Dr.jens.brede

Nature Physics. doi:10.1038/nphys4047

Authors: Vedran Jelic, Krzysztof Iwaszczuk, Peter H. Nguyen, Christopher Rathje, Graham J. Hornig, Haille M. Sharum, James R. Hoffman, Mark R. Freeman & Frank A. Hegmann

Nature Physics. doi:10.1038/nphys4080

Authors: Robert Drost, Teemu Ojanen, Ari Harju & Peter Liljeroth

Topological materials exhibit protected edge modes that have been proposed for applications in, for example, spintronics and quantum computation. Although a number of such systems exist, it would be desirable to be able to test theoretical proposals in an artificial system that allows precise control over the key parameters of the model. The essential physics of several topological systems can be captured by tight-binding models, which can also be implemented in artificial lattices. Here, we show that this method can be realized in a vacancy lattice in a chlorine monolayer on a Cu(100) surface. We use low-temperature scanning tunnelling microscopy (STM) to fabricate such lattices with atomic precision and probe the resulting local density of states (LDOS) with scanning tunnelling spectroscopy (STS). We create analogues of two tight-binding models of fundamental importance: the polyacetylene (dimer) chain with topological domain-wall states, and the Lieb lattice with a flat electron band. These results provide an important step forward in the ongoing effort to realize designer quantum materials with tailored properties.

Nature Physics. doi:10.1038/nphys4105

Authors: Marlou R. Slot, Thomas S. Gardenier, Peter H. Jacobse, Guido C. P. van Miert, Sander N. Kempkes, Stephan J. M. Zevenhuizen, Cristiane Morais Smith, Daniel Vanmaekelbergh & Ingmar Swart

Geometry, whether on the atomic or nanoscale, is a key factor for the electronic band structure of materials. Some specific geometries give rise to novel and potentially useful electronic bands. For example, a honeycomb lattice leads to Dirac-type bands where the charge carriers behave as massless particles. Theoretical predictions are triggering the exploration of novel two-dimensional (2D) geometries, such as graphynes and the kagomé and Lieb lattices. The Lieb lattice is the 2D analogue of the 3D lattice exhibited by perovskites; it is a square-depleted lattice, which is characterized by a band structure featuring Dirac cones intersected by a flat band. Whereas photonic and cold-atom Lieb lattices have been demonstrated, an electronic equivalent in 2D is difficult to realize in an existing material. Here, we report an electronic Lieb lattice formed by the surface state electrons of Cu(111) confined by an array of carbon monoxide molecules positioned with a scanning tunnelling microscope. Using scanning tunnelling microscopy, spectroscopy and wavefunction mapping, we confirm the predicted characteristic electronic structure of the Lieb lattice. The experimental findings are corroborated by muffin-tin and tight-binding calculations. At higher energies, second-order electronic patterns are observed, which are equivalent to a super-Lieb lattice.

Nature Physics. doi:10.1038/nphys4110

Authors: Landry Bretheau, Joel I-Jan Wang, Riccardo Pisoni, Kenji Watanabe, Takashi Taniguchi & Pablo Jarillo-Herrero

A normal conductor placed in good contact with a superconductor can inherit its remarkable electronic properties. This proximity effect microscopically originates from the formation in the conductor of entangled electron–hole states, called Andreev states. Spectroscopic studies of Andreev states have been performed in just a handful of systems. The unique geometry, electronic structure and high mobility of graphene make it a novel platform for studying Andreev physics in two dimensions. Here we use a full van der Waals heterostructure to perform tunnelling spectroscopy measurements of the proximity effect in superconductor–graphene–superconductor junctions. The measured energy spectra, which depend on the phase difference between the superconductors, reveal the presence of a continuum of Andreev bound states. Moreover, our device heterostructure geometry and materials enable us to measure the Andreev spectrum as a function of the graphene Fermi energy, showing a transition between different mesoscopic regimes. Furthermore, by experimentally introducing a novel concept, the supercurrent spectral density, we determine the supercurrent–phase relation in a tunnelling experiment, thus establishing the connection between Andreev physics at finite energy and the Josephson effect. This work opens up new avenues for probing exotic topological phases of matter in hybrid superconducting Dirac materials.

