Showing 1–36 of 36 results for author: Ren, G

PolyMon: A Unified Framework for Polymer Property Prediction

Authors: Gaopeng Ren, Yijie Yang, Jiajun Zhou, Kim E. Jelfs

Abstract: Accurate prediction of polymer properties is essential for materials design, but remains challenging due to data scarcity, diverse polymer representations, and the lack of systematic evaluation across modelling choices. Here, we present PolyMon, a unified and accessible framework that integrates multiple polymer representations, machine learning methods, and training strategies within a single, ac… ▽ More Accurate prediction of polymer properties is essential for materials design, but remains challenging due to data scarcity, diverse polymer representations, and the lack of systematic evaluation across modelling choices. Here, we present PolyMon, a unified and accessible framework that integrates multiple polymer representations, machine learning methods, and training strategies within a single, accessible platform. PolyMon supports various descriptors and graph construction strategies for polymer representations, and includes a wide range of models, from tabular models to graph neural networks, along with flexible training strategies including multi-fidelity learning, Δ-learning, active learning, and ensemble learning. Using five key polymer properties as benchmarks, we perform systematic evaluations to assess how representations and models affect predictive performance. These case studies further illustrate how different training strategies can be applied within a consistent workflow to leverage limited data and incorporate physical model derived information. Overall, PolyMon provides a comprehensive and extensible foundation for benchmarking and advancing machine learning-based polymer property prediction. The code is available at github.com/fate1997/polymon. △ Less

Submitted 2 March, 2026; originally announced March 2026.

Twist-induced Out-of-plane Ferroelectricity in Bilayer Hafnia

Authors: Jian Huang, Gwan Yeong Jung, Pravan Omprakash, Guodong Ren, Xin Li, Du Li, Xiaoshan Xu, Li Yang, Rohan Mishra

Abstract: Ferroelectric HfO2 is a promising candidate for next-generation memory devices due to its CMOS compatibility and ability to retain polarization at nanometer scales. However, the polar orthorhombic phase (Pca2_1) responsible for ferroelectricity is metastable and requires extrinsic stabilization, which makes it challenging for integration with silicon. We predict that bilayer 1T-HfO2 can exhibit ro… ▽ More Ferroelectric HfO2 is a promising candidate for next-generation memory devices due to its CMOS compatibility and ability to retain polarization at nanometer scales. However, the polar orthorhombic phase (Pca2_1) responsible for ferroelectricity is metastable and requires extrinsic stabilization, which makes it challenging for integration with silicon. We predict that bilayer 1T-HfO2 can exhibit robust and switchable out-of-plane (OOP) polarization arising from stacking-induced symmetry breaking. Using first-principles density functional theory, we predict that monolayer 1T-HfO2 can be cleaved from the (111) surface of cubic hafnia, and the monolayer is dynamically stable. When two aligned monolayers are twisted to form a moiré superlattice, it breaks the interlayer symmetry and allows the emergence of bistable OOP polarization. At a twist angle of 7.34o, the system exhibits a net polarization of ~16 μC/cm2. This sizeable polarization is due to the large polar displacements concentrated in AB stacking domains. Importantly, this polarization can be reversibly switched via interlayer sliding with a low energy barrier (~8 meV/formula unit) and comparable low coercive field (~0.2 V/nm), offering electric-field tunability. These findings establish twisted bilayer 1T-HfO2 as a scalable and robust 2D ferroelectric platform, enabling new pathways for integrating ferroelectric functionality into atomically thin memory and logic devices. △ Less

Submitted 25 March, 2026; v1 submitted 17 February, 2026; originally announced February 2026.

Hole-doping reduces the coercive field in ferroelectric hafnia

Authors: Pravan Omprakash, Gwan Yeong Jung, Guodong Ren, Rohan Mishra

Abstract: Ferroelectric hafnia (HfO2) holds promise for next-generation memory and logic applications because of its CMOS compatibility. However, the high coercive field required for polarization switching in HfO2 remains a critical challenge for efficient device operations. Using first-principles calculations and phenomenological modeling, we predict that hole doping can reduce the coercive field from 8 MV… ▽ More Ferroelectric hafnia (HfO2) holds promise for next-generation memory and logic applications because of its CMOS compatibility. However, the high coercive field required for polarization switching in HfO2 remains a critical challenge for efficient device operations. Using first-principles calculations and phenomenological modeling, we predict that hole doping can reduce the coercive field from 8 MV/cm in undoped hafnia to 6 MV/cm in hafnia doped with 0.2 holes per formula unit (f.u.). In the absence of doping, the reversal of polarization of the Pca21 phase is preferred through the non-polar, tetragonal P42/nmc phase. This switching pathway involves the coupling of three hard distortion modes that render undoped hafnia as an improper ferroelectric. The overall energy barrier through this pathway remains unchanged (80 meV/f.u.) upon hole doping. However, the introduction of holes hardens the polar distortion mode that connects the polar Pca21 phase to the non polar, orthorhombic Pbcm phase, and reduces the energy barrier from 180 meV/f.u. in undoped hafnia to 80 meV/f.u. at 0.2 holes/f.u.. The activation of the latter switching pathway through the Pbcm phase can lead to a reversal in the polarization direction. Overall, hole doping makes the switching pathway through the Pbcm phase competitive, and renders hafnia as a proper ferroelectric with a lower coercive field. △ Less

Submitted 12 March, 2026; v1 submitted 7 August, 2025; originally announced August 2025.

