Analog simulation provides a way to solve hard computational problems by building physical devices that mimic those problems. The history of such devices goes back at least 2000 years to the intricate clockwork mechanisms used to make complex astronomical predictions, before the advent of all-purpose digital computers. But today there remain many important problems that are intractable, even for the fastest supercomputers. An important class of such problems relates to simulating fundamental models of quantum matter, which underpin our understanding of nanoscale processes and bulk materials. Since universal quantum computers capable of tackling such problems are still far off, an emerging paradigm is to sacrifice generality for power by constructing devices with quantum components to perform analog quantum simulation.

However, a prerequisite for scaling up to simulators with meaningful power is to understand the basic nanoelectronic components from which they are built, and the interaction between such components in a quantum circuit. This involves characterizing simple devices in quantitative detail, validating theoretical models against experimental measurements -- in essence to simulate the simulators!

In this talk I discuss very recent experiments with nanoelectronics circuits involving coupled hybrid metal-semiconductor quantum dots, and the theoretical effort to model and understand them [1]. I show that novel interactions can be engineered in such systems, and this can be exploited to realize an exotic quantum impurity model that hosts a local free Z3 parafermion [2]. Distinctive conductance signatures predicted for the device are observed in the experiment. Finally, I discuss recent progress towards measuring the factional entropy associated with such fractional anyonic modes in quantum devices [3,4].

[1] arXiv:2108.12691
[2] arXiv:2210.04937
[3] PRL 129, 227702 (2022)
[4] PRL 128, 146803 (2022)

Recently, the rich phase diagrams found in few-layer materials have attracted much attention from the scientific community; competition of correlated states, including Mottness, superconductivity, and magnetic and charge order were observed in these systems [1-4]. One characteristic these distinct materials share is the Moiré structure originated by layer misalignment or intentional twist between them. However, a central point of the Moiré superlattice is the increase in the unit cell in real space, meaning smaller momentum space Brillouin Zones. This scenario can favor scattering between small-momentum electronic states, meaning that long-range real-space interactions might play a relevant role. Therefore, in this ongoing project, under the Random Phase Approximation (RPA), we investigate the influence of these longe-range interactions on the magnetic fluctuations of twisted bilayer graphene as a model system, in order to trigger the possible collective states resulting from the Moiré superlattice scenario. Finally, we find that long-range interactions might favor superconductivity up to a certain interaction strength, and then suppress it thereon depending on the doping of the sample. Also, in qualitative agreement with experiments, we also show both charge and magnetic order might be present, where the first one is closely related to the long-range interactions.

References: 

[1] - Science, 6539, 264-271 (2021). https://doi.org/10.1126/science.abc283

[2] - Wong et al., Nature 582, 198–202 (2020). https://doi.org/10.1038/s41586-020-2339-0

[3] - Devakul et al., Nat Commun 12, 6730 (2021). https://doi.org/10.1038/s41467-021-27042-9

[4] - Jiang et al., Nature 573, 91–95 (2019). https://doi.org/10.1038/s41586-019-1460-4

Van der Waals (vdW) heterostructures combining different two-dimensional (2D) materials have a great prospect for the realization of atomically thin devices with tailored properties. To achieve a high density, bottom-up integration, the synthesis of such heterostructures via vdW epitaxy (which is when 2D materials are grown on top of each other) is a promising alternative to sequential layer or flake transfer, which is problematic in terms of scaling and reproducibility. Nevertheless, due to the weak interaction between 2D crystals, vdW epitaxy is sensitive to various surface defects, usually leading to uncontrolled nucleation and thus non-uniform growth of polycrystalline material. In this talk, I will discuss this issue taking as an example the case of the 2D insulator hexagonal boron nitride (h-BN) grown by molecular beam epitaxy (MBE) directly on graphene/SiC(0001) substrates. Specifically, I will show how defect engineering in graphene can be employed to realize selective area growth of hBN/graphene heterosystems. Furthermore, I will present our recent studies on ferromagnetic vdW heterostructures with perpendicular magnetic anisotropy and high transition temperatures, two fundamental properties for spintronic applications. Such vdW heterostructures were realized via MBE growth of the novel 2D ferromagnetic metal Fe5-xGeTe2 (FGT, with 0 ≤ x ≤ 2) on graphene as well as h-BN templates.
 

The oxygen reduction reaction (ORR) is a crucial chemical process, which contributes significantly to the energy efficiency of fuel cells and metal-air batteries. Although Pt and its alloys are known as the most efficient catalysts for ORR, its high costs and scarcity limits their applications. Alternatively, Fe–N–C single-atom catalysts (SACs) have attracted the most interest owing to their low-cost and facile preparation methods. Metal macrocycles, such as iron phthalocyanines (FePc), are the precursors most widely used to prepare the ORR catalysts in fuel cells and metal-air batteries. However, limited progress has been made in improving their electrocatalytic activity and durability. Indeed, FePc has demonstrated to suffer from severe degradation in the acidic fuel cell environments, commonly attributed to demetallation and/or degradation by ORR intermediates. In this seminar, recent advances in the theoretical characterization of SAC for ORR will be highlighted, using DFT-based ab-initio calculations, including molecular dynamic simulations. The focus will be the interaction of FePc with oxygen molecules and mechanisms of FePc deactivation under operational conditions, including explicitly aqueous environments that reproduce acid and alkaline media.

Solid state battery technology has been hailed as the next frontier in terms of energy storage devices because it solves some of the crucial problems with Li-ion batteries, namely, flammability, limited voltage, unstable solid-electrolyte interphase formation, poor cycling performance and strength. In fact, hundreds of millions of dollars are being invested in companies that claim to exploit solid state technology, such as QuantumScape, creating a hype around solid state battery technology (albeit no products have been commercialized). In my talk, I will discuss the microscopic basis for the understanding of ionic motion in solids and show that traditional electrochemical concepts, used for centuries, are not useful for the understanding of ionic motion in crystals. Instead, only a solid state approach, based on quantum mechanical concepts, can be used as a guide for the creation of this new technology. In particular, I will provide a formula for the ionic conductivity of a crystal in terms of its basic microscopic elements that can be used as a guide for the development of super-ionic conductors.

In this seminar I will discuss the Majorana correlations in quantum impurities coupled to a topological superconducting wire. By means of the density matrix renormalization group (DMRG) approach, the Majorana correlation functions have been calculated (for the ground state of the system) for different coupling positions of the impurities along the wire and length of the wire. We have observed that the correlation decreases exponentially with the distance between the impurities and the ends of the wire. Moreover, different electron-electron interactions on the wire (and on the impurities) were also analyzed, and the results showed a stronger effect as the electron occupation in the wire is increased. In addition, while changing the occupation of the impurities by tuning their energy levels, we have observed that for specific values of ϵ (impurities' chemical potential), the absolute value of correlation is highly peaked when the chemical potential lies within the window of the topological regime. These peaks are associated to Coulomb blockade phenomena in the impurities, revealing unambiguously its effect on the Majorana bound states in the topological phase of the system.

Reference: G. S. Diniz and E. Vernek: arXiv:2208.09524.