18 research outputs found
Spectral and Diffusive Properties of Silver-Mean Quasicrystals in 1,2, and 3 Dimensions
Spectral properties and anomalous diffusion in the silver-mean (octonacci)
quasicrystals in d=1,2,3 are investigated using numerical simulations of the
return probability C(t) and the width of the wave packet w(t) for various
values of the hopping strength v. In all dimensions we find C(t)\sim
t^{-\delta}, with results suggesting a crossover from \delta<1 to \delta=1 when
v is varied in d=2,3, which is compatible with the change of the spectral
measure from singular continuous to absolute continuous; and we find w(t)\sim
t^{\beta} with 0<\beta(v)<1 corresponding to anomalous diffusion. Results
strongly suggest that \beta(v) is independent of d. The scaling of the inverse
participation ratio suggests that states remain delocalized even for very small
hopping amplitude v. A study of the dynamics of initially localized wavepackets
in large three-dimensional quasiperiodic structures furthermore reveals that
wavepackets composed of eigenstates from an interval around the band edge
diffuse faster than those composed of eigenstates from an interval of the
band-center states: while the former diffuse anomalously, the latter appear to
diffuse slower than any power law.Comment: 11 pages, 10 figures, 1 tabl
Spin and orbital effects in a 2D electron gas in a random magnetic field
Using the method of superbosonization we consider a model of a random
magnetic field (RMF) acting on both orbital motion and spin of electrons in two
dimensions. The method is based on exact integration over one particle degrees
of freedom and reduction of the problem to a functional integral over
supermatrices . We consider a general case when
both the direction of the RMF and the g-factor of the Zeeman splitting are
arbitrary. Integrating out fast variations of we come to a standard
collisional unitary non-linear -model. The collision term consists of
orbital, spin and effective spin-orbital parts. For a particular problem of a
fixed direction of RMF, we show that additional soft excitations identified
with spin modes should appear. Considering % -correlated weak RMF and
putting g=2 we find the transport time . This time is 2 times
smaller than that for spinless particles.Comment: 9 pages, no figure
Boundary hopping and the mobility edge in the Anderson model in three dimensions
It is shown, using high-precision numerical simulations, that the mobility
edge of the 3d Anderson model depends on the boundary hopping term t in the
infinite size limit. The critical exponent is independent of it. The
renormalized localization length at the critical point is also found to depend
on t but not on the distribution of on-site energies for box and Lorentzian
distributions. Implications of results for the description of the transition in
terms of a local order-parameter are discussed
Localization of non-interacting electrons in thin layered disordered systems
Localization of electronic states in disordered thin layered systems with b
layers is studied within the Anderson model of localization using the
transfer-matrix method and finite-size scaling of the inverse of the smallest
Lyapunov exponent. The results support the one-parameter scaling hypothesis for
disorder strengths W studied and b=1,...,6. The obtained results for the
localization length are in good agreement with both the analytical results of
the self-consistent theory of localization and the numerical scaling studies of
the two-dimensional Anderson model. The localization length near the band
center grows exponentially with b for fixed W but no
localization-delocalization transition takes place.Comment: 6 pages, 5 figure
Mehanizam gašenja fotoluminescencije u tankim filmovima N,N'-bis(3-methylphenyl)-N,N'-bis(phenyl)benzidine osvetljenih UV svetlošću u vazduhu
The mechanism of photoluminescence (PL) quenching of thin amorphous N,N'-bis(3-methylphenyl)- N,N'-bis(phenyl)benzidine (TPD) films exposed to UV light in air is studied. TPD is small organic molecule widely used in production of organic light emmiting devices (OLEDs). Photoluminescence of TPD films decays exponentially with time of irradiation, i.e. with the increase of concentration of impurities (photo-oxidized TPD molecules) generated by UV irradiation in air. Intensity of PL decreases to half of its original value when the concentration of impurities reaches 0.4%. Average distance between impurities (acceptors) is almost an order of magnitude larger than average distance between host TPD molecules (donors). Direct long range Forster energy transfer is ruled out as the mechanism of PL quenching, as the overlap between donor and acceptor is lacking, and exciton self-diffusion in TPD films is postulated for the mechanism. The presence of oxidation products is confirmed by infrared (IR) spectroscopy. Vibrational spectra of TPD molecule and few other possible products of photo-oxidation of TPD molecule, obtained by density functional theory, are compared to experimental IR spectra.U ovom radu je prikazana studija mehanizma gašenja fotoluminescencije (FL) tankih amorfnih filmova N,N'-bis(3-methylphenyl)-N,N'-bis(phenyl)benzidine (TPD) izloženih UV zračenju u vazduhu. TPD je organski molekul koji se često koristi u izradi organskih svetlećih dioda (OLED). Prilikom izlaganja TPD filmova UV zračenju u vazduhu, dolazi do fotooksidacije TPD molekula, te iz tog razloga fotoluminescencija TPD filmova opada eksponencijalno sa vremenom osvetljavanja filmova, odnosno sa povećanjem koncentracije nečistoća nastalih usled fotooksidacije. Intenzitet fotoluminescencije opadne na polovinu svoje početne vrednosti u slučaju kada je 0.4 % nečistoća prisutno u filmu. U tom slučaju je srednje rastojanje između nečistoća (akceptora) šest puta veće od srednjeg rastojanja između TPD molekula (donora). Direktan dugodometni Forsterov transfer energije je odbačen kao mehanizam gašenja fotoluminescencije jer je spektralno preklapanje emisije donora i apsorpcije akceptora zanemarljivo. Iz ovog razloga je postulirana ekscitonska difuzija u TPD filmovima, analogno nalazima u postojećoj literaturi. Prisustvo produkata oksidacije je potvrđeno uz pomoć infracrvene (IR) spektroskopije. Takodje, izračunat je IR spektar koristeći teoriju funkcionala gustine (DFT) i dobijeno je dobro slaganje sa eksperimentalnim rezultatima
