Abstracts
First session: 11:40 - 12:20
Impurity-induced resonant spinon zero modes in Dirac quantum spin-liquids
Guangze Chen, Aalto University
Guangze Chen and Jose Lado
Aalto University, Finland
Quantum spin-liquids are strongly correlated phases of matter, displaying a highly entangled ground state. Experimental identification of quantum spin-liquid states has proven to be a remarkable challenge due to their unconventional nature. Here we show that the effects of local impurities can provide strong signatures of a Dirac quantum spin-liquid state. Focusing on a gapless Dirac quantum spin-liquid state as realized in NaYbO2, we show that a single magnetic S=1/2 impurity coupled to the quantum spin-liquid state creates a resonant spinon peak at zero frequency, coexisting the original Dirac spinons. We explore the spatial dependence of this zero-bias resonance, and show how different zero modes stemming from several impurities interfere. Finally, we show that the spinon zero modes result in a zero frequency divergence in the spin structure factor, which can be probed by means of scanning tunnel spectroscopy and electrically-driven paramagnetic resonance. Our results put forward impurity engineering by scanning probe techniques as a simple method to probe quantum spin-liquid physics by a local real space measurement.
GuangzeChen.mp4Current imaging of graphene point contacts using single spin magnetometer
Myeongwon Lee, Korea University
Myeongwon Lee (1), Seong Jang (2), Woochan Jung (2), Yuhan Lee (1), Gil-Ho Lee (2), and Donghun Lee (1)
(1) Department of Physics, Korea University, Seoul 02841, Republic of Korea
(2) Department of Physics, Pohang University of Science and Technology (POSTECH), Pohang, 37673, Republic of Korea
Understanding electron transport in low dimensional materials such as two-dimensional electron gas and graphene has been of great interest in condensed material physics. Recently, there are increasing efforts of imaging electron trajectories in these materials as the direct visualization of carrier flow can provide useful insight of carrier dynamics that conventional transport measurement may not be able to provide. For instance, ballistic transport path in graphene by Andreev reflection was imaged by scanning gate microscopy and viscous flow of the Dirac fluid in graphene was imaged by diamond NV (Nitrogen-Vacancy) center. In this poster, we demonstrate current imaging of graphene point contact devices using single spin magnetometer based on diamond NV center. Point contacts (PCs) are point-like sources of current in graphene transport devices providing high gradient and local current profile that can be useful for the study of magnetic focusing, the Veselago lensing and relativistic hydrodynamics. Point contacts has been studied with the transport measurement but the relevant current flow has not been imaged. Here, we measure magnetic Oersted field generated by the current from PCs and reconstruct current profile from the obtained field map. We will discuss the basic working principle of scanning spin magnetometer and how to convert magnetic field information into the current imaging.
Myeongwon Lee, Teaser Video, Current imaging of graphene point contacts using single spin magnetometer.mp4Identification of the absorption geometries and electronic structures of VOPc molecules on Au(111) surface
Shinjae Nam, IBS Center for Quantum Nanoscience and Ewha Womans University
Shinjae Nam, Jinoh Jung, Chistoph Wolf, Andreas Heinrich, and Jungseok Chae
IBS Center for Quantum Nanoscience and Ewha Womans University, Seoul, Korea
Vanadyl Phthalocyanine (VOPc) molecules are considered as a potential candidate for the quantum manipulation using its 1/2 spin characteristics. VOPc is a planar molecule with vanadium atom at the center in addition to the oxygen atom on top of vanadium normal to molecular plane. To explore the spin structure of single VOPc molecules, we deposited the molecules on Au (111) surfaces with low coverage condition. The imaging of the single molecules is performed using low temperature scanning tunneling microscopy (STM). Molecules have preferred absorption sites, elbow or bulged sites of the reconstructed Au(111) surface. We figured out that VOPc have two geometric configurations when they are absorbed: Oxygen pointing toward the vacuum (O-up) with the Pc laying on the surface, and oxygen pointing toward the surface (O-down). To figure out which molecules are O-up or O-down configurations, we measured bias dependent topographic images, differential conductance maps, and tunneling spectroscopy. Using bias voltage of +100 mV, O-up is bright at the center and O-down is bright around Pc ligands. O-down molecule is tilted toward one direction in topographic image and mechanically less stable. In addition, configurations of the molecules were confirmed by the calculated results using density functional theory. In this poster, different electronic structures depending on the absorption geometries will be discussed.
Remote detection and recording of atomic-scale spin dynamics
R.J.G. Elbertse, Delft University of Technology
R.J.G. Elbertse, D. Coffey, J. Gobeil, and A.F. Otte
Delft University of Technology, The Netherlands
Atomic spin structures assembled by means of scanning tunneling microscopy (STM) provide valuable insight into the understanding of atomic-scale magnetism. Among the major challenges are the detection and subsequent read-out of ultrafast spin dynamics due to a dichotomy in travel speed of these dynamics and the probe tip. Here, we present a device composed of individual Fe atoms that allows for remote detection of spin dynamics. We have characterized the device and used it to detect the presence of spin waves originating from an excitation induced by the STM tip several nanometres away; this may be extended to much longer distances. The device contains a memory element that can be consulted seconds after detection, similar in functionality to e.g. a single photon detector. We performed statistical analysis of the responsiveness to remote spin excitations and corroborated the results using basic calculations of the free evolution of coupled quantum spins.
RobbieElbertse.m4vSTM studies of endohedral metallofullerene: Gd@C82 on Au(111)
Kyungju Noh, IBS Center for Quantum Nanoscience
Kyungju Noh (1,2), Jiyoon Hwang (1,2), Denis Krylov (2), Yujeong Bae (2), and Andreas Heinrich (1,2)
(1) Department of Physics, Ewha Womans University, Seoul 03760, Republic of Korea
(2) Center for Quantum Nanoscience, Institute for Basic Science, Seoul 03760, Republic of Korea
Endohedral metallofullerene is known for its property which protects inside spin system from environment and ensures longer coherence time [1,2]. Especially, Gd@C82 has various potential application such as magnetic resonance imaging (MRI) contrasting agents, cancer cures, and so on [3]. Here, we investigate the structural and electronic properties of Gd@C82 on Au(111) using scanning tunneling microscopy (STM). We investigate the adsorption of molecules on Au(111) depending on the coverage of Gd@C82 on the surface. In low coverage, we find isolated molecules at the step edge and the elbow sites of the Au(111) herringbone reconstruction. As increasing the density of molecules on the surface, Gd@C82 on step edge shows tendency to form dimer. When the step edge coverage is high enough to make molecular chains, we find the chain’s length is restricted by the periodicity of herringbone reconstruction since Gd@C82 prefers to sit on fcc sites more than hcp sites. To reveal the mechanism of intermolecular interaction, we compare the electronic states of Gd@C82 of monomer, dimer, and molecular chain using scanning tunneling spectroscopy (STS). And also the high-resolution STM images at different bias voltages show the intramolecular structure of Gd@C82 molecules, giving us understandings of orientational configuration and how it is affected from intermolecular interaction. These results may serve as a guide in the construction of surface architectures based on Au’s surface energy.
