arXiv subjects

Pankaj Sharma

Publications and source records attributed to Pankaj Sharma.

At least 19 recordsLinked to original sources

arXiv2026

Double-peak Majorana bound states in altermagnet--superconductor heterostructures

We study Majorana bound states in a planar Josephson junction in which the middle channel is a $d$-wave altermagnetic metal deposited on a proximitized two-dimensional electron gas. In the topological regime, the near-zero-energy states reveals a characteristic double-peak spatial profile, with the Majorana wavefunction localized near the altermagnet--superconductor interfaces. Using simplified theoretical models, we show that anisotropic hopping intrinsic to altermagnetism naturally generates interface-localized low-energy states, providing the natural explanation for the double-peak structure. In a nanowire geometry with extended normal metallic regions, the same feature persists but the Majorana bound states become more sensitive to the chemical potential compared to the case in planar Josephson junction. In a T-shaped Josephson junction, multiple near-zero-energy states appear, and the Majorana bound state expected at the crossing point is found to be localized near the interfaces, demonstrating that the localization of the Majorana bound states is primarily governed by interface boundaries rather than by the junction geometry. These results show that anisotropic hopping and interface structure play a central role in altermagnet-based topological superconductors and provide a promising route toward a network of controllable Majorana bound states without external magnetic fields.

cond-mat.supr-con↗

arXiv2026

Collective Phonon Mixing and Eigenvector Transport Under Isotope Substitution

Isotopic substitution modifies nuclear masses without altering the electronic potential energy surface to first order and is therefore often interpreted as a simple rescaling of vibrational frequencies. In solids with dense phonon manifolds, however, mass substitution acts as a parametric Hermitian deformation of the mass-weighted dynamical matrix, generating a continuous family of eigenproblems whose eigenvectors can undergo substantial rotation within coupled subspaces. Here we investigate protiated and deuterated ZIF-8 using inelastic neutron scattering and density functional theory lattice-dynamics calculations. While many vibrational modes exhibit near-ideal mass scaling and preserve their character across isotopic endpoints, modes embedded in spectrally congested regions display pronounced redistribution of vibrational character that cannot be inferred from frequency shifts alone. Because inelastic neutron scattering intensity is directly weighted by hydrogen displacement amplitude, spectral sparsity and congestion provide experimental indicators of predictable frequency renormalisation or susceptibility to qualitative eigenvector reorganisation under deuteration. To establish physically meaningful mode correspondence, we develop an adiabatic eigenvector-continuation framework with overlap-based tracking and explicit stability diagnostics. These results show that vibrational identity in complex framework materials is best understood as a continuous trajectory in eigenvector space and provide a general framework for analysing isotope-induced spectral flow in dense phonon systems.

cond-mat.mtrl-sci↗

arXiv2025

Diode effect in a skyrmion-coupled high-temperature Josephson junction

We show that a planar Josephson junction having $d$-wave superconducting regions, with a skyrmion crystal placed underneath, produces a robust gate-tunable superconducting diode effect. The spatially-varying exchange field of the skyrmion crystal breaks both inversion and time-reversal symmetries, leading to an asymmetric current-phase relation with an anomalous phase shift. Our theoretical calculations, obtained using resistively and capacitively shunted junction model combined with Bogoliubov-de Gennes method, reveal that the diode efficiency is largely tunable by controlling external gate voltage and skyrmion radius. Incorporation of a $d$-wave superconductor such as high-$T_c$ Cuprate enables the diode to function at higher operating temperatures. Our results establish a unique and practically-realizable mechanism for devising tunable field-free superconducting diodes based on magnetic texture-superconductor hybrid platforms.

cond-mat.supr-con↗

arXiv2025

Magnetic field-free braiding and nontrivial fusion of Majorana bound states in high-temperature planar Josephson junctions

