On the uniqueness of solutions for the Dirichlet boundary value problem of linear elastostatics in the circular domain

Summary The uniqueness and mathematical stability of the Dirichlet boundary value problem of linear elastostatics is studied. The problem is posed as a set of partial differential equations in terms of displacements and Dirichlet-type of boundary conditions (displacements) for arbitrary bounded domains. Then for the circular interior domain the closed form analytical solution is obtained, using an extended version of the method of separation of variables. This method with corresponding complete solution allows for the derivation of a necessary and sufficient condition for uniqueness. The results are compared with existing energy and uniqueness criteria. A parametric study of the elastic characteristics is performed to investigate the behaviour of the displacement field and the strain energy distribution, and to examine the mathematical stability of the solution. It is found that the solution for the circular element with hourglass-like boundary conditions will be unique for all v ≠ 0.5, 0.75, 1.0 and will be mathematically stable for all v ≠ 0.75. Locking of the circular element occurs for v = 0.75 as the energy tends to infinity.     

Spin–Lattice Relaxation of Mn Ions in Nanostructures with Semiconductor Quantum Dots

We use the combination of nonequilibrium phonon and exciton luminescence techniques to study the spin dynamics in diluted magnetic semiconductor structures with (Cd,Mn)Te and (Cd,Mn)Se quantum dots (QDs). We show that the spin–lattice relaxation (SLR) of Mn ions in these structures differs strongly from the SLR in quantum wells. We explain the results by a model where SLR process in structures with QDs is modified by the spin diffusion on Mn ions from the QD to a wetting layer.     

Magnetic Anisotropies and (Ga,Mn)As‐based Spintronic Devices

In this Review, we discuss the rich anisotropic properties of the ferromagnetic semiconductor (Ga,Mn)As, and their implications in transport studies. We review the various sources and types of anisotropy seen in the material, discuss its magnetization reversal process, and demonstrate how basic transport properties, such as resistivity and Hall measurements, can be used as very sensitive tools to investigate the magnetization properties of the material. We also discuss how the magnetic anisotropy, coupled with large spin–orbit coupling, leads to an anisotropy in the transport density of states, which in turn leads to fundamentally novel behavior such as tunneling anisotropic magnetoresistance (TAMR).     

Large room-temperature magnetoresistance in lateral organic spin valves fabricated by in situ shadow evaporation

We report the successful fabrication of lateral organic spin valves with a channel length in the sub 100 nm regime. The fabrication process is based on in situ shadow evaporation under UHV conditions and therefore yields clean and oxygen-free interfaces between the ferromagnetic metallic electrodes and the organic semiconductor. The spin valve devices consist of Nickel and Cobalt–Iron electrodes and the high mobility n-type organic semiconductor N,N^′-bis(heptafluorobutyl)-3,4:9,10-perylene diimide. Our studies comprise fundamental investigations of the process’ and materials’ suitability for the fabrication of lateral spin valve devices as well as magnetotransport measurements at room temperature. The best devices exhibit a magnetoresistance of up to 50%, the largest value for room temperature reported so far.     

Tunneling Anisotropic Magnetoresistance-Based Devices

The paper demonstrates the operation of several devices based on tunneling anisotropic magnetoresistance. This effect, which originates from the interplay between the magnetic and transport properties in magnetic materials with strong spin-orbit coupling such as the ferromagnetic semiconductor (Ga,Mn)As, leads to a dependence of the tunneling resistance of devices with respect to the direction of the magnetization in the (Ga,Mn)As layer. We show that such devices can be operated as either information storage elements or sensors. It was also demonstrated that they can be used in either volatile or nonvolatile mode and that they provide either two-state or multiple state devices in either of these modes. Lastly, we present experimental evidence that they can be coupled to traditional ferromagnetic materials to still further enhance the variety of possible device functionalities     

Controlling optical gain in semiconducting polymers with nanoscale chain positioning and alignment

We control the chain conformation of a semiconducting polymer by encapsulating it within the aligned nanopores of a silica host. The confinement leads to polarized, low-threshold amplified spontaneous emission from the polymer chains. The polymer enters the porous silica film from only one face and the filling of the pores is therefore graded. As a result, the profile of the index of refraction in the film is also graded, in the direction normal to the pores, so that the composite film forms a low-loss, graded-index waveguide. The aligned polymer chains plus naturally formed waveguide are ideally configured for optical gain, with a threshold for amplified spontaneous emission that is twenty times lower than in comparable unoriented polymer films. Moreover, the optimal conditions for ASE are met in only one spatial orientation and with one polarization. The results show that nanometre-scale control of semiconducting polymer chain orientation and position leads to novel and desirable optical properties.     

Temperature Peculiarities of Magnetic Anisotropy in (Ga,Mn)As: The Role of the Hole Concentration

It is demonstrated by SQUID measurements that (Ga,Mn)As films can exhibit perpendicular easy axis at low temperatures, even under compressive strain, provided that the hole concentration is sufficiently small. In such films, the easy axis assume a standard in-plane orientation when the temperature is increased towards the Curie temperature. The findings are shown to corroborate the predictions of the mean-field Zener model for strained (III,Mn)V ferromagnetic semiconductors.