arXiv subjects

Sumanta Tewari

Publications and source records attributed to Sumanta Tewari.

At least 19 recordsLinked to original sources

arXiv2026

Distinguishing Quantum Capacitance Signatures of a Topological Majorana Wire from a Normal Wire Segment

Majorana zero modes (MZMs) are spatially separated, near-zero-energy excitations expected at the ends of a topological superconducting (TS) wire. A quantum-dot interferometer can be used to probe the quantum capacitance of the TS wire, and the location and magnitude of the capacitance resonances provide information about the MZMs, while their magnetic flux dependence probes coherent coupling to the two Majorana modes. It has been recently shown that, a gapless (i.e., $\Delta=0$) wire segment can also exhibit flux-dependent quantum capacitance oscillations through Aharonov-Bohm interference and, with suitable tuning, can reproduce a Majorana-like response. Here we show that the two mechanisms can be distinguished experimentally. In the gapless normal wire segment, the two parity-dependent signals originate from separate energy resonances corresponding to the lack of generic zero-energy states. By contrast, in the topological wire, the pair of low energy levels with even and odd parity are nearly degenerate in energy, and therefore, the two parity branches remain within the same broader resonance region in the quantum-dot potential, and persist under independent variations of the dot potential and wire chemical potential. Our results show that the experimentally observed quantum capacitance response is distinguished not by a particular flux trace at one optimized point in parameter space, but by its persistence over a finite range of independently controlled parameters, including the quantum-dot potential. This parameter space stability provides a direct means of ruling out the gapless normal wire segment as the origin of the observed Majorana-like response.

cond-mat.mes-hall↗

arXiv2026

MEDA: Measurement-Efficient Disorder-Aware Majorana Zero Mode Detection in Realistic Devices

Fault-tolerant topological quantum computing relies on identifying Majorana zero modes (MZMs), but reliable detection in realistic devices remains challenging. Conventional topological indicators are inherently biased in finite, disordered systems, blurring the distinction between true MZMs and trivial states. Furthermore, attempts to map these indicators to real observables via machine learning require dense, expensive conductance measurements, creating a severe scaling bottleneck. To simultaneously address topological bias and measurement limitations, we present MEDA: a Measurement-Efficient, Disorder-Aware framework for MZM detection in realistic devices. MEDA maps sparse, practically obtainable observables directly to the robust periodic disorder invariant (PDI). Using a novel sparse parameter regime, MEDA reduces measurement volume by 10x while maintaining predictive quality, even in moderate to strong disorder regimes that limit conventional methods. Furthermore, MEDA naturally prioritizes input features consistent with the topological gap protocol, demonstrating strong physical interpretability.

cs.ET↗

arXiv2026

Floquet-Engineered Parity Anomaly Staircase in a Cold Atom Dirac Lattice

We propose a Floquet-engineered cold atom realization of a parity anomaly inspired anomalous Hall staircase in a two dimensional $\pi$-flux lattice. The effective model hosts massive Dirac fermions generated by the combined action of a time reversal symmetry breaking Floquet mass and a static inversion breaking mass offset. An additional momentum dependent scalar displacement term shifts different Dirac sectors in opposite energy directions without modifying their Bloch eigenvectors. As a result, the Berry curvature contribution associated with individual massive Dirac sectors can be selectively occupied, allowing the anomalous Hall response to evolve stepwise as a function of chemical potential or scalar displacement term. Evaluating the full lattice Berry curvature integral, we find plateau-like responses near $0$, $e^2/2h$, and $e^2/h$, corresponding respectively to the activation of zero, one, and two effective massive Dirac sector contributions. We analyze the associated low energy Dirac theory, band topology, Berry curvature structure, and two parameter response maps, and discuss a possible realization using Raman-assisted tunneling, off-resonant Floquet driving, and auxiliary AC-Stark dressing in ultracold atomic optical lattices.

cond-mat.quant-gas↗

arXiv2026

Intrinsic anomalous thermal hall effect as a signature of quantum metric in d-wave altermagnets

