prize in Physics, 1987).^[5] It was soon found that replacing the lanthanum with yttrium (i.e., making YBCO) raised the critical temperature to 92 K.^[37]

This temperature jump is particularly significant, since it allows liquid nitrogen as a refrigerant, replacing liquid helium.^[37] This can be important commercially because liquid nitrogen can be produced relatively cheaply, even on-site. Also, the higher temperatures help avoid some of the problems that arise at liquid helium temperatures, such as the formation of plugs of frozen air that can block cryogenic lines and cause unanticipated and potentially hazardous pressure buildup.^[38][39]

Many other cuprate superconductors have since been discovered, and the theory of superconductivity in these materials is one of the major outstanding challenges of theoretical condensed matter physics.^[40] There are currently two main hypotheses – the resonating-valence-bond theory, and spin fluctuation which has the most support in the research community.^[41] The second hypothesis proposed that electron pairing in high-temperature superconductors is mediated by short-range spin waves known as paramagnons.^{[42][43][dubious – discuss]}

Since about 1993, the highest-temperature superconductor has been a ceramic material consisting of mercury, barium, calcium, copper and oxygen (HgBa₂Ca₂Cu₃O_8+δ) with T_c = 133–138 K.^[44][45] The latter experiment (138 K) still awaits experimental confirmation, however.

In February 2008, an iron-based family of high-temperature superconductors was discovered.^[46][47] Hideo Hosono, of the Tokyo Institute of Technology, and colleagues found lanthanum oxygen fluorine iron arsenide (LaO_1−xF_xFeAs), an oxypnictide that superconducts below 26 K. Replacing the lanthanum in LaO_1−xF_xFeAs with samarium leads to superconductors that work at 55 K.^[48]

In May 2014, hydrogen sulfide (H
₂S) was predicted to be a high-temperature superconductor with a transition temperature of 80 K at 160 gigapascals of pressure.^[49] In 2015, H
₂S has been observed to exhibit superconductivity at below 203 K but at extremely high pressures — around 150 gigapascals.^[50]

Applications

 
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Video of superconducting levitation of YBCO

Superconducting magnets are some of the most powerful electromagnets known. They are used in MRI/NMR machines, mass spectrometers, the beam-steering magnets used in particle accelerators and plasma confining magnets in some tokamaks. 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 the pigment industries.

In the 1950s and 1960s, superconductors were used to build experimental digital computers using cryotron switches. More recently, superconductors have been used to make digital circuits based on rapid single flux quantum technology and RF and microwave filters for mobile phone base stations.

Superconductors are used to build Josephson junctions which are the building blocks of SQUIDs (superconducting quantum interference devices), the most sensitive magnetometers known. SQUIDs are used in scanning SQUID microscopes and magnetoencephalography. Series of Josephson devices are used to realize the SI volt. Depending on the particular mode of operation, a superconductor-insulator-superconductor Josephson junction can be used as a photon detector or as a mixer. The large resistance change at the transition from the normal- to the superconducting state is used to build thermometers in cryogenic micro-calorimeter photon detectors. The same effect is used in ultrasensitive bolometers made from superconducting materials.

Other early markets are arising where the relative efficiency, size and weight advantages of devices based on high-temperature superconductivity outweigh the additional costs involved. For example, in wind 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 total LCOE.^[51]

Promising future applications include high-performance smart grid, electric power transmission, transformers, power storage devices, electric motors (e.g. for vehicle propulsion, as in vactrains or maglev trains), magnetic levitation devices, fault current limiters, enhancing spintronic devices with superconducting materials,^[52] and superconducting magnetic refrigeration. However, superconductivity is sensitive to moving magnetic fields so applications that use alternating current (e.g. transformers) will be more difficult to develop than those that rely upon direct 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.^[53]

