We study the properties of flat-bands that appear in a heterostructure composed of strands of different widths of graphene armchair nanoribbons. One of the flat-bands is reminiscent of the one that appears in pristine armchair nanoribbons and has its origin in a quantum mechanical destructive interference effect, dubbed `Wannier orbital states' by Lin et al. in Phys. Rev. B 79, 035405 (2009). The additional flat bands present in the heterostructure, some reasonably closer to the Fermi level, seem to be generated by a similar interference process. After doing a thorough tight-binding analysis of the band structures of the different kinds of heterostructures, focusing in the properties of the flat-bands, we use Density Functional Theory to study the possibility of magnetic ground states when placing, through doping, the Fermi energy close to the different flat-bands. Our DFT results confirmed the expectation that these heterostructures, after being appropriately hole-doped, develop a ferromagnetic ground state that seems to require, as in the case of pristine armchair nanoribbons, the presence of a dispersive band crossing the flat-band. Currently, we are in the process of exploring different heterostructures to define which ones present more robust magnetism. In addition, we found a remarkable agreement between the tight-binding and DFT results for the charge density distribution of the so-called Wannier orbital states. Finally, it should be noted that some of these heterostructures were experimentally shown to have a topologically non-trivial ground state with a finite value for the winding number associated to the Su-Schrieffer-Heeger dimerized chain [Nature 560, 209 (2018)]. 

Crystallographic space group symmetry (CPGS) such as polar and nonpolar crystal classes have long been known to classify compounds that have spin-orbit-induced spin splitting. While taking a journey through the Brillouin Zone (BZ) from one k-point to another for a fixed CPGS, it is 
expected that the wavevector point group symmetry (WPGS) can change, and consequently a qualitative change in the texture of the spin polarization (the expectation value of spin operator S ⃗_(nk_0 ) in Bloch state u(n,k) and the wavevector k_0). However, the nature of the spin texture (ST) change is generally unsuspected. In this work, we determine a full classification of the linear-in-k spin texture patterns based on the polarity and chirality reflected in the WPGS at k_0. The spin-polarization vector S ⃗_(nk_0 ) controlling the ST is bound to be parallel to the rotation axis and perpendicular to the mirror planes and hence, symmetry operation types in WPGSs impose symmetry restriction to the ST. For instance, the ST is always parallel to the wavevector k in non-polar chiral WPGSs since they contain only rotational symmetries. Some consequences of the ST classification based on the symmetry operations in the WPGS include the observation of ST patterns that are unexpected according to the symmetry he crystal. For example, it is usually established that spin-momentum locking effect (spin vector always perpendicular to the wavevector) requires the crystal inversion symmetry breaking by an asymmetric electric potential. However, we find that polar WPGS can have this effect even in compounds without electric dipoles or external electric fields. We use the determined relation between WPGS and ST as a design principle to select compounds with multiple ST near band edges at different k-valleys. Based on high-throughput calculations for 1481 compounds, we find 37 previously fabricated materials with different ST near band edges.

There are many opportunities for physics graduates in the private sector. These in general fall into a few broad categories. I will give a general overview and present the pros and cons of each from a physicist point of view. In addition, I will describe the lessons from my experience as an interviewee and interviewer for private sector jobs, and give guidance for job interview preparation and the transition in general. In that I will include links to training and preparation resources.

The lateral resolution archived by Raman microscopy, as well as any conventional optical system, is diffraction limited to roughly half of the incident light wavelength. This means a resolution of few hundreds of nanometers when visible light is used as incident radiation, which is insufficient for resolving and properly characterize the morphology of any nanostructure. In such an optical system, the range of long wavevectors of the scattered optical components are lost at only few nanometers far from the sample as evanescent waves, limiting the detected optical information from the far-field and the archivable spatial resolution. As a way to go beyond this barrier, Tip Enhanced Raman Spectroscopy (TERS) combines the Raman spectroscopy system with a scanning probe microscope, which holds an optical nanoantenna few nanometers from the sample. The nanoantenna is designed to convert the propagation optical radiation into localized energy, and vice-versa, allowing Raman spectroscopy to perform with spatial resolution down to 10 nm. In this talk I will present and discuss the fundamental of TERS focusing on the development of new efficient optical nanoantennas made by Inmetro and UFMG. Also, I will give examples of applications of TERS in the characterization of two-dimensional materials and other nanomaterials, including our work recently published in Nature on the TERS characterization of low-angle twisted bilayer graphene.

We present a numerical approach for computing the properties of quantum dots (QDs) coupled to superconducting (SC) regions with finite charging energy Ec (e.g., SC grains, thin epitaxial SC layers on nanowires, etc.). It is based on the matrix product operator (MPO) representation of the Hamiltonian in terms of small 9x9 matrices. The low-lying excitations can be computed using the density matrix renormalization group (DMRG) and the (imaginary) time evolution using the time-dependent variational principle (TDVP), providing information on the dynamic response of the system (e.g., dynamical charge susceptibility of the QD). The method treats pairing interaction, electron-electron repulsion and the Kondo exchange interaction induced by the QD-SC hybridization on equal footing, and it is essentially exact.

