arXiv subjects

Santosh KC

Publications and source records attributed to Santosh KC.

13 recordsLinked to original sources

arXiv2026

Vibrational and Electronic Properties of Np2O5 from Experimental Spectroscopy and First Principles Calculations

High-valence actinide oxides are critical to understanding the behavior of 5f-electrons, yet their structural and electronic properties remain poorly understood due to challenges in synthesis and handling. We report the first Raman spectroscopic study of single-crystalline Np2O5 and the first scanning tunneling spectroscopy (STS) measurement on any neptunium-containing material. Hydrothermally synthesized crystals were structurally verified by X-ray diffraction. Raman spectra revealed sharply resolved vibrational features, including previously unreported low-frequency modes. STS measurements revealed a band gap of 1.5 eV. Density functional theory (DFT) enables vibrational mode assignments, reveals neptunium-dominated low-frequency phonons, oxygen-dominated high-frequency modes, and predicts an indirect band gap of 1.68 eV. This predicted value is in excellent agreement with the experimentally measured STS gap. This combined Raman, DFT, and STS approach provides a robust framework for correlating lattice dynamics and electronic structure in actinide materials, providing benchmark data for Np2O5, and opening new avenues for probing structure-property relationships in complex f-electron materials.

cond-mat.mtrl-sci↗

arXiv2022

Strong Effects of Interlayer Interaction on Valence-Band Splitting in Transition Metal Dichalcogenides

Understanding the origin of valence band maxima (VBM) splitting in transition metal dichalcogenides (TMDs) is important because it governs the unique spin and valley physics in monolayer and multilayer TMDs. In this work, we present our systematic study of VBM splitting ($Δ$) in atomically thin MoS$_2$ and WS$_2$ by employing photocurrent spectroscopy as we change the temperature and the layer numbers. We found that VBM splitting in monolayer MoS$_2$ and WS$_2$ depends strongly on temperature, which contradicts the theory that spin-orbit coupling solely determines the VBM splitting in monolayer TMDs. We also found that the rate of change of VBM splitting with respect to temperature ($m=\frac{\partialΔ}{\partial T}$) is the highest for monolayer (-0.14 meV/K for MoS$_2$) and the rate decreases as the layer number increases ($m ~ 0$ meV/K for 5 layers MoS$_2$). We performed density functional theory (DFT) and the GW with Bethe-Salpeter Equation (GW-BSE) calculations to determine the electronic band structure and optical absorption for a bilayer MoS$_2$ with different interlayer separations. Our simulations agree with the experimental observations and demonstrate that the temperature dependence of VBM splitting in atomically thin monolayer and multilayer TMDs originates from the changes in the interlayer coupling strength between the neighboring layers. By studying two different types of TMDs and many different layer thicknesses, we also demonstrate that VBM splitting also depends on the layer numbers and type of transition metals. Our study will help understand the role spin-orbit coupling and interlayer interaction play in determining the VBM splitting in quantum materials and develop next-generation devices based on spin-orbit interactions.

cond-mat.mes-hall↗

arXiv2022

First principle investigations of the structural, electronic, and phase stability in 2D layered ZnSb

Recently, the two dimensional (2D) materials have become a potential candidates for various technological applications in spintronics and optoelectronics. In the present study, the structural, electronic, and phase stability of 2D layered ZnSb compounds of four different phases viz. wurzite(w), tetragonal (t), hexagonal (h), and orthorhombic (o) have been tuned using the first principle calculations based on density functional theory (DFT). We invoked the Perdew-Burke-Ernzerhof (PBE) functional and the projected augmented wave (PAW) method during all the calculations. Based on our numerical results, we predicted the novel tetragonal phase as stable phase of ZnSb next to existing orthorhombic structure. We reported the pressure induced phase transition between orthorhombic to tetragonal phase at 12.48 GPa/atom. The projected density of states indicates the strong p-d hybridization between Sb-5p and Zn-3d states confirming the nature of strong covalent bonding between them. The electronic band structures suggest that t-ZnSb, w-ZnSb, and h-ZnSb are metallic in nature whereas o-ZnSb is semiconducting with narrow band gap of 0.03 eV using PBE. We predicted the possibility of extracting the two dimensional (2D) monolayer sheet in t-ZnSb and o-ZnSb according to the exfoliation energy criterion. In addition, the 2D monolayer (ML) of o-ZnSb has been predicted to be dynamically stable but that of t-ZnSb is not stable as manifested in phonon dispersion bands. Surprisingly, the semiconducting band gap nature of o-ZnSb changes from indirect and narrow to direct and sizable while going from 3D bulk to 2D ML structure. Further, we estimated the value of work functions for the surfaces of t-ZnSb and o-ZnSb as 4.61 eV and 4.04 eV respectively. Such materials can find the niche applications in next generation electronic devices utilizing 2D hetero-structures.

