arXiv subjects

Eva Zurek

Publications and source records attributed to Eva Zurek.

At least 19 recordsLinked to original sources

arXiv2026

Anharmonicity and Nonadiabaticity in Hydride Superconductors

We study superconductivity in representative hydrides using anharmonic phonons, electron-phonon vertex corrections, and full-bandwidth Eliashberg theory. The high-pressure binary hydrides H3S, YH6, and YH9 must be treated with both anharmonic and nonadiabatic corrections, whereas the ambient-pressure PdH/PdD/PdT series is strongly anharmonic but remains adiabatic, reproducing the inverse isotope effect without sizable vertex contributions. LaBeH8 exhibits weak anharmonicity, while vertex corrections reduce the critical temperature (Tc) by approximately 4 K, leaving the predicted Tc above experiment. To identify when treatments beyond harmonic, adiabatic Migdal-Eliashberg theory are required, we introduce the anharmonic renormalization $A_{\lambda}$ and the vertex ratio $R_{V}$ as material-specific diagnostics.

cond-mat.supr-con↗

arXiv2026

Geometry-Based Neural-Network Prediction of Electron Localization Function Topology in Dense Hydrogen

We develop a machine-learning framework to predict the electron localization function (ELF) of pure, dense hydrogen directly from atomic geometry, bypassing explicit electronic-structure calculations. Trained on first-principles data spanning multiple pressure regimes in dense fluid hydrogen, the model achieves high accuracy ($R^2 > 0.99$) and faithfully reproduces the global distribution of the ELF. A combined real- and reciprocal-space analysis reveals that the residual error is dominated by smooth, long-wavelength components with correlation lengths exceeding typical H--H bonding scales, and that the magnitude of these components increases systematically with pressure. Despite being trained exclusively on dense fluid hydrogen networks, the model transfers robustly to crystalline hydrogen configurations, preserving key features of ELF topology, including critical points and hydrogen-network connectivity. Taken together, these results suggest a viable route toward geometry-based, high-throughput evaluation of hydrogen-networking characteristics in both fluid and crystalline hydrogen.

cond-mat.mtrl-sci↗

arXiv2026

Metal Atom (Dis)Order and Superconductivity in YCaH$_{n}$ ($n=8-20$) High-Pressure Superhydrides

High-pressure superhydrides have attracted much attention due to their high superconducting critical temperatures ($T_\text{c}$s). Herein, density functional theory (DFT) calculations are used to study the structures and properties, including potential for metal atom disorder and doping-enhanced $T_\text{c}$, within Y-Ca superhydrides with YCaH$_{n}$ ($n=8-20$) compositions. For YCaH$_8$ numerous phases that differed in the arrangement of the metal atoms were found to be nearly isoenthalpic, suggesting the importance of configurational entropy on stability. The equimolar ratio of the two metal atoms brought the Fermi level to a peak in the density of states, enhancing $T_\text{c}$ to 149~K and 170~K for $P4/mmm$ and $Cmmm$ YCaH$_{8}$, respectively, at 180~GPa within the isotropic Eliashberg formalism. YCaH$_{12}$ was also predicted to be disordered, however the $T_\text{c}$s of the ordered variants spanned a wide range from 105-253~K at 200~GPa, showing that doping could either mildly enhance or drastically reduce $T_\text{c}$ from that of the parent compounds. For YCaH$_{18}$ and YCaH$_{20}$, only a single dynamically stable superhydride was predicted, which we attribute to the differences in the structures of the stable binary parents.

cond-mat.supr-con↗

arXiv2025

A Software Package for Generating Robust and Accurate Potentials using the Moment Tensor Potential Framework

We present the Plan for Robust and Accurate Potentials (PRAPs), a software package for training and using moment tensor potentials (MTPs) in concert with the Machine Learned Interatomic Potentials (MLIP) software package. PRAPs provides an automated workflow to train MTPs using active learning procedures, and a variety of utilities to ease and improve workflows when utilizing the MLIP software. PRAPs was originally developed in the context of crystal structure prediction, in which one calculates convex hulls and predicts low energy metastable and thermodynamically stable structures, but the potentials PRAPs develops are not limited to such applications. PRAPs produces two potentials, one capable of rough estimates of the energies, forces and stresses of almost any chemical structure in the specified compositional space -- the Robust Potential -- and a second potential intended to provide more accurate descriptions of ground state and metastable structures -- the Accurate Potential. We also present a Python library, mliputils, designed to assist users in working with the chemical structural files used by the MLIP package.

