Materials Science · Cross-listed from physics.app-ph
Slow dynamics in concrete: Effects of temperature, strength variation, and microcracking damage
Clayton Malone and Jinying Zhu
In this study, we investigated the slow dynamic behavior of concrete with varying compressive strengths ($f'_c = 32\sim56$ MPa) and alkali-silica reaction (ASR) damage. Concrete prisms were conditioned by compressive loading and recovery was monitored by coda wave interferometry (CWI). A self-referencing temperature correction technique was used to minimize the effect of ambient temperature changes, demonstrating the importance of careful temperature control and correction. For intact specimens, both the recovery rate, $m_v$, and velocity drop magnitude, $|c|$, generally increased with compressive strength. Recovery times were calculated from the fit parameters, and higher strength specimens were found to recover faster (9.9 h for $f'_c = 32$ MPa, 6.6 h for $f'_c = 56$ MPa). When the same stress was applied, ASR-damaged specimens had increased softening and faster recovery rates, but longer recovery times (up to 458 h). These findings demonstrate the potential of slow dynamics in concrete characterization, through its ability to evaluate the strength of intact samples and its sensitivity to microcracking in damaged samples.
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Materials Science
Sn-Doping in LPCVD-Grown (010) $\beta$-Ga$_2$O$_3$ Films
Saleh Ahmed Khan, Ahmed Ibreljic, Sourav Sarker et al. (+1 more)
In this work, Sn-doped (010) $\beta$-Ga$_2$O$_3$ homoepitaxial films were grown by low-pressure chemical vapor deposition (LPCVD), and the influence of Sn incorporation on their structural, morphological, and electrical properties was systematically investigated. Controlled room-temperature carrier concentrations ranging from $1.17 \times 10^{17}$ to $3.06 \times 10^{18}$ cm$^{-3}$ were achieved, with corresponding Hall mobilities decreasing from 113 to 63 cm$^2$ V$^{-1}$ s$^{-1}$. The films exhibited the monoclinic $\beta$-Ga$_2$O$_3$ phase, near-stoichiometric composition, and well-defined step-flow morphology, with a minimum rocking-curve FWHM of 68.4 arcsec and an RMS roughness of 2.63 nm. Film thicknesses ranging from 1.66 to 11.3 $\mu$m were obtained at growth rates of 6.4 to 16.6 $\mu$m h$^{-1}$, demonstrating the ability of LPCVD to produce thick epitaxial layers. The sample with a room-temperature carrier concentration of $1.17 \times 10^{17}$ cm$^{-3}$ exhibited room-temperature and low-temperature Hall mobilities of 113 cm$^2$ V$^{-1}$ s$^{-1}$ and 380 cm$^2$ V$^{-1}$ s$^{-1}$ at 84 K, respectively. Both represent the highest reported values for LPCVD-grown Sn-doped $\beta$-Ga$_2$O$_3$. Transport modeling of the same sample yielded a shallow donor activation energy of 32.7 meV, a deeper donor level at 95 meV, and a low compensating acceptor concentration of $2.0 \times 10^{16}$ cm$^{-3}$, indicating efficient donor activation and a low degree of compensation. These results demonstrate that LPCVD enables controlled Sn doping while maintaining excellent structural and electrical quality, providing a viable route for realizing thick $\beta$-Ga$_2$O$_3$ epitaxial drift layers.
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All authors Saleh Ahmed Khan, Ahmed Ibreljic, Sourav Sarker, Anhar Bhuiyan
Materials Science
Understanding the Oxygen Reduction Reaction and Oxygen Evolution Reaction in Metal Intercalated Biphenylene Bilayers
Henri G. Mendonça, Pedro H. Souza, Walter Orellana et al. (+1 more)
We conducted an ab initio study of the oxygen reduction reaction (ORR) and oxygen
evolution reaction (OER) in metal-encapsulated biphenylene bilayers, B/M/B, with M = Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Nb, Ru, W, Os and Pt. In most systems, the intercalated metal sits at the square carbon sites (C$^{468}$) of the biphenylene
lattice. Using a computational hydrogen electrode approach, we evaluated the reaction energetics at these active sites. Several B/M/B systems show competitive ORR and OER performance. Among the investigated systems, Cu, Pt, Ru, and Mn exhibit the lowest ORR overpotentials of 0.42, 0.44, 0.50, and 0.56 V, respectively, while Fe is identified as the most active catalyst for OER with an overpotential of 0.44 V. To understand the catalytic trends, we looked at the electronic structure through the metal $d-$band centers, the C$^{468}$ $p_z-$band centers, and the corresponding orbital charge populations. The band centers did not give a simple
polynomial dependence on the overpotentials, though they did point to favorable electronic ranges for the best catalysts. The $d-$orbital charge population of the encapsulated metal, however, correlated most clearly with activity-especially for OER-yielding volcano-type plots. From these, B/Fe/B emerges as the best OER catalyst, while B/Mn/B lies closest to the ORR optimum. The $p-$orbital population at the active carbon site also captures the main trends, albeit less strongly. Overall, these results show that straightforward electronic descriptors can predict catalytic behavior in metal-encapsulated biphenylene bilayers and guide the search for efficient catalysts where the carbon framework itself drives the reactivity.
