arXiv subjects

Lauro B. Braz

Publications and source records attributed to Lauro B. Braz.

9 recordsLinked to original sources

arXiv2026

Ni-O hybridization as a stabilizer for $s^{\pm}$ superconductivity in La$_3$Ni$_2$O$_7$: a DFT+RPA study

The superconducting gap symmetry of high-pressure bilayer nickelates remains under debate, with weak- and strong-coupling approaches yielding different pairing tendencies. In this work, we investigate how the weak-coupling treatment of electronic states away from the Fermi level influences magnetic fluctuations and superconductivity in La$_3$Ni$_2$O$_7$. We employ a full-spectrum model based on orthonormalized projections of Kohn-Sham states onto local Ni-$e_g$ orbitals, which preserves the density-functional band structure while redistributing spectral weight over a wide energy range. Compared to a low-energy description, this approach yields enhanced interlayer spin fluctuations and a commensurate magnetic instability. Within a spin-fluctuation framework, these features favor a sign-changing $s^\pm$ superconducting state, whereas low-energy models tend to stabilize $d$-wave pairing. Our results suggest that interlayer coupling in full-energy models may play an important role in shaping the predicted pairing symmetry of bilayer nickelates.

cond-mat.supr-con↗

arXiv2026

Density waves in low-pressure bilayer nickelates

The low-pressure phase diagram of La$_3$Ni$_2$O$_7$ provides an important reference for understanding its pressure-induced high-temperature superconductivity. While the spin-density-wave transition at $T_{\text{SDW}}\approx150$ K is increasingly well established, the origin of the second density-wave transition at $T_{\text{DW}}\approx130$ K has remained unresolved. Here, we perform unrestricted Hartree-Fock calculations to investigate the potential origin of the second transition. {Within the orthorhombic phase, the degeneracy between possible ordering wavevectors at $\boldsymbol{Q}_{Y}=(0,π)$ and at $\boldsymbol{Q}_{X}=(π,0)$ is lifted and the electronic system} develops a double-stripe spin-density wave with ordering vector $\boldsymbol{Q}_{Y}=(0,π)$. We identify that the pure double stripe spin state is unstable in La$_3$Ni$_2$O$_7$ towards a commensurate charge-density wave instability, which favors a spin-modulated double stripe order with intertwined charge and spin instabilities and establish the hierarchy of ordered states in La$_3$Ni$_2$O$_7$. We further discuss our results in the context of available experimental literature and propose further experimental tests to elucidate the origin of the SDW/DW states in this system.

cond-mat.str-el↗

arXiv2026

Onset of spin-valley order and Stoner boundaries in twisted WSe$_2$

We investigate spin-valley instabilities and their connection to the magnetically ordered states recently observed in the twisted bilayer dichalcogenide WSe$_2$ at a $5^o$ twist angle. Starting from an effective three-orbital faithful Wannier model for the spin-locked moiré bands, combined with orbital-dependent Hubbard interactions, we analyze the evolution of magnetic instabilities as a function of carrier density using the matrix random phase approximation (mRPA) approach. By computing the Stoner boundary lines from the spin-valley susceptibilities over the electric-field by hole filling phase diagram, we show that the spin-valley instabilities result in ordered states in the region close to the Lifshitz transition at the topmost moiré valence band, marked by crossing of the Van Hove singularity in the density of states. These spin-valley ordered states are dominated by interorbital spin-valley-flips involving the $MM$ and $MX$ moiré orbitals and occur at different momenta in each side of the Van Hove line, indicating a distinct spatial dependence of the spin-valley order parameter depending on the hole filling. Moreover, the corresponding Stoner boundaries exhibit strong fluctuations on its flanks, which can favor superconducting states in the regions close to the spin-valley-ordered ones. This mechanism provides a natural description for a reentrant superconducting dome consistent with the experimental results. As such, our results suggest spin-valley fluctuations near the Van Hove line as the microscopic origin of the reentrant superconductivity in twisted WSe$_2$.

cond-mat.supr-con↗

arXiv2026

Doping-dependent orbital magnetism in Chromium pnictides

We present results for the phase diagram of the parent compound LaCrAsO under electron doping using the matrix random-phase approximation. At low doping levels, the system stabilizes an antiferromagnetic state in which different Cr sublattices carry opposite spins, consistent with experimental observations. As the doping concentration increases, a stripe-type antiferromagnetic phase becomes favored. At even higher doping, the system repeats the two former magnetic states, but with incommensurate magnetic ordering vectors. The commensurate magnetic phases are associated with more localized electrons in the Cr $d_{3z^2-r^2}$ orbital, whereas the incommensurate phases are linked to the $d_{xy}$ orbital, whose stronger overlap favors itinerant-electron magnetism.

cond-mat.str-el↗

arXiv2025

Interlayer interactions in $\text{La}_3\text{Ni}_2\text{O}_7$ under pressure: from $s^{\pm}$ to $d_{xy}$-wave superconductivity

