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1. Unconventional supercurrent phase in Ising superconductor Josephson junction with atomically thin magnetic insulator

   https://doi.org/10.1038/s41467-021-25608-1  

   Idzuchi, H.; Pientka, F.; Huang, K.-F.; Harada, K.; Gül, Ö.; Shin, Y. J.; Nguyen, L. T.; Jo, N. H.; Shindo, D.; Cava, R. J.; et al (December 2021, Nature Communications)

   Abstract In two-dimensional (2D) NbSe 2 crystal, which lacks inversion symmetry, strong spin-orbit coupling aligns the spins of Cooper pairs to the orbital valleys, forming Ising Cooper pairs (ICPs). The unusual spin texture of ICPs can be further modulated by introducing magnetic exchange. Here, we report unconventional supercurrent phase in van der Waals heterostructure Josephson junctions (JJs) that couples NbSe 2 ICPs across an atomically thin magnetic insulator (MI) Cr 2 Ge 2 Te 6 . By constructing a superconducting quantum interference device (SQUID), we measure the phase of the transferred Cooper pairs in the MI JJ. We demonstrate a doubly degenerate nontrivial JJ phase ( ϕ ), formed by momentum-conserving tunneling of ICPs across magnetic domains in the barrier. The doubly degenerate ground states in MI JJs provide a two-level quantum system that can be utilized as a new dissipationless component for superconducting quantum devices. Our work boosts the study of various superconducting states with spin-orbit coupling, opening up an avenue to designing new superconducting phase-controlled quantum electronic devices. 

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2. Evidence for 4e charge of Cooper quartets in a biased multi-terminal graphene-based Josephson junction

   https://doi.org/10.1038/s41467-022-30732-7  

   Huang, Ko-Fan; Ronen, Yuval; Mélin, Régis; Feinberg, Denis; Watanabe, Kenji; Taniguchi, Takashi; Kim, Philip (December 2022, Nature Communications)

   Abstract In a Josephson junction (JJ) at zero bias, Cooper pairs are transported between two superconducting contacts via the Andreev bound states (ABSs) formed in the Josephson channel. Extending JJs to multiple superconducting contacts, the ABSs in the Josephson channel can coherently hybridize Cooper pairs among different superconducting electrodes. Biasing three-terminal JJs with antisymmetric voltages, for example, results in a direct current (DC) of Cooper quartet (CQ), which involves a four-fermion entanglement. Here, we report half a flux periodicity in the interference of CQ formed in graphene based multi-terminal (MT) JJs with a magnetic flux loop. We observe that the quartet differential conductance associated with supercurrent exhibits magneto-oscillations associated with a charge of 4 e , thereby presenting evidence for interference between different CQ processes. The CQ critical current shows non-monotonic bias dependent behavior, which can be modeled by transitions between Floquet-ABSs. Our experimental observation for voltage-tunable non-equilibrium CQ-ABS in flux-loop-JJs significantly extends our understanding of MT-JJs, enabling future design of topologically unique ABS spectrum. 

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3. Magnetic-field-free nonreciprocal transport in graphene multiterminal Josephson junctions

   https://doi.org/10.1103/PhysRevApplied.21.034011  

   Zhang, Fan; Rashid, Asmaul Smitha; Tanhayi Ahari, Mostafa; de Coster, George J.; Taniguchi, Takashi; Watanabe, Kenji; Gilbert, Matthew J.; Samarth, Nitin; Kayyalha, Morteza (March 2024, Physical Review Applied)

   Nonreciprocal superconducting devices have attracted growing interest in recent years as they potentially enable directional charge transport for applications in superconducting quantum circuits. Specifically, the superconducting diode effect has been explored in two-terminal devices that exhibit superconducting transport in one current direction while showing dissipative transport in the opposite direction. Here, we exploit multiterminal Josephson junctions (MTJJs) to engineer magnetic-field-free nonreciprocity in multiport networks. We show that when treated as a two-port electrical network, a three terminal Josephson junction (JJ) with an asymmetric graphene region exhibits reconfigurable two-port nonreciprocity. We observe nonreciprocal (reciprocal) transport between superconducting terminals with broken (preserved) spatial mirror symmetry. We explain our observations by considering a circuit network of JJs with different critical currents. 

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4. Fabrications of twisted moiré photonic crystal and random moiré photonic crystal and their potential applications in light extraction

   https://doi.org/10.1088/1361-6528/ad024a  

   Alnasser, Khadijah; Li, Shan; Sidhik, Siraj; Kamau, Steve; Hou, Jin; Hurley, Noah; Alzaid, Ayman; Wang, Sicheng; Yan, Hao; Deng, Jiangdong; et al (October 2023, Nanotechnology)

   Abstract Twisted moiré photonic crystal is an optical analog of twisted graphene or twisted transition metal dichalcogenide bilayers. In this paper, we report the fabrication of twisted moiré photonic crystals and randomized moiré photonic crystals and their use in enhanced extraction of light in light-emitting diodes (LEDs). Fractional diffraction orders from randomized moiré photonic crystals are more uniform than those from moiré photonic crystals. Extraction efficiencies of 76.5%, 77.8% and 79.5% into glass substrate are predicted in simulations of LED patterned with twisted moiré photonic crystals, defect-containing photonic crystals and random moiré photonic crystals, respectively, at 584 nm. Extraction efficiencies of optically pumped LEDs with 2D perovskite (BA)₂(MA)_n−1Pb_nI_3n+1ofn= 3 and (5-(2′-pyridyl)-tetrazolato)(3-CF₃−5-(2′-pyridyl)pyrazolato) platinum(II) (PtD) have been measured. 

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5. Anomalous metals: From “failed superconductor” to “failed insulator”

   https://doi.org/10.1073/pnas.2202496119  

   Zhang, Xinyang; Palevski, Alexander; Kapitulnik, Aharon (July 2022, Proceedings of the National Academy of Sciences)

   Resistivity saturation is found on both superconducting and insulating sides of an “avoided” magnetic-field-tuned superconductor-to-insulator transition (H-SIT) in a two-dimensional In/InO x composite, where the anomalous metallic behavior cuts off conductivity or resistivity divergence in the zero-temperature limit. The granular morphology of the material implies a system of Josephson junctions (JJs) with a broad distribution of Josephson coupling E J and charging energy E C , with an H-SIT determined by the competition between E J and E C . By virtue of self-duality across the true H-SIT, we invoke macroscopic quantum tunneling effects to explain the temperature-independent resistance where the “failed superconductor” side is a consequence of phase fluctuations and the “failed insulator” side results from charge fluctuations. While true self-duality is lost in the avoided transition, its vestiges are argued to persist, owing to the incipient duality of the percolative nature of the dissipative path in the underlying random JJ system. 

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