153 research outputs found
Experimental and theoretical investigation of the magnetization dynamics of an artificial square spin ice cluster
No full textPohlit M, Stockem I, Porrati F, Huth M, Schröder C, Müller J. Experimental and theoretical investigation of the magnetization dynamics of an artificial square spin ice cluster. Journal of Applied Physics. 2016;120(14): 142103
Highly-Packed Proximity-Coupled DC-Josephson Junction Arrays by a Direct-Write Approach
No full textFocused ion beam induced deposition (FIBID) is a direct-write technique enabling the growth of individual nanostructures of any shape and dimension with high lateral resolution. Moreover, the fast and reliable writing of periodically arranged nanostructures can be used to fabricate devices for the investigation of collective phenomena and to design novel functional metamaterials. Here, FIBID is employed to prepare dc-Josephson junction arrays (dc-JJA) consisting of superconducting NbC dots coupled through the proximity effect via a granular metal layer. The fabrication is straightforward and allows the preparation of dc-JJA within a few seconds. Microstructure and composition of the arrays are investigated by transmission electron microscopy and energy dispersive X-ray spectroscopy. The superconductor-to-metal transition of the prepared dc-JJA is studied in a direct way, by tuning the Josephson junction resistance in 70 nm-spaced superconducting NbC dots. The observed magnetoresistance oscillations with a period determined by the flux quantum give evidence for the coherent charge transport by paired electrons. Moreover, the measured resistance minima correspond to two fundamental matching configurations of fluxons in the dc-JJA, caused by magnetic frustration. The robust properties of the prepared dc-JJA demonstrate the opportunities for a fast preparation of complex device configurations using direct-write approaches
A note on the holographic beta and C functions
No full textThe holographic RG flow in AdS/CFT correspondence naturally defines a holographic scheme in which the central charge c and the beta function are related by the formula c = -2c beta(a)beta(b)G(ab), where G(ab) is the metric of the kinetic term of the supergravity scalars. In particular, the metric in the space of couplings is f(ab) = 2cG(ab). We perform some checks of that result and we compare it with the quantum field theory expectations. We discuss alternative definitions of the c-function. In particular, we compare, for a particular supersymmetric flow, the holographic c-function with the central charge computed directly from the two-point function of the stress-energy tensor. (C) 2000 Elsevier Science B.V. All rights reserved
Fabrication of FeSi and Fe3Si compounds by electron beam induced mixing of [Fe/Si]2 and [Fe3/Si]2 multilayers grown by focused electron beam induced deposition
No full textFe-Si binary compounds have been fabricated by focused electron beam induced deposition by the alternating use of iron pentacarbonyl, Fe(CO)5, and neopentasilane, Si5H12 as precursor gases. The fabrication procedure consisted in preparing multilayer structures which were treated by low-energy electron irradiation and annealing to induce atomic species intermixing. In this way, we are able to fabricate FeSi and Fe3Si binary compounds from [Fe/Si]2 and [Fe3/Si]2 multilayers, as shown by transmission electron microscopy investigations. This fabrication procedure is useful to obtain nanostructured binary alloys from precursors which compete for adsorption sites during growth and, therefore, cannot be used simultaneously
Alloy multilayers and ternary nanostructures by direct-write approach
No full textThe fabrication of nanopatterned multilayers, as used in optical and magnetic applications, is usually achieved by two independent steps, which consist in the preparation of multilayer films and in the successive patterning by means of lithography and etching processes. Here we show that multilayer nanostructures can be fabricated by using focused electron beam induced deposition (FEBID), which allows the direct writing of nanostructures of any desired shape with nanoscale resolution. In particular, [Co2Fe/Si]n multilayers are prepared by the alternating deposition from the metal carbonyl precursors, HFeCo3 (CO)12 and Fe (CO)5, and neopentasilane, Si5H12. The ability to fabricate nanopatterned multilayers by FEBID is of interest for the realization of hyperbolic metamaterials and related nanodevices. In a second experiment, we treated the multilayers by low-energy electron irradiation in order to induce atomic species intermixing with the purpose to obtain ternary nanostructured compounds. Transmission electron microscopy and electrical transport measurements indicate that in thick multilayers, (n = 12), the intermixing is only partial, taking place mainly in the upper part of the structures. However, for thin multilayers, (n = 2), the intermixing is such that a transformation into the L21 phase of the Co2FeSi Heusler compound takes place over the whole sample volume
Layer-resolved imaging of magnetic interlayer coupling by domain-wall stray fields
