arXiv subjects

Benjamin Geisler

Publications and source records attributed to Benjamin Geisler.

At least 19 recordsLinked to original sources

arXiv2026

Delafossites as an unexpected competing phase to infinite-layer oxides

Motivated by the discovery of superconductivity in Sr-doped infinite-layer nickelate films on SrTiO$_3$(001), we explore the broader landscape of $AB$O$_2$ oxides through comprehensive high-throughput first-principles simulations. Specifically, delafossites and their ordered rock-salt (111) variants stand out as intriguing layered oxides that share the infinite-layer $AB$O$_2$ stoichiometry and simultaneously retain a perovskite-like octahedral motif. This positions them as a unique structural bridge between these two phases and as promising candidates for novel correlated electronic states. We compile a phase diagram that compares the relative stability of these four distinct oxides across the periodic table. Surprisingly, we find that the delafossite structure rivals the infinite-layer phase in thermodynamic stability for the nickelates, and even more for the recently suggested palladate and platinate analogs. Comparison of the respective electronic structures reveals that the delafossite compounds, which we find to be characterized by reversed cation order, exhibit a strongly $d_{z^2}$-dominated Fermi surface, in stark contrast to the $d_{x^2-y^2}$ character observed in the infinite-layer phases. Among all candidates, the La-Ni combination stands out as a thermodynamic optimum for stabilizing the infinite-layer motif. Furthermore, we show that hole doping via Ca, Sr, and Ba systematically enhances the stability of the infinite-layer phase in all three transition-metal families. These results reveal fundamental challenges in realizing bulk substrate-free infinite-layer oxides, and simultaneously offer guidance for future experimental synthesis efforts targeting novel superconducting compounds.

cond-mat.mtrl-sci↗

arXiv2026

Towards the discovery of high critical magnetic field superconductors

Superconducting materials are of significant technological relevance for a broad range of applications, and intense research efforts aim at enhancing the critical temperature $T_{c}$. Intriguingly, while numerous studies have explored different computational and machine-learning routes to predict $T_{c}$, the fundamental role of the critical magnetic field has so far been overlooked. Here we open a new frontier in superconductor discovery by presenting a consistent computational database of critical fields $H_{c}$, $H_{c1}$, and $H_{c2}$ for over 7300 electron-phonon-paired superconductors covering distinct materials classes. A theoretical framework is developed that combines $\alpha^2F(\omega)$ spectral functions and highly accurate Fermi surfaces from density functional theory with clean-limit Eliashberg theory to obtain the coherence lengths, London penetration depths, and Ginzburg-Landau parameters. We discover an unexpectedly large number of Type-I superconductors and show that larger unit cells generically support higher critical fields and Type-II behavior. We identify the importance of going beyond BCS theory by including strong-coupling corrections to the superconducting gap and electron-phonon renormalizations of the effective mass for predictions of critical fields across materials. These results provide a framework for foundational AI models that realize the concept of inverse materials design for high-$T_{c}$ and high-critical-field superconductors.

cond-mat.supr-con↗

arXiv2025

Guided Diffusion for the Discovery of New Superconductors

The inverse design of materials with specific desired properties, such as high-temperature superconductivity, represents a formidable challenge in materials science due to the vastness of chemical and structural space. We present a guided diffusion framework to accelerate the discovery of novel superconductors. A DiffCSP foundation model is pretrained on the Alexandria Database and fine-tuned on 7,183 superconductors with first principles derived labels. Employing classifier-free guidance, we sample 200,000 structures, which lead to 34,027 unique candidates. A multistage screening process that combines machine learning and density functional theory (DFT) calculations to assess stability and electronic properties, identifies 773 candidates with DFT-calculated $T_\mathrm{c}>5$ K. Notably, our generative model demonstrates effective property-driven design. Our computational findings were validated against experimental synthesis and characterization performed as part of this work, which highlighted challenges in sparsely charted chemistries. This end-to-end workflow accelerates superconductor discovery while underscoring the challenge of predicting and synthesizing experimentally realizable materials.

