arXiv subjects

Tanay Nag

Publications and source records attributed to Tanay Nag.

At least 19 recordsLinked to original sources

arXiv2026

Localization and Transport in a Non-Hermitian Hexagonal Harper Model

We investigate a one-dimensional non-Hermitian hexagonal Harper model with quasiperiodically modulated hopping amplitudes. In the Hermitian limit, the model exhibits metallic, insulating, and multifractal phases characterized by distinct eigenstate properties. Upon introducing non- Hermiticity, the phase diagram is qualitatively altered, with an expansion of metallic regions and strong boundary sensitivity arising from the non-Hermitian skin effect. By analyzing wave-packet dynamics, we uncover qualitatively distinct transport signatures in metallic, multifractal, and in- sulating regimes. In the metallic region, nonreciprocal hopping induces finite sliding, resulting in ballistic center-of-mass motion that is absent in the Hermitian model, while wave-packet spreading is simultaneously suppressed and exhibits diffusive scaling. Interestingly, the multifractal regime emerges as a distinct dynamical phase supporting both enhanced spreading and finite sliding, both primarily of superdiffusive nature, in contrast to metallic regions where sliding (spreading) shows ballistic (diffusive) scaling. These features are markedly different from their Hermitian counterpart. On the other hand, in the insulating region, both the spreading and sliding are strongly suppressed. We reconfirm these intriguing transport characteristics by investigating the distinct growth profile of single-particle entanglement entropy where the effect of spreading of the wave-packet is clearly manifested. These results demonstrate that quasiperiodicity in hopping amplitudes, combined with non-Hermiticity, establishes the multifractal regime as a key mediator of transport.

cond-mat.dis-nn↗

arXiv2026

Topological Hall plateau in quasi-2D kagome magnet YMn$_6$Sn$_6$

We examine the impact of the Dzyaloshinskii-Moriya interaction (DMI) in kagome magnets and show that a predominantly planar DMI together with ferromagnetic exchange stabilizes a disordered skyrmion phase in quasi-two-dimensional (2D) YMn$_6$Sn$_6$. Within an ab initio framework combining density functional theory and spin-dynamics simulations, we generate realistic spin textures of disordered skyrmion and find that this phase persists for $B_{ext} < 0.5$ T, with a decreasing skyrmion size as magnetic field increases. We demonstrate the emergence of topological Hall plateau in the range $-0.5 \leq B_{ext} < 0.5$ T, driven by nearly uniform scalar spin chirality and the resulting constant real-space Berry curvature. This response is anti-symmetric with magnetic field while magnitude and sign of these plateau are determined by a complex interplay between Hund's coupling strength and chemical potential signifying the role of Dirac points and van Hove singularities. In addition, we reveal topological magnon excitations in the disordered skyrmion phase of quasi-2D YMn$_6$Sn$_6$.

cond-mat.mtrl-sci↗

arXiv2026

Spin-orbit coupling driven topological superconductivity in twisted bilayer graphene-WSe$_2$ heterostructures

Commencing from the low-energy Bistritzer-MacDonald continuum model of twisted bi-layer graphene (tBLG) with proximity-induced Ising, Rashba, and intrinsic spin-orbit couplings (SOCs), we construct the corresponding Bogoliubov-de Gennes Hamiltonian with conventional $s$-wave pairing and theoretically investigate the emergence of topological superconductivity in it. The latter can possibly be experimentally demonstrated in tBLG, Niobium and tungsten diselenide heterostructures. The topological superconducting phases, bearing an effective $p$-wave pairing profile and exhibiting inverted band dispersion, are protected by a bulk gap and are topologically characterized by Chern numbers. In the absence of intrinsic SOC, variation of the twist angle and other SOC strengths yield extended gapless, trivial, and topological phases, with phase boundaries exactly matching the closing of direct band gaps. The topological regime exhibits clear band inversion in the combined particle-hole and spin space, along with distinct Bloch localization profiles compared to the trivial phase. Including intrinsic SOC generates additional topological phases and eliminates the gapless phase, indicating the enhanced stability of the gapped topological superconducting regime in tBLG.

