Dr.jens.brede

Electronic properties of triangular and hexagonal nano-scale quantum dots (QDs) of Silicene and bilayer graphene are studied. It is shown that the low-energy edge-localized electronic states, existing within the size-quantized gap are easily tunable by electric field. The appearance and field evolution of the electronic gap in these zero energy states (ZES) is shown to be very sensitive to QD geometry that permits to design the field-effect scalable QD devices with electronic properties on-demand.

In-plane heterostructure of monolayer hexagonal boron nitride (h-BN) and graphene is of great interest for its tunable bandgap and other unique properties. Here, we reveal a H2-induced etching process to introduce triangular hole in triangular-shaped chemical vapor deposited individual h-BN crystal. In this study, we synthesized regular triangular-shaped h-BN crystals with the sizes around 2-10 μm on Cu foil by chemical vapor deposition (CVD). The etching behavior of individual h-BN crystal was investigated by annealing at different temperature in an H2:Ar atmosphere. Annealing at 900 °C, etching of h-BN was observed from crystal edges with no visible etching at the center of individual crystals. While, annealing at a temperature ≥950 °C, highly anisotropic etching was observed, where the etched areas were equilateral triangle-shaped with same orientation as that of original h-BN crystal. The etching process and well-defined triangular hole formation can be significant platform to fabricate planar heterostructure with graphene or other two-dimensional (2D) materials.

Scientific Reports 5 doi: 10.1038/srep10426

Electrons incident from a normal metal onto a superconductor are reflected back as holes - a process called Andreev reflection. In a normal metal where the Fermi energy is much larger than a typical superconducting gap, the reflected hole retraces the path taken by the incident electron. In graphene with ultra low disorder, however, the Fermi energy can be tuned to be smaller than the superconducting gap. In this unusual limit, the holes are expected to be reflected specularly at the superconductor-graphene interface due to the onset of interband Andreev processes, where the effective mass of the reflected holes change sign. Here we present measurements of gate modulated Andreev reflections across the low disorder van der Waals interface formed between graphene and the superconducting NbSe2. We find that the conductance across the graphene-superconductor interface exhibits a characteristic suppression when the Fermi energy is tuned to values smaller than the superconducting gap, a hallmark for the transition between intraband retro- and interband specular- Andreev reflections.

In recent years inelastic spin-flip spectroscopy using a lowtemperature scanning tunneling microscope has been a very successful tool for studying not only individual spins but also complex coupled systems. When these systems interact with the electrons of the supporting substrate correlated manyparticle states can emerge, making them ideal prototypical quantum systems. The spin systems, which can be constructed by arranging individual atoms on appropriate surfaces or embedded in synthesized molecular structures, can reveal very rich spectral features. Up to now the spectral complexity has only been partly described. This manuscript shows that perturbation theory enables one to describe the tunneling transport, reproducing the differential conductance with surprisingly high accuracy. Well established scattering models, which include Kondo-like spin-spin and potential interactions, are expanded to enable calculation of arbitrary complex spin systems in reasonable time scale and the extraction of important physical properties. The emergence of correlations between spins and, in particular, between the localized spins and the supporting bath electrons are discussed and related to experimentally tunable parameters. These results might stimulate new experiments by providing experimentalists with an easily applicable modeling tool.

Author(s): L. Zhang, A. Bagrets, D. Xenioti, R. Korytár, M. Schackert, T. Miyamachi, F. Schramm, O. Fuhr, R. Chandrasekar, M. Alouani, M. Ruben, W. Wulfhekel, and F. Evers

We report a combined experimental and theoretical study of the Kondo effect in a series of binuclear metal-organic complexes of the form [(Me(hfacac)2)2(bpym)]0, with Me = nickel (II), manganese (II), zinc (II); hfacac = hexafluoroacetylacetonate, and bpym = bipyrimidine, adsorbed on Cu(100) surface...

[Phys. Rev. B 91, 195424] Published Mon May 18, 2015

Author(s): Oleksandr V. Pylypovskyi, Volodymyr P. Kravchuk, Denis D. Sheka, Denys Makarov, Oliver G. Schmidt, and Yuri Gaididei

We show that the interaction of the magnetic subsystem of a curved magnet with the magnet curvature results in the coupling of a topologically nontrivial magnetization pattern and topology of the object. The mechanism of this coupling is explored and illustrated by an example of a ferromagnetic Möbi…

[Phys. Rev. Lett. 114, 197204] Published Fri May 15, 2015

Utilizing spin-polarized scanning tunneling microscopy and spectroscopy, we found coexistence of perpendicularly and in-plane magnetized cobalt nanoscale islands on the Ag(111) surface, and the relationship between the moire corrugation amplitude and the magnetization direction of the islands; the islands with the stronger moire corrugation show the perpendicular magnetization, and the ones with the weaker moire corrugation do the in-plane. Density functional theory calculations reproduce the relationship and explain the differences between the two types of the islands with an fcc stacking fault in the intrinsic hcp stacking of cobalt.

