Dr.jens.brede

We have used an efficient new quantum mechanical method for radical pair recombination reactions to study the spin-dependent charge recombination along PTZ$^{\bullet+}$--Ph$_n$--PDI$^{\bullet-}$ molecular wires. By comparing our results to the experimental data of E. Weiss {\em et al.} [J. Am. Chem. Soc. {\bf 126}, 5577 (2004)], we are able to extract the spin-dependent (singlet and triplet) charge recombination rate constants for wires with $n=2-5$. These spin-dependent rate constants have not been extracted previously from the experimental data because they require fitting its magnetic field-dependence to the results of quantum spin dynamics simulations. We find that the triplet recombination rate constant decreases exponentially with the length of the wire, consistent with the superexchange mechanism of charge recombination. However, the singlet recombination rate constant is nearly independent of the length of the wire, suggesting that the singlet pathway is dominated by an incoherent hopping mechanism. A simple qualitative explanation for the different behaviours of the two spin-selective charge recombination pathways is provided in terms of Marcus theory. We also find evidence for a magnetic field-independent background contribution to the triplet yield of the charge recombination reaction, and suggest several possible explanations for it. Since none of these explanations is especially compelling given the available experimental evidence, and since the result appears to apply more generally to other molecular wires, we hope that this aspect of our study will stimulate further experimental work.

Author(s): G. Drera, C. Cepek, L. L. Patera, F. Bondino, E. Magnano, S. Nappini, C. Africh, A. Lodi-Rizzini, N. Joshi, P. Ghosh, A. Barla, S. K. Mahatha, S. Pagliara, A. Giampietri, C. Pintossi, and L. Sangaletti

The graphene-Ni(111) (GrNi) growth via chemical vapor deposition has been explored by resonant, angle-resolved, and dichroic photoemission spectroscopy (soft x-ray Res-ARPES) in order to identify the possible contributions to the electronic structure deriving from different phases that can coexist i...

[Phys. Rev. B 96, 165442] Published Mon Oct 30, 2017

Author(s): Alexander Streltsov, Gerardo Adesso, and Martin B. Plenio

The dictum that “information is physical” indicates that we should understand how features of quantum physics, in particular, the phenomenon of quantum coherence, can be understood to be, and quantified as, a resource for the processing of information. This Colloquium discusses how to characterize, quantify, and manipulate quantum coherence, in application areas ranging from many-body and solid state physics to biological and nanoscale systems.

[Rev. Mod. Phys. 89, 041003] Published Mon Oct 30, 2017

Author(s): Jindong Ren, Haiming Guo, Jinbo Pan, Yan-Fang Zhang, Yifeng Yang, Xu Wu, Shixuan Du, Min Ouyang, and Hong-Jun Gao

Different interatomic spin interactions in graphene-regulated Mn atomic clusters are investigated by low-temperature scanning tunneling microscopy and magnetic-field-dependent inelastic spin excitation spectroscopy. All dimers observed exhibit an antiferromagnetic (AFM) singlet ground state and spin...

[Phys. Rev. Lett. 119, 176806] Published Fri Oct 27, 2017

Author(s): Chang Liu, Yunyi Zang, Wei Ruan, Yan Gong, Ke He, Xucun Ma, Qi-Kun Xue, and Yayu Wang

A topological variant of the Hall effect that relies on spin chirality is observed in a topological insulator at four quantum layers, but vanishes for fewer and more layers.

[Phys. Rev. Lett. 119, 176809] Published Fri Oct 27, 2017

Author(s): Jeison A. Fischer, Leonid M. Sandratskii, Soo-hyon Phark, Dirk Sander, and Stuart Parkin

We combine spin-polarized scanning tunneling microscopy (SP-STM) and first-principles calculations to demonstrate the control of the wavelength of helical spin textures in Fe nanoislands by varying their atomic structure. We make use of the complexity of submonolayer growth of Fe on Cu(111) to prepa...

[Phys. Rev. B 96, 140407(R)] Published Wed Oct 18, 2017

Experimental realization of two-dimensional Dirac nodal line fermions in monolayer Cu₂Si

Nature Communications, Published online: 18 October 2017; doi:10.1038/s41467-017-01108-z

Nodal line semimetals have been observed in three-dimensional materials but are missing in two-dimensional counterparts. Here, Feng et al. report two-dimensional Dirac nodal line fermions protected by mirror reflection symmetry in monolayer Cu₂Si.

Anomalous Kondo resonance mediated by semiconducting graphene nanoribbons in a molecular heterostructure

Nature Communications, Published online: 16 October 2017; doi:10.1038/s41467-017-00881-1

Semiconducting graphene nanoribbon provides a platform for band-gap engineering desired for electronic and optoelectronic applications. Here, Li et al. show that graphene nanoribbon can effectively mediate the interaction of molecular magnetic moment and electronic spin in underlying metallic substrates.

Experimental verification of the rotational type of chiral spin spiral structures by spin-polarized scanning tunneling microscopy

Scientific Reports, Published online: 16 October 2017; doi:10.1038/s41598-017-13329-9

Experimentally probing topological order and its breakdown through modular matrices

Nature Physics, Published online: 16 October 2017; doi:10.1038/nphys4281

Fundamental fingerprints of topological orders may be characterized uniquely and purely by experimental means. Here the authors provide a proof of principle demonstration using interferometric measurement in a two-dimensional lattice system.

Symmetry breaking by quantum coherence in single electron attachment

Nature Physics, Published online: 16 October 2017; doi:10.1038/nphys4289

Resonant electron attachment and subsequent dissociation of diatomic molecules is shown to exhibit spatial asymmetry as a consequence of coherent excitation and subsequent interference between reaction pathways.

