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Physics Condensed Matter Materials Science

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Friday 23 January

Materials Science

Dielectric, magnetic and lattice dynamics properties of double perovskite (Ca0.5Mn1.5)MnWO6

Hong Dang Nguyen, Alexei A. Belik, Petr Kužel et al. (+7 more)

Recent dielectric and magnetic studies of (Ca0.5Mn1.5)MnWO6 ceramics [A.A. Belik, Chem. Mater. 36, 7604 (2024)] have classified this material as a rare hybrid multiferroic, with both antiferromagnetic and (anti)ferroelectric ordering occurring at the same temperature of 22 K. The pronounced dielectric anomaly observed at this temperature indicated that the structural change is primarily induced by a phonon soft mode and not by a spin arrangement, as is usually the case in type II multiferroics. However, our comprehensive investigation involving new ceramic samples as well as the sample from the above-mentioned reference does not support this conclusion. Low-temperature polarization measurements revealed no evidence of either ferroelectric or antiferroelectric order in both sample series. The dielectric permittivity exhibits only a slight change at the antiferromagnetic transition, and phonon modes observed in IR and Raman spectra show no indication of a symmetry change at low temperatures. In the new samples the Neel temperature is shifted to TN = 18 K. XRD, SEM, EDS and WDS analyses confirmed the composition (Ca0.5Mn1.5)MnWO6 of both ceramics, but also indicated a small amount (percentage points) of MnO and CaO impurities in the sample from the previous publication and Mn3O4, CaWO4 secondary phases (<4%) in the new ceramics. The differences in dielectric and magnetic properties of the two samples can therefore be explained by their different chemical purity. The small dielectric anomaly of the new sample at the antiferromagnetic transition temperature is explained by a spin-phonon coupling. We conclude that (Ca0.5Mn1.5)MnWO6 is not a multiferroic, but a paraelectric antiferromagnet.
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All authors Hong Dang Nguyen, Alexei A. Belik, Petr Kužel, Fedir Borodavka, Maxim Savinov, Jan Drahokoupil, M. Jarošová, Petr Proschek, Bartoloměj Vaníček, Stanislav Kamba

Materials Science · Cross-listed from math.NA

Numerical efficiency of explicit time integrators for phase-field models

Marco Seiz and Tomohiro Takaki

Phase-field simulations are a practical but also expensive tool to calculate microstructural evolution. This work aims to compare explicit time integrators for a broad class of phase-field models involving coupling between the phase-field and concentration. Particular integrators are adapted to constraints on the phase-field as well as storage scheme implications. Reproducible benchmarks are defined with a focus on having exact sharp interface solutions, allowing for identification of dominant error terms. Speedups of 4 to 114 over the classic forward Euler integrator are achievable while still using a fully explicit scheme without appreciable accuracy loss. Application examples include final stage sintering with pores slowing down grain growth as they move and merge over time.
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Materials Science

Electrically Accessible Metamagnetic Transition via a Doping-Induced Low-Energy Magnetic State in Antiferromagnetic Insulator RFeO3

Wanting Yang, Haohuan Peng, Ziming Wang et al. (+7 more)

Low-energy antiferromagnetic phase transitions offer an appealing platform for low-power spintronic functionalities, yet their direct electrical access in insulating antiferromagnets remains challenging, particularly in the low-field regime where subtle Neeel vector reorientations dominate. Here, we demonstrate that targeted rare-earth-site engineering enables an electrically accessible metamagnetic transition in the insulating orthoferrite Ho0.5Dy0.5FeO3. By combining the distinct spin-reorientation sequences of DyFeO3 and HoFeO3, Dy substitution stabilizes a dual spin-reorientation pathway, hosting an intermediate state with a reduced energy barrier. This low-energy antiferromagnetic state can be tuned into the weak-ferromagnetic state under low magnetic fields. The critical field decreases with increasing temperature, providing a favorable window for functional manipulation. Both longitudinal and transverse spin Hall magnetoresistance channels exhibit clear and reproducible signatures of the metamagnetic transitions. Owing to the enhanced sensitivity of the transverse channel, additional low-field features are resolved, reflecting the projection of the Neel vector onto the spin-accumulation direction. Electrical transport measurements correlate directly with the magnetically determined phase boundaries, establishing a purely electrical access to low-energy phase transitions and to illustrate a viable pathway for exploring low-power spin dynamics in insulating oxide antiferromagnets.
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All authors Wanting Yang, Haohuan Peng, Ziming Wang, Xiaoxuan Ma, Baojuan Kang, Chang Xue, Rongrong Jia, Jun-Yi Ge, Jinrong Cheng, Shixun Cao

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

Superconductivity in non-centrosymmetric rhombohedral NbSe2

Zhengxian Li, Xiaoyu Shen, Kai Liu et al. (+14 more)

