Controlled formation of metallic nanowires via Au nanoparticle ac trapping

Applying ac voltages, we trapped gold nanoparticles between microfabricated electrodes under well-defined conditions. We demonstrate that the nanoparticles can be controllably fused together to form homogeneous gold nanowires with pre-defined diameters and conductance values. Whereas electromigration is known to form a gap when a dc voltage is applied, this ac technique achieves the opposite, thereby completing the toolkit for the fabrication of nanoscale junctions.Comment: Nanotechnology 18, 235202 (2007

Etiology of the membrane potential of rat white fat adipocytes

The plasma membrane potential (Vm) is key to many physiological processes, however its ionic aetiology in white fat adipocytes is poorly characterised. To address this question, we have employed the perforated patch current-clamp and cell-attached patch-clamp methods in isolated primary white fat adipocytes and their cellular model: 3T3-L1. The resting Vm of primary and 3T3-L1 adipocytes were -32.1±1.2mV (n=95) and -28.8±1.2mV (n=87), respectively. Vm was independent of cell size and fat content. Elevation of extracellular [K+] to 50mM by equimolar substitution of bath Na+ did not affect Vm, whereas substitution of bath Na+ with the membrane impermeant cation N-methyl-D-glucamine+ hyperpolarized Vm by 16mV, data indicative of a non-selective cation permeability. Substitution of 133mM extracellular Cl- with gluconate, depolarised Vm to +5.5, whereas Cl- substitution with I- caused a -9mV hyperpolarization. Isoprenaline (10µM) but not insulin (100nM) significantly depolarized Vm. Single-channel ion activity was voltage independent; currents were indicative for Cl- with an inward slope conductance of 16±1.3pS (n=11) and a reversal potential close to the Cl- equilibrium potential: -29±1.6mV. Reduction of extracellular Cl- elevated the intracellular Ca2+ of adipocytes. In conclusion, the Vm of white fat adipocyte is well described by the Goldman-Hodgkin-Katz equation with a predominant permeability to Cl-. Consequently, changes in serum Cl- homeostasis or the adipocyte’s permeability to this anion via drugs will affect its Vm, intracellular Ca2+ and ultimately its function and its role in metabolic control

Impact of Spacecraft Shielding on Direct Ionization Soft Error Rates

No abstract availabl

Dynamics of a ferromagnetic domain wall and the Barkhausen effect

We derive an equation of motion for the the dynamics of a ferromagnetic domain wall driven by an external magnetic field through a disordered medium and we study the associated depinning transition. The long-range dipolar interactions set the upper critical dimension to be $d_c=3$, so we suggest that mean-field exponents describe the Barkhausen effect for three-dimensional soft ferromagnetic materials. We analyze the scaling of the Barkhausen jumps as a function of the field driving rate and the intensity of the demagnetizing field, and find results in quantitative agreement with experiments on crystalline and amorphous soft ferromagnetic alloys.Comment: 4 RevTex pages, 3 ps figures embedde

Structural, Electronic, and Magnetic Properties of MnO

We calculate the structural, electronic, and magnetic properties of MnO from first principles, using the full-potential linearized augmented planewave method, with both local-density and generalized-gradient approximations to exchange and correlation. We find the ground state to be of rhombohedrally distorted B1 structure with compression along the [111] direction, antiferromagnetic with type-II ordering, and insulating, consistent with experiment. We show that the distortion can be understood in terms of a Heisenberg model with distance dependent nearest-neighbor and next-nearest-neighbor couplings determined from first principles. Finally, we show that magnetic ordering can induce significant charge anisotropy, and give predictions for electric field gradients in the ground-state rhombohedrally distorted structure.Comment: Submitted to Physical Review B. Replaced: regenerated figures to resolve font problems in automatically generated pd

Dynamics of a ferromagnetic domain wall: avalanches, depinning transition and the Barkhausen effect

We study the dynamics of a ferromagnetic domain wall driven by an external magnetic field through a disordered medium. The avalanche-like motion of the domain walls between pinned configurations produces a noise known as the Barkhausen effect. We discuss experimental results on soft ferromagnetic materials, with reference to the domain structure and the sample geometry, and report Barkhausen noise measurements on Fe$_{21}$Co$_{64}$B$_{15}$ amorphous alloy. We construct an equation of motion for a flexible domain wall, which displays a depinning transition as the field is increased. The long-range dipolar interactions are shown to set the upper critical dimension to $d_c=3$, which implies that mean-field exponents (with possible logarithmic correction) are expected to describe the Barkhausen effect. We introduce a mean-field infinite-range model and show that it is equivalent to a previously introduced single-degree-of-freedom model, known to reproduce several experimental results. We numerically simulate the equation in $d=3$, confirming the theoretical predictions. We compute the avalanche distributions as a function of the field driving rate and the intensity of the demagnetizing field. The scaling exponents change linearly with the driving rate, while the cutoff of the distribution is determined by the demagnetizing field, in remarkable agreement with experiments.Comment: 17 RevTeX pages, 19 embedded ps figures + 1 extra figure, submitted to Phys. Rev.

Saturation of electrical resistivity

Resistivity saturation is observed in many metallic systems with a large resistivity, i.e., when the resistivity has reached a critical value, its further increase with temperature is substantially reduced. This typically happens when the apparent mean free path is comparable to the interatomic separations - the Ioffe-Regel condition. Recently, several exceptions to this rule have been found. Here, we review experimental results and early theories of resistivity saturation. We then describe more recent theoretical work, addressing cases both where the Ioffe-Regel condition is satisfied and where it is violated. In particular we show how the (semiclassical) Ioffe-Regel condition can be derived quantum-mechanically under certain assumptions about the system and why these assumptions are violated for high-Tc cuprates and alkali-doped fullerides.Comment: 16 pages, RevTeX, 15 eps figures, additional material available at http://www.mpi-stuttgart.mpg.de/andersen/saturation

Low-Energy Proton Testing Using Cyclotrons Sources

No abstract availabl