Researchers from Boston College, the University of California Irvine and Boston University report evidence of long-range, non-disruptive quantum flow at the edges of an iron-chalcogenide superconductor. This material uniquely combines superconductivity with both band topology and spontaneous time-reversal-symmetry breaking, creating conditions expected to support topological superconducting boundary states. The work demonstrates an anomalous conductance plateau persisting over micrometer-scale separations, requiring sharp edges for sustained electron flow and exhibiting reduced thermal decoherence. This boundary-mediated transport is absent in comparable, topologically trivial materials.

TRSB Coexistence Defines Superconducting Symmetry Breaking

Iron-chalcogenide superconductors demonstrate a transport response dependent on both material topology and the sharpness of its edges. Conventional Andreev spectra are observed in topologically trivial materials regardless of edge quality, but a distinct conductance plateau emerges in topological samples only when connected to continuous, sharp edges. This finding establishes a stringent test for identifying boundary-mediated superconducting transport, differentiating it from localized responses.

The research focuses on iron-chalcogenide materials, specifically Fe(Te,Se), which uniquely combine superconductivity with a topologically nontrivial electronic structure and spontaneous time-reversal symmetry breaking (TRSB). The emergence of TRSB within the superconducting state of Fe(Te,Se) provides a critical symmetry-breaking scale directly comparable with transport measurements, and experiments reveal spontaneous magnetization coinciding with superconductivity across multiple compositions.

This co-existence is not merely coincidental. Temperature-dependent measurements show the edge-associated transport is governed by the TRSB state, with the conductance plateau exhibiting suppressed thermal broadening below $T_{K e r r} *$ and disappearing near $T_{K e r r}$ rather than at the bulk superconducting transition temperature, $T_{c}$ . The plateau’s width, approximately 0. 8 mV, aligns with the energy scale associated with $T_{K e r r}$ , further linking the phenomenon to the TRSB state. The methodology employed to reliably access boundary transport involved engineering crystallographically sharp, continuous edges and implementing side-surface-dominant contacts, validated through contact-size analysis and cross-sectional imaging.

This precise control over edge morphology and contact quality is essential, as rough or stepped edges in topologically nontrivial materials yield zero-bias conductance peaks instead of the observed plateau. The suppression of both the plateau and zero-bias conductance peak correlates strongly with the TRSB temperature scale, a result incompatible with conventional Andreev scenarios. “Experiments using different methods and samples revealed spontaneous magnetization in superconducting states of Fe(Te,Se),” the paper states, highlighting the robust nature of the observed TRSB.

The observed boundary-mediated transport extends over micrometer-scale separations, exceeding the bulk coherence length, and is absent in comparable topologically trivial materials, confirming its non-local character. This long-range nonlocality suggests a protected transport channel, potentially enabling chiral boundary modes through either splitting of bulk topological bands into a Weyl regime or generating an effectively layered chiral topological superconducting phase. The results indicate that the material’s topological phase and edge geometry combine to determine the observed phenomenology, opening avenues for manipulating and controlling these quantum states.

Fe(Te,Se) as Platform for Topological Superconductivity

Transport measurements reveal that a robust superconducting signal persists even when current flows across extended distances along the edges of Fe(Te,Se) devices, a behavior distinct from typical Andreev reflection spectra which smear at relatively low temperatures and persist up to the critical temperature. This anomalous conductance plateau, unlike trivial spectra, collapses around 10 K, indicating the plateau’s origin is not solely due to bulk superconductivity, and suggesting a unique mechanism governing electron flow at the material’s boundaries.

The observed thermal characteristics, nearly unchanged amplitude and width below 5 K, and subsequent vanishing at 10 K, point to a boundary-mediated transport channel with markedly reduced thermal smearing over an extended temperature range. The material hosts three intertwined ingredients relevant to temperature dependence: the nontrivial topological band structure, bulk superconductivity and spontaneous magnetic order below $T_{K e r r}$ .

Further theoretical work is needed to determine whether the observed boundary response arises from a Weyl regime, created by splitting bulk topological bands, or from this stacked chiral phase, but the current findings are broadly consistent with models of transport mediated by topological superconducting edge modes. Quantitative comparison, however, will likely require extensions to multi-channel or fully 3D geometries, and to account for the multiband superconductivity inherent in Fe(Te,Se).

“Although trivial Andreev spectra begin to smear at relatively low temperatures, they persist up to $T_{c}$ , unlike the plateau, which collapses already around 10 K,” indicating the plateau is not governed solely by bulk superconductivity, as stated in the research. Demonstrating such responses experimentally requires more than observing a local zero-bias feature, and a stringent test should distinguish a local response from a boundary-mediated signal.

The researchers note that transport-based demonstrations of such protected nonlocal responses remain challenging, requiring a feasible material platform combining superconductivity, topology and symmetry breaking, together with precise control over edge morphology, contact quality and measurement geometry.

