arXiv subjects

Anup Kumar Bera

Publications and source records attributed to Anup Kumar Bera.

13 recordsLinked to original sources

arXiv2025

Two Orders of Magnitude Enhancement in Oxide Ion Conductivity in Cu2P2O7 via Vanadium Substitution: A Pathway Toward SOFC Electrolytes

In the quest of green energy, Solid Oxide Fuel Cells (SOFC) have drawn considerable attention for chemical-to-electric energy conversion. In the present paper, we report an enhancement of ionic conductivity in Cu2P2-xVxO7 by vanadium substitution. The electrical (dc and ac conductivity, diffusivity, hopping rate, electric modulus and dielectric properties) and crystal structural properties of Cu2P2-xVxO7 (x = 0, 0.4, 0.6, 0.8 and 1) are investigated by impedance spectroscopy and neutron diffraction, respectively. X-ray photoelectron spectroscopy (XPS) study confirms the presence of Cu2+, P5+and V5+ mono-valence states. The dc conductivity results reveal a two orders of magnitude enhancement of ionic conductivity from ~3.81x10-5 S cm-1 for x =0 to ~2.08x10-3 S cm-1 for x =1 at 993 K, revealing a possible application in SOFCs. DC transport number studies reveal that the total conductivity is dominated by ionic conduction (> 95%). In addition, the diffusivity and hopping rate of oxide ions increase with increasing x. Besides, ac conductivity, electric modulus and dielectric properties have been investigated to illustrate the microscopic conduction mechanism. The derived results suggest that the mechanism for ionic conduction is the correlated barrier hopping (CBH) process. The soft-bond valence sum (BVS) analysis of the neutron diffraction patterns reveals the three-dimensional (3D) oxide ion conduction pathways within the crystal structure. The present study provides a pathway to enhance the ionic conductivity, as well as understanding of microscopic conduction mechanism, ionic conduction pathways and the role of crystal structure on the ionic conduction.

cond-mat.mtrl-sci↗

arXiv2025

Tailoring Magnetic Properties of Zigzag Structured Thin Films via Interface Engineering and Columnar Nano-structuring

We report the emergence of a novel interface-induced shape anisotropy component in zigzag-structured thin films fabricated via Sequential Oblique Angle Deposition (S-OAD). In this study, we systematically investigate cobalt (Co) and Co2FeAl (CFA) thin films by varying column length, number of bilayers, and magneto-crystalline anisotropy (MCA) to explore how structural modulation affects magnetic behavior. Using magneto-optical Kerr effect (MOKE) measurements in conjunction with synchrotron-based grazing-incidence small-angle X-ray scattering (GISAXS) and 2D X-ray diffraction (2DXRD), we reveal that the interplay between interface-induced, shape, and crystalline anisotropies allows for a tunable magnetic response, ranging from isotropic to anisotropic behavior. The observed uniaxial magnetic anisotropy (UMA) exceeds that of conventional OAD films, while column merging is effectively suppressed through precise multilayer engineering. Structural analysis confirms that periodic, high-density interfaces at the junctions of oppositely tilted columns are central to this anisotropy control. These findings demonstrate that interface engineering and columnar nanostructuring within zigzag nanostructures offer a powerful route for tailoring magnetic properties in zigzag thin films, enabling their application in next-generation spintronic and magnetic sensor technologies.

cond-mat.mtrl-sci↗

arXiv2025

Accessing quasi-flat $\textit{f}$-bands to harvest large Berry curvature in NdGaSi

