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1. Two-fold symmetric superconductivity in few-layer NbSe2

   https://doi.org/10.1038/s41567-021-01219-x  

   Hamill, Alex; Heischmidt, Brett; Sohn, Egon; Shaffer, Daniel; Tsai, Kan-Ting; Zhang, Xi; Xi, Xiaoxiang; Suslov, Alexey; Berger, Helmuth; Forró, László; et al (April 2021, Nature Physics)

   null (Ed.)

   The strong Ising spin–orbit coupling in certain two-dimensional transition metal dichalcogenides can profoundly affect the superconducting state in few-layer samples. For example, in NbSe2, this effect combines with the reduced dimensionality to stabilize the superconducting state against magnetic fields up to ~35 T, and could lead to topological superconductivity. Here we report a two-fold rotational symmetry of the superconducting state in few-layer NbSe2 under in-plane external magnetic fields, in contrast to the three-fold symmetry of the lattice. Both the magnetoresistance and critical field exhibit this two-fold symmetry, and it also manifests deep inside the superconducting state in NbSe2/CrBr3 superconductor-magnet tunnel junctions. In both cases, the anisotropy vanishes in the normal state, demonstrating that it is an intrinsic property of the superconducting phase. We attribute the behaviour to the mixing between two closely competing pairing instabilities, namely the conventional s-wave instability typical of bulk NbSe2 and an unconventional d- or p-wave channel that emerges in few-layer NbSe2. Our results demonstrate the unconventional character of the pairing interaction in few-layer transition metal dichalcogenides and highlight the exotic superconductivity in this family of two-dimensional materials. 

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2. Synthesis and Characterization of Three Multi‐Shell Metalloid Gold Clusters Au ₃₂ (R ₃ P) ₁₂ Cl ₈

   https://doi.org/10.1002/anie.201900644  

   Kenzler, Sebastian; Fetzer, Florian; Schrenk, Claudio; Pollard, Nia; Frojd, Andrew R.; Clayborne, Andre Z.; Schnepf, Andreas (March 2019, Angewandte Chemie International Edition)

   Abstract Three multi‐shell metalloid gold clusters of the composition Au₃₂(R₃P)₁₂Cl₈(R=Et,ⁿPr,ⁿBu) were synthesized in a straightforward fashion by reducing R₃PAuCl with NaBH₄in ethanol. The Au₃₂core comprises two shells, with the inner one constituting a tilted icosahedron and the outer one showing a distorted dodecahedral arrangement. The outer shell is completed by eight chloride atoms and twelve R₃P groups. The inner icosahedron shows bond lengths typical for elemental gold while the distances of the gold atoms in the dodecahedral arrangement are in the region of aurophilic interactions. Quantum‐chemical calculations illustrate that the Jahn–Teller effect observed within the cluster core can be attributed to the electronic shell filling. The easily reproducible synthesis, good solubility, and high yields of these clusters render them perfect starting points for further research. 

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3. Two superconducting states with broken time-reversal symmetry in FeSe _1−x S _x

   https://doi.org/10.1073/pnas.2208276120  

   Matsuura, Kohei; Roppongi, Masaki; Qiu, Mingwei; Sheng, Qi; Cai, Yipeng; Yamakawa, Kohtaro; Guguchia, Zurab; Day, Ryan P.; Kojima, Kenji M.; Damascelli, Andrea; et al (May 2023, Proceedings of the National Academy of Sciences)

