Room-Temperature Quantum Material Offers Path to Lossless Electronics

Researchers are actively pursuing room-temperature quantum phenomena to advance energy-efficient electronics and quantum technologies. Zhiqiang Hu, Yuqi Zhang from the Beijing Institute of Technology, and Yuyang Wang, Kebin Xiao et al. from Tsinghua University report the direct visualisation of robust quantum spin Hall (QSH) states in α-Bi4Br4 nanowires, persisting up to 300 K. Their work demonstrates a stair-stepped stacking configuration which enables this stability and scalability, circumventing limitations of previous designs and allowing for spatially decoupled edge states. This achievement establishes α-Bi4Br4 as a promising material for high-temperature applications and offers a generalisable strategy for developing scalable QSH systems.

This achievement overcomes a fundamental challenge in materials science, paving the way for energy-efficient electronics and advanced quantum technologies. Researchers employed microwave impedance microscopy to observe these QSH states, demonstrating their stability and scalability in a novel “stair-stepped” stacking configuration.

This multilayer geometry maintains spatial decoupling of QSH edge states from individual layers, circumventing limitations present in previous designs. The innovative stair-stepped QSH (SS-QSH) configuration allows for robust conduction in structures extending several micrometers in length and hundreds of nanometers in height.

This configuration bypasses the stringent alignment and layer number requirements that have hindered previous attempts to create scalable QSH systems. Magnetic field and temperature dependent measurements confirm the intrinsic topological nature of these states, solidifying their potential for practical applications.

Crucially, the SS-QSH and bulk signals exhibit a direct correlation with nanowire height, unequivocally verifying the stair-stepped origin of the observed conduction. Finite-element analysis simulations successfully reproduce the experimental results, further validating the theoretical understanding of this phenomenon.

This work establishes α-Bi4Br4 as a viable platform for high-temperature topological electronics, offering a significant advancement over existing materials. Furthermore, the demonstrated stacking strategy provides a generalizable approach for designing scalable QSH systems, opening new avenues for innovation in quantum materials and device fabrication. The research highlights a pathway toward realising dissipationless edge conduction at room temperature, a critical step for developing next-generation electronic devices.

Visualisation of robust quantum spin Hall states in stair-stepped α-Bi4Br4 nanowires

Microwave impedance microscopy directly visualised robust quantum spin Hall (QSH) states persisting up to 300 K in α-Bi4Br4 nanowires. Nanowires were synthesised via physical vapor deposition, forming structures several micrometers long and hundreds of nanometers high with a stair-stepped stacking configuration.

This multilayer geometry spatially decouples QSH edge states from individual layers, circumventing stringent alignment and layer number constraints present in previous designs. The research focused on α-Bi4Br4 crystals, exploiting their large band gap exceeding 200 meV and distinctive topological properties arising from van der Waals interlayer interactions with 180° rotation between adjacent layers.

Specifically, the study employed a stair-stepped stacking configuration comprising vertically stacked monolayer steps to create conductive side facets with a shallow inclination angle. This arrangement ensures that QSH states remain decoupled, largely independent of the total layer count, and potentially stable at room temperature.

Magnetic field and temperature dependent measurements were performed to confirm the intrinsic topological nature of the observed states. Crucially, the signals originating from the SS-QSH states and the bulk material both scaled proportionally with nanowire height, directly verifying the stair-stepped origin of the conduction.

Finite-element analysis simulations successfully reproduced the experimental results, validating the observed phenomena and the proposed mechanism. The microwave impedance microscopy technique mapped local conductivity, providing unambiguous imaging of dissipationless edge conduction at the mesoscale, unlike techniques that only probe electron density of states. This work establishes α-Bi4Br4 as a viable platform for high-temperature topological electronics and demonstrates a generalisable stacking strategy for designing scalable QSH systems.

Stair-stepped α-Bi4Br4 nanowires exhibit robust quantum spin Hall states up to 300 K

Researchers directly visualized robust quantum spin Hall (QSH) states persisting up to 300 K within α-Bi4Br4 nanowires. This stability and scalability are enabled by a stair-stepped stacking configuration, a multilayer geometry where QSH edge states from individual layers remain spatially decoupled. Structures achieved lengths of several micrometers and heights of hundreds of nanometers utilising this configuration.

Magnetic field and temperature dependent measurements confirmed the intrinsic topological nature of these states. The SS-QSH signals and bulk signals both scale proportionally with nanowire height, verifying the stair-stepped origin of the observed conduction. Finite-element analysis simulations successfully reproduced the experimental results, further validating the findings.

The crystal structure of α-Bi4Br4 consists of quasi-one-dimensional chains extending along the b-axis. Interlayer and intralayer interactions are governed by van der Waals forces, with adjacent layers rotated by 180°, contributing to the distinctive topological properties. Theoretical predictions indicate monolayer Bi4Br4 is a QSH insulator, with QSH states expected to persist at monolayer steps on bulk substrates.

Scanning tunneling microscopy measurements revealed an enhanced local density of states near the edges of monolayer Bi4Br4, consistent with the presence of QSH states. When layer edges are aligned along the c-axis, QSH states from adjacent layers can hybridize, creating a higher-order topological insulator (HOTI) phase.

This phase retains a single pair of QSH states localized at the intersection of surfaces, known as hinge states. However, maintaining strict edge alignment becomes increasingly difficult in larger structures, limiting the stability of these hinge states. The stair-stepped stacking configuration presented in this work comprises numerous vertically stacked monolayer steps, resulting in spatially separated and decoupled QSH states forming a conductive side facet.

Consequently, the behaviour of these SS-QSH states is largely independent of the precise layer count. Microwave impedance microscopy (MIM) was employed to resolve the local conductivity of SS-QSH states in micron-scale α-Bi4Br4 nanowires synthesized via physical vapor deposition. Unlike techniques probing density of states, MIM provides unambiguous conductance mapping, enabling real-space imaging of dissipationless conductive edges at the mesoscopic scale. This work establishes α-Bi4Br4 as a practical platform for high-temperature topological electronics and demonstrates a generalizable stacking strategy for designing scalable QSH systems.

Stair-stepped α-Bi4Br4 nanowires enable robust high-temperature quantum spin Hall states

Researchers have directly observed robust quantum spin Hall (QSH) states persisting up to 300 K within α-Bi4Br4 nanowires. This achievement circumvents previous limitations by utilising a stair-stepped stacking configuration, a multilayer geometry where edge states remain spatially decoupled. The configuration allows for stable and scalable QSH conduction in structures measuring several micrometers in length and hundreds of nanometers in height, demonstrating a significant advancement in materials science.

The stability and scalability of these states are attributed to the unique stair-stepped arrangement of the nanowires, confirmed through magnetic field and temperature-dependent measurements, as well as finite-element analysis simulations. The observed signals scale proportionally with nanowire height, further validating the stair-stepped origin of the QSH states.

This work establishes α-Bi4Br4 as a viable material for high-temperature electronics and introduces a generalizable stacking strategy applicable to other QSH systems. The authors acknowledge that further investigation into the influence of magnetic field orientation and microwave frequency dependence would be beneficial.

Future research may focus on extending this stacking model to other high-temperature QSH materials and integrating these structures with superconductors or magnets to explore advanced quantum phases. Scalable nanofabrication protocols are also needed to create interconnected networks of topological channels, potentially advancing large-scale quantum electronics and facilitating the development of energy-efficient topological circuits capable of hosting Majorana zero modes under ambient conditions.