세미나 일정:

Abstract:

In 1930, a resistance minimum observed in dilute magnetic alloys was the first experimental evidence for a new scattering mechanism of conduction electrons at magnetic impurities at low temperatures. More than 30 years later J. Kondo developed a theory that describes the effect as a consequence of the spin-flip scattering of conduction electrons at a localized magnetic impurity: The term “Kondo effect” was born for this phenomenon. About the same time P. W. Anderson developed his “single impurity model” where he calculated the implications of this scattering mechanism for the local density of states:

He found a strong singularity at the Fermi level, termed “Kondo resonance”. This prediction triggered numerous experimentalists to search and find this resonance in rare earth and transition metal alloys with techniques such as point contact measurements, photoelectron, and scanning tunneling spectroscopies (STS).

The latter technique enabled us to study the manifestations of the Kondo effect for individual atoms and molecules adsorbed on surfaces. An interesting case are the rare earth atoms. Their magnetic moment originates from electrons in the partially filled 4f orbitals which are well shielded by the outerlying orbitals leading to a very weak hybridization between them and the localized 4f electron. This is most probably the reason, why so far no Kondo resonance has been detected for a single Ce adatom on metallic and insulating surfaces. The Ce adatom remains spectroscopically “dark”. However, here we show that we can detect the magnetic moment of an individual Ce adatom adsorbed on a Cu2N ultra thin film on Cu(100) by using a sensor tip that has its apex functionalized with a Kondo screened spin system, a small Ce-cluster. We calibrate the sensor tip by deliberately coupling it to a well characterised Fe surface atom. Subsequently we use the splitting of the tip’s Kondo resonance when approaching a spectroscopically dark Ce atom to sense its magnetic moment [1]. Thus, the functionalized tip is used as a spin detector for single magnetic Ce-adatoms in the absence of an external magnetic field. This achievement indicates an alternative route to the study of magnetic nanostructures circumventing the application of spin-polarized STM tips.

When Ce-atoms self-assemble to create a superlattice on Ag(111) with an interatomic

lattice spacing of 3.2 nm [2], a Kondo lattice is formed. The differential conductance spectra

obtained on single Ce-adatoms within the superlattice reveal a considerably broadened Kondo

resonance as compared to the one of about 1 meV found for isolated Ce-clusters. This observation might indicate the presence of antiferromagnetic indirect exchange interactions (RKKY) in the superlattice. Depending on the distance between the Ce-adatoms in the superlattice the competition between Kondo singlet formation and RKKY interaction may lead to a coherent antiferromagnetic or ferromagnetic state when passing through a quantum critical point. Future studies at lower temperatures and with higher spectroscopic

resolution may provide new insights into the interplay between Kondo physics, localized spin-flip excitations, and the magnetic exchange interaction.

[1] M. Ternes, C. P. Lutz, A. J. Heinrich, and W.-D. Schneider, Phys. Rev. Lett. 124, 167202 (2020)

[2] F. Silly, M. Pivetta, M. Ternes, F. Patthey, J. P. Pelz, and W.-D. Schneider, Phys. Rev. Lett. 92, 016101 (2004)

Magnetic chains on s-wave superconductors evoke so-called Shiba bands inside the gap of the substrate. If these bands experience sufficiently strong spin-orbit coupling and overlap with the Fermi energy, a topologically nontrivial minigap can open up which protects zero energy Majorana bound states localized at the chains two ends. We study artificial spin chains, built atom-by-atom [1], with respect to such phenomena. By variation of substrate and adatom species and interatomic distances in the chain [2-5], we adjust the energies of the multi orbital YuShiba Rusinov states induced by the adatoms [2,3], their hybridizations [4], as well as the chains’ spin structures [5]. This enables us to tailor the multi-orbital Shiba bands formed by hybridizing Yu-Shiba Rusinov states such that topologically nontrivial minigaps open [6] and precursors of Majorana bound states appear [7]. Due to a narrow energetical width of these topological minigaps, the two components of the Majorana precursors from the two chain ends strongly hybridize, such that the desired protection by the topological minigap is not realized. In this talk, I will present our most recent experimental strategies in order to increase the width of the topological minigap [8-10] which will eventually lead to isolated and topologically protected Majorana bound states. I acknowledge funding by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) via the Cluster of Excellence ’Advanced Imaging of Matter’ (EXC 2056-project ID 390715994) and via the SFB-925- project 170620586.

