Coming Visitors and Research Stays:

Academic Affiliation: Ajou University

Date and Time of the Talk: May 27th, 2022; 10:15 – 11:15

First-principles theory of extending the spin qubit coherence time in hexagonal boron nitride

Negatively charged boron vacancies (VB–) in hexagonal boron nitride (h-BN) are a rapidly developing qubit platform in two-dimensional materials for solid-state quantum applications. However, their spin coherence time (T2) is very short, limited to a few microseconds owing to the inherently dense nuclear spin bath of the h-BN host. As the coherence time is one of the most fundamental properties of spin qubits, the short T2 time of VB– could significantly limit its potential as a promising spin qubit candidate. In this study, we theoretically proposed two materials engineering methods, which can substantially extend the T2 time of the VB– spin by four times more than its intrinsic T2. We performed quantum many-body computations by combining density functional theory and cluster correlation expansion and showed that replacing all the boron atoms in h-BN with the 10B isotope leads to the coherence enhancement of the VB– spin by a factor of three. In addition, the T2 time of the VB– can be enhanced by a factor of 1.3 by inducing a curvature around VB–. Herein, we elucidate that the curvature-induced inhomogeneous strain creates spatially varying quadrupole nuclear interactions, which effectively suppress the nuclear spin flip-flop dynamics in the bath. Importantly, we find that the combination of isotopic enrichment and strain engineering can maximize the VB– T2, yielding 207.2 and 161.9 μs for single- and multi-layer h-10BN, respectively. Furthermore, our results can be applied to any spin qubit in h-BN, strengthening their potential as material platforms to realize high-precision quantum sensors, quantum spin registers, and atomically thin quantum magnets.

[1] A. Gottscholl et al., Nat. Mater. 19, 540-545 (2020)

[2] A. Gottscholl et al., Sci. Adv. 7, eabf3630 (2021). 

[3] M. Ye, H. Seo, and G. Galli, npj Comp. Mater. 5, 44 (2019).

Academic Affiliation: Delft University of Technology

Research Area: Physical properties of atomic structures on surfaces for fundamental and engineering pursuits

Research stay period: September, 2021 – May, 2022

Ferdous Ara

Academic Affiliation: Centro De Fisica De Materiales, Spain

Research Area: Nanomagnetism combined with superconductivity

Research stay period: November 15, 2021 – January 31, 2022

Academic Affiliation: CSIC and DIPC, Spain

Research Area: Theory of Electronic Properties at the Nanoscale

Research stay period: November 1, 2021 – January 31, 2022

Academic Affiliation: Centro de Física de Materiales CFM/MPC (CSIC-UPV/EHU)

Fields of Specialisation: Study of magnetic impurities on superconducting surfaces.

Research stay period: November 1, 2021 – February 24, 2022

Academic Affiliation: Research associate, Institute for Solid State Physics, University of Tokyo

Fields of Specialisation: Heavy fermion compounds, Fe-based superconductors, Magnetism at interfaces and surfaces, Magnetic resonance

Research stay period: March 2021 – September 2021

Academic Affiliation: School of Physics and Astronomy, the University of Birmingham, Birmingham, UK

Fields of Specialisation: Quantum and frustrated magnetism; Quantum criticality; Nuclear Magnetic Resonance (NMR); Low temperature/high magnetic field experiments.

Research stay period: April 19, 2021 – May 21, 2021

Academic Affiliation: YST Research Fellow, Asia Pacific Center for Theoretical Physics (APCTP), POSTECH Campus, Korea

Fields of Specialisation: Calculating electronic, phonon, magnetic, optical, structural, ferroelectric, thermoelectric, elastic other related properties. Interfacial structures, surface slabs, nudged elastic band calculations

Research stay period: March 13 – 22, 2021

Academic Affiliation: Research associate, Institute for Solid State Physics, University of Tokyo

Fields of Specialisation: Heavy fermion compounds, Fe-based superconductors, Magnetism at interfaces and surfaces, Magnetic resonance

Research stay period: September 2020 – December 2020

Academic Affiliation: Centro De Fisica De Materiales, Spain

Research Area: Nanomagnetism combined with superconductivity

Research stay period: May 1 – September 30, 2020

Academic Affiliation: CSIC and DIPC, Spain

Research Area: Theory of Electronic Properties at the Nanoscale

Research stay period: July 15 – August 31, 2020

Academic Affiliation: C2N, Department of Nanoelectronics, CNRS, France

Research Area: STM spectroscopy, Superconductivity, Topological Insulators

Research stay period: February 2020 – July 2020

Academic Affiliation: Quantum Nanoscience Division, Peter Grünberg Institut, Forschungszentrum Jülich

Visit Dates: February 24-28, 2020

Academic Affiliation: Princeton University, USA

Research Area: At the forefront of quantum materials research is the goal to understand how new quantum phenomena can emerge from the topology of electronic wavefunctions or correlations arising from electron-electron interactions. The Yazdani Lab has contributed significantly to this research paradigm by harnessing the power of high-resolution scanning tunneling microscopy (STM) techniques to directly visualize electronic wavefunctions in topological and correlated quantum materials.

Research stay period: February 12 – 13, 2020

More detailed information about the visit will be updated soon!

Academic Affiliation: Research Center Jülich, Germany

Research Area: Understing fundamental properties on the nanoscale with the focus on magnetic excitations and correlated systems.

Research stay period: February 09 – 22, 2020

During the visit Prof. Ternes will give three talks in our Center.

Date: January 20th, 2020
Time: 12:30-15:30

QNS will meet Science Counselors and Deputies of EU Countries + Switzerland and Norway.
During their visit we present of our center and provide the labs tour.

More detailed schedule will be updated soon!

Academic Affiliation: Leiden University, Netherlands

Research Area: Develop and use novel techniques for STM to investigate quantum matter.