Nobel Prizes for superconductivity

• 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"
• John Bardeen, Leon N. Cooper, and J. Robert Schrieffer (1972), "for their jointly developed theory of superconductivity, usually called the BCS-theory"
• Leo Esaki, Ivar Giaever, and Brian 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 and K. Alex Müller (1987), "for their important break-through in the discovery of superconductivity in ceramic materials"
• Alexei A. Abrikosov, Vitaly L. Ginzburg, and Anthony J. Leggett (2003), "for pioneering contributions to the theory of superconductors and superfluids"^[54]
• Michael Kosterlitz, Duncan Haldane, David J. Thouless (2016)

Metallic hydrogen

From Wikipedia, the free encyclopedia

Gas giants such as Jupiter (pictured above) and Saturn might contain large amounts of metallic hydrogen (depicted in grey) and metallic helium.^[1]

A diagram of Jupiter showing a model of the planet's interior, with a rocky core overlaid by a deep layer of liquid metallic hydrogen and an outer layer predominantly of molecular hydrogen. Jupiter's true interior composition is uncertain. For instance, the core may have shrunk as convection currents of hot liquid metallic hydrogen mixed with the molten core and carried its contents to higher levels in the planetary interior. Furthermore, there is no clear physical boundary between the hydrogen layers—with increasing depth the gas increases smoothly in temperature and density, ultimately becoming liquid. Features are shown to scale except for the aurorae and the orbits of the Galilean moons.

Metallic hydrogen is a kind of degenerate matter, a phase of hydrogen in which it behaves like an electrical conductor. This phase was predicted in 1935 on theoretical grounds by Eugene Wigner and Hillard Bell Huntington.^[2]

At high pressure and temperatures, metallic hydrogen might exist as a liquid rather than a solid, and researchers think it is present in large quantities in the hot and gravitationally compressed interiors of Jupiter, Saturn, and in some extrasolar planets.^[3]

In October 2016, there were claims that metallic hydrogen had been observed in the laboratory at a pressure of around 495 gigapascals (4,950,000 bar; 4,890,000 atm; 71,800,000 psi).^[4] In January 2017, scientists at Harvard University reported the first creation of metallic hydrogen in a laboratory, using a diamond anvil cell.^[5] Several researchers in the field doubt this claim.^[6] Some observations consistent with metallic behavior had been reported earlier, such as the observation of new phases of solid hydrogen under static conditions^[7][8] and, in dense liquid deuterium, electrical insulator-to-conductor transitions associated with an increase in optical reflectivity.^[9]

Theoretical predictions

Metallization of hydrogen under pressure

Though often placed at the top of the alkali metal column in the periodic table, hydrogen does not, under ordinary conditions, exhibit the properties of an alkali metal. Instead, it forms diatomic H₂ molecules, analogous to halogens and non-metals in the second row of the periodic table, such as nitrogen and oxygen. Diatomic hydrogen is a gas that, at atmospheric pressure, liquefies and solidifies only at very low temperature (20 degrees and 14 degrees above absolute zero, respectively). Eugene Wigner and Hillard Bell Huntington predicted that under an immense pressure of around 25 GPa (250000 atm; 3600000 psi) hydrogen would display metallic properties: instead of discrete H₂ molecules (which consist of two electrons bound between two protons), a bulk phase would form with a solid lattice of protons and the electrons delocalized throughout.^[2] Since then, producing metallic hydrogen in the laboratory has been described as "...the holy grail of high-pressure physics."^[10]

The initial prediction about the amount of pressure needed was eventually shown to be too low.^[11] Since the first work by Wigner and Huntington, the more modern theoretical calculations were pointing toward higher but nonetheless potentially accessible metallization pressures of 100 GPa and higher.

Liquid metallic hydrogen

Helium-4 is a liquid at normal pressure near absolute zero, a consequence of its high zero-point energy (ZPE). The ZPE of protons in a dense state is also high, and a decline in the ordering energy (relative to the ZPE) is expected at high pressures. Arguments have been advanced by Neil Ashcroft and others that there is a melting point maximum in compressed hydrogen, but also that there might be a range of densities, at pressures around 400 GPa (3,900,000 atm), where hydrogen would be a liquid metal, even at low temperatures.^[12][13]