In typical rare-earth lanthanide compounds, the localized 4\textit{f}-electrons have a weak effect on the electrical conduction, limiting their influence on the Berry curvature and, hence, the intrinsic anomalous Hall effect. A comprehensive study of the magnetic, thermodynamic, and transport properties of single-crystalline NdGaSi, guided by first-principles calculations, reveals a ferromagnetic ground state that induces a splitting of quasi-flat 4\textit{f} electronic bands and positions them near the Fermi energy. The observation of an extraordinarily large intrinsic anomalous Hall conductivity of 1165 $\Omega^{-1}$ cm$^{-1}$ implies the direct involvement of localized states in the generation of non-trivial band crossings around the Fermi energy. The angle-resolved photoemission spectroscopy measurements provide direct evidence of non-trivial crossing of the 4\textit{f}-bands with dispersive bands. These results are remarkable when compared to ferrimagnetic NdAlSi, which differs only in a non-magnetic atom (a change in the principal quantum number \textit{n} of the outer \textit{p }orbital) with the same number of valence electrons and does not exhibit any measurable anomalous Hall conductivity.

cond-mat.mtrl-sci↗

arXiv2023

Field and Polarization Dependent Quantum Spin Dynamics in Honeycomb Magnet Na$_2$Co$_2$TeO$_6$: Magnetic Excitations and Continuum

We report terahertz spectroscopic measurements of quantum spin dynamics in the spin-1/2 honeycomb magnet Na$_2$Co$_2$TeO$_6$ as a function of applied magnetic field with different terahertz polarizations. Distinct field dependencies of the resolved spin dynamics are identified in three regimes, which are separated by two critical fields at $B_{c1}\approx 7$ and $B_{c2}\approx 10$ T. A polarization selective continuum is observed in the intermediate phase, featuring spin fluctuations of a proximate quantum spin liquid.

cond-mat.str-el↗

arXiv2023

Engineering magnetic anisotropy and the surface of epitaxial Fe films using ion beam erosion; unveiling self-assembly and tunability

The engineering of surface morphology and structure of the thin film is one of the essential technological assets for regulating the physical properties and functionalities of thin film-based devices. This study investigates the evolution of surface structure and magnetic anisotropy in epitaxially grown ultrathin Fe films on MgO (001) substrates subjected to multiple cycles of ion beam erosion (IBE) after growth. Ultrathin Fe film grows in 3D island mode and exhibits intrinsic fourfold magnetic anisotropy. After a few cycles of IBE, the film displays an induced uniaxial magnetic anisotropy that leads to a split in the hysteresis loop. In addition, clear and conclusive evidence of IBE mediated (2x2) reconstruction of the Fe surface has been observed. We also demonstrate that thermal annealing can reversibly tune the induced UMA and surface reconstruction. The feasibility of the IBE technique by properly selecting ion beam parameters for modification of surface structure has been highlighted apart from conventional methods of tailoring the morphology for tuning of UMA. Thus, the present work paves a way to explore the IBE-induced self-assembling phenomena further.

cond-mat.mtrl-sci↗

arXiv2023

In-situ Growth of Ultrathin Magnetic Films and Tuning the Magnetic Properties by Ion-sculpting

Magnetic anisotropy is a key parameter of magnetic materials as it decides the response in the presence of an external magnetic field. The artificial tailoring of magnetic anisotropy by manipulating surface and interface morphology is attracting widespread interest for its application in spintronic and magnetic memory devices. In this perspective, the primary focus of this thesis has been inducing and tailoring magnetic anisotropy in thin film-based systems via the engineering of structure and surface/interfacial morphology. For this purpose, instead of multistep, expensive lithographic techniques, we have utilized low-energy IBE as a handy, cost-effective, and useful tool for producing magnetic nanostructures and tailoring its surface structure, morphology and magnetic anisotropy. Furthermore, we have also proposed various new and unconventional methods (oblique angle deposition on the rippled substrate, sequential deposition-erosion) for enhancement of UMA and proved its effectiveness via experiments. We have performed most of the present thesis work in-situ utilizing a UHV chamber to get genuine characteristics and understand their interdependencies. However, advanced synchrotron-based techniques such as GISAXS and NFS were also utilized to complement the in-situ observations.

cond-mat.mtrl-sci↗

arXiv2023

Significantly increased magnetic anisotropy in Co nano-columnar multilayer structure via a unique sequential oblique-normal deposition approach

