Dr.jens.brede

Maths reality check resonates online

Nature 509, 7500 (2014). doi:10.1038/509263e

Author: Chris Woolston

Biologists of all stripes are sharing an essay by Harvard University mathematician-turned-biologist Jeremy Gunawardena that makes a sobering observation: the mathematical equations at the core of many biological models fail to reflect nature. He argues that the components of all quantitative models should be

Nature advance online publication 14 May 2014. doi:10.1038/nature13315

Authors: Takashi Kiuchi, Hikaru Koga, Munetaka Kawamoto, Keisuke Shoji, Hiroki Sakai, Yuji Arai, Genki Ishihara, Shinpei Kawaoka, Sumio Sugano, Toru Shimada, Yutaka Suzuki, Masataka G. Suzuki & Susumu Katsuma

The silkworm Bombyx mori uses a WZ sex determination system that is analogous to the one found in birds and some reptiles. In this system, males have two Z sex chromosomes, whereas females have Z and W sex chromosomes. The silkworm W chromosome has a dominant role in female determination, suggesting the existence of a dominant feminizing gene in this chromosome. However, the W chromosome is almost fully occupied by transposable element sequences, and no functional protein-coding gene has been identified so far. Female-enriched PIWI-interacting RNAs (piRNAs) are the only known transcripts that are produced from the sex-determining region of the W chromosome, but the function(s) of these piRNAs are unknown. Here we show that a W-chromosome-derived, female-specific piRNA is the feminizing factor of B. mori. This piRNA is produced from a piRNA precursor which we named Fem. Fem sequences were arranged in tandem in the sex-determining region of the W chromosome. Inhibition of Fem-derived piRNA-mediated signalling in female embryos led to the production of the male-specific splice variants of B. mori doublesex (Bmdsx), a gene which acts at the downstream end of the sex differentiation cascade. A target gene of Fem-derived piRNA was identified on the Z chromosome of B. mori. This gene, which we named Masc, encoded a CCCH-type zinc finger protein. We show that the silencing of Masc messenger RNA by Fem piRNA is required for the production of female-specific isoforms of Bmdsx in female embryos, and that Masc protein controls both dosage compensation and masculinization in male embryos. Our study characterizes a single small RNA that is responsible for primary sex determination in the WZ sex determination system.

Article

Rare-earth-doped crystals are prime candidates for qubit storage that could be easily interfaced with photonic systems. Towards this end, Siyushev et al. show the initialization, coherent manipulation and readout of single-electron spins on cerium ions embedded in YAG crystals.

Nature Communications doi: 10.1038/ncomms4895

Authors: P. Siyushev, K. Xia, R. Reuter, M. Jamali, N. Zhao, N. Yang, C. Duan, N. Kukharchyk, A. D. Wieck, R. Kolesov, J. Wrachtrup

We demonstrate single atom anisotropic magnetoresistance on the surface of a topological insulator, arising from the interplay between the helical spin-momentum-locked surface electronic structure and the hybridization of the magnetic adatom states. Our first-principles quantum transport calculations based on density functional theory for Mn on Bi$_2$Se$_3$ elucidate the underlying mechanism. We complement our findings with a two dimensional model valid for both single adatoms and magnetic clusters, which leads to a proposed device setup for experimental realization. Our results provide an explanation for the conflicting scattering experiments on magnetic adatoms on topological insulator surfaces, and reveal the real space spin texture around the magnetic impurity.

