Showing 1–50 of 90 results for author: Errea, I

Quantum geometry and critical temperature enhancement in MgB$_2$ superconductivity

Authors: Yi Jiang, Haoyu Hu, Dumitru Călugăru, Kaja H. Hiorth, Junze Deng, Hanqi Pi, Handong Chen, Maia G. Vergniory, Ion Errea, Emilia Morosan, Leslie M. Schoop, Claudia Felser, Miguel A. L. Marques, Päivi Törmä, Daniel Agterberg, B. Andrei Bernevig

Abstract: MgB$_2$, a phonon-mediated superconductor with record-high critical temperature $T_c\simeq 39$ K, is revisited to obtain a comprehensive theory of electrons, phonons, and their coupling with minimal ab initio input. We construct compact analytic models for the electronic structure, phonons, and electron-phonon coupling (EPC) of MgB$_2$. We show that strong in-plane B $sp^2$ bonding realizes an obs… ▽ More MgB$_2$, a phonon-mediated superconductor with record-high critical temperature $T_c\simeq 39$ K, is revisited to obtain a comprehensive theory of electrons, phonons, and their coupling with minimal ab initio input. We construct compact analytic models for the electronic structure, phonons, and electron-phonon coupling (EPC) of MgB$_2$. We show that strong in-plane B $sp^2$ bonding realizes an obstructed band structure whose natural description is a bond-centered kagome lattice, yielding small quasi-2D $σ$-band Fermi-surface cylinders and pronounced quantum-geometric effects. The phonon spectrum is found to closely track that of a graphene-like boron layer, but the heavy intercalated Mg atoms dominate the three acoustic branches and rigidly lift the boron modes into the optical sector, while the in-plane B-B bond-stretching mode exhibits a pronounced softening along $Γ$-A. By symmetry, this $Γ$-point bond-stretching mode is the only $Γ$ phonon that can couple to the $σ$ Fermi surface, explaining its dominant contribution to the EPC. Upon electron doping toward the doubly degenerate band edge of the $σ$ sheets, we find that a reduced density of states competes with enhanced EPC matrix elements. At light electron doping, ab initio calculations show that the EPC enhancement dominates, leading to an increase in $T_c$ (within the clean doping limit without disorder effects). Using the Gaussian approximation for the EPC tensor, we further show that this enhancement is overwhelmingly quantum geometric in origin, arising from a geometric EPC contribution of the small $σ$ Fermi surface peaked at $Γ$. Overall, our results provide a transparent, symmetry-based account of superconductivity in MgB$_2$ and suggest that quantum-geometric effects can be essential for shaping doping trends in phonon-mediated superconductors. △ Less

Submitted 21 July, 2026; originally announced July 2026.

Comments: 96 pages, 38 figures

Revealing magnetism in the distorted kagome $R$Ti$_3$Bi$_4$ ($R$ = Nd, Sm, Gd) via ARPES and XMCD

Authors: C. Lim, F. Ballester, A. Kar, M. Alkorta, D. Subires, J. Dai, M. Tallarida, E. Vescovo, T. K. Kim, C. Cacho, C. Yi, S. Roychowdhury, A. Kumar Sharma, Y. Choi, G. Fabbris, J. Strempfer, P. Gargiani, C. Shekhar, C. Felser, I. Errea, M. G. Vergniory, S. Blanco-Canosa

Abstract: Kagome materials are known for hosting emergent quantum phenomena driven by the interaction between different lattice, charge and spin orders. Here, we present a detailed angle resolved photoemission (ARPES), density functional theory (DFT) and x-ray magnetic circular dichroism (XMCD) study of the electronic and magnetic structure of $R$Ti$_3$Bi$_4$ ($R$ = Nd, Sm, Gd). ARPES and DFT demonstrate th… ▽ More Kagome materials are known for hosting emergent quantum phenomena driven by the interaction between different lattice, charge and spin orders. Here, we present a detailed angle resolved photoemission (ARPES), density functional theory (DFT) and x-ray magnetic circular dichroism (XMCD) study of the electronic and magnetic structure of $R$Ti$_3$Bi$_4$ ($R$ = Nd, Sm, Gd). ARPES and DFT demonstrate that the bulk electronic band structure is dominated by the hybridization of the Ti bands, and the weak electron-like pocket at $Γ$ is identified as a surface state. The isotropic XAS profile of the $M_{4,5}$-edge of the rare earth is consistent with the presence of $R^{3+}$ oxidation state. Using the XMCD sum rules, backed by the atomic multiplet theory calculations, we obtain the spin and orbital magnetic moments. The Ti $L_{2,3}$-edge XMCD reveals the presence of a small magnetic moment in GdTi$_3$Bi$_4$, presumably driven by the proximity of the {Ti} kagome layers to the $zigzag$ chains of Gd, while the total magnetic moment of Gd is shared by the $f$ and $d$ electrons. Our combined XMCD, ARPES and DFT study brings an important piece of information to understand the spin flip transitions and anomalous Hall effect observed in the $R$Ti$_3$Bi$_4$ kagome metals. △ Less

Submitted 29 April, 2026; originally announced April 2026.

Flat band driven competing charge and spin instabilities in the altermagnet CrSb

Authors: A. Korshunov, M. Alkorta, C. -Y. Lim, F. Ballester, Cong Li, Zhilin Li, D. Chernyshov, A. Bosak, M. G. Vergniory, Ion Errea, S. Blanco-Canosa

Abstract: The confinement of electronic wavefunctions in momentum space can give rise to flat electronic bands, where the quenching of kinetic energy enhances the density of states and amplifies interaction effects. Such conditions are fertile ground for emergent quantum phases, as spin, charge and lattice degrees of freedom become strongly entangled. In these regimes, subtle competitions between intertwine… ▽ More The confinement of electronic wavefunctions in momentum space can give rise to flat electronic bands, where the quenching of kinetic energy enhances the density of states and amplifies interaction effects. Such conditions are fertile ground for emergent quantum phases, as spin, charge and lattice degrees of freedom become strongly entangled. In these regimes, subtle competitions between intertwined order parameters often dictate the macroscopic ground state, producing complex and sometimes unexpected collective behavior. Here we show that the altermagnet CrSb provides a realization of this scenario, and uncover short-range charge-order fluctuations at the M point of the Brillouin zone, q*=(1/2 0), persisting above the Neel temperature (TN). Remarkably, these fluctuations collapse upon entering the magnetically ordered phase, revealing a direct and robust competition between charge and spin order. At TN, the phonon dispersion at q* develops a pronounced Kohn-like anomaly, signaling strong electron-phonon coupling in the vicinity of the magnetic transition. Below TN, exchange striction dramatically renormalizes the associated soft phonon mode by approximately ~6 meV, the largest spin-phonon coupling ever reported. First-principles calculations attribute this behavior to a strong coupling between nearly dispersionless electronic states and a phonon branch that appears unstable at the harmonic level only when no magnetic order is considered, revealing the large sensitivity of the lattice to magnetic symmetry breaking. The competition between charge and spin order parameters, amplified by flat-band physics, drives the observed phonon anomaly and its abrupt reconstruction at TN. With its chemically simple structure and symmetry-protected altermagnetic state, CrSb emerges as a model platform to explore how flat electronic bands mediate giant spin-phonon coupling and competing broken symmetries. △ Less

Submitted 26 March, 2026; originally announced March 2026.

Comments: 11 pages, 4 figures

Enhanced superconductivity in palladium hydrides by non-perturbative electron-phonon effects

Authors: Raffaello Bianco, Ion Errea

Abstract: Palladium hydrides exhibit the largest isotope-effect anomaly in superconductivity: replacing hydrogen with heavier isotopes increases the superconducting critical temperature. Although this behavior is commonly attributed to strong anharmonic hydrogen vibrations, \textit{ab initio} treatments have so far incorporated anharmonic effects only through phonon renormalization, neglecting non-linear co… ▽ More Palladium hydrides exhibit the largest isotope-effect anomaly in superconductivity: replacing hydrogen with heavier isotopes increases the superconducting critical temperature. Although this behavior is commonly attributed to strong anharmonic hydrogen vibrations, \textit{ab initio} treatments have so far incorporated anharmonic effects only through phonon renormalization, neglecting non-linear contributions to the electron-phonon interaction vertices. While such approaches reproduce the anomalous isotope trend, they severely underestimate the critical temperatures. Here, we show that non-linear electron-phonon coupling is essential in palladium hydrides. A straightforward inclusion of higher-order perturbative terms leads to a qualitative breakdown: the critical temperature is overestimated and the isotope anomaly is lost. We therefore adopt a non-perturbative framework based on an explicit evaluation of the ion-mediated electron-electron interaction, enabling anharmonic effects to be treated consistently in both the phonon spectra and the interaction vertices. Applied to PdH and PdD, it restores the anomalous isotope effect and brings calculated critical temperatures into significantly improved agreement with experiments. △ Less

Submitted 3 March, 2026; originally announced March 2026.

Iterative learning scheme for crystal structure prediction with anharmonic lattice dynamics

Authors: Hao Gao, Yue-Wen Fang, Ion Errea

Abstract: First-principles based crystal structure prediction (CSP) methods have revealed an essential tool for the discovery of new materials. However, in solids close to displacive phase transitions, which are common in ferroelectrics, thermoelectrics, charge-density wave systems, or superconducting hydrides, the ionic contribution to the free energy and lattice anharmonicity become essential, limiting th… ▽ More First-principles based crystal structure prediction (CSP) methods have revealed an essential tool for the discovery of new materials. However, in solids close to displacive phase transitions, which are common in ferroelectrics, thermoelectrics, charge-density wave systems, or superconducting hydrides, the ionic contribution to the free energy and lattice anharmonicity become essential, limiting the capacity of CSP techniques to determine the thermodynamical stability of competing phases. While variational methods like the stochastic self-consistent harmonic approximation (SSCHA) accurately account for anharmonic lattice dynamics \emph{ab initio}, their high computational cost makes them impractical for CSP. Machine-learning interatomic potentials offer accelerated sampling of the energy landscape compared to purely first-principles approaches, but their reliance on extensive training data and limited generalization restricts practical applications. Here, we propose an iterative learning framework combining evolutionary algorithms, atomic foundation models, and SSCHA to enable CSP with anharmonic lattice dynamics. Foundation models enable robust relaxations of random structures, drastically reducing required training data. Applied to the highly anharmonic H$_3$S system, our framework achieves good agreement with the benchmarks based on density functional theory, accurately predicting phase stability and vibrational properties from 50 to 200 GPa. Importantly, we find that the statistical averaging in the SSCHA reduces the error in the free energy evaluation, avoiding the need for extremely high accuracy of machine-learning potentials. This approach bridges the gap between data efficiency and predictive power, establishing a practical pathway for CSP with anharmonic lattice dynamics. △ Less

Submitted 23 December, 2025; originally announced December 2025.