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All authors Henri G. Mendonça, Pedro H. Souza, Walter Orellana, Roberto H. Miwa
Materials Science · Cross-listed from cond-mat.mes-hall
Ferroelectric-controllable spin-orbit torque in two-dimensional multiferroic heterostructure
Weiyi Pan, Gusthavo M.S. Brizolla and Jaroslav Fabian
Spin-orbit torque (SOT), which enables electrical control of magnetization, plays a crucial role in the development of next-generation spintronic devices. Realizing SOT in two-dimensional van der Waals systems, together with achieving efficient nonvolatile manipulation via ferroelectricity, would be highly beneficial for the implementation of tunable logic devices with enhanced storage density. In this work, based on first-principles calculation and using a multiferroic Fe$_{3}$GeTe$_{2}$/In$_{2}$Se$_{3}$
heterostructure as a representative example, we demonstrate that switching the ferroelectric polarization of the In$_{2}$Se$_{3}$ layer induces a pronounced modification in the magnetization-dependent distribution of torkance within the
heterostructure. Specifically, when the magnetization is in the plane, where the torque is maximal, reversing the polarization of In$_{2}$Se$_{3}$ from upward to downward enhances the total torkance to more than 150% of its original value. This substantial variation primarily originates from the polarization-induced modulation of the $z$ component of the time-reversal-odd torkance, which is mainly associated with an approximately 233% change in the atomic-resolved torque contributed from the middle Fe layer in Fe$_{3}$GeTe$_{2}$ layer. Further analysis reveals that the electronic states near $\Gamma$ on the Fermi surface undergo significant reconstruction upon polarization switching, which is responsible for the observed variation in the time-reversal-odd torque. Our results not only provide new insights into the functional potential of van der Waals multiferroic
heterostructures, but also offer a viable strategy for achieving electrically tunable SOT, paving the way for future programmable spintronic devices.
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Materials Science · Cross-listed from cond-mat.str-el
Engineering correlated phases through manipulation of Van Hove singularities
Thomas P. Sheerin, Maria Ramirez, Chris A. Hooley et al. (+1 more)
Controlling the ordered phases of correlated electron systems remains a central challenge in
quantum materials design. Divergences in the electronic density of states, known as Van Hove singularities (VHSs), are one obvious route to such control. It is clear from recent work that the exact functional form of these divergences can profoundly affect which phases are realized; a full picture, however, remains elusive. In this work, we use both the hot-spot parquet renormalization group and the truncated-unity functional renormalization group to theoretically study the emergent correlated states of a two-dimensional square-
lattice Hubbard model with VHSs at or near the Fermi level. By varying a single hopping parameter, $t_3$, we are able to change the strength of the VHS divergence in the density of states from logarithmic (for $t_3 < t_{3c}$) to power-law (for $t_3 = t_{3c}$). Further increase of $t_3$ ($t_3 > t_{3c}$) causes each original Van Hove point to split into two, both of the conventional logarithmic type. We show that which of these regimes we are in strongly influences the predicted ordered states. We also study the dependence on doping, and find that the ferromagnetic state that occurs at Van Hove filling in these models is unstable to very small shifts in the Fermi level, often giving way to distinct ordered states depending on whether the model is electron- or hole-doped. These results highlight the importance of tuning VHS properties to control ordered states in correlated materials, and offer design rules to engineer these phases in novel systems.
