arXiv subjects

Geoffroy Hautier

Publications and source records attributed to Geoffroy Hautier.

At least 19 recordsLinked to original sources

arXiv2026

Evaluating the Structural Basis for Polar Altermagnet Candidate Ca$_{3}$(Ru,Ti)$_{2}$O$_{7}$

The interplay between polar and altermagnetic orders remains largely unexplored in the broader landscape of correlated electron systems. Ca$_{3}$Ru$_{2}$O$_{7}$ has been proposed by density functional theory (DFT) as a polar altermagnet, reliant on the transformation of experimentally reported $Bb2_{1}m$ phase to a lower symmetry $Pn2_{1}a$ structure. Here, we perform a targeted search for the $Pn2_{1}a$ phase using synchrotron X-ray diffraction on single crystals of Ca$_{3}$Ru$_{2}$O$_{7}$ and Ca$_{3}$(Ru$_{0.99}$Ti$_{0.01}$)$_{2}$O$_{7}$. No diffraction signature of the $Pn2_{1}a$ structure is detected down to 20 K within experimental limits of $\sim$60-200 fm atomic displacements, significantly smaller than the DFT prediction of $\sim$1 pm. Combined with recent nonlinear transport measurements, our structural study suggests Ca$_{3}$Ru$_{2}$O$_{7}$ as a unique system where strong electron correlations drive an electronic phase transition without any measurable lattice symmetry change. With Ti substitution exceeding $\sim$3%, a chemically tunable altermagnetic phase with $Bb2_{1}m$ structure emerges. The study highlights the importance of sub-picometer metrology towards de-convolving structural versus electronic origins of altermagnets.

cond-mat.str-el↗

arXiv2026

Short-range order in the CoCrFeMnNi high-entropy alloy from cluster expansion

We investigate the short-range order (SRO) and phase stability of the equiatomic CoCrFeMnNi high-entropy alloy using cluster expansion supplemented by an eigen-decomposition analysis of the SRO parameters. Our results reveal that the primary ordering behavior is determined by strong Cr-Cr repulsive interactions, complemented by attractive heteroatomic Cr-$X$ pairs in the first nearest-neighbor shell. This chemical affinity is consistent with the emergence of ordered local environments and appears to be a major contributor to the primary order-disorder transition. At lower temperatures, the spectral SRO analysis suggests two additional lower-temperature instabilities: a collective ordering instability and an Fe-rich local clustering instability.

cond-mat.mtrl-sci↗

arXiv2026

Thin film synthesis of SrZn2P2 with SrI2 post-annealing for enhanced crystallinity and optoelectronic quality

Ternary Zintl phosphides are promising light-absorbing semiconductors for thin-film optoelectronic applications, but strategies for controlling their microstructure and optoelectronic quality remain underexplored. Here, we report the synthesis of phase-pure SrZn2P2 thin films using radio-frequency co-sputtering in a PH3 + Ar atmosphere and investigate the impact of post-growth processing on their structural and optical properties. Grazing-incidence X-ray scattering and Raman spectroscopy confirm the formation of crystalline SrZn2P2 films over a finite compositional window. Optical measurements reveal strong absorption near the direct-band-gap energy (~1.8 eV) and near-band-edge photoluminescence. Further, we have studied the effects of chemically compatible halide-assisted annealing. It is found that SrI2 treatments lead to pronounced grain growth and reduced diffraction peak broadening while preserving phase purity, in contrast to rapid thermal or forming-gas annealing. Notably, annealing with SrI2 at 450 {\deg}C significantly enhances both the intensity and spatial uniformity of the photoluminescence, thus connecting the observed microstructural consolidation with improved radiative recombination. Our study demonstrates that halide-assisted annealing provides an effective pathway for microstructural control in SrZn2P2 thin films and highlights a generalizable processing strategy for advancing Zintl phosphide semiconductors toward optoelectronic applications.

cond-mat.mtrl-sci↗

arXiv2026

Long photoexcited carrier lifetime in a stable and earth-abundant zinc polyphosphide

