204 research outputs found
Nonparabolicity effects and the spin-split electron dwell time in symmetric III-V double-barrier structures
We start from the fourth order nonparabolic and anisotropic conduction band bulk dispersion relation to obtain an one-band effective Hamiltonian which we apply to an AlGaSb symmetric double-barrier structure with resonant energies significantly (more than 200meV) above the well bottom. The spin-splitting is described by the k3 Dresselhaus spin-orbit coupling term modifying only the effective mass of the spin eigenstates in the investigated structure. Apart from the bulk-like resonant energy shift due to the band nonparabolicity, we obtain a substantial shift depending on the choice of boundary conditions for the envelope functions at interfaces between different materials. The shift of resonant energy levels leads to the change of spin-splitting and the magnitude of the dwell times. We attempt to explain the influence of both the nonparabolicity and boundary conditions choice by introducing various effective masses
Quantization of a gauge theory on a curved noncommutative space
We study quantization of a gauge analogon of the Grosse-Wulkenhaar model: we
find divergent one-loop contributions to the 1-point and 2-point Green
functions. We obtain that five counterterms are necessary for renormalization
and that all divergences are logarithmic.Comment: 23 pages, 3 figure
(Non)renormalizability of the D-deformed Wess-Zumino model
We continue the analysis of the -deformed Wess-Zumino model which was
started in the previous paper. The model is defined by a deformation which is
non-hermitian and given in terms of the covariant derivatives . We
calculate one-loop divergences in the two-point, three-point and four-point
Green functions. We find that the divergences in the four-point function cannot
be absorbed and thus our model is not renormalizable. We discuss possibilities
to render the model renormalizable.Comment: 19 pages; version accepted for publication in Phys.Rev.D; new section
with the detailed discussion on renormalizabilty added and a special choice
of coupling constants which renders the model renormalizable analyze
Time delay in thin slabs with self-focusing Kerr-type nonlinearity
Time delays for an intense transverse electric
(TE) wave propagating through a Kerr-type nonlinear slab are investigated.
The relation between the bidirectional group delay and the dwell time is
derived and it is shown that the difference between them can be separated
into three terms. The first one is the familiar self interference time, due
to the dispersion of the medium surrounding the slab. The other two terms
are caused by the nonlinearity and oblique incidence of the TE wave. It is
shown that the electric field distribution along the slab may be expressed
in terms of Jacobi elliptic functions while the phase difference introduced by the slab is given in terms of incomplete elliptic integrals. The expressions for the field intensity dependent complex reflection and transmission coefficients are derived and the multivalued oscillatory behavior of the delay times for the case of a thin slab is demonstrated
Non-commutative SU(N) gauge theories and asymptotic freedom
In this paper we analyze the one-loop renormalization of the
-expanded Yang-Mills theory. We show that the {\it freedom
parameter} , key to renormalization, originates from higher order
non-commutative gauge interaction, represented by a higher derivative term . The renormalization condition fixes the allowed values of the
parameter to one of the two solutions: or , i.e. to or to
, respectively. When the higher order interaction is switched on,
(), pure non-commutative SU(N) gauge theory at first order in
-expansion becomes one-loop renormalizable for various representations
of the gauge group. We also show that, in the case and the adjoint
representation of the gauge fields, the non-commutative deformation parameter
has to be renormalized and it is asymptotically free.Comment: 16 pages, no figure
Free planar vibration of structures composed of rigid bodies and elastic beam segments
Ovaj rad predstavlja analizu slobodnih vibracija struktura sastavljenih od krutih tela meÄusobno spojenih sa elastiÄnim gredama. Pretpostavlja se da se centri masa krutih tela ne nalaze na neutralnoj osi nedeformisanog elastiÄnog grednog segmenta kao i da kruta tela vrÅ”e ravno kretanje u istoj ravni i da se njihovi centri masa nalaze u toj istoj ravni. Za odreÄivanje frekvencija sistema, modifikacija klasiÄne 'CTMM' metode je upotrebljena. ElastiÄni gredni segmenti se tretiraju kao Ojler-Bernulijeve grede. Prikazan je numeriÄki primer.This article presents free vibration analysis of structures composed of rigid bodies connected with elastic beam segments. It is assumed that the mass centers of rigid bodies are not located on the neutral axes of undeformed elastic beam segments as well as rigid bodies perform planar motion in the same plane and their mass centers are located in that plane. For determination of natural frequencies of the system, modification of the conventional continuous-mass transfer matrix method has been performed. The elastic beam segments are treated as Euler-Bernoulli beams. Numerical example is presented
