One-magnon Raman scattering in La(2)CuO(4): the origin of the field-induced mode

We investigate the one-magnon Raman scattering in the layered antiferromagnetic La(2)CuO(4) compound. We find that the Raman signal is composed by two one-magnon peaks: one in the B1g channel, corresponding to the Dzyaloshinskii-Moryia (DM) mode, and another in the B3g channel, corresponding to the XY mode. Furthermore, we show that a peak corresponding to the XY mode can be induced in the planar (RR) geometry when a magnetic field is applied along the easy axis for the sublattice magnetization. The appearance of such field-induced mode (FIM) signals the existence of a new magnetic state above the Neel temperature T_N, where the direction of the weak-ferromagnetic moment (WFM) lies within the CuO(2) planes.Comment: 4 pages, 3 figure

Magnonic Crystal with Two-Dimensional Periodicity as a Waveguide for Spin Waves

We describe a simple method of including dissipation in the spin wave band structure of a periodic ferromagnetic composite, by solving the Landau-Lifshitz equation for the magnetization with the Gilbert damping term. We use this approach to calculate the band structure of square and triangular arrays of Ni nanocylinders embedded in an Fe host. The results show that there are certain bands and special directions in the Brillouin zone where the spin wave lifetime is increased by more than an order of magnitude above its average value. Thus, it may be possible to generate spin waves in such composites decay especially slowly, and propagate especially large distances, for certain frequencies and directions in ${\bf k}$-space.Comment: 13 pages, 4 figures, submitted to Phys Rev

Using social media to measure the contribution of Red List species to the nature-based tourism potential of African protected areas

Cultural ecosystem services are defined by people’s perception of the environment, which make them hard to quantify systematically. Methods to describe cultural benefits from ecosystems typically include resource-demanding survey techniques, which are not suitable to assess cultural ecosystem services for large areas. In this paper we explore a method to quantify cultural benefits through the enjoyment of natured-based tourism, by assessing the potential tourism attractiveness of species for each protected area in Africa using the IUCN’s Red List of Threatened Species. We use the number of pictures of wildlife posted on a photo sharing website as a proxy for charisma, popularity, and ease of observation, as these factors combined are assumed to determine how attractive species are for the global wildlife tourist. Based on photo counts of 2473 African animals and plants, species that seem most attractive to nature-based tourism are the Lion, African Elephant and Leopard. Combining the photo counts with species range data, African protected areas with the highest potential to attract wildlife tourists based on attractive species occurrence were Samburu National Reserve in Kenya, Mukogodo Forest Reserve located just north of Mount Kenya, and Addo Elephant National Park in South-Africa. The proposed method requires only three data sources which are freely accessible and available online, which could make the proposed index tractable for large scale quantitative ecosystem service assessments. The index directly links species presence to the tourism potential of protected areas, making the connection between nature and human benefits explicit, but excludes other important contributing factors for tourism, such as accessibility and safety. This social media based index provides a broad understanding of those species that are popular globally; in many cases these are not the species of highest conservation concern.JRC.H.5-Land Resources Managemen

Surface decoration of graphene with all-inorganic perovskite nanocrystals

Recent progress in the synthesis of high stability inorganic caesium lead halide perovskite nanocrystals (NCs) with unique optical and electronic properties has led to their increasing use in optoelectronic applications, such as broadband photodetectors. These NCs are of particular interest for the UV range as they have the potential to extend the wavelength range of photodetectors based on traditional materials. The physical properties of these nanostructures, such as the dynamics of charge carriers on different timescales and their effect on the optical recombination of carriers, are crucial for functional applications, but not yet fully understood. This work reports on a slow (> 1 s) reversible quenching of the NC photoluminescence due to a light-induced Stark effect involving defects on the surface of the NCs and the redistribution of photoexcited carriers onto the NC surface. We also demonstrate a defect-assisted high photoconductive gain (10^6 A/W) in the UV–Visible range for graphene transistors decorated with perovskite NCs, resulting from the charge transfer between the NCs and graphene. This is accompanied by a giant hysteresis of the graphene resistance that is strongly dependent on electrostatic gating and temperature. We summarise the properties of the perovskite/graphene transistor with 3 characteristic response times: optical (∼ 10 s); electrical (∼ 100 s); and magnetic (∼ 500 s). Our data are well described by a phenomenological macroscopic ‘two-capacitor’ model of the charge transfer from bound states in the NCs into the graphene layer, providing a useful tool for the design of high-photoresponsivity perovskite/graphene transistors. Finally, we investigate the prospects of using scalable additive manufacturing, specifically inkjet printing, of graphene (iGraphene) and other low-dimensional materials for development of fully printed optoelectronic devices compatible with a range of flexible and wearable substrates. We demonstrate a hybrid perovskite/iGraphene photodetector with responsivity ∼15 A/W and use iGraphene as Ohmic contacts to other 2D materials

