arXiv subjects

Kalpataru Pradhan

Publications and source records attributed to Kalpataru Pradhan.

At least 19 recordsLinked to original sources

arXiv2026

Quantum Destabilization of Skyrmions in Centrosymmetric Frustrated Magnets

We investigate the role of the spin quantum number $s$ on the stability of skyrmions in a $J_1-J_2-J_3$ centrosymmetric quantum Heisenberg model on a square lattice using the neural network quantum states method. Our results reveal that the skyrmion stability $Q$ is severely degraded when transitioning from the semiclassical regime to the extreme quantum limit ($s=1/2$), where it ultimately vanishes. We demonstrate that this destabilization is driven by quantum longitudinal fluctuations, with $Q$ exhibiting a power-law decay as a function of the reciprocal spin moment $1/s$. Notably, the extreme quantum limit ($s=1/2$) deviates drastically from this scaling behavior, exhibiting distinct physics compared to larger spin moments. Furthermore, we reveal the microscopic origin of this decay by establishing a quantitative correspondence between skyrmion stability, entanglement, and local spin magnitude: as the local second R\'enyi entropy (an indicator of entanglement) increases and the local spin magnitude is suppressed, the skyrmion stability vanishes linearly. This regime marks a quantum state where the skyrmion number $C$ remains as remanent geometric feature of the spin orientations, yet the skyrmion stability $Q$ vanishes due to the longitudinal suppression of the local spin magnitude. Our findings suggest that classically robust skyrmion phases in frustrated lattices are fundamentally restricted to high-spin materials, indicating that a spin moment must of at least $s = 3/2$ is required for the realization of stable, atomic-scale topological textures.

cond-mat.str-el↗

arXiv2026

Microwave-driven Floquet-Fano interference in a ring-chord quantum dot structure for enhanced spin-caloritronic performance

We investigate photon-assisted thermoelectric transport in four--quantum-dot nanostructures featuring ring and ring--chord geometries coupled to ferromagnetic leads. Focusing on the interplay between microwave-induced Floquet sidebands and geometry-driven Fano interference, we employ the nonequilibrium Green's function formalism combined with Floquet theory and a self-consistent Hartree treatment of electron-electron interactions within the linear response regime. The inclusion of a longitudinal interdot chord bridging the lead-coupled dots introduces a discrete interference pathway that competes with the continuum of ring-mediated states, giving rise to pronounced Fano resonances. Microwave irradiation dynamically reshapes these resonances through photon absorption and emission processes, enabling tunable control of electrical conductance, thermopower, and electronic thermal conductance. Quantitatively, at an intermediate temperature of $T=0.3\Gamma_0$ (where $\Gamma_0$ denotes the dot-lead coupling strength), the microwave-driven ring-chord geometry exhibits an exceptional thermoelectric figure of merit of ZT $\approx$ 12 and an optimal efficiency--power trade-off, reaching nearly $62\%$ of the Carnot efficiency with an output power of $6.24~\mathrm{fW}$. Crucially, the combination of spin-polarized injection from the leads and Zeeman splitting within the dots induces a robust spin-dependence within this Floquet--Fano interference. This cooperative interplay results in an enhanced spin Seebeck response and a maximum spin thermoelectric figure of merit of nearly $Z_sT \approx 18$. Our findings establish microwave-driven engineering of Fano interference as an effective strategy for modulating spin-caloritronic behavior in multi-quantum-dot devices.

cond-mat.mes-hall↗

arXiv2025

Strain-Controlled Magnetic Phase Transitions through Anisotropic Exchange Interactions: A Combined DFT and Monte Carlo Study

