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Scanning tunneling spectroscopy (STS) and angle-resolved photoemission spectroscopy (ARPES) have been investigated on single crystal samples of KFe2As2. A van Hove singularity (vHs) has been directly observed just a few meV below the Fermi level E_F of superconducting KFe2As2, which locates in the middle of the principle axes of the first Brillouin zone. The majority of the density-of-states at E_F, mainly contributed by the proximity effect of the saddle point to E_F, is non-gapped in the superconducting state. Our observation of nodal behavior of the momentum area close to the vHs points, while providing consistent explanations to many exotic behaviours previously observed in this material, suggests Cooper pairing induced by a strong coupling mechanism.
Comprehensive studies of the electronic states of Ir 5d and Te 5p have been performed to elucidate the origin of the structural phase transition in IrTe2 by combining angle-resolved photoemission spectroscopy and resonant inelastic X-ray scattering.
Using the functional renormalization group, we investigate the electron instability in the single-sheet BC$_3$ when the electron filling is near a type-II van Hove singularity. For a finite Hubbard interaction, the ferromagnetic-like spin density wav
A van der Waals coupled Weyl semimetal material NbIrTe4 is investigated by combining scanning tunneling microscopy/spectroscopy and first principles calculations. We observe a sharp peak in the tunneling conductance near the zero bias energy, and its
In the context of the relaxation time approximation to Boltzmann transport theory, we examine the behavior of the Hall number, $n_H$, of a metal in the neighborhood of a Lifshitz transition from a closed Fermi surface to open sheets. We find a univer
A topological Dirac semimetal is a novel state of quantum matter which has recently attracted much attention as an apparent 3D version of graphene. In this paper, we report critically important results on the electronic structure of the 3D Dirac semi