arXiv subjects

Satyaki Kar

Publications and source records attributed to Satyaki Kar.

At least 19 recordsLinked to original sources

arXiv2026

Transitions and Critical Divergences in Periodically Hopping Modulated Su-Schrieffer-Heeger Chains

We use a curvature renormalization group (CRG) approach to study the topological phase transitions in a Su-Schrieffer-Heeger chain and its extensions coming from periodic hopping modulations. A curvature function is defined in terms of system parameters near high-symmetry points where the divergence of this function at critical points, in analogy to usual phase transitions, signals a topological phase transition. According to this theory, the phase transition line for the two-site Su-Schrieffer-Heeger (SSH) model is visible at the critical line Δ= 0 where the curvature function diverges. Our study involves this model and also the modulated one with periodicity of four lattice spacing where the curvature function not only diverges at the topological phase transition point (Dirac-like) |Δ/t| = \sqrt(2) but also shows faster divergence at the non-topological gapless point Δ= 0. We further notice faster divergence of correlation length for Δ-> 0 as compared to that for the |Δ/t| -> \sqrt(2) resulting in two different sets of critical exponents making them lie in different universality classes. The edge state exhibits very slow decay into the bulk near the Δ= 0 point while a much quicker decay from edge into bulk is discernible around the |Δ/t| = \sqrt(2) point. We also continue similar analysis for a SSH model with hopping periodicity of eight lattice spacing.

cond-mat.str-el↗

arXiv2026

Interplay of Anisotropy, Dzyaloshinskii Moriya Interaction and Symmetry breaking Fields in a 2D XY Ferromagnet

A two dimensional classical ferromagnetic XY model with its bound vortex-antivortex dominated quasi long range ordered phase at low temperatures is a long standing as well as well studied problem of interest in the field of condensed matter. We conduct a detailed Monte Carlo study of such model in a square lattice with rather unexplored extensions where additional anisotropic exchange coupling and Dzyaloshinskii-Moriya interactions (DMI) together affect the Kosterlitz-Thouless (KT) transition in presence/ absence of symmetry breaking fields. Without DMI, the exchange term promotes collinear (ferromagnetic) order, whereas the DMI term induces spin cantings. By tuning anisotropy upto Ising limit, we document energy, specific-heat, magnetizations as well as helicity modulus and vortex densities for different temperatures and DMI strength. We also compute the 2nd moment of correlation lengths in order to probe the spatial correlation of the spins. Furthermore, the effect of U(1) symmetry breaking 4-fold and 8-fold symmetric h4 and h8 fields are explored which shows how the double-peaked specific heat profiles changes in presence of DMI. Overall, our findings append many important updates in the low temperature phases of a topological XY ferromagnet when additional DMI and isotropy-breaking exchange and/or field terms are considered thereby providing a few practical blueprints for suitably engineering topological spin systems.

cond-mat.str-el↗

arXiv2026

Topology and Localizations in a 2D Su-Schrieffer-Heeger Model with Domain Walls, Quasi-periodic Disorder and Periodic Hopping Modulations

We study a two dimensional (2D) Su-Schrieffer-Heeger (SSH) model on a square lattice in presence of domain walls (DW) / vortices or quasi-periodic disorders to investigate the nature of topology and localizations in its quantum states. While in a pure 2D SSH model, zero energy states (ZES) lie within the dispersion continuum and the bound states in continuum (BIC) are localized at the corners, a continuous distributions of DWs can produce localized ZES along the DW lines or at the DW center depending on the orientations of the DWs. Moreover with such DWs, one can witness nonzero energy in-gap states showing localizations at the edges, along the DWs or at the DW center. For probing disorder effect, we introduce on-site quasiperiodic potentials (QP) in such systems that show the usual tendency of the states to localize. But exotic reentrant localization behavior is also captured for judicious choice of the QP term. We also examine the scenario for different hopping periodicities in the SSH Hamiltonian. Interestingly for anisotropic hopping modulations, the bulk ZES gets exhausted leaving only topological boundary modes at zero energies. The fate of these states in presence of the DWs are also discussed. Our present study with its plethora of exotic outcomes can thus inspire varied applications in the field of topological quantum computations.

