Current induced hidden states in Josephson junctions

Article https://doi.org/10.1038/s41467-024-52271-z Current induced hidden states in Josephson junctions Received: 1 July 2024 Accepted: 28 August 2024 1234567890():,; 1234567890():,; Check for updates Shaowen Chen 1,6 , Seunghyun Park 1,6, Uri Vool 1,2, Nikola Maksimovic1, David A. Broadway3, Mykhailo Flaks3, Tony X. Zhou 1,5, Patrick Maletinsky 3, Ady Stern 4, Bertrand I. Halperin 1 & Amir Yacoby 1 Josephson junctions enable dissipation-less electrical current through metals and insulators below a critical current. Despite being central to quantum technology based on superconducting quantum bits and fundamental research into self-conjugate quasiparticles, the spatial distribution of super current flow at the junction and its predicted evolution with current bias and external magnetic field remain experimentally elusive. Revealing the hidden current flow, featureless in electrical resistance, helps understanding unconventional phenomena such as the nonreciprocal critical current, i.e., Josephson diode effect. Here we introduce a platform to visualize super current flow at the nanoscale. Utilizing a scanning magnetometer based on nitrogen vacancy centers in diamond, we uncover competing ground states electrically switchable within the zero-resistance regime. The competition results from the superconducting phase re-configuration induced by the Josephson current and kinetic inductance of thin-film superconductors. We further identify a new mechanism for the Josephson diode effect involving the Josephson currentinduced phase. The nanoscale super current flow emerges as a new experimental observable for elucidating unconventional superconductivity, and optimizing quantum computation and energy-efficient devices. Characterization and control over the super current flow is critical for Josephson junctions (JJs)1–3, which have become a building block in quantum and classical technology4–11 while remained a rich area of exploration into fundamental particles12–14 and unconventional superconductivity15–17. Compared to spectroscopic probes that measures the amplitude of the superconducting (SC) wave function18, the super current flow encodes the SC phase. Mapping the spatial distribution of super current has revealed the pairing symmetry of unconventional superconductors19,20, and recently identified screening current as the source of SC diode effect in SC/ferromagnet structures21. In addition, the local super current flow affects device parameters such as the impedance of SC circuits and anharmonicity of SC qubits due to the change in kinetic inductance22. Despite the scientific and technological relevance, direct visualization of the Josephson current flow and its response to external tuning knobs such as bias current and magnetic field remains experimentally beyond reach18,23–27. This is mostly due to the sensitive nature of the JJ, which responds to small perturbations and the nanoscale spatial resolution needed to resolve the evolution of the super current flow. To date, JJ characterization has primarily relied on indirect measurements such as the critical current that separates the dissipation-less (zero electrical resistance) and resistive states. However, this only provides insight into the resistive state while the ground state below the critical current stays hidden. 1 Department of Physics, Harvard University, Cambridge, MA 02138, USA. 2Max Planck Institute for Chemical Physics of Solids, 01187 Dresden, Germany. Department of Physics, University of Basel, Klingelbergstrasse 82, Basel CH-4056, Switzerland. 4Weizmann Institute of Science, Rehovot 76100, Israel. 5 Present address: Northrop Grumman Mission Systems, Linthicum, MD 21090, USA. 6These authors contributed equally: Shaowen Chen, Seunghyun Park. e-mail: ; 3 Nature Communications | (2024)15:8059 1 Article https://doi.org/10.1038/s41467-024-52271-z Here we quantitatively visualize the current flow in a JJ device with nanoscale resolution. The spatial distribution of Josephson current flow can be modulated by varying the SC phase difference between two sides of the junction. In any JJ, the SC phase difference is governed by three factors: (i) external magnetic field; (ii) external bias current; (iii) self-field or SC phase gradient induced by the finite Josephson current density. Our measurements reveal the evolution of Josephson current flow with all three factors, including features associated with the change of the number of current loops at the junction known as the Josephson vortex (JV). In particular, factors (i) and (ii) can affect (iii), altering the super current flow even without detectable transport features. We find two previously unidentified effects of the Josephson current-induced phase from factor (iii). First, hidden ground states with different numbers of JVs are found within the zero-resistance state, which can be electrically switched below the critical current. Second, a new mechanism for the Josephson diode effect is established based on the second harmonic phase terms induced by the Josephson current when time-reversal and inversion symmetry are broken. The measurement setup is shown in Fig. 1a. We employ a diamond tip containing a single nitrogen vacancy (NV) center to map the local magnetic field generated by the current flow28. The results are obtained from two devices with junction width W = 0.15 and 0.2 μm, length L = 1.5 μm and thickness t = 35 nm. The SC electrodes are measured to be in the thin-film limit L ≪ λp, where λp is the Pearl length (Supplementary Fig. 1). This suggests the factor (iii) contribution in our device comes from the Josephson current-induced phase associated with the kinetic inductance of the SC film, instead of the self-field c diamond tip y W Au J y ðxÞ = J c sin½ϕðxÞ, z x iθ Ψ=|Ψ|e induced phase can be neglected ("weak junction” limit). Φ0 is the flux quantum, λL is the London penetration length. In the weak-junction limit, external Bz controls the number of JV. The transport critical current Ic oscillates and reaches zero at nodes Bz = ±Bn (n is integer). It is known as the “Fraunhofer map”30,31. In each Ibias |Ic| f 0-JV 2 -1 1 +|Ic| 2 -|Ic| +|Ic| 0 Jy (x) / J c 0.5 dV/dI ( ) 1 0 g Idc ( A) e 0-JV 1-JV 2-JV 0-JV 3-JV |J | (a.u.) -B 2 -B 1 -B 0 B0 B1 B2 -2 0 B z (mT) 2 4 0 x /L 0.5 1-JV Yπ Xπ/2 X/Y±π/2 Microwave Jx Bias Current 0 τ/2 τ/2 bias I1 bias I2 Y X Y X 6 -φ2 y 3 π /2 θ Fig. 1 | Measurement setup and expected Josephson current flow. a Schematics showing SC-normal-SC junction measured by scanning NV center embedded in a diamond tip. The SC wave function can be described by an amplitude and phase Ψ = ∣Ψ∣eiθ. Under external magnetic field Bz, the screening current near the JJ (red lines) induces a phase difference ϕe(x). The bias current causes a phase difference between the SC electrodes ϕbias. b Measured differential resistance dV/dI versus perpendicular magnetic field Bz and bias current Idc, at T = 7 K. Das (...truncated)