Halide perovskites have revolutionized optoelectronics by demonstrating that long carrier lifetime can be achieved in materials processed in relatively uncontrolled environments, whereas conventional inorganic semiconductors typically suffer from short carrier lifetime unless very carefully prepared and postprocessed. Here, we report the discovery of exceptionally long photoexcited carrier lifetime in monoclinic ZnP2, effectively bridging the carrier lifetime gap between direct-gap inorganic semiconductors and halide perovskites. Through computational screening, ZnP2 is identified as a long carrier lifetime semiconductor characterized by an unconventional polyphosphide bonding, combining covalently bonded phosphorus chains and polar-covalent Zn-P tetrahedra. Experimentally, ZnP2 crystals synthesized from low-purity precursors exhibit bright band-to-band photoluminescence at 1.49 eV and carrier lifetimes of up to 1 $\mu$s. Further analysis reveals that the polyphosphide bonding of ZnP2 suppresses the formation of deep intrinsic defects, making it defect resistant. Combined with its remarkable environmental stability, ZnP2 presents a highly promising material for solar absorbers and light emitters. Our work illustrates that underexplored inorganic materials spaces with unusual chemical bonding hold great promise for discovering novel optoelectronic materials.

cond-mat.mtrl-sci↗

arXiv2026

Discovery of a new weberite-type antiferroelectric: La3NbO7

Antiferroelectrics are antipolar materials which possess an electric field-induced phase transition to a polar, ferroelectric phase and offer significant potential for sensing/actuation and energy-storage applications. Known antiferroelectrics are relatively scarce and mainly based on a limited set of perovskite materials and their alloys (e.g., PbZrO$_3$, AgNbO$_3$, NaNbO$_3$). Here, a new family of lead-free, weberite-type antiferroelectrics, identified through a large-scale, first-principles computational search is introduced. The screening methodology, which connects lattice dynamics to antipolar distortions, predicted that La$_3$NbO$_7$ could exhibit antiferroelectricity. We confirm the prediction through the synthesis and characterization of epitaxial La$_3$NbO$_7$ thin films, which display the signature double hysteresis loops of an antiferroelectric material as well as clear evidence of an antipolar ground state structure from transmission electron microscopy. The antiferroelectricity in La$_3$NbO$_7$ is simpler than most known antiferroelectrics and can be explained by a Kittel-type mechanism involving the movement of niobium atoms in an oxygen octahedron through a single phonon mode which results in a smaller change in the volume during the field-induced phase transition. Additionally, it is found that La$_3$NbO$_7$ combines a high threshold field with a high breakdown field ($\approx$ 6MV/cm) - which opens up opportunities for energy-storage applications. This new weberite-type family of materials offers many opportunities to tune electrical and temperature response especially through substitutions on the rare-earth site. Ultimately, this work demonstrates a successful data-driven theory-to-experiment discovery of an entirely new family of antiferroelectrics and provides a blueprint for the future design of ferroic materials.

cond-mat.mtrl-sci↗

arXiv2025

Silicon T centre hyperfine structure and memory protection schemes

Combining the long-coherence of spin qubits and the capability to transmit information and entanglement through photons, spin-photon interfaces (SPIs) are a promising platform for networked quantum computation and long-distance quantum communication. SPIs that possess local `memory' qubits in addition to the optically coupled `communication' qubit can improve remote entanglement fidelities through brokered entanglement schemes and entanglement purification. In these schemes, it is critical to protect the memory qubit from decoherence during entanglement operations on the communications qubit. Silicon, a platform with mature microelectronic and nanophotonic fabrication, is host to the T centre, an SPI with emission in the telecommunications O-band that directly integrates with silicon nanophotonics. Cavity-coupled T centres are a platform for brokered entanglement distribution in silicon photonic circuits and over long-distance optical fibre links. The T centre's electron and nuclear spin qubits are an intrinsic register of communication and memory qubits respectively, with anisotropic hyperfine coupling. In this work we determine the T centre's hydrogen hyperfine coupling tensor. We also introduce schemes to protect against dephasing or eliminate relaxation of the T centre's hydrogen memory qubit during optical excitation. These results address a key challenge for practical T centre quantum networks.

quant-ph↗

arXiv2025

On The Finetuning of MLIPs Through the Lens of Iterated Maps With BPTT

Accurate structural relaxation is critical for advanced materials design. Traditional approaches built on physics-derived first-principles calculations are computationally expensive, motivating the creation of machine-learning interatomic potentials (MLIPs), which strive to faithfully reproduce first-principles computed forces. We propose a fine-tuning method to be used on a pretrained MLIP in which we create a fully-differentiable end-to-end simulation loop that optimizes the predicted final structures directly. Trajectories are unrolled and gradients are tracked through the entire relaxation. We show that this method consistently improves performance across all evaluated pretrained models; resulting in an average of roughly 32% reduction in prediction error. Interestingly, we show the process is robust to substantial variation in the relaxation setup, achieving negligibly different results across varied hyperparameter and procedural modifications.