Soft Mode Origin of Charge Ordering in Superconducting Kagome CsV$_3$Sb$_5$

Authors: Philippa Helen McGuinness, Fabian Henssler, Manex Alkorta, Mark Joachim Graf von Westarp, Artem Korshunov, Alexei Bosak, Daisuke Ishikawa, Alfred Q. R. Baron, Michael Merz, Amir-Abbas Haghighirad, Maia G. Vergniory, Sofia-Michaela Souliou, Rolf Heid, Ion Errea, Matthieu Le Tacon

Abstract: Charge-density-wave (CDW) order and superconductivity coexist in the kagome metals AV$_3$Sb$_5$ (A=K, Cs, Rb), raising fundamental questions about the mechanisms driving their intertwined phases. Here we combine high-resolution inelastic X-ray scattering with first-principles calculations to uncover the origin of CDW formation in CsV$_3$Sb$_5$. Guided by structure factor analysis, we identify a so… ▽ More Charge-density-wave (CDW) order and superconductivity coexist in the kagome metals AV$_3$Sb$_5$ (A=K, Cs, Rb), raising fundamental questions about the mechanisms driving their intertwined phases. Here we combine high-resolution inelastic X-ray scattering with first-principles calculations to uncover the origin of CDW formation in CsV$_3$Sb$_5$. Guided by structure factor analysis, we identify a soft phonon mode along the reciprocal M-L direction, with the strongest effect at the L point, where the elastic scattering intensity also grows most rapidly upon cooling. First-principles calculations incorporating lattice anharmonicity and electron-phonon coupling reproduce these observations and establish a soft-mode instability at the L point as the driving mechanism of CDW formation. Despite the weakly first-order character of the transition, our results unambiguously demonstrate that the CDW in CsV$_3$Sb$_5$ originates from a softened phonon, clarifying its microscopic origin and highlighting the central role of lattice dynamics in kagome metals. △ Less

Submitted 22 October, 2025; originally announced October 2025.

Comments: 25 pages, 5 figures

Journal ref: Nature Communications 17 (1), 4817 (2026)

First-order phase transition driven by competing charge-order fluctuations in 1T'-TaTe$_{2}$

Authors: S. K. Mahatha, A. Kar, J. Corral-Sertal, Josu Diego, A. Korshunov, C. -Y. Lim, F. K. Diekmann, D. Subires, J. Phillips, T. Kim, D. Ishikawa, G. Marini, I. Vobornik, Ion Errea, S. Rohlf, M. Kalläne, V. Bellini, A. Q. R. Baron, Adolfo O. Fumega, A. Bosak, V. Pardo, K. Rossnagel, S. Blanco-Canosa

Abstract: First-order phase transitions, characterized by a discontinuous change in the order parameter, are intriguing phenomena in condensed matter physics. However, the underlying, material-specific, microscopic mechanisms often remain unclear. Here, we unveil a high-temperature incommensurate charge-order precursor with the wave vector $\mathbf{q}^* = (0, \frac{1}{4}+δ, \frac{1}{2})$ in the 1T' phase of… ▽ More First-order phase transitions, characterized by a discontinuous change in the order parameter, are intriguing phenomena in condensed matter physics. However, the underlying, material-specific, microscopic mechanisms often remain unclear. Here, we unveil a high-temperature incommensurate charge-order precursor with the wave vector $\mathbf{q}^* = (0, \frac{1}{4}+δ, \frac{1}{2})$ in the 1T' phase of TaTe$_2$, which competes with fluctuating high-temperature Ta trimer bonding states at $\mathbf{q}_\mathrm{CO} =(0, \frac{1}{3}, 0)$. The precursor state follows the temperature dependence of the hidden incommensurability of the $\textit{quasi}$-1D nested Fermi surface. In contrast, the low-temperature commensurate charge order at $\mathbf{q}_\mathrm{CO}$, characterized by a charge disproportionation of the inequivalent Ta sites, appears to be driven by local chemical bonding. Dynamical lattice calculations identify an imaginary optical mode at $\mathbf{q}^*$, involving an in-plane vibration of the Ta atoms forming a chain-like structure that renormalizes below $T_\mathrm{CO}$. Our experimental and theoretical observations suggest that the controversial first-order phase transition, as captured by phenomenological Ginzburg-Landau theory, results from the competition between two order parameters: one involving Fermi surface nesting and the other involving local chemical bonding. △ Less

Submitted 15 October, 2025; originally announced October 2025.

Search for thermodynamically stable ambient-pressure superconducting hydrides in GNoME database

Authors: Antonio Sanna, Tiago F. T. Cerqueira, Ekin Dogus Cubuk, Ion Errea, Yue-Wen Fang

Abstract: Hydrides are considered to be one of the most promising families of compounds for achieving high temperature superconductivity. However, there are very few experimental reports of ambient-pressure hydride superconductivity, and the superconducting critical temperatures ($T_{\rm c}$) are typically less than 10 K. At the same time several hydrides have been predicted to exhibit superconductivity aro… ▽ More Hydrides are considered to be one of the most promising families of compounds for achieving high temperature superconductivity. However, there are very few experimental reports of ambient-pressure hydride superconductivity, and the superconducting critical temperatures ($T_{\rm c}$) are typically less than 10 K. At the same time several hydrides have been predicted to exhibit superconductivity around 100 K at ambient pressure but in thermodynamically unfavorable phases. In this work we aim at assessing the superconducting properties of thermodynamically stable hydride superconductors at room pressure by investigating the GNoME material database, which has been recently released and includes thousands of hydrides thermodynamically stable at 0K. To scan this large material space we have adopted a multi stage approach which combines machine learning for a fast initial evaluation and cutting edge ab initio methods to obtain a reliable estimation of ($T_{\rm c}$). Ultimately we have identified 25 cubic hydrides with ($T_{\rm c}$) above 4.2~K and reach a maximum ($T_{\rm c}$) of 17 K. While these critical temperatures are modest in comparison to some recent predictions, the systems where they are found, being stable, are likely to be experimentally accessible and of potential technological relevance. △ Less

Submitted 17 March, 2026; v1 submitted 27 August, 2025; originally announced August 2025.

Comments: 11 pages, 5 figures in main text

Journal ref: Commun Phys 9, 94 (2026)

A Comprehensive Study on A$_2$PdH$_2$: From Ambient to High Pressure

Authors: Zahra Alizadeh, Yue-Wen Fang, Ion Errea, M. R. Mohammadizadeh

Abstract: We present a comprehensive first--principles study of the structural stability and superconducting behavior of Li$_2$PdH$_2$ under high pressure. Using random structure searching and phonon calculations, we identify a pressure--induced phase transition from a tetragonal I4/mmm structure, stable up to 5 GPa, to a monoclinic C2/m phase that remains thermodynamically stable up to 50 GPa. Superconduct… ▽ More We present a comprehensive first--principles study of the structural stability and superconducting behavior of Li$_2$PdH$_2$ under high pressure. Using random structure searching and phonon calculations, we identify a pressure--induced phase transition from a tetragonal I4/mmm structure, stable up to 5 GPa, to a monoclinic C2/m phase that remains thermodynamically stable up to 50 GPa. Superconductivity is absent in the tetragonal phase, even when anharmonic effects are considered, due to weak electron--phonon coupling and limited hydrogen involvement near the Fermi level. In contrast, the monoclinic phase exhibits a weak but pressure-enhanced superconducting transition, with Tc increasing from 0.6 K at 10 GPa to 4.7 K at 50 GPa, mainly driven by low--frequency Li and Pd-derived phonon modes. We further explore the isostructural A$_2$PdH$_2$ (A = Na, K, Rb, Cs) series to evaluate the impact of alkali-metal substitution on stability and superconductivity. Na, K, and Rb analogs retain dynamic stability at ambient pressure, with weak superconducting critical temperatures of 3.2 K, 2.1 K, and negligible Tc, respectively. Cs$_2$PdH$_2$, however, exhibits phonon instabilities, suggesting a need for external stabilization. These findings highlight the delicate balance between lattice dynamics, electronic structure, and atomic mass in tuning superconductivity in palladium-based hydrides. △ Less

Submitted 6 August, 2025; originally announced August 2025.

Simphony: A full tight-binding package for lattice vibrations and topological phonon analysis

Authors: Francesc Ballester, Ion Errea, Maia G. Vergniory

Abstract: Simphony is an open-source software package designed for the topological analysis of lattice vibrations based on Wannier tight-binding models. Its primary function is to classify the topology of novel materials by computing bulk and slab phonon band structures, extracting phonon surface spectra, and providing analysis tools such as Wilson loop calculations and Weyl node detection. The workflow is… ▽ More Simphony is an open-source software package designed for the topological analysis of lattice vibrations based on Wannier tight-binding models. Its primary function is to classify the topology of novel materials by computing bulk and slab phonon band structures, extracting phonon surface spectra, and providing analysis tools such as Wilson loop calculations and Weyl node detection. The workflow is analogous to that of established electronic topology codes like Wannier90 and WannierTools. It also incorporates long-range polar interactions during the wannierization process, making Simphony one of the first tools capable of diagnosing topology in polar insulators. △ Less

Submitted 24 July, 2025; v1 submitted 21 July, 2025; originally announced July 2025.

Comments: 34 pages, 5 figures, source code available at https://github.com/fballestermacia/simphony

Superconductivity in RbH$_{12}$ at low pressures: an \emph{ab initio} study

Authors: Đorđe Dangić, Manex Alkorta, Yuewen Fang, Ion Errea

Abstract: High-pressure polyhydrides are leading contenders for room temperature superconductivity. The next frontier lies in stabilizing them at ambient pressure, which would allow their practical applications. In this first-principles computational study, we investigate the potential for record-low pressure stabilization of binary superhydrides within the RbH$_{12}$ system including lattice quantum anharm… ▽ More High-pressure polyhydrides are leading contenders for room temperature superconductivity. The next frontier lies in stabilizing them at ambient pressure, which would allow their practical applications. In this first-principles computational study, we investigate the potential for record-low pressure stabilization of binary superhydrides within the RbH$_{12}$ system including lattice quantum anharmonic effects in the calculations. We identify five competing phases for the pressure range between 0 and 100 GPa. Incorporating anharmonic and quantum effects on ion dynamics, we find the $Immm$ and $P6_3/mmc$ phases to be the most probable, potentially metastable even at pressures as low as 10 GPa. Notably, all phases exhibit metallic properties, with critical temperatures between 46 and 111 K within the pressure range they are dynamically stable. These findings have the potential to inspire future experimental exploration of high-temperature superconductivity at low pressures in Rb-H binary compounds. △ Less

Submitted 15 May, 2026; v1 submitted 16 July, 2025; originally announced July 2025.

