arXiv subjects

Subrata Pachhal

Publications and source records attributed to Subrata Pachhal.

7 recordsLinked to original sources

arXiv2026

Topological random alloy

Topological phases of matter are often realized in crystalline materials. To extend their understanding beyond perfect stoichiometry, we introduce a minimal model of a topological random binary alloy and show that the system realizes an exotic form of impurity-band engineering. We reveal that, in contrast to Wannier charge centers pinned by impurities in conventional semiconductors, doping a proximate quantum anomalous Hall insulator results in dopant-centric chiral current loops. The nature of such current loops is intrinsically tied to the properties of both the host and the dopant. We demonstrate that, even at dilute dopant density, these current loops can form topological domains in an otherwise trivial host and trigger a topological phase transition. On the other hand, doping a topological host having chirality opposite to that of the dopants can unexpectedly stabilize a metallic phase in which bulk transport is mediated by inter-domain edge modes.

cond-mat.mes-hall↗

arXiv2025

Diffusive metal in a percolating Chern insulator

Two-dimensional non-interacting fermions without any anti-unitary symmetries generically get Anderson localized in the presence of disorder. In contrast, topological superconductors with their inherent particle-hole symmetry can host a thermal metallic phase, which is non-universal and depends on the nature of microscopic disorder. In this work, we demonstrate that in the presence of geometric disorders, such as random bond dilution, a robust metal can emerge in a Chern insulator with particle-hole symmetry. The metallic phase is realized when the broken links are weakly stitched via concomitant insertion of $\pi$ fluxes in the plaquettes. These nucleate low-energy manifolds, which can provide percolating conduction pathways for fermions to elude localization. This diffusive metal, unlike those in superconductors, can carry charge current and even anomalous Hall current. We investigate the transport properties and show that while the topological insulator to Anderson insulator transition exhibits the expected Dirac universality, the metal insulator transition displays a different critical exponent $\nu \approx 2$ compared to a disordered topological superconductor, where $\nu \approx 1.4$. Our work emphasizes the unique role of geometric disorder in engineering novel phases and their transitions in topological quantum matter.

cond-mat.mes-hall↗

arXiv2025

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

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.

cond-mat.str-el↗

arXiv2025

Symmetry-protected topological wire in a topological vacuum

Symmetry-protected topological phases host gapless modes at their boundary with a featureless environment of the same dimension or a trivial vacuum. In this study, we explore their behavior in a higher-dimensional environment, which itself is non-trivial - a topological vacuum. In particular, we embed a one-dimensional topological wire within a two-dimensional Chern insulator, allowing the zero-dimensional edge modes of the wire to interplay with the surrounding chiral boundary states created by the environment. In contrast to a trivial vacuum, we show depending on the nature of low energy modes, the topology of the environment selectively influences the topological phase transitions of the wire. Interestingly, such selectivity leads to scenarios where the environment trivializes the wire and even induces topological character in an otherwise trivial phase - an example of `proximity-induced topology'. Using both numerical and analytical approaches, we establish the general framework of such embedding and uncover the role of symmetries in shaping the fate of low-energy theories. Our findings will provide a deeper understanding of heterostructural topological systems, paving the way for their experimental exploration.

cond-mat.mes-hall↗

arXiv2024

Entanglement signatures of a percolating quantum system

Entanglement measures have emerged as one of the versatile probes to diagnose quantum phases and their transitions. Universal features in them expand their applicability to a range of systems, including those with quenched disorder. In this work, we show that when the underlying lattice has percolation disorder, free fermions at a finite density show interesting entanglement properties due to massively degenerate ground states. We define and calculate appropriate entanglement measures such as typical, annealed, and quenched entanglement entropy in both one and two dimensions, showing they can capture both geometrical aspects and electronic correlations of the percolated quantum system. In particular, while typical and annealed entanglement show volume law character directly dependent on the number of zero modes in the system, quenched entanglement is generally area law albeit showing characteristic signatures of the classical percolation transition. Our work presents an exotic interplay between the geometrical properties of a lattice and quantum entanglement in a many-body quantum system.

cond-mat.dis-nn↗

arXiv2023

Percolation Transition in a Topological Phase

Transition out of a topological phase is typically characterized by discontinuous changes in topological invariants along with bulk gap closings. However, as a clean system is geometrically punctured, it is natural to ask the fate of an underlying topological phase. To understand this physics we introduce and study both short and long-ranged toy models where a one dimensional topological phase is subjected to bond percolation protocols. We find that non-trivial boundary phenomena follow competing energy scales even while global topological response is governed via geometrical properties of the percolated lattice. Using numerical, analytical and appropriate mean-field studies we uncover the rich phenomenology and the various cross-over regimes of these systems. In particular, we discuss emergence of "fractured topological region" where an overall trivial system contains macroscopic number of topological clusters. Our study shows the interesting physics that can arise from an interplay of geometrical disorder within a topological phase.

cond-mat.dis-nn↗

arXiv2023

Spectral Form Factors of Topological Phases

Signatures of dynamical quantum phase transitions and chaos can be found in the time evolution of generalized partition functions such as spectral form factors (SFF) and Loschmidt echoes. While a lot of work has focused on the nature of such systems in a variety of strongly interacting quantum theories, in this work, we study their behavior in short-range entangled topological phases, particularly focusing on the role of symmetry-protected topological zero modes. We show, using both analytical and numerical methods, how the existence of such zero modes in any representative system can mask the SFF with large period (akin to generalized Rabi) oscillations, hiding any behavior arising from the bulk of the spectrum. Moreover, in a quenched disordered system, these zero modes fundamentally change the late-time universal behavior reflecting the chaotic signatures of the zero-energy manifold. Our study uncovers the rich physics underlying the interplay of chaotic signatures and topological characteristics in a quantum system.

cond-mat.str-el↗