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Band bending at semiconductor surfaces and interfaces is the key to applications ranging from classical transistors to topological quantum computing. A semiconductor particularly important for optical as well as microwave devices is GaN. What makes the material useful is not only its large bandgap but also that it can be heavily doped to become metallic. Here, we apply soft-x-ray angle-resolved photoelectron spectroscopy (ARPES) to metallic Si-doped GaN to explore the electron density and momentum-resolved band dispersions of the valence and conduction electrons varying through the surface band-bending region. We find an upward band bending, where the measured band occupation reduces toward the surface, as probed with low photon energies 1.4keV, where the photoelectron mean free path exceeds the spatial extent of the band-bending region. Our quantitative analysis of the experimental data describes the potential variation in the band-bending region via self-consistent Poisson-Schrödinger equations. We put forward an insightful model to simulate the ARPES spectra from this region through summing up the contribution from all atomic layers, weighted by the photoelectron mean free path, under in-phase conditions achieved at particular values of the photoelectron out-of-plane momentum. The model adequately describes the peculiarities of the ARPES spectra caused by the surface band bending, including the photon-energy dependence of the apparent band occupation and Fermi-surface area, and allows accurate determination of the band-bending profile and values of the photoelectron mean free path. Finally, comparison of our data with supercell density functional theory calculations reveals the preferential location of Si atoms as substitutional for Ga, with the doped electrons entering the GaN conduction bands without formation of separate impurity states as would occur for Si interstitials. Our theoretical and experimental results resolve fundamental questions underpinning device performance of the GaN-based and other semiconductor materials in general and demonstrate a general methodology for quantitative studies of electron states in the band-bending region.

Semiconductor heterostructures hosting two-dimensional electron gases are widely used today in high-electron-mobility transistors. Here, the authors probe the electronic structure in GaN/AlGaN, heterostructures, discovering planar anisotropy of the electron Fermi surface, offering new insights into transport properties.

Most in-hospital strokes occur after cardiac or vascular surgeries, during invasive therapeutic and diagnostic procedures on the heart and blood vessels. The incidence of stroke after coronary artery bypass grafting is about 2%; that after valve surgery and percutaneous cardiac intervention is 1 to 10% and 0.03 to 0.4%, respectively.Objective: to analyze the risk factors, pathogenetic, clinical, and laboratory characteristics of stroke after cardiac surgery and invasive interventions.Patients and methods. The investigation enrolled in-hospital stroke patients who had undergone invasive cardiac or open-heart surgery in Sverdlovsk Regional Hospital One in 2011 to 2015. A comparison group consisted of 196 patients with community-acquired stroke.Results. Stroke after cardiac surgical or endovascular interventions developed in 88 patients (44.9% of all the in-hospital stroke cases registered in the above period). The incidence of stroke after open-heart surgery was 0.97% (63 cases), depending on the type of surgery. Stroke following open-heart surgery was recorded after combined correction (1.4%), two-valve replacement surgery (1.98%), aortic and mammary coronary artery bypass grafting under extracorporeal circulation (1%), and beating heart bypass surgery (0.25%). The severity of stroke and the degree of disability did not differ in patients after invasive cardiac and open-heart surgeries.Conclusion. Patients who have undergone cardiac surgery and are at high risk for in-hospital stroke should be followed up and should receive preventive treatment for stroke in the early postoperative period.

Introduction. Strokes in patients managed at the department of cardiovascular surgery hold a leading position among all in-hospital strokes in multispeciality hospitals. According to the literature, the prevalence of stroke ranges from 0.20.4% for percutaneous cardiac interventions to 16% after heart valve surgeries. Objective. To reveal the stroke risk factors in patients managed at the department of cardiovascular surgery, including depending on the type of surgical intervention. Materials and methods. The study group included 58 cases of acute cerebrovascular accidents (ACVA) in patients at the department of cardiovascular surgery, amounting to 30.5% of the total amount of in-hospital strokes recorded over 5 years (20112016). Results. Ischemic stroke was prevalent in the study group (54 patients; 93.1%, р0.001); four (6.9%) patients had transient ischemic attacks. The largest number of strokes occurred in patients who had undergone heart bypass (23 patients, 41.1%) and heart valve replacement surgeries (25 patients, 44.6%); in 12 (21.4%) patients, ACVA occurred after mitral valve replacement combined with tricuspid annuloplasty. In most cases, stroke developed within the first three days after surgical intervention (36 patients, 64.3%, р0.05). Conclusions. Patients who had undergone cardiovascular interventions, especially after heart bypass and heart valve replacement surgeries, require hemodynamic monitoring and thromboelastography during the first 3 days to prevent a stroke. Despite the early detection of in-hospital strokes, systemic thrombolytic therapy was contraindicated for all patients. Mechanical thrombus extraction should be treatment of choice for this category of patients.