PECULIARITIES OF ACOUSTIC INDUCED CHANGES OF ELECTROPHYSICAL CHARACTERISTICS IN GaN/Al0,2Ga0,8N/GaN/AlN HETEROSTRUCTURES

Optoelektronìka ta napìvprovìdnikova tehnìka ◽

10.15407/iopt.2020.55.109 ◽

2020 ◽

Vol 55 ◽

pp. 109-116

Author(s):

Ya.M. Olikh ◽

Keyword(s):

Charge Carrier ◽

Low Temperatures ◽

Point Of View ◽

Charge Carriers ◽

Forced Oscillations ◽

Physical Parameters ◽

Alternative Mechanism ◽

Linear Charge Density ◽

Time Dependencies ◽

Induced Changes

We studied temperature, amplitude and time dependencies of electrophysical parameters in GaN/Al0,2Ga0,8N/GaN/AlN structures when the ultrasound (US) was switched on/off (fUS = 9 MHz). We found out the charge carriers concentration n(Т) increases and the mobility μH(Т) decreases under the ultrasonic loading. With decreasing the temperature, the effect of acoustic induced changes increases. When US switches on/off, long-term (up to ~ 500 s) relaxation of the acoustic conductivity US(t) is observed, with increasing the amplitude of the ultrasound at low temperatures the acoustic conductivity increases exponentially, and at high ones decreases exponentially. It is established that the main mechanisms of charge carrier scattering at low temperatures (T ≤ 150 K) are ionized centers scattering and dislocations scattering; at high temperatures (T > 200 K) the charge carriers mobility is limited by polar optical phonons scattering. An acoustic deformation mechanism of charge carrier redistribution as a result of acoustic lattice deformation and corresponding additional structure piezopolarization is proposed. In our opinion, the determining factor that contributes to these effects, is the high density of boundary dislocations, as well as the change in the linear charge density on the dislocations in the process of their forced oscillations in the field of an external ultrasonic deformation. We have also considered an alternative mechanism that related with an acoustic induced (AI) transformation of metastable DX centers and can occur simultaneously. The mechanism of AI concentration n(Т) increase in this model is associated with a decrease of the barrier for an electron capture in DX0-state as a result of a periodic change distance between possible positions of the donor atoms (at the lattice node and non-central DX¯-state). To study the effect allows to obtain information both about such defect structure of the material and about the nature of changes in its macroscopic characteristics under the ultrasonic loading influence. From a practical point of view, it leads to the search for new opportunities for using of ultrasound for control the physical parameters kinetics of semiconductor structures.

Download Full-text

Interpretation of irradiation-induced changes in electron diffractograms and images of extinction contours at low temperatures

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100111458 ◽

1984 ◽

Vol 42 ◽

pp. 268-269

Author(s):

E. Knapek ◽

H. Formanek ◽

G. Lefranc ◽

I. Dietrich

Keyword(s):

Low Temperatures ◽

Carbon Films ◽

Organic Crystals ◽

Wide Spread ◽

Lens System ◽

Preparation Conditions ◽

Thin Carbon ◽

Electron Diffractograms ◽

Diffraction Patterns ◽

Induced Changes

A few years ago results on cryoprotection of L-valine were reported, where the values of the critical fluence De i.e, the electron exposure which decreases the intensity of the diffraction reflections by a factor e, amounted to the order of 2000 + 1000 e/nm2. In the meantime a discrepancy arose, since several groups published De values between 100 e/nm2 and 1200 e/nm2 /1 - 4/. This disagreement and particularly the wide spread of the results induced us to investigate more thoroughly the behaviour of organic crystals at very low temperatures during electron irradiation.For this purpose large L-valine crystals with homogenuous thickness were deposited on holey carbon films, thin carbon films or Au-coated holey carbon films. These specimens were cooled down to nearly liquid helium temperature in an electron microscope with a superconducting lens system and irradiated with 200 keU-electrons. The progress of radiation damage under different preparation conditions has been observed with series of electron diffraction patterns and direct images of extinction contours.

Download Full-text

Patient’s point of view on the use of telemedicine in multiple sclerosis: a web-based survey

Neurological Sciences ◽

10.1007/s10072-021-05398-6 ◽

2021 ◽

Author(s):

Doriana Landi ◽

Marta Ponzano ◽

Carolina Gabri Nicoletti ◽

Gaia Cola ◽

Gianluca Cecchi ◽

...

