arXiv subjects

S. S. Banerjee

Publications and source records attributed to S. S. Banerjee.

At least 19 recordsLinked to original sources

arXiv2025

Unravelling magnetic vortex-like excitations through rapid thermal quenching in low-carbon steel

Steel, traditionally valued for its structural strength, emerges in this study as a remarkable material for exploring novel magnetic phenomena. We investigate how common processing techniques-thermal treatments and mechanical strain-significantly affect the magnetic properties of low-carbon steels (0.05 percent by weight). Our findings show that slow annealing enlarges the grain size, enhancing magnetic susceptibility, while rapid quenching reduces grain size, resulting in a decreased magnetic response. Quenching low-carbon steel produces significant increase in the fraction of high-angle grain boundaries and a rapid spatial variation of local magnetic anisotropy between grains, a feature which is unachievable with mechanical straining even up to the material's ultimate tensile strength. Tensile-straining of low-carbon steel enhances magnetic susceptibility through altered magnetic anisotropy, contrary to the observed decrease of susceptibility in quenched low-carbon steel. Magnetic force microscopy and micromagnetic modelling of our data reveal that, the reduced magnetic susceptibility in quenched steel is a result of the presence of intriguing magnetic excitations akin to magnetic vortices. These localized structures act as strong magnetic domain wall pinning centres, causing the observed decrease in magnetic susceptibility in these quenched low-carbon steels. Beyond its established structural utility, low-carbon steel combines mechanical stability with favourable magnetic properties, positioning it as a strong platform for magnetic device applications.

cond-mat.mtrl-sci↗

arXiv2025

Low Temperature Two Fluid State in SmB6

Comprehensive study using DC transport, specific heat, magnetization, and two-coil mutual inductance measurements unveils an understanding of three temperature regimes in SmB$_6$: (i) $T \geq T^{*}$ ($\sim66$K), (ii) $T_g$ ($\sim40$ K) $\leq T < T^{*}$, and (iii) $T < T_g$. Onset of Kondo breakdown below $T^{*}$ releases disorder-driven magnetic fluctuations, which splits the bulk ($\sim116$K) and surface Kondo temperature ($T_k^{s} \approx 7$ K). Below $T_g$, as magnetic fluctuations subside, surface Kondo screening revives, stabilizing the topological surface state and generating an in-gap feature ($\sim2.2$ meV) across which Dirac-like carriers are excited. Nyquist impedance analysis reveals a crossover from purely capacitive to capacitive-inductive behavior, signalling a disorder-driven two-fluid phase of heavy quasiparticles and light, high-mobility carriers below $T_g$. We identify a characteristic length scale, $L_{ν_0}(T)$, associated with the high-mobility phase, exhibiting an almost divergent trend below $T_k^{s}$. These findings underscore the complex nature of the surface conducting state in SmB$_6$.

cond-mat.str-el↗

arXiv2024

Dimension-Dependent Critical Scaling Analysis and Emergent Competing Interaction Scales in a 2D Van der Waals magnet Cr$_{2}$Ge$_{2}$Te$_{6}$

We investigate thickness-dependent transformation from a paramagnetic to ferromagnetic phase in Cr$_{2}$Ge$_{2}$Te$_{6}$ (CGT) in bulk and few-layer flake forms. 2D Ising-like critical transition in bulk CGT occurs at $T_{c}$ = 67 K with out-of-plane magnetic anisotropy. Few-layer CGT on hBN/SiO$_{2}$/Si substrate displays the same $T_{c}$ but also exhibits a new critical transition at $T^{\prime}_c$ = 14.2 K. Here, critical scaling analysis reveals the critical exponents differ significantly from those in bulk and do not align with the known universality classes. Our Density Functional Theory (DFT) and classical calculations indicate competition between magnetocrystalline and dipolar anisotropy emerges with reduced dimensions. The observed behavior is due to minor structural distortions in low dimensional CGT, which modify the balance between spin-orbit coupling, exchange interactions and dipolar anisotropy. This triggers a critical crossover at $T^{\prime}_c$. Our study shows the emergence of a complex interplay of short- and long-range interactions below $T^{\prime}_c$ as CGT approaches the 2D limit.