Demonstration of non-Abelian statistics of Majorana bound states (MBS) is crucial for the realization of fault-tolerant topological quantum computation. Two-dimensional platforms such as planar Josephson junctions require an in-plane magnetic field to generate a pair of MBS at its non-superconducting channel ends; however, the fixed direction of the in-plane magnetic field puts a constraint on the realization of a multi-terminal topological planar junction, and hence its ability to physically move multiple MBS -- which is necessary for performing the fusion and braiding operations. Here we show that in a planar Josephson junction coupled to a skyrmion crystal, which can generate multiple pairs of MBS in the absence of any external magnetic field, the non-trivial fusion and braiding operations can be performed. Our numerical calculations, designed for realistic two-dimensional quantum systems, certify the feasibility of experimental realization of the proposed device schemes. We find that both $s$-wave and $d$-wave superconducting leads can generate the MBS; indicating that the MBS movement operations can be performed at higher temperatures using $d$-wave superconducting leads. Our results establish that the skyrmion crystal-coupled planar Josephson junction is a viable platform for the generation and controlled movement of the MBS.

cond-mat.supr-con↗

arXiv2024

Eclipse Dynamics and X-ray Burst Characteristics in the Low-Mass X-ray Binary EXO 0748-676

This study investigates the timing and spectral characteristics of X-ray bursts from the neutron star system EXO 0748-676 using the NuSTAR observatory's FPMA and FPMB instruments in the 3-79 keV range. We identify Type I X-ray bursts driven by thermonuclear explosions on the neutron star's surface, notably a significant burst at $X = 18,479.97$, indicating rapid energy release, followed by a recoil burst at $X = 19,463.97$, reflecting stabilization. The correlation between burst timing and the neutron star's optical period suggests modulation by its rotation and periodic accretion dynamics. Spectral modeling reveals a photon index of $ \Gamma = 1.24 \pm 0.014 $ and a cutoff energy of $ E_C = 36.20 \pm 1.04 \; \text{keV} $, indicating a hot corona around the neutron star. The measured flux of approximately $ (381.17 \pm 0.014) \times 10^{-12} \; \text{erg cm}^{-2} \text{s}^{-1} $ underscores the dynamic nature of accretion-driven systems. Calculated luminosities derived from distance estimates range from $ (3.86 \pm 0.239) \times 10^{36} \; \text{erg/s} $ to $ (2.3 \pm 0.177) \times 10^{36} \; \text{erg/s} $. Comparative analysis with prior observations from the IBIS/ISGRI instrument on the INTEGRAL satellite shows variability in emission characteristics, including softer photon indices and higher cutoff energies in 2003 and 2004. Our examination of smaller energy gaps (3-7 keV, 7-12 keV, etc.) reveals energy-dependent behavior in burst characteristics, enhancing our understanding of nuclear burning phases. Overall, these findings validate models describing Type I X-ray bursts and lay the groundwork for future investigations into similar astrophysical systems and stellar evolution processes in extreme environments.

astro-ph.HE↗

arXiv2024

Multiple Majorana bound states and their resilience against disorder in planar Josephson junctions

Planar Josephson junctions are theoretically predicted to harbor zero-energy Majorana bound states (MBS) in a tunable two-dimensional geometry, at the two ends of the middle metallic channel. Here we show that three distinct topological superconducting regimes, governing the localization of the near-zero-energy MBS, appear in these planar Josephson junctions. The topologically-protected MBS appear near the narrow edges of the junction -- not only in the middle metallic channel but also in the superconducting leads which have widths similar to the values used in recent experiments. We incorporate random fluctuation in the chemical potential to investigate the influence of non-magnetic disorder on the localization of the MBS in different topological regimes and find that the MBS are quite robust against disorder because of the two-dimensional geometry. Interestingly, moderate amount of disorder reduces the splitting between the MBS pairs, possibly by minimizing the wave function overlap of the MBS. We also discuss the changes in the topological superconducting phases when the superconducting lead width is varied. Our results reveal a rich structure of the localization of topologically protected multiple MBS in experimentally-accessible planar Josephson junctions, and call for their experimental confirmation.

cond-mat.supr-con↗

arXiv2024

Imaging topological polar structures in marginally twisted 2D semiconductors

Moire superlattices formed in van der Waals heterostructures due to twisting, lattice mismatch and strain present an opportunity for creating novel metamaterials with unique properties not present in the individual layers themselves. Ferroelectricity for example, arises due to broken inversion symmetry in twisted and strained bilayers of 2D semiconductors with stacking domains of alternating out-of-plane polarization. However, understanding the individual contributions of twist and strain to the formation of topological polar nanostructures remains to be established and has proven to be experimentally challenging. Inversion symmetry breaking has been predicted to give rise to an in-plane component of polarization along the domain walls, leading to the formation of a network of topologically non-trivial merons (half-skyrmions) that are Bloch-type for twisted and Neel-type for strained systems. Here we utilise angle-resolved, high-resolution vector piezoresponse force microscopy (PFM) to spatially resolve polarization components and topological polar nanostructures in marginally twisted bilayer WSe2, and provide experimental proof for the existence of topologically non-trivial meron/antimeron structures. We observe both Bloch-type and Neel-type merons, allowing us to differentiate between moire superlattices formed due to twist or heterogeneous strain. This first demonstration of non-trivial real-space topology in a twisted van der Waals heterostructure opens pathways for exploring the connection between twist and topology in engineered nano-devices.

