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Physics Condensed Matter Materials Science

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Thursday 26 February

Materials Science

Substrate induced optimization of the Electrocatalytic Hydrogen Evolution Reaction (HER) performances of MoS2 thin film

Hafiz Sami-Ur-Rehman, Arpana Singh, Nunzia Coppola et al. (+11 more)

Molybdenum disulfide (MoS2) has emerged as a promising, cost-effective catalyst for hydrogen production via water splitting. We investigate the structural and electrocatalytic properties of MoS2 thin films deposited on different substrates (Al2O3, SiC, STO) to study their hydrogen evolution reaction (HER) activity. In particular, in order to study the substrate influence on the stabilization of different polymorphic MoS2 phases, the films are synthesised using pulsed laser deposition on substrates with different crystal symmetries and lattice parameters. All the deposited samples are characterized by X-Ray Diffraction, Raman Spectroscopy, Linear Sweep Voltammetry and Electrochemical Impedance Spectroscopy analyses. The films grown on Al2O3 substrates exhibit the best HER performance, likely due to the stabilization of the metastable 1T phase through the interfacial interactions between film and substrate. Presence of the 1T phase in the samples grown on Al2O3 improves the charge transfer efficiency and the electrochemically active surface with a better response to the applied potential, demonstrating their enhanced catalytic behaviour for hydrogen evolution.
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All authors Hafiz Sami-Ur-Rehman, Arpana Singh, Nunzia Coppola, Pierpaolo Polverino, Sandeep Kumar Chaluvadi, Shyni Punathum-Chalil, Heinrich-Christoph Neitzert, Diana Sannino, Pasquale Orgiani, Alice Galdi, Cesare Pianese, Paolo Barone, Carmela Aruta, Luigi Maritato

Materials Science

Self-foaming, Sintering-resistant Iron-Tungsten Powders Enable High-Cycle Thermochemical Hydrogen Storage

Jie Qi and David C. Dunand

H2-H2O redox cycling of iron powder beds at 650-800 {\deg}C offers a compact, safe, economical hydrogen storage method, but sintering-induced capacity loss has stalled its scalability for decades. Here, we show that adding redox-active tungsten to Fe powders solves this problem in static powder beds: Fe-25W (at%) alloyed powder self-foams during redox cycling via W gas-phase transport, increasing porosity and preserving capacity. In a custom automated reactor, a kilogram-scale powder bed reversibly stores 42g H2 and sustains 96+/-3% capacity utilization over 30 redox cycles. Temperature-resolved in-situ X-ray diffraction reveals a chemical-vapor-transport-mediated self-foaming mechanism that redistributes W to refine the microstructure, complemented by a contact-barrier stabilization mechanism during high-temperature holds. Partial-capacity cycling up to 90 cycles further confirms sintering resistance under incomplete redox conditions. These results establish Fe-W powder beds as a robust, scalable, and compact platform for safe, stationary hydrogen storage.
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Materials Science · Cross-listed from cond-mat.mes-hall

First-principles and tight-binding analysis of thermoelectricity in irradiated WSe$_2$

Cynthia Ihuoma Osuala, Tanu Choudhary, Raju K. Biswas et al. (+2 more)

Electronic and thermoelectric transport in zigzag monolayer WSe$_2$ nanoribbons are studied under monochromatic irradiation. The electronic structure is described within a six-orbital tight-binding framework constructed from the relevant tungsten and selenium orbitals, with atomic spin-orbit coupling included explicitly. Periodic driving is incorporated via the Peierls substitution, and in the high-frequency limit the system is mapped onto an effective static Floquet Hamiltonian with polarization-dependent renormalized hoppings. Coherent transport is evaluated using wave-function matching within the Landauer-Büttiker formalism. The lattice thermal conductivity is obtained independently from density functional perturbation theory combined with an iterative solution of the phonon Boltzmann transport equation. Light-induced hopping renormalization reshapes the band dispersion and transmission spectrum near the Fermi level, modifying the Landauer transport integrals that determine electrical and thermal conductances and the Seebeck coefficient. Together with spin-orbit-driven band splitting and reduced lattice thermal conductivity from enhanced anharmonic scattering, this leads to a thermoelectric figure of merit $ZT$ exceeding unity over a broad temperature range.
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All authors Cynthia Ihuoma Osuala, Tanu Choudhary, Raju K. Biswas, Sudin Ganguly, Santanu K. Maiti

Materials Science

Anti-Thermal Quenching Phosphors based on Metal Halides

Baowei Zhang and Liberato Manna

Thermal quenching (TQ) generally occurs in phosphors and is ascribed to the activation of non-radiative transitions at elevated temperatures. This effect limits the use of most phosphors in high-power/high-temperature applications, such as outdoor lighting and laser systems. To achieve anti-TQ properties, structural design of phosphors is required. This usually follows two guidelines: (1) increasing lattice rigidity to minimize thermal expansion; (2) converting thermal energy into radiative transitions to compensate for the non-radiative losses. While metal oxides and metal nitrides dominate the field of commercial anti-TQ phosphors, metal halides - despite their inherently soft lattices - have shown remarkable progress as anti-TQ phosphors in recent years. Here, we review the advances in anti-TQ metal halides (covering the time span from 2017, when first reports have appeared, till today) and discuss their mechanisms and applications. We argue that the low synthesis temperatures of metal halides and their high photoluminescence quantum yields (PLQYs) make them promising candidates as anti-TQ phosphors. Furthermore, since the rich optical-physical processes underlying the anti-TQ effect in soft-lattice in metal halides are only now beginning to be unraveled, this creates opportunities for many fundamental investigations.
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Materials Science