cond-mat.mtrl-sci↗

arXiv2025

Alloying to Tune the Bandgap of the AM2Pn2 Zintl Compounds

The AM2Pn2 Zintl compounds are a large class of semiconductor materials that have a wide range of bandgaps and are mostly stable in the same crystal structure. Representative compounds BaCd2P2 and CaZn2P2 have recently been found to exhibit high visible light absorption and long carrier lifetime. Here we use high throughput first-principles calculations to study AM2Pn2 alloys for applications as tandem top cell absorbers (i.e., bandgaps around 1.8 eV) and far infrared detector materials (i.e., bandgaps lower than 0.5 eV). Using a first-principles computational screening workflow for assessing stability and electronic structure of alloys, we identify several promising candidates. These include Ca(Cd0.8Mg0.2)2P2 with a suitable direct bandgap for use in tandem top cells on silicon bottom cells and SrCd2(Sb1-xBix)2 for far infrared detectors. We demonstrate that alloys of AM2Pn2 materials can be realized by experimentally synthesizing Ca(Zn0.8Mg0.2)2P2.

cond-mat.mtrl-sci↗

arXiv2025

ADAPT: Lightweight, Long-Range Machine Learning Force Fields Without Graphs

Point defects play a central role in driving the properties of materials. First-principles methods are widely used to compute defect energetics and structures, including at scale for high-throughput defect databases. However, these methods are computationally expensive, making machine-learning force fields (MLFFs) an attractive alternative for accelerating structural relaxations. Most existing MLFFs are based on graph neural networks (GNNs), which can suffer from oversmoothing and poor representation of long-range interactions. Both of these issues are especially of concern when modeling point defects. To address these challenges, we introduce the Accelerated Deep Atomic Potential Transformer (ADAPT), an MLFF that replaces graph representations with a direct coordinates-in-space formulation and explicitly considers all pairwise atomic interactions. Atoms are treated as tokens, with a Transformer encoder modeling their interactions. Applied to a dataset of silicon point defects, ADAPT achieves a roughly 33 percent reduction in both force and energy prediction errors relative to a state-of-the-art GNN-based model, while requiring only a fraction of the computational cost.

cs.LG↗

arXiv2025

First principles and scanning tunneling spectroscopical evidences for thermodynamically stable "on-top" sulfur divacancy in monolayer WS$_{2}$

Chalcogen vacancies in monolayer transition metal dichalcogenides (TMDs), such as WS$_{2}$, play a crucial role in various applications ranging from optoelectronics and catalysis to quantum information science (QIS), making their identification and control essential. This study focuses on WS$_{2}$ single vacancy and vacancy pairs. Using first principles computations, we investigate their thermodynamic stabilities and electronic structures. We identify an "on-top" divacancy configuration where two vacancies sit on top of each other to be the only energetically stable complex with a binding energy of 160 meV. We compute a small difference in electronic structure with a shift of the unoccupied state by 140 meV for the divacancy complex and observe electronic state shift during Scanning Tunneling Spectroscopy of a series of vacancy in WS$_2$ providing spectroscopical evidence for the presence of this defect.

cond-mat.mtrl-sci↗

arXiv2025

CaCd$_2$P$_2$: A Visible-Light Absorbing Zintl Phosphide Stable under Photoelectrochemical Water Oxidation

A key bottleneck to solar fuels is the absence of stable and strongly absorbing photoelectrode materials for the oxygen evolution reaction (OER). Modern approaches generally trade off between stable but weakly absorbing materials, such as wide bandgap oxides, or strongly absorbing materials that rely on encapsulation for stability and are weakly catalytic, such as the III-V family of semiconductors. Of interest are materials like transition metal phosphides, such as FeP$_2$, that are known to undergo beneficial in situ surface transformations in the oxidative environment of OER, though stability has remained a primary hurdle. Here we report on CaCd$_2$P$_2$, a Zintl phase visible-light absorber with favorable 1.6 eV bandgap, that we identified using high-throughput computational screening. Using a combination of photoelectrochemical measurements, microscopy, and spectroscopy, we show that CaCd$_2$P$_2$ undergoes a light-stabilized surface transformation that renders it stable under alkaline OER conditions. We also show that the well known OER catalyst CoPi can act as a stable co-catalyst in synergy with the \textit{in-situ} CaCd$_2$P$_2$ surface. The light-induced stabilization that CaCd$_2$P$_2$ displays is in sharp contrast to the photocorrosion commonly observed in visible light-absorbing photoelectrodes. The broader AM$_2$P$_2$ family of Zintl phases offers a significant opportunity to explore stabilizing interface chemistry and re-design the manner in which low-bandgap semiconductors are used for photoelectrochemical energy conversion.

