More Like this

1. 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 (May 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 4e, 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. 

   more » « less
2. 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. 

   more » « less
3. 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. 

   more » « less
4. Low-Cost Superconducting Fan-Out With Cell $$\text{I}_\text{C}$$ Ranking

   https://doi.org/10.1109/TASC.2023.3256797  

   Volk, Jennifer; Tzimpragos, Georgios; Wynn, Alex; Golden, Evan; Sherwood, Timothy (September 2023, IEEE Transactions on Applied Superconductivity)

   Superconductor electronics (SCE) promise computer systems with orders of magnitude higher speeds and lower energy consumption than their complementary metal–oxide semiconductor (CMOS) counterparts. At the same time, the scalability and resource utilization of superconducting systems are major concerns. Some of these concerns come from device-level challenges and the gap between SCE and CMOS technology nodes, and others come from the way Josephson junctions (JJs) are used. Toward this end, we notice that a considerable fraction of hardware resources are not involved in logic operations, but rather are used for fan-out and buffering purposes. In this article, we ask if there is a way to reduce these overheads, propose the use of JJs at the cell boundaries to increase the number of outputs that a single stage can drive, and establish a set of rules to discretize critical currents in a way that is conducive to this assignment. Finally, we explore the design trade-offs that the presented approach opens up and demonstrate its promise through detailed analog simulations and modeling analyses. Our experiments indicate that the introduced method leads to a 48% savings in the JJ count for a tree with a fan-out of 1024, as well as an average of 43% of the JJ count for signal splitting and 32% for clock splitting in ISCAS’85 benchmarks. 

   more » « less
5. Controllable superconducting to semiconducting phase transition in topological superconductor 2M-WS₂

   https://doi.org/10.1088/2053-1583/ad10bb  

   Gautam, Sabin; McBride, Joseph; Scougale, William R; Samarawickrama, Piumi I; De_Camargo_Branco, Danilo; Yang, Peilin; Fu, ZhuangEn; Wang, Wenyong; Tang, Jinke; Cheng, Gary J; et al (December 2023, 2D Materials)

   Abstract The investigation of exotic properties in two-dimensional (2D) topological superconductors has garnered increasing attention in condensed matter physics, particularly for applications in topological qubits. Despite this interest, a reliable way of fabricating topological Josephson junctions (JJs) utilizing topological superconductors has yet to be demonstrated. Controllable structural phase transition presents a unique approach to achieving topological JJs in atomically thin 2D topological superconductors. In this work, we report the pioneering demonstration of a structural phase transition from the superconducting to the semiconducting phase in the 2D topological superconductor 2M-WS₂. We reveal that the metastable 2M phase of WS₂remains stable in ambient conditions but transitions to the 2H phase when subjected to temperatures above 150 °C. We further locally induced the 2H phase within 2M-WS₂nanolayers using laser irradiation. Notably, the 2H phase region exhibits a hexagonal shape, and scanning tunneling microscopy uncovers an atomically sharp crystal structural transition between the 2H and 2M phase regions. Moreover, the 2M to 2H phase transition can be induced at the nanometer scale by a 200 kV electron beam. The electrical transport measurements further confirmed the superconductivity of the pristine 2M-WS₂and the semiconducting behavior of the laser-irradiated 2M-WS₂. Our results establish a novel approach for controllable topological phase change in 2D topological superconductors, significantly impacting the development of atomically scaled planar topological JJs. 

   more » « less