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Beutel O.,University of Osnabrück | Nikolaus J.,Humboldt University of Berlin | Birkholz O.,University of Osnabrück | You C.,University of Osnabrück | And 3 more authors.
Angewandte Chemie - International Edition | Year: 2014

Lipid analogues carrying three nitrilotriacetic acid (tris-NTA) head groups were developed for the selective targeting of His-tagged proteins into liquid ordered (lo) or liquid disordered (ld) lipid phases. Strong partitioning into the lo phase of His-tagged proteins bound to tris-NTA conjugated to saturated alkyl chains (tris-NTA DODA) was achieved, while tris-NTA conjugated to an unsaturated alkyl chain (tris- NTA SOA) predominantly resided in the ld phase. Interestingly, His-tag-mediated lipid crosslinking turned out to be required for efficient targeting into the lo phase by tris-NTA DODA. Robust partitioning into lo phases was confirmed by using viral lipid mixtures and giant plasma membrane vesicles. Moreover, efficient protein targeting into lo and ld domains within the plasma membrane of living cells was demonstrated by singlemolecule tracking, thus establishing a highly generic approach for exploring lipid microdomains in situ. © 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.


Giesbers A.J.M.,TU Eindhoven | Uhlirova K.,Leiden Institute of Physics | Konecny M.,TU Eindhoven | Peters E.C.,Max Planck Institute for Solid State Research | And 3 more authors.
Physical Review Letters | Year: 2013

We show ferromagnetic properties of hydrogen-functionalized epitaxial graphene on SiC. Ferromagnetism in such a material is not directly evident as it is inherently composed of only nonmagnetic constituents. Our results nevertheless show strong ferromagnetism with a saturation of 0.9μB/hexagon projected area, which cannot be explained by simple magnetic impurities. The ferromagnetism is unique to hydrogenated epitaxial graphene on SiC, where interactions with the interfacial buffer layer play a crucial role. We argue that the origin of the observed ferromagnetism is governed by electron correlation effects of the narrow Si dangling bond states in the buffer layer exchange coupled to localized states in the hydrogenated graphene layer. This forms a quasi-three-dimensional ferromagnet with a Curie temperature higher than 300 K. © 2013 American Physical Society.


Faez S.,Max Planck Institute for the Science of Light | Faez S.,Leiden Institute of Physics | Turschmann P.,Max Planck Institute for the Science of Light | Haakh H.R.,Max Planck Institute for the Science of Light | And 4 more authors.
Physical Review Letters | Year: 2014

Many of the currently pursued experiments in quantum optics would greatly benefit from a strong interaction between light and matter. Here, we present a simple new scheme for the efficient coupling of single molecules and photons. A glass capillary with a diameter of 600 nm filled with an organic crystal tightly guides the excitation light and provides a maximum spontaneous emission coupling factor (β) of 18% for the dye molecules doped in the organic crystal. A combination of extinction, fluorescence excitation, and resonance fluorescence spectroscopy with microscopy provides high-resolution spatiospectral access to a very large number of single molecules in a linear geometry. We discuss strategies for exploring a range of quantum-optical phenomena, including polaritonic interactions in a mesoscopic ensemble of molecules mediated by a single mode of propagating photons. © 2014 American Physical Society.


Tromp R.M.,IBM | Tromp R.M.,Leiden Institute of Physics | Schramm S.M.,Leiden Institute of Physics
Ultramicroscopy | Year: 2013

The Contrast Transfer Function (CTF) describes the manner in which the electron microscope modifies the object exit wave function as a result of objective lens aberrations. For optimum resolution in C3-corrected microscopes it is well established that a small negative value of C3, offset by positive values of C5 and defocus C1 results in the most optimal instrument resolution, and optimization of the CTF has been the subject of several studies. Here we describe a simple design procedure for the CTF that results in a most even transfer of information below the resolution limit. We address not only the resolution of the instrument, but also the stability of the CTF in the presence of small disturbances in C1 and C3. We show that resolution can be traded for stability in a rational and transparent fashion. These topics are discussed quantitatively for both weak-phase and strong-phase (or amplitude) objects. The results apply equally to instruments at high electron energy (TEM) and at very low electron energy (LEEM), as the basic optical properties of the imaging lenses are essentially identical. © 2012 Elsevier B.V.