Role of the Nephelauxetic Effect in Engineering Mn4+ Luminescence Kinetics for Lifetime-Based Thermometry

A. Basheer, M. Szymczak, M. Piasecki et al. (+3 more)

Although the considerable potential of luminescence thermometry based on emission kinetics has been widely demonstrated, reliable tools for the intentional prediction of thermometric performance remain limited. To address this challenge, the present work introduces an approach that enables a theoretical description of the 2E state lifetime of Mn4+ ions, as well as the absolute and relative sensitivities, in terms of the nephelauxetic effect within the group of double perovskites: Sr2InNbO6, Sr2InTaO6, Ba2InTaO6, and Ba2InNbO6. Our results clearly show that, contrary to common assumptions, the Dq/B ratio is not the primary factor governing either the spectroscopic behavior of Mn4+ ions or the thermometric performance of Mn4+-doped phosphors. Instead, the nephelauxetic beta1 parameter plays the dominant role. The empirical analysis carried out in this study led to the development of a predictive model that enables estimation of SAMAX and SRMAX values based exclusively on beta1. This methodology represents a significant step toward the rational design of lifetime-based luminescence thermometers with predefined thermometric characteristics tailored to the requirements of specific applications.
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All authors A. Basheer, M. Szymczak, M. Piasecki, A. M. Srivastava, M.G. Brik, L. Marciniak

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

Phonons reflect dynamic spin-state order in LaCoO$_3$

Alsu Ivashko, Taishun Manjo, Maximilian Kauth et al. (+17 more)

We investigate lattice dynamics in LaCoO$_3$ using inelastic neutron and x-ray scattering over $T = 2\mbox{-}650\,\mathrm{K}$, spanning the spin-state crossover at $T_{1} \approx 100\,\mathrm{K}$ and the insulator–metal transition at $T_{2} \approx 550\,\mathrm{K}$. Comparison with quasi-harmonic $ab-initio$ lattice-dynamical calculations helps reveal anomalous softening of a $\approx 10\,\mathrm{meV}$ oxygen phonon, confined to the temperature interval $T_{1} \leq T \leq T_{2}$ and localized in momentum space at $\boldsymbol{q}_{\mathrm{SSO}} = \left( \frac{1}{2},\frac{1}{2},\frac{1}{2} \right)_{c}$. This wave vector corresponds to the spin-state ordering originally proposed by Goodenough [J. Phys. Chem. Solids 6, 287-297 (1958)]. Our results therefore provide momentum-resolved evidence for dynamic correlations of high-spin and low-spin Co$^{3+}$ states in LaCoO$_{3}$, linking spin-state fluctuations to anomalous phonon renormalization.
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All authors Alsu Ivashko, Taishun Manjo, Maximilian Kauth, Yuliia Tymoshenko, Adrian M. Merritt, Klaus-Peter Bohnen, Rolf Heid, Michael Merz, Andreas Eich, John-Paul Castellan, Alexandre Ivanov, Nathaniel Schreiber, Hong Zheng, J. F. Mitchell, Martin Meven, Jitae T. Park, Daisuke Ishikawa, Yuiga Nakamura, Alfred Q. Baron, Frank Weber

Materials Science

LLM-driven discovery for carbon allotropes with bond-network entropy

Yuzhou Hao, Yujie Liu, Xuejie Li et al. (+4 more)

The discovery of novel carbon allotropes with tailored thermal and mechanical properties is critical for advanced thermal management. However, exploring the vast configurational space of carbon using ab initio calculations remains computationally prohibitive. Driven by the rich topological landscape of carbon, where the competition between $sp, sp^2,$ and $sp^3$ hybridization states dictates material performance, we establish a closed-loop AI framework to explore this complex configurational space. We introduce a hybridization entropy descriptor to guide the search beyond conventional forms. Here, we establish a closed-loop AI framework that synergizes a Large Language Model (LLM) for structural generation with a Machine Learning Potential (MLP) for accelerated evaluation. Leveraging CrystaLLM to generate candidates and an iteratively refined MLP for high-fidelity validation, we screened thousands of structures to identify several stable allotropes with exotic properties. Specifically, we report “yne-diamond C$_{12}$” and “yne-hex-diamond C$_{8}$”, which exhibit extreme thermal anisotropy and ultralow in-plane shear stiffness arising from their mixed $sp$-$sp^3$ hybridization. Furthermore, we discovered a complex $sp$-$sp^2$-$sp^3$ hybridized C$_{12}$ phase that combines metallic conductivity with an anomalous negative Poisson's ratio. Notably, we identified a superhard phase (C16_3) possessing a calculated Vickers hardness (103.3 GPa) exceeding that of diamond 96 GPa). Microscopic analysis reveals that thermal transport in these materials is governed by the interplay between rigid frameworks and flexible linkers. This work expands the known carbon phase space and demonstrates the efficacy of coupling generative AI with machine learning potentials for the accelerated inverse design of functional materials.
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All authors Yuzhou Hao, Yujie Liu, Xuejie Li, Turab Lookman, Xiangdong Ding, Jun Sun, Zhibin Gao

