arXiv subjects

Qiang-Jun Cheng

Publications and source records attributed to Qiang-Jun Cheng.

8 recordsLinked to original sources

arXiv2026

Distinct Surface and Bulk Superconductivity in the Kagome Superconductor SrSn$_3$

Surface and bulk superconductivity may possess fundamentally different superconducting properties in quantum materials with nontrivial electronic structures, yet their superimposed spectroscopic signatures often prevent direct experimental access to each superconducting channel. Here we reveal, in epitaxial films of the kagome superconductor SrSn$_3$, distinct surface and bulk superconducting channels with markedly different superconducting gaps, upper critical fields, and vortex-core electronic states by tuning the tunneling junction resistance in scanning tunneling spectroscopy. The surface superconductivity is characterized by a thickness-independent superconducting gap and an enhanced upper critical field, whereas the bulk superconducting channel exhibits a larger superconducting gap that decreases with reducing film thickness and a much lower upper critical field. Within magnetic vortex cores, robust non-split zero-bias conductance peaks are observed exclusively in the surface superconducting channel, while pronounced zero-bias suppression is consistently associated with the bulk superconducting channel. These findings demonstrate that the vortex-core electronic structure depends sensitively on the underlying superconducting channel, providing new insight into vortex-bound states in topological quantum materials.

cond-mat.supr-con↗

arXiv2026

Spectroscopic Studies of two-dimensional Superconductivity

Two-dimensional superconductivity has become a major frontier in condensed matter physics. It holds the key to the mechanism of high-temperature superconductors and offers an exceptional arena to stabilize emergent quantum states enabled by enhanced electron correlations in reduced dimensionality. These states are frequently characterized by spatial modulations and intertwined with competing orders, calling for studies that combine real-space imaging with local spectroscopy. Scanning tunneling microscopy and spectroscopy meets this need by directly accessing local density of states with lattice-scale resolution. In this review, we summarize recent advances of the study on several representative unconventional superconductors using this technique, focusing on direct characterization of high-temperature superconducting planes, pair-density waves, and topological superconductivity in both artificial heterostructures and intrinsic materials. We conclude by outlining current challenges and future directions motivated by the microscopic insights.

cond-mat.supr-con↗

arXiv2026

Intrinsic translational symmetry-breaking charge stripes in underdoped iron pnictides

Despite being well established in cuprates, an intrinsic translational symmetry-breaking charge order has not been clearly identified in iron-based superconductors. Using spectroscopic-imaging scanning tunneling microscopy on epitaxial Ca(Fe1-xCox)2As2 (x = 0 ~ 0.055) thin films, we observe smectic, near-commensurate charge-stripe order in the underdoped regime that intervenes between the nematic parent phase and optimally doped superconductivity. Distinct from the bidirectional checkerboard-like order in cuprates, these charge stripes are unidirectional along the antiferromagnetic Fe-Fe bond direction and are accompanied by a van Hove singularity near the Fermi level, inherited from the Fermi surface reconstruction driven by intertwined antiferromagnetic and nematic correlations. Both local and global suppression of the charge-stripe instability enhance superconductivity, tunable via epitaxial strain and Co doping. These results establish charge-stripe order as an intermediate electronic phase in iron pnictides and reveal a coherent pathway from nematicity to superconductivity. Our findings highlight charge ordering as a unifying element across different families of high-temperature superconductors.

cond-mat.supr-con↗

arXiv2026

Intertwined Charge Stripes and Majorana Zero Modes in An Iron-Based Superconductor

In type-II superconductors, magnetic fields modulate the amplitude and phase of the superconducting order parameter, forming quantized vortices where superconductivity is locally suppressed and exotic bound states or competing electronic orders emerge. Using spectroscopic-imaging scanning tunneling microscopy on epitaxial Ba(Fe$_{0.94}Co$_{0.06})$_2$As$_2$ films, we discover an incommensurate charge-stripe order aligned with the Fe-Fe bond direction. These charge modulations become strongly enhanced at the vortex core and extend far into the vortex halo. Strikingly, the charge order modulates Andreev bound states of vortices at non-zero energies, producing abelian vortices with half-odd-integer level quantization and non-abelian vortices with integer-quantized core states that host a Majorana zero mode. The distinct vortex types are distinguished by the registry of their centers relative to the charge-stripe pattern and remain robust in ultrathin (2.5-unit-cell) films. Our findings reveal a density-wave-textured vortex matter and provide fresh insights into intertwined phenomena in iron-based superconductors.

cond-mat.supr-con↗

arXiv2026

Topological Semimetal Transport Modulated by Interstitial Fe in Ba(Fe$_{1-x}$Co$_x$)$_{2+\delta}As$_2$ Superconductors