Real-space understanding of electron-phonon coupling in superconducting hydrides

Authors: Trinidad Novoa, Raffaello Bianco, Julia Contreras-García, Ion Errea

Abstract: Electron-phonon coupling is at the origin of conventional superconductivity, enabling the pairing of electrons into Cooper pairs. The electron-phonon matrix elements depend on the electronic eigenstates and, in the standard linear approximation, on the first derivative of the potential felt by the electrons with respect to ionic perturbations. Here, we focus on the derivatives of the potential wit… ▽ More Electron-phonon coupling is at the origin of conventional superconductivity, enabling the pairing of electrons into Cooper pairs. The electron-phonon matrix elements depend on the electronic eigenstates and, in the standard linear approximation, on the first derivative of the potential felt by the electrons with respect to ionic perturbations. Here, we focus on the derivatives of the potential with a twofold aim: to assess their contribution to the overall coupling and to analyze the limitations of neglecting higher-order derivatives. Several real-space functions are proposed to do the analysis, and are computed for some well-known superconductors. Our results show that, in hydrides, the derivatives of the potential tend to be larger in regions of high electron localization, explaining the success of electronic descriptors previously described to correlate with the critical temperature. The new functions introduced here are able to tell apart structures with similar types of bonding but very different critical temperatures, such as H3S and H3Se Im-3m phases, where electronic descriptors alone fail. Interestingly, our descriptors are capable of easily estimating the impact of higher-order terms in the electron-phonon coupling. In fact, we capture the limitations of the linear approximation expected for PdH, and predict an even more important non-linear behavior in LaH10. △ Less

Submitted 9 July, 2025; originally announced July 2025.

Symmetry-broken ground state and phonon mediated superconductivity in Kagome CsV$_3$Sb$_5$

Authors: Manex Alkorta, Martin Gutierrez-Amigo, Ðorđe Dangić, Chunyu Guo, Philip J. W. Moll, Maia G. Vergniory, Ion Errea

Abstract: The newly discovered family of non-magnetic Kagome metals AV$_3$Sb$_5$ (A=K,Rb,Cs) provides a unique platform for exploring the interplay between charge density wave (CDW) order, superconductivity, non-trivial topology, and spontaneous time-reversal symmetry breaking. Although characterizing the CDW phase is essential for understanding and modeling these exotic phenomena, its nature remains unreso… ▽ More The newly discovered family of non-magnetic Kagome metals AV$_3$Sb$_5$ (A=K,Rb,Cs) provides a unique platform for exploring the interplay between charge density wave (CDW) order, superconductivity, non-trivial topology, and spontaneous time-reversal symmetry breaking. Although characterizing the CDW phase is essential for understanding and modeling these exotic phenomena, its nature remains unresolved. In this work, we employ first-principles free-energy calculations, accounting for both ionic kinetic energy and anharmonic effects, to resolve the atomistic phase diagram of CsV$_3$Sb$_5$ and its charge ordering structure. Our results uncover that the CDW ground state is formed by reconstructed vanadium Kagome layers in a triangular hexagonal pattern, featuring energetically degenerate different stacking orders. This accounts for the various out-of-plane modulations observed experimentally and supports the coexistence of multiple domains. The discovered symmetry-broken ground state is consistent with the absence of any electronic anisotropy in transport experiments. By combining anharmonic phonons with the calculation of electron-phonon matrix elements, we predict a superconducting critical temperature for the CDW phase in agreement with experiments, showing that superconductivity is phonon mediated. These findings not only resolve a long-standing structural puzzle, but also clarify the impact of the CDW in superconductivity, highlighting its fundamental importance in shaping the low-temperature quantum phase diagram of Kagome metals. △ Less

Submitted 21 August, 2025; v1 submitted 26 May, 2025; originally announced May 2025.

Comments: 8 pages, 5 figures

Journal ref: Phys. Rev. Lett. 136, 206401 (2026)

Emergent chirality and enantiomeric selectivity in layered NbOX$_2$ crystals

Authors: Martin Gutierrez-Amigo, Claudia Felser, Ion Errea, Maia G. Vergniory

Abstract: The spontaneous emergence of chirality in crystalline solids has profound implications for electronic, optical, and topological properties, making the control of chiral phases a central challenge in materials design. Here, we investigate the structural and electronic properties of a new family of layered compounds, $\mathrm{NbOX_2}$, and explore the connection between their achiral $I m m m$ phase… ▽ More The spontaneous emergence of chirality in crystalline solids has profound implications for electronic, optical, and topological properties, making the control of chiral phases a central challenge in materials design. Here, we investigate the structural and electronic properties of a new family of layered compounds, $\mathrm{NbOX_2}$, and explore the connection between their achiral $I m m m$ phase and chiral $C 2$. Through first-principles calculations, we identify an intermediate achiral $C 2/m$ phase that bridges the high- and low-symmetry phases within a three-dimensional order parameter space. By analyzing the Born-Oppenheimer energy surfaces, we find that the shallow energy minima of the $C2/m$ phase suggest it may be stabilized either by external factors such as pressure, as demonstrated here, or by ionic quantum or thermal fluctuations and the resulting lattice anharmonicity. Additionally, we show how an external electric field, by breaking the necessary symmetries, biases the system toward a preferred chirality by lifting the energy degeneracy between the two enantiomers. This, combined with the small energy barrier between the enantiomers in the $C 2$ phase, enables handedness control and allows us to propose a mechanism for selective handedness stabilization by leveraging electric fields and pressure or temperature-dependent anharmonic effects. Our findings establish a framework for understanding chirality emergence in layered materials and offer a pathway for designing systems with tunable enantiomeric populations. △ Less

Submitted 14 May, 2026; v1 submitted 14 May, 2025; originally announced May 2025.

Comments: 16 pages, 10 figures

Journal ref: Phys. Rev. Lett. 136, 166605 (2026)

Long-range electron coherence in Kagome metals

Authors: Chunyu, Guo, Kaize Wang, Ling Zhang, Carsten Putzke, Dong Chen, Maarten R. van Delft, Steffen Wiedmann, Fedor F. Balakirev, Ross D. McDonald, Martin Gutierrez-Amigo, Manex Alkorta, Ion Errea, Maia G. Vergniory, Takashi Oka, Roderich Moessner, Mark H. Fischer, Titus Neupert, Claudia Felser, Philip J. W. Moll

Abstract: The wave-like nature of electrons lies at the core of quantum mechanics, distinguishing them from classical particles. Landmark experiments have revealed phase coherence of mobile electrons within solids, such as Aharonov-Bohm interference in mesoscopic rings. However, this coherence is typically limited by numerous environmental interactions. Controlling and ideally mitigating such decoherence re… ▽ More The wave-like nature of electrons lies at the core of quantum mechanics, distinguishing them from classical particles. Landmark experiments have revealed phase coherence of mobile electrons within solids, such as Aharonov-Bohm interference in mesoscopic rings. However, this coherence is typically limited by numerous environmental interactions. Controlling and ideally mitigating such decoherence remains a central challenge in condensed matter physics. Here, we report magnetoresistance oscillations in mesoscopic pillars of the Kagome metal CsV$_3$Sb$_5$ for fields applied parallel to the Kagome planes. Their periodicity is independent of materials parameters, simply given by the number of flux quanta $h/e$ threading between adjacent Kagome layers akin to an atomic-scale Aharonov-Bohm interferometer. Intriguingly they occur under conditions not favorable for typical interference in solids, at temperatures above 20 K and in micrometer-scale devices well exceeding the single-particle mean free path. Further, the oscillations exhibit non-analytic field-angle dependence and scale consistently with a broad range of key electronic responses in CsV$_3$Sb$_5$, pointing to a cooperative mechanism that establishes intrinsic coherence. Our findings provide new insights into the debated origin of correlated order in CsV$_3$Sb$_5$ and establish Kagome metals as a promising platform for interaction-stabilized long-range electron coherence - crucial for both fundamental studies and technological advancements in quantum interference in metallic systems. △ Less

Submitted 18 April, 2025; originally announced April 2025.

The Maximum $T_c$ of Conventional Superconductors at Ambient Pressure

Authors: Kun Gao, Tiago F. T. Cerqueira, Antonio Sanna, Yue-Wen Fang, Đorđe Dangić, Ion Errea, Hai-Chen Wang, Silvana Botti, Miguel A. L. Marques

Abstract: The theoretical maximum critical temperature ($T_c$) for conventional superconductors at ambient pressure remains a fundamental question in condensed matter physics. Through analysis of electron-phonon calculations for over 20,000 metals, we critically examine this question. We find that while hydride metals can exhibit maximum phonon frequencies of more than 5000 K, the crucial logarithmic averag… ▽ More The theoretical maximum critical temperature ($T_c$) for conventional superconductors at ambient pressure remains a fundamental question in condensed matter physics. Through analysis of electron-phonon calculations for over 20,000 metals, we critically examine this question. We find that while hydride metals can exhibit maximum phonon frequencies of more than 5000 K, the crucial logarithmic average frequency $ω_\text{log}$ rarely exceeds 1800 K. Our data reveals an inherent trade-off between $ω_\text{log}$ and the electron-phonon coupling constant $λ$, suggesting that the optimal Eliashberg function that maximizes $T_c$ is unphysical. Based on our calculations, we identify Li$_2$AgH$_6$ and its sibling Li$_2$AuH$_6$ as theoretical materials that likely approach the practical limit for conventional superconductivity at ambient pressure. Analysis of thermodynamic stability indicates that compounds with higher predicted $T_c$ values are increasingly unstable, making their synthesis challenging. While fundamental physical laws do not strictly limit $T_c$ to low-temperatures, our analysis suggests that achieving room-temperature conventional superconductivity at ambient pressure is extremely unlikely. △ Less

Submitted 25 February, 2025; originally announced February 2025.

Journal ref: Nat Commun 16, 8253 (2025)

From superconductivity to non-superconductivity in LiPdH: a first principle approach

Authors: Zahra Alizadeh, Yue-Wen Fang, Ion Errea, M. R. Mohammadizadeh

Abstract: The layered structure of LiPdH was theoretically suggested to be a superconductor as a result of its larger electron-phonon coupling constant compared to that of PdH. However, the experimental results reported contrary findings, with no trace of superconductivity. We study the electronic, vibrational, and superconducting properties of the ambient pressure tetragonal phase of LiPdH ($P4/mmm$) withi… ▽ More The layered structure of LiPdH was theoretically suggested to be a superconductor as a result of its larger electron-phonon coupling constant compared to that of PdH. However, the experimental results reported contrary findings, with no trace of superconductivity. We study the electronic, vibrational, and superconducting properties of the ambient pressure tetragonal phase of LiPdH ($P4/mmm$) within first principles density functional theory methods, both in the harmonic and anharmonic approximations for the lattice dynamics, and conclude that it does not show any superconducting behavior. High-pressure crystal structure prediction calculations indicate that no structural transition is expected to occur under pressure up to 100 GPa in LiPdH. Our theoretical calculations demonstrate that increasing pressure reduces the density of states at the Fermi surface and consequently weakens electron-phonon interactions, leading to a further suppression of the superconducting critical temperature. △ Less

Submitted 4 February, 2025; originally announced February 2025.

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

Authors: Francesco Belli, Eva Zurek, Ion Errea

Abstract: 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,… ▽ More 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. △ Less

Submitted 24 January, 2025; originally announced January 2025.

