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Physics Condensed Matter Materials Science

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Thursday 6 August

Materials Science

Anatomy of Spin–Orbit Torques in Monolayer Fe$_3$GeTe$_2$ and Fe$_3$GaTe$_2$: Insights from atomistic and momentum-space decompositions

Gusthavo M. S. Brizolla, Stepan S. Tsirkin, Yaroslav Zhumagulov et al. (+1 more)

We present a systematic first-principles study of the spin-orbit torques in the ferromagnetic monolayers Fe$_3$GeTe$_2$ (FGT) and Fe$_3$GaTe$_2$ (FGaT). Despite sharing the same crystal structure (point group $D_{3h}$) and predominantly Fe~$3d$ spin-polarized bands, the two materials exhibit markedly different current-induced torques. We reveal these differences by computing the full angular dependence of the torkance—the torque per unit applied electric field—using linear-response theory with symmetry-adapted spin–orbit-coupled Wannier functions. FGaT may be viewed as a hole-doped analogue of FGT, since Ga contributes one valence electron fewer than Ge. Although the work functions differ by only about $28$~meV, the band filling near $K$ and $K'$ changes substantially: the density of states at $\varepsilon_F$ is reduced by a factor of three and its spin polarization reverses from minority in FGT to majority in FGaT. These electronic changes are reflected in the torques resolved by time-reversal parity, sublattice, and momentum. In particular, we identify pronounced hidden torques in FGaT and relate the suppression of its fourth-harmonic Fermi-sea component to the evolution of momentum-space pockets. Finally, we discuss the emergence of such self-torques, which are not captured by the conventional picture of current-induced spin accumulation, within a symmetry-based phenomenological framework. Our results provide microscopic insight into current-induced torques in two-dimensional ferromagnets and offer guidance for defect and van der Waals engineering of layered magnetic materials.
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All authors Gusthavo M. S. Brizolla, Stepan S. Tsirkin, Yaroslav Zhumagulov, Jaroslav Fabian

Materials Science · Cross-listed from quant-ph

Multi-cavity strong coupling to an electron spin ensemble: spectral and dark-state signatures

P. Oehrl, B. Pérez González, A. Dunaev et al. (+5 more)

Spin ensembles are considered as potential candidates for quantum memory and quantum enhanced sensing applications. Here, we explore the controlled coupling of multiple superconducting microwave cavities to a spin ensemble, which shows signatures of strong coupling and, due to the multi-mode character, the formation of dark states. In particular, the latter are of interest, as they provide a potential pathway to enhance memory times and enable protected storage of non-classical states in spin ensembles due to the suppressed coupling to the circuit environment. We model the spin multi-cavity hybrid to reproduce the spectra and extract characteristic coupling strengths using the input-output formalism.
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All authors P. Oehrl, B. Pérez González, A. Dunaev, M. Althammer, T. S. Parvini, F. Piazza, M. Benito, H. Huebl

Materials Science · Cross-listed from physics.optics

Transverse quantum-state characterization of programmable electron optics

Shengbo You, Paolo Rosi, Enzo Rotunno et al. (+6 more)

Programmable electron optics – electronically controlled phase plates – underpin proposals from dose-efficient phase imaging to shaped-electron X-ray sources, nearly all assuming a pure, fully coherent delivered wave whose purity has never been measured. Here we reconstruct the transverse density matrix of a microelectromechanical electrostatic spiral phase plate by mixed-state ptychography, from one four-dimensional STEM scan per state and without added hardware. The delivered beam is substantially mixed: its purity falls from approximately 0.47 to approximately 0.24 as the applied bias grows, inconsistent with a fixed lateral source-blur model, while the real-space coherence width stays near 1 nm. The same scans calibrate the device in situ, allow virtual orbital-angular-momentum sorting and, through a partial-coherence-aware transfer theory, indicate that purifying the output could improve dose efficiency roughly threefold. One acquisition thus becomes a quantum-state acceptance test for programmable electron optics, supplying the purity and coherence that emerging phase-plate and diffractive-imaging schemes assume but leave unquantified.
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All authors Shengbo You, Paolo Rosi, Enzo Rotunno, Alberto Roncaglia, Luca Belsito, Amir H. Tavabi, Rafal E. Dunin-Borkowski, Vincenzo Grillo, Philipp M. Pelz

Materials Science · Cross-listed from cond-mat.str-el

Helical-to-Fan Transitions under Magnetic Fields in the Noncentrosymmetric Tetragonal Magnet EuRhGe$_3$

Takeshi Matsumura, Hiroto Nitta, Hironori Nakao et al. (+4 more)

The magnetic structure of EuRhGe$_3$, a noncentrosymmetric body-centered tetragonal magnet with the space group $I4mm$, has been investigated by resonant X-ray diffraction. Below $T_{\text{N}}=12$ K, EuRhGe$_3$ undergoes a helical magnetic ordering with an incommensurate propagation vector $q=(0, 0, 0.809)$, in which the magnetic moments lie in the $ab$ plane and rotate by a constant turn angle of $145.8^{\circ}$ between adjacent layers. When a magnetic field is applied along the $a$ axis at 2 K, a second-harmonic $2q$ peak develops, indicating that the circular helix is gradually distorted into a helimagnetic soliton-lattice state, which eventually undergoes a lock-in transition to the commensurate structure with $q=0.8$ at 3.8 T. Above the subsequent phase boundary at 5 T, the helicity is lost, and a spin-flop $xyz$-fan (elliptic conical) state is realized, in which the moments oscillate predominantly along the $b$ axis but are accompanied by a small $c$-axis component. At higher fields, the system enters a conventional planar $xy$-fan phase without a $c$-axis component. EuRhGe$_3$ provides a prototypical example of a helimagnet that exhibits a full sequence of field-induced structures, evolving from a circular helix to a spin-flop $xyz$-fan (elliptic conical), and finally to a planar $xy$-fan structure, which has been theoretically predicted.
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All authors Takeshi Matsumura, Hiroto Nitta, Hironori Nakao, Masashi Kakihana, Masato Hedo, Takao Nakama, Yoshichika Ōnuki

