41 research outputs found
Correlated electron-hole plasma in organometal perovskites
Organic-inorganic perovskites are a class of solution-processed semiconductors holding promise for the realization of low-cost efficient solar cells and on-chip lasers. Despite the recent attention they have attracted, fundamental aspects of the photophysics underlying device operation still remain elusive. Here we use photoluminescence and transmission spectroscopy to show that photoexcitations give rise to a conducting plasma of unbound but Coulomb-correlated electron-hole pairs at all excitations of interest for light-energy conversion and stimulated optical amplification. The conductive nature of the photoexcited plasma has crucial consequences for perovskite-based devices: in solar cells, it ensures efficient charge separation and ambipolar transport while, concerning lasing, it provides a low threshold for light amplification and justifies a favourable outlook for the demonstration of an electrically driven laser. We find a significant trap density, whose cross-section for carrier capture is however low, yielding a minor impact on device performance
Magnetic properties of pseudomorphic epitaxial films of Pr_{0.7}Ca_{0.3}MnO_3 under different biaxial tensile stresses
In order to analyse the effect of strain on the magnetic properties of
narrow-band manganites, the temperature and field dependent susceptibilities of
about 8.5 nm thick epitaxial Pr0.7Ca0.3MnO3 films, respectively grown on (001)
and (110) SrTiO3 substrates, have been compared. For ultrathin samples grown on
(001) SrTiO3, a bulk-like cluster-glass magnetic behaviour is found, indicative
of the possible coexistence of antiferromagnetic and ferromagnetic phases. On
the contrary, ultrathin films grown on (110) substrates show a robust
ferromagnetism, with a strong spontaneous magnetization of about 3.4 mB /Mn
atom along the easy axis. On the base of high resolution reciprocal space
mapping analyses performed by x-ray diffraction, the different behaviours are
discussed in terms of the crystallographic constraints imposed by the epitaxy
of Pr0.7Ca0.3MnO3 on SrTiO3. We suggest that for growth on (110) SrTiO3, the
tensile strain on the film c-axis, lying within the substrate plane, favours
the ferromagnetic phase, possibly by allowing a mixed occupancy and
hybridization of both in-plane and out-of-plane eg orbitals. Our data allow to
shed some physics of inhomogeneous states in manganites and on the nature of
their ferromagnetic insulating state.Comment: 9 pages, 9 figure
Low-temperature magnetic behavior of perovskite compounds PbFe1/2Ta1/2O3 and PbFe1/2Nb1/2O3
The isostructural perovskite compounds PbFe(1/2)Ta(1/2)O3 and PbFe(1/2)Nb(1/2)O3 have been known for long time, and they are part of the important class of materials called multiferroic, where ferroelasticity, ferroelectricity, and ferromagnetism coexist. In the literature regarding PbFe(1/2)Ta(1/2)O3 and PbFe(1/2)Nb(1/2)O3, an "anomaly" of their low-temperature magnetic behavior has not always been reported. Moreover, both the origin of this behavior, and the cause for which it was not always observed, were never completely explained. In this paper, the magnetic behavior of the two compounds at low temperature has been extensively studied and explained as the occurring of a spin-glasslike transition