Enhanced superconductivity in surface-electron-doped iron pnictide Ba(Fe1.94Co0.06)2As2

Momentum-resolved visualization of electronic evolution in doping a Mott insulator

High temperature superconductivity in cuprates arises from doping a parent Mott insulator by electrons or holes. A central issue is how the Mott gap evolves and the low-energy states emerge with doping. Here we report angle-resolved photoemission spectroscopy measurements on a cuprate parent compound by sequential in situ electron doping. The chemical potential jumps to the bottom of the upper Hubbard band upon a slight electron doping, making it possible to directly visualize the charge transfer band and the full Mott gap region. With increasing doping, the Mott gap rapidly collapses due to the spectral weight transfer from the charge transfer band to the gapped region and the induced low-energy states emerge in a wide energy range inside the Mott gap. These results provide key information on the electronic evolution in doping a Mott insulator and establish a basis for developing microscopic theories for cuprate superconductivity.

How a Mott insulating state evolves into a conducting or superconducting state is a central issue in doping a Mott insulator and important to understand the physics in high temperature cuprate superconductors. Here, the authors visualize the electronic structure evolution of a Mott insulator within the full Mott gap region and address the fundamental issues.

A new Majorana platform in an Fe-As bilayer superconductor

Iron-chalcogenide superconductors have emerged as a promising Majorana platform for topological quantum computation. By combining topological band and superconductivity in a single material, they provide significant advantage to realize isolated Majorana zero modes. However, iron-chalcogenide superconductors, especially Fe(Te,Se), suffer from strong inhomogeneity which may hamper their practical application. In addition, some iron-pnictide superconductors have been demonstrated to have topological surface states, yet no Majorana zero mode has been observed inside their vortices, raising a question of universality about this new Majorana platform. In this work, through angle-resolved photoemission spectroscopy and scanning tunneling microscopy/spectroscopy measurement, we identify Dirac surface states and Majorana zero modes, respectively, for the first time in an iron-pnictide superconductor, CaKFe₄As₄. More strikingly, the multiple vortex bound states with integer-quantization sequences can be accurately reproduced by our model calculation, firmly establishing Majorana nature of the zero mode.

Iron-pnictide superconductors share similar topological band structure with iron-chalcogenide superconductors, but no Majorana modes have been observed in the former. Here, the authors observe both the superconducting Dirac surface states and Majorana zero modes inside its vortex cores in CaKFe₄As₄.

Visualizing Orbital Content of Electronic Bands in Anisotropic 2D Semiconducting ReSe2

Many properties of layered materials change as they are thinned from their bulk forms down to single layers, with examples including indirect-to-direct band gap transition in 2H semiconducting transition metal dichalcogenides as well as thickness-dependent changes in the valence band structure in post-transition-metal monochalcogenides and black phosphorus. Here, we use angle-resolved photoemission spectroscopy to study the electronic band structure of monolayer ReSe₂, a semiconductor with a distorted 1T structure and in-plane anisotropy. By changing the polarization of incoming photons, we demonstrate that for ReSe₂, in contrast to the 2H materials, the out-of-plane transition metal d_z² and chalcogen p_z orbitals do not contribute significantly to the top of the valence band, which explains the reported weak changes in the electronic structure of this compound as a function of layer number. We estimate a band gap of 1.7 eV in pristine ReSe₂ using scanning tunneling spectroscopy and explore the implications on the gap following surface doping with potassium. A lower bound of 1.4 eV is estimated for the gap in the fully doped case, suggesting that doping-dependent many-body effects significantly affect the electronic properties of ReSe₂. Our results, supported by density functional theory calculations, provide insight into the mechanisms behind polarization-dependent optical properties of rhenium dichalcogenides and highlight their place among two-dimensional crystals.

Enhanced superconductivity of Ba0.5K0.5Fe2As2 under surface potassium dosing

Surface potassium dosing has been proven to be an effective method in tuning the electron doping and enhancing the superconducting transition temperatures in both iron chalcogenides and electron doped iron pnictides. However, it is not clear how surface potassium dosing affects the hole doping and superconductivity in hole doped Fe-based superconductors. Here we performed K-dosing on Ba_0.5K_0.5Fe₂As₂, a prototypical hole-doped iron pnictide compound, and explored the electronic structure by in situ angle-resolved photoemission spectroscopy measurements. Starting from the slightly over-doped Ba_0.5K_0.5Fe₂As₂, surface K-dosing effectively reduces the hole concentration towards optimal doping and enhances the superconductivity. Intriguingly, the enhancement of superconductivity does not slow down at optimal doping, and the gap further increases with K dosing even when the carrier doping effect is saturated. Meanwhile, the quasiparticle coherence of the inner hole pockets is enhanced by higher K dosing. Our results uncover a novel scattering-reduction effect of K-dosing in Ba_1-x K _x Fe₂As₂, which collaborates with the carrier doping effect and enhances superconductivity.

Origin of charge transfer and enhanced electron-phonon coupling in single unit-cell FeSe films on SrTiO3

Interface charge transfer and electron–phonon coupling have been suggested to play a crucial role in the recently discovered high-temperature superconductivity of single unit-cell FeSe films on SrTiO₃. However, their origin remains elusive. Here, using ultraviolet photoemission spectroscopy and element-sensitive X-ray photoemission spectroscopy, we identify the strengthened Ti–O bond that contributes to the interface enhanced electron–phonon coupling and unveil the band bending at the FeSe/SrTiO₃ interface that leads to the charge transfer from SrTiO₃ to FeSe films. We also observe band renormalization that accompanies the onset of superconductivity. Our results not only provide valuable insights into the mechanism of the interface-enhanced superconductivity, but also point out a promising route toward designing novel superconductors in heterostructures with band bending-induced charge transfer and interfacial enhanced electron–phonon coupling.

The origin of interface charge transfer and electron-phonon coupling in single unit-cell FeSe on SrTiO₃ remains elusive. Here, Zhang et al. report strengthened Ti-O bond and band bending at the FeSe/SrTiO₃ interface, which leads to several important processes.