Discovery of X-ray absorption features from the dipping low-mass X-ray binary XB 1916-053 with XMM-Newton

We report the discovery of narrow Fe XXV and Fe XXVI K alpha X-ray absorption lines at 6.65 and 6.95 keV in the persistent emission of the dipping low-mass X-ray binary (LMXB) XB 1916-053 during an XMM-Newton observation performed in September 2002. In addition, there is marginal evidence for absorption features at 1.48 keV, 2.67 kev, 7.82 keV and 8.29 keV consistent with Mg XII, S XVI, Ni XXVII K alpha and Fe XXVI K beta transitions, respectively. Such absorption lines from highly ionized ions are now observed in a number of high inclination (ie. close to edge-on) LMXBs, such as XB 1916-053, where the inclination is estimated to be between 60-80 degrees. This, together with the lack of any orbital phase dependence of the features (except during dips), suggests that the highly ionized plasma responsible for the absorption lines is located in a cylindrical geometry around the compact object. Using the ratio of Fe XXV and Fe XXVI column densities, we estimate the photo-ionization parameter of the absorbing material to be 10^{3.92} erg cm s^{-1}. Only the Fe XXV line is observed during dipping intervals and the upper-limits to the Fe XXVI column density are consistent with a decrease in the amount of ionization during dipping intervals. This implies the presence of cooler material in the line of sight during dipping. We also report the discovery of a 0.98 keV absorption edge in the persistent emission spectrum. The edge energy decreases to 0.87 keV during deep dipping intervals. The detected feature may result from edges of moderately ionized Ne and/or Fe with the average ionization level decreasing from persistent emission to deep dipping. This is again consistent with the presence of cooler material in the line of sight during dipping.Comment: 13 pages, accepted for publication in Astronomy and Astrophysic

Surface decoration of graphene with all-inorganic perovskite nanocrystals

Recent progress in the synthesis of high stability inorganic caesium lead halide perovskite nanocrystals (NCs) with unique optical and electronic properties has led to their increasing use in optoelectronic applications, such as broadband photodetectors. These NCs are of particular interest for the UV range as they have the potential to extend the wavelength range of photodetectors based on traditional materials. The physical properties of these nanostructures, such as the dynamics of charge carriers on different timescales and their effect on the optical recombination of carriers, are crucial for functional applications, but not yet fully understood. This work reports on a slow (> 1 s) reversible quenching of the NC photoluminescence due to a light-induced Stark effect involving defects on the surface of the NCs and the redistribution of photoexcited carriers onto the NC surface. We also demonstrate a defect-assisted high photoconductive gain (10^6 A/W) in the UV–Visible range for graphene transistors decorated with perovskite NCs, resulting from the charge transfer between the NCs and graphene. This is accompanied by a giant hysteresis of the graphene resistance that is strongly dependent on electrostatic gating and temperature. We summarise the properties of the perovskite/graphene transistor with 3 characteristic response times: optical (∼ 10 s); electrical (∼ 100 s); and magnetic (∼ 500 s). Our data are well described by a phenomenological macroscopic ‘two-capacitor’ model of the charge transfer from bound states in the NCs into the graphene layer, providing a useful tool for the design of high-photoresponsivity perovskite/graphene transistors. Finally, we investigate the prospects of using scalable additive manufacturing, specifically inkjet printing, of graphene (iGraphene) and other low-dimensional materials for development of fully printed optoelectronic devices compatible with a range of flexible and wearable substrates. We demonstrate a hybrid perovskite/iGraphene photodetector with responsivity ∼15 A/W and use iGraphene as Ohmic contacts to other 2D materials