arXiv subjects

Alessio Cucciari

Publications and source records attributed to Alessio Cucciari.

5 recordsLinked to original sources

arXiv2026

Capturing nuclear quantum effects in high-pressure superconducting hydrides and ice with nuclear-electronic orbital theory

Nuclear quantum effects are essential for correctly describing hydrogen-rich materials at high pressures. Superconducting hydrides and ice are prime examples of such systems, requiring the inclusion of lattice anharmonicity and nuclear quantum effects to correctly predict and describe the structures and phase transition pressures observed experimentally. Herein, we show that the nuclear-electronic orbital density functional theory (NEO-DFT) method, which treats specified nuclei quantum mechanically on the same level as the electrons, is capable of accurately describing nuclear quantum effects in superconducting hydrides and ice. NEO-DFT predicts the hydrogen-bond symmetrization pressure in H$_3$S and D$_3$S, benchmarking against the more expensive stochastic self-consistent harmonic approximation (SSCHA) method, and predicts the correct symmetric Fm$\bar{3}$m structure for LaH$_{10}$ at a wide range of pressures. NEO-DFT also predicts the ice VIII to ice X phase transition pressures for H$_2$O and D$_2$O in agreement with experimental measurements. The accuracy, computational efficiency, and broad applicability of the NEO method opens the door for expanded large-scale studies into these types of systems.

cond-mat.supr-con↗

arXiv2025

An $ab\;initio$ answer to long-debated questions about superconducting Nb$_3$Sn

We present the first fully $ab\;initio$ microscopic description of cubic and tetragonal Nb$_3$Sn. We compute the anharmonic free energy surface, phonon spectra, and solve the full-bandwidth anisotropic Migdal-Eliashberg equations for the superconducting gap of the two phases. Our results show that anharmonic effects are crucial to stabilize both the cubic and tetragonal structures, yielding phonon spectra in excellent agreement with neutron scattering data. We find that the martensitic transition is weakly first-order and that the superconducting gap is strongly anisotropic yet fully-open, with contributions from both longitudinal and transverse Nb $d$-orbitals, revealing an unexpected three-dimensional pairing mechanism. We also find that the experimentally observed reduction of the upper critical field $H_{c2}$ across the transition is explained by a combination of overall weaker electron-phonon coupling and a redistribution of Fermi velocities, which shifts parts of the Fermi surface to longer coherence lengths and limits $H_{c2}$. Based on these insights, we propose that Sn-site doping could enhance transverse-state coupling and gap isotropy, potentially improving both $T_c$ and $H_{c2}$, while Nb-site doping reinforce $H_{c2}$ at the cost of lowering $T_c$.

cond-mat.supr-con↗

arXiv2024

NbTi: a nontrivial puzzle for the conventional theory of superconductivity

We present the first $ab$-$initio$ study of superconductivity in NbTi, the workhorse for many applications. Despite its apparent simplicity, NbTi turns out to be a major challenge for computational superconductivity. In fact, anharmonic effects are crucial to obtain dynamically stable phonons for the ordered bcc phase, unstable at the harmonic level, and beyond-Morel Anderson effects in the Coulomb interaction reduce the Tc by more than 20%. Lattice disorder causes an additional large discrepancy in $T_c$ compared to experiment. Our results imply that a quantitative description of technologically-relevant superconductors requires methodological developments beyond the current standards.

cond-mat.supr-con↗

arXiv2023

Search for ambient superconductivity in the Lu-N-H system

Motivated by the recent report of room-temperature superconductivity at near-ambient pressure in N-doped lutetium hydride by Dasenbrock et al. [Nature 615, 244 (2023)], we performed a comprehensive, detailed study of the phase diagram of the Lu-N-H system, looking for superconducting phases. We combined ab initio crystal structure prediction with ephemeral data-derived interatomic potentials to sample over 200,000 different structures. Out of the more than 150 structures predicted to be metastable within $\sim$ 50 meV from the convex hull we identify 52 viable candidates for conventional superconductivity, for which we computed their superconducting properties from Density Functional Perturbation Theory. Although for some of these structures we do predict a finite superconducting $T_{\text{c}}$, none is even remotely compatible with room-temperature superconductivity as reported by Dasenbrock et al. Our work joins the broader community effort that has followed the report of near-ambient superconductivity, confirming beyond reasonable doubt that no conventional mechanism can explain the reported $T_{\text{c}}$ in Lu-N-H.

cond-mat.supr-con↗

arXiv2022

Mapping Superconductivity in High-Pressure Hydrides: The $Superhydra$ Project

The discovery of high-$T_c$ conventional superconductivity in high-pressure hydrides has helped establish computational methods as a formidable tool to guide material discoveries in a field traditionally dominated by serendipitous experimental search. This paves the way to an ever-increasing use of data-driven approaches to the study and design of superconductors. In this work, we propose a new method to generate meaningful datasets of superconductors, based on element substitution into a small set of representative structural templates, generated by crystal structure prediction methods (MultiTemplate-HighThroughput approach). Our approach realizes an optimal compromise between structural variety and computational efficiency, and can be easily generalized to other elements and compositions. As a first application, we apply it to binary hydrides at high pressure, realizing a database of 880 hypothetical structures, characterized with a set of electronic, vibrational and chemical descriptors. 139 structures of our $Superhydra$ Database are superconducting according to the McMillan-Allen-Dynes approximation. Studying the distribution of $T_c$ and other properties across the database with advanced statistical and visualization techniques, we are able to obtain comprehensive material maps of the phase space of binary hydrides. The $Superhydra$ database can be thought as a first step of a generalized effort to map conventional superconductivity.

cond-mat.supr-con↗