cond-mat.mtrl-sci↗

arXiv2024

Impact of Co2C Nanoparticles on Enhancing the Critical Current Density of Bi-2223 Superconductor

We have investigated the superconducting properties of nanocomposite pellets made from Bi-2223 and Co2C powders. There is loss of superconducting fraction in the nanocomposites, but the retained superconducting fraction exhibits robust bulk superconducting properties, having Tc ~ 109 K which was found to be comparable to that of the pure Bi-2223 pellet. We found that the composites net magnetization response is a superposition of ferromagnetic and superconducting fractions contributions. We also found the surviving superconducting fraction exhibits a robust Meissner response. In the nanocomposite the irreversibility field of the superconducting fraction at 77 K is found to increase by almost three times compared to the pristine material, thereby showing strong vortex pinning features. We also find a broadened magnetic field regime over which we observe a single vortex pinning regime sustained in the nanocomposite. The critical current density, Jc, of the nanocomposite was found to be approximately five times higher than that of the pristine Bi-2223 pellet at low T. In fact, the enhancement in Jc is most significant in the high T regime, where at temperatures close to Tc in the nanocomposite we see almost two orders of magnitude increase of Jc compared to the pristine Bi-2223 pellet. The larger sized agglomeration of magnetic nanoparticles of Co2C leads to loss of superconductivity in the nanocomposite. However, there are also unagglomerated Co2C nanoparticles distributed uniformly throughout the nanocomposite which acts as efficient pinning centres allowing for collective vortex pinning centres to be retained, even upto temperatures near Tc, and these nanoparticles also do not compromise the bulk Tc of the superconducting fraction. Our study shows that these nanocomposites exhibit enhanced Jc especially in the high T regime are potentially useful for high current applications.

cond-mat.supr-con↗

arXiv2023

Probing the strongly correlated magnetic state of Co$_2$C nanoparticles at low temperatures using $μ$SR

Co$_2$C nanoparticles (NPs) are amongst transition metal carbides whose magnetic properties have not been well explored. A recent study by Nirmal Roy et al. [1] showed that a collection of Co$_2$C NPs exhibit an exchange bias (EB) effect below T$_{EB}$ = 50 K and also a spin glass (SG) state below T$_{SG}$ = 5 K. We use magnetic, electrical transport, specific heat, and muon spin rotation ($μ$SR) measurements to explore further the magnetic properties of these NPs. We uncover the onset of Kondo localization at Kondo temperature T$_K$ (= 40.1 K), near the onset of EB effect. A crossover from the Kondo-screened scenario to an RKKY interaction-dominated regime is also observed for T < T$_K$. Specific heat measurements confirm Kondo localization and heavy fermionic nature in Co$_2$C at low T. At low T, zero field $μ$SR spectra reveal a dominant magnetically disordered fraction with slow relaxation and a smaller fraction with short-range order exhibiting fast relaxation, with no evidence of long-range magnetic order. We observe an increase in this fast relaxation rate between T$_{EB}$ and T$_{SG}$, suggesting a slowing down of the fluctuating local magnetic environment around muons. Transverse field $μ$SR spectra show the emergence of a stable, multi-peaked local magnetic field distribution below T$_{EB}$. Longitudinal field $μ$SR spectra shows distinct changes in the dynamics of fluctuations suggesting the presence of a frozen glassy like state below 6 K. Our results suggest that below T$_{EB}$, Co$_2$C NPs pellet develops a magnetic interface, separating disordered and short-range order fractions. The Exchange interaction that sets in below T$_{EB}$ at the interface couples them and suppresses the fluctuations. With the suppression of magnetic fluctuations below T$_{EB}$, strong correlation effects in the electronic state of Co$_2$C lead to Kondo localization.

cond-mat.str-el↗

arXiv2023

Exploration of strongly correlated states in SmB6 through a comparison of its two-coil pick-up response to that of Bi2Se3