Research stay period: January 19 – 23, 2020

During the stay Dr. K. Bastiaans gives two talks in our Center:

1. Charge trapping and super-Poissonian noise centers in a cuprate superconductor
January 21, 2020; 15:00-16:00

2. A strongly inhomogeneous superfluid in an iron- based superconductor
January 22, 2020; 15:00-16:00

Academic Affiliation: Wigner Research Center for Physics, Hungary

Talk: December 13, 2019

First-principles-based simulation of scanning tunneling microscopy: From magnetic surfaces to molecular structures

Understanding and engineering scanning tunneling microscopy (STM) image contrasts is of crucial importance in wide areas of surface science and related technologies, ranging from magnetic surfaces to molecular structures. In the talk different STM tip effects on the image contrast are highlighted based on first principles calculations, going beyond the Tersoff-Hamann model, e.g., within 3D-WKB tunneling theory [1]. Examples include highly oriented pyrolytic graphite [2], which is commonly used for STM calibration, and complex surface magnetic structures exhibiting non-collinear magnetic order, like recently popular topologically protected skyrmions [3]. By comparing STM topographic data between experiment and large scale simulations, a statistical analysis of the tip apex structure is demonstrated for the first time [2]. A combination of STM and X-ray photodiffraction helps the understanding of chirality transfer from molecules to crystal surfaces [4]. Furthermore, two recent developments of STM theories are presented: (i) an extension of Chen’s derivative rule [5] for STM simulations including tip-orbital interference effects with demonstrated importance of such effects on the STM contrast for two surface structures: N-doped graphene and a magnetic Mn2H complex on the Ag(111) surface [6]; (ii) a combined tunneling electron charge and vector spin transport theory, which provides the first steps toward the theoretical modeling of high-resolution spin transfer torque imaging [3,7].

References:
[1] K. Palotás et al., Front. Phys. 9, 711 (2014)
[2] G. Mándi et al., J. Phys.: Condens. Matter 26, 485007 (2014), Prog. Surf. Sci. 90, 223 (2015)
[3] K. Palotás et al., Phys. Rev. B 96, 024410 (2017), Phys. Rev. B 97, 174402 (2018), Phys. Rev. B 98, 094409 (2018)
[4] W. Xiao et al., Nature Chem. 8, 326 (2016)
[5] C. J. Chen, Phys. Rev. B 42, 8841 (1990)
[6] G. Mándi, K. Palotás, Phys. Rev. B 91, 165406 (2015)
[7] K. Palotás et al., Phys. Rev. B 94, 064434 (2016)

Academic Affiliation: Institute of Chemistry and Biochemistry, Freie Universität Berlin

Talk: December 13, 2019

Charge transfer processes on well-defined heterogeneous model catalysts: an EPR perspective

Surfaces and their interaction with the surrounding environment play a pivotal role in a large variety of technical applications ranging from heterogeneous catalysis, coatings all the way to biological systems. Gaining insight into these systems at the atomic level is one of the key goals of today’s research, however, it is still challenging for most of the complex technological systems. With respect to heterogeneous catalysis oxides play an important role both as support material for metal nano-particles or as catalysts themselves. It is well established that much of the chemical processes at such surfaces do not happen on ideal stoichiometric low index surfaces. Instead higher index surfaces, structural defects like steps, vacancies in the lattice or thin oxide films grown on metal nano-particles -commonly referred to as the strong metal support (SMSI) effect- are important to understand the properties of the surfaces. Even though this is known for quite some time a characterization at the atomic level and an elucidation of the impact on the chemical properties is still challenging. One strategy to gain insight into these properties is using well-defined model systems which grasp essential aspects of the systems of interest but can be studied with the rigor of modern surface science methodology. In this presentation it will be shown how electron paramagnetic resonance (EPR) spectroscopy, a tool rarely used in surface science, can help to gain insight into the properties of defect sites as well as their impact on chemical and physical properties of the surfaces.

The presentation will focus on results obtained on single crystalline, epitaxial MgO oxide films grown on metal single crystal surfaces and discuss charge transfer processes that are unexpected to take place in stochiometric MgO, but become important in case paramagnetic point defects such as color center or transition metal dopants are present in the system. The role of these sites for the activation of small molecules by charge transfer will be elucidated using molecular oxygen a key ingredient for oxidation catalysis. Furthermore, it will be shown that such charge transfer processes, which are considered to be important for catalysis cannot only be induced by defect sites, but may also happen on stoichiometric oxide surfaces in case the oxide is present as an ultrathin film on a metal surface. The evidence based on EPR spectroscopy as well as tunneling microscopy for these processes and the implication of this for catalysis will be discussed.

Academic Affiliation: The Ohio State University

Research Area: Low-Temperature Scanning Tunneling Microscopy

Period of Stay: December 9 – 21, 2019

During the stay Mr. Repicky will give a talk in our center:

Date and Location: December 18th, 2019 / Saturn Seminar Room

Magnetic Textures in Chiral Magnet MnGe Observed with SP-STM

Materials with non-centrosymmetric crystal structures can host chiral spin states including magnetic skyrmions. Bulk MnGe hosts a short period magnetic state (3 nm), whose structure depends strongly on atomic lattice strain, and shows a large emergent transport signature associated with the skyrmion phase. Here, we use low-temperature (5 K) spin-polarized scanning tunneling microscopy (SP-STM) to image the magnetic textures in MnGe thin films grown via molecular beam epitaxy and study the influence of the surface on those textures. Most microscopic locations show a spin spiral phase with a 6-8 nm period and a propagation direction that is influenced by step edges, surface termination, and strain. We also report the presence of isolated target skyrmions that likely form due to local modulations in the atomic structure. The skyrmions have a triangular shape which appears to be set by the in-plane lattice vectors, and a core size of approximately 15 nm. We observe the target state is significantly more sensitive to magnetic fields than the spiral phase, and that local voltage and current pulses with the STM tip imply the texture can be ‘switched’ between states with different topological charge. To fully understand the magnetic textures in MnGe we will expand this study by investigating films of different thicknesses to vary the magnetic anisotropy.