Crystal stacking offers a powerful yet underexplored route to engineer symmetry in layered superconductors. Here we report superconductivity in rhombohedral-stacked NbSe2 (3R-NbSe2), a non-centrosymmetric polytype in which global inversion symmetry is removed by stacking alone. Using comprehensive structural, transport, magnetic, and thermodynamic measurements, we establish superconductivity as a bulk property of the 3R phase and find that the in-plane upper critical field exceeds the Pauli paramagnetic limit, indicating the persistence of strong Ising-type spin-orbit coupling. Unlike the thickness-dependent superconductivity in centrosymmetric 2H-NbSe2, the superconducting transition temperature in 3R-NbSe2 shows little dependence on layer number but exhibits an unusually strong sensitivity to disorder. We further observe strongly enhanced nonlinear optical and electrical responses near the superconducting transition, consistent with stacking-induced inversion-symmetry breaking. Our results identify 3R-NbSe2 as a single-phase platform in which stacking engineering reshapes superconductivity and enables nonlinear transport phenomena in layered materials.
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All authors Zhengxian Li, Xiaoyu Shen, Kai Liu, Yating Sha, Tianyang Wang, Feng Liu, Qingchen Duan, Kenji Watanabe, Takashi Taniguchi, Peng Chen, Shiyong Wang, Ruidan Zhong, Dong Qian, Shengwei Jiang, Yufan Li, Noah F. Q. Yuan, Guorui Chen

Materials Science

Simulations of High Temperature Decomposition of Metal-Organic Frameworks to form Amorphous Catalysts

Connor W. Edwards, Oliver M. Linder-Patton and Jack D. Evans

Metal-organic framework (MOF) derived materials formed through high temperature processes show great potential as catalysts. However, understanding of structure-property relationships between the initial MOF and the resulting MOF-derived catalyst is limited because the amorphous nature of the catalyst challenges standard structural characterization methods. Neural network approaches that learn interatomic potentials from density functional theory offer a promising solution. We simulated the pyrolysis of UiO-66, UiO-67 and MIP-206 using both foundational and fine-tuned machine learned interatomic potentials (MLIPs). To mimic experimental conditions, an atmosphere of CO2 and H2 was introduced and the structures were doped with 20 wt% copper to probe the effect of copper on the structural evolution of MOFs. These simulations provide atomistic insights into gas evolution, metal nanoparticle formation, and linker decomposition that were compared to available experimental data. Overall, this work demonstrates the potential of MLIPs to accurately model high temperature MOF dynamics under experimentally relevant conditions and guide the design of new catalytic materials.
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Materials Science

Anharmonic thermodynamics redefines metastability and parent phases in ferroelectric HfO2

Yiheng Shen, Chang Liu, Wei Xie et al. (+1 more)

Hafnia (HfO2) is a silicon-compatible dielectric material, yet stabilizing its desired but metastable ferroelectric phase remains challenging. Phase stability predictions by density functional theory (DFT) have provided crucial guidance, but most simulations neglected or only treated finite temperature effects with (quasi-)harmonic approximation due to high computational cost of DFT. Here, we develop a machine learning force field and perform thermodynamic calculations for HfO2 using self-consistent phonon theory to address growing evidence of anharmonicity. Our results reveal that the ferroelectric orthorhombic phase oIII exhibits metastability below 0.1kBT under most conditions within the simulated regime of temperature and pressure (600 K <= T <= 1500 K and 0 <= p <= 7.5 GPa), contradicting previous harmonic predictions of metastability above 1500 K at ambient pressure. We further report evidence for temperature- and pressure-dependent ferroelectric parent phase despite efforts to identify a universal one. This study highlights the importance of anharmonicity and provides an effective approach for its treatment in the design of HfO2-based ferroelectrics.
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All authors Yiheng Shen, Chang Liu, Wei Xie, Wei Ren

Materials Science

Photoinduced metastable cation disorder in metal halide double perovskites

Shunran Li, Burak Guzelturk, Conrad A. Kocoj et al. (+18 more)

Lead-free perovskites have emerged as environmentally benign alternatives to lead-halide counterparts for optoelectronics. Among them, the double perovskite Cs2AgInCl6 family exhibits remarkable white-light emission with proper composition engineering, enabled by strong electron-phonon coupling and the formation of self-trapped excitons (STEs). Despite these advantages, the fundamental photo- and structural dynamics governing their excited-state behavior remain poorly understood. Here, we report a long-lived metastable phase in the Cs2AgInCl6 double perovskite family and unravel this process and the concomitant electronic and structural evolution using a suite of tools including transient optical spectroscopy, time-resolved X-ray diffraction (TR-XRD) and X-ray absorption (TR-XAS). We show that the photoinduced, transient metastable phase is associated with B-site (Ag-In) disorder, which induces a dramatically reduced optical bandgap. Supported by TR-XRD and first-principles calculations, the Ag-In disorder drives the formation of Ag-rich and In-rich domains with millisecond lifetimes, with lifetimes increasing at lower temperatures. TR-XAS further reveals that photogenerated STEs oxidize Ag+ to Ag2+, facilitating this highly temporally asymmetric order-disorder transition. Our findings demonstrate a new mechanism, mediated by hole-localized STE formation, that enables prolongation of transient light-induced states to the multi-millisecond regime in double perovskites, opening possibilities to harvesting the functional properties of metastable phases of these materials.
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All authors Shunran Li, Burak Guzelturk, Conrad A. Kocoj, Donald A. Walko, Du Chen, Haidan Wen, Xian Xu, Xiaoming Wang, Bongjun Choi, Borui Li, Zhibo Kang, Cunming Liu, Suchismita Sarker, Benjamin T. Diroll, Xiaoyi Zhang, Yong Q. Cai, Yu He, Deep Jariwala, Yanfa Yan, Diana Y. Qiu, Peijun Guo