cond-mat.supr-con↗

arXiv2025

Developing a Complete AI-Accelerated Workflow for Superconductor Discovery

The quest to identify new superconducting materials with enhanced properties is hindered by the prohibitive cost of computing electron-phonon spectral functions, severely limiting the materials space that can be explored. Here, we introduce a Bootstrapped Ensemble of Equivariant Graph Neural Networks (BEE-NET), a machine-learning model trained to predict the Eliashberg spectral function and superconducting critical temperature with a mean-absolute-error of 0.87 K relative to DFT-based Allen-Dynes calculations. Intriguingly, BEE-NET achieves a true-negative-rate of 99.4\%, enabling highly efficient screening for the rare property of superconductivity. Integrated into a multi-stage, AI-accelerated discovery pipeline that incorporates elemental-substitution strategies and machine-learned interatomic potentials, our workflow reduced over 1.3 million candidate structures to 741 dynamically and thermodynamically stable compounds with DFT-confirmed $T_{\mathrm{c}} > 5$ K. We report the successful synthesis and experimental confirmation of superconductivity in two of these previously unreported compounds. This study establishes a data-driven framework that integrates machine learning, quantum calculations, and experiments to systematically accelerate superconductor discovery.

cond-mat.supr-con↗

arXiv2025

Electronic reconstruction and interface engineering of emergent spin fluctuations in compressively strained La$_3$Ni$_2$O$_7$ on SrLaAlO$_4$(001)

Motivated by the recent observation of ambient-pressure superconductivity with $T_c \sim 40$ K in La3Ni2O7 on SrLaAlO4(001) (SLAO), we explore the structural and electronic properties as well as the spin-spin correlation function of this bilayer nickelate system by using density functional theory including a Coulomb repulsion term. We find that the compressive strain exerted by this substrate leads to an unconventional occupation of the antibonding Ni $3d_{z^2}$ states around the $\Gamma$ point, distinct from the superconducting bulk compound under pressure. While pure strain effects rather modestly enhance the dynamical spin susceptibility, investigation of a reconstructed interface composition as observed in transmission electron microscopy uncovers a strong amplification of the spin fluctuations due to Fermi surface nesting of the antibonding Ni $3d_{z^2}$ states near the interface. These results provide insights into the emergence of superconductivity in strained La$_3$Ni$_2$O$_7$, suggest a possible key role of the interface, and highlight fundamental differences from the hydrostatic pressure scenario.

cond-mat.supr-con↗

arXiv2024

Fermi surface reconstruction and enhanced spin fluctuations in strained La$_3$Ni$_2$O$_{7}$ on LaAlO$_3$(001) and SrTiO$_3$(001)

We explore the structural and electronic properties of the bilayer nickelate La3Ni2O7 on LaAlO3(001) and SrTiO3(001) by using density functional theory including a Coulomb repulsion term. For La$_3$Ni$_2$O$_{7}$/LaAlO$_3$(001), we find that compressive strain and electron doping across the interface result in the unconventional occupation of the antibonding Ni $3d_{z^2}$ states. In sharp contrast, no charge transfer is observed for La$_3$Ni$_2$O$_{7}$/SrTiO$_3$(001). Surprisingly, tensile strain drives a metallization of the bonding Ni $3d_{z^2}$ states, rendering a Fermi surface topology akin to superconducting bulk La$_3$Ni$_2$O$_{7}$ under high pressure, yet with spin fluctuations enhanced considerably beyond pressure effects. Concomitantly, significant octahedral rotations are retained. We discuss the fundamental differences between hydrostatic pressure versus epitaxial strain and establish that strain provides a much stronger control over the Ni $e_g$ orbital polarization. The results suggest epitaxial La$_3$Ni$_2$O$_{7}$, particularly under tensile strain, as interesting system to provide novel insights into the physics of bilayer nickelates and possibly induce superconductivity without external pressure.

cond-mat.supr-con↗

arXiv2024

Accelerating superconductor discovery through tempered deep learning of the electron-phonon spectral function