physics.chem-ph↗

arXiv2025

Non-local Chemistry Driven by Cation-Anion Size Disparity in Helium Inserted Compounds under High Pressure

Opposing the theory that Helium (He) cannot be inserted into AB-type ionic compounds due to the Madelung energy increase, our crystal structure search and first-principles calculations found that He can form stable compounds with sodium halides (NaX, X=Cl, Br, I) under high-pressure. These reactions are driven by the non-local chemistry arising from the cation-anion size disparity, distinctly different from the He insertion reaction with A2B-type compounds. The large size differences between Na+ and X- enable structures that can effectively host He insertions through volume and inter-atomic distance disproportionation. Furthermore, the insertion of He atoms can significantly relieve the elevated Madelung energy that builds up in NaX under high pressure. This energy increase arises from structural transitions driven by cation-anion size disparity, which are necessary for reducing volume under pressure. The insertion of He allows the reduction of the total volume under high pressure without increasing the Madelung energy. Our predicted compounds and stability analysis reveal a new example of He reactivity governed not by local chemical bond formation, but by long-range electrostatic interactions.

cond-mat.mtrl-sci↗

arXiv2025

Superionicity in Ammonium Polyhydrides at Extreme Pressures

Polyhydrides have been shown to form novel structures at high pressure, which may be found in the interiors of giant planets. With density functional molecular dynamics simulations we studied the behavior of ammonium polyhydride compounds with stoichiometries of NH$_7$, NH$_9$, NH$_{10}$, NH$_{11}$, NH$_{14}$, NH$_{20}$, and NH$_{24}$ which were predicted with crystal structure search methods to be metastable at 100-300~GPa. For every compound, we performed simulations at a range of temperatures (and for several compounds, pressures) covering the solid, superionic and liquid phases. We show that when heated, high pressure ammonium polyhydride compounds exhibit hydrogen superionic diffusion. We demonstrate a number of metrics by which the solid-to-superionic and superionic-to-liquid transitions can be detected from simulation data, including changes in the internal energy and pressure, formation of new chemical species, and atomic diffusion rates. We find that both the solid-to-superionic and the superionic-to-liquid transitions decrease in temperature as proton fraction increases. These trends indicate that above a proton fraction of $\sim$0.97, ammonium hydride structures are likely to directly melt instead of first exhibiting a superionic phase. Our observed melting trend further indicates that at the extreme conditions of ice giant interiors, hydrogen rich ammonium hydrides such as those studied in this work would exist predominantly as liquids rather than exhibiting a superionic phase.

cond-mat.mtrl-sci↗

arXiv2025

Novel Transformations of PbTiO3 with Pressure and Temperature

We investigated the behavior of lead titanate (PbTiO3) up to 100 GPa, both at room temperature and upon laser heating, using synchrotron X ray diffraction combined with density functional theory (DFT) computations. At the high pressure temperature (PT) conditions produced in laser heated diamond anvil cells, PbTiO3 dissociates into PbO and TiO2, consistent with our DFT computations showing that decomposition becomes enthalpically favored above 65 GPa. In contrast, on room temperature compression, PbTiO3 persists in the tetragonal I4mcm phase up to at least 100 GPa. Laser heating produces distinct PbO phases: a compressed form of alpha PbO and a previously unreported delta PbO polymorph, both of which transform to beta PbO on decompression. The calculations predict that alpha PbO undergoes pressure-induced band gap closure, metallizing above 70 GPa, whereas the delta and beta phases remain semiconducting with a band gap above 1 eV even at megabar pressures. The experimental and confirming theoretical results reveal an unanticipated dimension of the behavior of PbTiO3, showing that distinct equilibrium and metastable phases can be stabilized along different PT synthesis paths.

