Dr.jens.brede

Article

Electrons and holes trapped in a molecular crystal couple to form excitons. Here, the authors use scanning tunnelling microscopy to inject current with submolecular precision into structural defects in solid C 60 and demonstrate single photon emission from the excitons trapped there.

Nature Communications doi: 10.1038/ncomms9461

Authors: P. Merino, C. Große, A. Rosławska, K. Kuhnke, K. Kern

Author(s): D. Di Castro, C. Cantoni, F. Ridolfi, C. Aruta, A. Tebano, N. Yang, and G. Balestrino

CaCuO2/SrTiO3 heterostructures show interface superconductivity at 40K due to extra oxygen ions in the interface Ca plane, which act as doping holes.

[Phys. Rev. Lett. 115, 147001] Published Mon Sep 28, 2015

Author(s): S. Omar, M. Gurram, I. J. Vera-Marun, X. Zhang, E. H. Huisman, A. Kaverzin, B. L. Feringa, and B. J. van Wees

In graphene spintronics, interaction of localized magnetic moments with the electron spins paves a new way to explore the underlying spin-relaxation mechanism. A self-assembled layer of organic cobalt porphyrin (CoPP) molecules on graphene provides a desired platform for such studies via the magneti…

[Phys. Rev. B 92, 115442] Published Fri Sep 25, 2015

Tunneling spectroscopy of close-spaced dangling-bond pairs in Si(001):H

Scientific Reports, Published online: 25 September 2015; doi:10.1038/srep14496

We show that the spin-orbit torque induced magnetization switching in nanomagnets presenting Dzyaloshinskii-Moriya (DMI) interaction is governed by a chiral domain nucleation at the edges. The nucleation is induced by the DMI and the applied in-plane magnetic field followed by domain wall propagation. Our micromagnetic simulations show that the DC switching current can be defined as the edge nucleation current, which decreases strongly with increasing amplitude of the DMI. This description allows us to build a simple analytical model to quantitatively predict the switching current. We find that domain nucleation occurs down to a lateral size of 15 nm, defined by the length-scale of the DMI, beyond which the reversal mechanism approaches a macrospin behavior. The switching is deterministic and bipolar.

Author(s): Conor Hogan, Stefano Colonna, Roberto Flammini, Antonio Cricenti, and Fabio Ronci

Using a combination of scanning tunneling microscopy (STM) imaging and total-energy calculations within density functional theory, we evaluate a range of new and existing reconstruction models for Si nanoribbons that form on Ag(110). We propose two models that are consistent with experimental estima…

[Phys. Rev. B 92, 115439] Published Thu Sep 24, 2015

Author(s): Matthew Yankowitz, Devin McKenzie, and Brian J. LeRoy

Tunneling spectroscopy exhibits suppression of intervalley electronic scattering in the valence band of a monolayer of the transition metal dichalcogenide WSe2.

[Phys. Rev. Lett. 115, 136803] Published Thu Sep 24, 2015

Organic-inorganic hybrid perovskites, which have proved to be promising semiconductor materials for photovoltaic applications, have been made into atomically thin two-dimensional (2D) sheets. We report the solution-phase growth of single- and few-unit-cell-thick single-crystalline 2D hybrid perovskites of (C4H9NH3)2PbBr4 with well-defined square shape and large size. In contrast to other 2D materials, the hybrid perovskite sheets exhibit an unusual structural relaxation, and this structural change leads to a band gap shift as compared to the bulk crystal. The high-quality 2D crystals exhibit efficient photoluminescence, and color tuning could be achieved by changing sheet thickness as well as composition via the synthesis of related materials. Authors: Letian Dou, Andrew B. Wong, Yi Yu, Minliang Lai, Nikolay Kornienko, Samuel W. Eaton, Anthony Fu, Connor G. Bischak, Jie Ma, Tina Ding, Naomi S. Ginsberg, Lin-Wang Wang, A. Paul Alivisatos, Peidong Yang

Author(s): Cui-Zu Chang, Peizhe Tang, Xiao Feng, Kang Li, Xu-Cun Ma, Wenhui Duan, Ke He, and Qi-Kun Xue

The existence of a gapless Dirac surface band of a three dimensional (3D) topological insulator (TI) is guaranteed by the nontrivial topological character of the bulk band, yet the surface band dispersion is mainly determined by the environment near the surface. In this Letter, through in situ angle…

