Dr.thedudeman

Motivated by the striking promise of quantum computation, Majorana bound states (MBSs) in solid-state systems have attracted wide attention in recent years. In particular, the wavefunction localization of MBSs is a key feature and crucial for their future implementation as qubits. Here, we investigate the spatial and electronic characteristics of topological superconducting chains of iron atoms on the surface of Pb(110) by combining scanning tunneling microscopy (STM) and atomic force microscopy (AFM). We demonstrate that the Fe chains are mono-atomic, structured in a linear fashion, and exhibit zero-bias conductance peaks at their ends which we interprete as signature for a Majorana bound state. Spatially resolved conductance maps of the atomic chains reveal that the MBSs are well localized at the chain ends (below 25 nm), with two localization lengths as predicted by theory. Our observation lends strong support to use MBSs in Fe chains as qubits for quantum computing devices.

We report on a temperature-induced transition from a conventional semiconductor to a two-dimensional topological insulator investigated by means of magnetotransport experiments on HgTe/CdTe quantum well structures. At low temperatures, we are in the regime of the quantum spin Hall effect and observe an ambipolar quantized Hall resistance by tuning the Fermi energy through the bulk band gap. At room temperature, we find electron and hole conduction that can be described by a classical two-carrier model. Above the onset of quantized magnetotransport at low temperature, we observe a pronounced linear magnetoresistance that develops from a classical quadratic low-field magnetoresistance if electrons and holes coexist. Temperature-dependent bulk band structure calculations predict a transition from a conventional semiconductor to a topological insulator in the regime where the linear magnetoresistance occurs.

The control of non-equilibrium phenomena in complex solids is an important research frontier, encompassing new effects like light induced superconductivity. Here, we show that coherent optical excitation of molecular vibrations in the organic conductor K3C60 can induce a non-equilibrium state with the optical properties of a superconductor. A transient gap in the real part of the optical conductivity and a low-frequency divergence of the imaginary part are measured for base temperatures far above equilibrium Tc=20 K. These findings underscore the role of coherent light fields in inducing emergent order.

Two new thiol compounds with {\sigma}-{\pi}-{\sigma} structure were synthesized and self-assembled on gold substrates. The morphology and the structural characterization of SAMs assessed by infrared spectroscopy, contact angle, XPS, electrochemistry and scanning tunneling microscopy (STM) show the formation of monolayers. SAMs with a terthiophene (3TSH) core as conjugated system are much better organized compared to those with a naphthalene carbodiimide (NaphSH) core as demonstrated by the cyclic voltammetry and STM studies. The surface concentration of 3TSH and NaphSH is respectively three and six times lower than ordered SAMs of pure alkyl chains. A large number of I/V characteristics have been studied either by STS measurements on gold substrates or by C-AFM on gold nanodots. Transition Voltage Spectroscopy (TVS) was used to clearly identify the transport in these partially organized monolayers. The chemical nature of the conjugated system, donor for 3TSH and acceptor for NaphSH, involves an opposite rectification associated to the asymmetrical coupling of the molecular orbitals and the electrodes. The conductance histograms show that the 3TSH junctions are less dispersed than those of NaphSH junctions. This is explained by a better control of the molecular organization in the molecular junctions.

Author(s): Jun-Ping Peng, Jia-Qi Guan, Hui-Min Zhang, Can-Li Song, Lili Wang, Ke He, Qi-Kun Xue, and Xu-Cun Ma

Molecular beam epitaxy is used to grow TiSe₂ ultrathin films on a graphitized SiC(0001) substrate. TiSe₂ films proceed via a nearly layer-by-layer growth mode and exhibit two dominant types of defects, identified as Se vacancy and interstitial, respectively. By means of scanning tunneling microscopy...

[Phys. Rev. B 91, 121113] Published Tue Mar 31, 2015

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.