cond-mat.mes-hall↗

arXiv2023

Spin-Orbital Coupling in All-Inorganic Metal-Halide Perovskites: the Hidden Force that Matters

Highlighted with improved long-term thermal and environmental stability, all-inorganic metal halide perovskites exhibit tunable physical properties, cost-effective synthesis, and satisfactory optoelectronic performance, attracting increasing research interests worldwide. However, a less explored feature of these materials is their strong spin-orbit coupling (SOC), which is the hidden force influencing not only band structure but also properties including magnetoresistance, spin lifetime and singlet-triplet splitting. This review provides an overview of the fundamental aspects and the latest progress of the SOC and debate regarding Rashba effects in all-inorganic metal halide perovskites, providing critical insights into the physical phenomena and potential applications. Meanwhile, crystal structures and photophysics of all-inorganic perovskite are discussed in the context of SOC, along with the related experimental and characterization techniques. Furthermore, a recent understanding of the band topology in the all-inorganic halide perovskites is introduced to push the boundary even further for the novel applications of all-inorganic halide perovskites. Finally, an outlook is given on the potential directions of breakthroughs via leveraging the SOC in halide perovskites.

cond-mat.mtrl-sci↗

arXiv2023

Challenges in detecting topological superconducting transitions via supercurrent and phase probes in planar Josephson junctions

Topological superconductors harbor, at their boundaries and vortex cores, zero-energy Majorana bound states, which can be the building blocks in fault-tolerant topological quantum computing. Planar Josephson junctions host such topological superconducting phases, highly tunable by external magnetic field or phase difference between the superconducting leads. Despite many theoretical and experimental studies, the signatures of the transition to a topological superconducting phase, based on minima in the critical supercurrent $I_c$ flowing across the junction, $0$-$\pi$ transition in the ground state junction phase and their anisotropic magnetic-field response have remained unsettled. Using rigorous numerical calculations with several experimentally-relevant parameter settings, we show that $I_c$ and $\varphi_{_{\rm GS}}$ cannot indicate unambiguously topological transition in any realistic planar junctions. Furthermore, the anisotropic variations of $I_c$ and $\varphi_{_{\rm GS}}$ with in-plane magnetic field appear in junctions that are undoubtedly in trivial superconducting phase, raising concerns on the effectiveness of these probes in identifying topological transitions in planar junctions. We discuss possible strategies to confirm a topological superconducting phase in these platforms.

cond-mat.supr-con↗

arXiv2022

Meta Learning for Few-Shot Medical Text Classification

Medical professionals frequently work in a data constrained setting to provide insights across a unique demographic. A few medical observations, for instance, informs the diagnosis and treatment of a patient. This suggests a unique setting for meta-learning, a method to learn models quickly on new tasks, to provide insights unattainable by other methods. We investigate the use of meta-learning and robustness techniques on a broad corpus of benchmark text and medical data. To do this, we developed new data pipelines, combined language models with meta-learning approaches, and extended existing meta-learning algorithms to minimize worst case loss. We find that meta-learning on text is a suitable framework for text-based data, providing better data efficiency and comparable performance to few-shot language models and can be successfully applied to medical note data. Furthermore, meta-learning models coupled with DRO can improve worst case loss across disease codes.