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

Diffusive and hydrodynamic magnetotransport around a density perturbation in a two-dimensional electron gas

P. S. Parashar and M. M. Fogler

We study current flow around a density inhomogeneity in a two-dimensional electron gas in the presence of a strong magnetic field. The inhomogeneity is parametrized by a power-law tail with an exponent $\beta > 2$. We show that current and electrochemical potential are exponentially suppressed inside a surrounding area much larger than the geometric size of the perturbation. The corresponding “no-go” radius grows as a certain power of the magnetic field. Residual current and potential exhibit spiraling patterns inside the no-go region. Outside of it, they acquire corrections inversely proportional to the distance, which is known as the Landauer resistivity dipole. The Landauer dipole is rotated by the angle $\pi (1 - 1 / \beta)$ with respect to the average electric field. The rotation direction depends on whether the local density is raised or lowered. We also consider the effect of electron viscosity and show that the variation of the no-go radius with magnetic field becomes more rapid if viscosity is large enough. The Landauer dipole size is set by the Gurzhi length, which is much larger than the no-go radius, which is in turn much larger than the geometric size of the perturbation. Our results may be useful for interpreting nanoimaging of current distribution in graphene and other two-dimensional systems.
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Materials Science · Cross-listed from cond-mat.str-el

Realization of a triangular spin necklace in a verdazyl-based Ni complex

Itsuki Shimamura, Risa Yagura, Takanori Kida et al. (+6 more)

We successfully synthesized a verdazyl-based complex, ($m$-Py-V)$_3$[Ni(NO$_3$)$_2$], in which Ni$^{2+}$ ions and verdazyl radicals form a one-dimensional, triangular spin necklace consisting of spin-1/2 and spin-1 units. Molecular orbital calculations reveal strong antiferromagnetic (AF) interactions between inversion-related radical pairs that form spin-1/2 singlet dimers. The remaining verdazyl and Ni$^{2+}$ spins form frustrated triangular units, creating a distinctive spin network. Magnetic susceptibility and specific heat measurements identify a phase transition to an AF order. The application of magnetic fields suppresses the phase transition signal, suggesting field-induced decoupling of the spin-1 moments. Electron spin resonance measurements are used to evaluate the easy-axis anisotropy of spin-1, which may promote the AF order. This work provides a rare example of a geometrically frustrated quantum spin chain realized via molecular design, thereby offering a platform for exploring frustration-driven quantum phases in low-dimensional materials.
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All authors Itsuki Shimamura, Risa Yagura, Takanori Kida, Masayuki Hagiwara, Koji Araki, Yoshiki Iwasaki, Yuko Hosokoshi, Kenta Kimura, Hironori Yamaguchi

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

Emergence of Kondo-assisted Néel order in a Kondo necklace model

Hironori Yamaguchi, Shunsuke C. Furuya, Yu Tominaga et al. (+3 more)

The interplay between Kondo screening and magnetic order has long been a central issue in the physics of strongly correlated systems. While the Kondo effect has traditionally been understood to suppress magnetism through the formation of local singlets, recent studies suggest that Kondo interactions may enhance magnetic order under certain conditions. However, these scenarios often rely on complex electronic structures, including orbital and charge degrees of freedom, making the essential mechanisms difficult to isolate. Here we report the realization of a spin-(1/2,1) Kondo necklace model in a Ni-based complex-a minimal spin-only analog of the Kondo lattice that isolates quantum spin correlations by eliminating charge degrees of freedom. Thermodynamic measurements identify a magnetic phase transition and a field-induced quantum phase transition. Perturbative analysis reveals that the Kondo coupling mediates effective antiferromagnetic interactions between the spin-1 sites, stabilizing the Néel order across the entire chain. Our results establish a universal boundary in Kondo physics, where coupling to spin-1/2 moments yields singlets, but to spin-1 and higher stabilizes magnetic order.
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All authors Hironori Yamaguchi, Shunsuke C. Furuya, Yu Tominaga, Takanori Kida, Koji Araki, Masayuki Hagiwara

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

Ab Initio Many Body Quantum Embedding and Local Correlation in Crystalline Materials using Interpolative Separable Density Fitting

Junjie Yang, Ning Zhang, Shunyue Yuan et al. (+3 more)

We present an efficient implementation of ab initio many-body quantum embedding and local correlation methods for infinite periodic systems through translational symmetry adapted interpolative separable density fitting, an approach which reduces the scaling of the calculations to only linear with the number of k-points. Employing this methodology, we compute correlated ground-state coupled cluster energies within density matrix embedding and local natural orbital correlation frameworks for both weakly and strongly correlated solids, using up to 1000 k-points. By extrapolating the local correlation domains and k-point sampling we further obtain estimates of the full coupled cluster with singles, doubles, and perturbative triples ground-state energies in the thermodynamic limit.
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All authors Junjie Yang, Ning Zhang, Shunyue Yuan, Jincheng Yu, Hong-Zhou Ye, Garnet Chan

Materials Science

Energy Eigenstates of Electrons, Magnons and Phonons in Fe$_3$O$_4$ (magnetite), MnFe$_2$O$_4$ (jacobsite), and mixed Mn-Zn ferrites

