25,279 research outputs found
Theory of band gap bowing of disordered substitutional II-VI and III-V semiconductor alloys
For a wide class of technologically relevant compound III-V and II-VI
semiconductor materials AC and BC mixed crystals (alloys) of the type
A(x)B(1-x)C can be realized. As the electronic properties like the bulk band
gap vary continuously with x, any band gap in between that of the pure AC and
BC systems can be obtained by choosing the appropriate concentration x, granted
that the respective ratio is miscible and thermodynamically stable. In most
cases the band gap does not vary linearly with x, but a pronounced bowing
behavior as a function of the concentration is observed. In this paper we show
that the electronic properties of such A(x)B(1-x)C semiconductors and, in
particular, the band gap bowing can well be described and understood starting
from empirical tight binding models for the pure AC and BC systems. The
electronic properties of the A(x)B(1-x)C system can be described by choosing
the tight-binding parameters of the AC or BC system with probabilities x and
1-x, respectively. We demonstrate this by exact diagonalization of finite but
large supercells and by means of calculations within the established coherent
potential approximation (CPA). We apply this treatment to the II-VI system
Cd(x)Zn(1-x)Se, to the III-V system In(x)Ga(1-x)As and to the III-nitride
system Ga(x)Al(1-x)N.Comment: 14 pages, 10 figure
Chaos and energy spreading for time-Dependent Hamiltonians, and the various Regimes in the theory of Quantum Dissipation
We make the first steps towards a generic theory for energy spreading and
quantum dissipation. The Wall formula for the calculation of friction in
nuclear physics and the Drude formula for the calculation of conductivity in
mesoscopic physics can be regarded as two special results of the general
formulation. We assume a time-dependent Hamiltonian with
, where is slow in a classical sense. The rate-of-change is
not necessarily slow in the quantum-mechanical sense. Dissipation means an
irreversible systematic growth of the (average) energy. It is associated with
the stochastic spreading of energy across levels. The latter can be
characterized by a transition probability kernel where and
are level indices. This kernel is the main object of the present study. In the
classical limit, due to the (assumed) chaotic nature of the dynamics, the
second moment of exhibits a crossover from ballistic to diffusive
behavior. We define the regimes where either perturbation theory or
semiclassical considerations are applicable in order to establish this
crossover in the quantal case. In the limit perturbation theory
does not apply but semiclassical considerations can be used in order to argue
that there is detailed correspondence, during the crossover time. In the
perturbative regime there is a lack of such correspondence. Namely,
is characterized by a perturbative core-tail structure that persists during the
crossover time. In spite of this lack of (detailed) correspondence there may be
still a restricted correspondence as far as the second-moment is concerned.
Such restricted correspondence is essential in order to establish the universal
fluctuation-dissipation relation.Comment: 46 pages, 6 figures, 4 Tables. To be published in Annals of Physics.
Appendix F improve
Multiplexing regulated traffic streams: design and performance
The main network solutions for supporting QoS rely on traf- fic policing (conditioning, shaping). In particular, for IP networks the IETF has developed Intserv (individual flows regulated) and Diffserv (only ag- gregates regulated). The regulator proposed could be based on the (dual) leaky-bucket mechanism. This explains the interest in network element per- formance (loss, delay) for leaky-bucket regulated traffic. This paper describes a novel approach to the above problem. Explicitly using the correlation structure of the sources’ traffic, we derive approxi- mations for both small and large buffers. Importantly, for small (large) buffers the short-term (long-term) correlations are dominant. The large buffer result decomposes the traffic stream in a stream of constant rate and a periodic impulse stream, allowing direct application of the Brownian bridge approximation. Combining the small and large buffer results by a concave majorization, we propose a simple, fast and accurate technique to statistically multiplex homogeneous regulated sources. To address heterogeneous inputs, we present similarly efficient tech- niques to evaluate the performance of multiple classes of traffic, each with distinct characteristics and QoS requirements. These techniques, applica- ble under more general conditions, are based on optimal resource (band- width and buffer) partitioning. They can also be directly applied to set GPS (Generalized Processor Sharing) weights and buffer thresholds in a shared resource system
Resource dimensioning through buffer sampling
Link dimensioning, i.e., selecting a (minimal) link capacity such that the users’ performance requirements are met, is a crucial component of network design. It requires insight into the interrelationship among the traffic offered (in terms of the mean offered load , but also its fluctuation around the mean, i.e., ‘burstiness’), the envisioned performance level, and the capacity needed. We first derive, for different performance criteria, theoretical dimensioning formulas that estimate the required capacity as a function of the input traffic and the performance target. For the special case of Gaussian input traffic, these formulas reduce to , where directly relates to the performance requirement (as agreed upon in a service level agreement) and reflects the burstiness (at the timescale of interest). We also observe that Gaussianity applies for virtually all realistic scenarios; notably, already for a relatively low aggregation level, the Gaussianity assumption is justified.\ud