The Impact of Ionic Anharmonicity on Superconductivity in Metal-Stuffed B-C Clathrates

Authors: Wenbo Zhao, Ying Sun, Jiaxiang Li, Peng Yuan, Toshiaki Iitaka, Xin Zhong, Hefei Li, Yue-Wen Fang, Hanyu Liu, Ion Errea, Yu Xie

Abstract: Metal-stuffed B$-$C compounds with sodalite clathrate structure have captured increasing attention due to their predicted exceptional superconductivity above liquid nitrogen temperature at ambient pressure. However, by neglecting the quantum lattice anharmonicity, the existing studies may result in an incomplete understanding of such a lightweight system. Here, using state-of-the-art ab initio met… ▽ More Metal-stuffed B$-$C compounds with sodalite clathrate structure have captured increasing attention due to their predicted exceptional superconductivity above liquid nitrogen temperature at ambient pressure. However, by neglecting the quantum lattice anharmonicity, the existing studies may result in an incomplete understanding of such a lightweight system. Here, using state-of-the-art ab initio methods incorporating quantum effects and machine learning potentials, we revisit the properties of a series of $XY$$\text{B}_{6}\text{C}_{6}$ clathrates where $X$ and $Y$ are metals. Our findings show that ionic quantum and anharmonic effects can harden the $E_g$ and $E_u$ vibrational modes, enabling the dynamical stability of 15 materials previously considered unstable in the harmonic approximation, including materials with previously unreported ($XY$)$^{1+}$ state, which is demonstrated here to be crucial to reach high critical temperatures. Further calculations based on the anisotropic Migdal-Eliashberg equation demonstrate that the $T_\text{c}$ values for KRb$\text{B}_{6}\text{C}_{6}$ and Rb$\text{B}_{3}\text{C}_{3}$ among these stabilized compounds are 102 and 115 K at 0 and 15 GPa, respectively, both being higher than $T_\text{c}$ of 92 K of KPb$\text{B}_{6}\text{C}_{6}$ at the anharmonic level. These record-high $T_\text{c}$ values, surpassing liquid nitrogen temperatures, emphasize the importance of anharmonic effects in stabilizing B-C clathrates with large electron-phonon coupling strength and advancing the search for high-$T_\text{c}$ superconductivity at (near) ambient pressure. △ Less

Submitted 10 January, 2026; v1 submitted 21 January, 2025; originally announced January 2025.

Journal ref: npj Computational Materials volume 11, Article number: 347 (2025)

Broad Spectral Tuning of Ultra-Low Loss Polaritons in a van der Waals Crystal by Intercalation

Authors: Javier Taboada-Gutiérrez, Gonzalo Álvarez-Pérez, Jiahua Duan, Weiliang Ma, Kyle Crowley, Iván Prieto, Andrei Bylinkin, Marta Autore, Halyna Volkova, Kenta Kimura, Tsuyoshi Kimura, M. -H. Berger, Shaojuan Li, Qiaoliang Bao, Xuan P. A. Gao, Ion Errea, Alexey Nikitin, Rainer Hillenbrand, Javier Martín-Sánchez, Pablo Alonso-González

Abstract: Phonon polaritons (PhPs) -- light coupled to lattice vibrations -- in polar van der Waals (vdW) crystals are promising candidates for controlling the flow of energy at the nanoscale due to their strong field confinement, anisotropic propagation, and ultra-long lifetime in the picosecond range \cite{ref1,ref2,ref3,ref4,ref5}. However, the lack of tunability in their narrow and material-specific spe… ▽ More Phonon polaritons (PhPs) -- light coupled to lattice vibrations -- in polar van der Waals (vdW) crystals are promising candidates for controlling the flow of energy at the nanoscale due to their strong field confinement, anisotropic propagation, and ultra-long lifetime in the picosecond range \cite{ref1,ref2,ref3,ref4,ref5}. However, the lack of tunability in their narrow and material-specific spectral range -- the Reststrahlen Band (RB) -- severely limits their technological implementation. Here, we demonstrate that the intercalation of Na atoms in the vdW semiconductor $α$-V$_2$O$_5$ enables a broad spectral shift of RBs, and that the PhPs excited exhibit ultra-low losses (lifetime of $4 \pm 1$~ps), similar to PhPs in the non-intercalated crystal (lifetime of $6 \pm 1$ ps). We expect our intercalation method to be applicable to other vdW crystals, opening the door for the use of PhPs in broad spectral bands in the mid-infrared domain. △ Less

Submitted 15 January, 2025; originally announced January 2025.

Comments: 15 pages, 5 figures

Journal ref: Nature Materials 19, 964-968 (2020)

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

Authors: Pugeng Hou, Francesco Belli, Tiange Bi, Eva Zurek, Ion Errea

Abstract: 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… ▽ More 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. △ Less

Submitted 24 December, 2024; originally announced December 2024.

Comments: arXiv admin note: substantial text overlap with arXiv:2107.08669; text overlap with arXiv:2102.00072

ACM Class: J.2.9

Superconductivity in barium hydrides via incorporation of light elements

Authors: Yue-Wen Fang, Ion Errea

Abstract: Barium hydrides are of interest for their potential in both ionic conductivity and superconductivity. Recently, a superconducting hydride BaH$_{12}$ containing H$_2$ and H${_3}^{-1}$ molecular units was experimentally reported with a critical temperature $T_\text{c}$ of 20 K at 140 GPa [Nat Commun 12, 273 (2021)]. Herein, we combine ab initio methods with a rapid calculator of $T_\text{c}$ based o… ▽ More Barium hydrides are of interest for their potential in both ionic conductivity and superconductivity. Recently, a superconducting hydride BaH$_{12}$ containing H$_2$ and H${_3}^{-1}$ molecular units was experimentally reported with a critical temperature $T_\text{c}$ of 20 K at 140 GPa [Nat Commun 12, 273 (2021)]. Herein, we combine ab initio methods with a rapid calculator of $T_\text{c}$ based on the networking value model to predict that the introduction of light elements, such as Be, can effectively expand the structure diversity and structure space of barium hydrides. Although molecular hydrogen units are still widely present in thermodynamically stable and metastable crystal structures, we find that a metastable phase of BeBaH$_8$ shows a high $T_\text{c}$ of 49 K at 100 GPa, which is only 38 meV/atom above the thermodynamic stability energy. This BeBaH$_8$ remains dynamically stable at 15 GPa. Furthermore, our study shows that increasing pressure can further elevate $T_\text{c}$ beyond 100 K by enhancing the electron-phonon coupling constant. Our study proposes a feasible method for broadening the structural landscape in the exploration of superconducting phases of barium hydrides. △ Less

Submitted 5 September, 2025; v1 submitted 12 December, 2024; originally announced December 2024.

Comments: 9 pages and 5 figures in main text

Journal ref: Phys. Rev. B 112, 125204 (2025)

Ambient pressure high temperature superconductivity in RbPH$_3$ facilitated by ionic anharmonicity

Authors: Đorđe Dangić, Yue-Wen Fang, Tiago F. T. Cerqueira, Antonio Sanna, Miguel A. L. Marques, Ion Errea

Abstract: Recent predictions of metastable high-temperature hydride superconductors give hope that superconductivity at ambient conditions is within reach. In this work, we predict RbPH$_3$ as a new compound with a superconducting critical temperature around 100 K at ambient pressure, dynamically stabilized thanks to ionic quantum anharmonic effects. RbPH$_3$ is thermodynamically stable at 30 GPa in a perov… ▽ More Recent predictions of metastable high-temperature hydride superconductors give hope that superconductivity at ambient conditions is within reach. In this work, we predict RbPH$_3$ as a new compound with a superconducting critical temperature around 100 K at ambient pressure, dynamically stabilized thanks to ionic quantum anharmonic effects. RbPH$_3$ is thermodynamically stable at 30 GPa in a perovskite $Pm\bar{3}m$ phase, allowing its experimental synthesis at moderate pressures far from the megabar regime. With lowering pressure it is expected to transform to a $R3m$ phase that should stay dynamically stable thanks to quantum fluctuations down to ambient pressures. Both phases are metallic, with the $R3m$ phase having three distinct Fermi surfaces, composed mostly of states with phosphorus and hydrogen character. The structures are held together by strong P-H covalent bonds, resembling the pattern observed in the high-temperature superconducting H$_3$S, with extra electrons donated by rubidium. These results demonstrate that quantum ionic fluctuations, neglected thus far in high-throughput calculations, can stabilize at ambient pressure hydride superconductors with a high critical temperature. △ Less

Submitted 6 November, 2024; originally announced November 2024.

Lattice thermal conductivity in the anharmonic overdamped regime

Authors: Đorđe Dangić, Giovanni Caldarelli, Raffaello Bianco, Ivana Savić, Ion Errea

Abstract: In crystalline materials, low lattice thermal conductivity is often associated with strong anharmonicity, which can cause significant deviations from the expected Lorentzian lineshape of phonon spectral functions. These deviations, occurring in an overdamped regime, raise questions about the applicability of the Boltzmann transport equation. Furthermore, strong anharmonicity can trigger structural… ▽ More In crystalline materials, low lattice thermal conductivity is often associated with strong anharmonicity, which can cause significant deviations from the expected Lorentzian lineshape of phonon spectral functions. These deviations, occurring in an overdamped regime, raise questions about the applicability of the Boltzmann transport equation. Furthermore, strong anharmonicity can trigger structural phase transitions with temperature, which cannot be adequately described by the standard harmonic approximation. To address these challenges, we propose a novel approach for computing the lattice thermal conductivity. Our method combines the Green-Kubo linear response theory with the stochastic self-consistent harmonic approximation. The latter allows us to describe the temperature-dependent evolution of the crystal structure, including first- and second-order phase transitions, as well as the vibrational properties in highly anharmonic materials. The Green-Kubo method considers the entire lineshapes of phonon spectral functions in the calculation of the lattice thermal conductivity, thus eliminating the questionable use of phonon lifetimes in the overdamped regime, as well as naturally including coherent transport effects. Additionally, we extend our theory to model complex dynamical lattice thermal conductivity, enhancing our understanding of time-dependent thermoreflectance experiments. As a practical application, we employ this approach to calculate the lattice thermal conductivity of CsPbBr$_3$, a complex crystal known for its anomalous thermal transport behavior with a complex phase diagram. Our method is able to determine the thermal conductivity across different phases in good agreement with experiments. △ Less

Submitted 19 March, 2025; v1 submitted 17 October, 2024; originally announced October 2024.

