arXiv subjects

Varsha Subramanyan

Publications and source records attributed to Varsha Subramanyan.

17 recordsLinked to original sources

arXiv2026

A field theory approach to Breit-type Hamiltonians in gapped Dirac systems

We develop a path-integral-based field theory for deriving Breit-type low-energy Hamiltonians for gapped Dirac systems coupled to both vector and axial gauge fields. Treating the mass gap as the large energy scale, we integrate out the high-energy component of the Dirac spinor and obtain a canonical Schrödinger description for the remaining low-energy degrees of freedom. For the ordinary Dirac equation, our method reproduces the conventional Breit Hamiltonian order by order. In the presence of axial gauge fields, however, the resulting Hamiltonian contains additional vector-axial couplings that have no analogue in the traditional electromagnetic case. We illustrate the method using three- and two-dimensional Dirac models and discuss its relevance to gapped Weyl systems, where dynamical axial fields can generate distinctive low-energy transport signatures.

cond-mat.mes-hall↗

arXiv2026

Sound Induced Hall Currents in Weyl Exciton Insulators

Weyl semimetals exhibit anomalous transport controlled by their gapless chiral nodes, while gapped Weyl systems are often expected to lose such distinctive responses. Here we show that Weyl excitonic insulators instead host a new form of axial-field-driven transport that exists only in the massive phase. Starting from the low-energy Breit-type Hamiltonian for a gapped Weyl system coupled to strain-induced axial potentials, we develop a semiclassical wave-packet theory and identify a dissipative transverse current generated by a dynamical axial potential. This response is qualitatively distinct from both ordinary vector-potential transport in gapped systems and axial responses in gapless Weyl semimetals. Physically, it originates from a mass-induced Berry structure of the reconstructed Weyl bands, which becomes active when the system is driven out of equilibrium by a chiral chemical-potential imbalance. We show that transverse sound waves provide a natural route to generate the required dynamical axial field and estimate the resulting current for realistic material parameters. Our results reveal sound-induced Hall transport as a direct probe of Weyl excitonic order and provide a transport signature of interaction-generated mass in Weyl materials.

cond-mat.mes-hall↗

arXiv2026

Regression to the Mean can Explain Saturation of Geomagnetic Storms

Extreme space weather events on Earth occur during intervals of strong solar wind driving. The solar wind drives plasma convection and currents in the near-Earth space environment. For low values of the driver, the Earth's response is linear, estimated by parameters such as the polar cap index based on ground magnetometer activity. Curiously, for extreme solar wind driving, the Earth's response appears not to increase beyond a saturation limit. Theorists have advanced a host of explanations for this saturation effect, but there is no consensus. Here, we demonstrate that this saturation is a manifestation of the regression to the mean effect arising from random uncertainty in the timing and magnitude of solar wind measurements. Our results reveal that data analysis underpinning the saturation theories is non-linearly biased, thereby challenging the validity of the theories. Correcting for the uncertainties reveals that the Earth's response to solar wind driving is linear throughout, and that the impact of extreme geomagnetic storms can be twice as large as previously thought. We show that regression to the mean is a fundamental property of the relationship between measurement and the truth, where the truth corresponding to the measurement is closer to the mean. This effect is particularly pronounced for uncertain measurements of extreme values and is likely to manifest across various fields, from extreme climate studies to chronic medical pain.

physics.space-ph↗

arXiv2026

Geometry contribution to sound attenuation in double-Weyl semimetals

The axial coupling of strain to the nodes of the simple Weyl semimetals leads to anomalous contributions to sound attenuation in such materials. However, in double Weyl semimetals, there is no such axial coupling. Strain instead couples as a symmetry-breaking director field that deforms the Fermi surface around each Weyl node. In this work, we show that absence of axial coupling in double Weyl semimetals implies a very different mechanism of relaxation due to sound. The deformed geometry of the Fermi surface is the only source of sound attenuation under these conditions. Thus, we identify a geometric contribution to sound attenuation in double Weyl semimetals that is entirely absent in simple Weyl semimetals.

cond-mat.mes-hall↗

arXiv2026

From atomic to global connectivity in the structure of the SARS-CoV2-Human ACE2 receptor complex

