Room-temperature superconductor

A room-temperature superconductor is a material that is capable of exhibiting superconductivity at operating temperatures above 0 °C (273 K; 32 °F), that is, temperatures that can be reached and easily maintained in an everyday environment. As of 2020 the material with the highest accepted superconducting temperature is an extremely pressurized carbonaceous sulfur hydride with a critical transition temperature of +15°C at 267 GPa.[1]

At atmospheric pressure the temperature record is still held by cuprates, which have demonstrated superconductivity at temperatures as high as 138 K (−135 °C).[2]

Although researchers once doubted whether room-temperature superconductivity was actually achievable,[3][4] superconductivity has repeatedly been discovered at temperatures that were previously unexpected or held to be impossible.

Claims of "near-room temperature" transient effects date from the early 1950s. Finding a room temperature superconductor "would have enormous technological importance and, for example, help to solve the world's energy problems, provide for faster computers, allow for novel memory-storage devices, and enable ultra-sensitive sensors, among many other possibilities."[4][5]

Unsolved problem in physics:
Is it possible to make a material that is a superconductor at room temperature and atmospheric pressure?
(more unsolved problems in physics)

Reports

Since the discovery of high-temperature superconductors, several materials have been reported to be room-temperature superconductors, although most of these reports have not been confirmed.

In 2000, while extracting electrons from diamond during ion implantation work, Johan Prins claimed to have observed a phenomenon that he explained as room-temperature superconductivity within a phase formed on the surface of oxygen-doped type IIa diamonds in a 10−6 mbar vacuum.[6]

In 2003, a group of researchers published results on high-temperature superconductivity in palladium hydride (PdHx: x>1)[7] and an explanation in 2004.[8] In 2007 the same group published results suggesting a superconducting transition temperature of 260 K.[9] The superconducting critical temperature increases as the density of hydrogen inside the palladium lattice increases. This work has not been corroborated by other groups.

In 2012, an Advanced Materials article claimed superconducting behavior of graphite powder after treatment with pure water at temperatures as high as 300 K and above.[10] So far, the authors have not been able to demonstrate the occurrence of a clear Meissner phase and the vanishing of the material's resistance.

In 2014, an article published in Nature suggested that some materials, notably YBCO (yttrium barium copper oxide), could be made to superconduct at room temperature using infrared laser pulses.[11]

In 2015, an article published in Nature by researchers of the Max Planck Institute suggested that under certain conditions such as extreme pressure H2S transitioned to a superconductive form H3S at around 1.5 million times atmospheric pressure in a diamond anvil cell. The critical temperature is 203 K (−70 °C) which would be the highest Tc ever recorded and their research suggests that other hydrogen compounds could superconduct at up to 260 K (−13 °C) which would match up with the original research of Ashcroft.[12][13]

In 2018, Dev Kumar Thapa and Anshu Pandey from the Solid State and Structural Chemistry Unit of the Indian Institute of Science in Bangalore claimed the observation of superconductivity at ambient pressure and room temperature in films and pellets of a nanostructured material that is composed of silver particles embedded in a gold matrix.[14] Due to similar noise patterns of supposedly independent plots and the publication's lack of peer review, the results have been called into question.[15] Although the researchers validated their findings in a later paper in 2019,[16] this claim is yet to be verified and confirmed.

Also in 2018, researchers noted a possible superconducting phase at 260 K (−13 °C) in lanthanum decahydride at elevated (200 GPa) pressure.[17]

In 2019 the material with the highest accepted superconducting temperature was highly pressurized lanthanum decahydride (LaH10), whose transition temperature is approximately 250 K (−23 °C).[18][19]

In September 2020 new research showed a completely new mechanism of superconductivity in SrRuO4 known as "g-wave".[20]

In October 2020, room-temperature superconductivity (at 15 °C) was reported in a carbonaceous sulfur hydride at very high pressure (267 GPa) triggered into crystallisation via green laser. [21][22]

Theories

Theoretical work by British physicist Neil Ashcroft predicted that solid metallic hydrogen at extremely high pressure (~500 GPa) should become superconducting at approximately room-temperature because of its extremely high speed of sound and expected strong coupling between the conduction electrons and the lattice vibrations (phonons).[23] This prediction is yet to be experimentally verified, as the pressure to achieve metallic hydrogen is not known but may be on the order of 500 GPa.

