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Ipsita Mandal

Publications and source records attributed to Ipsita Mandal.

At least 19 recordsLinked to original sources
arXiv2026

Plasmon modes in triple-point semimetal, its quadratic cousin, and Hopf semimetal

We study collective charge excitations in pseudospin-1 triple-point semimetals within the random-phase approximation, considering both the linearised model with a flat-band and its quadratic-corrected counterpart. We analyse their density response, Drude weights, screening, plasmon dispersion and damping, particle-hole continua, and Friedel oscillations from the long-wavelength to the arbitrary-momentum regime. The quadratic correction renders the flat-band dispersive and endows it with a Fermi surface, thereby affecting the overall response. Comparisons with Weyl and Hopf semimetals reveal how bandstructure and quantum geometry jointly govern collective charge dynamics. While the Drude weights are determined by the Fermi surface and band-velocities, interband screening, plasmon dispersion, and Friedel oscillations reflect the quantum geometry of the eigenstates.

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arXiv2026

Plasmon modes in tilted three-dimensional nodal-ring semimetals. I. $\mathcal{PT}$-symmetric nodal ring and its gapped cousin

We study collective charge excitations in three-dimensional $\mathcal{PT}$-symmetric nodal-ring semimetals (PTNRs) and their gapped counterparts (GNRs) in the presence of band tilt, together with Weyl semimetals (WSMs). Within a low-energy two-band description, we consider untilted systems as well as in-plane and axial tilts. Using the random-phase approximation, we obtain the density of states, Drude weights, dielectric functions, and plasmon dispersions at low doping and in the intrinsic limit. The mass gap and the two inequivalent tilt directions generate distinct Fermi-surface topologies and thereby control the formation and damping of plasmons. By comparing the collective modes with the particle-hole continuum, we identify the regimes in which plasmons remain well defined. We also calculate the response of untilted and tilted WSMs and contrast it with that of PTNRs and GNRs. The two classes of semimetals differ in the carrier-density dependence and isotropy of the plasmon frequency as well as in interband screening and their response to tilt. These differences reflect their distinct Fermi-surface geometries and interband structures. Our results establish how mass and tilt govern collective charge dynamics through their interplay with intraband spectral weight and interband particle-hole excitations.

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arXiv2026

Plasmon modes in tilted three-dimensional nodal-ring semimetals. II. Vortex nodal ring

We investigate collective charge excitations in three-dimensional semimetals with dipole-like Berry-curvature (BC) profiles, focusing on vortex nodal-ring semimetals (VNRs) and Hopf semimetals (HSMs). The VNR hosts a continuous ring of dipole-like BC sources generated by its winding pseudospin texture, whereas the HSM hosts an ideal BC dipole. This allows us to examine how distinct realisations of dipole-like BC structure affect collective charge dynamics. While plasmon modes in $\mathcal{PT}$-symmetric nodal-ring semimetals (PTNRs) and their gapped cousins were studied in a companion work, arXiv:2609.35373, here we analyse the density response, Drude weights, dielectric function, and plasmon modes of VNRs and HSMs at low doping within the random-phase approximation. For VNRs, we consider untilted, in-plane tilted, and axially tilted systems. Although VNR and PTNR have identical energy dispersions, their distinct eigenstates introduce a toroidal-angle dependence in the band-overlap factor, yielding additional interband contributions for in-plane wavevectors and weak corrections to the screened plasmon frequencies. The toroidal Fermi surface also produces intrinsically anisotropic bare plasmons. In contrast, HSMs have isotropic bare plasmons, with anisotropy arising only through their interband response. Our results show that collective charge dynamics can distinguish different realisations of dipole-like BC structure and reveal pseudospin information beyond the single-particle dispersion.

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arXiv2026

Puiseux series about exceptional singularities dictated by symmetry-allowed Hessenberg forms of perturbation matrices