cond-mat.mtrl-sci↗

arXiv2025

-Quantum Effects or Theoretical Artifacts? A Computational Reanalysis of Hydrogen's High-Pressure Phase Stability and Properties

Experimental and theoretical studies of the high-pressure phases of hydrogen are highly sensitive to methodological choices. We compare the cold phase diagram of hydrogen calculated between 400-700 GPa with meta-GGA functionals (R2SCAN and SCAN0) with the commonly employed GGA-PBE functional. Molecular phases are stabilized over the atomic phase to higher pressures with the meta-GGA, in closer agreement with diffusion Monte Carlo calculations. R2SCAN phonon spectra show that the dynamical instabilities and anharmonic signatures predicted at the GGA level vanish, indicating that such effects may partly arise from functional deficiencies rather than quantum nuclear effects. Bonding analysis reveals that PBE artificially weakens intramolecular H-H bonds and enhances intermolecular interactions through charge delocalization, whereas meta-GGA preserves a more localized molecular character.

cond-mat.mtrl-sci↗

arXiv2025

Single layer clathrane: A potential superconducting two-dimensional (2D) hydrogenated metal borocarbide

We propose a new family of two-dimensional (2D) metal-borocarbide clathrane superconductors, derived from three-dimensional (3D) MM$^\prime$B$_6$C$_6$ clathrates. First-principles calculations reveal that hydrogen passivation and surface metal decoration stabilize the M$_2$M$^\prime$B$_8$C$_8$H$_8$ monolayers. These 2D systems exhibit tunable superconductivity governed by hole concentration, structural anisotropy, and electron-phonon coupling. We find that in-plane anisotropy competes with superconductivity, reducing \tc\ despite favorable doping. Biaxial strain mitigates this anisotropy, enhances Fermi surface nesting, and increases \tc\ by an average of 15.5~K. For example, the \tc\ of Sr$_3$B$_8$C$_8$H$_8$ is predicted to increase from 11.3~K to 22.2~K with strain engineering. These findings identify 2D clathranes as promising, strain-tunable superconductors and highlight design principles for optimizing low-dimensional superconducting materials.

cond-mat.supr-con↗

arXiv2025

Using Topology to Predict Electrides in the Solid State

Electrides are characterized by electron density highly localized in interstitial sites, which do not coincide with the interatomic contacts. The rigorous quantum mechanical definition of electrides is based upon topological criteria derived from the electron density, and in particular the presence of non-nuclear attractors (NNAs). We employ these topological criteria in combination with crystal structure prediction methods (the XtalOpt evolutionary algorithm), to accelerate the discovery of crystalline electrides at ambient and non-ambient pressures. The localization and quantification of NNAs is used as the primary discriminator for the electride character of a solid within a multi-objective evolutionary structure search. We demonstrate the reliability of this approach through a comprehensive crystal structure prediction study of Ca5Pb3 at 20 GPa, a system previously theorized to exhibit electride character under compression. Our strategy could predict, and sort on-the-fly, several unknown low-enthalpy phases that possess NNAs in interstitial loci, such as the newly discovered P4/mmm structure. These results demonstrate how evolutionary algorithms, guided by rigorous topological descriptors, can be relied upon to effectively survey complex phases to find new electride candidates.

cond-mat.mtrl-sci↗

arXiv2025

Refining Tc Prediction in Hydrides via Symbolic-Regression-Enhanced Electron-Localization-Function-Based Descriptors

Hydrogen-based materials are able to possess extremely high superconducting critical temperatures, \tc s, due to hydrogen's low atomic mass and strong electron-phonon interaction. Recently, a descriptor based on the Electron Localization Function (ELF) has enabled the rapid estimation of the \tc\ of hydrogen-containing compounds from electronic networking properties, but its applicability has been limited by the small size and homogeneity of the training dataset used. Herein, the model is re-examined compiling a publicly available combined dataset of 244 binary and ternary hydride superconductors. Our analysis shows that though ELF-based networking remains a valuable descriptor, its predictive power declines with increasing compositional complexity. However, by introducing the molecularity index, defined as the highest value of the ELF at which two hydrogen atoms connect, and applying symbolic regression, the accuracy of the predictions can be substantially enhanced. These results establish a more robust framework for assessing superconductivity in hydride materials, facilitating accelerated screening of novel candidates through integration with crystal structure prediction methods or high-throughput searches.