A team at Harvard University has claimed to make metallic hydrogen and reports a pressure of 495 GPa.[24] Though the exact critical temperature has not yet been determined, weak signs of a possible Meissner effect and changes in magnetic susceptibility at 250K may have appeared in early magnetometer tests on the original now-lost sample and is being analyzed by the French team working with doughnut shapes rather than planar at the diamond culet tips.[25]

In 1964, William A. Little proposed the possibility of high temperature superconductivity in organic polymers.[26] This proposal is based on the exciton-mediated electron pairing, as opposed to phonon-mediated pairing in BCS theory. This may have inadvertently been confirmed by OLED experiments where light emission due to electron-hole recombination happens after several non-emissive steps requiring transport along the molecules so a crossover between OLED and condensed matter physics is quite feasible. This link was suggested by an independent researcher and as yet has not been peer reviewed though given what we now know it seems likely.

It is also possible that advanced magnetic scanners may be able to detect weak signs of the Meissner effect in materials not previously tested, one example is an atomic magnetometer based on a rubidium or cesium nuclear resonance chamber sometimes used for oil surveys.[27] OLED panels are also unusual in that the degradation to non-emission or non-radiative recombination similar to the Auger mechanism is sometimes reversible with the right tools though this depends on which mechanism has caused the majority of the damage: generally speaking indium migration cannot usually be repaired as easily.[28]

In 2016, research suggested a link between the palladium hydride containing small impurities of sulfur nanoparticles as a plausible explanation for the anomalous transient resistance drops seen during some experiments, and hydrogen absorption by cuprates was suggested in light of the 2015 results in H2S as a plausible explanation for transient resistance drops or "USO" noticed in the 1990s by Chu et al. during research after the discovery of YBCO.[29] It is also possible that if the bipolaron explanation is correct, a normally semiconducting material can transition under some conditions into a superconductor if a critical level of alternating spin coupling in a single plane within the lattice is exceeded; this may have been documented in very early experiments from 1986. The best analogy here would be anisotropic magnetoresistance, but in this case the outcome is a drop to zero rather than a decrease within a very narrow temperature range for the compounds tested similar to "re-entrant superconductivity".

In 2018, support was found for electrons having anomalous 3/2 spin states in YPtBi.[30] Though YPtBi is a relatively low temperature superconductor, this does suggest another approach to creating superconductors.

It was also discovered that many superconductors, including the cuprates and iron pnictides, have two or more competing mechanisms fighting for dominance (Charge density wave) and excitonic states so, as with organic light emitting diodes and other quantum systems, adding the right spin catalyst may by itself increase Tc. A possible candidate would be iridium or gold placed in some of the adjacent molecules or as a thin surface layer so the correct mechanism then propagates throughout the entire lattice similar to a phase transition. As yet, this is speculative; some efforts have been made, notably adding lead to BSCCO, which is well known to help promote high Tc phases by chemistry alone. However, relativistic effects similar to those found in lead-acid batteries might be responsible suggesting that a similar mechanism in mercury- or thallium-based cuprates may be possible using a related metal such as tin.

Any such catalyst would need to be nonreactive chemically but have properties that affect one mechanism but not the others, and also not interfere with subsequent annealing and oxygenation steps nor change the lattice resonances excessively. A possible workaround for the issues discussed would be to use strong electrostatic fields to hold the molecules in place during one of the steps until the lattice is formed.

Some research efforts are currently moving towards ternary superhydrides, where it has been predicted that Li2MgH16 would have a Tc of 473 K (200 °C) at 250 GPa[31][32] (much hotter than what is normally considered room temperature).

See also
  • Persistent current – Perpetual electric current, not requiring an external power source

References
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  2. Dai, P.; Chakoumakos, B.C.; Sun, G.F.; Wong, K.W.; Xin, Y.; Lu, D.F. (1995). "Synthesis and neutron powder diffraction study of the superconductor HgBa2Ca2Cu3O8+δ by Tl substitution". Physica C. 243 (3–4): 201–206. Bibcode:1995PhyC..243..201D. doi:10.1016/0921-4534(94)02461-8.
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  31. Sun, Ying; Lv, Jian; Xie, Yu; Liu, Hanyu; Ma, Yanming (26 August 2019). "Route to a Superconducting Phase above Room Temperature in Electron-Doped Hydride Compounds under High Pressure". Physical Review Letters. 123 (9): 097001. Bibcode:2019PhRvL.123i7001S. doi:10.1103/PhysRevLett.123.097001. PMID 31524448. The recent theory-orientated discovery of record high-temperature superconductivity (Tc∼250 K) in sodalitelike clathrate LaH10 is an important advance toward room-temperature superconductors. Here, we identify an alternative clathrate structure in ternary Li2MgH16 with a remarkably high estimated Tc of ∼473 K at 250 GPa, which may allow us to obtain room-temperature or even higher-temperature superconductivity.
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