physics.comp-ph↗

arXiv2022

Towards the Ionizing Radiation Induced Bond Dissociation Mechanism in Oxygen, Water, Guanine and DNA Fragmentation: A Density Functional Theory Simulation

The radiation-induced damages in bio-molecules are ubiquitous processes in radiotherapy, radio-biology and critical to space-projects. In this study we present a precise quantification of the fragmentation mechanisms of deoxyribonucleic acid (DNA) and the molecules surrounding DNA such as oxygen and water under non-equilibrium conditions using the first-principle calculations based on density functional theory (DFT). Our results reveal the structural stability of DNA-bases and backbone that withstand up to a combined threshold of charge and hydrogen abstraction owing to simultaneous direct and indirect ionization processes. We show the hydrogen contents of the molecules significantly control the stability in the presence of radiation. This study provides comprehensive information on the impact of the direct and indirect induced bond dissociations and DNA damage, and introduces a systematic methodology in fine-tuning of the input parameters necessary for the large-scale Monte Carlo simulations of radio-biological responses and mitigation of detrimental effects of ionizing radiation.

physics.bio-ph↗

arXiv2021

A Novel Data Pre-processing Technique: Making Data Mining Robust to Different Units and Scales of Measurement

Many existing data mining algorithms use feature values directly in their model, making them sensitive to units/scales used to measure/represent data. Pre-processing of data based on rank transformation has been suggested as a potential solution to overcome this issue. However, the resulting data after pre-processing with rank transformation is uniformly distributed, which may not be very useful in many data mining applications. In this paper, we present a better and effective alternative based on ranks over multiple sub-samples of data. We call the proposed pre-processing technique as ARES | Average Rank over an Ensemble of Sub-samples. Our empirical results of widely used data mining algorithms for classification and anomaly detection in a wide range of data sets suggest that ARES results in more consistent task specific? outcome across various algorithms and data sets. In addition to this, it results in better or competitive outcome most of the time compared to the most widely used min-max normalisation and the traditional rank transformation.

cs.LG↗

arXiv2021

Predicting spin orbit coupling effect in the electronic and magnetic properties of cobalt (Co) doped WSe2 monolayer

The electronic and magnetic properties of cobalt (Co) doped monolayer (ML) tungsten diselenide (WSe2) are investigated using the density functional theory with the on-site Hubbard potential correction (DFT+U) for the localized d orbitals of Co atom taking into account the spin orbit coupling (SOC) interaction. The results show that the substitution of Co at the W sites of ML WSe2 is energetically favorable under Se rich environment. We noticed that the Hund's exchange splitting (\Delta H_{ex}) is dominant over the crystal field splitting (\Delta_{cf}). The induced magnetic moment due to the Co-doped defect is ~3.00 \mu_B per Co atom. The magnetic interaction between two Co atoms at the nearest neighbor separation depends mainly on the concentration of the impurity atoms. The calculated value of curie temperature (TC) is increasing with increasing impurity concentration satisfying the Zener model. Based on the results, it can be proposed that the Co-doped WSe2 monolayer is potential candidate to apply in spintronics, optoelectronics, and magnetic storage devices.

