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Abhijit Ghosh

Publications and source records attributed to Abhijit Ghosh.

14 recordsLinked to original sources
arXiv2026

Earthquake Detection Using Benford's Law

Reliable detection of local earthquakes and accurate identification of P-wave onsets are fundamental tasks in seismology, yet many existing methods to accomplish them require extensive parameter tuning or large training datasets. In this study, we investigate the applicability of Benford's Law - a logarithmic distribution governing the occurrence of leading digits in naturally occurring data - as a statistical framework for local earthquake detection. We analyze continuous seismic waveform data from two contrasting tectonic environments in the Indian subcontinent: the intraplate Deccan Volcanic Province and the actively deforming Himalayan region. Using a sliding-window approach, we quantify the temporal conformity of first-digit distributions of seismic amplitudes to Benford's Law and apply an adaptive normalization scheme to account for station-specific noise. Our results show that local earthquake waveforms consistently conform to Benford's Law during the onset of seismic energy, while pre-event noise does not. Benford-derived statistical anomalies align closely with theoretical P-wave arrival times, with detection performance primarily controlled by window length. These results establish Benford's Law as a computationally inexpensive, parameter-light, and training-free tool for local earthquake detection.

physics.geo-ph↗
arXiv2024

Room Temperature Strong Orbital Moments in Perpendicularly Magnetized Magnetic Insulator

The balance between the orbital and spin magnetic moments in a magnetic system is the heart of many intriguing phenomena. Here, we show experimental evidence of a large orbital moment, which competes with its spin counterpart in a ferrimagnetic insulator thulium iron garnet, Tm3Fe5O12. Leveraging element-specific X-ray magnetic circular dichroism (XMCD), we establish that the dominant contribution to the orbital moment originates from 4f orbitals of Tm. Besides the large Tm orbital moment, intriguingly, our results also reveal a smaller but evident non-zero XMCD signal in the O K edge, suggesting additional spin-orbit coupling and exchange interactions with the nearest neighbour Fe atoms. The unquenched orbital moment is primarily responsible for a significant reduction in g-factor, typically 2 in transition metals, as determined independently using ferromagnetic resonance spectroscopy. Our findings reveal a non-linear reduction in the g-factor from 1.7 at 300 K to 1.56 at 200 K in Tm3Fe5O12 thin films. These results provide critical insights into the role of the f orbitals in long-range magnetic order and stimulate further exploration in orbitronics.

cond-mat.mtrl-sci↗
arXiv2021

Crystallographic evaluation of low cycle fatigue crack growth in a polycrystalline Ni based superalloy

The present work discusses the micro-mechanism of low cycle fatigue (LCF) crack growth in smooth bar specimens of Haynes 282. Two parametric approaches, i.e. crack tip opening angle (CTOA) and maximum tangential stress (θMTS) have been opted to characterize the cracks. CTOA variations along with a propagating crack, exhibit a non-linear decay followed by a stabilized regime. Mixicity of local KI and KII fields is directly proportional to θMTS and that can be assessed by measuring local deflections. Around the crack, the role of grain incompatibility has been addressed through EBSD and slip transfer analysis. There is a critical bound for Elastic Modulus (EM) and Schmid factor (SF) for the grains favouring subsurface crack propagation, and these values exist beyond a limiting threshold. The SF-EM maps mark the regions of cracked and uncracked grains in the material. The favourable twin-matrix incompatibility of the microstructure has also been identified about the fatigue crack growth and twins in (211) plane is abundant in the cracked region. A detailed slip transfer analysis based on the Luster-Morris parameter (LMP) has been carried out for investigating the interrelation between slip activity, elasto-plastic incompatibility, and grain boundary geometry.

cond-mat.mtrl-sci↗
arXiv2017

Interface enhanced spin-orbit torques and current-induced magnetization switching of Pd/Co/AlO$_x$ layers

Magnetic heterostructures that combine large spin-orbit torque efficiency, perpendicular magnetic anisotropy, and low resistivity are key to develop electrically-controlled memory and logic devices. Here we report on vector measurements of the current-induced spin orbit torques and magnetization switching in perpendicularly magnetized Pd/Co/AlO$_x$ layers as a function of Pd thickness. We find sizeable damping-like (DL) and field-like (FL) torques, of the order of 1~mT per $10^7$~A/cm$^2$, which have different thickness and magnetization angle dependence. The analysis of the DL torque efficiency per unit current density and electric field using drift-diffusion theory leads to an effective spin Hall angle and spin diffusion length of Pd larger than 0.03 and 7~nm, respectively. The FL SOT includes a significant interface contribution, is larger than estimated using drift-diffusion parameters, and is further strongly enhanced upon rotation of the magnetization from the out-of-plane to the in-plane direction. Finally, taking advantage of the large spin-orbit torques in this system, we demonstrate bipolar magnetization switching of Pd/Co/AlO$_x$ layers with similar current density as used for Pt/Co layers with comparable perpendicular magnetic anisotropy.

