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Non-local detection of collective atomic-scale spin dynamics
The ability to position individual magnetic atoms in an arrangement of choice makes STM an ideal tool for studying quantum magnetism at the fundamental level. Recent years have shown tremendous progress in building, probing and understanding various coupled spin geometries, some of which I will review in this colloquium.

But the STM also has an inherent limitation: due to the locality of the probe tip, excitation and read-out always happen at the same location on a spin structure. In order to be able to study excitations traveling away from the tip, we devised an atomic spin chain with a remote detector. The detector is equipped with a memory, capable of retaining information until the tip has time to visit and read it out.

I will also address recent experiments that shed light on the mechanism underlying such excitations. By positioning atoms on a highly symmetric binding site, we have been able to completely separate excitations involving the spin and the unquenched orbital moment degrees of freedom. The latter results in excitations of Δm = 4, which may intuitively seem forbidden.

Scanning Tunneling Spectroscopy of Yu-Shiba-Rusinov states

Magnetic impurities in conventional superconductors induce a pair-breaking potential, which leads to bound states inside the superconducting energy gap. These states are called Yu-Shiba-Rusinov states, or short: Shiba states. Although theoretical predictions on the properties of these states date reach back by several decades, their characterization by experiments has only been possible with the advent of high-performance low-temperature scanning tunneling microscopes (STM). STM allows for a characterization of the spectroscopic signatures and spatial extent of the Shiba states.

This colloquium will review the fundamental properties of Shiba states and discuss some real examples probed by STM.

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Studying topological states of matter with artificial lattices

The topology of the electronic structure of a material can give rise to unique and potentially useful phenomena. A prime example is the emergence of so-called topological states at zero-energy that are localized at the boundaries of otherwise insulating materials. Such topological boundary modes can be brought about by strong spin-orbit coupling, as well as by certain lattice symmetries. These states are present as long as certain symmetries of the system are not broken. For example, for spin-orbit based topological insulators, the boundary modes do not depend on the exact geometry of the edges of the material.

After I have reviewed the origin of topological states brought about by lattice symmetries, I will show how artificial lattices, created by patterning the Cu(111) surface with CO molecules using the tip of a scanning tunneling microscope, can be used to study topological states of matter. First, our recent efforts to create robust zero-energy modes localized at the 0D corners of a 2D lattice will be discussed. Finally, I will show that in contrast to spin-orbit based topological insulators, the emergence of robust boundary states in systems protected by lattice symmetries depend sensitively on the edge geometry.

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Engineering intrinsic π-electron magnetism in atomically-precise carbon nanostructures

Topologies of the edge bonds and π-electron network critically influence the electronic structure of finite size graphene fragments such as nanographenes and graphene nanoribbons. Among various properties that arise in such carbon nanomaterials, intrinsic magnetism is a particularly attractive one. Given the weak spin-orbit and hyperfine couplings in carbon and the possibility of electric-field control of spin transport, realization of magnetic carbon nanomaterials may offer unique opportunities in spintronic applications.

In this presentation, I will discuss the on-surface synthesis and scanning probe microscopy / spectroscopy characterization of nanographenes with structural topologies entailing intrinsic π-magnetism. I will present a non-Kekulé compound in which magnetism arises due to topological frustration of the π-electron network, and report on recent progress towards all-zigzag nanographenes. Finally, two complementary approaches to molecular spin chains will be discussed. While the first approach is based on the covalent interlinking of triangular nanographenes, a promising alternative builds on topological electronic quantum phases in edge-extended graphene nanoribbons.

Stochastic dynamics of spins and electrons on surfaces

At the atomic scale most time-dependent processes exhibit a certain degree of randomness; they are governed by transition amplitudes and scattering probabilities. Imaging and possibly controlling such stochastic dynamics is to gain microscopic understanding of quantum excitations in materials.

I will introduce recent advances in time-resolved scanning tunneling microscopy and then focus on two applications where external voltage drive signals synchronize the evolution of spin and charge excitations. This induces oscillatory modes in otherwise highly dissipative systems of individual magnetic atoms on surfaces and charge-density waves in transition metal dichalcogenides.