Materials Science · Cross-listed from cond-mat.supr-con

Moire Engineering of Cooper-Pair Density Modulation States

Zihao Wang, Bing Xia, Stephen Paolini et al. (+10 more)

Cooper-pair density modulation (CPDM) states are superconducting phases in which the order parameter varies periodically in real space without breaking translational symmetry. Recently, moire superlattices in layered materials have emerged as powerful platforms for engineering charge density with tunable lattice symmetry, offering a new route to creating and controlling CPDM states. In this work, we demonstrate moire-induced CPDM states in a bilayer heterostructure formed by epitaxially stacking one quintuple layer (1 QL) of topological insulator Sb2Te3 on a six-unit-cell (6 UC) antiferromagnetic FeTe layer. Scanning tunneling microscopy and spectroscopy (STM/S) measurements reveal a moiré superlattice formed between the hexagonal Te lattice of Sb2Te3 and the square Te lattice of FeTe, which spatially modulates the two superconducting gaps of the 1 QL Sb2Te3/6 UC FeTe bilayer. Our Josephson STM/S measurements provide direct real-space imaging of the CPDM states with a wavelength corresponding to the periodicity of the moire superlattice. By substituting Sb2Te3 with Bi2Te3, we achieve control over both the periodicity and magnitude of the CPDM states. Our work demonstrates an epitaxial strategy for synthesizing moire superlattices from materials with different crystal symmetries and reveals a new mechanism for engineering CPDM states in designer bilayer heterostructures.
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All authors Zihao Wang, Bing Xia, Stephen Paolini, Zi-Jie Yan, Pu Xiao, Jiatao Song, Veer Gowda, Hongtao Rong, Di Xiao, Xiaodong Xu, Weida Wu, Ziqiang Wang, Cui-Zu Chang

Materials Science

Single Pair of Charge-two Weyl Fermions in Chiral Boron Allotropes

Hui-Jing Zheng, Yan Gao, Yanfeng Ge et al. (+2 more)

The realization of a minimal Weyl semimetal (WSM) hosting a single pair of Weyl points (WPs) has thus far been restricted to magnetic systems, since time-reversal symmetry generally enforces a minimum of four WPs in nonmagnetic materials. Here, combining first-principles calculations with symmetry analysis, we identify two stable boron allotropes, chiral HDSBC-B$_{20}$ and cage-like CR-B$_{12}$, as the first nonmagnetic electronic materials realizing a single pair of WPs in the spinless regime. We show that the interplay between time-reversal symmetry and crystallographic rotation symmetry ($C_4$ or $C_3$) stabilizes exactly one pair of $C=2$ WPs pinned at time-reversal-invariant momenta, thereby circumventing the conventional node-quartet constraint. These double-WPs exhibit linear dispersion along the rotation axis and quadratic dispersion in the perpendicular plane. In HDSBC-B$_{20}$, the sign of the topological charge is directly correlated with structural chirality. Both materials host exceptionally long double Fermi arcs spanning the surface Brillouin zone, providing experimentally accessible signatures. Our findings establish nonmagnetic material platforms for minimal double-Weyl fermions and broaden the landscape of unconventional WSMs.
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All authors Hui-Jing Zheng, Yan Gao, Yanfeng Ge, Yong Liu, Zhong-Yi Lu

Materials Science

Symmetry-Protected Minimum of Four Conventional Weyl Points in Nonmagnetic Crystals

Ze-Xin Xue, Ke-Xin Pang, Yun-Yun Bai et al. (+3 more)

Realizing nonmagnetic Weyl semimetals (WSMs) with the minimal number of conventional Weyl points (WPs) and a clean Fermi surface remains a central challenge. Here, combining symmetry analysis with first-principles calculations, we establish the definitive conditions under which a nonmagnetic crystal can host exactly four conventional ($C = \pm 1$) WPs, identifying 76 space groups in the spinless limit and 83 in the spinful case that allow this minimal configuration. Guided by this framework, we predict two previously unknown boron allotropes, P6-B$_{48}$ and TBIN-B$_{48}$, as ideal WSMs. Both exhibits precisely four isolated WPs near the Fermi level, with exceptionally clean electronic structures. Notably, the WPs in P6-B$_{48}$ are pinned to high-symmetry points, while those in TBIN-B$_{48}$ lie along high-symmetry lines, leading to distinct and experimentally accessible surface states, including single and double Fermi arcs. Our work provides a complete symmetry-based foundation and pristine material platforms for minimal Weyl physics.
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All authors Ze-Xin Xue, Ke-Xin Pang, Yun-Yun Bai, Yanfeng Ge, Yong Liu, Yan Gao

Materials Science

Interpretable self-driving sputter epitaxy: from black-box optimization to human-usable growth rules

Yuki K. Wakabayashi, Yui Ogawa, Franz Benedict Romero et al. (+2 more)

