arXiv subjects

Daniel D. Rivera

Publications and source records attributed to Daniel D. Rivera.

4 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

Alloy engineering of Magnetic phases in two-dimensional Chromium Trihalides

Two-dimensional magnetic materials offer unique opportunities for exploring low-dimensional spin phenomena and next-generation spintronic devices. Chromium trihalides CrX3 (X = Cl, Br, I) belong to an important family of these materials, where alloying opens pathways for tailoring their electronic, magnetic, optical properties, and thermodynamic stability. In this work, we present a density functional theory study of CrX3 compounds and their ternary alloys. Our results show that for the pure compounds, the ground state is ferromagnetic (FM), with the antiferromagnetic-zigzag (AFM-Z) and paramagnetic (PM) phases being close in energy. For these pure systems, the band gap variation among different magnetic phases does not exceed 0.16 eV, and the average magnetic moments on Cr atoms increase from Cl to Br to I. For the alloys, the FM state remains the lowest-energy configuration, but the energy difference towards the AFM-Z phase decreases for compounds with lower iodine concentration. The calculated band gaps reveal a pronounced bowing along the compositional edge connecting CrCl3 and CrI3. The Curie temperatures show a smooth variation across compositions, consistent with the nearly linear behavior of the magnetic exchange parameters. Based on the calculated mixing enthalpy and configurational entropy, the approximate Gibbs free energy indicates that alloy formation becomes thermodynamically favorable at finite temperatures, which is important to overcome the intrinsic experimental instability of these compounds.

cond-mat.mtrl-sci↗

arXiv2026

Elusive Exciton Insulator States in 1T-HfTe2: Exciton softening, and Symmetry Breaking by Ab Initio Methods

Recent experiments have provided evidence for excitonic insulator (EI) states in 1T HfTe2. In this work, we investigate EI states in monolayer, bilayer, trilayer, and bulk 1T HfTe2 using advanced meta generalized gradient approximation (meta GGA) calculations and a model Bethe-Salpeter equation (BSE) approach, together with structural and electronic symmetry breaking analyses. Our results show that both the monolayer and bilayer exhibit negative exciton energies, leading to the spontaneous formation of bound excitons and EI states, whereas the trilayer and bulk display positive exciton energies and do not support EI states. Structural symmetry-breaking calculations show very small in-plane displacements of the Hf atoms from their symmetric positions in the monolayer and multilayers, consistent with experimental observations. Interestingly, electronic symmetry-breaking calculations for the monolayer, performed using a symmetric structure and a hybrid functional, show a pronounced unfolded valence-band feature at the M point and no unfolded conduction-band states near the Fermi level at Gamma, in good agreement with experimental results. Overall, our findings support the existence of EI states in low dimensional 1T HfTe2. The methodology developed here can be readily extended to investigate EI behavior in other related quantum material systems.

cond-mat.mtrl-sci↗

arXiv2026

Identifying strong correlation using only the Kohn-Sham density of one-electron states

Strongly correlated systems have long been a central and highly non-trivial topic in condensed matter physics. At the non-interacting level, strong correlation can be associated with powerful (near) degeneracies between occupied and unoccupied states, which leads to a high density of states near the Fermi level in metallic configurations. Such regimes are commonly treated with beyond-density functional theory (DFT) approaches, such as DFT+U or DFT+DMFT while maintaining symmetric configurations. Here, we explore the hypothesis that symmetry breaking in the Kohn-Sham (KS) non-interacting system can qualitatively account for the energetic effects of strong correlation in the corresponding interacting system within standard DFT. By lifting near-degeneracies around the Fermi level, symmetry breaking diminishes the potential correlation effects, reducing the need for an explicit treatment of electron correlation, transforming an otherwise strongly correlated symmetric configuration into a normally correlated one, thus avoiding the need for interacting methods beyond DFT. This naturally connects nonmagnetic to magnetic states. We apply this idea to both strongly and normally correlated metals and observe that spin symmetry breaking leads to a pronounced reduction of the density of states at the Fermi level and a significant lowering of the total energy in strongly correlated cases. To describe the degree of correlation that the interacting system would have relative to the KS state, we introduce a correlation parameter ($\Gamma$), defined as the ratio between the Kohn-Sham density of one-electron states at the Fermi level and that of a corresponding uniform electron gas. This parameter distinguishes strongly correlated systems, which would require explicit treatment, from normally correlated ones, which do not.

cond-mat.str-el↗