Author(s): N. Néel, C. Steinke, T. O. Wehling, and J. Kröger

Wrinkles and blisters of graphene on Ir(111) give rise to remarkably high signals in inelastic electron tunneling spectroscopy due to graphene phonons. Spatially resolved spectra unravel the gradual increase of the graphene phonon signatures with progressive delamination of graphene from the metal s…

[Phys. Rev. B 95, 161410(R)] Published Fri Apr 28, 2017

Nature Physics. doi:10.1038/nphys4110

Authors: Landry Bretheau, Joel I-Jan Wang, Riccardo Pisoni, Kenji Watanabe, Takashi Taniguchi & Pablo Jarillo-Herrero

A normal conductor placed in good contact with a superconductor can inherit its remarkable electronic properties. This proximity effect microscopically originates from the formation in the conductor of entangled electron–hole states, called Andreev states. Spectroscopic studies of Andreev states have been performed in just a handful of systems. The unique geometry, electronic structure and high mobility of graphene make it a novel platform for studying Andreev physics in two dimensions. Here we use a full van der Waals heterostructure to perform tunnelling spectroscopy measurements of the proximity effect in superconductor–graphene–superconductor junctions. The measured energy spectra, which depend on the phase difference between the superconductors, reveal the presence of a continuum of Andreev bound states. Moreover, our device heterostructure geometry and materials enable us to measure the Andreev spectrum as a function of the graphene Fermi energy, showing a transition between different mesoscopic regimes. Furthermore, by experimentally introducing a novel concept, the supercurrent spectral density, we determine the supercurrent–phase relation in a tunnelling experiment, thus establishing the connection between Andreev physics at finite energy and the Josephson effect. This work opens up new avenues for probing exotic topological phases of matter in hybrid superconducting Dirac materials.

Author(s): Judith Donner, Jan-Philipp Broschinski, Bastian Feldscher, Anja Stammler, Hartmut Bögge, Thorsten Glaser, and Daniel Wegner

In an effort to improve the spin coupling in single-molecule magnets, we rationally designed a new building-block molecule with significantly enhanced spin coupling compared to a previously established molecule. We relate this to a stabilization of aromaticity in the central connecting carbon ring, …

[Phys. Rev. B 95, 165441] Published Mon Apr 24, 2017

Nature Physics. doi:10.1038/nphys4105

Authors: Marlou R. Slot, Thomas S. Gardenier, Peter H. Jacobse, Guido C. P. van Miert, Sander N. Kempkes, Stephan J. M. Zevenhuizen, Cristiane Morais Smith, Daniel Vanmaekelbergh & Ingmar Swart

Geometry, whether on the atomic or nanoscale, is a key factor for the electronic band structure of materials. Some specific geometries give rise to novel and potentially useful electronic bands. For example, a honeycomb lattice leads to Dirac-type bands where the charge carriers behave as massless particles. Theoretical predictions are triggering the exploration of novel two-dimensional (2D) geometries, such as graphynes and the kagomé and Lieb lattices. The Lieb lattice is the 2D analogue of the 3D lattice exhibited by perovskites; it is a square-depleted lattice, which is characterized by a band structure featuring Dirac cones intersected by a flat band. Whereas photonic and cold-atom Lieb lattices have been demonstrated, an electronic equivalent in 2D is difficult to realize in an existing material. Here, we report an electronic Lieb lattice formed by the surface state electrons of Cu(111) confined by an array of carbon monoxide molecules positioned with a scanning tunnelling microscope. Using scanning tunnelling microscopy, spectroscopy and wavefunction mapping, we confirm the predicted characteristic electronic structure of the Lieb lattice. The experimental findings are corroborated by muffin-tin and tight-binding calculations. At higher energies, second-order electronic patterns are observed, which are equivalent to a super-Lieb lattice.