We investigate connectivity properties of the SARS-CoV2 spike protein-human ACE2-receptor complex employing a protein side chain-based network method that allows us to span a range from atomic to global protein scales. We analyze network topology in terms of clusters and cliques obtained from averaging over snapshots of MD simulations (from D.E. Shaw Research). We demonstrate that SARS-CoV2 forms a more dominant, robust connection with the ACE2-receptor as compared to the less virulent SARS-CoV1. Globally, this stronger connectivity is reflected by our percolation analysis where the interface cluster for the SARS-CoV2-ACE2 complex persists when restricted to stronger and stronger bonds, as compared to the SARSCoV1- ACE2 complex. At the atomic level, interface clique structure reflects a stronger connectivity in the former complex. We pinpoint key functional residues in SARSCoV2 that play important roles in establishing this higher connectivity. Thus, our studies provide an objective method to map spatial connectivity of atomic level non-covalent interactions to global connectivity between any two amino acids in the complex. We also analyze specific snapshots of the MD simulations to highlight prominent variations in network topology that explore diverse conformational landscapes. Finally, we demonstrate that a majority of mutations that occur in the SARSCoV2 spike protein in variants of concern/interest (including the currently circulating JN.1) have been observed at the interface with the ACE2 receptor. Our analyses highlight the importance of interface interactions and provide a rationale for designing receptor-like peptides and proteins to combat immunity-escaping variants.

physics.bio-ph↗

arXiv2025

Dynamics of "Classical" Bosons, Fermions, and beyond

We study the classical mechanics and dynamics of particles that retains some memory of quantum statistics. Our work builds on earlier work on the statistical mechanics and thermodynamics of such particles. Starting from the effective classical manifold associated with two-particle bosonic and fermionic coherent states, we show how their exchange statistics is reflected in the symplectic form of the manifold. We demonstrate the classical analogues of exclusion or bunching behavior expected in such states by studying their trajectories in various quadratic potentials. Our examples are two-particle coherent states in one dimension and two-particle vortex motion in the lowest Landau level. We finally compare and contrast our results with previous simulations of the full quantum system, and with existing results on the geometric interpretations of quantum mechanics.

quant-ph↗

arXiv2025

Symmetries and dynamics of quantum Hall bulk anyons in quadratic potentials

We study two-particle coherent states and their dynamics in the lowest Landau level (LLL) under the influence of quadratic potentials. We focus on generalized coherent states that describe Abelian anyons in the LLL and are associated with the $\mathfrak{su}(1,1)$ Lie algebra. We draw on parallels with quantum optics and symmetry properties of the coherent states considered here to analytically calculate quantities such as a bunching parameter, which depends on quantum statistics, as well as coherent state trajectories under the influence of generic quadratic potentials. Our results show that in unbounded saddle potentials, the bunching parameter governs the trajectories which show exponentially diverging behavior in a manner that depends on quantum statistics. In bounded elliptical potentials, the bunching parameter is oscillatory and its maximum magnitude depends on the eccentricity of the applied potential. We draw connections between our analyses and the key concepts that underlie anyon detection in recent experiments.

cond-mat.mes-hall↗

arXiv2025

Geometric transport signatures of strained multi-Weyl semimetals

The minimal coupling of strain to Dirac and Weyl semimetals, and its modeling as a pseudo-gauge field has been extensively studied, resulting in several proposed topological transport signatures. In this work, we study the effects of strain on higher winding number Weyl semimetals and show that strain is not a pseudo-gauge field for any winding number larger than one. We focus on the double-Weyl semimetal as an illustrative example to show that the application of strain splits the higher winding number Weyl nodes and produces an anisotropic Fermi surface. Specifically, the Fermi surface of the double-Weyl semimetal acquires nematic order. By extending chiral kinetic theory for such nematic fields, we determine the effective gauge fields acting on the system and show how strain induces anisotropy and affects the geometry of the semi-classical phase space of the double-Weyl semimetal. Further, the strain-induced deformation of the Weyl nodes results in transport signatures related to the covariant coupling of the strain tensor to the geometric tensor associated with the Weyl nodes giving rise to strain-dependent dissipative corrections to the longitudinal as well as the Hall conductance. Thus, by extension, we show that in multi-Weyl semimetals, strain produces geometric signatures rather than topological signatures. Further, we highlight that the most general way to view strain is as a symmetry-breaking field rather than a pseudo-gauge field.