We investigate the intrinsic anomalous thermal Hall effect in d-wave altermagnets, where a transverse heat current is generated by a longitudinal temperature gradient in the absence of a magnetic field, with the leading response proportional to $(\nabla T)^3$. In these systems, the intrinsic Berry curvature-driven linear and thermal quantum-metric-driven second-order anomalous thermal Hall currents vanish as a consequence of crystalline symmetry. We show that the first nonvanishing contribution arises at third order in the temperature gradient and is governed by a nonlinear thermal Berry-connection polarizability, a quantity introduced in this work. Our analysis reveals a distinctive angular dependence of the anomalous thermal Hall conductance as the applied thermal gradient is rotated with respect to the crystal axes. We also find characteristic temperature and chemical-potential dependences that can be tested experimentally. These results identify unique quantum geometry-induced thermal responses and establish altermagnets as a promising platform for exploring intrinsic (i.e., scattering-time-independent) geometric transport phenomena.

cond-mat.mes-hall↗

arXiv2026

Quantum Inductance as a Phase-Sensitive Probe of Fermion Parity Switching in Majorana Nanowires

We study the flux-dependent quantum inductance of a one-dimensional (1D) semiconductor-superconductor (SM-SC) Majorana nanowire coupled to a quantum dot in an interferometric setup. Although quantum capacitance in this setup enables fast fermion parity readout, as has been demonstrated experimentally, it cannot by itself reliably confirm a protected fermion parity switch, a key signature of non-trivial topology and the existence of Majorana zero modes (MZMs). In realistic devices, disorder can produce avoided crossings or narrow double crossings between the two parity sectors that can mimic the behavior of a protected single parity switching, leading to false positives for non-trivial topological behavior. We show that quantum inductance provides a complementary probe that is directly sensitive to the phase structure of the low energy spectrum, allowing us to distinguish genuine fermion-parity crossings from avoided crossings or narrow double crossings. Using a general Lehmann framework applied to both effective models and full microscopic simulations with disorder, we demonstrate that only a true fermion-parity switch produces the characteristic inductive response of a protected crossing. In contrast, topologically trivial avoided crossings or narrow double crossings yield quantum inductance signatures that are markedly different from those of topologically nontrivial fermion parity crossings. Therefore, our results show that combined measurements of quantum capacitance and quantum inductance provide a robust and experimentally accessible means to identify true fermion-parity switches, corresponding to a nontrivial Pfaffian invariant.

cond-mat.mes-hall↗

arXiv2026

Pfaffian-based topological invariants for one dimensional semiconductor-superconductor heterostructures

We review the Pfaffian-based $\mathbb{Z}_2$ topological invariants in one dimensional semiconductor-superconductor (SM-SC) nanowire heterostructures and clarify their validity in finite and disordered systems. For the clean nanowire, the product of the Pfaffians of the Hamiltonian at particle-hole symmetric momenta $k=0,\pi$ changes sign at the topological phase transition defined by the bulk gap closing, leading to the definition of $\mathbb{Z}_2$ Kitaev invariant also known as Majorana number. We show that this momentum-space invariant is equivalent to a real space construction based on twisted boundary conditions, in which the sign of the product of the Pfaffians of the Hamiltonian under periodic and anti-periodic boundary conditions defines the $\mathbb{Z}_2$ index. By introducing a superlattice description of periodically repeated disorder, we demonstrate that the real space Pfaffian invariant defined as the sign of the Pfaffians of the Hamiltonian with periodic and anti-periodic boundary conditions, remains a well defined invariant even in the absence of microscopic translational symmetry. Within this framework, it is also equivalent to the recently defined periodic disorder invariant (PDI), which constitutes an integer valued ($\mathbb{Z}$) topological invariant in the presence of chiral symmetry. Finally, we prove that the sign of the Pfaffian of a quadratic Hamiltonian equals the fermion parity of its ground state, establishing a direct physical interpretation of the invariant, in terms of sign of the product of the ground state fermion parity with periodic and anti-periodic boundary conditions. Numerical results confirm the correspondence between sign of the Pfaffian reversals, flux-induced level crossings, and ground-state parity switching in clean and disordered nanowires.