Directionally Locked Heteroepitaxy with a Structurally Modulated van der Waals Material

Authors: Nitish Mathur, Guangming Cheng, Francesc Ballester, Gabrielle Carrel, Vincent M. Plisson, Fang Yuan, Jiangchang Zheng, Caiyun Chen, Scott B. Lee, Ratnadwip Singha, Sudipta Chatterjee, Kenji Watanabe, Takashi Taniguchi, Kenneth S. Burch, Berthold Jäck, Ion Errea, Maia G. Vergniory, Nan Yao, Sanfeng Wu, Leslie M. Schoop

Abstract: Precise orientation of symmetry-mismatched epilayers on van der Waals (vdW) substrates via heteroepitaxy has commonly been achieved through surface treatment processes to accommodate weak interlayer registry and bonding strength, thereby limiting the range of material combinations for heterostructure design. In this study, we investigate the influence of lattice instabilities in a structurally mod… ▽ More Precise orientation of symmetry-mismatched epilayers on van der Waals (vdW) substrates via heteroepitaxy has commonly been achieved through surface treatment processes to accommodate weak interlayer registry and bonding strength, thereby limiting the range of material combinations for heterostructure design. In this study, we investigate the influence of lattice instabilities in a structurally modulated vdW TaCo2Te2 substrate on the growth and alignment of a symmetry-mismatched bulk CoxTey epilayer using in situ heating in a transmission electron microscope (TEM). We show that a Peierls-like lattice instability occurs in TaCo2Te2 at a transition temperature of ~523 K, which was corroborated by phonon calculations. Post-heat-treated samples reveal a thermally induced surface diffusion process and the dominant lateral growth of the CoxTey epilayer on the TaCo2Te2 vdW layers, as observed in cross-sectional TEM images. Temperature-dependent selected area electron diffraction (SAED) patterns reveal that the quasi-vdW CoxTey/TaCo2Te2 heterointerface acquires directional locking by aligning larger interlayer lattice mismatch along the lattice instability axis of TaCo2Te2, while preserving a strong lattice matching along the orthogonal direction. This heterostructure exhibits precise interlayer registry with one-dimensional lattice incommensuration along the lattice instability axis, resulting from structural distortion to accommodate lattice-mismatch strain. Moreover, the interfacial reconstruction of TaCo2Te2 back to the distorted phase stabilizes the lattice-locking of the quasi-vdW heterointerface at elevated temperatures. These findings encourage the expansion of material diversity for designing and predicting novel multi-dimensional heterostructures by leveraging lattice instabilities to guide epitaxy. △ Less

Submitted 16 April, 2026; v1 submitted 29 September, 2024; originally announced September 2024.

Comments: 25 pages, 6 figures

Unveiling the Mechanism of Phonon-Polariton Damping in α-MoO_3

Authors: Javier Taboada-Gutiérrez, Yixi Zhou, Ana I. F. Tresguerres-Mata, Christian Lanza, Abel Martínez-Suárez, Gonzalo Álvarez-Pérez, Jiahua Duan, José Ignacio Martín, María Vélez, Iván Prieto, Adrien Bercher, Jérémie Teyssier, Ion Errea, Alexey Y. Nikitin, Javier Martín-Sánchez, Alexey B. Kuzmenko, Pablo Alonso-González

Abstract: Phonon polaritons (PhPs) (light coupled to lattice vibrations) in the highly anisotropic polar layered material molybdenum trioxide (α-MoO_3) are currently the focus of intense research efforts due to their extreme subwavelength field confinement, directional propagation and unprecedented low losses. Nevertheless, prior research has primarily concentrated on exploiting the squeezing and steering c… ▽ More Phonon polaritons (PhPs) (light coupled to lattice vibrations) in the highly anisotropic polar layered material molybdenum trioxide (α-MoO_3) are currently the focus of intense research efforts due to their extreme subwavelength field confinement, directional propagation and unprecedented low losses. Nevertheless, prior research has primarily concentrated on exploiting the squeezing and steering capabilities of α-MoO_3 PhPs, without inquiring much into the dominant microscopic mechanism that determines their long lifetimes, key for their implementation in nanophotonic applications. This study delves into the fundamental processes that govern PhP damping in α-MoO_3 by combining ab initio calculations with scattering-type scanning near-field optical microscopy (s-SNOM) and Fourier-transform infrared (FTIR) spectroscopy measurements across a broad temperature range (from 8 to 300 K). The remarkable agreement between our theoretical predictions and experimental observations allows us to identify third-order anharmonic phonon-phonon scattering as the main damping mechanism of α-MoO_3 PhPs. These findings shed light on the fundamental limits of low-loss PhPs, a crucial factor for assessing their implementation into nanophotonic devices. △ Less

Submitted 19 August, 2024; originally announced August 2024.

Comments: 24 pages, 4 figures

Searching Materials Space for Hydride Superconductors at Ambient Pressure

Authors: Tiago F. T. Cerqueira, Yue-Wen Fang, Ion Errea, Antonio Sanna, Miguel A. L. Marques

Abstract: We employed a machine-learning assisted approach to search for superconducting hydrides under ambient pressure within an extensive dataset comprising over 150 000 compounds. Our investigation yielded around 50 systems with transition temperatures surpassing 20 K, and some even reaching above 70 K. These compounds have very different crystal structures, with different dimensionality, chemical compo… ▽ More We employed a machine-learning assisted approach to search for superconducting hydrides under ambient pressure within an extensive dataset comprising over 150 000 compounds. Our investigation yielded around 50 systems with transition temperatures surpassing 20 K, and some even reaching above 70 K. These compounds have very different crystal structures, with different dimensionality, chemical composition, stoichiometry, and arrangement of the hydrogens. Interestingly, most of these systems displayed slight thermodynamic instability, implying that their synthesis would require conditions beyond ambient equilibrium. Moreover, we found a consistent chemical composition in the majority of these systems, which combines alkali or alkali-earth elements with noble metals. This observation suggests a promising avenue for future experimental investigations into high-temperature superconductivity within hydrides at ambient pressure. △ Less

Submitted 20 March, 2024; originally announced March 2024.

Molecularity: a fast and efficient criterion for probing superconductivity

Authors: Matías E. di Mauro, Benoît Braïda, Ion Errea, Trinidad Novoa, Julia Contreras-García

Abstract: We present an efficient criterion for probing the critical temperature of hydrogen based superconductors. We start by expanding the applicability of 3D descriptors of electron localization to superconducting states within the framework of superconducting DFT. We first apply this descriptor to a model system, the hydrogen chain, which allows to prove two main concepts: i) that the electron localiza… ▽ More We present an efficient criterion for probing the critical temperature of hydrogen based superconductors. We start by expanding the applicability of 3D descriptors of electron localization to superconducting states within the framework of superconducting DFT. We first apply this descriptor to a model system, the hydrogen chain, which allows to prove two main concepts: i) that the electron localization changes very little when the transition from the normal to the superconducting state takes place, i.e. that it can be described at the DFT level from the normal state; and ii) that the formation of molecules can be characterized within this theoretical framework, enabling to filter out systems with marked molecular character and hence with low potential to be good superconductors. These two ideas, are then exploited in real binary and ternary systems, showing i) that the bonding type can be characterized automatically; and ii) that this provides a new index which enables to feed machine learning algorithms for a better prediction of critical temperatures. Overall, this sets a grounded theoretical scenario for an automatic and efficient high-throughput of potential hydrogen based superconductors. △ Less

Submitted 12 March, 2024; originally announced March 2024.

Observation of Chiral Surface State in Superconducting NbGe$_2$

Authors: Mengyu Yao, Martin Gutierrez-Amigo, Subhajit Roychowdhury, Ion Errea, Alexander Fedorov, Vladimir N. Strocov, Maia G. Vergniory, Claudia Felser

Abstract: The interplay between topology and superconductivity in quantum materials harbors rich physics ripe for discovery. In this study, we investigate the topological properties and superconductivity of the nonsymmorphic chiral superconductor NbGe$_2$ using high-resolution angle-resolved pho-toemission spectroscopy (ARPES), transport measurements, and ab initio calculations. The ARPES data revealed exot… ▽ More The interplay between topology and superconductivity in quantum materials harbors rich physics ripe for discovery. In this study, we investigate the topological properties and superconductivity of the nonsymmorphic chiral superconductor NbGe$_2$ using high-resolution angle-resolved pho-toemission spectroscopy (ARPES), transport measurements, and ab initio calculations. The ARPES data revealed exotic chiral surface states on the (100) surface originating from the inherent chiral crystal structure. Supporting calculations indicate that NbGe$_2$ likely hosts elusive Weyl fermions in its bulk electronic structure. Furthermore, we uncovered the signatures of van Hove singularities that can enhance many-body interactions. Additionally, transport measurements demonstrated that NbGe$_2$ exhibits superconductivity below 2K. Overall, our comprehensive results provide the first concrete evidence that NbGe$_2$ is a promising platform for investigating the interplay between non-trivial band topology, possible Weyl fermions, van Hove singularities, and superconductivity in chiral quantum materials. △ Less

Submitted 4 April, 2024; v1 submitted 5 March, 2024; originally announced March 2024.

Phonon collapse and anharmonic melting of the 3D charge-density wave in kagome metals

Authors: Martin Gutierrez-Amigo, Ðorđe Dangić, Chunyu Guo, Claudia Felser, Philip J. W. Moll, Maia G. Vergniory, Ion Errea

Abstract: The charge-density wave (CDW) mechanism and resulting structure of the AV3Sb5 family of kagome metals has posed a puzzling challenge since their discovery four years ago. In fact, the lack of consensus on the origin and structure of the CDW hinders the understanding of the emerging phenomena. Here, by employing a non-perturbative treatment of anharmonicity from first-principles calculations, we re… ▽ More The charge-density wave (CDW) mechanism and resulting structure of the AV3Sb5 family of kagome metals has posed a puzzling challenge since their discovery four years ago. In fact, the lack of consensus on the origin and structure of the CDW hinders the understanding of the emerging phenomena. Here, by employing a non-perturbative treatment of anharmonicity from first-principles calculations, we reveal that the charge-density transition in CsV3Sb5 is driven by the large electron-phonon coupling of the material and that the melting of the CDW state is attributed to ionic entropy and lattice anharmonicity. The calculated transition temperature is in very good agreement with experiments, implying that soft mode physics are at the core of the charge-density wave transition. Contrary to the standard assumption associated with a pure kagome lattice, the CDW is essentially three-dimensional as it is triggered by an unstable phonon at the L point. The absence of involvement of phonons at the M point enables us to constrain the resulting symmetries to six possible space groups. The unusually large electron-phonon linewidth of the soft mode explains why inelastic scattering experiments did not observe any softened phonon. We foresee that large anharmonic effects are ubiquitous and could be fundamental to understand the observed phenomena also in other kagome families. △ Less

Submitted 13 May, 2026; v1 submitted 23 November, 2023; originally announced November 2023.

Comments: 8 pages, 5 figures

Journal ref: Communications Materials 5, 234 (2024)

Prediction of Ambient Pressure Conventional Superconductivity above 80K in Thermodynamically Stable Hydride Compounds

Authors: Antonio Sanna, Tiago F. T. Cerqueira, Yue-Wen Fang, Ion Errea, Alfred Ludwig, Miguel A. L. Marques

Abstract: The primary challenge in the field of high-temperature superconductivity in hydrides is to achieve a superconducting state at ambient pressure rather than the extreme pressures that have been required in experiments so far. Here, we propose a family of compounds, of composition Mg$_2$XH$_6$ with X$=$Rh, Ir, Pd, or Pt, that achieves this goal. These materials were identified by scrutinizing more th… ▽ More The primary challenge in the field of high-temperature superconductivity in hydrides is to achieve a superconducting state at ambient pressure rather than the extreme pressures that have been required in experiments so far. Here, we propose a family of compounds, of composition Mg$_2$XH$_6$ with X$=$Rh, Ir, Pd, or Pt, that achieves this goal. These materials were identified by scrutinizing more than a million compounds using a machine-learning accelerated high-throughput workflow. They are thermodynamically stable, indicating that they are serious candidates for experimental synthesis. We predict that their superconducting transition temperatures are in the range of 45-80K, or even above 100K with appropriate electron doping of the Pt compound. These results indicate that, although very rare, high-temperature superconductivity in thermodynamically stable hydrides is achievable at room pressure. △ Less

Submitted 10 October, 2023; originally announced October 2023.