Earlier studies on the Kondo insulator SmB6 reveal the presence of a bulk Kondo insulating gap between 30 - 50 K, and the emergence of a conducting surface state only below 4 K. Here, we compare the two-coil mutual inductance pick-up response of SmB6 single crystal with that of a conventional topological insulator (TI), Bi2Se3 single crystal. From these studies we identify three distinct temperature regimes for SmB6, viz., (i) T >= T*(~ 66 K), (ii) (40 K~) T_g <= T < T*, and (iii) T < T_g. At T* in SmB6, we observe a peak in the temperature-dependent AC pickup signal which corresponds to the peak in the broad hump feature in the bulk DC susceptibility measurements and features in the resistivity measurements. A dip in the pickup signal at T_g in SmB6 correlates with the evidence for the opening of a bulk Kondo gap in transport measurements. Our study of the pickup signal in SmB6 suggests the presence of a thin (submicron order thickness) high conducting surface layer from a temperature just below T_g. In this T regime in SmB6, the pickup signal shows a distinct square root frequency (f) dependence compared to the linear f dependence found in Bi2Se3. Across all the different T regimes, distinct AC frequency dependence and scaling properties are observed. Our results suggest that above T*, weak exchange interactions cause electrons to scatter from random ion sites. Electronic correlations gradually strengthen with the onset of Kondo like hybridization, setting in from below T*, and at T_g, a strongly correlated Kondo gap opens up in the bulk of the material. The appearance of the thin high conducting surface layer is nearly coincident with the onset of bulk Kondo insulating state below T_g in SmB6.

cond-mat.str-el↗

arXiv2022

Coexistence of different pinning mechanisms in Bi-2223 superconductor and its implications for using the material for high current applications

We investigate the pinning mechanism in high-critical-current polycrystalline samples of Bi-2223 (Bi2Sr2Ca2Cu3O10) utilized in high current applications. Using differential magneto-optical (DMO) imaging technique, we track the magnetic field penetration in the sample. Our DMO imaging studies show circular regions with an average diameter of 20 um with dark contrast. We identify these as strong-pinning regions with a substantially higher local penetration field than the surrounding regions. A unique feature of these strong-pinning centers is that they survive upto high temperatures (near Tc) and produce a non-Gaussian distribution of the penetration field strength. By analysing the field dependence of the pinning force behaviour, we identify two distinct pinning mechanisms: at low temperatures, well below Tc, it is predominantly surface pinning mechanism and at higher temperatures near Tc, we see a crossover into a purely delTc pinning mechanism. Our studies show that surface pinning effects are related to grain alignment, grain boundary, and voids in the sample. The effect of these diminishes near Tc, and the strong-pinning regions here are related to local stoichiometric fluctuations. We investigate the impact of these pinning centers on the current distribution in a macroscopic Bi-2223 superconducting cylindrical tube. We map the current distribution across the macroscopic cylindrical tube using an array of hall sensors distributed around the cylinder. The map shows that the current distribution is non-uniform across the tube at high currents. The non-uniformity reveals an inhomogeneous distribution of strong-pinning centers across large length scales in superconductors used for applications.

cond-mat.supr-con↗

arXiv2021

Demonstration of a three-dimensional current mapping technique around a superconductor in a prototype of a conventional superconducting fault current limiter

Here we describe a three-dimensional current mapping technology developed for a superconductor using an array of Hall sensors distributed around it. We demonstrate this in a prototype similar to a conventional resistive superconducting fault current limiter (SCFCL). By calibrating the Hall sensor voltage, we can directly measure the distribution of the currents in the superconductor and the shunt. Using pulsed measurements, we measure the fractions of current distributed between the superconductor and shunt resistor parallel combination when a fault-like condition is mimicked in the system. Using the Hall array measurements, we generate a real-time three-dimensional map of local average current distribution around the superconductor used in our prototype of SCFCL. Our measurements show that, even for currents less than the critical current a non-uniform current flow pattern exists around the superconductor which we have used in the prototype. The capability of real-time, three-dimensional monitoring of the average local current distribution offers a way for the early detection of instabilities like hotspots developing in a superconductor. We discuss the use of this technique to not only show how it offers early detection and protection against instabilities developing in the superconductor, but also how it offers an added flexibility, namely, a user-settable fault current threshold.

cond-mat.supr-con↗

arXiv2021

Negative differential resistance state in the free-flux-flow regime of driven vortices in a single crystal of 2H-NbS$_2$

