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      Volume 84, Issue 6

      June 2015,   pages  945-1128

    • Guest Editorial

      Ranjani Viswanatha Umesh V Waghmare

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    • Electronic structure of Fe-based superconductors

      Kalobaran Maiti

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      Fe-based superconductors have drawn much attention during the last decade due to the presence of superconductivity in materials containing the magnetic element, Fe, and the coexistence of superconductivity and magnetism. Extensive study of the electronic structure of these systems suggested the dominant role of 𝑑 states in their electronic properties, which is significantly different from the cuprate superconductors. In this article, some of our studies of the electronic structure of these fascinating systems employing high-resolution photoemission spectroscopy is reviewed. The combined effect of electron correlation and covalency reveals an interesting scenario in their electronic structure. The contribution of ligand 𝑝 states at the Fermi level is found to be much more significant than indicated in earlier studies. Temperature evolution of the energy bands reveals the signature of transition akin to Lifshitz transition in these systems.

    • Understanding metal–insulator transition in sodium tungsten bronze

      Sanhita Paul Satyabrata Raj

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      We have carried out angle-resolved photoemission spectroscopy (ARPES) and spectromicroscopy studies to understand the metal–insulator transition (MIT) observed in sodium tungsten bronzes, Na𝑥WO3. The experimentally determined band structure is compared with the theoretical calculation based on full-potential linear augmented plane-wave method. It has been found that there is a good gross agreement between experiment and theory. ARPES spectra on the insulating sample show that the states near 𝐸F are localized due to the random distribution of Na in WO3 lattice which causes strong disorder in the system. Our spectromicroscopy measurements on both insulating and metallic samples do not approve percolation model to explain MIT in Na𝑥WO3. Photoemission spectroscopy on metallic samples does not show any Na-induced impurity band (level), which was one of the models to explain MIT. Electron-like Fermi surface(s) has been found from our experiment for metallic samples at the 𝛤(X) point which shows good agreement with band calculation.

    • Magnetoresistance stories of double perovskites

      Abhishek Nag Sugata Ray

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      Tunnelling magnetoresistance (TMR) in polycrystalline double perovskites has been an important research topic for more than a decade now, where the nature of the insulating tunnel barrier is the core issue of debate. Other than the nonmagnetic grain boundaries as conventional tunnel barriers, intragrain magnetic antiphase boundaries (APB) as well as magnetically frustrated grain surfaces have also been proposed to act as tunnel barriers in Sr2FeMoO6. In this review, the present state of the debate has been discussed briefly and how the physical state of the material can affect the magnetoresistance signal of double perovskites in many different ways has been pointed out.

    • Magnetocaloric effect in rare-earth intermetallics: Recent trends

      R Nirmala A V Morozkin S K Malik

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      Magnetocaloric effect (MCE) is the change in isothermal magnetic entropy (𝛥𝑆m)and adiabatic temperature (𝛥𝑇ad) that accompany magnetic transitions in materials during the application or the removal of magnetic field under adiabatic conditions. The physics of MCE gets enriched by correlated spin-lattice degrees of freedom. This phenomenon has been actively investigated over the past few decades as it holds a promise for an alternate method of refrigeration/heat pumping. This has already resulted in several reviews on this topic. This paper focusses on some recent trends in this field and prospects of using rare-earth-based materials as active magnetic refrigerants over a broad temperature range that includes gas liquefaction and near-room temperature refrigeration/heating.

    • Site-specific doping, tunable dielectric properties and intrinsic paramagnetism in Mn-doped SrTiO3

      D Choudhury

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      Mn doping in SrTiO3 leads to the emergence of qualitatively distinct and novel physical properties. We show that Mn ions can be controllably doped at either of the perovskite 𝐴(Sr) or 𝐵(Ti) site as well as at both sites simultaneously and the resultant physical properties depend intimately on the particular dopant site. We critically review the recent literature on various Mn-doped SrTiO3 systems, which includes reports of dielectric glass, spin-glass and ferromagnetism and by combining experiments with first-principles calculations, we demonstrate that depending on the particular dopant site for Mn ions, the dielectric properties can be widely tuned from a quantum paraelectric to a dielectric glass. However, the intrinsic magnetism in all these cases remains essentially paramagnetic for phase-pure systems.

