arXiv subjects

Walber H. Brito

Publications and source records attributed to Walber H. Brito.

9 recordsLinked to original sources

arXiv2025

Low-energy structure and topology of the two-band Hubbard-Kanamori model

We investigate the Mott transition in a two-band Hubbard-Kanamori model using Dynamical Mean-Field Theory (DMFT) with the Density Matrix Renormalization Group (DMRG) and the Numerical Renormalization Group (NRG) as impurity solvers. Our study focuses on the case where the intraorbital and interorbital Coulomb interactions are equal (U = U2) and the Hund's coupling is absent (J = 0). Spectral analysis confirms the absence of an orbital-selective Mott transition (OSMT), even in systems with significantly different bandwidths (t1 and t2 for the wide and narrow bands, respectively), indicating a simultaneous Mott transition in both bands. Notably, the NRG results reveal the emergence of a pseudo-gap-like feature and a central peak in the narrow band, whose characteristics depend on the hopping parameter t2. These spectral features may serve as precursors to OSMT in more realistic systems with finite Hund's coupling (J > 0). Furthermore, in the Mott insulating phase, the self-energies of both bands diverge, suggesting that the Mott transition represents a topological phase transition. Our results highlight the crucial role of accurate impurity solvers in capturing the density of states and detailed spectral structures.

cond-mat.str-el↗

arXiv2024

Role of native point defects and Hg impurities in the electronic properties of Bi$_4$I$_4$

We studied the effects of point defects and Hg impurities in the electronic properties of bismuth iodide (Bi$_4$I$_4$). Our transport measurements after annealing at different temperatures show that the resistivity of Bi$_4$I$_4$ depends on its thermal history, suggesting that the formation of native defects and impurities can shape the temperature dependence of electrical resistivity. Our density functional theory calculations indicate that the bismuth and iodine antisites, and bismuth vacancies are the dominant native point defects. We find that bismuth antisites introduce resonant states in the band-edges, while iodine antisites and bismuth vacancies lead to a $n$-type and $p$-type doping of Bi$_4$I$_4$, respectively. The Hg impurities are likely to be found at Bi substitutional sites, giving rise to the $p$-type doping of Bi$_4$I$_4$. Overall, our findings indicate that the presence of native point defects and impurities can significantly modify the electronic properties, and, thus, impact the resistivity profile of Bi$_4$I$_4$ due to modifications in the amount and type of carriers, and the associated defect(impurity) scattering. Our results suggest possible routes for pursuing fine-tuning of the electronic properties of quasi-one-dimensional quantum materials.

cond-mat.mtrl-sci↗

arXiv2024

Electronic and magnetic properties of (NdNiO$_3$)/(La$_{2/3}$Sr$_{1/3}$MnO$_3$) superlattices: a DFT+U perspective

The electronic and magnetic properties of NdNiO$_3$ (NNO) thin films are very sensitive to epitaxial strain and to the proximity to ferromagnetic oxides. In this work, we investigate the structural, electronic, and magnetic properties of NdNiO$_3$/La$_{2/3}$Sr$_{1/3}$MnO$_3$ (NNO/LSMO) superlattices under the epitaxial strain of NdGaO$_3$ (NGO) using density functional theory with Hubbard U correction (DFT+U) calculations. Our findings reveal that LSMO induces insignificant structural distortions on the NiO$_6$ octahedra, and that there is a negligible charge transfer between LSMO and NNO. More importantly, we find an intricate magnetic order regarding the interfacial Ni local moments, where we observe a coexistence of ferromagnetic interactions between interfacial Ni and Mn atoms, and antiferromagnetic interactions between inner Ni ions, which results in an overall ferromagnetic coupling across the interface. We also observe the metallization of NNO through the emergence of a half-metallic density of states in the same spin channel as LSMO, which becomes suppressed the further we move away from the interface. Our theoretical findings agrees well with recent experimental data, shedding light on the complex interplay between the induced magnetic order and the electronic properties of NNO.

cond-mat.mtrl-sci↗

arXiv2024

Hole doping and electronic correlations in Cr-substituted BaFe$_{2}$As$_{2}$

Superconductivity (SC) is absent in Cr-substituted BaFe$_{2}$As$_{2}$ (CrBFA), a well-established but poorly understood topic. Additionally, the suppression of the spin density wave transition temperature ($T_{\text{SDW}}$) in CrBFA and Mn-substituted BaFe$_{2}$As$_{2}$ (MnBFA) coincides as a function of Cr/Mn content, despite the distinct electronic effects of these substitutions. In this work, we employ angle-resolved photoemission spectroscopy (ARPES) and combined density functional theory plus dynamical mean field theory calculations (DFT+DMFT) to address the evolution of the Fermi surface (FS) and electronic correlations in CrBFA. Our findings reveal that incorporating Cr leads to an effective hole doping of the states near the FS, which is well described within the virtual crystal approximation (VCA). We analyzed the electronic band spectra with main $d_{yz}$-orbital character and found a fractional scaling of the imaginary part of self-energy as a function of the binding energy, a signature property of Hund's correlations. We conclude that CrBFA is a correlated electron system and the changes in the FS as a function of Cr are unrelated to the suppression of $T_{\text{SDW}}$. We suggest that the absence of SC is primarily due to the competition between Cr local moments and the Fe-derived itinerant spin fluctuations.

cond-mat.str-el↗

arXiv2024

Structural investigation of the quasi-one-dimensional topological insulator Bi$_4$I$_4$

