Loveleen Kaur Brar

Electronic transport in nanostructured films of La0.67Sr0.33MnO3

In this paper we report electronic transport in nanostructured films of the rare-earth manganite La0.67Sr0.33MnO3. The films were grown by chemical solution deposition. The films show a resistivity peak in the temperature range of 250–265 K and have average grain size (∼50–60nm). The grain size can be controlled by postdeposition annealing. The films also show a rise in resistivity at low temperature (T<40K), reasonable low-field magnetoresistance up to 200 K, and nonlinear conductivity that shows up below 30 K. We ascribe these behaviors to the large number of natural grain boundaries that are present in these nanostructured films. We were also able to map the inhomogeneous local electronic properties arising from these grain boundaries using a variable-temperature scanning-tunneling microscope. We found that as the temperature is lowered, due to differences between the electronic properties of the grains and grain boundaries, the transport becomes more inhomogeneous. The nonlinear conduction as well as ...

Growth of oriented films of La0.67Ca0.33MnO3 and La0.67Sr0.33MnO3 on SrTiO3 using chemical solution deposition

Oriented thin films of La 0.67 Sr 0.33 MnO 3 and La 0.67 Ca 0.33 MnO 3 were fabricated on a SrTiO 3 (002) single crystal substrate by the chemical solution deposition (CSD) process. The CSD grown films have electronic and magnetotransport properties which are comparable to those of films prepared by the pulsed laser technique and to those of the bulk single crystal samples. The magnetoresistance of the films was found to be very similar to those of pulsed laser deposition grown films and they show no contribution of grain boundaries unlike polycrystalline films. The atomic force microscopy study of the roughness and its scaling with length shows that the surfaces of the films are self affine.

Evolution of structural and thermal properties of carbon-coated TaC nanopowder synthesized by single step reduction of Ta-ethoxide

Carbon-coated nano tantalum carbide (TaC) has been synthesized at 800 °C using tantalum-ethoxide precursor by the single step chemical reaction route without using any external carbon source. The XRD results of the synthesized samples indicate that formation of TaC starts immediately upon heating but the complete transformation is observed only after 10 h of holding at 800 °C. The 10h sample shows distinct decarburization and oxidation peaks in DSC/DTG. The surface weighted sizes obtained from double-Voigt method were confirmed by BET. The BET analysis shows that synthesized powders have large surface area and contain a mixture of micropores and mesopores. The morphology and particle size distribution analysis shows that the powders are of faceted to spherical shape with thin carbon coating having size variation primarily between 20–40 nm. DSC/TG, XRD and microstructure analysis results have been used to predict the mechanism for the formation of the carbon coated nano-TaC particles.

The role of carbon in structural evolution during single step synthesis of nano tantalum carbide

Cubic phase carbon-coated nano tantalum carbide (TaC) has been synthesized at 800 °C in a single step from tantalum oxide using the carbon and hydrogen produced in situ via decomposition of acetone in an autoclave. In the product phase(s) carbon exists: (a) inside the carbide, (b) on the surface of the carbide particles and (c) as free carbon (amorphous as well as graphitic). The effects of initial carbon concentration on the final carbon content inside as well as outside the TaC have been studied. The structural features of the final product have a complex dependency on the initial carbon concentration. The thermal behaviour of the final product clearly delineates the effects of internal and external carbon content. The soaking time studies show that the grain growth of TaC within the autoclave follows the simultaneous grain boundary migration and grain rotation model. The DSC/TG, XRD and microstructure analysis results along with thermal calculations have been used to predict the formation mechanism for the carbide particles. The reaction mechanism analysis brings forth the role of Mg in lowering the reaction temperature. In this process the carbon content of TaC, the size as well as the strain of the synthesized powders and the %free carbon content can be tailored as per the requirement for the given application.

Distinct levels in the nanoscale organization of DNA-histone complex revealed by its mechanical unfolding

Mechanical unfolding of nanoscale DNA-histone complex, using an atomic force microscope, shows a stepwise disassembly of histones from the nucleosome. A quantitative analysis of the rupture jump statistics and the length released per jump reveals insights into the possible histone contacts within the octamer complex. The measured ruptures correlate with the breakage of multiple contacts that stabilize the histone octamer. These results provide a mechanistic basis by which stepwise disassembly of histone proteins may result from an external force exerted by the adenosinetriphosphate ATP dependent chromatin remodeling machines to access regulatory sites on DNA.

Mapping of local electronic properties in nanostructured CMR thin films by Scanning Tunneling Microscopy (STM) and Local Conductance Map (LCMAP)

We have investigated the local electronic properties and the spatially resolved magnetoresistance of a nanostructured film of a colossal magnetoresistive (CMR) material by local conductance mapping (LCMAP) using a variable temperature Scanning Tunneling Microscope (STM) operating in a magnetic field. The nanostructured thin films (thickness ≈500nm) of the CMR material La0.67Sr0.33MnO3 (LSMO) on quartz substrates were prepared using chemical solution deposition (CSD) process. The CSD grown films were imaged by both STM and atomic force microscopy (AFM). Due to the presence of a large number of grain boundaries (GBs), these films show low field magnetoresistance (LFMR) which increases at lower temperatures. The measurement of spatially resolved electronic properties reveal the extent of variation of the density of states (DOS) at and close to the Fermi level (EF) across the grain boundaries and its role in the electrical resistance of the GB. Measurement of the local conductance maps (LCMAP) as a function of magnetic field as well as temperature reveals that the LFMR occurs at the GB. While it was known that LFMR in CMR films originates from the GB, this is the first investigation that maps the local electronic properties at a GB in a magnetic field and traces the origin of LFMR at the GB.