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DDC: 530 × DDC: 620 × Has files: Yes × Publisher: London : Nature Publishing Group ×

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    Comprehensive scan for nonmagnetic Weyl semimetals with nonlinear optical response
    (London : Nature Publishing Group, 2020) Xu, Q.; Zhang, Y.; Koepernik, K.; Shi, W.; van den Brink, J.; Felser, C.; Sun, Y.
    First-principles calculations have recently been used to develop comprehensive databases of nonmagnetic topological materials that are protected by time-reversal or crystalline symmetry. However, owing to the low symmetry requirement of Weyl points, a symmetry-based approach to identifying topological states cannot be applied to Weyl semimetals (WSMs). To date, WSMs with Weyl points in arbitrary positions are absent from the well-known databases. In this work, we develop an efficient algorithm to search for Weyl points automatically and establish a database of nonmagnetic WSMs with Weyl points near the Fermi level based on the experimental non-centrosymmetric crystal structures in the Inorganic Crystal Structure Database (ICSD). In total, 46 Weyl semimetals were discovered to have nearly clean Fermi surfaces and Weyl points within 300 meV of the Fermi level. Nine of them are chiral structures which may exhibit the quantized circular photogalvanic effect. In addition, the nonlinear optical response is studied and the giant shift current is explored. Besides nonmagnetic WSMs, our powerful tools can also be used in the discovery of magnetic topological materials.
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    Separate tuning of nematicity and spin fluctuations to unravel the origin of superconductivity in FeSe
    (London : Nature Publishing Group, 2020) Baek, S.-H.; Ok, J.M.; Kim, J.S.; Aswartham, S.; Morozov, I.; Chareev, D.; Urata, T.; Tanigaki, K.; Tanabe, Y.; Büchner, B.; Efremov, D.V.
    The interplay of orbital and spin degrees of freedom is the fundamental characteristic in numerous condensed matter phenomena, including high-temperature superconductivity, quantum spin liquids, and topological semimetals. In iron-based superconductors (FeSCs), this causes superconductivity to emerge in the vicinity of two other instabilities: nematic and magnetic. Unveiling the mutual relationship among nematic order, spin fluctuations, and superconductivity has been a major challenge for research in FeSCs, but it is still controversial. Here, by carrying out 77Se nuclear magnetic resonance (NMR) measurements on FeSe single crystals, doped by cobalt and sulfur that serve as control parameters, we demonstrate that the superconducting transition temperature Tc increases in proportion to the strength of spin fluctuations, while it is independent of the nematic transition temperature Tnem. Our observation therefore directly implies that superconductivity in FeSe is essentially driven by spin fluctuations in the intermediate coupling regime, while nematic fluctuations have a marginal impact on Tc.
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    Turning charge-density waves into Cooper pairs
    (London : Nature Publishing Group, 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.