Self-driving laboratories have emerged as powerful tools for navigating high-dimensional process spaces, yet systems remain black-box optimizers that yield limited transferable process understanding. Here, we demonstrate an interpretable self-driving laboratory framework that transforms autonomous optimization into human-usable growth rules. As a stringent benchmark, we apply this framework to RF magnetron sputtering, addressing a long-standing challenge of achieving high-quality beta-Ga2O3 heteroepitaxy and single-crystalline beta-Ga2O3 homoepitaxy via sputtering. By combining Bayesian optimization with automated optical evaluation of the Urbach energy as a metric of sub-bandgap disorder, the self-driving system efficiently identifies heteroepitaxial growth conditions yielding a minimum Urbach energy of 182 meV, the lowest value for sputtered beta-Ga2O3 films. Importantly, the optimized growth window is transferable, realizing single-crystalline beta-Ga2O3 homoepitaxy without further optimization, corroborated by scanning transmission electron microscopy. To convert the closed-loop dataset into interpretable growth rules, we train a random forest surrogate and distill it into response curves and quantified pairwise interactions across the four-dimensional growth-parameter space. This analysis identifies substrate temperature as the primary control knob, with RF power and gas flows acting largely additively and only a modest temperature-oxygen coupling delineating the narrow window for high-quality growth, establishing a general route from autonomous experimentation to transferable growth rules.
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All authors Yuki K. Wakabayashi, Yui Ogawa, Franz Benedict Romero, Takuma Otsuka, Yoshitaka Taniyasu

Materials Science

The Surface Sensitivity of X-ray Second Harmonic Generation as a Function of Energy

Daniel Schacher, Tod A. Pascal, Keith V. Lawler et al. (+1 more)

The surface sensitivity and probe depth in the x-ray regime of diamond for second harmonic generation (SHG) was investigated both analytically and computationally with velocity gauge real-time time-dependent density functional theory (VG-RT-TDDFT), which includes a full multipole expansion. This was accomplished using two different approaches, by changing the number and location of layers that can generate SHG computationally and by exploiting the symmetry of a crystal, a similar pattern emerged. We find that by 1000 eV, well above the ~285 eV of the C $K$-edge, the SHG of diamond is dominated by the bulk, quadrupole response, in agreement with our analytic calculations. The bulk response continues to grow as the energy is increased, becoming overwhelming by 7000 eV. Near the C $K$-edge the measurement is quite surface sensitive, however, this surface sensitivity reduces as the energy increases such that by 1000 eV (and certainly by 3500 eV) SHG is largely bulk sensitive. Moreover, we find that the specific details of the crystal orientation (i.e., comparing a (001)-terminated and (111)-terminated surface) appear to have significant effects on the surface sensitivity.
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All authors Daniel Schacher, Tod A. Pascal, Keith V. Lawler, Craig P. Schwartz

Wednesday 25 February

Materials Science · Cross-listed from physics.optics

Correlated inhomogeneous absorption profiles across distinct optical transitions in a rare-earth doped crystal

Flora Segur, Sacha Welinski, Alban Ferrier et al. (+2 more)

In rare-earth ion-doped crystals, inhomogeneous absorption profiles reflect the distribution of local environments experienced by individual ions. While each optical transition probes this distribution differently, their fine spectral structures may retain correlations arising from shared local perturbations. In this paper, we present a low-temperature, high-resolution spectroscopic study in Er$^{3+}$:YSO of the transition $^{4}I_{15/2}$ - $^{4}I_{11/2}$ at 980 nm and compare it to the well-known transition $^{4}I_{15/2}$ - $^{4}I_{13/2}$ at 1.5 $\mu$m. Using spectral hole burning on one transition while monitoring the other, we uncover for the first time spectral correlations between two optical transitions, providing new insight into the microscopic origin of inhomogeneous distributions in rare-earth-doped crystals.
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All authors Flora Segur, Sacha Welinski, Alban Ferrier, Perrine Berger, Anne Louchet-Chauvet

Materials Science

Many-Body effects beyond excitons in second-harmonic generation of monolayer MoS$_{2}$

Peio Garcia-Goiricelaya and Julen Ibañez-Azpiroz

We present a quantitative study of many-body effects including the three-particle level on second-harmonic generation in monolayer MoS$_2$. Our approach combines many-body perturbation theory with time-dependent current-density-functional theory within an ab initio framework in the optical limit. The inclusion of two-particle excitonic effects via a dynamical long-range linear exchange-correlation kernel reproduces the qualitative features of the second-harmonic response, but differs from the experimentally reported magnitude by nearly a factor of two. By incorporating three-particle trionic correlations through a static long-range quadratic exchange-correlation kernel, we achieve significantly improved quantitative agreement with experiment for the three main bound excitonic resonances. These findings suggest that many-body interactions beyond the two-body excitonic level can play an important role in accurately describing second-order optical responses in two-dimensional semiconductors.
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Materials Science

Mimicking the earth core conditions with ultrafast laser materials interaction

Mohamed Yaseen Noor, Aram Yedigaryan, Gabriel Calderon et al. (+6 more)

Ultrafast lasers create extreme, non-equilibrium thermodynamic conditions that can transiently reach pressures and temperatures comparable to interior core of the earth. Here we show that femtosecond excitation of amorphous silica-hafnia multilayer dielectrics drives the formation of high-pressure crystalline phases of silica including stishovite, seifertite, and the pyrite-type high density structure, within confined subsurface regions.Using TEM, SAED, and 4D-STEM, we directly map nanoscale phase evolution and identify crystalline motifs embedded inside laser generated blisters.Complementary molecular dynamics simualtions reveal the thermodynamic pathway underlying these transformations, where rapid electronic pressure initiates densification and octahedral coordination, followed by temperature driven crystallization and displacive transitions during ultrafast quenching. The resulting polymorphs reflects a dual-stage pathway inaccessible under equilibrium processing. Our results establish femtosecond laser excitation as a viable route to synthesize and stabilize ultrahigh-density high pressure silica phases under ambient conditions, without a diamond anvil cell, with implications for laser-damage mechanisms, high-energy-density materials, and planetary physics.
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All authors Mohamed Yaseen Noor, Aram Yedigaryan, Gabriel Calderon, Arshak Tsaturyan, Elena Kanchan, Jinwoo Hwang, Carmen S. Menoni, Jean-Philippe Colombier, Enam Chowdhury