Epitaxial strain provides a powerful, non-chemical route to tune the properties of functional materials by manipulating the coupling between spin, charge, and lattice degrees of freedom. Using density functional theory (DFT) calculations and $\rm BiFeO_3$ as a model system, we first demonstrate how epitaxial strain exactly leads to anisotropic magnetic interactions where the exchange coupling along the $c$-axis differs from that in the $ab$-plane. We show that subtle structural modifications, specifically the distortion from a cubic to a tetragonal lattice, drive a magnetic phase transition from a G-type to a C-type antiferromagnetic (AF) phase. The anisotropy in magnetic interactions, which becomes prominent in the lower symmetry tetragonal phase, provides a direct link between the structural distortion and the potential change in magnetic ordering. For a more comprehensive study, we next investigate the role of strain in driving magnetic phase transitions within a half-filled one-band Hubbard model in three dimensions. In this framework, strain is introduced through anisotropic hopping processes between nearest- and next-nearest-neighbor sites, inspired by the DFT calculations. Using a semiclassical Monte Carlo (s-MC) approach, we construct ground state phase diagrams in the nonperturbative regime, which show how uniaxial strain stabilizes distinct magnetic ground states: Compressive strain drives a transition from a G-type to a C-type AF insulator, whereas tensile strain suppresses the C-type AF order, favoring an A-type AF phase. Overall, our combined DFT and s-MC calculations highlight that strain is a powerful tuning parameter for controlling competing magnetic phases by governing exchange coupling mechanisms in correlated systems, offering valuable insights for the design of strain-controlled materials.

cond-mat.str-el↗

arXiv2025

Impact of Random Bond Disorder on Quantum Skyrmions in a spin-half Quantum Heisenberg Model

We investigate the impact of random bond disorder on quantum skyrmions using a spin-half quantum Heisenberg model on the square lattice with Dzyaloshinskii-Moriya interaction, Heisenberg anisotropy, and boundary-pinned magnetic field. Utilizing the neural network quantum state technique, we explore the influence of disorder on spin textures, topological properties, and quantum entanglement. We show that the disorder reduces the stability of quantum skyrmions, ultimately causing them to collapse at high disorder strengths. In addition, our results reveal two key insights. First, the presence of disorder, rather than simply degrading skyrmion order, significantly enhances local quantum entanglement, as evidenced by the rise in second R\'enyi entropy. Second, our calculations show that the topological entanglement entropy calculated using the second R\'enyi entropy remains negligible across all the disorder strengths. This suggests long-range entanglement is absent and the skyrmion phase is not detectable using this specific probe. Overall, our work provides new insights into how disorder constructively influences quantum materials.

cond-mat.str-el↗

arXiv2025

Combine effect of site dilution and long-range interaction on magnetic and transport properties in the half-filled Hubbard model

We investigate the magnetotransport properties of a diluted half-filled one-band Hubbard model with second-nearest-neighbor hopping on a simple cubic lattice, aiming to explore the possibility of metallicity in diluted antiferromagnetic systems. Our semiclassical Monte Carlo calculations reveal an antiferromagnetic metallic regime in diluted correlated materials. This unexpected metallic regime naturally leads to a central question: how does the introduction of dilution into an antiferromagnetic material, especially with long-range magnetic interactions, induce metallicity -- a feature not commonly associated with antiferromagnets? To address this question, we demonstrate that when the on-site repulsive Hubbard interaction strength is set to zero on a percentage of the sites (site dilution), the insulating state weakens due to percolative conduction among the diluted sites at low temperatures. Remarkably, this occurs without any significant alteration to the underlying long-range antiferromagnetic ordering in the system, thereby providing a pathway to realize antiferromagnetic metals. In addition, we show how the sublattice-dependent hopping can be exploited to engineer spin-polarized half-metallic antiferromagnets. Overall, our numerical results collectively provide a basis for understanding the combined effect of site dilution and competing interactions, which will assist in the design of new antiferromagnetic metals for future spintronic applications.

cond-mat.str-el↗

arXiv2025

Exploring the magnetic states in the one-band Hubbard model: Impact of long-range hoppings

Correlated electron systems with competing interactions provide a valuable platform for examining exotic magnetic phases. Theoretical models often focus on nearest-neighbor interactions, although long-range interactions can have a considerable impact on the behavior of the system, creating distinct and complicated magnetic phases. We investigate the consequences of competing interactions in a half-filled one-band Hubbard model on a simple cubic lattice, incorporating hopping processes up to third-nearest-neighbors, to explore the underlying magnetotransport properties. Our magnetic phase diagrams at low temperatures, obtained using semi-classical Monte Carlo analysis, reveal that the long-range interactions can disrupt one form of magnetic phase while creating a new type of magnetic order. For the nonperturbative regime (on-site Hubbard repulsive strength $U \sim$ bandwidth) the C-type antiferromagnetic ground state is preferred over the G-type antiferromagnetic phase when the interaction between second-nearest neighbor sites becomes significant to the nearest-neighbor interactions. However, interactions beyond the second-nearest-neighbors are required to stabilize the A-type antiferromagnetic ground state. Remarkably, at low temperatures, a highly correlated paramagnetic insulating phase develops at the intersection between the antiferromagnetic phases, which might promote a three-dimensional spin-liquid state.

