Research lines at van der Zant Lab

We employ and develop different experimental techniques to explore quantum transport through single molecules, nanoparticles, graphene nanoribbons, biological nanowires and confined structures in 2D materials. The techniques involve the use of mechanically controlled break junctions, electromigrated break junctions and direct e-beam writing for sub-10 electrode spacings. Some of the current topics of interest include:

Quantum interference plays an important role in charge transport through single-molecule junctions, even at room temperature. Of special interest is the measurement of the destructive quantum interference dip itself. We use the mechanosensitivity of specific molecules to reconstruct the destructive quantum interference dip of conductance versus displacement. Calculations that include electrode distance and energy alignment variations explain the observations quantitatively, emphasizing the crucial role of thermal fluctuations for measurements under ambient conditions. Furthermore, quantum interference can be used to create switches with well-defined conductance states, of interest for neuromorphic applications.

Papers: paper Sebastiaan, Nat. Commun. Xx (2024)

Project (Funding): TUDelft

Collaborations: Marcel Mayor (Basel), Cina Foroutan-Nejad (Warsaw)

Magnetic fingerprints of the molecules manifest themselves typically at low temperatures by e.g. zero-bias peaks in the differential conductance spectra attributed to Kondo resonances arising and spin-flip inelastic electron tunnelling spectroscopy (IETS) steps. Different spin systems are under study: all-organic mono- and diradicals, spin crossover components (molecules and nanoparticles) and single molecule magnets. Of special interest is the mechanical manipulation and electrostatic gating of the spin state of the molecule (spin switching).

Papers: T.Y. Baum, S. Fernandez, D. Peña and H.S.J. van der Zant. Magnetic fingerprints in an all-organic radical molecular break junction , Nano Letters 22 (2022)

Project (Funding): Spring (EU project)

Collaborations: Marcel Mayor (Basel), Talah Mallah (Paris), Abhishake Mondal (Bangalore), Eugenio Coronado (Valencia), Diego Peña (San Sebastian)

By subjecting a nanoobject such as a molecule to a temperature gradient, a current will flow through it without the application of a bias voltage. This thermocurrent contains important information about the nanoobject under study such as the spin entropy and universal signatures of Kondo physics. Of special interest therefore are high-spin molecules such as single-molecule magnets. Furthermore, by connecting in series to a load resistance, a particle exchange heat engine is created. Such an engine does not have any moving parts and is a few nanometer in size. It emerges as an ideal candidate for low-temperature energy harvesting applications where miniaturization is of paramount importance.

Part of the focus of the van der Zant Lab is in contacting and measuring electronic transport through atomically precise graphene nanoribbons. These carbon-based conductors offer promising prospects for future electronics and spintronics applications. An exciting part of atomically precise graphene nanoribbons is the ability to manipulate their structure, which allows for making metallic and magnetic graphene nanoribbons or ribbons with controlled defects for use as single-photon emitters. We also study graphene nanoribbons by coupling them to superconducting electrodes.

Papers: D. Bouwmeester et al., MoRe Electrodes with 10 nm Nanogaps for Electrical Contact to Atomically Precise Nanoribbons. ACS Appl. Nano Mater. (2023)

Project (Funding): Atypical (EU Pathfinder)

Collaborations: Michael Perrin and Gabriela Borin Barin (EMPA)

We study transport through a variety of biological systems including single amino acids, peptides, ferritin particles and biological nanowires. Especially the latter have captured our attention recently. Specifically, multicellular cable bacteria display an exceptional form of biological conduction sustaining electrical currents across centimeter distances through a regular network of protein fibers embedded in the cell envelope. The fiber conductivity is among the highest recorded for biomaterials, providing a promising outlook for the emerging field of bio-based electronics, but the underlying mechanism of electron transport remains elusive. Low-temperature measurements indicate that quantum effects can manifest themselves in biological systems, revealing a high degree of spatial ordering in their fundamental conducting structures.

Papers: J.A. Labra-Muñoz and H.S.J. van der Zant, J. Ferritin single-electron transistor, Phys. Chem. B 128 (2024);
J.R. van der Veen, et al. Temperature-Dependent Characterization of Long-Range Conduction in Conductive Protein Fibers of Cable Bacteria, ACS Nano (2024).

Project (Funding): PRINGLE (EU Pathfinder project)

Collaborations: Filip Meysman (Antwerp)

The properties of graphene can be enriched by the proximity to other two-dimensional (2D) materials in van der Waals heterostructures. We study quantum transport in graphene which is magnetized by the neighboring 2D magnet. The transport measurements provide evidence for the spin Hall effect and modified graphene band structure which unlocks its potential for future quantum-coherent spintronic and valleytronic devices.
Bilayer graphene (BLG) is a zero-bandgap semimetal that can be gapped under the application of a perpendicular electric field. We exploit this effect to create quantum point contacts and employ the spin and valley degree of freedom present in BLG to create devices with demonstrating specular electron focusing and valley-resolved ballistic electron transport.

Papers: T. Ghiasi, et al. Quantum Anomalous Hall and Spin Hall Effects in Magnetic Graphene. Arxiv (2024)
J. Ingla-Aynés, et al., Specular electron Focusing between Gate Defined Quantum Point Contacts in Bilayer Graphene. Nanoletters (2023)

Collaborations: Eugenio Coronado (Valencia), Quantum Tinkerer (Delft)

The Van der Zant Lab has had a long history with nanomechanical resonators, starting with studies on suspended carbon nanotubes in 2003. Now we focus on suspended 2D materials, where we develop novel ways of detecting phase transitions, measure their thermodynamic properties, or develop sensors outperforming state-of-the-art technology. This research line aims to get a deep understanding of magnetism towards the two-dimensional limit, where, for isotropic materials, we lack extensive experimental evidence of magnetic phase stabilizing mechanisms such as the Berezinskii-Kosterlitz-Thouless transition.

This project investigates the interplay between magnetic and mechanical excitations in nanomechanical magnetic devices, focusing on understanding their coupling and its role in spin dynamics. One avenue of exploration involves dissipation mechanisms, where phenomena like thermoelasticity and magnetoelasticity provide insights into this coupling via the quality factor of resonators. Additionally, we integrate van der Waals magnet resonators with superconducting cavities to probe magnetic excitations through ferromagnetic resonance (FMR), aiming to achieve strong coupling between electromagnetic and spin wave modes in thin vdW magnets—a key challenge in the field. If magnetoelastic interactions are sufficiently strong, the system could exhibit dynamics similar to optomechanics, enabling to explore the effects of magnetostriction in the nanoscale.

Papers: MJA Houmes, S Mañas‐Valero, A Bermejillo‐Seco, et al. Highly Anisotropic Mechanical Response of the Van der Waals Magnet CrPS4. Advanced Functional Materials 34 (2024).

Project (Funding): NWO XL

Collaborations: Yaroslav Blanter (Delft), Peter Steeneken (Delft), Gary Steele (Delft), Eugenio Coronado (Valencia)

We will fabricate suspended membranes of 2D ferroelectric and multiferroic quantum materials and heterostructures, and measure their nanomechanical properties by probing the resonance frequency of the membranes as a function of temperature and layer thickness. The study of van der Waals ferroelectricity is a recent development with many challenges. We will focus on new methods to characterize it by nanomechanical means and study its phase transitions (in combination with possible magnetic transitions in multiferroic compounds).

Papers: Work in progress

Project (Funding): Kavli Foundation

Collaborations: Mazhar Ali (Delft), Yaroslav Blanter (Delft), Peter Steeneken (Delft)