[1] D.-J. Choi, N. Lorente, J. Wiebe, K. von Bergmann, A. F. Otte, A. J. Heinrich, Rev. Mod. Phys. 91, 041001 (2019).
[2] L. Schneider, M. Steinbrecher L. Rózsa, J. Bouaziz, K. Palotás, M. dos Santos Dias, S. Lounis, J. Wiebe, R. Wiesendanger, npj Quantum
Materials 4, 42 (2019).
[3] L. Schneider, S. Brinker, M. Steinbrecher, J. Hermenau, Th. Posske, M. dos Santos Dias, S. Lounis, R. Wiesendanger, J. Wiebe, Nature
Commun. 11, 4707 (2020).
[4] P. Beck, L. Schneider, L. Rózsa, K. Palotás, A. Lászlóffy, L. Szunyogh, J. Wiebe, R. Wiesendanger, Nature Commun. 12, 2040 (2021).
[5] L. Schneider, P. Beck, J. Wiebe, R. Wiesendanger, Science Advances 7, eabd7302 (2021).
[6] L. Schneider, P. Beck, T. Posske, D. Crawford, E. Mascot, S. Rachel, R. Wiesendanger, J. Wiebe, Nat. Phys. 17, 943 (2021).
[7] L. Schneider, P. Beck, J. Neuhaus-Steinmetz, T. Posske, J. Wiebe, R. Wiesendanger, Nature Nanotechnology 17, 384 (2022).
[8] P. Beck, L. Schneider, L. Bachmann, J. Wiebe, R. Wiesendanger, Phys. Rev. Materials 6, 024801 (2022).
[9] P. Beck, L. Schneider, R. Wiesendanger, J. Wiebe, arXiv:2205.10062 [cond-mat.supr-con] (2022).
[10] P. Beck, L. Schneider, R. Wiesendanger, J. Wiebe, arXiv:2205.10073 [cond-mat.supr-con] (2022.)

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: Department of Chemistry, School of Life Sciences, University of Sussex, UK

Date and Time of the Talk: Nov 17th, 2021; 17:00 – 18:00 (KST)

Axial Crystal Fields and Single-Molecule Magnetism in Dysprosium Sandwich Compounds

Organometallic sandwich compounds play a leading role in the development of f-element chemistry. In addition to making significant contributions to our fundamental understanding of structure, bonding and reactivity in lanthanide and actinide compounds, organometallic sandwiches are used in catalysis and a variety of small-molecule activation processes.

The reactivity of f-element sandwich compounds often has no parallel with transition metals or main group elements and offers many opportunities for development. In contrast, the magnetic properties of f-element organometallics have not been studied extensively, which is surprising given the many applications of lanthanides in magnetic materials and related areas, such as MRI. This is particularly true in the case of single-molecule magnets (SMMs), a family of compounds that show magnet-like behaviour below a characteristic blocking temperature.

Since 2010, we have developed a large family of dysprosium SMMs based on the metallocene structural unit. We have applied our findings to develop a working model that allows the magnetic blocking temperature to be increased in a rational way. The culmination of our work is a dysprosium metallocene SMM with a blocking temperature of 80 K, which is (currently) the only example to function above liquid nitrogen temperatures.

Using our structure-property relationship, we are now focused on improving the SMM properties further. To do this, our attention has shifted toward a ligand that is well-known in transition metal chemistry but is extremely rare in the f-block: the strained ring species cyclobutadienyl (see image, above right). In this seminar I will present our recent findings in this area, showing how the beastly [4-C4(SiMe3)4]2– ligand can be tamed with appropriate control of the chemical conditions.

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

Date and Time of the Talk: Sept 14th, 2021; 17:00 – 19:00 (KST)

New insights into spin dynamics – Stochastic resonance as a tool

Stochastic scattering between magnetic atoms and electrons of a metal shortens excitation lifetimes and accelerates decoherence. However, this scattering also creates characteristic time scales, that carry information about dynamics of the system. Modulating the scattering close to a threshold leads to frequency-dependent synchronization of the spin of the magnetic atoms and the modulation [1]. This
effect, known as Stochastic Resonance, amplifies the dynamic response to the modulation and can be used to investigate quantum fluctuations of the system [2]. Here we introduce a measurement technique which uses the synchronization effect that underlies Stochastic Resonance to control and measure characteristic time scales of magnetic atoms with a scanning tunneling microscope. This enables the investigation of spin dynamics in a bandwidth ranging from milliseconds to picoseconds. In some spin structures this reveals multiple switching paths between higher excited states, thus providing insight into new, previously inaccessible dynamics.