cond-mat.mes-hall↗

arXiv2026

Superconducting diode effect via Floquet topological Fulde-Ferrell phase in driven Rashba nanowire

Much has been studied on Floquet engineering in Rashba nanowire model regarding topological superconductivity hosting Majorana $0$- and anomalous $\pi$-modes, here we theoretically investigate the possible emergence of finite momentum Fulde-Ferrell (FF) superconducting state in quasi-energy of the above model under the periodic modulation of in-plane and out-of-plane magnetic fields while the static limit does not host a FF ground state. We demonstrate controllable switching between Floquet Majorana $0$- and $\pi$-modes via reversal of the supercurrent direction, revealing pronounced nonreciprocal supercurrent signatures which is a manifestation of the FF pairing. We validate the onset of FF pairing following a self-consistent mean-field analysis where externally applied supercurrent facilitates nonreciprocal signatures in quasi-energy spectra. The above findings directly indicates to the intriguing phenomenon of superconducting diode effect (SDE). The drive amplitude serves as a parameter to regulate the diode efficiency with chemical potential. Our study thus reveals a rich interplay between Floquet topological superconductivity and finite-momentum FF pairing, providing a tunable way to switch between different Floquet Majorana modes and realize the SDE with high efficiency.

cond-mat.mes-hall↗

arXiv2026

Quantized orbital and spin Hall transport: interplay between $sp$-hybridization, altermagnetism and spin-orbit coupling

We here explore the emergence of orbital and spin Hall effects, originating beyond the $L$-$S$ coupling, and investigate the interplay between inter-orbit hybridization, relativistic Rashba spin-orbit coupling (SOC), and non-relativistic SOC, namely altermagnetic (AM) order, in a two-dimensional model Hamiltonian. The orbital (spin) Hall responses are remarkably found to be quantized within a window of Fermi energy when the strength of AM order (Rashba SOC) exceeds (falls below) the scale set by $sp$-hybridization. Importantly, orbital and spin Hall quantizations are independent of Rashba SOC and AM order, respectively, while the uniform profiles of finite orbital and vanishingly small spin moments of bands around the Fermi energy. The microscopic origin of such quantization comes from the Fermi surface-activated orbital and spin Berry curvatures. The extent of the quantized regime is strongly controlled by the intra-orbital coupling strength. As the temperature increases, the quantization is significantly compromised in the spin Hall case. We extend our analysis to the orbital and spin Nernst coefficients where the pronounced dip-peak structures signal the existence of the quantization leading to experimental relevance.

cond-mat.mes-hall↗

arXiv2026

Finite-size scaling properties of classical random walk on various two-dimensional lattices

We consider various two-dimensional lattices such as square, Kagome, Lieb, honeycomb, dice lattices of finite extent, to study the effect of lattice profile in terms of the number of nearest neighbour and connectivity patterns on the classical random walk in the unbiased scenario. We find that the standard deviation of distance travelled by the walker is insensitive to the non-uniformity of the lattice profile leading to diffusive transport even in the finite size lattices. Our study indicates that the mass fractal dimension varies within a window $1.50\pm 0.03$ for all finite-size lattices. A weak ordering within the above window, correlated with the average coordination number, is observed, while Lieb and square lattices yielding the minimum and maximum values, respectively. However, confidence intervals reveal substantial statistical overlap for several lattice pairs even though the lattice profiles vary as far as the average number of connecting bonds and directionality of bonds are concerned. We also study the scaling complexity of the circumference of the closed curve traced by the walker while investigating the hull dimension. We find similar trend for hull fractal dimension as well and that was found to within the window $1.37\pm 0.03$ for finite-size lattices. Within the above window, the ordering remains qualitatively unaltered as compared to mass dimension while the confidence interval rectifies the order quantitatively. The square lattice clearly exhibits the upper bound for hull fractal dimension and the remaining lattices show extensive statistical overlap within the above window. We exhibit a tendency of the mass and hull fractal dimension to reach their thermodynamic values given by Brownian motion when we allow more number of steps within the finite size of the lattice, as confirmed by a data collapse analysis.

