High-temperature superconductivity
High-temperature superconductivity is superconductivity in materials with a critical temperature above, the boiling point of liquid nitrogen. They are "high-temperature" only relative to previously known superconductors, which function only closer to absolute zero. The first high-temperature superconductor was discovered in 1986 by IBM researchers Georg Bednorz and K. Alex Müller. Although the critical temperature is around, this material was modified by Ching-Wu Chu to make the first high-temperature superconductor with critical temperature. Bednorz and Müller were awarded the Nobel Prize in Physics in 1987 "for their important break-through in the discovery of superconductivity in ceramic materials". Most high-c materials are type-II superconductors.
The major advantage of high-temperature superconductors is that they can be cooled using liquid nitrogen, in contrast to previously known superconductors, which require expensive and hard-to-handle coolants, primarily liquid helium. A second advantage of high-c materials is they retain their superconductivity in higher magnetic fields than previous materials. This is important when constructing superconducting magnets, a primary application of high-c materials.
The majority of high-temperature superconductors are ceramics, rather than the previously known metallic materials. Ceramic superconductors are suitable for some practical uses but encounter manufacturing issues. For example, most ceramics are brittle, which complicates wire fabrication.
The main class of high-temperature superconductors is copper oxides combined with other metals, especially the rare-earth barium copper oxides such as yttrium barium copper oxide. The second class of high-temperature superconductors in the practical classification is the iron-based compounds. Magnesium diboride is sometimes included in high-temperature superconductors: It is relatively simple to manufacture, but it superconducts only below, which makes it unsuitable for liquid nitrogen cooling.
History
Superconductivity was discovered by Kamerlingh Onnes in 1911, in a metal solid. Ever since, researchers have attempted to create superconductivity at higher temperatures with the goal of finding a room-temperature superconductor. By the late 1970s, superconductivity was observed in several metallic compounds at temperatures that were much higher than those for elemental metals and which could even exceed.In 1986, at the IBM research lab near Zürich in Switzerland, Bednorz and Müller were looking for superconductivity in a new class of ceramics: the copper oxides, or cuprates. In that year, Bednorz and Müller discovered superconductivity in lanthanum barium copper oxide, a lanthanum-based cuprate perovskite material, which had a transition temperature of 35 K. It was soon found that replacing the lanthanum with yttrium raised the critical temperature above 90 K. Their results were soon confirmed by many groups.
In 1987, Philip W. Anderson gave the first theoretical description of these materials, based on the resonating valence bond theory, but a full understanding of these materials is still developing today. These superconductors are now known to possess a d-wave pair symmetry. The first proposal that high-temperature cuprate superconductivity involves d-wave pairing was made in 1987 by N. E. Bickers, Douglas James Scalapino and R. T. Scalettar, followed by three subsequent theories in 1988 by Masahiko Inui, Sebastian Doniach, Peter J. Hirschfeld and Andrei E. Ruckenstein, using spin-fluctuation theory, and by Claudius Gros, Didier Poilblanc, Maurice T. Rice and FC. Zhang, and by Gabriel Kotliar and Jialin Liu identifying d-wave pairing as a natural consequence of the RVB theory. The confirmation of the d-wave nature of the cuprate superconductors was made by a variety of experiments, including the direct observation of the d-wave nodes in the excitation spectrum through angle resolved photoemission spectroscopy, the observation of a half-integer flux in tunneling experiments, and indirectly from the temperature dependence of the penetration depth, specific heat and thermal conductivity.
Until 2001 the cuprates were thought be the only true high temperature superconductors. In that year MgB2 with Tc of 39K was discovered by Akimitsu and colleagues. This was followed in 2006 by Hosono and coworkers with iron-based layered oxypnictide compound with Tc of 56K. These temperature are below the cuprates but well above the conventional superconductors.
In 2014, evidence showing that fractional particles can happen in quasi two-dimensional magnetic materials was reported by École Polytechnique Fédérale de Lausanne scientists lending support for Anderson's theory of high-temperature superconductivity. In 2014 and 2015, hydrogen sulfide at extremely high pressures was first predicted and then confirmed to be a high-temperature superconductor with a transition temperature of 80 K.
In 2018, a research team from the Department of Physics, Massachusetts Institute of Technology, discovered superconductivity in bilayer graphene with one layer twisted at an angle of approximately 1.1 degrees with cooling and applying a small electric charge. Even if the experiments were not carried out in a high-temperature environment, the results are correlated less to classical but high temperature superconductors, given that no foreign atoms needed to be introduced. The superconductivity effect came about as a result of electrons twisted into a vortex between the graphene layers, called "skyrmions". These act as a single particle and can pair up across the graphene's layers, leading to the basic conditions required for superconductivity.