Comments: 9 pages, 6 figures

Journal ref: npj Comput Mater 10, 44 (2024)

On the dynamical stability of copper-doped lead apatite

Authors: Sun-Woo Kim, Kang Wang, Siyu Chen, Lewis J. Conway, G. Lucian Pascut, Ion Errea, Chris J. Pickard, Bartomeu Monserrat

Abstract: The recent claim of room temperature superconductivity in a copper-doped lead apatite compound, called LK-99, has sparked remarkable interest and controversy. Subsequent experiments have largely failed to reproduce the claimed superconductivity, while theoretical works have identified multiple key features including strong electronic correlation, structural instabilities, and dopability constraint… ▽ More The recent claim of room temperature superconductivity in a copper-doped lead apatite compound, called LK-99, has sparked remarkable interest and controversy. Subsequent experiments have largely failed to reproduce the claimed superconductivity, while theoretical works have identified multiple key features including strong electronic correlation, structural instabilities, and dopability constraints. A puzzling claim of several recent theoretical studies is that both parent and copper-doped lead apatite structures are dynamically unstable at the harmonic level, questioning decades of experimental reports of the parent compound structures and the recently proposed copper-doped structures. In this work, we demonstrate that both parent and copper-doped lead apatite structures are dynamically stable at room temperature. Anharmonic phonon-phonon interactions play a key role in stabilizing some copper-doped phases, while most phases are largely stable even at the harmonic level. We also show that dynamical stability depends on both volume and correlation strength, suggesting controllable ways of exploring the copper-doped lead apatite structural phase diagram. Our results fully reconcile the theoretical description of the structures of both parent and copper-doped lead apatite with experiment. △ Less

Submitted 22 January, 2024; v1 submitted 20 September, 2023; originally announced September 2023.

Comments: 4 figures + Supplementary Information, published version

Journal ref: npj Computational Materials 10, 16 (2024)

Purely anharmonic charge-density wave in the 2D Dirac semimetal SnP

Authors: Martin Gutierrez-Amigo, Fang Yuan, Davide Campi, Leslie M. Schoop, Maia G. Vergniory, Ion Errea

Abstract: Charge density waves (CDWs) in two-dimensional (2D) materials have been a major focus of research in condensed matter physics for several decades due to their potential for quantum-based technologies. In particular, CDWs can induce a metal-insulator transition by coupling two Dirac fermions, resulting in the emergence of a topological phase. Following this idea, here we explore the behavior of thr… ▽ More Charge density waves (CDWs) in two-dimensional (2D) materials have been a major focus of research in condensed matter physics for several decades due to their potential for quantum-based technologies. In particular, CDWs can induce a metal-insulator transition by coupling two Dirac fermions, resulting in the emergence of a topological phase. Following this idea, here we explore the behavior of three different CDWs in a new 2D layered material, SnP, using both density functional theory calculations and experimental synthesis to study its stability. The layered structure of its bulk counterpart, Sn4P3, suggests that the structure can be synthesized down to the monolayer by exfoliation or chemical means. However, despite the stability of the bulk, the monolayer shows unstable phonons at Γ, K, and M points of the Brillouin zone, which lead to three possible charge-density-wave phases. All three CDWs lead to metastable insulating phases, with the one driven by the the active phonon in the K point being topologically non-trivial under strain. Strikingly, the ground-state structure is only revealed due to the presence of strong anharmonic effects. This, underscores the importance of studying CDWs beyond the conventional harmonic picture, where the system's ground state can be elucidated solely from the harmonic phonon spectra. △ Less

Submitted 13 May, 2026; v1 submitted 11 September, 2023; originally announced September 2023.

Journal ref: Physical Review B 109, 174112 (2024)

Electronic structure and lattice dynamics of 1T-VSe$_2$: origin of the 3D-CDW

Authors: Josu Diego, D. Subires, A. H. Said, D. A. Chaney, A. Korshunov, G. Garbarino, F. Diekmann, K. Mahatha, V. Pardo, J. Strempfer, Pablo J. Bereciartua Perez, S. Francoual, C. Popescu, M. Tallarida, J. Dai, Raffaello Bianco, Lorenzo Monacelli, Matteo Calandra, A. Bosak, Francesco Mauri, K. Rossnagel, Adolfo O. Fumega, Ion Errea, S. Blanco-Canosa

Abstract: In order to characterize in detail the charge density wave (CDW) transition of 1$T$-VSe$_2$, its electronic structure and lattice dynamics are comprehensively studied by means of x-ray diffraction, angle resolved photoemission (ARPES), diffuse and inelastic x-ray scattering (IXS), and state-of-the-art first principles density functional theory calculations. Resonant elastic x-ray scattering (REXS)… ▽ More In order to characterize in detail the charge density wave (CDW) transition of 1$T$-VSe$_2$, its electronic structure and lattice dynamics are comprehensively studied by means of x-ray diffraction, angle resolved photoemission (ARPES), diffuse and inelastic x-ray scattering (IXS), and state-of-the-art first principles density functional theory calculations. Resonant elastic x-ray scattering (REXS) does not show any resonant enhancement at either V or Se K-edges, indicating that the CDW peak describes a purely structural modulation of the electronic ordering. ARPES identifies (i) a pseudogap at T$>$T$_{CDW}$, which leads to a depletion of the density of states in the $ML-M'L'$ plane at T$<$T$_{CDW}$, and (ii) anomalies in the electronic dispersion reflecting a sizable impact of phonons on it. A diffuse scattering precursor, characteristic of soft phonons, is observed at room temperature (RT) and leads to the full collapse of the low-energy phonon ($ω_1$) with propagation vector (0.25 0 -0.3) r.l.u. We show that the frequency and linewidth of this mode are anisotropic in momentum space, reflecting the momentum dependence of the electron-phonon interaction (EPI), hence demonstrating that the origin of the CDW is, to a much larger extent, due to the momentum dependence EPI with a small contribution from nesting. The pressure dependence of the $ω_1$ soft mode remains nearly constant up to 13 GPa at RT, with only a modest softening before the transition to the high-pressure monoclinic $C2/m$ phase. The wide set of experimental data are well captured by our state-of-the art first-principles anharmonic calculations with the inclusion of van der Waals (vdW) corrections in the exchange-correlation functional. The description of the electronics and dynamics of VSe$_2$ reported here adds important pieces of information to the understanding of the electronic modulations of TMDs. △ Less

Submitted 28 July, 2023; originally announced July 2023.

Assessing the feasibility of near-ambient conditions superconductivity in the Lu-N-H system

Authors: Yue-Wen Fang, Ðorđe Dangić, Ion Errea

Abstract: The recent report of near-ambient superconductivity in nitrogen-doped lutetium hydrides (Lu-N-H) has generated a great interest. However, conflicting results have raised doubts regarding superconductivity. Here, we combine high-throughput crystal structure predictions with a fast predictor of the superconducting critical temperature ($T_c$) to shed light on the properties of Lu-N-H at 1 GPa. None… ▽ More The recent report of near-ambient superconductivity in nitrogen-doped lutetium hydrides (Lu-N-H) has generated a great interest. However, conflicting results have raised doubts regarding superconductivity. Here, we combine high-throughput crystal structure predictions with a fast predictor of the superconducting critical temperature ($T_c$) to shed light on the properties of Lu-N-H at 1 GPa. None of the predicted structures shows the potential to support high-temperature superconductivity and the inclusion of nitrogen favors the appearance of insulating phases. Despite the lack of near-ambient superconductivity, we consider alternative metastable templates and study their $T_c$ and dynamical stability including quantum anharmonic effects. The cubic Lu$_4$H$_{11}$N exhibits a high $T_c$ of 100 K at 20 GPa, a large increase compared to 30 K obtained in its parent LuH$_3$. Interestingly, it has a similar X-ray pattern to the experimentally observed one. The LaH$_{10}$-like LuH$_{10}$ and CaH$_6$-like LuH$_6$ become high-temperature superconductors at 175 GPa and 100 GPa, with $T_c$ of 286 K and 246 K, respectively. Our findings suggest that high-temperature superconductivity is not possible in stable phases at near-ambient pressure, but metastable high-$T_c$ templates exist at moderate and high pressures. △ Less

Submitted 20 July, 2023; originally announced July 2023.

Comments: 52 pages, 5 figures and 1 table in main text, 11 figures and 2 tables in Supplementary

Journal ref: Communications Materials 5, 61 (2024)

Reply to "Comment on 'Nontrivial Quantum Geometry and the Strength of Electron-Phonon Coupling', arXiv:2305.02340, J. Yu, C. J. Ciccarino, R. Bianco, I. Errea, P. Narang, B. A. Bernevig"

Authors: Jiabin Yu, Christopher J. Ciccarino, Raffaello Bianco, Ion Errea, Prineha Narang, B. Andrei Bernevig

Abstract: This is our reply to "Comment on 'Nontrivial Quantum Geometry and the Strength of Electron-Phonon Coupling', arXiv:2305.02340, J. Yu, C. J. Ciccarino, R. Bianco, I. Errea, P. Narang, B. A. Bernevig" by Prof. Pickett, which focuses on the MgB$_2$ part of our work. We show that the entirety of the criticism in Prof. Pickett's comment pertaining to our work (arXiv:2305.02340) is invalid. This is our reply to "Comment on 'Nontrivial Quantum Geometry and the Strength of Electron-Phonon Coupling', arXiv:2305.02340, J. Yu, C. J. Ciccarino, R. Bianco, I. Errea, P. Narang, B. A. Bernevig" by Prof. Pickett, which focuses on the MgB$_2$ part of our work. We show that the entirety of the criticism in Prof. Pickett's comment pertaining to our work (arXiv:2305.02340) is invalid. △ Less

Submitted 6 June, 2023; originally announced June 2023.

Comments: 3+3 pages, 1 figure

Distinct switching of chiral transport in the kagome metals KV$_3$Sb$_5$ and CsV$_3$Sb$_5$

Authors: Chunyu Guo, Maarten R. van Delft, Martin Gutierrez-Amigo, Dong Chen, Carsten Putzke, Glenn Wagner, Mark H. Fischer, Titus Neupert, Ion Errea, Maia G. Vergniory, Steffen Wiedmann, Claudia Felser, Philip J. W. Moll

Abstract: The kagome metals AV$_3$Sb$_5$ (A=K,Rb,Cs) present an ideal sandbox to study the interrelation between multiple coexisting correlated phases such as charge order and superconductivity. So far, no consensus on the microscopic nature of these states has been reached as the proposals struggle to explain all their exotic physical properties. Among these, field-switchable electric magneto-chiral anisot… ▽ More The kagome metals AV$_3$Sb$_5$ (A=K,Rb,Cs) present an ideal sandbox to study the interrelation between multiple coexisting correlated phases such as charge order and superconductivity. So far, no consensus on the microscopic nature of these states has been reached as the proposals struggle to explain all their exotic physical properties. Among these, field-switchable electric magneto-chiral anisotropy (eMChA) in CsV$_3$Sb$_5$ provides intriguing evidence for a rewindable electronic chirality, yet the other family members have not been likewise investigated. Here, we present a comparative study of magneto-chiral transport between CsV$_3$Sb$_5$ and KV$_3$Sb$_5$. Despite their similar electronic structure, KV$_3$Sb$_5$ displays negligible eMChA, if any, and with no field switchability. This is in stark contrast to the non-saturating eMChA in CsV$_3$Sb$_5$ even in high fields up to 35 T. In light of their similar band structures, the stark difference in eMChA suggests its origin in the correlated states. Clearly, the V kagome nets alone are not sufficient to describe the physics and the interactions with their environment are crucial in determining the nature of their low-temperature state. △ Less

Submitted 1 June, 2023; originally announced June 2023.