Nature advance online publication 07 May 2014. doi:10.1038/nature13252

Authors: Wenkai Zhang, Roberto Alonso-Mori, Uwe Bergmann, Christian Bressler, Matthieu Chollet, Andreas Galler, Wojciech Gawelda, Ryan G. Hadt, Robert W. Hartsock, Thomas Kroll, Kasper S. Kjær, Katharina Kubiček, Henrik T. Lemke, Huiyang W. Liang, Drew A. Meyer, Martin M. Nielsen, Carola Purser, Joseph S. Robinson, Edward I. Solomon, Zheng Sun, Dimosthenis Sokaras, Tim B. van Driel, György Vankó, Tsu-Chien Weng, Diling Zhu & Kelly J. Gaffney

Crucial to many light-driven processes in transition metal complexes is the absorption and dissipation of energy by 3d electrons. But a detailed understanding of such non-equilibrium excited-state dynamics and their interplay with structural changes is challenging: a multitude of excited states and possible transitions result in phenomena too complex to unravel when faced with the indirect sensitivity of optical spectroscopy to spin dynamics and the flux limitations of ultrafast X-ray sources. Such a situation exists for archetypal polypyridyl iron complexes, such as [Fe(2,2′-bipyridine)3]2+, where the excited-state charge and spin dynamics involved in the transition from a low- to a high-spin state (spin crossover) have long been a source of interest and controversy. Here we demonstrate that femtosecond resolution X-ray fluorescence spectroscopy, with its sensitivity to spin state, can elucidate the spin crossover dynamics of [Fe(2,2′-bipyridine)3]2+ on photoinduced metal-to-ligand charge transfer excitation. We are able to track the charge and spin dynamics, and establish the critical role of intermediate spin states in the crossover mechanism. We anticipate that these capabilities will make our method a valuable tool for mapping in unprecedented detail the fundamental electronic excited-state dynamics that underpin many useful light-triggered molecular phenomena involving 3d transition metal complexes.

Neuroscience: White matter matters in autism

Nature 509, 7499 (2014). doi:10.1038/509136b

A genetic mutation that causes defects in the insulation of neurons may contribute to the cognitive deficits seen in autism and other neuropsychiatric disorders.Pratik Mukherjee of the University of California in San Francisco and his colleagues used magnetic resonance imaging to study the brains

Author(s): Sabrina Haas, Eric Heintze, Sina Zapf, Boris Gorshunov, Martin Dressel, and Lapo Bogani

The far-infrared optical transmission has been studied for two lanthanide-based single-chain magnets DyPhOPh and TbPhOPh in the frequency range between 3 and 80 cm−1. The spectra were acquired at temperatures between 2 and 80 K and magnetic fields up to 6 T. Based on their magnetic field dependence ...

[Phys. Rev. B 89, 174409] Published Wed May 07, 2014

Author(s): T.-H. Chuang, Kh. Zakeri, A. Ernst, Y. Zhang, H. J. Qin, Y. Meng, Y.-J. Chen, and J. Kirschner

We present a comprehensive study of magnetic properties and high wave-vector magnon excitations in epitaxial Fe(001) films grown on Ir(001) substrate. The magnetic properties are investigated by magneto-optical Kerr effect for various thicknesses of Fe film from 4 up to 27 monolayers. The magnon dis...

[Phys. Rev. B 89, 174404] Published Fri May 02, 2014

Recent advances in experimental techniques emphasize the usefulness of multiple scanning probe techniques when analyzing nanoscale samples. Here, we analyze theoretically dual-probe setups with probe separations in the nanometer range, i.e., in a regime where quantum coherence effects can be observed at low temperatures. In a dual-probe setup the electrons are injected at one probe and collected at the other. The measured conductance reflects the local transport properties on the nanoscale, thereby yielding information complementary to that obtained with a standard one-probe setup (the local density-of-states). In this work we develop a real space Green's function method to compute the conductance. This requires an extension of the standard calculation schemes, which typically address a finite sample between the probes. In contrast, the developed method makes no assumption on the sample size (e.g., an extended graphene sheet). Applying this method, we study the transport anisotropies in pristine graphene sheets, and analyze the spectroscopic fingerprints arising from quantum interference around single-site defects, such as vacancies and adatoms. Furthermore, we demonstrate that the dual-probe setup is a useful tool for characterizing the electronic transport properties of extended defects or designed nanostructures. In particular, we show that nanoscale perforations, or antidots, in a graphene sheet display Fano-type resonances with a strong dependence on the edge geometry of the perforation.