Integrating deep learning with the search for new electron-phonon superconductors represents a burgeoning field of research, where the primary challenge lies in the computational intensity of calculating the electron-phonon spectral function, $\alpha^2F(\omega)$, the essential ingredient of Midgal-Eliashberg theory of superconductivity. To overcome this challenge, we adopt a two-step approach. First, we compute $\alpha^2F(\omega)$ for 818 dynamically stable materials. We then train a deep-learning model to predict $\alpha^2F(\omega)$, using an unconventional training strategy to temper the model's overfitting, enhancing predictions. Specifically, we train a Bootstrapped Ensemble of Tempered Equivariant graph neural NETworks (BETE-NET), obtaining an MAE of 0.21, 45 K, and 43 K for the Eliashberg moments derived from $\alpha^2F(\omega)$: $\lambda$, $\omega_{\log}$, and $\omega_{2}$, respectively, yielding an MAE of 2.5 K for the critical temperature, $T_c$. Further, we incorporate domain knowledge of the site-projected phonon density of states to impose inductive bias into the model's node attributes and enhance predictions. This methodological innovation decreases the MAE to 0.18, 29 K, and 28 K, respectively, yielding an MAE of 2.1 K for $T_c$. We illustrate the practical application of our model in high-throughput screening for high-$T_c$ materials. The model demonstrates an average precision nearly five times higher than random screening, highlighting the potential of ML in accelerating superconductor discovery. BETE-NET accelerates the search for high-$T_c$ superconductors while setting a precedent for applying ML in materials discovery, particularly when data is limited.

cond-mat.supr-con↗

arXiv2024

Optical properties and electronic correlations in La$_3$Ni$_2$O$_7$ bilayer nickelates under high pressure

We explore the optical properties of La3Ni2O7 bilayer nickelates by using density functional theory including a Coulomb repulsion term. Convincing agreement with recent experimental ambient-pressure spectra is achieved for U=3eV, which permits tracing the microscopic origin of the characteristic features. Simultaneous consistency with angle-resolved photoemission spectroscopy and x-ray diffraction suggests the notion of rather moderate electronic correlations in this novel high-Tc superconductor. Oxygen vacancies form predominantly at the inner apical sites and renormalize the optical spectrum quantitatively, while the released electrons are largely accommodated by a defect state. We show that the structural transition occurring under high pressure coincides with a significant enhancement of the Drude weight and a reduction of the out-of-plane interband contribution that act as a fingerprint of the emerging hole pocket. We further calculate the optical spectra for various possible magnetic phases including spin-density waves and discuss the results in the context of experiment. Finally, we investigate the role of the 2-2 versus 1-3 layer stacking and compare the bilayer nickelate to La4Ni3O10, La3Ni2O6, and NdNiO2, unveiling general trends in the optical spectrum as a function of the formal Ni valence in Ruddlesden-Popper versus reduced Ruddlesden-Popper nickelates.

cond-mat.supr-con↗

arXiv2023

Structural transitions, octahedral rotations, and electronic properties of $A_3$Ni$_2$O$_7$ rare-earth nickelates under high pressure

Motivated by the recent observation of superconductivity with $T_c \sim 80$ K in pressurized La3Ni2O7 [Nature 621, 493 (2023)], we explore the structural and electronic properties in A3Ni2O7 bilayer nickelates (A=La-Lu, Y, Sc) as a function of hydrostatic pressure (0-150 GPa) from first principles including a Coulomb repulsion term. At $\sim 20$ GPa, we observe an orthorhombic-to-tetragonal transition in La$_3$Ni$_2$O$_7$ at variance with recent x-ray diffraction data, which points to so-far unresolved complexities at the onset of superconductivity, e.g., charge doping by variations in the oxygen stoichiometry. We compile a structural phase diagram with particular emphasis on the $b/a$ ratio, octahedral anisotropy, and octahedral rotations. Intriguingly, chemical and external pressure emerge as two distinct and counteracting control parameters. We find unexpected correlations between $T_c$ and the in-plane Ni-O-Ni bond angles for La$_3$Ni$_2$O$_7$. Moreover, two novel structural phases with significant $c^+$ octahedral rotations and in-plane bond disproportionations are uncovered for A=Nd-Lu, Y, Sc that exhibit a surprising pressure-driven electronic reconstruction in the Ni $e_g$ manifold. By disentangling the involvement of basal versus apical oxygen states at the Fermi surface, we identify Tb$_3$Ni$_2$O$_7$ as an interesting candidate for superconductivity at ambient pressure. These results suggest a profound tunability of the structural and electronic phases in this novel materials class and are key for a fundamental understanding of the superconductivity mechanism.