We develop a systematic framework for determining the nature of exceptional points of $n^{\rm th}$ order (EP$_n$s) in non-Hermitian (NH) systems, represented by complex square matrices. By expressing symmetry-preserving perturbations ($\propto ε$) in the Jordan-normal basis of the defective matrix at an EP$_n$, we show that the upper-$\ell$ Hessenberg structure of the perturbation matrix directly dictates the leading-order eigenvalue- and eigenvector-splitting to be $\propto ε^{1/\ell}$, when expanded in a Puiseux series. Applying this to three-band NH models invariant under parity (P), charge-conjugation (C), or parity-time-reversal (PT), we find that EP$_3$s in P- and C-symmetric systems are restricted to at most $\sim ε^{1/2}$ branch points, while PT-symmetric systems generically support EP$_3$s with the strongest possible singularities (viz. $\sim ε^{1/3}$). We illustrate these results with concrete three-dimensional models in which exceptional curves and surfaces emerge. We further show that fine-tuned perturbations can suppress the leading-order branch point to a less-singular splitting, which have implications for designing direction-dependent EP-based sensors. The appendix extends the analysis to four-band P- and C-symmetric models, establishing the existence of EP$_4$s with $\sim ε^{1/4}$ and $\sim ε^{1/2}$ singularities, respectively.

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arXiv2026

Magnetoconductivity of two-dimensional Dirac cones and gapped nodal-rings under impurity-potentials in the ultraquantum limit

We investigate the dc magnetoconductivity of two-dimensional Dirac cones and gapped nodal rings (GNRs) subjected to a perpendicular magnetic field, which quantises the electronic spectrum into Landau levels (LLs). Working in the ultraquantum limit, where only the lowest LL (LLL) is partially occupied, we employ the Kubo--Bastin formalism to compute the transport coefficients for pointlike, Gaussian, and Yukawa impurity-potentials. For the Dirac case, the longitudinal conductivity is field-independent for pointlike impurities and a monotonic function of $B$ for the Gaussian and Yukawa potentials, while the Hall conductivity vanishes identically owing to the particle-hole symmetry of the two neighbouring LLs. The GNR case is qualitatively different: its non-monotonic stretched-checkmark LL spectrum causes the effective LLL to migrate to successively lower indices as the field increases, producing a pronounced oscillatory structure in both conductivities. The longitudinal response develops resonant peaks at LLL degeneracies, while the Hall conductivity traces out a sawtooth pattern with sharp zero-crossings at these same points. These results establish distinct transport fingerprints for the two systems in the extreme quantum limit, and provide a theoretical framework for interpreting magnetotransport experiments on GNRs.

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arXiv2026

Unconventional magnetoelectric conductivity and electrochemical response from dipole-like sources of Berry curvature

We investigate the longitudinal magnetoelectric conductivity and nonlinear electrochemical response (ECR) of three-dimensional nodal-ring semimetals, including vortex nodal-rings (VNRs) and $\mathcal{PT}$-symmetric nodal-rings (PTNRs), together with three-band Hopf semimetals (HSMs), using the semiclassical Boltzmann formalism. The nodal rings of the VNRs and PTNRs are taken to lie in the $k_z=0$ plane, whereas the nodal points of HSMs host isolated Berry-curvature (BC) dipoles whose axes are aligned along the $k_z$ direction. In VNRs, such dipoles may be viewed as forming a continuous distribution along the nodal ring, providing a useful connection between the two classes of systems. For collinear electric and magnetic ($\mathcal{B}$) fields applied along the anisotropy axis $\boldsymbol {\hat{z}}$, the linear response is computed exactly, displaying the presence of odd powers of $B_z$. For a two-node HSM with oppositely oriented BC-dipole axes, a competition between intranode and internode scattering plays out. We further analyse the nonlinear ECR, described by third-rank response tensors that relate the electrical current to the combined effects of an external electric field and a chemical-potential gradient. The surviving tensor components are shown to be completely determined by the tilt direction of the band structure. A comparison of the three systems reveals a close correspondence between the responses of VNRs and HSMs, while PTNRs display qualitatively different behavior. We attribute these similarities and differences to the topology of the BC field, namely dipole-like sources in VNRs and HSMs versus the ring-confined BC structure characteristic of PTNRs.

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arXiv2026

Direction-dependent magnetoelectric conductivity from dipolar topological semimetals