Superconductivity

In 1968, Neil Ashcroft suggested that metallic hydrogen might be a superconductor, up to room temperature (290 K or 17 °C), far higher than any other known candidate material. This hypothesis is based on an expected strong coupling between conduction electrons and lattice vibrations.^[14]

Possibility of novel types of quantum fluid

Presently known "super" states of matter are superconductors, superfluid liquids and gases, and supersolids. Egor Babaev predicted that if hydrogen and deuterium have liquid metallic states, they might have quantum ordered states that cannot be classified as superconducting or superfluid in the usual sense. Instead, they might represent two possible novel types of quantum fluids: superconducting superfluids and metallic superfluids. Such fluids were predicted to have highly unusual reactions to external magnetic fields and rotations, which might provide a means for experimental verification of Babaev's predictions. It has also been suggested that, under the influence of magnetic field, hydrogen might exhibit phase transitions from superconductivity to superfluidity and vice versa.^[15][16][17]

Lithium alloying reduces requisite pressure

In 2009, Zurek et al. predicted that the alloy LiH₆ would be a stable metal at only one quarter of the pressure required to metallize hydrogen, and that similar effects should hold for alloys of type LiH_n and possibly other related alloys of type Li_n.^[18]

Experimental pursuit

Shock-wave compression, 1996

In March 1996, a group of scientists at Lawrence Livermore National Laboratory reported that they had serendipitously produced the first identifiably metallic hydrogen^[19] for about a microsecond at temperatures of thousands of kelvins, pressures of over 1000000 atm (100 GPa), and densities of approximately 0.6 g/cm³.^[20] The team did not expect to produce metallic hydrogen, as it was not using solid hydrogen, thought to be necessary, and was working at temperatures above those specified by metallization theory. Previous studies in which solid hydrogen was compressed inside diamond anvils to pressures of up to 2500000 atm (250 GPa), did not confirm detectable metallization. The team had sought simply to measure the less extreme electrical conductivity changes they expected. The researchers used a 1960s-era light-gas gun, originally employed in guided missile studies, to shoot an impactor plate into a sealed container containing a half-millimeter thick sample of liquid hydrogen. The liquid hydrogen was in contact with wires leading to a device measuring electrical resistance. The scientists found that, as pressure rose to 1400000 atm (140 GPa), the electronic energy band gap, a measure of electrical resistance, fell to almost zero. The band-gap of hydrogen in its uncompressed state is about 15 eV, making it an insulator but, as the pressure increases significantly, the band-gap gradually fell to 0.3 eV. Because the thermal energy of the fluid (the temperature became about 3000 K or 2730 °C due to compression of the sample) was above 0.3 eV, the hydrogen might be considered metallic.

Other experimental research, 1996–2004

Many experiments are continuing in the production of metallic hydrogen in laboratory conditions at static compression and low temperature. Arthur Ruoff and Chandrabhas Narayana from Cornell University in 1998,^[21] and later Paul Loubeyre and René LeToullec from Commissariat à l'Énergie Atomique, France in 2002, have shown that at pressures close to those at the center of the Earth (3200000–3400000 atm or 320–340 GPa) and temperatures of 100–300 K (−173–27 °C), hydrogen is still not a true alkali metal, because of the non-zero band gap. The quest to see metallic hydrogen in laboratory at low temperature and static compression continues. Studies are also ongoing on deuterium.^[22] Shahriar Badiei and Leif Holmlid from the University of Gothenburg have shown in 2004 that condensed metallic states made of excited hydrogen atoms (Rydberg matter) are effective promoters to metallic hydrogen.^[23]

Pulsed laser heating experiment, 2008

The theoretically predicted maximum of the melting curve (the prerequisite for the liquid metallic hydrogen) was discovered by Shanti Deemyad and Isaac F. Silvera by using pulsed laser heating.^[24] Hydrogen-rich molecular silane (SiH₄) was claimed to be metallized and become superconducting by M.I. Eremets et al..^[25] This claim is disputed, and their results have not been repeated.^[26][27]

Observation of liquid metallic hydrogen, 2011

In 2011 Eremets and Troyan reported observing the liquid metallic state of hydrogen and deuterium at static pressures of 2600000–3000000 atm (260–300 GPa).^[7] This claim was questioned by other researchers in 2012.^[28][29]