cond-mat.mtrl-sci↗

arXiv2020

Predicting the phase stability of multi-component high entropy compounds

A generic method to estimate the relative feasibility of formation of high entropy compounds in a single phase, directly from first principles, is developed. As a first step, the relative formation abilities of 56 multi-component, AO, oxides were evaluated. These were constructed from 5 cation combinations chosen from A={Ca, Co, Cu, Fe, Mg, Mn, Ni, Zn}. Candidates for multi-component oxides are predicted from descriptors related to the enthalpy and configurational entropy obtained from the mixing enthalpies of two component oxides. The utility of this approach is evaluated by comparing the predicted combinations with the experimentally realized entropy stabilized oxide, (MgCoCuNiZn)O. In the second step, Monte Carlo simulations are utilized to investigate the phase composition and local ionic segregation as a function of temperature. This approach allows for the evaluation of potential secondary phases, thereby making realistic predictions of novel multi-component compounds that can be synthesized.

cond-mat.mtrl-sci↗

arXiv2020

On the Elastic Anisotropy of the Entropy-Stabilized Oxide (Mg, Co, Ni, Cu, Zn)O

In this paper, we study the elastic properties of the entropy-stabilized oxide (Mg, Co, Ni, Cu, Zn)O, using experimental and first principles techniques. Our measurements of the indentation modulus on grains with a wide range of crystallographic orientations of the entropy-stabilized oxide revealed a high degree of elastic isotropy at ambient conditions. First principles calculations predict mild elastic anisotropy for the paramagnetic structure, which decreases when the system is considered to be non-magnetic. When the antiferromagnetic state of CoO, CuO and NiO is accounted for in the calculations, a slight increase in the elastic anisotropy is observed, suggesting a coupling between magnetic ordering and the orientation dependent elastic properties. Furthermore, an examination of the local structure reveals that the isotropy is favored through local ionic distortions of Cu and Zn - due to their tendency to form tenorite and wurtzite phases. The relationships between the elastic properties of the multicomponent oxide and those of its constituent binary oxides are reviewed. These insights open up new avenues for controlling isotropy for technological applications through tuning composition and structure in the entropy-stabilized oxide or the high entropy compounds in general.

cond-mat.mtrl-sci↗

arXiv2019

Self-Assembled Room Temperature Multiferroic BiFeO3-LiFe5O8 Nanocomposites

Multiferroic materials have driven significant research interest due to their promising technological potential. Developing new room-temperature multiferroics and understanding their fundamental properties are important to reveal unanticipated physical phenomena and potential applications. Here, a new room temperature multiferroic nanocomposite comprised of an ordered ferrimagnetic spinel LiFe5O8 (LFO) and a ferroelectric perovskite BiFeO3 (BFO) is presented. We observed that lithium (Li)-doping in BFO favors the formation of LFO spinel as a secondary phase during the synthesis of LixBi1-xFeO3 nanoceramics. Multimodal functional and chemical imaging methods are used to map the relationship between doping-induced phase separation and local ferroic properties in both the BFO-LFO composite ceramics and self-assembled nanocomposite thin films. The energetics of phase separation in Li doped BFO and the formation of BFO-LFO composites is supported by first principles calculations. These findings shed light on Li-ion role in the formation of a functionally important room temperature multiferroic and open a new approach in the synthesis of light element doped nanocomposites.

cond-mat.mtrl-sci↗

arXiv2017

High-temperature magnetostructural transition in van der Waals-layered a-MoCl3

The crystallographic and magnetic properties of the cleavable 4d3 transition metal compound a-MoCl3 are reported, with a focus on the behavior above room temperature. Crystals were grown by chemical vapor transport and characterized using temperature dependent x-ray diffraction, Raman spectroscopy, and magnetization measurements. A structural phase transition occurs near 585 K, at which the Mo-Mo dimers present at room temperature are broken. A nearly regular honeycomb net of Mo is observed above the transition, and an optical phonon associated with the dimerization instability is identified in the Raman data and in first-principles calculations. The crystals are diamagnetic at room temperature in the dimerized state, and the magnetic susceptibility increases sharply at the structural transition. Moderately strong paramagnetism in the high-temperature structure indicates the presence of local moments on Mo. This is consistent with results of spin-polarized density functional theory calculations using the low- and high-temperature structures. Above the magnetostructural phase transition the magnetic susceptibility continues to increase gradually up to the maximum measurement temperature of 780 K, with a temperature dependence that suggests two-dimensional antiferromagnetic correlations.