cond-mat.supr-con↗

arXiv2023

Nature of the magnetic coupling in infinite-layer nickelates versus cuprates

In contrast to the cuprates, where the proximity of antiferromagnetism (AFM) and superconductivity is well established, first indications for AFM interactions in superconducting infinite-layer nickelates were only recently obtained. Here, we explore, based on first-principles simulations, the nature of the magnetic coupling in NdNiO2 as a function of the on-site Coulomb and exchange interaction, varying the explicit hole doping and the treatment of the Nd $4f$ electrons. The $U$-$J$ phase diagrams for undoped nickelates and cuprates indicate $G$-type ordering, yet show different $U$ dependency. By either Sr hole doping or explicit treatment of the Nd $4f$ electrons, we find a transition to a Ni $C$-type AFM ground state. We trace the effect of Sr doping back to a distinct accommodation of the holes by the Ni versus Cu $e_g$ orbitals. The interaction between Nd $4f$ and Ni $3d$ states stabilizes $C$-type AFM order on both sublattices. Though spin-orbit interactions induce a band splitting near the Fermi energy, the bad-metal state is retained even under epitaxial strain. These results establish the distinct role of the magnetic interactions in the nickelates versus the cuprates and suggest the former as a unique platform to investigate the relation to unconventional superconductivity.

cond-mat.supr-con↗

arXiv2023

Rashba spin-orbit coupling in infinite-layer nickelate films on SrTiO3(001) and KTaO3(001)

The impact of spin-orbit interactions in NdNiO2/SrTiO3(001) and NdNiO2/KTaO3(001) is explored by performing density functional theory simulations including a Coulomb repulsion term. Polarity mismatch drives the emergence of an interfacial two-dimensional electron gas in NdNiO2/KTaO3(001) involving the occupation of Ta $5d$ conduction-band states, which is twice as pronounced as in NdNiO2/SrTiO3(001). We identify a significant anisotropic $k^3$ Rashba spin splitting of the respective $d_{xy}$ states in both systems that results from the broken inversion symmetry at the nickelate-substrate interface and exceeds the width of the superconducting gap. In NdNiO2/KTaO3(001), the splitting reaches 210 meV, which is comparable to Bi(111) surface states. At the surface, the Ni $3d_{x^2-y^2}$-derived states exhibit a linear Rashba effect with $\alpha_\text{R} \sim$ 125 meV \r{A}, exemplifying its orbital selectivity. The corresponding Fermi sheets present a reconstructed circular shape due to the electrostatic doping, but undergo a Lifshitz transition towards a cuprate-like topology deeper in the film that coincides with a realignment of their spin texture. These results promote surface and interface polarity as interesting design parameters to control spin-orbit physics in infinite-layer nickelate heterostructures.

cond-mat.supr-con↗

arXiv2022

Oxygen vacancy formation and electronic reconstruction in strained LaNiO$_3$ and LaNiO$_3$/LaAlO$_3$ superlattices

By using DFT+U, we explore the formation of oxygen vacancies and their impact on the electronic and magnetic structure in strained bulk LaNiO3 and (LaNiO3)$_1$/(LaAlO3)$_1$(001) superlattices. For bulk LaNiO3, we find that epitaxial strain induces a substantial anisotropy in the oxygen vacancy formation energy. In particular, tensile strain promotes the selective reduction of apical oxygen, which may explain why the recently observed superconductivity of infinite-layer nickelates is limited to strained films. For (LaNiO3)$_1$/(LaAlO3)$_1$(001) superlattices, the simulations reveal that the NiO2 layer is most prone to vacancy formation, whereas the AlO2 layer exhibits generally the highest formation energies. The reduction is consistently endothermic, and a largely repulsive vacancy-vacancy interaction is identified as a function of the vacancy concentration. The released electrons are accommodated exclusively in the NiO2 layer, reducing the vacancy formation energy in the AlO2 layer by 70% with respect to bulk LaAlO3. By varying the vacancy concentration from 0% to 8.3% in the NiO2 layer at tensile strain, we observe an unexpected transition from a localized site-disproportionated (0.5%) to a delocalized (2.1%) charge accommodation, a re-entrant site disproportionation leading to a metal-to-insulator transition despite a half-filled majority-spin Ni $e_g$ manifold (4.2%), and finally a magnetic phase transition (8.3%). While a band gap of up to 0.5 eV opens at 4.2% for compressive strain, it is smaller for tensile strain or the system is metallic, which is in sharp contrast to the defect-free superlattice. The strong interplay of electronic reconstructions and structural modifications induced by oxygen vacancies in this system highlights the key role of an explicit supercell treatment and exemplifies the complex response to defects in artificial transition metal oxides.