cond-mat.stat-mech↗

arXiv2026

Semiclassical theory of frequency dependent linear magneto-optical transport in Weyl semimetals

We develop a semiclassical Boltzmann theory for frequency-dependent magneto-optical transport in Weyl semimetals (WSMs), incorporating momentum-dependent relaxation via a scattering matrix approach. The interplay of orbital magnetic moment, Weyl cone tilt, intervalley scattering, and electromagnetic driving is analyzed to obtain the full conductivity tensor in the presence of a static magnetic field. For untilted WSMs with orbital magnetic moment, strong intervalley scattering in the weak ac regime induces a sign reversal of the longitudinal magneto-optical conductivity (LMOC), thereby suppressing the chiral anomaly. In contrast, in the strong ac regime, intervalley scattering fails to neutralize the chiral imbalance within a driving cycle, and no sign reversal is observed. Orbital magnetic moment induces linear magnetic-field contributions, while chiral anomaly yields quadratic response accompanied by expected angular profiles. Tilt direction and orientation strongly affect LMOC such as, transverse tilt gives symmetric non-monotonic behavior, whereas parallel tilt leads to asymmetric, nearly monotonic response. Notably, negative LMOC arises intrinsically for parallel tilt, but requires orbital magnetic moment for transverse tilt. These results highlight frequency-dependent conductivity as a sensitive probe of chiral relaxation in MHz-THz magneto-optical experiments.

cond-mat.mes-hall↗

arXiv2025

Chiral topological superconductivity in twisted bilayer and double bilayer graphene

We present a theoretical investigation of the emergence of chiral topological superconductivity in small-angle twisted bilayer graphene (tBLG) and twisted double bilayer graphene (tDBLG). Using the low-energy continuum model and incorporating spin-triplet $p_{x}+i p_{y}$ pairing in each graphene layer, we construct the effective models for both tBLG and tDBLG with superconductivity. By varying the chemical potential, superconducting order parameter, and twist angle, we explore the emergence of topological superconducting phases via the calculation of Chern numbers. Our phase diagrams for tBLG and tDBLG (both AB-AB and AB-BA stackings) reveal distinct topological transitions, which are consistently marked by bulk gap-closing points. To gain further insight, we analyze the evolution of Chern numbers by tracking the number and location of gap closings within the moir\'e Brillouin zone. Additionally, we illustrate representative squared amplitude of Bloch states corresponding to different topological phases. In the later part of our study, the effect of trigonal warping on the topological superconducting properties is also discussed. Beyond the quantitative results, our study highlights how the interplay between moir\'e band structure and unconventional pairing symmetries enriches the landscape of possible superconducting states in twisted graphene systems. The framework developed here may also be extended to other multilayer moir\'e materials, offering a route towards engineering exotic topological superconductivity with tunable parameters.

cond-mat.mes-hall↗

arXiv2025

Field-free diode effects in one-dimensional superconductor: a complex interplay between Fulde-Ferrell pairing and altermagnetism

We investigate the emergence of nonreciprocal dissipationless supercurrents in one-dimension manifested through the superconducting diode effect (SDE) and the Josephson diode effect (JDE) in the absence of any external magnetic field, where inversion symmetry (IS) and time-reversal symmetry (TRS) can be intrinsically broken by spin-orbit coupling (SOC), and altermagnetism (AM), respectively. We investigate Ising and Rashba SOC separately in two models where two-component AM, assembled with crystallographic angle, can lead to qualitatively similar indirect band-gap closing and non-reciprocal supercurrent in a Fulde-Ferrell (FF) superconductor. Interestingly, in the absence of the above SOCs, SDE persists and the sign of efficiency can be altered by tuning the angle only. Parallel spin components with complementary momentum functions, ensuring the breaking of IS and TRS, can induce SDE in the presence of FF pairing. Continuing the analysis in the context of JDE, we explore the interplay between SOCs and AM with the p-wave and Fulde-Ferrell superconductivity in three different setups. The bulk bands contributes to the non-reciprocity in the case of p-wave superconductivity while JDE is dominated by Andreev bound states for FF superconductivity. Importantly, JDE continues to exist due to finite momentum Cooper pair even without AM and SOC unlike the p-wave superconductivity. The sign of JD efficiency can be tuned with angle for p-wave superconductivity while absence of such sign reversal is a hallmark signature of FF superconductivity. Similar to SDE, parallel spin components in conjunction with p-wave superconductivity can lead to JDE that can also be mediated by only FF pairing in the absence of SOC and AM.