The idea to utilize not only the charge but also the spin of electrons in the operation of electronic devices has led to the development of spintronics, causing a revolution in how information is stored and processed. A novel advancement would be to develop ultrafast spintronics using femtosecond laser pulses. Employing terahertz (10$^{12}$ Hz) emission spectroscopy, we demonstrate optical generation of spin-polarized electric currents at the interfaces of metallic ferromagnetic heterostructures at the femtosecond timescale. The direction of the photocurrent is controlled by the helicity of the circularly polarized light. These results open up new opportunities for realizing spintronics in the unprecedented terahertz regime and provide new insights in all-optical control of magnetism.

To study the magnetic dynamics of superparamagnetic nanoparticles we use scanning probe relaxometry and dephasing of the nitrogen-vacancy (NV) center in diamond, characterizing the spin-noise of a single 10-nm magnetite particle. Additionally, we show the anisotropy of the NV sensitivity's dependence on the applied decoherence measurement method. By comparing the change in relaxation (T 1 ) and dephasing (T 2 ) time in the NV center when scanning a nanoparticle over it, we are able to extract the nanoparticle's diameter and distance from the NV center using an Ornstein-Uhlenbeck model for the nanoparticle's fluctuations. This scanning-probe technique can be used in the future to characterize different spin label substitutes for both medical applications and basic magnetic nanoparticle behavior.

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.

We report scanning tunneling microscopy (STM) and spectroscopy (STS) measurements of monolayer and bilayer WSe$_2$. We measure a band gap of 2.21 $\pm$ 0.08 eV in monolayer WSe$_2$, which is much larger than the energy of the photoluminescence peak indicating a large excitonic binding energy. We additionally observe significant electronic scattering arising from atomic-scale defects. Using Fourier transform STS (FT-STS), we map the energy versus momentum dispersion relations for monolayer and bilayer WSe$_2$. Further, by tracking allowed and forbidden scattering channels as a function of energy we infer the spin texture of both the conduction and valence bands. We observe a large spin-splitting of the valence band due to strong spin-orbit coupling, and additionally observe spin-valley-layer coupling in the conduction band of bilayer WSe$_2$.

Nature Physics 11, 307 (2015). doi:10.1038/nphys3242

Authors: Jacob Linder & Jason W. A. Robinson

Clusters built from individual iron atoms adsorbed on surfaces (adatoms) were investigated by atomic force microscopy (AFM) with subatomic resolution. Single copper and iron adatoms appeared as toroidal structures and multiatom clusters as connected structures, showing each individual atom as a torus. For single adatoms, the toroidal shape of the AFM image depends on the bonding symmetry of the adatom to the underlying structure [twofold for copper on copper(110) and threefold for iron on copper(111)]. Density functional theory calculations support the experimental data. The findings correct our previous work, in which multiple minima in the AFM signal were interpreted as a reflection of the orientation of a single front atom, and suggest that dual and triple minima in the force signal are caused by dimer and trimer tips, respectively. Authors: Matthias Emmrich, Ferdinand Huber, Florian Pielmeier, Joachim Welker, Thomas Hofmann, Maximilian Schneiderbauer, Daniel Meuer, Svitlana Polesya, Sergiy Mankovsky, Diemo Ködderitzsch, Hubert Ebert, Franz J. Giessibl

Author(s): Q. Dubout, F. Donati, C. Wäckerlin, F. Calleja, M. Etzkorn, A. Lehnert, L. Claude, P. Gambardella, and H. Brune

Cobalt atoms exposed to hydrogen gas have higher spins, an effect that could be used to build magnetic nanostructures and lattices.

[Phys. Rev. Lett. 114, 106807] Published Wed Mar 11, 2015

Author(s): Liwei Liu, Kai Yang, Yuhang Jiang, Boqun Song, Wende Xiao, Shiru Song, Shixuan Du, Min Ouyang, Werner A. Hofer, Antonio H. Castro Neto, and Hong-Jun Gao

The magnetic structure of a magnetic metal-organic complex on a gold surface can be mapped by exploring the field dependence of the extended Kondo effect.

