A room-temperature superconductor is a hypothetical material that would be capable of exhibiting superconductivity at operating temperatures above 0° C (273.15 K). While this is not strictly "room temperature", which would be approximately 20–25 °C, it is the temperature at which ice forms and can be reached and easily maintained in an everyday environment. The highest temperature known superconducting material is hydrogen sulfide, whose critical temperature reaches 203 K (-70 °C) the highest accepted superconducting critical temperature as of 2015. By substituting a small part of sulfur with phosphorus and using even higher pressures, it has been predicted that it may be possible to raise the critical temperature to above 0 °C and achieve room-temperature superconductivity. Previously the record was held by the cuprates, which have demonstrated superconductivity at atmospheric pressure at temperatures as high as ‑135 °C (138 K) and 164K under high pressure.
Although some researchers doubt whether room-temperature superconductivity is actually achievable, 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 and some suggest that in fact the breakthrough might have been made more than once but could not be made stable enough and/or reproducible as the relationship between isotope number and Tc was not known at the time.
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."
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.
In 2003, a group of researchers published results on high-temperature superconductivity in palladium hydride (PdHx: x>1) and an explanation in 2004. In 2007 the same group published results suggesting a superconducting transition temperature of 260 K. 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. 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 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 203K which would be the highest Tc ever recorded and their research suggests that other hydrogen compounds could superconduct at up to 260K which would match up with the original research of Ashcroft.  It is also worth noting that there is a clear and obvious link between neutron number (ie HD2S or H2DS) and Tc in that the higher the deuterium loading the lower the observed Tc, further supporting the BCS theory.
Other research also suggests a link between the palladium hydride containing small impurities of sulphur as a plausible explanation for the anomalous resistance drops noticed by other researchers, and hydrogen adsorption by cuprates has been suggested in light of the recent results in H2S as a plausible explanation for transient resistance drops or "USO" noticed in the 1990s during research after the discovery of YBCO.
Theoretical work by 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). This prediction is yet to be experimentally verified, as yet the pressure to achieve metallic hydrogen is not known but may be of the order of 500 GPa.
In 1964, William A. Little proposed the possibility of high temperature superconductivity in organic polymers. This proposal is based on the exciton-mediated electron pairing, as opposed to phonon-mediated pairing in BCS theory.
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