Z machine, 2015

In 2015, scientists at the Z Pulsed Power Facility announced the creation of metallic deuterium.^[30]

Claimed observation of solid metallic hydrogen, 2016

On October 5, 2016, Ranga Dias and Isaac F. Silvera of Harvard University released claims of experimental evidence that solid metallic hydrogen had been synthesised in the laboratory. This manuscript was available in October 2016,^[31] and a revised version was subsequently published in the journal Science in January 2017.^[4][5]

In the preprint version of the paper, Dias and Silvera writes:

With increasing pressure we observe changes in the sample, going from transparent, to black, to a reflective metal, the latter studied at a pressure of 495 GPa... the reflectance using a Drude free electron model to determine the plasma frequency of 30.1 eV at T = 5.5 K, with a corresponding electron carrier density of 6.7×10²³ particles/cm³, consistent with theoretical estimates. The properties are those of a metal. Solid metallic hydrogen has been produced in the laboratory.

— Dias & Silvera (2016) ^[31]

Silvera stated that they did not repeat their experiment, since more tests could damage or destroy their existing sample, but assured the scientific community that more tests are coming.^[32][6] He also stated that the pressure would eventually be released, in order to find out whether the sample was metastable (i.e., whether it would persist in its metallic state even after the pressure was released).^[33]

Shortly after the claim was published in Science, Nature's news division published an article stating that some other physicists regarded the result with skepticism. Recently, prominent members of the high pressure research community have criticised the claimed results,^[34][35][36] questioning the claimed pressures or the presence of metallic hydrogen at the pressures claimed.

In February 2017, it was reported that the sample of claimed metallic hydrogen was lost, after the diamond anvils it was contained between broke.^[37]

In August 2017, Silvera and Dias issued an erratum^[38] to the Science article, regarding corrected reflectance values due to variations between the optical density of stressed natural diamonds and the synthetic diamonds used in their pre-compression diamond anvil cell.

Quark star

From Wikipedia, the free encyclopedia

A quark star is a hypothetical type of compact exotic star, where extremely high temperature and pressure has forced nuclear particles to form a continuous state of matter that consists primarily of free quarks.

It is well known that massive stars can collapse to form neutron stars, under extreme temperatures and pressures. In simple terms, neutrons usually have space separating them, due to degeneracy pressure keeping them apart. Under extreme conditions such as a neutron star, the pressure separating nucleons is overwhelmed by gravity, and the separation between them breaks down, causing them to be packed extremely densely and form an immensely hot and dense state known as neutron matter. Because these neutrons are made of quarks, it is hypothesized that under even more extreme conditions, the degeneracy pressure keeping the quarks apart within the neutrons might break down in much the same way, creating an ultra-dense phase of degenerate matter based on densely packed quarks. This is seen as plausible, but is very hard to prove, as scientists cannot easily create the conditions needed to investigate the properties of quark matter, so it is not yet certain whether or not it actually happens in the universe.

If quark stars can form, then the most likely place to find quark star matter would be inside neutron stars that exceed the internal pressure needed for quark degeneracy - the point at which neutrons (which are formed from quarks bound together) break down into a form of dense quark matter. They could also form if a massive star collapses at the end of its life, provided that it is possible for a star to be large enough to collapse beyond a neutron star but not large enough to form a black hole. However, as scientists are unable so far to explore most properties of quark matter, the exact conditions and nature of quark stars, and their existence, remain hypothetical and unproven. The question whether such stars exist and their exact structure and behavior is actively studied within astrophysics and particle physics.

If they exist, quark stars would resemble and be easily mistaken for neutron stars: they would form in the death of a massive star in a Type II supernova, they would be extremely dense and small, and possess a very high gravitational field. They would also lack some features of neutron stars, unless they also contained a shell of neutron matter, because free quarks are not expected to have properties matching degenerate neutron matter. For example, they might be radio-silent, or not have typical size, electromagnetic, or temperature measurements, compared to other neutron stars.

