Variable range cotunneling and conductivity of a granular metal

The Efros-Shklovskii law for the conductivity of granular metals is interpreted as a result of a variable range cotunneling process. The cotunneling between distant resonant grains is predominantly elastic at low T << T_c, while it is inelastic (i.e., accompanied by creation of electron-hole pairs on a string of intermediate non-resonant grains) at T > T_c. The corresponding E-S temperature T_ES in the latter case is slightly (logarithmically) T-dependent. The magnetoresistance in the two cases is different: it may be relatively strong and negative at T much below T_c, while at T>T_c it is suppressed due to inelastic processes which destroy the interference.Comment: Submitted to JETP Letter

Digitalization of the Educational Environment: Risk Assessment of Distance Education by Russian and Vietnamese Students

The digitalization of higher education is a long-term trend that gained a new impetus for further development because of the forced transition to distance learning during the COVID-19 pandemic. The aim of the article is to analyze the impact of digital transformation on the educational process in universities and to describe the risks through the students’ eyes. The analysis is based on the results of a survey conducted by the authors in 2020 among students of two universities - Moscow State Institute of International Relations (MGIMO) and the Institute of the Trade Union Movement of the General Confederation of Labor of Vietnam (IPLV).The article describes the methodology of the online survey. The attitude towards distance learning is interpreted as a three-level attitude with emotional, cognitive and behavioral aspects. It is suggested that students’ readiness to switch to distance learning is determined by a set of macro and micro factors. Among the macro factors are the national specifics of the educational system, traditions in the field of higher education, infrastructure, the national educational policy, and the mobilization potential of the population. Among the micro factors are the cognitive and other psychological characteristics of the students, the socio-psychological openness to innovation.According to the survey results, there is certain similarity in the way Russian and Vietnamese students assess their experience of distance learning. At the same time, significant differences in the perception of the outcome of the digital transformation of education have been revealed. For MGIMO students, major risks are associated with dehumanization, the severing of social ties, and the possible loss of student status. For Vietnamese students the most significant risks are mainly associated with the fears of the decreasing quality of education. It is therefore concluded that distance learning is both the field of opportunities and possible source of individual and institutional risks

Negative hopping magnetoresistance of two-dimensional electron gas in a smooth random potential

We show that the tunnel coupling between semiclassical states localized in different minima of a smooth random potential increases when magnetic field is applied. This increase originates from the difference in gauge factors which electron wave functions belonging to different electron ``lakes'' acquire in the presence of the field. We illustrate the increase of coupling by a model calculation of tunneling through a saddle point separating two adjacent lakes. In the common case, when the barrier between two lakes is much narrower than their size, the characteristic magnetic field is determined by the area of the lakes, and thus may be quite small. The effect of the field on coupling constants leads to a negative magnetoresistance in low-temperature conduction.Comment: 9 pages RevTe

Электрофизические свойства пленок оксида ванадия, нанесенных методом реактивного магнетронного распыления

The aim of this work was to study the effect of the gas composition during sputtering on the electrophysical properties of vanadium oxide films deposited by pulsed reactive magnetron sputtering of a vanadium target in an Ar/O2 medium of working gases.The dependences of the magnetron discharge voltage, deposition rate, resistivity, temperature coefficient of resistance (TCR), and the band gap of vanadium oxide films on the oxygen concentration in the gas mixture are obtained. It was found that amorphous films of vanadium oxide are formed during reactive magnetron sputtering. It is shown that the properties of the deposited vanadium oxide films have a strong dependence on the oxygen concentration in the Ar/O2 gas mixture, which is associated with the formation of a mixture of various intermediate vanadium oxides in the film. It was found that from the point of view of using vanadium oxide films as thermosensitive layers of microbolometers, the films must be deposited at oxygen concentrations in the gas mixture of 17 to 25 %. At the given oxygen concentrations without heating the substrates, vanadium oxide films with a resistivity (0.6–4.0)·10-2 Ohm·m, TCR 2.2–2.3%/°C and a band gap for direct transitions of 3.7–3.78 eV. The obtained characteristics make it possible to use these films as thermosensitive layers of microbolometers.Целью работы являлось исследование влияния состава газовой среды в процессе распыления на электрофизические характеристики пленок оксида ванадия, нанесенных методом импульсного реактивного магнетронного распыления ванадиевоaй мишени в среде Ar/O2 рабочих газов.Получены зависимости напряжения разряда магнетрона, скорости нанесения, удельного сопротивления, температурного коэффициента сопротивления (ТКС), ширины оптической запрещенной зоны пленок оксида ванадия от концентрации кислорода в смеси газов. Установлено, что при реактивном магнетроном распылении формируются аморфные пленки оксида ванадия. Показано, что электрофизические свойства нанесенных пленок оксида ванадия имеют сильную зависимость от концентрации кислорода в Ar/O2 смеси газов, что связано с образованием в пленке смеси различных промежуточных оксидов ванадия. Установлено, что с точки зрения использования пленок оксида ванадия в качестве термочувствительных слоев пленки необходимо наносить при концентрациях кислорода в смеси газов от 17 до 25 %. При данных концентрациях кислорода без нагрева подложек получены пленки оксида ванадия с удельным сопротивлением (0,6–4,0)·10-2 Ом·м, ТКС 2,2–2,3 %/°C и шириной запрещенной зоны для прямых переходов 3,7–3,78 эВ. Полученные характеристики позволяют использовать данные пленки в качестве термочувствительных слоев микроболометров