Materials Science

Magnetic susceptibility of diluted magnetic semiconductors at low carrier densities

Adel Kassaian and Mona Berciu

We calculate the static longitudinal and the transverse dynamic magnetic susceptibilities of (III,Mn)V diluted magnetic semiconductors, using the random phase approximation, for a simple impurity band model appropriate for the low charge carrier concentration regime. The magnetic susceptibilities are shown to depend sensitively on the amount of positional disorder of the Mn impurities. The results we obtain are consistent with previous studies of the spin wave spectrum and of the spatially inhomogeneous ferromagnetic state of these materials.
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Materials Science

Interface Engineering of Helium Confinement in Argon-Preplated MCM-41 Nanopores

Rahul Soni, Nathan S. Nichols, Sutirtha Paul et al. (+3 more)

Atomic-scale modification of mesopore interfaces provides a route to tune the confinement experienced by adsorbed fluids, but how a specific interface preparation translates into the resulting microscopic confinement potential remains unclear. Here, we show that preplating MCM-41 with an argon monolayer modifies the effective pore interface by occupying strongly attractive regions of the heterogeneous silica surface and screening its atomic-scale corrugation. Grand-canonical Monte Carlo simulations of argon adsorption, low-temperature molecular dynamics, and helium test-particle insertion are combined with adsorption isotherms and neutron-scattering measurements to characterize the preplated pore at the atomic scale. Helium test-particle insertion calculations show that the modified interface shifts the helium adsorption minimum to an annular region inside the pore and produces a confinement landscape dominated by a smooth radial component. The resulting radial confinement potential can be described by a continuum cylindrical model, providing microscopic support for the effective potential used in earlier quantum Monte Carlo studies. Residual corrugation persists over multiple spatial scales and is accurately captured by a Gaussian process surrogate. These results demonstrate how atomic preplating can tailor nanopore confinement and provide an experimentally constrained microscopic potential for predictive studies of confined quantum fluids.
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All authors Rahul Soni, Nathan S. Nichols, Sutirtha Paul, Garfield Warren, Paul Sokol, Adrian Del Maestro

Materials Science · Cross-listed from cond-mat.str-el

Strongly Enhanced Charge-Density Waves and Correlated Insulating State in Atomically Thin 1$T$-TaS$_2$

Gan Liu, Yulu Liu, Qiling Luo et al. (+7 more)

We investigate thickness-dependent charge-density-wave (CDW) transitions in 1$T$-TaS$_2$ using temperature-dependent Raman spectroscopy and electrical transport. Raman measurements show that the incommensurate, nearly commensurate, and commensurate CDW phases persist down to the monolayer limit. As the thickness is reduced, the transition temperatures increase, accompanied by an orders-of-magnitude rise in sheet resistance and a sharp reduction in the carrier localization length. The first-order hysteretic CCDW-NCCDW transition is uniquely absent in the monolayer. Calculations suggest that the enhanced CDW in thin layers originates from strengthened Coulomb interactions due to reduced out-of-plane screening, particularly in the nonlocal component. These findings highlight the cooperative roles of electron correlation, electron-phonon interaction, and interlayer coupling in shaping the ground state and transition dynamics of atomically thin 1$T$-TaS$_2$, opening pathways for engineering correlated phases in two-dimensional CDW systems.
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All authors Gan Liu, Yulu Liu, Qiling Luo, Zhentao Huang, Kenji Watanabe, Takashi Taniguchi, Meiyu Wang, Jinsheng Wen, Yi Lu, Xiaoxiang Xi

Materials Science

Emergent Surface Altermagnetism

Yuzhong Hu, Pan Zhou, Baoru Pan et al. (+3 more)

Research on altermagnetism has thus far primarily focused on spin-polarized bulk electronic states in magnetic materials. In this work, we advance the field by introducing the concept of surface altermagnetism (SAM), wherein altermagnetic spin polarization emerges at the surfaces of collinear antiferromagnets (AFMs) or altermagnets (AMs). To lay the theoretical groundwork for this phenomenon, we construct a thorough symmetry-based framework that systematically connects bulk spin groups to surface spin groups for both types of systems. Through symmetry analysis, we identify all symmetry-breaking surfaces capable of supporting SAM, identifying 35 for $PT$-symmetric AFMs and 61 distinct cases for bulk AMs. Moreover, we show that 203 collinear spin space groups—including 100 without and 103 with the $[C_2 \Vert P]$ operation—permit the appearance of SAM on the surface of $tT$-symmetric AFMs via the breaking of fractional translational symmetries. The proposed framework is verified using tight-binding models and first-principles calculations, with practical material implementations shown in representative compounds like NaMnP, LiMnAs, and CrSb. Our results establish SAM as a robust, symmetry-protected magnetic state, extending altermagnetic phenomena to material surfaces and paving the way for advanced, field-free spin manipulation in next-generation spintronic technologies.
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All authors Yuzhong Hu, Pan Zhou, Baoru Pan, Songmin Liu, Binchang Zhou, Lizhong Sun

