We present STM/STS, ARPES and magnetotransport studies of the surface topography and electronic structure of pristine Bi₂Se₃ in comparison to Bi_1.96Mg_0.04Se₃ and Bi_1.98Fe_0.02Se₃. The topography images reveal a large number of complex, triangle-shaped defects at the surface. The local electronic structure of both the defected and non-defected regions is examined by STS. The defect-related states shift together with the Dirac point observed in the undefected area, suggesting that the local electronic structure at the defects is influenced by doping in the same way as the electronic structure of the undefected surface. Additional information about the electronic structure of the samples is provided by ARPES, which reveals the dependence of the bulk and surface electronic bands on doping, including such parameters as the Fermi wave vector. The subtle changes of the surface electronic structure by doping are verified with magneto-transport measurements at low temperatures (200 mK) allowing the detection of Shubnikov-de Haas (SdH) quantum oscillations.

The origin of nematicity and its relation to superconductivity in iron pnictide high-temperature superconductors remains unclear. One of its possible sources is the uneven occupation of Fe $d_{xz}$ and $d_{yz}$ orbitals. Here, by using x-ray linear dichroism technique, we show that such an imbalance is present in $\mathrm{Eu}{\left({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x\right)}_2{\mathrm{As}}_2$ single crystals deep in the tetragonal phase and also in the superconducting state, where we find that the $d_{xz}$ orbital has a higher occupation as indicated by an x-ray linear dichroism magnitude of 1.5%. Our result shows the importance of orbital polarization in the theoretical description of nematicity and superconductivity, particularly for determination of the superconducting gap symmetry, which is affected by orbital fluctuations.

The temperature dependence of resistance and of current–voltage characteristics I − V are studied for two underdoped La_1.952Sr_0.048CuO₄ films with different thickness and different degrees of built-in compressive strain. Strain induces an inhomogeneous superconducting (SC) state in the films, with two-step SC transition, consistent with the presence of SC islands coupled by the proximity effect. On further cooling, superfluid density is suppressed and global phase coherence is never reached. Instead, a reentrant increase of resistance appears, reaching several orders of magnitude in the thinner, more strongly strained film, and saturating at lowest temperatures. The saturation is accompanied by Coulomb blockade, which resembles the blockade reported for Josephson junction (JJ) arrays. However, increasing magnetic field quickly diminishes the influence of Coulomb blockade on transport, which is distinct from the behavior observed for JJ arrays. The observation of Coulomb blockade indicates that Cooper pairs survive in the insulating state, suggesting the local character of pairing.