cond-mat.mes-hall↗

arXiv2025

Spin-Triplet Excitonic Insulator in the Ultra-Quantum Limit of HfTe5

More than fifty years ago, excitonic insulators, formed by the pairing of electrons and holes due to Coulomb interactions, were first predicted. Since then, excitonic insulators have been observed in various classes of materials, including quantum Hall bilayers, graphite, transition metal chalcogenides, and more recently in moire superlattices. In these excitonic insulators, an electron and a hole with the same spin bind together and the resulting exciton is a spin singlet. Here, we report the experimental observation of a spin-triplet exciton insulator in the ultra-quantum limit of a three-dimensional topological material HfTe5. We observe that the spin-polarized zeroth Landau bands, dispersing along the field direction, cross each other beyond a characteristic magnetic field in HfTe5, forming the one-dimensional Weyl mode. Transport measurements reveal the emergence of a gap of about 250 μeV when the field surpasses a critical threshold. By performing the material-specific modeling, we identify this gap as a consequence of a spin-triplet exciton formation, where electrons and holes with opposite spin form bound states, and the translational symmetry is preserved. The system reaches charge neutrality following the gap opening, as evidenced by the zero Hall conductivity over a wide magnetic field range (10 - 72 T). Our finding of the spin-triplet excitonic insulator paves the way for studying novel spin transport including spin superfluidity, spin Josephson currents, and Coulomb drag of spin currents in analogy to the transport properties associated with the layer pseudospin in quantum Hall bilayers.

cond-mat.str-el↗

arXiv2024

Majorana bound state parity exchanges in planar Josephson junctions

We describe a scheme to exchange fermion parity between two pairs of Majorana bound states mediated by coupling with a centralized quantum dot. We specifically formulate such a scheme for Majorana bound states nucleated in the Josephson vortices formed in a four-fold crossroads junction of planar topological superconductors in the presence of a perpendicular magnetic field. This platform yields several advantages to the execution of our scheme as compared to similar ideas proposed in wire geometries, including control over the positions of the MBS and hence, a tunable coupling with the quantum dot. We show that moving the MBS along the junctions through voltage pulses can facilitate parity exchange via a two-step process, with intermediate projective measurements of the quantum dot charge. Thus, we formulate a way to achieve single qubit operations for MBS in extended Josephson junctions through projective measurements of quantum dot charge. We also discuss the physical viability of our scheme with a particular focus on changes in quantum dot energy levels as a measurable indicator of the success of the scheme.

cond-mat.mes-hall↗

arXiv2022

Signatures of Majorana bound states and parity effects in two-dimensional chiral p-wave Josephson junctions

We characterize topological features of Josephson junctions formed by coupled mesoscopic chiral $p$-wave superconducting islands. Through analytic and numerical studies of the low-lying BdG (Bogoliubov-de Gennes) spectrum, we identify localized MBS (Majorana bound states) nucleated in Josephson vortices by the application of a perpendicular magnetic field. Additionally, we demonstrate the existence of an extended MBS that is delocalized around the outer perimeter of the coupled islands, which has measurable consequences on the Josephson supercurrent and phase dynamics of the junction. In particular, we predict a change in the critical current diffraction pattern in which the odd integer-flux nodes are lifted in a fermion parity-dependent fashion. We model the competing stochastic effects of thermal noise and macroscopic quantum tunneling within the RCSJ framework and show the emergence of a bimodal critical current distribution. We demonstrate that increasing the parity transition rate suppresses the bimodal nature of the distribution, thus strongly emphasizing the non-trivial parity dependent nature of the many-body ground state. Finally, we consider a trijunction geometry with three islands and discuss possible schemes to braid Majorana bound states by moving the Josephson vortices to which they are bound.

cond-mat.supr-con↗

arXiv2021

Microtubules as electron-based topological insulators

The microtubule is a cylindrical biological polymer that plays key roles in cellular structure, transport, and signalling. In this work, based on studies of electronic properties of polyacetelene and mechanical properties of microtubules themselves (see Phys. Rev. Lett. 103, 248101), we explore the possibility that microtubules could act as topological insulators that are gapped to electronic excitations in the bulk but possess robust electronic bounds states at the tube ends. Through analyses of structural and electronic properties, we model the microtubule as a cylindrical stack of Su-Schrieffer-Heeger chains (originally proposed in the context of polyacetylene) describing electron hopping between the underlying dimerized tubulin lattice sites. We postulate that the microtubule is mostly uniform, dominated purely by GDP-bound dimers, and is capped by a disordered regime due to the presence of GTP-bound dimers as well. In the uniform region, we identify the electron hopping parameter regime in which the microtubule is a topological insulator. We then show the manner in which these topological features remain robust when the hopping parameters are disordered. We briefly mention possible biological implications for these microtubules to possess topologically robust electronic bound states.

cond-mat.mes-hall↗

arXiv2020

Physics of the Inverted Harmonic Oscillator: From the lowest Landau level to event horizons

