Critical temperature, written Tc, is the temperature below which a material enters its superconducting state. Below this threshold, it has zero direct-current (DC) electrical resistance and expels magnetic fields as it transitions—two behaviors that distinguish superconductivity from ordinary low-resistance conduction.
What happens below a superconductor’s critical temperature?
In an ordinary conductor, resistance generally decreases as the material cools. A superconductor undergoes a distinct transition: below its Tc, its DC electrical resistance is zero. The U.S. Department of Energy (DOE) describes superconductors as also expelling magnetic fields during the transition. That magnetic response is called the Meissner effect, and it is a defining feature beyond simply having very low resistance. DOE; National Academies.
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Does a material superconduct at any current or magnetic field?
No. Critical temperature is one operating limit, not a guarantee that superconductivity persists under every condition. A material can leave the superconducting state if the current exceeds its critical current or the magnetic field exceeds the applicable critical-field limit. Practical comparisons therefore need to consider all three: Tc, critical current, and magnetic-field performance. DOE; National Academies.
Does “high-temperature superconductor” mean room temperature?
No. “High-temperature” is a relative label within superconductivity, not a claim that the material works at ordinary room temperature. The DOE notes that some high-temperature superconductors can operate above the boiling-temperature range of liquid nitrogen, but they still require cooling. DOE.
How cold is a superconductor’s critical temperature?
There is no single critical temperature shared by all superconductors: Tc depends on the material. Mercury was the first known superconductor. The Nobel Prize educational material reports its transition at about 4 kelvin (K), while the National Institute of Standards and Technology (NIST) manual gives 4.2 K. These are historical figures for mercury, not a universal threshold or a current record for superconductors. Nobel Prize; NIST.
Why does critical temperature matter in practice?
Tc helps determine how a material can be cooled and used in a system. Superconducting magnets are used in applications including MRI and particle accelerators, but a higher Tc alone does not establish that one material is better for a particular application. The current and magnetic field it can tolerate also matter. DOE; National Academies.
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What causes superconductivity?
The discovery of superconductivity in mercury dates to 1911, according to the DOE. In 1957, John Bardeen, Leon Cooper, and Robert Schrieffer proposed the conventional electron-pairing account known as BCS theory. That account does not fully explain every newer material: the DOE says the mechanism in many high-temperature superconductors remains incompletely understood. DOE; Nobel Prize.
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