cs.CL↗

arXiv2022

Singularity: Planet-Scale, Preemptive and Elastic Scheduling of AI Workloads

Lowering costs by driving high utilization across deep learning workloads is a crucial lever for cloud providers. We present Singularity, Microsoft's globally distributed scheduling service for highly-efficient and reliable execution of deep learning training and inference workloads. At the heart of Singularity is a novel, workload-aware scheduler that can transparently preempt and elastically scale deep learning workloads to drive high utilization without impacting their correctness or performance, across a global fleet of AI accelerators (e.g., GPUs, FPGAs). All jobs in Singularity are preemptable, migratable, and dynamically resizable (elastic) by default: a live job can be dynamically and transparently (a) preempted and migrated to a different set of nodes, cluster, data center or a region and resumed exactly from the point where the execution was preempted, and (b) resized (i.e., elastically scaled-up/down) on a varying set of accelerators of a given type. Our mechanisms are transparent in that they do not require the user to make any changes to their code or require using any custom libraries that may limit flexibility. Additionally, our approach significantly improves the reliability of deep learning workloads. We show that the resulting efficiency and reliability gains with Singularity are achieved with negligible impact on the steady-state performance. Finally, our design approach is agnostic of DNN architectures and handles a variety of parallelism strategies (e.g., data/pipeline/model parallelism).

cs.DC↗

arXiv2020

Brain volume: An important determinant of functional outcome after acute ischemic stroke

Objective: To determine whether brain volume is associated with functional outcome after acute ischemic stroke (AIS). Methods: We analyzed cross-sectional data of the multi-site, international hospital-based MRI-GENetics Interface Exploration (MRI-GENIE) study (July 1, 2014- March 16, 2019) with clinical brain magnetic resonance imaging (MRI) obtained on admission for index stroke and functional outcome assessment. Post-stroke outcome was determined using the modified Rankin Scale (mRS) score (0-6; 0: asymptomatic; 6 death) recorded between 60-190 days after stroke. Demographics and other clinical variables including acute stroke severity (measured as National Institutes of Health Stroke Scale score), vascular risk factors, and etiologic stroke subtypes (Causative Classification of Stroke) were recorded during index admission. Results: Utilizing the data from 912 acute ischemic stroke (AIS) patients (65+/-15 years of age, 58% male, 57% history of smoking, and 65% hypertensive) in a generalized linear model, brain volume (per 155.1cm^3 ) was associated with age (beta -0.3 (per 14.4 years)), male sex (beta 1.0) and prior stroke (beta -0.2). In the multivariable outcome model, brain volume was an independent predictor of mRS (beta -0.233), with reduced odds of worse long-term functional outcomes (OR: 0.8, 95% CI 0.7-0.9) in those with larger brain volumes. Conclusions: Larger brain volume quantified on clinical MRI of AIS patients at time of stroke purports a protective mechanism. The role of brain volume as a prognostic, protective biomarker has the potential to forge new areas of research and advance current knowledge of mechanisms of post-stroke recovery.

q-bio.NC↗

arXiv2020

Fractal basins of attraction in a binary quasar model

The present paper investigates the binary system of quasars in the framework of the Circular Restricted Three-Body Problem. The parametric evolution of libration points, the geometry of zero-velocity curves are one of the crucial aspects of our study. The multivariate form of NR method is applied to study the basin of attraction connected with libration points. The algorithm for using the Newton-Raphson method is slightly modified in order to avoid the unnecessary delay in the convergence of initial conditions. The impact of parameters on the shape of the basin of attraction and the number of iterations needed for the convergence of initial conditions are explored. We carry out an exhaustive (numerical) study to show the influence of these parameters on converging regions in basins of convergence. We unveil the existence of fractal structure in the basin of attraction using the method of basin entropy. In almost all cases, the existence of fractal structure is found throughout the basins of attraction.

nlin.CD↗

arXiv2019

Top polarisation as a probe of CP-mixing top-Higgs coupling in $tjh$ signals

In this letter we explore beyond the Standard Model top-Higgs Yukawa couplings as a function of a CP-mixing parameter $ξ_t$ at the 14 TeV HL-LHC in the process $pp\to thj$. We observe that angular variables of the decay products of the top are non-trivially sensitive to $ξ_t$. This fact is exploited in a full detector level analysis that employs machine learning techniques to optimize signal sensitivity on a suite of variables, including lepton azimuthal angle. The key result of this study is an improved projected exclusion limit on $ξ_t$ even when including the realistic effects of detector smearing and a conservative estimate of systematic error.

