Global Analysis of the Flows of Fluids with Pressure-Dependent Viscosities

 To describe the flows of fluids over a wide range of pressures, it is necessary to take into account the fact that the viscosity of the fluid depends on the pressure. That the viscosity depends on the pressure has been verified by numerous careful experiments. While the existence of solutions local-in-time to the equations governing the flows of such fluids are available for small, special data and rather unrealistic dependence of the viscosity on the pressure, no global existence results are in place. Our interest here is to establish the existence of weak solutions for spatially periodic three-dimensional flows that are global in time, for a large class of physically meaningful viscosity-pressure relationships. (Accepted May 1, 2002) Published online November 15, 2002 Communicated by S. S. ANTMAN     

Edge misfit dislocations in the GeSi/Si(001) pair: Conditions and specific features of high-quantity generation

Dislocation structure of Ge_xSi_1?x films (x=0.4?0.8) grown by molecular-beam epitaxy on Si(001) substrates was studied by means of transmission electron microscopy. It was found that the density of edge MDs formed at the early stage of plastic strain relaxation in the films could exceed the density of 60° MDs. In our previous publications, a predominant mechanism underlying the early formation of edge misfit dislocations (MD) in Ge_xSi_1?x/Si films with x>0.4 was identified; this mechanism involves the following processes. A 60° glissile MD provokes nucleation of a complementary 60° MD gliding on a mirror-like tilted plane (111). A new edge MD forms as a result of interaction of the two complementary 60° MDs, and the length of the newly formed edge MD can then be increased following the motion of the “arms” of the complementary 60° MDs. Based on this scenario of the edge MD generation process, we have calculated the critical thickness of insertion of an edge MD into GeSi layers of different compositions using the force balance model. The obtained values were found to be more than twice lower than the similar values for 60° MDs. This result suggests that a promising strategy towards obtaining dislocation arrays dominated by 90° dislocations in MBE-grown Ge_xSi_1?x/Si films can be implemented through preliminary growth on the substrate of a thin, slightly relaxed buffer layer with 60° MDs present in this layer. The dislocated buffer layer, acting as a source of threading dislocations, promotes the strain relaxation in the main growing film through nucleation of edge MDs in the film/buffer interface. It was shown that in the presence of threading dislocations penetrating from the relaxed buffer into the film nucleation of edge MDs in the stressed film can be initiated even if the film thickness remains small in comparison with the critical thickness for insertion of 60° MDs. Examples of such unusual MD generation processes are found in the literature.     

Edge misfit dislocations in GexSi1 ? x/Si(001) (x ～ 1) heterostructures: role of buffer GeySi1 ? y (y < x) interlayer in their formation

The structure of dislocations in Ge _x Si_{1 − x} (x ∼ 0.4–0.8) films grown by molecular beam epitaxy on Si(001) substrates tilted by 6° toward the nearest (111) plane has been studied. The epitaxy of GeSi films on substrates deviating from the exact (001) orientation has allowed us to establish the main mechanism of formation of edge misfit dislocations (MDs), which most effectively (for heterostructures of the given composition) relieve stresses caused by the mismatch between lattice parameters of the film and substrate. Despite the edge MDs being defined as immobile (sessile) dislocations, their formation proceeds according to the gliding mechanism proposed by Kvam et al. [J. Mater. Res. 5, 1900 (1990)]. A comparative estimation of the propagation velocities of the primary and induced 60° dislocations, as well as the resulting 90° MDs, has been performed. It has been established that the condition providing for the most effective edge MD formation by the induced nucleation mechanism is the appearance of 60° MDs in a stressed film immediately after it reached a critical thickness. A source of these dislocations can be provided by a preliminarily grown buffer GeSi layer that occurs in a metastable state at the initial stage of plastic relaxation.     

Structural state of Ge/Si heterosystems with (001), (111), and (7 7 10) interfaces

It is shown for (111) and (001) interfaces that at an identical degree of strain relaxation in semi-conductor epitaxial films, the ratio of distances D between neighboring dislocations is D ₍₁₁₁₎/D ₍₀₀₁₎ = 1.5. This allows us to establish that dislocation interface (7 7 10) contains partial 90° Shockley dislocations lying in three directions of 〈110〉.     

Photoluminescence kinetics of wurtzite GaN quantum dots in an AlN matrix

The photoluminescence (PL) of GaN quantum dots in an AlN matrix is studied. It is found that the maximum of the PL line does not shift when the laser excitation power varies. The transient PL spectra indicate that the quenching kinetics is nonexponential and the quenching law depends on the spectral range. The experimental data are explained in the framework of a model taking into account the strong built-in electric field in wurtzite structures and the transport of charge carriers between them.     