cond-mat.mes-hall↗

arXiv2025

Majorana modes in graphene strips: polarization, wavefunctions, disorder, and Andreev states

Topologically protected Majorana zero modes (MZMs) have attracted intense interest due to their potential application in fault-tolerant quantum computation (TQC). Graphene nanoribbons, with tunable edge terminations and compatibility with planar device architectures, offer a promising alternative to semiconductor nanowires. Here we present a comprehensive theoretical study of finite graphene strips with armchair, zigzag, and nearly square geometries, proximitized by an s-wave superconductor and subject to Rashba spin-orbit coupling, Zeeman fields, and disorder. Using exact diagonalization of the Bogoliubov-de Gennes tight-binding Hamiltonian, we analyze Majorana polarization, low-energy spectra, and real-space wavefunctions to identify the non-trivial topological phases supporting MZMs and distinguish them from from partially separated Andreev bound states (psABS) or the quasi-Majoranas. We systematically chart the robustness of these modes across geometries and disorder regimes, finding that armchair strips with short zigzag edges provide the most stable platform. Our results unify polarization diagnostics with spatial wavefunction analysis and disorder effects, yielding concrete design guidelines for graphene-based topological superconductors.

cond-mat.mes-hall↗

arXiv2025

Electric field induced Berry curvature dipole and non-linear anomalous Hall effects in higher wave symmetric unconventional magnets

We investigate the second-order anomalous Hall response in two-dimensional higher-wave symmetric magnets, including the recently discovered class of collinear magnets known as altermagnets, when subjected to a symmetry-breaking external electric field. In these systems, the first- and second-order anomalous Hall responses mediated by the first- and second-order multipoles of the Berry curvature over the occupied states vanish by symmetry. However, a symmetry-breaking dc electric field can induce a nonzero Berry curvature dipole by coupling to a non-vanishing quantum metric, also known as the Berry connection polarizability. An applied ac electric field can then generate a finite nonlinear transverse Hall effect characterized by a second harmonic response. In addition, the dc field itself can generate a finite third-order transverse Hall response. We discuss this remarkable effect in a class of higher-order symmetric unconventional magnets (of $p$, $d$, $f$, $g$, $i$ symmetry), including the subclass of altermagnets. We demonstrate that the electric-field-induced anomalous Hall effect in the higher-wave-symmetric magnets can serve not only as a probe of the underlying quantum metric of the occupied states but also as a means to distinguish the even ($d$-,$g$-wave) and odd ($p$-wave) order parameter symmetries defined on the square lattice.

cond-mat.mes-hall↗

arXiv2025

Topological invariant for finite systems in the presence of disorder

Topological invariants, rigorously defined only in the thermodynamic limit, have been generalized to topological indicators applicable to finite-size disordered systems. However, in many experimentally relevant situations, such as semiconductor-superconductor (SM-SC) hybrid nanowires hosting Majorana zero modes, the interplay between strong disorder and finite-size effects renders these indicators (e.g., the so-called topological visibility) biased and ill-defined, significantly limiting their usefulness. In this paper, we propose the topological invariant rigorously defined for an infinite system constructed by periodically repeating the original finite disordered system, as a topological indicator. Using the one-dimensional SM-SC hybrid nanowire as an example, we show that this general and transparent approach yields faithful topological indicators free from the biases affecting commonly used finite-size indicators, capturing the nature (topological or trivial) of the phase at generic points in parameter space, and providing a reliable tool for interpreting experimental results.

cond-mat.mes-hall↗

arXiv2025

Antibonding and Electronic Instabilities in GdRu2X2 (X = Si, Ge, Sn): A New Pathway Toward Developing Centrosymmetric Skyrmion Materials