Topological semimetals are renowned for exhibiting large, unsaturated magnetoresistance arising from ultrahigh carrier mobility and electron-hole compensation. However, such behaviors remain poorly understood in iron-based superconductors that have been recently recognized to harbor rich nontrivial topology. Here, we combine angle-resolved magneto-transport measurements with first principles calculations to reveal the emergence and tunability of topological semimetals in ferropnictide Ba(Fe$_{1-x}$Co$_x$)$_{2+\delta}As$_2$ epitaxial films, modulated by interstitial Fe. These states exhibit ultralow residual resistivity, coexisting high-mobility electron and hole carriers, and linear positive magnetoresistance below 110 K. Remarkably, the magnetoresistance becomes more pronounced when the magnetic field is applied parallel to the film plane, reaching an unsaturated 1206% at 56 T. Furthermore, superconductivity persists in these ferropnictide films, establishing them as a tunable platform for investigating the interplay among electron correlation, topology, and superconductivity.

cond-mat.supr-con↗

arXiv2025

Atomic-scale imaging of electronic nematicity in ferropnictides

Electronic nematicity, a correlated state characterized by broken rotational symmetry, has been recognized as a ubiquitous feature intertwined with unconventional electron pairing in various iron-based superconductors. Here we employ spectroscopic-imaging scanning tunneling microscopy to visualize atomic-scale electronic nematicity directly on FeAs planes of a prototypical ferropnictide BaFe$_2$As$_2$. Spatially, the nematic order appears as 4$a_{\textrm{Fe}}$-spaced stripes ($a_{\textrm{Fe}} \sim $ 0.28 nm is the in-plane Fe-Fe distance) within homogeneously and orthogonally oriented nano-domains. The energy-resolved conductance maps reveal a pronounced energy-dependence of the nematic order parameter that experiences a sign change at approximately 30 meV. This characteristic behavior coincides with energy-dependent orbital splitting previously identified in momentum space, but is remarkably visualized in real space for the first time in our study. Moreover, the electronic nematicity exhibits pronounced sensitivity to single impurities and is notably suppressed by cobalt substitution for Fe atoms, promoting optimal superconductivity when nematic fluctuations are strongest. Our results provide pivotal experimental insights for developing a microscopic model of nematic order, thus paving the way to study its complex relationship with unconventional superconductivity.

cond-mat.supr-con↗

arXiv2025

Full-Gap Superconductivity in BaAs/Ferropnictide Heterostructures

Interfacial interactions often promote the emergence of unusual phenomena in two-dimensional systems, including high-temperature superconductivity. Here, we report the observation of full-gap superconductivity with a maximal spectroscopic temperature up to 26 K in a BaAs monolayer grown on ferropnictide Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ (abbreviated as BFCA) epitaxial films. The superconducting gap remains robust even when the thickness of underlying BFCA is reduced to the monolayer limit, in contrast to the rapid suppression of $T_\textrm{c}$ in standalone BFCA thin films. We reveal that the exceptional crystallinity of the BaAs/BFCA heterostructures, featured by their remarkable electronic and geometric uniformities, is crucial for the emergent full-gap superconductivity with mean-field temperature dependence and pronounced bound states within magnetic vortices. Our findings open up new avenues to unravel the mysteries of unconventional superconductivity in ferropnictides and advance the development of FeAs-based heterostructures.

cond-mat.supr-con↗

arXiv2024

Quantum spin driven Yu-Shiba-Rusinov multiplets and fermion-parity-preserving phase transition in K$_3$C$_{60}$

Magnetic impurities in superconductors are of increasing interest due to emergent Yu-Shiba-Rusinov (YSR) states and Majorana zero modes for fault-tolerant quantum computation. However, a direct relationship between the YSR multiple states and magnetic anisotropy splitting of quantum impurity spins remains poorly characterized. By using scanning tunneling microscopy, we resolve systematically individual transition-metal (Fe, Cr and Ni) impurities induced YSR multiplets as well as their Zeeman effects in K$_3$C$_{60}$ superconductor. The YSR multiplets show identical $d$ orbital-like wave functions that are symmetry-mismatched to the threefold K$_3$C$_{60}$(111) host surface, breaking point-group symmetries of the spatial distribution of YSR bound states in real space. Remarkably, we identify an unprecedented fermion-parity-preserving quantum phase transition between ground states with opposite signs of the uniaxial magnetic anisotropy that can be manipulated by an external magnetic field. These findings can be readily understood in terms of anisotropy splitting of quantum impurity spins, and thus elucidate the intricate interplay between the magnetic anisotropy and YSR multiplets.

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