A superconductor’s critical temperature, written Tc, is the temperature threshold below which it enters a superconducting state. In that state, it has zero direct-current (DC) electrical resistance and expels magnetic fields as it transitions. The threshold is not a stand-alone guarantee of performance: current and magnetic field also affect whether superconductivity can be maintained.
What happens below the critical temperature?
In an ordinary conductor, resistance generally decreases as the material cools. A superconductor does something different: when it crosses its Tc, it undergoes a transition into a state with zero DC resistance. The U.S. Department of Energy (DOE) describes this as conducting DC electricity without energy loss when cooled below the critical temperature (DOE’s superconductivity explainer).
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Superconductivity also involves a magnetic response. During the transition, a superconductor expels magnetic fields, an effect known as the Meissner effect. This helps distinguish superconductivity from a material that simply has very low resistance (National Academies discussion of superconductivity).
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No. A material can leave its superconducting state if the current through it exceeds its critical current or if the magnetic field exceeds the applicable critical-field limit. These limits depend on the material and operating conditions. A useful comparison therefore considers Tc, current capacity, and magnetic-field performance together—not just which material has the highest transition temperature (DOE; National Academies).
Does “high-temperature superconductor” mean room temperature?
No. “High-temperature” is a relative label within superconductivity, not a claim that a material works at ordinary room temperature. DOE notes that some high-temperature superconducting materials can operate above the boiling-temperature range of liquid nitrogen, but they still require cooling. The term describes a higher operating temperature than many other superconductors, not room-temperature operation.
Why does the critical temperature matter in practice?
Tc helps determine how a superconductor must be cooled before it can be used. That matters in systems such as superconducting magnets, which enable technologies including MRI machines and particle accelerators. But the cooling requirement is only part of the engineering choice: the material must also tolerate the system’s current and magnetic field.
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How cold was the first known superconductor?
Superconductivity was discovered in mercury in 1911. NIST’s WebHTS manual gives mercury’s transition temperature as 4.2 kelvin (K), while Nobel Prize educational material describes it as about 4 K. These are rounded descriptions of mercury’s transition, not a universal critical temperature or a current record for superconductors (NIST WebHTS manual; Nobel Prize educational material).
What explains superconductivity?
For conventional superconductors, John Bardeen, Leon Cooper, and Robert Schrieffer proposed an electron-pairing theory in 1957. That account does not fully explain all high-temperature superconductors: DOE notes that the mechanism in many newer materials remains incompletely understood. The practical meaning of Tc—the threshold for the superconducting transition—does not depend on having a complete explanation for every material.
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