Materials Science · Cross-listed from cond-mat.stat-mech

An Information-theoretic Collective Variable for Configurational Entropy

Ashley Z. Guo, Kaelyn Chang and Nicholas J. Corrente

Entropy governs molecular self-assembly, phase transitions, and material stability, yet remains challenging to quantify and directly control in molecular systems. Here, we demonstrate that the computable information density (CID), a data compression-based information theoretic metric, provides a general per-configuration structural descriptor that tracks configurational entropy changes in molecular dynamics simulations, reflecting both local and long-range structural organization. We validate the CID across systems of increasing complexity, beginning with single-component Lennard-Jones melting before examining binary phase separation, polymer condensation and dispersion, and assembly of amorphous carbon networks at multiple densities. Unlike conventional order parameters, CID requires no a priori knowledge of relevant structural features and captures organizational signatures across a variety of molecular systems and discretization resolutions. By establishing a data compression-based structural complexity metric as a practical proxy for configurational entropy, this framework lays a foundation for future entropy-driven materials design and optimization strategies.
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Materials Science · Cross-listed from math.NA

A Reduced Order Model approach for First-Principles Molecular Dynamics Computations

Siu Wun Cheung, Youngsoo Choi, Jean-Luc Fattebert et al. (+2 more)

To leverage the redundancy between the electronic structure computed at each step of first-principles molecular dynamics, we present a data-driven modeling framework for Kohn-Sham Density Functional Theory that bypasses the explicit optimization of electronic wavefunctions. We sample a priori representative atomic configurations and construct a low-dimensional basis that efficiently approximates the electronic structure subspace. Subsequently, we employ this reduced basis in a direct solver for the electronic single particle density matrix, thereby enabling the efficient determination of ground state without iterative wavefunction optimization. We demonstrate the efficacy of our approach in a Born-Oppenheimer molecular dynamics of a water molecule, showing that the resulting simulations accurately reproduce key structural properties, such as bond lengths and bond angle, obtained from full first-principles molecular dynamics. This work highlights the potential of data-driven approaches to develop efficient electronic structure solvers for first-principles simulations.
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All authors Siu Wun Cheung, Youngsoo Choi, Jean-Luc Fattebert, Jonas Kaufman, Daniel Osei-Kuffuor

Materials Science · Cross-listed from cond-mat.supr-con

Inhomogeneous superconductivity in (001), (110), and (111) KTaO$_3$ two-dimensional electronic gases: effect of electronic confinement on the transition temperature

Mattia Trama, Roberta Citro and Carmine Antonio Perroni

Experimental observations on KTaO$_3$ superconductivity report widely different critical temperatures with a clear hierarchy among different crystallographic orientations. Accounting for the first time for spatial confinement, we connect the inhomogeneous superconducting order parameter with the spatial extent of the two-dimensional electron gas. We reproduce the observed hierarchy within a unified framework, based on a self-consistent tight-binding slab model with local spin-singlet $s$-wave pairing. The methods developed here can readily be extended to include more general interaction models, while still consistently including the characteristic spatial structure of the electron gas.
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Materials Science · Cross-listed from cond-mat.mes-hall

Atomic-Scale Quantum Control of Single Spin Defects in a Two-Dimensional Semiconductor

Kwan Ho Au-Yeung, Wantong Huang, Johanna Matusche et al. (+15 more)

Individual spin defects in solids are promising building blocks for quantum technologies, but their deterministic creation, individual addressability, and operation near surfaces remain major challenges. Two-dimensional materials provide an attractive alternative, as their single-layer thickness enables direct atomic-scale access to defect states. Here, we demonstrate single-spin control of solid-state defects in a two-dimensional semiconductor by a combination of scanning tunneling microscopy and electron spin resonance. We create and manipulate individual sulfur vacancies and carbon substitution defects in monolayer molybdenum disulfide and characterize their spin dynamics, including coherent control, at the single-defect level. Using atomic manipulation, we further engineer and probe spin-spin interactions between defect pairs. Our results demonstrate deterministic creation, addressability, coherent manipulation, and controlled coupling of individual spin defects within a single experimental platform. This establishes atomically engineered spin defects in two-dimensional semiconductors as a versatile class of controllable solid-state quantum systems and opens a route towards tailored quantum sensing experiments.
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All authors Kwan Ho Au-Yeung, Wantong Huang, Johanna Matusche, Paul Greule, Jonas Arnold, Lovis Hardeweg, Máté Stark, Luise Renz, Affan Safeer, Daniel Jansen, Thomas Michely, Jeison Fischer, Wolfgang Wernsdorfer, Christoph Sürgers, Hannu-Pekka Komsa, Johannes Schwenk, Wouter Jolie, Philip Willke

Materials Science · Cross-listed from cond-mat.mes-hall

Phononic enhancement and detection of hidden spin-nematicity and dynamics in quantum magnets