[Phys. Rev. Lett. 115, 136801] Published Wed Sep 23, 2015

The understanding of orbital hybridization and spin-polarization at the organic-ferromagnetic interface is essential in the search for efficient hybrid spintronic devices. Here, using first-principles calculations, we report a systematic study of spin-split hybrid states of C$_{60}$ deposited on various ferromagnetic surfaces: bcc-Cr(001), bcc-Fe(001), bcc-Co(001), fcc-Co(001) and hcp-Co(0001). We show that the adsorption geometry of the molecule with respect to the surface crystallographic orientation of the magnetic substrate as well as the strength of the interaction play an intricate role in the spin-polarization of the hybrid orbitals. We find that a large spin-polarization in vacuum above the buckyball can only be achieved if the molecule is adsorbed upon a bcc-(001) surface by its pentagonal ring. Therefore bcc-Cr(001), bcc-Fe(001) and bcc-Co(001) are the optimal candidates. Spin-polarized scanning tunneling spectroscopy measurements on single C$_{60}$ adsorbed on Cr(001) and Co/Pt(111) also confirm that both the symmetry of the substrate and of the molecular conformation have a strong influence on the induced spin polarization. Our finding may give valuable insights for further engineering of spin filtering devices through single molecular orbitals.

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Author(s): J. L. Lado and J. Fernández-Rossier

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. Her…

[Phys. Rev. B 92, 115433] Published Mon Sep 21, 2015

The existence of gapless Dirac surface band of a three dimensional (3D) topological insulator (TI) is guaranteed by the non-trivial topological character of the bulk band, yet the surface band dispersion is mainly determined by the environment near the surface. In this Letter, through in-situ angle-resolved photoemission spectroscopy (ARPES) and the first-principles calculation on 3D TI-based van der Waals heterostructures, we demonstrate that one can engineer the surface band structures of 3D TIs by surface modifications without destroying their topological non-trivial property. The result provides an accessible method to independently control the surface and bulk electronic structures of 3D TIs, and sheds lights in designing artificial topological materials for electronic and spintronic purposes.

Article

Skyrmions—magnetic vortices with an additional twist—have only been observed in a small number of chiral magnets, all with specific non-centrosymmetric structure. Here, the authors suggest that skyrmions can be found in many frustrated magnets as long as they meet a specific set of criteria.

Nature Communications doi: 10.1038/ncomms9275

Authors: A. O. Leonov, M. Mostovoy

High quality chromium (Cr) doped three-dimensional topological insulator (TI) Sb2Te3 films are grown via molecular beam epitaxy on heat-treated insulating SrTiO3(111) substrates. We report that the Dirac surface states are insensitive to Cr doping, and a perfect robust long-range ferromagnetic order is unveiled in epitaxial Sb2-xCrxTe3 films. The anomalous Hall effect is modulated by applying a bottom gate, contrary to the ferromagnetism in conventional diluted magnetic semiconductors (DMSs), here the coercivity field is not significantly changed with decreasing carrier density. Carrier-independent ferromagnetism heralds Sb2-xCrxTe3 films as the base candidate TI material to realize the quantum anomalous Hall (QAH) effect. These results also indicate the potential of controlling anomalous Hall voltage in future TI-based magneto-electronics and spintronics.

Article

Routes towards inducing strong spin–orbit coupling in graphene have been hindered by detrimental effects on its electronic properties and material quality. Here, the authors demonstrate a possible solution by exploiting interfacial interactions between graphene and a tungsten disulfide substrate.

Nature Communications doi: 10.1038/ncomms9339

Authors: Zhe Wang, Dong–Keun Ki, Hua Chen, Helmuth Berger, Allan H. MacDonald, Alberto F. Morpurgo

Monolayer graphene exhibits many spectacular electronic properties, with superconductivity being arguably the most notable exception. It was theoretically proposed that superconductivity might be induced by enhancing the electron-phonon coupling through the decoration of graphene with an alkali adatom superlattice [Profeta et al. Nat. Phys. 8, 131-134 (2012)]. While experiments have indeed demonstrated an adatom-induced enhancement of the electron-phonon coupling, superconductivity has never been observed. Using angle-resolved photoemission spectroscopy (ARPES) we show that lithium deposited on graphene at low temperature strongly modifies the phonon density of states, leading to an enhancement of the electron-phonon coupling of up to $\lambda\!\simeq\!0.58$. On part of the graphene-derived $\pi^*$-band Fermi surface, we then observe the opening of a $\Delta\!\simeq\!0.9$ meV temperature-dependent pairing gap. This suggests, for the first time, that Li-decorated monolayer graphene is superconducting at 3.5 K.

Alkali-metal (potassium) adsorption on FeSe thin films with thickness from two unit cells (UC) to 4-UC on SrTiO3 grown by molecular beam epitaxy is investigated with a low-temperature scanning tunneling microscope. At appropriate potassium coverage (0.2-0.3 monolayer), the tunneling spectra of the films all exhibit a superconducting-like gap larger than 11 meV (five times the gap value of bulk FeSe), and two distinct features of characteristic phonon modes at 11 meV and 21 meV. The results reveal the critical role of the interface enhanced electron-phonon coupling for possible high temperature superconductivity in the system and is consistent with recent theories. Our study provides compelling evidence for the conventional pairing mechanism for this type of heterostructure superconducting systems.