cond-mat.str-el↗

arXiv2024

Microscopic study of interlayer magnetic coupling across the interface in antiferromagnetic bilayers

The enhancement of Neel temperature ($T_N$) of low-$T_{N}$ antiferromagnets in antiferromagnetic bilayers AF1/AF2, where the $T_N$ of AF1 is larger than AF2 (for example enhancement of $T_{N}$ of CoO in CoO/NiO or FeO in FeO/CoO), is a subject of considerable interest. One essential question needs to be answered in these bilayers: is the interfacial coupling a short-range one or long-range that mediates the effect of the AF1 layers on the magnetic properties of AF2 layer? To understand the systematics of the magnetic coupling across the interface, we investigate the plane-resolved magnetotransport properties of antiferromagnetic bilayers using an electron-hole symmetric one-band Hubbard model at half-filling, employing a semi-classical Monte Carlo method. In our model Hamiltonian calculations, we set Coulomb repulsion $U_{1} = 8$ to mimic high-$T_{N}$ AF1 layer, whereas we use $U_{2}$ = $2\times U_{1}$ to mimic the low-$T_{N}$ AF2 layer. Our calculations show that the $T_{N}$ of the low-$T_{N}$ antiferromagnet enhances substantially when it's thickness is small, similar to experiments, giving rise to single magnetic transition temperature of the bilayer system. These findings are well supported by a single peak in temperature-dependent specific heat. However, for larger thicknesses, the $T_{N}$ of the low-$T_{N}$ antiferromagnet approaches towards its bulk value and constituent antiferromagnetic layers align antiferromagnetically at two separate temperatures and two maxima are observed in specific heat data. Our calculations also show that the delocalization of moments is more or less confined near the interface indicating the short-ranged nature of the proximity effect. Our obtained results are consistent with the experimental observations. A detailed discussion of the modifications that will occur if we use $U_{1} = 8$ and $U_{2}$ = $0.5\times U_{1}$ will also be addressed.

cond-mat.str-el↗

arXiv2024

Evidence of spin reorientation transition below 150 K from magnetic force microscopy in a ferromagnetic BiFeO$_3$ thin film

We investigated the magnetic transitions in BiFeO$_3$ at low temperature (5-300 K) and observed nearly 90$^o$ rotation of magnetic domains (imaged by vertical magnetic force microscopy) across 150 K in an epitaxial thin film of thickness $\sim$36 nm. It offers a clear evidence of spin reorientation transition. It also corroborates the transition observed below $\sim$150 K in the zero-field-cooled and field-cooled magnetization versus temperature data. The field-driven 180$^o$ domain switching at room temperature, on the other hand, signifies presence of ferromagnetism. Since bulk antiferromagnetic BiFeO$_3$ does not exhibit such a transition, this observation in ferromagnetic thin film of BiFeO$_3$ indicates a radical effect because of epitaxial strain. Density functional theory based first-principles calculations too reveal that combined in- and out-of-plane epitaxial strain induces magnetic transition from G- to C-type structure in BiFeO$_3$.

cond-mat.mtrl-sci↗

arXiv2024

Glucose Sensing Using Pristine and Co-doped Hematite Fiber-Optic sensors: Experimental and DFT Analysis

Glucose monitoring plays a critical role in managing diabetes, one of the most prevalent diseases globally. The development of fast-responsive, cost-effective, and biocompatible glucose sensors is essential for improving patient care. In this study, a comparative analysis is conducted between pristine and Co-doped hematite samples, synthesized via the hydrothermal method, to evaluate their structural, morphological, and optical properties. The glucose sensing performance of both samples is assessed using a fiber-optic evanescent wave (FOEW) setup. While the sensitivity remains comparable for both pristine and Co-doped hematite, a reduction in the Limit of Detection (LoD) is observed in the Co-doped sample, suggesting enhanced interactions with glucose molecules at the surface. To gain further insights into the glucose adsorption mechanisms, Density Functional Theory (DFT) calculations are performed, revealing key details regarding charge transfer, electronic delocalization, and glucose binding on the hematite surfaces. These findings highlight the potential of Co-doped hematite for advanced glucose sensing applications, offering a valuable synergy between experimental and theoretical approaches for further exploration in biosensing technologies.