Junyu Tang, Hong-hao Song and Gang v. Chen

The spin nematic phase, characterized by long-range order of spin quadrupole moments in the absence of dipolar magnetism, presents a significant challenge for conventional experimental detection. We propose a novel method to detect this elusive order in quantum magnets with an illustration in the spin-1 triangular lattice Mott insulator. By integrating out the phonon degrees of freedom, we obtain a phase diagram with substantially enlarged regions for the spin-nematic and spin-nematic-supersolid phases. We then demonstrate that through the spin-lattice coupling, the emergence of spin nematic order imprints a distinctive signature onto the phonon spectra, providing a clear spectroscopic signature for the quadrupolar order accessible via Raman or inelastic X-ray scattering. Our formalism offers a direct and powerful method to uncover the hidden spin nematicity, opening a new pathway for diagnosing multipolar orders in quantum magnets.
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Materials Science

High-pressure single-crystal X-ray diffraction study of ErVO4

Josu Sanchez-Martin, Gaston Garbarino, Samuel Gallego-Parra et al. (+5 more)

We present an investigation into the crystal structure of ErVO4 under variable pressure conditions. The high-pressure single crystal X-ray diffraction experiments performed employing helium as the pressure medium facilitated structure refinements up to 24.1(2) GPa. The transition from zircon to scheelite was observed at a pressure of 7.9(1) GPa. In contrast to previous reports, we did not detect any sign of phase coexistence. We also did not observe the second phase transitions previously predicted by density-functional theory to occur below 20 GPa. The determination of the pressure dependence of unit-cell parameters and volume yields precise values for linear compressibility of each axis and the pressure-volume equation of state for both the zircon and scheelite phases. Additional information on the mechanical properties of ErVO4, obtained from density-functional theory calculations, is also reported.
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All authors Josu Sanchez-Martin, Gaston Garbarino, Samuel Gallego-Parra, Alfonso Munoz, Sushree Sarita Sahoo, Kanchana Venkatakrishnan, Ganapathy Vaitheeswaran, Daniel Errandonea

Materials Science

High-Pressure X-Ray Diffraction Study of Scheelite-type Perrhenates

Neha Bura, Pablo Botella, Catalin Popescu et al. (+7 more)

The effects of pressure on the crystal structure of scheelite-type perrhenates were studied using synchrotron powder X-ray diffraction and density-functional theory. At ambient conditions, the studied materials AgReO4, KReO4, and RbReO4, exhibit a tetragonal scheelite-type crystal structure described by space group I41/a. Under compression, a transition from scheelite-to-M${\prime}$-fergusonite (space group P21/c) was observed at 1.6 and 7.4 GPa for RbReO4 and KReO4, respectively. The transition involves a relative volume decrease. On the other hand, AgReO4 underwent a phase transition to the M-fergusonite structure (space group I2/a) at 13.6 GPa. In this case there is no appreciable volume discontinuity. The room-temperature pressure-volume equation of state for the three studied perrhenates was estimated using a second-order Birch-Murnaghan equation of state. The results for the low-pressure phase are confirmed by density-functional theory calculations. The analysis of the bulk modulus shows that the compressibility of the compounds decreases following the sequence RbReO4 > KReO4 > AgReO4, which is related to the compressibility of the RbO8, KO8, and AgO8 bidisphenoid units. Density-functional theory also offers valuable insights into the elastic constants. Despite giving a good description for the low-pressure phase in the three compounds, density-functional theory cannot catch the structural phase transition observed in experiments. Reasons for it are discussed in the manuscript.
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All authors Neha Bura, Pablo Botella, Catalin Popescu, Frederico Alabarse, Ganapathy Vaitheeswaran, Alfonso Munoz, Brendan J. Kennedy, Jose Luis Rodrigo Ramon, Josu Sanchez-Martin, Daniel Errandonea

Materials Science · Cross-listed from quant-ph

Loss Mechanisms in High-coherence Multimode Mechanical Resonators Coupled to Superconducting Circuits

Raquel Garcia Belles, Alexander Anferov, Lukas F. Deeg et al. (+14 more)

Circuit quantum acoustodynamics (cQAD) devices have a wide range of applications in quantum science, all of which depend crucially on the quantum coherence of the mechanical subsystem. In this context, high-overtone bulk acoustic-wave resonators (HBARs) are particularly promising, since they have shown very high quality factors with negligible dephasing. However, the introduction of piezoelectric films, which are necessary for coupling to a superconducting circuit, can lead to additional loss channels, such as surface scattering and two-level systems (TLS). Here, we study the acoustic dissipation of HBAR resonators in cQAD systems and find that the defect density of the piezoelectric material and its interface with the bulk are limiting factors for the coherence. We measure acoustic modes with phonon lifetimes up to 400 $\mu$s and lifetime-limited coherence times approaching one millisecond in the quantum regime. When coupled to a superconducting qubit, this leads to a hybrid system with a large quantum coherence cooperativity of $C_{T_2}=1.1\times10^5$. These results represent a new milestone for the performance of cQAD devices and offer concrete paths forward for further improvements.
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All authors Raquel Garcia Belles, Alexander Anferov, Lukas F. Deeg, Loris Colicchio, Arianne Brooks, Tom Schatteburg, Maxwell Drimmer, Ines C. Rodrigues, Rodrigo Benevides, Marco Liffredo, Jyotish Patidar, Oleksandr Pshyk, Matteo Fadel, Luis Guillermo Villanueva, Sebastian Siol, Gerhard Kirchmair, Yiwen Chu

