arXiv subjects

D. Jana

Publications and source records attributed to D. Jana.

12 recordsLinked to original sources

arXiv2024

Tuning THz magnons in a mixed van-der-Waals antiferromagnet

Alloying stands out as a pivotal technological method employed across various compounds, be they metallic, magnetic, or semiconducting, serving to fine-tune their properties to meet specific requirements. Ternary semiconductors represent a prominent example of such alloys. They offer fine-tuning of electronic bands, the band gap in particular, thus granting the technology of semiconductor heterostructures devices, key elements in current electronics and optoelectronics. In the realm of magnetically ordered systems, akin to electronic bands in solids, spin waves exhibit characteristic dispersion relations, featuring sizeable magnon gaps in many antiferromagnets. The engineering of the magnon gap constitutes a relevant direction in current research on antiferromagnets, aiming to leverage their distinct properties for THz technologies, spintronics, or magnonics. In this study, we showcase the tunability of the magnon gap across the THz spectral range within an alloy comprising representative semiconducting van-der-Waals antiferromagnets FePS$_3$ and NiPS$_3$. These constituents share identical in-plane crystal structures, magnetic unit cells and the direction of the magnetic anisotropy, but differ in the amplitude and sign of the latter. Altogether these attributes result in the wide tunability of the magnon gap in the Fe$_{1-x}$Ni$_x$PS$_3$ alloy in which the magnetic order is imposed by stronger, perpendicular anisotropy of iron.

cond-mat.mtrl-sci↗

arXiv2024

The temperature influence on the brightening of neutral and charged dark excitons in WSe$_2$ monolayer

The optically dark states play an important role in the electronic and optical properties of monolayers (MLs) of semiconducting transition metal dichalcogenides. The effect of temperature on the in-plane-field activation of the neutral and charged dark excitons is investigated in a WSe$_2$ ML encapsulated in hexagonal BN flakes. The brightening rates of the neutral dark (X$^D$) and grey (X$^G$) excitons and the negative dark trion (T$^D$) differ substantially at a particular temperature. More importantly, they vanish considerably by about 3 -- 4 orders of magnitude with the temperature increased from 4.2 K to 100 K. The quenching of the dark-related emissions is accompanied by the two-order-of-magnitude increase in the emissions of their neutral bright counterparts, $i.e.$ neutral bright exciton (X$^B$) and spin-singlet (T$^S$) and spin-triplet (T$^T$) negative trions, due to the thermal activations of dark states. Furthermore, the energy splittings between the dark X$^D$ and T$^D$ complexes and the corresponding bright X$^B$, T$^S$, and T$^T$ ones vary with temperature rises from 4.2 K to 100 K. This can be explained in terms of the different exciton-phonon couplings for the bright and dark excitons stemming from their distinct symmetry properties.

cond-mat.mes-hall↗

arXiv2024

Transverse and longitudinal magnons in strongly anisotropic antiferromagnet FePSe3

FePSe3 is a collinear honeycomb antiferromagnet with an easy-axis anisotropy and large spins S=2. It belongs to a family of magnetic van der Waals materials, which recently attracted a considerable attention. In this work we present an experimental magneto-optical study of the low-energy excitation spectrum in FePSe3, together with its theoretical description. The observed response contains several types of magnon excitations. Two of them are conventional transverse magnons described by a classical theory of antiferromagnetic resonance. Two other modes are identified as multimagnon hexadecapole excitations with an anomalous g factor approximately equal to four times the g factor of a single Fe^2+ ion. These quasiparticles correspond to full reversals of iron spins that coherently propagate in the up-down antiferromagnetic structure. They constitute a novel type of collective excitations in anisotropic magnetic solids, called longitudinal magnons. Comparison between theory and experiment allows us to estimate the microscopic parameters of FePSe3 including exchange coupling constants and the single-ion anisotropy.

cond-mat.str-el↗

arXiv2023

Magneto-optical sensing of the pressure driven magnetic ground states in bulk CrSBr