Chemical bonding is key to unlocking the potential of magnetic materials for future information technology. Magnetic skyrmions are topologically protected nano-sized spin textures that can enable high-density low-power spin-based electronics. Despite increasing interest in the discovery of new skyrmion hosts and their characterization, the electronic origins of the skyrmion formation remain unknown. Here, we study GdRu2X2 (X = Si, Ge, Sn) as a model system to study the connection among chemical bonding, electronic instability, and the critical temperature and magnetic field at which skyrmions evolve. The nature of the electronic structure of GdRu2X2 is characterized by chemical bonding, Fermi surface analysis, and density of energy function. As X-p orbitals become more extended from Si-3p to Ge-4p and Sn-5p, improved interactions between the Gd spins and the [Ru2X2] conduction layer and increased destabilizing energy contributions are obtained. GdRu2Si2 possesses a Fermi surface nesting (FSN) vector [Q = (q, 0, 0)], whereas GdRu2Ge2 displays two inequivalent FSN vectors [Q = (q, 0, 0); QA = (q, q, 0)] and GdRu2Sn2 features multiple Q vectors. In addition, competing ferromagnetic and antiferromagnetic exchange interactions in the Gd plane become more pronounced as a function of X. These results reveal some correlation among the electronic instability, the competing interaction strength, and the temperature and magnetic field conditions at which the skyrmions emerge. This work demonstrates how chemical bonding and electronic structure enable a new framework for understanding and developing skyrmions under desired conditions that would otherwise be impossible.

cond-mat.str-el↗

arXiv2025

Approximate half-integer quantization in anomalous planar transport in $d$-wave altermagnets

We investigate anomalous planar transport phenomena in a recently identified class of collinear magnetic materials known as $d$-wave altermagnets. The anomalous planar effects manifest in a configuration when the applied electric field/temperature gradient, magnetic field, and the measured Hall voltage are all co-planar, but the planar magnetic field is instrumental in breaking $\hat{C}_{4z}\hat{\mathcal{T}}$ symmetry of the $d$-wave altermagnet, where $\hat{\mathcal{T}}$ is the time reversal operator, resulting in a Zeeman gap at a shifted Dirac node and a nonzero Berry curvature monopole. We demonstrate that these systems exhibit nearly half-quantized anomalous planar Hall and planar thermal Hall effects at low temperatures that persist over a range of magnetic fields. The angular dependence of the planar transport reveals a $\cos2\phi$ dependence on the magnetic field direction, where $\phi$ is the azimuthal angle made by the magnetic field. We also discuss the anomalous planar Nernst effect, or transverse thermopower, and demonstrate that the Nernst conductivity peaks when the chemical potential lies just outside the induced Zeeman gap and vanishes within the gap. We further explore the dependence of all three coefficients on the polar and the azimuthal angle of the magnetic field when it is rotated in the full $3D$ space. Our results reveal the presence of approximately half-quantized anomalous planar thermal Hall plateau for a range of in-plane magnetic fields without requiring topological superconductivity and conducting Majorana modes, and can be probed in experiments in $d$-wave altermagnets.

cond-mat.mes-hall↗

arXiv2025

Fermion parity and quantum capacitance oscillation with partially separated Majorana and quasi-Majorana modes

In a recent experiment, flux dependent oscillations of the quantum capacitance were observed in a one dimensional spin-orbit coupled semiconductor superconductor heterostructure connected end to end via a quantum dot and threaded by a magnetic flux. In the topological superconducting phase of the heterostructure, the oscillations corresponding to different fermion parity sectors are shifted by half a period and can serve as a mechanism for fermion parity readout or fusion operations involving a pair of localized, well separated Majorana modes. In this work, we demonstrate that flux induced fermion parity dependent oscillations of the quantum capacitance in a disordered semiconductor superconductor quantum dot system can originate not only from topologically protected, spatially well separated Majorana zero modes (MZMs) localized at the wire ends, but also, generically, from partially separated Majorana modes with significant overlap, as well as from quasi-Majorana modes in the topologically trivial phase, which can be viewed as Andreev bound states whose constituent Majorana wave functions are slightly shifted relative to each other and have nonzero amplitude at opposite ends of the wire. Therefore, while the detection of flux dependent oscillations of quantum capacitance marks an important experimental advance, such observations alone do not constitute evidence of the presence of topological Majorana zero modes.