Materials Science · Cross-listed from physics.comp-ph

Phase-Dependent Excitonic Light Harvesting and Photovoltaic Limits in Monolayer Y2TeO2 MOenes

Bill D. A. Huacarpuma, Jose A. dos S. Laranjeira, Nicolas F. Martins et al. (+5 more)

We investigate phase-dependent electronic and excitonic phenomena in monolayer Y2TeO2 MOenes in the 1T and 2H polymorphs using first-principles theory and an effective many-body framework. Phonon spectra and elastic stability criteria establish both phases as dynamically and mechanically stable. Quasiparticle band structures reveal direct gaps in the near-infrared to visible range, with gap values increasing systematically from semilocal to hybrid exchange treatments. Optical spectra computed using a tight-binding Bethe-Salpeter approach demonstrate pronounced excitonic resonances arising from reduced dimensionality and weak dielectric screening. The exciton binding energies reach 152 meV in the 1T phase and 126 meV in the 2H phase, reflecting enhanced quantum confinement in the structurally denser phase. Our results identify Y2TeO2monolayers as a rare class of stable, direct-gap MOenes with strong excitonic effects, providing a platform for exploring many-body physics in low-dimensional oxychalcogenide systems especially for photovoltaic applications.
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All authors Bill D. A. Huacarpuma, Jose A. dos S. Laranjeira, Nicolas F. Martins, Julio R. Sambrano, Kleuton A. L. Lima, Santosh K. Tiwari, Alexandre C. Dias, Luiz A. Ribeiro Jr

Materials Science · Cross-listed from physics.optics

Purcell-enhanced Bright and Dark Exciton Emission from Perovskite Quantum Dots in Micro-ring Resonators

Lanyin Luo, Mohit Khurana, Ian M. Murray et al. (+5 more)

Colloidal quantum dots (QDs) integrated with waveguide-coupled dielectric resonators are promising building blocks for compact on-chip light sources. However, deterministic placement of QDs with strong mode overlap at the desired location remains a challenge. Here, we demonstrate a simple and scalable strategy for integrating colloidal QDs with a waveguide-coupled Si3N4 micro-ring resonator platform and for controlling the radiative dynamics of both bright and dark excitons via Purcell enhancement. We use strongly quantum-confined CsPbBr3 QDs, which exhibit bright-exciton emission at room-temperature, while emission at cryogenic temperatures originates from both bright and dark excitons. The CsPbBr3 QDs are selectively retained on the Si3N4 micro-ring cavities through a spin-coating/rinsing process, enabling efficient overlap with whispering-gallery modes and routing of the emission through integrated waveguides. We confirm accelerated decay of emission from both bright and dark excitons for CsPbBr3 QDs coupled to the micro-ring cavities. These results demonstrate an effective route to integrate colloidal QDs with Si3N4 micro-ring cavities and to leverage cavity-enhanced emission in scalable integrated photonic devices.
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All authors Lanyin Luo, Mohit Khurana, Ian M. Murray, Sina Baghbani Kordmahale, Akanksha Pandey, Xiaohan Liu, Alexei V. Sokolov, Dong Hee Son

Materials Science · Cross-listed from physics.chem-ph

MBD-ML: Many-body dispersion from machine learning for molecules and materials

Evgeny Moerman, Adil Kabylda, Almaz Khabibrakhmanov et al. (+1 more)

Van der Waals (vdW) interactions are essential for describing molecules and materials, from drug design and catalysis to battery applications. These omnipresent interactions must also be accurately included in machine-learned force fields. The many-body dispersion (MBD) method stands out as one of the most accurate and transferable approaches to capture vdW interactions, requiring only atomic $C_6$ coefficients and polarizabilities as input. We present MBD-ML, a pretrained message passing neural network that predicts these atomic properties directly from atomic structures. Through seamless integration with libMBD, our method enables the immediate calculation of MBD-inclusive total energies, forces, and stress tensors. By eliminating the need for intermediate electronic structure calculations, MBD-ML offers a practical and streamlined tool that simplifies the incorporation of state-of-the-art vdW interactions into any electronic structure code, as well as empirical and machine-learned force fields.
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All authors Evgeny Moerman, Adil Kabylda, Almaz Khabibrakhmanov, Alexandre Tkatchenko

Materials Science

Discovering new photovoltaics using optimal transport theory

Matthew A. H. Walker, Zibo Zhou, Junayd Ul Islam et al. (+1 more)