cond-mat.mtrl-sci↗
arXiv2016

Faceted particles formed by the frustrated packing of anisotropic colloids on curved surfaces

We use computer simulations and simple theoretical models to analyze the morphologies that result when rod-like particles end-attach onto a curved surface, creating a finite-thickness monolayer aligned with the surface normal. This geometry leads to two forms of frustration, one associated with the incompatibility of hexagonal order on surfaces with Gaussian curvature, and the second reflecting the deformation of a layer with finite thickness on a surface with non-zero mean curvature. We show that the latter effect leads to a faceting mechanism. Above threshold values of the inter-particle attraction strength and surface mean curvature, the adsorbed layer undergoes a transition from orientational disorder to an ordered state that is demarcated by reproducible patterns of line defects. The number of facets is controlled by the competition between line defect energy and intra-facet strain. Tuning control parameters thus leads to a rich variety of morphologies, including icosahedral particles and irregular polyhedra. In addition to suggesting a new strategy for the synthesis of aspherical particles with tunable symmetries, our results may shed light on recent experiments in which rod-like HIV GAG proteins assemble around nanoscale particles.

cond-mat.soft↗
arXiv2015

Magnetoresistance of heavy and light metal/ferromagnet bilayers

We studied the magnetoresistance of normal metal (NM)/ferromagnet (FM) bilayers in the linear and nonlinear (current-dependent) regimes and compared it with the amplitude of the spin-orbit torques and thermally induced electric fields. Our experiments reveal that the magnetoresistance of the heavy NM/Co bilayers (NM = Ta, W, Pt) is phenomenologically similar to the spin Hall magnetoresistance (SMR) of YIG/Pt, but has a much larger anisotropy, of the order of 0.5%, which increases with the atomic number of the NM. This SMR-like behavior is absent in light NM/Co bilayers (NM = Ti, Cu), which present the standard AMR expected of polycrystalline FM layers. In the Ta, W, Pt/Co bilayers we find an additional magnetoresistance, directly proportional to the current and to the transverse component of the magnetization. This so-called unidirectional SMR, of the order of 0.005%, is largest in W and correlates with the amplitude of the antidamping spin-orbit torque. The unidirectional SMR is below the accuracy of our measurements in YIG/Pt.

cond-mat.mes-hall↗
arXiv2015

Unidirectional spin Hall magnetoresistance in ferromagnet/normal metal bilayers

Magnetoresistive effects are usually invariant upon inversion of the magnetization direction. In noncentrosymmetric conductors, however, nonlinear resistive terms can give rise to a current dependence that is quadratic in the applied voltage and linear in the magnetization. Here we demonstrate that such conditions are realized in simple bilayer metal films where the spin-orbit interaction and spin-dependent scattering couple the current-induced spin accumulation to the electrical conductivity. We show that the longitudinal resistance of Ta|Co and Pt|Co bilayers changes when reversing the polarity of the current or the sign of the magnetization. This unidirectional magnetoresistance scales linearly with current density and has opposite sign in Ta and Pt, which we associate with the modification of the interface scattering potential induced by the spin Hall effect in these materials. Our results suggest a route to control the resistance and detect magnetization switching in spintronic devices using a two-terminal geometry, which applies also to heterostructures including topological insulators.

cond-mat.mes-hall↗
arXiv2014

Interplay of spin-orbit torque and thermoelectric effects in ferromagnet/normal metal bilayers

We present harmonic transverse voltage measurements of current-induced thermoelectric and spin-orbit torque (SOT) effects in ferromagnet/normal metal bilayers, in which thermal gradients produced by Joule heating and SOT coexist and give rise to ac transverse signals with comparable symmetry and magnitude. Based on the symmetry and field-dependence of the transverse resistance, we develop a consistent method to separate thermoelectric and SOT measurements. By addressing first ferromagnet/light metal bilayers with negligible spin-orbit coupling, we show that in-plane current injection induces a vertical thermal gradient whose sign and magnitude are determined by the resistivity difference and stacking order of the magnetic and nonmagnetic layers. We then study ferromagnet/heavy metal bilayers with strong spin-orbit coupling, showing that second harmonic thermoelectric contributions to the transverse voltage may lead to a significant overestimation of the antidamping SOT. We find that thermoelectric effects are very strong in Ta(6nm)/Co(2.5nm) and negligible in Pt(6nm)/Co(2.5nm) bilayers. After including these effects in the analysis of the transverse voltage, we find that the antidamping SOTs in these bilayers, after normalization to the magnetization volume, are comparable to those found in thinner Co layers with perpendicular magnetization, whereas the field-like SOTs are about an order of magnitude smaller.