Oblique/normal sequential deposition technique is used to create Co based unique multilayer structure [Co-oblique(4.4nm)/Co-normal (4.2 nm)]x10, where each Co-oblique layer is deposited at an oblique angle of 75deg, to induce large in-plane uniaxial magnetic anisotropy (UMA). Compared to the previous ripple, stress and oblique angle deposition (OAD) related studies on Cobalt in literature, one-order higher UMA with the easy axis of magnetization along the projection of the tilted nano-columns in the multilayer plane is observed. The multilayer retains magnetic anisotropy even after annealing at 450C. The in-plane UMA in this multilayer is found to be the combination of shape, and magneto-crystalline anisotropy (MCA) confirmed by the temperature-dependent grazing incidence small angle X-ray scattering (GISAXS), in situ reflection high energy electron diffraction (RHEED) and grazing incidence X-ray diffraction (GIXRD) measurements. The crystalline texturing of hcp Co in the multilayer minimizes spin-orbit coupling energy along the column direction, which couples with the shape anisotropy energies and results in preferential orientation of the easy magnetic axis along the projection of the columns in the multilayer plane. Reduction in UMA after annealing is attributed to diffusion/merging of columns and annihilating crystallographic texturing. The obtained one-order high UMA demonstrates the potential application of the unique structure engineering technique, which may have far-reaching advantages in magnetic thin films/multilayers and spintronic devices.

cond-mat.mtrl-sci↗

arXiv2023

Effect of surface morphology on magnetization dynamics of cobalt ultrathin films

Growth of Co film on SiO2 substrates with the surface roughness of 0.5 nm and 1.6 nm has been studied in situ using the magneto-optical Kerr effect (MOKE) and four probe resistivity measurements. In-situ measurements jointly suggest the Volmer-Weber growth process and proceed via a nonmagnetic, superparamagnetic and ferromagnetic phase formation on both substrates. Islands are found to coalesce at film thicknesses of 0.6 nm and at 1.5 nm with continuous film formation around film thicknesses of 1.5 nm and 3.0 nm for smooth and rough substrates, respectively. Ferromagnetic long-range ordering i.e., the appearance of a magnetic hysteresis loop in both films, is observed just after coalescing stage. Observed azimuthal angular dependence of coercivity confirmed the presence of a weak uniaxial magnetic anisotropy in both films, whereas the difference in uniaxial magnetic anisotropy with substrate roughness is interpreted in terms of the combined effect of domain wall pinning and internal stresses in the films. The origin of much higher UMA in the case of the Co film deposited on a ripple-patterned substrate of similar root means square roughness is attributed to the modified long-range dipolar stray fields on the surface.

cond-mat.mtrl-sci↗

arXiv2022

Enhancing the limit of uniaxial magnetic anisotropy induced by ion beam erosion

The artificial tailoring of magnetic anisotropy by manipulation of interfacial morphology and film structure are of fundamental interest from application point of view in spintronic and magnetic memory devices. This letter reports an approach of engineering and enhancing the strength of oblique incidence ion beam erosion (IBE) induced in-plane uniaxial magnetic anisotropy (UMA) by simultaneous modification of film morphology as well as film texture. To meet this objective, Cobalt film and Si substrate have been taken as a model system. Unlike conventional post growth IBE of film, we direct our effort to the sequential deposition and subsequent IBE of the film. Detailed in-situ investigation insights that the film grows in highly biaxially textured polycrystalline state with formation of nanometric surface ripples. The film also exhibits pronounced UMA with easy axis oriented parallel to the surface ripple direction. Remarkably, the induced UMA is about one order of magnitude larger than the reported similar kind of earlier studies. The possibility of imposing in-plane crystallographic texture giving rise to magneto-crystalline anisotropy, along with long-range dipolar interaction throughout ripple crests enhances the strength of the UMA. The present findings can be further extended to systems characterized by different crystallographic structure and magnetic properties and show the general applicability of the present method.

cond-mat.mtrl-sci↗

arXiv2021

Non-Abelian statistics in light scattering processes across interacting Haldane chains