Deepak Dhariwal, Michael R. von Spakovsky and William T. Reynolds Jr

We report first-principles calculations of the electronic structure, magnon excitations, and phonons in magnetite (Fe$_3$O$_4$), jacobsite (MnFe$_2$O$_4$), and mixed manganese-zinc ferrites (Mn$_{x}$,Zn$_{1-x}$)Fe$_2$O$_4$ for representative compositions ($0\le x \le 1$) and A/B-site cation arrangements. Electronic structures are computed using density functional theory (DFT) augmented by rotationally invariant DFT+U+J, with on-site Hubbard and Hund's parameters, $U$ and $J$, respectively, determined self-consistently by spin-polarized linear-response perturbations of the chosen correlated subspaces (including, where applied, the ligand $2p$ subspace). A classical Heisenberg spin Hamiltonian is parameterized by mapping DFT+U+J total energies for multiple collinear spin configurations onto nearest-neighbor exchange couplings, which are then used to obtain magnon dispersions and magnon densities of states within linear spin-wave theory. Phonon spectra and densities of states are obtained from finite-displacement force constants and dynamical matrices computed on the same DFT+U+J-relaxed structures. Overall, the workflow provides a consistent, composition- and configuration-aware route to electronic, vibrational, and magnetic excitation spectra across the Mn/Zn ferrite space.
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Thursday 22 January

Materials Science · Cross-listed from cond-mat.soft

Spherical Balls Settling Through a Quiescent Cement Paste Measured by X-ray Tomography: Influence of the Paste Thixotropy

Subhransu Dhar, Eduardo Machado-Charry, Robert Schennach et al. (+2 more)

The settling of spherical balls in quiescent cement pastes of increasing age is studied. Metallic spheres with radii of 2, 2.5 and 3mm are dropped into the paste and allowed to settle, while their position is tracked using X-ray tomography. The instantaneous velocity of the spheres, calculated from their movement, is observed to be quasi-constant during their fall, and an average is estimated. The results show that the average velocity of the balls decreases logarithmically with paste age until ball stoppage, for all three ball sizes. In parallel, the rheological properties of the cement paste are measured using a rheometer with a vane geometry. The evolution of the paste static yield stress over time is evaluated, and proves to be a reliable predictor for ball stoppage. Finally, thixotropic models of increasing complexity are evaluated. These models consider four forms of structural growth and breakdown parameters, and their ability to capture the ball settling velocity as a function of paste age is compared. This emphasizes the importance of considering paste breakdown in relation to shearing of the paste when the ball passes through it.
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All authors Subhransu Dhar, Eduardo Machado-Charry, Robert Schennach, Teresa Liberto, Agathe Robisson

Materials Science · Cross-listed from quant-ph

Reducing TLS loss in tantalum CPW resonators using titanium sacrificial layers

Zachary Degnan, Chun-Ching Chiu, Yi-Hsun Chen et al. (+5 more)

We demonstrate a substantial reduction in two-level system loss in tantalum coplanar waveguide resonators fabricated on high-resistivity silicon substrates through the use of an ultrathin titanium sacrificial layer. A 0.2nm titanium film, deposited atop pre-sputtered {\alpha}-tantalum, acts as a solid-state oxygen getter that chemically modifies the native Ta oxide at the metal-air interface. After device fabrication, the titanium layer is removed using buffered oxide etchant, leaving behind a chemically reduced Ta oxide surface. Subsequent high-vacuum annealing further suppresses two-level system loss. Resonators treated with this process exhibit internal quality factors Qi exceeding an average of 1.5 million in the single-photon regime across ten devices, over three times higher than otherwise identical devices lacking the titanium layer. These results highlight the critical role of interfacial oxide chemistry in superconducting loss and reinforce atomic-scale surface engineering as an effective approach to improving coherence in tantalum-based quantum circuits. The method is compatible with existing fabrication workflows applicable to tantalum films, offering a practical route to further extending T1 lifetimes in superconducting qubits.
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All authors Zachary Degnan, Chun-Ching Chiu, Yi-Hsun Chen, David Sommers, Leonid Abdurakhimov, Lihuang Zhu, Arkady Fedorov, Peter Jacobson

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

Magnetic structure of EuZn$_2$Sb$_2$ single-crystal thin-film

Yu Wei Soh, Hsiang Lee, Eugen Weschke et al. (+4 more)

Magnetic topological materials are a class of compounds which can host massless electrons controlled by the magnetic order. One such compound is EuZn$_2$Sb$_2$, which has recently garnered interest due to its strong interplay between the Eu magnetism and charge carriers. However the topology of the electronic band structure, which depends on the ground state magnetic configuration of the europium sublattice, has not been determined. Based on our ab-initio calculations, we find that an in-plane and out-of-plane A-type antiferromagnetic (AFM) order generates a topological crystalline insulator and Dirac semimetal respectively, whereas a ferromagnetic (FM) order stabilizes a Weyl semimetal. Our resonant x-ray elastic scattering measurements of single-crystal thin film EuZn$_2$Sb$_2$ reveal both a sharp magnetic peak at $\textit{\textbf{Q}}$=$(0,0,\frac{1}{2})$ and broad $\textit{\textbf{Q}}$=$(0,0,1)$ below $T_{\mathrm{N}}=12.9$\,K, which is associated with an A-type AFM and FM order, respectively. Our measurements indicate that the FM and AFM layers are spatially separated along the crystal $c$ axis, with the former limited to the top three atomic layers. We propose that EuZn$_2$Sb$_2$ behaves as a Weyl semimetal in the surface FM layers, and as a topological crystalline insulator in the lower AFM layers.
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All authors Yu Wei Soh, Hsiang Lee, Eugen Weschke, Shinichi Nishihaya, Mikhael T. Sayat, Masaki Uchida, Jian-Rui Soh

