2020, Borisenko, Sergey, Bezguba, Volodymyr, Fedorov, Alexander, Kushnirenko, Yevhen, Voroshin, Vladimir, Sturza, Mihai, Aswartham, Saicharan

Topological superconductors should be able to provide essential ingredients for quantum computing, but are very challenging to realize. Spin–orbit interaction in iron-based superconductors opens the energy gap between the p-states of pnictogen and d-states of iron very close to the Fermi level, and such p-states have been recently experimentally detected. Density-functional theory predicts existence of topological surface states within this gap in FeTe1−xSex making it an attractive candidate material. Here we use synchrotron-based angle-resolved photoemission spectroscopy and band structure calculations to demonstrate that FeTe1−xSex (x = 0.45) is a superconducting 3D Dirac semimetal hosting type-I and type-II Dirac points and that its electronic structure remains topologically trivial. We show that the inverted band gap in FeTe1−xSex can possibly be realized by further increase of Te content, but strong correlations reduce it to a sub-meV size, making the experimental detection of this gap and corresponding topological surface states very challenging, not to mention exact matching with the Fermi level. On the other hand, the p–d and d–d interactions are responsible for the formation of extremely flat band at the Fermi level pointing to its intimate relation with the mechanism of high-Tc superconductivity in IBS.

2020, Chikina, Alla, Fedorov, Alexander, Bhoi, Dilipkumar, Voroshnin, Vladimir, Haubold, Erik, Kushnirenko, Yevhen, Kim, Kee Hoon, Borisenko, Sergey

[ no abstract available] Correction to: npj Quantum Materials https://doi.org/10.1038/s41535-020-0225-5, published online 14 April 2020

2021, Thirupathaiah, Setti, Kushnirenko, Yevhen, Koepernik, Klaus, Piening, Boy Roman, Büchner, Bernd, Aswartham, Saicharan, van den Brink, Jeroen, Borisenko, Sergey, Fulga, Ion Cosma

We show that the cubic compound PtBi2, is a topological semimetal hosting a sixfold band touching point in close proximity to the Fermi level. Using angle-resolved photoemission spectroscopy, we map the bandstructure of the system, which is in good agreement with results from density functional theory. Further, by employing a low energy effective Hamiltonian valid close to the crossing point, we study the effect of a magnetic field on the sixfold fermion. The latter splits into a total of twenty Weyl cones for a Zeeman field oriented in the diagonal, [111] direction. Our results mark cubic PtBi2, as an ideal candidate to study the transport properties of gapless topological systems beyond Dirac and Weyl semimetals.