Nodeless d-wave pairing in monolayer FeSe revealed by intra-unit-cell Josephson interferometry
Authors: Runxin Li, Yuchen Huo, Jiong Mei, Ming-Qiang Ren, Miaomiao Wang, Quanxing Wei, Fengyu Yao, Maowen Zheng, Qing Qu, Cui Ding, Hongtao Yuan, Zhuoyu Chen, Wei-Qiang Chen, Kun Jiang, Qi-Kun Xue, Yu-Jie Sun
Abstract
The Josephson effect provides a direct phase-sensitive probe of superconducting order and has played a central role in identifying unconventional pairing symmetries1-6. Conventional Josephson interferometry, however, extracts phase information through engineered junction geometries and crystallographic orientations, rather than by resolving the internal phase structure of the pair state within a single unit cell. Here, we use a superconducting Nb tip to realize atomic-scale Josephson interferometry in monolayer FeSe/SrTiO3. Despite a fully gapped quasiparticle spectrum, the local Josephson response exhibits a striking intra-unit-cell pattern, with maxima centred on Fe-Fe bonds and robust nodal lines along the Fe-Se directions. We show that these nodal lines arise from destructive interference between Josephson tunnelling amplitudes associated with opposite-sign pairing components. Microscopic calculations reproduce the observed spatial pattern and identify it as a fingerprint of nodeless d-wave pairing. Our results establish intra-unit-cell Josephson interferometry as a local phase-sensitive probe of pairing symmetry in unconventional superconductors.
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Direct real-space imaging of sign-reversal superconducting state by scanning Josephson tunnelingTian-Heng Wei, Yan-Zhao Liu, Wei Ren et al. · 2026 · Not summarized yet
Direct real-space imaging uses scanning Josephson tunnelling to identify sign-reversing pairing through spatial modulations in monolayer Fe(Te,Se).
Abstract
Phase-sensitive experiments based on Josephson tunneling have been considered as a pivotal method to determine the pairing symmetry in high-temperature (high-Tc) cuprate superconductors. However, in iron-based superconductors, traditional phase-sensitive experiments face setbacks because of the complexity of pairing symmetry in multiband systems. Here, using scanning Josephson tunneling microscopy (SJTM), we report the half-unit-cell periodic Josephson tunneling modulations in monolayer high-Tc Fe(Te,Se) film epitaxially grown on SrTiO3(001). The Josephson tunneling almost vanishes along the lines connecting next-nearest-neighbor Fe atoms under an s-wave superconducting tip. Combined with si…
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Interband-Pairing-Boosted Supercurrent Diode Effect in Multiband SuperconductorsJiong Mei, Sheng-Shan Qin, Jiang-Ping Hu · 2025 · cited 4 times · Not summarized yet
Interband-pairing theory predicts a supercurrent diode effect in FeSe/STO, including for the nodeless d-wave state studied here.
Abstract
We unveil a mechanism that enables a robust supercurrent diode effect in Josephson junctions based on multiband superconductors. We predict that interband pairing can significantly amplifies this effect, even under weak spin-orbit coupling while intraband pairing alone would render it negligible. To illustrate this, we examine monolayer FeSe/STO, a system where recent experiments suggest interband pairing in either a nodeless $d$-wave or $\eta$ pairing state. Using experimentally derived parameters, we predict that FeSe/STO can serve as a high-temperature platform for realizing a substantial supercurrent diode effect, with efficiencies reaching up to $30\%$ for $d$-wave pairing and $12\%$ fo…
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Resilient Nodeless d-Wave Superconductivity in Monolayer FeSe.D. Agterberg, T. Shishidou, Joseph O'Halloran et al. · 2017 · Physical Review Letters · cited 51 times · Not summarized yet
Resilient Nodeless d-Wave Superconductivity develops a theory predicting fully gapped nodeless d-wave pairing in monolayer FeSe.
Abstract
Monolayer FeSe exhibits the highest transition temperature among the iron based superconductors and appears to be fully gapped, seemingly consistent with s-wave superconductivity. Here, we develop a theory for the superconductivity based on coupling to fluctuations of checkerboard magnetic order (which has the same translation symmetry as the lattice). The electronic states are described by a symmetry based k·p-like theory and naturally account for the states observed by angle resolved photoemission spectroscopy. We show that a prediction of this theory is that the resultant superconducting state is a fully gapped, nodeless, d-wave state. This state, which would usually have nodes, stays nod…
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Magnetization and Josephson current coupling drives the Josephson diode effect in FeTe0.55Se0.45 van der Waals junctionsSeung-Hyun Shin, C. Ahn, Chang-Won Choi et al. · 2026 · Not summarized yet
Magnetization and Josephson-current coupling explains a diode response in FeTeSe junctions, rather than measuring FeSe pairing symmetry.