Strained germanium films in Ge/InGaAs/GaAs heterostructures: Formation of edge misfit dislocations at the Ge/InGaAs interface

Heterostructures of the “strained Ge film/artificial InGaAs layer/GaAs substrate” type have been grown by molecular beam epitaxy. A specific feature of these structures is that the plastically relaxed (buffer) InGaAs layer has the density of threading dislocations on a level of 10⁵–10⁶ cm⁻². These dislocations penetrate into the strained Ge layer to become sources of both 60° and 90° (edge) misfit dislocations (MDs). Using the transmission electron microscopy, both MD types have been found at the Ge/InGaAs interface. It has been shown that the presence of threading dislocations inherited from the buffer layer in a tensile-strained Ge film favors the formation of edge dislocations at the Ge/InGaAs interface even in the case of small elastic deformations in the strained film. Possible mechanisms of the formation of edge MDs have been considered, including (i) accidental collision of complementary parallel 60° MDs propagating in the mirror-tilted {111} planes, (ii) induced nucleation of a second 60° MD and its interaction with the primary 60° MD, and (iii) interaction of two complementary MDs after a cross-slip of one of them. Calculations have demonstrated that a critical layer thickness (h _c ) for the appearance of edge MDs is considerably smaller than h _c for 60° MDs.     

Specific features of formation and propagation of 60° and 90° misfit dislocations in GexSi1−x/Si films with x>0.4

The dislocation structure at the initial stage of relaxation of Ge_xSi_1−x films (x∼0.4–0.8) grown on Si (0 0 1) substrates tilted at 6° to the nearest (1 1 1) plane is studied. The use of Si substrates tilted away from the exact (0 0 1) orientation for epitaxial growth of Ge_xSi_1−x films (x≥0.4) allowed finding the basic mechanism of formation of edge dislocations that eliminate the mismatch stresses. Though the edge dislocations are defined as sessile dislocations, they are formed in accordance with the slipping mechanism proposed previously by Kvam et al. (1990). It is highly probable that a 60° misfit dislocation (MD) propagating by the slipping mechanism provokes the nucleation of a complementary 60° MD slipping in a mirror-like tilted plane (1 1 1). The reaction between these dislocations leads to the formation of an edge MD that ensures more effective reconciliation of the discrepancy. Comparative estimation of the slip velocities of the primary and induced 60° MDs and also of the resultant 90° MD is fulfilled. The slip velocity of the induced 60° MD is appreciably greater than the velocity of the primary 60° MD. Therefore, the induced MD “catches up” with the second front of the primary MD, thus forming a 90° MD propagating to both sides due to slipping of the 60° MDs forming it. The propagation velocity of the 90° MD is also greater than the slip velocity of a single 60° MD. For these reasons, 90° MDs under certain conditions that favor their formation and propagation can become the main defects responsible for plastic relaxation of GeSi films close to Ge in terms of their composition.     

Guided modes of integrated optical guides. A mathematical study

A waveguide in integrated optics is defined by its refractiveindex. The guide is assumed to be invariant in the propagationdirection while in the transverse direction it is supposed tobe a compact perturbation of an unbounded stratified medium.We are interested in the modes guided by this device, whichare waves with a transverse energy confined in a neighbourhoodof the perturbation. Our goal is to analyse the existence of such guided modes. Underthe assumptions of weak guidance the problem reduces to a two-dimensionaleigenvalue problem for a scalar field. The associated operatoris unbounded, selfadjoint, and bounded from below. Its spectrumconsists of the discrete spectrum corresponding to the guidedmodes and of the essential spectrum corresponding to the radiationmodes. We present existence results of guided modes and an asymptoticstudy at high frequencies, which shows that contrarily to thecase of optical fibers, the number of guided modes can remainbounded. The major tools are the min-max principle and comparisonof results between different eigenvalue problems. The originalityof the present study lies in the stratified character of theunbounded reference medium.     

Interaction of the mu-opioid receptor with GPR177 (Wntless) inhibits Wnt secretion: potential implications for opioid dependence

Background  

Opioid agonist drugs produce analgesia. However, long-term exposure to opioid agonists may lead to opioid dependence. The analgesic and addictive properties of opioid agonist drugs are mediated primarily via the mu-opioid receptor (MOR). Opioid agonists appear to alter neuronal morphology in key brain regions implicated in the development of opioid dependence. However, the precise role of the MOR in the development of these neuronal alterations remains elusive. We hypothesize that identifying and characterizing novel MOR interacting proteins (MORIPs) may help to elucidate the underlying mechanisms involved in the development of opioid dependence.