The winners of the SPS Awards 2016

Electron spin resonance on single atoms

Susanne Baumann ^1,2, William Paul ¹, Taeyoung Choi ¹, Christopher P. Lutz ¹, Arzhang Ardavan ³, Andreas J. Heinrich ^1
1 IBM Almaden Research Center, 650 Harry Road, San Jose, CA 95120, USA
² Departement of Physics, University of Basel, Klingelbergstrasse 82, 4056 Basel, Switzerland
³ Clarendon Laboratory, Department of Physics, University of Oxford, OXI 3PU, United Kingdom

In a spin resonance experiment, radio-frequency radiation drives transitions between low-energy spin states. Electron Spin Resonance (ESR) experiments generally require a large ensemble of nearly identical spins, but magnetic resonance has, in certain systems, been detected also on individual spins. Here we present how electron spin resonance experiments can be performed on single iron atoms in a scanning tunnelling microscope (STM) by using a spin-polarized tip to readout the atom’s quantum state. In contrast to conventional ESR experiments, we use an oscillating electric field (20 to 30 GHz) to excite the spin resonance in the individual iron atoms placed on a magnesium oxide film. We determine an energy relaxation time of T₁ ~ 100 microseconds and a phase-coherence time of T₂ ~ 210 nanoseconds. This approach combines the high energy resolution of conventional spin resonance (here ~10 neV) with the unique strengths of the tunnelling microscope to characterize and modify an individual magnetic nanostructure and its atomic-scale environment.

The study of a unique magnetic coupling at oxide interfaces

M. Gibert ¹, M. Viret ², P. Zubko ³, N. Jaouen ⁴, J.-M. Tonnerre ⁵, A. Torres-Pardo ^6,7, S. Catalano ¹, J. Fowlie ¹, A. Gloter ⁶, O. Stéphan ⁶ and J.-M. Triscone ^1
1 Département de Physique de la Matière Quantique, University of Geneva, Geneva, Switzerland
² Service de Physique de l'Etat Condensé, CEA/DSM/IRAMIS, CEA Saclay, France
³ London Centre for Nanotechnology, University College London, London, UK
⁴ Synchrotron SOLEIL, L'Orme des Merisiers, France
⁵ Institut Néel, CNRS et Université Joseph Fourier, Grenoble, France
⁶ Laboratoire de Physique des Solides, Université Paris-Sud, Orsay, France
⁷ Departamento de Química Inorgánica, Facultad de Químicas, Universidad Complutense Madrid, Spain

Transition metal oxides display a wide range of physical properties arising from the complex interplay between their spin, charge, orbital and lattice degrees of freedom. In recent years, complex-oxide heterostructures have garnered much attention due to the many routes they offer for the engineering of novel functionalities and the discovery of fascinating and often unexpected phenomena. The emergence of new phases due to reduced dimensionality or at interfaces between chemically distinct compounds have led to some of the most interesting findings.
We report on how interface engineering can be used to induce a new magnetic phase in the otherwise non-magnetic material LaNiO₃ [1]. We show that an induced antiferromagnetic order can be stabilized in LaNiO₃ by interfacial coupling to the insulating ferromagnet LaMnO₃ in (111)-oriented LaNiO₃/LaMnO₃ superlattices. The emergent magnetism is used to generate an interlayer magnetic coupling in the heterostructures of a nature that depends on the exact number of LaNiO₃ monolayers [2]. For 7-monolayer-thick LaNiO₃/LaMnO₃ superlattices, negative and positive exchange bias is observed at low temperature before the stabilization of an antiferromagnetically coupled state between the LaMnO₃ layers above the blocking temperature. All these behaviours are explained by the onset of an antiferromagnetic spiral order of (1/4, 1/4, 1/4)-wavevector in the ultrathin LaNiO₃ layer, akin to that of all other insulating nickelates, and the presence of a structural interface asymmetry with LaMnO₃ [3].