We investigate the role of \emph{interlayer} interaction terms in the competition between different superconducting gap symmetries in the bilayer nickelate $\text{La}_3\text{Ni}_2\text{O}_7$ under high pressure. We study a two-layer, two-orbital electron model that encompasses both intra- and interlayer Coulomb interaction terms within the matrix random-phase approximation. We find that interlayer interactions favor a $d_{xy}$-wave superconducting pairing symmetry over the $s^{\pm}$-wave symmetry, which has been found to prevail when interlayer interactions are disregarded. Moreover, our findings indicate that interlayer interactions enhance the interorbital pairing, incorporating contributions from all three electron pockets, arising from both $d_{3z^2-r^2}$ and $d_{x^2-y^2}$ orbital character, resulting in nodes within the gap function (not present in the $s^{\pm}$-wave state) and consequently favoring the $d_{xy}$-wave pairing.

cond-mat.supr-con↗

arXiv2024

Competing magnetic states on the surface of multilayer ABC-stacked graphene

We study interaction-mediated magnetism on the surface of ABC-multilayer graphene driven by its zero-energy topological flat bands. Using the random-phase approximation we treat onsite Hubbard repulsion and find multiple competing magnetic states, due to both intra- and inter-valley scattering, with the latter causing an enlarged magnetic unit cell. At half-filling and when the Hubbard repulsion is weak, we observe two different ferromagnetic orders. Once the Hubbard repulsion becomes more realistic, new ferrimagnetic orders arise with distinct incommensurate intra- or inter-valley scattering vectors depending on interaction strength and doping, leading to a multitude of competing magnetic states.

cond-mat.mes-hall↗

arXiv2024

Superconductivity from spin fluctuations and long-range interactions in magic-angle twisted bilayer graphene

Magic-angle twisted bilayer graphene (MATBG) has been extensively explored both theoretically and experimentally as a suitable platform for a rich and tunable phase diagram that includes ferromagnetism, charge order, broken symmetries, and unconventional superconductivity. In this work, we investigate the intricate interplay between long-range electron-electron interactions, spin fluctuations, and superconductivity in MATBG. By employing a low-energy model for MATBG that captures the correct shape of the flat bands, we explore the effects of short- and long-range interactions on spin fluctuations and their impact on the superconducting (SC) pairing vertex in the Random Phase Approximation (RPA). We find that the SC state is notably influenced by the strength of long-range Coulomb interactions. Interestingly, our RPA calculations indicate that there is a regime where the system can traverse from a magnetic phase to the SC phase by \emph{increasing} the relative strength of long-range interactions compared to the on-site ones. These findings underscore the relevance of electron-electron interactions in shaping the intriguing properties of MATBG and offer a pathway for designing and controlling its SC phase.

cond-mat.mes-hall↗

arXiv2024

Charge and spin fluctuations in superconductors with intersublattice and interorbital interactions

Multiband superconductors have featured one of the main challenges to achieve a comprehensive understanding of unconventional superconductivity. Here, the multiband character is studied separately as orbital and sublattice degrees of freedom, as they have different effects for the superconducting and magnetic or charge orders. We build on the framework of the matrix random-phase approximation (RPA), which accounts for the RPA Feynman diagrams and also vertex corrections, to treat the electron-electron interactions in an off-site degenerate Hubbard model. As a result, systems without a sublattice degree of freedom tend to be dominated by spin fluctuations, while systems with multiple sublattice sites and orbitals have the charge fluctuations favored. Finally, we explicitly demonstrate that the known suppression of the superconducting pairing strength $λ$ by spin fluctuations from repulsive interactions at zero momentum transfer $\boldsymbol{q}$ is countered by the finite-$\boldsymbol{q}$ pairing, which always improves $λ$.

cond-mat.supr-con↗

arXiv2022

A semi-empirical analysis of the paramagnetic susceptibility of solid state magnetic clusters

Recent developments in the synthesis of new magnetic materials lead to the discovery of new quantum paramagnets. Many of these materials, such as the perovskites Ba$_{4}$LnMn$_{4}$O$_{12}$ (Ln = Sc or Nb), Ba$_{3}$Mn$_{2}$O$_{8}$, and Sr$_{3}$Cr$_{2}$O$_{8}$ present isolated magnetic clusters with strong intracluster interactions but weak intercluster interactions, which delays the onset of order to lower temperatures ($T$). This offset between the local energy scale and the magnetic ordering temperature is the hallmark of magnetic frustration. At sufficient high-$T$, the paramagnetic susceptibility ($χ$) of frustrated cluster magnets can be fit to a Curie-Weiss law, but the derived microscopic parameters cannot in general be reconciled with those obtained from other methods. In this work, we present an analytical microscopic theory to obtain $χ$ of dimer and trimer cluster magnets, the two most commonly found in literature, making use of suitable Heisenberg-type Hamiltonians. We also add intercluster interactions in a mean-field level, thus obtaining an expression to the critical temperature of the system and defining a new effective frustration parameter $f_{\text{eff}}$. Our method is exemplified by treating the $χ$ data of some selected materials.

cond-mat.mtrl-sci↗