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All authors Thomas P. Sheerin, Maria Ramirez, Chris A. Hooley, Luke C. Rhodes
Materials Science · Cross-listed from quant-ph
Quantum Relaxometry Under Continuous Wave Excitation
Vladimir Verkhovlyuk, Chayma Bouchair, Oleg A. Anisimov et al. (+2 more)
Quantum relaxometry is one of the most successful applications of nitrogen-vacancy (NV) centers in diamond and, more broadly, solid-state
spin qubits, enabling ultrasensitive detection of
magnetic noise and paramagnetic species via measurements of the
spin-
lattice relaxation time $T_1$. Conventional pulsed protocols, however, probe $T_1$ efficiently only over a limited temporal range, which restricts the scope and throughput of the technique. Here we introduce a continuous-wave
quantum relaxometry protocol that operates in the frequency domain. By measuring the frequency response of the optically detected
magnetic resonance signal under low-frequency microwave amplitude modulation, we extract $T_1$ from the characteristic response time of the
spin system. The method enables efficient $T_1$ measurements spanning more than three orders of magnitude – directly demonstrated from 60 $\mu$s to 200 ms in our experiments – across a broad temperature range and under substantial ensemble inhomogeneity. We further show that this protocol enables quantitative relaxometry-based sensing in nanodiamonds, achieving a substantial speed-up over the pulsed methods and offering a practical approach to optimizing nanodiamond size for enhanced sensitivity.
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All authors Vladimir Verkhovlyuk, Chayma Bouchair, Oleg A. Anisimov, Anton Pershin, Adam Gali
Materials Science · Cross-listed from physics.app-ph
Determination of acoustic nonlinearity parameters using thermal modulation of ultrasonic waves
Hongbin Sun and Jinying Zhu
This study presents a test method and its theoretical framework to determine the acoustic nonlinearity parameters ($\alpha,\beta,\delta$) of material using thermal modulation of ultrasonic waves. Temperature change induced thermal strain excites the nonlinear response of the material and modulates the ultrasonic wave propagating in it. Experimental results showed a strong correlation between the relative wave velocity change and the temperature change. With a quadratic
polynomial model, the acoustic nonlinearity parameters were obtained from the
polynomial coefficients by curve fitting the experimental curves. Their effects on thermal-induced velocity change were discussed. The parameters $\alpha,\beta,\delta$ govern the hysteretic gap, average slope, and
curvature of the correlation curve, respectively. The proposed theory was validated on aluminum, steel, intact and damaged concrete samples. The obtained nonlinear parameters show reasonable agreements with values reported in the literature. Compared to other nonlinear acoustic methods using vibration or acoustic excitation, the thermal modulation method generates more uniform, slow changing, and larger strain field in the test sample. Employing thermal effect as the driving force for nonlinearity instead of an undesired influencing factor, this method can measure the absolute values of $\alpha,\beta,\delta$ with good accuracy using a simple ultrasonic test setup.
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Materials Science · Cross-listed from cond-mat.mes-hall
Optimally embedded tight binding for reproducing geometry dependent observables
Jonas J Telle and Gunnar F Lange
In tight-binding models, the position operator is reduced to intra-
cell orbital positions (
embeddings). While accurately reproducing band structures, such models often fail for geometry dependent responses depending on the position operator. To address this, we investigate the role of these
embeddings and introduce the general framework of optimally embedded tight binding. Treating the
embeddings as
geometric tuning parameters to be fixed against a reference response (obtained from ab-initio computation or experiment), we obtain tight-binding models of GaAs and CdS which quantitatively reproduce non-linear optical responses at no cost to band structure accuracy. The optimal
embeddings are determined efficiently using position derivatives obtained from decomposing tight-binding observables into a geometry independent and dependent part, and explicit derivatives are provided for key quantities such as the
quantum geometric tensor. The decomposition reveals where
geometric effects dominate, and we show in both toy models and in the Chern
insulator V2O3 how geometry can dramatically alter the local metric trace. Our results highlight that orbital
embeddings should be treated as a genuine model parameter which should be explicitly fixed against physical data to get accurate minimal models.