hep-ph↗

arXiv2019

Exploring CP phase in $τ$-lepton Yukawa coupling in Higgs decays at the LHC

We study the prospect of determining the CP violating phase in $τ$-lepton Yukawa coupling at the Large Hadron Collider (LHC). While the current run is already exploring the production of a pair of the third generation $τ$ leptons from Higgs decay, these measurements are not sensitive enough to constrain the CP violating phase. In this paper, several CP odd observables are proposed and analyzed utilizing the dominant channels with the semi-invisible hadronic decay of $τ$. Several asymmetries corresponding to the T odd momentum correlations are also studied and their sensitivities to the CP violating phase in tau-lepton Yukawa couplings are estimated at 13 TeV LHC with 1000 ${fb}^{-1}$ of integrated luminosity. We also present a novel way to reconstruct $τ$ momentum at the LHC utilizing the information of impact parameter. Finally, we obtain that the asymmetries can be as large as 35$\%$ for a case of maximal CP violation in the $τ$ Yukawa couplings.

hep-ph↗

arXiv2018

The Effect of a Light Sterile Neutrino at NO$ν$A and DUNE

Now that the NOvA experiment is approaching two years run time and has released some preliminary data, some constraints for the oscillation parameters can be inferred. Currently the best fits for NOvA are three separate results, the reason they are indistinct is that they produce almost degenerate probability curves. It has been postulated that these degeneracies can be resolved by running antineutrinos at NOvA and/or combining its data with T2K. However, this degeneracy resolution power can be compromised if sterile neutrinos are present due to additional degrees of freedom that can significantly alter the oscillation probability for any of the current best fits. We aim to investigate this degradation in predictive power and the effect of the DUNE experiment on it. Now that the NOvA experiment has been running for a few years and has released some preliminary data, some constraints for the oscillation parameters can be inferred. The best fits for NOvA include three degenerate results, the reason they are indistinct is that they produce almost degenerate probability curves. It has been postulated that these degeneracies can be resolved by running antineutrinos at NOvA and/or combining its data with T2K. However, this degeneracy resolution power can be compromised if sterile neutrinos are present due to additional degrees of freedom that can significantly alter the oscillation probability for any of these best fits. We aim to investigate this degradation in predictive power and the effect of the DUNE experiment on it. In light of the 2018 NOvA data we also consider the same fits but with theta_{23}=45 to see if the sensitivity results are different.

hep-ph↗

arXiv2017

Parameter degeneracy and hierarchy sensitivity of NO$ν$A in presence of sterile neutrino

The first hint of neutrino mass hierarchy is expected to come from the NO$ν$A experiment in Fermilab as the present best-fit parameter space i.e., normal hierarchy and $δ_{CP}=-90^\circ$ is the favourable parameter space for NO$ν$A where there is no degeneracy. But this situation may change if the standard three flavour framework is not complete and there is existence of new physics. In this work we consider the presence of an extra light sterile neutrino at the eV scale and study the new degeneracies which are absent in the standard three flavour framework. We also study the effect of these new degeneracies on the hierarchy measurement of NO$ν$A.

hep-ph↗

arXiv2017

Signal to background interference in $pp\to t H^-\to t W^- b\bar b$ at the LHC Run-II

We investigate in the Large Hadron Collider (LHC) environment the possibility that sizeable interference effects between a {\sl heavy} charged Higgs boson signal produced via $bg\to tH^-$ (+ c.c.) scattering and decaying via $H^-\to W^- A\to W^- b\bar b$ (+ c.c.) and the irreducible background given by $bg\to t W^- b\bar b$ topologies could spoil current search approaches where the former and latter channels are treated separately. The rationale for this comes from the fact that a heavy charged Higgs state can have a large width, and so can happen for the CP-odd neutral Higgs state emerging in the ensuing decays well, which in turn enables such interferences. We conclude that effects are very significant, both at the inclusive and exclusive level (i.e., both before and after $H^\pm$ selection cuts are enforced, respectively) and typically of a destructive nature. This therefore implies that currently established LHC reaches for heavy charged Higgs bosons require some level of rescaling. However, this is possible a posteriori, as the aforementioned $H^\pm$ selection cuts shape the interference contributions at the differential level in a way similar to that of the isolated $H^\pm$ signal, so there is no need to reassess the efficiency of the individual cuts. We show such effects quantitatively by borrowing benchmarks points from different Yukawa types of a 2-Higgs Doublet Model parameter space for $H^\pm$ values starting from around 200 GeV.

hep-ph↗