Signatures of emergent surface states across a displacive topological phase transition in Bi$_4$I$_4$

Deep Singha Roy; Sk Kalimuddin; Subrata Pachhal; Saikat Mondal; Soham Das; Sukanya Jana; Arnab Bera; Satyabrata Bera; Tuhin Debnath; Ankan Bag; Souvik Pramanik; Sudipta Chatterjee; Sanjib Naskar; Shishir Kumar Pandey; Adhip Agarwala; Mintu Mondal

arXiv · 2509.03469

Signatures of emergent surface states across a displacive topological phase transition in Bi$_4$I$_4$

Deep Singha Roy·Sk Kalimuddin·Subrata Pachhal·Saikat Mondal·Soham Das·Sukanya Jana·Arnab Bera·Satyabrata Bera·Tuhin Debnath·Ankan Bag·Souvik Pramanik·Sudipta Chatterjee·Sanjib Naskar·Shishir Kumar Pandey·Adhip Agarwala·Mintu Mondal

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Abstract

Topological phase transitions involving crystalline symmetry breaking provide a fertile ground to explore the interplay between symmetry, topology, and emergent quantum phenomena. Recently discovered quasi-one-dimensional topological material, Bi$_4$I$_4$, has been predicted to host topologically non-trivial gapless surfaces at high temperature, which undergo a finite temperature phase transition to a low temperature gapped phase. Here we present experimental signatures of this room temperature phase transition from a high-temperature $\beta$-phase with a surface state to a gapped $\alpha$-phase hosting hinge states. Using real-space current mapping and resistance fluctuation spectroscopy, we identify signatures of a displacive topological phase transition mediated by a first-order thermodynamic structural change. Near the emergence of $\beta$-phase, we observe pronounced telegraphic noise, indicating fluctuating phase domains with topological surface states. The spatially resolved current map reveals electron transport via the gapless surface states in the $\beta$-phase, which vanishes upon transitioning to the $\alpha$-phase with localized conduction channels (or hinge modes). Our experimental results, supported by first principles estimates and effective theory of a topological displacive phase transition, establish Bi$_4$I$_4$ as a candidate material showing intricate interplay of classical thermodynamic phase transitions with topological quantum phenomena.

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Deep Singha Roy, Sk Kalimuddin, Subrata Pachhal, Saikat Mondal, Soham Das, Sukanya Jana, Arnab Bera, Satyabrata Bera, Tuhin Debnath, Ankan Bag, Souvik Pramanik, Sudipta Chatterjee, Sanjib Naskar, Shishir Kumar Pandey, Adhip Agarwala, Mintu Mondal. 2025-09-03. Signatures of emergent surface states across a displacive topological phase transition in Bi$_4$I$_4$. https://arxiv.org/abs/2509.03469

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