Dr.jens.brede

Abstract

Author(s): Johannes Mielke, Jesús Martínez-Blanco, Maike V. Peters, Stefan Hecht, and Leonhard Grill

The dynamics at the interface between a close-packed porphyrin monolayer and Au(111) is investigated by time-dependent scanning tunneling microscopy, detecting the motion of single-interface adatoms in real space. Imaging sequences reveal predominant switching of the molecular appearance in adjacent…

[Phys. Rev. B 94, 035416] Published Tue Jul 12, 2016

Author(s): Cyril Laplane, Emmanuel Zambrini Cruzeiro, Florian Fröwis, Philippe Goldner, and Mikael Afzelius

Electron spin resonance measurements on Nd3+ doped single crystals of YVO4 directly reveal the pairwise anti-symmetric exchange interaction known as the Dzyaloshinsky-Moriya interaction.

[Phys. Rev. Lett. 117, 037203] Published Tue Jul 12, 2016

Author(s): J. Stigloher, M. Decker, H. S. Körner, K. Tanabe, T. Moriyama, T. Taniguchi, H. Hata, M. Madami, G. Gubbiotti, K. Kobayashi, T. Ono, and C. H. Back

Researchers have verified experimentally that the reflection and refraction of spin waves at an interface follow a Snell’s-like law.

[Phys. Rev. Lett. 117, 037204] Published Tue Jul 12, 2016

Nature Materials. doi:10.1038/nmat4686

Authors: Z. F. Wang, Huimin Zhang, Defa Liu, Chong Liu, Chenjia Tang, Canli Song, Yong Zhong, Junping Peng, Fangsen Li, Caina Nie, Lili Wang, X. J. Zhou, Xucun Ma, Q. K. Xue & Feng Liu

Abstract

To push commercial electronics beyond its current size limits, atomic-scale communication channels and logic units need to be designed, making the use of quantum entities an imperative. In this regime, quantum fluctuations naturally become prominent, and are generally considered to be detrimental. Here we show that for spin-based information processing, these fluctuations can be uniquely exploited to gate the flow of classical binary information across a magnetic chain. Moreover, this information flow can be controlled with a modest external magnetic field that drives the system through different many-body quantum phases in which the orientation of the final spin does or does not reflect the orientation of the initial input. Our results are general for a wide class of anisotropic spin chains that act as magnetic cellular automata, and suggest that quantum fluctuations may play a unique role in driving classical information flow at the atomic scale.

Author(s): Hyungju Oh, Sinisa Coh, Young-Woo Son, and Marvin L. Cohen

We study by first-principles calculations a densely packed island of organic molecules (F4TCNQ) adsorbed on graphene. We find that with electron doping the island naturally forms a p−n junction in the graphene sheet. For example, a doping level of ∼3×1013  electrons per cm2 results in a p−n junction…

[Phys. Rev. Lett. 117, 016804] Published Thu Jun 30, 2016

Attosecond spectroscopic techniques have made it possible to measure differences in transport times for photoelectrons from localized core levels and delocalized valence bands in solids. We report the application of attosecond pulse trains to directly and unambiguously measure the difference in lifetimes between photoelectrons born into free electron–like states and those excited into unoccupied excited states in the band structure of nickel (111). An enormous increase in lifetime of 212 ± 30 attoseconds occurs when the final state coincides with a short-lived excited state. Moreover, a strong dependence of this lifetime on emission angle is directly related to the final-state band dispersion as a function of electron transverse momentum. This finding underscores the importance of the material band structure in determining photoelectron lifetimes and corresponding electron escape depths. Authors: Zhensheng Tao, Cong Chen, Tibor Szilvási, Mark Keller, Manos Mavrikakis, Henry Kapteyn, Margaret Murnane

