Dr.jens.brede

Author(s): F. Donati, A. Singha, S. Stepanow, C. Wäckerlin, J. Dreiser, P. Gambardella, S. Rusponi, and H. Brune

We investigated the magnetic properties of individual Ho atoms adsorbed on the (111) surface of Pt, which have been recently claimed to display single ion magnetic behavior. By combining x-ray absorption spectroscopy and magnetic dichroism measurements with ligand field multiplet calculations, we re...

[Phys. Rev. Lett. 113, 237201] Published Wed Dec 03, 2014

Each year, $1.4 trillion are invested in research by governments, foundations, and corporations. Hundreds if not thousands of high-profile prizes and medals are awarded to the best researchers, boosting their careers. Therefore, establishing a reliable predictor of future performance is a trillion-dollar matter. Last month, the Alexander von Humboldt Foundation convened an international assembly of leaders in academia, research management, and policy to discuss “Beyond Bibliometrics: Identifying the Best.” Current assessment is largely based on counting publications, counting citations, taking note of the impact factor of the journals where researchers publish, and derivatives of these such as the h-index. These approaches were severely criticized for numerous reasons, with shortcomings particularly apparent when assessing young scientists for prestigious, interdisciplinary awards. It is time to develop more appropriate measures and to use the scientific method itself to help in this endeavor. Author: Marcia McNutt

By merging bottom-up and top-down strategies we tailor graphene's electronic properties within nanometer accuracy, which opens up the possibility to design optical and plasmonic circuitries at will. In a first step, graphene electronic properties are macroscopically modified exploiting the periodic potential generated by the self assembly of metal cluster superlattices on a graphene/Ir(111) surface. We then demonstrate that individual metal clusters can be selectively removed by a STM tip with perfect reproducibility and that the structures so created are stable even at room temperature. This enables one to nanopattern circuits down to the 2.5 nm only limited by the periodicity of the Moiré-pattern, i.e., by the distance between neighbouring clusters, and different electronic and optical properties should prevail in the covered and uncovered regions. The method can be carried out on micro-meter-sized regions with clusters of different materials permitting to tune the strength of the periodic potential.

Scientific Reports 4 doi: 10.1038/srep07314

Author(s): Philipp Leicht, Julia Tesch, Samuel Bouvron, Felix Blumenschein, Philipp Erler, Luca Gragnaniello, and Mikhail Fonin

We report on low-temperature scanning tunneling spectroscopy measurements on epitaxial graphene flakes on Au(111). We show that using quasiparticle interference (QPI) mapping, we can discriminate between the electronic systems of graphene and Au(111). Beyond the scattering vectors, which can be ascr...

[Phys. Rev. B 90, 241406] Published Wed Dec 03, 2014

We consider the interplay between magnetic skyrmions in an insulating thin film and the Dirac surface states of a 3D topological insulator (TI), coupled by proximity effect. The magnetic texture of skyrmions can lead to confinement of Dirac states at the skyrmion radius, where out of plane magnetization vanishes. This confinement can result in charging of the skyrmion texture. The presence of bound states is robust in an external magnetic field, which is needed to stabilize skyrmions. It is expected that for relevant experimental parameters skyrmions will have a few bound states that can be tuned using an external magnetic field. We argue that these charged skyrmions can be manipulated directly by an electric field, with skyrmion mobility proportional to the number of bound states at the skyrmion radius. Coupling skyrmionic thin films to a TI surface can provide a more direct and efficient way of controlling skyrmion motion in insulating materials. This provides a new dimension in the study of skyrmion manipulation.

Abstract

We propose that constructing a molecule super-lattice on a superconducting ultrathin film is a promising way to manipulate superconductivity in experiment. We theoretically study superconductivity in a molecule graphene system, which is built by fabricating a hexagonal molecule super-lattice on 2-dimensional electron gas. The super-lattice potential dramatically changes the electron density of states, which oscillates as function of the energy. We show that such a molecular graphene may increase superconducting gap by a few times, which may open a new route to realize high temperature superconductivity.

