Structure and Rheology of the Defect-gel States of Pure and Particle-dispersed Lyotropic Lamellar Phases

We present important new results from light-microscopy and rheometry on a moderately concentrated lyotropic smectic, with and without particulate additives. Shear-treatment aligns the phase rapidly, except for a striking network of oily-streak defects, which anneals out much more slowly. If spherical particles several microns in diameter are dispersed in the lamellar medium, part of the defect network persists under shear-treatment, its nodes anchored on the particles. The sample as prepared has substantial storage and loss moduli, both of which decrease steadily under shear-treatment. Adding particles enhances the moduli and retards their decay under shear. The data for the frequency-dependent storage modulus after various durations of shear-treatment can be scaled to collapse onto a single curve. The elasticity and dissipation in these samples thus arises mainly from the defect network, not directly from the smectic elasticity and hydrodynamics.Comment: 19 pages inclusive of 12 PostScript figures, uses revtex, psfrag and epsfig. Revised version, accepted for publication in Euro. Phys. J. B, with improved images of defect structure and theoretical estimates of network elasticity and scalin

The Density of States of hole-doped Manganites: A Scanning Tunneling Microscopy/Spectroscopy study

Variable temperature scanning tunneling microscopy/spectroscopy studies on single crystals and epitaxial thin films of hole-doped manganites, which show colossal magnetoresistance, have been done. We have investigated the variation of the density of states, at and near the Fermi energy ($E_f$), as a function of temperature. Simple calculations have been carried out, to find out the effect of temperature on the tunneling spectra and extract the variation of density of states with temperature, from the observed data. We also report here, atomic resolution images, on the single crystals and larger range images showing the growth patterns on thin films. Our investigation shows unambiguously that there is a rapid variation in density of states for temperatures near the Curie temperature ($T_c$). While for temperatures below $T_c$, a finite DOS is observed at $E_f$, for temperatures near $T_c$ a hard gap opens up in the density of states near $E_f$. For temperatures much higher than $T_c$, this gap most likely gives way to a soft gap. The observed hard gap for temperatures near $T_c$, is somewhat higher than the transport gap for all the materials. For different materials, we find that the magnitude of the hard gap decreases as the $T_c$ of the material increases and eventually, for materials with a $T_c$ close to 400 K, the value of the gap approaches zero.Comment: 9 pages RevTeX, 12 postscript figures, 1 table included in text, submitted to Physical Review

Nanostructure of giant magnetoresistive oxide film Nd2/3Sr1/3MnO3 by scanning tunneling microscopy

Scanning tunneling microscopy was used to study the surface nanostructure of the epitaxial film Nd2/3Sr1/3MnO3 that shows giant magnetoresistance. The surface morphology of the film consists of a number of overlapping platelets of about 30–35 Å diameter that grow at an angle of 35°–45° to the surface normal. The peak to peak height of the platelets are multiples of the c‐axis lattice parameter of 7.85 Å showing that the growth of the platelets takes place by the layer by layer addition of one formula unit. The mean surface roughness is about 10 Å. In the range of a few microns the film exhibits no defects or dislocations. The film is unstable in ambient atmosphere and tends to get covered by an adsorbate layer. Tip‐surface interactions cause the adsorbate to be dislodged exposing the surface nanostructure. The degradation of the film in real time when imaged in air was recorded. The adsorbates increase the surface roughness of the film

Nanostructure of giant magnetoresistive oxide film Nd2/3Sr1/3MnO3Nd_{2/3}Sr_{1/3}MnO_3 by scanning tunneling microscopy

Scanning tunneling microscopy was used to study the surface nanostructure of the epitaxial film $Nd_{2/3}Sr_{1/3}MnO_3$ that shows giant magnetoresistance. The surface morphology of the film consists of a number of overlapping platelets of about 30–35 \AA diameter that grow at an angle of 35°–45° to the surface normal. The peak to peak height of the platelets are multiples of the c-axis lattice parameter of 7.85 \AA showing that the growth of the platelets takes place by the layer by layer addition of one formula unit. The mean surface roughness is about 10 \AA. In the range of a few microns the film exhibits no defects or dislocations. The film is unstable in ambient atmosphere and tends to get covered by an adsorbate layer. Tip-surface interactions cause the adsorbate to be dislodged exposing the surface nanostructure. The degradation of the film in real time when imaged in air was recorded. The adsorbates increase the surface roughness of the film

Field and potential around local scatterers in thin metal films studied by scanning tunneling potentiometry

We report the direct observation of electrochemical potential and local transport field variations near scatterers like grain boundaries, triple points, and voids in thin platinum films studied by scanning tunneling potentiometry. The field is highest at a void, followed by a triple point and a grain boundary. The local transport field near a void can even be four orders of magnitude higher than the macroscopic field, indicating that the void is the most likely place for an electromigration induced failure. The field build up for a particular type of scatterer depends on the grain connectivity. We estimate an average grain boundary reflection coefficient for the film from the temperature dependence of its resistivity

Nanostructure of giant magnetoresistive oxide film Nd<SUB>&#x2154;</SUB>Sr<SUB>1/3</SUB>MnO<SUB>3</SUB> by scanning tunneling microscopy

Scanning tunneling microscopy was used to study the surface nanostructure of the epitaxial film Nd&#x2154;Sr1/3MnO3 that shows giant magnetoresistance. The surface morphology of the film consists of a number of overlapping platelets of about 30-35 &#197; diameter that grow at an angle of 35&#176;-45&#176; to the surface normal. The peak to peak height of the platelets are multiples of the c-axis lattice parameter of 7.85 &#197; showing that the growth of the platelets takes place by the layer by layer addition of one formula unit. The mean surface roughness is about 10 &#197;. In the range of a few microns the film exhibits no defects or dislocations. The film is unstable in ambient atmosphere and tends to get covered by an adsorbate layer. Tip-surface interactions cause the adsorbate to be dislodged exposing the surface nanostructure. The degradation of the film in real time when imaged in air was recorded. The adsorbates increase the surface roughness of the film

Improving Aerodynamic Performance of a Truck: a Numerical Based Analysis

With a change in EU-legislation regarding the dimensions of heavy trucks [1] coming up, specifically allowing aerodynamic devices fitted to the back of trailers, the transportation industry is set to largely decrease its environmental impact in the near future. These aerodynamic devices have been researched for quite some time [2] but have not yet gained widespread market acceptance, partly because of the transportation industry’s complex owner structures and partly because of size regulations of the past. Though with this new legislation and other expected new EU-legislation on CO2 emissions, development of these aerodynamic devices looks set to become a field of intense study. The study explores different options regarding the influence of changing the rear shape of a generalized ground transportation system (GTS), a simplified model of a truck geometry. By changing the rear shape the drag force induced by the oncoming air flow was reduced, and the overall coefficient of drag (CD) value lowered. There is also an investigation as to how an added suction slot, and that slots location, affects the CD. Computational Fluid Dynamics (CFD) in the form of the commercial computer software STAR-CCM+ was used to simulate the flow around the GTS, and the results were verified with similar studies and experiments [3] on the same geometry. RANS equations and Menter k-! SST turbulence model was used for all simulations. The results show that CD can be lowered by 21% compared to a baseline case. Further on the added suction slots can reduce drag, but depending on slot location also can add to the drag force experienced by the GTS