Competition between exchange interactions and magnetocrystalline anisotropy may bring new magnetic states that are of great current interest. An applied hydrostatic pressure can further be used to tune their balance. In this work we investigate the magnetization process of a biaxial antiferromagnet in an external magnetic field applied along the easy axis. We find that the single metamagnetic transition of the Ising type observed in this material under ambient pressure transforms under hydrostatic pressure into two transitions, a first-order spin flop transition followed by a second order transition towards a polarized ferromagnetic state near saturation. This reversible tuning into a new magnetic phase is obtained in layered bulk CrSBr at low temperature by varying the interlayer distance using high hydrostatic pressure, which efficiently acts on the interlayer magnetic exchange, and is probed by magneto-optical spectroscopy.

cond-mat.mes-hall↗

arXiv2023

Enhancement of photocatalytic performance of V2O5 by rare-earth ions doping, synthesized by facile hydrothermal technique

The rare-earth (RE) elements [Holmium (Ho) and Ytterbium (Yb)] doped vanadium pentoxide (V2O5) with a series of doping concentrations (1 mol.%, 3 mol.%, and 5 mol.%) have been successfully synthesized using environment-friendly facile hydrothermal method. The effect of RE ions on the photocatalytic efficiency of doped V2O5 has also been analyzed. The stable orthorhombic crystal structure of doped V2O5 confirms by the X-ray diffraction with no secondary phase, and high-stressed conditions are generated for the 3 mol.%. The crystallite size, strain, and dislocation density are calculated to perceive the doping effect on the bare V2O5. The optical characteristics have been measured using UV-vis spectroscopy. The absorptions are found to be increased with increasing doping concentrations; however, the bandgap remains in the visible range. The photocatalytic properties are examined for the compounds with varying pH, and it is observed that higher efficiency is exhibited for the pH 7 and catalyst concentration 500 ppm. The highest degradation efficiency is found to be 93% and 95% for the 3 mol.% of Ho and Yb-doped V2O5 samples within 2 hours, respectively. It is elucidated that the RE ions significantly impact the catalytic behavior of V2O5, and the mechanism behind these extraordinary efficiencies has been explained thoroughly.

physics.chem-ph↗

arXiv2022

Impact of reaction temperatures on the particle size of V2O5 synthesized by facile hydrothermal technique and their auspicious photocatalytic performance in dye degradation

In this study, a complete study of the effect of hydrothermal reaction temperatures on the synthesis and physical properties of V2O5 using the green facile mild hydrothermal method has been performed with six different temperatures 100 °C to 200 °C, in the step of 20 °C. . The XRD pattern confirm the stable orthorhombic crystal structure of the synthesized samples at all reaction temperatures. The SEM and TEM images demonstrate the particle-like morphology, and these characterizations affirmed that the particles size became larger with the increase of reaction temperatures. The FTIR analysis is employed to study the functional groups, and the obtained results are consistent with the XRD analysis. The bandgap has been estimated at various reaction temperatures using UV-vis diffuse reflectance spectra (UV-DRS) and was found to be varied 2.09 eV to 2.15 eV that are suitable range to absorb a significant amount of visible light. The photocatalysis of methylene blue (MB) with synthesized samples has been accomplished to investigate photocatalytic efficiency. The pure V2O5 synthesized at lower reaction temperature (100 °C) possess a lower bandgap and, accordingly, higher photocatalytic efficiency.

cond-mat.mtrl-sci↗

arXiv2020

DFT insights into new B-containing 212 MAX phases: Hf2AB2 (A = In, Sn)

212 MAX phase borides are new additions to the MAX phase family with enhanced thermo-mechanical properties. In this article, first-principles calculations have been carried out to explore the mechanical properties, Vickers hardness, elastic anisotropy, thermal properties, and optical properties of predicted thermodynamically stable MAX compounds Hf2AB2 (A = In, Sn). The structural properties are compared with the available data to assess the validity of calculations. The mechanical stability of Hf2AB2 (A = In, Sn) compounds is established with the help of the computed stiffness constants (Cij). The possible reason for enhanced mechanical properties and Vickers hardness of Hf2SnB2 is explained based on the analysis of bonding strength, followed by the electronic density of states. Higher mechanical strength and Vickers hardness of Hf2AB2 (A = In, Sn) compared to Hf2AC (A = In, Sn) are also indicated in the light of charge density mapping. The values of Pugh ratio, Poissons ratio and Cauchy pressure predict brittle character of the studied compounds. Besides, the anisotropic nature of the titled borides is investigated by 2D and 3D plots of elastic moduli along with some well established anisotropy indices. Thermal properties were investigated by calculating the Debye temperature, minimum thermal conductivity, Grüneisen parameter, and melting temperature. The thermal properties of Hf2AB2 (A = In, Sn) are also superior to Hf2AC (A = In, Sn). The optical constants such as real and imaginary parts of the dielectric function, refractive index, extinction coefficient, absorption coefficient, photoconductivity, reflectivity, and loss function are investigated.