Engineered Edge Geometry Enables Boundary Transport

Establishing a clear distinction between boundary-mediated transport originating from topological properties and conventional bulk superconductivity presented a significant methodological challenge, prompting a focused examination of differential conductance spectra in iron-chalcogenide devices. The team discovered that the observed spectra were highly sensitive to both material composition and the morphology of the device edges, revealing a critical link between geometry and transport behavior.

Edges aligned with specific crystallographic directions proved essential, as discontinuities and roughness interrupted the pathways for propagating edge modes, as revealed by atomic force microscopy showing nanometer-scale height variations on less-ideal edges. This dependence on edge geometry was notably absent in topologically trivial iron-chalcogenide devices, solidifying the conclusion that the observed conductance plateau was not simply a byproduct of superconductivity or contact quality.

Three distinct transport regimes emerged from the experiments: conventional Andreev reflection-like spectra in trivial materials, a zero-bias conductance peak associated with rough edges. The robust, bias-independent conductance plateau exclusively observed when sharp edges were contacted. The team’s work demonstrates that a continuous boundary is, in principle, an essential geometric condition for observing transport responses mediated by propagating edge modes.

To further validate the boundary-mediated nature of the conductance plateau, researchers designed experiments probing the necessity of edge contacts in the transport process. By maintaining a constant source contact while selectively positioning the drain electrode either on an edge or on the device surface, they distinguished between local tunneling processes and nonlocal, edge-mediated transport.

The results showed a clear dependence on the drain’s location, confirming that the plateau’s persistence required both source and drain electrodes to be attached to uninterrupted, sharp edges. “A local process at the source interface must not depend on where the drain electrode is placed,” the team noted, highlighting the non-local character of the observed transport.

Further investigation into the role of the bulk in transport is planned, with researchers suggesting phase-sensitive interferometry and controlled edge constrictions as potential avenues for future exploration. Complementary thermal-transport measurements and spatially resolved Kerr imaging could also provide valuable insights into the interplay between magnetic order and boundary channel behavior, potentially unlocking a deeper understanding of the underlying mechanisms driving this unique superconducting transport.

Nonlocal Conductance Plateau Signals Boundary States

Statistical analysis of iron-chalcogenide devices reveals a robust conductance plateau appearing exclusively on edges exhibiting macroscopic in-plane tortuosity and a microscopic edge transition-width-to-thickness ratio below 10. Exceeding these geometric thresholds introduces a terraced morphology that disrupts continuous boundaries, eliminating the transport signature and driving the system into a zero-bias conductance peak. These findings establish a direct link between edge geometry and the observed superconducting transport, suggesting a boundary-mediated mechanism at play.

Experiments designed to probe the requirement for edge contacts in the transport process further support this interpretation. The persistence of the conductance plateau in double-edge configurations, coupled with its absence in single-edge setups, emphasises the necessity of continuous edge pathways for this phenomenon. This dependence on edge continuity distinguishes the observed transport from trivial Andreev bound states, which can arise from disorder or contact inhomogeneity.

Temperature evolution studies reveal the plateau’s resilience to decoherence mechanisms, contrasting sharply with conventional Andreev reflection spectra. While increasing temperature rapidly smears out features in conventional spectra, the conductance plateau remains remarkably stable until approaching the critical temperature. “If the plateau were governed solely by superconductivity, its characteristic scale should track the superconducting gap and vanish at $T_{c}$ ,” the researchers explain, highlighting the potential for an additional stabilizing factor.

The observed temperature dependence suggests the plateau’s emergence may correlate with the symmetry-breaking scale established by magnetic order, as measured by the Kerr effect. Further investigation is needed to determine whether bulk-state Andreev processes or conversion between boundary modes and bulk superconducting states contribute to the measured conductance. A comprehensive theoretical model, along with experimental studies across a range of flake thicknesses and contact geometries, will be important in evaluating specific scenarios.

Polar Kerr Measurements Confirm TRSB Emergence

Polar Kerr measurements reveal a critical temperature below which time-reversal symmetry breaks in iron-chalcogenide superconductors, a threshold distinct from the onset of superconductivity itself. The observed emergence of this symmetry breaking, denoted $T_{K e r r}$ , provides an independent scale for comparison with transport measurements and reinforces the link between magnetism and the unusual electronic behavior at the material’s edges.

This separation of $T_{K e r r}$ from the superconducting transition temperature, $T_{c}$ , is a key finding, as conventional superconducting spectra continue to exhibit features up to $T_{c}$ , while the boundary-mediated conductance plateau vanishes at $T_{K e r r}$ . This correlation further strengthens the connection between the plateau and the spontaneous breaking of time-reversal symmetry, suggesting the plateau’s emergence is not solely governed by the superconducting gap.