[1] W. Harneit, Phys. Rev. A 65, 032322 (2002)
[2] Z. Hu et al., J. Am. Chem. Soc. 140, 1123−1130 (2017)
[3] J. Zhang et al., Nat. Chem. 5, 880−885 (2013)
Magnetic resonance imaging of single atoms on a surface
Philip Willke, Karlsruhe Institute of Technology
Philip Willke (1,2,3) Kai Yang (3), Yujeong Bae (1,2,3), Andreas J. Heinrich (1,2) and Christopher P. Lutz (3)
(1) Center for Quantum Nanoscience, Institute for Basic Science (IBS), Seoul 03760, Republic of Korea
(2) Ewha Womans University, Seoul 03760, Republic of Korea
(3) IBM Almaden Research Center, San Jose, CA 95120, USA
Magnetic resonance imaging (MRI) revolutionized diagnostic medicine and biomedical research by allowing non-invasive access to spin ensembles. To enhance MRI resolution to the nanometer scale, new approaches including scanning probe methods have been used in recent years, which culminated in the detection of individual spins. This allowed for the visualization of organic samples and magnetic structures, as well as identifying the location of electron and nuclear spins. Here, we demonstrate the MRI of individual atoms on a surface [1]. The set-up, implemented in a cryogenic scanning tunnelling microscope, uses single-atom electron spin resonance [2] to achieve subångström resolution, exceeding the spatial resolution of previous MRI experiments by one to two orders of magnitude [3]. In this approach, the magnetic-field gradient, the electric readout as well as the driving field are all combined in the STM tip apex. We additionally focus on the 3D magnetic interaction potential between tip spin and surface atom spin, which we can derive from the MRI data. For different tip configurations and atomic species on the surface, we find fingerprints in the MRI scans that reflect the magnetic properties of both tip and surface atoms. Thus, MRI-STM has the ability to reveal the unique spin signature of individual atomic species and locate their position and energy even on the nanoscale making the magnetic properties of for instance large planar molecules and spin structures directly accessible.
References
[1] P. Willke et al. Magnetic Resonance Imaging of Single Atoms, Nat. Phys. 15, 1005–1010 (2019).
[2] S. Baumann et al. Electron paramagnetic resonance of individual atoms on a surface. Science 350, 417-420 (2015).
[3] M. S. Grinolds et al. Quantum control of proximal spins using nanoscale magnetic resonance imaging. Nat. Phys. 7, 687 (2011).
Poster_PhilipWillke.mp4How a traveling salesperson teaches fast quasiparticle interference mapping
Fabian Natterer, University of Zurich
Jens Oppliger and Fabian D. Natterer
Department of Physics, University of Zurich, Switzerland
The successful traveling salesperson knows two things very well: Which customers add to the bottom line, and how to get to them quickly. We apply this wisdom to an oddly similar scenario encountered in scanning tunneling microscopy in which the large scale mapping of the local density of states could last several days using conventional grid scanning. Here we use a sparse sampling approach to remove data redundancies and a traveling salesperson path routing for efficient trip-planning to fundamentally speed up quasiparticle interference mapping. The faster mapping allows the exploration of a vaster parameter space and the improvement of momentum or energy resolution. We further introduce drift-correction schemes and windowing functions that improve the QPI quality and mitigate Fourier artifacts.
FabianNatterer.mp4Induced excitation of molecular hydrogen at a tunneling junction on a single VOPc molecule
Jinoh Jung, IBS Center for Quantum Nanoscience
Jinoh Jung (1,2), Shinjae Nam (1,3), Christoph Wolf (1,4), Andreas Heinrich (1,3), and Jungseok Chae (1,4)
1 Center for Quantum Nanoscience, Institute for Basic Science (IBS), Seoul 03760, Korea
2 Department of Physics, KAIST, Daejeon, Korea
3 Physics Department, Ewha Womans University, Seoul 03760, Korea
4 Ewha Womans University, Seoul 03760, Korea
Molecular magnets with highly coherent spins are considered as candidates for a building block for quantum technology. Vanadyl phthalocyanine (VOPc) molecules have benefits of the spin S = ½ and long coherence time even at room temperature in its crystalline form. Here, we focus on the interactions of VOPc molecules deposited on Au(111) substrate with high hydrogen partial pressure environment using scanning tunneling microscopy (STM) and atomic force microscopy (AFM). VOPc preferred to adsorbed on Herringbone edge of Au(111) with the same probability of oxygen pointing toward vacuum or gold. STM images shows clear distinction between two configurations.We measured two level fluctuations in tunneling conductance, which is similar to hydrogen adsorbed metallic substrates. However, different spectral line shapes and different excitation energies of hydrogen vibration at the center, carbon-nitrogen (C-N) ring, benzene rings and outside of VOPc molecule due to the interactions of hydrogen with it. Moreover, non-contact AFM measurement confirmed the frequency shift around C-N ring by the interaction with hydrogen.
