Search

Atomic-scale manipulation in scanning tunneling microscopy has enabled the creation of quantum states of matter based on artificial structures and extreme miniaturization of computational circuitry based on individual atoms. The ability to autonomously arrange atomic structures with precision will enable the scaling up of nanoscale fabrication and expand the range of artificial structures hosting exotic quantum states. However, the a priori unknown manipulation parameters, the possibility of spontaneous tip apex changes, and the difficulty of modeling tip-atom interactions make it challenging to select manipulation parameters that can achieve atomic precision throughout extended operations. Here we use deep reinforcement learning (DRL) to control the real-world atom manipulation process. Several state-of-the-art reinforcement learning (RL) techniques are used jointly to boost data efficiency. The DRL agent learns to manipulate Ag adatoms on Ag(111) surfaces with optimal precision and is integrated with path planning algorithms to complete an autonomous atomic assembly system. The results demonstrate that state-of-the-art DRL can offer effective solutions to real-world challenges in nanofabrication and powerful approaches to increasingly complex scientific experiments at the atomic scale. Engineering quantum states requires precise manipulations at the atomic level. Here, the authors use deep reinforcement learning to manipulate Ag adatoms on Ag surfaces, which combined with path planning algorithms enables autonomous atomic assembly. [ABSTRACT FROM AUTHOR]

Heavy-fermion systems represent one of the paradigmatic strongly correlated states of matter1–5. They have been used as a platform for investigating exotic behaviour ranging from quantum criticality and non-Fermi liquid behaviour to unconventional topological superconductivity4–12. The heavy-fermion phenomenon arises from the exchange interaction between localized magnetic moments and conduction electrons leading to Kondo lattice physics, and represents one of the long-standing open problems in quantum materials3. In a Kondo lattice, the exchange interaction gives rise to a band with heavy effective mass. This intriguing phenomenology has so far been realized only in compounds containing rare-earth elements with 4f or 5f electrons1,4,13,14. Here we realize a designer van der Waals heterostructure where artificial heavy fermions emerge from the Kondo coupling between a lattice of localized magnetic moments and itinerant electrons in a 1T/1H-TaS2 heterostructure. We study the heterostructure using scanning tunnelling microscopy and spectroscopy and show that depending on the stacking order of the monolayers, we can reveal either the localized magnetic moments and the associated Kondo effect, or the conduction electrons with a heavy-fermion hybridization gap. Our experiments realize an ultimately tunable platform for future experiments probing enhanced many-body correlations, dimensional tuning of quantum criticality and unconventional superconductivity in two-dimensional artificial heavy-fermion systems15–17.A study demonstrates the synthesis and characterization of a two-dimensional van der Waals heterostructure hosting artificial heavy fermions, providing a tunable platform for investigations of heavy-fermion physics. [ABSTRACT FROM AUTHOR]

Controlling the properties of organic/inorganic materials requires detailed knowledge of their molecular adsorption geometries. This is often unattainable, even with current state‐of‐the‐art tools. Visualizing the structure of complex non‐planar adsorbates with atomic force microscopy (AFM) is challenging, and identifying it computationally is intractable with conventional structure search. In this fresh approach, cross‐disciplinary tools are integrated for a robust and automated identification of 3D adsorbate configurations. Bayesian optimization is employed with first‐principles simulations for accurate and unbiased structure inference of multiple adsorbates. The corresponding AFM simulations then allow fingerprinting adsorbate structures that appear in AFM experimental images. In the instance of bulky (1S)‐camphor adsorbed on the Cu(111) surface, three matching AFM image contrasts are found, which allow correlating experimental image features to distinct cases of molecular adsorption. [ABSTRACT FROM AUTHOR]

