Electronic structures and photoemission spectroscopy

Abstract:

Shandite kagome materials have attracted great research attention due to their intriguing properties, such as the magnetic Weyl semimetal phase, endless nodal lines, and pressure-induced superconductivity. In this work, by combining angle-resolved photoemission spectroscopy and ab initio calculation, we systematically investigate the electronic band structure of the shandite kagome compound Ni₃In₂Se₂. The measured band structure is in good agreement with ab initio calculations including the spin-orbit coupling effect. The experimental spectra are predominantly characterized by surface resonance states exhibiting highly anisotropic band dispersions near the Fermi level. These features dominate the electronic states near the Fermi level, which are likely associated with the anisotropic transport properties observed in Ni₃In₂Se₂. Notably, the large spin-orbit coupling in this material leads to the formation of a massive Dirac-like band dispersion in the surface resonance states, contrasting with the gapless Dirac dispersion found in the surface states of its sister compound Ni₃In₂Se₂. Our work will help understand the influence of the spin-orbit coupling effect on both the surface and bulk electronic states of shandite compounds. Furthermore, it establishes a foundation for exploring the potential applications of surface resonance states in surface science.

Abstract:

Fe-based superconductors represent a fascinating class of materials, extensively studied for their complex interplay of superconductivity, magnetism, spin density waves, and nematicity, along with the interactions among these orders. An intriguing yet unexplained phenomenon observed in Fe-based superconductors is the emergence of superconductivity below 25 K in the non-superconducting parent compound SrFe₂As₂ following exposure to water at its surface. In this study, we employed in situ angle-resolved photoemission spectroscopy and low-energy electron diffraction to meticulously examine the electronic structure evolution of SrFe₂As₂ upon in situ water dosing. Our findings indicate that water dosing markedly attenuates the spin density wave phase and surface Sr reconstruction while preserving the nematic order in SrFe₂As₂. Furthermore, we detected an enhancement in the spectral weight of bands near the Fermi level. Our observations highlight the critical role of the intricate interplay among various orders induced by water dosing, which effectively modifies the band structure and favors the emergence of superconductivity in SrFe₂As₂.

Abstract:

Band renormalizations comprise crucial insights for understanding the intricate roles of electron-boson coupling and electron correlation in emergent phenomena such as superconductivity. In this Letter, by combining high-resolution angle-resolved photoemission spectroscopy and theoretical calculations, we systematically investigate the electronic structure of the trilayer nickelate superconductor La₄Ni₃O₁₀ at ambient pressure. We reveal a dichotomy in the electronic band renormalizations of La₄Ni₃O₁₀ in comparison to cuprate superconductors. At a high energy scale of hundreds of meV, its band structure is strongly renormalized by an electron correlation effect enhanced by Hund's coupling. The resultant waterfall-like dispersions resemble the high-energy kinks in cuprate superconductors. However, at low-energy scales of tens of meV, the dispersive bands are nearly featureless and devoid of any resolvable electron-boson interactions, in drastic contrast to the low-energy kinks observed in cuprates and other correlated 3d transition-metal compounds. The dichotomic band renormalizations highlight the disparity between nickelate and cuprate superconductors and emphasize the importance of strong electron correlation in the superconductivity of Ruddlesden-Popper phase nickelates.

Abstract:

The recently discovered heavy-fermion superconductor, UTe2, is an excellent candidate for spin-triplet superconductors in which electrons form spin-triplet Cooper pairs with spin S = 1 and odd parity. Unconventional superconductivity often hosts unconventional vortices. Yet, the vortex core and lattice in UTe2 have not been directly visualized and characterized. Here, by using ultralow-temperature scanning tunnelling microscopy and spectroscopy, we study the superconducting vortices on the (0−11) surface termination of UTe2 with an out-of-plane external magnetic field. At the centre of the vortex core, we observe a robust zero-energy vortex-core state that exhibits a cigar-shaped spatial distribution and extends to ∼30 nm along the [100] direction (crystallographic a-axis) of UTe2. Along the direction perpendicular to [100], the superconducting gap is deeper and the coherence peak on one side of the vortex core is stronger than on the opposite side, and they are even enhanced in comparison with those under zero field. Due to the anisotropy of magnetic susceptibility in UTe2, the asymmetric dI/dV spectra on the two sides of the vortex core result from the interplay between the magnetization-induced bound current and supercurrent around the vortex core. Our work reveals the important role of magnetization in the vortex behaviours of UTe2 and provides essential microscopic information for understanding its superconducting properties in a magnetic field.