Structural and phase characteristics of vanadium oxide films

Проведены исследования влияния параметров процесса нанесения и последующего отжига на структуру и фазовый состав пленок оксида ванадия (VOx ). Пленки VOx наносились методом импульсного реактивного магнетронного распыления V мишени в Ar/O2 смеси газов и подвергались отжигу в атмосфере O2 при давлении 10 5 Па. Температура отжига изменялась от 100 до 450 °С. Время отжига варьировалось от 10 до 120 мин. Установлено, что при отжиге начальные процессы кристаллизации наблюдаются при температурах 250–275 °С. При этом, в зависимости от концентрации кислорода в процессе распыления, формируются поликристаллические пленки кубической или смешанной моноклинной/кубической кристаллической решеткой. При увеличении температуры отжига происходит переход от промежуточного оксида V 4 O 9 к смешанной фазе VO 2 /VO x /V 2 O 5 и далее к высшему оксиду V 2 O 5 . При увеличении времени отжига формирование высшего оксида V 2 O 5 наблюдается при более низких температурах и его концентрация увеличивается. The article The influence of the parameters of the deposition and subsequent annealing on the structure and phase composition of vanadium oxide (VO x ) films has been studied. VO x films were deposited by pulsed reactive magnetron sputtering of a V target in an Ar/O 2 gas mixture and annealed in an O 2 atmosphere at a pressure of 105 Pa. The temperature of the annealing varied from 100 to 450 °C. The time of the annealing varied from 10 to 120 min. It was found that during annealing, the initial crystallization processes are observed at temperatures of 250–275 °C. In this case, depending on the oxygen concentration during sputtering, polycrystalline films of a cubic or mixed monoclinic (cubic) crystal lattice are formed. As the temperature of the annealing rises, a transition occurs from the intermediate oxide V 4 O 9 to the mixed phase VO 2 /VO x /V 2 O 5 and then to the higher oxide V 2 O 5. With an increase in the annealing time, the formation of the higher oxide V 2 O 5 is observed at lower temperatures and its concentration increases

Why do adult women in Vietnam take iron tablets?

BACKGROUND: Conducting iron supplementation programs has been a major strategy to reduce iron deficiency anemia in pregnancy. However, only a few countries have reported improvements in the anemia rate at a national level. The strategies used for control of nutrition problems need regular review to maintain and improve their effectiveness. The objective of this study was to analyze the factors in compliance with taking iron tablets, where daily doses of iron (60 mg) and folic acid (400 μg) were distributed in rural Vietnamese communes. METHODS: A cross sectional survey was conducted in Nghe An province, Vietnam in January, 2003. The study population was adult women aged less than 35 years who delivered babies between August 1(st )2001 and December 1(st )2002 (n = 205), of which 159 took part in the study. Data for the study were collected from a series of workshops with community leaders, focus group discussions with community members and a questionnaire survey. RESULTS: Improvements in the rate of anemia was not given a high priority as one of the commune's needs, but the participants still made efforts to continue taking iron tablets. Two major factors motivated the participants to continue taking iron tablets; their experience of fewer spells of dizziness (50%), and their concern for the health of their newborn baby (54%). When examining the reasons for taking iron tablets for at least 5–9 months, the most important factor was identified as 'a frequent supply of iron tablets' (OR = 11.93, 95% CI: 4.33–32.85). CONCLUSION: The study found that multiple poor environmental risk factors discouraged women from taking iron tablets continuously. The availability (frequent supply) of iron tablets was the most effective way to help adult women to continue taking iron tablets

Влияние отжига на структурно-фазовые и электрофизические свойства пленок оксида ванадия