Materials Science

Nuclear quadrupole interaction and zero first-order Zeeman transitions of $^{167}$Er$^{3+}$ in CaWO$_4$

Lewin Marsh, Yikai Yang, Cesare Mattiroli et al. (+5 more)

We report microwave spectroscopy of $^{167}$Er$^{3+}$ doped in CaWO$_4$ which reveals the hyperfine splitting of the erbium electronic ground state ($Z_1$, $J_\mathrm{eff.}$=15/2) induced by the $I$=7/2 nuclear spin. From spectra measured below$\sim$50 mK in magnetic fields up to 200 mT, we extract spin Hamiltonian parameters including the electron $\textbf{g}$, hyperfine $\textbf{A}$, and nuclear electric quadrupolar $\textbf{Q}$ tensors. Crucially, our analysis demonstrate unambiguously, that the previously unobserved nuclear electric quadrupolar moment is essential to reproduce the experimental data. With these refined parameters, we identify zero first-order Zeeman (ZEFOZ) transitions at zero magnetic field. Extending the analysis to finite fields, we uncover that ZEFOZ points lie either along the $c$ axis or within the $a$-$b$ plane. These results establish CaWO$_4$ as a promising host for long lifetime quantum memories.
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All authors Lewin Marsh, Yikai Yang, Cesare Mattiroli, Mikhael T. Sayat, \DJàm Minh Trí, Henrik M. Rønnow, Jevon J. Longdell, Jian-Rui Soh

Materials Science

Thermally-Activated Epitaxy of NbO

Sandra Glotzer, Jeong Rae Kim and Joseph Falson

We demonstrate a thermally-activated epitaxy window for the growth of NbO at temperatures exceeding 1000 $^o$C. NbO films grown in this mode display superior structural and transport properties, which are reproducible across a window of oxygen partial pressure. Through comprehensive analysis, we propose the prototypical electrical properties of NbO, for which a consensus has not yet been made. This study unequivocally demonstrates the utility of high temperatures in the thin film synthesis of refractory metal compounds.
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Materials Science

Towards a single-junction non-concentrator metal halide perovskite hot carrier solar cell: review of current gaps and opportunities in understanding slow hot carrier cooling

Gabriel J. Man

The photovoltaic solar cell is a mature technology, with silicon-based technologies deployed at scale, yet current technologies are limited by the Shockley-Queisser thermodynamic limit, known since the early 1960s. The single-junction non-concentrator hot carrier solar cell operating at ambient temperature - with its theoretically predicted ultimate power conversion efficiency limit of nearly 70% that is twice the Shockley-Queisser limit and is higher than what can be achieved with even n=6 multijunction solar cells - has remained an elusive yet hot research target since the early 1980s. Metal halide perovskite semiconductors were discovered in the late 1970s and photovoltaic applications have been intensively researched and developed since the early 2010s. Current technology development of perovskite solar cells is heavily motivated by their expected cheap processing costs relative to other Shockley-Queisser limited technologies. History has shown that very few absorber materials develop into viable solar cell technologies, and it has been recognized that given the declining costs of silicon-based technologies, a new material must offer potential for both lower cost and higher efficiencies than the Shockley-Queisser limit. Slow cooling of photocarriers with energy in excess of the band edges (hot carriers), which is the first prerequisite of a solar absorbing material for building a hot carrier solar cell technology, has been reported in perovskites since the 2010s. The goal of this review is to illuminate the path towards a single-junction perovskite hot carrier solar cell technology by emphasizing uncertainties in understanding slow hot carrier cooling and recommending approaches to resolve them.
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Materials Science · Cross-listed from physics.optics

Active learning for photonic crystals

Ryan Lopez, Charlotte Loh, Rumen Dangovski et al. (+1 more)

Active learning for photonic crystals explores the integration of analytic approximate Bayesian last layer neural networks (LL-BNNs) with uncertainty-driven sample selection to accelerate photonic band gap prediction. We employ an analytic LL-BNN formulation, corresponding to the infinite Monte Carlo sample limit, to obtain uncertainty estimates that are strongly correlated with the true predictive error on unlabeled candidate structures. These uncertainty scores drive an active learning strategy that prioritizes the most informative simulations during training. Applied to the task of predicting band gap sizes in two-dimensional, two-tone photonic crystals, our approach achieves up to a 2.7x reduction on average in required training data compared to a random sampling baseline while maintaining predictive accuracy. The efficiency gains arise from concentrating computational resources on high uncertainty regions of the design space rather than sampling uniformly. Given the substantial cost of full band structure simulations, especially in three dimensions, this data efficiency enables rapid and scalable surrogate modeling. Our results suggest that analytic LL-BNN based active learning can substantially accelerate topological optimization and inverse design workflows for photonic crystals, and more broadly, offers a general framework for data efficient regression across scientific machine learning domains.
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All authors Ryan Lopez, Charlotte Loh, Rumen Dangovski, Marin Soljačić

