Conference Programme

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P-01: Magnetic Junctions
Monday, 19/Jun/2017:
1:30pm - 3:30pm

Session Chair: Elke Scheer, University of Konstanz
Session Chair: Marco Carlotti, University of Groningen
Location: Rm 302

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1:30pm - 2:00pm

Magnetic Exchange Coupling Modifications of High-Spin Organic Polyradicals by Mechanical Deformations and Electrochemical Gating


Institut de Ciència de Materials de Barcelona/CIBER-BBN, Spain

Organic (poly)radicals are neutral molecules exhibiting intrinsic magnetic moment(s) due to the presence of unpaired electron(s) in the molecule in their ground states. This property, added to the low spin-orbit couplings and weak hyperfine interactions, make neutral organic radicals good candidates for molecular spintronics insofar the radical character is preserved in solid state electronic devices. Here, we report on the maintenance of the paramagnetism of radicals of the polychlorotriphenylmethyl (PTM) family when they are embedded in two- and three-terminal solid-state devices, regardless of mechanical and electrostatic external perturbations.[1] Remarkable is the observation of a sample-dependent sign inversion of the magnetic exchange coupling between the three unpaired spins of a PTM organic triradical molecule embedded in a three-terminal device. The observed ferro-to-antiferromagnetic transition is due to structural distortions and results in a high-to-low spin ground state change in this molecule that has been traditionally considered to be a robust high-spin quartet. [2] Interestingly is also the possibility of controlling through a reversible and stable charging of a single PTM organic diradical molecule with one additional electron. By means of inelastic electron tunnel spectroscopy (IETS) we show that the added electron occupies a molecular orbital distinct from those containing the two radical electrons, forming a system of three antiferromagnetically coupled spins. Changing the redox state of the molecule therefore effectively switches on and off the spin-exchange couplings between the added electron and the two radical spins. [3]


[1] R. Frisenda, et al.,, Nano Lett. 2015, 15, 3109-3114

[2] R. Gaudenzi, et al. Nano Lett. 2016, 16, 2066-2071

[3] R. Gaudenzi, et al., submitted 2017.

2:00pm - 2:15pm

Molecular Half Metallicity: Extreme Intramolecular Spin Filtering and Magnetoresistance

Atindra Nath PAL1,2, Sudipto CHAKRABARTI2, Soumyajit SARKAR3, Nadav GENOSSAR2, Lev KHMELNITSKY2, Leeor KRONIK3, Oren TAL2

1Department of Physics, Indian Institute of Technology Kharagpur, India; 2Department of Chemical Physics, Weizmann Institute of Science, Israel; 3Department of Materials and Interfaces, Weizmann Institute of Science, Israel

Perhaps the most essential requirement for spintronic manipulations is the generation of highly spin polarised currents. This property is achieved in half metals that act as metals for one spin type and as insulators for the opposite spin. Half metallic compounds were first predicted and then found already two decades ago, however half metallicity was never demonstrated in nanoscale structures, despite the great promise for highly efficient nanoscale spin manipulations. Here, we show that half metallicity can be realized at the level of a single molecule in a molecular junction based on a molecular magnet suspended between two non-magnetic electrodes. The junction show over 90% spin filtering and 1-3 orders of magnitude magnetoresistance. The results are explained with the aid of DFT calculations and control experiments in the framework of spin splitting of energy levels and molecular anisotropic magnetoresistance.

2:15pm - 2:30pm

New Fundamentals, Effects and Applications in Single-Molecule Circuitry

Albert C. ARAGONÈS1, Nadim DARWISH2, Fausto SANZ1, Eliseo RUIZ3, Ismel DÍEZ-PÉREZ1

1Physical-Chemistry Department and The Institute of Theoretical and Computational Chemistry of the Universitat de Barcelona (IQTCUB) , University of Barcelona, Spain; 2Nanochemistry Research Institute, Department of Chemistry, Faculty of Science & Engineering, Curtin University, Australia; 3Inorganic Chemsitry Department and The Institute of Theoretical and Computational Chemistry of the Universitat de Barcelona (IQTCUB) , University of Barcelona, Spain

Inspired by the proposal that single molecules will be functional elements of future nanoelectronic and photovoltaic devices, there exists considerable interest in understanding charge transport in individual molecular backbones.[1] To investigate charge transport in single-molecule devices, we exploit scanning tunneling microscopy-based approaches in the break-junction mode.

The first block of this seminar will present a novel way to form highly conductive and tunable molecular wires exploiting supramolecular chemistry schemes. Single metalloporphyrin rings are wired from its metallic center by using strong Lewis bases, resulting in an increase of the conductivity of three orders of magnitude versus previous single-porphyrin wires. This novel platform of wiring individual porphyrins mimics the way nature exploits these systems by orienting the perpendicular porphyrin axis as the easy axis for electron/energy transfer.[2]

In the second block, we will demonstrate the use of such approaches to study basic mechanisms in chemical catalysis at the nanoscale. We have designed a surface model system to probe electric field catalysis of a Diels-Alder reaction by delivering an oriented electrical field-stimulus across two reactants. This method enable studying chemical reactions at the single-molecule level.[3]

For the last block, we will focus on spin-dependent transport in such single-molecule devices. We will show that the interfacial magnetism or spinterface, resulting from the interaction between a magnetic or chiral molecule and a metal surface, becomes the key pillar to engineering nanoscale molecular devices with novel functionalities, such as a spinfilter-based switch.[4,5]


[1] N. J. Tao, Nat. Nanotechnol. 1, 173 (2006).