physics.comp-ph↗

arXiv2025

XtalOpt Version 14: Variable-Composition Crystal Structure Search for Functional Materials Through Pareto Optimization

Version 14 of XtalOpt, an evolutionary multi-objective global optimization algorithm for crystal structure prediction, is now available for download from its official website https://xtalopt.github.io, and the Computer Physics Communications Library. The new version of the code is designed to perform a ground state search for crystal structures with variable compositions by integrating a suite of ab initio methods alongside classical and machine-learning potentials for structural relaxation. The multi-objective search framework has been enhanced through the introduction of Pareto optimization, enabling efficient discovery of functional materials. Herein, we describe the newly implemented methodologies, provide detailed instructions for their use, and present an overview of additional improvements included in the latest version of the code.

physics.comp-ph↗

arXiv2025

Roadmap for warm dense matter physics

This roadmap presents the state-of-the-art, current challenges and near future developments anticipated in the thriving field of warm dense matter physics. Originating from strongly coupled plasma physics, high pressure physics and high energy density science, the warm dense matter physics community has recently taken a giant leap forward. This is due to spectacular developments in laser technology, diagnostic capabilities, and computer simulation techniques. Only in the last decade has it become possible to perform accurate enough simulations \& experiments to truly verify theoretical results as well as to reliably design experiments based on predictions. Consequently, this roadmap discusses recent developments and contemporary challenges that are faced by theoretical methods, and experimental techniques needed to create and diagnose warm dense matter. A large part of this roadmap is dedicated to specific warm dense matter systems and applications in astrophysics, inertial confinement fusion and novel material synthesis.

physics.plasm-ph↗

arXiv2025

Theoretical Predictions of MB5N5: Atom-Stuffed Boronitride Clathrate Cages Derived from the High-Pressure Superhydride

This study investigates 198 MX5Y5 (X, Y = B, C, or N) clathrate-like structures derived from MH10 superhydrides using high-throughput Density Functional Theory (DFT) geometry optimizations and phonon calculations. A wide variety of electropositive and electronegative encapsulated atoms were considered. From all of the studied systems only 34 MB5N5 phases were found to be dynamically stable at ambient pressure. The highest 1-atmosphere superconducting critical transition temperature was predicted for FB5N5. However, ab initio molecular dynamics simulations revealed that all of the identified superconducting phases decompose by 300~K at 1~atm, while only eleven semiconducting phases remained thermally stable. Our findings underscore the critical role of kinetic and thermal stability in predicting viable superconductors. The electronic structure of the MB5N5 compounds were rationalized in terms of electron donating and withdrawing intercalants, and machine-learning based predictions of their mechanical properties were compared with those of an empty boronitride cage.

cond-mat.supr-con↗

arXiv2025

Creation, stabilization, and study at ambient pressure of pressure-induced superconductivity in Bi$_{0.5}$Sb$_{1.5}$Te$_3$

In light of breakthroughs in superconductivity under high pressure, and considering that record critical temperatures (T$_c$s) across various systems have been achieved under high pressure, the primary challenge for higher Tc should no longer solely be to increase T$_c$ under extreme conditions but also to reduce, or ideally eliminate, the need for applied pressure in retaining pressure-induced or -enhanced superconductivity. The topological semiconductor Bi$_{0.5}$Sb$_{1.5}$Te$_3$ (BST) was chosen to demonstrate our approach to addressing this challenge and exploring its intriguing physics. Under pressures up to ~ 50 GPa, three superconducting phases (BST-I, -II, and -III) were observed. A superconducting phase in BST-I appears at ~ 4 GPa, without a structural transition, suggesting the possible topological nature of this phase. Using the pressure-quench protocol (PQP) recently developed by us, we successfully retained this pressure-induced phase at ambient pressure and revealed the bulk nature of the state. Significantly, this demonstrates recovery of a pressure-quenched sample from a diamond anvil cell at room temperature with the pressure-induced phase retained at ambient pressure. Other superconducting phases were retained in BST-II and -III at ambient pressure and subjected to thermal and temporal stability testing. Superconductivity was also found in BST with T$_c$ up to 10.2 K, the record for this compound series. While PQP maintains superconducting phases in BST at ambient pressure, both depressurization and PQP enhance its T$_c$, possibly due to microstructures formed during these processes, offering an added avenue to raise T$_c$. These findings are supported by our density-functional theory calculations.