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Materials Science
Defect-Controlled Multiferroicity via Stacking Control in Nonmagnetic van der Waals Bilayers
Bumseop Kim, Sayed Ali Akbar Ghorashi and Andrew M. Rappe
We present a general paradigm that directly couples vacancy-localized magnetism to interfacial sliding ferroelectricity in nonmagnetic van der Waals (vdWs) bilayers. Utilizing bilayer hexagonal boron nitride (hBN) as a prototypical vdWs host system, we use first-principles calculations to show that a single vacancy acts as a local registry sensor, lifting the degeneracy between polar sliding partners via a defect-centered polarization offset. For interlayer vacancy pairs, we discover a defect selectivity where the hosting sublattice fully dictates the interlayer exchange, stabilizing either ferrimagnetic or antiferromagnetic configurations. Applying an out-of-plane electric field selects the polar registry and drives an amplitude modulation of the compensated Néel order parameter. These findings establish a robust, sublattice-dependent engineering of multiferroic functionality via stacking control across a wide class of nonmagnetic 2D
heterostructures.
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Materials Science · Cross-listed from physics.flu-dyn
Fluid-Structure Interaction and Underwater Hydrostatic Implosion of Thin-Walled Metallic Cylinders in Semi-Confined Conditions
Bolaji Oladipo, Helio Matos, Arun Shukla et al. (+1 more)
This study presents a comprehensive numerical investigation of the dynamic behavior and fluid-structure interactions (FSI) of metallic cylinders undergoing hydrostatic collapse in semi-confined fluid environments using a structured Arbitrary Eulerian-Lagrangian (ALE) formulation in LS-DYNA. The numerical model reproduces the experimentally measured collapse pressure of 3.69 MPa and predicts the first water hammer peak with a 1.01% error, demonstrating high predictive fidelity. Following validation, the effects of material type (aluminum and titanium), cylinder slenderness ratio (L/D), and confinement diameter on collapse behavior, pressure
evolution, and fluid motion are examined. Titanium cylinders exhibited sharper collapses, higher water hammer pressures exceeding 70 MPa, and greater kinetic and strain energy accumulation than aluminum due to their higher stiffness and yield strength. Lower L/D ratios produced more abrupt collapses, whereas higher L/D ratios promoted more gradual, axisymmetric deformation. Larger confinement diameters intensified jet formation and increased fluid velocities. The simulations provide mechanistic insight into the coupling between structural deformation and surrounding fluid, showing that geometry, material stiffness, and confinement govern collapse-induced energy transfer. Full-field FSI analysis captures key phenomena, including radial jetting, peak fluid velocities, and internal cavitation, that are not evident from pressure-time histories alone. These findings provide quantitative guidance for the design and safety assessment of subsea pressure housings, marine pipelines, and other underwater structures subjected to extreme hydrostatic loading.
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All authors Bolaji Oladipo, Helio Matos, Arun Shukla, Sumanta Das
Materials Science · Cross-listed from cs.AI
Agent-MD: Selective LLM Intervention with Event-Driven Escalation for Stateful GCMC–MD Campaigns
Yijie Wang, Zhen-Yu Yin, Zhenheng Tang et al. (+1 more)
Long-running molecular simulation campaigns require repeated continuation from saved states, provenance-aware progression, adaptive assessment, and occasional interpretation of workflow conditions that cannot be resolved safely by fixed rules. Here, we present
Agent-MD, a framework that places large
language model (LLM)
reasoning selectively at campaign construction and event-triggered review, while routine simulation, analysis, continuation, archiving, and state progression are handled by a persistent rule-based campaign
agent using approved policies and explicit state records.
Agent-MD was demonstrated in a grand canonical Monte Carlo-molecular dynamics (GCMC-MD) water-vapor desorption campaign comprising five montmorillonite systems and three sequential relative-humidity states (RH = 0.9-0.3-0.1). Across 15 system-RH states, the workflow completed 120 segmented simulation cycles with state-specific sampling lengths and provenance-aware restart inheritance. Routine production required no live
reasoning-
agent invocation, while one state reached a review boundary; two preserved incidents were subsequently evaluated through blinded
reasoning-
agent replay, which identified the underlying workflow problems and recommended appropriate follow-up actions. The simulations also revealed distinct composition-dependent low-RH responses, with Ca-bearing montmorillonite retaining more interlayer water and maintaining a larger basal spacing than the Na- and K-bearing systems, while the highest-charge Na system retained more residual water under dry conditions. These results demonstrate that long-running scientific workflows need not place every operation inside an LLM
reasoning loop: selective
reasoning can instead be combined with deterministic execution, structured evidence, and validated control handoffs to provide reproducible and auditable
agent-assisted molecular simulation.