In this work, we present the inverted harmonic oscillator (IHO) Hamiltonian as a paradigm to understand the quantum mechanics of scattering and time-decay in a diverse set of physical systems. As one of the generators of area preserving transformations, the IHO Hamiltonian can be studied as a dilatation generator, squeeze generator, a Lorentz boost generator, or a scattering potential. In establishing these different forms, we demonstrate the physics of the IHO that underlies phenomena as disparate as the Hawking-Unruh effect and scattering in the lowest Landau level(LLL) in quantum Hall systems. We derive the emergence of the IHO Hamiltonian in the LLL in a gauge invariant way and show its exact parallels with the Rindler Hamiltonian that describes quantum mechanics near event horizons. This approach of studying distinct physical systems with symmetries described by isomorphic Lie algebras through the emergent IHO Hamiltonian enables us to reinterpret geometric response in the lowest Landau level in terms of relativistic effects such as Wigner rotation. Further, the analytic scattering matrix of the IHO points to the existence of quasinormal modes (QNMs) in the spectrum, which have quantized time-decay rates. We present a way to access these QNMs through wave packet scattering, thus proposing a novel effect in quantum Hall point contact geometries that parallels those found in black holes.

cond-mat.mes-hall↗

arXiv2020

Surveying the side-chain network approach to protein structure and dynamics: The SARS-CoV-2 spike protein as an illustrative case

Network theory-based approaches provide valuable insights into the variations in global structural connectivity between differing dynamical states of proteins. Our objective is to review network-based analyses to elucidate such variations, especially in the context of subtle conformational changes. We present technical details of the construction and analyses of protein structure networks, encompassing both the non-covalent connectivity and dynamics. We examine the selection of optimal criteria for connectivity based on the physical concept of percolation. We highlight the advantages of using side-chain based network metrics in contrast to backbone measurements. As an illustrative example, we apply the described network approach to investigate the global conformational change between the closed and partially open states of the SARS-CoV-2 spike protein. This conformational change in the spike protein is crucial for coronavirus entry and fusion into human cells. Our analysis reveals global structural reorientations between the two states of the spike protein despite small changes between the two states at the backbone level. We also observe some differences at strategic locations in the structures, correlating with their functions, asserting the advantages of the side-chain network analysis. Finally we present a view of allostery as a subtle synergistic-global change between the ligand and the receptor, the incorporation of which would enhance the drug design strategies.

q-bio.BM↗

arXiv2019

Quasinormal Modes and Hawking-Unruh effect in Quantum Hall Systems: Lessons from Black Hole Phenomena

In this work, we propose the quantum Hall system as a platform for exploring black hole phenomena. By exhibiting deep rooted commonalities between lowest Landau level and spacetime symmetries, we show that features of both quantum Hall and gravitational systems can be elegantly captured by a simple quantum mechanical model, the inverted harmonic oscillator. Through this correspondence, we argue that radiation phenomena in gravitational situations, such as presented by W. G. Unruh and S. Hawking, bears a parallel with saddle-potential scattering of quantum Hall quasiparticles. We also find that scattering by the quantum Hall saddle potential can mimic the signature quasinormal modes in black holes, such as theoretically demonstrated through Gaussian scattering off a Schwarzschild black hole by C. V. Vishveshwara. We propose a realistic quantum Hall point contact setup for probing these temporally decaying modes in quasiparticle tunneling, offering a new mesoscopic parallel for black hole ringdown.

cond-mat.mes-hall↗

arXiv2019

Correlations, Dynamics, and Interferometry of Anyons in the Lowest Landau Level

In this article, we present a systematic study of quantum statistics and dynamics of a pair of anyons in the lowerst Landau level (LLL), of direct relevance to quasiparticle excitations in the quantum Hall bulk. We develop the formalism for such a two-dimensional setting of two charged particles subject to a transverse field, including fractional angular momentum states and the related algebra stemming from the anyonic boundary condition, coherent state descriptions of localized anyons, and bunching features associated with such anyons. We analyze the dynamic motion of the anyons in a harmonic trap, emphasizing phase factors emerging from exchange statistics. We then describe non-equilibrium dynamics upon the application of a saddle potential, elaborating on its role as a squeezing operator acting on LLL coherent states, and its action as a beam splitter for anyons. Employing these potential landscapes as building blocks, we analyze anyon dynamics in a quantum Hall bulk interferometer. We discuss parallels between the presented LLL setting and other realms, extensively in the context of quantum optics, whose formalism we heavily borrow from, and briefly in that of black hole phenomena.

cond-mat.mes-hall↗

arXiv2016

On a square-ice analogue of plane partitions

We study a one-parameter family ($\ell=1,2,3,\ldots$) of configurations that are square-ice analogues of plane partitions. Using an algorithm due to Bratley and McKay, we carry out exact enumerations in order to study their asymptotic behaviour and establish, via Monte Carlo simulations as well as explicit bounds, that the asymptotic behaviour is similar to that of plane partitions. We finally carry out a series analysis and provide independent estimates for the asymptotic behaviour.

cond-mat.stat-mech↗