Journal ref: npj Quantum Materials, 9, 20 (2024)

Ab initio study of the structural, vibrational and optical properties of potential parent structures of nitrogen-doped lutetium hydride

Authors: Đorđe Dangić, Peio Garcia-Goiricelaya, Yue-Wen Fang, Julen Ibañez-Azpiroz, Ion Errea

Abstract: The recent report of near-ambient conditions superconductivity in a nitrogen-doped lutetium hydride has inspired a large number of experimental studies with contradictory results. We model from first principles the physical properties of the possible parent structures of the reported superconductor, LuH$_2$ and LuH$_3$. We show that only the phonon band structure of LuH$_3$ can explain the reporte… ▽ More The recent report of near-ambient conditions superconductivity in a nitrogen-doped lutetium hydride has inspired a large number of experimental studies with contradictory results. We model from first principles the physical properties of the possible parent structures of the reported superconductor, LuH$_2$ and LuH$_3$. We show that only the phonon band structure of LuH$_3$ can explain the reported Raman spectra due to the presence of hydrogens at the interstitial octahedral sites. However, this structure is stabilized by anharmonicity only above 6 GPa. We find that the intriguing color change with pressure in the reported superconductor is consistent with the optical properties of LuH$_2$, which are determined by the presence of an undamped interband plasmon. The plasmon blue-shifts with pressure and modifies the color of the sample without requiring any structural phase transition. Our findings suggest that the main component in the experiments is LuH$_2$ with some extra hydrogen atoms at octahedral sites. None of LuH$_2$ and LuH$_3$ superconduct at high temperatures. △ Less

Submitted 20 July, 2023; v1 submitted 11 May, 2023; originally announced May 2023.

Nontrivial Quantum Geometry and the Strength of Electron-Phonon Coupling

Authors: Jiabin Yu, Christopher J. Ciccarino, Raffaello Bianco, Ion Errea, Prineha Narang, B. Andrei Bernevig

Abstract: The coupling of electrons to phonons (electron-phonon coupling) is crucial for the existence of various phases of matter, in particular superconductivity and density waves. Here, we devise a theory that incorporates the quantum geometry of the electron bands into the electron-phonon coupling, demonstrating the crucial contributions of the Fubini-Study metric or its orbital selective version to the… ▽ More The coupling of electrons to phonons (electron-phonon coupling) is crucial for the existence of various phases of matter, in particular superconductivity and density waves. Here, we devise a theory that incorporates the quantum geometry of the electron bands into the electron-phonon coupling, demonstrating the crucial contributions of the Fubini-Study metric or its orbital selective version to the dimensionless electron-phonon coupling constant. We apply the theory to two materials, graphene and MgB$_2$ where the geometric contributions account for approximately 50\% and 90\% of the total electron-phonon coupling constant, respectively. The quantum geometric contributions in the two systems are further bounded from below by topological contributions. Our results suggest that the nontrivial electron band geometry/topology might favor superconductivity with relatively high critical temperature. △ Less

Submitted 27 May, 2024; v1 submitted 3 May, 2023; originally announced May 2023.

Comments: submitted on 04/14/2023; close to the published version

Journal ref: Nat. Phys. (2024)

Correlated order at the tipping point in the kagome metal CsV$_3$Sb$_5$

Authors: Chunyu Guo, Glenn Wagner, Carsten Putzke, Dong Chen, Kaize Wang, Ling Zhang, Martin Gutierrez-Amigo, Ion Errea, Maia G. Vergniory, Claudia Felser, Mark H. Fischer, Titus Neupert, Philip J. W. Moll

Abstract: Spontaneously broken symmetries are at the heart of many phenomena of quantum matter and physics more generally. However, determining the exact symmetries broken can be challenging due to imperfections such as strain, in particular when multiple electronic orders form complex interactions. This is exemplified by charge order in some kagome systems, which are speculated to show nematicity and flux… ▽ More Spontaneously broken symmetries are at the heart of many phenomena of quantum matter and physics more generally. However, determining the exact symmetries broken can be challenging due to imperfections such as strain, in particular when multiple electronic orders form complex interactions. This is exemplified by charge order in some kagome systems, which are speculated to show nematicity and flux order from orbital currents. We fabricated highly symmetric samples of a member of this family, CsV$_3$Sb$_5$, and measured their transport properties. We find the absence of measurable anisotropy at any temperature in the unperturbed material, however, a striking in-plane transport anisotropy appears when either weak magnetic fields or strains are present. A symmetry analysis indicates that a perpendicular magnetic field can indeed lead to in-plane anisotropy by inducing a flux order coexisting with more conventional bond order. Our results provide a unifying picture for the controversial charge order in kagome metals and highlight the need for microscopic materials control in the identification of broken symmetries. △ Less

Submitted 3 April, 2023; originally announced April 2023.

Journal ref: Nature Physics 20, 579 (2024)

Large impact of phonon lineshapes on the superconductivity of solid hydrogen

Authors: Đorđe Dangić, Lorenzo Monacelli, Raffaello Bianco, Francesco Mauri, Ion Errea

Abstract: Phonon anharmonicity plays a crucial role in determining the stability and vibrational properties of high-pressure hydrides. Furthermore, strong anharmonicity can render phonon quasiparticle picture obsolete questioning standard approaches for modeling superconductivity in these material systems. In this work, we show the effects of non-Lorentzian phonon lineshapes on the superconductivity of high… ▽ More Phonon anharmonicity plays a crucial role in determining the stability and vibrational properties of high-pressure hydrides. Furthermore, strong anharmonicity can render phonon quasiparticle picture obsolete questioning standard approaches for modeling superconductivity in these material systems. In this work, we show the effects of non-Lorentzian phonon lineshapes on the superconductivity of high-pressure solid hydrogen. We calculate the superconducting critical temperature T$_\mathrm{C}$ \emph{ab initio} considering the full phonon spectral function and show that it overall enhances the T$_\mathrm{C}$ estimate. The anharmonicity-induced phonon softening exhibited in spectral functions increases the estimate of the critical temperature, while the broadening of phonon lines due to phonon-phonon interaction decreases it. Our calculations also reveal that superconductivity emerges in hydrogen in the $Cmca-12$ molecular phase VI at pressures between 450 and 500 GPa and explain the disagreement between the previous theoretical results and experiments. △ Less

Submitted 22 March, 2024; v1 submitted 14 March, 2023; originally announced March 2023.

Non-perturbative theory of the electron-phonon coupling and its first-principles implementation

Authors: Raffaello Bianco, Ion Errea

Abstract: The harmonic approximation of ionic fluctuations and the linear coupling between phonons and electrons provide the standard framework to compute, from first principles, the contribution of nuclear dynamics and its interaction with electrons to materials properties. These approaches become questionable when quantum and anharmonic effects are significant, such as in hydrogenous systems, high-$T_c$ s… ▽ More The harmonic approximation of ionic fluctuations and the linear coupling between phonons and electrons provide the standard framework to compute, from first principles, the contribution of nuclear dynamics and its interaction with electrons to materials properties. These approaches become questionable when quantum and anharmonic effects are significant, such as in hydrogenous systems, high-$T_c$ superconductors, and systems close to displacive phase transitions. Here we propose a novel non-perturbative approach to compute the electron-phonon interaction from first principles, including non-linear effects and accounting for the quantum nature of nuclei. The method is based on the $GW^{ph}$ approximation for the electron self-energy, given by the nuclei-mediated electron-electron interaction $W^{ph}$ and the electron Green's function $G$. Electrons are treated at a mean-field level, while nuclear dynamics is described by a Gaussian distribution function that captures anharmonic effects, for example within the self-consistent harmonic approximation. The key quantities of the Gaussian $GW^{ph}$ self-energy are renormalized average vertices, computed in supercells using a stochastic approach based on self-consistent electronic potentials for distorted configurations. To validate the method, $GW^{ph}$ calculations are performed on aluminum, where the results reproduce standard linear electron-phonon theory, and on palladium hydride, where strong non-linear contributions emerge, with corrections comparable in magnitude to the linear-order result. △ Less

Submitted 3 March, 2026; v1 submitted 5 March, 2023; originally announced March 2023.

Superconductivity in the doped polymerized fullerite clathrate from first principles

Authors: Jorge Laranjeira, Ion Errea, Dorde Dangic, Leonel Marques, Manuel Melle-Franco, Karol Strutynski

Abstract: Superconductivity in the new polymerized C60 fullerite clathrate doped with simple metals was investigated through density functional theory methods. The phonon dispersion curves were systematically computed for the clathrate structures containing the guest dopants (Li, Na, K, Be, Mg, Ca, Al, Ga, Ge) in one of the two distinct cages, fullerene-like C60 and sodalite-like C24, exhibited by this stru… ▽ More Superconductivity in the new polymerized C60 fullerite clathrate doped with simple metals was investigated through density functional theory methods. The phonon dispersion curves were systematically computed for the clathrate structures containing the guest dopants (Li, Na, K, Be, Mg, Ca, Al, Ga, Ge) in one of the two distinct cages, fullerene-like C60 and sodalite-like C24, exhibited by this structure. Only four of these structures, Li@C24, Na@C24, Ga@C24 and Be@C60, are dynamically stable in the harmonic approximation. They all show superconducting behavior, although their critical temperatures are predicted to be below 2 K. △ Less

Submitted 20 February, 2023; v1 submitted 14 February, 2023; originally announced February 2023.