Materials Science

High-resolution neutron diffraction determination of noncollinear antiferromagnetic order in the honeycomb magnetoelectric Fe$_{4}$Nb$_{2}$O$_{9}$

Raktim Datta, Kapil Kumar, Dong Gun Oh et al. (+7 more)

Magnetoelectric systems offer potential for device applications exploiting coupled states between electric and magnetic properties. Among magnetoelectric materials, \FNO has attracted special attention because of its pronounced dielectric signal at high magnetic transition temperatures. However, the magnetic ground state, which is essential information for understanding its unusual magnetoelectricity, remains unclarified. Here, we report a noncollinear magnetic ground state of Fe$_{4}$Nb$_{2}$O$_{9}$. To examine the magnetoelectric effect associated with sequential magnetic and structural transitions upon cooling, we conducted combined x-ray diffraction, magnetic susceptibility, magnetization, dielectric constant, and magnetodielectric experiments. Powder neutron diffraction experiments revealed a series of magnetic Bragg peaks and clear splitting of peaks via structural transition. Magnetic Rietveld refinements, combined with group theory analysis, determined a noncollinear antiferromagnetic structure including a significant $c$-axis moment component at 1.5 K. This study provides insights into the understanding of its magnetoelectric properties.
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All authors Raktim Datta, Kapil Kumar, Dong Gun Oh, Dongwook Kim, Rahul Goel, Nara Lee, Ara Go, Young Jai Choi, Valery Kiryukhin, Sungkyun Choi

Materials Science

Biexcitons in Ruddlesden-Popper Metal Halides Probed by Nonlinear Coherent Spectroscopy

Katherine A. Koch, Carlos Silva-Acuña and Ajay Ram Srimath Kandada

Excitons and their correlated complexes underpin the rich photophysics of quantum-confined semiconductors. Among these, biexcitons – bound states of two electrons and two holes – provide a sensitive probe of Coulomb correlations, exciton-exciton interactions, and the role of the dielectric environment. In Ruddlesden-Popper metal halide materials (RPMHs), strong quantum and dielectric confinement stabilize excitons with binding energies of hundreds of meV, creating an ideal platform for multi-exciton phenomena. Conventional linear spectroscopies, such as photoluminescence and transient absorption, reveal biexciton signatures but suffer from spectral congestion and reabsorption artifacts. Two-dimensional coherent spectroscopies, particularly two-quantum (2Q) multidimensional techniques, uniquely access multi-exciton coherences and provide unambiguous estimates of biexciton binding energies. This minireview surveys the spectroscopic evidence for biexcitons in RPMHs, highlights the advantages of nonlinear multidimensional approaches, and situates biexciton physics within the broader context of excitonic materials, including GaAs quantum wells, quantum dots, and transition-metal dichalcogenides. By emphasizing the interplay of exciton-exciton annihilation, excitation-induced dephasing, and biexciton formation, we argue that multidimensional coherent spectroscopy offers the most reliable pathway to disentangle many-body interactions in quantum-well derivatives of metal-halide perovskites.
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Materials Science · Cross-listed from cond-mat.str-el

Interface Spin-orbit Coupling Induced Room-temperature Ferromagnetic Insulator

Yuhao Hong, Shilin Hu, Ziyue Shen et al. (+10 more)

Fabricating room-temperature ferromagnetic insulators, which are crucial candidates for next-generation dissipation-free quantum and spintronic devices, remains a significant challenge. In this Letter, we report on the epitaxial synthesis of novel room-temperature ferromagnetic insulating thin films created through the precise construction of (111)-oriented 3d/5d interfaces. Our analysis indicates that, unlike conventional doping methods, the (111)-oriented SrIrO3/La2/3Sr1/3MnO3 (SIO/LSMO) interfaces exhibit markedly enhanced spin-orbit coupling. This enhanced interfacial spin-orbit coupling strengthens the electron-phonon coupling in LSMO, thereby shortening the electronic mean free path. As a result, the intrinsic metallicity of LSMO is suppressed, giving rise to a new ferromagnetic insulating phase that emerges between the ferromagnetic metal and paramagnetic insulator regimes of the LSMO phase diagram. Furthermore, the temperature window of the ferromagnetic insulating phase can be tuned by precisely controlling the thickness of the LSMO layers. Our Letter reveals a new strategy for developing ferromagnetic insulators by engineering 3d/5d interfaces and orientations, paving a way for the development of novel dissipation-free quantum and spintronic devices.
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All authors Yuhao Hong, Shilin Hu, Ziyue Shen, Chao Deng, Xiaodong Zhang, Lei Wang, Long Wei, Qinghua Zhang, Lingfei Wang, Liang Si, Yulin Gan, Kai Chen, Zhaoliang Liao

Materials Science

Physical and Dielectric Properties of Polycrystalline LaV$_{0.5}$Nb$_{0.5}$O$_4$

Ashok Kumar, Simranjot K. Sapra, Ramcharan Meena et al. (+3 more)