    • Correlation effects driven by reduced dimensionality in magnetic surface alloys

      U Manju

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      The evolution of electronic properties and correlation effects in manganese-based two-dimensional magnetic surface alloys are discussed. Enhanced correlations resulting from the reduced dimensionality of the surface alloys lead to the modification of the core level and valence band electronic structures resulting in the appearance of distinct satellite features. Apart from this, surface alloying-induced strong modifications in the substrate surface states arising from charge reorganization and electron transfer to the surface states as well as band-gap openings are also discussed.

    • Growth and photoemission spectroscopic studies of ultrathin noble metal films on graphite

      S K Mahatha Krishnakumar S R Menon

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      Growth of Cu, Ag and Au thin films on graphite(0 0 0 1)surface and possible formation of quantum well (QW) states originating due to the confinement of thin film sp electrons within the band gap of graphite along 𝛤 M symmetry direction are investigated using low-energy electron diffraction (LEED) and angle-resolved photoemission spectroscopy (ARPES). Higher surface diffusivity and surface energy of Cu on graphite surface led to cluster growth and does not reveal any quantum size effect, while Ag and Au films grow epitaxially in spite of large lattice mismatch. However, better surface ordering has been achieved by growing Ag and Au at low temperature (LT), followed by room-temperature (RT) annealing which are evident from LEED and the presence of sharp Shockley-type surface state (SS) at Fermi level (𝐸F). ARPES study of Ag films on graphite does not show any QW states, whereas Au films demonstrate a very sharp SS, Au bulk bands and well-resolved QW states or resonances. The observed low in-plane dispersions of these Au QW states or resonances are compared with the dispersions obtained in the previous Au QW state studies as well as for free-standing Au films.

    • Quantum confinement effects in low-dimensional systems

      D Topwal

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      The confinement effects of electrons in ultrathin films and nanowires grown on metallic and semiconducting substrates investigated using band mapping of their electronic structures using angle-resolved photoemission spectroscopy is discussed here. It has been shown that finite electron reflectivity at the interface is sufficient to sustain the formation of quantum well states and weak quantum well resonance states even in closely matched metals. The expected parabolic dispersion of sp-derived quantum well states for free-standing layers undergoes deviations from parabolic behaviour and modifications due to the underlying substrate bands, suggesting the effects of strong hybridization between the quantum well states and the substrate bands. Electron confinement effects in low dimensions as observed from the dispersionless features in the band structures are also discussed.

    • A model for the direct-to-indirect band-gap transition in monolayer MoSe2 under strain

      Ruma Das Priya Mahadevan

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      A monolayer of MoSe2 is found to be a direct band-gap semiconductor. We show, within ab-initio electronic structure calculations, that a modest biaxial tensile strain of 3% can drive it into an indirect band-gap semiconductor with the valence band maximum (VBM) shifting from 𝐾 point to 𝛤 point. An analysis of the charge density reveals that while Mo–Mo interactions contribute to the VBM at 0% strain, Mo–Se interactions contribute to the highest occupied band at 𝛤 point. A scaling of the hopping interaction strengths within an appropriate tight binding model can capture the transition.

    • Mechanical behaviour of nanoparticles: Elasticity and plastic deformation mechanisms

      Celine Gerard Laurent Pizzagalli

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      Nano-objects often exhibit drastically different properties compared to their bulk counterpart, opening avenues for new applications in many fields, such as in advanced composite materials, nanomanufacturing, nanoelectromechanical systems etc. As such, related research topics have become increasingly prominent in recent years. In this review on the mechanical behaviour of nanoparticles, the main investigation approaches are first briefly presented. The main results in terms of elasticity and plastic deformation mechanisms are then reported and discussed.

    • Narrowing the size distribution of CdTe nanocrystals using digestive ripening

      Mona Mittal Sameer Sapra

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      Digestive ripening of polydispersed colloidal CdTe nanocrystals is performed which results in monodispersed nanocrystals (NCs) as studied by optical spectroscopy. Optimization of ligand and refluxing time is carried out. Monodispersed NCs are obtained using mercaptopropionic acid (MPA) as a digestive ripening agent at a refluxing time of 1–2 h. Digestive ripening of CdTe NCs, which are less polydispersed, is also executed and it leads to more monodispersed NCs. In all the cases, there is a shift of maximum emission wavelength of CdTe NCs after digestive ripening that may be due to Ostwald ripening along with digestive ripening.