   Iron-chalcogenide superconductors FeSe_1−xS_xpossess unique electronic properties such as nonmagnetic nematic order and its quantum critical point. The nature of superconductivity with such nematicity is important for understanding the mechanism of unconventional superconductivity. A recent theory suggested the possible emergence of a fundamentally new class of superconductivity with the so-called Bogoliubov Fermi surfaces (BFSs) in this system. However, such an ultranodal pair state requires broken time-reversal symmetry (TRS) in the superconducting state, which has not been observed experimentally. Here, we report muon spin relaxation (μSR) measurements in FeSe_1−xS_xsuperconductors for0≤x≤0.22covering both orthorhombic (nematic) and tetragonal phases. We find that the zero-field muon relaxation rate is enhanced below the superconducting transition temperatureT_cfor all compositions, indicating that the superconducting state breaks TRS both in the nematic and tetragonal phases. Moreover, the transverse-fieldμSR measurements reveal that the superfluid density shows an unexpected and substantial reduction in the tetragonal phase (x>0.17). This implies that a significant fraction of electrons remain unpaired in the zero-temperature limit, which cannot be explained by the known unconventional superconducting states with point or line nodes. The TRS breaking and the suppressed superfluid density in the tetragonal phase, together with the reported enhanced zero-energy excitations, are consistent with the ultranodal pair state with BFSs. The present results reveal two different superconducting states with broken TRS separated by the nematic critical point in FeSe_1−xS_x, which calls for the theory of microscopic origins that account for the relation between nematicity and superconductivity. 

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4. Anomalous superconductivity in twisted MoTe ₂ nanojunctions

   https://doi.org/10.1126/sciadv.adq5712  

   Jia, Yanyu; Song, Tiancheng; Zheng, Zhaoyi Joy; Cheng, Guangming; Uzan, Ayelet J; Yu, Guo; Tang, Yue; Pollak, Connor J; Yuan, Fang; Onyszczak, Michael; et al (January 2025, Science Advances)

   Introducing superconductivity in topological materials can lead to innovative electronic phases and device functionalities. Here, we present a unique strategy for quantum engineering of superconducting junctions in moiré materials through direct, on-chip, and fully encapsulated 2D crystal growth. We achieve robust and designable superconductivity in Pd-metalized twisted bilayer molybdenum ditelluride (MoTe₂) and observe anomalous superconducting effects in high-quality junctions across ~20 moiré cells. Unexpectedly, the junction develops enhanced, instead of weakened, superconducting behaviors, exhibiting fluctuations to a higher critical magnetic field compared to its adjacent Pd₇MoTe₂superconductor. In addition, the critical current further exhibits a notable V-shaped minimum at zero magnetic field. These features are unexpected in conventional Josephson junctions and absent in junctions of natural bilayer MoTe₂created using the same approach. We discuss implications of these observations, including the possible formation of mixed even- and odd-parity superconductivity at the moiré junctions. Our results also demonstrate a pathway to engineer and investigate superconductivity in fractional Chern insulators. 

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5. Evidence for even parity unconventional superconductivity in Sr ₂ RuO ₄

   https://doi.org/10.1073/pnas.2025313118  

   Chronister, Aaron; Pustogow, Andrej; Kikugawa, Naoki; Sokolov, Dmitry A.; Jerzembeck, Fabian; Hicks, Clifford W.; Mackenzie, Andrew P.; Bauer, Eric D.; Brown, Stuart E. (June 2021, Proceedings of the National Academy of Sciences)

   Unambiguous identification of the superconducting order parameter symmetry in ${\text{Sr}}_2{\text{RuO}}_4$has remained elusive for more than a quarter century. While a chiral p-wave ground state analogue to superfluid³He-A was ruled out only very recently, other proposed triplet-pairing scenarios are still viable. Establishing the condensate magnetic susceptibility reveals a sharp distinction between even-parity (singlet) and odd-parity (triplet) pairing since the superconducting condensate is magnetically polarizable only in the latter case. Here field-dependent¹⁷O Knight shift measurements, being sensitive to the spin polarization, are compared to previously reported specific heat measurements for the purpose of distinguishing the condensate contribution from that due to quasiparticles. We conclude that the shift results can be accounted for entirely by the expected field-induced quasiparticle response. An upper bound for the condensate magnetic response of <10% of the normal state susceptibility is sufficient to exclude all purely odd-parity candidates. 

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