Abstract
The Josephson diode effect (JDE) reflects symmetry breaking in the current-phase relation (CPR) of Josephson junctions, yet establishing its microscopic origin is often challenging. We investigate the mechanism of the JDE in van der Waals Josephson junctions based on FeTe0.55Se0.45, a platform where superconductivity, topology, and magnetism may coexist at the interface. Our devices exhibit a robust zero-field diode response with asymmetric switching currents. Magnetic-field measurements reveal a hysteretic Fraunhofer pattern and magnetic-resonance-induced voltage feature, evidencing interfacial magnetism coupled to the Josephson current. Microwave response further shows half-integer Shapiro…
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d-wave pairing from spin fluctuations in the K x Fe 2-y Se 2 superconductorsThomas Maier, S. Graser, P. Hirschfeld et al. · 2011 · Physical Review B · cited 119 times · Not summarized yet
A spin-fluctuation calculation proposes d-wave pairing for KxFe2-ySe2, a different iron-based superconductor.
Abstract
Angle-resolved photoemission spectroscopy measurements on the recently discovered superconduc- tors in the KFe2Se2 family with critical temperatures up to - 33K suggest that no Fermi pockets of hole character centered on the {\Gamma} point of the Brillouin zone are present, in contrast to all other known ferropnictide and ferrochalcogenide superconductors. Using a fluctuation exchange approximation and a 5-orbital tight-binding description of the band structure, we calculate the effective pairing interaction. We find that the pairing state in this system is most likely to have d-wave symmetry due to pair scattering between the remaining electron Fermi pockets at wave vector q - ({\pi}, {\pi}…
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Atomic-scale frustrated Josephson coupling and multicondensate visualization in FeSeNileema Sharma, Matthew Toole, James McKenzie et al. · 2025 · Nature Materials · cited 6 times · Not summarized yet
Atomic-scale Josephson microscopy maps frustrated tunnelling channels and condensates in FeSe, using spatial Josephson interference.
Abstract
In a Josephson junction involving multiband superconductors, competition between interband and interjunction Josephson couplings gives rise to frustration and spatial disjunction of superfluid densities among superconducting condensates1, 2, 3, 4, 5, 6–7. Such frustrated coupling manifests as the quantum interference of Josephson currents from different tunnelling channels and becomes tunable if channel transparency can be varied5, 6, 7–8. To explore these unconventional effects in the prototypical s±-wave superconductor FeSe (ref. 9), we use atomic-resolution scanned Josephson tunnelling microscopy10, 11, 12–13 for condensate-resolved imaging and junction tuning—capabilities unattainable in…
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Plain s-wave superconductivity in single-layer FeSe on SrTiO3 probed by scanning tunnelling microscopyQ. Fan, W.-H. Zhang, X. Liu et al. · 2015 · Nature Physics · cited 156 times · Not summarized yet
Scanning tunnelling measurements report a fully gapped spectrum in monolayer FeSe/STO, while leaving its pairing symmetry unclear.
Abstract
Single-layer FeSe film on SrTiO3(001) has recently come to the fore as an interfacial superconducting system, with a transition temperature that is significantly enhanced with respect to bulk FeSe. The mechanism for this enhancement, and indeed for the superconductivity itself, is therefore of great interest. Although the film has a simple Fermi surface topology, its pairing symmetry is unclear. By using low-temperature scanning tunnelling microscopy, we have systematically investigated the superconductivity of single-layer FeSe/SrTiO3 films, and report a fully gapped tunnelling spectrum and magnetic vortex lattice in the film. Quasi-particle interference patterns reveal scatterings between …
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Molecule-intercalated van der Waals Josephson junctions uncover complex multiband superconductivityDaniel Margineda, David Caldevilla-Asenjo, Covadonga Álvarez-García et al. · 2026 · Not summarized yet
Molecular intercalation tunes multiband superconductivity and Josephson diode effects in TaS2/NbSe2 junctions.
