968 research outputs found
Electronic states in a graphene flake strained by a Gaussian bump
The effect of strain in graphene is usually modeled by a pseudo-magnetic
vector potential which is, however, derived in the limit of small strain. In
realistic cases deviations are expected in view of graphene's very high strain
tolerance, which can be up to 25%. Here we investigate the pseudo-magnetic
field generated by a Gaussian bump and we show that it exhibits significant
differences with numerical tight-binding results. Furthermore, we calculate the
electronic states in the strained region for a hexagon shaped flake with
armchair edges. We find that the six-fold symmetry of the wave functions inside
the Gaussian bump is directly related to the different effect of strain along
the fundamental directions of graphene: zigzag and armchair. Low energy
electrons are strongly confined in the armchair directions and are localized on
the carbon atoms of a single sublattice
Veselago lensing in graphene with a p-n junction: classical versus quantum effects
The feasibility of Veselago lensing in graphene with a p-n junction is
investigated numerically for realistic injection leads. Two different set-ups
with two narrow leads are considered with absorbing or reflecting side edges.
This allows us to separately determine the influence of scattering on electron
focusing for the edges and the p-n interface. Both semiclassical and
tight-binding simulations show a distinctive peak in the transmission
probability that is attributed to the Veselago lensing effect. We investigate
the robustness of this peak on the width of the injector, the position of the
p-n interface and different gate potential profiles. Furthermore, the influence
of scattering by both short- and long-range impurities is considered.Comment: 10 pages, 7 figure
Electronic properties of bilayer phosphorene quantum dots in the presence of perpendicular electric and magnetic fields
Using the tight-binding approach, we investigate the electronic properties of
bilayer phosphorene (BLP) quantum dots (QDs) in the presence of perpendicular
electric and magnetic fields. Since BLP consists of two coupled phosphorene
layers, it is of interest to examine the layer-dependent electronic properties
of BLP QDs, such as the electronic distributions over the two layers and the
so-produced layer-polarization features, and to see how these properties are
affected by the magnetic field and the bias potential. We find that in the
absence of a bias potential only edge states are layer-polarized while the bulk
states are not, and the layer-polarization degree (LPD) of the unbiased edge
states increases with increasing magnetic field. However, in the presence of a
bias potential both the edge and bulk states are layer-polarized, and the LPD
of the bulk (edge) states depends strongly (weakly) on the interplay of the
bias potential and the interlayer coupling. At high magnetic fields, applying a
bias potential renders the bulk electrons in a BLP QD to be mainly distributed
over the top or bottom layer, resulting in layer-polarized bulk Landau levels
(LLs). In the presence of a large bias potential that can drive a
semiconductor-to-semimetal transition in BLP, these bulk LLs exhibit different
magnetic-field dependences, i.e., the zeroth LLs exhibit a linear-like
dependence on the magnetic field while the other LLs exhibit a square-root-like
dependence.Comment: 11 pages, 6 figure
THERMAL VARIATIONS IN OCTOBER 2013 IN NORTH-WESTERN ROMANIA
The present analysis refers two weather situations in October 2013 in north-western Romania, represented by a period of cold weather in the first part of the month, and, respectively, a warm period in the last decade of the month. The cold wave produced minimum daily temperatures ranging between -6.4 and -1.9°C, in the lower areas, while in the mountainous region they were between -9.4 and -7.2°C. These values are by 0.2 to 5.6°C lower than the absolute daily minimum temperatures registered between 1961-2012 period. Positive deviations from the maximum daily absolute temperatures up to 4.0°C were recorded in the warm period at the end of the month. The data base used in the study was made up of minimum and maximum daily temperatures for the periods 3-8 and 22-30 October 2013, registered at 14 meteorological stations situated in north-western Romania. Other data used were the air temperature at standard isobaric levels of 850, 700 and 500 hPa, in the period 1973-2013. Synoptic reanalysis maps for the period 1961-2013 were also used
Some aspects of the wind in the northern area of the depression of Transsylvania
Összefoglalás - A jelen tanulmány az Erdélyi Medence északi részén előforduló talajmenti és magaslati szél egyes
vonásait mutatja be. A szél paramétereinek az elemzése 5 meteorológiai állomás (Torda, Kolozsvár, Dézs, Beszterce és
Marosvásárhely) és a Kolozsvári Aerológiai Állomás által szolgáltatott, 1961 és 1980 között feljegyzett adatok alapján
történt. A szél irányának és sebességének az évi, évszaki és havi középértékeit a talajszinten és az 5 standard izobár szinten
számítottuk ki. A felszín morfológiai sajátosságai és ennek a Kárpátok vonulatához viszonyított elhelyezkedése
meghatározza a talajmenti szél paramétereinek regionális jellemzőit. A Kárpátok vonulatának a légmozgásra gyakorolt
hatása körülbelül 3000 m magasságig érződik, fölötte a nyugati széljárás jellemző.