Author(s): Sven Runte, Predrag Lazić, Chi Vo-Van, Johann Coraux, Jörg Zegenhagen, and Carsten Busse

Graphene on Ir(111) is studied using scanning tunneling microscopy (STM) and x-ray standing waves (XSW). The graphene layer has a corrugated shape due to the moiré pattern formed as a result of a spatially varying interaction strength between graphene and its substrate. The coherent fraction F(111) ...

[Phys. Rev. B 89, 155427] Published Fri Apr 25, 2014

Quantum communications leap out of the lab

Nature 508, 7497 (2014). http://www.nature.com/doifinder/10.1038/508441a

Author: Jane Qiu

China begins work on super-secure network as ‘real-world’ trial successfully sends quantum keys and data.

We describe the design, construction, and performance of an ultra-low temperature, high-field scanning tunneling microscope (STM) with two independent tips. The STM is mounted on a dilution refrigerator and operates at a base temperature of 30 mK with magnetic fields of up to 13.5 T. We focus on the design of the two-tip STM head, as well as the sample transfer mechanism, which allows \textit{in situ} transfer from an ultra high vacuum (UHV) preparation chamber while the STM is at 1.5 K. Other design details such as the vibration isolation and rf-filtered wiring are also described. Their effectiveness is demonstrated via spectral current noise characteristics and the root mean square roughness of atomic resolution images. The high-field capability is shown by the magnetic field dependence of the superconducting gap of $\text{Cu}_x\text{Bi}_2\text{Se}_3$. Finally, we present images and spectroscopy taken with superconducting Nb tips with the refrigerator at 35 mK that indicate that the effective temperature of our tips/sample is approximately 184 mK, corresponding to an energy resolution of 16 $\mu$eV.

Author(s): Jae-Hyun Park and Hu-Jong Lee

Out-of-plane spin-injection and detection through naturally stacked graphene layers were investigated in ferromagnet/graphene/ferromagnet (FGF) junctions. We obtained a maximum magnetoresistance (MR) of 4.6% at T=4.2 K in the junction of a four-layer graphene insertion, having a very small area−junc...

[Phys. Rev. B 89, 165417] Published Mon Apr 21, 2014

Author(s): D. Klar, S. Bhandary, A. Candini, L. Joly, P. Ohresser, S. Klyatskaya, M. Schleberger, M. Ruben, M. Affronte, O. Eriksson, B. Sanyal, and H. Wende

Different magnetic coupling mechanisms have been identified for a few monolayers of Co-porphyrin molecules deposited on a graphene-covered Ni(111) single crystal. A relatively strong antiferromagnetic coupling of the first molecular layer via graphene to the Ni crystal in comparison to a weaker inte...

[Phys. Rev. B 89, 144411] Published Mon Apr 14, 2014

Author(s): F. Djeghloul, P. Dey, A. Hallal, E. Urbain, S. Mahiddine, M. Gruber, D. Spor, M. Alouani, H. Bulou, F. Scheurer, and W. Weber

Spin-polarized electron scattering experiments on different metal-organic and metal-carbon interfaces are performed. A completely unexpected behavior of the spin-motion angles as well as of related quantities as a function of the organic layer or carbon coverage is observed. In fact, by deposition o...

[Phys. Rev. B 89, 134411] Published Mon Apr 14, 2014

Abstract

Author(s): Andreas Krönlein, Jeannette Kemmer, Pin-Jui Hsu, and Matthias Bode

We report on a detailed quasiparticle interference (QPI) scattering study of the Ni(111) surface by low-temperature scanning tunneling spectroscopy (LT-STS). While conventional constant-separation STS shows two broad features, which are interpreted as the Λ3 bulk band and sp-like Shockley-type surfa...