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All authors Yijie Wang, Zhen-Yu Yin, Zhenheng Tang, Xiaowen Chu
Materials Science
Permutation invariant neural network prediction of vacancy formation under deformation and varying chemical environment in FCC high entropy alloys
Tanvir Sohail and Swarnava Ghosh
Vacancy formation energies govern
diffusion, irradiation damage, phase stability, and dynamic failure in high-
entropy alloys (HEAs), yet their strong dependence on local chemical environments and mechanical deformation makes atomistic calculations prohibitively expensive for large-scale studies. Here, we develop an atomistically informed permutation invariant machine learning framework for predicting strain dependent vacancy formation energies in FCC HEAs from local atomic environments. The model employs a vacancy-centered representation constructed from objective
geometric descriptors together with invariants of the local deformation gradient, enabling the coupled effects of chemical disorder and finite deformation to be learned within a unified framework. Atomistic simulations reveal that volumetric deformation is the dominant factor controlling the average variation in vacancy formation energy, whereas shear deformation has a comparatively minor influence. At the same time, substantial site to site variability persists under identical macroscopic loading, demonstrating that local chemical environments govern the statistical distribution of vacancy energetics beyond species-averaged trends. The proposed framework accurately predicts vacancy formation energies across diverse deformation states while providing orders-of-magnitude faster evaluation than direct atomistic simulations. These results establish an efficient route for incorporating stress-dependent defect energetics into multiscale models of
diffusion, irradiation damage, and dynamic failure in chemically complex
alloys.
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Materials Science
DynaCrys: Crystal Generation with Dynamic Space-Group Diffusion
Zhuotao Jin, Xiaoyun Wang, Nicholas Brawand et al. (+5 more)
The search for new crystalline materials spans an enormous compositional and structural space. Generating candidates in this space requires jointly modeling discrete crystallographic
symmetry, elemental composition, and continuous geometry. We introduce DynaCrys, a generative model for
crystals in which the space group co-evolves with Wyckoff occupations and elements through a coupled symbolic
diffusion process. The structured space-group transitions follow crystallographic group-subgroup relations. As the space group changes, a shared, pretrained
symmetry codebook provides both the legality-constrained
stochastic decoder and the
symmetry-constrained
crystal-geometry model with a common representation of the corresponding Wyckoff vocabulary. Across large-scale evaluations using two independent relaxation-and-evaluation engines, DynaCrys achieves best-in-class performance in
symmetry-aware discovery of stable, unique, and novel
crystals, both overall and under the additional requirement of nontrivial post-relaxation
symmetry. It also enables fast sampling while generating structures with consistently low relaxation-induced structural displacements.
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All authors Zhuotao Jin, Xiaoyun Wang, Nicholas Brawand, Roman Zubatyuk, Atul Thakur, Eric Qu, Boris Kozinsky, Justin Smith
Materials Science · Cross-listed from cond-mat.mes-hall
Type-II Mirror Chern Insulator in Altermagnets
Amrita Mukherjee, Pritesh Srivastava, Rahul Verma et al. (+1 more)
Altermagnets with momentum-dependent
spin splitting despite zero net magnetization can support unique topological states under broken time-reversal
symmetry. We predict a mirror-
symmetry-protected topological crystalline
insulator with momentum-separated edge modes in a two-dimensional altermagnet. Using a square-octagon
lattice model, we show that altermagnetic order generates
symmetry-related valley-polarized Dirac nodes, which are gapped by
spin-orbit coupling to yield a mirror Chern
insulator with $C_{\mathcal{M}}=2$. In contrast to conventional mirror Chern
insulators, where the two mirror-protected edge modes cross at the same momentum to form a Dirac cone, altermagnetic
spin splitting and valley-selective band inversion separate these edge modes in momentum. We refer to this phase as a type-II mirror Chern
insulator. We further propose a PbSe/$\mathrm{V_2Se_2O}$ heterobilayer as a candidate material for realizing this phase through the altermagnetic proximity effect. Our results establish altermagnetism as a route to mirror-protected topological phases with momentum-separated edge modes.