Materials Science

Intra-unit-cell resolved intertwining of multi-$Q$ charge and spin textures in an itinerant skyrmion magnet

Christopher J. Butler, Katsuki Nihongi, Haruto Yoshimochi et al. (+4 more)

The mechanisms stabilizing non-collinear magnetism in centrosymmetric crystals remain unclear, but likely involve spin-spin interactions mediated by itinerant electrons, such as the RKKY interaction. Finite-$Q$ magnetic order may then be accompanied by electronic modulations that are observable using a scanning tunneling microscope. In five successive magnetic phases of GdRu$_{2}$Ge$_{2}$, including two nano-scale skyrmion crystal phases, we show that multi-$Q$ magnetism among Gd 4$f$ spins entails a corresponding multi-$Q$ texture among the Ru 4$d$ orbitals that contribute itinerant electron bands. With atomically-resolved images of each electronic texture's motif, and a simple numerical modeling scheme drawing on the underlying spin structures, we infer their key relationship: The alignment between nearest-neighbor Gd spins tightly correlates with the local density-of-states of the Ru 4$d$ orbitals on the two bond-centered sublattices of the Gd square net. These analyses offer a microscopic view of the atomic-scale intertwining of charge and spin degrees-of-freedom in non-collinear itinerant magnets.
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All authors Christopher J. Butler, Katsuki Nihongi, Haruto Yoshimochi, Nguyen Duy Khanh, Rina Takagi, Tetsuo Hanaguri, Shinichiro Seki

Materials Science · Cross-listed from math-ph

On the synthesis of complete two-dimensional second-gradient continua: Tri-pantographic fabrics

Casey Rodriguez, Emilio Barchiesi, Simon R. Eugster et al. (+2 more)

We introduce a notion of completeness for two-dimensional second-gradient elastic continua and propose a microstructural route toward its synthesis. A continuum is said to be complete if the Hessian of the stored energy with respect to the second-gradient variable is locally positive definite, so that every nonzero admissible increment of the placement second gradient is quadratically controlled in the highest-order part of the energy about each configuration. Starting from the Principle of Virtual Work, we derive the constitutive relations, equilibrium equations, and admissible boundary interactions for a broad class of fibrous second-gradient continua whose stored energies depend on fiber stretch, stretch gradients, and curvature. The theory is then applied to several continua motivated by pantographic microstructures. Classical pantographic sheets are shown to be incomplete, while bi-pantographic fabrics enlarge the class of components of the second gradient detected by the energy but remain incomplete. Finally, we formulate a tri-pantographic continuum associated with a proposed three-family architecture and prove that it is complete. The examples illustrate how microstructural architecture influences both the completeness properties of an effective continuum and the pointwise form of its higher-order boundary interactions.
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All authors Casey Rodriguez, Emilio Barchiesi, Simon R. Eugster, Ivan Giorgio, Francesco dell'Isola

Wednesday 5 August

Materials Science

Antiferromagnetic Phases in Zr-Fe-Ge Kagome Systems

Peter Minch, Shiya Chen, Weiyi Xia et al. (+4 more)

A wide variety of chemical substitutions in ferromagnetic Kagome systems can lead to diverse magnetic phases with electronic structures suitable for topological or quantum material properties. Here, we study the electronic structure and magnetic orderings using first-principles calculations for the magnetic Kagome compounds ZrFe6Ge6, ZrFe6Ge4, and ZrFe6Ge5. For ZrFe6Ge6, the obtained ground-state magnetic structure is A-type antiferromagnetic (AFM), in agreement with existing experiments. We predicted that the magnetic ground states of ZrFe6Ge4 and ZrFe6Ge5 are collinear A-type bilayer AFM structures with long-period ordering that involves a mix of FM and AFM interlayer orientations. The formation of such long-range magnetic structures appears to be a general feature and is not tied to specific substitutions. The magnetic moments in these systems are largely local and only weakly dependent on the magnetic configuration, with magnitudes in good agreement with available experimental estimates. Neutron scattering experiments, which could provide direct verification of these predictions, are therefore of particular importance.
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All authors Peter Minch, Shiya Chen, Weiyi Xia, Wei-Shen Tee, Yang Sun, Cai-Zhuang Wang, Vladimir Antropov

Materials Science

Low-resistivity nitrogen-doped p-type Cu2O thin films enabled by millisecond flash lamp annealing

Jan Koloros, Pavel Baroch, Thomas Preußner et al. (+3 more)

