Session DD01: V: Bilayers

CVD-grown ultralong graphene nanoribbons for high-performance electronics

Graphene nanoribbons (GNRs) with widths of a few nanometers are promising candidates for future nano-electronic applications due to their structurally tunable bandgaps, ultrahigh carrier mobilities, and exceptional stability. However, the direct growth of micrometer-long GNRs on insulating substrates, which is essential for the fabrication of nano-electronic devices, remains an immense challenge. Here, we report the growth of GNRs on an insulating hexagonal boron nitride (hBN) substrate through nanoparticle-catalyzed chemical vapor deposition (CVD). The as-grown GNRs exhibit highly desirable features being ultra-long (micrometers), ultra-narrow (a few nm), and atomically smooth edges. Remarkably, the as-grown GNRs are crystallographically aligned with the hBN substrate, forming one-dimensional (1D) moiré superlattices. The ultranarrow nature and bandgaps are revealed by high-resolution microscopic techniques and spectroscopy. Fully atomistic computational simulations support the experimental results and reveal a competition between the formation of GNRs and carbon nanotubes (CNTs) during the nucleation stage, and van der Waals sliding of the GNRs throughout the growth stage. Using the grown structures, we demonstrate the transfer-free fabrication of GNR field-effect devices that exhibit excellent performance at room temperature. Our study provides a scalable, single-step method for growing micrometer-long narrow GNRs on insulating substrates, thus opening a route to explore the performance of high-quality GNR devices and the fundamental physics of 1D moiré superlattices.   

Nano-electromechanical quantum simulator

Electron-phonon interactions lead to a plethora of phenomena in strongly correlated solid-state systems such as superconductivity and charge-density waves. However, the complex dynamics manifesting these phases can be beyond the reach of computational modelling, especially when taking into account electron-electron interaction. Therefore, one of the outstanding challenges in the field of correlated-electron physics is a widely tuneable model system that can mutually couple several electronic and phononic degrees of freedom. To date, no such system has been experimentally realized.

While previous efforts have mostly focused on cold-atom configurations, nano-electromechanical systems are naturally suited to address this challenge. One of the most challenging requirements to engineering such a system is the achievement of ultrastrong electromechanical coupling, which has been recently demonstrated by our research group in a capacitively coupled carbon nanotube. Leveraging this capability, we work to engineer a model system in which electronic degrees of freedom are defined within four quantum dots and coupled to vibrational modes of a carbon nanotube. If successful, the project will enable the first experimental platform for quantum simulation of electron-phonon coupling.   

Quasiperiodicity enabled quasi-fractal order in narrow-band moir'e systems

The observation of correlated phenomena and unconventional superconductivity in twisted bilayer graphene (tBLG), a moiré system, is one of the most interesting findings in condensed matter physics in recent years. However, twisted bilayer graphene, even in a pristine form, is extremely peculiar: for most twist angles it lacks translational invariance, having instead a quasiperiodic structure. Quasiperiodicity in 1D is known to strongly impact the electronic properties. However, the role of quasiperiodicity in twisted bilayer graphene and other moiré materials has been mostly ignored so far.

In this talk, we will present recent theoretical efforts towards the understanding of the role played by quasiperiodity in moiré electron systems. We have theoretically demonstrated that quasiperiodicity in tBLG leads to the emergence of narrow band single-particle critical states (delocalized in both real and momentum space) with subbalistic transport properties [1].

To unveil the interplay between interactions and quasiperiodicity, we resort to a 1D illustrative example to demonstrate that quasiperiodicity can radically change the ground state properties of moiré systems [2]. While narrow bands play a significant role in enhancing interactions both for commensurate and incommensurate structures, only quasiperiodicity concomitant with critical states is able to extend the ordered phase down to an infinitesimal interaction strength. In this regime, the quasiperiodic-enabled state has contributions from infinitely many wave vectors. This quasi-fractal regime cannot be stabilized in the commensurate case even in the presence of a narrow band.