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All authors Amrita Mukherjee, Pritesh Srivastava, Rahul Verma, Bahadur Singh
Materials Science · Cross-listed from cond-mat.soft
Segmental Dynamics in the Strain-hardening Regime for Poly(methyl methacrylate) Glasses with and without Melt-stretching
Enran Xing, Trevor Bennin, Masoud Razavi et al. (+1 more)
Strain-hardening is a feature of
polymer glasses during large deformation, which helps to stabilize the glasses against breakage. Experimentally, little is known about the segmental dynamics during strain-hardening, and such data is important for building a molecular-level theory of
polymer glasses deformed in this regime. Here, using a photobleaching technique, we measured the segmental dynamics of lightly-crosslinked poly(methyl methacrylate) (PMMA) glasses with and without melt-stretching, which were deformed into the strain-hardening regime with local engineering strain rates from 10^-4.6 s^-1 to 10^-4 s^-1 at Tg-23K and Tg-33K. We find that melt-stretched PMMA glasses show a more prominent strain-hardening feature and faster segmental dynamics by a factor of about 15% compared to PMMA without melt-stretching. At a given true strain rate, the segmental dynamics of PMMA without melt-stretching are accelerated in the deep strain-hardening regime from the value just beyond yield, by up to 40% at 0.8 true strain. Our observations are in agreement with previously published simulation results.
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All authors Enran Xing, Trevor Bennin, Masoud Razavi, M. D. Ediger
Materials Science · Cross-listed from cond-mat.str-el
Entwined lattice of atoms and anionic electrons in layered electride LaCl
Songyuan Geng, Xin Wang, Jianqi Zhong et al. (+14 more)
Controlling the
lattice geometry that governs electronic structure is a central theme in condensed-matter physics, yet in crystalline solids this geometry is usually fixed by the atomic framework. Electrides offer an alternative route to electronic structure design in which their excess electrons can organize into anionic electron
lattice (AEL) and provide a
lattice-like degree of freedom. Recent work has highlighted the standalone limit, where the AEL in YCl yields bands well described by the dice-
lattice model. Here, using angle-resolved photoemission spectroscopy (ARPES), we show that LaCl, although isostructural to YCl, realizes a qualitatively different regime where the AEL is entwined with the La cation framework, producing a fully reconstructed electronic structure. Combining the ARPES result with tight-binding model analysis, we demonstrate that this radical divergence stems from the activation of direct hopping channels between the AEL and the La atomic
lattice. This coupling reshapes the effective
lattice geometry, reconstructs the electronic states, and modifies the associated Chern band
topology, transforming the bipartite dice-
lattice network in YCl into a tripartite structure in LaCl. Our findings demonstrate that the coupling between the AEL and the atomic
lattice can actively shape the effective
lattice geometry that governs the electronic structure. This coupling can act as a powerful tuning knob for electronic structure design that is inaccessible in conventional materials.
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All authors Songyuan Geng, Xin Wang, Jianqi Zhong, Risi Guo, Fangjie Chen, Qun Wang, Kangjie Li, Keyu An, Teng-Fei Ying, Chen Qiu, Hanpu Liang, Zhengtai Liu, Mao Ye, Sungsoo Hahn, Balasubramanian Thiagarajan, Benjamin T. Zhou, Haoxiang Li
Materials Science
Surface Equilibration Mechanism Controls the Stability of a Model Co-deposited Glass Mixture of Organic Semiconductors
Shinian Cheng, Yejung Lee, Junguang Yu et al. (+2 more)
While previous work has identified the conditions for preparing ultrastable single-component organic glasses by physical vapor deposition (PVD), little is known about the stability of co-deposited mixtures. Here, we prepared binary PVD glasses of organic
semiconductors, TPD (N,N-Bis(3-methylphenyl)-N,N-diphenylbenzidine) and m-MTDATA (4,4,4-Tris[phenyl(m-tolyl)amino]triphenylamine), with 50:50 mass concentration over a wide range of substrate temperatures (Tsub). The enthalpy and kinetic stability are evaluated with differential scanning calorimetry and spectroscopic ellipsometry. Binary organic
semiconductor glasses with exceptional thermodynamic and kinetic stability comparable to the most stable single-component organic glasses are obtained when deposited at Tsub=0.78-0.90Tg (where Tg is the conventional glass transition temperature). When deposited at 0.94Tg, the enthalpy of m-MTDATA/TPD glass equals that expected for the
equilibrium liquid at that temperature. Thus, the surface equilibration mechanism previously advanced for single-component PVD glasses is also applicable for these co-deposited glasses. These results provide an avenue for designing high-performance organic electronic devices.