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arXiv2026

Electrical and Thermal conductance through a Nodal Surface Semimetal-Insulator-Superconductor junction

Motivated by the unique dispersions close to the two dimensional band crossing in a topologically charged nodal surface semimetal (NSSM) spectrum, we perform theoretical analysis of quantum tunnelling through a junction consisting of such NSSM, an insulator and a s-wave superconductor (acronymed NSSM-I-SC junction). In particular, for excitation energies both more and less than the superconducting gap potential $Δ$ we probe the normal and Andreev conductance for different incident orientations and thereby find the tunnelling electrical conductance through the heterostructure. The present work considers only the thin barrier limit which witness the conductance G to oscillate periodically with frequency $π$ as a function of the barrier strength, both in high and low doping limit. Such periodic behavior is also observed while calculating the thermal conductance $κ$ through the junction. Novelty of this problem is that the behavior of these G or $κ$ with insulator width are, in many respect, different compared to that from a normal metal - insulator - superconductor (NIS) junction on graphene or silicene. The findings can thus motivate experimentalists to culture renewed control over electric or thermal transport on topological materials.

cond-mat.mes-hall↗

arXiv2025

Transport through Nodal Surface Semimetal-Superconductor junction in absence/presence of light irradiation

We study quantum tunnelling via s-wave superconductor (SC) junction with a topologically charged nodal surface semimetal (NSSM) where a nonsymmorphic symmetry forces the nodal surfaces to stick to the Brillouin Zone boundary. Due to their unique dispersions close to the two dimensional band crossing, the charge carriers in the NSSM display many unorthodox behavior in the nature of Andreev as well as normal reflections at the SC junction interface. We investigate such behaviors for different incident orientations for both subgap and supergap energies where monotonic decays/rises of reflectance with incident energy or angle of incidences are often not followed. We also consider irradiation via light with circular and linear polarization on such systems following a Floquet approach in the limit of high frequency irradiation and probe the stroboscopic temporal evolution of the transport parameters. Our results indicate many nontrivial Andreev transport features including near-depletion of the subgap conductivities. All these nontrivialities can be tested in a cold atom set-up on optical lattices and well experimented for quantum information processing purpose.

cond-mat.supr-con↗

arXiv2025

Reviewing Current-Driven Dynamics and Monte Carlo based Analysis of Thermodynamic Properties of a Magnetic Skyrmion Crystal

Magnetic skyrmions with its topologically protected, nano-sized spin textures have already earned immense fame as information carriers due to their stability and low-current mobility. While ferromagnetic skyrmions suffer from a transverse deflection (i.e., the skyrmion Hall effect), their anti-ferromagnetic counterparts promise straight-line motion and ultrafast dynamics. Here we present a numerical study of the dynamics of lattice-based antiferromagnetic skyrmions driven by spin-transfer torque for which the Landau-Lifshitz-Gilbert-Slonczewski (LLGS) equation is solved using a fourth-order Range-Kutta integration. Then we conduct a detailed Monte Carlo study of the two-dimensional classical XY model to quantify how spatial anisotropy and Dzyaloshinskii-Moriya (DM) coupling reshape its thermal response across multiple lattice sizes. By tuning the ratio Jy/Jx, we document a systematic evolution of the specific-heat anomaly. For example, in the quasi-one-dimensional limit, CV exhibits a broad, low-temperature hump, whereas stronger anisotropy yields sharper peaks that migrate to higher inverse temperature. Finite-size scaling confirms the crossover from quasi-1D fluctuations to a two-dimensional Kosterlitz-Thouless transition. Incorporating a DM interaction further enriches this landscape. At D/J = 0.1, the main peak shifts modestly upward and is slightly suppressed; raising D/J elevates the peak magnitudes and creates a pronounced low-temperature plateau. This residual CV signals enduring chiral excitations and complex spin-twist textures beyond simple vortex unbinding. Our findings chart how directional and chiral couplings can be harnessed to tune pseudo-critical temperatures and thermodynamic signatures in two-dimensional magnets, providing a practical blueprint for engineering topological spin systems.

cond-mat.str-el↗

arXiv2024

Floquet Analysis on an Irradiated Nodal Surface Semimetal with Non-Symmorphic Symmetry

A nodal surface semimetal (NSSM) features symmetry enforced band crossings along a surface within the three-dimensional (3D) Brillouin zone (BZ) and a presence of a nonsymmorphic symmetry there pushes such surfaces to stick to the BZ center or boundaries. The topological robustness of the same does not always come with nonzero Berry fluxes. We consider two such nodal surfaces (NS), one with zero and another with nonzero topological charges and investigate the effect of light irradiation on them. We find that depending on the state of polarization, one can obtain additional Weyl points/ nodal surfaces in the corresponding Floquet Hamiltonians. Particularly, using a simple two band spinless/spin polarized models with no spin orbit coupling, we emphasize the low energy behavior of the continuum Hamiltonians close to the band crossings and its evolution in a Floquet system in the high frequency limit. In the Floquet system, we also find the nodal surfaces to perish or new multi Weyl points to get popped up for different polarization scenario or different NSSM Hamiltonians. Our findings open up important avenues on what out of equilibrium NSSM systems can offer in many active fields including quantum computations.

