Pressure can tune material's electronic properties and control its quantum state, making some systems present disconnected superconducting region as observed in iron chalcogenides and heavy fermion CeCu2Si2. For CaC6 superconductor (Tc of 11.5 K), applying pressure first Tc increases and then suppresses and the superconductivity of this compound is eventually disappeared at about 18 GPa. Here, ...

We examine a contact between a superconductor whose order parameter changes sign across the Brillioun zone, and an ordinary, uniform-sign superconductor. Within a Ginzburg-Landau-type model, we find that if the barrier between the two superconductors is not too high, the frustration of the Josephson coupling between different portions of the Fermi surface across the contact can lead to surprisi...

We have performed angle-resolved photoemission spectroscopy of the iron-chalcogenide superconductor Fe1.03Te0.7Se0.3 to investigate the electronic structure relevant to superconductivity. We observed a holelike Fermi surface (FS) and an electronlike FS at the Brillouin zone center and corner, respectively, which are nearly nested by the Q∼(π,π) wave vector. We do not find evidence for the nesti...

The electronic nematic phase emerging with spontaneous rotation symmetry breaking is a central issue of modern condensed matter physics. In particular, various phases in iron-based superconductors and high-$T_{\rm c}$ cuprate are extensively studied recently. Electric quadrupole moments (EQMs) one the order parameters characterizing these unified way, elucidating EQMs key to understanding phase...

We perform scanning tunneling microscopy/spectroscopy study on the layered iron based superconductor KCa$_2$Fe$_4$As$_4$F$_2$ with a critical temperature of about 33.5 K. Two types terminated surfaces are generally observed after cleaving samples in vacuum. On one commonly obtained surface, we observe full gap feature energy values close to 4.6 meV. This type spectrum shows clean and uniform sp...

We perform scanning tunneling microscopy/spectroscopy study on the layered iron based superconductor KCa$_2$Fe$_4$As$_4$F$_2$ with a critical temperature of about 33.5 K. Two types terminated surfaces are generally observed after cleaving samples in vacuum. On one commonly obtained surface, we observe full gap feature energy values close to 4.6 meV. This type spectrum shows clean and uniform sp...

$A{\mathrm{Co}}_{2}{\mathrm{Se}}_{2}$ ($A$=K, Rb, Cs) is a homologue of the iron-based superconductor $A{\mathrm{Fe}}_{2}{\mathrm{Se}}_{2}$. From comprehensive study ${\mathrm{RbCo}}_{2}{\mathrm{Se}}_{2}$ via measurements magnetization, transport, neutron diffraction, angle-resolved photoemission spectroscopy, and first-principles calculations, we identify ferromagnetic order accompanied by an ...