cond-mat.supr-con↗

arXiv2022

Reconstructing the polar interface of infinite-layer nickelate thin films

Nickel-based superconductors provide a long-awaited experimental platform to explore possible cuprate-like superconductivity. Despite similar crystal structure and $d$ electron filling, these systems exhibit several differences. Nickelates are the most polar layered oxide superconductor, raising questions about the interface between substrate and thin film -- thus far the only sample geometry to successfully stabilize superconductivity. We conduct a detailed experimental and theoretical study of the prototypical interface between Nd$_{1-x}$Sr$_x$NiO$_2$ and SrTiO$_3$. Atomic-resolution electron energy loss spectroscopy in the scanning transmission electron microscope reveals the formation of a single intermediate Nd(Ti,Ni)O$_3$ layer. Density functional theory calculations with a Hubbard $U$ term show how the observed structure alleviates the strong polar discontinuity. We explore effects of oxygen occupancy, hole doping, and cation structure to disentangle the contributions of each for reducing interface charge density. Resolving the nontrivial interface structure will be instructive for future synthesis of nickelate films on other substrates and in vertical heterostructures.

cond-mat.supr-con↗

arXiv2021

Active learning and element embedding approach in neural networks for infinite-layer versus perovskite oxides

Combining density functional theory simulations and active learning of neural networks, we explore formation energies of oxygen vacancy layers, lattice parameters, and their correlations in infinite-layer versus perovskite oxides across the periodic table, and place the superconducting nickelate and cuprate families in a comprehensive statistical context. We show that neural networks predict these observables with high precision, using only 30-50% of the data for training. Element embedding autonomously identifies concepts of chemical similarity between the individual elements in line with human knowledge. Based on the fundamental concepts of entropy and information, active learning composes the training set by an optimal strategy without a priori knowledge and provides systematic control over the prediction accuracy. This offers key ingredients to considerably accelerate scans of large parameter spaces and exemplifies how artificial intelligence may assist on the quantum scale in finding novel materials with optimized properties.

cond-mat.supr-con↗

arXiv2021

Correlated interface electron gas in infinite-layer nickelate versus cuprate films on SrTiO$_3$(001)

Based on first-principles calculations including a Coulomb repulsion term, we identify trends in the electronic reconstruction of $A$NiO$_2$/SrTiO$_3$(001) ($A=$ Pr, La) and $A$CuO$_2$/SrTiO$_3$(001) ($A=$ Ca, Sr). Common to all cases is the emergence of a quasi-two-dimensional electron gas (q2DEG) in SrTiO$_3$(001), albeit the higher polarity mismatch at the interface of nickelates vs. cuprates to the nonpolar SrTiO$_3(001)$ substrate (${3+}/0$ vs. ${2+}/0$) results in an enhanced q2DEG carrier density. The simulations reveal a significant dependence of the interfacial Ti $3d_{xy}$ band bending on the rare-earth ion in the nickelate films, being $20$-$30\%$ larger for PrNiO$_2$ and NdNiO$_2$ than for LaNiO$_2$. Contrary to expectations from the formal polarity mismatch, the electrostatic doping in the films is twice as strong in cuprates as in nickelates. We demonstrate that the depletion of the self-doping rare-earth $5d$ states enhances the similarity of nickelate and cuprate Fermi surfaces in film geometry, reflecting a single hole in the Ni and Cu $3d_{x^2-y^2}$ orbitals. Finally, we show that NdNiO$_2$ films grown on a polar NdGaO$_3(001)$ substrate feature a simultaneous suppression of q2DEG formation as well as Nd~$5d$ self-doping.

cond-mat.supr-con↗

arXiv2020

A statistical model to assess risk for supporting SARS-CoV-2 quarantine decisions