There are many interests to achieve long-range magnetic order in topological insulators of Bi2Se3 or Bi2Te3 by doping magnetic transition metals such as Fe and Mn. The transition metals act as not only magnetic dopants but also electric dopants because they are usually divalent. However, if the doping elements are rare-earth metals such as Gd, which are trivalent, only magnetic moments can be introduced. We fabricated single crystals of Bi2-xGdxTe3 (0 ≤ × ≤ 0.2), in which we observed magnetic phase change from paramagnetic (PM) to antiferromagnetic (AFM) phase by increasing x. This PM-to-AFM phase transition agrees with the density functional theory calculations showing a weak and short-ranged Gd-Gd AFM coupling via the intervening Te ions. The critical point corresponding to the magnetic phase transition is x = 0.09, where large linear magnetoresistance and highly anisotropic Shubnikov-de Haas oscillations are observed. These results are discussed with two-dimensional properties of topological surface state electrons.

Scientific Reports 5 doi: 10.1038/srep10309

We report the achievement of the first atomically resolved scanning tunneling microscope (STM) imaging in a water-cooled magnet (WM), where the extremely harsh vibrations and noises have been the major challenge. This homebuilt WM-STM features an ultra-rigid and compact scan head in which the coarse approach is driven by our new design of the TunaDrive piezoelectric motor. A three-level spring hanging system is exploited for vibration isolation. Room-temperature raw-data images of graphite with quality atomic resolution were obtained in very high magnetic fields up to 27 T in a 32 mm bore WM whose absolute maximum field is 27.5 T at the power rating of 10 MW. This record of 27 T has exceeded the maximum field strength of the conventional superconducting magnets. Besides, our WM-STM has also paved the way to the STM imaging in the 45 T, 32 mm bore hybrid magnet, which is the world's flagship magnet and can produces the highest steady magnetic field at present.

Author(s): Kelvin J. A. Ooi, H. S. Chu, C. Y. Hsieh, Dawn T. H. Tan, and L. K. Ang

The discovery of light emission from metal-insulator-metal tunnel junctions in the 1970s suggested a low-energy, broadband source of visible light. Presently this technology is also of interest for generating subwavelength surface plasmons electrically. The authors show that inelastic electron tunneling excitation is potentially 105 times as efficient for producing mid-infrared plasmons in graphene than in metal, offering great promise for on-chip integrated nanophotonics.

[Phys. Rev. Applied 3, 054001] Published Fri May 08, 2015

We show that in the presence of ferromagnetic electronic reservoirs and spin-dependent tunnel couplings, molecular vibrations in nonmagnetic single molecular transistors induce an effective intramolecular exchange magnetic field. It generates a finite spin-accumulation and -precession for the electrons confined on the molecular bridge and occurs under (non)equilibrium conditions. The effective exchange magnetic field is calculated here to lowest order in the tunnel coupling for a nonequilibrium transport setup. Coulomb interaction between electrons is taken into account as well as a finite electron-phonon coupling. We show that for realistic physical parameters, an effective spin-phonon coupling emerges. It is induced by quantum many-body interactions, which are either electron-phonon or Coulomb-like. We investigate the precession and accumulation of the confined spins as function of bias- and gate-voltages as well as their dependence on the angle enclosed by the magnetizations between the left and right reservoir.

Single molecule magnets and single spin centers can be individually addressed when coupled to contacts forming an electrical junction. In order to control and engineer the magnetism of quantum devices, it is necessary to quantify how the structural and chemical environment of the junction affects the spin center. Metrics such as coordination number or symmetry provide a simple method to quantify the local environment, but neglect the many-body interactions of an impurity spin when coupled to contacts. Here, we utilize a highly corrugated hexagonal boron nitride (h-BN) monolayer to mediate the coupling between a cobalt spin in CoHx (x=1,2) complexes and the metal contact. While the hydrogen atoms control the total effective spin, the corrugation is found to smoothly tune the Kondo exchange interaction between the spin and the underlying metal. Using scanning tunneling microscopy and spectroscopy together with numerical simulations, we quantitatively demonstrate how the Kondo exchange interaction mimics chemical tailoring and changes the magnetic anisotropy.