The hypothesis about quark stars was first proposed in 1965 by Soviet physicists D. D. Ivanenko and D. F. Kurdgelaidze.^[1][2] Their existence has not been confirmed. The equation of state of quark matter is uncertain, as is the transition point between neutron-degenerate matter and quark matter. Theoretical uncertainties have precluded making predictions from first principles. Experimentally, the behaviour of quark matter is being actively studied with particle colliders, but this can only produce very hot (above 10¹² K) quark-gluon plasma blobs the size of atomic nuclei, which decay immediately after formation. The conditions inside compact stars with extremely high densities and temperatures well below 10¹² K can not be recreated artificially, so there are no known methods to produce, store or study "cold" quark matter directly as it would be found inside quark stars. The theory predicts quark matter to possess some peculiar characteristics under these conditions.

Creation

It is theorized that when the neutron-degenerate matter, which makes up neutron stars, is put under sufficient pressure from the star's own gravity or the initial supernova creating it, the individual neutrons break down into their constituent quarks (up quarks and down quarks), forming what is known as quark matter. This conversion might be confined to the neutron star's center or it might transform the entire star, depending on the physical circumstances. Such a star is known as a quark star.^[3][4]

Stability and strange quark matter

Ordinary quark matter consisting of up and down quarks (also referred to as u and d quarks) has a very high Fermi energy compared to ordinary atomic matter and is only stable under extreme temperatures and/or pressures. This suggests that the only stable quark stars will be neutron stars with a quark matter core, while quark stars consisting entirely of ordinary quark matter will be highly unstable and dissolve spontaneously.^[5][6]

It has been shown that the high Fermi energy making ordinary quark matter unstable at low temperatures and pressures can be lowered substantially by the transformation of a sufficient number of u and d quarks into strange quarks, as strange quarks are, relatively speaking, a very heavy type of quark particle.^[5] This kind of quark matter is known specifically as strange quark matter and it is speculated and subject to current scientific investigation whether it might in fact be stable under the conditions of interstellar space (i.e. near zero external pressure and temperature). If this is the case (known as the Bodmer–Witten assumption), quark stars made entirely of quark matter would be stable if they quickly transform into strange quark matter.^[7]

Strange stars

Quark stars made of strange quark matter are known as strange stars, and they form a subgroup under the quark star category.^[7]

Strange stars might exist without regard of the Bodmer–Witten assumption of stability at near-zero temperatures and pressures, as strange quark matter might form and remain stable at the core of neutron stars, in the same way as ordinary quark matter could.^[3] Such strange stars will naturally have a crust layer of neutron star material. The depth of the crust layer will depend on the physical conditions and circumstances of the entire star and on the properties of strange quark matter in general.^[8] Stars partially made up of quark matter (including strange quark matter) are also referred to as hybrid stars.^{[9][10][11][12]}

Theoretical investigations have revealed that quark stars might not only be produced from neutron stars and powerful supernovas, they could also be created in the early cosmic phase separations following the Big Bang.^[5] If these primordial quark stars transform into strange quark matter before the external temperature and pressure conditions of the early Universe makes them unstable, they might turn out stable, if the Bodmer–Witten assumption holds true. Such primordial strange stars could survive to this day.^[5]

Characteristics

Quark stars have some special characteristics that separate them from ordinary neutron stars.

Under the physical conditions found inside neutron stars, with extremely high densities but temperatures well below 10¹² K, quark matter is predicted to exhibit some peculiar characteristics. It is expected to behave as a Fermi liquid and enter a so-called color-flavor-locked (CFL) phase of color superconductivity, where "color" refers to the six "charges" exhibited in the strong interaction, instead of the positive and the negative charges in electromagnetism. At slightly lower densities, corresponding to higher layers closer to the surface of the compact star, the quark matter will behave as a non-CFL quark liquid, a phase that is even more mysterious than CFL and might include color conductivity and/or several additional yet undiscovered phases. None of these extreme conditions can currently be recreated in laboratories so nothing can be inferred about these phases from direct experiments.^[13]

If the conversion of neutron-degenerate matter to (strange) quark matter is total, a quark star can to some extent be imagined as a single gigantic hadron. But this "hadron" will be bound by gravity, rather than the strong force that binds ordinary hadrons.