[1] Hänze et al., Sci. Adv. 2021, 7 (2021).
[2] R. Löfstedt, S. N. Coppersmith, Phys Rev. Lett. 72, 1947 (1994)

Academic Affiliation: Forschungszentrum Jülich

Date and Time of the Talk: July 28th, 2021; 16:00 – 17:00 (KST)

New mkSTM in Jülich: design, perfomance and first results

In my talk, I will introduce our new UHV STM operating at temperatures down to 26 millikelvin and high magnetic fields of up to 8 Tesla. The peculiarity of our setup is that it uses an adiabatic demagnetisation refrigerator (ADR) to reach its base temperature. Since this is the first instance of ADR use for STM, I will describe our microscope’s characteristics and modes of operation in some detail, after which I will demonstrate the first measurements we performed on superconducting Al(100) surface in the attempt to clarify the issue of our mkSTM junction’s effective electronic temperature.

Academic Affiliation: IQIM Postdoctoral Scholar, California Institute of Technology

Date and Time of the Talk: June 18th, 2021; 13:30 – 14:30

Quantum Metrology with Strongly Interacting Spin Systems

Quantum metrology is a powerful tool for explorations of fundamental physical phenomena and applications in material science and biochemical analysis. While in principle the sensitivity can be improved by increasing the density of sensing particles, in practice this improvement is severely hindered by interactions between them. Here, using a dense ensemble of interacting electronic spins in diamond, we demonstrate a novel approach to quantum metrology to surpass such limitations. It is based on a new method of robust quantum control, which allows us to simultaneously suppress the undesired effects associated with spin-spin interactions, disorder, and control imperfections, enabling a significant enhancement in coherence time compared to the state-of-the-art control sequences. Combined with optimal spin state initialization and readout directions, this allows us to achieve an ac magnetic field sensitivity well beyond the previous limit imposed by interactions, opening a new regime of high-sensitivity solid-state ensemble magnetometers.

Date and Time of the Talk: June 1st, 2021; 14:00 – 15:00

Our ‘Spin’ invites you to our kick-off talk of the ‘Spin Art’ contest. We will introduce spin and quantum mechanics to the general public and in addition to that, there is a Q&A session. Jinkyung Kim who wrote the article about Spin and Magritte will tell you a nice story about that world. Don’t miss the talk!

Academic Affiliation: Synchrotron SOLEIL – L’orme des merisiers Saint-Aubin

Date and Time of the Talk: May 20th, 2021; 15:30 – 16:30

In the maze of the interaction between magnetic molecules and superconductivity

During the last decade, research on superconductivity found renewed interest since several theoretical works have announced the possibility to “fabricate” new topological phases by coupling superconducting solid-state systems with local magnetism and spin-orbit interaction [1]. Stabilization of such phases would be accompanied by the emergence of so-called Majorana edge states [2]. Experimental observation of such Majorana edge states would have a huge technological impact because they are considered good candidates for the realization of single qubits and their manipulation would provide a platform for the development of physical quantum computational systems.

To address this challenge, I focused on a hybrid system obtained by coupling self-assembled two-dimensional arrays of magnetic Metallo-organic molecules, i.e. manganese phthalocyanines (MnPcs), with thin films of lead (1 and 3 monolayers) grown on Si(111). In the first part of the talk, I will show the results [3] of the in-situ grown MnPcs/Pb/Si(111) system that has been characterized by Scanning TunnelingMicroscopy (STM) and spectroscopy (STS).

In the second part of the talk, I will focus on the results I obtained during the postdoc. One of the objectives of research in the field of organic spintronics is the understanding of the different paths through which localized spins in molecular lattices can interact to stabilize magnetic order, e.g. ferromagnetism or antiferromagnetism. In this regard, hybrid systems based on self-assembled layers of magnetic molecules, e.g.transition-metal phthalocyanines, supported by metallic substrates have been the subject of a huge number of studies [4,5]. When considering such organic/metal interface many types of coupling are at plays: in addition to direct exchange and superexchange coupling, so-called indirect exchange interactions that are mediated by the conduction electrons of the metallic substrate are also possible [6]. The interacting picture becomes dramatically more complicated if the substrate is allowed to be a superconductor. Magnetic atoms/molecules coupled to a superconductor induce bound states (Yu-Shiba-Rusinov(YSR) states [7]) within the superconducting gap. Under certain conditions, an overlap of YSR states associated to different magnetic atoms/molecules can mediate an indirect exchange interaction [8].