cond-mat.mes-hall↗

arXiv2025

Topological superconductivity and superconducting diode effect mediated via unconventional magnet and Ising spin-orbit coupling

We propose a theoretical framework in which a one-dimensional (1D) tight-binding model incorporating unconventional magnetic order together with Rashba and Ising spin-orbit couplings are considered to realize two key phenomena in condensed matter systems: topological superconductivity and the superconducting diode effect (SDE). We first elucidate the underlying band topology of the normal-state Hamiltonian and subsequently introduce an on-site attractive Hubbard interaction. Performing a a self-consistent mean-field analysis, we establish superconducting order parameters in both the conventional Bardeen-Cooper-Schrieffer (BCS) and finite-momentum Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) pairing channels. Intriguingly, both pairing states can support topological superconductivity, characterized by a nontrivial winding number, and lead to the emergence of four zero-energy Majorana modes localized at the ends of the 1D chain. The FFLO state further gives rise to an intrinsic field-free SDE, manifested as a nonreciprocal supercurrent and quantified by the diode efficiency $\eta$. Notably, our model yields a large diode efficiency $\eta \sim 65\%$, highlighting its potential for realising topological superconductivity and highly efficient superconducting devices.

cond-mat.mes-hall↗

arXiv2025

Generation of concurrence in a generalized central spin model with a three-spin interacting environment

We consider the three-spin Ising model to study the effect of three-spin interacting term on bi-partitie entanglement between adjacent spins. The three-dominated disordered region has tri-partite entanglement causing a vanishingly small concurrence, while it acquires maximum value around the critical points. Considering the above model as an environment, we construct a generalized central spin model where two central spins, initially in an unentangled pure state, are coupled locally to two distinct sites of the environmental spin chain. We study the generation of mixed state entanglement between the central spins when the transverse field of the environment is kept fixed, and suddenly quenched, referring to equilibrium and non-equilibrium dynamics of the central spins, respectively. For the critical environment in the equilibrium, the concurrence shows a dip-revival structure governed by quasi-particle movement. In the non-equilibrium study, we find an initial growth of concurrence followed by a two-stage fall for the inter-phase quench which is governed by dynamic decoherence channels. The central spins are maximally entangled for a quench in the vicinity of a multicritical point, which arises due to three-spin interaction only. The concurrence becomes long-lived for an intra-phase quench, and this sustainability depends on the strength of the three-spin interaction. Therefore, the three-spin interaction indeed helps in generating bi-partite entanglement in the central spins.

cond-mat.stat-mech↗

arXiv2025

Adiabatic charge transport through non-Bloch bands

We explore the non-reciprocal intracell hopping mediated non-Hermitian topological phases of an extended Su-Schrieffer-Heeger model hosting second-nearest-neighbour hopping. We microscopically analyze the phase boundaries using the non-Bloch momentum while the off-critical (critical) phases are directly associated with the gapped (gapless) nature of the non-Bloch bands that we derive from the characteristic equation using the gauge freedom. The non-Bloch momentum accurately reflects the bulk boundary correspondence (BBC) explaining the winding number profile under open boundary conditions. We examine the adiabatic dynamics to promote the concept of adiabatic charge transport in a non-Hermitian scenario justifying the BBC in spatio-temporal Bott index and non-Bloch Chern number. Once the non-Bloch bands experience no (a) gap-closing during the evolution of time, quantized flow of is preserved (broken). Our study systematically unifies the concept of non-Bloch bands for both static and driven situations.

