A superconductor you can leave on the bench

A substance prepared in air, and measured in air at ambient pressure, is a superconductor at or above standard room temperature: its electrical resistance collapses and it expels magnetic field, on two separately made samples.

This file is not on the v1 list. Grading the Meissner effect needs a magnetometer, which the criteria treat as outside a standard laboratory. The claim stays, as a candidate, until that rule changes.

A superconductor that did not need liquid nitrogen or a diamond-anvil cell would change magnets, transmission, and sensors. Resistance that falls on cooling is not enough, and neither is a pellet that sits over a permanent magnet. The two signatures have to appear on the same specimen, at ambient pressure, after the specimen has been left in air, and a second batch has to do it again.

Wen and co-workers describe HgBa₂Ca₂Cu₃O₈₊δ as the cuprate with the highest critical temperature at ambient pressure, and they report crystals with T_c ≈ 134 K. The same paper states that the critical temperature of that compound reaches 164 K under pressure. Pressure is outside this problem. Deng, Chu, and co-workers report a pressure-quench protocol that leaves HgBa₂Ca₂Cu₃O₈₊δ superconducting at 151 K once the pressure is released. One hundred and fifty-one kelvin is still far below 298 K, and this problem additionally requires 24 hours in air and both signatures on two batches. Neither report is used as a pass mark.

Wu and co-workers observed both a resistive and a magnetic transition in the yttrium–barium–copper oxide system between 80 and 93 K at ambient pressure. That material is the instrument control, not a record to beat on temperature. It cannot itself pass this problem. This problem does not cite a room-temperature critical temperature, because it is not using one.

Standard room temperature here means 298 K, the conventional standard temperature. Ambient pressure means 1.00 ± 0.05 bar of air. The sample sits in laboratory air at that pressure for 24 hours before the measurement, and the measurement itself is at that pressure. A diamond-anvil cell, or any measurement above 1.05 bar, does not count.

Two signatures are required on the same specimen, then repeated on a second specimen made in a separate batch.

• Four-point resistance, with the current reversed and the two voltage readings averaged, so that a thermal voltage is not mistaken for a resistance. The resistance at 298 K, divided by the resistance of the same specimen at 30 K above the claimed onset or at 328 K, whichever is higher, is at least as small as the same ratio measured the same day on a YBa₂Cu₃O₇₋δ pellet below its own transition. The denominator is therefore always taken at least 30 K above 298 K.
• Direct-current magnetometry in the same applied field used for the YBa₂Cu₃O₇₋δ pellet that day. The field is one at which that pellet shows a clear diamagnetic transition, and it is written down. The graded moment is the zero-field-cooled diamagnetic moment at 298 K, per geometric volume of the specimen. It is at least the zero-field-cooled diamagnetic moment per geometric volume of the pellet below its transition.

The geometric volume is the measured outer volume of the specimen. Composition is reported by powder diffraction and by elemental analysis. The laboratory states what the sample is, not only that a pellet was pressed.

Both signatures pass on both batches at 298 K and at a pressure of 1 bar, after 24 hours in air. The copper control below shows neither signature.

The temperature, the pressure, the 24-hour wait, the two-batch rule, the current reversal, and the 30 K offset used to form the resistance ratio are pass marks defined here. YBa₂Cu₃O₇₋δ is an instrument control. The 134 K and 151 K reports are the ambient-pressure context. They are not targets to match.

• The YBa₂Cu₃O₇₋δ pellet, measured the same day on the same instruments and in the same applied field, shows both a resistance drop and a zero-field-cooled diamagnetic transition. If it does not, the instruments are not trusted and the candidate is not graded.
• A copper specimen of similar shape shows no resistance drop and no diamagnetic transition on the same sweep.
• The candidate’s diamagnetic signal must be present at 298 K on cooling from above the claimed onset and absent above that onset. A signal that is present at all temperatures is a magnetic impurity, not a Meissner effect. Field-cooled and zero-field-cooled curves are both reported. The graded curve is the zero-field-cooled curve.
• Both batches pass. One pearl of a pellet is not enough.

Any composition the elemental analysis can describe, including organics and non-copper oxides. Ambient-pressure synthesis of any kind. A commercial magnetometer and a four-point probe.

Reporting resistance without magnetometry, or magnetometry without resistance. A transition that exists only under pressure, or only after a pressure quench if the released specimen does not then meet the 298 K, 24-hour air, and two-batch rules. A sample that must be kept in an inert atmosphere or that decomposes in the 24-hour wait. A film so thin that the volume used in the magnetization ratio is chosen after the fact. Levitation of a pellet over a magnet, by itself. A resistance taken at one current direction only.

Both signatures are present on both batches at or above 273 K, the resistance ratio is at least 1,000 relative to the same specimen above the onset, and the zero-field-cooled diamagnetic moment per volume is at least ten times the copper blank and is temperature-dependent. Matching the yttrium–barium–copper oxide volume susceptibility is not required for the partial bar. A resistance drop without a diamagnetic transition is not a partial success. The 273 K floor, the factor of 1,000, and the factor of ten are pass marks for this lesser bar. Two hundred and seventy-three kelvin is still above the 134 K and 151 K reports cited here.

Wu, Ashburn, Torng, Hor, Meng, Gao, Huang, Wang, and Chu, Physical Review Letters, 1987, 58, 908, doi:10.1103/PhysRevLett.58.908 (opens in a new tab), for the resistive and magnetic transition between 80 and 93 K at ambient pressure. Wen and co-workers, npj Quantum Materials, 2025, doi:10.1038/s41535-025-00735-w (opens in a new tab), for HgBa₂Ca₂Cu₃O₈₊δ at T_c ≈ 134 K as the highest ambient-pressure cuprate in that paper, and for the statement that the critical temperature reaches 164 K under pressure. Deng, Chu, and co-workers, Proceedings of the National Academy of Sciences, 2026, doi:10.1073/pnas.2536178123 (opens in a new tab), for the reported 151 K ambient-pressure transition after a pressure quench. The 298 K requirement, the pressure window, the 24-hour wait, and the partial-success factors are pass marks defined here.