Achieving large‐area uniform 2D metal‐organic frameworks (MOFs) and controlling their electronic properties on inert surfaces is a big step toward future applications in electronic devices. Here a 2D monolayer Cu‐dicyanoanthracene MOF with long‐range order is successfully fabricated on an epitaxial graphene surface. Its structural and electronic properties are studied by low‐temperature scanning tunneling microscopy and spectroscopy complemented by density‐functional theory calculations. Access to multiple molecular charge states in the 2D MOF is demonstrated using tip‐induced local electric fields. It is expected that a similar strategy could be applied to fabricate and characterize 2D MOFs with exotic, engineered electronic states. [ABSTRACT FROM AUTHOR]

Exotic states such as topological insulators, superconductors and quantum spin liquids are often challenging or impossible to create in a single material1–3. For example, it is unclear whether topological superconductivity, which has been suggested to be a key ingredient for topological quantum computing, exists in any naturally occurring material4–9. The problem can be circumvented by deliberately selecting the combination of materials in heterostructures so that the desired physics emerges from interactions between the different components1,10–15. Here we use this designer approach to fabricate van der Waals heterostructures that combine a two-dimensional (2D) ferromagnet with a superconductor, and we observe 2D topological superconductivity in the system. We use molecular-beam epitaxy to grow 2D islands of ferromagnetic chromium tribromide16 on superconducting niobium diselenide. We then use low-temperature scanning tunnelling microscopy and spectroscopy to reveal the signatures of one-dimensional Majorana edge modes. The fabricated 2D van der Waals heterostructure provides a high-quality, tunable system that can be readily integrated into device structures that use topological superconductivity. The layered heterostructures can be readily accessed by various external stimuli, potentially allowing external control of 2D topological superconductivity through electrical17, mechanical18, chemical19 or optical means20. A van der Waals structure based on a two-dimensional magnet and layered superconductor offers a potential system in which topological superconductivity could be easily tuned and integrated into devices. [ABSTRACT FROM AUTHOR]

There has been enormous recent interest in heterostructures of two-dimensional van der Waals materials. Integrating materials with different quantum ground states in vertical heterostructures is predicted to lead to novel electronic properties that are not found in the constituent layers. Here, we present direct synthesis of a superconductor-magnet hybrid heterostructure by combining superconducting niobium diselenide (NbSe2) with the monolayer vanadium diselenide (VSe2). Molecular-beam epitaxy growth in ultra-high vacuum yields clean and atomically sharp interfaces. Combining different characterization techniques and density-functional theory calculations, we investigate the electronic and magnetic properties of VSe2 on NbSe2. Low temperature scanning tunneling microscopy measurements show an absence of the typical charge density wave on VSe2 and demonstrate a reduction of the superconducting gap of NbSe2 on the VSe2 layer. This suggests magnetization of the VSe2 sheet, at least on the local scale. Our work demonstrates superconducting-magnetic hybrid materials with potential applications in future electronics devices. Whether two dimensional magnetic ordering exists in monolayers of VSe2 has been the subject of recent debate. Here, the authors investigate monolayers of VSe2 grown on an NbSe2 substrate and demonstrate a reduction in the superconducting gap of the NbSe2 and absence of charge density wave formation supporting the presence of a magnetic ground state in the VSe2. [ABSTRACT FROM AUTHOR]

Topological modes in one- and two-dimensional systems have been proposed for numerous applications utilizing their exotic electronic responses. The 1D, zero-energy, topologically protected end modes can be realized in structures implementing the Su–Schrieffer–Heeger (SSH) model. While the edge modes in the SSH model are at exactly the mid-gap energy, other paradigmatic 1D models such as trimer and coupled dimer chains have non-zero energy boundary states. However, these structures have not been realized in an atomically tuneable system that would allow explicit control of the edge modes. Here, we demonstrate atomically controlled trimer and coupled dimer chains realized using chlorine vacancies in the c(2 × 2) adsorption layer on Cu(100). This system allows wide tuneability of the domain wall modes that we experimentally demonstrate using low-temperature scanning tunneling microscopy (STM). [ABSTRACT FROM AUTHOR]

