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Why do changes in oxygen content affect the transition temperature of cuprate superconductors?

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Figure 1: Specific heat in the normal (Cen) and superconducting (Ces) states of a classic superconductor as a function of absolute temperature. The two functions are identical at the transition temperature (Tc) and above Tc. — superconductivity Figure 1: Specific heat in the normal (C_en) and superconducting (C_es) states of a classic superconductor as a function of absolute temperature. The two functions are identical at the transition temperature (T_c) and above T_c.

In cuprate superconductors, the crystal structure consists of planes of copper and oxygen atoms, which play a critical role in the material's superconducting properties. Varying the oxygen content or the heat treatment of these compounds dramatically alters their transition temperatures (T_c), critical magnetic fields, and other physical characteristics.1

While the exact mechanism remains a subject of intense scientific investigation, these materials are characterized by strong correlations between electron motions and significant magnetic phenomena. Most experts believe that interactions among the electrons themselves—rather than solely the lattice vibrations (phonons) identified in classic BCS theory—generate high-temperature superconductivity. Because the oxygen atoms are integral to the crystal planes where these electronic interactions occur, changing the oxygen concentration directly influences the electronic environment, thereby shifting the temperature at which the material transitions into a superconducting state.1