Final answer:
The critical temperature (Tc) generally means the temperature below which a material becomes superconducting, which varies among materials, and not all materials achieve superconductivity. High-temperature superconductors have critical temperatures high enough to utilize less expensive cooling methods such as liquid nitrogen instead of liquid helium.
Step-by-step explanation:
The critical temperature (Tc) refers to the temperature below which a material becomes superconducting. This property is pivotal for understanding superconductors and their applications. From a broader perspective, the term critical temperature can also refer to the point in the phase diagram of a substance where it can no longer exist as a liquid, regardless of pressure. While all materials have some sort of critical point when considering the latter definition, not all materials become superconductors; therefore, not all materials have a critical temperature for superconductivity.
In the context of superconductors, superconductivity is a phenomenon where a material exhibits zero electrical resistance below a certain critical temperature. This temperature is different for each superconductor and can range from a fraction of a Kelvin to values much higher than 100 K. High-temperature superconductors have a critical temperature near or above the boiling point of liquid nitrogen (77 K), which makes them extremely valuable for practical applications because liquid nitrogen is more affordable and accessible than liquid helium, which boils at 4.2 K.
High-temperature superconductors are those with verifiable Tcs greater than 125 K, a temperature easily achieved with liquid nitrogen cooling. These are particularly significant in reducing the costs associated with cooling superconductive materials for their use in various technologies.