Measuring superconducting phase transitions with Nitrogen-Vacancy centers in diamond
Domenico Paone, Max Planck Institute for Solid State Research
D. Paone (1,2) , D. Pinto (1,3), G. Kim (1), L. Feng (1,4,) M.-J. Kim (1,4), R. Stöhr (2), A. Singha (1), S. Kaiser (1,4), G. Logvenov (1), J. Wrachtrup (2) and K. Kern (1,3)
(1) Max Planck Institute for Solid State Research, Stuttgart, Germany
(2) 3rd Institute of Physics and Research Center SCoPE, University Stuttgart, Germany
(3) Institut de Physique, Ecole Polytechnique Fédérale de Lausanne, Switzerland
(4) 4th Institute of Physics and Research Center SCoPE, University Stuttgart, Germany
The magnetism in superconductors is often accompanied by exotic electronic phases such as the vortex formation in high temperature type II superconductors. Several experimental tools have been developed to study magnetic phase transitions within these systems. However, the local sensing of dynamical mechanisms in superconductors still remains a challenge. A sensor scheme based on an atomic sized quantum sensor, the nitrogen-vacancy (NV) center in diamond, pushes the sensitivity to the local read out of weak magnetic fields. Here, we present a spatially resolved study of the Meissner effect by using the fluorescence count rate of an NV center ensemble. We are able to observe a correlation between the NV fluorescence and the Meissner state of a Lanthanum Strontium Cooper Oxide (LSCO) thin film. Implementing the NV center emission into time resolved pump probe experiments could enable the local measurement of dynamical processes.
Magnetism of surface-embedded lanthanide single ions with room temperature structural stability
Safa Lamia Ahmed, Ewha Womans University
S. L. Ahmed (1,2), A. Singha (1,2), D. Krylov (2), S. Rusponi (3), M. Pivetta (3), C. Wolf (1,2), A. Lodesani (4), A. Picone (4), A. Brambilla (4), A. Barla (5), A. J. Heinrich (1,2), and F. Donati (1,2)
(1) Center for Quantum Nanoscience, Institute for Basic Science (IBS), Seoul 03760, Republic of Korea
(2) Department of Physics, Ewha Womans University, Seoul 03760, Republic of Korea
(3) Institute of Physics, Ecole Polytechnique Fédérale de Lausanne, 1015 Lausanne, Switzerland
(4) Dipartimento di Fisica, Politecnico di Milano, 20133 Milano, Italy
(5) Istituto di Struttura della Materia (ISM), Consiglio Nazionale delle Ricerche (CNR), I-34149 Trieste, Italy
We investigate surface embedded lanthanide ions in ultra-thin MgO film on Ag (100) as a perspective material to realize single atom magnets [Science 352, 318 (2016)] with room temperature (RT) structural stability. RT scanning tunneling microscopy images show negligible surface diffusion and nucleation of the embedded ions. Using X-ray magnetic circular dichroism, we reveal out-of-plane magnetic anisotropy for Tm and Sm; in-plane anisotropy for Ho and Dy and negligible magnetic anisotropy for Er and Gd. For all systems, we observed paramagnetic loops at 2.5 K. Combining these results with DFT and multiplet calculations, we rationalize interaction between the 4f electrons and their ligand environment and determine the ions’ magnetic level structure.
Vibron-assisted spin excitation in a magnetically anisotropic molecule
Leo Garnier, IPCMS/CNRS Strasbourg
N. Bachellier (1), B. Verlhac (1), L. Garnier (1), J. Zaldívar (2), C. Rubio-Verdú (2), P. Abufager (3), M. Ormaza (1), D.-J. Choi (4), M.-L. Bocquet (5), J.I. Pascual (2), N. Lorente (4), and L. Limot (1)
(1) Université de Strasbourg, CNRS, IPCMS, UMR 7504, F-67000 Strasbourg, France
(2) CIC nanoGUNE, 20018 Donostia-San Sebastián, Spain
(3) Instituto de Física de Rosario, CONICET and Universidad Nacional de Rosario, Av. Pellegrini 250 (2000) Rosario, Argentina
(4) Centro de Física de Materiales (CFM), 20018 Donostia-San San Sebastián, Spain
(5) PASTEUR, ENS, Sorbonne Universités, CNRS, 75005 Paris, France
The control and readout of molecular spin states through the electron current is key for high-density storage. The coupling of electrons with the vibrational modes, or vibrons, of the molecule can, however, modify or even suppress the molecular conductance, thus indirectly impacting spin manipulation. Here, we use inelastic electron tunneling spectroscopy to promote and detect the excited spin states of a prototypical molecule with magnetic anisotropy. We demonstrate the existence of a vibron-assisted spin excitation spatially displaced off the molecular orbital carrying the spin, that can exceed in energy and in amplitude a simple excitation among spin states. The excitation, which can be quenched by modifying the molecular spin, is explained using first-principles calculations and model calculations that include dynamical correlations. This vibron-assisted spin excitation should be common to molecular systems possessing magnetic anisotropy.
Ab initio multiplet calculations for magnetic adatoms on surfaces
Christoph Wolf, Center for Quantum Nanoscience (QNS), Seoul
Christoph Wolf (1), Fernando Delgado (2), Jose Reina (3) and Nicolas Lorente (3)
(1) Center for Quantum Nanoscience (QNS), Seoul, Korea
(2) Departamento de Fı́sica, Universidad de La Laguna, Spain
(3) Centro de Fı́sica de Materiales, Centro Mixto CSIC-UPV/EHU, Spain
Recent advances in scanning probe spectroscopy to single-atom electron spin resonance (STM-ESR) have greatly improved our understanding of magnetic impurities on clean surfaces [1-3]. Here, we present our most recent effort to better predict the behavior of such spins on surfaces from an ab initio perspective. Our method is based on density functional theory (DFT) but is able to accurately calculate the many-electron multiplet based on a local Wannier basis and a multi-orbital Hubbard model [4]. This method is unbiased and truly ab initio, i.e. free of experimental input and therefore suitable for the systematic discovery of surface spin systems with favorable properties for single atom based data storage or quantum computing.