In order to explore and extract information about the magnetic signature of this mechanism, I carried out X-ray Magnetic circular dichroism (XMCD) experiments at 2K on different systems. In particular, I will present the results about (sub-) monolayers of MnPcs self-assembled on then on-superconducting HOPG and the superconducting 2H-NbSe2 [9].

References:

[1] Brauneckeret al., Phys. Rev. Lett. 111, 147202 (2013) / Hoffman S. et al., Phys. Rev. B92, 125422 (2015)
[2] SauJ.D. et al., Phys. Rev. B 82, 214509 (2010)
[3] D. Longo et al. J. Phys. Chem. C 36,19829 (2020)
[4] Wäckerlin C. et al., Chem. Commun. 51, 12958 (2015)
[5] Wu W. et al. , Phys. Rev. B 77, 184403 (2008)
[6] Wu W. et al. , Phys. Rev. B 77, 184403 (2008)
[7] Yu L., Acta Phys. Sin. 21, 75 (1965) / Shiba H., Prog. Theor. Phys. 40, 435 (1968) /Rusinov A.I., Sov. Phys. JETP Lett. 9, 85 (1969)
[8] Yao N.Y. et al. Phys. Rev. Lett. 113, 087202 (2014) / Hoffman S. etal., Phys. Rev. B 92, 125422 (2015) /Kezilebieke S. et al. Nano Lett. 2018, 18, 2311–2315 (2018)
[9] D.Longo et al., to be published

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

Date and Time of the Talk: May 14th, 2021; 14:00 – 15:00

Quantum Spin liquids in One, Two, and Three Dimensions

Quantum spin liquids (QSLs) are exotic states of matter characterized by a long-range entanglement and fractionalization. DefyingLandau’s lesson on spontaneous symmetry breaking, the interacting spins in a QSL state do not develop any long-range order nor freezing down to absolute zero. During the last couple of decades, significant progress has been made in finding QSLs both in theory and materials realization of two-dimensional models.

However, there is yet a consensus whether the best-known candidates are truly QSLs as a smoking-gun experiment is lacking. On the other hand, the QSLis a rule than an exception in one dimension due to strong fluctuations and is well established. In this talk I will present some results of QSLs in one- and two-dimensional systems comparatively. It is also worth noting that an increasing number of candidates are found for three-dimensional QSL which will be discussed. It is my hope that I will be able to sketch some ideas to pursue together with QNS.

Visit Dates: April 20-23, 2020

Location: International Iberian Nanotechnology Laboratory, Portugal

The discovery of Scanning Tunneling Microscope (STM) Electron Spin Resonance (ESR) is opening new venues in surface quantum nanoscience. This workshop will bring together experts in this emerging area to present latest developments and discuss future work.

Participation only by invitation

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The Talk is postponed. The new day and time will be updated soon

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Academic Affiliation: Center for Quantum Nanoscience

Date: will be updated soon

Time: will be updated soon

ESR-STM studies of Dy and FePc on MgO

In 2016 it was discovered that individual Ho atoms on MgO can show magnetic bistability with record breaking temporal, thermal and magnetic field stability. During our first research project as part of my stay at QNS, we discovered that Dy atoms in many ways exceed the stability of Ho. We find anisotropy barriers roughly twice as large as in Ho, as well as superior robustness against low field demagnetization processes as inferred from a novel ESR-STM technique. We believe that this is a result of a similar physical environment as found for Ho but with additional symmetry protection due to a non-integer spin ground state in the 4f⁹ configuration of Dy. Furthermore we demonstrate facile atomic manipulation, which allows to engineer precise local magnetic fields with sub-nanometer precision.

During our second research project, we demonstrate for the first time electron spin resonance measurements of individually addressable molecular spins on a surface. Using the spin-1/2 system FePc, we carry out ESR measurements in a vector field and find an isotropic moment of 0.97muB. We coherently manipulate the spin and find coherence times of 42 ns in Rabi experiments. Using a Hahn echo scheme, we show that the intrinsic coherence time is 180 ns with tunneling electrons presenting the dominant source of decoherence. Finally we demonstrate that FePc molecules self-assemble in a commensurate lattice while at the same time retaining the spin properties of individual molecules. We spatially resolve the molecular interactions using local magnetic resonance imaging and demonstrate that the phase coherence time is fully preserved in molecules within a lattice.

