arXiv subjects

Simone Di Cataldo

Publications and source records attributed to Simone Di Cataldo.

31 records · Page 2Linked to original sources

arXiv2023

Quantum lattice dynamics and their importance in ternary superhydride clathrates

The quantum nature of the hydrogen lattice in superconducting hydrides can have crucial effects on the material's properties. Taking a detailed look at the dynamic stability of the recently predicted BaSiH$_8$ phase, we find that the inclusion of anharmonic quantum ionic effects leads to an increase in the critical dynamical pressure to $20\,\text{GPa}$ as compared to $5\,\text{GPa}$ within the harmonic approximation. We identify the change in the crystal structure due to quantum ionic effects to be the main driving force for this increase and demonstrate that this can already be understood at the harmonic level by considering zero-point energy corrections to the total electronic energy. In fact, the previously determined critical pressure of kinetic stability $p_\text{kin} = 30\,\text{GPa}$ still poses a stricter bound for the synthesizability of BaSiH$_8$ and similar hydride materials than the dynamical stability and therefore constitutes a more rigorous and accurate estimate for the experimental realizability of these structures.

cond-mat.mtrl-sci↗

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↗

arXiv2023

Phase diagram and superconductivity of Calcium Alanates under pressure

In this paper we present a first-principles study of the high-pressure superconducting phase diagram of calcium alanates (Ca-Al-H), based on ab-initio crystal structure prediction and anisotropic Migdal-Eliashberg Theory. Calcium alanates have been intensively studied at ambient pressure for their hydrogen-storage properties, but their high-pressure behavior is largely unknown. By performing a full scan of the ternary convex hull at several pressures between 0 and 300 GPa, we identify several new structural motifs, characterized by a high Al-H coordination, where Al--$d$ orbitals participate in the bonding. Among all new phases thus identified, we focus in particular on a phase with CaAlH$_7$ composition, which lies on the convex hull at 300 GPa, and remains dynamically stable down to 50 GPa, with a predicted superconducting T$_c$ of 82 K, which likely represents a new promising template to achieve increase chemical precompression in ternary hydrides. Our findings reveal important insights into the structure-property relationships of calcium alanates under high pressure, and highlight a possible strategy to achieve conventional superconductivity at low pressures.

cond-mat.supr-con↗

arXiv2023

Absence of electron-phonon-mediated superconductivity in hydrogen-intercalated nickelates

A recent experiment [X. Ding et al., Nature 615, 50 (2023)] indicates that superconductivity in nickelates is restricted to a narrow window of hydrogen concentration: 0.22 < x < 0.28 in Nd$_{0.8}$Sr$_{0.2}$NiO$_{2}$H$_{x}$. This reported necessity of hydrogen suggests that it plays a crucial role for superconductivity, as it does in the vast field of hydride superconductors. Using density-functional theory and its extensions, we explore the effect of topotactic hydrogen on the electronic structure and phonon-mediated superconductivity in nickelate superconductors. Our calculations show that the electron-phonon coupling in hydrogen-intercalated nickelates is not strong enough to drive the electron pairing, and thus cannot explain the reported superconductivity.

cond-mat.supr-con↗

arXiv2022

Metal Borohydrides as high-$T_{c}$ ambient pressure superconductors

The extreme pressures required to stabilize the recently discovered superhydrides represent a major obstacle to their practical application. In this paper, we propose a novel route to attain high-temperature superconductivity in hydrides at ambient pressure, by doping commercial metal borohydrides. Using first-principles calculations based on Density Functional Theory and Migdal-Eliashberg theory, we demonstrate that in Ca(BH$_4$)$_2$ a moderate hole doping of 0.03 holes per formula unit, obtained through a partial replacement of Ca with monovalent K, is sufficient to achieve $T_c$'s as high as 110 K. The high-$T_c$ arises because of the strong electron-phonon coupling between the B-H $σ$ molecular orbitals and bond-stretching phonons. Using a random sampling of large supercells to estimate the local effects of doping, we show that the required doping can be achieved without significant disruption of the electronic structure and at moderate energetic cost. Given the wide commercial availability of metal borohydrides, the ideas presented here can find prompt experimental confirmation. If successful, the synthesis of high-$T_c$ doped borohydrides will represent a formidable advancement towards technological exploitation of conventional superconductors.

cond-mat.supr-con↗

arXiv2022

In-silico synthesis of lowest-pressure high-$T_\text{c}$ ternary superhydrides

We report the theoretical prediction of two high-performing hydride superconductors BaSiH$_8$ and SrSiH$_8$. They are thermodynamically stable above pressures of $130$ and $174\,\text{GPa}$, respectively, and metastable below that. Employing anharmonic phonon calculations, we determine the minimum pressures of dynamical stability to be around $3\,\text{GPa}$ for BaSiH$_8$ and $27\,\text{GPa}$ for SrSiH$_8$, and using the fully anisotropic Migdal-Eliashberg theory, we predict $T_\text{c}$'s around $71$ and $126\,\text{K}$, respectively. We also introduce a method to estimate the lowest pressure of synthesis, based on the calculation of the enthalpy barriers protecting the BaSiH$_8$ $Fm\bar{3}m$ structure from decomposition at various pressures. This $kinetic$ pressure threshold is sensibly higher than the one based on $dynamic$ stability, but gives a much more rigorous limit for synthesizability.

