2020, Chikina, A., Fedorov, A., Bhoi, D., Voroshnin, V., Haubold, E., Kushnirenko, Y., Kim, K.H., Borisenko, S.

The relationship between charge-density waves (CDWs) and superconductivity is a long-standing debate. Often observed as neighbors in phase diagrams, it is still unclear whether they cooperate, compete, or simply coexist. Using angle-resolved photoemission spectroscopy, we demonstrate here that by tuning the energy position of the van Hove singularity in Pd-doped 2H-TaSe2, one is able to suppress CDW and enhance superconductivity by more than an order of magnitude. We argue that it is particular fermiology of the material that is responsible for each phenomenon, thus explaining their persistent proximity as phases.

2016, Güttler, M., Generalov, A., Otrokov, M.M., Kummer, K., Kliemt, K., Fedorov, A., Chikina, A., Danzenbächer, S., Schulz, S., Chulkov, E.V., Koroteev, Yu. M., Caroca-Canales, N., Shi, M., Radovic, M., Geibel, C., Laubschat, C., Dudin, P., Kim, T.K., Hoesch, M., Krellner, C., Vyalikh, D.V.

Spin-polarized two-dimensional electron states (2DESs) at surfaces and interfaces of magnetically active materials attract immense interest because of the idea of exploiting fermion spins rather than charge in next generation electronics. Applying angle-resolved photoelectron spectroscopy, we show that the silicon surface of GdRh2Si2 bears two distinct 2DESs, one being a Shockley surface state, and the other a Dirac surface resonance. Both are subject to strong exchange interaction with the ordered 4f-moments lying underneath the Si-Rh-Si trilayer. The spin degeneracy of the Shockley state breaks down below ~90 K, and the splitting of the resulting subbands saturates upon cooling at values as high as ~185 meV. The spin splitting of the Dirac state becomes clearly visible around ~60 K, reaching a maximum of ~70 meV. An abrupt increase of surface magnetization at around the same temperature suggests that the Dirac state contributes significantly to the magnetic properties at the Si surface. We also show the possibility to tune the properties of 2DESs by depositing alkali metal atoms. The unique temperature-dependent ferromagnetic properties of the Si-terminated surface in GdRh2Si2 could be exploited when combined with functional adlayers deposited on top for which novel phenomena related to magnetism can be anticipated.