cond-mat.mes-hall↗

arXiv2025

The orbital-driven topological phase transition and planar Hall responses in ternary tellurides Weyl semi-metals

We study electronic properties of the ternary tellurides TaXTe$_4$ (X=Rh, Ir) using density functional theory and investigate chiral anomaly mediated planar Hall response from ab initio calculations. We show that TaRhTe$_4$ is a hybrid Weyl semimetal (WSM), hosting Weyl points (WPs) of both type-I, type-II, and TaIrTe$_4$ is a type-I WSM in absence of spin-orbit coupling (SOC). TaRhTe$_4$ continues to remain a hybrid WSM while TaIrTe$_4$ converts into a type-II WSM under the application of SOC. We observe long Fermi arcs connecting WPs of opposite chirality. We report orbital-driven topological phase transition in ternary tellurides. The WSM phases in TaXTe$_4$ are controlled by the orbital character of the $d_{xz}$ and $d_{z^2}$ states of X=Ir/Rh atoms. Replacing Rh with Ir enhances the $d_{z^2}$ orbital contribution near the Fermi level at the expense of $d_{xz}$ states. This transforms the type-I WPs into type-II resulting in a conversion of hybrid WSM TaRhTe$_4$ to type-II WSM TaIrTe$_4$. This systematic study opens new routes for engineering topological materials relying beyond strong SOC and sheds light on the effect of orbital degree of freedom on the electronic properties of tellurides. We further report an enhancement of planar Hall effects due to orbital-driven topological phase transition in TaXTe$_4$ and we make resort to a tight-binding model to correlate the above findings with the velocity modulated off-diagonal effective mass anisotropy in different types of WSMs.

cond-mat.mtrl-sci↗

arXiv2025

Topological energy pumping in a quasi-periodically driven four-level system

We investigate a quasi-periodically driven four-level system that serves as a temporal analog of topological phenomena found in four-band models with intertwined spin and orbital degrees of freedom. Under a two-tone drive in the strong-driving regime, the system realizes a two-dimensional synthetic Floquet lattice, thus facilitating the realization of topological energy pumping. For a temporal quantum spin Hall insulator, we find that the rates of emission and absorption of energy between the two drives are not exactly opposite for a given band. However, when contributions from two chiral symmetric partner bands are added, they become exactly opposite. This quantized rate of energy exchange is a direct consequence of propagating edge modes in the real-space model, which we further characterize by computing the spin-Chern number. Interestingly, our analysis yields zero rate of exchange of energy between the drives for a temporal higher-order topological insulator, suggesting the presence of localized corner modes that we characterize by the mid-gap Wannier spectra. {Our findings uncover the role of chiral, particle-hole and time reversal symmetries on the energy dynamics in temporal quantum spin Hall and higher-order topological insulators.} Finally, we demonstrate that the perfect (imperfect) nature of the fidelity during the time-evolution of the system serves as a characteristic signature of a topological (trivial) phase.

cond-mat.mes-hall↗

arXiv2025

Dissipation induced Majarona $0$- and $\pi$-modes in a driven Rashba nanowire

Periodic drive is an intriguing way of creating topological phases in a non-topological setup. However, most systems are often studied as a closed system, despite being always in contact with the environment, which induces dissipation. Here, we investigate a periodically driven Rashba nanowire in proximity to an $s$-wave superconductor in a dissipative background. The system's dynamics is governed by a periodic Liouvillian operator, from which we construct the Liouvillian time-evolution operator and use the third-quantization method to obtain the `Floquet damping matrix', which captures the spectral and topological properties of the system. We show that the system exhibits edge-localized topological Majorana $0$-modes (MZMs) and $\pi$-modes (MPMs). Additionally, the system also supports a trivial $0$-modes (TZMs) and $\pi$-modes (TPMs), which are also localized at the edges of the system. The MZMs and the MPMs are connected to the bulk topology and carry a bulk topological invariant, while the emergence of TZMs and TPMs is primarily tied to exceptional points and is topologically trivial. We show that both the topological (MZMs and MPMs) and trivial (TZMs and TPMs) edge modes are robust against onsite disorder. We study the topological phase diagrams in terms of the topological invariants and show that the dissipation can modify the topological phase diagram substantially and even induce topological phases in the system. Our work extends the understanding of a driven-dissipative topological superconductor.