cond-mat.mes-hall↗

arXiv2025

Chiral Anomaly Induced Transverse Planar Transport Phenomena in Three Dimensional Spin-Orbit Coupled Metals

We investigate linear and nonlinear transverse planar transport phenomena (viz. linear and nonlinear Hall and Nernst coefficients) induced by chiral anomaly in three-dimensional spin-orbit coupled metallic systems. Unlike Weyl semimetals, these systems do not possess multiple Weyl nodes located at isolated points in the momentum space but instead host a pair of Fermi surfaces characterized by opposite Berry curvature fluxes enclosing the same band-degeneracy point. Using semiclassical Boltzmann transport formalism within the relaxation time approximation, we derive first- and second-order transverse planar transport coefficients induced by electrical and thermal gradients in the presence of an in-plane magnetic field. Our analysis reveals distinctive angular dependencies of the transport coefficients, along with characteristic scaling behavior with the magnetic field strength. Furthermore, we demonstrate that the anomaly-induced transport coefficients exhibit an exponential temperature dependence. This unconventional behavior leads to the violations of the Mott relation at comparatively low temperatures, highlighting unique thermoelectric signatures that can be probed experimentally in 3D spin-orbit coupled metallic systems.

cond-mat.mes-hall↗

arXiv2025

Majorana polarization in disordered heterostructures

Recent studies propose Majorana polarization (MP) as a tool for identifying topological Majorana bound states (MBS). We analyze MP in two systems: a one-dimensional (1d) semiconducting nanowire and a quasi-1d system, both with Rashba spin-orbit coupling, proximity-induced superconductivity, and disorder. While MP reflects topological features, it does not always distinguish true MBS from partially separated Andreev bound states (psABS) or quasi-Majorana modes. True MBS are expected to satisfy $\mathcal{P}^*_\mathrm{left} \cdot \mathcal{P}_\mathrm{right} \sim -1$, but two additional criteria - a topological bandgap and edge-localized wavefunctions - are also necessary for their identification and use in topological quantum computation (TQC). We highlight cases where the polarization product condition holds without meeting these extra requirements. Our conclusions remain robust irrespective of the chosen definition of Majorana polarization, whether based on chiral or the particle-hole framework. Robust MBS identification for TQC demands combining MP with energy spectra and wavefunction localization, especially in disordered systems.

cond-mat.mes-hall↗

arXiv2024

Topological superfluid phases of attractive Fermi-Hubbard model in narrow-band cold-atom optical lattices

We investigate the effects of attractive Hubbard interaction on two-component fermionic atoms in narrow two-dimensional (2D) energy bands that exhibit Rashba spin-orbit coupling (SOC) in the presence of an applied Zeeman field. This narrow-band 2D spin-orbit coupled attractive Fermi-Hubbard model can potentially be realized in cold atom systems in optical lattices with artificially engineered Rashba SOC and Zeeman field. Employing a self-consistent mean field approximation for the pairing potential, we uncover a complex phase diagram featuring various topological superfluid (TS) phases, dependent on the chemical potential and the Zeeman field. We focus on the pairing potential and the corresponding quasiparticle gap characterizing the TS phases, which are notably small for a wide-band model with quadratic dispersion near the $\Gamma$-point, as found in earlier work, and we identify the parameter regimes that maximize the gap. We find that, while generally the value of the pairing potential increases with the reduction of the fermionic bandwidth, as expected for narrow- or flat-band systems, the magnitude of the topological gap characterizing the TS phases reaches a maximum of about $10-12.5\%$ of the interaction strength at finite values of the hopping amplitude, Rashba coupling, and Zeeman field.