Nitrogen doping is a promising method of engineering the electronic structure of a metal oxide to modify its optical and electrical properties; however, the doping effect strongly depends on the types of defects introduced. Herein, we report a comparative study of nitrogen-doping-induced defects in Cu2O. Even in the lightly doped samples, a considerable number of nitrogen interstitials (Ni) formed, accompanied by nitrogen substitutions (NO) and oxygen vacancies (VO). In the course of high-temperature annealing, these Ni atoms interacted with VO, resulting in an increase in NO and decreases in Ni and VO. The properties of the annealed sample were significantly modified as a result. Our results suggest that Ni is a significant defect type in nitrogen-doped Cu2O.

Scientific Reports 4 doi: 10.1038/srep07240

Near-surface two-dimensional electron gases on the topological insulator Bi$_2$Te$_2$Se are induced by electron doping and studied by angle-resolved photoemission spectroscopy. A pronounced spin-orbit splitting is observed for these states. The $k$-dependent splitting is strongly anisotropic to a degree where a large splitting ($\approx 0.06$ \AA$^{-1}$) can be found in the $\bar{\Gamma}\bar{M}$ direction while the states are hardly split along $\bar{\Gamma}\bar{K}$. The direction of the anisotropy is found to be qualitatively inconsistent with results expected for a third-order anisotropic Rashba Hamiltonian. However, a $\mathbf{k} \cdot \mathbf{p}$ model that includes the possibility of band structure anisotropy as well as both isotropic and anisotropic third order Rashba splitting can explain the results. The isotropic third order contribution to the Rashba Hamiltonian is found to be negative, reducing the energy splitting at high $k$. The interplay of band structure, higher order Rashba effect and tuneable doping offers the opportunity to engineer not only the size of the spin-orbit splitting but also its direction.

Article

Friedel oscillations are ripples in the electron density surrounding a charge impurity. Bouhassoune et al . now use first-principle calculations to show that Friedel oscillation surrounding an oxygen impurity in a ferromagnetic film can be engineered and amplified by choice of substrate and film thickness

Nature Communications doi: 10.1038/ncomms6558

Authors: Mohammed Bouhassoune, Bernd Zimmermann, Phivos Mavropoulos, Daniel Wortmann, Peter H. Dederichs, Stefan Blügel, Samir Lounis

Author(s): D. Pacilé, S. Lisi, I. Di Bernardo, M. Papagno, L. Ferrari, M. Pisarra, M. Caputo, S. K. Mahatha, P. M. Sheverdyaeva, P. Moras, P. Lacovig, S. Lizzit, A. Baraldi, M. G. Betti, and C. Carbone

Photoemission, from core levels and valence band, and low-energy electron diffraction (LEED) have been employed to investigate the electronic and structural properties of graphene-ferromagnetic (G-FM) systems, obtained by intercalation of one monolayer (1 ML) and several layers (4 ML) of Co on G gro...

[Phys. Rev. B 90, 195446] Published Wed Nov 26, 2014

Author(s): Marco Cammarata, Roman Bertoni, Maciej Lorenc, Hervé Cailleau, Sergio Di Matteo, Cindy Mauriac, Samir F. Matar, Henrik Lemke, Matthieu Chollet, Sylvain Ravy, Claire Laulhé, Jean-François Létard, and Eric Collet

We study the basic mechanisms allowing light to photoswitch at the molecular scale a spin-crossover material from a low- to a high-spin state. Combined femtosecond x-ray absorption performed at LCLS X-FEL and optical spectroscopy reveal that the structural stabilization of the photoinduced high-spin...

[Phys. Rev. Lett. 113, 227402] Published Wed Nov 26, 2014

Author(s): Y. R. Song, Y. Y. Zhang, F. Yang, K. F. Zhang, Canhua Liu, Dong Qian, C. L. Gao, S. B. Zhang, and Jin-Feng Jia

A self-assembled iron(II) phthalocyanine single layer adsorbed on the topological insulator Bi₂Te₃ was investigated by spin-polarized scanning tunneling microscopy and density functional theory calculations. Although the molecule-substrate interaction is dominated by a relatively weak van der Waals ...