Flash lamp annealing (FLA) provides millisecond-scale thermal processing, but its effects on p-type Cu2O and nitrogen-related defects remain poorly understood. Reactively sputtered Cu2O:N films with different nitrogen content were deposited using reactive high-power impulse magnetron sputtering and exposed to a single 1.9 ms FLA pulse at 4.9 - 11.7 J cm-2. Their compositional,morphological, structural, vibrational, electrical, and optical responses were evaluated. WDS showed no statistically significant change in total elemental composition, and XRD confirmed retention of cubic Cu2O. Nitrogen-containing films exhibited surface coarsening, shifts of the Cu2O reflections, and non-monotonic changes in the Raman band assigned to molecular N2. Nitrogen incorporation substantially reduced the as-deposited resistivity. The very low value of 0.045 {\Omega}cm was obtained after FLA at 4.9 J cm-2, whereas higher energy densities markedly increased resistivity. Hall measurements showed increasing mobility but decreasing hole concentration. At high energy densities, the optical band gap of nitrogen-rich films widened. The results define a narrow low-energy processing window with a positive effect on electrical properties, whereas high-energy FLA modifies the structure and optical absorption edge but degrades electrical conductivity.
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All authors Jan Koloros, Pavel Baroch, Thomas Preußner, Matthias Fahland, Michaela Červená, Jiří Rezek

Materials Science

Fine-Tuning Small Language Models for Reliable VASP INCAR Generation

Xinyue Zhang, Jixiang Li, Bin Shao et al. (+3 more)

Language models can prepare VASP INCAR files from natural-language requests, but so far only large proprietary cloud models come close to handling the tightly coupled, physics-sensitive settings reliably, a dependence that fits poorly with local, high-throughput materials workflows where privacy, cost, and offline deployment matter. We show that a small language model (SLM) can close this gap. The SLM is fine-tuned on reference VASP calculations and paired with VASPGuard, a deterministic post-processor that checks syntax, workflow, and material-dependent constraints; we call the combined model INCAR-SLM. On INCARBench, a benchmark for VASP INCAR generation, INCAR-SLM built on Qwen3-4B outperforms every general-purpose LLM evaluated, exceeding GPT-5.4 by 15.55 points on the 100-point INCAR Score. Most of this gain comes from fine-tuning, with VASPGuard correcting the errors that remain. We further find that model size matters less than expected: once fine-tuning and post-processing are applied, performance saturates at a few billion parameters, and Qwen3-4B outperforms larger models in the same family.
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All authors Xinyue Zhang, Jixiang Li, Bin Shao, Baishun Yang, Zhiyang Liu, Weichao Wang

Materials Science

Nitrogen Vacancy Centers in Hexagonal Diamond Exhibit Long Coherence Times

Gabriel Kumar, Siyuan Chen, Victor Wen-zhe Yu et al. (+1 more)

We show that negatively charged nitrogen-vacancy (NV) centers in the hexagonal diamond polymorph lonsdaleite offer a route to spin qubits with enhanced coherence relative to their cubic-diamond counterparts. Using first-principles calculations, we examine two distinct defect configurations, AA, with the same symmetry as in cubic diamond and AB, with reduced symmetry. We find that the AB configuration of the NV center exhibits a finite transverse zero-field splitting, giving rise to an approximate fourfold enhancement of the Hahn-echo coherence time $T_2$ at zero magnetic field. The AA configuration, by contrast, closely reproduces the electronic structure and coherence properties of the cubic NV center. We further characterize the many-body electronic structure, vertical excitation energies, and photoluminescence spectra of both configurations, providing spectral fingerprints for their experimental identification. Our results establish symmetry-broken NV centers in lonsdaleite as promising candidates for quantum sensing and information science applications.
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All authors Gabriel Kumar, Siyuan Chen, Victor Wen-zhe Yu, Giulia Galli

Materials Science

Cooperative adsorption and diffusion trapping induced by AlF3 intercalation in graphite

H. Betancourt-Infante, G. Ruano, F. Bonetto et al. (+1 more)

Graphite's structural and electronic response to molecular intercalation is central to its performance as a carbon-based electrode material, yet the microscopic coupling between subsurface intercalation and surface adsorption remains poorly understood. We present a first-principles investigation of AlF3 adsorption and intercalation in graphite to explain the microscopic origin of a recently observed two-step self-limiting sorption mechanism. Using density functional theory (DFT-D3), we show that a single intercalated AlF3 molecule locally transforms the structure, electronic properties, and diffusion behavior of graphite through a blister-like surface deformation. Comparing pristine graphite with a graphite surface containing a subsurface intercalated molecule, coverage-dependent adsorption energetics reveal a crossover from repulsive lateral interactions to cooperative binding above the blister, driven by local curvature and intercalation-induced charge redistribution. Diffusion-barrier calculations show that the blister simultaneously acts as a kinetic trap, raising diffusion barriers and transitioning surface mobility from a quasi-barrierless to a thermally activated regime. Charge-density difference and Mulliken population analyses identify the intercalant as a stable electronic reservoir that deepens the surface potential landscape, kinetically immobilizing adsorbed species. Together, these results establish a structure-property relationship for intercalation-induced deformation in graphite, offering a quantitative framework for controlling intercalation efficiency in carbon-based energy storage and conversion systems.
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All authors H. Betancourt-Infante, G. Ruano, F. Bonetto, S. J. Rodríguez-Sotelo

Materials Science · Cross-listed from hep-ph

First-principles upper bounds on dark matter-electron scattering rates from condensed matter sum rules

Bradford A. Barker, Jay Epstein, Luke James et al. (+5 more)

