arXiv subjects

Srijata Lahiri

Publications and source records attributed to Srijata Lahiri.

13 recordsLinked to original sources

arXiv2026

Large nonlinear Hall effect in strained moiré structures hosting pseudospin-3/2 fermions

We investigate the linear and nonlinear Hall response of a moiré \emph{watermill lattice}, in which stacking and twisting generate a four-band manifold near the Fermi level with suppressed group velocities at discrete magic angles. Including an inversion symmetry breaking onsite mass breaks the interlayer symmetry, opening a gap in this manifold and driving the system into a non-trivial bulk topological phase. We map the resulting phase diagram as a function of the strength of the mass and twist angle $θ$, revealing several sectors with high Chern numbers. We then introduce strain to break the residual $C_3$ symmetry of the lattice which activates a finite Berry curvature dipole and correspondingly, a nonlinear Hall response. The dipole reverses sign sharply across topological phase boundaries, producing butterfly like features when plotted against the relevant system parameters. Its magnitude substantially exceeds that reported for symmetry-broken transition metal dichalcogenides, consistent with the elevated Wilson-loop winding and enhanced quantum geometry associated with the lattice's pseudospin-$3/2$ character. We conclude by incorporating thermal effects on the Berry curvature dipole, asserting that it is an important tool for discerning topology at low temperatures.

cond-mat.mes-hall↗

arXiv2026

Probing topological phase transitions via nonlinear Hall response in strained moiré dice lattice

Valley polarized twisted bilayer dice lattice hosts topologically nontrivial flat bands far from charge neutrality due to broken time reversal symmetry, whereas the ones in the vicinity of it remain topologically trivial. However, when both valleys are taken into consideration, the time reversal symmetry is preserved, which poses a serious hindrance to enumerate the valley specific topological phases that rely on the detection of the Berry curvature. In this work, we demonstrate that such a twisted structure with an applied uniaxial strain exhibits a nonlinear Hall effect far from charge neutrality. We ascertain that the nonlinear anomalous Hall signals can serve as a probe for topological phase transitions associated with a specific energy state that is constrained to reside at the lower edge of the middle subband and controlled via a staggered mass. Specifically, we show that the nonlinear anomalous Hall response undergoes a sign reversal across the topological phase boundaries. By tuning the carrier density, we compute the nonlinear Hall response obtained from the Berry curvature dipole, both in the chiral limit, and also when the chiral symmetry is broken. It is further seen that the nonlinear Hall effect is significantly enhanced in the broken chiral symmetry regime.

cond-mat.mes-hall↗

arXiv2026

Emergent topology of flat bands in a twisted bilayer $α$-$T_3$ lattice

We investigate an interesting interplay of destructive interference due to lattice geometry and band folding due to enlargement of the Brillouin zone in generating and subsequently modifying the band topology in a twisted bilayer $α$-$T_3$ system. The pronounced degeneracy of the emergent flat band in the dice limit of the $α$-$T_3$ lattice is removed on alignment with h-BN layers, resulting in the formation of sub-bands with varying topological characteristics. Remarkably, while the sub-band near charge neutrality exhibits a trivial behavior, a topologically non-degenerate singular sub-band emerges away from charge neutrality. The topological band remains isolated from the rest of the bands for a substantial area of the $α- θ$ plane (where $α$ and $θ$ correspond to the hopping ratio and twist angle respectively) while exhibiting multiple phase transitions as a function of the aforementioned parameters via hybridization with its nearest bands. We study the evolution of the hybrid Wannier charge center and the Chern number to characterize the different emergent topological phases. Finally, the degree of flatness of the topological band is studied as a function of both $α$ and $θ$ to explicitly show the influence of quantum interference and band folding on the width of the topological band.

cond-mat.mes-hall↗

arXiv2026

Emergence of Non-Hermitian Magic Angles and Topological Phase Transitions in Twisted Bilayer $α$-$T_3$ Lattices