physics.med-ph↗

arXiv2024

Emergent scalar-chirality \& colossal transverse-magnetoresponse in strongly correlated nodal-line half-metal

Understanding the interplay of strong correlation and temperature in nodal-line semimetals can offer novel ways to control spin currents. Here we consider the 3d-5d double-perovskite Ba$_{2}$CoWO$_{6}$, which features mirror-symmetry-protected nodal-lines, strong Co-site interactions, and spin-orbit coupling (SOC) at W sites. Our first principles and exact diagonalization results reveal a half-metallic ground state with high-spin Co and topologically non-trivial bands. We demonstrate that SOC gaps out nodal points, causes band-inversion and generates anomalous Hall response. A semi-classical Monte Carlo finite-temperature simulation of five-orbital Hubbard model uncovers an emergent Co-spin scalar chirality and colossal positive transverse-magnetoresponse. We predict the temperature and magnetic field scales for the tunability of scalar-chirality and magnetoresponse.

cond-mat.str-el↗

arXiv2024

Layer Resolved Magnetotransport Properties in Antiferromagnetic/Paramagnetic Superlattices

We investigate the layer resolved magnetotransport properties of the antiferromagnetic/paramagnetic superlattices based on one band half-filled Hubbard model in three dimensions. In our set up the correlated layers (with on-site repulsion strength $U \ne$ 0) are intercalated between the uncorrelated (U = 0) layers. Our calculations based on the semi-classical Monte-Carlo technique show that the magnetic moments are induced in the uncorrelated layers at low temperatures due to kinetic hopping of the carriers across the interface. The average induced magnetic moment in the uncorrelated layer varies nonmonotonically with the $U$ values of the correlated layer. Interestingly, the induced magnetic moments are antiferromagnetically arranged in uncorrelated layers and mediates the antiferromagnetic ordering between correlated layers. As a result the whole SL system turns out to be antiferromagnetic insulating at low temperatures. For $U \sim$ bandwidth the local moments in the correlated planes increases as a function of the distance from the interface. Expectedly our in-plane resistivity calculations show that the metal insulator transition temperature of the central plane is larger than the edge planes in the correlated layers. On the other hand, although the induced moments in uncorrelated planes decreases considerably as move from edge planes to center planes the metal insulator transition temperature remains more or less same for all planes. The induced moments in uncorrelated layers gradually dissipates with increasing the thickness of uncorrelated layer and as a result the long range antiferromagnetic ordering vanishes in the superlattices similar to the experiments.

cond-mat.str-el↗

arXiv2024

Magnetotransport Properties of Ferromagnetic/Antiferromagnetic Superlattices: Probing the role of induced magnetization in antiferromagnetic layer

We investigate the magnetic and transport properties of the $La_{1-x}Sr_{x}MnO_{3}$ (LSMO)/$Pr_{1-x}Ca_{x}MnO_{3}$ (PCMO) like ferromagnetic/antiferromagnetic superlattices in three dimensions using a two orbitals double exchange model incorporating the Jahn-Teller lattice distortions, superexchange interactions and long-range Coulomb interactions. In our simulations we primarily focus on periodic arrangement of $w_L$ planes of ferromagnetic LSMO and $w_P$ planes of antiferromagnetic PCMO manganites, and set $w_L$+$w_P$ = 10. The induced ferromagnetic correlations in the parent PCMO layer decreases monotonically with increasing the PCMO layer width $w_P$ at high temperatures for both half-doping ($n =0.5$) and off-half-doping ($n = 0.55$) scenarios. As we decrease the temperature further the induced ferromagnetic moments in PCMO layer disappears or decreases considerably at half-doping due to the onset of charge ordering in antiferromagnetic PCMO layers. Overall, the magnetization in PCMO layer decreases at low temperatures and the metal-insulator transition temperature of the LSMO/PCMO superlattices increases with increase of the PCMO layer width $w_P$, similar to the experiments. On the other hand, at off-half-doping, the induced ferromagnetic moment survives even at low temperatures due to weakened charge ordering in PCMO layer and interestingly, varies nonmonotonically with PCMO layer width, in agreement with experiments. The nonmonotonic trend of the conductivity of the superlattice with increase of the PCMO layer width $w_P$ shows an one-to-one correspondence between conductivity of the superlattice and the induced ferromagnetic moments in the PCMO layer. We highlight the key role of induced ferromagnetic moment in PCMO layer in analyzing the magnetotransport properties of the LSMO/PCMO superlattices.