Keyword(s):

Multiple Sclerosis ◽

Point Of View ◽

Physical Parameters ◽

Chronic Medical Conditions ◽

Socioeconomic Conditions ◽

Web Based ◽

Healthcare Facilities ◽

Advantages And Disadvantages ◽

Disease Characteristics ◽

Degree Of Disability

AbstractRestrictions in the access to healthcare facilities during COVID-19 pandemic have raised the need for remote monitoring of chronic medical conditions, including multiple sclerosis (MS). In order to enable the continuity of care in these circumstances, many telemedicine applications are currently tested. While physicians’ preferences are commonly investigated, data regarding the patients’ point of view are still lacking. We built a 37 items web-based survey exploring patients’ propensity, awareness, and opinions on telemedicine with the aim to evaluate the sustainability of this approach in MS. Analysing 613 questionnaires out of 1093 that were sent to persons with MS followed at the Multiple Sclerosis Center of Tor Vergata University, Rome, we found that more than half of respondents (54%) were open to having a televisit. Propensity toward telemedicine significantly depended on having a higher income (p = 0.037), living farther from the center (p = 0.038), using computer and tablet (p = 0.010) and using the Internet for other remote activities (p < 0.001), conversely it was not influenced by any specific disease characteristics (i.e. degree of disability). The main advantages and disadvantages of televisit reported by participants were respectively saving time (70%) and impossibility to measure physical parameters (71%). Although the majority of respondents are in favour of televisit, so far this approach is restricted to those displaying better socioeconomic conditions and higher familiarity with technology. Implications of the study are that telemedicine platforms should be better tailored to patients’ demands in order to spread the use of telemedicine, to enhance usability and to increase patients’ adherence.

Download Full-text

Influence of Random Forest Hyperparameterization on Short-Term Runoff Forecasting in an Andean Mountain Catchment

Atmosphere ◽

10.3390/atmos12020238 ◽

2021 ◽

Vol 12 (2) ◽

pp. 238

Author(s):

Pablo Contreras ◽

Johanna Orellana-Alvear ◽

Paul Muñoz ◽

Jörg Bendix ◽

Rolando Célleri

Keyword(s):

Random Forest ◽

Model Performance ◽

Early Warning Systems ◽

Point Of View ◽

Lead Times ◽

Physical Parameters ◽

Runoff Forecasting ◽

Spatio Temporal ◽

Improved Model ◽

Search Approach

The Random Forest (RF) algorithm, a decision-tree-based technique, has become a promising approach for applications addressing runoff forecasting in remote areas. This machine learning approach can overcome the limitations of scarce spatio-temporal data and physical parameters needed for process-based hydrological models. However, the influence of RF hyperparameters is still uncertain and needs to be explored. Therefore, the aim of this study is to analyze the sensitivity of RF runoff forecasting models of varying lead time to the hyperparameters of the algorithm. For this, models were trained by using (a) default and (b) extensive hyperparameter combinations through a grid-search approach that allow reaching the optimal set. Model performances were assessed based on the R2, %Bias, and RMSE metrics. We found that: (i) The most influencing hyperparameter is the number of trees in the forest, however the combination of the depth of the tree and the number of features hyperparameters produced the highest variability-instability on the models. (ii) Hyperparameter optimization significantly improved model performance for higher lead times (12- and 24-h). For instance, the performance of the 12-h forecasting model under default RF hyperparameters improved to R2 = 0.41 after optimization (gain of 0.17). However, for short lead times (4-h) there was no significant model improvement (0.69 < R2 < 0.70). (iii) There is a range of values for each hyperparameter in which the performance of the model is not significantly affected but remains close to the optimal. Thus, a compromise between hyperparameter interactions (i.e., their values) can produce similar high model performances. Model improvements after optimization can be explained from a hydrological point of view, the generalization ability for lead times larger than the concentration time of the catchment tend to rely more on hyperparameterization than in what they can learn from the input data. This insight can help in the development of operational early warning systems.