cond-mat.mes-hall↗

arXiv2025

Josephson current signature of Floquet Majorana and topological accidental zero modes in altermagnet heterostructures

We theoretically investigate the generation and Josephson current signatures of Floquet Majorana end modes (FMEMs) in a periodically driven altermagnet (AM) heterostructure. Considering a one-dimensional (1D) Rashba nanowire (RNW) proximitized to a regular $s$-wave superconductor and a $d$-wave AM, we generate both $0$- and $\pi$-FMEMs by driving the nontopological phase of the static system. While the static counterpart hosts both topological Majorana zero modes (MZMs) and nontopological accidental zero modes (AZMs), the drive can gap out the static AZMs and generate robust $\pi$-FMEMs, termed as topological AZMs (TAZMs). We topologically characterize the emergent FMEMs via dynamical winding numbers exploiting chiral symmetry of the system. Moreover, we consider a periodically driven Josephson junction comprising of RNW/AM-based 1D topological superconduting setup. We identify the signature of MZMs and FMEMs utilizing $4\pi$-periodic Josephson effect, distinguishing them from trivial AZMs exhibiting $2\pi$-periodicty, in both static and driven platforms. This Josephson current signal due to Majorana modes survives even in presence of finite disorder. Our work establishes a route to realize and identify FMEMs in AM-based platforms through Floquet engineering and Josephson current response.

cond-mat.mes-hall↗

arXiv2025

Adiabatic charge transport in extended SSH models

We explore the topological properties of extended SSH models, considering four sub-lattices in a unit cell and second-nearest-neighbor intercell hopping for SSH4 and SSH long-range (SSHLR) models, respectively. The additional tuning parameters cause the SSH4 (SSHLR) model to host chiral symmetry protected two (two and four) zero-energy modes producing a richer phase diagram that we characterize by momentum space, periodic-bulk and open-bulk real space winding numbers. We introduce time to study charge transport in the periodically driven SSH4 and SSHLR models under the adiabatic limit. We remarkably find that the whole parameter space turned topological for a certain choice of the remaining parameters leading to always finite quantized value of pumped charge at the end of a complete cycle. Considering time as another variable, we characterize these new phases of the driven models by momentum space Chern number, periodic-bulk and open-bulk real space Bott index. We also investigate the time evolution of pumped charge for these models and connect it with the intriguing windings of the mid-gap energy levels with time. Interestingly, the maximum value of Chern number or Bott index for the driven models is more than that of the winding number associated with the static model indicating the fact that there exist more zero-energy modes during the full course of a driving cycle compared to the underlying static models. We further extend our study to the quantum metric where the fluctuations in the above quantity can identify the presence of a topological phase boundary.

cond-mat.mes-hall↗

arXiv2025

Extended Haldane model -- a modern gateway to topological insulators

The seminal Haldane model brings up a paradigm beyond the quantum Hall effect to look for a plethora of topological phases in the honeycomb and other lattices. Here we dwell into this model considering a full parameter space in the presence of spin-orbit interaction as well as Zeeman field such that the flavour of Kane-Mele model is invoked. Adopting this extended Haldane model as an example, we elucidate, in a transparent manner, a number of topological features in a pedagogical manner. First, we describe various first order topological insulator phases and their characterizations while explaining various anomalous quantum Hall effects and quantum spin Hall effects in the extended Haldane model. Second, we demonstrate the concepts of higher order topological insulator phases along with the topological invariants in the anisotropic limit of the extended Haldane model. At the end, we discuss various open issues involving \textcolor{black}{emergent or extended} symmetries that might lead to a broader understanding of various topological phases and the associated criteria behind their emergence.

cond-mat.mes-hall↗