Dr.jens.brede

The layered antiferromagnetism of parallel nanowire (NW) arrays self-assembled on Si(110) have been observed at room temperature by direct imaging of both the topographies and magnetic domains using spin-polarized scanning tunneling microscopy/spectroscopy (SP-STM/STS). The topographic STM images reveal that the self-assembled unidirectional and parallel NiSi NWs grow into the Si(110) substrate along the ##IMG## [http://ej.iop.org/images/0957-4484/29/9/095706/nanoaaa6eaieqn1.gif] {$[\bar{1}10]$} direction (i.e. the endotaxial growth) and exhibit multiple-layer growth. The spatially-resolved SP-STS maps show that these parallel NiSi NWs of different heights produce two opposite magnetic domains, depending on the heights of either even or odd layers in the layer stack of the NiSi NWs. This layer-wise antiferromagnetic structure can be attributed to an antiferromagnetic interlayer exchange coupling between the adjacent layers in the multiple-layer NiSi NW with...

Author(s): Zhen-Hua Wang, Eduardo V. Castro, and Hai-Qing Lin

Graphene nanoribbons with armchair edges are studied for externally enhanced but realistic parameter values: enhanced Rashba spin-orbit coupling due to proximity to a transition-metal dichalcogenide, such as WS2, and enhanced Zeeman field due to exchange coupling with a magnetic insulator, such as E...

[Phys. Rev. B 97, 041414(R)] Published Tue Jan 30, 2018

Author(s): Sylvain Ravets, Patrick Knüppel, Stefan Faelt, Ovidiu Cotlet, Martin Kroner, Werner Wegscheider, and Atac Imamoglu

Quantum Hall polaritons in the strong coupling regime are studied experimentally in GaAs. These polaritons are expected to be useful in implementing a photonic quantum simulator.

[Phys. Rev. Lett. 120, 057401] Published Tue Jan 30, 2018

Spin-Orbit Effects on the Dynamical Properties of Polarons in Graphene Nanoribbons

Spin-Orbit Effects on the Dynamical Properties of Polarons in Graphene Nanoribbons, Published online: 30 January 2018; doi:10.1038/s41598-018-19893-y

Spin-Orbit Effects on the Dynamical Properties of Polarons in Graphene Nanoribbons

Certain lattice wave systems in translationally invariant settings have one or more spectral bands that are strictly flat or independent of momentum in the tight binding approximation, arising from either internal symmetries or fine-tuned coupling. These flat bands display remarkable strongly-interacting phases of matter. Originally considered as a theoretical convenience useful for obtaining exact analytical solutions of ferromagnetism, flat bands have now been observed in a variety of settings, ranging from electronic systems to ultracold atomic gases and photonic devices. Here we review the design and implementation of flat bands and chart future directions of this exciting field.

A large enough piece of ferromagnet is usually not magnetized uniformly, but develops a magnetization texture. In thin films these textures can be doubly-periodic. Such are the well known magnetic bubble domains and the recently observed "skyrmion" magnetization textures in MnSi. In this paper we develop a theory of periodic magnetization textures, based on complex calculus to answer the question -- is there a difference between those two textures even if they seem to carry the same topological winding number (or topological charge) ? We find that such difference exists, facilitated by a different role played by the magnetization vector's in-plane phase. We separate classical-like and quantum-like features of magnetization textures and highlight the role of magnetic anisotropy in favouring either of these cases.

In addition to the well-known anti-ferromagnetic and ferromagnetic edge states in zigzag graphene nanoribbons (GNR), we find that there also exist some excited spin density wave (ESDW) states, the energies of which are close to the anti-ferromagnetic state (ground state). We thus argue that these ESDW states may coexist in experiment. Our numerical results from the self-consistent mean-field method as well as the first-principles calculations indicate that the allowed ESDWs are commensurate; and their dispersion curves are linear in the long wave limit. The coupling of the two edge portions in an ESDW becomes very weak in a wide GNR, in which case these ESDW portions may have different phases. Finally our calculations in the non-equilibrium Green’s function theory combined with the Hubbard model show that these ESDW states can also exist in some localized (middle or terminal) region of GNR.