cond-mat.quant-gas↗

arXiv2024

Stabilizing topological superconductivity in disordered spin-orbit coupled semiconductor-superconductor heterostructures

We investigate theoretically a one-dimensional semiconductor-superconductor (SM-SC) heterostructure with Rashba spin-orbit coupling and parallel Zeeman field in the presence of disorder generated by random charged impurities and identify the optimal regimes for realizing topological superconductivity and Majorana zero modes. Using a Green's function approach, we show that upon increasing the disorder strength the stable topological superconducting phase characterized by robust end-to-end Majorana correlations "migrates" toward larger values of the Zeeman field and can be stabilized by increasing the effective SM-SC coupling. Based on these findings, we propose a strategy for accessing a regime characterized by well-separated Majorana zero modes that is based on (a) enhancing the strength of the effective SM-SC coupling (e.g., through interface engineering) and (b) expanding the range of accessible Zeeman fields (e.g., by enhancing the gyromagnetic ratio or optimizing the parent superconductor, to enable the application of larger magnetic fields). While this strategy may still require some reduction of the disorder strength, this requirement is significantly less strict than the corresponding requirement in a strategy that focuses exclusively on disorder reduction.

cond-mat.mes-hall↗

arXiv2024

Nonlinear anomalous transverse responses induced by Berry curvature quadrupole in systems with broken time-reversal symmetry

Recent theoretical work has shown that higher-order moments of the Berry curvature, e.g., Berry curvature quadrupole and hexapole moments, can produce the leading order nonlinear anomalous Hall response (NLAH) in systems with special magnetic point group symmetry. Recent experimental work has reported the observation of the Berry curvature quadrupole-induced third-order NLAH (i.e., Hall voltage proportional to the third power of the external electric field) from cryogenic conditions to room temperature in an epitaxially grown material platform with broken time-reversal symmetry. In this paper, using semiclassical Boltzmann formalism in the relaxation time approximation, we compute the Berry curvature quadrupole-induced nonlinear anomalous thermal Hall and Nernst coefficients in time-reversal broken systems. In systems where Berry curvature monopole and dipole moments vanish by symmetry, our results predict the behavior of the leading order anomalous thermal Hall and Nernst coefficients proportional to the third power of the applied longitudinal temperature gradient. They are guaranteed to exist in systems that have already exhibited the third-order nonlinear anomalous Hall effect in recent experiments.

cond-mat.mes-hall↗

arXiv2023

Hydrogen induces chiral conduction channels in the topological magnet

Chirality, a characteristic handedness that distinguishes 'left' from 'right', cuts widely across all of nature$^1$, from the structure of DNA$^2$ to opposite chirality of particles and antiparticles$^3$. In condensed matter chiral fermions have been identified in Weyl semimetals$^4$ through their unconventional electrodynamics arising from 'axial' charge imbalance between chiral Weyl nodes of topologically nontrivial electronic bands. Up to now it has been challenging or impossible to create transport channels of Weyl fermions in a single material that could be easily configured for advancing chiral logic or spintronics$^{5,6}$. Here we generate chirality-directed conduction channels in inversion-symmetric Weyl ferromagnet (FM) $MnSb_2Te_4$, emergent from a deep connection between chirality in reciprocal and real space. We alter the bandstructure on-demand with an intake and a subsequent release of ionic hydrogen ($H^+$) $-$ a process we show to induce the tilt and rotation of Weyl bands. The transformed Weyl FM states feature a doubled Curie temperature $\geq50K$ and an enhanced angular transport chirality synchronous with a rare field-antisymmetric longitudinal resistance $-$ a low-field tunable 'chiral switch' that roots in the interplay of Berry curvature$^7$, chiral anomaly$^8$ and hydrogen-engendered mutation of Weyl nodes.

cond-mat.mtrl-sci↗