cond-mat.str-el↗

arXiv2023

Spin order dependent skyrmion stabilization in MnFeCoGe hexagonal magnets

Topological magnetic skyrmions in centrosymmetric systems exhibit a higher degrees of freedom in their helicity, hence possess a great potential in the advanced spintronics including skyrmion based quantum computation. However, the centrosymmetric magnets also display non-topological trivial bubbles along with the topological skyrmions. Hence it is utmost priority to investigate the impact of different magnetic ground states and their underlying interactions on the stabilization of magnetic skyrmions in cetrosymmetric magnets. Here, we present a combined theoretical and experimental study on the role of non-collinear magnetic ground state on the skyrmion stabilization in a series of exchange frustrated non-collinear ferromagnetic system MnFe1-xCoxGe. With the help of neutron diffraction (ND) and Lorentz transmission electron microscopy (LTEM) studies, we show that hexagonal skyrmions lattice emerges as a stable field driven state only when the underlying magnetic ground state is collinear with easy-axis anisotropy. In contrast, non-topological type-II bubbles are found to be stable state in the case of non-collinear magnetic ordering with partial in-plane anisotropy. Furthermore, we also find that the skyrmions transform to the non-topological bubbles when the system undergoes a spin reorientation transition from the easy-axis to easy-cone ferromagnetic phase. Our results categorically establish the significant role of in-plane magnetic moment/anisotropy that hinders the stability of skyrmion both in the case of collinear and non-collinear magnets. Thus, the present study offers a wide range of opportunities to manipulate the stability of dipolar skyrmions by changing the intrinsic characteristics of the materials.

cond-mat.mtrl-sci↗

arXiv2023

Interfacial-antiferromagnetic-coupling driven magneto-transport properties in ferromagnetic superlattices

We explore the role of interfacial antiferromagnetic interaction in coupled soft and hard ferromagnetic layers to ascribe the complex variety of magneto-transport phenomena observed in $La_{0.7}Sr_{0.3}MnO_3/SrRuO_3$ (LSMO/SRO) superlattices (SLs) within a one-band double exchange model using Monte-Carlo simulations. Our calculations incorporate the magneto-crystalline anisotropy interactions and super-exchange interactions of the constituent materials, and two types of antiferromagnetic interactions between Mn and Ru ions at the interface: (i) carrier-driven and (ii) Mn-O-Ru bond super-exchange in the model Hamiltonian to investigate the properties along the hysteresis loop. We find that the antiferromagnetic coupling at the interface induces the LSMO and SRO layers to align in anti-parallel orientation at low temperatures. Our results reproduce the positive exchange bias of the minor loop and inverted hysteresis loop of LSMO/SRO SL at low temperatures as reported in experiments. In addition, conductivity calculations show that the carrier-driven antiferromagnetic coupling between the two ferromagnetic layers steers the SL towards a metallic (insulating) state when LSMO and SRO are aligned in anti-parallel (parallel) configuration, in good agreement with the experimental data. This demonstrate the necessity of carrier-driven antiferromagnetic interactions at the interface to understand the one-to-one correlation between the magnetic and transport properties observed in experiments. For high temperature, just below the ferromagnetic $T_C$ of SRO, we unveiled the unconventional three-step flipping process along the magnetic hysteresis loop. We emphasize the key role of interfacial antiferromagnetic coupling between LSMO and SRO to understand these multiple-step flipping processes along the hysteresis loop.

cond-mat.str-el↗

arXiv2023

Tailoring the interfacial magnetic interaction in epitaxial La$_{0.7}$Sr$_{0.3}$MnO$_3$/Sm$_{0.5}$Ca$_{0.5}$MnO$_3$ heterostructures

