Dr.jens.brede

The ability to detect and distinguish quantum interference signatures is important for both fundamental research and for the realization of devices including electron resonators, interferometers and interference-based spin filters. Consistent with the principles of subwavelength optics, the wave nature of electrons can give rise to various types of interference effects, such as Fabry-P\'erot resonances, Fano resonances and the Aharonov-Bohm effect. Quantum-interference conductance oscillations have indeed been predicted for multiwall carbon nanotube shuttles and telescopes, and arise from atomic-scale displacements between the inner and outer tubes. Previous theoretical work on graphene bilayers indicates that these systems may display similar interference features as a function of the relative position of the two sheets. Experimental verification is, however, still lacking. Graphene nanoconstrictions represent an ideal model system to study quantum transport phenomena due to the electronic coherence and the transverse confinement of the carriers. Here, we demonstrate the fabrication of bowtie-shaped nanoconstrictions with mechanically controlled break junctions (MCBJs) made from a single layer of graphene. We find that their electrical conductance displays pronounced oscillations at room temperature, with amplitudes that modulate over an order of magnitude as a function of sub-nanometer displacements. Surprisingly, the oscillations exhibit a period larger than the graphene lattice constant. Charge-transport calculations show that the periodicity originates from a combination of quantum-interference and lattice-commensuration effects of two graphene layers that slide across each other. Our results provide direct experimental observation of Fabry-P\'erot-like interference of electron waves that are partially reflected/transmitted at the edges of the graphene bilayer overlap region.

The use of magnetic insulators is attracting a lot of interest due to a rich variety of spin-dependent phenomena with potential applications to spintronic devices. Here we report ultra-thin yttrium iron garnet (YIG) / gadolinium iron garnet (GdIG) insulating bilayers on gadolinium iron garnet (GGG). From spin Hall magnetoresistance (SMR) and X-ray magnetic circular dichroism measurements, we show that the YIG and GdIG magnetically couple antiparallel even in moderate in-plane magnetic fields. The results demonstrate an all-insulating equivalent of a synthetic antiferromagnet in a garnet-based thin film heterostructure and could open new venues for insulators in magnetic devices. As an example, we demonstrate a memory element with orthogonal magnetization switching that can be read by SMR.

Single adatoms are expected to participate in many processes occurring at solid surfaces, such as the growth of graphene on metals. We demonstrate, both experimentally and theoretically, the catalytic role played by single metal adatoms during the technologically relevant process of graphene growth on nickel (Ni). The catalytic action of individual Ni atoms at the edges of a growing graphene flake was directly captured by scanning tunneling microscopy imaging at the millisecond time scale, while force field molecular dynamics and density functional theory calculations rationalize the experimental observations. Our results unveil the mechanism governing the activity of a single-atom catalyst at work.

Author(s): Jian Li, Sangjun Jeon, Yonglong Xie, Ali Yazdani, and B. Andrei Bernevig

Magnetic atomic chains grown on top of conventional superconductors may form an interesting and promising platform for the study of Majorana zero modes. Despite the strong evidence already provided with a state-of-art scanning tunneling microscope (STM) technique, clear features beyond the energetic and spatial ones are required to exclude the only remaining alternative interpretation of the zero modes as trivial Shiba states accidentally occurring at zero energy. The authors here propose a robust spin signature for this purpose. This signature is rooted in two sum rules that dictate the distribution of spin densities in a superconducting state with respect to a normal state, and has been recently detected with spin-polarized STM technique that implicitly takes advantage of the sum rules.

[Phys. Rev. B 97, 125119] Published Thu Mar 15, 2018

Author(s): G. Berruto, I. Madan, Y. Murooka, G. M. Vanacore, E. Pomarico, J. Rajeswari, R. Lamb, P. Huang, A. J. Kruchkov, Y. Togawa, T. LaGrange, D. McGrouther, H. M. Rønnow, and F. Carbone

Experiments on the optical writing and erasing of magnetic skyrmions in FeGe indicate that more efficient skyrmion generation can be achieved in a cooled sample.

