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We investigate the influence of noise upon the dynamics of the current density distribution in a model of a semiconductor nanostructure, namely, a double barrier resonant tunneling diode. We fix the parameters of the device below the Hopf bifurcation, where the only stable state of the system is a spatially inhomogeneous "filamentary" steady state. We show that the addition of weak Gaussian white noise to the system gives rise to spatially inhomogeneous oscillations that can be quite coherent. As the noise intensity grows, the oscillations tend to become more and more spatially homogeneous, while simultaneously the temporal correlation of the oscillations decreases. Thus, while on one hand noise destroys temporal coherence, on the other hand it enhances the spatial coherence of the current density pattern.

Phase transitions in the behavior of neuron populations depending on electromagnetic input as in repetitive transcranial magnetic stimulation ("rTMS") are calculated, based on mesoscopic evolution equations, which are generated from stochastic single neuron models capturing a variety of inputs. Wherever synchronization of firing in neuron populations is measurable, the above model-based quantitative predictions of effects of rTMS on brain activity can, for the first time, be tested experimentally.

Time-delayed feedback is exploited for controlling noise-induced motion in coherence resonance oscillators. Namely, under the proper choice of time delay, one can either increase or decrease the regularity of motion. It is shown that in an excitable system, delayed feedback can stabilize the frequency of oscillations against variation of noise strength. Also, for fixed noise intensity, the phenomenon of entrainment of the basic oscillation period by the delayed feedback occurs. This allows one to steer the time scales of noise-induced motion by changing the time delay., (Copyright 2004 The American Physical Society)

We present a scheme to stabilize high-frequency domain oscillations in semiconductor superlattices by a time-delayed feedback loop. Applying concepts from chaos control theory we propose to control the spatiotemporal dynamics of fronts of accumulation and depletion layers which are generated at the emitter and may collide and annihilate during their transit, and thereby suppress chaos. The proposed method only requires the feedback of internal global electrical variables, viz., current and voltage, which makes the practical implementation very easy.

We present a general method for studying front propagation in nonlinear systems with a global constraint in the language of hybrid tank models. The method is illustrated in the case of semiconductor superlattices, where the dynamics of the electron accumulation and depletion fronts shows complex spatiotemporal patterns, including chaos. We show that this behavior may be elegantly explained by a tank model, for which analytical results on the emergence of chaos are available. In particular, for the case of three tanks the bifurcation scenario is characterized by a modified version of the one-dimensional iterated tent map.

The double barrier resonant tunneling diode exhibits complex spatiotemporal patterns including low-dimensional chaos when operated in an active external circuit. We demonstrate how autosynchronization by time-delayed feedback control can be used to select and stabilize specific current density patterns in a noninvasive way. We compare the efficiency of different control schemes involving feedback in either local spatial or global degrees of freedom. The numerically obtained Floquet exponents are explained by analytical results from linear stability analysis.

We investigate time-delayed feedback control schemes which are based on the unstable modes of the target state, to stabilize unstable periodic orbits. The periodic time dependence of these modes introduces an external time scale in the control process. Phase shifts that develop between these modes and the controlled periodic orbit may lead to a huge increase of the control performance. We illustrate such a feature on a nonlinear reaction diffusion system with global coupling and give a detailed investigation for the Rössler model. In addition we provide the analytical explanation for the observed control features.

Control of spatio-temporal chaos by the time-delay autosynchronization method is improved by several orders of magnitude. Unstable time periodic patterns are efficiently stabilized if one employs filters and couplings which originate from the Floquet eigenvalue problem of the unstable orbit. We illustrate our scheme by an application to a globally coupled reaction-diffusion model which describes charge transport in semiconductor devices.

Time-delayed feedback control for stabilizing time periodic spatial patterns is investigated in a generic reaction-diffusion system with global coupling. We focus on the case of low-dimensional chaos where unstable patterns admit only a single unstable mode. Spatial degrees of freedom are taken into account to define different control schemes. The efficiency of these schemes is discussed, where control forces are motivated by physical requirements as well as by the possibility of obtaining analytically exact results. We find that control schemes that contain the full feedback of the inhibitor variable may finally destroy the control performance. Thus schemes that omit the inhibitor might be more efficient. Our numerical findings are explained in terms of Floquet spectra and compared with analytical solutions of particular coupling schemes.

By means of kinetic Monte Carlo simulations of the self-organized growth of quantum dots in strained semiconductor systems we resolve the seemingly contradictory features of kinetic versus thermodynamic behavior, e.g., with respect to the temperature dependence of the average dot size and their dispersion. We show that the size distribution immediately after deposition is kinetically controlled, with smaller islands for lower temperatures and larger islands for higher temperatures. For longer simulation times the kinetics leads to equilibration, and a crossover effect between the size distributions occurs, which is in good agreement with the predictions of thermodynamics.

Spatio-temporal bifurcations and complex dynamics in globally coupled intrinsically bistable electrochemical systems with an S-shaped current-voltage characteristic under galvanostatic control are studied theoretically on a one-dimensional domain. The results are compared with the dynamics and the bifurcation scenarios occurring in a closely related model which describes pattern formation in semiconductors. Under galvanostatic control both systems are unstable with respect to the formation of stationary large amplitude current domains. The current domains as well as the homogeneous steady state exhibit oscillatory instabilities for slow dynamics of the potential drop across the double layer, or across the semiconductor device, respectively. The interplay of the different instabilities leads to complex spatio-temporal behavior. We find breathing current domains and chaotic spatio-temporal dynamics in the electrochemical system. Comparing these findings with the results obtained earlier for the semiconductor system, we outline bifurcation scenarios leading to complex dynamics in globally coupled bistable systems with subcritical spatial bifurcations.