Recent experiments demonstrate a temperature control of the electric conduction through a ferrocene-based molecular junction. Here we examine the results in view of determining means to distinguish between transport through single-particle molecular levels or via transport channels split by Coulomb repulsion. Both transport mechanisms are similar in molecular junctions given the similarities between molecular intralevel energies and the charging energy. We propose an experimentally testable way to identify the main transport process. By applying a magnetic field to the molecule, we observe that an interacting theory predicts a shift of the conductance resonances of the molecule whereas in the noninteracting case each resonance is split into two peaks. The interaction model works well in explaining our experimental results obtained in a ferrocene-based single-molecule junction, where the charge degeneracy peaks shift (but do not split) under the action of an applied 7-Tesla magnetic field. This method is useful for a proper characterization of the transport properties of molecular tunnel junctions.

A lucid Fourier analysis based description of the photoemission process is presented that directly relates photon energy (hv) dependent ARPES response of two-dimensional (2D) electron states to their wavefunctions. The states formed by quantum confinement of bulk Bloch waves (including Shockley-Tamm type surface and interface states, and quantum-well states) show periodic peaks of ARPES intensity as a function of hv. Amplitudes of these peaks reflect Fourier series of the oscillating Bloch-wave component of the wavefunction, and their broadening spatial confinement of its envelope function. In contrast, the 2D formed by local orbitals (dangling bonds and defects at the surface or interface) show aperiodic hv-dependence, where the rate of decay reflects localization of these states in the out-of-plane direction. This formalism sets up a straightforward methodology to access fundamental properties of different 2D states, as illustrated by analysis of previous photoemission experimental data including the paradigm Al(100) surface state, quantum-well states in multilayer graphene and at the buried GaAlN/GaN interface, and molecular orbitals.

The detection of fluorescence with submolecular resolution enables the exploration of spatially varying photon yields and vibronic properties at the single-molecule level. By placing individual polycyclic aromatic hydrocarbon molecules into the plasmon cavity formed by the tip of a scanning tunneling microscope and a NaCl-covered Ag(111) surface, molecular light emission spectra are obtained that unravel vibrational progression. In addition, light spectra unveil a signature of the molecule even when the tunneling current is injected well separated from the molecular emitter. This signature exhibits a distance-dependent Fano profile that reflects the subtle interplay between inelastic tunneling electrons, the molecular exciton and localized plasmons in at-distance as well as on-molecule fluorescence. The presented findings open the path to luminescence of a different class of molecules than investigated before and contribute to the understanding of single-molecule luminescence at surfaces in a unified picture.

Author(s): N. P. Armitage, E. J. Mele, and Ashvin Vishwanath

In recent years many three-dimensional crystals have been discovered whose low energy electronic properties are described by the Dirac or Weyl equations for relativistic fermions. This leads to many unusual physical properties and potentially to new applications. This review explains the theory behind these developments, their material realizations, and the current experimental status.

[Rev. Mod. Phys. 90, 015001] Published Mon Jan 22, 2018

Author(s): Johannes Schöneberg, Paolo Ferriani, Stefan Heinze, Alexander Weismann, and Richard Berndt

Spin-orbit coupling (SOC) links spin space and real space and leads to solids with intriguing spin topologies and transport properties, such as anisotropic magnetoresistance (AMR). The orientation of a real-space symmetry axis with respect to the spin direction determines the size of SOC-induced changes of the electronic structure. To show this effect at the single-molecule level, the authors arrange Pb dimers on a ferromagnetic Fe layer and observe that the AMR resulting from their molecular orbitals depends strongly on the dimer orientation.

[Phys. Rev. B 97, 041114(R)] Published Mon Jan 22, 2018

Abstract

Ultrathin iron oxide films epitaxially grown on the (111)- and (0001)-oriented metal single crystal supports exhibit unique electronic, catalytic and magnetic properties not observed for the corresponding bulk oxides. These properties originate mainly from the presence of Moir\'e superstructures which, in turn, disqualify ultrathin films as model systems imitating bulk materials. We present a route for the preparation of a close-packed Moir\'e-free ultrathin iron oxide film, namely FeO(111) on Ag(111). Experimental scanning tunneling microscopy (STM), low energy electron diffraction (LEED) and x-ray photoelectron spectroscopy (XPS) results confirm perfect structural order in the film. Density functional theory (DFT)-based calculations suggest full relaxation of the oxide layer that adopts the atomic lattice of the crystalline support and exhibits properties similar to those of a free-standing FeO. The results open new pathways for model-type studies of electronic, catalytic and magnetic properties of fully-relaxed iron oxide films and related systems.