cond-mat.mes-hall↗

arXiv2024

Topology and $\mathcal{PT}$ Symmetry in a Non-Hermitian Su-Schrieffer-Heeger Chain with Periodic Hopping Modulation

We study the effect of periodic but commensurate hopping modulation on a Su-Schrieffer-Heeger (SSH) chain with an additional onsite staggered imaginary potential. Such dissipative, non-Hermitian (NH) extension amply modifies the features of the topological trivial phase (TTP) and the topological nontrivial phase (TNP) of the SSH chain, more so with the periodic hopping distribution. Generally a weak potential can respect the parity-time (PT ) symmetry keeping the energy eigenvalues real, while a strong potential breaks PT conservation leading to imaginary end state and complex bulk state energies in the system. We find that this PT breaking with imaginary potential strength γshow interesting dependence on the hopping modulation Δfor different hoping modulations. In-gap states, that appear also in the γ= 0 limit, take either purely real or purely imaginary eigenvalues depending on the strength of both γand Δ. The localization of end states (in-gap states) at the boundaries are investigated which show extended nature not only near topological transitions (further away from |Δ/t| = 1) but also near the unmodulated limit of Δ= 0. Moreover, localization of the bulk states is observed at the maximally dimerized limit of |Δ/t| = 1, which also have a γ dependence. Analyzing further the dissipation caused by the complex eigenvalues in this problem with different hopping periodicity can be essential in modulating the gain-loss contrast in optical systems or in designing various quantum information processing and storage devices.

cond-mat.mes-hall↗

arXiv2024

Topological Solitons in Su-Schrieffer-Heeger Chain with periodic hopping modulation, domain walls and disorder

A chiral symmetric Su-Schrieffer-Heeger (SSH) chain features topological end states in one of its dimerized configurations. Those mid-gap zero energy states show interesting modifications upon a periodic tuning of the hopping modulations. Besides, more and more in-gap end modes appear at nonzero energies for further partitioning of the Brillouin zone (BZ) due to increased hopping periodicity. The new topological phases are identified with a detailed analysis of the topological invariants namely, winding number and Zak phases. The spectra and topology of these systems with periodically modulated hopping are studied also in the presence of a single static domain wall, separating two topologically inequivalent dimerized structures. The domain wall causes additional in-gap modes in the spectrum as well as zero energy domain wall solitonic states for specific hopping periodicities. We also study the effect of disorder, particularly the chirality breaking onsite ones, on the edge and domain wall states. Other than the SSH type we also consider random, Rice-Mele or AI type disorder to do a comparative analysis of the evolution of chirality and zero energy states as the strength of disorder and hopping periodicity is varied. Our findings can add important feedback in utilizing topological phases in various fields including quantum computations while the results can be easily verified in a cold atom set up within optical lattices.

cond-mat.str-el↗

arXiv2023

Edge state behavior in a Su-Schrieffer-Heeger like model with periodically modulated hopping

Su-Schrieffer-Heeger (SSH) model is one of the simplest models to show topological end/edge states and the existence of Majorana fermions. Here we consider a SSH like model both in one and two dimensions where a nearest neighbor hopping features spatially periodic modulations. In the 1D chain, we witness appearance of new in-gap end states apart from a pair of Majorana zero modes (MZM) when the hopping periodicity go beyond two lattice spacings. The pair of MZMs, that appear in the topological regime, characterise the end modes each existing in either end of the chain. These, however, crossover to both-end end modes for small hopping modulation strength in a finite chain. Contrarily in a 2D SSH model with symmetric hopping that we consider, both non-zero and zero energy topological states appear in a finite square lattice even with a simple staggered hopping, though the zero energy modes disappear in a ribbon configuration. Apart from edge modes, the 2D system also features corner modes as well as modes with satellite peaks distributed non-randomly within the lattice. In both the dimensions, an increase in the periodicity of hopping modulation causes the zero energy Majorana modes to become available for either sign of the modulation. But interestingly with different periodicity for hopping modulations in the two directions, the zero energy modes in a 2D model become rarer and does not appear for all strength and sign of the modulation.