Time series measurements in 2H-NbS$_2$ crystal had unravelled a drive induced transition wherein the critical current (Ic) changes from a low to a high Ic jammed vortex state, via a negative differential resistance (NDR) transition. Here, using multiple current (I) - voltage (V) measurement cycles, we explore the statistical nature of observing the NDR transition in the free-flux-flow (FF) regime in a single crystal of 2H-NbS$_2$. The probability of observing the NDR transition always remains finite for a vortex state created with either fast or slow rate of magnetic field. The probability of observing the NDR transition in the FF regime is found to systematically increase with magnetic field (B) in weak collective pinning regime. In the strong pinning regime, the said probability becomes B-independent. We show that the higher Ic state is unique and cannot be accessed via any conventional route. While the I-V curves do not distinguish between zero field cooled (ZFC) and field cooled (FC) modes of preparing the vortex state, the probability for observing an NDR transition has different B-dependences for the vortex matter prepared in the ZFC and FC modes. We find that the NDR occurs in a high dissipation regime, where the flow resistivity is well above the theoretical value expected in the FF regime. We understand our results on the basis of a rapid drop in vortex viscosity at high drives in 2H-NbS$_2$, which triggers a rapid increase in the vortex velocity and reorganization in the moving vortex matter leading to a dynamical unstable vortex flow. This dynamical instability leads to the NDR transition into a high entropy vortex state with high Ic.

cond-mat.supr-con↗

arXiv2020

Macroscopic, layered onion shell like magnetic domain structure generated in YIG film using ultrashort, megagauss magnetic pulses

Study of the formation and evolution of large scale, ordered structures is an enduring theme in science. The generation, evolution and control of large sized magnetic domains are intriguing and challenging tasks, given the complex nature of competing interactions present in any magnetic system. Here, we demonstrate large scale non-coplanar ordering of spins, driven by picosecond, megagauss magnetic pulses derived from a high intensity, femtosecond laser. Our studies on a specially designed Yttrium Iron Garnet (YIG)/dielectric/metal film sandwich target, show the creation of complex, large, concentric, elliptical shaped magnetic domains which resemble the layered shell structure of an onion. The largest shell has a major axis of over hundreds of micrometers, in stark contrast to conventional sub micrometer scale polygonal, striped or bubble shaped magnetic domains found in magnetic materials, or the large dumbbell shaped domains produced in magnetic films irradiated with accelerator based relativistic electron beams. Through micromagnetic simulations, we show that the giant magnetic field pulses create ultrafast terahertz (THz) spin waves. A snapshot of these fast propagating spin waves is stored as the layered onion shell shaped domains in the YIG film. Typically, information transport via spin waves in magnonic devices occurs in the gigahertz (GHz) regime, where the devices are susceptible to thermal disturbances at room temperature. Our intense laser light pulse - YIG sandwich target combination, paves the way for room temperature table-top THz spin wave devices, which operate just above or in the range of the thermal noise floor. This dissipation-less device offers ultrafast control of spin information over distances of few hundreds of microns.

cond-mat.mes-hall↗

arXiv2020

Imaging the topological current carrying state and the surface to bulk transformation, in Bi2Se3 single crystal and thin film

Magneto-optics based current imaging technique compares the nature of topological current distribution in a single crystal and thin film of topological insulator material, Bi2Se3. The single crystal, at low temperatures, has uniform topological surface current sheets which are about 3.6 nm thick. With increasing temperature, the current partially diverts into the crystal bulk and concomitantly, the sheet break up into a patchy network of high and low current density regions. The temperature dependence of the high current density areas shows that the surface to bulk transformation in the crystal has features of classical phase transition phenomena. The surface area fraction with topological high current density behaves like an order parameter. This phase transition is driven by disorder. In Bi2Se3 thin film we show the presence of quasi one-dimensional topological edge currents which are suppressed with a weak applied magnetic field. The edge current transforms into a uniform bulk current in the film.

cond-mat.mtrl-sci↗

arXiv2020

Exploring the non-equilibrium fluctuation relation for quantum mechanical tunneling of electrons across a modulating barrier