We compute the linear magnetoelectric conductivity of a two-band vortex nodal-ring (VNR) semimetal and a three-band Hopf semimetal (HSM) within the semiclassical Boltzmann formalism in the relaxation-time approximation. We consider three inequivalent planar-Hall configurations, in which the electric ($\boldsymbol E$) and magnetic ($\boldsymbol B$) fields are oriented differently with respect to the nodal-ring plane and the BC-dipole axis. Unlike Weyl and multifold semimetals, where Berry-curvature (BC) flux originates from monopole-like sources, both systems host dipole-like BC sources and vanishing Chern numbers. Working consistently to cubic order in the magnetic field, we evaluate the Drude, BC, orbital-magnetic-moment (OMM), anomalous-Hall, and Lorentz-force contributions to the conductivity. In both systems, the dipolar BC and OMM generate odd powers of $|\boldsymbol B|$ in in-plane response channels that are symmetry-forbidden in systems with monopole-like BC sources. For the HSM, the internode-scattering contribution to the conductivity vanishes identically. By comparing the VNR with a $\mathcal{PT}$-broken gapped nodal ring and the HSM with a pseudospin-1 triple-point semimetal, we demonstrate that the dipolar nature of the BC gives rise to distinctive features in magnetoelectric response that are absent in semimetals with monopole-like BC flux.

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arXiv2026

Fermi arcs for generic nodal points hosting monopoles or dipoles

Surface-localised states in the form of Fermi arcs emerge at the interfaces between three-dimensional topological semimetals and their surrounding vacuum, serving as primary signatures of momentum-space topology accessible to modern spectroscopic methods. These arcs encode information about topological singularities --- nodal points where multiple bands converge --- through their geometric properties. Specifically, the arcs terminate tangentially at the edges of Fermi-surface projections, delineating where surface modes transition into bulk extended states. The arcs' multiplicity at each node corresponds directly to the underlying topological charge (Berry-curvature monopole magnitude), thereby offering a window into momentum-space invariants via angle-resolved photoemission experiments. This work establishes a systematic framework for computing surface state configurations applicable to arbitrary nodal degeneracies and band dispersions: (1) nodes with twofold through multifold band multiplicities; and (2) bands exhibiting diverse momentum-dependence patterns, including linear, nonlinear, isotropic, and anisotropic regimes. We additionally examine topologically distinct nodal types where monopole charges vanish but dipolar Berry-curvature distributions appear, clarifying the surface-state landscape in such unconventional scenarios.

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arXiv2026

Interplay of strain-induced axial gauge fields and intrinsic band-topology in the magnetoelectric conductivity of gapped nodal rings

We compute the magnetoelectric conductivity of a semimetal hosting an ideal gapped nodal ring (GNR) in three distinct planar-Hall configurations, in the simultaneous presence of an external electric field $\boldsymbol{E}$, a magnetic field $\boldsymbol{B}$, and a strain-induced axial pseudomagnetic field $\boldsymbol{B}_5$. The latter arises from a nonuniform lattice deformation and couples to antipodal points on the toroidal Fermi surface with opposite signs, reflecting its chiral nature. Extending our earlier analysis to include $\boldsymbol{B}_5$, we demonstrate how its vortex-like field lines --- co-aligned with the Berry curvature (BC) and orbital magnetic moment (OMM) --- imprint qualitatively distinct signatures on the conductivity tensor. In particular, this alignment causes the dot product of $\boldsymbol{B}_5$ with the BC or OMM-induced quantities to be angle-independent on the Fermi surface, generating a nonvanishing integral linear-in-$B_5$, which is not possible for isotropic nodal points harbouring BC-monopoles. We show that a part of the planar-Hall conductivity in the first set-up remains completely immune to strain, providing a strain-insensitive internal reference for topological transport. Our explicit analytical expressions offer concrete and experimentally-testable predictions for identifying strain-induced signatures in transport measurements on GNR materials.

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arXiv2026

Floquet scattering and Fano resonances in nodal-ring and multi-Weyl semimetals: Role of propagating and evanescent modes

We develop a comprehensive Floquet scattering theory for quantum transport in periodically driven nodal-ring semimetals (NRSs) and multi-Weyl semimetals (mWSMs), extending our earlier study reported in Annalen der Physik 535, 2200460 (2023), in which evanescent modes were neglected, to a complete formalism that includes both propagating and evanescent channels. By solving the full boundary-value problem, we obtain the complete set of scattering states and show that, although evanescent modes are indispensable for satisfying the matching conditions at the potential interfaces, they carry zero net probability current and do not contribute to any observable transport quantity. We identify a previously unexplored transport regime in NRSs in which two propagating channels coexist and participate in coherent scattering, producing multi-channel quantum interference and Floquet-induced Fano resonances. The transmission, reflection, pumped shot noise, and the associated Fano resonance features are determined entirely by the propagating channels, and the resonance energies coincide with those of the corresponding quasi-bound states of the static potential well. Our results establish a unified framework for Floquet transport in anisotropic topological semimetals, and confirm that the approximation of neglecting evanescent modes in our earlier work is justified for all measurable transport properties.

