The Trans-Neptunian Object (50000) Quaoar, classified as a cubewano, is a dwarf planet candidate with a diameter of 1,110 km, a semi-major axis of 43.7 au, and an orbital eccentricity of 0.04. Its satellite Weywot orbits at 13,300 km from the primary object, and its diameter is about 90 km, derived from its flux, assuming the same albedo as Quaoar. It allows the determination of Quaoar's mass. Over the years, several campaigns were conducted within the ERC Lucky Star project to observe stellar occultations by Quaoar and Weywot. Besides measuring Quaoar's and Weywot's sizes and shapes, those campaigns aimed at searching for material around this TNO. The events analyzed in this work were observed between 2018 and 2021, and in these high-quality occultation light curves, besides the main body occultation, secondary events were also observed, and they could be explained as a dense ring surrounding Quaoar at about 4,100 km (7.4 Quaoar radii).  One important detail is that this region is well outside the Roche limit of the central body of 1,780 km (3.2 Quaoar radii), assuming that the bulk density of particles would be around 400 kg m-3 (typical of Saturnian small satellites). This discovery will be discussed in detail during the presentation.

The advent of nanomaterials has brought several challenges on the materials' characterization framework. These challenges open for opportunities on the development of instruments capable to overcome today's technological limitations. In optical spectroscopy, diffraction mimics the capacity of conventional optical setups to extract spectral information at the nanoscale. In this seminar, I will present recent advances on the development of a near-field Raman spectroscopy system, taking place in LabNS, the Nanospectroscopy Lab of the Department of Physics, UFMG. The instrument allows for the investigation of local properties in individual nano-objects, and the information extracted from this local analysis is useful to understand statistical results extracted from measurements performed in the micro and macro scales. Supporting the instrument's technology, we have developed a new scattering-type near-field probe formed by a micro-pyramidal body whose length L is scalable to fine-tune localized surface plasmon resonance (LSPR) modes. These so-called plasmon-tunable tip pyramids (PTTPs) act as monopole antennas, as revealed by electron energy loss spectroscopy (EELS). The monopole character of the PTTP is a consequence of its geometry: the nanopiramidal part is electrically grounded on a flat metallic plateau that acts like a mirror providing the monopole's image that closes the dipole system. The talk ends with a discussion on the coherence properties of scattered fields in the proximity of the source (a material system illuminated by strongly focused by optical fields). I will demonstrate that the spatial extent of near-field correlations relies on local properties of the source which are inaccessible in the far field zone.

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.
 

Dynamical mean-field theory (DMFT) obtains an exact solution of the metal-insulator transition (MIT) of the ½-filled Hubbard model on the Bethe lattice with infinite coordination number, in terms of the local MIT of an Anderson impurity model that is hybridized with a conduction bath of correlated electrons whose properties (e.g., the electronic density of states) are determined self-consistently. The MIT is observed via the impurity/bath local spectral function, in terms of a dynamical transfer of spectral weight from the Kondo resonance at zero frequency to Hubbard sidebands at higher frequencies as the on-site Hubbard repulsion (U) on the Anderson impurity is tuned towards strong coupling. However, the numerical implementation of self-consistency precludes a deeper understanding of the physics mechanism responsible for the breakdown of the Kondo screening process, and the related dynamical spectral weight transfer. In meeting this goal, we develop a minimal impurity model by extending the standard Anderson impurity model to include an additional local spin exchange coupling between the impurity and the conduction bath, as well as a weak local on-site correlation in the conduction bath site just adjacent to the impurity. We analyse this extended Anderson impurity model by employing the recently developed unitary renormalization group (URG) method [1-4]. This reveals that local holon-doublon fluctuations on the bath sites nearby the impurity destabilise the local Fermi liquid metal phase, and lead to the emergence of the repulsion-driven local moment Mott insulating phase of the impurity. Interestingly, a local non-Fermi liquid metal is found to exist precisely at the transition, and marks a partial breakdown of the Kondo screening. We will present the understanding obtained of the well-studied phenomenology of the MIT within DMFT, as well as some novel predictions on the universal nature of the metal-insulator transition in this model. Implications for DMFT studies of more realistic models of strongly correlated electronic systems are considered.

