arXiv subjects

Arijit Saha

Publications and source records attributed to Arijit Saha.

At least 19 recordsLinked to original sources

arXiv2026

Hierarchy of topological superconductivity generated via heterostructures of unconventional $p$-wave magnets

A theoretical framework is proposed to engineer both first and second-order topological superconducting phases in a two-dimensional (2D) heterostructure, consisting of a quantum spin Hall insulator (QSHI) and an unconventional $p$-wave magnet in presence of proximity-induced $s$-wave superconducting pairing. Our analysis establishes that the transitions between the trivial and topological superconducting (TSC) phases can be regulated though the parameters of $p$-wave magnet. Presence of chiral symmetry leads to the characterization of both types of TSC phases by the respective invariants, one-dimensional winding number and quadrupolar winding number. These results are supplemented by an analytical effective low-energy edge theory that yields a deeper insight into the emergence of the different topological phases of the system. Bulk pairing analysis reveals the competition between the effective $(p_x+p_y)$ and $(p_x+ip_y)$ type pairings that are governed by the intrinsic spin-orbit coupling inherited to the QSHI and spin-split bands of the $p$-wave magnet, respectively.

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arXiv2026

Quantum geometric signatures of Link-Unlink transitions and nonlinear Hall response in Hopf-link semimetals

Quantum geometry, comprising of quantum metric and Berry curvature, plays a significant role in the electronic transport properties of solids. In this work, we theoretically investigate the quantum geometric properties of a Hopf-link semimetal, a distinct topological class that is charecterized by a nodal link-unlink transition. We compute the interband optical conductivity of the Hopf link, which effectively distinguishes between linked and trivial phases. While recent studies establish quantum metric dipole-mediated scattering-free nonlinear Hall effect, this effect becomes even more fascinating in systems where the Berry-curvature-dipole contribution to nonlinear Hall conductivity vanishes. Owing to the underlying $PT$ symmetry of the Hopf-link semimetal, the Berry curvature and its corresponding contribution to the nonlinear Hall effect are entirely suppressed. Consequently, by introducing an appropriate perturbation, a finite nonlinear Hall conductivity emerges solely due to the quantum metric in the Hopf semimetal. Notably, this purely intrinsic, symmetry-driven nonlinear response remains entirely unmixed with extrinsic components.

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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.

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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.

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arXiv2026

Generation and time evolution of anomalous Floquet Majorana flat edge modes in two-dimensional noncolinear magnet-superconductor heterostructures

We theoretically investigate the realization of gapless Floquet topological superconducting phases in a two-dimensional magnet-superconductor heterostructure (2D Shiba lattice) in the presence of a harmonic drive implemented in the chemical potential. Employing a real-space tight-binding model, we obtain both the regular $0$- and anomalous $\pi$-Floquet Majorana flat edge modes (FMFEMs) in the quasi-energy spectrum. We also study the real-time evolution of the FMFEMs and analyze their local density of states in the presence of such a periodic drive. The topological characterization is performed using the winding number, exploiting the chiral symmetry of the equivalent bulk effective momentum-space Hamiltonian. This is also supported by the corresponding edge state spectra. Furthermore, we employ the Brillouin-Wigner (BW) and Floquet perturbation theory (FPT) to gain analytical insight into the problem. We compare our exact (numerical), BW, and FPT results in terms of the quasi-energy spectra obtained across different frequency regimes. We find good agreement between the exact numerical, BW, and FPT results in the higher-frequency and high-amplitude domain, particularly close to the $0$-quasi-energy modes.