The photoemission of electrons from atoms, molecules, and condensed matter provides the experimental basis of our understanding of electronic structure. During the process of photoemission, a sufficiently large quantum of electromagnetic radiation (a photon) is absorbed by matter and converted into an electronic excitation, promoting a bound electron into a final state above the vacuum energy Evac. In photoemission spectroscopy, the kinetic energy and momentum of electrons in such final states are analyzed after their propagation to a distant detector. To determine the electronic structure of the sample, the “sudden approximation” has to be fulfilled, whereby the photoelectron leaves the sample fast enough, without further interaction with the remaining electronic structure. On page 62 of this issue, Tao et al. (1) provide unprecedented insight into final-state dynamics by measuring the time a photoelectron takes to leave a solid material for characteristically different final states. By comparing an electron excited to a final state of a nickel solid Ψ Nif with one excited to a state of vacuum Ψ vacf, they establish that a photoelectron resides in the final state for 200 attoseconds (as) (2 × 10−16 s) before it leaves the nickel (see the figure). Such time scales would still allow for the electron to interact with its surroundings and, thus, are relevant for the validity of the sudden approximation. Authors: Uwe Bovensiepen, Manuel Ligges

Author(s): A. Bansil, Hsin Lin, and Tanmoy Das

First-principles band theory, properly augmented by topological considerations, has provided a remarkably successful framework for predicting new classes of topological materials. This Colloquium discusses the underpinnings of the topological band theory and its materials applications.

[Rev. Mod. Phys. 88, 021004] Published Wed Jun 29, 2016

Abstract

We design spin filters for particles with potentially arbitrary spin ##IMG## [http://ej.iop.org/images/0953-8984/28/33/335301/cmaa2b01ieqn001.gif] {$S\left(=1/2,1,3/2,\ldots \right)$} using a one-dimensional periodic chain of magnetic atoms as a quantum device. Describing the system within a tight-binding formalism we present an analytical method to unravel the analogy between a one-dimensional magnetic chain and a multi-strand ladder network. This analogy is crucial, and is subsequently exploited to engineer gaps in the energy spectrum by an appropriate choice of the magnetic substrate. We obtain an exact correlation between the magnitude of the spin of the incoming beam of particles and the magnetic moment of the substrate atoms in the chain desired for opening up of a spectral gap. Results of spin polarized transport, calculated within a transfer matrix formalism, are presented for particles having half-integer as well as higher spin states. We find tha...

This article reviews new superconducting phases of carbon-based materials. During the past decade, new carbon-based superconductors have been extensively developed through the use of intercalation chemistry, electrostatic carrier doping, and surface-proving techniques. The superconducting transition temperature T c of these materials has been rapidly elevated, and the variety of superconductors has been increased. This review fully introduces graphite, graphene, and hydrocarbon superconductors and future perspectives of high- T c superconductors based on these materials, including present problems. Carbon-based superconductors show various types of interesting behavior, such as a positive pressure dependence of T c . At present, experimental information on superconductors is still insufficient, and theoretical treatment is also incomplete. In particular, experimental results are still lacking for graphene and hydrocarbon superc...

Nature Physics. doi:10.1038/nphys3806

Authors: Christopher Gutiérrez, Lola Brown, Cheol-Joo Kim, Jiwoong Park & Abhay N. Pasupathy

Nature Physics. doi:10.1038/nphys3805

Authors: Juwon Lee, Dillon Wong, Jairo Velasco Jr, Joaquin F. Rodriguez-Nieva, Salman Kahn, Hsin-Zon Tsai, Takashi Taniguchi, Kenji Watanabe, Alex Zettl, Feng Wang, Leonid S. Levitov & Michael F. Crommie

Electrostatic confinement of charge carriers in graphene is governed by Klein tunnelling, a relativistic quantum process in which particle–hole transmutation leads to unusual anisotropic transmission at p–n junction boundaries. Reflection and transmission at these boundaries affect the quantum interference of electronic waves, enabling the formation of novel quasi-bound states. Here we report the use of scanning tunnelling microscopy to map the electronic structure of Dirac fermions confined in quantum dots defined by circular graphene p–n junctions. The quantum dots were fabricated using a technique involving local manipulation of defect charge within the insulating substrate beneath a graphene monolayer. Inside such graphene quantum dots we observe resonances due to quasi-bound states and directly visualize the quantum interference patterns arising from these states. Outside the quantum dots Dirac fermions exhibit Friedel oscillation-like behaviour. Bolstered by a theoretical model describing relativistic particles in a harmonic oscillator potential, our findings yield insights into the spatial behaviour of electrostatically confined Dirac fermions.

