Atomic-scale visualization of multiferroicity in monolayer NiI2_2

Progress in layered van der Waals materials has resulted in the discovery of ferromagnetic and ferroelectric materials down to the monolayer limit. Recently, evidence of the first purely two-dimensional multiferroic material was reported in monolayer NiI$_2$. However, probing multiferroicity with scattering-based and optical bulk techniques is challenging on 2D materials, and experiments on the atomic scale are needed to fully characterize the multiferroic order at the monolayer limit. Here, we use scanning tunneling microscopy (STM) supported by theoretical calculations based on density functional theory (DFT) to probe and characterize the multiferroic order in monolayer NiI$_2$. We demonstrate that the type-II multiferroic order displayed by NiI$_2$, arising from the combination of a magnetic spin spiral order and a strong spin-orbit coupling, allows probing the multiferroic order in the STM experiments. Moreover, we directly probe the magnetoelectric coupling of NiI$_2$ by external electric field manipulation of the multiferroic domains. Our findings establish a novel point of view to analyse magnetoelectric effects at the microscopic level, paving the way towards engineering new multiferroic orders in van der Waals materials and their heterostructures

Electronic and magnetic characterization of epitaxial CrBr3_3 monolayers

The ability to imprint a given material property to another through proximity effect in layered two-dimensional materials has opened the way to the creation of designer materials. Here, we use molecular-beam epitaxy (MBE) for a direct synthesis of a superconductor-magnet hybrid heterostructure by combining superconducting niobium diselenide (NbSe$_2$) with the monolayer ferromagnetic chromium tribromide (CrBr$_3$). Using different characterization techniques and density-functional theory (DFT) calculations, we have confirmed that the CrBr$_3$ monolayer retains its ferromagnetic ordering with a magnetocrystalline anisotropy favoring an out-of-plane spin orientation. Low-temperature scanning tunneling microscopy (STM) measurements show a slight reduction of the superconducting gap of NbSe$_2$ and the formation of a vortex lattice on the CrBr$_3$ layer in experiments under an external magnetic field. Our results contribute to the broader framework of exploiting proximity effects to realize novel phenomena in 2D heterostructures

Emergence of Exotic Spin Texture in Supramolecular Metal Complexes on a 2D Superconductor

Designer heterostructures, where the desired physics emerges from the controlled interactions between different components, represent one of the most powerful strategies to realize unconventional electronic states. This approach has been particularly fruitful in combining magnetism and superconductivity to create exotic superconducting states. In this work, we use a heterostructure platform combining supramolecular metal complexes (SMCs) with a quasi-2D van der Waals (vdW) superconductor NbSe$_2$. Our scanning tunneling microscopy (STM) measurements demonstrate the emergence of Yu-Shiba-Rusinov (YSR) bands arising from the interaction between the SMC magnetism and the NbSe$_2$ superconductivity. Using X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) measurements, we show the presence of antiferromagnetic coupling between the SMC units. These result in the emergence of an unconventional $3\times3$ reconstruction in the magnetic ground state that is directly reflected in real space modulation of the YSR bands. The combination of flexible molecular building blocks, frustrated magnetic textures, and superconductivity in heterostructures establishes a fertile starting point to fabricating tunable quantum materials, including unconventional superconductors and quantum spin liquids