Strange stars

Recent theoretical research has found mechanisms by which quark stars with "strange quark nuggets" may decrease the objects' electric fields and densities from previous theoretical expectations, causing such stars to appear very much like—nearly indistinguishable from—ordinary neutron stars. This suggests that many, or even all, known neutron stars might in fact be strange stars. However, the investigating team of Prashanth Jaikumar, Sanjay Reddy, and Andrew W. Steiner made some fundamental assumptions that led to uncertainties in their results large enough that the case is not finally settled. More research, both observational and theoretical, remains to be done on strange stars in the future.^[14]

Other theoretical work^[15] contends that, "A sharp interface between quark matter and the vacuum would have very different properties from the surface of a neutron star"; and, addressing key parameters like surface tension and electrical forces that were neglected in the original study, the results show that as long as the surface tension is below a low critical value, the large strangelets are indeed unstable to fragmentation and strange stars naturally come with complex strangelet crusts, analogous to those of neutron stars.

Observed overdense neutron stars

At least under the assumptions mentioned above, the probability of a given neutron star being a quark star is low,^{[citation needed]} so in the Milky Way there would only be a small population of quark stars. If it is correct however, that overdense neutron stars can turn into quark stars, that makes the possible number of quark stars higher than was originally thought, as observers would be looking for the wrong type of star.

Quark stars and strange stars are entirely hypothetical as of 2018, but there are several candidates.

Observations released by the Chandra X-ray Observatory on April 10, 2002 detected two possible quark stars, designated RX J1856.5-3754 and 3C58, which had previously been thought to be neutron stars. Based on the known laws of physics, the former appeared much smaller and the latter much colder than it should be, suggesting that they are composed of material denser than neutron-degenerate matter. However, these observations are met with skepticism by researchers who say the results were not conclusive;^[16] and since the late 2000s, the possibility that RX J1856 is a quark star has been excluded.

Another star, XTE J1739-285,^[17] has been observed by a team led by Philip Kaaret of the University of Iowa and reported as a possible quark star candidate.

In 2006, Y. L. Yue et al., from Peking University, suggested that PSR B0943+10 may in fact be a low-mass quark star.^[18]

It was reported in 2008 that observations of supernovae SN2006gy, SN2005gj and SN2005ap also suggest the existence of quark stars.^[19] It has been suggested that the collapsed core of supernova SN1987A may be a quark star.^[20][21]

In 2015, Z.G. Dai et al. from Nanjing University suggested that Supernova ASASSN-15lh is a newborn strange quark star.^[22]

Other theorized quark formations

Apart from ordinary quark matter and strange quark matter, other types of quark-gluon plasma might theoretically occur or be formed inside neutron stars and quark stars. This includes the following, some of which has been observed and studied in laboratories:

• Jaffe 1977, suggested a four-quark state with strangeness (qsqs).
• Jaffe 1977 suggested the H dibaryon, a six-quark state with equal numbers of up-, down-, and strange quarks (represented as uuddss or udsuds).
• Bound multi-quark systems with heavy quarks (QQqq).
• In 1987, a pentaquark state was first proposed with a charm anti-quark (qqqsc).
• Pentaquark state with an antistrange quark and four light quarks consisting of up- and down-quarks only (qqqqs).
• Light pentaquarks are grouped within an antidecuplet, the lightest candidate, Ө⁺.
  • This can also be described by the diquark model of Jaffe and Wilczek (QCD).
• Ө⁺⁺ and antiparticle Ө⁻⁻.
• Doubly strange pentaquark (ssddu), member of the light pentaquark antidecuplet.
• Charmed pentaquark Ө_c(3100) (uuddc) state was detected by the H1 collaboration.^[23]
• Tetra quark particles might form inside neutron stars and under other extreme conditions. In 2008, 2013 and 2014 the tetra quark particle of Z(4430), was discovered and investigated in laboratories on Earth.^[24]