[1] Natterer et al., Nature 2017, 543, 226– 228
[2] Donati et al., Science 2016, 352, 318-321
[3] Willke et al., Nature Physics, 2019, 15, 1005-1010
[4] Wolf et al., J. Phys. Chem. A, 2020, 124, 2318-2327
Nematicity, checkerboard charge order and magnetic-field tuning of a van-Hove singularity in the surface layer of Sr2RuO4
Carolina de Almeida Marques, School of Physics and Astronomy, University of St Andrews
Carolina A. Marques (1), Luke C. Rhodes (1), Rosalba Fittipaldi (2), Veronica Granata (3), Chi Ming Yim (1), Renato Buzio (4), Andrea Gerbi (4), Antonio Vecchione (2), Andreas W. Rost (1, 5), and Peter Wahl (1)
(1) SUPA, School of Physics and Astronomy, University of St Andrews, North Haugh, St Andrews, KY16 9SS, United Kingdom
(2) CNR-SPIN, UOS Salerno, Via Giovanni Paolo II 132, Fisciano, I-84084, Italy
(3) Dipartimento di Fisica “E. R. Caianiello”, Università di Salerno, I-84084 Fisciano, Salerno, Italy
(4) CNR-SPIN, Corso F.M. Perrone 24, Genova, 16152, Italy
(5) Max-Planck-Institute for Solid State Research, Heisenbergstr. 1, 70569 Stuttgart, Germany
Strongly correlated materials such as high temperature superconductors, heavy fermion materials and the metamagnetic Sr3Ru2O7 have been widely associated with quantum critical behaviour. One mechanism that could lead to a quantum phase transition is the tuning of a van Hove singularity in the electronic structure towards the Fermi level by, for example, applying a magnetic field. However, spectroscopic evidence of how the electronic states change close to a field-tuned quantum phase transition is not available so far, though there is large evidence that exotic orders, including checkerboard charge order and nematicity, become important. In Sr2RuO4, the bulk of the material is a well-known unconventional superconductor. However, upon cleaving, the surface undergoes a reconstruction with a rotation of the RuO6 octahedra. Here, using ultra-low temperature Scanning tunnelling microscopy, we study the low energy electronic properties of the surface of Sr2RuO4. We establish the existence of four van Hove singularities within 5 mV of the Fermi energy, linked intricately to checkerboard order and nematicity of the electronic states. Comparison with tight binding calculations shows that a nematic order parameter together with a weak intraband hybridization on the dxy band reproduces the measured low energy density of states and leads to the formation of those four van Hove singularities. Additionally, by applying a magnetic field up to 14T, we observe one of the van Hove singularities to Zeeman split, with one of its branches extrapolated to reach the Fermi level at ~32T. This is the first spectroscopic observation of a van Hove singularity moving towards the Fermi energy under magnetic field in a strongly correlated system. Our results show that the surface of Sr2RuO4 has all the ingredients for quantum criticality.
Self-assembly of rare-earth based metallorganic coordination complexes
Sébastien Reynaud, Ecole polytechnique fédérale de Lausanne
Sébastien Reynaud, Marina Pivetta and Harald Brune
Institut de Physique, Ecole polytechnique fédérale de Lausanne, Switzerland
Rare earth single atoms adsorbed on decoupling layers grown on metallic substrates have shown remarkable magnetic properties [1,2]. The crystal field generated by the bonding to the surface determines the magnetic level diagram of the adatom and is therefore a key parameter for the adatom magnetic stability. In the case of Ho on MgO, the individual adatoms have stable magnetization up to 45 K [3] which is close to the onset temperature of diffusion of around 50 K [4]. In order to realize systems where the rare earth atoms are immobilized, a possible strategy is to create metal-organic coordination complexes (MOCC) with rare earth atoms [5]. Using different molecular ligands, this approach also allows to tune the crystal field experienced by the atoms. Here we present the results of scanning tunneling microscopy experiments aiming at synthesizing MOCCs using terbium and biphenyl-dicarboxylic acid on Ag(100) and on MgO/Ag(100).
[1] Donati et al., Science 603, 318 (2016)
[2] Baltic et al., Nano Lett. 16, 7610 (2016)
[3] Natterer et al., Phys. Rev. Lett. 121, 027201 (2018)
[4] E. Fernandes, PhD thesis, Ecole polytechnique fédérale de Lausanne (2017)
[5] Ecija et al., Acc. Chem. Res. 51, 365 (2018)
Electron spin resonance in a dilution refrigerator based scanning tunneling microscope
Jinkyung Kim, Center for Quantum Nanoscience (QNS)
Jinkyung Kim (1,2), Thi Hong Bui (1,2), Denis Krylov (1,2), Soonhyeong Lee (1,2), Deung-jang Choi (3), Won-jun Jang (1,2), Yujeong Bae (1,2), Andreas J. Heinrich (1,2)
(1) Center for Quantum Nanoscience (QNS), Institute for Basic Science, Seoul 03760, Korea
(2) Department of Physics, Ewha Womans University, Seoul 03760, Korea
(3) Institut de Physique et Chimie des Matériaux de Strasbourg, Université de Strasbourg, CNRS, 67034 Strasbourg, France
Scanning tunneling microscopy combined with electron spin resonance (ESR) technique [1] enables a direct access to the quantum states of individual magnetic atoms or molecules on surfaces. Here, we introduce our newly designed STM with ESR capabilities operated at ~10 mK in a dilution refrigerator. With the capabilities to transfer high-frequency signals to both sides of STM tip and sample, the transmission of radio frequency (RF) signals to the STM junction is characterized in the wide range of frequency: 5 to 30 GHz. Using hydrogenated Ti (TiH) atoms on MgO as our model system, we optimize the ESR signals to have a larger signal-to-noise ratio and narrower line width. We further investigate the magnetic interaction and quantum states of Fe-TiH dimers at different vector magnetic fields. Our results provide deeper understanding on the quantum states of magnetically coupled atoms and thus the direction to engineer and design the spin-based quantum states.
[1] S. Baumann et al., Electron paramagnetic resonance of individual atoms on a surface, Science, 350, 6259, 417-420 (2015)
Second session: 12:20 - 13:00
Edge channels of broken symmetry Quantum Hall states in graphene probed by atomic force microscopy
Sungmin Kim, National Institute of Standards and Technology and University of Maryland
Sungmin Kim (1,2), Johannes Schwenk (1,2), Daniel Walkup (1,2), Yihang Zeng (3), Fereshte Ghahari (1,2), Son T. Le (1,4), Marlou R. Slot (1,5), Julian Berwanger (6), Steven R. Blankenship (1), Kenji Watanabe (7), Takashi Taniguchi (7), Franz J. Giessibl (6), Nikolai B. Zhitenev (1), Cory R. Dean (3), and Joseph A. Stroscio (1)
(1) Physical Measurement Laboratory, National Institute of Standards and Technology, Gaithersburg, MD 20899, USA.
(2) Institute for Research in Electronics and Applied Physics, University of Maryland, College Park, MD 20742, USA.
(3) Department of Physics, Columbia University, New York, NY 10027, USA.
(4) Theiss Research, La Jolla, CA 92037, USA.
(5) Department of Physics, Georgetown University, Washington, DC 20007, USA.