cond-mat.mes-hall↗

arXiv2022

Chiral anomaly induced magnetoconductances in an irradiated Type-I Weyl Semimetal

Magneto conductivities in Weyl semimetals (WSM) in presence of small fields are studied using quasi-classical Boltzmann transport equations (BTE). Following such formalism here we consider irradiation via circularly polarized light on a two-node time reversal breaking WSM already under a dc/static electric field and study the magneto-transport properties due to the presence of chiral anomaly. Chiral anomaly affects both longitudinal magnetoconductivity as well as planar Hall conductivity. {As our field set-up causes continuous time variation in the relative orientation between the fields, one naturally expects interesting magneto-transport behavior for different field strengths and tilting.} The type-I tilting that we study here displays both positive and negative magnetoconductances depending on the field strengths and time. Furthermore, we find that a direct temporal tuning of the irradiated field strengths can {lead to fluctuating} magneto-transport behavior which can be easily improvised and checked in the laboratories.

cond-mat.str-el↗

arXiv2021

Fermi Level Fluctuations, Reduced Effective Masses and Zeeman Effect during Quantum Oscillations in Nodal Line Semimetals

We probe quantum oscillations in nodal line semimetals (NLSM) by considering a NLSM continuum model under strong magnetic field and report the characteristics of the Landau level spectra and the fluctuations in the Fermi level as the field in a direction perpendicular to the nodal plane is varied through. Based on the results on parallel magnetization, we demonstrate the growth of quantum oscillation with field strength as well as its constancy in period when plotted against 1/B. We find that the density of states which show series of peaks in succession, witness bifurcation of those peaks due to Zeeman effect. For field normal to nodal plane, such bifurcations are discernible only if the electron effective mass is considerably smaller than its free value, which usually happens in these systems. Though a reduced effective mass $m^*$ causes the Zeeman splitting to become small compared to Landau level spacing, experimental results indicate a manyfold increase in the Lande $g$ factor which again amplifies the Zeeman contribution. We also consider magnetic field in the nodal plane for which the density of state peaks do not repeat periodically with energy anymore. The spectra become more spread out and the Zeeman splittings become less prominent. We find the low energy topological regime, that appears with such in-plane field set up, to shrink further with reduced $m^*$ values. However, such topological regime can be stretched out in case there are smaller Fermi velocities for electrons in the direction normal to the nodal plane.

cond-mat.str-el↗

arXiv2021

Quantum Oscillation and Landau-Zener transition in Untilted Nodal line semimetals under a time-periodic magnetic field

Nodal line semimetals (NLSM) exhibit interesting quantum oscillation characteristics when acted upon by a strong magnetic field. We study the combined effect of strong direct (dc) and alternating (ac) magnetic field, perpendicular to the nodal plane in an untilted NLSM in order to probe the behavior of the low lying Landau level (LL) states that can periodically become gapless for suitably chosen field parameters. The oscillatory field variation, as opposed to a steady one, has interesting impact on the quantum oscillation phenomena with the Landau tubes crossing the Fermi surface extremally two times per cycle. Furthermore, the low energy modes can witness Landau-Zener like transitions between valence and conduction band providing further routes to conduction. We discuss such transition phenomena following the framework of adiabatic-impulse approximation for slow quenches. Next we also investigate the effect of oscillating magnetic field acting parallel to the nodal loop where topologically nontrivial magnetic oscillations at low energies can be witnessed. Therefore, with proper parameters chosen, one can engineer topological transitions to occur periodically in such systems as the oscillating field is swept through its cycles.

cond-mat.str-el↗

arXiv2020

A Primer on Weyl Semimetals: Down the Discovery of Topological Phases

Recently discovered Weyl semimetals (WSM) have found special place in topological condensed matter studies for they represent first example of massless Weyl fermions found in condensed matter systems. A WSM shows gapless bulk energy spectra with Dirac-like point degeneracies, famously called Weyl nodes, which carry with themselves well defined chiralities and topologically protected chiral charges. One finds the Berry curvature of the Bloch bands to become singular, like in a magnetic monopole, at these Weyl nodes. Moreover, these systems feature topological surface states in the form of open Fermi arcs. In this review, we undergo a concise journey from graphene based Dirac physics to Weyl semimetals: the underlying Hamiltonians, their basic features and their unique response to external electric and magnetic fields in order to provide a basic walk-through of how the Weyl physics unfolded with time starting from the discovery of Graphene.

