Dr.jens.brede

We use a low-temperature scanning tunneling microscope to study the interplay between the Kondo effect of a single-atom contact and a spin current. To this end, a nickel tip is coated by a thick layer of copper and brought into contact with a single Co atom adsorbed on a Cu(100) surface. We show that upon contact the Kondo resonance of Co is spin split and attribute the splitting to the spin current produced by the nickel tip and flowing across the copper spacer. A quantitative line shape analysis indicates that the spin polarization of the junction amounts up to 18%, but decreases when a pristine nickel tip is directly contacted to the Co atom.

Nature Physics. doi:10.1038/nphys2637

Authors: Marco Koschorreck, Daniel Pertot, Enrico Vogt & Michael Köhl

Harnessing spins as information carriers has emerged as an elegant extension to the transport of electrical charges. The coherence of such spin transport in spintronic circuits is determined by the lifetime of spin excitations and by spin diffusion. Fermionic quantum gases allow the study of spin transport from first principles because interactions can be precisely tailored and the dynamics is on directly observable timescales. In particular, at unitarity, spin transport is dictated by diffusion and the spin diffusivity is expected to reach a universal, quantum-limited value on the order of the reduced Planck constant ħ divided by the mass m. Here, we study a two-dimensional Fermi gas after a quench into a metastable, transversely polarized state. Using the spin-echo technique, for strong interactions, we measure the lowest transverse spin diffusion constant so far 0.25(3) ħ/m. For weak interactions, we observe a collective transverse spin-wave mode that exhibits mode softening when approaching the strongly interacting regime.

Author(s): Yan Li, Wei Zhang, Markus Morgenstern, and Riccardo Mazzarello

We carry out an ab initio study of the structural, electronic, and magnetic properties of zigzag graphene nanoribbons on Cu(111), Ag(111), and Au(111). Both, H-free and H-terminated nanoribbons are considered revealing that the nanoribbons invariably possess edge states when deposited on these surfa...

[Phys. Rev. Lett. 110, 216804] Published Fri May 24, 2013

Nature Materials. doi:10.1038/nmat3654

Authors: Shiyong Tan, Yan Zhang, Miao Xia, Zirong Ye, Fei Chen, Xin Xie, Rui Peng, Difei Xu, Qin Fan, Haichao Xu, Juan Jiang, Tong Zhang, Xinchun Lai, Tao Xiang, Jiangping Hu, Binping Xie & Donglai Feng

Author(s): Takaaki Tominaga, Takaki Sakamoto, Howon Kim, Takahiro Nishio, Toyoaki Eguchi, and Yukio Hasegawa

Using low-temperature scanning tunneling microscopy and spectroscopy, we have studied quantized magnetic fluxes (vortices) that penetrated nanosized superconductors with void structures inside. The void structures act as a pinning center for the vortices, and the trapped vortices are confined and sq...

[Phys. Rev. B 87, 195434] Published Mon May 20, 2013

We describe the evolution of the SU(4) Kondo effect as the number of magnetic centers increases from one impurity to the two-dimensional (2D) lattice. We derive a Hubbard-Anderson model which describes a 2D array of atoms or molecules with two-fold orbital degeneracy, acting as magnetic impurities and interacting with a metallic host. We calculate the differential conductance, observed typically in experiments of scanning tunneling spectroscopy, for different arrangements of impurities on a metallic surface: a single impurity, a periodic square lattice, and several sites of a rectangular cluster. Our results point towards the crucial importance of the orbital degeneracy and agree well with recent experiments in different systems of iron(II) phtalocyanine molecules deposited on top of Au(111) [N. Tsukahara et al., Phys. Rev. Lett. 106, 187201 (2011)], indicating that this would be the first experimental realization of an artificial 2D SU(4) Kondo-lattice system.

We study and classify the proximity-induced superconducting pairing in a topological insulator (TI)-superconductor (SC) hybrid structure for SCs with different symmetries. The Dirac surface state gives a coupling between spin-singlet and spin-triplet pairing amplitudes as well as pairing that is odd in frequency for p-wave SCs. We also find that all SCs induce pairing that is odd in both frequency and orbital (band) index, with oddness in frequency and orbital index being completely interchangeable. The different induced pairing amplitudes significantly modifies the density of states in the TI surface layer.

An information-theoretic principle implies that any discrete physical theory is classical

Nature Communications 4, 1851 (2013). doi:10.1038/ncomms2821

Authors: Corsin Pfister & Stephanie Wehner

Lab life: Don't bristle at blunders

Nature 497, 7449 (2013). doi:10.1038/497309a

Author: Mario Livio

Embrace mistakes, urges Mario Livio — they are portals to scientific progress.

Here we report the direct observation of single electron charging of a single atomic Dangling Bond (DB) on the H-Si(100) 2x1 surface. The tip of a scanning tunneling microscope is placed adjacent to the DB to serve as a single electron sensitive charge-detector. Three distinct charge states of the dangling bond, positive, neutral, and negative, are discerned. Charge state probabilities are extracted from the data, and analysis of current traces reveals the characteristic single electron charging dynamics. Filling rates are found to decay exponentially with increasing tip-DB separation, but are not a function of sample bias, while emptying rates show a very weak dependence on tip position, but a strong dependence on sample bias, consistent with the notion of an atomic quantum dot tunnel coupled to the tip on one side and the bulk silicon on the other.