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arXiv2026

Emergent superconducting phases in unconventional $p$-wave magnets: Topological superconductivity, Bogoliubov Fermi surfaces and superconducting diode effect

The recent discovery of unconventional momentum-dependent magnetic orders has expanded the landscape of magnetism beyond conventional ferromagnetism and antiferromagnetism. Among them, $p$-wave magnets ($p$WMs) represent a novel class of odd-parity, non-collinear compensated magnetic order that generates spin-split electronic bands. In this work, our theoretical investigation establishes $p$WMs as a versatile platform for realizing intriguing superconducting phases including topological superconductivity (TSC), Bogoliubov Fermi surfaces (BFSs), and superconducting diode effect (SDE), within a unified microscopic framework. Employing a minimal model incorporating $p$-wave magnetic order, exchange coupling, and Zeeman fields, we perform a self-consistent mean-field analysis and uncover a rich phase diagram featuring unconventional finite-momentum Fulde-Ferrell (FF) and Larkin-Ovchinnikov (LO) superconducting phases. Remarkably, we also show that $p$WMs can undergo a transition to a TSC phase anchoring Majorana flat edge modes, a hallmark of two-dimensional TSCs, even without Rashba spin-orbit coupling and Zeeman field. Upon applying a Zeeman field, gapless FF and LO phases emerge with BFSs characterized by the appearance of finite zero-energy quasiparticle density of states. Furthermore, we demonstrate that SDE arises naturally in the asymmetric FF phase. Our analysis manifests that $p$WMs serve as a unique and novel platform to host TSC phase, gapless superconducting states, and non-reciprocal transport phenomena.

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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.

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arXiv2025

Current switching behavior mediated via hinge modes in higher-order topological phases using altermagnets

We propose a theoretical framework to engineer hybrid-order and higher-order topological phases in three-dimensional topological insulators by coupling to $d$-wave altermagnets (AMs). Presence of only $d_{x^2-y^2}$-type AM drives the system into a hybrid-order topological phase where both first-order and second-order topological phases coexist. This phase is characterized by spectral analysis, low-energy surface theory, dipolar and quadrupolar winding numbers, and it's signature is further confirmed by two-terminal differential conductance calculations. Incorporation of the $d_{x^2-z^2}$-type AM drives the system into two second-order topological insulator phases hosting distinct type of hinge modes. These two variants of second-order topological phases are also topologically characterized by spectral analysis, topological invariants, low-energy surface thoery, and transport calculations. Importantly, the localization and direction of propagation of these one-dimensional hinge modes are controllable by tuning the relative strengths of the alermagnetic exchange orders. We utilize this feature to propose a tunable current-switching behaviour mediated via the hinge modes. Our results establish AMs based hybrid structure as a versatile platform for controllable higher-order topology and hinge-mediated device applications.

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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.

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arXiv2025

Proximity-induced superconductivity and emerging topological phases in altermagnet-based heterostructures

We present a theoretical framework for investigating superconducting proximity effect in altermagnet (AM)-superconductor (SC) heterostructures. In general, AMs, characterized by vanishing net magnetization but spin-split electronic spectra, provide a promising platform for realizing unconventional magnetic phases. We consider a two-dimensional $d$-wave AM proximity coupled to a three dimensional ordinary $s$-wave SC. By integrating out the superconducting degrees of freedom, we derive an effective Hamiltonian that describes the proximity-induced modifications in the AM layer in the form of a self-energy. We then derive an effective Green's function to obtain the proximity-induced pairing amplitudes in the AM layer and classify the induced pairing amplitudes according to their parity, frequency, and spin. We find the presence of even-parity singlet and triplet pairing amplitudes in the AM layer. To achieve the odd-parity triplet components, important to realize topological superconductivity, we introduce a layer of Rashba spin-orbit coupling (RSOC) in the heterostructure. We analyse the band topology of this proximity-induced AM-RSOC layer and demonstrate the emergence of both weak and strong topological superconducting phases with edge-localized modes, characterized by winding number and Chern number. These findings highlight the role of AM-SC hybrid setup as a versatile platform for realizing odd-parity triplet pairings and engineering topological superconductivity in two-dimension.

