arXiv subjects

Aditi Chakrabarty

Publications and source records attributed to Aditi Chakrabarty.

8 recordsLinked to original sources

arXiv2026

Monitored free fermions under periodic driving

We investigate analytically and numerically a one-dimensional periodically driven free-fermionic system subjected to monitoring of the local particle density. Based on the analytical approach that describes the long-wavelength physics of the time-dependent Hamiltonian in the field-theoretical language using the nonlinear sigma-model (NLSM), we reveal that driving does not alter the universality class of the problem. As a consequence, the system retains the area-law behavior in the thermodynamic limit, with an intermediate diffusive regime giving rise to logarithmic growth of entanglement entropy for a small monitoring rate. At the same time, driving leads to a renormalization of the bare coupling constant of the NLSM, which controls the space-time ``conductivity'' in the diffusive regime. We derive the analytic form of this renormalization, which becomes particularly strong in the case of a ``maximally symmetric'' drive and sufficiently short driving period. In addition, we employ the Wiener-Hopf method to investigate the ballistic-diffusive crossover. These analytical predictions are corroborated by numerical simulations of the von-Neumann entanglement entropy and the density correlation function. Our numerical results clearly demonstrate that, with an increase in the system size, there are successive crossovers from ballistic to diffusive behavior and ultimately to localization. Furthermore, in the diffusive regime, we observe weak-localization corrections that are in agreement with the analytical predictions of the NLSM. Overall, our results provide a unified analytical and numerical framework for understanding the effects of monitoring in time-modulated fermionic systems, paving a way for broader investigations of driven quantum matter.

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arXiv2025

Topological boundaries in non-Hermitian p-wave Kitaev chains with Rashba spin-orbit coupling

In this work, we investigate the combined effects of Rashba spin-orbit coupling (RSOC) and non-Hermiticity on topological phase transitions in spinful p-wave Kitaev chains. While previous studies have separately examined non-Hermitian (NH) extensions of Kitaev chains and the effects of RSOC in Hermitian systems, the interplay between these two mechanisms remains largely unexplored. We analyze this interplay by considering two distinct types of complex on-site potentials: (i) a uniform gain/loss term and (ii) a complex quasiperiodic potential. We demonstrate that the impact of RSOC is highly model-dependent. In particular, RSOC does not affect the topological phase boundary in the Hermitian limit of the uniform gain/loss model (provided the spin-flip hopping is weaker than the pairing strength), but significantly alters the topological landscape in the NH regime. In contrast, for the quasiperiodic model, RSOC modifies the phase boundaries in both the Hermitian and non-Hermitian cases. Notably, we find that the combined interplay of non-Hermiticity and RSOC drives topological transitions at significantly lower potential strengths compared to the Hermitian limit. We derive analytical expressions for the topological phase transitions in both cases and validate our predictions through numerical calculations of energy spectra and real-space winding numbers. This work provides a comprehensive understanding of how non-Hermiticity and RSOC cooperatively reshape topological phase diagrams in one-dimensional superconducting systems.

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arXiv2025

Emergence of non-trivial phases in interacting non-Hermitian quasiperiodic chains with power-law hopping

In the last few years, several works have identified the concurrence of the spectral, delocalization-localization and topological phase transitions in non-Hermitian quasiperiodic systems in the presence of time-reversal symmetry (TRS), with or without interaction. In this work, we investigate one-dimensional interacting non-Hermitian quasiperiodic lattices with asymmetric power-law hopping and unveil that although the Hamiltonian respects the TRS, the reality of the eigenspectrum does not necessarily indicate a topologically trivial non-Hermitian many-body localization (NHMBL) regime. In fact, we reveal the emergence of a topologically trivial intermediate regime, where the states that are primarily multifractal in nature can also possess a fully real spectrum, thereby restoring the TRS before crossing over to the NHMBL phase. Moreover, in the entire intermediate regime, the interaction completely destroys the multifractal and mobility edges observed in the non-interacting counterpart. Besides, we unveil that due to the long-range nature of the hopping, the entire topologically non-trivial ergodic regime under the periodic boundary condition does not always give rise to boundary localized skin modes under the open boundary condition. Our findings thus advances and deepens the understanding about the emergence of non-trivial phases due to the interplay of interaction and long-range hopping in non-Hermitian quasiperiodic systems.

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arXiv2024

The fate of Wannier-Stark localization and skin effect in periodically driven non-Hermitian quasiperiodic lattices

The eigenstates of one-dimensional Hermitian and non-Hermitian tight-binding systems (in the presence/absence of quasiperiodic potential) and an external electric field undergo complete localization with equally spaced eigenenergies, known as the Wannier-Stark (WS) localization. In this work, we demonstrate that when the electric field is slowly modulated with time, new non-trivial phases with multiple mobility edges emerge in place of WS localized phase, which persists up to a certain strength of the non-Hermiticity. On the other hand, for a large driving frequency, we retrieve the usual sharp delocalization-localization transition to the usual (no WS) localized phase, similar to the static non-Hermitian Aubry-Andr\'e-Harper type without any electric field. This vanishing of WS localization can be attributed solely to the time-periodic drive and occurs irrespective of the non-Hermiticity. Interestingly, under the open boundary condition (OBC), we find that contrary to the undriven systems where an external electric field destroys the SE completely, the SE appears in certain regime of the parameter space when the electric field is temporally driven. This appearance of SE is closely related to the absence of extended unitarity. In addition, in the presence of the drive, the skin states are found to be multifractal, contrary to its usual nature in such non-Hermitian systems. An in-depth understanding about the behavior of the states in the driven system is established from the long-time dynamics of an initial excitation.