[Phys. Rev. Lett. 120, 117201] Published Wed Mar 14, 2018

Author(s): Nadine Hauptmann, Melanie Dupé, Tzu-Chao Hung, Alexander K. Lemmens, Daniel Wegner, Bertrand Dupé, and Alexander A. Khajetoorians

We image simultaneously the geometric, the electronic, and the magnetic structures of a buckled iron bilayer film that exhibits chiral magnetic order. We achieve this by combining spin-polarized scanning tunneling microscopy and magnetic exchange force microscopy (SPEX) to independently characterize...

[Phys. Rev. B 97, 100401(R)] Published Fri Mar 09, 2018

Long coherence times of single spins in silicon quantum dots make these systems highly attractive for quantum computation, but how to scale up spin qubit systems remains an open question. As a first step to address this issue, we demonstrate the strong coupling of a single electron spin and a single microwave photon. The electron spin is trapped in a silicon double quantum dot, and the microwave photon is stored in an on-chip high-impedance superconducting resonator. The electric field component of the cavity photon couples directly to the charge dipole of the electron in the double dot, and indirectly to the electron spin, through a strong local magnetic field gradient from a nearby micromagnet. Our results provide a route to realizing large networks of quantum dot–based spin qubit registers.

In this paper, we theoretically investigate the highest possible expected performance for graphene nanoribbon field effect transistors (GNRFETs) for a wide range of operation voltages and device structure parameters, such as the width of the graphene nanoribbon and gate length. We formulated a self-consistent, non-equilibrium Green’s function method in conjunction with the Poisson equation and modeled the operation of nanometer sized GNRFETs, of which GNR channels have finite bandgaps so that the GNRFET can operate as a switch. We propose a metric for competing with the current silicon CMOS high performance or low power devices and explain that this can vary greatly depending on the GNRFET structure parameters.

On-surface synthesis has rapidly emerged as a most promising approach to prepare functional molecular structures directly on a support surface. Compared to solution synthesis, performing chemical reactions on a surface offers several exciting new options: due to the absence of a solvent, reactions can be envisioned that are otherwise not feasible due to the insolubility of the reaction product. Perhaps even more important, the confinement to a two-dimensional surface might enable reaction pathways that are not accessible otherwise. Consequently, on-surface synthesis has attracted great attention in the last decade, with an impressive number of classical reactions transferred to a surface as well as new reactions demonstrated that have no classical analogue. So far, the majority of the work has been carried out on conducting surfaces. However, when aiming for electronic decoupling of the resulting structures, e.g. for the use in future molecular electronic devices, non-con...

Author(s): Zlatko Papić, Roger S. K. Mong, Ali Yazdani, and Michael P. Zaletel

A scanning tunneling microscope might detect unambiguous signatures of anyons in graphene.

[Phys. Rev. X 8, 011037] Published Tue Mar 06, 2018

The observation of surface phonon dispersion using local probes can provide important information related to local structural and thermal properties. In this study, surface phonon modes on a Cu(100) surface were measured using the inelastic tunneling spectroscopy of scanning tunneling microscopy (STM-IETS) with atomically sharp tips. Different phonon modes were selectively measured depending on the structures of the probing tips or the surfaces. Two different surface phonon modes, at 19.0 meV on a clean Cu(100) surface and at 13.5 meV on an oxygen-adsorbed Cu(100) surface, are explained by the selection rules. Additionally, the spatial variation in STM-IETS showed surface stress relaxation.

Author(s): H. Wu, L. Huang, C. Fang, B. S. Yang, C. H. Wan, G. Q. Yu, J. F. Feng, H. X. Wei, and X. F. Han

Three new transistors for spin-based currents may lead to a new type of circuitry that is faster and more efficient than traditional electronics.