    • Effect of transition metal dopants on the optical and magnetic properties of semiconductor nanocrystals

      Ranjani Viswanatha

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      This review discusses the recent developments in doped semiconductor nanocrystals with a special emphasis on the effect of dopant on the electronic structure of the host nanocrystals. The review focusses on 3𝑑 transition metal dopants with unique electronic structure making them receptive for dramatic changes in magnetism, absorption and photoluminescence properties by the successful introduction of a small percentage of dopants into the nanocrystals. Many of these properties are shown to be qualitatively different from that of the bulk properties, leading to challenges in understanding the nature and effects of the confinement of the host. The optical and magnetic changes induced by Mn doping is first reviewed, followed by the use of Cu as a probe to understand the bulk and surface electronic structure of the host. The review concludes with a short section on photomagnetism induced by Cu on the host nanocrystal and a summary of the work with other transition metal ions.

    • Recent advances in the preparation of nanocrystal solids

      Rekha Mahadevu Dev Kumar Thapa Anshu Pandey

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      Colloidal chemistry offers several tools to synthesize and manipulate nanoscopic objects. Despite the existence of a plethora of tools to design building blocks, methods for assembling these components into functional macroscopic materials are still in their infancy. This review discusses the recent progress made towards assembling rudimentary nanoscale building blocks into functional macroscopic materials.

    • High-capacity electrode materials for electrochemical energy storage: Role of nanoscale effects

      Jagjit Nanda Surendra K Martha Ramki Kalyanaraman

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      This review summarizes the current state-of-the art electrode materials used for high-capacity lithium-ion-based batteries and their significant role towards revolutionizing the electrochemical energy storage landscape in the area of consumer electronics, transportation and grid storage application. We discuss the role of nanoscale effects on the electrochemical performance of high-capacity battery electrode materials. Decrease in the particle size of the primary electrode materials from micron to nanometre size improves the ionic and electronic diffusion rates significantly. Nanometre-thick solid electrolyte (such as lithium phosphorous oxynitride) and oxides (such as Al2O3, ZnO, TiO2 etc.) material coatings also improve the interfacial stability and rate capability of a number of battery chemistries. We elucidate these effects in terms of different high-capacity battery chemistries based on intercalation and conversion mechanism.

    • Colloidal transparent conducting oxide nanocrystals: A new infrared plasmonic material

      Bharat Tandon Aswathi Ashok Angshuman Nag

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      Thin films of transparent conducting oxides (TCO) are technologically important for applications as a visible light transparent electrode in a wide variety of optoelectronic devices. In the last few years, researchers started to explore novel size- and shape-dependent properties of TCO, where the crystallite size is ∼10 nm. So far, the localized surface plasmon resonance (LSPR) properties of TCO nanocrystals (NCs) have been found to be the most interesting. TCOs like Sn-doped In2O3, Al-doped ZnO and In-doped CdO NCs, exhibit LSPR band in near- to mid-infrared region. LSPR from a TCO NC exhibits many intrinsic differences with that of a metal NC. Carrier density in a TCO NC can easily be tuned by controlling the dopant concentration, which allows the LSPR band to be tuned over a range of ∼2000 nm (∼0.62 eV) in the near- to mid-infrared region. This review discusses recent advances in the understanding of plasmonic properties of various TCO NCs and highlights the potential applications of such unique plasmonic properties.

    • Carbon nanostructure composite for electromagnetic interference shielding

      Anupama Joshi Suwarna Datar

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      This communication reviews current developments in carbon nanostructure-based composite materials for electromagnetic interference (EMI) shielding. With more and more electronic gadgets being used at different frequencies, there is a need for shielding them from one another to avoid interference. Conventionally, metal-based shielding materials have been used. But due to the requirement of light weight, corrosion resistive materials, lot of work is being done on composite materials. In this research the forerunner is the nanocarbon-based composite material whose different forms add different characteristics to the composite. The article focusses on composites based on graphene, graphene oxide, carbon nanotubes, and several other novel forms of carbon.

    • Subject Index

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    • Author Index

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