We experimentally explore the phenomenon of electron tunneling across a modulated tunneling barrier which is created between an STM tip and an Au film deposited on a vibrating piezo surface. Measurements of the time series of the quantum mechanical tunneling current across the modulating barrier show large fluctuations. Analysis of the average work done in establishing tunneling current in finite time interval shows a distribution of both positive and negative work events. The negative work events suggest tunneling against the bias voltage direction. We show that these distributions obey the Gallavotti Cohen Non-equilibrium Fluctuation Relations (GC-NEFR) valid for systems driven through a dissipating environment. Typically, while the GC-NEFR has been shown for non -equilibrium classical systems we show its validity for the quantum mechanical tunneling process too. The GC-NEFR analysis also gives us a way to measure the dissipation present in this quantum tunneling system. We propose the modulated barrier behaves like a lossy scattering medium for the tunneling electrons resulting in a tendency to randomize of the tunneling process.

cond-mat.mes-hall↗

arXiv2020

The Exchange Bias effect in pure Co2C nanoparticles

We study the low temperature magnetic properties of nanoparticles of pure transition metal carbide, viz., Co2C, with an average particle diameter of $40 \pm 10$ nm. These Co2C nanoparticles are ferromagnetic (FM) up to room temperature with blocking temperatures above room temperature. The coercive field shows abrupt deviation from the Kneller law below 50 K. In this low temperature regime the magnetization hysteresis loop shows shifts due to exchange bias (EB) effect, with an exchange field of ~ 250 Oe. Analysis of training of the EB effect and ac and dc magnetic measurements suggest that EB arises in the nanoparticles due to a core-shell structure with a FM core and a cluster glass shell. The shell contains uncompensated spins, some of which are freely rotatable while some are frozen. DFT calculations of structural and magnetic properties of small Co2C clusters of diameter of few Angstroms confirm a core-shell structure, where the structurally ordered core has uniform magnetic moment distribution and the structurally disordered shell has non-uniform moment distribution.

cond-mat.str-el↗

arXiv2020

Coupling-decoupling of conducting topological surface states in thick Bi$_2$Se$_3$ single crystals

Sensitive ac susceptibility measurements of a topological insulator, Bi$_2$Se$_3$ single crystal, using mutual two coil inductance technique (Ref. 32) shows coupling and decoupling of high conducting surface states. The coupling of the surface states exists upto thickness of 70 microns, which is much larger than the direct coupling limit of thickness approximately 5 to 10 nanometers found in thin films. The high conducting topological surface states are coupled through the crystal via high electrically conducting channels, generated by Selenium vacancies. These conducting channels through the bulk disintegrate beyond 70 micron thickness and at high temperatures, thereby leading to decoupling of the topological surface states. We show the decoupled surface states persist upto room temperature in the topological insulator. Analysis of Nyquist plot of ac-susceptibility response of the TI using a resistor (R) Inductor (L) model shows an inductive nature of the coupling between surface states found in these Bi$_2$Se$_3$ crystals.

physics.app-ph↗

arXiv2020

Imaging the effect of drive on the low-field vortex melting phenomenon in Ba0.6K0.4Fe2As2 single crystal

Self-field imaging of current distribution in Ba0.6K0.4Fe2As2 superconductor is used to study the effect of drive on the low-field vortex solid to liquid melting phase transformation. At low fields, the current induced drive on the vortices aids in thermally destabilizing the vortex state thereby shifting the low field melting phase boundary. We show that the current induced drive shifts solid-liquid boundaries and prepones the vortex melting phenomenon compared to the equilibrium situation. The analysis shows that for currents above 50 mA, Joule heating effects shift the melting line while below 50 mA, an effective temperature concept for driven system viz., a drive dependent shaking temperature, explains the shift. The observation of a transformation from inhomogeneous to homogeneous current flow in the sample at low fields as a function of the driving force is reconciled via an inverse dependence of the shaking temperature on vortex velocity, which is incorporated in our analysis.

