Search

In this paper, we report the growth of pure {\alpha}-MoO3 micro-flakes by CVD technique and their structural, electronic, optical, and magnetic properties. Samples are annealed at various temperatures in an H2 atmosphere to induce ferromagnetism. All the samples exhibit ferromagnetism at room temperature, and 250oC annealed sample shows the highest magnetic moment of 0.087 emu/g. It is evident from PL data that pristine as well as annealed samples contain different types of defects like oxygen vacancies, surface defects, interstitial oxygen, etc. It is deduced from the analysis of Mo3d and O1s core-level XPS spectra that oxygen vacancies increase up to an annealing temperature of 250oC that correlates with the magnetic moment. Significant changes in the total density of states and also in the magnetic moment for two and three oxygen vacancies are noticed through first-principle-based calculations. It is concluded that the magnetic moment is produced by oxygen vacancies or vacancy clusters, which is consistent with our experimental findings.

Semiconducting oxides possess a variety of intriguing electronic, optical, and magnetic properties, and native defects play a crucial role in these systems. In this study, we study the influence of native defects on these properties of $\alpha$-MoO$_{3}$ using the first-principles density functional theory (DFT) calculations. From the formation energy calculations, it is concluded that Mo vacancies are difficult to form in the system, while O and Mo-O co-vacancies are energetically quite favorable. We further find that vacancies give rise to mid-gap states (trap states) that remarkably affect the magneto-optoelectronic properties of the material. Our calculations indicate that a single Mo vacancy leads to half-metallic behavior, and also induces a large magnetic moment of 5.98 $\mu_{B}$. On the other hand, for the single O vacancy case, the band gap disappears completely, but the system remains in a non-magnetic state. For Mo-O co-vacancies of two types considered in this work, a reduced band gap is found, along with an induced magnetic moment of 2.0 $\mu_{B}$. Furthermore, a few finite peaks below the main band edge are observed in the absorption spectra of configurations with Mo and O vacancies, while they are absent in the Mo-O co-vacancies of both types, just like in the pristine state. From the ab-initio molecular dynamics simulations, stability and sustainability of induced magnetic moment at room temperate is verified. Our findings will enable the development of defect strategies that maximize the functionality of the system, and further help in designing highly efficient magneto-optoelectronic and spintronic devices.

Low-dimensional transition metal dichalcogenide, TMDC, materials are heralding a new era in optoelectronics and valleytronics owing to their unique properties. Photo-induced dynamics in these systems has mostly been studied from the perspective of individual quasi-particles, including excitons, bi-excitons or, even, trions. Their formation, evolution and decay. The role of multi-body and exciton dynamics, the associated collective behaviour, condensation and inter-excitonic interactions remain intriguing and seek attention, especially in room-temperature scenarios which are relevant for device applications. In this work we evidence the formation and decay of an unexpected electron-hole quantum liquid phase at room-temperature on ultrafast picosecond timescales in multi-layer MoS2 nanoparticles through femtosecond broadband transient absorption spectroscopy. Our studies reveal the complete dynamical picture: the initial electron-hole plasma, EHP, condenses into an quantum electron-hole liquid, EHL, phase which typically lasts as long as 10 ps, revealing its robustness, whereafter the system decays through phonons., Comment: 26 pages, 4 figures

Electrochemical capacitors with pure electric double layer capacitance are so far largely been limited to carbon materials only. Conventional metal oxides with Faradaic pseudocapacitance substantially suffer from material instability at high temperatures and thus there is a demand for novel metal oxides exhibiting thermally improved high electric double layer capacitance with material stability. In this study, titanium sesquioxide, Ti$_2$O$_3$, a Mott insulator in its metallic state at 300$^o$ C showed an anomalous increase of $\approx$ 560% in the solid-state electrochemical capacitance as compared to its pristine response at room temperature (RT). In aqueous electrolyte, the maximum capacitance was obtained up to 1053 $\mu$F/cm$^2$, which is 307% higher than with solid-state electrolyte. Moreover, the semiconducting state of Ti$_2$O$_3$ demonstrated $\approx$ 3500% enhancement in its electrochemical capacitance upon infrared illumination at RT. The observed electrochemical responses in Ti$_2$O$_3$ are attributed to the transition of semiconducting to the metallic state by redistribution of the localized electrons. Our experimental results find a good agreement with the first-principles density-functional theory calculations that revealed an increase in the charge density with the rise in temperature, which is mainly attributed to the surface Ti atoms. The study open has wide avenues to engineer the electrochemical double layer capacitance of theory calculations that revealed an increase in the charge density with the rise in temperature, which is mainly attributed to the surface Ti atoms. The study opens wide avenues to engineer the electrochemical double layer capacitance of Ti$_2$O$_3$ with high chemical stability., Comment: 27 pages, 10 figures (included)

We report an observation of room temperature magnetodielectric (RTMD) effect in Mn doped LaGaO3. Results of frequency dependent magnetoresistance (FDMR) measurements discards the possibility of any magnetoresistive contribution in the observed MD effect. The intrinsic nature of MD coupling has been validated/evidenced by means of magnetic field dependent Raman spectroscopy and explained in terms of modified volume strain governed by magnetic field induced rerotation of spin coupled Mn-orbitals. Ultimately, present RTMD effect is attributed to magneto-compression/magnetostriction associated with spin-phonon coupling as evidenced in the form of magnetic field induced hardening of symmetric stretching (SS) MnO6 octahedral Raman modes. Presently studied Mn doped LaGaO3 can be a candidate for magnetodielectric applications.