[Phys. Rev. Lett. 114, 126601] Published Tue Mar 24, 2015

We combine scanning-tunneling-spectroscopy experiments probing magnetic impurities on a superconducting surface with a theoretical analysis of the tunneling processes between (superconducting) tip and substrate. We show that the current is carried by single-electron tunneling at large tip-substrate distances and Andreev reflections at smaller distances. The single-electron current requires relaxation processes between the impurity-induced Shiba bound state and the quasiparticle continuum, allowing us to extract information on such relaxation processes from our analysis.

Author(s): Kirsten von Bergmann, Markus Ternes, Sebastian Loth, Christopher P. Lutz, and Andreas J. Heinrich

Spin-resolved scanning tunneling microscopy is employed to quantitatively determine the spin polarization of the magnetic field-split Kondo state. Tunneling conductance spectra of a Kondo-screened magnetic atom are evaluated within a simple model taking into account inelastic tunneling due to spin e...

[Phys. Rev. Lett. 114, 076601] Published Fri Feb 20, 2015

Both quantum and classical behavior of single atomic spins on surfaces is determined by the local anisotropy of adatoms and their coupling to the immediate electronic environment. Yet adatoms seldom reside on surfaces alone and it is generally acknowledged that substrated-mediated interactions can couple single spins among each other impacting their magnetic behavior. Here we show that also magnetic anisotropy, which is usually considered to be a constant determined by the local crystal field, can be extremely sensitive to such interactions. By the example of Co dimers on Cu(001) and Pt(001) surfaces we highlight the intricate interplay of exchange coupling and magnetic anisotropy providing a much sought possibility to tune the latter through deliberate adjustment of the adatoms' separation. As a technologically relevant implication we demonstrate the impact of such emergent non-local anisotropy on the hysterectic properties of single-atom magnetization curves.

Author(s): Mikhail V. Kuznetsov, Lada V. Yashina, Jaime Sánchez-Barriga, Ilya I. Ogorodnikov, Andrey S. Vorokh, Andrey A. Volykhov, Roland J. Koch, Vera S. Neudachina, Marina E. Tamm, Anna P. Sirotina, Andrei Yu. Varykhalov, Gunther Springholz, Günther Bauer, John D. Riley, and Oliver Rader

Understanding how topologically protected surface states behave at surfaces and interfaces requires knowledge of the atomic structure. Whether the (111) surfaces of the prototypical topological insulators Bi₂Se₃ and Bi₂Te₃ are Bi or chalcogen terminated is the subject of current controversies. We em...

[Phys. Rev. B 91, 085402] Published Mon Feb 02, 2015

Nature Physics. doi:10.1038/nphys3201

Authors: G. Aivazian, Zhirui Gong, Aaron M. Jones, Rui-Lin Chu, J. Yan, D. G. Mandrus, Chuanwei Zhang, David Cobden, Wang Yao & X. Xu

Local energy extrema of the bands in momentum space, or valleys, can endow electrons in solids with pseudospin in addition to real spin. In transition metal dichalcogenides this valley pseudospin, like real spin, is associated with a magnetic moment that underlies the valley-dependent circular dichroism that allows optical generation of valley polarization, intervalley quantum coherence and the valley Hall effect. However, magnetic manipulation of valley pseudospin via this magnetic moment, analogous to what is possible with real spin, has not been shown before. Here we report observation of the valley Zeeman splitting and magnetic tuning of polarization and coherence of the excitonic valley pseudospin, by performing polarization-resolved magneto-photoluminescence on monolayer WSe2. Our measurements reveal both the atomic orbital and lattice contributions to the valley orbital magnetic moment; demonstrate the deviation of the band edges in the valleys from an exact massive Dirac fermion model; and reveal a striking difference between the magnetic responses of neutral and charged valley excitons that is explained by renormalization of the excitonic spectrum due to strong exchange interactions.

