arXiv subjects

Jiong Mei

Publications and source records attributed to Jiong Mei.

9 recordsLinked to original sources

arXiv2026

Intra-unit-cell Josephson scanning tunneling spectroscopy of the cuprate Bi$_{1.2}$Pb$_{0.8}$Sr$_2$CaCu$_2$O$_{8+δ}$

The predominantly $d_{x^2-y^2}$-wave pairing symmetry of cuprate superconductors is well established, yet its microscopic manifestation in local Cooper-pair tunneling remains less explored. Here, we investigate Bi$_{1.2}$Pb$_{0.8}$Sr$_2$CaCu$_2$O$_{8+δ}$, using Josephson scanning tunneling spectroscopy with a conventional $s$-wave superconducting Pb tip. We observe a zero-bias conductance enhancement whose approximately quadratic dependence on junction conductance is consistent with phase-diffusive Josephson tunneling. Atomic-resolution conductance maps and site-resolved spectra reveal a pronounced intra-unit-cell modulation, with enhanced Josephson response near oxygen sites and strong suppression near copper sites and along diagonal directions. Microscopic calculations incorporating the bilayer electronic structure and realistic Wannier-based tunneling matrix elements reproduce the observed spatial pattern through interference between pairing contributions of opposite signs. Our results reveal how the sign-changing $d$-wave superconducting order parameter manifests itself in atomic-scale Josephson tunneling, providing complementary evidence for unconventional pairing symmetry in cuprates.

cond-mat.supr-con↗

arXiv2026

Nodeless d-wave pairing in monolayer FeSe revealed by intra-unit-cell Josephson interferometry

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.

cond-mat.supr-con↗

arXiv2026

Frustration from Localized Zhang-Rice States: A Unified Theory of Doping-Driven Magnetic Transitions in Cuprates

The microscopic mechanism by which doped holes disrupt the antiferromagnetic (AFM) order is one of the fundamental questions in cuprates. In this work, we propose a unified microscopic theory in which doped holes form spatially localized Zhang-Rice singlets which actively mediate emergent spin exchange. Rather than acting as simple non-magnetic vacancies, these localized states introduce emergent next-nearest $J_2$ and third-nearest $J_3$ neighbor superexchanges. This dopant-induced exchange pathway generates significant magnetic frustration, naturally explaining the rapid collapse of the Néel AFM order and the emergence of a spin-glass phase on the hole-doped side. In contrast, electron doping primarily leads to spin dilution, allowing AFM order to persist to substantially higher doping levels, consistent with the experimentally observed electron-hole asymmetry. Our findings provide a unified microscopic framework for understanding doping-driven magnetic transitions in lightly doped cuprates.

cond-mat.str-el↗

arXiv2026

Itinerant Nature of Spin-Density-Wave Order in Ruddlesden-Popper Nickelates

The nature of magnetism in layered Ruddlesden-Popper nickelates remains a central open question, particularly in light of recent observations of spin-wave-like magnetic excitations in metallic multilayer compounds. Here, we develop a unified itinerant description of spin-density-wave (SDW) order and magnetic excitations in La$_3$Ni$_2$O$_7$ and La$_4$Ni$_3$O$_{10}$. The essential ingredient is the multilayer mirror structure of the NiO$_2$ blocks, which organizes the low-energy electronic states into mirror-even and mirror-odd sectors. We show that dominant interband nesting between mirror-opposite bands drives a mirror-selective itinerant SDW instability, whose collective modes naturally reproduce the experimentally observed spin-wave-like spectra. In La$_4$Ni$_3$O$_{10}$, the SDW further induces a secondary mirror-even charge density wave, yielding intertwined spin and charge textures. Our results demonstrate that magnetism in multilayer nickelates is fundamentally itinerant rather than local-moment in origin, and establish mirror-selective interband SDW order as a unifying organizing principle for magnetic correlations in these systems.

