Solute-diffusion-controlled dislocation creep in pure aluminium containing 0.026 at.% Fe

Pure aluminium containing about 200?at.ppm Fe in solution is shown to creep about 10⁶ times slower at 200°C than the same aluminium containing a negligible amount of iron in solution. The high creep resistance of the Al–200?at.ppm?Fe alloy is attributed to the presence of subgrain boundaries containing iron solute atoms. It is proposed that the opposing stress fields from subgrain boundaries and from the piled-up dislocations during creep are cyclically relaxed, by iron solute diffusion, to allow climb of the lead dislocation in the pile-up. The mechanism is a form of mechanical ratcheting. The model is applied to Al–Fe alloys and correctly predicts that the creep rate is controlled by the rate of iron solute diffusion and by a temperature dependence equal to the activation energy for iron diffusion, namely Q _c?=?221?kJ?mol^?1. Basic creep studies on solid-solution alloying with solute atoms that diffuse slowly in the lattice of aluminium (e.g. manganese, chromium, titanium and vanadium) appear worthy of study as a way of enhancing creep strength and of understanding creep mechanisms involving solute-atom-containing subgrain boundaries.     

On stable geometries

Within the concept of projective lattice geometry we are considering the class of stable geometries which have also been introduced in [14]. The investigation of their basic properties will result in fundamental structure theorems which especially give a lattice-geometric characterization of free left modules of rank 6 over proper right Bezout rings of stable rank 2. This yields a proper generalization of previous results of ours.     

Preparing, manipulating, and measuring quantum states on a chip

We use gate voltage control of the exchange interaction to prepare, manipulate, and measure two-electron spin states in a GaAs double quantum dot. By placing two electrons in a single dot at low temperatures we prepare the system in a spin singlet state. The spin singlet is spatially separated by transferring an electron to an adjacent dot. The spatially separated spin singlet state dephases in due to the contact hyperfine interaction with the GaAs host nuclei. To combat the hyperfine dephasing, we develop quantum control techniques based on fast electrical control of the exchange interaction. We demonstrate coherent spin-state rotations in a singlet–triplet qubit and harness the coherent rotations to implement a singlet–triplet spin echo refocusing pulse sequence. The singlet–triplet spin echo extends the spin coherence time to .     

Quantitative model of price diffusion and market friction based on trading as a mechanistic random process

We model trading and price formation in a market under the assumption that order arrival and cancellations are Poisson random processes. This model makes testable predictions for the most basic properties of markets, such as the diffusion rate of prices (which is the standard measure of financial risk) and the spread and price impact functions (which are the main determinants of transaction cost). Guided by dimensional analysis, simulation, and mean-field theory, we find scaling relations in terms of order flow rates. We show that even under completely random order flow the need to store supply and demand to facilitate trading induces anomalous diffusion and temporal structure in prices.     

Experimental realization of a quantum spin pump

We demonstrate the operation of a quantum spin pump based on cyclic radio-frequency excitation of a GaAs quantum dot, including the ability to pump pure spin without pumping charge. The device takes advantage of bidirectional mesoscopic fluctuations of pumped current, made spin dependent by the application of an in-plane Zeeman field. Spin currents are measured by placing the pump in a focusing geometry with a spin-selective collector.     

Exponential decay lifetimes of excitons in individual single-walled carbon nanotubes

The dynamics of excitons in individual semiconducting single-walled carbon nanotubes was studied using time-resolved photoluminescence (PL) spectroscopy. The PL decay from tubes of the same (n,m) type was found to be monoexponential, however, with lifetimes varying between less than 20 and 200 ps from tube to tube. Competition of nonradiative decay of excitons is facilitated by a thermally activated process, most likely a transition to a low-lying optically inactive trap state that is promoted by a low-frequency phonon mode.     

Auger carrier leakage in III‐nitride quantum‐well light emitting diodes

Auger induced leakage is shown to be a contributing factor for the internal quantum efficiency (IQE) droop in III‐nitride quantum‐well light emitting diodes (LEDs). The mechanism is based on leakage current from carrier spill‐out of the well originating from energy transfer during Auger recombination. Adding this leakage reduces the Auger coefficient by 50% when compared to a standard Auger model with cubic density dependence. As reference, experimental data of a green quantum‐well LED are taken. Direct leakage due to non‐ideal carrier capture and re‐emission out of the well affects the IQE at current densities much larger than the maximum IQE point. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Boundary value problems for nonuniformly elliptic equations with measurable coefficients

Summary Let A be a symmetric N × N real-matrix-valued function on a connected region in Rⁿ, with A positive definite a.e. and A, A⁻¹ locally integrable. Let b and c be locally integrable, non-negative, real-valued functions on Ω, with c positive a.e. Put a(u, v) = = ((A∇u, ∇v) + buv) dx. We consider in X the weak boundary value problem a(u, v) = = fvcdx, all v ε X; where X is a suitable Hilbert space contained in H _loc ^1,1 (Ω). Criteria are given in order that the Green's operator for this problem have an integral representation and bounded eigenfunctions; in addition, criteria for compactness are given. Entrata in Redazione il 21 giugno 1975. Research was partially supported by the National Science Foundation under Grant GP-28377A2.