Author(s): G. D. Scott and T.-C. Hu

We present low-temperature transport measurements of C60-based single-molecule transistors fabricated using ferromagnetic break junction devices with planar elliptical leads, revealing a gate-modulated single-channel spin-12 Kondo effect. The shape anisotropy and dipole interaction of the source and...

[Phys. Rev. B 96, 144416] Published Fri Oct 13, 2017

Layer-by-layer assembly of two-dimensional materials into wafer-scale heterostructures

Nature 550, 7675 (2017). doi:10.1038/nature23905

Authors: Kibum Kang, Kan-Heng Lee, Yimo Han, Hui Gao, Saien Xie, David A. Muller & Jiwoong Park

High-performance semiconductor films with vertical compositions that are designed to atomic-scale precision provide the foundation for modern integrated circuitry and novel materials discovery. One approach to realizing such films is sequential layer-by-layer assembly, whereby atomically thin two-dimensional building blocks are vertically stacked, and held together by van der Waals interactions. With this approach, graphene and transition-metal dichalcogenides—which represent one- and three-atom-thick two-dimensional building blocks, respectively—have been used to realize previously inaccessible heterostructures with interesting physical properties. However, no large-scale assembly method exists at present that maintains the intrinsic properties of these two-dimensional building blocks while producing pristine interlayer interfaces, thus limiting the layer-by-layer assembly method to small-scale proof-of-concept demonstrations. Here we report the generation of wafer-scale semiconductor films with a very high level of spatial uniformity and pristine interfaces. The vertical composition and properties of these films are designed at the atomic scale using layer-by-layer assembly of two-dimensional building blocks under vacuum. We fabricate several large-scale, high-quality heterostructure films and devices, including superlattice films with vertical compositions designed layer-by-layer, batch-fabricated tunnel device arrays with resistances that can be tuned over four orders of magnitude, band-engineered heterostructure tunnel diodes, and millimetre-scale ultrathin membranes and windows. The stacked films are detachable, suspendable and compatible with water or plastic surfaces, which will enable their integration with advanced optical and mechanical systems.

Non-collinear spin states with unique rotational sense, such as chiral spin-spirals, are recently heavily investigated because of advantages for future applications in spintronics and information technology and as potential hosts for Majorana Fermions when coupled to a superconductor. Tuning the properties of such spin states, e.g., the rotational period and sense, is a highly desirable yet difficult task. Here, we experimentally demonstrate the bottom-up assembly of a spin-spiral derived from a chain of Fe atoms on a Pt substrate using the magnetic tip of a scanning tunneling microscope as a tool. We show that the spin-spiral is induced by the interplay of the Heisenberg and Dzyaloshinskii-Moriya components of the Ruderman-Kittel-Kasuya-Yosida interaction between the Fe atoms. The relative strengths and signs of these two components can be adjusted by the interatomic Fe distance, which enables tailoring of the rotational period and sense of the spin-spiral.

Author(s): F. Finocchiaro, J. L. Lado, and J. Fernandez-Rossier

We study a graphene Hall probe located on top of a magnetic surface as a detector of skyrmions, using as a working principle the anomalous Hall effect produced by the exchange interaction of the graphene electrons with the noncoplanar magnetization of the skyrmion. We study the magnitude of the effe...

[Phys. Rev. B 96, 155422] Published Mon Oct 09, 2017

Organic spintronics: Window of opportunity

Nature Physics, Published online: 4 October 2017; doi:10.1038/nphys4284

A crystalline organic semiconductor that combines the long spin-relaxation times of organic semiconductors with the high charge-carrier mobilities typically found in inorganic semiconductors provides unprecedented prospects for organic spintronics.

Nature Nanotechnology 12, 953 (2017). doi:10.1038/nnano.2017.149

Authors: Zuocheng Zhang, Xiao Feng, Jing Wang, Biao Lian, Jinsong Zhang, Cuizu Chang, Minghua Guo, Yunbo Ou, Yang Feng, Shou-Cheng Zhang, Ke He, Xucun Ma, Qi-Kun Xue & Yayu Wang

The recent experimental observation of the quantum anomalous Hall effect has cast significant attention on magnetic topological insulators. In these magnetic counterparts of conventional topological insulators such as Bi2Te3, a long-range ferromagnetic state can be established by chemical doping with transition-metal elements. However, a much richer electronic phase diagram can emerge and, in the specific case of Cr-doped Bi2(SexTe1−x)3, a magnetic quantum phase transition tuned by the actual chemical composition has been reported. From an application-oriented perspective, the relevance of these results hinges on the possibility to manipulate magnetism and electronic band topology by external perturbations such as an electric field generated by gate electrodes—similar to what has been achieved in conventional diluted magnetic semiconductors. Here, we investigate the magneto-transport properties of Cr-doped Bi2(SexTe1−x)3 with different compositions under the effect of a gate voltage. The electric field has a negligible effect on magnetic order for all investigated compositions, with the remarkable exception of the sample close to the topological quantum critical point, where the gate voltage reversibly drives a ferromagnetic-to-paramagnetic phase transition. Theoretical calculations show that a perpendicular electric field causes a shift in the electronic energy levels due to the Stark effect, which induces a topological quantum phase transition and, in turn, a magnetic phase transition.

Precise engineering of quantum dot array coupling through their barrier widths

Nature Communications, Published online: 5 October 2017; doi:10.1038/s41467-017-00872-2

Arrays of quantum dots can exhibit a variety of quantum properties, being sensitive to their spacing. Here, the authors fine tune interdot coupling using hexagonal molecular networks in which the dots are separated by single or double haloaromatic compounds, structurally identical but for a single atom.