Academic Affiliation: Institute of Physics, Chinese Academy of Sciences

Research area: Scanning Tunneling/Probe microscopy, Specially Instrumentation

Design of a bath-type cryostat and a tip-etching unit

In this talk I will mainly show a bath-type cryostat and a tip-etching unit that we designed. Detailed designing processes, like FEA (finite element analysis), circuit design, will be discussed. I will also show some of very new results from our home-made systems.

Affiliation: Department of Physics, Yonsei University

Research Field: Transition metal dichalcogenides, Topological insulators, Unconventional superconductors, High frequency measurements

Scanning noise spectroscopy

Scanning tunneling spectroscopy has become indispensable for investigating the local electronic structure of correlated electron systems. However, valuable information about the dynamics in electric charge transport cannot be accessed by conventional time-averaged spectroscopy techniques. An example is the granularity of charge that leads to current fluctuations; so called shot noise. Correlations can lead to deviations from Poissonian noise which are smeared out in the averaged current value. In mesoscopic systems, noise-spectroscopy measurements have been widely used to study the dynamics of strongly correlated phenomena. Here, we present a newly developed noise spectroscopy technique, for which we combine a Scanning Tunneling Microscope (STM) with a novel MHz amplifier to bring noise-spectroscopy measurements to the atomic scale. We demonstrate the Poissonian tunneling process on a Au(111) surface and the Andreev reflection induced multiple charge tunneling in a Josephson junction. In addition, we observe unexpected non-Poissonian tunneling process on a cuprate high temperature uperconductor with atomic resolution. This provides us a new way to unveil electronic properties hidden in the time-averaged transport measurements on exotic quantum materials

Affiliation: Max-Planck-Institute for Solid State Research, Germany

Research Field: Quantum Limits in Scanning Tunneling Microscopy, Superconductivity

During his stay Dr. Christian Ast will give 3 seminars in our center:

1. Introduction to Superconductivity and Scanning Tunneling Microscopy

November 11, 14:00 – 15:00

Scanning tunneling microscopy (STM) is the most important experimental tool to explore the world of nanoscience due to its unique capabilities to resolve single atomic structures. STM exploits the tunneling effect to feel the topography of the sample surface at picometer length scales. Beyond the imaging functionality, the tunneling electrons themselves carry a wealth of information about the electronic properties of the substrate. This becomes particularly interesting when superconductivity is involved, because the elementary excitation in a superconductor is a Bogoliubov quasiparticle, a linear superposition of an electron and a hole, but only electrons or holes can tunnel. I will give an introduction to the basics of tunneling in the STM with a particular focus on tunneling between superconductors.

2. Experimental Details of a Dilution Fridge STM

November 12, 14:00 – 15:00

Reaching lowest temperatures in scanning tunneling microscopy (STM) requires combining the microscope with a dilution refrigerator, which triggers interesting technical. Most importantly, a dilution fridge is not a quiet piece of equipment, but STM requires the lowest noise possible. Even measuring temperature becomes a non-trivial problem. Also, the choice of materials that perform well at lowest temperatures, but are at the same time suitable for sample preparation at high temperatures is very limited. Furthermore, modeling the tunnel junction to interpret the experimental data at mK temperatures requires to go beyond the tunnel junction itself to macroscopic scales. The lower the temperature, the more the experiment becomes part of the experiment. I will discuss the challenges of building a mK-STM and interpreting the experimental data, which in the end leads to a much better understanding of the tunneling process as a whole.

3. Sensing the Quantum Limits in Scanning Tunneling Spectroscopy

November 14, 14:00 – 15:00

Sixty years ago in December 1959, when Richard Feynman said that there is “Plenty of Room at the Bottom”, he envisioned the seemingly endless possibilities of the microscopic world, which we now refer to as Nanoscience. However, his statement also implies that there is a bottom meaning that there is a limit to the miniaturization of every process. I will discuss the fundamental limits of transport in scanning tunneling microscopy. This concerns, for example, the changes in the tunneling process imposed by revealing the quantum nature of the electron charge at extremely low temperatures. Further, we exploit magnetic impurities coupled to a superconductor to create single quasiparticle levels inside the superconducting gap (Yu-Shiba-Rusinov states). I will show how we use these states to demonstrate the bare minimum of what is necessary to create a tunneling current.

Affiliation: Tohoku University, Sendai, Japan

Scanning Tunneling Micrososcopy(STM) Study of Supramolecule Formation and Spin Properties of Radical Molecules of Cobalt Tetrakis(1,2,5-thiadizole) Porphyrazine(CoTTDPz) and 1,3,5-Trithia-2,4,6-Triazapentalenyl(TTTA)

Organic molecule films adsorbed on metal substrates have been widely reported because of the potential application on electronic devices. In this context, metallo-phthalocyanines (MPcs) and their derivatives are intensively studied. However, most of related works above are based on weak molecule-molecule interaction system while a series of Pc derivatives with strong intermolecular force have been synthesized, called metallo-tetrakis(1,2,5-thiadiazole) porphyrazines (MTTDPz). LT-STM study of CoTTDPz on Au(111) from sub-monolayer to multilayer will be introduced in this talk. Image and STS are applied to characterize the bonding configuration and electronic properties of molecules. In addition, the spin properties of CoTTDPz have been studied by detecting Kondo feature on molecules, which show different behaviours form CoPc.

STM observation of TTTA on Au(111) is also described. It belongs to heterocyclic π-radicals family which have been widely studied in recent years, because of the reversible process that the dimers are disposed to thermally driven dimer (S=0) to radical (S=1/2). In my experiment, coverage of TTTA is controlled by adjusting exposing time and annealing. When coverage is near 1 monolayer, two types of TTTA-Au coordination are confirmed and both of them have chirality, electrostatic bonding was also observed in lattice which only appeared between two identical arrays. At low coverage, hierarchical structures made up by threefold units were dominating, the height of Au adatoms are higher than those in arrays. While the mixture of the two structures occurred without initial annealing, this work was probably the first real space observation of TTTA.