[Phys. Rev. B 89, 155413] Published Tue Apr 08, 2014

Author(s): Maxim Ziatdinov, Shintaro Fujii, Koichi Kusakabe, Manabu Kiguchi, Takehiko Mori, and Toshiaki Enoki

Understanding how foreign chemical species bond to atomic vacancies in graphene layers can advance our ability to tailor the electronic and magnetic properties of defective graphenic materials. Here, we use ultrahigh-vacuum scanning tunneling microscopy (UHV-STM) and density functional theory to ide...

[Phys. Rev. B 89, 155405] Published Thu Apr 03, 2014

This lecture has been given at the 45th Spring School: Computing Solids: Models, Ab-initio Methods and Supercomputing organized at the Forschungszentrum J\"ulich. The goal of this manuscript is to review the basics behind the theory accompanying Scanning Tunneling Microscopy.

Author(s): T. Eelbo, M. Waśniowska, M. Sikora, M. Dobrzański, A. Kozłowski, A. Pulkin, G. Autès, I. Miotkowski, O. V. Yazyev, and R. Wiesendanger

The electronic and magnetic properties of individual Fe atoms adsorbed on the surface of the topological insulator Bi2Te3(111) are investigated. Scanning tunneling microscopy and spectroscopy prove the existence of two distinct types of Fe species, while our first-principles calculations assign them...

[Phys. Rev. B 89, 104424] Published Fri Mar 28, 2014

Models of exchange-bias in thin films have been able to describe various aspects of this technologically relevant effect. Through appropriate choices of free parameters the modelled hysteresis loops adequately match experiment, and typical domain structures can be simulated. However, the use of these parameters, notably the coupling strength between the systems' ferromagnetic (F) and antiferromagnetic (AF) layers, obscures conclusions about their influence on the magnetization reversal processes. Here we develop a 2D phase-field model of the magnetization process in exchange-biased CoO/(Co/Pt)n that incorporates the 10nm-resolved measured local biasing characteristics of the antiferromagnet. Just three interrelated parameters set to measured physical quantities of the ferromagnet and the measured density of uncompensated spins thus suffice to match the experiment in microscopic and macroscopic detail. We use the model to study changes in bias and coercivity caused by different distributions of pinned uncompensated spins of the antiferromagnet, in application-relevant situations where domain wall motion dominates the ferromagnetic reversal. We show the excess coercivity can arise solely from inhomogeneity in the density of biasing- and anti-biasing pinned uncompensated spins in the antiferromagnet. Counter to conventional wisdom, irreversible processes in the latter are not essential.

Scientific Reports 4 doi: 10.1038/srep04508

The ongoing miniaturization in nanoscience and -technology challenges the sensitivity and selectivity of experimental analysis methods to the ultimate level of single atoms and molecules. A promising new approach, addressed here, focuses on the combination of two well-established complementary techniques that have proven to be very successful in their own fields: (i) low-temperature scanning tunneling microscopy (STM), offering high spatial resolution for imaging and spectroscopy together with the capability of manipulating single atoms and molecules in a well-controlled manner; (ii) radio-frequency (rf) magnetic resonance techniques, providing paramount analytical power based on a high energy resolution combined with the versatility of being sensitive to a great variety of different properties of matter. Here, we demonstrate the successful resonant excitation and detection of nuclear and electronic magnetic transitions of a single quantum spin in a single molecule by rf tunneling of electrons applied through the tip of a modified STM instrument operated at 5 K. The presented rf-STM approach allows the unrivalled spectroscopic investigation of electronic hyperfine levels in single molecules with simultaneous sub-molecular spatial resolution. The achieved single-spin sensitivity represents a ten orders of magnitude improvement compared to existing methods of magnetic resonance - offering, atom-by-atom, unprecedented analytical power and spin control with impact to physics, chemistry, biology, medicine, nanoscience and -technology.