For the QD-SC problem, we examined the transition from the regime dominated by Yu-Shiba-Rusinov (YSR) physics to the regime governed by the Coulomb blockade, with the nature of the low-lying excitations continuously evolving between the two limits [1]. For Ec>D, the method also allows investigating the case of odd-occupancy of the SC island [1], where a new type of subgap excitation is found with no counterpart in the Ec<D range. These predictions have been confirmed in recent experiments [2].

[1] Pavešić, L., Bauernfeind D., Žitko, R. (2021). Yu-Shiba-Rusinov states in superconducting islands with finite charging energy, arXiv:2101.10168.

[2] Estrada Saldaña, J. C., Vekris, A., Pavešić, L., Krogstrup, P., Žitko, R., Grove-Rasmussen, K., Nygård, J. (2021). Bias asymmetric subgap states mimicking Majorana signatures. arXiv, 2101.10794.

In recent years, thanks to the progress in the field of quantum simulations with ultracold gases, there has been a growing interest in pattern formation. The search for spontaneous pattern formation in equilibrium phases with genuine 
quantum properties is a leading direction of current research. We investigate the effect of quantum fluctuations - zero point motion and exchange interactions - on the phases of an ensemble of bosonic particles with isotropic hard-soft 
corona interactions. We perform extensive path-integral Monte Carlo simulations  to determine their ground state properties. A rich phase diagram, parametrized by the density of particles and the interaction strength of the soft-corona 
potential, reveals supersolid stripes, kagome and triangular crystals in the low-density regime. In the high-density limit we observe patterns with 12-fold rotational symmetry compatible with periodic approximants of quasicrystalline 
phases. We characterize these quantum phases by computing the superfluid density and the bond-orientational order parameter. Finally, we highlight the qualitative and quantitative differences of our findings with the classical equilibrium phases 
for the same parameter regimes.

A central theme in condensed matter physics in the last few decades has been the search for topological phases that harbor particles known as Majorana zero modes. The main motivation stems from the realization that these particles behave as non-abelian anyons, whose manipulation could provide a platform for topological quantum computation. In this talk I will discuss the theoretical proposals and experimental evidence that suggest that Majorana fermions may emerge from the fractionalization of spin excitations in Kitaev spin liquids. In particular, I will talk about how the application of electric fields allows one to detect and manipulate Majorana zero modes in magnetic insulators. 

In this seminar, I will discuss the main features of a well-written scientific paper, with tips related to the anatomy of a paper and how to convey the contributions of a scientific work. I will also present a strategy based on corpus linguistics for multilingual authors to acquire the discourse of 
scientific writing in English. 

A valence-bond solid is a quantum paramagnetic phase that can be realized in a quantum spin system,  characterized by the absence of magnetic long-range order, but broken lattice symmetries. The elementary excitations of a dimerized VBS phase correspond to singlets turned into triplets,  the so-called triplons. Such excitations can be analytically described  within the bond-operator representation, where spin operators are expanded in terms of singlet and triplet boson operators. In this talk, we will discussed the stability and properties of  many-triplon states.  We will concentrate on the intermediate parameter region of the square lattice spin-1/2  J1--J2 antiferromagnetic Heisenberg model, where a quantum paramagnetic phase sets in. An introduction to the model, valence-bond solid phases, and the bond-operator formalism will be presented.  We will show our mean-field results for the excitation spectrum above a many-triplon state, in addition to spin-spin and dimer-dimer correlation functions, dimer order parameters,  and the bipartite von Neumann entanglement entropy as a function of the triplon number. We also comment on possible relations between many-triplon states with large triplon number and gapped spin-liquid states.

In the past couple of years, machine learning has permeated many areas of physics and found numerous applications in condensed matter and chemistry. In particular, we have witnessed remarkable progress toward developing computational methods using neural networks as variational estimators. Variational representations of quantum states abound and have successfully been used to guess ground-state properties of quantum many-body systems. Some are based on partial physical insight (Jastrow, Gutzwiller projected, and fractional quantum Hall states, for instance), and others operate as a black box that may contain information about the underlying structure of entanglement and correlations (tensor networks, neural networks) and offer the advantage of a large set of variational parameters that can be efficiently optimized. However, using variational approaches to study excited states and, in particular, calculating the excitation spectrum, remains a challenge.

In this talk, I present two variational methods to calculate the dynamical properties and spectral functions of quantum many-body systems in the frequency domain: The first one consists of encoding the Green's function of the problem in the form of a neural network. We introduce a natural gradient descent approach to solve linear systems of equations and use Monte Carlo to obtain the dynamical correlation function. The second approach is based on a Chebyshev expansion of the spectral function and a neural network representation for the wave functions. The Chebyshev moments are obtained by recursively applying the Hamiltonian and projecting on the space of variational states. We compare this approach with a modified approximation of the spectral function which uses a Krylov subspace constructed from the ``Chebyshev wave-functions''. I will present results for the one-dimensional and two-dimensional Heisenberg model on the square lattice, and compare to those obtained by other methods in the literature.

[1] “Chebyshev expansion of spectral functions using restricted Boltzmann machines” D. Hendry, Hongwei Chen, Phillip Weinberg, A. E. Feiguin arXiv: 2103.08804.

[2] “A machine learning approach to dynamical properties of quantum many-body systems” Douglas Hendry, Adrian E. Feiguin Phys. Rev. B 100, 245123 (2019).