References
1.    A. Mukherjee and S. Lal. Nucl. Phys. B 960, 115170 (2020);
ibid, Nucl. Phys. B 960, 115163 (2020)
2.    A. Mukherjee and S. Lal. New Journal of Physics. 22, 063007
(2020); ibid, New Journal of Physics. 22, 063008 (2020).
3.    A. Mukherjee et al., Physical Review B 105, 085119 (2022)
4.    S. Patra et al., arXiv:2205.00790

The volume of data generated in the world has increased exponentially, and in the last 10 years,
it is no longer possible to store all the data generated. In 2020, about 40 trillion gigabytes of
data were generated. But, after all, what does it matter in the academic world? What does this
impact on the business of a small, medium or large company? How can this impact our life?
The answer starts with the search for people with technical training to work in the processing of
all this volume of data. They are professionals with the ability to translate information into
strategic decisions, essential within companies, in research and innovation. At this moment, we
are faced with a major problem: there are not enough people in the world with enough technical
skills, today and in the next 10 years, to work in the areas of data and generate value with them.
Why such an intense search for professionals who work with data?
Companies have already understood that the data driven culture generates an increase in
revenue, cost reduction and improvement in people's quality of life.
And when we say “companies” we mean all market segments: industry, health, retail, finance,
e-commerce, wholesale, among others.
There are almost 20 million companies in Brazil alone. Imagine 1% of them hiring technology
professionals, we are talking about 2 thousand vacancies, but we already know that the
projection is 700 thousand vacancies from today to the next 5 years.
With the information mentioned, the question remains about how the academic area,
universities, faculties, technical institutes and schools can be structured to technically train all
this volume of people. In addition, how to generate a greater bond between educational
institutions and companies so that this process can be accelerated and optimized for people
who intend to enter the job market.
The idea of this seminar is to present how big data and analytics are being implemented in real
cases in companies from different sectors. How Data Science, Data Engineering, Machine
Learning Engineering, DevSecOps Engineering MLOps, DataOps and Cloud Architecture are
being structured.
 

Two-dimensional heterostructures with nanometer scale unit cells realized via moiré and nanopatterned engineering provide tunable platforms to observe electronic Hofstadter bands in lattices pierced by magnetic flux per unit cell comparable to the flux quantum h/e. The celebrated fractal spectrum and the topological properties of the single particle states have long been associated with quantum Hall phenomena. In this talk, I will theoretically discuss new states of matter that result when Hofstadter electrons form Cooper pairs. I will begin by providing a general symmetry-based classification of such Hofstadter superconductors that highlights the role of magnetic translation symmetries in determining the irreducible representations of such paired states [1]. This classification shows that Hofstadter superconductors are described by multi-component charge 2e order parameter supporting rich phase diagram, and that pairing implies breaking of magnetic translation symmetries.  

Second, I will employ a renormalization group (RG) analysis on the square lattice Hofstadter-Hubbard model to demonstrate that the combination of repulsive interactions with the presence of a tunable manifold of Van Hove singularities provides a new mechanism for driving unconventional superconductivity in Hofstadter bands [2]. Specifically, the number of Van Hove singularities at the Fermi energy can be controlled by varying the flux per unit cell and the electronic filling, leading to instabilities toward nodal superconductivity and chiral topological superconductivity with Chern number C = ±6. The latter is characterized by a self-similar fixed trajectory of the RG flow and an emerging self-similarity symmetry of the order parameter. This analysis uncovers new potentialities for the realization of unconventional symmetry broken and topological orders in Hofstadter quantum materials.

[1] Theory of Hofstadter Superconductors, D. Shaffer, J. Wang and L.H. Santos, Phys. Rev. B 104, 184501 (2021)

[2] Unconventional Self-Similar Hofstadter Superconductivity from Repulsive Interactions, D. Shaffer, J. Wang and L.H. Santos, arXiv:2204.13116  

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.