Author(s): Jill A. Miwa, Søren Ulstrup, Signe G. Sørensen, Maciej Dendzik, Antonija Grubišić Čabo, Marco Bianchi, Jeppe Vang Lauritsen, and Philip Hofmann

The electronic structure of epitaxial single-layer MoS₂ on Au(111) is investigated by angle-resolved photoemission spectroscopy. Pristine and potassium-doped layers are studied in order to gain access to the conduction band. The potassium-doped layer is found to have a (1.39±0.05)  eV direct band ga...

[Phys. Rev. Lett. 114, 046802] Published Thu Jan 29, 2015

Low temperature scanning tunneling microscopy is used to image the doped topological insulator Bi$_{2-x}$Fe$_x$Se$_3$. Interstitial Fe defects allow the detection of quasiparticle interference (QPI), and the reconstruction of the empty state band structure. Quantitative comparison between measured data and density functional theory calculations reveals the unexpected coexistence of quantum well states (QWS) with topological surface states (TSS) on the atomically clean surface of Bi$_{2-x}$Fe$_x$Se$_3$. Spectroscopic measurements quantify the breakdown of linear dispersion due to hexagonal warping. Nonetheless, both QWS and TSS remain spin-polarized and protected from backscattering to almost 1 eV above the Dirac point, suggesting their utility for spin-based applications.

We present a first-principles theory of dynamical spin excitations in the presence of spin-orbit coupling. The broken global spin rotational invariance leads to a new sum rule. We explore the competition between the magnetic anisotropy energy and the external magnetic field, as well as the role of electron-hole excitations, through calculations for 3$d$-metal adatoms on the Cu(111) surface. The spin excitation resonance energy and lifetime display non-trivial behavior, establishing the strong impact of relativistic effects. We legitimate the use of the Landau-Lifshitz-Gilbert equation down to the atomic limit, but with parameters that differ from a stationary theory.

Author(s): David MacNeill, Colin Heikes, Kin Fai Mak, Zachary Anderson, Andor Kormányos, Viktor Zólyomi, Jiwoong Park, and Daniel C. Ralph

Using polarization-resolved photoluminescence spectroscopy, we investigate the breaking of valley degeneracy by an out-of-plane magnetic field in back-gated monolayer MoSe₂ devices. We observe a linear splitting of -0.22  meV/T between luminescence peak energies in σ₊ and σ_- emission for both neutra...

[Phys. Rev. Lett. 114, 037401] Published Thu Jan 22, 2015

Author(s): W. Steurer, J. Repp, L. Gross, I. Scivetti, M. Persson, and G. Meyer

We show charge-state manipulation of single Au adatoms on 2–11 monolayer (ML) thick NaCl films on Cu surfaces by attaching or detaching single electrons via the tip of an atomic force microscope (AFM). Tristate charge control (neutral, negatively charged, and positively charged) is achieved. On Cu(1...

[Phys. Rev. Lett. 114, 036801] Published Thu Jan 22, 2015

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

Nature Physics. doi:10.1038/nphys3212

Authors: Fabian Schulz, Mari Ijäs, Robert Drost, Sampsa K. Hämäläinen, Ari Harju, Ari P. Seitsonen & Peter Liljeroth

Many-body effects arise from the collective behaviour of large numbers of interacting particles, for example, electrons, and the properties of such a system cannot be understood considering only single or non-interacting particles. Despite the generality of the many-body picture, there are only a few examples of experimentally observing such effects in molecular systems. Measurements of the local density of states of single molecules by scanning tunnelling spectroscopy is usually interpreted in terms of single-particle molecular orbitals. Here, we show that the simple single-particle picture fails qualitatively to account for the resonances in the tunnelling spectra of different charge states of cobalt phthalocyanine molecules. Instead, these resonances can be understood as a series of many-body excitations of the different ground states of the molecule. Our theoretical approach opens an accessible route beyond the single-particle picture in quantifying many-body states in molecules.