Many-body effects arise from the collective behaviour of large numbers of interacting particles, for example, electrons, and the properties of such a system cannot be understood considering only single or non-interacting particles. Despite the generality of the many-body picture, there are only a few examples of experimentally observing such effects in molecular systems. Measurements of the local density of states of single molecules by scanning tunnelling spectroscopy is usually interpreted in terms of single-particle molecular orbitals. Here, we show that the simple single-particle picture fails qualitatively to account for the resonances in the tunnelling spectra of different charge states of cobalt phthalocyanine molecules. Instead, these resonances can be understood as a series of many-body excitations of the different ground states of the molecule. Our theoretical approach opens an accessible route beyond the single-particle picture in quantifying many-body states in molecules. [ABSTRACT FROM AUTHOR]

It is well known that scanning probe techniques such as scanning tunnelling microscopy (STM) and atomic force microscopy (AFM) routinely offer atomic scale information on the geometric and the electronic structure of solids. Recent developments in STM and especially in non-contact AFM have allowed imaging and spectroscopy of individual molecules on surfaces with unprecedented spatial resolution, which makes it possible to study chemistry and physics at the single molecule level. In this feature article, we first review the physical concepts underlying image contrast in STM and AFM. We then focus on the key experimental considerations and use selected examples to demonstrate the capabilities of modern day low-temperature scanning probe microscopy in providing chemical insight at the single molecule level. [ABSTRACT FROM AUTHOR]

In molecular electronics individual molecules serve as electronic devices. In these systems, electron-vibron (e-ν) coupling can be expected to lead to new physical phenomena and potential device functions. In previous studies of molecular wires, the e-ν coupling occurred as a result of the well-known Franck-Condon principle, for which the Born-Oppenheimer approximation holds. This means that after a vibronic excitation, the electrons and the vibrations evolve independently from each other. Here we show that this simple picture changes markedly when two electronic levels in a molecule are coupled by a molecular vibration. In molecular wires we observe a non-Born-Oppenheimer regime, for which a coherent coupling of electronic and nuclear motion emerges. This phenomenon should occur in all systems with strong electron-vibration coupling and an electronic level spacing of the order of vibrational energies. The coherent coupling of electronic and nuclear motion could be used to implement mechanical control of electron transport in molecular electronics. [ABSTRACT FROM AUTHOR]

Integrating Bayesian Inference with Scanning Probe Experiments for Robust Identification of Surface Adsorbate Configurations (Adv. Funct. Atomic force microscopy, Bayesian inference, density-functional theory, organic surface adsorbates, structure search Keywords: atomic force microscopy; Bayesian inference; density-functional theory; organic surface adsorbates; structure search EN atomic force microscopy Bayesian inference density-functional theory organic surface adsorbates structure search 1 1 1 08/12/21 20210809 NES 210809 B Bayesian Inference b In article number 2010853, Patrick Rinke and co-workers combine artificial intelligence enhanced ab-initio structure search with atomic force microscopy simulations (SIM) and experiments (EXP) to detect configurations of bulky 3D adsorbates. [Extracted from the article]

The article reports on the study conducted by Leonhard Grill and co-workers which uses scanning tunneling microscopy (STM) to examine porphycene molecules adsorbed on a copper surface. It shows that individual copper atoms can modify the reaction substantially. It confirms that placing two adatoms symmetrically around the porphycene has no effect on the overall reaction rate.

The article discusses a study by Willi Auwärter and colleagues at the Technical University in Munich, Germany on the creation of a four-level molecular conductance switch using porphyrin. Noted is the design challenge presented by the interfacing of molecular components with the macroscopic world. It is said that molecular switches should be protected from changes of the atomic structure of the molecule because it can alter the electronic properties of the whole molecule.

The combination of several materials into heterostructures is a powerful method for controlling material properties. The integration of graphene (G) with hexagonal boron nitride (BN) in particular has been heralded as a way to engineer the graphene band structure and implement spin- and valleytronics in 2D materials. Despite recent efforts, fabrication methods for well-defined G-BN structures on a large scale are still lacking. We report on a new method for producing atomically well-defined G-BN structures on an unprecedented length scale by exploiting the interaction of G and BN edges with a Ni(111) surface as well as each other. [ABSTRACT FROM AUTHOR]