cond-mat.str-el↗

arXiv2025

Magnetic electron-hole asymmetry in cuprates: a computational revisit

In this work, we revisit the electron-hole asymmetry of antiferromagnetism in cuprates by studying the three-band Emery model. Using parameters relevant to La$_2$CuO$_4$, we benchmark the anti-ferromagnetic response for a large range of dopings with variational Monte Carlo, determinant quantum Monte Carlo, constrained-path auxiliary-field quantum Monte Carlo, density-matrix embedding theory, and the Gutzwiller approximation. Across methods and accessible sizes/temperatures, we find no significant electron-hole asymmetry if we consider only Neel anti-ferronagnetic response and ignore other possible orders such as stripe state. This result is robust to a moderate oxygen-site repulsion $U_p$ and to parameter sets of Nd$_2$CuO$_4$. Incorporating dopant-induced local potentials reveals an extrinsic route to asymmetry: Cu-site defects enhance AFM on the electron-doped side, whereas O-site defects suppress it on the hole-doped side. These results indicate that dopant-driven effects make a non-negligible contribution to apparent electron-hole asymmetry in the general phase diagram of cuprates and should be included when analyzing competing orders in cuprates.

cond-mat.str-el↗

arXiv2025

Interband-Pairing-Boosted Supercurrent Diode Effect in Multiband Superconductors

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 $η$ 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 $12\%$ for $η$ pairing. These results demonstrate that measuring the supercurrent diode effect can provides a powerful probe of the pairing symmetry in monolayer FeSe/STO, offering critical insights into its superconducting state.

cond-mat.supr-con↗

arXiv2024

Inversion-symmetric Electron Gases as New Platforms for Topological Planar Josephson Junctions

Intrinsic Rashba spin-orbital coupling (SOC) can exist in centrosymmetric materials with local inversion symmetry breaking. Here we show that such a SOC can induce topological superconductivity together with an in-plane Zeeman field in planar Josephson junctions formed by the centrosymmetric materials. A single Majorana mode can be created at each end of the junction. We demonstrate this result in a model based on iron-based superconductors. We derive the necessary Fermi surface condition for the topological planar junction and calculate the topological phase diagram with respect to the in-plane Zeeman field and the phase difference between the two superconductors. We provide experimental characteristics for the topological superconductivity, including the differential conductance and the Fano factor tomography which can be measured in the scanning tunneling spectroscopy. Our study reveals that the centrosymmetric systems with local-inversion-symmetry breaking can serve as new platforms for the topological planar Josephson junctions, and help to find more experimentally feasible materials for the topological superconductors.

cond-mat.supr-con↗

arXiv2024

Electronic and magnetic excitations in La$_3$Ni$_2$O$_7$

The striking discovery of high-temperature superconductivity (HTSC) of 80 K in a bilayer nickelate La$_3$Ni$_2$O$_7$ under a moderately high pressure of about 14 GPa ignited a new wave of studying HTSC in nickelates. The properties of the parental phase at ambient pressure may contain key information on basic interactions therein and bosons that may mediate pairing giving birth to superconductivity. Moreover, the bilayer structure of La$_3$Ni$_2$O$_7$ may suggest a distinct minimal model in comparison to cuprate superconductors. Here using X-ray absorption spectroscopy and resonant inelastic X-ray scattering, we studied La$_3$Ni$_2$O$_7$ at ambient pressure, and found that Ni 3$d_{x^2-y^2}$, Ni 3$d_{z^2}$, and ligand oxygen 2$p$ orbitals dominate the low-energy physics with a small charge-transfer energy. Remarkably, well-defined optical-like magnetic excitations were found to soften into a quasi-static spin-density-wave ordering, evidencing the strong electronic correlations and rich magnetic properties. Based on a Heisenberg spin model, we found that the inter-layer effective magnetic superexchange interaction is much larger than the intra-layer ones, and proposed two viable magnetic structures. Our results set the foundation for further exploration of La$_3$Ni$_2$O$_7$ superconductor.

cond-mat.supr-con↗

arXiv2023

Identifying Majorana Zero Modes in Vortex Lattices Using Fano Factor Tomography

In this work, we investigate the tunneling characteristics of Majorana zero modes (MZMs) in vortex lattices based on scanning tunneling microscopy measurement. We find that zero bias conductance does not reach the quantized value owing to the coupling between the MZMs. On the contrary, the Fano factor measured in the high voltage regime reflects the local particle-hole asymmetry of the bound states and is insensitive to the energy splitting between them. We propose using spatially resolved Fano factor tomography as a tool to identify the existence of MZMs. In both cases of isolated MZM or MZMs forming bands, there is a spatially resolved Fano factor plateau at one in the vicinity of a vortex core, regardless of the tunneling parameter details, which is in stark contrast to other trivial bound states. These results reveal new tunneling properties of MZMs in vortex lattices and provide measurement tools for topological quantum devices.

cond-mat.supr-con↗