Entity

Time filter

Source Type


Black-Schaffer A.M.,Uppsala University | Balatsky A.V.,NORDITA | Balatsky A.V.,Institute for Materials Science | Fransson J.,Uppsala University
Physical Review B - Condensed Matter and Materials Physics | Year: 2015

We show that the energy gap induced by ferromagnetically aligned magnetic impurities on the surface of a topological insulator can be filled, due to scattering off the nonmagnetic potential of the impurities. In both a continuum surface model and a three-dimensional tight-binding lattice model, we find that the energy gap disappears already at weak potential scattering as impurity resonances add spectral weight at the Dirac point. This can help explain seemingly contradictory experimental results as to the existence of a gap. © 2015 American Physical Society. Source


Steiner J.,Ecole Polytechnique - Palaiseau | Schafer R.,Leibniz Institute for Solid State and Materials Research | Wieczoreck H.,Leibniz Institute for Solid State and Materials Research | McCord J.,Institute for Materials Science | Otto F.,Max Planck Institute for Mathematics in the Sciences
Physical Review B - Condensed Matter and Materials Physics | Year: 2012

The concertina is a magnetization pattern in elongated thin-film elements of a soft ferromagnetic material. It is a ubiquitous domain pattern that occurs in the process of magnetization reversal in the direction of the long axis of the small element. Van den Berg and Vatvani argued that this pattern grows out of the flux-closure domains at the sample's tips as the external field is reduced. Based on experimental observations and theory, we argue that in sufficiently elongated thin-film elements the concertina pattern rather bifurcates from an oscillatory buckling mode. Typical sample widths and thicknesses are of the order of 10-100 μm and of the order of 10-150 nm, respectively. Using a reduced model that is derived by asymptotic analysis from the micromagnetic energy and that is also investigated by means of numerical simulation, we quantitatively predict the average period of the concertina pattern and qualitatively predict its hysteresis. In particular, we argue that the experimentally observed coarsening of the concertina pattern is due to secondary bifurcations related to an Eckhaus instability. We also link the concertina pattern to the magnetization ripple and discuss the effect of a weak (crystalline or induced) anisotropy. © 2012 American Physical Society. Source


Avasthi D.K.,Inter University Accelerator Center | Mishra Y.K.,Institute for Materials Science | Singh F.,Inter University Accelerator Center | Stoquert J.P.,Institute of Electronics of Solids and Systems
Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms | Year: 2010

Swift heavy ions have unique feature of creating ion tracks in insulators of dimension from a few nm to about 10 nm. This particular feature of the swift heavy ions is used to engineer the size and shape of the nanoparticles embedded in silica matrix. On the basis of several experiments, it is evidenced that the embedded nanoparticles either grow in size or reduce in size, if they are smaller than or comparable to the ion track size. The shape transformation from spherical to elongated along the beam direction occurs, when the nanoparticle size is larger than the ion track diameter in silica. The reduction, growth and elongation of Au nanoparticles embedded in silica matrix under swift heavy ion irradiation have been discussed in the frame work of thermal spike model. © 2010 Elsevier B.V. All rights reserved. Source


Through this "nanoscale-sculpturing" process, metals such as aluminium, titanium, or zinc can permanently be joined with nearly all other materials, become water-repellent, or improve their biocompatibility. The potential spectrum of applications of these "super connections" is extremely broad, ranging from metalwork in industry right through to safer implants in medical technology. Their results have now been published in the prestigious journal Nanoscale Horizons of the Royal Society of Chemistry. "We have now applied a technology to metals that was previously only known from semiconductors. To use this process in such a way is completely new," said Dr. Jürgen Carstensen, co-author of the publication. In the process, the surface of a metal is converted into a semiconductor, which can be chemically etched and thereby specifically modified as desired. "As such, we have developed a process which - unlike other etching processes - does not damage the metals, and does not affect their stability," emphasised Professor Rainer Adelung, head of the "Functional Nanomaterials" team at the Institute for Materials Science. Adelung stressed the importance of the discovery: "In this way, we can permanently connect metals which could previously not be directly joined, such as copper and aluminium." How does the "nanoscale-sculpturing" process work exactly? The surfaces of metals consist of many different crystals and grains, some of which are less chemically stable than others. These unstable particles can be specifically removed from the surface of a metal by a targeted etching. The top surface layer is roughened by the etching process, creating a three-dimensional surface structure. This changes the properties of the surface, but not of the metal as a whole. This is because the etching is only 10 to 20 micrometers deep - a layer as thin as a quarter of the diameter of human hair. The research team has therefore named the process "nanoscale-sculpturing". The change due to etching is visible to the naked eye: the treated surface becomes matt. "If, for example, we treat a metal with sandpaper, we also achieve a noticeable change in appearance, but this is only two-dimensional, and does not change the characteristics of the surface," explained Dr. Mark-Daniel Gerngross of the research team on materials sciences from Kiel. Through the etching process, a 3D-structure with tiny hooks is created. If a bonding polymer is then applied between two treated metals, the surfaces inter-lock with each other in all directions like a three-dimensional puzzle. "These 3-D puzzle connections are practically unbreakable. In our experiments, it was usually the metal or polymer that broke, but not the connection itself," said Melike Baytekin-Gerngross, lead author of the publication. Even a thin layer of fat, such as that left by a fingerprint on a surface, does not affect the connection. "In our tests, we even smeared gearbox oil on metal surfaces. The connection still held," explained Baytekin-Gerngross. Laborious cleaning of surfaces, such as the pre-treatment of ships' hulls before they can be painted, could thus be rendered unnecessary. In addition, the research team exposed the puzzle connections to extreme heat and moisture, to simulate weather conditions. This also did not affect their stability. Carstensen emphasised: "Our connections are extremely robust and weather-resistant." A beneficial side-effect of the process is that the etching makes the surfaces of metal water-repellent. The resulting hook structure functions like a closely-interlocked 3D labyrinth, without holes which can be penetrated by water. The metals therefore possess a kind of built-in corrosion protection. "We actually don't know this kind of behaviour from metals like aluminium. A lotus effect with pure metals, i.e. without applying a water-repellent coating, that is new," said Adelung. "The range of potential applications is extremely broad, from metalworking industries such as ship-building or aviation, to printing technology and fire protection, right through to medical applications," said Gerngross. Because the "nanoscale-sculpturing" process not only creates a 3D surface structure, which can be purely physically bonded without chemicals, the targeted etching can also remove harmful particles from the surface, which is of particularly great interest in medical technology. Titanium is often used for medical implants. To mechanically fix the titanium in place, small quantities of aluminium are added. However, the aluminium can trigger undesirable side-effects in the body. "With our process, we can remove aluminium particles from the surface layer, and thereby obtain a significantly purer surface, which is much more tolerable for the human body. Because we only etch the uppermost layer on a micrometer scale, the stability of the whole implant remains unaffected," explained Carstensen. The researchers have so far applied for four patents for the process. Businesses have already shown substantial interest in the potential applications. "And our specialist colleagues in materials sciences have also reacted enthusiastically to our discoveries," said a delighted Adelung. Explore further: New generation of orthopedic, dental and cardiovascular prostheses More information: M. Baytekin-Gerngross et al, Making metal surfaces strong, resistant, and multifunctional by nanoscale-sculpturing, Nanoscale Horiz. (2016). DOI: 10.1039/C6NH00140H