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↗

arXiv2021

High-T$_c$ Superconductivity in doped boron-carbon clathrates

We report a high-throughput ab-initio study of the thermodynamic and superconducting proper- ties of the recently synthesized XB$_3$C$_3$ clathrates. These compounds, in which boron and carbon form a sponge-like network of interconnected cages each enclosing a central X atom, are attractive candidates to achieve high-Tc conventional superconductivity at ambient pressure, due to the simultaneous presence of a stiff B-C covalent network and a tunable charge reservoir, provided by the guest atom. Ternary compounds like CaB$_3$C$_3$, SrB$_3$C$_3$ and BaB$_3$C$_3$ are predicted to exhibit T$_c$ $\lt$ 50 K at moderate or ambient pressures, which may further increase up to 77 K if the original compounds are hole-doped by replacing the divalent alkaline earth with a monovalent alkali metal to form ordered $XY$B$_6$C$_6$ alloys.

cond-mat.supr-con↗

arXiv2021

La-$X$-H hydrides: is hot superconductivity possible?

Motivated by the recent claim of hot superconductivity with critical temperatures up to 550 K in La + x hydrides (arXiv:2006.03004), we investigate the high-pressure phase diagram of possible compounds that may have formed in the experiment, using first-principles calculations for evolutionary crystal structure prediction and superconductivity. Starting from the hypothesis that the observed T$_c$ may be realized by successive heating upon a pre-formed LaH$_{10}$ phase, we examine plausible ternaries of lanthanum, hydrogen and other elements present in the diamond anvil cell: boron, nitrogen, carbon, platinum, gallium, gold. We find that only boron forms superhydride-like structures that can host high-Tc superconductivity, but the predicted Tc are incompatibe with the experimental reports. Our results indicate that, while the claims of hot superconductivity should be reconsidered, it is very likely that unkwown H-rich ternary or multinary phases containing lanthanum, boron and hydrogen may have formed under the reported experimental conditions, and that these may exhibit superconducting properties comparable, or even superior, to those of currently known hydrides.

cond-mat.supr-con↗

arXiv2021

LaBH$_{8}$: the first high-T$_{c}$ low-pressure superhydride

In the last five years a large number of new high-temperature superconductors have been predicted and experimentally discovered among hydrogen-rich crystals, at pressures which are way too high to meet any practical application. In this work, we report the computational prediction of a hydride superconductor, LaBH$_{8}$, with a T$_{c}$ of 126 K at a pressure of 50 GPa, thermodynamically stable above 100 GPa, and dynamically stable down to 40 GPa, an unprecedentedly low pressure for high-T$_{c}$ hydrides. LaBH$_{8}$ can be seen as a ternary sodalite-like hydride, in which a metallic hydrogen sublattice is stabilized by the chemical pressure exerted by the guest elements. The combination of two elements with different atomic sizes in LaBH$_{8}$ realizes a more efficient packing of atoms than in binary sodalite hydrides. A suitable choice of elements may be exploited to further reduce the stabilization pressure to ambient conditions.

cond-mat.supr-con↗

arXiv2021

Importance of feature engineering and database selection in a machine learning model: A case study on carbon crystal structures

Drive towards improved performance of machine learning models has led to the creation of complex features representing a database of condensed matter systems. The complex features, however, do not offer an intuitive explanation on which physical attributes do improve the performance. The effect of the database on the performance of the trained model is often neglected. In this work we seek to understand in depth the effect that the choice of features and the properties of the database have on a machine learning application. In our experiments, we consider the complex phase space of carbon as a test case, for which we use a set of simple, human understandable and cheaply computable features for the aim of predicting the total energy of the crystal structure. Our study shows that (i) the performance of the machine learning model varies depending on the set of features and the database, (ii) is not transferable to every structure in the phase space and (iii) depends on how well structures are represented in the database.

cond-mat.mtrl-sci↗

arXiv2020

Phase Diagram and Superconductivity of Calcium Borohyrides at Extreme Pressures

Motivated by the recent discovery of near-room temperature superconductivity in high-pressure superhydrides, we investigate from first-principles the high-pressure superconducting phase diagram of the ternary Ca-B-H system, using ab-initio evolutionary crystal structure prediction, and Density Functional Perturbation Theory. We find that below 100 GPa all stable and weakly metastable phases are insulating. This pressure marks the appearance of several new chemically-forbidden phases on the hull of stability, and the first onset of metalization in CaBH$_5$. Metallization is then gradually achieved at higher pressure at different compositions. Among the metallic phases stable in the Megabar regime, we predict two high-$T_c$ superconducting phases with CaBH$_6$ and Ca$_2$B$_2$H$_{13}$ compositions, with critical temperatures of 119 and 89 K at 300 GPa, respectively, surviving to lower pressures. Ternary hydrides will most likely play a major role in superconductivity research in the coming years; our study suggests that, in order to reduce the pressure for the onset of metallicity and superconductivity, further explorations of ternary hydrides should focus on elements less electronegative than boron.

cond-mat.supr-con↗

arXiv2020

High-Temperature Conventional Superconductivity in the Boron-Carbon system: Material Trends

In this work we probe the possibility of high-temperature conventional superconductivity in the boron-carbon system, using ab-initio screening. A database of 320 metastable structures with fixed composition (50$\%$/50$\%$) is generated with the Minima-Hopping method, and characterized with electronic and vibrational descriptors. Full electron-phonon calculations on sixteen representative structures allow to identify general trends in $T_{\textrm{c}}$ across and within the four families in the energy landscape, and to construct an approximate $T_{\textrm{c}}$ predictor, based on transparently interpretable and easily computable electronic and vibrational descriptors. Based on these, we estimate that around 10$\%$ of all metallic structures should exhibit $T_{\textrm{c}}$'s above 30 $K$. This work is a first step towards ab-initio design of new high-$T_{\textrm{c}}$ superconductors.

cond-mat.supr-con↗