Coherence and single-particle excitations in the high-temperature superconductors

Area of Science:

• Condensed Matter Physics
• Materials Science
• Superconductivity

Background:

• The pseudogap phenomenon in underdoped copper oxide superconductors is a key area of high-temperature superconductivity research.
• Unlike conventional BCS superconductors, the pseudogap appears above the critical temperature (Tc) and can exceed the BCS gap energy.

Purpose of the Study:

• To investigate and compare the distinct energy scales associated with the pseudogap in copper oxide superconductors.
• To elucidate the relationship between these energy scales and the superconducting state across different doping regimes.

Main Methods:

• Comparison of gap energies using angle-resolved photoemission spectroscopy (ARPES) and tunneling experiments to determine the single-particle excitation energy (Δp).
• Utilizing Andreev reflection experiments to measure the coherence energy scale of the superconducting state (Δc).

Main Results:

• Two distinct energy scales, Δp (single-particle excitation) and Δc (superconducting coherence), were identified.
• In the overdoped regime, Δp and Δc converge, consistent with BCS theory.
• In the underdoped regime, Δp and Δc diverge, with Δp being larger than Δc, indicating pairing above the condensation temperature.

Conclusions:

• The divergence of Δp and Δc in underdoped cuprates suggests pairing occurs above the superconducting condensation temperature.
• This behavior may be explained by models involving crossovers from BCS to Bose-Einstein condensation or the formation of striped phases.