cond-mat.supr-con↗

arXiv2019

Localized spin waves at low temperatures in a Cobalt Carbide nanocomposite

We study magnetic, transport and thermal properties of Cobalt carbide nanocomposite with a mixture of Co2C and Co3C phases in 1:1 ratio, with an average particle diameter of 40$\pm 15$ nm. We show that the behavior of the nanocomposite is completely different from that of either Co3C or Co2C. We observed that with decreasing temperature the saturation magnetization MS(T) increases, however, below 100 K, there is a steep rise. A detail analysis shows the increase in MS(T) down to 100 K is explained via the surface spin freezing model. However, below 100 K the steep increase in MS(T) is explained by a finite size effect related to a confinement of spin waves within the nano particles. The measurement of heat capacity shows broad peak at 100 K along with presence of another anomaly at a lower temperature 43 K(=Tex). Resistance measurement in the nanocomposite shows metallic behavior at high T with an unusual anomaly appearing at Tex, which is near the T regime where MS(T) begins to increase steeply. A measurement of the temperature gradients across the sample thickness indicates an abrupt change in thermal conductivity at Tex which suggests a phase transition at Tex. Our results are explained in terms of a transformation from a magnetically coupled state with a continuous spectrum of spin waves into a magnetically decoupled state below 100 K with confined spin waves.

cond-mat.str-el↗

arXiv2019

Planar pinning induced, lowering of vortex dimensionality and low field melting in a single crystal of Ba0.6K0.4Fe2As2

Theoretically, the vortex melting phenomenon occurs at both low and high magnetic fields at a fixed temperature. While the high field melting has been extensively investigated in high Tc cuprates, the low field melting phenomena in the presence of disorder hasn't been well explored. Using bulk magnetization measurement and high-sensitivity differential magneto-optical imaging technique, we detect a low-field vortex melting phenomenon in a single crystal of Ba0.6K0.4Fe2As2. The low field melting is accompanied by a significant change in local magnetization ~ 3 G, which decreases with increasing applied field. The observed vortex melting phenomena is traced on a field temperature phase diagram and which lies very close to theoretically predicted Lindemann criteria based low field melting line. Our analysis shows a Lindemann number cL = 0.14 associated with the low field melting. Imaging of low-field vortex melting features shows the process nucleates via formation of extended finger-like projections which spreads across the sample with increasing field or temperature, before entering into an interaction-dominated vortex solid phase regime. Magnetization scaling analysis shows that the dimensionality of melting vortex state is close to one. Angular dependence of bulk magnetization hysteresis loop in our sample shows the presence of extended defects. From our studies, we propose the sample contains a peculiar geometry of extended defects arranged in a plane in the sample, with these planes extending through the sample thickness. In the weak intervortex interaction limit, we argue that reduced vortex dimensionality due to pinning by these peculiar extended defect planes strongly enhances thermal fluctuations. It is these extended defects planes, which we propose are promoting low dimensional vortex melting in the pnictide system.

cond-mat.supr-con↗

arXiv2019

Non-contact mutual inductance based measurement of an inhomogeneous topological insulating state in Bi2Se3 single crystals with defects

Pure Topological Insulating materials preserve a unique electronic state comprising of bulk insulating gap and conducting surface states. Here we use bulk Bi2Se3 single crystals possessing Se vacancy defects as a prototype topological insulator (TI) material for exploring the effect of non-magnetic disorder on the conducting properties of TIs. We employ a sensitive, non-contact, mutual inductance based technique for measuring the surface and bulk contribution to electrical conductivity in the TI. We discern the different contributions, by observing that predominant surface electrical conduction produces linear frequency dependence of the pickup signal while bulk conductivity gives rise to quadratic frequency dependence. We also see an algebraic temperature dependent surface conductivity while an activated bulk conductivity. Using the above we uncover an interplay between surface and bulk contribution to electrical conductivity in the TI as a function of temperatures. In the Bi2Se3 crystals the transformation from surface to bulk dominated electrical transport is found to occur close to 70 K. This temperature range matches well with our results from activated bulk electrical transport results which shows an activation energy scale, delta which is in the millieV range. The gap delta is much less than the bulk band gap in Bi2Se3, and which we argue is associated with defect states in the TI material. To understand our results, we propose a model of a TI comprising of an inhomogeneous low electrically conducting medium (bulk) which is sandwiched between thin two high electrically conducting sheets (surface). The inhomogeneous TI state we argue is generated by Selenium vacancies defects in Bi2Se3, which is responsible for producing an interplay between bulk and surface conductivity.

cond-mat.mtrl-sci↗