cond-mat.str-el↗

arXiv2019

Mixed-order transition and tricritical point associated with checkerboard supersolidity in the two-dimensional $t_2-V_1$ model

We use Quantum Monte Carlo method employing stochastic-series-expansion technique to study the ground state properties of the $t_2-V_1$ model on a square lattice. We find that, away from half-fillings, the minimal combination of nearest-neighbor repulsion $V_1$ and next-nearest-neighbor hopping $t_2$ may give rise to checkerboard supersolidity. The nature of the quantum phase transition, where the superfluid changes to a checkerboard supersolid, depends on the relative strength of $V_1/t_2$ and the average site occupancy. Interestingly, the model exhibits a mixed-order transition near half filling; at a higher (lower) filling, tricriticality is witnessed followed by a second-order transition at densities even further away from half filling. Close to half filling, the model displays the extreme Thouless effect and transits from a superfluid to a checkerboard solid.

cond-mat.str-el↗

arXiv2019

Magnon excitations in $Cs_2CuAl_4O_8$ - a bond alternating S=1/2 spin chain with next nearest neighbor coupling

A recent density functional theory (DFT) based analysis, complemented with Quantum Monte Carlo calculations revealed a highly spin-frustrating nature of the one-dimensional spin-$\frac{1}{2}$ compound $Cs_2CuAl_4O_8$ that comprises of unique bond alternations and relatively strong next nearest neighbor interactions. This article gives a brief account on possible magnon excitations that can appear in the ground state of such systems. We find that the spin waves obtained on top of coplanar helical reference states show multiple magnon modes (both acoustic and optical). However, those magnon modes turn out to be stable only in the absence of bond alternations.

cond-mat.str-el↗

arXiv2018

Photo-induced Entanglement in a Magnonic Floquet Topological Insulator

When irradiated via high frequency circularly polarized light, the stroboscopic dynamics in a Heisenberg spin system on a honeycomb lattice develops a next nearest neighbor (NNN) Dzyaloshinskii-Moriya (DM) type term\cite{owerre}, making it a magnonic Floquet topological insulator. We investigate the entanglement generation and its evolution on such systems - particularly an irradiated ferromagnetic XXZ spin-$\frac{1}{2}$ model in a honeycomb lattice as the system parameters are optically tuned. In the high frequency limit, we compute the lowest quasi-energy state entanglement in terms of the concurrence between nearest neighbor (NN) and NNN pair of spins and witness the entanglement transitions occurring there. For the easy axis scenario, the unirradiated system forms a product state but entanglement grows between the NNN spin pairs beyond some cut-off DM strength. Contrarily in easy planar case, NN and NNN spins remain already entangled in the unirradiated limit. It then goes through an entanglement transition which causes decrease (increase) of the NN (NNN) concurrences down to zero (up to some higher value) at some critical finite DM interaction strength. For a high frequency of irradiation and a suitably chosen anisotropy parameter, we can vary the field strength to witness sudden death and revival of entanglement in the Floquet system. Both exact diagonalization and modified Lanczos techniques are used to obtain the results upto 24 site lattice. We also calculate the thermal entanglement and obtain estimates for the threshold temperatures below which non-zero concurrence can be expected in the system.

cond-mat.str-el↗

arXiv2018

Andreev tunnelling and Josephson current in light irradiated graphene

We investigate the Andreev tunneling and Josephson current in graphene irradiated with high-frequency linearly polarized light. The corresponding stroboscopic dynamics can be solved using Floquet mechanism which results in an effective stationary theory to the problem. It exhibits anisotropy in the Dirac spectrum and modifies the so-called pseudospin-momentum locking in graphene. The Andreev reflection at a normal graphene - superconductor (NS) interface becomes an oscillatory function of the optical strength. Specifically we find that, by varying the polarization direction we can both suppress AR considerably or cause the Andreev transport to remain maximum at sub-gap excitation energies even in the presence of Fermi level mismatch. Furthermore, we study the optical effect on the Andreev bound states (ABS) within a short normal-graphene sheet, sandwiched between two s-wave superconductors. It shows redistribution of the low energy regime in the ABS spectrum, which in turn, has major effect in shaping the Josephson super-current. Subjected to efficient tuning, such current can be sufficiently altered even at the charge neutrality point. Our observations provide useful feedback in regulating the quantum transport in Dirac-like systems, achieved via controlled off-resonant optical irradiation on them.

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