Summary - The study presents some aspects of the ground level and altitudinal wind in the northern area of the
Depression of Transsylvania. The analysis of the wind parameters is based on the data provided by 5 meteorological
stations (Turda, Cluj-Napoca, Dej, Bistrija and Tárgu-Mure$) and the Cluj-Napoca Aerological Observatory, covering the
interval between 1961-1980. The annual, seasonal and monthly average values of the direction and speed of the wind at
ground level and at the five standard isobaric levels have been calculated. The regional differences of the wind parameters
at ground level are affected by the morphologic peculiarities of the area and by its position in the vicinity of the Carpathian
bow. In the altitude the influence of the Carpathians upon the circulation of the air is felt as high as 3000 m. Above this
level the western circulation is predominant
Anomalies Observed in VLF and LF Radio Signals on the Occasion of the Western Turkey Earthquake (Mw = 5.7) on May 19, 2011
VLF radio signals lie in the 10 - 60 kHz frequency band. These radio signals are used for worldwide navigation support, time signals and for military purposes. They are propagated in the earth-ionosphere wave-guide mode along great circle propagation paths. So, their propaga-tion is strongly affected by the ionosphere conditions. LF signals lie in 150 - 300 kHz frequency band. They are used for long way broadcasting by the few (this type of broadcasting is going into disuse) transmitters located in the world. These radio signals are characterized by the ground wave and the sky wave propagation modes [1]. The first generates a stable signal that propagates in the channel Earth-troposphere and is affected by the surface ground and troposphere condition. The second instead gives rise to a signal which varies greatly between day and night, and between summer and winter, and which propagates using the lower ionosphere as a reflector; its propagation is mainly affected by the ionosphere condi-tion, particularly in the zone located in the middle of the transmitter-receiver path. The propagation of the VLF/LF radio signals is affected by different factors such as the meteorological condition, the solar bursts and the geo-magnetic activity. At the same time, variations of some parameters in the ground, in the atmosphere and in the ionosphere occurring during the preparatory phase of earthquakes can produce disturbances in the above men-tioned signals. As already reported by many previous studies [2-18] the disturbances are classified as anoma-lies and different methods of analysis as the residual dA/ dP [15], the terminator time TT [9], the Wavelet spectra and the Principal Component Analysis have been used [6,7].
Here the analysis carried out on LF and VLF radio signals using three different methods on the occasion of a strong earthquake occurred recently in Turkey is pre-sented
Wavelet analysis of the LF radio signals collected by the European VLF/LF network from July 2009 to April 2011
In 2008, a radio receiver that works in very low frequency (VLF; 20-60 kHz) and LF (150-300 kHz) bands was developed by an Italian factory. The
receiver can monitor 10 frequencies distributed in these bands, with the measurement for each of them of the electric field intensity. Since 2009, to
date, six of these radio receivers have been installed throughout Europe to establish a ‘European VLF/LF Network’. At present, two of these are into
operation in Italy, and the remaining four are located in Greece, Turkey, Portugal and Romania. For the present study, the LF radio data collected
over about two years were analysed. At first, the day-time data and the night-time data were separated for each radio signal. Taking into account
that the LF signals are characterized by ground-wave and sky-wave propagation modes, the day-time data are related to the ground wave and
the night-time data to the sky wave. In this framework, the effects of solar activity and storm activity were defined in the different trends. Then, the
earthquakes with M ≥5.0 that occurred over the same period were selected, as those located in a 300-km radius around each receiver/transmitter and
within the 5th Fresnel zone related to each transmitter-receiver path. Where possible, the wavelet analysis was applied on the time series of the radio
signal intensity, and some anomalies related to previous earthquakes were revealed. Except for some doubt in one case, success appears to have been obtained in all of the cases related to the 300 km circles in for the ground waves and the sky waves. For the Fresnel cases, success in two cases and one
failure were seen in analysing the sky waves. The failure occurred in August/September, and might be related to the disturbed conditions of the ionosphere in summer
Pseudo magnetic field in strained graphene: revisited
We revisit the theory of the pseudo magnetic field as induced by strain in
graphene using the tight-binding approach. A systematic expansion of the
hopping parameter and the deformation of the lattice vectors is presented from
which we obtain an expression for the pseudo magnetic field for low energy
electrons. We generalize and discuss previous results and propose a novel
effective Hamiltonian. The contributions of the different terms to the pseudo
magnetic field expression is investigated for a model triaxial strain profile
and are compared with the full solution. Our work suggests that the previous
proposed pseudo magnetic field expression is valid up to reasonably high strain
15 % and there is no K-dependent pseudo-magnetic field.Comment: 11 pages, 4 figure
Resonant valley filtering of massive Dirac electrons
Electrons in graphene, in addition to their spin, have two pseudospin degrees
of freedom: sublattice and valley pseudospin. Valleytronics uses the valley
degree of freedom as a carrier of information similar to the way spintronics
uses electron spin. We show how a double barrier structure consisting of
electric and vector potentials can be used to filter massive Dirac electrons
based on their valley index. We study the resonant transmission through a
finite number of barriers and we obtain the energy spectrum of a superlattice
consisting of electric and vector potentials. When a mass term is included the
energy bands and energy gaps at the K and K' points are different and they can
be tuned by changing the potential.Comment: 20 figure
„AIR AND WATER – COMPONENTS OF THE ENVIRONMENT” CONFERENCE AT 10 YEARS OF EXISTENCE
The Faculty of Geography has experienced a constant and accelerated development since 1994, when it became an independent subunit of Babeş-Bolyai University. In this context, in 2006, the fifth and the newest specialization of the faculty, Hydrology and Meteorology, was launched as Bachelor studies level. This specialization has as main aim to meet to practical needs, namely the training of specialists for several fields of the economic development, mainly for the national hydrological and meteorological networks.
In order to provide a superior level of education for this specialization, in 2009 was established the Master studies program, under the title Resources and risks in the hydro-atmospheric environment. This ensured the complex training of future hydrologists and meteorologists. Over the 10-years period, the two study programs have been quite attractive, the number of students in the 1st year oscillating between 20 and 30 students, at Bachelor level and between 10 and 20 students, at Master level
(Pandi, 2014).
The opportunity and success of the specializations resulted also from the relatively high number of graduates who have been employed in the hydrological and meteorological national networks, as well as in other institutions. Moreover, many employees from the branches of the „Romanian Waters” National Administration and the National Meteorological Administration have completed their studies at a bachelor or master level. Most graduates work as hydrologists or meteorologists in weather and hydrological stations, research institutes, water basin monitoring offices, water management departments including leadership positions etc
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