Four-level approximation in disordered medium

Abstract

The adsorption of superfluid ⁴He into a disordered medium, the disappearance of superfluidity due to adsorption, and the subsequent appearance of superfluidity when more ⁴He flows into the disordered medium have been described by the four-level approximation. The adsorption of superfluid ⁴He into a disordered medium depletes the ZMS as the particles shift to higher excited levels resulting in ‘four-level approximation’. Superfluidity reappears on further adsorption but at a lower temperature implying that the <i>T</i>_cvalue of a four-level system should be lower than the <i>T</i>_c value for a three-level system, which is in agreement with the experimental observations.