For magnetic atoms we find that a harmonic modulation of the tunnel current can induce a stochastic resonance which makes the spin evolve more predictably than seemingly made possible by the quantum nature of the spin relaxation. On charge-density wave materials we employ broadband excitation by THz-pulses to induce ultrafast alternating electric fields under the STM tip and find that these electric fields locally excite collective modes.

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Imaging Spinons in a 2D Gapless Quantum Spin Liquid

Two-dimensional triangular-lattice antiferromagnets are predicted under some conditions to exhibit a quantum spin liquid ground state whose low-energy behavior is described by a spinon Fermi surface [1]. This “ghost” Fermi surface (in an otherwise insulating material) is a key concept for understanding spin liquids and their relationship to other quantum phases. Directly imaging the spinon Fermi surface, however, is difficult due to the fractional and chargeless nature of spinons. I will discuss how we have used scanning tunneling microscopy (STM) to image density fluctuations arising from a spin liquid Fermi surface in single-layer 1T-TaSe2, a two-dimensional Mott insulator [2]. Quantum spin liquid behavior was observed in isolated single layers of 1T-TaSe2 through long-wavelength modulations of the local density of states at Hubbard band energies. These modulations reflect a spinon Fermi surface instability in single-layer 1T-TaSe2 and allow direct experimental measurement of the spinon Fermi wavevector, in good agreement with theoretical predictions for a 2D quantum spin liquid [3]. Our results establish single-layer 1T-TaSe2 as a new platform for studying novel two-dimensional quantum spin liquid phenomena.

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Electron Spin Resonance of single atoms on a surface observed with STM

Scanning Tunneling Microscopy (STM) can be combined with electron spin resonance [1]. The major advantage of spin resonance is the fact that the energy resolution is independent of the temperature and thus can be much higher than a Fermi-function limited spectroscopy technique such as STM tunneling. In ESR STM we apply a microwave-frequency electric field to the STM tunnel junction and convert this AC electric field into a driving field for the ESR [2]. We find an energy resolution in ESR STM, which is about 10,000 times better than low-temperature STM.

Here we will focus on two examples: Fe and Ti atoms on MgO on Ag(100). Fe on MgO has a spin of S=2 with a strong out-of-plane easy-axis magnetic anisotropy. ESR active Fe atoms can be used to measure the local magnetic field very precisely and with atomic-scale spatial resolution. We will use this to measure the magnetic field emanating from a stable single-atom magnet nearby: Ho on MgO [3].

Ti atoms on MgO are an S=1/2 electron system (when a single Hydrogen is attached) with an interesting nuclear spin system, consisting of several isotopes (I=0, I=5/2 and I=7/2). ESR STM can measure the hyperfine interaction of the electron spin with the nuclear spin of the Ti atom [4]. The hyperfine interaction is a sensitive measure of the local bonding geometry.

ESR STM is just in its infancy with many groups joining this research effort.

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Tinkering with Majoranas

The appearance and the signatures of Majoranas—the simplest non-Abelian states with potential applications in quantum computing—are already captured by a simple 1D model combining spin-orbit coupling, Zeeman splitting, and superconductivity. A trained researcher can likely solve this model in a few minutes. Using this model as a baseline, I am going to demonstrate the rich physics that this minimal description sweeps under the rug. In expanding the description of the system, we are going to encounter the interplay between the roles of symmetry analysis, dimensionality of the system, quasiclassical, and geometric effects. The last one is my personal favorite—I will demonstrate how a geometry change can improve the Majorana properties by more than an order of magnitude.

The magic of atomically thin crystals

The discovery of two dimensional (2D) atomically thin crystals has changed the way we think about materials. Because all the atoms in any 2D crystal are exposed to our three-dimensional world, it has become possible for the first time to tune the electronic properties of a material without changing its chemical composition, for example by introducing strain, plucking out atoms, or intentionally stacking of 2D crystals in various ways. One of the simplest techniques, changing the relative orientation of superposed 2D crystals, has proven to be especially impactful and has taken center stage recently following the surprising discovery of interaction induced insulating states and superconductivity in twisted bilayer graphene. In this talk I will discuss the rapidly evolving field of twisted 2D crystals from its serendipitous discovery to recent developments.