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Register a new userA study of uranium-based multilayers: I. Fabrication and structural characterisation
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This paper addresses the structural characterisation of a series of U/Fe, U/Co and U/Gd multilayers. X-ray reflectivity has been employed to investigate the layer thickness and roughness parameters along the growth direction and high-angle diffraction measurements have been used to determine the crystal structure and orientation of the layers. For the case of uranium/transition metal systems, the interfaces are diffuse and the transition metals are present in a polycrystalline form of their common bulk phases with a preferred orientation along the closest packed planes; Fe, bcc (110) and Co, hcp (001), respectively. The uranium is present in a poorly crystalline orthorhombic, alpha-U state. In contrast, the U/Gd multilayers have sharp interfaces with negligible intermixing of atomic species, and have a roughness, which is strongly dependent on the gadolinium layer thickness. Diffraction spectra indicate a high degree of crystallinity in both U and Gd layers with intensities consistent with the growth of a novel hcp U phase, stabilised by the hcp gadolinium layers.
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SQUID magnetometry and polarised neutron reflectivity measurements have been employed to characterise the magnetic properties of U/Fe, U/Co and U/Gd multilayers. The field dependence of the magnetisation was measured at 10K in magnetic fields from -70kOe to 70kOe. A temperature dependent study of the magnetisation evolution was undertaken for a selection of U/Gd samples. PNR was carried out in a field of 4.4kOe for U/Fe and U/Co samples (at room temperature) and for U/Gd samples (at 10K). Magnetic dead layers of about 15 Angstrom were observed for U/Fe and U/Co samples, consistent with a picture of interdiffused interfaces. A large reduction in the magnetic moment, constant over a wide range of Gd layer thicknesses, was found for the U/Gd system (about 4 Bohr magnetons compared with 7.63 for the bulk metal). This could be understood on the basis of a pinning of Gd moments arising from a column-like growth mechanism of the Gd layers. A study of the effective anisotropy suggests that perpendicular magnetic anisotropy could occur in multilayers consisting of thick U and thin Gd layers. A reduction in the Curie temperature was observed as a function of Gd layer thickness, consistent with a finite-size scaling behaviour.
Graphene and graphene-based materials exhibit exceptional optical and electrical properties with great promise for novel applications in light detection. However, several challenges prevent the full exploitation of these properties in commercial devices. Such challenges include the limited linear dynamic range (LDR) of graphene-based photodetectors, the lack of efficient generation and extraction of photoexcited charges, the smearing of photoactive junctions due to hot-carriers effects, large-scale fabrication and ultimately the environmental stability of the constituent materials. In order to overcome the aforementioned limits, different approaches to tune the properties of graphene have been explored. A new class of graphene-based devices has emerged where chemical functionalisation, hybridisation with light-sensitising materials and the formation of heterostructures with other 2D materials have led to improved performance, stability or versatility. For example, intercalation of graphene with FeCl$_3$ is highly stable in ambient conditions and can be used to define photo-active junctions characterized by an unprecedented LDR while graphene oxide (GO) is a very scalable and versatile material which supports the photodetection from UV to THz frequencies. Nanoparticles and quantum dots have been used to enhance the absorption of pristine graphene and to enable high gain thanks to the photogating effect. In the same way, hybrid detectors made from stacked sequences of graphene and layered transition-metal dichalcogenides enabled a class of detectors with high gain and responsivity. In this work we will review the performance and advances in functionalised graphene and hybrid photodetectors, with particular focus on the physical mechanisms governing the photoresponse in these materials, their performance and possible future paths of investigation.
We discuss the structural and electronic properties of tetragonal CuO grown on SrTiO3(100) by means of hybrid density functional theory. Our analysis explains the anomalously large Cu-O vertical distance observed in the experiments (~2.7 A) in terms of a peculiar frustration between two competing local Cu-O environments characterized by different in-plane and out-of-plane bond lengths and Cu electronic populations. The proper inclusion of substrate effects is crucial to understand the tetragonal expansion and to reproduce correctly the measured valence band spectrum for a CuO thickness of 3-3.5 unit cells, in agreement with the experimentally estimated thickness.
Surface diffusion has an impact on the lateral resolution of nanostructures in bottom-up atom nanofabrication. In this paper we study the effects of the gallium atoms self-assembled on silicon surfaces (100) patterned with trenches at different slopes. These particular substrate morphologies have been made to enable an effective deposition rate variation along the surface. In this way we experimentally mimic the effect of the atomic flux modulation created by standing wave during an atom nanofabrication experiment. Even if we observe self organization of gallium atoms on the surface, we conclude that the nano-islands are not affected by surface diffusion processes and the effective variation of the deposition rate per unit area is the dominant factor affecting the growth differences along the surface. This result demonstrates that the gallium atoms self-organization should not prevent the observation of a periodic nano-patterning created by atom nano-fabrication techniques.
All-Heusler multilayer structures have been investigated by means of high kinetic x-ray photoelectron spectroscopy and x-ray magnetic circular dichroism, aiming to address the amount of disorder and interface diffusion induced by annealing of the multilayer structure. The studied multilayers consist of ferromagnetic Co$_2$MnGe and non-magnetic Rh$_2$CuSn layers with varying thicknesses. We find that diffusion begins already at comparably low temperatures between 200 $^{circ}$C and 250 $^{circ}$C, where Mn appears to be most prone to diffusion. We also find evidence for a 4 {AA} thick magnetically dead layer that, together with the identified interlayer diffusion, are likely reasons for the small magnetoresistance found for current-perpendicular-to-plane giant magneto-resistance devices based on this all-Heusler system.
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