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arXiv2025

Spin polarization and diode effect in thermoelectric current through altermagnet-based superconductor heterostructures

The recent advent of a new class of magnetic material named as {\it altermagnet} (AM), characterized by a combination of momentum-dependent spin splitting with zero net magnetization, has opened up promising prospects for spintronic applications. We theoretically explore how the altermagnetic spin splitting affects the thermoelectric quasiparticle current in AM-based superconducting heterostructures. Our setup comprises of a bilayer system where a $d$-wave AM is proximity coupled to an ordinary $s$-wave superconductor (SC). We calculate the thermoelectric current carried by the quasiparticles applying a finite thermal bias across the junction. The behavior of the thermoelectric current with the system's base temperature and chemical potential is very similar to that in traditional SC heterostructures. Remarkably, the dissipative thermoelectric current found in the AM junction is spin split and thus generates finite spin polarization in the AM-based junction, which can approach $100\%$ spin polarization in the strong altermagnetic phase. We further investigate the thermoelectric current in AM-based Josephson junction (JJ) and illustrate how to achieve almost perfect diode effect in this AM-based JJ characterized by its efficiency $\sim 100\%$ with its sign decided by the strength of the AM, enhancing the potential for spin-caloritronics applications.

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arXiv2025

Field-free superconducting diode effect in two-dimensional Shiba lattices

The superconducting diode effect (SDE) refers to non-reciprocal transport, where current flows without resistance in one direction but becomes resistive in the opposite direction, but its typical reliance on magnetic field hinders scalability and device integration. In this article, we present a theoretical framework for realizing a field-free SDE based on a two-dimensional (2D) Shiba lattice featuring a conical spin texture. Using the real-space Bogoliubov-de Gennes (BdG) calculations, we illustrate that the conical spin configuration alone is sufficient to break the necessary inversion and time reversal symmetries, enabling nonreciprocal supercurrent flow without any external magnetic field, yielding diode efficiency exceeding 40%. Furthermore, we find that the efficiency of such a diode effect becomes strongly dependent on the direction of current flow, revealing a pronounced angular dependence that can be tuned by varying the pitches of the spin texture along the two spatial lattice directions. Our findings offer a pathway toward scalable, field-free superconducting components for non-dissipative electronics and quantum technologies.

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arXiv2025

Engineering second order topological superconductor hosting tunable Majorana corner modes in magnet/$d$-wave superconductor hybrid platform

We theoretically study the noncollinear magnetic texture effect on second-order topological superconductor (SOTSC) phase generated in unconventional $d$-wave superconductors and two-dimensional (2D) quantum spin Hall insulators (QSHI). While the interplay of the $d$-wave superconductor and QSHI has been studied as a platform to realize Majorana corner modes (MCMs), we show that the addition of the spin texture enables the tunability of these MCMs. Each corner of this hybrid system can host one or two Majorana modes depending on the system parameters, in particular, exchange strength and pitch vector of the spin texture. To characterize the higher order bulk topology, we compute the quadrupolar winding number, which directly corresponds to the number of MCMs acquiring a value of one for four corner modes and two for eight corner modes. We investigate and show the close resemblance in the topological phase diagrams obtained from the low energy effective Hamiltonian that reveals an emergent in-plane Zeeman field and spin-orbit coupling induced by the spin texture, and the real space tight binding lattice model. The microscopic pairing mechanism responsible for the appearance of SOTSC phase is investigated via an effective bulk pairing analysis, while a low-energy edge theory captures the mechanism behind tunability of MCMs. Our result paves the way for realizing SOTC with multiple MCMs which can be tuned via system parameters.

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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.

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arXiv2025

Non-Hermitian band topology in twisted bilayer graphene aligned with hexagonal boron nitride

Utilizing the established Bistritzer-MacDonald model for twisted bilayer graphene (tBLG), we theoretically investigate the non-Hermitian (NH) topological properties of this in the presence of non-reciprocal (NR) hopping on both layers and hexagonal boron nitride (hBN) induced mass term incorporated only on the top layer of the tBLG system. It is well known that the hBN mass term breaks the $C_{2}$ symmetry of tBLG and gaps out the Dirac cones inducing a valley Hall insulating phase. However, when NR hopping is introduced, this system transits into a NH valley Hall insulator (NH-VHI). Our analysis reveals that, in the chiral limit, the bandwidth of the system vanishes under NH effects for a wide range of twist angles. Such range can be visibly expanded as we enhance the degree of non-Hermiticity ($\beta$). At the magic angle, we observe that enhancement of $\beta$ inflates the robustness of the gapless Dirac points, requiring a progressively larger mass term to induce a gap in the NH tBLG system. Additionally, for a fixed NH parameter, we identify a range of twist angles where gap formation is significantly obstructed. To explore the topological aspects of the NH tBLG, we analyze the direct band gap in the Moir\'e Brillouin zone (mBZ) and compute the Chern number for the NH system. We find that the corresponding topological phase transitions are associated with corresponding direct band gap closings in the mBZ.