Article

Manipulation of single molecules can be achieved using scanning probe microscopy but the influence of molecular conformation on this process has, until now, been unclear. Here, the authors probe two different types of porphyrin conformer on a surface and see strong differences in their mechanochemical response.

Nature Communications doi: 10.1038/ncomms9338

Authors: S. P. Jarvis, S. Taylor, J. D. Baran, N. R. Champness, J. A. Larsson, P. Moriarty

Article

Hot electrons—electrons with very high kinetic energies—are important in many processes but difficult to observe due to their short lifetimes. Here, the authors analyse STM based nonlocal manipulation of organic molecules on a surface, showing that the measurements probe hot electron dynamics in real space.

Nature Communications doi: 10.1038/ncomms9365

Authors: D. Lock, K. R. Rusimova, T. L. Pan, R. E. Palmer, P. A. Sloan

Article

Detection and manipulation of single charges in molecules are fundamental cornerstones in molecular electronics. Here, Steurer et al . demonstrate a technique with the use of an atomic force microscope that is able to resolve charge states and single electron charge transfer between molecules.

Nature Communications doi: 10.1038/ncomms9353

Authors: Wolfram Steurer, Shadi Fatayer, Leo Gross, Gerhard Meyer

We present an analysis of electron transport through two weakly coupled precision placed phosphorus donors in silicon. In particular, we examine the (1,1) to (0,2) charge transition where we predict a new type of current blockade driven entirely by the nuclear spin dynamics. Using this nuclear spin blockade mechanism we devise a protocol to readout the state of single nuclear spins using electron transport measurements only. We extend our model to include realistic effects such as Stark shifted hyperfine interactions and multi-donor clusters. In the case of multi-donor clusters we show how nuclear spin blockade can be alleviated allowing for low magnetic field electron spin measurements.

Author(s): S. K. Panda, I. Dasgupta, and T. Saha-Dasgupta

We theoretically investigate the electronic and magnetic structure of vacancy bearing K-intercalated Fe chalcogenides (Ch=Se and Te) using first-principles calculations. Motivated by experimental report on the compositions and suggested Fe valences of the parent insulating and superconducting phases…

[Phys. Rev. B 92, 104421] Published Fri Sep 18, 2015

Author(s): Madhab Neupane, Nasser Alidoust, Ilya Belopolski, Guang Bian, Su-Yang Xu, Dae-Jeong Kim, Pavel P. Shibayev, Daniel S. Sanchez, Hao Zheng, Tay-Rong Chang, Horng-Tay Jeng, Peter S. Riseborough, Hsin Lin, Arun Bansil, Tomasz Durakiewicz, Zachary Fisk, and M. Zahid Hasan

Rare-earth hexaborides have attracted considerable attention recently in connection to a variety of correlated phenomena including heavy fermions, superconductivity, and low-temperature magnetic phases. Here, we present high-resolution angle-resolved photoemission spectroscopy studies of trivalent C…

[Phys. Rev. B 92, 104420] Published Fri Sep 18, 2015

Author(s): Jay D. Sau and P. M. R. Brydon

We consider the problem of bound states in strongly anisotropic ferromagnetic impurities in a superconductor, motivated by recent experiments that claim to observe Majorana modes at the ends of ferromagnetic wires on a superconducting substrate [S. Nadj-Perge et al., Science 346, 602 (2014)]. Genera…

[Phys. Rev. Lett. 115, 127003] Published Fri Sep 18, 2015

Abstract

Large organic molecules are of important interest for organic-based devices such as hybrid photovoltaics or molecular electronics. Knowing their adsorption geometries and electronic structures allows to design and predict macroscopic device properties. Fundamental investigations in ultra-high vacuum (UHV) are thus mandatory to analyze and engineer processes in this prospects. With increasing size, complexity or chemical reactivity, depositing molecules by thermal evaporation becomes challenging. A recent way to deposit molecules in clean conditions is Electrospray Ionization (ESI). ESI keeps the possibility to work with large molecules, to introduce them in vacuum, and to deposit them on a large variety of surfaces. Here, ESI has been successfully applied to deposit triply fused porphyrin molecules on an insulating KBr(001) surface in UHV environment. Different deposition coverages have been obtained and characterization of the surface by in-situ atomic force microscopy working in the non-contact mode shows details of the molecular structures adsorbed on the surface. We show that UHV-ESI, can be performed on insulating surfaces in the sub-monolayer regime and to single molecules which opens the possibility to study a variety of complex molecules.

Beilstein J. Nanotechnol. 2015, 6, 1927–1934. doi:10.3762/bjnano.6.195