Abstract
Intercalation of molecular species provides a route to tailor the electronic structure in van der Waals superconductors, yet intercalated compounds have rarely been integrated into phase-sensitive quantum devices. Here, we realize hybrid molecular Josephson junctions based on TaS2/NbSe2 heterostructures. Selective intercalation of the TaS2 electrode induces monolayer-like Ising superconductivity, while the junctions exhibit asymmetric Fraunhofer interference, field-induced and spontaneous Josephson diode effects. The zero-field diode disappears upon reversing the electrode stacking and re-emerges after intercalating the lower NbSe2 electrode. These findings indicate that the hybrid supercond…
Prior work it builds on
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A strongly inhomogeneous superfluid in an iron-based superconductorD. Cho, K. M. Bastiaans, D. Chatzopoulos et al. · 2019 · Nature · cited 80 times · Not summarized yet
Atomic-resolution Josephson microscopy maps spatial variations in the superfluid of FeTeSe, another iron-based superconductor.
Abstract
Although the possibility of spatial variations in the superfluid of unconventional, strongly correlated superconductors has been suggested1–7, it is not known whether such inhomogeneities—if they exist—are driven by disorder, strong scattering or other factors. Here we use atomic-resolution Josephson scanning tunnelling microscopy to reveal a strongly inhomogeneous superfluid in the iron-based superconductor FeTe0.55Se0.45. By simultaneously measuring the topographic and electronic properties of the superconductor, we find that this inhomogeneity in the superfluid is not caused by structural disorder or strong inter-pocket scattering and is not correlated with variations in the energy requir…
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Pairing mechanism in Fe-based superconductorsA. Chubukov · 2011 · Annual Review of Condensed Matter Physics · cited 422 times · Not summarized yet
Pairing mechanism in Fe-based superconductors reviews gap symmetries and pairing mechanisms across iron-based superconductors.
Abstract
I review recent works on the symmetry and the structure of the superconducting gap in Fe-based superconductors (FeSCs) and on the underlying pairing mechanism in these systems. The experimental data on superconductivity show very rich behavior, with potentially different symmetry of a superconducting state for different compositions of the same material. The variety of different pairing states raises the issue of whether the physics of FeSCs is model dependent or is universal, governed by a single underlying pairing mechanism. I argue that the physics is universal and that all pairing states obtained so far can be understood within the same universal pairing scenario and are well described b…
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Imaging the spatial form of a superconducting order parameter via Josephson scanning tunneling spectroscopyM. Graham, D. Morr · 2017 · Physical Review B · cited 22 times · Not summarized yet
Imaging the spatial form of a superconducting order parameter theoretically shows how local critical Josephson current can reveal spatial pairing variations.
Abstract
Motivated by recent experiments, we investigate Josephson scanning tunneling spectroscopy in an s-wave superconductor. We demonstrate that the spatial oscillations in the superconducting order parameter induced by defects can be spatially imaged through local measurements of the critical Josephson current, providing unprecedented insight into the nature of superconductivity. The spatial form of the Josephson current reflects the nature of the defects, and can be used to probe defect-induced phase transitions from an $S=0$ to an $S=1/2$ ground state.
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Electron pairing in the presence of incipient bands in iron-based superconductorsA. Linscheid, Xiao Chen, S. Maiti et al. · 2015 · Bulletin of the American Physical Society · cited 65 times · Not summarized yet
Electron pairing in the presence of incipient bands analyzes multiband pairing in iron-based superconductors, including pairing on bands near the Fermi level.
Abstract
Recent experiments on certain Fe-based superconductors have hinted at a role for paired electrons in "incipient" bands that are close to, but do not cross the Fermi level. Related theoretical works disagree on whether or not strong-coupling superconductivity is required to explain such effects, and whether a critical interaction strength exists. In this work, we consider various versions of the model problem of pairing of electrons in the presence of an incipient band, within a simple multiband weak-coupling BCS approximation. We categorize the problem into two cases: case(I) where superconductivity arises from the "incipient band pairing" alone, and case(II) where it is induced on an incipi…
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Symmetry analysis of possible superconducting states in KxFeySe2superconductorsI. Mazin · 2011 · Physical Review B · cited 76 times · Not summarized yet
Symmetry analysis evaluates which superconducting states are allowed in KxFeySe2, a different material family from monolayer FeSe.
Abstract
A newly discovered family of the Fe-based superconductors is isostructural with the so-called 122 family of Fe pnictides, but has a qualitatively different doping state. Early experiments indicate that superconductivity is nodeless, yet prerequisites for the $s_\pm$ nodeless state (generally believed to be realized in Fe superconductors) are missing. It is tempting to assign a $d-$ wave symmetry to the new materials, and it does seem at first glance that such a state may be nodeless. Yet a more careful analysis shows that it is not possible, given the particular 122 crystallography, and that the possible choice of admissible symmetries is severly limited: it is either a conventional single-s…
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