(6) Institute of Experimental and Applied Physics, University of Regensburg, Regensburg 93040, Germany.
(7) National Institute for Materials Science, Tsukuba, Ibaraki 305-0044, Japan.
The quantum Hall (QH) effect, a topologically non-trivial quantum phase has brought into focus the concept of topological order in physics. The topologically protected quantum Hall edge states are the essential features of the QH effect, however microscopic local probe studies of edge states have been limited. The QH edge states in graphene are special since they emerge from four-fold nearly-degenerate Landau levels. In this talk, we present a microscopic study a of the QH edge states originating from the broken symmetry zero Landau level (zLL) in graphene using a combination of scanning tunneling microscopy (STM), atomic force microscopy (AFM), and magnetotransport measurements. The Kelvin probe force microscopy (KPFM) mode of AFM detects the chemical potential transitions when Landau levels are being filled or emptied as a function of back gate potential and show the same fidelity for Landau level spectroscopy as STS measurements. In particular, symmetry breaking states can be resolved at filling factors ν = ±1 inside the N=0 Landau level manifold, showing the lifting of the graphene four-fold degeneracy due to spin and valley. The edge states emerging from integer filling factors of ν = 0, ±1 are spatially mapped across the quantum Hall edge boundary using atomic force microscopy (AFM) and Kelvin probe force microscopy (KPFM). The microscopic properties of quantum Hall edge states can now be correlated with macroscopic magnetotransport measurements.
Resolving the topological classification of bismuth with topological defects
Abhay Kumar Nayak, Weizmann Institute of Science, Rehovot
Abhay Kumar Nayak (1), Jonathan Reiner (1), Raquel Queiroz (1), Huixia Fu (1), Chandra Sekhar (2), Binghai Yan (1), Claudia Felser (2), Nurit Avraham (1), and Haim Beidenkopf (1)
(1) Dept. of Condensed Matter Physics, Weizmann Institute of Science, Rehovot, Israel
(2) Max Planck Institute for Chemical Physics of Solids, Dresden, Germany
Bulk boundary correspondence has allowed the study of electronic bulk properties through the investigation of their topological boundary modes. However, for some materials the growing diversity of topological classes leads to ambiguity between classes sharing similar boundary phenomenology. Such is the current status of bismuth, for which recent studies have suggested several non-trivial classifications, such as a strong or higher order TI, birth of which hosts helical 1D modes on their boundaries. We use a novel approach to resolve the topological classification of bismuth by spectroscopically mapping the response of its boundary modes to a topological defect in the form of a screw dislocation [1]. We find a 1D mode bound to the step edges of bismuth, spanning over a wide energy range than previously identified and extends along the step edge up to the core of the screw dislocation without gapping out. This signifies that the edge mode also binds to the topological defect, characteristic of a material with non-zero weak indices. We argue that the small scale of bulk energy gap at the TRIM L point, positions bismuth within the critical region of a topological phase transition between a higher order TI and a strong TI with weak indices.
[1] Nayak, A. K. et al. Resolving the topological classification of bismuth with topological defects. Sci. Adv. 5, eaax6996 (2019).
AbhayKumarNayak.mp4Electronic transport through 1D coordination polymers
Christian Wäckerlin, Institute of Physics of the Czech Academy of Sciences and Empa
Christian Wäckerlin (1,2), Oleksander Stetsovych (1), Santhini Vijai Meena (1), Aleš Cahlík (1), Simon Pascal (3), Martin Švec (1), Jesús Mendieta (1), Pingo Mutombo (1), Olivier Siri (3), Pavel Jelínek (1)
(1) Institute of Physics of the Czech Academy of Sciences, Praha, Czech Republic
(2) Empa - Swiss Federal Laboratories for Materials Science and Technology, Dübendorf, Switzerland
(3) Aix Marseille Université, CNRS, Marseille, France
We report on the electric transport through transition metal based 1D coordination polymers (wires) suspended between the tip of a scanning probe microscope (SPM) and a Au(111) substrate. The wires are synthesized in-situ by co-deposition of the metal atoms and the quinonediimine (2,5-diamino-1,4-benzoquinone-diimine, 2HQ) ligand. Isostructural Ni, Co and Fe wires with lengths over 200 nm are obtained. We are able to lift these wires quite far from the surface and to perform detailed transport measurements as a function of height and bias voltage.
The transition metal element contained in the organometallic wire has a profound impact on its transport properties: Fe and Ni wires exhibit a band gap at heights larger than 3 to 5 nm, but Co containing polymers are conductive without gap up to more than 12 nm.
The conductance of suspended CoQ wires can be switched by ~3 orders of magnitude by the electric bias and by irradiation with visible light. Switching by light is reversible (conductive ↔ non-conductive) and can be performed for thousands of cycles. Switching by bias occurs only in one way (conductive → non-conductive), but the conductive state can be recovered by irradiation with light.
waeckerlin.mp42D materials and heterostructures on metal substrates: a combined Pulsed Laser Deposition and Scanning Tunneling Microscopy approach
Francesco Tumino, Politecnico di Milano
Francesco Tumino (1), Paolo D'Agosta (1), Andi Rabia (1), Valeria Russo (1), Andrea Li Bassi (1), Sergio Tosoni (2), and Carlo Spartaco Casari (1)
(1) Department of Energy, Politecnico di Milano, Milano, Italy
(2) Department of Materials Science, Università di Milano-Bicocca, Milano, Italy
The last decade has witnessed the emergence of 2D materials and van der Waals heterostructures (vdWH) as novel platforms for fundamental research on low-dimensional systems and for engineering a new generation of ultrathin devices.
The experimental study of vdWH and their application in future nanotechnology pose fundamental challenges, such as the development of large-scale bottom-up synthesis approaches and the detailed investigation of structural and electronic properties at the sub-nanometer scale. Addressing these issues requires the controlled synthesis of model systems and their in-situ characterization by high-resolution techniques. In our work, we adopted an experimental approach based on the non-conventional combination of Pulsed Laser Deposition (PLD) (rarely used for the synthesis of 2D materials) and in-situ Scanning Tunneling Microscopy (STM), under ultra-high vacuum conditions.