[Phys. Rev. Lett. 120, 097205] Published Fri Mar 02, 2018

Author(s): L. J. Cornelissen, J. Liu, B. J. van Wees, and R. A. Duine

Three new transistors for spin-based currents may lead to a new type of circuitry that is faster and more efficient than traditional electronics.

[Phys. Rev. Lett. 120, 097702] Published Fri Mar 02, 2018

Author(s): Christopher E. Patrick, Santosh Kumar, Geetha Balakrishnan, Rachel S. Edwards, Martin R. Lees, Leon Petit, and Julie B. Staunton

Magnetocrystalline anisotropy, the microscopic origin of permanent magnetism, is often explained in terms of ferromagnets. However, the best performing permanent magnets based on rare earths and transition metals (RE-TM) are in fact ferrimagnets, consisting of a number of magnetic sublattices. Here ...

[Phys. Rev. Lett. 120, 097202] Published Wed Feb 28, 2018

Author(s): Ivo A. Maceira and Frédéric Mila

Motivated by the recent observation that exponentially long coherence times can be achieved for edge spins in models with strong zero modes, we study the impact of level crossings in finite-length spin chains on the dynamics of the edge spins. Focusing on the XY spin-1/2 chain with a transverse or l...

[Phys. Rev. B 97, 064424] Published Wed Feb 28, 2018

Author(s): Y. Zhou, L. Miao, P. Wang, F. F. Zhu, W. X. Jiang, S. W. Jiang, Y. Zhang, B. Lei, X. H. Chen, H. F. Ding, Hao Zheng, W. T. Zhang, Jin-feng Jia, Dong Qian, and D. Wu

Single monolayer FeSe film grown on a Nb-doped SrTiO3(001) substrate shows the highest superconducting transition temperature (TC∼100  K) among the iron-based superconductors (iron pnictides), while the TC value of bulk FeSe is only ∼8  K. Although bulk FeSe does not show antiferromagnetic order, ca...

[Phys. Rev. Lett. 120, 097001] Published Tue Feb 27, 2018

Abstract

Author(s): Dorsa Komijani, Alberto Ghirri, Claudio Bonizzoni, Svetlana Klyatskaya, Eufemio Moreno-Pineda, Mario Ruben, Alessandro Soncini, Marco Affronte, and Stephen Hill

The use of spin-bearing organic linkers (radicals) as a means of mediating magnetic interactions to lanthanide ions has become of recent interest within the molecular nanomagnetism community. For example, charge transport through organic ligands provides a means of addressing electron and nuclear quantum states associate with lanthanide containing single-molecule devices. In this work, a neutral terbium bis-phthalocyaninato metalorganic complex, (TbPc2)0, was studied using an angle-resolved, single-crystal high-frequency electron paramagnetic resonance technique. The results provide important insights into the anisotropic coupling between the unpaired spin density delocalized over the coordinating Pc2 radical and the Ising moment associated with the Tb ion.

[Phys. Rev. Materials 2, 024405] Published Fri Feb 23, 2018

Author(s): Koji Miyamoto, Hirokazu Miyahara, Kenta Kuroda, Takamasa Maegawa, Akio Kimura, and Taichi Okuda

The spin texture of the spin-split surface states on Bi(111) has been comprehensively investigated by high energy and angular resolution spin- and angle-resolved photoelectron spectroscopy. A large out-of-plane spin component that alternately changes its sign is clearly observed. There is no evidenc...

[Phys. Rev. B 97, 085433] Published Fri Feb 23, 2018

Abstract

The graphene and phosphorene nanostructures have a big potential application in a large area of actuals research in physics. However, their methods of synthesis still do not allow the production of perfect materials with an intact molecular structure. In this paper, the occurrence of atomic vacancies was considered in the edge structure of the zigzag phosphorene and graphene nanoribbons. For different concentrations of these edge vacancies, their influence on the metallic properties was investigated. The calculations were performed for different sizes of the unit cell. Furthermore, for a smaller size, the influence of a uniform magnetic field was added.