Author(s): Jean-Yves Chauleau, William Legrand, Nicolas Reyren, Davide Maccariello, Sophie Collin, Horia Popescu, Karim Bouzehouane, Vincent Cros, Nicolas Jaouen, and Albert Fert

Chirality in condensed matter has recently become a topic of the utmost importance because of its significant role in the understanding and mastering of a large variety of new fundamental physical mechanisms. Versatile experimental approaches, capable to reveal easily the exact winding of order para...

[Phys. Rev. Lett. 120, 037202] Published Thu Jan 18, 2018

Author(s): Daniel Braithwaite, Dai Aoki, Jean-Pascal Brison, Jacques Flouquet, Georg Knebel, Ai Nakamura, and Alexandre Pourret

In most unconventional superconductors, like the high-Tc cuprates, iron pnictides, or heavy-fermion systems, superconductivity emerges in the proximity of an electronic instability. Identifying unambiguously the pairing mechanism remains nevertheless an enormous challenge. Among these systems, the o...

[Phys. Rev. Lett. 120, 037001] Published Wed Jan 17, 2018

The oxidation and spin state of a metal-organic molecule determine its chemical reactivity and magnetic properties. Here, we demonstrate the reversible control of the oxidation and spin state in a single Fe-porphyrin molecule in the force field of the tip of a scanning tunneling microscope. Within the regimes of half-integer and integer spin state, we can further track the evolution of the magnetocrystalline anisotropy. Our experimental results are corroborated by density functional theory and wave function theory. This combined analysis allows us to draw a complete picture of the molecular states over a large range of intramolecular deformations.

Author(s): Timofey Balashov, Christian Karlewski, Tobias Märkl, Gerd Schön, and Wulf Wulfhekel

Magnetic excitations of single atoms on surfaces have been widely studied experimentally in the past decade. Lately, systems with unprecedented magnetic stability started to emerge. Here, we present a general theoretical investigation of the stability of rare-earth magnetic atoms exposed to crystal ...

[Phys. Rev. B 97, 024412] Published Fri Jan 12, 2018

Author(s): Q. L. Li, C. Zheng, R. Wang, B. F. Miao, R. X. Cao, L. Sun, D. Wu, Y. Z. Wu, S. C. Li, B. G. Wang, and H. F. Ding

We address the long-term controversy on the fundamental question of the role of the surface state on the Kondo effect with Co adatoms on a Ag(111) surface. The width of the Kondo resonance oscillates with the same period of half Fermi wavelength of the surface state. But the amplitude increases for ...

[Phys. Rev. B 97, 035417] Published Fri Jan 12, 2018

In normal metals, the magnetic moment of impurity spins disappears below a characteristic Kondo temperature, TK, where coupling with the conduction-band electrons produces an entangled state that screens the local moment. In contrast, moments embedded in insulators remain unscreened at all temperatures. This raises the question about the fate of magnetic moments in intermediate, pseudogap systems, such as graphene. In these systems theory predicts a quantum phase-transition at a critical coupling strength which separates a local magnetic-moment phase from a Kondo screened phase. However, attempts to experimentally confirm these predictions and their intriguing consequences such as the ability to electrostatically control magnetic moments, have thus far been elusive. Here we report the observation of Kondo screening and the quantum phase-transition between screened and unscreened phases of vacancy magnetic-moments in graphene. Using scanning tunneling microscopy (STM), spectroscopy (STS) and numerical renormalization group (NRG) calculations, we identified Kondo screening by its spectroscopic signature and mapped the phase-transition as a function of coupling strength and chemical potential. We show that this transition makes it possible to turn the magnetic-moment on and off electrostatically through a gate voltage or mechanically through variations in local curvature.