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arXiv2024

Engineering unique localization transition with coupled Hatano-Nelson chains

The Hatano-Nelson (HN) Hamiltonian has played a pivotal role in catalyzing research interest in non-Hermitian systems, primarily because it showcases unique physical phenomena that arise solely due to non-Hermiticity. The non-Hermiticity in the HN Hamiltonian, driven by asymmetric hopping amplitudes, induces a delocalization-localization (DL) transition in a one-dimensional (1D) lattice with random disorder, sharply contrasting with its Hermitian counterpart. A similar DL transition occurs in a 1D quasiperiodic HN (QHN) lattice, where a critical quasiperiodic potential strength separates metallic and insulating states, akin to the Hermitian case. In these systems, all states below the critical potential are delocalized, while those above are localized. In this study, we reveal that coupling two 1D QHN lattices can significantly alter the nature of the DL transition. We identify two critical points, $V_{c1} < V_{c2}$, when the nearest neighbors of the two 1D QHN lattices are cross-coupled with strong hopping amplitudes under periodic boundary conditions (PBC). Generally, all states are completely delocalized below $ V_{c1}$ and completely localized above $V_{c2}$, while two mobility edges symmetrically emerge about $\rm{Re[E]} = 0$ between $V_{c1}$ and $V_{c2}$. Notably, under specific asymmetric cross-hopping amplitudes, $V_{c1}$ approaches zero, resulting in localized states even for infinitesimally weak potential. Remarkably, we also find that the mobility edges precisely divide the delocalized and localized states in equal proportions. Furthermore, we observe that the conventional one-to-one correspondence between electronic states under PBC and open boundary conditions (OBC) in 1D HN lattices breaks

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arXiv2024

Extended unitarity and absence of skin effect in periodically driven systems

One of the most striking features of non-Hermitian quasiperiodic systems with arbitrarily small asymmetry in the hopping amplitudes and open boundaries is the accumulation of all the bulk eigenstates at one of the edges of the system, termed in literature as the skin effect, below a critical strength of the potential. In this Letter, we uncover that a time-periodic drive in such systems can eliminate the SE up to a finite strength of this asymmetry. Remarkably, the critical value for the onset of SE is independent of the driving frequency and approaches to the static behavior in the thermodynamic limit. We find that the absence of SE is intricately linked to the emergence of extended unitarity in the delocalized phase, providing dynamical stability to the system. Interestingly, under periodic boundary condition, our non-Hermitian system can be mapped to a Hermitian analogue in the large driving frequency limit that leads to the extended unitarity irrespective of the hopping asymmetry and the strength of the quasiperiodic potential, in stark contrast to the static limit. Additionally, we numerically verify that this behavior persists Based on our findings, we propose a possible experimental realization of our driven system, which could be used as a switch to control the light funneling mechanism.

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arXiv2022

Skin effect and dynamical delocalization in non-Hermitian quasicrystals with spin-orbit interaction

The investigations of the spectral and dynamical delocalization-localization (DL) transition have revealed intriguing features in a wide range of non-Hermitian systems. The present study aims at exploring the spectral and dynamical properties in a non-Hermitian quasiperiodic system with asymmetric hopping in the presence of Rashba Spin-Orbit (RSO) interaction. In particular, in such systems, we have identified that the DL transition is associated with a concurrent change in the energy spectrum, where the eigenstates always break the time-reversal symmetry for all strenghts of the quasiperiodic potential, contrary to the systems without RSO interaction. Remarkably, we find that the reality of energy spectrum under the open boundary condition that is frequently symbolised as a hallmark of the skin-effect, is a system-size dependent phenomena, and appears even when the associated energies are indeed complex. In addition, it is demonstrated that the spin-flip term in the RSO interaction in fact possesses a tendency to diminish the directionality of the skin-effect. On scrutinizing the dynamical attributes in our non-Hermitian system, we unveil that in spite of the fact that the spectral DL transition accords with the dynamical phase transition, interestingly, the system comes across hyper-diffusive and negative diffusion dynamical regimes depending upon the strength of the RSO interaction, in the spectrally localized regime.

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arXiv2021

Localization, $\mathcal{PT}$-Symmetry Breaking and Topological Transitions in non-Hermitian Quasicrystals

According to the topological band theory of a Hermitian system, the different electronic phases are classified in terms of topological invariants, wherein the transition between the two phases characterized by a different topological invariant is the primary signature of a topological phase transition. Recently, it has been argued that the delocalization-localization transition in a quasicrystal, described by the non-Hermitian $\mathcal{PT}$-symmetric extension of the Aubry-André-Harper (AAH) Hamiltonian can also be identified as a topological phase transition. Interestingly, the $\mathcal{PT}$-symmetry also breaks down at the same critical point. However, in this article, we have shown that the delocalization-localization transition and the $\mathcal{PT}$-symmetry breaking are not connected to a topological phase transition. To demonstrate this, we have studied the non-Hermitian $\mathcal{PT}$-symmetric AAH Hamiltonian in the presence of Rashba Spin-Orbit (RSO) coupling. We have obtained an analytical expression of the topological transition point and compared it with the numerically obtained critical points. We have found that, except in some special cases, the critical point and the topological transition point are not the same. In fact, the delocalization-localization transition takes place earlier than the topological transition whenever they do not coincide.

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