A wide variety of condensed matter systems are used or proposed as detectors to search for dark matter-electron scattering. In general, the scattering rate depends on detailed knowledge of the electronic properties of these systems. However, when dark matter couples to electron density, the dark matter-electron scattering rate can be related to the electron energy loss function, whose integrals are bounded by first-principles sum rules that rely on only a few macroscopic target properties. In this paper, we use these first-principles sum rules to derive upper bounds on the dark matter-electron scattering rate depending on only a few material properties: the plasma frequency $\omega_\text{p}$, the target mass density $\rho_T$, and the static (longitudinal) dielectric function at finite momentum transfer, $\varepsilon(q, 0)$. The bulk material properties $\omega_\text{p}$ and $\rho_T$ vary only over a limited range across a wide variety of materials, and to a good approximation, the generic large-$q$ dependence of $\varepsilon(q, 0)$ can be understood from a simple scaling law depending only on $\omega_\text{p}$ which we verify with analytic and numerical examples. Thus, our upper bounds are largely material-agnostic, and place a fundamental limit on the sensitivity of any dark matter-electron direct detection experiment probing the coupling to electron density.
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All authors Bradford A. Barker, Jay Epstein, Luke James, Yonatan Kahn, Elizabeth A. Peterson, Anirudh Prabhu, Tanner Trickle, Samuel L. Watkins

Materials Science · Cross-listed from cs.AI

Agentic self-driving microscopy benchmarks support qualification but do not necessarily generalize to unseen tasks

Nathan S Johnson and Ian Abshire

Large language model agents are increasingly being developed to control a wide range of scientific characterization tools including microscopes and synchrotron beamlines. Research into agentic control of physical infrastructure is nascent and there are few well-established paradigms for how to engineer an agentic system. There are many choices to make when designing a microscopy agent, including the choice of LLM, the number of agents to use, agent responsibilities and delegation rules, retrieval-augmented generation parameters, and more. When designing and optimizing an agentic microscope controller, researchers not only want to ensure that the agent can correctly perform known tasks but also that the agent can generalize to new tasks that it has not encountered before. In this study, we develop a benchmark and trace-logging framework that reveals a) how different choices of agent architecture impact performance at microscopy tasks and b) the limitations of benchmarks for predicting if a particular agent will perform well on unseen microscopy tasks. The framework was used to evaluate one-, two-, and three-agent graph topologies, five LLMs, RAG and context parameters, and operational constraints across 53 microscopy benchmark tests. In total, 105 agent configurations, 1,949 individual test runs, and 49,109 RAG retrievals were recorded. Direct comparisons showed clear differences in latency, token use, cost, and failure mode between configurations. However, surrogate models trained on agent architecture and test results did not reliably predict an agent's performance on new, unseen tasks. These results show that these benchmarks are useful for qualification, regression testing, diagnosis, and direct comparison, but the current heterogeneous test suite does not support a task-independent global configuration model.
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Materials Science

The crossmetric tensor and the geometrical meaning of the imaginary numbers

Cyril Cayron

The product of two Cartesian quaternions can be written as a quadratic form based on a 4x4 matrix made of the symbols 1,i,j,k where i,j,k are imaginary numbers introduced by Hamilton. We generalized this matrix to non-Cartesian bases, and showed that the matrix is made of the metric and the cross tensors. We called it crossmetric tensor. Its symbols are s,a,b,c; they are the elementary crystallographic quaternions. We determined the crossmetric tensors for the six crystal families. We also showed that any unit crystallographic quaternion can be geometrically represented by an infinity of pairs of oriented planes intersecting along the vectorial component of the quaternion such that the angle between them is the semiangle of the rotation. The composition of quaternions follows the intuitive source-target rule. The elementary quaternions a,b,c are geometrically represented by pair of perpendicular and oriented planes intersecting along the axis a,b,c, respectively. Other complementary quaternions noted a',b',c' were also introduced. They are the pairs of planes ma, mb, mc. The elementary quaternions a,b,c follow Hamilton rules on the squares of imaginary numbers. The complementary quaternions follow Hamilton rules on the bi and tri-products. For Cartesian basis, the quaternions i,j,k appear as a specific case of crystallographic quaternions. They are formed by the pairs of perpendicular faces of the cube mx, my, mz. Since these planes intersect along the x, y and z axis, respectively, the elementary and complementary quaternions are equal, i.e. i=i',j=j',k=k', which explains why the square, bi and tri-product Hamilton rules are satisfied all together with only three quaternions i,j,k.
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Materials Science

Effective single particle picture for anharmonic lattice dynamics: a Rosetta stone for electronic and ionic response

Giovanni Caldarelli and Francesco Mauri

We establish a theoretical framework for the dynamics of a lattice of ions in a mean-field approach, where anharmonicity is included via self-consistency. In this picture, the many-body dynamics of a system of $N$ atoms in three dimensions is mapped onto two kinds of $6N$-dimensional vectors: the phonon condensate, describing the evolution of the average atomic positions, and the phonon spinors, describing the evolution of the atomic elastic constants. The phonon spinors are classified by a quantum number that behaves as a spin: the phonon pseudospin. The many-body Liouville equation is replaced by two wave equations equivalent to the time-dependent Schrödinger equation for the electronic wave function in density functional theory. Exploiting this parallelism, we formulate the response of the anharmonic lattice in one-to-one correspondence with time-dependent density functional theory for electrons. In complete analogy with the electronic case, we express the ionic response in terms of matrix elements of operators representing external fields and forces. We show how anharmonicity screens external perturbations through a phonon analogue of the Hartree-exchange-correlation kernel. We provide expressions for the lattice optical and thermal conductivity, showing how thermal conductivity depends on the phonon pseudospin. By approximating the density matrix as a Gaussian, we recover the equations of the time-dependent self-consistent harmonic approximation. In this case, the linear-response equations are formulated in terms of an anharmonic kernel including three- and four-phonon scattering. By translating anharmonic lattice dynamics into the language of density functional theory, this work shows how theoretical and computational advances in modeling the dynamical response of interacting electrons can be directly applied to interacting ions.
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Materials Science · Cross-listed from cond-mat.str-el