Spectral Statistics in Chiral-Orthogonal Disordered Systems
We describe the singularities in the averaged density of states and the
corresponding statistics of the energy levels in two- (2D) and
three-dimensional (3D) chiral symmetric and time-reversal invariant disordered
systems, realized in bipartite lattices with real off-diagonal disorder. For
off-diagonal disorder of zero mean we obtain a singular density of states in 2D
which becomes much less pronounced in 3D, while the level-statistics can be
described by semi-Poisson distribution with mostly critical fractal states in
2D and Wigner surmise with mostly delocalized states in 3D. For logarithmic
off-diagonal disorder of large strength we find indistinguishable behavior from
ordinary disorder with strong localization in any dimension but in addition
one-dimensional Dyson-like asymptotic spectral singularities. The
off-diagonal disorder is also shown to enhance the propagation of two
interacting particles similarly to systems with diagonal disorder. Although
disordered models with chiral symmetry differ from non-chiral ones due to the
presence of spectral singularities, both share the same qualitative
localization properties except at the chiral symmetry point E=0 which is
critical.Comment: 13 pages, Revtex file, 8 postscript files. It will appear in the
special edition of J. Phys. A for Random Matrix Theor
Magnetization of Gd
Theoretical studies of the temperature (T) dependence of magnetization of
Gd13 clusters have been carried out within a classical Heisenberg model
using Monte-Carlo simulations. It is shown that for a broad range of values
of γ, defined as the ratio between competing ferro and anti-ferro
magnetic couplings, the cluster magnetization increases with T in the low
T region, as seen in experiment.
The clusters are also shown to exhibit a wide distribution of moments
at a given T, which broadens significantly with increasing T.
It is suggested that this may affect the observed magnetic behavior
of magnetic clusters in Stern-Gerlach experiments
Fast photoluminescence quenching in thin films of 4,4 -bis(2,2-diphenylvinyl)-1,1 -biphenyl exposed to air
The photoluminescence (PL) quenching mechanism of UV light and air-exposed amorphous thin films of 4,4-bis(2,2-diphenylvinyl)-1,1-biphenyl (DPVBi), a well-known hole-transport material used in organic light-emitting diodes, is studied. Thin films of DPVBi are stable when exposed to UV light in vacuum but tend to degrade if oxygen is present simultaneously. This is evident from the changes in UV-vis absorption spectra of the latter, showing that degradation rate of DPVBi films is linearly proportional to both oxygen concentration and UV light intensity. Mass spectrometry study of such films revealed a number of different oxygen-containing molecules and fragments of DPVBi thus confirming apparent photo-oxidation process. Also, DFT study of molecular DPVBi with and without oxygen was carried out, the IR spectra calculated for the lowest energy molecules found and the results are compared with the experiment. The most sensitive to photo-oxidation is DPVBi photoluminescence, which decays exponentially with respect to the concentration of photo-oxidized DPVBi molecules (impurities). The PL quantum yield of DPVBi thin film drops to a half of its original value for 0.2% of the impurities present, at which point an average distance between DPVBi molecules (the donors) and photo-oxidized DPVBi species (acceptors) is an order of magnitude larger than the separation between two adjacent molecules. This implies a need for a long-range Forster energy transfer, which we rule out based on the lack of a donor-acceptor spectral overlap. The apparent discrepancy can be removed by postulating exciton self-diffusion in DPVBi thin films, for which there is supporting evidence in existing literature. (C) 2015 Elsevier B.V. All rights reserved
Fast photoluminescence quenching in thin films of 4,4 '-bis(2,2-diphenylvinyl)-1,1 '-biphenyl exposed to air
The photoluminescence (PL) quenching mechanism of UV light and air-exposed amorphous thin films of 4,4'-bis(2,2-diphenylvinyl)-1,1'-biphenyl (DPVBi), a well-known hole-transport material used in organic light-emitting diodes, is studied. Thin films of DPVBi are stable when exposed to UV light in vacuum but tend to degrade if oxygen is present simultaneously. This is evident from the changes in UV-vis absorption spectra of the latter, showing that degradation rate of DPVBi films is linearly proportional to both oxygen concentration and UV light intensity. Mass spectrometry study of such films revealed a number of different oxygen-containing molecules and fragments of DPVBi thus confirming apparent photo-oxidation process. Also, DFT study of molecular DPVBi with and without oxygen was carried out, the IR spectra calculated for the lowest energy molecules found and the results are compared with the experiment. The most sensitive to photo-oxidation is DPVBi photoluminescence, which decays exponentially with respect to the concentration of photo-oxidized DPVBi molecules (impurities). The PL quantum yield of DPVBi thin film drops to a half of its original value for 0.2% of the impurities present, at which point an average distance between DPVBi molecules (the donors) and photo-oxidized DPVBi species (acceptors) is an order of magnitude larger than the separation between two adjacent molecules. This implies a need for a long-range Forster energy transfer, which we rule out based on the lack of a donor-acceptor spectral overlap. The apparent discrepancy can be removed by postulating exciton self-diffusion in DPVBi thin films, for which there is supporting evidence in existing literature