Flokstra M.,Leiden Institute of Physics | Van Der Knaap J.M.,Leiden Institute of Physics | Aarts J.,Leiden Institute of Physics
Physical Review B - Condensed Matter and Materials Physics | Year: 2010

We investigate the magnetotransport behavior of ferromagnet (F)/superconductor/ferromagnet trilayers made of ferromagnetic Ni80 Fe20 (Permalloy, Py) and superconducting Nb for temperatures both above and below the superconducting transition temperature Tc. In such devices, and for weak ferromagnets, Tc depends on the relative magnetization directions of the two F layers in such a way that TcP of the parallel (P) alignment is lower than Tc AP of the antiparallel (AP) alignment (the so-called superconducting spin-valve effect). For strong magnets, the suppression of Andreev reflection may alter this picture, but also stray field effects become important, as is known from earlier work. We compare large-area samples with microstructured ones, and find blocklike switching in the latter. We show this not to be due to a switch between the P and AP states, but rather to dipolar coupling between domains which are forming in the two Py layers, making a stray-field scenario likely. We also present measurements of the depairing (critical) current Idp and show that a similar depression of superconductivity exists far below Tc as is found around T c. © 2010 The American Physical Society.


Van Wezel J.,University of Cambridge | Van Wezel J.,Argonne National Laboratory | Oosterkamp T.H.,Leiden Institute of Physics
Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences | Year: 2012

Modern, state-of-the-art nanomechanical devices are capable of creating spatial superpositions that are massive enough to begin to experimentally access the quantum to classical crossover, and thus force us to consider the possible ways in which the usual quantum dynamics may be affected. One recent theoretical proposal describes the crossover from unitary quantum mechanics to classical dynamics as a form of spontaneous symmetry breaking. Here, we propose a specific experimental set-up capable of identifying the source of unitarity breaking in such a mechanism. The experiment is aimed specifically at clarifying the role played by gravity, and distinguishes the resulting dynamics from that suggested by alternative scenarios for the quantum to classical crossover. We give both a theoretical description of the expected dynamics, and a discussion of the involved experimental parameter values and the proposed experimental protocol. © 2011 The Royal Society.


News Article | November 25, 2016
Site: www.rdmag.com

NewsThis emerging research may lead to new ways of designing geared devices like satellite trackers or watches. Contributed Author: Leiden Institute of Physics Topics: Physics


News Article | November 8, 2016
Site: phys.org

In a flowchart, Alessandra Silvestri and Levon Pogosian answer questions with ‘yes’ and ‘no’, leading to the subsequent conclusion. For example, if μ is greater than one and Σ is less, then a large collection of models is ruled out; the so-called Horndeski class. Credit: Leiden Institute of Physics Cosmologists have many possible models for the universe, of which only one can be true. A new flowchart detailed in Physical Review D on November 7 will eliminate some of them when two specific universe features are accurately measured. Cosmologists try to understand how the entire universe formed and evolves. In short, cosmology is the science of everything above the scale of our pale blue dot floating around in the vastness of space. By aiming their telescopes at distant galaxies and the afterglow of the Big Bang, cosmologists look back in time and pick up pieces of the puzzle. They used these pieces as parameters in the many possible models they have created for the universe. The more precisely parameters are measured, the more models can be excluded. Lately, astrophysicists have done many observations to measure two specific parameters. These are called μ and Σ. They represent how fast galaxies formed from the irregularities in the universe just after the Big Bang and how much distant light is bent by gravitational lensing. Recently measured values of μ and Σ generally show a mild tension with the leading cosmological model called ΛCDM. More and better observations will characterize an accurate value of the two parameters. Things would get very interesting if the observed μ and Σ indeed did not agree with the values expected in the popular ΛCDM model. So then what? Leiden University physicist Alessandra Silvestri and Levon Pogosian from Simon Fraser University published a paper in Physical Review D with an overview of all the models ruled out by each value measured. In a flowchart, they answered questions with 'yes' and 'no,' leading to the subsequent conclusion. For example, if μ is greater than one and Σ is less, then a large collection of models is ruled out; the so-called Horndeski class. With their paper, they add more significance to future cosmological observations, as it lends concrete meaning to the measurement of otherwise abstract values. Explore further: Physicists reveal the role of diffusion in the early universe More information: Levon Pogosian et al. What can cosmology tell us about gravity? Constraining Horndeski gravity withand, Physical Review D (2016). DOI: 10.1103/PhysRevD.94.104014


