Jian‐Xin Zhu

This review presents recent developments in the understanding of how impurities influence the electronic states in the bulk properties of superconductors. The focus is on quasilocalized states in the vicinity of impurity sites in conventional and unconventional superconductors and the goal is to provide a unified framework for their description. The nonmagnetic impurity resonances in unconventional superconductors are directly related to the Yu-Shiba-Rusinov states around magnetic impurities in conventional $s$-wave systems. The physics behind these states, including the quantum phase transition between screened and unscreened impurities, are reviewed and recent work on $d$-wave superconductors is emphasized. The bound states are seen in scanning-tunneling spectroscopy measurements on high-${T}_{c}$ cuprates, which are described in detail. This paper discusses very recent progress in our understanding of states coupled to impurity sites, which have their own dynamics. Also reviewed are inelastic electron-tunneling spectroscopy features that could be seen by scanning-tunneling microscopy in real space and their Fourier-transformed images and impurity resonances in the presence of an order competing with superconductivity. The last part of the review is devoted to the influence of local deviations of the impurity concentration from its average value on the density of states in $s$-wave superconductors. Discussed is how these fluctuations affect the density of states and it is shown that $s$-wave superconductors are, strictly speaking, gapless in the presence of an arbitrarily small concentration of magnetic impurities.

Abstract Three dimensional topological insulators, as a new phase of quantum matters, are characterized by an insulating gap in the bulk and a metallic state on the surface. Particularly, most of the topological insulators have narrow band gaps, and hence have promising applications in the area of terahertz optoelectronics. In this work, we experimentally demonstrate an electronically-tunable terahertz intensity modulator based on Bi 1:5 Sb 0:5 Te 1:8 Se 1:2 single crystal, one of the most insulating topological insulators. A relative frequency-independent modulation depth of ~62% over a wide frequency range from 0.3 to 1.4 THz has been achieved at room temperature, by applying a bias current of 100 mA. The modulation in the low current regime can be further enhanced at low temperature. We propose that the extraordinarily large modulation is a consequence of thermally-activated carrier absorption in the semiconducting bulk states. Our work provides a new application of topological insulators for terahertz technology.

Data from $\mathrm{ab}$-oriented ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7}/\mathrm{I}/\mathrm{Cu}$ tunnel junctions are presented. Self-assembled monolayers form the insulating tunnel barrier, I. The ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7}$ features in the tunneling conductance match those of low-leakage $\mathrm{ab}$-oriented ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7}/\mathrm{Pb}$ junctions. Results show that the zero-bias conductance peak is an Andreev bound state (ABS) of a $d$-wave order parameter. In zero magnetic field, the ABS splits below $\ensuremath{\sim}7\mathrm{K}$, consistent with the presence of a subdominant order parameter at the surface. An applied magnetic field induces further splitting that grows nonlinearly with increasing field.

Establishing the appropriate theoretical framework for unconventional superconductivity in the iron-based materials requires correct understanding of both the electron correlation strength and the role of Fermi surfaces. This fundamental issue becomes especially relevant with the discovery of the iron chalcogenide superconductors. Here, we use angle-resolved photoemission spectroscopy to measure three representative iron chalcogenides, FeTe0.56Se0.44, monolayer FeSe grown on SrTiO3 and K0.76Fe1.72Se2. We show that these superconductors are all strongly correlated, with an orbital-selective strong renormalization in the dxy bands despite having drastically different Fermi surface topologies. Furthermore, raising temperature brings all three compounds from a metallic state to a phase where the dxy orbital loses all spectral weight while other orbitals remain itinerant. These observations establish that iron chalcogenides display universal orbital-selective strong correlations that are insensitive to the Fermi surface topology, and are close to an orbital-selective Mott phase, hence placing strong constraints for theoretical understanding of iron-based superconductors.