cond-mat.mes-hall↗

arXiv2025

Identifying Majorana edge and end modes in a Josephson junction of a $p$-wave superconductor with a magnetic barrier

We propose a theoretical model describing a Josephson junction featuring a magnetically textured barrier within two-dimensional (2D) $p$-wave superconductor, considering both $p_x + p_y$ and $p_x + ip_y$ type pairing symmetries. Our study reveals the influence of the magnetic barrier strength and its spatial periodicity on the system's topological properties, in terms of local density of states and Josephson current calculations. Notably, we demonstrate that these parameters regulate the number of Majorana zero modes at the junction in the topological regime. Our setup further allows for the identification of three distinct topological phases, the differentiation between one-dimensional (1D) Majorana edge (either flat/dispersive and arising from intrinsic 2D $p$-wave pairing) and localized Majorana end modes, and an analysis of their hybridization through the Josephson current. In particular, the Josephson current exhibits a discontinuous jump due to the edge modes and pronounced hump in the $p_x + ip_y$ pairing case, directly linked to the hybridized Majorana modes. Moreover, our study opens a possible interesting avenue to distinguish between 1D Majorana edge modes and zero-dimensional end modes via Josephson current signatures.

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arXiv2025

Thermoelectric properties of magic angle twisted bilayer graphene-superconductor hetero-junction: effect of valley polarization and trigonal warping

We theoretically investigate the thermoelectric properties (electronic contribution) of a normal-superconductor (NS) hybrid junction, where the normal region consists of magic-angle twisted bilayer graphene (MATBG). The superconducting region is characterized by a common $s$-wave superconductor closely proximitized to the MATBG. We compute various thermoelectric coefficients, including thermal conductance, thermopower, and the figure of merit ($zT$), using the scattering matrix formalism. These results are further supported by calculations based on a lattice-regularized version of the effective Hamiltonian. Additionally, we explore the impact of trigonal warping and valley polarization on the thermoelectric coefficients. Notably, we find a significant variation in $zT$ as a function of these parameters, reaching values as high as 2.5. Interestingly, we observe a violation of the Wiedemann-Franz law near the charge neutrality point with the superconducting correlation, indicating that MATBG electrons behave as slow Dirac fermions in this regime. This observation is further confirmed by the damped oscillatory behavior of the thermal conductance as a function of the barrier strength when an insulating barrier is modelled at the interface of the NS junction. Beyond theoretical insights, our findings suggest new possibilities for thermoelectric applications using MATBG based NS junctions.

cond-mat.mes-hall↗

arXiv2024

Topological Majorana zero modes and the superconducting diode effect driven by Fulde-Ferrell-Larkin-Ovchinnikov pairing in a helical Shiba chain

We propose a theoretical framework for the realization of Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) pairing in a helical Shiba chain subjected to an out-of-plane Zeeman field, analyzed through a self-consistent Bogoliubov-de-Gennes (BdG) mean-field formalism approach. A chain of magnetic adatoms with helical spin texture deposited on the surface of a common $s$-wave superconductor, has emerged as a pivotal platform for realizing topological Majorana zero modes (MZMs). Our study reveals the crucial role of finite momentum pairing of Cooper pairs in the form of FFLO state which also supports topological MZMs at the ends of the chain. Interestingly, we demonstrate that FFLO pairing facilitates non-reciprocal charge transport, giving rise to superconducting diode effect in our system where both time-reversal and inversion symmetries are broken. Such diode effect stems directly from the presence of finite Cooper pair momentum of the FFLO ground state. Our comprehensive analysis highlights the intricate interplay between the richness of helical Shiba chain, the out-of-plane Zeeman field, and FFLO pairing in the emergence of MZMs and driving the superconducting diode effect. These findings offer valuable insights into the design and realization of topological superconducting devices with diode-like properties, potentially advancing technological applications in quantum computing and superconducting electronics.

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