arXiv subjects

Álvaro Adrián Carrasco Álvarez

Publications and source records attributed to Álvaro Adrián Carrasco Álvarez.

3 recordsLinked to original sources

arXiv2026

First-Principles Spin-Lattice Coupling from Downfolded Electron-Phonon Interaction

We present a method to calculate spin-phonon coupling parameters from first-principles perturbation theory by downfolding the electron-phonon coupling (EPC). We exploit the localized nature of magnetic moments and atomic displacements by working in the Wannier representation of the electronic Hamiltonian and the EPC matrix. The spin system is mapped to a classical Heisenberg Hamiltonian, whose parameters are obtained by treating local spin rotations as a perturbation within a Green's-function formalism. The spin and phonon perturbations are connected through the EPC parameters, which enter as lattice-induced perturbations to the tight-binding Hamiltonian. By combining these lattice perturbations with local spin rotations, we obtain real-space derivatives of magnetic exchange parameters without performing displaced magnetic supercell calculations. We illustrate the method on SrMnO$_3$ and show that it can be integrated directly into standard workflows.

cond-mat.mtrl-sci↗

arXiv2023

Charge ordering as the driving mechanism for superconductivity in rare-earth nickel oxides

Superconductivity is one of the most intriguing properties of matter described by an attractive interaction that bounds electrons into Cooper pairs. To date, the highest critical temperature at ambient conditions is achieved in copper oxides. While layered nickel oxides were long proposed to be analogous to cuprates, superconductivity was only demonstrated in 2019 albeit without clarifying the pairing mechanism. Here we use Density Functional Theory (DFT) to show that superconductivity in nickelates is driven by an electron-phonon coupling originating from a charge ordering. Due to an intrinsic electronic instability in half-doped compounds, Ni$^{1.5+}$ cations dismutate into more stable Ni$^+$ and Ni$^{2+}$ cations, which is accompanied by a bond disproportionation of NiO$_4$ complexes producing an insulating charge ordered state. Once doping suppresses the instability, the bond disproportionation vibration is sufficient to reproduce the key characteristic of nickelates observed experimentally, notably the dome of T$_c$ as a function of doping content. These phenomena are identified if relevant degrees of freedom as well as an exchange correlation functional that sufficiently amends self-interaction errors are involved in the simulations. Finally, despite the presence of correlation effects inherent to $3d$ elements, nickelates superconductors appear similar to non-magnetic bismuth oxide superconductors.

cond-mat.supr-con↗

arXiv2021

Structural instabilities of infinite-layer nickelates from first-principles simulations

Rare-earth nickelates RNiO$_2$ adopting an infinite-layer phase show superconductivity once La, Pr or Nd are substituted by a divalent cation. Either in the pristine or doped form, these materials are reported to adopt a high symmetry, perfectly symmetric, P4/mmm tetragonal cell. Nevertheless, bulk compounds are scarce, hindering a full understanding of the role of chemical pressure or strain on lattice distortions that in turn could alter magnetic and electronic properties of the 2D nickelates. Here, by performing a full analysis of the prototypical YNiO$_2$ compound with first-principles simulations, we identify that these materials are prone to exhibit O$_4$ group rotations whose type and amplitude are governed by the usual R-to-Ni cation size mismatch. We further show that these rotations can be easily tuned by external stimuli modifying lattice parameters such as pressure or strain. Finally, we reveal that H intercalation is favored for any infinite-layer nickelate member and pushes the propensity of the compounds to exhibit octahedra rotations.

cond-mat.mtrl-sci↗