Affiliation: Michigan State University, USA

Seminar Time: October 4th, 2019, 11:00-12:00

Ultrafast terahertz scanning tunneling microscopy of atomically precise nanostructures

Terahertz scanning tunneling microscopy (THz-STM) is a new ultrafast imaging technique that provides access to femtosecond dynamics on the atomic scale for the first time. Its versatility enables numerous scientific opportunities, including ultrafast studies of electron dynamics within single molecules and nanostructures. In this talk, I will introduce a conceptual basis for ultrafast terahertz scanning tunneling spectroscopy, show THz-STM snapshot imaging of local electron densities, and discuss future directions in the field.

Affiliation: ALBA Synchrotron Light Source, Barcelona, Spain

Research Field: Electronic and magnetic ground state of spin-orbit active transition metal compounds using x-ray spectroscopy (XAS, XMCD, RIXS)

Date: September 25 – 27, 2019

Location: Center for Quantum Nanoscience at Ewha Womans University in Seoul, South Korea

Website: https://icqns.org/

Our Invited Speakers:

• Andreas Heinrich – Director of IBS Center for Quantum Nanoscience, Korea
• Andrew Dzurak – School of Electrical Engineering and Telecommunications, UNSW, Australia
• Ania Jayich – Department of Physics, UCSB, USA
• Arzhang Ardavan – Department of Physics, Oxford Univ., UK
• Daniel Loss – Department of Physics, Univ. of Basel, Switzerland
• Donghun Lee – Physics Department of Korea Univ., Korea
• Fabio Donati – Center for QNS / Department of Physics at Ewha Univ. Korea
• Jelena Klinovaja – Department of Physics, Univ. of Basel, Switzerland
• Jörg Wrachtrup – Physics Department of University of Stuttgart, Germany
• Martin Plenio – Institute of Theoretical Physics in University of Ulm, Germany
• Roberta Sessoli – Department of Chemistry, Univ. of Florence, Italy
• Taeyoung Choi – Center for QNS / Department of Physics at Ewha Univ., Korea
• Taner Esat – Center for QNS, Korea
• William D. Oliver – Department of Physics, Director at MIT Lincoln Lab, USA
• Wolfgang Wernsdorfer – Department of Physics, KIT Karlsruhe, Germany
• Yonuk Chong – University of Science and Technology(UST), Korea
• Yujeong Bae – Center for QNS, Korea

Affiliation: Centro de Física de Materiales (CSIC-UPV/EHU)

Period of Stay: September 1 ~ September 30

Research Field: Nanomagnetism combined with superconductivity

Academic Affiliation: École polytechnique fédérale de Lausanne

Research area: Investigation of magnetic properties of nanostructures with low-temperature scanning tunneling microscopy and spectroscopy

Visit Dates: August 31 – September 2

Affiliation: Freie Universität Berlin

Period of Stay: August 12 ~ September 19

Research Field: Scanning Tunneling Microscopy of magnetic adatoms on superconductors, Molecules on surfaces

– Seminar 1: August 14, 13:00-14:00 – “Tuning the exchange and potential scattering strength of individual magnetic adsorbates on superconductors”
– Seminar 2: August 21, 11:00-12:00 – “Transport properties through Shiba states”

Affiliation: CNRS – University Paris-Saclay

Talk: August 12th,2019

Research Field: STM – spectroscopy, Quantum effects on superconductivity

Hybrid semiconducting-superconducting systems and superconductivity at the Anderson limit

Hybrid semiconducting-superconducting systems have attracted intense interest because of their potential applications to the fields of superconducting spintronic and topological superconductivity. After a brief overview of hybrid superconducting-semiconducting systems, including some recent works in our group [1,2], I will present a scanning tunneling spectroscopy study of superconducting lead (Pb) nanocrystals grown on the (110) surface of InAs.

The existence of Cooper pairing and the nature of superfluid order in clusters with a small number of Fermions are questions of fundamental interest that have implications for various systems, from atomic nucleus to Helium clusters and superconducting nanocrystals. With its atomic resolution capabilities, STM is an ideal instrument for the study of the electronic spectrum of small size superconducting nanocrystals. Yet, so far, such studies have been hampered by the two antagonistic requirements that the substrate must be conducting and the nanocrystals only weakly coupled to this substrate.Instead of a simple metallic substrate, we found that the 2D electron gas of large Fermi wavelength at the surface of InAs provides an ideal substrate for the growth and study of superconducting lead (Pb) nanocrystals in the regime of quantum confinement.

This discovery allowed to study, for the first time by STM, the superconducting parity effect in nanocrystals as function of their volume and to demonstrate unambiguously the validity of the Anderson criterion, conjectured in 1959, for the existence of superconductivity in nanocrystals submitted to quantum size effects [3].

References:
1. Assouline, A. et al. Spin-Orbit induced phase-shift in Bi2Se3 Josephson junctions.
Nat. Commun. 10, 126 (2019).
2. Assouline, A. et al. Shiba Bound States across the Mobility Edge in Doped InAs Nanowires.
Phys. Rev. Lett. 119, 097701 (2017).
3. Vlaic, S. et al. Superconducting parity effect across the Anderson limit.
Nat. Commun. 8, 14549 (2017).