We report a detailed investigation of the structural, electronic, vibrational, and dielectric properties of polycrystalline LaV$_{0.5}$Nb$_{0.5}$O$_4$ samples, prepared at two sintering temperatures (1000\degree C and 1250\degree C). The introduction of Nb$^{5+}$ at the V$^{5+}$ site leads to notable structural and vibrational changes, which can be attributed to their isoelectronic nature and the comparatively larger ionic radius of Nb$^{5+}$. The Rietveld refinement of the X-ray diffraction patterns confirms a coexistence of monoclinic ($P$2$_{1}$/$n$) and scheelite-type tetragonal ($I$4$_{1}$/$a$) phases; for example, with a fraction of 4% and 96% for the sample annealed at 1250\degree C. The particle morphology has altered from spherical (1000\degree C) to irregular-shaped (1250\degree C) as a result of increase in annealing temperature. The Raman spectroscopy, Fourier Transform Infrared spectroscopy and X-ray Photoemission Spectroscopy have been used to understand the vibrational and electronic properties. An optical band gap of 2.7~eV for the sample sintered at 1250\degree C is calculated using Ultraviolet-vis diffuse reflectance spectroscopy measurements. The dielectric studies shows the higher dielectric permittivity ($\epsilon$$_{r}$) and lower dielectric loss for the sample annealed at 1250\degree C.
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All authors Ashok Kumar, Simranjot K. Sapra, Ramcharan Meena, Vinod Singh, Anita Dhaka, Rajendra S. Dhaka

Materials Science

Facile Optimization of Combinatorial Sputtering Processes with Arbitrary Numbers of Components for Targeted Compositions

Shelby Sutton Fields, Christopher David White, Keith Knipling et al. (+1 more)

Combinatorial sputtering is a physical vapor deposition method that enables the high-throughput synthesis of compositionally varied thin films. Using this technique, the effects of stoichiometry on specific properties of alloy thin films with analog composition gradients can be mapped using high-throughput characterization. To obtain specific stoichiometries, such as those desired for an equiatomic, intermetallic, or doped compounds, the sputter power of each target must be simultaneously tuned to optimize the deposition rate of each component. This optimization problem increases in complexity with the number of components, which commonly leads to iterative guess-and-check processing and can limit the intrinsic high-throughput advantages of this synthesis method. To circumvent this challenge, this work introduces a composition optimization procedure that enables the facile synthesis of sputtered combinatorial films with targeted compositions. This procedure leverages the expeditious mapping of composition using wavelength dispersive x-ray fluorescence and is capable of optimizing processing for an arbitrary number of components. As a demonstration, this method is leveraged to sputter a combinatorial Cr$_{v}$Fe$_{w}$Mo$_{x}$Nb$_{y}$Ta$_{z}$ film with an equiatomic composition near the wafer center.
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All authors Shelby Sutton Fields, Christopher David White, Keith Knipling, Steven Bennett

Materials Science

Electric-Switchable Chiral Magnons in PT-Symmetric Antiferromagnets

Jinyang Ni, Congzhe Yan, Peiyuan Cui et al. (+3 more)

The magnons in antiferromagnetic insulators (AFIs) exhibit dual chirality, each carrying opposite spin angular momentum. However, in PT-symmetric AFIs, the magnon bands remain degenerate. In this work, we introduce a new class of PT-preserving AFIs in which the giant chiral splitting of magnons can be induced and controlled by an external electric field. Unlike conventional cases, such AFIs host a hidden dipole coupled to the antiferromagnetic order, which allows an external electric field to break the magnon sublattice symmetry and thereby largely lift the band degeneracy. Group theoretical analysis identifies the possible magnetic layer groups, while first-principles calculations and spin-wave theory reveal band splittings up to 20meV in Cr2CCl2 and Cr2CBr2 under the electric field of 0.2 V/Å, corresponding to an effective magnetic field of 200T. In addition, the electrically controlled magnon chiral splitting enables reversible switching of magnon-mediated spin currents. These findings open a new route toward nonvolatile spintronics based on magnons.
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All authors Jinyang Ni, Congzhe Yan, Peiyuan Cui, Zhijun Jiang, Yuanjun Jin, Guoqing Chang

Materials Science

Towards a Modern Theory of Chiralization

Nicola A. Spaldin

The Modern Theory of Polarization, which rigorously defines the spontaneous electric polarization of a periodic solid and provides a recipe for its computation in electronic structure codes, transformed our understanding of ferroelectricity and related dielectric properties. Here we call for the development of an analogous Modern Theory of Chiralization. We review earlier attempts to quantify chirality, highlight the fundamental and practical developments that a modern theory would facilitate, and suggest possible promising routes to its establishment.
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Materials Science

CVD grown bilayer MoS2 based artificial optoelectronic synapses for arithmetic computing and image recognition applications

Umakanta Patra, Subhrajit Sikdar, Roshan Padhan et al. (+3 more)