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All authors Shinian Cheng, Yejung Lee, Junguang Yu, Lian Yu, M. D. Ediger
Materials Science · Cross-listed from cond-mat.soft
Overaging with stress in polymer glasses? Faster segmental dynamics despite larger yield stress!
Masoud Razavi, Enran Xing and M.D. Ediger
It is well known that physical aging of
polymer glasses increases their yield stress and affects their failure behavior. Studies indicate that application of moderate levels of stress during aging results in higher yield stress compared to aging in the absence of stress (quiescent aging). This has been interpreted to indicate that stress accelerates physical aging, and has been described as overaging. In this study, we age PMMA glasses under stress, and carry out direct measurement of segmental dynamics during and after aging by using a probe reorientation technique. We observe that samples aged under stress, despite having higher yield stress, have faster segmental dynamics after stress release than quiescently aged samples. This contradicts the overaging interpretation, for the range of conditions explored here. Our results indicate that yield stress is not a simple function of structural relaxation time and theoretical models based on this understanding need to be revised.
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Materials Science
Fractional Spin Ferroelectric and Sliding Spin Current in Magnetic Sliding Ferroelectrics
Yilin Han, Lei Li, Chaoxi Cui et al. (+3 more)
We investigate the fractional
spin ferroelectric (FSFE) in
magnetic sliding ferroelectrics (SFEs), where ferroelectric switching is characterized not only by the reversal of the out-of-plane electric polarization but also by a variation of fractional in-plane
spin electronic polarization. We show that interlayer sliding in FSFEs can naturally lead to a
symmetry-protected pure
spin current, termed the sliding
spin current here. The underlying mechanism is that, during switching, the contributions of valence electrons and ions to the in-plane charge transfer cancel each other, whereas the in-plane
spin transfer, which stems solely from valence electrons, persists, leading to a pure
spin current. We demonstrate our ideas in various material candidates, including $H$-stacked bilayer CrI$_3$, whose few-layer form has been experimentally confirmed to be a
magnetic SFE, and $R$-stacked bilayers $2H$-V$X_2$ ($X=$ S, Se, Te), which have been experimentally synthesised. For a typical switching time of about $1$ ns, the estimated
spin-current densities for bilayer CrI$_3$ and V$X_2$ reach $10^9 (\hbar/2e)\mathrm{A/m^2}$ and $10^8 (\hbar/2e)\mathrm{A/m^2}$, respectively. This means that by applying a periodic out-of-plane electric field, a significant alternating
spin current can be generated in
magnetic SFEs. Thus, our findings propose a compelling new mechanism for the all-electrical generation of pure
spin current, and predict concrete realistic materials for experimental verification.
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All authors Yilin Han, Lei Li, Chaoxi Cui, Run-Wu Zhang, Zhi-Ming Yu, Yugui Yao
Materials Science
Homojunction-induced thermopower enhancement in polymer films
Zhen Xu, Hui Li, Guangzheng Zuo et al. (+7 more)
It has been more than twenty years since conductive
polymers began to receive
attention as an emerging thermoelectric material. However, the trade-off between electrical conductivity ({\sigma}) and thermopower (S) has proven to be a major challenge that has obstructed their use in actual devices. Here we report the discovery that the thermopower of the p- and n-type legs of organic thermogenerators can be substantially enhanced, without significant deterioration of {\sigma}, by constructing an in-plane segmented structure consisting of a homojunction with different doping levels on either side. In such segmented layers, the S is abnormally higher than the average value of the constituent parts when applying a forward temperature gradient (heating the heavily doped counterpart), while it is lower upon a reverse temperature gradient. Typically, for a two-stage segmented film of p-type PDPP-Se, an abnormally large S of 210 uV K-1 and {\sigma} of 2.5*10^4 S m-1 are obtained, resulting in a large power factor (PF) of 1100 uW m-1 K-2 and a record ZT of 1.36 at room temperature. The enhanced thermopower is attributed to an additional voltage developed at the homojunction under heating as explained by kinetic Monte Carlo simulations. This finding provides a breakthrough approach to the modulation of thermoelectric transport properties of conductive
polymers.
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All authors Zhen Xu, Hui Li, Guangzheng Zuo, Xiaojuan Dai, Jincheng Liao, Guofeng Cheng, Jian Song, Wenqing Zhang, Martijn Kemerink, Lidong Chen