Affiliation: Zurich Instruments AG

Talk: August 9, 2019

Research Field: Scanning Probe Microsopy, Digital Signal Processing

Smart versus Big Data in SPM: from frequency to time domain analysis

Non-linearity or transient phenomena in Scanning Probe Microscopy (SPM) tip-sample interaction are responsible for large deviations in simple harmonic oscillator models that necessitate various multifrequency techniques or timeresolved analysis. By now, many different methods of data deconvolution have been used, some of which may be platform dependent to a single SPM manufacturer. In this talk, I would like to focus on smart and universal data acquisition using FPGA-based hardware and server-based software, which can work with any third-party instruments. Both time and frequency domain analysis can then be performed in realtime or streamed to a usable data set for further calculation. The aim is to either crunch down data to its essential components using lock-in techniques in open or closed loops or to accurately align large digitized data streams in a useable matrix format [1]. The ability to bring Arbitrary Waveform Generator (AWG) in the SPM path allows for precise control of delays for either pure electrical pump-probe approach or in combination with optical methods [2]. Extension of this to the manipulation of qubit state will also be addressed.

References:

[1] Ehsan Nasr Esfahani, PhD dissertation, University of Washington, Seatle, 2018
[2] Time-resolved SPM

Affiliation: National Institute of Standards and Technology, MD, United States

Talk: July 4, 2019

Improvement of milli-Kelvin Scanning Probe Microscope System and Edge State Studyof Graphene Quantum Hall Device

In this talk, I present the improvement of functionality and energy resolution of tunneling spectroscopy of the NIST milliKelvin SPM system, and recent studies performed by this system on graphene quantum Hall device. The NIST millikelvin SPM system was constructed in 2010 as the first generation of millikelvin STM by using dilution refrigerator, and it has been operated to conduct the number of researches on 2D materials such as graphene, topological insulators. Recently, implementation of qPlus sensor AFM on the SPM module made it overcome the limitation of tunneling current measurement. By applying multi-contact sample holder and auto navigating algorithm we accomplished the in-situ transport measurement. In addition, by combining RF filters and proper electrical grounds, we achieved an ultra-high energy resolution tunneling spectroscopy – better than 10ueV energy resolution – for measuring Josephson peak spectrum on superconductor-superconductor junction, which is the best level of energy resolution in tunneling spectroscopy measurement by using SPM so far. For the analysis of high energy resolution spectroscopy, P(E) model and Maki’s theory are applied. In addition, recent studies of graphene quantum Hall edge state by AFM and KPFM measurements will be introduced.

The information about time and location of the talk will be updated soon.

Affiliation: Institute for Quantum Computing/University of Waterloo & the Perimeter Institute for Theoretical Physics

Talk: July 1, 2019

Many-body quantum electrodynamics (QED) with atoms and photons: A new platform for quantum optics

An exciting frontier in quantum information science is the creation and manipulation of quantum systems that are built and controlled quanta by quanta. In this context, there is an active research worldwide to achieve strong and coherent coupling between light and matter as the building block of complex quantum systems. Despite the range of physical behaviours accessible by these QED systems, the low-energy description is inherently masked by mean fields or renormalization groups

In contrast, I describe our comprehensive theory/experiment program towards synthetic quantum matter, where highly-correlated Rydberg quantum material is strongly coupled to quantized optical cavity fields. We call this new domain of quantum optics, “many-body quantum electrodynamics,” where locally gauged quantum materials are entirely driven by quantum optical fluctuations. I describe our initial laboratory effort towards stabilizing a 2D U(1) quantum spin liquid and the observation of cavity-mediated multiparticle plaquette dynamics, corresponding to an emergent visonic topological defect excitation, for the magnetic flux of the compact dynamical gauge field.

From a broader perspective, our work towards many-body QED will open exhilarating opportunities in atomic and condensed matter physics, and quantum information science to explore the consequential features of macroscopic universes living within strongly-correlated systems, and to help answer some of the most profound questions in physics and computer science — from Baryonic asymmetry, to quantum gravity, and to the quantum Church-Turing thesis. We believe that many-body QED is an essential endeavour for our very instinct to explore genuinely surprising phenomena arising from highly-entangled quantum matter, by light.

Academic Affiliation: Institute of Physics, Chinese Academy of Sciences

Research area: Scan tunneling/probe microscopy, specially instrumentation

Visit Dates: June 13, 2019

Recent R&D progresses on UHV SPM in IOP, CAS – Components and systems

This talk will mainly focus on instrumentation. I will introduce our R&D progresses on UHV-SPM systems and related components, which includes evaporators, electronics, cryostat, scanner heads, the update of a 4-probe STM, Variable-temperature STM, Low-temperature SPM combined with MBE and optical accesses, and Low-temperature STM combined with Combi-LMBE et. al. Some of the test results and research results from these systems will also be shown.

Academic Affiliation: Radboud University Nijmegen

Research area: STM – spectroscopy, quantum effects on superconductivity

Visit Dates: June 9 – 15, 2019

Coupled atom magnetism

In modern times more and more technology is used during the day. All this technology is based on logic operators that work with electric currents. One of the ways to make these logic operators more energy efficient is to make magnetic logic operators. But, to make magnetic logic operators applicable in everyday life, we still need to learn a lot about them. Therefore I’m giving a summary of different types of coupled atom magnetism on surfaces, which eventually could be used to make logic operators consisting of only a few atoms. This is an improvement in both size and energy efficiency, but there is still a long way to go.

Academic Affiliation: Ruhr-Universität Bochum

Research area: Surface Science

Visit Dates: June 5, 2019

STM investigation of laser driven processes at surfaces

While photochemistry in the gas phase demands a resonant excitation of the molecules, the presence of a metal surface in surface chemistry opens a different pathway via the creation of hot electrons in the metal and subsequent attachment of these energetic electrons to adsorbed molecules. We use two set-ups that combine a low-temperature scanning tunneling microscope operating below 10 K with a frequency doubled femto-second laser and a tunable pico-second laser, respectively, to investigate processes induced by these electrons on a single molecule basis.