Academic Affiliation: Center for Quantum Nanoscience

Date: April 9, 2020

Time: 16:00 – 17:00

Upgrade of a scanning tunneling microscope for spin resonance

The addition of electron spin resonance to the STM-toolbox greatly increased the physical phenomena accessible in STM experiments. For this reason we upgraded throughout 2017-2018 a homebuilt, low temperature, high magnetic field STM to become world’s second ESR-STM. I will give a general introduction into ESR-STM and describe in detail the technical steps needed for the upgrade.

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

Research Area: STM spectroscopy, Superconductivity, Topological Insulators

Research stay period: February 2020 – July 2020

Talk: February 25th, 16:00 – 17:00; Saturn Seminar room (3F-367)

Anomalous Phase Shift and Spectral Signature of Equal Spin Triplet

A description of the talk will be updated soon!

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

Visit Dates: February 24-28, 2020

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

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

Talk: February 21, 2020

Fitting Model for Spin Excitation

In this introductory lecture Prof. Markus Ternes will outline the fundamental considerations necessary to calculate the current transport through magnetic structures in a scanning tunneling setup using well established perturbation models. He will discuss the influence of higher order scattering on the spectrum and the limitations of the perturbative approach in this respect. Using the below provided scripts Prof.Ternes will discuss how one can use the ‘Graphical User Interface’ to simulate and fit single and complex spin structures in the “zero-current” and dynamical limit.

Participants interested in this lecture should bring their own laptop with them to directly use the scripts. For this, please install beforehand Scilab_6.0.2 (available without charge for Windows, Mac, and Linux from https://www.scilab.org/).

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.

Talk: February 13, 2020, 16:00-17:00

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

Talk: February 11, 16:00 – 17:00

Sensing dark spins with a scanning tunneling microscope

Probing the spin of individual atoms and molecules on surfaces by means of inelastic electron tunneling spectroscopy (IETS) has since its introduction [1] developed to a very successful and widely used method in scanning tunneling microscopy [2]. Recently, IETS measurements have been also taken simultaneously on two magnetic impurities, one attached to the apex at the tip, the other one on the sample surface, revealing correlation-driven transport asymmetries, reminiscent of spinpolarized transport in a magnetic field [3]. In this experiment only the spin on the surface was spectroscopically active while the one on the tip was spectroscopically dark.

Here now I will show how the transport is significantly altered when the tunneling electron is interacting with both spins simultaneously; the one on sample and the one on the tip apex. Different to measurements on two S = 1spins which only showed the expected steps in the differential conductance at each individual excitation and the sum of both excitations [4] we observe complex IETS data when we use a functionalized tip apex with a high S = 3/2 Kondo system to probe a S = 2 Fe adatom on the CuN surface. To successfully simulate such spectrum, Kondo as well as potential scattering processes have to be taken into account whereby strong interference effects due to the fermionic nature of the tunneling electrons lead to novel selection rules [5].

The understanding of the transport rules is important because it enables one to use well understood surface supported spins to calibrate the spin at the ill defined tip apex. Successively, such spin can then be used as a well characterized mobile sensor with unprecedented spacial resolution [6].

References:
[1] A.J. Heinrich, J.A. Gupta, C.P. Lutz, and D.M. Eigler, Science 306, 466 (2004).
[2] M. Ternes, New J. Phys. 17, 063016 (2015).
[3] M. Muenks, P. Jacobson, M. Ternes, and K. Kern, Nature Comm. 8, 14119 (2017).
[4] M. Ormaza, et al., Nano Lett. 17, 1877 (2017).
[5] M. Ternes, C.P. Lutz, A.J. Heinrich, and W.-D. Schneider, arXiv:1908.08267 [cond-mat.meshall].
[6] B. Verlhac, et al., Science 366, 623 (2019).

Academic Affiliation: Leiden University, Netherlands

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

Talk date: January 22, 2020

A strongly inhomogeneous superfluid in an iron-based superconductor

Although the possibility of spatial variations in the superfluid of unconventional, strongly correlated superconductors has been suggested, it is not known whether such inhomogeneities – if they exist – are driven by disorder, strong scattering, or other factors. In this talk I will show how we use Josephson scanning tunneling microscopy to reveal a strongly inhomogeneous superfluid in the iron-based superconductor FeTe_0.55Se_0.45. By simultaneously acquiring the topographic and electronic properties, we find that this inhomogeneity in the superfluid is not caused by structural disorder or strong inter-pocket scattering, and does not correlate with variations in the energy of the Cooper pair-breaking gap. Instead, we see a clear spatial correlation between superfluid density and the quasiparticle strength, defined as the height of the coherence peak, on a local scale. [1]