Demand for lower computing power has rapidly increased. In this context, brain-inspired neuromorphic computing, which integrate data storage and processing, has attracted significant attention. Here, our study reveals that field effect transistors fabricated on chemical vapor deposited bilayer (2L) MoS2 films can mimic the functions of biological synapse. These devices demonstrate high level of pair pulse facilitation (PPF), short term to long term memory (STM-to-LTM) transition as well as learning-forgetting-relearning properties. Effect of light intensity, pulse number, pulse width and photon energy on the STM-to-LTM transition is studied. It has been found that the rate of depression of the memory state can be controlled using the gate bias. Electrical and optical energy consumptions per synaptic event are estimated to be as low as 280 fJ and 20 nJ, respectively. Furthermore, photocurrent in these devices is observed to increase linearly with the number of the excitation pulses. This property has been exploited to demonstrate different arithmetic operations by the device. Moreover, these devices show great potential for image recognition. Artificial neural network simulation has returned an image recognition accuracy of ~85%. All these findings show a great prospect of 2L-MoS2 for developing low power, transparent and flexible neuromorphic devices.
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All authors Umakanta Patra, Subhrajit Sikdar, Roshan Padhan, Amandeep Kaur, Satyaprakash Sahoo, Subhabrata Dhar

Materials Science

Strain as a topological selector in altermagnetic CrSb

Sumohan Giri and Nirmal Ganguli

Altermagnetism combines fully compensated magnetic order with a magnetic symmetry that relates inequivalent spin sublattices, offering a promising, still underexplored platform for unconventional topological phases. Here we show that both isotropic tensile strain and electron localization, controlled by an effective Hubbard interaction $U_{\text{eff}}$, can act as efficient and systematic topological control parameters in the altermagnetic Weyl semimetal CrSb. While CrSb hosts Weyl fermions at equilibrium, modest tensile strain of 4-5% stabilizes additional symmetry allowed Dirac crossings and triple-point fermions, with further strain selectively favoring the triple-point phase. We propose a 3D low-energy Hamiltonian that captures the interplay between the Hubbard interaction $U$ and the sublattice symmetry of the altermagnet, giving rise to an interaction-driven Dirac crossing. Our results establish CrSb as a model altermagnet in which either strain or electron localization can selectively access and control the distinct topologies inherent to the altermagnets.
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Materials Science · Cross-listed from physics.optics

Size-dependent Dielectric Permittivity of Perovskite Nanocrystals

Jehyeok Ryu, Victor Krivenkov, Vitaly Goryashko et al. (+2 more)

Perovskite nanocrystals (PNCs) are promising building blocks for quantum photonic devices. Optical properties of PNCs can be enhanced by integration with optical cavities or nanoantennas. Designing such structures requires accurate size dependent dielectric permittivity of PNCs. However, current reports provide primarily ensemble averaged values with limited access to the intrinsic response of individual PNCs. Here we suggest a methodology to reconstruct the size dependent complex dielectric permittivity of CsPbBr3 PNCs from the measured absorbance spectrum of colloidal solution. The permittivity of PNCs is modeled as a sum of Voigt profile oscillators, with the size dependent transition energies governed by the exciton effective mass. Using a transmission electron microscopy derived size distribution of the PNCs, the solution permittivity is obtained via Maxwell Garnett effective medium approximation. This permittivity is used in a transfer matrix method to simulate and fit the absorbance spectrum, from which the permittivity of PNCs is reconstructed. The extracted spectral linewidth from the imaginary part of the permittivity (78.4 meV) is consistent with single nanocrystal emission linewidths at room temperature. Finite element simulations show enhanced absorption cross section of a single PNC coupled to a nanoantenna, demonstrating applicability of the extracted permittivity. More generally, these findings provide a route to extract intrinsic permittivity of individual nanocrystals from absorbance measurements of their ensembles.
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All authors Jehyeok Ryu, Victor Krivenkov, Vitaly Goryashko, Yury Rakovich, Alexey Y. Nikitin

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

Topological Semimetal Transport Modulated by Interstitial Fe in Ba(Fe$_{1-x}$Co$_x$)$_{2+\delta}As$_2$ Superconductors

Ze-Xian Deng, Qiang-Jun Cheng, Jing Jiang et al. (+14 more)

Topological semimetals are renowned for exhibiting large, unsaturated magnetoresistance arising from ultrahigh carrier mobility and electron-hole compensation. However, such behaviors remain poorly understood in iron-based superconductors that have been recently recognized to harbor rich nontrivial topology. Here, we combine angle-resolved magneto-transport measurements with first principles calculations to reveal the emergence and tunability of topological semimetals in ferropnictide Ba(Fe$_{1-x}$Co$_x$)$_{2+\delta}As$_2$ epitaxial films, modulated by interstitial Fe. These states exhibit ultralow residual resistivity, coexisting high-mobility electron and hole carriers, and linear positive magnetoresistance below 110 K. Remarkably, the magnetoresistance becomes more pronounced when the magnetic field is applied parallel to the film plane, reaching an unsaturated 1206% at 56 T. Furthermore, superconductivity persists in these ferropnictide films, establishing them as a tunable platform for investigating the interplay among electron correlation, topology, and superconductivity.
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All authors Ze-Xian Deng, Qiang-Jun Cheng, Jing Jiang, Yong-Wei Wang, Xi Zhou, Ming-Qiang Ren, Cong Cong Lou, Xiao-Xiang Chen, Bin-Jie Wu, Zeng-Wei Zhu, Qing-Hua Zhang, Lin Gu, Ding Zhang, Kai Liu, Xu-Cun Ma, Qi-Kun Xue, Can-Li Song