We focused on the transition metal dichalcogenides (TMDs) MoS2 and WS2. By properly tuning PLD parameters, we produced single-layer nanostructures of both materials, over cm-scale metal substrates suitable for STM investigation, i.e. Au(111) and Ag(111) [1]. In-situ STM revealed the morphological and structural properties down to the atomic scale, e.g. resolving surface atomic defects [2]. These results set the basis to study the PLD growth of MoS2-WS2 vdWH on metal substrates. In addition to STM, ex-situ Raman spectroscopy was used to characterize the vibrational properties and to monitor the stability under ambient conditions. Our work opens new perspectives in the synthesis and surface characterization of vdWH. For instance, by combining PLD with organic-MBE, we approached the study of atomically thin hybrid vdWH between pentacene and MoS2, a prototypical mixed-dimensional vdWH of high technological potential.
[1] F. Tumino et al., Nanoscale Advances 1, 643 (2019).
[2] F. Tumino et al., J Phys Chem C 124, 12424 (2020).
Unveiling the radiative local density of optical states of a plasmonic nanocavity by STM
Roberto Otero, Universidad Autónoma de Madrid and IMDEA Nanoscience
Alberto Martín-Jiménez (1), Antonio I. Fernández-Domínguez (2), Koen Lauwaet (1), Daniel Granados (1), Rodolfo Miranda (1,3), Francisco J. García-Vidal (2,4) and Roberto Otero (1,3)
(1) IMDEA Nanoscience, Madrid, Spain
(2) Dep. of Theoretical Condensed Matter Physics and IFIMAC, Universidad Autónoma de Madrid, Madrid, Spain
(3) Dep. of Condensed Matter Physics and IFIMAC, Universidad Autónoma de Madrid, Madrid, Spain
(4) DIPC, Donostia-San Sebastián, Spain.
Atomically-sharp tips in close proximity of metal surfaces create plasmonic nanocavities supporting both radiative (bright) and non-radiative (dark) localized surface plasmon modes. Disentangling their respective contributions to the total density of optical states remains a challenge. Electroluminescence due to tunneling through the tip-substrate gap could allow the identification of the radiative component, but this information is inherently convoluted with that of the electronic structure of the system. In this contribution, we present a fully experimental procedure to eliminate the electronic-structure factors from the scanning tunneling microscope luminescence spectra by confronting them with spectroscopic information extracted from elastic current measurements [1]. Comparison against electromagnetic calculations demonstrates that this procedure allows the characterization of the meV shifts experienced by the nanocavity plasmonic modes under atomic-scale gap size changes. Therefore, the method gives access to the frequency-dependent radiative Purcell enhancement that a microscopic light emitter would undergo when placed at such nanocavity.
[1] A. Martín-Jiménez et al., Nature Communications, 11 1021 (2020).
Otero-videoSPM.mp4Spatially dispersing Yu-Shiba-Rusinov states in the unconventional superconductor FeTe0.55Se0.45
Damianos Chatzopoulos, Leiden University
Damianos Chatzopoulos (1), Doohee Cho (1,2), Koen M. Bastiaans (1), Gorm O. Steffensen (3), Damian Bouwmeester (1,4), Alireza Akbari (5,6), Genda Gu (7), Jens Paaske (3), Brian M. Andersen (3) and Milan P. Allan (1)
(1) Leiden University, The Netherlands
(2) Yonsei University, Korea
(3) University of Copenhagen, Denmark
(4) Delft University of Technology, The Netherlands
(5) Max Planck Institute for the Chemical Physics of Solids, Dresden, Germany
(6) Max Planck POSTECH Center for Complex Phase Materials and Department of Physics, POSTECH, Korea
(7) Brookhaven National Laboratory, USA
Using a scanning tunneling microscope (STM) we find dispersive, in-gap states in the iron-based superconductor FeTe0.55Se0.45, a material that exhibits signatures of topological superconductivity, and Majorana bound states at vortex cores or at impurity locations. We use a superconducting STM tip for enhanced energy resolution, which enables us to show that impurity states can be tuned through the Fermi level with varying tip-sample distance. We find that the impurity state is of the Yu-Shiba-Rusinov (YSR) type, and argue that the energy shift is caused by the low carrier density in FeTe0.55Se0.45, which allows the electric field of the tip to slightly penetrate the sample. We model this tip-gating scenario within the single-impurity Anderson model and find good agreement to the experimental data.
2H- and 1T'- MoTe2 islands on graphene/Ir(111): Growth, topography and electronic structure
Pablo Casado Aguilar, Universidad Autónoma de Madrid
P. Casado Aguilar (1,2), A. L. Vázquez de Parga (1,2), R. Miranda (1,2) and M. Garnica (2)
(1) Departamento de Física de la Materia Condensada and IFIMAC, Universidad Autónoma de Madrid, Madrid, Spain
(2) Instituto Madrileño de Estudios Avanzados en Nanociencia (IMDEA-Nanociencia), Madrid, Spain
Transition Metal Dichalcogenides (TMDs) are layered materials with a MX2 composition, where M denotes a transition metal (such as Mo, W, Pt, Pd…) and X a chalcogen element (S, Se or Te). They present different electronic properties ranging from insulators (e.g. HfS2), semiconductors (e.g., MoS2 and WS2), metals (e.g., TaSe2), semimetals (e.g. WTe2) as well as superconductor candidates (e.g. NbSe2). Interestingly, most of these properties depend on their thickness and phase, such as the direct semiconducting hexagonal phase (2H) ranging from indirect bandgap in bulk to a direct bandgap at the monolayer [1] or the semimetallic distorted octahedral phase (1T′) predicted to exhibit quantum spin Hall (QSH) effect in the monolayer regime [2]. Here, we report the growth of 2D-islands of MoTe2 on graphene grown on the (111) face of an Iridium single crystal by molecular beam epitaxy. Their structural characteristics as well as their electronic properties are studied by means of scanning tunneling microscopy/spectroscopy (STM/STS). We obtain both 2H- and 1T'-phases, which remain decoupled from the substrate due to the weak interaction with graphene revealing their different electronic nature. Apart from their characteristic electronic structure, other electronic effects are observed as a result of the appearance of defects, island edges or borders between structures.
[1] I. Gutiérrez Lezama et al. Surface transport and band gap structure of exfoliated 2H- MoTe2 crystals. 2D Materials, 2014, 1, 2.
[2] X. Qian et al. Quantum spin Hall effect in two-dimensional transition metal dichalcogenides. Science, 2014, 346, 6215, 1344-1347.