[1] Incommensurability-induced sub-ballistic narrow-band-states in twisted bilayer graphene

Miguel Gonçalves, Hadi Z. Olyaei, Bruno Amorim, Rubem Mondaini, Pedro Ribeiro, and Eduardo V. Castro

2D Mater. 9 011001 (2022)

doi: 10.1088/2053-1583/ac3259

[2] Incommensurability enabled quasi-fractal order in 1D narrow-band moiré systems

Miguel Gonçalves, Bruno Amorim, Flávio Riche, Eduardo V. Castro, and Pedro Ribeiro

arXiv: 2305.03800

https://arxiv.org/abs/2305.03800   

Mirror-protected Majorana zero modes in f-wave multilayer graphene superconductors

Inspired by recent experimental discoveries of superconductivity in multilayer graphene, we study models of f-wave superconductivity on the honeycomb lattice with arbitrary numbers of layers. For odd numbers of layers, these systems are topologically nontrivial, characterized by a mirror-projected winding number ν_± = ±1. Along each mirror-preserving edge in armchair nanoribbons, there are two protected Majorana zero modes. These modes are present even if the sample is finite in both directions, such as in rectangular and hexagonal flakes. Crucially, zero modes can also be confined to vortex cores. Finally, we apply these models to twisted bilayer and trilayer systems, which also feature boundary-projected and vortex-confined zero modes. Since vortices are experimentally accessible by local scanning probes, our study suggests that superconducting multilayer graphene systems are promising platforms to create and manipulate Majorana zero modes.   

Electrically tunable flat bands with layer-resolved charge distribution in twisted monolayer-bilayer graphene

At a small twist angle, exotic electronic properties emerge in twisted monolayer-bilayer graphene (TMBG), including electrically switchable magnetic order and correlated insulating states. These fascinating many-body phenomena manifest when the low-energy bands feature a narrow bandwidth. In this study, we examine the electronic structure of TMBG using a tight-binding model with accurate parameters. Our results show that the low-energy bands of TMBG reach a minimum width at a quasi-magic angle, strongly correlated with the magic angle in twisted bilayer graphene. In addition, we find charge localization in the adjacent twisted layers and delocalization in the outer Bernal layer. Furthermore, in the presence of an electric field, an energy gap opens if lattice relaxation is taken into account. The particle-hole asymmetry in TMBG further leads to flatter conduction bands compared with the valence bands, with an electrically tunable band width and band gap.   

Effect of Local Environment on the Vibrational Properties of Twisted Bilayer Graphene: A Machine Learning Approach

In recent years, there has been growing interest in twisted bilayer structures in which two single atom thick sheets are rotated with respect to each other around their normal axes. This interest is due to the unusual behavior exhibited by these systems which their single layer or untwisted counterparts fail to demonstrate. While the emerging effects are most probably associated with moiré lattices-the new type of periodicity appearing in twisted structures; the relationship between the superlattice parameters and the rotation angle is highly non-linear in nature, thus making it very challenging to build a model which relates the effect of moiré lattices to vibrational properties. While the Bernal stacked bilayer graphene bears two distinct atom types in its lattice, there exists no mathematical framework addressing the question of what this number would be in a given twisted bilayer graphene moiré superlattice. We first address this question and identify atoms with a unique environment. Subsequently, we calculate their local phonon density of states to establish a database, which is used to train a machine learning model. We show that our machine learning model effectively predicts the vibrational properties of any given twisted bilayer graphene.   

Long-distance spin transport in indirect excitons in MoSe2/WSe2 heterostructure

We studied spatially indirect excitons (IXs), also known as interlayer excitons, in van der Waals heterostructures composed of atomically thin layers of transition-metal dichalcogenides (TMD). In the IX system, we observed the long-distance spin transport with the decay distances exceeding 100 microns and diverging so spin currents show no decay in the HS. With increasing IX density, we observed spin localization, then long-distance spin transport, and then reentrant spin localization, in agreement with the Bose-Hubbard theory prediction for superfluid and insulating phases in periodic potentials due to moire superlattices.   

Numerical calculations of superheating field in superconductors with nanostructured surfaces.

We report the numerical calculation of a DC superheating field H_sh by solving the Ginzburg-Landau (GL) equations for superconductors with inhomogeneous impurity concentration at the surface and superconductor-insulator-superconductor (S-I-S) multilayers.  The superheating field was calculated as the field onset of instability of the Meissner state with respect to perturbations with a finite wavelength along the surface for the GL parameters κ typical of Nb and Nb₃Sn.  We show that optimizing the thicknesses of the impurity diffusion layer and the overlayer in the S-I-S structure can significantly increase H_sh beyond the bulk superheating fields of both materials. For instance, a S-I-S structure comprised of a 90 nm thick Nb₃Sn overlayer on Nb can boost H_sh up to 500 mT while blocking dendritic thermomagnetic avalanches triggered by local penetration of vortices. This H_sh is about 2.2 times higher than the H_sh of Nb and ≃ 5.3 % higher than the H_sh of Nb₃Sn.