We investigate the flat-band properties and topological phase transitions in a non-Hermitian twisted bilayer $α-T_3$ lattice. Here, non-Hermiticity is introduced via Hatano-Nelson-type asymmetric hopping, while an aligned hexagonal boron nitride substrate provides a staggered sublattice mass to the system. We find that the introduction of non-reciprocal hopping splits the conventional single magic angle into three distinct non-Hermitian magic angles (NHMAs). Unlike the exceptional magic angles driven by spectral singularities, these NHMAs host perfectly isolated flat bands where the real and imaginary parts of the bandwidth simultaneously vanish. By mapping the complex eigenspectrum across the moiré Brillouin zone, we show that the scattered energy eigenvalues coalesce into well-defined, closed loop-like structures as the non-Hermitian parameter strength increases, indicating emergence of a nontrivial point-gap topology and hence the non-Hermitian skin effect. Furthermore, we characterize the topological phases by computing the direct band gap and the biorthogonal Chern number. While the system exhibits a transition to a higher topological phase at weak non-Hermiticity, we demonstrate that stronger non-Hermiticity drives the gap-closing boundaries to merge and their topological charges to mutually annihilate. This convergence results in a trivial gap closing and a complete suppression of the intermediate topological phase, confirming that non-Hermiticity fundamentally plays a crucial role with regard to destabilizing the robust topological features of this moiré system.

cond-mat.mes-hall↗

arXiv2026

Second-order Skin Effect in a Brick-Wall Lattice

Non-Hermitian skin effect, which is a unique feature of non-Hermitian systems, exhibits the formation of an extensive number of boundary modes under open boundary conditions. However, its manifestation in higher dimensions remains elusive. In our work, we demonstrate a hybrid skin-topological effect arising from the interplay between first-order band topology and non-reciprocal hopping in an engineered two-dimensional brick-wall geometry. The non-Hermitian brick-wall lattice under open boundary conditions in both directions exhibits several unconventional spectral features. Notably, the eigenvalues associated with the corner skin modes do not exhibit non-trivial windings in the complex energy plane; instead, they exhibit dynamically stable exceptional point-like features that do not originate from eigenvector coalescence. In contrast, the remaining modes accumulate at the opposite pair. Of all the corner skin modes, only the two that originate from the topological corner states of the Hermitian brick-wall lattice remain localized at individual corners, while the rest accumulate at the pair of opposite corners. This spatial distribution contrasts sharply with the second-order skin effect, where corner skin modes are more uniformly distributed. Finally, for the non-Hermitian Brick-wall lattice, we design and implement the corresponding topolectrical circuit (circuit for a square lattice is included for comparison) to directly visualize the hybrid skin-topological modes.

cond-mat.mes-hall↗

arXiv2025

Topological defect-mediated corner states and higher-order bulk topology in a two-dimensional crystalline insulator

We report appearance of non-trivial zero energy corner modes in the form of topological defects (trimers) in a carefully designed 2D crystalline topological insulator. The proposed scenario is developed via an unconventional stacking of 1D topological atomic chains with crystalline mirror symmetry along the diagonal (y=x) line. Our analysis shows that by systematically varying the hopping parameters t (intra-chain), v (within the unit cell) and w (between the unit cells) the system exhibits more than one distinct non-trivial second order topological phases. These phases are distinguished by the zero energy corner modes. In one of these phases the system supports four zero modes. Two of them reside on the trimers and the rest on isolated sites situated at the corner along the diagonal line. However, in the second case, the zero modes on the isolated sites persist at the corners while the zero modes on the trimers vanish. A critical look at the phase evolution of the Bloch states helps in investigating the topology of these phases via using winding numbers. Our work also shows the bulk-corner correspondence that exist between the invariants and the zero modes at the corners. With four zero modes at the corners and a winding number as 2, we conclude that the system has transformed into a second order topological insulator via tuning of the hopping amplitudes.

cond-mat.str-el↗

arXiv2025

Emergent topological phases and coexistence of gapless and spectral-localized Floquet quantum spin Hall states via electron-phonon interaction

In this work, a thorough exploration has been carried out to unravel the role of electron-phonon interaction (EPI) in a Bernevig-Hughes-Zhang (BHZ) quantum spin Hall (QSH) insulator subjected to a time-periodic step drive. It is observed that upon inclusion of the EPI, the system demonstrates emergent Floquet QSH (FQSH) phases and several topological phase transitions therein, mediated solely by the interaction strength. Quite intriguingly, the emergence of topological zero ($π$) modes in the bulk that remains otherwise gapless in the vicinity of the $π$ (zero) energy sector is observed, thus serving as a prime candidate of robust topology in gapless systems. With other invariants being found to be deficient in characterizing such coexistent phases, a spectral localizer (SL) is employed, which distinctly ascertains the nature of the (zero or $π$) edge modes. Following the SL prescription, a real-space Chern marker computed by us further provides support to such \textit{gapless} Floquet topological scenario. Our results can be realized in advanced optical setups that may underscore the importance of EPI-induced Floquet features.