We have employed spin-polarized scanning tunneling microscopy and Monte-Carlo simulations to investigate the effect of lateral confinement onto the nanoskyrmion lattice in Fe/Ir(111). We find a strong coupling of one diagonal of the square magnetic unit cell to the close-packed edges of Fe nanostructures. In triangular islands this coupling in combination with the mismatching symmetries of the islands and of the square nanoskyrmion lattice leads to frustration and triple-domain states. In direct vicinity to ferromagnetic NiFe islands, the surrounding skyrmion lattice forms additional domains. In this case a side of the square magnetic unit cell prefers a parallel orientation to the ferromagnetic edge. These experimental findings can be reproduced and explained by Monte-Carlo simulations. Here, the single-domain state of a triangular island is lower in energy, but nevertheless multi-domain states occur due to the combined effect of entropy and an intrinsic domain wall pinning arising from the skyrmionic character of the spin texture.

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Article

Magnetic impurities break time reversal symmetry in topological insulators, but there has been disagreement between theory and experiment. Here, the authors study the response of topological states to magnetic dopants at the atomic level and show that, contrary to what generally believed, magnetic order and gapless states can coexist.

Nature Communications doi: 10.1038/ncomms12027

Authors: Paolo Sessi, Rudro R. Biswas, Thomas Bathon, Oliver Storz, Stefan Wilfert, Alessandro Barla, Konstantin A. Kokh, Oleg E. Tereshchenko, Kai Fauth, Matthias Bode, Alexander V. Balatsky

Article

Carotenoids are photoactive organic pigments found in many plants. Here, the authors report the self-assembly of carotenoid suprastructures by forming a pillararene host-guest complex, and show that these structures display functionalities including stimuli responsiveness and photocatalytic activity.

Nature Communications doi: 10.1038/ncomms12042

Authors: Yan Sun, Fang Guo, Tongfei Zuo, Jingjing Hua, Guowang Diao

Article

Emergent phenomena at the interface between a topological insulator and a ferromanget reflect broken symmetry of topological state. Here, Lee et al. report direct measurement of induced magnetism at the Bi 2 Se 3 -EnS interface, paving the way to understand emergent orders in topological material with broken time reversal symmetry.

Nature Communications doi: 10.1038/ncomms12014

Authors: Changmin Lee, Ferhat Katmis, Pablo Jarillo-Herrero, Jagadeesh S. Moodera, Nuh Gedik

We consider the details of the near-surface electronic band structure of a prototypical ferromagnet, Fe(001). Using high resolution angle-resolved photoemission spectroscopy we demonstrate openings of the spin-orbit induced electronic band gaps near the Fermi level. The band gaps and thus the Fermi surface can be manipulated by changing the remanent magnetization direction. The effect is of the order of $\Delta$E = 100 meV and $\Delta \text {k} = 0.1\,\text{\AA}^{-1}$. We show that the observed dispersions are dominated by the bulk band structure. First-principles calculations and one-step photoemission calculations suggest that the effect is related to changes in the electronic ground state, rather than caused by the photoemission process itself. The symmetry of the effect indicates that the observed electronic bulk states are influenced by the presence of the surface, which might be understood as related to a Rashba-type effect. By pinpointing the regions in the electronic band structure where the switchable band gaps occur, we demonstrate the significance of spin-orbit interaction even for elements as light as 3d ferromagnets.

Author(s): Rico Friedrich, Vasile Caciuc, Nicolae Atodiresei, and Stefan Blügel

In this theoretical investigation we demonstrate that the adsorption of spatially extended two-dimensional (2D) π systems such as graphene and hexagonal boron nitride on the ferromagnetic fcc Co(111) surface leads to a specific behavior of the in-plane and interlayer Co-Co magnetic exchange interact…

[Phys. Rev. B 93, 220406(R)] Published Wed Jun 22, 2016

The design of an atomic force microscope with an all-fiber interferometric detection scheme capable of atomic resolution at about 500 mK is presented. The microscope body is connected to a small pumped 3He reservoir with a base temperature of about 300 mK. The bakeable insert with the cooling stage can be moved from its measurement position inside the bore of a superconducting 10 T magnet into an ultra-high vacuum chamber, where tip and sample can be exchanged in-situ. Moreover, single atoms or molecules can be evaporated onto a cold substrate located inside the microscope. Two side chambers are equipped with standard surface preparation and surface analysis tools. The performance of the microscope at low temperatures is demonstrated by resolving single Co atoms on Mn/W(110) and by showing atomic resolution on NaCl(001).