In this paper, we present the first non-contact atomic force microscopy (nc-AFM) of a silicene on a silver (Ag) surface, obtained by combining non-contact atomic force microscopy and scanning tunneling microscopy (STM). STM images over large areas of silicene grown on the Ag(111) surface show both (√13 × √13)R13.9° and (4 × 4) superstructures. For the widely observed (4 × 4) structure, the observed nc-AFM image is very similar to the one recorded by STM. The structure resolved by nc-AFM is compatible with only one out of two silicon atoms being visible. This indicates unambiguously a strong buckling of the silicene honeycomb layer.

We have found a perpendicular magnetic anisotropy of iron adatoms on a surface of the prototypical three-dimensional topological insulator Bi 2 Se 3 by using x-ray magnetic circular dichroism measurements. The orbital magnetic moment of Fe is strongly enhanced at lower coverage, where angle-resolved photoemission spectroscopy shows coexistence of non-trivial topological states at the surface.

Symmetry study deemed a fraud

Nature 497, 7448 (2013). http://www.nature.com/doifinder/10.1038/497170a

Author: Eugenie Samuel Reich

University finds evidence of fakery in Jamaican dance data.

Author(s): M. Urdampilleta, S. Klyatskaya, M. Ruben, and W. Wernsdorfer

A multiterminal device based on a carbon nanotube quantum dot was used at very low temperature to probe a single electronic and nuclear spin embedded in a bis-(phthalocyaninato) terbium (III) complex (TbPc₂). A spin-valve signature with large conductance jumps was found when two molecules were stron...

[Phys. Rev. B 87, 195412] Published Thu May 09, 2013

The massless charge carriers in graphene interact with highly charged defects to create an analog of atomic collapse states.

Authors: Yang Wang, Dillon Wong, Andrey V. Shytov, Victor W. Brar, Sangkook Choi, Qiong Wu, Hsin-Zon Tsai, William Regan, Alex Zettl, Roland K. Kawakami, Steven G. Louie, Leonid S. Levitov, Michael F. Crommie

An analytical framework bolstered by attosecond spectroscopy conveys a clear understanding of asymmetric spectral line shapes. [Also see Perspective by Lin and Chu]

Authors: Christian Ott, Andreas Kaldun, Philipp Raith, Kristina Meyer, Martin Laux, Jörg Evers, Christoph H. Keitel, Chris H. Greene, Thomas Pfeifer

Individual Fe atoms on a Cu2N/Cu(100) surface exhibit strong magnetic anisotropy due to the crystal field. Using atom manipulation in a low-temperature STM we demonstrate that the anisotropy of one Fe atom is significantly influenced by local strain due to a second Fe atom placed nearby. Depending on the relative positions of the two atoms on the Cu2N lattice we can controllably enhance or reduce the uniaxial anisotropy. We present a model that explains the observed behavior qualitatively in terms of first principles.

Author(s): S. Di Napoli, A. Weichselbaum, P. Roura-Bas, A. A. Aligia, Y. Mokrousov, and S. Blügel

We calculate the conductance as a function of temperature G(T) through Au monatomic chains containing one Co atom as a magnetic impurity, and connected to two conducting leads with a fourfold symmetry axis. Using the information derived from ab initio calculations, we construct an effective model Ĥ_...

[Phys. Rev. Lett. 110, 196402] Published Mon May 06, 2013

Author(s): Z. F. Wang, Zheng Liu, and Feng Liu

The quantum anomalous Hall effect (QAHE) is a fundamental transport phenomenon in the field of condensed-matter physics. Without an external magnetic field, spontaneous magnetization combined with spin-orbit coupling gives rise to a quantized Hall conductivity. So far, a number of theoretical propos...

[Phys. Rev. Lett. 110, 196801] Published Mon May 06, 2013

Magnetic monopoles have provided a rich field of study, leading to a wide area of research in particle physics, solid state physics, ultra-cold gases, superconductors, cosmology, and gauge theory. So far, no true magnetic monopoles were found experimentally. Using the Aharonov-Bohm effect, one of the central results of quantum physics, shows however, that an effective monopole field can be produced. Understanding the effects of such a monopole field on its surroundings is crucial to its observation and provides a better grasp of fundamental physical theory. We realize the diffraction of fast electrons at a magnetic monopole field generated by a nanoscopic magnetized ferromagnetic needle. Previous studies have been limited to theoretical semiclassical optical calculations of the motion of electrons in such a monopole field. Solid state systems like the recently studied 'spin ice' provide a constrained system to study similar fields, but make it impossible to separate the monopole from the material. Free space diffraction helps to understand the dynamics of the electron-monopole system without the complexity of a solid state system. The use of a simple object such as a magnetized needle will allow various areas of physics to use the general dynamical effects of monopole fields without requiring a monopole particle or specific solids which have internal monopole-like properties. The experiment performed here shows that even without a true magnetic monopole particle, the theoretical background on monopoles serves as a basis for experiments and can be applied to efficiently create electron vortices. Various predictions about angular momentum and general field effects can readily be studied using the available equipment. This realization provides insights for the scientific community on how to detect magnetic monopoles in high energy collisions, cosmological processes, or novel materials.