[3] R. Lv et al. Transition Metal Dichalcogenides and Beyond: Synthesis, Properties, and Applications of Single- and Few-Layer Nanosheets. Acc. Chem. Res., 2015, 48 (1), 56–64.
PabloCasado.mp4Discrete electronic subbands due to Bragg scattering at molecular edges
Alberto Martín-Jiménez, Instituto Madrileño de Estudios Avanzados en Nanociencia
A. Martín-Jiménez (1), J. M. Gallego (2), R. Miranda (3,1) and R. Otero (3,1)
(1) Instituto Madrileño de Estudios Avanzados en Nanociencia (IMDEA-NANO), 28049 Madrid, Spain
(2) Instituto de Ciencia de Materiales de Madrid (ICMM-CSIC), 28049 Madrid, Spain
(3) Dep. de Física de la Materia Condensada and Condensed Matter Physics Center (IFIMAC), Universidad Autónoma de Madrid, 28049 Madrid, Spain
The discretization of the electronic structure of nanometer-size solid systems due to quantum confinement and the concomitant modification of their physical properties is one of the cornerstones for the development of nanoscience and nanotechnology. In this Letter we demonstrate that the Bragg scattering of Cu(111) surface-state electrons by the periodic arrangement of tetracyanoquinodimethane molecules at the edges of self-assembled molecular islands, along with the dominant contribution of backscattering processes to the electronic density of states, discretizes the possible values of the electron momentum parallel to the island edge. The electronic structure consists thus of a discrete number of subbands which occur in a nonclosed space, and therefore without quantum confinement.
A. Martín-Jiménez et al. Phys. Rev. Lett. 122, 176801 (2019)
AMJ_Bragg_TCNQ_Cu111.mp4Spin dynamics in single Dy adatoms on graphene/Ir(111) studied by SP-STM
Alberto Curcella and Dante Sblendorio, Ecole polytechnique fédérale de Lausanne
A. Curcella, D. Sblendorio, S. Rusponi, M. Pivetta, F. Patthey and H. Brune
Institut de Physique, Ecole polytechnique fédérale de Lausanne, Switzerland
Manipulation of magnetic states in nano-objects and in surface adsorbed single atoms offers new avenues for future spintronic applications and the realization of ultra-high-density magnetic memory devices. Spin-polarized scanning tunneling microscopy (SP-STM) provides information about morphology as well as the magnetization state of the targeted nanomagnet. We use SP-STM with a MnNi anti-ferromagnetic tip [1] to investigate the stability of the magnetization in single Dy adatoms on graphene/Ir(111) at liquid He temperature. Telegraph-noise traces demonstrate the bistability of the Dy magnetization. They indicate long relaxation times of a few minutes, consistent with previous XMCD measurements [2]. What is new compared to former SP-STM studies on rare earth adatoms is that we find an extremely high magneto-resistance [3]. Moreover, the occupancy of the magnetic states as a function of tunnel voltage is evidence for spin-torque [4]. We attempt to reproduce the experimental observations using a model accounting for scattering of the Dy spin with tunneling electrons [5] and with substrate phonons [6].
[1] P. R. Forrester et al., Rev. Sci. Instrum. 89, 123706 (2018)
[2] R. Baltic et al., Nano Lett. 16, 7610-7615 (2016)
[3] M. Pivetta et al., submitted (2020)
[4] A. A. Khajetoorians et al., Science 339, 55-59 (2013)
[5] F. Delgado et al., Phys. Rev. B 82, 134414 (2010)
[6] A. Fort et al., Phys. Rev. Lett. 80, 612 (1998)
Sblendorio.mp4Chiral single-molecule magnets: the investigation of a Dy-based system sublimated on metal surfaces
Andrea Luigi Sorrentino, University of Florence
Andrea Luigi Sorrentino (1,2), Jeanne Crassous (3), Lorenzo Poggini (1), Giulia Serrano (1,2), Brunetto Cortigiani (1), Fabrice Pointillart (3), Roberta Sessoli (1), and Matteo Mannini (1)
(1) Department of Chemistry “Ugo Schiff” and INSTM RU, University of Florence, Via della Lastruccia 3, 50019, Sesto Fiorentino (FI), Italy
(2) Department of Industrial Engineering, University of Florence, Via S. Marta 3, 50139, Firenze
(3) Institut des Sciences Chimiques de Rennes, UMR 6226 CNRS – Universite´ de Rennes 1, 263 Avenue du Général Leclerc, 35042 Rennes Cedex, France
Single-molecule magnets (SMMs) that exhibit magnetic bistability and quantum tunneling of the magnetization, are significant candidates to be exploited for the improvement of molecular spintronics devices [1]. Earlier studies on the assembly of SMM complexes on metal substrates, fundamental prerequisite for the development of hybrid devices, evidenced that the persistence of the peculiar SMMs properties is strongly dependent on the molecule-surface interactions but this disclosed another perspective, the possibility of using this interaction to influence and to study the properties of magnetic and superconductive surfaces and to develop novel spintronics devices [2]. Furthermore, in the last years chiral molecules have aroused interest thanks to the “CISS effect” (Chiral Induced Spin Selectivity) that opens the possibility of using a chiral system as spin filtering agent in spintronic devices. A novel class of molecular magnets has been growing interest as they feature combining chirality and SMM behavior thus representing the ideal candidate in this perspective. Among them the Dy(hfac)3L complex with L=3-(2-pyridyl)-4-aza[6]-helicene do not show, in crystals, hysteresis opening in the racemic phase, at variance with enantiomerically pure crystals featuring slow dynamics of the magnetization at a temperature of 0.5 K [3]. Here, we report the spectroscopic and structural study of Dy(hfac)3L complex sublimated on different metal surfaces (i.e. Au (111) and Cu (100)). Thanks to a multi-technique approach, employing Scanning Tunneling Microscopy (STM) and X-Ray Photoelectron Spectroscopy (XPS) it has been possible to demonstrate the non-innocent role of the substrate on the bi-dimensional structure of sub-monolayer coverage both in structural and electronic properties.
[1] D. Gatteschi et al., “Molecular Nanomagnets” 2006.
[2] G. Serrano et al., Nat. Mater., vol. 19, pp. 546–551, 2020.
[3] N. Saleh et al., Chem. Commun., vol. 52, pp. 14474–14477, 2016.
