128 research outputs found
Geometry of good sets in n-fold Cartesian product
We propose here a multidimensional generalisation of the notion of link
introduced in our previous papers and we discuss some consequences for
simplicial measures and sums of function algebras.Comment: 17 pages, no figures, no table
Probing the inter-layer exciton physics in a MoS/MoSe/MoS van der Waals heterostructure
Stacking atomic monolayers of semiconducting transition metal dichalcogenides
(TMDs) has emerged as an effective way to engineer their properties. In
principle, the staggered band alignment of TMD heterostructures should result
in the formation of inter-layer excitons with long lifetimes and robust valley
polarization. However, these features have been observed simultaneously only in
MoSe/WSe heterostructures. Here we report on the observation of long
lived inter-layer exciton emission in a MoS/MoSe/MoS trilayer van
der Waals heterostructure. The inter-layer nature of the observed transition is
confirmed by photoluminescence spectroscopy, as well as by analyzing the
temporal, excitation power and temperature dependence of the inter-layer
emission peak. The observed complex photoluminescence dynamics suggests the
presence of quasi-degenerate momentum-direct and momentum-indirect bandgaps. We
show that circularly polarized optical pumping results in long lived valley
polarization of inter-layer exciton. Intriguingly, the inter-layer exciton
photoluminescence has helicity opposite to the excitation. Our results show
that through a careful choice of the TMDs forming the van der Waals
heterostructure it is possible to control the circular polarization of the
inter-layer exciton emission.Comment: 19 pages, 3 figures. Just accepted for publication in Nano Letters
(http://pubs.acs.org/doi/10.1021/acs.nanolett.7b03184
On the existence of automorphisms with simple Lebesgue spectrum
It is shown that if T is a measure preserving automorphism on a probability space (Ω,B, m) which admits a random variable X0 with mean zero such that the stochastic sequence X0 o Tn,n ε Z is orthonormal and spans L02(Ω,B,m), then for any integer k ≠ 0, the random variables X o Tnk,n ε Z generate B modulom
Non equilibrium anisotropic excitons in atomically thin ReS
We present a systematic investigation of the electronic properties of bulk
and few layer ReS van der Waals crystals using low temperature optical
spectroscopy. Weak photoluminescence emission is observed from two
non-degenerate band edge excitonic transitions separated by 20 meV. The
comparable emission intensity of both excitonic transitions is incompatible
with a fully thermalized (Boltzmann) distribution of excitons, indicating the
hot nature of the emission. While DFT calculations predict bilayer ReS to
have a direct fundamental band gap, our optical data suggests that the
fundamental gap is indirect in all cases
Influence of Trapping on the Exciton Dynamics of Al_xGa_1-xAs Films
We present a systematic study on the exciton relaxation in high purity AlGaAs
epilayers. The time for the excitonic photoluminescence to reach its maximum
intensity (t_max) shows a non-monotonic dependence on excitation density which
is attributed to a competition between exciton localization and carrier-carrier
scattering. A phenomenological four level model fully describes the influence
of exciton localization on t_max. This localization effect is enhanced by the
increase of the Al content in the alloy and disappears when localization is
hindered by rising the lattice temperature above the exciton trapping energy.Comment: 4 pages, 3 figures, 16 ref
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