The team’s experiments also imposed a critical constraint on potential explanations for the plateau, confirming its disappearance at $T_{K e r r}$ and not at the higher temperature of $T_{c}$ . This observation rules out scenarios where the plateau is simply a consequence of bulk superconductivity. “Our experiments impose a key constraint on any such scenario,” the researchers state, “both the plateau and the edge-contact ZBCP vanish at $T_{K e r r}$ , well below $T_{c}$ .” The team’s work establishes experimental criteria collectively supporting an edge-state interpretation of the observed transport, a finding that could be essential for realizing robust topological quantum computation.

The distinct temperature dependence of the plateau, tracking the Kerr rotation rather than the superconducting gap, provides compelling evidence that the low-energy transport scale is governed by the symmetry-breaking order, not solely by superconductivity. This subtle understanding of the interplay between superconductivity, topology, and magnetism opens new avenues for exploring and manipulating quantum phenomena in iron-chalcogenide materials.

Temperature Dependence Links Plateau to TRSB

The temperature at which spontaneous magnetization emerges in iron-chalcogenide superconductors directly correlates with the anomalous conductance plateau observed at the material’s edges, establishing a link between time-reversal symmetry breaking and boundary-mediated transport. Measurements using the Sagnac magneto-optical Kerr effect and muon spin rotation show that this symmetry breaking occurs within the bulk of the material, providing a key setting for exploring new topological superconducting phases.

This finding moves beyond simply identifying superconductivity, topology and magnetism coexisting in the same material, and begins to map their interplay. Establishing a clear distinction between edge-state transport and conventional mechanisms required careful examination of potential artifacts and measurement configurations. Researchers systematically tested whether the conductance plateau could arise from the experimental setup itself, finding that the plateau’s persistence over micrometer-scale separations and reduced thermal decoherence strongly suggest a protected, boundary-mediated origin.

The team’s work demonstrates that the plateau’s low-energy response exhibits markedly reduced thermal smearing, extending over a broader temperature range than would be expected from Andreev spectra or trivial edge states. This is further supported by the observation that the plateau vanishes at the temperature at which time-reversal symmetry breaks, denoted $T_{K e r r}$ , which is notably lower than the bulk superconducting transition temperature, $T_{c}$ . The correlation between the plateau width and the polar Kerr rotation, $θ_{K}$ , across varying temperatures provides independent confirmation of this link.

The experimental criteria established by this work collectively support an edge-state interpretation, with the plateau’s suppression temperature coinciding with the onset of time-reversal symmetry breaking. Sharp, continuous edges are essential for producing the plateau, while rough or stepped edges yield a zero-bias conductance peak, indicating that the observed transport is intrinsically linked to the material’s topological properties and edge geometry. This combination of topology, superconductivity, and time-reversal symmetry breaking creates a unique platform for isolating boundary-mediated superconducting transport, offering a pathway toward understanding and using topological quantum materials.

Long-Range Transport Distinguishes Topological Response

The iron-chalcogenide superconductor exhibits a transport response unlike conventional materials, maintaining a consistent conductance level across micrometer-scale separations, distances far exceeding the material’s inherent capacity for sustaining coherence within its bulk structure. This long-range nonlocality suggests that electrons are propagating along the edges of the material in a way that bypasses typical disruption, a phenomenon not observed in topologically trivial counterparts. Researchers engineered devices from exfoliated Fe(Te,Se) to isolate and study this edge-based conductivity, focusing on the interplay between superconductivity, band topology and spontaneous time-reversal symmetry breaking.

Fabrication of these devices required a meticulous process, shielding samples from environmental interference throughout preparation. This careful approach allowed for the creation of high-quality edge contacts on crystallographically defined flakes, essential for observing the unique transport characteristics. The team systematically tested criteria to differentiate true boundary-mediated superconductivity from other potential sources of conductance, including Andreev bound states, disorder and even heating effects.

A key finding was the emergence of a conductance plateau only when contacts were made on sharp, continuous edges, while rough edges yielded a zero-bias conductance peak, a previously documented effect. The observed conductance plateau exhibits markedly reduced thermal smearing, meaning its signal remains clear even at higher temperatures, a characteristic inconsistent with trivial Andreev spectra.

This correlation provides an independent measure of symmetry breaking that aligns with the observed transport behavior, and the plateau and edge-contact zero-bias conductance peak both vanish at $T_{K e r r}$ . Establishing this boundary-mediated transport required careful examination of the experimental setup itself, systematically testing whether the plateau could arise from artifacts of the measurement process.

👉 More information
🗞 Nontrivial Boundary-Mediated Superconducting Transport in a TRSB Topological Iron-Based Superconductor
✍️ Wenyao Liu et al.
🧠 DOI: http://link.aps.org/doi/10.1103/8vgt-twh5

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