cond-mat.mes-hall↗

arXiv2024

Competing topological phases in a non-Hermitian time-reversal symmetry-broken Bernevig-Hughes-Zhang model

The Bernevig-Hughes-Zhang (BHZ) model, which serves as a cornerstone in the study of the quantum spin Hall insulators, showcases robust spin-filtered helical edge states in a nanoribbon geometry. In the presence of an in-plane magnetic field, these (first-order) helical states gap out to be replaced by second-order corner states under suitable open-boundary conditions. Here, we show that the inclusion of a spin-dependent non-Hermitian balanced gain/loss potential induces a competition between these first and second-order topological phases. Surprisingly, the previously dormant first-order helical edge states in the nanoribbon resurface as the non-Hermitian effect intensifies, effectively neutralizing the role played by the magnetic field. By employing the projected spin spectra and the spin Chern number, we conclusively explain the resurgence of the first-order topological properties in the time-reversal symmetry-broken BHZ model in presence of non-Hermiticity. Finally, the biorthogonal spin-resolved Berry phase, exhibiting a non-trivial winding, definitively establishes the topological nature of these revived edge states, emphasizing the dominance of non-Hermiticity over the magnetic field.

cond-mat.mes-hall↗

arXiv2024

Holstein polaron in a pseudospin-$1$ quantum spin Hall system: first and second order topological phase transitions

We theoretically propose the occurrence of a quantum spin Hall (QSH) and a second order topological phase transition (TPT) driven by electron-phonon (e-p) coupling in a pseudospin-$1$ fermionic system on an $α$-$T_3$ lattice. Our model is formulated in the spirit of the Kane-Mele model modified by the Holstein Hamiltonian. The Lang-Firsov approach is employed to describe polarons reasonably well in the anti-adiabatic (high frequency) limit and to obtain an effective electronic Hamiltonian. It is shown that the system possesses topologically nontrivial phases up to a critical e-p coupling, $λ_c$ and are characterized by the helical QSH edge states along with a non-zero $\mathbb{Z}_2$ invariant for a certain range of $α$. The topological phase vanishes beyond $λ_c$ and is accompanied by a bulk gap closing transition at $λ_c$, manifesting a TPT. We observe a more intriguing phenomenon for higher values of $α$, where the system exhibits TPTs supported by two distinct gap closing transitions at $λ_{c_1}$ and $λ_{c_2}$, while a slim region at slightly lower values hosts a semi-metallic signature below $λ_{c_1}$. Subsequently, to explore more intricate features, we introduce a time reversal symmetry breaking magnetic field to trigger the formation of a second order topological phase. The magnetic field, by construction causes a boundary dependent gapping out of the edge states, consequently giving rise to robust corner modes in a tailored open boundary conditions. We justify the formation of the higher order phase by employing an appropriate invariant, namely the projected spin Chern number. Finally, we show that the e-p coupling significantly influences the corner modes (and also the real space energy bandstructure), corroborating a higher order TPT as we tune $λ$ beyond a critical value for a given value of $α$.

cond-mat.mes-hall↗

arXiv2024

Quasiperiodic potential induced corner states in a quadrupolar insulator

We systematically investigate the topological and localization properties of a quadrupolar insulator represented by the celebrated Benalcazar-Bernevig-Hughes model in presence of a quasiperiodic disorder instilled in its hopping amplitude. While disorder can be detrimental to the existence of the topological order in a system, we observe the emergence of a disorder driven topological phase where the original (clean) system demonstrates trivial behavior. This phenomenon is confirmed by the re-emergence of zero energy states in the bandstructure together with a non-zero bulk quadrupole moment, which in turn establishes the bulk boundary correspondence (BBC). Furthermore, the distribution of the excess electronic charge shows a pattern that is reminiscent of the bulk quadrupole topology. To delve into the localization properties of the mid-band states, we compute the inverse participation and normalized participation ratios. It is observed that the in-gap states become critical (multifractal) at the point that discerns a transition from a topological localized to a trivial localized phase. Finally, we carry out a similar investigation to ascertain the effect of the quasiperiodic disorder on the quadrupolar insulator when the model exhibits topological properties in the absence of disorder. Again, we note a multifractal behavior of the eigenstates in the vicinity of the transition.