As estimating is relatively straightforward, the remaining open issue concerns the estimation of . We argue that particularly if corresponds to small time-scales, it may be inaccurate to estimate it directly from the traffic traces. Therefore, we propose an indirect method that samples the buffer content, estimates the buffer content distribution, and ‘inverts’ this to the variance. We validate the inversion through extensive numerical experiments (using a sizeable collection of traffic traces from various representative locations); the resulting estimate of is then inserted in the dimensioning formula. These experiments show that both the inversion and the dimensioning formula are remarkably accurate
Impurity effects in a two--dimensional system with Dirac spectrum
It is demonstrated that in a two--band 2D system the resonance state is
manifested close to the energy of the Dirac point in the electron spectrum for
the sufficiently large impurity perturbation. With increasing the impurity
concentration, the electron spectrum undergoes the rearrangement, which is
characterized by the opening of the broad quasi--gap in the vicinity of the
nodal point. If the critical concentration for the spectrum rearrangement is
not reached, the domain of localized states remains exponentially small
compared to the bandwidth.Comment: 4 pages, 1 figur
Disorder and Impurities in Hubbard-Antiferromagnets
We study the influence of disorder and randomly distributed impurities on the
properties of correlated antiferromagnets. To this end the Hubbard model with
(i) random potentials, (ii) random hopping elements, and (iii) randomly
distributed values of interaction is treated using quantum Monte Carlo and
dynamical mean-field theory. In cases (i) and (iii) weak disorder can lead to
an enhancement of antiferromagnetic (AF) order: in case (i) by a
disorder-induced delocalization, in case (iii) by binding of free carriers at
the impurities. For strong disorder or large impurity concentration
antiferromagnetism is eventually destroyed. Random hopping leaves the local
moment stable but AF order is suppressed by local singlet formation. Random
potentials induce impurity states within the charge gap until it eventually
closes. Impurities with weak interaction values shift the Hubbard gap to a
density off half-filling. In both cases an antiferromagnetic phase without
charge gap is observed.Comment: 16 pages, 9 figures, latex using vieweg.sty (enclosed); typos
corrected, references updated; to appear in "Advances in Solid State
Physics", Vol. 3
Resource dimensioning through buffer sampling
Link dimensioning, i.e., selecting a (minimal) link capacity such that the users’ performance requirements are met, is a crucial component of network design. It requires insight into the interrelationship between the traffic offered (in terms of the mean offered load M, but also its fluctuation around the mean, i.e., ‘burstiness’), the envisioned performance level, and the capacity needed. We first derive, for different performance criteria, theoretical dimensioning formulae that estimate the required capacity C as a function of the input traffic and the performance target. For the special case of Gaussian input traffic these formulae reduce to C = M+V , where directly relates to the performance requirement (as agreed upon in a service level agreement) and V reflects the burstiness (at the timescale of interest). We also observe that Gaussianity applies for virtually all realistic scenarios; notably, already for a relatively low aggregation level the Gaussianity assumption is justified.\ud
As estimating M is relatively straightforward, the remaining open issue concerns the estimation of V . We argue that, particularly if V corresponds to small time-scales, it may be inaccurate to estimate it directly from the traffic traces. Therefore, we propose an indirect method that samples the buffer content, estimates the buffer content distribution, and ‘inverts’ this to the variance. We validate the inversion through extensive numerical experiments (using a sizeable collection of traffic traces from various representative locations); the resulting estimate of V is then inserted in the dimensioning formula. These experiments show that both the inversion and the dimensioning formula are remarkably accurate
Single hole motion in LaMnO
We study single hole motion in LaMnO using the classical approximation
for JT lattice distortions, a modified Lang-Firsov approximation for dynamical
breathing-mode phonons, and the self-consistent Born approximation (verified by
exact diagonalization) for hole-orbital-excitation scattering. We show that in
the realistic parameter space for LaMnO, quantum effects of electron-phonon
interaction are small. The quasiparticle bandwidth in the
purely orbital t-J model. It is strikingly broadened to be of order by
strong static Jahn-Teller lattice distortions even when the polaronic band
narrowing is taken into account.Comment: 4 pages, 4 eps figure
Mott transition in the asymmetric Hubbard model at half-filling within dynamical mean-field theory
We apply the approximate analytic methods to the investigation of the band
structure of the asymmetric Hubbard model where the chemical potentials and
electron transfer parameters depend on the electron spin (type of
quasiparticles). The Hubbard-I and alloy-analogy approximations are the
simplest approximations which are used. Within the alloy-analogy approximation,
the energy band of particles does not depend on the transfer parameter of
particles of another sort. It means that the gap in the spectrum opens at the
critical value that is the same in two different limiting cases: the
Falicov-Kimball model and the standard Hubbard model. The approximate analytic
scheme of the dynamical mean-field theory is developed to include into the
theory the scattering of particles responsible for the additional mechanism
(due to the transfer of particles of another sort) of the band formation. We
use the so-called GH3 approach that is a generalization of the Hubbard-III
approximation. The approach describes the continuous Mott transition with the
value dependent on a ratio of transfer parameters of different
particles.Comment: 10 pages, 10 figure
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