[2] A. C. Aragonès et al., Nano Lett. 14, 4751 (2014).

[3] A. C. Aragonès et al., Nature 531, 88 (2016).

[4] A. C. Aragonès et al., Nano Lett. 16, 218 (2016).

[5] A. C. Aragonès et al., Small 13, 1602519 (2017).

2:30pm - 3:00pm

Large Magnetoresistance in Single Radical Molecular Junctions

Ryoma HAYAKAWA1,2, M. Amin KARIMI2, Sebastian HAMBSCH2, Jannic WOLF2, Thomas HUHN2, Martin S. ZÖLLNER3, Carmen HERRMANN3, Elke SCHEER2

1National Institute for Materials Science, Japan; 2University of Konstanz, Germany; 3University of Hamburg, Germany

We present the charge transport properties of single radical molecule junctions formed by a break junction technique at 4.2 K in magnetic field B. In this study, stable and neutral radical molecules based on an oligo(p-phenylene ethynylene) (OPE) backbone (TEMPO-OPE) were placed on a freestanding gold (Au) bridge. We observe large positive magnetoresistance (MR) up to 287 % at 4T from TEMPO-OPE molecules when B is applied perpendicular to the sample plane [1]. The average MR amplitude is one order of magnitude larger than that of the analogous non-radical OPE molecule and two orders of magnitude larger than for gold single atom contacts fabricated with the same technique. In parallel field a step-like and hysteretic negative MR occurs with similar amplitude at fields of 1.5 to 2 T. Again the non-radical OPE or single-atom contacts do not show this effect. The analysis of the MR, of IVs and of inelastic electron tunneling spectra (IETS) in perpendicular field reveal an effective reduction of the electronic coupling between the current-carrying molecular orbital and the electrodes with increasing B while the origin of the step-like negative MR in parallel field might be attributed to collective ffetcs causded by the surrounding molecules. Our findings thus provide a new physical approach for tuning the charge transport via radical molecules.


[1] R. Hayakawa et al. Nano Lett. 16, 4960 (2016).

3:00pm - 3:15pm

Pathways in Molecular Conductance and Spin Coupling


University of Hamburg, Germany

For understanding both electron transport through molecular bridges and (exchange) spin coupling between local spin centers within in a molecule, it is interesting to know which parts of the molecule are responsible for mediating transport or spin interactions. In the case of spin coupling, ferro- and antiferromagnetic pathways may add up or partially cancel, which is hidden if only the total spin coupling is considered. A new approach to decomposing spin coupling [1] based on Green's functions [2-4, compare 5] will be presented and discussed in comparison with local contributions to electron transmission through molecular junctions [6]. This allows not only identifying which molecular parts are responsible for spin coupling in isolated molecules, but may also allow for distinguishing, e.g., between intramolecular and through-surface contributions. This is essential for understanding and designing surface-based nanospintronics systems.


[1] T. Steenbock, C. Herrmann, to be submitted.

[2] A.I. Liechtenstein, M.I. Katnelson, V.P. Antropov, V.A. Gubanov, J. Magn. Magn. Mater., 67, 65 (1987).

[3] M.J. Han, T. Ozaki, J. Yu, Phys. Rev. B, 70, 184421 (2004).

[4] T. Steenbock, J. Tasche, A.I. Lichtenstein, C. Herrmann, J. Chem. Theory Comput., 11 5651–5664 (2015).

[5] J. J. Phillips, J. E. Peralta, J. Chem. Phys. 138, 174115 (2013).

[6] G.C. Solomon, C. Herrmann, T. Hansen, V. Mujica, M.A. Ratner, Nature Chem. 2 223-228 (2010).

3:15pm - 3:30pm

Exploring Through-Space Conjugation in Molecular Junctions


University of Groningen, The Netherlands

Understanding how molecular structure-function characteristics relate to the electrical properties of molecule-templated tunneling junctions is highly important to the realization of nanoscale electronic devices. In this study, we made use of a liquid top electrode made of eutectic Ga-In alloy for probing the transport properties across tunneling junctions obtained by contacting Self-Assembled Monolayers (SAMs) on different flat gold surfaces.

In particular we investigated tunneling transport in systems characterized by two phenyl rings in either a vertical, face-on or a horizontal, edge-on arrangement and that are held in close proximity with C-C bonds. For these latter, intra-molecular through space conjugation can give rise to quantum interference effects. The measurements revealed to be remarkably sensible to the nature and the geometry of the molecules in the junction which can be ascribed to the packing in the SAM.

Studying the behavior of different compounds in the SAM environment (compared to single molecule studies) helps us to better understand collective effects of molecules on a surface and the possible characteristics of a final molecular electronic device.

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