Author(s): Yuxuan Wang and Andrey Chubukov

We present a comparative analysis of superconducting and charge-density-wave orders in the spin-fluctuation scenario for the cuprates. That spin-fluctuation exchange gives rise to d-wave superconductivity is well known. Several groups recently argued that the same spin-mediated interaction may also ...

[Phys. Rev. B 91, 195113] Published Mon May 11, 2015

The quantum anomalous Hall (QAH) effect is predicted to possess, at zero magnetic field, chiral edge channels that conduct spin polarized current without dissipation. While edge channels have been observed in previous experimental studies of the QAH effect, their dissipationless nature at a zero magnetic field has not been convincingly demonstrated. By a comprehensive experimental study of the gate and temperature dependences of local and nonlocal magnetoresistance, we unambiguously establish the dissipationless edge transport. By studying the onset of dissipation, we also identify the origin of dissipative channels and clarify the surprising observation that the critical temperature of the QAH effect is two orders of magnitude smaller than the Curie temperature of ferromagnetism.

Article

Molecular magnets are among the smallest structures that may be exploited for quantum information processing. Here, Guidi et al . use polarized neutron scattering to observe finite size effects and a noncollinear spin arrangement in a Cr 8 Cd ring molecule, an even-numbered open antiferromagnetic spin-3/2 chain.

Nature Communications doi: 10.1038/ncomms8061

Authors: T. Guidi, B. Gillon, S. A. Mason, E. Garlatti, S. Carretta, P. Santini, A. Stunault, R. Caciuffo, J. van Slageren, B. Klemke, A. Cousson, G. A. Timco, R. E. P. Winpenny

The design of high-finesse resonant cavities for electronic waves faces challenges due to short electron coherence lengths in solids. Complementing previous approaches to confine electronic waves by carefully positioned adatoms at clean metallic surfaces, we demonstrate an approach inspired by the peculiar acoustic phenomena in whispering galleries. Taking advantage of graphene’s gate-tunable light-like carriers, we create whispering-gallery mode (WGM) resonators defined by circular pn junctions, induced by a scanning tunneling probe. We can tune the resonator size and the carrier concentration under the probe in a back-gated graphene device over a wide range. The WGM-type confinement and associated resonances are a new addition to the quantum electron-optics toolbox, paving the way to develop electronic lenses and resonators. Authors: Yue Zhao, Jonathan Wyrick, Fabian D. Natterer, Joaquin F. Rodriguez-Nieva, Cyprian Lewandowski, Kenji Watanabe, Takashi Taniguchi, Leonid S. Levitov, Nikolai B. Zhitenev, Joseph A. Stroscio

Author(s): Adam Argondizzo, Xuefeng Cui, Cong Wang, Huijuan Sun, Honghui Shang, Jin Zhao, and Hrvoje Petek

We investigate the spectroscopy and photoinduced electron dynamics within the conduction band of reduced rutile TiO2(110) surface by multiphoton photoemission (mPP) spectroscopy with wavelength tunable ultrafast (∼20fs) laser pulse excitation. Tuning the mPP photon excitation energy between 2.9 and ...

[Phys. Rev. B 91, 155429] Published Mon Apr 27, 2015

Author(s): Can-Li Song, Lili Wang, Ke He, Shuai-Hua Ji, Xi Chen, Xu-Cun Ma, and Qi-Kun Xue

Scanning tunneling microscopy and spectroscopy have been used to investigate the femtosecond dynamics of Dirac fermions in the topological insulator Bi2Se3 ultrathin films. At the two-dimensional limit, bulk electrons become quantized and the quantization can be controlled by the film thickness at a...

[Phys. Rev. Lett. 114, 176602] Published Tue Apr 28, 2015

Author(s): Niklas Romming, André Kubetzka, Christian Hanneken, Kirsten von Bergmann, and Roland Wiesendanger

Atomic-scale imaging reveals the shape and size of a technologically interesting magnetic quasiparticle.

[Phys. Rev. Lett. 114, 177203] Published Fri May 01, 2015

Skyrmions are topologically protected spin textures, characterized by a topological winding number N , that occur spontaneously in some magnetic materials. Recent experiments have demonstrated the capability to grow graphene on top Fe/Ir, a system that exhibits a two dimensional Skyrmion lattice. Here we show that a weak exchange coupling between the Dirac electrons in graphene and a two dimensional Skyrmion lattice with $N = \pm 1$ drives graphene into a quantum anomalous Hall phase, with a band-gap in bulk, a Chern number ${\cal C}=2N$ and chiral edge states with perfect quantization of conductance $G = 2N\frac{e^2}{h}$ . Our findings imply that the topological properties of the Skyrmion lattice can be imprinted in the Dirac electrons of graphene.