In this talk, I will first present recent results on the effect of intense fs-laser illumination on the structure of metal surfaces, in particular Ag(100) and Cu(111). Then I will compare laser-induced diffusion to thermal diffusion of CO molecules on Cu(111). In particular the long-range interaction between diffusing molecules differs in the two cases. Finally, I will present results obtained by electrons attaching to and solvating in different ice structures adsorbed on Cu(111) without and with halogenated (chlorine or bromine) benzene molecules adsorbed on them. These model systems enhance our understanding of the impact of ionizing radiation on the chemical composition of Earth’s upper atmosphere (e.g. its ozone density), which is known to trigger chemical processes at the surfaces of cold ice covered grain particles.

Academic Affiliation: Technische Universität München

Prof. Reuter’s (b. 1970) research activities are mainly focused on quantitative modeling of material properties and functionalities. Heterogeneous catalysis is a particular area of interest. He makes widespread use of modern multiscale modeling, which unites methods and concepts from the disciplines of physics and chemistry as well as materials science and engineering.

Talk: June 4, 2019

Knowledge-Based Approaches in Catalysis and Energy Modelling

Reflecting the general data revolution, knowledge-based methods are now also entering theoretical catalysis and energy related research with full might. Automatized workflows and the training of machine learning approaches with first-principles data generate predictive-quality insight into elementary processes and process energetics at undreamed-of pace. Computational screening and data mining allows to explore these data bases for promising materials and extract correlations like structure-property relationships. At present, these efforts are still largely based on highly reductionist models that break down the complex interdependencies of working catalyst and energy conversion systems into a tractable number of so-called descriptors, i.e. microscopic parameters that are believed to govern the macroscopic function. For certain classes of materials like transition metal catalysts, corresponding human-designed models have indeed established trend understanding and spurred a targeted materials design. Future efforts will concentrate on using artificial intelligence also in the actual generation and reinforced improvement of the reductionist models. This is expected to better capture complexities like incomplete understanding or operando changes of interfacial morphology, to provide access to structured and compound materials classes, or ultimately to even fulfill the dream of an inverse (de novo) design from function to structure. In this talk, I will briefly survey these developments, providing examples from our own research, in particular on adsorption energetics at bimetallic catalysts and data mining for the design of organic semiconductors.

Date: May 27 – 30, 2019

Location: Center for Quantum Nanoscience at Ewha Womans University in Seoul, South Korea

Invited Speakers:

Europe

• Daniel Loss (University of Basel)
• Jelena Klinovaja (University of Basel)
• Nicolas Lorente (CFM/DIPC)
• G. Burkard (Universität Konstanz)
• Hugo Ribeiro (Max Planck Institute for the Science of Light)
• Samir Lounis (Forschungszentrum Juelich)
• Fernando Delgado (Universidad de La Laguna)
• Joaquín Fernández Rossier (Theory of Quantum Nanostructure/INL)
• Pascal Simon (University of Paris-Sud)

Japan

• Peter Stano (RIKEN)
• Takahiro Sagawa (University of Tokyo)

China

• Mircea Trif (Tsinghua University)
• Ying-Dan WANG (UCAS)
• Fu Chun Zhang (UCAS)

Korea

• Mahn Soo Choi (Korea University)
• Gun Sang Jeon (Ewha Womans University, Seoul)
• Dong-Hee Kim (GIST, Gwangju)

Committee:

• Andreas Heinrich (IBS QNS)
• Christoph Wolf (Center for Quantum Nanoscience)
• Pascal Simon (CNRS, Université Paris Sud)

Academic Affiliation: IBM Research – Almaden

Talk: June 4, 2019

Chiral Spintronics

Chirality is one of the fundamental asymmetries in nature. Objects with chirality cannot be superimposed onto each other after mirror reflection operation like right and left hands. Archetypal examples of chiral objects are chiral molecules in stereroisomers and chiral magnetic domain walls whose chiralities are set by e.g. Dzyaloshinskii-Moriya interaction. Most importantly moving spins become chiral such that they can reciprocally interact with chiral objects thus allowing “Chiral Spintronics”. This means that spins traveling in a chiral object can be polarized or reversely spin angular momentum can be transferred to the chiral structure. In this talk I will review this emerging new field, the marriage of spintronics to chirality, by discussing recently discovered various novel torques and phenomena from chiral magnetic nanostructures to chiral molecules: spin-orbit torques from perpendicularly magnetized ultrathin films [1], exchange coupling torque [2] and novel new phase of domain wall dynamic, chiral exchange drag [3] from synthetic antiferromagnets, and efficient separation of chiral molecules, enantiomers, by magnetic structure [4]. In the end I will give an overall outlook on Chiral Spintronics and discuss potential applications from the new findings.

[1] K.-S. Ryu, L. Thomas, S.-H. Yang, and S. S. P. Parkin, “Chiral spin torque at magnetic domain walls”, Nat. Nano. 8, 527 (2013).
[2] S.-H. Yang, K.-S. Ryu, and S. S. P. Parkin, “Domain-wall velocities of up to 750 m s−1 driven by exchange-coupling torque in synthetic antiferromagnets“, Nat. Nano. 10, 221 (2015).
[3] S.-H. Yang, C. Garg, and S. S. P. Parkin, “Chiral Exchange Drag and Chirality Oscillations in Synthetic Antiferromagnets”, Nature Physics 10.1038/s41567-019-04383 (2019).
[4] K. Banerjee-Ghosh, O.B. Dor, F. Tassinari, E. Capua, S. Yochelis, A. Capua, S.-H. Yang, S.S.P. Parkin, S. Sarkar, L. Kronik, L. T. Baczewski, R. Naaman, and Y. Paltiel, “Separation of enantiomers by their enantiospecific interaction with achiral magnetic substrates”, Science 360, 1331 (2018).