Download Full-text

Magnetic Quantum Oscillations from Surface States of Bi Nanowires

MRS Proceedings ◽

10.1557/opl.2011.1051 ◽

2011 ◽

Vol 1350 ◽

Author(s):

L. A. Konopko ◽

T. E. Huber ◽

A. A. Nikolaeva

Keyword(s):

Single Crystal ◽

Low Temperatures ◽

Surface States ◽

Charge Carriers ◽

Quantum Oscillations ◽

Wide Range ◽

Spin Orbit Interactions ◽

Conducting Tube ◽

Magnetic Quantum Oscillations ◽

Strong Spin

ABSTRACTIn this work, we report the results of studies of the transverse magnetoresistance (MR) of single-crystal Bi nanowires with diameter d<80 nm. The single-crystal nanowire samples were prepared by the Taylor-Ulitovsky technique. Due to the semimetal-to-semiconductor transformation and high density of surface states with strong spin-orbit interactions, the charge carriers are confined to the conducting tube made of surface states. The non monotonic changes of transverse MR that are equidistant in a direct magnetic field were observed at low temperatures in a wide range of magnetic fields up to 14 T. The period of oscillations depends on the wire diameter d as for the case of longitudinal MR. An interpretation of transverse MR oscillations is presented.

Download Full-text

Optical Transitions in Chalcogenide Glasses From the Point of View of a Barrier-Cluster Model

Advanced Materials Research ◽

10.4028/www.scientific.net/amr.39-40.253 ◽

2008 ◽

Vol 39-40 ◽

pp. 253-256

Author(s):

Ivan Baník

Keyword(s):

Optical Absorption ◽

Low Temperatures ◽

Cluster Model ◽

Chalcogenide Glasses ◽

Temperature Shift ◽

Point Of View ◽

Optical Transitions ◽

Influence Of Temperature ◽

Exponential Tails ◽

Influence Of Temperature On

A barrier-cluster model of chalcogenide glasses is employed to analyze optical transitions near the absorption edge. The influence of temperature on the optical absorption is studied. The model is used to explain the temperature shift of exponential tails of the optical absorption and the temperature dependence of the optical forbidden-band width at low temperatures.

Download Full-text

Intention, Principle, Outputs and Aims of the Experimental Pavilion Research of Building Envelopes Including Windows for Wooden Buildings

Civil and Environmental Engineering ◽

10.1515/cee-2017-0005 ◽

2017 ◽

Vol 13 (1) ◽

pp. 42-51 ◽

Cited By ~ 3

Author(s):

Daniela Štaffenová ◽

Ján Rybárik ◽

Miroslav Jakubčík

Keyword(s):

Experimental Research ◽

Climatic Conditions ◽

Point Of View ◽

Physical Parameters ◽

Data Sets ◽

Data Set ◽

Building Envelopes ◽

Climate Conditions ◽

Time Period

AbstractThe aim of experimental research in the area of exterior walls and windows suitable for wooden buildings was to build special pavilion laboratories. These laboratories are ideally isolated from the surrounding environment, airtight and controlled by the constant internal climate. The principle of experimental research is measuring and recording of required physical parameters (e.g. temperature or relative humidity). This is done in layers of experimental fragment sections in the direction from exterior to interior, as well as in critical places by stable interior and real exterior climatic conditions. The outputs are evaluations of experimental structures behaviour during the specified time period, possibly during the whole year by stable interior and real exterior boundary conditions. The main aim of this experimental research is processing of long-term measurements of experimental structures and the subsequent analysis. The next part of the research consists of collecting measurements obtained with assistance of the experimental detached weather station, analysis, evaluation for later setting up of reference data set for the research locality, from the point of view of its comparison to the data sets from Slovak Hydrometeorological Institute (SHMU) and to localities with similar climate conditions. Later on, the data sets could lead to recommendations for design of wooden buildings.

Download Full-text

Multi-Criteria Evaluation of Coal Properties in Terms of Gasification

Archives of Mining Sciences ◽

10.2478/amsc-2014-0047 ◽

2014 ◽

Vol 59 (3) ◽

pp. 677-690 ◽

Cited By ~ 1

Author(s):

Jolanta Marciniak-Kowalska ◽

Tomasz Niedoba ◽

Agnieszka Surowiak ◽

Tadeusz Tumidajski

Keyword(s):