[Phys. Rev. B 90, 180408] Published Tue Nov 25, 2014

Abstract

For potential applications in spintronics and quantum computing, it is desirable to place a quantum spin Hall insulator [i.e., a 2D topological insulator (TI)] on a substrate while maintaining a large energy gap. Here, we demonstrate a unique approach to create the large-gap 2D TI state on a semiconductor surface, based on first-principles calculations and effective Hamiltonian analysis. We show that when heavy elements with strong spin orbit coupling (SOC) such as Bi and Pb atoms are deposited on a patterned H-Si(111) surface into a hexagonal lattice, they exhibit a 2D TI state with a large energy gap of ≥0.5 eV. The TI state arises from an intriguing substrate orbital filtering effect that selects a suitable orbital composition around the Fermi level, so that the system can be matched onto a four-band effective model Hamiltonian. Furthermore, it is found that within this model, the SOC gap does not increase monotonically with the increasing strength of SOC. These interesting results may shed new light in future design and fabrication of large-gap topological quantum states.

Scientific Reports 4 doi: 10.1038/srep07102

Author(s): Panjin Kim, Kyeong Tae Kang, Gyungchoon Go, and Jung Hoon Han

Tight-binding models for the recently observed surface electronic bands of SrTiO3 and KTaO3 are analyzed with a view to bringing out the relevance of momentum-space chiral angular momentum structures of both orbital and spin characters. The orbital and the accompanying spin angular momentum structur...

[Phys. Rev. B 90, 205423] Published Tue Nov 18, 2014

A system of two exchange-coupled Kondo impurities in a magnetic field gives rise to a rich phase space hosting a multitude of correlated phenomena. Magnetic atoms on surfaces probed through scanning tunnelling microscopy provide an excellent platform to investigate coupled impurities, but typical high Kondo temperatures prevent field-dependent studies from being performed, rendering large parts of the phase space inaccessible. We present an integral study of pairs of Co atoms on insulating Cu2N/Cu(100), which each have a Kondo temperature of only 2.6 K. In order to cover the different regions of the phase space, the pairs are designed to have interaction strengths similar to the Kondo temperature. By applying a sufficiently strong magnetic field, we are able to access a new phase in which the two coupled impurities are simultaneously screened. Comparison of differential conductance spectra taken on the atoms to simulated curves, calculated using a third order transport model, allows us to independently determine the degree of Kondo screening in each phase.

Interfacial mode coupling as the origin of the enhancement of Tc in FeSe films on SrTiO3

Nature 515, 7526 (2014). doi:10.1038/nature13894

Authors: J. J. Lee, F. T. Schmitt, R. G. Moore, S. Johnston, Y.-T. Cui, W. Li, M. Yi, Z. K. Liu, M. Hashimoto, Y. Zhang, D. H. Lu, T. P. Devereaux, D.-H. Lee & Z.-X. Shen

Films of iron selenide (FeSe) one unit cell thick grown on strontium titanate (SrTiO3 or STO) substrates have recently shown superconducting energy gaps opening at temperatures close to the boiling point of liquid nitrogen (77 kelvin), which is a record for the iron-based superconductors. The gap opening temperature usually sets the superconducting transition temperature Tc, as the gap signals the formation of Cooper pairs, the bound electron states responsible for superconductivity. To understand why Cooper pairs form at such high temperatures, we examine the role of the SrTiO3 substrate. Here we report high-resolution angle-resolved photoemission spectroscopy results that reveal an unexpected characteristic of the single-unit-cell FeSe/SrTiO3 system: shake-off bands suggesting the presence of bosonic modes, most probably oxygen optical phonons in SrTiO3 (refs 5, 6, 7), which couple to the FeSe electrons with only a small momentum transfer. Such interfacial coupling assists superconductivity in most channels, including those mediated by spin fluctuations. Our calculations suggest that this coupling is responsible for raising the superconducting gap opening temperature in single-unit-cell FeSe/SrTiO3.

