pseudogap

The pseudogap is a partial suppression of the electronic density of states at the Fermi energy that appears in the normal state above the superconducting transition temperature in some strongly correlated materials, most notably the underdoped cuprate superconductors. It is characterized by a reduction of spectral weight near the Fermi level without the emergence of a full superconducting gap or long-range phase coherence.

In cuprates, the pseudogap is typically most pronounced at underdoping and often exhibits momentum dependence, being stronger in the antinodal regions of the Brillouin zone. It is observed by multiple experimental probes, including angle-resolved photoemission spectroscopy (ARPES), scanning tunneling microscopy (STM), nuclear magnetic resonance (NMR), and infrared and optical conductivity measurements. ARPES can reveal Fermi-surface sections with diminished spectral weight, sometimes described as Fermi arcs, while STM detects a partial gap in the local density of states above Tc.

The characteristic temperature scale, T*, marks the onset of pseudogap behavior and generally decreases with increased hole doping, sometimes approaching Tc near optimal doping. The relationship between the pseudogap and superconductivity remains a major open question. Competing theories invoke different origins: a precursor superconducting state with preformed pairs lacking phase coherence, or a competing order such as charge-density wave, spin-density wave, loop-current order, or other forms of electronic order and fluctuations.

While most prominently studied in cuprates, pseudogap-like phenomena have been discussed in other correlated-electron systems, though the evidence is less uniform. The pseudogap continues to be a central topic in the study of high-temperature superconductivity and strongly correlated materials.