Academic Affiliation: Department of Electrical & Computer Engineering, University of Minnesota, Minneapolis, US

Talk: March 13, 2019

Manipulating light flow with 2D materials plasmons

Recent years have observed a plethora of strong dipole type polaritonic excitations in 2D materials owing to the reduced screening. These polaritons can be sustained as electromagnetic modes at the interface between a positive and negative permittivity material. In the case of the plasmon-polaritons (e.g. in semi-metallic graphene), the negative permittivity is provided by the coherent oscillations of the free carriers. For exciton-polaritons (e.g. in semiconducting transition metal dichalcogenides, TMD) and phonon-polaritons (e.g. in diatomic hexagonal boron nitride, hBN), it is associated with their resonant optical absorption, resulting from a highly dispersive permittivity. These optical resonances can also result in a negative permittivity, albeit over a narrow spectral window.
In this talk, I will discuss our recent efforts in understanding plasmons behavior in 2D materials and using them to control the flow of light both in the far- and near-field. The general constitutive materials response of 2D materials, in conjunction with metasurface approaches, can potentially enable arbitrary control of phase, amplitude, polarization of light. The flow of light within the 2D materials can also exhibit rich transport behavior, such as hyperbolic rays, non-reciprocal chiral propagation, time reversal of waves and coupling of light spin to induce one-way propagation.

Academic Affiliation: Institute for Functional Matter and Quantum Technologies, University of Stuttgart, Germany

Talks:
February 26, 2019 – “State-of-the-art in ultrafast scanning tunneling microscopy”
February 27, 2019 – “Ultrafast Charge-Density Wave Dynamics at the Atomic Scale”

Academic Affiliation: International Center for Quantum Materials, School of Physics, Peking University

Talk: February 15, 2019

Peering into interfacial water by scanning probe microscopy

Water at surfaces is ubiquitous in nature and plays an essential role in a broad spectrum of physics, chemistry, biology, energy and material sciences. One of the most fundamental issues is the characterization of H-bonding structure and related dynamics at molecular level. Ideally, attacking this problem requires the access to the internal degrees of freedom of water molecules, i.e. the directionality of OH bonds. However, it remains a great challenge due to the small mass and size of hydrogen. In this talk, I will present our recent progress on the development of scanning probe microscopy/spectroscopy (SPM/S) with ultrahigh sensitivity and resolution, and its application to surface water. I will first focus on how to achieve sub-molecular-resolution imaging (1,2) and single-bond vibrational spectroscopy of single water molecules (3). In the following, I will discuss the application of those techniques to water clusters, ion hydrates and two-dimensional ice layers on insulating and metal surfaces. Some important issues, including H-bonding topology (4), proton dynamics (5), nuclear quantum effects (3), ion transport (6), ice edge structure and growth, will be addressed. In the end, I will present an outlook on the future directions of water-solid interfaces as well as the challenges faced by this field.

Academic Affiliation: University of Stuttgart, Germany

Talk: February 15, 2019

Atomically resolved vibrational spectroscopy and ultrafast dynamics in 2D materials

In this talk, first, I will show the results of tip-enhanced Raman spectroscopy (TERS) on emerging 2D borophene and silicene. The highly enhanced electric field perpendicular to the sample surface makes TERS a selective-modes enhancement behavior. The local structures and vibrational properties at defects, domain walls, and strain areas also could be resolved with sub-nanometer spatial resolution. Second, using ultrafast THz-coupled STM, we studied the ultrafast electron dynamics in the correlated material of 1T-TaS2 at the atomic scale. The pump-probe spectra vary strongly on the scale of one unit cell of the David star. The atomically resolved phonon spectroscopy and THz pump-probe spectroscopy are expected to shed new light on some crucial issues, such as charge density wave states and interface enhanced superconductivity.

(References)
1. Sheng, S.; Ma, R.; Wu, J. B.; Li, W.; Kong, L.; Cong, X.; Cao, D.; Hu, W.; Gou, J.; Luo, J. W.; Cheng, P.; Tan, P. H.; Jiang, Y.; Chen, L.; Wu, K., The Pentagonal Nature of Self-Assembled Silicon Chains and Magic Clusters on Ag(110). Nano Lett. 18, 2937-2942 (2018).
2. Sheng, S.; Wu, J.; Cong, X.; Li, W.; Gou, J.; Zhong, Q.; Cheng, P.; Tan, P.; Chen, L.; Wu, K., Vibrational Properties of a Monolayer Silicene Sheet Studied by Tip-Enhanced Raman Spectroscopy. Phys. Rev. Lett. 119, 196803 (2017).

Academic Affiliation: Center for Quantum Information, KIST

Talk: January 9, 2019

Color centers in wide bandgap semiconductors for quantum applications

High purity single crystalline solids can be used as a robust platform for quantum information research since isolated point-defects, which resemble isolated single atoms when phonon coupling is efficiently suppressed, can be created in the middle of the almost noise-free environment. Among point-defects which can be created artificially, color center in wide-bandgap semiconductors such as diamond and silicon carbide have been used to demonstrate various proof-of-idea experiments for quantum information and also quantum applications such as quantum metrology. Even though small size integrated qubit devices consisting of a few entangled spin qubits have been demonstrated, there remain open questions about whether this physical system will allow building large quantum devices in which many qubits can interact with each other without losing their coherent properties.
Managing spin-spin interaction among defects spins near each other is a typical method to enable quantum connection among spin qubits. In addition, the optical transitions strongly correlated with spin states of the color centers allow another pathway for integrating individual qubits. One example is the photonic quantum network which can meditate entanglement among distant spin qubits via spin-selective optical transitions. The spin-to-photon interface based on a color center in diamond has shown the entanglement rate exceeding the decoherence rate of two distant spin qubits. In parallel, there have been efforts for finding novel color centers in various large bandgap materials whose ability as a spin-to-photon interface may exceed that of the diamond color centers. The color centers in silicon carbide are candidates since well-developed device fabrication techniques exist and efficient spin-to-photon interfaces have been found recently. In this presentation, a broad review of the quantum technology based on color centers in solids will be provided, and a summary about recent progress will be presented as well.