cond-mat.mtrl-sci↗

arXiv2017

Clustered vacancies in ZnO: Chemical aspects and consequences on physical properties

Chemical nature of point defects, their segregation, cluster or complex formation in ZnO is an important area of investigation. In this report, 1.2 MeV Ar ion beam is used to incorporate defects in granular ZnO. Evolution of defective state with irradiation fluence 1 x 10^14 and 1 x 10^16 ions/cm2 has been monitored using XPS, PL and Raman spectroscopic study. XPS study shows presence of oxygen vacancies (VO) in the Ar ion irradiated ZnO. Zn(LMM) Auger spectra clearly identifies transition involving metallic zinc in the irradiated samples. Intense PL emission from IZn related shallow donor bound excitons (DBX) is visible in the 10 K spectra for all samples. Although overall PL is largely reduced with irradiation disorder, DBX intensity is increased for the highest fluence irradiated sample. Raman study indicates damage in both zinc and oxygen sub-lattice by energetic ion beam. Representative Raman modes from defect complexes involving VO, IZn and IO are visible after irradiation with intermediate fluence. Further increase of fluence shows, to some extent, a homogenization of disorder. Huge reduction of resistance is also noted for this sample. Certainly, high irradiation fluence induces a qualitative modification of the conventional (and highly resistive) grain boundary (GB) structure of granular ZnO. Low resistive path, involving IZn related shallow donors, across the GB can be presumed to explain resistance reduction. Open volumes (VZn and VO) agglomerate more and more with increasing irradiation fluence and finally get transformed to voids. Results as a whole have been elucidated with a model which emphasizes possible evolution of new defect microstructure that is distinctively different from the GB related disorder. Based on the model, qualitative explanations of commonly observed radiation hardness, colouration and ferromagnetism in disordered ZnO have been put forward.

cond-mat.mtrl-sci↗

arXiv2016

Shallow acceptor state in ZnO realized by simple irradiation and annealing route

Activation of shallow acceptor state has been observed in ion irradiated and subsequently air annealed polycrystalline ZnO material. Low temperature photoluminescence (PL) spectrum of the sample exhibits clear signature of acceptor bound exciton (ABX) emission at 3.360 eV. The other two samples, pristine and ion irradiated (without annealing), however, do not show acceptor related PL emission in the nearby energy region. Electron transition from shallow donor (most probable site is interstitial zinc for undoped ZnO) to such newly formed shallow acceptor state creates new donor-acceptor pair (DAP) luminescence peak ~ 3.229 eV. ABX and DAP peak energy positions confirm that the acceptor is N related. The acceptor exciton binding energy has been estimated to be 180 +/- 15 meV which is in conformity with earlier reports. The activation of shallow acceptors without any source of atomic nitrogen can only be possible through diffusion of molecular nitrogen inside the sample during annealing. The N2 molecules get trapped at bulk defect sites incorporated by ion irradiation and subsequent annealing. X-ray diffraction (XRD) and Raman spectroscopic (RS) investigation have been employed to probe the changing defective nature of the ZnO samples. Irradiation induced increased disorder has been detected (both by XRD and RS) which is partially removed/modified by annealing at 300 oC. Simultaneous activation of molecular nitrogen acceptor in purposefully defective ZnO is the key finding of this work. Results presented here provide a simple but controlled way of producing shallow acceptor state in ZnO. If optimized through suitable choice of ion, its energy and fluence as well as the annealing temperature, this methodology can trigger further scope to fabricate devices using ZnO epitaxial thin films or nanowires.