Designing quantum matter in two dimensions

Experimental condensed matter research is experiencing a paradigm shift. In the past, the need for 3D crystalline samples with a specific structure and doping imposed strict requirements for achieving the desired properties. Now, it is possible to create tunable 2D samples in the lab, overcoming the previous limitations and accelerating the research. In this thesis, I will demonstrate how we utilized the molecular beam epitaxy (MBE) technique to design and fabricate van der Waals (vdW) heterostructures. The vdW nature of the MBE-grown materials enabled us to combine physical properties of different materials without altering them. I will showcase how we engineered two phases of matter that are central to condensed matter physics, and probed them using scanning tunneling microscope. The first is heavy fermions, which we artificially created by combining magnetic and metallic vdW materials. Here, a lattice of magnetic moments in 1T-TaS2 couples via the Kondo exchange coupling to conduction electrons in 1H-TaS2, resulting in the emergence of Kondo lattice physics. The flat band of Kondo lattice hybridized with the dispersive electrons, leading to a gap at the Fermi level of 1H-TaS2. In this special case of separated magnetic moments and conduction electrons, we reproduce the expected observables in the spectral function: Kondo peak on the side of magnetic lattice and heavy-fermion hybridization gap on the side of conduction electrons. The second is unconventional superconductivity, which we attempted to achieve in two different systems: a monolayer vdW material and a vdW heterostructure composed of a superconductor and a magnet. We observe strong signatures of nodal superconductivity in 1H-TaS2: V-shaped spectral gap, many-body fluctuations and a pseudogap, which is the first evidence of unconventional superconductivity in a monolayer vdW material. We also create topological superconductivity in a heterostructure of magnetic CrBr3 and superconducting 2H-NbSe2, and observe the edge state and the Shiba bands. Futhermore, we show that the topology of this system arises from the moiré pattern. These results highlight the benefits of the designer approach and open new opportunities in their respective fields

Control of Molecular Orbital Ordering Using a van der Waals Monolayer Ferroelectric

Two-dimensional (2D) ferroelectric materials provide a promising platform for the electrical control of quantum states. In particular, due to their 2D nature, they are suitable for influencing the quantum states of deposited molecules via the proximity effect. Here, we report electrically controllable molecular states in phthalocyanine molecules adsorbed on monolayer ferroelectric material SnTe. In particular, we demonstrate that the strain and ferroelectric order in SnTe creates a transition between two distinct orbital orders in the adsorbed phthalocyanine molecules. By controlling the polarization of the ferroelectric domain using scanning tunneling microscopy (STM), we have successfully demonstrated that orbital order can be manipulated electrically. Our results show how ferroelastic coupling in 2D systems allows control of molecular states, providing a starting point for ferroelectrically switchable molecular orbital ordering and ultimately, electrical control of molecular magnetism.peerReviewe

Moiré-Enabled Topological Superconductivity

The search for artificial topological superconductivity has been limited by the stringent conditions required for its emergence. As exemplified by the recent discoveries of various correlated electronic states in twisted van der Waals materials, moiré patterns can act as a powerful knob to create artificial electronic structures. Here, we demonstrate that a moiré pattern between a van der Waals superconductor and a monolayer ferromagnet creates a periodic potential modulation that enables the realization of a topological superconducting state that would not be accessible in the absence of the moiré. The magnetic moiré pattern gives rise to Yu–Shiba–Rusinov minibands and periodic modulation of the Majorana edge modes that we detect using low-temperature scanning tunneling microscopy (STM) and spectroscopy (STS). Moiré patterns and, more broadly, periodic potential modulations are powerful tools to overcome the conventional constraints for realizing and controlling topological superconductivity.peerReviewe

Evidence of nodal f-wave superconductivity in monolayer 1H-TaS2_2 with hidden order fluctuations

Unconventional superconductors represent one of the fundamental directions in modern quantum materials research. In particular, nodal superconductors are known to appear naturally in strongly correlated systems, including cuprate superconductors and heavy-fermion systems. Van der Waals materials hosting superconducting states are well known, yet nodal monolayer van der Waals superconductors have remained elusive. Here, we show that pristine monolayer 1H-TaS$_2$ realizes a nodal superconducting state with f-wave spin-triplet symmetry. We show that including non-magnetic disorder drives the nodal superconducting state to a conventional gapped s-wave state. Furthermore, we observe the emergence of many-body excitations potentially associated to the unconventional pairing mechanism. Our results demonstrate the emergence of nodal superconductivity in a van der Waals monolayer, providing a building block for van der Waals heterostructures exploiting unconventional superconducting states

Single-gap superconductivity in Mo8Ga41

International audienc