Particle Size ◽

Coal Gasification ◽

Three Dimensional ◽

Point Of View ◽

Gasification Process ◽

Multi Criteria Evaluation ◽

Depth Analysis ◽

Coal Properties ◽

Induced Changes ◽

Dimensional Surface

Abstract This paper presents a comparative analysis of two types of coal taken from the ZG Janina and ZG Wieczorek coalmines. The aim of this study has been to analyze the suitability of the coal in the context of the gasification process. The types of coal vary considerably in terms of their characteristics. Each of them was subjected to treatment in a ten-ringed annular jig. A particle size of 0-18 mm constituted the feed. The separated coal was divided into five layers, each of them containing material from two additional annular jigs. Analysis of their characteristics was carried out for each of the five layers and for both types of coal obtained, taking into account both their physicochemical properties as well as chemical ones. Each of these characteristics was then presented in three-dimensional surface diagrams, where the ordinate (or Y-axis) and abscissa (X-axis) was the particle size and height in which the material ended up in the jig (expressed as a percentage of the total height of the device). On the basis of observations, it was found that the types of coal have different potential for gasification, although both types are within the limits specified on the basis of previous studies. A correlation analysis between particle size and remaining characteristics of coal was carried out for each of the layers, allowing to determine which of the studied characteristics induced changes significant from the point of view of the coal gasification process. The entire research and observation was supported by conclusions and findings, which shall form the basis for further, in-depth analysis of coal.

Download Full-text

Phase Segregation via Etching-Induced Cation Migration in CoSx-ZnS Nanoarchitectures for Solar Hydrogen Evolution

Catalysis Science & Technology ◽

10.1039/d1cy02158c ◽

2022 ◽

Author(s):

Zirong Shen ◽

Junmin Huang ◽

Junying Chen ◽

Yingwei Li

Keyword(s):

Photocatalytic Activity ◽

Charge Carrier ◽

Hydrogen Evolution ◽

Carrier Mobility ◽

Phase Segregation ◽

Charge Carrier Mobility ◽

Charge Carriers ◽

Solar Hydrogen ◽

Cation Migration ◽

Low Charge

Low charge carrier mobility limits the development of highly efficient semiconductor-based photocatalysis. Heterointerface engineering is a promising approach to spatially separate the photoexcited charge carriers and thus enhance photocatalytic activity....

Download Full-text

Excitation of decay-less transverse oscillations of coronal loops by random motions

Astronomy and Astrophysics ◽

10.1051/0004-6361/201937187 ◽

2020 ◽

Vol 633 ◽

pp. L8 ◽

Cited By ~ 6

Author(s):

A. N. Afanasyev ◽

T. Van Doorsselaere ◽

V. M. Nakariakov

Keyword(s):

Broad Band ◽

Random Excitation ◽

Forced Oscillations ◽

Oscillatory Process ◽

Coronal Loops ◽

Alternative Mechanism ◽

Steady Flows ◽

Kink Waves ◽

Random Motions ◽

Transverse Oscillations

Context. The relatively large-amplitude decaying regime of transverse oscillations of coronal loops has been known for two decades and has been interpreted in terms of magnetohydrodynamic kink modes of cylindrical plasma waveguides. In this regime oscillations decay in several cycles. Recent observational analysis has revealed so-called decay-less, small-amplitude oscillations, in which a multi-harmonic structure has been detected. Several models have been proposed to explain these oscillations. In particular, decay-less oscillations have been described in terms of standing kink waves driven with continuous mono-periodic motions of loop footpoints, in terms of a simple oscillator model of forced oscillations due to harmonic external force, and as a self-oscillatory process due to the interaction of a loop with quasi-steady flows. However, an alternative mechanism is needed to explain the simultaneous excitation of several longitudinal harmonics of the oscillation. Aims. We study the mechanism of random excitation of decay-less transverse oscillations of coronal loops. Methods. With a spatially one-dimensional and time-dependent analytical model taking into account effects of the wave damping and kink speed variation along the loop, we considered transverse loop oscillations driven by random motions of footpoints. The footpoint motions were modelled by broad-band coloured noise. Results. We found the excitation of loop eigenmodes and analysed their frequency ratios as well as the spatial structure of the oscillations along the loop. The obtained results successfully reproduce the observed properties of decay-less oscillations. In particular, excitation of eigenmodes of a loop as a resonator can explain the observed quasi-monochromatic nature of decay-less oscillations and the generation of multiple harmonics detected recently. Conclusions. We propose a mechanism that can interpret decay-less transverse oscillations of coronal loops in terms of kink waves randomly driven at the loop footpoints.

Download Full-text