Author(s): Maximilian Staab, Dmytro Kutnyakhov, Robert Wallauer, Sergey Chernov, Katerina Medjanik, Hans Joachim Elmers, Mathias Kläui, and Gerd Schönhense

The magnetic circular dichroism in threshold photoemission (TPMCD) for perpendicularly magnetized fcc Co films on Pt(111) has been revisited. A complete mapping of the spectral function I(EB,kx,ky) (binding energy EB, momentum parallel to surface kx, ky) and the corresponding TPMCD asymmetry distrib…

[Phys. Rev. B 95, 165437] Published Thu Apr 20, 2017

Author(s): R. S. Deacon, J. Wiedenmann, E. Bocquillon, F. Domínguez, T. M. Klapwijk, P. Leubner, C. Brüne, E. M. Hankiewicz, S. Tarucha, K. Ishibashi, H. Buhmann, and L. W. Molenkamp

Majorana particles, which are their own antiparticles, offer great potential for future quantum computers, but significant experimental challenges hamper proof of their existence and properties. New measurements of electrical supercurrents in an HgTe quantum well provide a way to gain insight into the induced superconductivity required for these experiments.

[Phys. Rev. X 7, 021011] Published Thu Apr 20, 2017

We report europium (Eu)-induced changes in the π -band of graphene (G) formed on the 6H-SiC(0001) surface by a combined study of photoemission measurements and density functional theory (DFT) calculations. Our photoemission data reveal that Eu intercalates upon annealing at 120 °C into the region between the graphene and the buffer layer (BL) to form a G/Eu/BL system, where a band gap of 0.29 eV opens at room temperature. This band gap is found to increase further to 0.48 eV upon cooling down to 60 K. Our DFT calculations suggest that the increased band gap originates from the enhanced hybridization of the graphene π- band with the Eu 4 f band due to the increased magnetic ordering upon cooling. These Eu atoms continue to intercalate further down below the BL to produce bilayer graphene (G/BL/Eu) upon annealing at 300 °C. The π -band stemming from the BL then exhibits another band gap of 0.37 eV, which appears to be due to the strong hybridization between...

Linear, suspended chains of magnetic atoms proximity coupled to an s-wave superconductor are predicted to host Majorana zero modes at the chain ends in the presence of strong spin-orbit coupling. Specifically, iron (Fe) chains on Pb(110) have been explored as a possible system to exhibit topological superconductivity and host Majorana zero-modes [Nadj-Perge, et al., Science 346, 602 (2014)]. Here, we study chains of the transition metal cobalt (Co) on Pb(110) and check for topological signatures. Using spin-polarized scanning tunneling spectroscopy, we resolve ferromagnetic order in the $d$ bands of the chains. Interestingly, also the subgap Yu-Shiba-Rusinov (YSR) bands carry a spin polarization as was predicted decades ago. Superconducting tips allow us to resolve further details of the YSR bands and in particular resonances at zero energy. We map the spatial distribution of the zero-energy signal and find it delocalized along the chain. Hence, despite of the ferromagnetic coupling within the chains and the strong-spin orbit coupling in the superconductor, we do not find clear evidence of Majorana modes. Simple tight-binding calculations suggest that the spin-orbit-split bands may cross the Fermi level four times which suppresses the zero-energy modes.

Enhanced superconductivity accompanying a Lifshitz transition in electron-doped FeSe monolayer

Nature Communications, Published online: 19 April 2017; doi:10.1038/ncomms14988

The origin of superconductivity enhancement in FeSe monolayer grown on SrTiO₃ compared to bulk FeSe is still a debated issue. Here, Shi et al. report a further 15 K jump of T_c accompanying a second Lifshitz transition triggered by electron doping in FeSe/SrTiO₃ monolayer films.