Direct Evidence for Robust Bulk Band Gap Across the Charge Density Wave Transition in TiSe2

Turgut Yilmaz, Anil Rajapitamahuni, Muhammad Awais Fiaz et al. (+7 more)

The mechanism driving the charge density wave (CDW) transition in TiSe2 has been debated for decades, with proposals ranging from an excitonic insulator to a lattice-driven instability. A central question remains whether the transition involves an opening or enhancement of the bulk band gap. Using high-resolution angle-resolved photoemission spectroscopy, we directly track the temperature evolution of the bulk band edges across the CDW transition at TCDW = 200 K. Contrary to the expectation of a gap-opening transition, we find that the size of the fundamental band gap remains constant from the high-temperature normal phase down to 160 K. While the CDW induces clear band-folding signatures and spectral weight redistribution, the underlying band extrema are unperturbed. These results demonstrate that TiSe2 does not undergo a temperature-driven electronic gap opening. Instead, they support a scenario where the transition is governed by a lattice symmetry-breaking reconstruction that folds, but does not gap, the electronic structure of a pre-existing band insulator.
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All authors Turgut Yilmaz, Anil Rajapitamahuni, Muhammad Awais Fiaz, Zhenxian Liu, Jerzy T. Sadowski, Abdullah Al-Mahboob, Asish K. Kundu, Shawna M. Hollen, Boris Sinkovic, Elio Vescovo

Materials Science

HPHT growth of centimeter-sized cubic boron nitride crystals

Andrey Katrusha, Weihua Peng, Jianguo Peng et al. (+1 more)

Single crystals of cubic boron nitride (cBN) exceeding 10 mm in size were grown by the high-pressure high-temperature (HPHT) temperature-gradient method using a Ni-Cr-based solvent catalyst. Compared with the previously reported maximum crystal size of approximately 3 mm, this improvement was achieved by maintaining a stable precursor flux during one week of growth at a source temperature of 1950 {\deg}C. In contrast to diamonds, which were grown with the same HPHT cell and showed nearly isometric shapes, the cBN crystals had elongated shapes. We attribute this cBN morphology to a localized growth near the BN source due to the relatively low effective diffusivity of boron and nitrogen species in the metallic solvent.
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All authors Andrey Katrusha, Weihua Peng, Jianguo Peng, Konstantin Iakoubovskii

Materials Science

X-ray Thermal diffuse scattering from real-space displacement correlations

Benjamin Fahl, Jonathan Bulled, Artem Korshunov et al. (+3 more)

We introduce a method for calculating X-ray thermal diffuse scattering based on the three-dimensional difference pair distribution function (3D-dPDF). Within the harmonic approximation the method is exact, as it includes all orders of multi-phonon scattering. A single Fourier transform delivers the diffuse intensity over large volumes of reciprocal space. We tested the method against experimental diffuse scattering measured on a silicon single crystal. With nothing refined beyond a scale and a background, the calculation reproduces the measured intensity to an R2 residual below 5%. Because the method uses the same pair-correlation language as the established Yell program, thermal and static correlated disorder enter the analysis on equal footing.
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All authors Benjamin Fahl, Jonathan Bulled, Artem Korshunov, Dmitry Chernyshov, Yevheniia Kholina, Arkadiy Simonov

Materials Science

Delocalized Coupled-Cluster Theory for Polaron Structure and Dynamics

Hamlin Wu, Moritz K.A. Baumgarten, Tong Jiang et al. (+1 more)

Polaron ground states and finite-temperature dynamics remain challenging to simulate because existing methods struggle to combine nonperturbative accuracy, systematic improvability, and scalability from models to materials-specific Hamiltonians. We introduce a translationally invariant variational coupled-cluster (CC) theory for polarons, termed delocalized CC (dCC), with closed-form energies at cost as low as $\mathcal{O}(N^3)$ and no phonon-number cutoff. dCC accurately describes the ground states of the one- and two-dimensional Holstein and Su–Schrieffer–Heeger (optical and bond) models and the Fröhlich model, in close agreement with density matrix renormalization group (DMRG) and diagrammatic Monte Carlo benchmarks. A projected tangent-space response formalism built on the same ansatz yields electron-addition spectral functions and optical conductivities at zero and finite temperature. The resulting spectra agree well with DMRG, Lanczos, and neural-network quantum-state benchmarks while retaining a physically interpretable excitation hierarchy, and extend to two-dimensional lattices at finite temperature beyond the practical reach of these methods. The same framework applies directly to ab initio electron–phonon matrix elements, yielding LiF electron- and hole-polaron binding energies that match state-of-the-art many-body calculations. These results establish dCC as a unified variational framework for polaron ground states and dynamics, from model systems to real materials.
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All authors Hamlin Wu, Moritz K.A. Baumgarten, Tong Jiang, Joonho Lee