In February 2020 the first human infection with SARS-CoV-2 was reported in Germany. Since then the local public health offices have been responsible to monitor and react to the dynamics of the pandemic. One of their major tasks is to contain the spread of the virus after potential spreading events, for example when one or multiple participants have a positive test result after a group meeting (e.g. at school, at a sports event or at work). In this case, contacts of the infected person have to be traced and potentially are quarantined (at home) for a period of time. When all relevant contact persons obtain a negative polymerase chain reaction (PCR) test result, the quarantine may be stopped. However, tracing and testing of all contacts is time-consuming, costly and (thus) not always feasible. This motivates our work, in which we present a statistical model for the probability that no transmission of Sars-CoV-2 occurred given an arbitrary number of test results at potentially different timepoints. Hereby, the time-dependent sensitivity and specificity of the conducted PCR test are taken in account. We employ a parametric Bayesian model which can be adopted to different situations when specific prior knowledge is available. This is illustrated for group events in German school classes and applied to exemplary real-world data from this context. Our approach has the potential to support important quarantine decisions with the goal to achieve a better balance between necessary containment of the pandemic and preservation of social and economic life. The focus of future work should be on further refinement and evaluation of quarantine decisions based on our statistical model.

stat.AP↗

arXiv2020

Competition of defect ordering and site disproportionation in strained LaCoO$_{3}$ on SrTiO$_3$(001)

The origin of the $3 \times 1$ reconstruction observed in epitaxial LaCoO$_{3}$ films on SrTiO$_3(001)$ is assessed by using first-principles calculations including a Coulomb repulsion term. We compile a phase diagram as a function of the oxygen pressure, which shows that ($3 \times 1$)-ordered oxygen vacancies (LaCoO$_{2.67}$) are favored under commonly used growth conditions, while stoichiometric films emerge under oxygen-rich conditions. Growth of further reduced LaCoO$_{2.5}$ brownmillerite films is impeded by phase separation. We report two competing ground-state candidates for stoichiometric films: a semimetallic phase with $3 \times 1$ low-spin/intermediate-spin/intermediate-spin magnetic order and a semiconducting phase with intermediate-spin magnetic order. This demonstrates that tensile strain induces ferromagnetism even in the absence of oxygen vacancies. Both phases exhibit an intriguing ($3 \times 1$)-reconstructed octahedral rotation pattern and accordingly modulated La-La distances. In particular, charge and bond disproportionation and concomitant orbital order of the $t_{2g}$ hole emerge at the Co sites that are also observed for unstrained bulk LaCoO$_3$ in the intermediate-spin state and explain structural data obtained by x-ray diffraction at elevated temperature. Site disproportionation drives a metal-to-semiconductor transition that reconciles the intermediate-spin state with the experimentally observed low conductivity during spin-state crossover without Jahn-Teller distortions.

cond-mat.mtrl-sci↗

arXiv2020

Fundamental difference in the electronic reconstruction of infinite-layer vs. perovskite neodymium nickelate films on SrTiO$_3$(001)

Motivated by recent reports of superconductivity in Sr-doped NdNiO$_2$ films on SrTiO$_3$(001) [Nature (London) 572, 624 (2019)], we explore the role of the polar interface on the structural and electronic properties of NdNiO$_n$/SrTiO$_3$(001) ($n=2,3$) by performing first-principles calculations including a Coulomb repulsion term. For infinite-layer nickelate films ($n=2$), electronic reconstruction drives the surprising emergence of a two-dimensional electron gas (2DEG) at the interface involving a strong occupation of the Ti $3d$ states. This effect is more pronounced than in LaAlO$_3$/SrTiO$_3$(001) and accompanied by a substantial reconstruction of the Fermi surface: a depletion of the self-doping Nd $5d$ states and an enhanced Ni $e_g$ orbital polarization reaching up to $35\%$ at the surface, reflecting a single hole in the $3d_{x^2-y^2}$ states, i.e., cuprate-like behavior. In contrast, no 2DEG forms for perovskite films ($n=3$) or if a single perovskite layer persists at the interface. We show that the topotactic reaction from the perovskite to the infinite-layer phase is confined to the nickelate film, whereas the SrTiO$_3$ substrate remains intact.

cond-mat.supr-con↗