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arXiv2026

Distinguishing features of longitudinal magnetoconductivity for a Rarita-Schwinger-Weyl node

The band-degeneracy points in the Brillouin zones of chiral crystals exist in multiple avatars, with the high-symmetry points being able to host multifold nodes of distinct characters. A class of such crystals, assisted by the spin-orbit coupling, harbours fourfold degeneracy in the form of Rarita-Schwinger-Weyl node (RSWN) at the $Γ$-point. Our aim is to explore the nature of longitudinal magnetoconductivity, arising from applying collinear electric and magnetic fields, for such systems. Adjusting the chemical potential to lie near the intrinsic energy-location of the RSWN, the multifold nature of the RSWN is revealed by an interplay of intraband and interband scatterings, which would not arise in twofold degeneracies like the conventional Weyl nodes. The current study fills up the much-needed gap in obtaining the linear response from an exact computation, rather than the insufficient relaxation-time approximation employed earlier.

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arXiv2026

Linear response from tilted Dirac cones under strain-induced pseudomagnetic fields

We investigate the transport signatures of pseudo-Landau levels (PLLs) in two-dimensional anisotropic Dirac systems with tilted cones, whose effective bandstructure results from strain-induced pseudogauge fields. In contrast to conventional Landau quantisation, the PLLs exhibit explicit momentum-dependence by being dispersive, leading to finite longitudinal group-velocities. We analyse the transport properties within the semiclassical Boltzmann framework by computing the electrical, thermoelectric, and thermal response in the linear regime, which acquire nonzero longitudinal components. We also check the validity of the Mott relation and Wiedemann-Franz law in our system. Our results provide a unified framework for understanding the interplay between tilted spectrum and structural deformation in affecting quantum transport, and suggest unambiguous experimental signatures in strain-engineered systems.

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arXiv2026

Direction-dependent linear response for gapped nodal-line semimetals in planar-Hall configurations

We compute the magnetoelectric conductivity for ideal nodal-line semimetals (NLSMs), with a finite but tiny mass-gap, in distinct planar-Hall set-ups. Each differing configuration results from the relative orientation of the nodal-ring's plane with respect to the plane spanned by the electric ($\mathbf E $) and magnetic ($\mathbf B$) fields. The net conductivity tensor has components comprising the Drude, anomalous-Hall, in-plane (with $\mathbf E$ and $\mathbf B$) longitudinal and transverse, and Lorentz-force-operator-induced parts. Our results feature the signatures of the inherent topology of a gapped NLSM, revealed through nonzero values of the Berry curvature and the orbital magnetic moment. In particular, we show that both of these vector fields, arising in the momentum space, give rise to terms of comparable magnitudes in the resulting response. Our explicit theoretical expressions will help identify unique signatures of NLSMs in contemporary experiments.

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arXiv2026

Generic deformation channels for critical Fermi surfaces including the impact of collisions

This paper constitutes a sequel to our theoretical efforts to determine the nature of generic low-energy deformations of the Fermi surface of a quantum-critical metal, which arises at the stable non-Fermi liquid (NFL) fixed point of a quantum phase transition. The emergent critical Fermi surface, arising right at the Ising-nematic quantum critical point (QCP), is a paradigmatic example where an NFL behaviour is induced by the strong interactions of the fermionic degrees of freedom with those of the bosonic order parameter. It is an artifact of the bosonic modes becoming massless at the QCP, thus undergoing Landau-damping at the level of one-loop self-energy. We resort to the well-tested formalism of the quantum Boltzmann equations (QBEs) for identifying the excitations. While in our earlier works, we have focused on the collisionless regime by neglecting the collision integral and assuming the bosons to be in equilibrium, here we embark on a full analysis. In particular, we take into account the bosonic part of the QBEs as well, which, however, turn out to have no effect on the solutions. Decomposing the master equation into angular-momentum ($\ell$) channels, the emergent modes are of two types: Fermi-surface deformations with discrete spectra and particle-hole excitations forming a continuous band. The long-lived zero-sound mode, which corresponds to $\ell = 0$, is found to be robust against damping effects. Intriguingly, we have an infinite family of discrete modes corresponding to higher-order harmonics of the net deformation.