Transition from Yu-Shiva-Rusinov states to Kondo screening of a charged TCNQ molecule on a Nb STM tip
Cosme. G. Ayani, IMDEA Nanociencia & Universidad Autonoma Madrid
C.G. Ayani (1,2), F. Calleja (1), P. Casado (1,2), M. Garnica (1), A. L. Vazquez de Parga (1,2), R. Miranda (1,2)
(1) Instituto Madrileño de Estudios Avanzados en Nanociencia (IMDEA Nanociencia), Cantoblanco 28049, Madrid, Spain.
(2) Departamento de Física de la Materia Condensada, Instituto Nicolás Cabrera and IFIMAC, Universidad Autónoma de Madrid, Cantoblanco 28049, Madrid, Spain.
One of the main fields of research in condense matter physics is the interaction of magnetic impurities with their hosts, as they can deeply modify their original properties [1]. Two of the most studied are: the screening of the magnetic moment by the normal metal electrons giving rise to the well know Kondo effect [2], and the interaction with Cooper pair quasiparticles producing the Yu-Shiva-Rushinov bound states [3]. In this work we explore the transition from YSR bound states to Kondo screening by employing the magnetic field (Bmax=3 T) of a commercial JT-STM. The magnetic impurity is provided by a charged TCNQ molecule absorbed on a SC Nb STM tip with a critical magnetic field of BC=1.2 T. As the TCNQs are physisorphed to the substrate, gr/Ir(111) [4], they can be vertically manipulated from the surface to the STM tip. The fingerprint that a molecule has jumped to the tip apex is the appearance of two sub gap states. The appearance of one state or two and the relative height of the pair states depend on the coupling of the magnetic impurity to the SC [3]. Finally, we can observe the transition from Shiba bound states to Kondo screening by ramping the magnetic field up to 1.5 T were the SC state of the tip is supressed.
[1] Pervin R, Krishnan M, Rana A K, Kannan M, Arumugam S and Shirage P M 2017 Enhancement of superconducting critical current density by Fe impurity substitution in NbSe2 single crystals and the vortex pinning mechanism Phys. Chem. Chem. Phys. 19 11230–8
[2] Ternes M, Heinrich A J and Schneider W D 2009 Spectroscopic manifestations of the Kondo effect on single adatoms J. Phys. Condens. Matter 21 053001
[3] Heinrich B W, Pascual J I and Franke K J 2018 Single magnetic adsorbates on s -wave superconductors Prog. Surf. Sci. 93 1–19
[4] Garnica Alonso M 2013 ELECTRON ACCEPTOR MOLECULES DEPOSITED ON EPITAXIAL GRAPHENE STUDIED BY MEANS OF LOW TEMPERATURE SCANNING TUNNELING MICROSCOPY/SPECTROSCOPY phd thesis.
CosmeGonzalez.mp4Molecular structure elucidation with charge-state control
Shadi Fatayer, IBM Research - Zurich
Shadi Fatayer, Florian Albrecht, Nikolaj Moll, Leo Gross
IBM Research, Zurich, Switzerland
The charge state of an adsorbed molecule significantly affects its physical as well as its chemical properties, for example, adsorption position, molecular conformation and aromaticity. The proven capabilities of single-electron sensitivity [1,2] and atomic-resolution [3] of atomic force microscopy (AFM) make it an ideal tool to perform charge-state manipulation experiments while atomically resolving the induced changes within the molecule. Here, we present AFM-based results that simultaneously demonstrate both the control in charge-state and the capability of resolving the atomic structure of a single molecule for different molecules [4].
[1] L. Gross, et al. Science 324, 5933 (2009)
[2] S. Fatayer, et al. Nature Nanotechnology 13, 376 (2018)
[3] L. Gross, et al. Science 325, 5944 (2009)
[4] S. Fatayer, et al. Science 365, 6449 (2019)
ShadiFatayer_virtualSPMfront.pdfOn-surface synthesis of low-dimensional sp-sp2 hybrid carbon systems on Au(111): atomic-scale structure, electronic and vibrational properties
Andi Rabia, Politecnico di Milano
A. Rabia (1), F. Tumino (1), V. Russo (1), A. Milani (1), A. Li Bassi (1), S. Achilli (2), G. Fratesi (2), G. Onida (2), N. Manini (2), Q. Sun (3,4), W. Xu (4), C. Casari (1)
(1) Politecnico di Milano, Department of Energy, Milan, Italy
(2) Università degli Studi di Milano, Department of Physics, Milan, Italy
(3) Empa- Swiss Federal Laboratories for Materials Science and Technology, nanotech@surfaces Laboratory, Dübendorf, Switzerland
(4) Tongji University, College of Materials Science & Engineering, Shanghai, China
Low-dimensional carbon allotropes, e.g. carbon atomic wires (CAWs) and mixed sp-sp2 hybridized nanostructures (graphyne, graphdiyne), obtained a growing importance in nanoscience for the outstanding functional properties predicted by theory [1]. However, synthesis of highly ordered sp-sp2 carbon-based systems has been achieved only recently in ultra-high vacuum (UHV), exploiting chemical reactions of molecular precursors on the surface of metallic substrates and allowing Scanning Tunneling Microscopy/Spectroscopy (STM/STS) techniques to image the structure and provide insights into local electronic properties.
In this context, our work focuses on the synthesis and characterization of nanostructures based on sp-sp2 carbon, both as 1D molecular wires and 2D networks, and the investigation of their structure, electronic (by STM/STS) and vibrational properties (by Raman spectroscopy).
By depositing onto Au(111) in UHV specifically designed molecular precursors consisting in bromoethynyl moieties linked to benzene rings, we were able to synthesize and investigate the 1D [2] and 2D system by means of in-situ STM analyzing their structural changes after heating the samples at temperatures >400 K. STS measurement, supported by theoretical calculations based on Density Functional Theory (DFT), reported the surface electronic properties as a result of the hybridization between molecular and substrate states. In addition, we have performed the characterization of the vibrational properties of sp-sp2 carbon nanostructures by means of Raman spectroscopy, supported again by DFT calculations on suitable molecular models.
Our combined experimental/computational approach represents a powerful and non-destructive strategy to study the fundamental properties of surface-supported sp-sp2 carbon nanostructures, such as graphyne and graphdiyne.
[1] C. S. Casari et. al., Nanoscale 2016, 8, 4414–4435
[2] A. Rabia et. al., Nanoscale 2019, 11, 18191–18200
AndiRabia.mp4