cond-mat.mtrl-sci↗

arXiv2010

Optical property modification of ZnO: Effect of 1.2 MeV Ar irradiation

We report a systematic study on 1.2 MeV Ar^8+ irradiated ZnO by x-ray diffraction (XRD), room temperature photoluminescence (PL) and ultraviolet-visible (UV-Vis) absorption measurements. ZnO retains its wurtzite crystal structure up to maximum fluence of 5 x 10^16 ions/cm^2. Even, the width of the XRD peaks changes little with irradiation. The UV-Vis absorption spectra of the samples, unirradiated and irradiated with lowest fluence (1 x 10^15 ions/cm^2), are nearly same. However, the PL emission is largely quenched for this irradiated sample. Red shift of the absorption edge has been noticed for higher fluence. It has been found that red shift is due to at least two defect centers. The PL emission is recovered for 5 x 10^15 ions/cm^2 fluence. The sample colour is changed to orange and then to dark brown with increasing irradiation fluence. Huge resistivity decrease is observed for the sample irradiated with 5 x 10^15 ions/cm^2 fluence. Results altogether indicate the evolution of stable oxygen vacancies and zinc interstitials as dominant defects for high fluence irradiation.

cond-mat.mtrl-sci↗

arXiv2009

Controlled defects in ZnO by low energy Ar irradiation

We report interesting observations in 1.2 MeV Ar8+ ion irradiated ZnO which, to the best of our knowledge, have not been published earlier and will be useful for the scientific community engaged in research on ZnO. Irradiation with the initial fluence 1 X 10^15 ions/cm^2 changes the colour of the sample from white to orange while the highest irradiation fluence makes it dark reddish brown that appears as black. Such changes in colour can be correlated with the oxygen vacancy type defects. No significant change in the grain size of the irradiated samples, as revealed from the x-ray diffraction (XRD) line width broadening, has been observed. Increase of surface roughness due to sputtering is clearly visible in scanning electron micrographs (SEM) with highest fluence of irradiation. Room temperature Photoluminescence (PL) spectrum of the unirradiated sample shows intense ultra-violet (UV) emission (~ 3.27 eV) and less prominent defect level emissions (2-3 eV). The overall emission is largely quenched due to initial irradiation fluence. But with increasing fluence UV emission is enhanced along with prominent defect level emissions. Remarkably, the resistivity of the irradiated sample with highest fluence is reduced by four orders of magnitude compared to that of the unirradiated sample. This indicates increase of donor concentration as well as their mobility due to irradiation. Oxygen vacancies are deep donors in ZnO, but surely they influence the stability of the shallow donors (presumably zinc interstitial related) and vice versa. This is in conformity with recent theoretical calculations.

cond-mat.mtrl-sci↗

arXiv2009

Synthesis and characterization of single phase Mn doped ZnO

Different samples of Zn1-xMnxO series have been prepared by conventional solid state sintering method. It has been identified, up to what extent of doping enable us to synthesize single-phase polycrystalline Mn doped ZnO samples which is one of the prerequisite for dilute magnetic semiconductor and we have analyzed its certain other physical aspects. In synthesizing the samples proportion of Mn varies from 1 at% to 5 at%. However the milling times have been varied (6, 12, 24, 48 & 96 hours) for only 2 at% Mn doped samples while for other samples (1, 3, 4 & 5 at% Mn doped) the milling time has been kept fixed at 96 hours. Room temperature X-Ray diffraction (XRD) data reveal that all of the prepared samples up to 3 at% of Mn doping exhibit wurtzite-type structure, no segregation of Mn and/or its oxides has been found. The 4 at% Mn doped samples show a weak peak of ZnMn2O4 apart from usual other peaks of ZnO and the intensity of this impurity peak has been further increased for 5 at% of Mn doping. So beyond 3 at% doping single-phase behavior is destroyed. Band gap for all the 2 at% Mn doped samples have been estimated as between 3.21 to 3.19 eV and reason for this low band gap values has been explained through the grain boundary trapping model. The room temperature resistivity measurement shows increase of resistivity up to 48 hours of milling and with further milling it saturates. The defect state of these samples has been investigated by using positron annihilation lifetime (PAL) spectroscopy technique. Here all the relevant lifetime parameters of positron i.e. free annihilation (tau 1), at defect site (tau 2) and average (tau av) increases with milling time.

cond-mat.mtrl-sci↗