Experimental realization of the topological Haldane model with ultracold fermions

Nature 515, 7526 (2014). doi:10.1038/nature13915

Authors: Gregor Jotzu, Michael Messer, Rémi Desbuquois, Martin Lebrat, Thomas Uehlinger, Daniel Greif & Tilman Esslinger

The Haldane model on a honeycomb lattice is a paradigmatic example of a Hamiltonian featuring topologically distinct phases of matter. It describes a mechanism through which a quantum Hall effect can appear as an intrinsic property of a band structure, rather than being caused by an external magnetic field. Although physical implementation has been considered unlikely, the Haldane model has provided the conceptual basis for theoretical and experimental research exploring topological insulators and superconductors. Here we report the experimental realization of the Haldane model and the characterization of its topological band structure, using ultracold fermionic atoms in a periodically modulated optical honeycomb lattice. The Haldane model is based on breaking both time-reversal symmetry and inversion symmetry. To break time-reversal symmetry, we introduce complex next-nearest-neighbour tunnelling terms, which we induce through circular modulation of the lattice position. To break inversion symmetry, we create an energy offset between neighbouring sites. Breaking either of these symmetries opens a gap in the band structure, which we probe using momentum-resolved interband transitions. We explore the resulting Berry curvatures, which characterize the topology of the lowest band, by applying a constant force to the atoms and find orthogonal drifts analogous to a Hall current. The competition between the two broken symmetries gives rise to a transition between topologically distinct regimes. By identifying the vanishing gap at a single Dirac point, we map out this transition line experimentally and quantitatively compare it to calculations using Floquet theory without free parameters. We verify that our approach, which allows us to tune the topological properties dynamically, is suitable even for interacting fermionic systems. Furthermore, we propose a direct extension to realize spin-dependent topological Hamiltonians.

We investigate how the electron-vibron coupling influences electron transport via an anisotropic magnetic molecule, such as a single-molecule magnet (SMM) Fe$_4$, by using a model Hamiltonian with parameter values obtained from density-functional theory (DFT). Magnetic anisotropy parameters, vibrational energies, and electron-vibron coupling strengths of the Fe$_4$ are computed using DFT. A giant spin model is applied to the Fe$_4$ with only two charge states, specifically a neutral state with the total spin $S=5$ and a singly charged state with $S=9/2$, which is consistent with our DFT result and experiments on Fe$_4$ single-molecule transistors. In sequential electron tunneling, we find that the magnetic anisotropy gives rise to new features in conductance peaks arising from vibrational excitations. In particular, the peak height shows a strong, unusual dependence on the direction as well as magnitude of applied B field. The magnetic anisotropy also introduces vibrational satellite peaks whose position and height are modified with the direction and magnitude of applied B field. Furthermore, when multiple vibrational modes with considerable electron-vibron coupling have energies close to one another, a low-bias current is suppressed, independently of gate voltage and applied B field, although that is not the case for a single mode with the similar electron-vibron coupling. In the former case, the conductance peaks reveal a stronger B-field dependence than in the latter case. The new features appear because the magnetic anisotropy barrier is of the same order of magnitude as the energies of vibrational modes with significant electron-vibron coupling. Our findings clearly show the interesting interplay between magnetic anisotropy and electron-vibron coupling in electron transport via the Fe$_4$. The similar behavior can be observed in transport via other anisotropic magnetic molecules.

Author(s): B. F. Miao, L. Sun, Y. W. Wu, X. D. Tao, X. Xiong, Y. Wen, R. X. Cao, P. Wang, D. Wu, Q. F. Zhan, B. You, J. Du, R. W. Li, and H. F. Ding

We report the creation of an artificial skyrmion crystal, which is configurable reliably at room temperature. The samples are fabricated by embedding lithography-patterned arrays of micron-sized Co disks onto Co/Pt multilayer films that have perpendicular magnetic anisotropy. Kerr microscopy and mag...

[Phys. Rev. B 90, 174411] Published Tue Nov 11, 2014

Article

One-dimensional boundaries in lateral heterostructures of two-dimensional materials are expected to have interesting properties. Park et al. probe the electronic properties of the graphene/hexagonal-boron-nitride interface, revealing the spatial and energetic distributions of one-dimensional boundary states.

Nature Communications doi: 10.1038/ncomms6403

Authors: Jewook Park, Jaekwang Lee, Lei Liu, Kendal W. Clark, Corentin Durand, Changwon Park, Bobby G. Sumpter, Arthur P. Baddorf, Ali Mohsin, Mina Yoon, Gong Gu, An-Ping Li