High-Tc · pseudogap · d-wave pairing symmetry · phase diagram
Cuprate superconductors (e.g., YBCO, BSCCO, LSCO) exhibit the highest known Tc values (up to ~135 K at ambient pressure) with unconventional d_x²−y² pairing symmetry: the gap Δ(k) = Δ₀(cos kx − cos ky)/2 has four nodes along the (±1,±1) directions. Their phase diagram is exceptionally rich: parent compound is a Mott insulator with antiferromagnetic order; hole doping creates a pseudogap phase (T* line) where a partial gap opens on the Fermi surface ("Fermi arcs") before full superconducting coherence; optimal doping (~0.16 holes/Cu) maximizes Tc. The mechanism — likely spin-fluctuation mediated pairing — remains one of the deepest open problems in condensed matter physics.