Materials Science · Cross-listed from cs.LG

CheMLFlow: An Open-Source Platform for Cheminformatics and Materials Informatics Applications

Brendan Smith, Susana Lopez-Moreno, Eric Dolores-Cuenca et al. (+3 more)

CheMLFlow is an open-source platform for building and executing end-to-end, high-throughput, and agentic workflows for scientific and technological applications. CheMLFlow targets a common bottleneck in scientific machine learning development, where researchers often need to assemble data acquisition, curation, representation, model training, validation, screening, interpretation, and reporting into a reproducible pipeline, even when their primary research contribution concerns only one stage. CheMLFlow provides modular workflow components, ready-to-run reference pipelines, standardized artifacts, and evaluation outputs that reduce orchestration overhead and support benchmarking across methods and datasets. The platform is designed to be extensible, reproducible, and automation friendly, with pluggable representations and models, deterministic splits, explicit run artifacts, batch execution, and report generation. As scientific software increasingly moves toward agent assisted experimentation, CheMLFlow's configuration driven workflows and structured outputs also provide a practical interface for coding agents to help users construct experiments, inspect results, and summarize findings under human supervision. This article describes the system architecture, core workflows, and benchmarks that reach literature performance for quantum mechanical, physicochemical and bioactivity property prediction, and use cases involving time series datasets demonstrating applications beyond molecular chemistry datasets.
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All authors Brendan Smith, Susana Lopez-Moreno, Eric Dolores-Cuenca, Sangil Kim, Jose L. Mendoza-Cortes, Nijamudheen Abdulrahiman

Materials Science

Effects of Interfacial States and Strain on Tunnel Magnetoresistance in van der Waals Magnetic Tunnel Junctions

Sakshi Goel, Arti Kashyap, Keisuke Masuda et al. (+1 more)

All-two-dimensional magnetic tunnel junctions promise atomically sharp interfaces, yet the role of interface-induced states in their spin transport is not fully understood. Here, we theoretically investigate spin-dependent transport in van der Waals magnetic tunnel junctions of the structure Cr$_2$C/$MY_2$/Cr$_2$C ($M$ = Mo, W; $Y$ = S, Se) with barrier thicknesses of 3, 5, 7, and 9 layers. The broad features of the $\mathbf{k}_{\parallel}$-resolved conductances, namely suppression near the $\Gamma$ point and enhancement at six off-$\Gamma$ hot spots, are consistent with the decay of evanescent states in the barrier. However, trilayer WS$_2$, MoSe$_2$, and WSe$_2$ barriers exhibit conductances of the order of $e^2/h$ at $\mathbf{k}_{\parallel}$ points within the hot spots. We attribute these near-unity transmission channels to resonant coupling between the interfacial states at the two electrode–barrier interfaces, as evidenced by their weak but finite residual weight at the barrier center. For thicker barriers, this coupling weakens, which suppresses the residual weight, thereby reducing the tunnel magnetoresistance (TMR) ratio of the MoS$_2$ junction while enhancing those of the other junctions. To exploit the interfacial states for spin-selective tunneling, we further examine biaxial tensile strain applied to the trilayer junctions. At 4% strain, the TMR ratio increases from 176% to 540% for MoS$_2$ and from 98% to 496% for WS$_2$, whereas MoSe$_2$ and WSe$_2$ exhibit comparatively weaker enhancement. Our results establish interfacial-state engineering via strain and barrier thickness as effective routes for enhancing the TMR effect in all-two-dimensional magnetic tunnel junctions.
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All authors Sakshi Goel, Arti Kashyap, Keisuke Masuda, Terumasa Tadano

Materials Science

Structural Chirality from Short-Range Order in Heteroanionic Materials

Benjamin J. Morgan

Established routes to structural chirality in inorganic crystals depend on symmetry-lowering atomic displacements of an achiral parent structure. We show that chirality can instead be forced by the ordering of two anion species over the sites of an achiral parent, independent of atomic displacements. ReO$_3$-type oxyfluorides with a 2:1 anion stoichiometry, such as NbO$_2$F, exhibit two ordering patterns: cis octahedral coordination and period-three anion-chain ordering, both rooted in the off-centring of $d^0$ cations. By direct enumeration, we prove that every configuration combining both orderings on the smallest commensurate cell is chiral, belonging to a Sohncke space group. For the specific case of NbO$_2$F, density-functional theory calculations predict that the ground state is the maximally symmetric chiral configuration. Wang-Landau Monte Carlo simulations of a DFT-trained cluster-expansion model predict an equilibrium chiral phase up to a first-order transition at 494 K. These results establish configurational ordering as a chemically realisable route to structural chirality, and mark heteroanionic materials with analogous ordering chemistry as candidates for a new class of chiral functional materials.
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Materials Science · Cross-listed from cond-mat.str-el

Ghost-RISB for Correlated Electron-Phonon Systems: Application to the Hubbard-Holstein Model

Samuele Giuli, Ricardo J. Campos-Lopes, Emin Moghadas et al. (+1 more)