Excitonic Insulator to Superconductor Phase Transition in Ultra-Compressed Helium

Authors: Cong Liu, Ion Errea, Chris Pickard, Lewis J. Conway, Bartomeu Monserrat, Yue-Wen Fang, Chi Ding, Qing Lu, Jian Sun, Jordi Boronat, Claudio Cazorla

Abstract: Helium, the second most abundant element in the universe, exhibits an extremely large electronic band gap of about $20$ eV at low pressures ($\le 0.1$ GPa). While the metallization pressure of hcp helium has been accurately predicted, thus far little attention has been paid to the specific mechanisms driving the band-gap closure and electronic properties of this quantum crystal in the terapascal r… ▽ More Helium, the second most abundant element in the universe, exhibits an extremely large electronic band gap of about $20$ eV at low pressures ($\le 0.1$ GPa). While the metallization pressure of hcp helium has been accurately predicted, thus far little attention has been paid to the specific mechanisms driving the band-gap closure and electronic properties of this quantum crystal in the terapascal regime (1 TPa $= 1,000$ GPa). Here, we employ state-of-the-art density functional theory and many-body perturbation theory calculations to fill up this knowledge gap. It is found that prior to reaching metallicity bulk solid helium becomes an excitonic insulator (EI), an exotic state of matter typically observed in low-dimensional systems in which electrostatically bound electron-hole pairs form spontaneously. Furthermore, it is shown that electron-phonon coupling (EPC) is significantly enhanced across the EI to metal phase transition as signaled by prominent phonon softening and giant EPC strength values ($λ\sim 10-100$) estimated at specific reciprocal space points. Accordingly, we predict metallic helium to be a superconductor with a critical temperature of $\approx 30$ K at $20$ TPa and of $\approx 100$ K at $100$ TPa. These unforeseen phenomena have important consequences on the elastic, thermodynamic and transport properties of metallic helium hence may be critical for improving our fundamental understanding and modelling of celestial bodies. △ Less

Submitted 17 January, 2023; originally announced January 2023.

Comments: 25 pages, 18 figures

Topological phonon analysis of the 2D buckled honeycomb lattice: an application to real materials

Authors: Martin Gutierrez-Amigo, Maia G. Vergniory, Ion Errea, J. L. Mañes

Abstract: By means of group theory, topological quantum chemistry, first-principles and Monte Carlo calculations, we analyze the topology of the 2D buckled honeycomb lattice phonon spectra. Taking the pure crystal structure as an input, we show that eleven distinct phases are possible, five of which necessarily have non-trivial topology according to topological quantum chemistry. Another four of them are al… ▽ More By means of group theory, topological quantum chemistry, first-principles and Monte Carlo calculations, we analyze the topology of the 2D buckled honeycomb lattice phonon spectra. Taking the pure crystal structure as an input, we show that eleven distinct phases are possible, five of which necessarily have non-trivial topology according to topological quantum chemistry. Another four of them are also identified as topological using Wilson loops in an analytical model that includes all the symmetry allowed force constants up to third nearest neighbors, making a total of nine topological phases. We then compute the ab initio phonon spectra for the two-dimensional crystals of Si, Ge, P, As and Sb in this structure and construct its phase diagram. Despite the large proportion of topological phases found in the analytical model, all of the crystals lie in a trivial phase. By analyzing the force constants space using Monte Carlo calculations, we elucidate why topological phonon phases are physically difficult to realize in real materials with this crystal structure. △ Less

Submitted 13 May, 2026; v1 submitted 23 November, 2022; originally announced November 2022.

Journal ref: Physical Review B 107, 144307 (2023)

Anharmonicity reveals the tunability of the charge density wave orders in monolayer VSe$_2$

Authors: Adolfo O. Fumega, Josu Diego, V. Pardo, S. Blanco-Canosa, Ion Errea

Abstract: VSe$_2$ is a layered compound that has attracted great attention due to its proximity to a ferromagnetic state that is quenched by the presence of a charge density wave (CDW) phase. In the monolayer limit, unrelated experiments have reported different CDW orders with transition temperatures in the range of 130 to 220 K, making this monolayer very controversial. Here we perform first-principles non… ▽ More VSe$_2$ is a layered compound that has attracted great attention due to its proximity to a ferromagnetic state that is quenched by the presence of a charge density wave (CDW) phase. In the monolayer limit, unrelated experiments have reported different CDW orders with transition temperatures in the range of 130 to 220 K, making this monolayer very controversial. Here we perform first-principles non-perturbative anharmonic phonon calculations in monolayer VSe$_2$ in order to estimate the CDW order and the corresponding transition temperature. Our analysis solves previous experimental contradictions as we reveal that monolayer VSe$_2$ develops two independent charge density wave orders associated to $\sqrt{3} \times \sqrt{7}$ and $4 \times 4$ modulations that compete as a function of strain. In fact, tiny variations of only 1.5% in the lattice parameter are enough to stabilize one order or the other, which underlines that the CDW order becomes substrate-dependent. The predicted CDW temperature is strain-dependent and has a value of around 220 K, in good agreement with experiments. Moreover, we analyze the impact of external Lennard-Jones interactions on the CDW. We show that these can act together with the anharmonicity to suppress the CDW orders. In the particular case of monolayer VSe$_2$, this may give rise to the emergence of a ferromagnetic order. △ Less

Submitted 17 November, 2022; originally announced November 2022.

Comments: 8 pages, 4 figures, and supplemental material

Journal ref: Nano Letters, 23, 5, 1794-1800 (2023)

Ab initio study of metastable occupation of tetrahedral sites in palladium hydrides and its impact on superconductivity

Authors: Antonella Meninno, Ion Errea

Abstract: A recent experimental work on palladium hydrides suggested that metastable structures with hydrogen atoms occupying tetrahedral sites could lead to superconductivity above 50 K, a huge increase compared to the 10 K critical temperature of the stable structure with all hydrogen atoms occupying octahedral sites. By generating many structures with hydrogen atoms randomly occupying the octahedral and… ▽ More A recent experimental work on palladium hydrides suggested that metastable structures with hydrogen atoms occupying tetrahedral sites could lead to superconductivity above 50 K, a huge increase compared to the 10 K critical temperature of the stable structure with all hydrogen atoms occupying octahedral sites. By generating many structures with hydrogen atoms randomly occupying the octahedral and tetrahedral sites of the face-centered cubic lattice and calculating their energy at different theoretical levels from first principles, we determine that metastable structures with partial or full occupation of tetrahedral sites are possible, even when the ionic quantum zero-point energy and anharmonicity are included in the calculations. Anharmonicity is crucial in palladium hydrides when hydrogen atoms occupy octahedral sites and, in fact, makes the structure with full octahedral occupation the ground state. Despite the metastable existence of structures with full or partial tetrahedral sites occupation, the superconducting critical temperature is reduced with the number of tetrahedral sites occupied. Our calculations discard that the occupation of tetrahedral sites can increase the critical temperature in palladium hydrides. △ Less

Submitted 29 September, 2022; originally announced September 2022.

Evidence for ground state coherence in a two-dimensional Kondo lattice

Authors: Wen Wan, Rishav Harsh, Antonella Meninno, Paul Dreher, Sandra Sajan, Haojie Guo, Ion Errea, Fernando de Juan, Miguel M. Ugeda

Abstract: Kondo lattices are ideal testbeds for the exploration of heavy-fermion quantum phases of matter. While our understanding of Kondo lattices has traditionally relied on complex bulk f-electron systems, transition metal dichalcogenide heterobilayers have recently emerged as simple, accessible and tunable 2D Kondo lattice platforms where, however, their ground state remains to be established. Here we… ▽ More Kondo lattices are ideal testbeds for the exploration of heavy-fermion quantum phases of matter. While our understanding of Kondo lattices has traditionally relied on complex bulk f-electron systems, transition metal dichalcogenide heterobilayers have recently emerged as simple, accessible and tunable 2D Kondo lattice platforms where, however, their ground state remains to be established. Here we present evidence of a coherent ground state in the 1T/1H-TaSe2 heterobilayer by means of scanning tunneling microscopy/spectroscopy at 340 mK. Our measurements reveal the existence of two symmetric electronic resonances around the Fermi energy, a hallmark of coherence in the spin lattice. Spectroscopic imaging locates both resonances at the central Ta atom of the charge density wave of the 1T phase, where the localized magnetic moment is held. Furthermore, the evolution of the electronic structure with the magnetic field reveals a non-linear increase of the energy separation between the electronic resonances. Aided by ab initio and auxiliary-fermion mean-field calculations, we demonstrate that this behavior is inconsistent with a fully screened Kondo lattice, and suggests a ground state with magnetic order mediated by conduction electrons. The manifestation of magnetic coherence in TMD-based 2D Kondo lattices enables the exploration of magnetic quantum criticality, Kondo breakdown transitions and unconventional superconductivity in the strict two-dimensional limit. △ Less

Submitted 10 October, 2023; v1 submitted 30 June, 2022; originally announced July 2022.

Journal ref: Nature Communications 2023

Impact of ionic quantum fluctuations on the thermodynamic stability and superconductivity of LaBH$_8$

Authors: Francesco Belli, Ion Errea

Abstract: The recent prediction of a metastable high-symmetry Fm$\bar{3}$m phase of LaBH$_8$ gives hopes to reach high superconducting critical temperatures at affordable pressures among ternary hydrogen-rich compounds. Making use of first-principles calculations within density functional theory and the stochastic self-consistent harmonic approximation, we determine that ionic quantum fluctuations drive the… ▽ More The recent prediction of a metastable high-symmetry Fm$\bar{3}$m phase of LaBH$_8$ gives hopes to reach high superconducting critical temperatures at affordable pressures among ternary hydrogen-rich compounds. Making use of first-principles calculations within density functional theory and the stochastic self-consistent harmonic approximation, we determine that ionic quantum fluctuations drive the system dynamically unstable below 77 GPa, a much higher pressure than the 45 GPa expected classically. Quantum anharmonic effects stretch the covalent B-H bond in the BH$_8$ units of the structure and, consequently, soften all hydrogen-character modes. Above 77 GPa Fm$\bar{3}$m LaBH$_8$ remains metastable and, interestingly, its superconducting critical temperature is largely enhanced by quantum anharmonic effects, reaching critical temperatures around 170 K at the verge of the dynamical instability. Our results suggest that low pressure metastable phases with covalently bonded symmetric XH$_8$ units will be destabilized by ionic quantum fluctuations. △ Less

Submitted 15 June, 2022; originally announced June 2022.

Comments: 10 pages, 6 figures, 1 table

MSC Class: 82Dxx ACM Class: J.2

Absence of sizable superconductivity in hydrogen boride: A first principles study

Authors: Antonella Meninno, Ion Errea

Abstract: The recently synthesized hydrogen boride monolayer in the Cmmm phase is a promising super-conductor due to its similarity to MgB2 and the large hydrogen content in its structure. Making use of first-principles calculations based on density functional theory, we study its electronic, vibrational,and superconducting properties and conclude that, despite the expectations, hydrogen boride does not hav… ▽ More The recently synthesized hydrogen boride monolayer in the Cmmm phase is a promising super-conductor due to its similarity to MgB2 and the large hydrogen content in its structure. Making use of first-principles calculations based on density functional theory, we study its electronic, vibrational,and superconducting properties and conclude that, despite the expectations, hydrogen boride does not have a sizable superconducting critical temperature. The presence of hydrogen in the system alters the boron-boron bonding, weakening the electron-phonon interaction. We have studied the effect of enhancing the critical temperature by doping the system, but the inclusion of electrons or holes reveals ineffective. We attribute the small critical temperature of this system to the vanishing hydrogen character of the states at the Fermi level, which are dominated by boron p states. Our results determine that a large proportion of hydrogen-like states are needed at the Fermi level to attain a large superconducting critical temperature in hydrogenated monolayers. △ Less

Submitted 16 May, 2022; originally announced May 2022.