Search

The observation of quantum oscillations in the electrical resistance of YBa₂Cu₃O_6.5, is reported, establishing the existence of a well-defined Fermi surface in the ground state of underdoped copper oxides (once superconductivity is suppressed by a magnetic field). The low oscillation frequency reveals a Fermi surface made of small pockets, in contrast to the large cylinder characteristic of the overdoped regime.

Transport measurements in a high-temperature superconductor provide evidence that the so-called pseudogap phase ends at a quantum critical point located inside the superconducting dome in the phase diagram of cuprates.

The observation of a negative Hall resistance in the magnetic-field-induced normal state of underdoped 'YBCO'materials, which reveals that these pockets are electron-like rather than hole-like. It is proposed that these electron pockets most probably arise from a reconstruction of the Fermi surface caused by the onset of a density-wave phase, as is thought to occur in the electron-doped copper oxides near the onset of antiferromagnetic order.

In the study of high-transition-temperature (high-T_c) copper oxide superconductors, a fundamental question is what symmetries are broken when the pseudogap phase sets in below a temperature T*. A large in-plane anisotropy of the Nernst effect is now observed in a high-T_c copper oxide superconductor that sets in precisely at T* throughout the doping phase diagram. It is concluded that the pseudogap phase is an electronic state that strongly breaks four-fold rotational symmetry.

The Nernst effect in metals is sensitive to superconductivity and density-wave order. The large positive Nernst signal observed in hole-doped high-transition-temperature superconductors has been attributed to fluctuating superconductivity but Olivier Cyr-Choiniere and colleagues report that the Nernst signal can be caused by stripe order. In LSCO doped with Nd or Eu, the onset of stripe order causes the Nernst signal to go from small and negative to large and positive.

The point at which a magnetic field kills superconductivity in the cuprates has been difficult to measure. Grissonnanche et al. use thermal conductivity measurements to reliably determine this field and find that it drops suddenly below some critical doping, suggesting the onset of a new competing phase.

High-temperature superconductors exhibit pseudogap behaviour that remains of unknown origin, despite many years of intensive study. Here the authors study the onset of the pseudogap under pressure, providing evidence that it requires a hole-like Fermi surface and constraining future theoretical developments.