cond-mat.supr-con↗

arXiv2025

A chemical bonding based descriptor for predicting the impact of quantum nuclear and anharmonic effects on hydrogen-based superconductors

Quantum nuclear effects (QNEs) can significantly alter a material's crystal structure and phonon spectra, impacting properties such as thermal conductivity and superconductivity. However, predicting a priori whether these effects will enhance or suppress superconductivity, or destabilize a structure, remains a grand challenge. Herein, we address this unresolved problem by introducing a descriptor, based upon the integrated crystal orbital bonding index (iCOBI), to predict the influence of QNEs on a crystal lattice's dynamic stability, phonon spectra and superconducting properties. We find that structures with atoms in symmetric chemical bonding environments exhibit greater resilience to structural perturbations induced by QNEs, while those with atoms in asymmetric bonding environments are more susceptible to structural alterations, resulting in enhanced superconducting critical temperatures.

cond-mat.supr-con↗

arXiv2024

Quantum Anharmonic Effects on the Superconductivity of I-43m CH4-H3S at High Pressures: a First-Principles Study

Making use of first-principles calculations we analyze the effect of quantum ionic fluctuations and lattice anharmonicity on the crystal structure and superconductivity of I-43m CH4-H3S, one of the lowest enthalpy structures in the C-S-H system, in the 150-300 GPa pressure range within the stochastic self-consistent harmonic approximation. We predict a correction to the crystal structure, which is formed by an H3S lattice and CH4 molecules, the phonon spectra, and the pressure-dependent superconducting critical temperatures, which have been estimated in previous calculations without considering ionic fluctuations on the crystal structure and assuming the harmonic approximation for the lattice dynamics. Our results show that quantum ionic fluctuations have an impact on the distance between H atoms and S atoms in the H3S host lattice, pushing it towards more symmetric bonds, while the methane molecules are barely affected. According to our anharmonic phonon spectra, this structure is dynamically stable above 150 GPa, which is 30 GPa lower than the pressure at which the harmonic approximation predicts the emergence of an instability. As a consequence of the strong anharmonic enhancement of the phonon frequencies, the electron-phonon coupling constant is suppressed by 46% at 200 GPa, and even more at lower pressures. As a result, the superconducting critical temperature is overestimated by around 50 K at 200 GPa, such that it falls below 150 K in the whole pressure range studied. Our results underline that ternary hydrides are subject to strong anharmonic effects on their structural, vibrational, and superconducting properties.

cond-mat.mtrl-sci↗

arXiv2024

Why Mg$_2$IrH$_6$ is predicted to be a high temperature superconductor, but Ca$_2$IrH$_6$ is not

The X$_2$MH$_6$ family, consisting of an electropositive cation X and a main group metal M octahedrally coordinated by hydrogen, has been predicted to hold promise for high-temperature conventional superconductivity. Herein, we analyze the electronic structure of two members of this family, Mg$_2$IrH$_6$ and Ca$_2$IrH$_6$, showing why the former may possess superconducting properties rivaling those of the cuprates, whereas the latter does not. Within Mg$_2$IrH$_6$ the vibrations of the IrH$_6^{4-}$ anions are key for the superconducting mechanism, and they induce coupling in the $e_g^*$ set, which are antibonding between the H 1$s$ and the Ir $d_{x^2-y^2}$ or $d_{z^2}$ orbitals. Because calcium possesses low-lying d-orbitals, $e_g^* \rightarrow$ Ca $d$ back-donation is preferred, quenching the superconductivity. Our analysis explains why high critical temperatures were only predicted for second or third row X metal atoms, and may hold implications for superconductivity in other systems where the antibonding anionic states are filled.

cond-mat.supr-con↗