We develop a generalization of the ghost-rotationally-invariant slave-boson (ghost-RISB) method that incorporates local phonon modes coupled to arbitrary on-site electronic degrees of freedom, enabling a nonperturbative treatment of electron-electron and electron-phonon interactions within an efficient variational framework. The method extends the ghost-orbital construction to capture dynamical self-energy effects and phonon-induced renormalizations beyond static slave-boson approaches. Benchmarking against dynamical mean-field theory (DMFT) results for the Hubbard-Holstein model, we find excellent quantitative agreement for electron quasiparticle weights and phonon properties across a wide range of coupling strengths, and it captures accurately the competition between electron-electron and electron-phonon interactions. We show that the inclusion of the ghost orbitals is crucial to accurately describe the regime of low-frequency, strongly dynamical, phonons. The extended ghost-RISB achieves this accuracy at a fraction of the computational cost of DMFT, due to its self-consistency rooted in static observables instead of dynamical ones, enabling rapid exploration of correlated electron-phonon phase diagrams. We exploit this advantage to characterize the most demanding regime of strong coupling and adiabatic phonons. Our analysis shows a suppression of the superconducting order parameter, which is interpreted as a Franck-Condon-like reduction of the overlap between the phonon wavefunctions associated with empty and doubly-occupied sites in the bipolaronic regime.
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All authors Samuele Giuli, Ricardo J. Campos-Lopes, Emin Moghadas, Massimo Capone

Materials Science

Growth and characterization of GaN/Ga2O3 Nanowire Heterostructures for Ultraviolet Optoelectronics

Edgars Butanovs, Eriks Dipans, Martins Zubkins et al. (+10 more)

Ultraviolet-range GaN/b-Ga2O3 heterostructures were fabricated and investigated in both planar and nanowire geometries using pulsed laser deposition and reactive magnetron sputtering from a liquid gallium target for b-Ga2O3 deposition, while both GaN nanowire arrays and planar p-type Mg-doped GaN layers were grown by metal-organic chemical vapor deposition. Precise control of film uniformity and thickness was achieved as confirmed by structural and morphology studies using X-ray diffraction, X-ray photoelectron spectroscopy, atomic force microscopy and scanning electron microscopy. Planar n-Ga2O3/p-GaN heterojunction diodes were electrically and photoelectronically characterized, exhibiting pronounced rectifying behavior, high forward current and a visible-blind ultraviolet photoresponse under zero external bias, demonstrating intrinsic self-powered operation. Furthermore, GaN/b-Ga2O3 core/shell nanowire heterostructures were developed and systematically studied with a focus on morphology control and process optimization. The influence of deposition parameters on shell thickness, uniformity, and tapering was investigated, enabling improved conformality of the b-Ga2O3 coating on the M-plane facets of GaN nanowires. The results highlight the viability of physical vapor deposition techniques for forming GaN/b-Ga2O3 heterostructures and establish a pathway toward nanowire-based ultraviolet optoelectronic devices.
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All authors Edgars Butanovs, Eriks Dipans, Martins Zubkins, Edvards Strods, Anatolijs Sarakovskis, Juris Purans, Ashutosh Kumar, Sergiy Khartsev, Martin Berg, Anders Hallen, Qin Wang, Joseph le Pluart, Peter Ramvall

Materials Science

An Exchange-Correlation Functional for Fast and Accurate Modeling of Ferroelectric Perovskites

Owain T. Beynon and Chiara Gattinoni

We present a novel exchange correlation functional, C09x-PBEc, which combines C09 exchange with PBE correlation, to accurately model the ferroelectric properties of perovskites while retaining the computational efficiency of GGA functionals. With a growing interest in developing machine learning interatomic potentials (MLIPs) to model large-scale ferroelectric systems of technological relevance, it is important to scrutinise the density functional theory exchange-correlation functionals which are used to compute the forces, energies and stresses the MLIP is trained on. Using the example of the prototypical ferroelectrics lead titanate, PbTiO3, and barium titanate, BaTiO3 we show that many widely used functionals tend to overestimate their lattice constants and spontaneous polarization. Conversely, non-local van der Waals functionals with C09 exchange accurately capture these properties compared to experiment, but with a larger computational overhead than, for example, GGA. We show that C09x-PBEc combines the accuracy provided by the C09 exchange with the computational affordability of GGA, making it an excellent candidate to be used in the training of MLIPs for ferroelectric perovskites. We also demonstrate that an MLIP trained using C09x-PBEc accurately reproduces the ferroelectric-to-paraelectric phase transition temperature of PbTiO3 with respect to experiment, showing a marked improvement on MLIPs trained using GGA.
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Materials Science

A phase field model of coupled crack and dislocations: emission, blunting, and the necessity of dissipative toughening

Khanh Chau Le and Thi My Kieu Tran

We propose a phase field model of a macrocracked single crystal in which the crack and the geometrically necessary dislocations descend from a single energy functional. Energy minimization alone then decides dislocation nucleation, through an integral criterion evaluated in closed form along slip chords. The criterion yields a size effect inaccessible to point-wise strength conditions: a grain-size-dependent yield stress. With slip suppressed the model reproduces Griffith fracture; with fracture suppressed, the nucleation load measured by the full non-smooth solver agrees with the closed-form nucleation criterion to four percent. The coupled computations produce a two-stage response: at loads an order of magnitude below cleavage, dislocation bands emitted from the notch tip blunt and shield it, raising the initiation load; once the crack grows, the bands heal; a compact cluster of like-signed dislocations travels with the tip, its canceling partner walls pinned at the grain boundary, and the dissipated fracture resistance equals the elastic one. In the purely energetic, dissipationless limit, emission shields the crack but does not toughen it; toughening requires dissipation, incorporated in the sequel through the threshold resistance to dislocation motion.
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