Change of quasiparticle dispersion in crossing T_c in the underdoped cuprates

One of the most remarkable properties of the high-temperature superconductors is a pseudogap regime appearing in the underdoped cuprates above the superconducting transition temperature T_c. The pseudogap continously develops out of the superconducting gap. In this paper, we demonstrate by means of a detailed comparison between theory and experiment that the characteristic change of quasiparticle dispersion in crossing T_c in the underdoped cuprates can be understood as being due to phase fluctuations of the superconducting order parameter. In particular, we show that within a phase fluctuation model the characteristic back-turning BCS bands disappear above T_c whereas the gap remains open. Furthermore, the pseudogap rather has a U shape instead of the characteristic V shape of a d_x²−y²-wave pairing symmetry and starts closing from the nodal k⃗=(π∕2,π∕2) directions, whereas it rather fills in at the antinodal k⃗=(π,0) regions, yielding further support to the phase fluctuation scenario.