Novel Superconducting Phases in Unconventional Superconductors: FFLO state and Ising Superconductivity
일 시 : 2022년 04월 26일 화요일 14:00
연 사 : 조 창 우 박사 (Grenoble High Magnetic Field Laboratory)
장 소 : 자연과학관 226호 & 온라인 ( 회의 ID: 893 6038 1750 , 암호: 164860 )
The recent discovery of a room-temperature superconductor in an extremely high-pressurized environment has been attracted lots of interest in condensed matter society [Nature 586, 373 (2020)]. Also, there have been unexpected superconductivity emergence in an infinity-layer Nickelate [Nature 572, 624 (2019)] as well as magic-angle graphene superlattices [Nature 556, 43 (2018)]. Such an observation led us to reexplore the deep sea of unconventional superconductivity (UnSC) research. In a broad topic of UnSC, without a doubt, uncovering the mechanism of UnSC is of primary interest. In this regard, I will introduce some exotic and novel superconducting phases in Fe-based and transition-metal-dichalcogenide (TMDC) superconductors, which gives a clue for a more profound understanding of UnSC.
The Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state is a prominent example of novel superconducting phases in UnSC. The FFLO state is an unusual superconducting state that helps a superconductor to overcome the ultimate magnetic-field limit for superconductivity, the Pauli limit. It was theoretically predicted in the middle of the last century, but has so far been confirmed in only very few superconductors. In our experiments, we successfully observe FFLO state signatures in KFe2As2 and NbS2 superconductors by adjusting the B-field orientation with the help of a high-resolution piezo-rotator. These observations provide us that Fe-based and TMDC superconductors can also be a good platform for studying the FFLO state.
Next, I will present the pairing gap symmetry study in the newly discovered Ising superconductor NbSe2. We observe a transformation from a six-fold nodal symmetry near Hc2(T) to a two-fold symmetry at lower temperatures. While the first phase agrees with theoretical predictions of a nodal topological superconducting phase, the observation of a second distinct superconducting phase with two-fold symmetry is unexpected and contradicts the crystalline symmetry. It may therefore be another example of an unconventional nematic superconducting phase besides superconducting Bi2Se3, and we demonstrate that in NbSe2 such a nematic state can indeed arise from the presence of several competing superconducting channels.
 Thermodynamic Evidence for the Fulde-Ferrell-Larkin-Ovchinnikov state in the KFe2As2 Superconductor, Chang-woo Cho et. al., Phys. Rev. Lett. 119, 217002 (2017)
 Evidence for the Fulde-Ferrell-Ovchinnikove state in bulk NbS2, Chang-woo Cho et. al., Nat. Commun. 12, 3676 (2021)
 Nodal and nematic superconducting phases in NbSe2 monolayers from competing superconducting channels, Chang-woo Cho et. al., preprint arXiv:2003.12467
 Competition between orbital effects, Pauli limiting, and Fulde-Ferrell-Larkin-Ovchinnikov states in 2D transition metal dichalcogenide superconductors, Chang-woo Cho et. al., preprint arXiv:2201.09428