[1] Gibert et al., Nat. Mater. 11, 195 (2012).
[2] Gibert et al. Nat. Commun. 7, 11227 (2016).
[3] Gibert et al., Nano Letters 15, 7355 (2015).

Characterizing Single Molecules by Scanning Probe Microscopy using Functionalized Tips

B. Schuler, G. Meyer and L. Gross, IBM Research – Zurich, Säumerstrasse 4, 8803 Rüschlikon, Switzerland

Physical and chemical interactions between single molecules and their environment form the basis for a plethora of processes in nature and are key for many technological applications. Using a combination of low-temperature scanning tunneling microscopy (STM) and atomic force microscopy (AFM) we measure complementary molecular properties with atomic-scale precision exploiting the enhanced resolution obtained with functionalized tips. From atomically resolved AFM images one can learn about the chemical structure, the adsorption geometry [1] and even small details like bond order differences in single bonds can be resolved. With STM, one can directly image the frontier molecular orbitals. This can be used for instance for chemical structure identification, which we consider as an important emerging application for high-resolution scanning probe microscopy. By atomic manipulation it is also possible to change the molecular conformation and to trigger chemical reactions [2]. Detailed insights about molecules and their interaction help us to understand and control the underlying physical and chemical mechanisms to eventually build custom-designed molecular networks.

[1] B. Schuler et al. Phys. Rev. Lett. 111, 106103 (2013)
[2] B. Schuler et al. Nature Chem. 8, 220, (2016)

Scanning probe thermometry of nanosystems

F. Menges ¹, H. Riel ¹, A. Stemmer ² and B. Gotsmann ^1
1 IBM Research - Zurich, Rüschlikon, Switzerland
² ETH Zurich, Rüschlikon, Switzerland

Nanoscopic hot spots, such as those observed in integrated circuits and plasmonic nanostructures can locally affect the physical properties of matter and have tremendous impact on the performances and reliability of scaled devices. Experimental techniques to quantify temperature fields at the nanoscale, however, are rarely established making the development of tools and methods for nanoscale thermometry of central relevance for various areas of science and technology.
Addressing this challenge, we developed a novel high-vacuum scanning thermal microscope to characterize thermal non-equilibrium processes in electronic devices, and thermal transport across nanoscopic contacts and interfaces. We demonstrated the quantification of thermal conductance with sub-10 nm spatial and sub-nW heat flux resolution [1], which enabled realspace characterization of local heat transport properties with sensitivity for single atomic layers [1]. Most recently, we presented a novel technique to characterize local temperature fields using scanning probe thermometry [2]. In contrast to previous scanning-probe-based thermometry approaches, we simultaneously measure a steady-state and a transient heat flux signal between a self-heated scanning probe sensor and a temperature modulated sample, which enables to separate signals related to variations of the temperature potential from that related to changes of tip-sample thermal contact resistance. Our approach facilitates the elimination of tip-sample contact-related artifacts, a major hurdle that so far has limited the use of scanning probe microscopy for nanoscale thermometry. We applied the technique to visualize local Peltier effects at the metal-semiconductor contacts to indium arsenide nanowires and nanoscopic hot spots in self-heated metal interconnects with 7 mK and sub-10 nm spatial temperature resolution [2].
In summary, we have developed both a novel instrument and method to study the particularities of nanoscopic thermal conversion and transport processes in real-space by enabling the measurement of local temperature variations with nanometer-scale spatial resolution.

[1] F. Menges, P. Mensch, H. Schmid, H. Riel, A. Stmmer, and B. Gotsmann, Temperature mapping in operating nanoscale devices by scanning probe thermometry, Nature Communications 7, (2016).
[2] F. Menges, H. Riel, A. Stemmer, C. Dimitrakopoulos, and B. Gotsmann, Thermal transport into graphene through nanoscopic contacts, Physical Review Letters 111, 205901, (2013).