Pattern formation in semiconductor heterostructures is studied on the basis of a spatially two-dimensional model of reaction-diffusion type. In particular, we investigate the neighborhood of a codimension-two Turing-Hopf instability by analytical methods. Amplitude equations are derived which predict the absence of mixed modes but extended ranges of bistability between homogeneous oscillatory states and hexagonal Turing patterns. Our results are confirmed by numerical simulations. The features are not confined to a neighborhood of the bifurcation point so that the conclusions of the weakly nonlinear analysis explain the observations in large portions of the parameter space at least qualitatively

We analyze the dynamics of charge distributions in weakly coupled, doped, dc voltage biased semiconductor superlattices subject to voltage steps of different sizes. Qualitatively different current responses to voltage switching processes have been observed experimentally. We explain them by invoking distinct scenarios for electric-field domain formation, validated by numerical simulations. Furthermore, we investigate the transient from an unstable to a stable point in the current-voltage characteristics after a steplike or ramplike increase of the external voltage.

Image processing and archiving are of increasing importance in the practice of modern medicine. Particularly due to the introduction of computer-based investigation methods, physicians are dealing with a wide variety of analogue and digital picture archives. On the other hand, clinical information is stored in various text-based information systems without integration of image components. The link between such traditional medical databases and picture archives is a prerequisite for efficient data management as well as for continuous quality control and medical education. At the Department of Orthopedic Surgery, University of Berne, a software program was developed to create a complete multimedia electronic patient record. The client-server system contains all patients' data, questionnaire-based quality control, and a digital picture archive. Different interfaces guarantee the integration into the hospital's data network. This article describes our experiences in the development and introduction of a comprehensive image archiving system at a large orthopedic center.

We evaluated 132 consecutively implanted cemented titanium alloy stems (Ti6-Al7-Nb, SLS-88) after a mean follow-up of 6.6 years (range 5-7 years). Almost 30% of the stems showed significant osteolysis, mainly in the proximal stem area. The amount of osteolysis correlated significantly to a typical pain pattern: dull, circular, and occurring at rest. Six stems had already been revised, and another 7 stems were loose clinically and radiologically, which resulted in a Kaplan-Meier survivorship of 95% for stem revisions and 88% for stem failures (revisions plus loose stems). We found that smaller stem sizes tend to increase the development of osteolysis and loosening. Two factors seem to play a role in early loosening of cemented titanium alloy stems: i) increased elasticity of titanium, which leads in small stem sizes to micromotion between the stem and cement and to cement breakage, and ii) corrosion of the cemented titanium alloy stem and subsequent osteolysis in the adjacent bone. The combination of both factors (high elasticity plus corrosion) accelerates the degradation of the bone anchorage of the stem and initiates early loosening. We advise against the further use of cemented SLS-88 titanium alloy stems.

A global time-delayed feedback control is applied to a globally coupled reaction-diffusion system describing charge transport in a bistable semiconductor. We demonstrate that a variety of spatiotemporal unstable periodic orbits (UPOs) embedded in a chaotic attractor of the spatially extended system can be stabilized using an extended time-delay autosynchronization algorithm. These UPOs correspond to spiking current filaments. We critically evaluate analytical approximations for the limits of control originally developed for low-dimensional temporal chaos and show that the delay time can be extrapolated with high accuracy, while the theoretical limit for the control of UPOs in terms of the product of the period and the largest Lyapunov exponent is not reached. If the global feedback is modified by a spatial filter, we achieve stabilization of different spatial patterns.

Seventy-five symptomatic dysplastic hip joints (63 patients) were treated with the Bernese periacetabular osteotomy during a period of 44 months. The mean patients' age was 29 years (range, 13-56 years) and the female:male ratio was 3.4:1. Group III dysplasia according to Severin was seen in 50% and Group IV dysplasia was seen in 44% of the patients. Osteoarthritis was present in 58% of the patients. Followup was obtained at a mean of 11.3 years (range, 10-13.8 years) in 71 hip joints (95%). Radiographic measurements of the lateral center edge angle, anterior center edge angle, acetabular index, lateralization of the femoral head, and intactness of Shenton's line showed a high correction potential of this type of osteotomy. In 58 patients (82%) the hip joint was preserved at last followup with a good to excellent result in 73%. Unfavorable outcome was significantly associated with higher age of the patient, moderate to severe osteoarthritis at surgery, a labral lesion, less anterior coverage correction, and a suboptimal acetabular index. Major complications were encountered in the first 18 patients including an intraarticular cut in two, excessive lateralization in one, secondary loss of correction in two and femoral head subluxation in three patients.

From 1986 to 1994 18 patients with 19 IIIB open tibial fractures were treated following similar therapeutic management. This included early and radical primary debridement, early and wherever possible immediate internal fixation of the bone and coverage with a local muscle flap (hemisoleus or gastrocnemius). At the time of follow-up all fractures had consolidated clinically as well as radiographically. The mean time to bony union was 7 months (3-20). Twelve patients did not have any severe pain and were able to walk for more than 4 km. The time to return to work averaged 8 months. In 2 cases a permanent disability of 50% was established. Fourteen of 18 patients were satisfied with the function of the leg. On the other hand, 9 didn't like the aesthetic appearance and were inhibited because of it. The following complications occurred within a period of 2 years: non-union (3), flap necrosis (2), osteomyelitis (2), delayed union (4). The mean number of reoperations was 3 (0-8). In no case did an amputation have to be performed.