The aim of this work was to study the effect of the parameters of deposition process and subsequent annealing on the properties of vanadium oxide VOx films deposited by the pulsed reactive magnetron sputtering of a V target in an Ar/O2 gas  mixture.  The  dependences  of  the  structure,  phase,  temperature  coefficient of resistance (TCR), resistivity p, band gap Egof the films on the oxygen concentration in Ar/O2 gas mixture during the deposition ГO2, and the temperature of annealing in an O2 atmosphere were obtained. The films were found to have an amorphous structure after deposition. Crystallization processes are observed at temperatures above  275 °C.  In  this  case,  depending  on  the  temperature,  polycrystalline  films  with  a  monoclinic,  cubic or mixed crystal lattice are formed and a transition occurs from the intermediate oxide V4O9 to the mixed phase VO2/VOx/V2O5 and then to the higher oxide V2O5. The character of changes in p, TCR and Egof films coming from the change in the annealing temperature is complex and largely determined by ГO2. It was established that with the view of using VOx films as thermosensitive layers, the following conditions of deposition and annealing would be preferable: films deposited at the oxygen concentration 25 % in Ar/O2 gas mixture and annealed at a  temperature  of  250–275 °C  in  an  O2 atmosphere  for  10  min.  Under  these  conditions  VOx films  with  the following properties were obtained: p= (1.0 – 3.0).10-2 Ohm.m, TCR = 2.05 %/°C, and Eg= 3.76–3.78 eV.Целью  работы  являлось  исследование  влияния  параметров  процесса нанесения  и последующего  отжига  на  свойства  пленок  оксида  ванадия  VOx,  осажденных  методом  реактивного магнетронного распыления V мишени в Ar/O2 смеси газов. Получены зависимости структуры, фазового состава,  температурного  коэффициента  сопротивления  (ТКС),  удельного  сопротивления p,  ширины запрещенной зоны Eg пленок от концентрации кислорода в Ar/O2 смеси газов в процессе нанесения ГO2и температуры  отжига  пленок  в  атмосфере  O2.  Установлено,  что  после  нанесения  пленки  имеют аморфную структуру. Процессы кристаллизации наблюдаются при температурах более 275 °С. При этом формируются  поликристаллические  пленки  с  моноклинной,  кубической  или  смешанной кристаллической  решеткой  и  происходит  переход  от  промежуточногооксида V4O9 к  смешанной  фазе VO2/VOx/V2O5 и далее к высшему оксиду V2O5. Характер изменения , ТКС и Eg пленок при изменении температуры отжига имеют сложный характер и во многом определяется ГO2. Установлено, что с точки зрения использования VOxпленок в качестве термочувствительных слоев предпочтительными являются следующие условия нанесения и отжига: пленки наносятся при концентрации кислорода 25 % в Ar/O2 смеси  газов  и  отжигаются  при  температуре  250–275 °С  в  атмосфере кислорода  10  мин.  При  данных условиях получены пленки VOx с p= (1,0 – 3,0).10–2 Ом.м, ТКС = 2,05 %/°С и Eg= 3,76–3,78 эВ

Luminescence spectra and kinetics of disordered solid solutions

We have studied both theoretically and experimentally the luminescence spectra and kinetics of crystalline, disordered solid solutions after pulsed excitation. First, we present the model calculations of the steady-state luminescence band shape caused by recombination of excitons localized in the wells of random potential induced by disorder. Classification of optically active tail states of the main exciton band into two groups is proposed. The majority of the states responsible for the optical absorption corresponds to the group of extended states belonging to the percolation cluster, whereas only a relatively small group of “radiative” states forms the steady-state luminescence band. The continuum percolation theory is applied to distinguish the “radiative” localized states, which are isolated in space and have no ways for nonradiative transitions along the tail states. It is found that the analysis of the exciton-phonon interaction gives the information about the character of the localization of excitons. We have shown that the model used describes quite well the experimental cw spectra of CdS(1−c)Sec and ZnSe(1−c)Tec solid solutions. Further, the experimental results are presented for the temporal evolution of the luminescence band. It is shown that the changes of band shape with time come from the interplay of population dynamics of extended states and spatially isolated “radiative” states. Finally, the measurements of the decay of the spectrally integrated luminescence intensity at long delay times are presented. It is shown that the observed temporal behavior can be described in terms of relaxation of separated pairs followed by subsequent exciton formation and radiative recombination. Electron tunneling processes are supposed to be responsible for the luminescence in the long-time limit at excitation below the exciton mobility edge. At excitation by photons with higher energies the diffusion of electrons can account for the observed behavior of the luminescence