« Ricardo supplying single-cylinder diesel research engine with TVCS to DongFeng Commercial Vehicles | Main | Jaguar unveils Formula E team’s official name, title sponsor, driver line-up and electric racing livery » Researchers at the University of Kiel (Germany) have developed a new process—which they call “nanoscale-sculpturing”—for the surface preparation of metals. Nanoscale-sculpturing, which is based on knowledge from semiconductor etching, turns surfaces of everyday metals into their most stable configuration, but leaves the bulk properties unaffected. Thus, nanoscale-sculpturing ensures stronger, reliable joints to nearly all materials, reduces corrosion vastly, and generates a multitude of multifunctional surface properties. An open-access paper on their work is published in the RSC journal Nanoscale Horizons. In strong contrast to nearly all relevant technical surface treatments on metals and semiconductors, the sculpturing approach utilizes the intrinsic features of the surface-near grain structure on the nanoscale. The (electro-)chemistry is tuned to selectively etch out entire or at least large parts of grains on the nanolevel in a coordinated manner introducing an intrinsic micro 3D-character into the resulting surfaces. Deep cavities with undercuts allowing for mechanical interlocking are thus an intrinsic feature of sculpturing. Due to the 3D-character the preserved grains, plains, and facets, i.e., the bulk structure is extremely stable, since, e.g., no grain boundaries are widened weakening the surface microstructure. The surfaces of metals consist of many different crystals and grains, some of which are less chemically stable than others. These unstable particles can be specifically removed from the surface of a metal by a targeted etching. The top surface layer is roughened by the etching process, creating a three-dimensional surface structure. This changes the properties of the surface, but not of the metal as a whole. This is because the etching is only 10 to 20 micrometers deep—a layer as thin as a quarter of the diameter of human hair. The research team has therefore named the process “nanoscale-sculpturing”. To use this process in such a way is completely new, said Dr. Jürgen Carstensen, co-author of the publication. As such, we have developed a process which—unlike other etching processes—does not damage the metals, and does not affect their stability. In this way, we can permanently connect metals which could previously not be directly joined, such as copper and aluminium. —Professor Rainer Adelung, head of the Functional Nanomaterials team at the Institute for Materials Science The change due to etching is visible to the naked eye: the treated surface becomes matt. Through the etching process, a 3D-structure with tiny hooks is created. If a bonding polymer is then applied between two treated metals, the surfaces inter-lock with each other in all directions like a three-dimensional puzzle. Even a thin layer of fat, such as that left by a fingerprint on a surface, does not affect the connection. The researchers even smeared gearbox oil on metal surfaces, and found that the connection still held, said Baytekin-Gerngroß. Laborious cleaning of surfaces, such as the pre-treatment of ships’ hulls before they can be painted, could thus be rendered unnecessary. Extreme heat and moisture also did not affect the joins. A beneficial side-effect of the process is that the etching makes the surfaces of metal water-repellent. The resulting hook structure functions like a closely-interlocked 3D labyrinth, without holes which can be penetrated by water. The metals therefore possess a kind of built-in corrosion protection. Because the nanoscale-sculpturing process not only creates a 3D surface structure which can be purely physically bonded without chemicals, the targeted etching can also remove harmful particles from the surface, which is of particularly great interest in medical technology. The researchers have so far applied for four patents for the process. Businesses have already shown substantial interest in the potential applications.

Discover hidden collaborations