Temperature correction to the Casimir force in cryogenic range and anomalous skin effect

The temperature correction to the Casimir force is considered for real metals at low temperatures. With the temperature decrease, the mean free path for electrons becomes larger than the field penetration depth. In this condition, description of metals with the impedance of anomalous skin effect is shown to be more appropriate than with the permittivity. The effect is crucial for the temperature correction. It is demonstrated that in the zero-frequency limit, the reflection coefficients should coincide with those of ideal metal if we demand the entropy to be zero at T=0. All the other prescriptions discussed in the literature for the n=0 term in the Lifshitz formula give negative entropy. It is shown that the temperature correction in the region of anomalous skin effect is not suppressed as it happens in the plasma model. This correction will be important in the future cryogenic measurements of the Casimir force.