Fire protection problems with large forest fires in Deliblatska peÅ”Äara (Serbia)
Purpose - The aim of the paper was to investigate the major forest fires in Deliblatska peÅ”Äara, as well as to analyze the efficiency of fire protection measures. Design / methodology / approach - The analysis included interviews with participants in extinguishing the major fires. The questions referred to the course of fires, as well as to the main fire protection disadvantages. The documentation of the 'Banat' Forest Management - PanÄevo (Public Enterprise 'VojvodinaÅ”ume') was also used in the paper for the period 1948-2012. Findings - The major forest fires in Deliblatska peÅ”Äara in the studied period were recorded in 1973, 1990, 1996 and 2007. Although they account for only about 1.5 % of the total number of fires, they collectively have invaded more than half of the total burnt area and more than two-thirds of the forest area. According to the surveys, the main characteristics of these fires were: frequent appearance of new fire hot spots, strong wind variable in direction which crucially affects the spread of fire and the impossibility of direct action on fire. The main disadvantages of fire protection were: inefficiency of fire breaks, blockage of forest roads for the passage of vehicles and the lack of the modern means of fire protection equipment. Research limitations / implications - Given the specificities of the studied area (the absence of surface water, sandy soils, microclimate conditions and vegetation composition), the research results cannot be fully generalized for Serbia. Practical implications - The research results indicate the need for making changes in the fire prevention system, as well as the possibility of fire danger forecast based on the heliocentric hypothesis. Originality / value - What has been the importance of the paper is that it provides the basis for a new approach to the planning of fire prevention measures
Towards automated design of quantum cascade lasers
We present an advanced technique for the design and optimization of GaAs/AlGaAs quantum cascade laser structures. It is based on the implementation of the simulated annealing algorithm with the purpose of determining a set of design parameters that satisfy predefined conditions, leading to an enhancement of the device output characteristics. Two important design aspects have been addressed: improved thermal behavior, achieved by the use of higher conduction band offset materials, and a more efficient extraction mechanism, realized via a ladder of three lower laser states, with subsequent pairs separated by the optical phonon energy. A detailed analysis of performance of the obtained structures is carried out within a full self-consistent rate equations model of the carrier dynamics. The latter uses wave functions calculated by the transfer matrix method, and evaluates all relevant carrierāphonon and carrierācarrier scattering rates from each quantized state to all others within the same and neighboring periods of the cascade. These values are then used to form a set of rate equations for the carrier density in each state, enabling further calculation of the current density and gain as a function of the applied field and temperature. This paper addresses the application of the described procedure to the design of lambda~9 Āµm GaAs-based mid-infrared quantum cascade lasers and presents the output characteristics of some of the designed optimized structures. Ā© 2005 American Institute of Physic
On analytical solutions of f(R) modified gravity theories in FLRW cosmologies
A novel analytical method for f(R) modified theories without matter in
Friedmann-Lemaitre-Robertson-Walker spacetimes is introduced. The equation of
motion for the scale factor in terms of cosmic time is reduced to the equation
for the evolution of the Ricci scalar R with the Hubble parameter H. The
solution of equation of motion for actions of the form of power law in Ricci
scalar R, is presented with a detailed elaboration of the action quadratic in
R. The reverse use of the introduced method is exemplified in finding
functional forms f(R) which lead to specified scale factor functions. The
analytical solutions are corroborated by numerical calculations with excellent
agreement. Possible further applications to the phases of inflationary
expansion and late-time acceleration as well as f(R) theories with radiation
are outlined.Comment: 16 pages, 6 figures. v2: minor changes, references added. v3: minor
changes, more references added. v4: version to appear in IJMPD. v5: DOI and
journal reference adde
- ā¦