cond-mat.mtrl-sci↗

arXiv2025

Direct determination of antiferroelectric-to-ferroelectric phase transition pathways in PbZrO$_3$ with Operando Electron Microscopy

Under a sufficiently high applied electric field, a non-polar antiferroelectric material, such as \ce{PbZrO3}, can undergo a rapid transformation to a polar ferroelectric phase. While this behavior is promising for energy storage and electromechanical applications, a complete understanding of the atomic-scale mechanisms governing the phase transition remain elusive. Here, we employ \textit{operando} scanning transmission electron microscopy electric field biasing to directly resolve the antiferroelectric-to-ferroelectric transition pathway in \ce{PbZrO3} thin films under device-relevant conditions. Atomic-resolution imaging reveals a multi-step transition that includes several metastable phases. Complementary nano-beam electron diffraction and atomic scale analysis further show that this pathway and its end states can be modulated, leading to the formation of a \quotes{dead layer} near the substrate with suppressed switching behavior. Taking advantage of this depth-dependent heterogeneity, dynamic phase transformations are observed between coexisting antiferroelectric and metastable ferroelectric phases. At this dynamic transition front, repeated phase interconversion is shown to be driven by competing internal (due to substrate clamping and extended defects) and external fields, allowing the relative energies of intermediate phases to be compared as a function of electric field. This work highlights the critical role of local energetics in phase stability and provides key experimental insights into field-induced phase transitions, guiding the design of antiferroelectric-based devices.

cond-mat.mtrl-sci↗

arXiv2025

BaCd2P2: a promising impurity-tolerant counterpart of GaAs for photovoltaics

BaCd2P2 (BCP) has been recently identified as a new solar absorber with promising optoelectronic properties. This work demonstrates that, despite having a low precursor purity (98.90% to 99.95%), synthesized BCP samples exhibit a promising photoconductive carrier lifetime up to 300 ns, an implied open-circuit voltage exceeding 1 V, and photoluminescence quantum yield in the order of 0.2%, comparable to a high-purity single-crystalline GaAs wafer. To better understand the underlying mechanisms of BCP's promising properties, its tolerance to intrinsic defects and extrinsic impurities is investigated using first-principles defect modeling and compared with that of the well-studied GaAs. The results show that the nonradiative recombination rates induced by dominant deep-level intrinsic antisite defects are lower in BCP than in GaAs under typical growth conditions. Further exploration of the impact of transition metal impurities in the raw materials used to make BCP and impurities introduced during its synthesis shows that most of these do not form deep-level nonradiative recombination centers. As an impurity-tolerant counterpart of GaAs, BCP demonstrates great potential to improve the cost-to-performance ratio of photovoltaics.

cond-mat.mtrl-sci↗

arXiv2025

First principles computations of the Stark shift of a defect-bound exciton: the case of the T center in silicon

The T center in silicon has recently drawn a lot of attention for its potential in quantum information science. The sensitivity of the zero-phonon line (ZPL) to electrical field was recently investigated by a combination of different experimental methods but there is still no first principles study on the Stark shift of the T center. Dealing with the defect-bound exciton nature of the excited state is particularly challenging using density functional theory because of the large spatial delocalization associated with the wavefunction. Here, we tackle this issue by performing a convergence study over the supercell size. We obtain an exciton binding energy of 28.5meV, in good agreement with experimental results. We then calculate the Stark shift through the dipole moment change of the ZPL transition of the T center using the modern theory of polarization formalism and find a modest linear coefficient of $\Delta \mu$=0.79D along X and $\Delta \mu$=0.03D along Y. We discuss our results in light of the recent experimental measurements of the Stark shift. Our analysis suggests that bound-exciton defects could be particularly sensitive to local field effect as a result of their large spatial extent.

cond-mat.mtrl-sci↗

arXiv2025

$\Delta$SCF in \texttt{VASP} for excited-state defect computations: tips and pitfalls