The temperature dependent diffuse reflectance spectroscopy measurements were carried out on the polycrystalline samples of BaTiO3 across the tetragonal to cubic structural phase transition temperature. The values of various optical parameters such as band gap (Eg), Urbach energy (EU) and Urbach focus (E0) are estimated in the range of 300 K to 470K. It is observed that near structural phase transition temperatures there exists two value of E0, suggesting presence of electronic heterogeneity over wide temperature range. Further near transition temperature EU shows metastability i.e. value of EU at temperature T is not constant but is a function of time (t). Interestingly it is observed that the ratio of EU(t=0)/ EU(t = tm), is almost remains constant at 295 K (pure tetragonal phase) and at 450 K (pure cubic phase), whereas this ratio shows decreasing behavior close to structural phase transition temperature, which confirms the presence of electronic metastibility in the pure BaTiO3. The observed metastibility can be fitted with the stretch exponents relaxation behavior, suggesting the presence of dynamic heterogeneous electronic disorder present in the sample across the transition. Further it appears that these metastable Urbach tail states (electronic disorder) may couple with the soft phonon modes and responsible for the observed terahertz dielectric relaxation (Phys. Rev. Lett. 101, 167402 (2008)). Further; present studies suggest that the optical studies appear to be more sensitive to probe the disorder/heterogeneity present in the sample.

With the growing popularity, the number of data sources and the amount of data has been growing very fast in recent years. The distribution of operational data on disperse data sources impose a challenge on processing user queries. In such database systems, the database relations required by a query to answer may be stored at multiple sites. This leads to an exponential increase in the number of possible equivalent or alternatives of a user query. Though it is not computationally reasonable to explore exhaustively all possible query plans in a large search space, thus a strategy is requisite to produce optimal query plans in distributed database systems. The query plan with most cost-effective option for query processing is measured necessary and must be generated for a given query. This paper attempts to generate such optimal query plans using a parameter less optimization technique Teaching-Learner Based Optimization(TLBO). The TLBO algorithm was experiential to go one better than the other optimization algorithms for the multi objective unconstrained and constrained benchmark problems. Experimental comparisons of TLBO based optimal plan generation with the multiobjective genetic algorithm based distributed query plan generation algorithm shows that for higher number of relations, the TLBO based algorithm is able to generate comparatively better quality Top K query plans., Comment: 12 pages

Using density functional theory, we predict half-metallicity in edge hydrogenated armchair boron nitride nanoribbons (ABNNRs). The predicted spin polarization is analyzed in detail by calculating electronic and magnetic properties of these hydrogenated ABNNRs by means of first-principles calculations within the local spin-density approximation (LSDA). ABNNRs with only edge B atoms passivated by H atoms are found to be half-metallic (regardless of their width) with a half-metal gap of 0.26 eV. Upto 100% spin polarized charge transport is predicted across the Fermi level owing to the giant spin splitting. Transmission spectrum analysis also confirms the separation of spin up and spindown electronic channels. It is revealed that H-passivation of only edge N atoms transforms non-magnetic bare ribbons into energetically stable magnetic semiconductors whereas hydrogenation of both the edges does not affect the electronic and magnetic state of bare ribbons significantly. The results are promising towards the realization of inorganic spintronic devices., Comment: arXiv admin note: substantial text overlap with arXiv:1501.01473

Theoretical Raman line shape functions have been studied to take care of quantum confinement effect and Fano effect individually and jointly. The characteristics of various Raman line shapes have been studied in terms of the broadening and asymmetry of Raman line shapes. It is shown that the asymmetry in the Raman line-shape function caused by these two effects individually does not add linearly to give asymmetry of line-shape generated by considering the combined effect. This indicates existence of interplay between the two effects. The origin of interplay lies in the fact that Fano effect itself depends on quantum confinement effect and in turn provides an asymmetry. This can not be explained by considering the two effects contribution independent of each other., Comment: 10 Pages

We predict intrinsic half-metallicity in armchair boron nitride nanoribbons (ABNNRs) via edge fluorination. The stability, electronic and magnetic properties of bare and edge fluorinated ABNNRs have been systematically analyzed by means of first-principles calculations within the local spin-density approximation (LSDA). The ribbons whose only edge-B atoms passivated with F atoms (i.e., edge-N atoms are un-passivated), regardless of width, are found half-metallic with a half-metal gap of 0.3 eV. A 100 \% spin polarized charge transport across the Fermi level is expected for such ribbons as the spin polarized states are $\sim$0.4 eV more stable than the spin un-polarized states and only single-spin conducting channels are present across the Fermi level owing to the gigantic spin splitting. The existence of half-metallicity is attributed to the localization of electronic charge at bare edge-N atoms as revealed from the analysis of Bloch states and projected density of states (PDOS).The sufficiently large half-metal gap (0.3 eV) with huge difference in the energies ($\sim$ 0.4 eV) of spin polarized and spin compensated states projects these half-metallic ABNNRs as potential candidate for spintronics applications., Comment: Error in Typing in older version