The bismuth-halide Bi$_4$I$_4$ undergoes a structural transition around $T_P\sim 300$K, which separates a high-temperature $β$ phase ($T>T_P$) from a low-temperature $α$ phase ($T<T_P$). $α$ and $β$ phases are suggested to host electronic band structures with distinct topological classifications. Rapid quenching was reported to stabilize a metastable $β$-Bi$_4$I$_4$ at $T<T_P$, making possible a comparative study of the physical properties of the two phases in the same low-temperature range. In this work, we present a structural investigation of the Bi$_4$I$_4$ before and after quenching together with electrical resistivity measurements. We found that rapid cooling does not consistently lead to a metastable $β$-Bi$_4$I$_4$, and a quick transition to $α$-Bi$_4$I$_4$ is observed. As a result, the comparison of putative signatures of different topologies attributed to a specific structural phase should be carefully considered. The observed phase instability is accompanied by an increase in iodine vacancies and by a change in the temperature dependence of electrical resistivity, pointing to native defects as a possible origin of our finding. Density functional theory (DFT) calculations support the scenario that iodine vacancies, together with bismuth antisites and interstitials, are among the defects that are more likely to occur in Bi$_4$I$_4$ during the growth.

cond-mat.mtrl-sci↗

arXiv2023

Incoherent electronic band states in Mn-substituted BaFe$_{2}$As$_{2}$

Chemical substitution is commonly used to explore new ground states in materials, yet the role of disorder is often overlooked. In Mn-substituted BaFe$_{2}$As$_{2}$ (MnBFA), superconductivity (SC) is absent, despite being observed for nominal hole-doped phases. Instead, a glassy magnetic phase emerges, associated with the $S=5/2$ Mn local spins. In this work, we present a comprehensive investigation of the electronic structure of MnBFA using angle-resolved photoemission spectroscopy (ARPES). We find that Mn causes a small and orbital-specific reduction of the electron pockets, only partially disrupting nesting conditions. Based upon the analysis of the spectral properties, we observe, for all bands, an increase in the electronic scattering rate as a function of Mn content. This is interpreted as increasing band incoherence, which we propose as the primary contributor to the suppression of the magnetic order in MnBFA. This finding connects the MnBFA electronic band structure properties to the glassy magnetic behavior observed in these materials and suggests that SC is absent because of the collective magnetic impurity behavior that scatters the Fe-derived excitations. Additionally, our analysis shows that the binding energy ($E_{B}$) dependence of the imaginary part of the self-energy [$\text{Im}Σ(E_{B})$] is best described by a fractional scaling ($\text{Im}Σ(E_{B})\propto\sqrt{-E_{B}}$). These results indicate that Mn tunes MnBFA into an electronic disordered phase between the correlated Hund's metal in BaFe$_{2}$As$_{2}$ and the Hund's insulator in BaMn$_{2}$As$_{2}$.

cond-mat.supr-con↗

arXiv2022

Formation of spin and charge ordering in the extended Hubbard model during a finite-time quantum quench

We investigate the formation of charge and spin ordering by starting from a non-interacting state and studying how it evolves in time under a Hamiltonian with finite electronic interactions. We consider the one-dimensional, half-filled extended Hubbard model, which we solve within time-dependent density matrix renormalization group. By employing linear finite-time quenches in the onsite and nearest-neighbor interactions, we find the existence of impulse, intermediate, and adiabatic regimes of time evolution. For the quenches we analyze, we observe that the adiabatic regime is reached with distinct ramping time scales depending on whether the charge density wave (CDW) or the spin density wave (SDW) is formed. The former needs to be slower than the latter to prevent entangled excited states from being accessed during the quench. More interestingly, in the intermediate regime, we observe an enhancement of the entanglement entropy with respect to its initial value, which precedes the formation of the CDW ordering; a similar enhancement is not seen in the quench towards SDW. Our findings also show that the breaking of the system integrability, by turning on the nearest-neighbor interactions, does not give rise to significant changes in the non-equilibrium behavior within the adiabatic approximation.

cond-mat.str-el↗

arXiv2022

Reconstruction of low dimensional electronic states by altering the chemical arrangement at the SrTiO3 surface

Developing reliable methods for modulating the electronic structure of the two-dimensional electron gas (2DEG) in SrTiO3 is crucial for utilizing its full potential and inducing novel properties. Here, we show that relatively simple surface preparation reconstructs the 2DEG of SrTiO3 (STO) surface, leading to a Lifshitz-like transition. Combining experimental methods, such as angle-resolved photoemission spectroscopy (ARPES) and X-ray photoemission spectroscopy (XPS) with ab initio calculations, we find that the modulation of the surface band structures is primarily attributed to the reorganization of the chemical composition. In addition, ARPES experiments demonstrate that vacuum ultraviolet (VUV) light can be efficiently employed to alter the band renormalization of the 2DEG system and control the electron-phonon interaction (EPI). Our study provides a robust and straightforward route to stabilize and tune the low-dimensional electronic structure via the chemical degeneracy of the STO surface.

cond-mat.str-el↗

arXiv2021

Universal Structural Influence on the 2D Electron Gas at SrTiO$_3$ Surfaces

The two-dimensional electron gas found at the surface of SrTiO$_3$ and related interfaces has attracted significant attention as a promising basis for oxide electronics. In order to utilize its full potential, the response of this 2DEG to structural changes and surface modification must be understood in detail. Here, we present a study of the detailed electronic structure evolution of the 2DEG as a function of sample temperature and surface step density. By comparing our experimental results with \textit{ab initio} calculations, we found that a SrO-rich surface layer is a prerequisite for electronic confinement. We also show that local structure relaxations cause a metal-insulator transition of the system around 135~K. Our study presents a new and simple way of tuning the 2DEG via surface vicinality and identifies how the operation of prospective devices will respond to changes in temperature.

cond-mat.mtrl-sci↗