$\Delta$SCF with constrained occupations have been wildly used to investigate the excited-state and optical properties of defects. Recent studies have demonstrated that combining $\Delta$SCF with hybrid functionals yields good accuracy in predicting defect properties. The Vienna Ab initio Simulation Package (\texttt{VASP}) is one of the most widely used quantum mechanical packages based on plane-wave methods. Despite the increasing application of $\Delta$SCF as implemented in \texttt{VASP} for defect studies, detailed walkthroughs explaining how to conduct these calculations remain limited, making this approach a nontrivial task. Applying $\Delta$SCF with hybrid functionals can present convergence challenges; worse, it may sometimes converge to incorrect excited states and can go largely unnoticed. This document aims to serve as a concise guide outlining what we think might be the appropriate approach for performing $\Delta$SCF calculations in \texttt{VASP}. We benchmark this method by simulating excited states for a particularly challenging system: the neutral charge state of the silicon vacancy (SiV$^0$) defect in diamond. By highlighting potential pitfalls, we hope this document encourages further discussion within the community and assists researchers experiencing difficulties with this technique. The guidelines provided here are largely based on private discussions with Oscar Bulancea Lindvall from Link{\"o}ping University and Chris Ciccarino from Stanford University.

cond-mat.mtrl-sci↗

arXiv2025

Entanglement of a nuclear spin qubit register in silicon photonics

Color centers provide an optical interface to quantum registers based on electron and nuclear spin qubits in solids. The T center in silicon is an emerging spin-photon interface that combines telecom O-band optical transitions and an electron spin in a scalable photonics platform. In this work, we demonstrate the initialization, coherent control, and state readout of a three-qubit register based on the electron spin of a T center coupled to a hydrogen and a silicon nuclear spin. The spin register exhibits spin echo coherence times of $0.41(2)$~ms for the electron spin, $112(12)$~ms for the hydrogen nuclear spin, and $67(7)$~ms for the silicon nuclear spin. We use nuclear-nuclear two-qubit gates to generate entanglement between the two nuclear spins with a fidelity of $F=0.77(3)$ and a coherence time of $T^*_2=2.60(8)$~ms. Our results show that a T center in silicon photonics can realize a multi-qubit register with an optical interface for quantum communication.

quant-ph↗

arXiv2025

Identifying high performance spectrally-stable quantum defects in diamond

Point defects in semiconductors are becoming central to quantum technologies. They can be used as spin qubits interfacing with photons, which are fundamental for building quantum networks. Currently, the most prominent quantum defect in diamond is the nitrogen-vacancy (NV) center. However, it suffers from spectral diffusion that negatively impacts optical coherence and is due to the coupling of the emission energy with uncontrolled electric fields. The group IV vacancy complexes on the other hand have shown to be significantly more spectrally-stable as they are centrosymmetric and thus immune to the linear Stark shift. They however suffer from several issues ranging from low operation temperature to low optical efficiency due to dark states and difficulty in stabilizing the right defect charge state. Here we search for alternative to the group IV vacancy complex in diamond by systematically evaluating all possible vacancy complex using high-throughput first-principles computational screening. We identify the defects that combine centrosymmetry, emission in the visible range, as well as favorable and achievable electronic structure promoting higher operation temperature and defect levels well within the band gap. We find Zn$V^{-2}$ to be especially appealing.

cond-mat.mtrl-sci↗

arXiv2025

A Map of the Zintl AM2Pn2 Compounds: Influence of Chemistry on Stability and Electronic Structure

The AM2Pn2 (A= Ca, Sr, Ba, Yb, Mg; M= Mn, Zn, Cd, Mg; and Pn=N, P, As, Sb, Bi) family of Zintl phases has been known as thermoelectric materials and has recently gained much attention for highly promising materials for solar absorbers in single junction and tandem solar cells. In this paper we will, from first-principles, explore the entire family of AM2Pn2 compounds in terms of their ground state structure, thermodynamic stability, and electronic structure. We also perform photoluminescence spectroscopy on bulk powder and thin film samples to verify our results, including the first measurements of the bandgaps of SrCd2P2 and CaCd2P2. The AM2Pn2 compounds exhibit broad stability, are mostly isostructural in the CaAl2Si2-type structure (P3m1), and cover a wide range of bandgaps from 0 to beyond 3 eV. This could make them useful for a variety of purposes, for which we propose several candidates, such as CaZn2N2 for tandem top cell solar absorbers and SrCd2Sb2 and CaZn2Sb2 for infrared detectors. By examining the band structures of the AM2Pn2, we find that Mg3Sb2 has the most promise as a thermoelectric material due to several off-{\Gamma} valence band pockets which are unique to it among the compositions studied here.

cond-mat.mtrl-sci↗