References:
[1] D. Cho*, K.M. Bastiaans* et al. Nature 571, 541 (2019)

Academic Affiliation: Leiden University, Netherlands

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

Talk date: January 21, 2020

Charge trapping and super-Poissonian noise centers in a cuprate superconductor

In this talk I will present new insight into the mystery of highly anisotropic transport in a cuprate high-temperature superconductor and show that these materials can trap additional charges. Above the superconducting transition these materials are perfectly metallic along the crystal planes (ab-plane), but are insulating in the c-axis, with ratios between in-plane and perpendicular resistance exceeding 104. This anisotropy has been identified as one of the key mysteries in the cuprates and has been connected to the mechanism of high-temperature superconductivity. I will show how we employ our newly developed scanning noise spectroscopy technique, for which we combined a scanning tunneling microscope (STM) with a novel MHz frequency amplifier to bring noise-spectroscopy measurements to the atomic scale
[1]. We discover surprisingly large deviations from the expected Poissonian noise of uncorrelated electron transport in this strongly correlated material. A behavior that is familiar from highly polarizable insulators and represents strong evidence for trapping of charge in the charge reservoir layers of the cuprates. Our measurements suggest a picture of metallic layers separated by polarizable insulators within a three-dimensional superconducting state. We will show how these new observations connect to the mystery of the anisotropic transport in this high-temperature superconductor, shedding new light onto this issue [2].

References:
[1] K.M. Bastiaans et al. Rev. Sci. Instrum. 89, 093709 (2018).
[2] K.M. Bastiaans et al. Nature Physics 14, 1183 (2018).

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!

Affiliation: The University of New South Wales

During the stay Prof. Sven Rogge will give a talk:

Building Quantum Matter Atom by Atom

Atomic-scale engineering reached a level of control where single-atom devices can be reproducibly fabricated. This talk focuses on single dopant atom placement in the context of engineered matter for quantum simulation and computation. Silicon offers an interesting platform for engineered quantum matter because when isotopically purified it acts as a “semiconductor vacuum” for spins. After a general introduction of quantum simulation and computation a first step towards engineered Hamiltonians for Fermionic systems in the form of atomic chains will be presented. Here strongly interacting dopants were employed to simulate a two-site Hubbard Hamiltonian at low effective temperatures with single-site resolution which allows the quantification of the entanglement entropy and Hubbard interaction strengths. To scale this approach to larger systems in-situ multi-electrode devices have been fabricated by a scanning probe hydrogen depassivation and decoration technique. Spatially resolved gated single-electron spectroscopy maps obtained ion these devices n ultra-high vacuum will be presented. Such quantum-state images of two-donor devices led to a donor based two qubit gate design that is robust in regard to variability in dopant placement. In addition to the work on donors I will also present work on single defects in silicon with a spin-orbit interaction for electrical manipulation and coupling.

The talk will be on Monday (November 18th) from 13:00 till 14:30 in the Saturn seminar room.

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: 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

Experimental Details of a Dilution Fridge STM

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.

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

Research Field: Quantum Limits in Scanning Tunneling Microscopy, Superconductivity

Introduction to Superconductivity and Scanning Tunneling Microscopy

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.

Academic Affiliation: The Ohio State University

Research Area: Low-Temperature Scanning Tunneling Microscopy

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.

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: 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)

Introduction to multiplet theory

The first part of the lecture will deal with the theory of full atomic multiplet interaction and its effect on the lineshape of the XAS spectrum. The term symbols will be introduced in order to properly describe the ground state and the final states that can be reached in the absorption process. In the second part the crystal field and ligand field theories will be introduced in order to properly describe the L2,3 spectra of transition metal compounds. Examples of orbital degeneracy breaking induced by the crystal field and its effect on the ground state symmetry will be shown.

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)

Introduction to x-ray absorption spectroscopy and x-ray magnetic circular dichroism

The lecture will first provide a general introduction to the absorption process and the methods used to measure an x-ray absorption spectrum (XAS). I will present the key features that makes this technique so powerful and simple examples of the variety of information about the local electronic and magnetic structure that can be determined from XAS. In the second part I will discuss the spectral shape of XAS and the need of a multielectron approach to describe it. Finally, I will explain the application of the sum rules to x-ray magnetic circular dichroism (XMCD) data in order to determine the spin and orbital moments. Examples of experimental studies using XMCD will be shown.