No abstract

Magnetorheological (MR) fluids, which can rapidly be changed from a liquid state to a solid state and vice versa by a magnetic field, have the potential to revolutionize several industrial sectors. The key issue is to enhance their yield shear stress. This paper reviews the physical mechanism and microstructure of MR fluids. It finds that the weak points of the MR microstructure under a shear force are at the chains' ends. Hence, a general technique, a compression-assisted-aggregation process, is developed to change the induced MR structure to a structure that consists of robust thick columns with strong ends. The scanning electronic micrographic (SEM) images confirm such a structure change. With this approach, MR fluids become super-strong. The enhanced yield stress of MR fluids reaches 800 kPa at a moderate magnetic field.

It is sho~n how the correlation energy of a system of two-dimensional electrons in a strong magnetic field may be enhanced if the electrons are in a regular array of the Landau orbitals. This gives an energy gap if the proportion v of occupied orbitals is a simple fraction without giving rise to charge-density waves which may pin the system. The gap is determined self-consistently.Such a state can give rise to the plateaus in the Hall conductance observed at fractional multiples of e /h.

The yield stress of magnetorheological ͑MR͒ fluids depends on the induced solid structure. Since thick columns have a yield stress much higher than a single-chain structure, we improve the yield stress of MR fluids by changing the fluid microstructure. Immediately after a magnetic field is applied, we compress the MR fluid along the field direction. Scanning electron microscopy images show that particle chains are pushed together to form thick columns. The shear force measured after the compression shows that the structure-enhanced static yield stress can reach as high as 800 kPa under a moderate magnetic field, while the same MR fluid has a yield stress of 80 kPa without compression. This improved yield stress increases with the magnetic field and compression pressure and has an upper limit well above 800 kPa. The method may also be useful for electrorheological fluids.

Sr 2 IrO 4 was predicted to be a high-temperature superconductor upon electron doping since it highly resembles the cuprates in crystal structure, electronic structure, and magnetic coupling constants. Here, we report a scanning tunneling microscopy/spectroscopy (STM/STS) study of Sr 2 IrO 4 with surface electron doping by depositing potassium (K) atoms. We find that as the electron doping increases, the system gradually evolves from an insulating state to a normal metallic state, via a pseudogaplike phase, and a phase with a sharp, V-shaped low-energy gap with about 95% loss of density of state (DOS) at E F . At certain K coverage (0.5-0.6 monolayer), the magnitude of the low-energy gap is 25-30 meV, and it closes at around 50 K. Our observations show that the electron-doped Sr 2 IrO 4 remarkably resembles hole-doped cuprate superconductors.