cond-mat.mes-hall↗
arXiv2014

Ultrafast magnetization switching by spin-orbit torques

Spin-orbit torques induced by spin Hall and interfacial effects in heavy metal/ferromagnetic bilayers allow for a switching geometry based on in-plane current injection. Using this geometry, we demonstrate deterministic magnetization reversal by current pulses ranging from 180~ps to ms in Pt/Co/AlOx dots with lateral dimensions of 90~nm. We characterize the switching probability and critical current $I_c$ as function of pulse length, amplitude, and external field. Our data evidence two distinct regimes: a short-time intrinsic regime, where $I_c$ scales linearly with the inverse of the pulse length, and a long-time thermally assisted regime where $I_c$ varies weakly. Both regimes are consistent with magnetization reversal proceeding by nucleation and fast propagation of domains. We find that $I_c$ is a factor 3-4 smaller compared to a single domain model and that the incubation time is negligibly small, which is a hallmark feature of spin-orbit torques.

cond-mat.mes-hall↗
arXiv2014

Fieldlike and antidamping spin-orbit torques in as-grown and annealed Ta/CoFeB/MgO layers

We present a comprehensive study of the current-induced spin-orbit torques in perpendicularly magnetized Ta/CoFeB/MgO layers. The samples were annealed in steps up to 300 degrees C and characterized using x-ray absorption spectroscopy, transmission electron microscopy, resistivity, and Hall effect measurements. By performing adiabatic harmonic Hall voltage measurements, we show that the transverse (field-like) and longitudinal (antidamping-like) spin-orbit torques are composed of constant and magnetization-dependent contributions, both of which vary strongly with annealing. Such variations correlate with changes of the saturation magnetization and magnetic anisotropy and are assigned to chemical and structural modifications of the layers. The relative variation of the constant and anisotropic torque terms as a function of annealing temperature is opposite for the field-like and antidamping torques. Measurements of the switching probability using sub-μs current pulses show that the critical current increases with the magnetic anisotropy of the layers, whereas the switching efficiency, measured as the ratio of magnetic anisotropy energy and pulse energy, decreases. The optimal annealing temperature to achieve maximum magnetic anisotropy, saturation magnetization, and switching efficiency is determined to be between 240 degrees and 270 degrees C.

cond-mat.mes-hall↗
arXiv2013

A "Gaussian" for diffusion on the sphere

We present an analytical closed form expression, which gives a good approximate propagator for diffusion on the sphere. Our formula is the spherical counterpart of the Gaussian propagator for diffusion on the plane. While the analytical formula is derived using saddle point methods for short times, it works well even for intermediate times. Our formula goes beyond conventional "short time heat kernel expansions" in that it is nonperturbative in the spatial coordinate, a feature that is ideal for studying large deviations. Our work suggests a new and efficient algorithm for numerical integration of the diffusion equation on a sphere. We perform Monte Carlo simulations to compare the numerical efficiency of the new algorithm with the older Gaussian one.

cond-mat.stat-mech↗
arXiv2007

Elasticity of Stiff Biopolymers

We present a statistical mechanical study of stiff polymers, motivated by experiments on actin filaments and the considerable current interest in polymer networks. We obtain simple, approximate analytical forms for the force-extension relations and compare these with numerical treatments. We note the important role of boundary conditions in determining force-extension relations. The theoretical predictions presented here can be tested against single molecule experiments on neurofilaments and cytoskeletal filaments like actin and microtubules. Our work is motivated by the buckling of the cytoskeleton of a cell under compression, a phenomenon of interest to biology.

cond-mat.soft↗
arXiv2006

DNA Elasticity : Topology of Self-Avoidance

We present a theoretical treatment of DNA stretching and twisting experiments, in which we discuss global topological subtleties of self avoiding ribbons and provide an underlying justification for the worm like rod chain (WLRC) model proposed by Bouchiat and Mezard. Some theoretical points regarding the WLRC model are clarified: the writhe of open curves and the use of an adjustable cutoff parameter to ``regularise'' the model. Our treatment brings out the precise relation between the worm like chain (WLC), the paraxial worm like chain (PWLC) and the WLRC models. We describe the phenomenon of ``topological untwisting'' and the resulting collapse of link sectors in the WLC model and note that this leads to a free energy profile {\it{periodic}} in the applied link. This periodicity disappears when one takes into account the topology of self avoidance or at large stretch forces (paraxial limit). We note that the difficult nonlocal notion of self avoidance can be replaced (in an approximation) by the simpler local notion of ``south avoidance'' in the WLRC model. This gives an explanation for the efficacy of the approach of Bouchiat and Mezard in explaining the `hat curves' using the WLRC model. We propose a new class of experiments to probe the continuous transition between the periodic and aperiodic behavior of the free energy.

cond-mat.soft↗