The $S=1$ Haldane state is constructed from a product of local singlet dimers in the bulk and topological states at the edges of a chain. It is a fundamental representative of topological quantum matter. Its well-known representative, the quasi-one-dimensional SrNi$_2$V$_2$O$_8$ shows both conventional as well as unconventional magnetic Raman scattering. The former is observed as one- and two-triplet excitations with small linewidths and energies corresponding to the Haldane gap $Δ_H$ and the exchange coupling $J_c$ along the chain, respectively. Well-defined magnetic quasiparticles are assumed to be stabilized by interchain interactions and uniaxial single-ion anisotropy. Unconventional scattering exists as broad continua of scattering with an intensity $I(T)$ that shows a mixed bosonic / fermionic statistic. Such a mixed statistic has also been observed in Kitaev spin liquids and could point to a non-Abelian symmetry. As the ground state in the bulk of SrNi$_2$V$_2$O$_8$ is topologically trivial, we suggest its fractionalization to be due to light-induced interchain exchange processes. These processes are supposed to be enhanced due to a proximity to an Ising ordered state with a quantum critical point. A comparison with SrCo$_2$V$_2$O$_8$, the $S=1/2$ analogue to our title compound, supports these statements.

cond-mat.str-el↗

arXiv2019

Dispersions of Many-Body Bethe strings

Complex bound states of magnetic excitations, known as Bethe string, were predicted almost a century ago to exist in one-dimensional quantum magnets 1. The dispersions of the string states have so far remained the subject of intensive theoretical studies 2-7. By performing neutron scattering experiments on the one-dimensional Heisenberg-Ising antiferromagnet SrCo2V2O8 in high longitudinal magnetic fields, we reveal in detail the dispersion relations of the string states over the full Brillouin zone, as well as their magnetic field dependences. Furthermore the characteristic energy, the scattering intensity and linewidth of the observed string states exhibit excellent agreement with our precise Bethe Ansatz calculations. Our results establish the important role of string states in the quantum spin dynamics of one-dimensional systems, and will invoke studies of their dynamical properties in more general many-body systems.

cond-mat.str-el↗

arXiv2017

Experimental Observation of Bethe Strings

Almost one century ago, string states - complex bound states (Wellenkomplexe) of magnetic excitations - have been predicted to exist in one-dimensional quantum magnets and since then become a subject of intensive theoretical study. However, experimental realization and identification of string states in condensed-matter systems remains an unsolved challenge up to date. Here we use high-resolution terahertz spectroscopy to identify string states in the antiferromagnetic Heisenberg-Ising chain SrCo2V2O8 in strong longitudinal magnetic fields. We observe complex bound states (strings) and fractional magnetic excitations (psinons and antipsinons) in the field-induced critical regime, which are precisely described by the Bethe ansatz. Our study reveals that two-string and three-string states govern the quantum spin dynamics close to the quantum criticality, while the fractional excitations are dominant at low energies, reflecting the antiferromagnetic quantum fluctuations.

cond-mat.str-el↗

arXiv2016

From confined spinons to emergent fermions: Observation of elementary magnetic excitations in a transverse-field Ising chain

We report on spectroscopy study of elementary magnetic excitations in an Ising-like antiferromagnetic chain compound SrCo$_2$V$_2$O$_8$ as a function of temperature and applied transverse magnetic field up to 25 T. An optical as well as an acoustic branch of confined spinons, the elementary excitations at zero field, are identified in the antiferromagnetic phase below the Néel temperature of 5 K and described by a one-dimensional Schrödinger equation. The confinement can be suppressed by an applied transverse field and a quantum disordered phase is induced at 7 T. In this disordered paramagnetic phase, we observe three emergent fermionic excitations with different transverse-field dependencies. The nature of these modes is clarified by studying spin dynamic structure factor of a 1D transverse-field Heisenberg-Ising (XXZ) model using the method of infinite time evolving block decimation. Our work reveals emergent quantum phenomena and provides a concrete system for testifying theoretical predications of one-dimension quantum spin models.

cond-mat.str-el↗