Searching by chemical and structural analogy is one of the most commonly used and successful approaches to materials discovery. However, formulating this task for algorithmic implementation raises the question of how we define similar materials. Methods have been proposed for searching materials space using vectors based on chemical composition and functional fragments in the material. Descriptors for structural similarity have also been proposed. However, the question of how to incorporate and balance structural and compositional similarity measures in a single metric remains open. Here, we adapt methods developed for calculating distances between undirected graphs and apply them to crystalline materials similarity. The Fused Gromov-Wasserstein (FGW) metric uses optimal transport theory to map between two graphs considering a balance of the graph structure and the information present at the nodes of the graph (atoms in crystals). We apply the method to exploring new photovoltaic materials. We demonstrate that FGW is competitive with embeddings from an equivariant graph neural network, trained on $> 10^6$ materials, despite minimal training. We then apply FGW to a discovery campaign to identify materials from the Materials Project database that have not previously been explored as photovoltaics, but have similarities to known high-efficiency materials. After validating predictions with hybrid density functional theory, we identify seven previously unexplored high-efficiency photovoltaic absorber candidates, including Cs$_5$Sb$_8$, which is found to have a predicted SLME of $> 30\%$ and to be thermodynamically stable. The FGW approach demonstrates the power of strong inductive biases for developing metrics for materials exploration with minimal training data.
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All authors Matthew A. H. Walker, Zibo Zhou, Junayd Ul Islam, Keith T. Butler

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

Crystallography-driven molecularization of a two-dimensional spin-$3/2$ magnet

Hari Borutta, Tobias Müller, Ronny Thomale et al. (+2 more)

Large-spin two-dimensional magnets are generally expected to develop conventional long-range order once the dominant exchange scale becomes appreciable. The layered spin-$3/2$ maple-leaf compound Na$_2$Mn$_3$O$_7$ defies this expectation: despite sizable antiferromagnetic interactions and no evident disorder, it exhibits no magnetic ordering and displays two well-separated thermodynamic crossover scales. We show that this behavior originates from a crystallography-driven molecularization of the magnetic degrees of freedom. The low-symmetry structure partitions the Mn sublattice into inequivalent exchange pathways, generating a pronounced hierarchy that nearly isolates antiferromagnetic hexagons. Magnetic correlations therefore develop in two stages: first within individual hexagons at a scale set by the dominant exchange, and only at much lower temperatures do frustrated inter-hexagon couplings attempt to establish coherence across the lattice. While isolated hexagons reproduce the two-step thermodynamic structure, the experimentally relevant temperature scales emerge only once the hexagons are embedded in the frustrated two-dimensional network. The resulting quantum ground state is magnetically disordered, characterized by strong intra-hexagon correlations and rapidly decaying inter-hexagon correlations. These results identify crystallographic inequivalence as a materials-level mechanism for stabilizing molecularized and quantum-disordered states even in large-spin two-dimensional magnets.
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All authors Hari Borutta, Tobias Müller, Ronny Thomale, Harald O. Jeschke, Yasir Iqbal

Materials Science

Intrinsic Instabilities and Mechanical Anisotropy in Halide Perovskite Monolayers

Gabriel X. Pereira, Lucas M. Farigliano, Roberto H. Miwa et al. (+1 more)

Halide perovskites have been extensively studied owing to their excellent optoelectronic properties and their unique lattice characteristics, that are very soft and anharmonic. Recent studies indicate the importance of a deep understanding of their surfaces and, in the limit, the properties of low-dimensional structures based on these materials. To investigate the structural and electronic properties of halide perovskite monolayers (i.e., perovskenes), this work uses first-principles simulations. We have studied three different stoichiometries (ABX3, ABX4, and A2BX4) and structural phases for iodide, bromide, and chloride perovskite monolayers. Their thermodynamic behavior was evaluated through the construction of phase diagrams, highlighting the instability of the ABX4 stoichiometry, which was further supported by its mechanical instability. Structurally, the covalent characteristics of the Pb–X bond, in contrast to the Cs–X bonds, induce a strong anisotropy in the Young's modulus and Poisson's ratio along different crystallographic directions, and also account for the lower stiffness observed in the phases where the octahedra are not aligned. The electronic properties are somewhat similar to those of their 3D counterparts, but with a slightly larger band gap; in the monolayers, the band gap increases with halogen electronegativity (I, Br, Cl) and octahedral tilting. Moreover, the non-symmetric ABX3 stoichiometry exhibited a spin splitting due to the internal dipole moment in these layers. Overall, our work lays the groundwork for a deeper understanding of low-dimensional structures based on halide perovskites.
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All authors Gabriel X. Pereira, Lucas M. Farigliano, Roberto H. Miwa, Gustavo M. Dalpian

Materials Science

Intrinsic (non)-Gilbert damping in magnetic insulators calculated from a minimal model and ab initio spin Hamiltonians

Andrei Shumilin, Diego López-Alcalá, Nassima Benchtaber et al. (+2 more)

We present an analytically solvable minimal model for the relaxation of low-frequency magnons in magnetic insulators arising from magnon-phonon and magnon-magnon interactions. The model establishes a direct connection between microscopic relaxation processes and Gilbert damping, and reveals how magnon decay evolves from bulk systems to the monolayer limit. We find that magnon-phonon coupling produces Gilbert damping of comparable magnitude in three- and two-dimensional magnets, with qualitative differences between flexural phonons in free-standing monolayers and three-dimensional phonons in substrate-supported layers. By contrast, non-Gilbert damping due to four-magnon scattering is strongly enhanced in two dimensions, where it becomes independent of spin-orbit coupling. To benchmark the model against real materials, we introduce a numerical approach for computing magnon damping from ab initio-derived spin Hamiltonians. We demonstrate that the central conclusions of the model remain valid for magnons in bulk YIG and several van der Waals magnetic insulators: CrSBr, CrI3 and CrPS4.
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All authors Andrei Shumilin, Diego López-Alcalá, Nassima Benchtaber, Alberto M. Ruiz, José J. Baldoví