Nature Physics. doi:10.1038/nphys4124

Authors: Marc Gabay & Jean-Marc Triscone

Ferroelectricity and superconductivity do not have much in common. Now, a superconducting and a ferroelectric-like state have been found to coexist in a doped perovskite oxide.

Author(s): Patrik Hlobil, Jasmin Jandke, Wulf Wulfhekel, and Jörg Schmalian

Scanning tunneling microscopy has been shown to be a powerful experimental probe to detect electronic excitations and further allows us to deduce fingerprints of bosonic collective modes in superconductors. Here, we demonstrate that the inclusion of inelastic tunnel events is crucial for the interpr…

[Phys. Rev. Lett. 118, 167001] Published Mon Apr 17, 2017

Because the backbone of most organic molecules is composed primarily of carbon-carbon bonds, the development of efficient methods for their construction is one of the central challenges of organic synthesis. Transition metal–catalyzed cross-coupling reactions between organic electrophiles and nucleophiles serve as particularly powerful tools for achieving carbon-carbon bond formation. Until recently, the vast majority of cross-coupling processes had used either aryl or alkenyl electrophiles as one of the coupling partners. In the past 15 years, versatile new methods have been developed that effect cross-couplings of an array of alkyl electrophiles, thereby greatly expanding the diversity of target molecules that are readily accessible. The ability to couple alkyl electrophiles opens the door to a stereochemical dimension—specifically, enantioconvergent couplings of racemic electrophiles—that substantially enhances the already remarkable utility of cross-coupling processes. Authors: Junwon Choi, Gregory C. Fu

Understanding the binding mechanisms for aromatic molecules on transition-metal surfaces, especially with defects such as vacancies, steps and kinks, is a major challenge in designing functional interfaces for organic devices. One important parameter in the performance of organic/inorganic devices is the barrier of charge carrier injection. In the case of a metallic electrode, tuning the electronic interface potential or the work function for electronic level alignment is crucial. Here, we use density-functional theory (DFT) calculations with van der Waals (vdW) interactions treated with both screened pairwise (vdW surf ) and many-body dispersion (MBD) methods, to systematically study the interactions of benzene with a variety of stepped surfaces. Our calculations confirm the physisorptive character of Ag(2 1 1), Ag(5 3 3), Ag(3 2 2), Ag(7 5 5) and Ag(5 4 4) surfaces upon the adsorption of benzene. The MBD effects reduce the adsorption energies by about 0.15 eV per mole...

Strongly bound excitons in anatase TiO₂ single crystals and nanoparticles

Nature Communications, Published online: 13 April 2017; doi:10.1038/s41467-017-00016-6

Here the authors combine steady-state angle-resolved photoemission spectroscopy, ellipsometry and ultrafast two-dimensional ultraviolet spectroscopy to examine the role of many-body correlations in anatase TiO₂, revealing the existence of strongly bound excitons in single crystals and nanoparticles.

A promising route to the realization of Majorana fermions is in non-centrosymmetric superconductors, in which spin-orbit-coupling lifts the spin degeneracy of both bulk and surface bands. A detailed assessment of the electronic structure is critical to evaluate their suitability for this through establishing the topological properties of the electronic structure. This requires correct identification of the time-reversal-invariant momenta. One such material is BiPd, a recently rediscovered non-centrosymmetric superconductor which can be grown in large, high-quality single crystals and has been studied by several groups using angular resolved photoemission to establish its surface electronic structure. Many of the published electronic structure studies on this material are based on a reciprocal unit cell which is not the actual Brillouin zone of the material. We show here the consequences of this for the electronic structures and show how the inferred topological nature of the material is affected.

Topological superconductivity in monolayer transition metal dichalcogenides

Nature Communications, Published online: 11 April 2017; doi:10.1038/ncomms14985

Conditions to realize topological superconductivity have long been known, but the materialization remains rare. Here, Hsu et al. report a strategy towards possible topological superconductivity in monolayer hole-doped transition metal dichalcogenide by splitting the spin degeneracy in momentum space.