Interface engineering in complex oxide heterostructures has developed into a flourishing field as various intriguing physical phenomena can be demonstrated which are otherwise absent in their constituent bulk compounds. Here we present La$_{0.7}$Sr$_{0.3}$MnO$_3$ (LSMO) / Sm$_{0.5}$Ca$_{0.5}$MnO$_3$ (SCMO) based heterostructures showcasing the dominance of antiferromagnetic interaction with increasing interfaces. In particular, we demonstrate that exchange bias can be tuned by increasing the number of interfaces; while, on the other hand, electronic phase separation can be mimicked by creating epitaxial multilayers of such robust charge ordered antiferromagnetic (CO-AF) and ferromagnetic (FM) manganites with increased AF nature, which otherwise would require intrinsically disordered mixed phase materials. The origin of these phenomena is discussed in terms of magnetic interactions between the interfacial layers of the LSMO/SCMO. A theoretical model has been utilized to account for the experimentally observed magnetization curves in order to draw out the complex interplay between FM and AF spins at interfaces with the onset of charge ordering.

cond-mat.mtrl-sci↗

arXiv2022

Itinerant ferromagnetism in a spin-fermion model for diluted spin systems

We investigate the itinerant ferromagnetism using a diluted spin-fermion model, derived from a repulsive Hubbard model, where itinerant fermions are coupled antiferromagnetically to auxiliary fields in a three-dimensional simple cubic lattice. We focus, in particular, on understanding the spin-dependent transport properties of the itinerant fermions in the impurity band by taking positional disorder of the auxiliary fields into account. For on-site repulsion $U$ $\sim$ bandwidth the density of the itinerant carriers confined to the impurity band, play a key role in determining the kinetic energy of the system and consequently the carrier spin polarization. Our semi-classical Monte Carlo calculations show that the ferromagnetic transition temperature of the carrier spins indeed shows an optimization behavior with the carrier density. We calculate the transport properties in details to establish a one-to-one correspondence between the magnetic and transport properties of the carriers. Our results obtained beyond the perturbative regime are significant for understanding the ferromagnetism in diluted magnetic semiconductors.

cond-mat.str-el↗

arXiv2021

Antiferromagnetism beyond classical percolation threshold in the site-diluted half-filled one-band Hubbard model in three dimensions

We investigate the impact of site dilution by setting the on-site repulsion strength ($U$) to zero at a fraction of sites in the half-filled Hubbard model on a simple cubic lattice. We employ a semi-classical Monte-Carlo approach first to recover the zero dilution (undiluted $x=1$) properties, including $U$ dependence of insulator to metal crossover temperature scale $T^*$ and long-range staggered antiferromagnetic ordering temperature ($T_N$). For the non-perturbative regime of $U \sim$ bandwidth, we find a rapid suppression of $T^*$ with reducing $x$ from 1 to 0.7. However, $T_N$ remains unchanged in this dilution range, showing a weakening of the insulating state but not of the magnetic order. At $x \leq 0.7$, $T^*$ and $T_N$ coincide and are suppressed together with further increase in site-dilution. Finally, the system loses the magnetic order and the insulating state for $x=0.15$, significantly below the classical percolation threshold $x_p^{sc} (\sim 0.31$). We show that the induced moments on $U=0$ sites drive the magnetic order below the classical percolation limit by studying local moment systematics and finite-size analysis of magnetic order. At the end, we show that either increasing $U$ to large values or raising temperature beyond a $U$ dependent critical value, suppresses the induced local moments of the $U=0$ sites and recovers the classical percolation threshold.

cond-mat.str-el↗

arXiv2021

Electric Field Control of Magnetism of Mn dimer supported on Carbon-doped-h-BN surface

Using density functional theory we show that the interaction between two Mn atoms can be tuned from anti-ferromagnetic (AFM) to ferromagnetic (FM) state by creating charge disproportion between the two on a 2D surface. The non-metallic planar heterostructures, the 2D surface, in our work is designed by doping carbon hexagon rings in a hexagonal boron nitride (h-BN) sheet. In addition, we show that an external electric field can be used to control the charge disproportion and hence the magnetism. In fact, our calculations demonstrate that the magnetic states of the dimer can be switched from AFM to FM or vice versa in an external electric field. The origin of this magnetic switching is explained using the charge transfer from (or to) the Mn dimer to (or from) the 2D material. The switching between anti-ferromagnetic to ferromagnetic states can be useful for future spintronic applications.

cond-mat.mtrl-sci↗