No full textLayer-resolved magnetic domain images of epitaxially grown Co/Cu/Ni trilayers on Cu(001) have been studied, taken by photoelectron emission microscopy using x-ray magnetic circular dichroism as a magnetic contrast mechanism. In these trilayers the Ni layers are magnetized perpendicularly to the film plane, whereas the Co magnetization is in the film plane. Comparison of the as-grown magnetic domain images of the Co and Ni layers reveals the influence of the magnetostatic stray fields from Ni domain walls on the Co domain pattern as a lateral displacement of the Co domain wall position compared to the Ni domain walls. The effect is quantified by comparing to the effect of external magnetic fields, and is found to be equivalent to about 250 Oe. Micromagnetic simulations using the Landau-Lifshitz-Gilbert equation confirm that size of the Ni domain wall stray field interaction
Site-Selective Chemical Vapor Deposition on Direct-Write 3D Nanoarchitectures
No full textRecent advancements in additive manufacturing have enabled the preparation of free-shaped 3D objects with feature sizes down to and below the micrometer scale. Among the fabrication methods, focused electron beam-and focused ion beam-induced deposition (FEBID and FIBID, respectively) associate high flexibility and unmatched accuracy in 3D writing with a wide material portfolio, thereby allowing for the growth of metallic to insulating materials. The combination of the free-shaped 3D nanowriting with established chemical vapor deposition (CVD) techniques provides attractive opportunities to synthesize complex 3D core-shell heterostructures. Hence, this hybrid approach enables the fabrication of morphologically tunable layer-based nanostructures with the great potential of unlocking further functionalities. Here, the fundamentals of such a hybrid approach are demonstrated by preparing core-shell heterostructures using 3D FEBID scaffolds for site-selective CVD. In particular, 3D microbridges are printed by FEBID with the (CH3)3CH3C5H4Pt precursor and coated by thermal CVD using the Nb(NMe2)3(N-t-Bu) and HFeCo3(CO)12 precursors. Two model systems on the basis of CVD layers consisting of a superconducting NbC-based layer and a ferromagnetic Co3Fe layer are prepared and characterized with regard to their composition, microstructure, and magneto-transport properties
Minimal constrained supergravity
No full textAbstractWe describe minimal supergravity models where supersymmetry is non-linearly realized via constrained superfields. We show that the resulting actions differ from the so called “de Sitter” supergravities because we consider constraints eliminating directly the auxiliary fields of the gravity multiplet
Brane-World Gravity
No full textThe observable universe could be a 1+3-surface (the “brane”) embedded in a 1+3+d-dimensional spacetime (the “bulk”), with Standard Model particles and fields trapped on the brane while gravity is free to access the bulk. At least one of the d extra spatial dimensions could be very large relative to the Planck scale, which lowers the fundamental gravity scale, possibly even down to the electroweak (∼ TeV) level. This revolutionary picture arises in the framework of recent developments in M theory. The 1+10-dimensional M theory encompasses the known 1+9-dimensional superstring theories, and is widely considered to be a promising potential route to quantum gravity. At low energies, gravity is localized at the brane and general relativity is recovered, but at high energies gravity “leaks” into the bulk, behaving in a truly higher-dimensional way. This introduces significant changes to gravitational dynamics and perturbations, with interesting and potentially testable implications for high-energy astrophysics, black holes, and cosmology. Brane-world models offer a phenomenological way to test some of the novel predictions and corrections to general relativity that are implied by M theory. This review analyzes the geometry, dynamics and perturbations of simple brane-world models for cosmology and astrophysics, mainly focusing on warped 5-dimensional brane-worlds based on the Randall–Sundrum models. We also cover the simplest brane-world models in which 4-dimensional gravity on the brane is modified at low energies – the 5-dimensional Dvali–Gabadadze–Porrati models. Then we discuss co-dimension two branes in 6-dimensional models
A tunable strain sensor using nanogranular metals
No full textThis paper introduces a new methodology for the fabrication of strain-sensor elements for MEMS and NEMS applications based on the tunneling effect in nano-granular metals. The strain-sensor elements are prepared by the maskless lithography technique of focused electron-beam-induced deposition (FEBID) employing the precursor trimethylmethylcyclopentadienyl platinum [MeCpPt(Me)3]. We use a cantilever-based deflection technique to determine the sensitivity (gauge factor) of the sensor element. We find that its sensitivity depends on the electrical conductivity and can be continuously tuned, either by the thickness of the deposit or by electron-beam irradiation leading to a distinct maximum in the sensitivity. This maximum finds a theoretical rationale in recent advances in the understanding of electronic charge transport in nano-granular metals
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