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arXiv2026

Magneto-transport signatures in periodically-driven Weyl and multi-Weyl semimetals

We investigate the influence of a time-periodic driving (for example, by shining circularly polarized light) on three-dimensional Weyl and multi-Weyl semimetals, in the planar Hall and planar thermal Hall set-ups. We incorporate the effects of the drive by using the Floquet formalism in the large frequency limit. We evaluate the longitudinal magneto conductivity, planar Hall conductivity, longitudinal thermo-electric coefficient, and transverse thermo-electric coefficient, using the semi-classical Boltzmann transport equations. We demonstrate the explicit expressions of these transport coefficients in certain limits of the parameters, where it is possible to perform the integrals analytically. We cross-check our analytical approximations by comparing the physical values with the numerical results, obtained directly from the numerical integration of the integrals. The answers obtained show that the topological charges of the corresponding semimetals have profound signatures in these transport properties, which can be observed in experiments.

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arXiv2026

Longitudinal magnetoconductivity in chiral multifold semimetals exemplified by pseudospin-1 nodal points

We embark on computing the longitudinal magnetoconductivity within the semiclassical Boltzmann formalism, where an isotropic triple-point semimetal (TSM) is subjected to collinear electric ($\boldsymbol E $) and magnetic ($\boldsymbol B$) fields. Except for the Drude part, the $B$-dependence arises exclusively from topological properties like the Berry curvature and the orbital magnetic moment. We solve the Boltzmann equations exactly in the linear-response regime, applicable in the limit of weak/nonquantising magnetic fields. The novelty of our investigation lies in the consideration of the truly multifold character of the TSMs, where the so-called flat-band (flatness being merely an artefact of linear-order approximations) is made dispersive by incorporating the appropriate quadratic-in-momentum correction in the effective Hamiltonian. It necessitates the consideration of interband scatterings within the same node as well, providing a complex interplay of intraband, interband, intranode, and internode processes, offering an overwhelmingly rich set of possibilities. The exact results are compared with those obtained from a naive relaxation-time approximation.

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arXiv2026

Non-Fermi liquid behaviour of CDW instabilities in fractionally-filled moiré flatbands

Spin- and valley-polarized fractionally-filled moiré flatbands are known to host emergent Fermi-liquid phases, when analysed with the help of a dual description in terms of holes. The dominant Coulomb interactions in an almost flatband endow the fermions with a nontrivial dispersion, when the system is described in terms of the hole operators (rather than the particle operators). In particular, for one-fourth filling, the Fermi surface takes a quasi-triangular shape, which brings about the possibility of charge-density-wave (CDW) ordering in the ground state, characterised by the nesting vectors ($ \mathbf{Q}_n $). The $\mathbf{Q}_n$'s connect antipodal points of the Fermi surface (designated as hot-spots) and are found to belong to the space of reciprocal vectors of the underlying honeycomb structure. The resulting CDW order can be described in terms of instabilities caused by bosonic fields with momenta centred at $\lbrace \mathbf{Q}_n \rbrace $, coupling with the fermions residing in the vicinity of a pair of antipodal hot-spots. When there is a transition from a Fermi liquid to a CDW state, the bosons become massless (or critical), effectuating a non-Fermi liquid behaviour. We set out to identify such non-Fermi liquid phases after constructing a minimal effective action.

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arXiv2025

Disentangling contributions to longitudinal magnetoconductivity for Kramers-Weyl nodes

We set out to compute the longitudinal magnetoconductivity for an isolated and isotropic Kramers-Weyl node (KWN), existing in chiral crystals, which forms an exotic cousin of the conventional Weyl nodes resulting from band-inversions. The peculiarities of KWNs are many, the principal one being the presence of two concentric Fermi surfaces at any positive chemical potential ($μ$) with respect to the nodal point. This is caused by a dominant quadratic-in-momentum dispersion, with the linear-in-momentum Dirac- or Weyl-like terms relegated to a secondary status. In a KWN, the chirally-conjugate node typically serves as a mere doppelgänger, being significantly separated in energy. Hence, when $μ$ is set near such a node, the signatures of a lone node are probed in the transport-measurements. The intrinsic topological quantities in the forms of Berry curvature and orbital magnetic moment contribute to the linear response, which we determine by exactly solving the semiclassical Bolzmann equations. Another crucial feature is that the two bands at the same KWN node carry actual spin-quantum numbers, thus providing an additional coupling to an external magnetic field ($\boldsymbol B$), and affecting the conductivity. We take this into account as well, and demonstrate that it causes a linear-in-$B$ dependence, on top of the usual $B^2$-dependence.

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