The presence or absence of a quantum critical point and its location in the phase diagram of high-temperature superconductors have been subjects of intense scrutiny. Clear evidence for quantum criticality, particularly in the transport properties, has proved elusive because the important low-temperature region is masked by the onset of superconductivity. We present measurements of the low-temperature in-plane resistivity of several highly doped La _{2– x} Sr _x CuO ₄ single crystals in which the superconductivity had been stripped away by using high magnetic fields. In contrast to other quantum critical systems, the resistivity varies linearly with temperature over a wide doping range with a gradient that scales monotonically with the superconducting transition temperature. It is maximal at a critical doping level ( p _c ) ∼ 0.19 at which superconductivity is most robust. Moreover, its value at p _c corresponds to the onset of quasi-particle incoherence along specific momentum directions, implying that the interaction that first promotes high-temperature superconductivity may ultimately destroy the very quasi-particle states involved in the superconducting pairing.

We have performed high-resolution angle-resolved photoemission spectroscopy on an FeSe superconductor ($T_c\sim 8\mathrm{K}$), which exhibits a tetragonal-to-orthorhombic structural transition at $T_s\sim 90\mathrm{K}$. At low temperature, we found splitting of the energy bands as large as 50 meV at the $M$ point in the Brillouin zone, likely caused by the formation of electronically driven nematic states. This band splitting persists up to $T\sim 110\mathrm{K}$, slightly above $T_s$, suggesting that the structural transition is triggered by the electronic nematicity. We have also revealed that at low temperature the band splitting gives rise to a van Hove singularity within 5 meV of the Fermi energy. The present result strongly suggests that this unusual electronic state is responsible for the unconventional superconductivity in FeSe.

Quantum transport of Dirac cone states in the iron pnictide Ba(FeAs)2 with a d-multiband system is studied by using single crystal samples. Transverse magnetoresistance develops linearly against the magnetic field at low temperatures. The transport phenomena are interpreted in terms of the zeroth Landau level by applying the theory predicted by Abrikosov. The results of the semiclassical analyses of a two carrier system in a low magnetic field limit show that both the electron and hole reside as the high mobility states. Our results show that pairs of electron and hole Dirac cone states must be taken into account for an accurate interpretation in iron pnictides, which is in contrast with previous studies.