In 2019 it was discovered that lanthanum hydride becomes a superconductor at 250 K under a pressure of 170 gigapascals.
In 2020, a room-temperature superconductor made from hydrogen, carbon and sulfur under pressures of around 270 gigapascals was described in a paper in Nature. However, in 2022 the article was retracted by the editors because the validity of background subtraction procedures had been called into question. All nine authors maintain that the raw data strongly support the main claims of the paper.
In 2023 a study reported superconductivity at room temperature and ambient pressure in highly oriented pyrolytic graphite with dense arrays of nearly parallel line defects.
As of 2021, the superconductor with the highest transition temperature at ambient pressure was the cuprate of mercury, barium, and calcium, at around. Other superconductors have higher recorded transition temperaturesfor example lanthanum superhydride at, but these only occur at high pressure.
Selected list of superconductors
Properties
The "high-temperature" superconductor class has had many definitions.The label high-c should be reserved for materials with critical temperatures greater than the boiling point of liquid nitrogen. However, a number of materialsincluding the original discovery and recently discovered pnictide superconductorshave critical temperatures below but nonetheless are commonly referred to in publications as high-c class.
A substance with a critical temperature above the boiling point of liquid nitrogen, together with a high critical magnetic field and critical current density, would greatly benefit technological applications. In magnet applications, the high critical magnetic field may prove more valuable than the high c itself. Some cuprates have an upper critical field of about 100 tesla. However, cuprate materials are brittle ceramics that are expensive to manufacture and not easily turned into wires or other useful shapes. Furthermore, high-temperature superconductors do not form large, continuous superconducting domains, rather clusters of microdomains within which superconductivity occurs. They are therefore unsuitable for applications requiring actual superconductive currents, such as magnets for magnetic resonance spectrometers. For a solution to this, see HTS wire.
There has been considerable debate regarding high-temperature superconductivity coexisting with magnetic ordering in YBCO, iron-based superconductors, several ruthenocuprates and other exotic superconductors, and the search continues for other families of materials. HTS are Type-II superconductors, which allow magnetic fields to penetrate their interior in quantized units of flux, meaning that much higher magnetic fields are required to suppress superconductivity. The layered structure also gives a directional dependence to the magnetic field response.
All known high-c superconductors are Type-II superconductors. In contrast to Type-I superconductors, which expel all magnetic fields due to the Meissner effect, Type-II superconductors allow magnetic fields to penetrate their interior in quantized units of flux, creating "holes" or "tubes" of normal metallic regions in the superconducting bulk called vortices. Consequently, high-c superconductors can sustain much higher magnetic fields.
Cuprates
Iron-based
Iron-based superconductors contain layers of iron and a pnictogensuch as arsenic or phosphorus, a chalcogen, or a crystallogen. This is currently the family with the second highest critical temperature, behind the cuprates. Interest in their superconducting properties began in 2006 with the discovery of superconductivity in LaFePO at and gained much greater attention in 2008 after the analogous material LaFeAs was found to superconduct at up to under pressure.The highest critical temperatures in the iron-based superconductor family exist in thin films of FeSe, where a critical temperature in excess of was reported in 2014.
Since the original discoveries several families of iron-based superconductors have emerged:
- LnFeAs or LnFeAsO1−x with c up to, referred to as 1111 materials. A fluoride variant of these materials was subsequently found with similar c values.
- Fe2As2 and related materials with pairs of iron-arsenide layers, referred to as 122 compounds. c values range up to. These materials also superconduct when iron is replaced with cobalt.
- LiFeAs and NaFeAs with c up to around. These materials superconduct close to stoichiometric composition and are referred to as 111 compounds.
- FeSe with small off-stoichiometry or tellurium doping.
- LaFeSiH with c around in its stoichiometric composition. This superconducting crystallogenide has oxide and fluoride variants LaFeSiOx and LaFeSiFx.
The phase diagram emerging as the iron-arsenide layers are doped is remarkably similar, with the superconducting phase close to or overlapping the magnetic phase. Strong evidence that the c value varies with the As–Fe–As bond angles has already emerged and shows that the optimal c value is obtained with undistorted FeAs4 tetrahedra. The symmetry of the pairing wavefunction is still widely debated, but an extended s-wave scenario is currently favoured.