Academic Affiliation: Institute of Advanced Composite Materials, Korea Institute of Science and Technology (KIST)

Talk: January 7, 2019

Wafer-scale single-crystal hexagonal boron nitride film via self-collimated grain formation

Although polycrystalline hexagonal boron nitride (PC-hBN) has been realized, defects and grain boundaries still cause charge scatterings and trap sites, impeding high-performance electronics. Here, we report a method of synthesizing wafer-scale single- crystalline hBN (SC-hBN) monolayer films by chemical vapor deposition. The limited solubility of boron (B) and nitrogen (N) atoms in liquid gold promotes high diffusion of adatoms on the surface of liquid at high temperature to provoke the circular hBN grains. These further evolve into closely packed unimodal grains by means of self-collimation of B and N edges inherited by electrostatic interaction between grains, eventually forming an SC-hBN film on a wafer scale. This SC-hBN film also allows for the synthesis of wafer-scale graphene/hBN heterostructure and single-crystalline tungsten disulfide.

Reference:
[1] Wafer-scale single-crystal hexagonal boron nitride film via self-collimated grain formation, Science, 362, 817 (2018)

Academic Affiliation: Korea University

Talk: December 11, 2018

Quantum Phase Transition on Quantum Dot Systems: Turning Competition into Coalition

We consider a quantum dot coupled to both superconducting and ferromagnetic electrodes, and study the triad interplay of the Kondo effect, superconductivity, and ferromagnetism, any pair of which compete with and suppress each other. We find that the interplay leads to a mixed-valence quantum phase transition, which is usually a crossover rather than a true transition. At the transition, the system changes from the spin doublet to singlet state. The singlet phase is adiabatically connected (through crossovers) to the so-called ‘charge Kondo state’ and to the superconducting state. We analyze in detail the physical characteristics of different states and propose that the measurement of the cross-current correlation and the charge relaxation resistance can clearly distinguish between them.

Research area: Applying and developing SPM tools for challenging questions of fundamental research in this field investigating e.g. metalsemiconductor interfaces, transport in graphene, charge dynamics of single dopants in GaAs, Kondo effect and material classes like pnictides, hexaborides and iridates.

Academic Affiliation: Universität Göttingen

Period of Stay: October 1 – December 31, 2018

Academic Affiliation: IBM Research

Talk: November 5, 2018

Charged individual molecules on insulating films by atomic force microscopy

Here, we present results concerning molecules and their charge-state determination and control on multilayer NaCl films. We demonstrate lateral single electron transfer between molecules [1] and gain insight into charging processes via single-electron tunneling spectroscopy, where we quantify the reorganization energy of a molecule on a NaCl substrate [2]. Finally, we show results where the charge state of a complex on top of an insulator plays a key role for its on-surface chemical reaction.

Reference:
[1] W. Steurer et al. Nature Communications 114, 036801 (2015)
[2] S. Fatayer et al. Nature Nanotechnology 13, 376 (2018)

Academic Affiliation: IBM Research

Talk: November 5, 2018

On-surface synthesis by atom manipulation studied with atomic force microscopy

Elusive molecules can be created using atom manipulation with a combined atomic force/scanning tunneling microscope (AFM/STM). Molecules that are highly reactive and shortlived under ambient conditions can be stabilized at low temperature and by using inert surfaces. Employing high-resolution AFM with functionalized tips [1] provides insights into the structure, geometry, aromaticity and bond orders of the molecules created and into the reactions performed [2].

We created radicals, diradicals [3], non-Kekulé molecules [4], anti-aromatics [5], and polyynes [6] and studied their structural and electronic properties. We expanded the toolbox for the synthesis of molecules by atom manipulation, demonstrating reversible cyclisation reactions [3] and, most recently, skeletal rearrangements (see figure) [6]

Figure: Polyyne formation via a skeletal rearrangement induced by atom manipulation. Chemical reaction scheme (top). CO-tip AFM data (bottom), showing the different reaction steps on bilayer NaCl on Cu(111). All reaction steps are induced by bias voltage pulses with the AFM tip. [6]

Reference:
[1] L. Gross et al. Science 325, 1110 (2009)
[2] L. Gross et al. Angew. Chem. Int. Ed 57, 3888 (2018)
[3] B. Schuler et al. Nat. Chem. 8, 220 (2016)
[4] N. Pavliček et al. Nat. Nano. 12, 308 (2017)
[5] Z. Majzik et al. Nat. Comm. 9, 1198 (2018)
[6] N. Pavliček et al. Nat. Chem. 10, 853 (2018)

Academic Affiliation: Swiss Federal Institute of Technology in Lausanne (EPFL)

Visiting Period: August  6-12, 2018

Talk: August  9, 2018

Academic Affiliation: Leibniz-Institute for Solid State and Materials Research

Visiting Period: August  6, 2018

Research area: Theoretical Studies of Quantum Magnets

Academic Affiliation: Virginia Tech, USA

Period of Stay: June 20 – July 20, 2018

Academic Affiliation: Princeton University

Visiting Period: June  11-15, 2018

Group Leader, Nanosciences

Academic Affiliation: CIC Nanoguene, France

Visiting Period: June  8, 2018

Alessandra Apicella, Head of Science and Technology
Ji Hyun Lim, Head of Science and Technology
Orlando Schaerer, Associate Director CGI, IBS, UNIST
Jean-Luc Barras, Director fo the Swiss National Research Foundation
Matt Kimmich, Scientific Officer of the Swiss National Research Foundation

Visiting Period: June 5, 2018

Senior Admin Manager

Academic Affiliation: Centre for Quantum Technologies, NUS, Singapore

Visiting Period: May  30-April 2, 2018

Research Director of Nanomagnetism

Academic Affiliation: CIC Nanoguene, France

Visiting Period: May  30, 2018

Research Scientist

Research area: Theory of Electronic Properties at the Nanoscale

Academic Affiliation: CSIC and DIPC

Period of Stay: October 1-31 2017