News Article | February 3, 2016
Site: phys.org

Leiden physicists sent short ultraviolet laserpulses of two picoseconds through a crystal. This leads to the creation of four photons that are entangled in their orbital angular momentum—here depicted as red blue spirals. The rainbow colored circles illustrate the phase (color) and intensity (brightness) of the photon’s cross section. Credit: Leiden Institute of Physics For the first time, scientists have entangled four photons in their orbital angular momentum. Leiden physicists sent a laser through a crystal, thereby creating four photons with coupled 'rotation'. So far this has only been achieved with two photons. The discovery makes uncrackable secret communication of complex information possible between multiple parties. The report is forthcoming in Physical Review Letters. Entanglement holds great promise for perfectly secret communication and quantum computing. If two photons are created simultaneously, they are counterparts—their rotation is always reversed with respect to the other. If we measure left rotation for one photon, then the other will always rotate to the right after measurement with a similar filter. This is called entanglement. Before the measurement, each photon's rotation is undetermined. Such rotation is a property of photons that scientists discovered in 1992 in Leiden; physicists call this orbital angular momentum. And this property has more than two values. It covers an infinitely large alphabet of information. Via rotation, it is possible to transfer much more information per photon than with a property like polarization, which contains only two possible values. In 2001, scientists managed to entangle two photons in orbital angular momentum for the first time. Now, Leiden physicist Wolfgang Löffler and his colleagues are the first to entangle four photons in this way. The discovery offers additional possibilities, like sending an uncrackable encrypted message to more than one party. During their successful experiment, the researchers sent short ultraviolet laser pulses of two picoseconds through a crystal. Occasionally, this leads to the creation of four entangled photons. This is extremely rare, but by generating 80 million pulses per second, they managed to detect on average two so-called photon quadruplets each second. To confirm these were, indeed, entangled in orbital angular momentum, the team used a spatial phase modulator that converts this rotation back to light traveling as a plane wave. They registered this 'normal' light with single photon detectors. More information: Observation of four-photon orbital angular momentum entanglement, B. C. Hiesmayr, M, J. A. de Dood, W. Löffler, Physical Review Letters, 2016. On Arxiv: arxiv.org/abs/1508.01480


News Article | November 14, 2016
Site: phys.org

Electron microscope image of a chromium dioxide devices based on wires. The green wire is the chromium dioxide ferromagnet. The orange wires are superconductors and are necessary to produce a superconducting current through the green wire. Credit: Leiden Institute of Physics Researchers have discovered that electrons that spin synchronously around their axes remain superconductive across large distances within magnetic chrome dioxide. Electric current from these electrons can flip small magnets, and its superconductive version could form the basis of a hard drive without energy loss. The study has been published in Physical Review X. In Leiden in 1911, Nobel Prize-winner Heike Kamerlingh Onnes discovered the principle of superconduction; electric current flowing through ice-cold metal without any resistance. This super-current can transport electricity or power an electromagnet without energy loss, an essential property for MRI scanners, maglev trains and nuclear fusion reactors. Half a century later, scientists discovered that electrons appear to form pairs, enabling the super-current to escape the classical rules of electricity. Physicists assumed that both electrons spin around their axes in opposite directions, so that the pairs have a net 'spin' of zero. Around the turn of the century, that assumption proved to be premature. Super-currents can, indeed, have a net 'spin,' and even possibly manipulate small magnets. Leiden physicist Prof. Jan Aarts and his group have now created a wire made of chrome dioxide, which only carries currents with 'spin.' They cooled it to a superconducting state and measured a particularly strong current of a billion A/m2. That's powerful enough to flip magnets, potentially facilitating future hard drives without energy loss. Moreover, the super-current covered a record distance of 600 nanometer. This seems like a small stretch—bacteria are bigger—but it lets electron pairs survive long enough for practical use. More information: Amrita Singh et al. High-QualityNanowires for Dissipation-less Spintronics, Physical Review X (2016). DOI: 10.1103/PhysRevX.6.041012

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