Spectroscopic evidence for a pseudogap in the normal state of underdoped high-Tc superconductors

Chemical potential oscillations from nodal Fermi surface pocket in the underdoped high-temperature superconductor YBa2Cu3O6+x

“…The nodal locations refer to k nodal = [π/2a, π/2a] in the Brillouin zone, and the antinodal locations refer to k antinodal = [π/a, 0] in the Brillouin zone, where a is the lattice constant. This picture, however, is in direct contrast with the observation made by photoemission experiments of a significant (~50 meV) gap in density-ofstates at the Fermi level at the antinodes [14][15][16] .…”

“…Complementary photoemission experiments at zero field [14][15][16] , optical conductivity experiments up to 8 T, and high field heat capacity 27 experiments indicate a concentration of density-ofstates at the Fermi level solely at the nodal location in momentum space. Taken together with the persistence of quantum oscillations unaltered in form down to low fields ~22 T where the d-wave superconducting gap is well developed, the quasi-two-dimensional Fermi surface pocket that gives rise to multiple frequency components observed by quantum oscillations in underdoped YBa 2 Cu 3 O 6 + x is likely connected with the nodal region of the Brillouin zone.…”

“…The Fermi surface pocket responsible for quantum oscillations is likely located at the nodal region, given that these oscillations persist down to low magnetic fields, where the d-wave superconducting gap is well developed, whereas complementary experiments up to these fields [14][15][16]27 find a concentration in the density-of-states at the Fermi level at the nodes of the superconducting wavefunction. Translational symmetry-breaking models proposed thus far are either nodal-antinodal (for example, refs 11,12), which would be inconsistent with the current experimental results, or comprise solely nodal hole pockets 13 , which would be difficult to reconcile with negative Hall and Seebeck effect 4,20 , leaving us with a dilemma.…”

“…The next question relates to the location of this quasi-two-dimensional pocket. We consider complementary experiments: photoemission experiments at zero field [14][15][16] , optical conductivity experiments up to 8 T (refs 25,26), and high field heat capacity experiments (ref. 27) indicate a solely nodal concentration of the density-ofstates at the Fermi level.…”

Fermi surface dichotomy of the superconducting gap and pseudogap in underdoped pnictides

“…For this reason, much attention has been devoted in the past to underdoped (UD) cuprates. In addition to the SC gap, evidence for a large doping-dependent pseudogap (PG) above T c at the antinodes of UD cuprates has existed for some time 15,16 . Nevertheless, debates on the interplay between superconductivity, antiferromagnetism and PG in cuprates are ongoing.…”