arXiv subjects

Cristóbal Méndez

Publications and source records attributed to Cristóbal Méndez.

5 recordsLinked to original sources

arXiv2026

A geometric basis for materials families in inorganic solids

The thermodynamic stability of inorganic solids spans a vast compositional space, yet materials scientists have long organized their intuition around a manageable number of materials families. Here we show that this organization has a precise geometric basis. The formation-energy convex hull of all inorganic compounds from the Materials Project, spanning 92-dimensional elemental composition space, is captured to near DFT accuracy by a polyhedron with only seven facets. Each facet corresponds to a family of materials sharing similar chemical potentials. This low-dimensional structure is not merely an economical description of energies: without retraining or structural input, the same framework reproduces trends in DFT-calculated defect energies and elemental spatial correlations in high-entropy nanoparticles. These results reveal that a small number of material families, corresponding to geometric features of composition-energy space, govern bulk stability, defect energetics, and elemental mixing, and provide a unified, interpretable framework for rapid screening across diverse materials systems.

cond-mat.mtrl-sci↗

arXiv2026

$\textit{Ab initio}$ Identification of Hydrogen Tunneling as Two-Level Systems in Nb$_2$O$_5$ and Ta$_2$O$_5$

Two-level systems (TLS) in native Nb and Ta oxides limit superconducting-qubit coherence and SRF-cavity quality factors in the microwave frequency range, yet their microscopic origin remains unclear. We combine MLIP-accelerated sampling of hydrogen configurations and diffusion pathways in amorphous Nb and Ta pentoxides with targeted $\textit{ab initio}$ validation. Hydrogen is the only light interstitial with barrier-distance combinations near the $\sim0.1-10$ GHz tunneling regime, and its ensemble statistics in amorphous oxides produce effective TLS densities and loss estimates consistent with the experimentally observed higher loss in Nb oxide than in Ta oxide. Our results point to H tunneling as a plausible microscopic TLS source in these materials.

cond-mat.supr-con↗

arXiv2026

Active-Learning Inspired $\textit{Ab Initio}$ Theory-Experiment Loop Approach for Management of Material Defects: Application to Superconducting Qubits

Surface oxides are associated with two-level systems (TLSs) that degrade the performance of niobium-based superconducting quantum computing devices. To address this, we introduce a predictive framework for selecting metal capping layers that inhibit niobium oxide formation. Using DFT-calculated oxygen interstitial and vacancy energies as thermodynamic descriptors, we train a logistic regression model on a limited set of experimental outcomes to successfully predict the likelihood of oxide formation beneath different capping materials. This approach identifies Zr, Hf, and Ta as effective diffusion barriers. Our analysis further reveals that the oxide formation energy per oxygen atom serves as an excellent standalone descriptor for predicting barrier performance. By combining this new descriptor with lattice mismatch as a secondary criterion to promote structurally coherent interfaces, we identify Zr, Ta, and Sc as especially promising candidates. This closed-loop strategy integrates first-principles theory, machine learning, and limited experimental data to enable rational design of next-generation materials.

cond-mat.mtrl-sci↗

arXiv2026

$\textit{Ab initio}$ Theory of Eliminating Surface Oxides of Superconductors with Noble-Metal Encapsulation

Nanometer-scale surface chemistry limits the performance of SRF cavities and quantum circuits. We present an ab initio framework connecting DFT interfacial energetics with strong-coupling Eliashberg theory for capped Nb and Ta surfaces. This approach identifies Au and Au-based alloys (AuPd, AuPt) as effective passivation layers. Our model further predicts that combining a noble-metal capping layer with an appropriate wetting/adhesion layer (WAL) yields far more robust adhesion than a capping layer alone under realistic conditions, enabling thinner caps, and thereby addressing a central challenge in superconducting surface passivation.

cond-mat.supr-con↗

arXiv2024

$\textit{Ab initio}$ electron-phonon coupling theory of elastic helium atom scattering

We propose a fully $ \textit{ab initio} $ approach to predicting thermal attenuation in elastic helium atom scattering amplitudes, validated through strong agreement with experiments on Nb(100) and (3$\times$1)-O/Nb(100) surfaces. Our results reveal the relative contributions from bulk, resonant, and surface phonon modes, as well as from different surface mode polarizations, providing insights into differences between smooth and corrugated surfaces. These findings advance understanding of surface dynamics and electron-phonon coupling, laying groundwork for future studies on surface superconductivity.

cond-mat.mtrl-sci↗