cond-mat.mtrl-sci↗

arXiv2017

Magnetic Behavior and Spin-Lattice Coupling in Cleavable, van der Waals Layered CrCl3 Crystals

CrCl3 is a layered insulator that undergoes a crystallographic phase transition below room temperature and orders antiferromagnetically at low temperature. Weak van der Waals bonding between the layers and ferromagnetic in-plane magnetic order make it a promising material for obtaining atomically thin magnets and creating van der Waals heterostructures. In this work we have grown crystals of CrCl3, revisited the structural and thermodynamic properties of the bulk material, and explored mechanical exfoliation of the crystals. We find two distinct anomalies in the heat capacity at 14 and 17 K confirming that the magnetic order develops in two stages on cooling, with ferromagnetic correlations forming before long range antiferromagnetic order develops between them. This scenario is supported by magnetization data. A magnetic phase diagram is constructed from the heat capacity and magnetization results. We also find an anomaly in the magnetic susceptibility at the crystallographic phase transition, indicating some coupling between the magnetism and the lattice. First principles calculations accounting for van der Waals interactions also indicate spin-lattice coupling, and find multiple nearly degenerate crystallographic and magnetic structures consistent with the experimental observations. Finally, we demonstrate that monolayer and few-layer CrCl3 specimens can be produced from the bulk crystals by exfoliation, providing a path for the study of heterostructures and magnetism in ultrathin crystals down to the monolayer limit.

cond-mat.other↗

arXiv2017

Antiferromagnetism in the van der Waals layered spin-lozenge semiconductor CrTe3

The crystallographic, magnetic, and transport properties of the van der Waals bonded, layered compound CrTe3 have been investigated on single crystal and polycrystalline materials. The crystal structure contains layers made up of lozenge shaped Cr_4 tetramers. Electrical resistivity measurements show the crystals to be semiconducting, with a temperature dependence consistent with a band gap of 0.3eV. The magnetic susceptibility exhibits a broad maximum near 300K characteristic of low dimensional magnetic systems. Weak anomalies are observed in the susceptibility and heat capacity near 55K, and single crystal neutron diffraction reveals the onset of long range antiferromagnetic order at this temperature. Strongly dispersive spin-waves are observed in the ordered state. Significant magneto-elastic coupling is indicated by the anomalous temperature dependence of the lattice parameters and is apparent in structural optimization in van der Waals density functional theory calculations for different magnetic configurations. The cleavability of the compound is apparent from its handling and is confirmed by first principles calculations, which predict a cleavage energy 0.5J/m^2, similar to graphite. Based on these results, CrTe3 is identified as a promising compound for studies of low dimensional magnetism in bulk crystals as well as magnetic order in monolayer materials and van der Waals heterostructures.

cond-mat.str-el↗

arXiv2014

Electronic Properties of MoS2/HfO2 Interface: Impact of Interfacial Impurities during Atomic Layer Deposition Growth

Using ab-initio calculations within the framework of Density Functional Theory (DFT), atomic structures and electronic properties of MoS2/HfO2 interface are investigated. The impact of interfacial oxygen concentration on the MoS2/HfO2 interface electronic structure is examined in order to mimic the atomic layer deposition growth at ambient conditions. Then, the effect on band offsets and the thermodynamic stability of those interfaces is investigated and compared with available relevant experimental data. These results seem to indicate that, for a well-prepared interface, the electronic device performance should be better than other interfaces like III-V/high-k due to the absence of interface defect states. However, any unpassivated defects, if present during oxide growth, strongly influence the subsequent electronic properties of the interface.

cond-mat.mtrl-sci↗