cond-mat.mes-hall↗

arXiv2023

Wannier charge center, spin resolved bulk polarization and corner modes in a strained quantum spin Hall insulator

Topological invariants are a significant ingredient in the study of topological phases of matter that intertwines the supposedly contradicting concepts of bulk and boundary. The nature of the invariants differ depending on the dimension of the boundary at which the topological states manifest themselves. The primary motivation of this work is to study two distinct scenarios of topological phase, differing in the dimensionality of their boundary states and study the associated bulk topological invariants that characterize them. In this regard, we study the band engineered Kane Mele model which originally is a prototypical example of a system that hosts quantum spin Hall effect on a honeycomb lattice. Under a smooth band deformation caused by varying one of the nearest neighbor hopping amplitudes (say $t_1$) as compared to the other two (say $t$), we observe that the system transits from its first order topological insulating state (or quantum spin Hall state) to a second order topological insulating (SOTI) state via a gap closing transition. This transition occurs when the system crosses a particular threshold of the deformation parameter $t_1\mathbin{/}t$ (namely $t_1\mathbin{/}t=2$). We show the presence of edge and corner modes as a signature of first and second order topology respectively. Further, we observe the evolution of the Wannier charge center (WCC), a bulk property as a function of the deformation parameter ${t_1}\mathbin{/}{t}$. We also find that, while the $\mathbb{Z}_2$ invariant successfully characterizes the QSH state, it cannot characterize higher order topology (second order here). The model being mirror invariant, we also calculate mirror winding number to show that it is rendered trivial in the SOTI phase as well, while being non-trivial in the QSH phase. Finally, spin resolved bulk polarization establishes the SOTI phase as obstructed atomic insulator.

cond-mat.mes-hall↗

arXiv2023

Higher order topology in a band deformed Haldane model

Haldane model is a celebrated tight binding toy model in a 2D honeycomb lattice that exhibits quantized Hall conductance in the absence of an external magnetic field. In our work, we deform the bands of the Haldane model smoothly by varying one of its three nearest neighbour hopping amplitudes ($t_1$), while keeping the other two ($t$) fixed. This breaks the $C_3$ symmetry of the Hamiltonian, while the $M_x*T$ symmetry is preserved. The symmetry breaking causes the Dirac cones to shift from the K and the K' points in the Brillouin zone (BZ) to an intermediate M point. This is evident from the Berry curvature plots which show a similar shift in the corresponding values as a function of $\frac{t_1}{t}$. We observe two different topological phases, one being a topological insulator (TI) phase and the other is a higher order topological insulator (HOTI). The Chern number ($C$) remains perfectly quantized at a value of $C=1$ for the TI phase and goes to zero in the HOTI phase. Furthermore the evolution of the Wannier charge center (WCC) as the band is deformed shows a jump in the TI phase indicating a non-trivial bulk. We also study the HOTI phase and diagonalize the real space Hamiltonian on a rhombic supercell to show the presence of in-gap zero energy corner modes. The polarization of the system, namely $p_x$ and $p_y$, are evaluated, along the $x$ and the $y$ directions respectively. We see that both $p_x$ and $p_y$ are quantized in the HOTI phase owing to the presence of the inversion symmetry of the system.

cond-mat.mes-hall↗

arXiv2022

Higher order topology in a Creutz ladder

A Creutz ladder, is a quasi one dimensional system hosting robust topological phases with localized edge modes protected by different symmetries such as inversion, chiral and particle-hole symmetries. Non-trivial topology is observed in a large region of the parameter space defined by the horizontal, diagonal and vertical hopping ampitudes and a transverse magnetic flux that threads through the ladder. In this work, we investigate higher order topology in a two dimensional extrapolated version of the Creutz ladder. To explore the topological phases, we consider two different configurations, namely a torus (periodic boundary) and a ribbon (open boundary) to look for hints of gap closing phase transitions. We also associate suitable topological invariants to characterize the non-trivial sectors. Further, we find that the resultant phase diagram hosts two different topological phases, one where the higher order topological excitations are realized in the form of robust corner modes, along with (usual) first order excitations demonstrated via the presence of edge modes in a finite lattice, for the other.

cond-mat.other↗