Sub-nm2 ferroelectric domains via charged 180 degree walls in ZrO2

Authors: Nashrah Afroze, Hamoon Fahrvandi, Guodong Ren, Pawan Kumar, Christopher Nelson, Sarah Lombardo, Mengkun Tian, Ping-Che Lee, Jiayi Chen, Manifa Noor, Kisung Chae, Sanghyun Kang, Prasanna Venkat Ravindran, Matthew Bergschneider, Gwan Yeong Jung, Pravan Omprakash, Gardy K. Ligonde, Nujhat Tasneem, Dina Triyoso, Steven Consiglio, Kanda Tapily, Robert Clark, Gert Leusink, Jayakanth Ravichandran, Shimeng Yu , et al. (9 additional authors not shown)

Abstract: Flat phonon bands in fluorite ferroelectrics (HfO2 or ZrO2) shrink polar domains laterally to an irreducible half-unit-cell width (0.27 nm) within which the vertical arrangement of dipoles is expected to remain uniform. We report on the direct observation of nonuniform and nearly discrete vertical arrangements of dipoles in ZrO2 thin films consisting of closely spaced head-to-head (HH) and tail-to… ▽ More Flat phonon bands in fluorite ferroelectrics (HfO2 or ZrO2) shrink polar domains laterally to an irreducible half-unit-cell width (0.27 nm) within which the vertical arrangement of dipoles is expected to remain uniform. We report on the direct observation of nonuniform and nearly discrete vertical arrangements of dipoles in ZrO2 thin films consisting of closely spaced head-to-head (HH) and tail-to-tail (TT) charged 180 degree walls, each exhibiting a distinct bulk-like structure. These charged domain walls (CDWs) further compress the irreducibly narrow, laterally stacked domains vertically to a thickness of 1-2.75 nm, yielding in-plane domains with sub-nm2 footprints-among the smallest ever reported for any ferroelectric material. The HH and TT walls form due to their flat longitudinal optical (LO) polar bands and are electrostatically stabilized by bound-charge compensation via interstitial oxygen atoms, which act as natural structural defects at the HH walls. Moreover, these walls are predicted to be conducting and to exhibit ultralow propagation barriers, with HH walls (1.6 meV) being far more mobile than TT walls (22.3 meV), indicating strong potential for low-voltage, domain-wall-based nanoelectronics. △ Less

Submitted 5 February, 2026; v1 submitted 24 July, 2025; originally announced July 2025.

Comments: 4 main figures, 16 supplementary figures

Design Principles and Identification of Birefringent Materials

Authors: Gwan Yeong Jung, Guodong Ren, Pravan Omprakash, Jayakanth Ravichandran, Rohan Mishra

Abstract: Birefringence ($Δn$) is the dependence of the refractive index of a material on the polarization of light travelling through it. Birefringent materials are used as polarizers, waveplates, and for novel light-matter coupling. While several birefringent materials exist, only a handful of them show large $Δn$ > 0.3, and are primarily limited to the infrared region. The variation of $Δn$ across divers… ▽ More Birefringence ($Δn$) is the dependence of the refractive index of a material on the polarization of light travelling through it. Birefringent materials are used as polarizers, waveplates, and for novel light-matter coupling. While several birefringent materials exist, only a handful of them show large $Δn$ > 0.3, and are primarily limited to the infrared region. The variation of $Δn$ across diverse materials classes and strategies to achieve highly birefringent materials with transparency covering different regions of the electromagnetic spectrum are missing. We have calculated the $Δn$ of 967 non-cubic, formable crystals having vastly different structures, polyhedral connectivity and chemical compositions. From this set of compounds, we have screened highly birefringent crystals ($Δn$ greater than 0.3) having transparency in different regions of the electromagnetic spectrum. The screened compounds belong to several families such as A3'MN3, AMO2, AN3, and A'N6 (A = Li, Na, K; A'= Ca, Sr, Ba; M = V, Nb, Ta). By analyzing the electronic structures of these compounds, we have distilled rules to enable the design of crystals with large $Δn$. △ Less

Submitted 23 July, 2025; originally announced July 2025.

Comments: 27 pages, 5 figures

Single Vanadium Dioxide Nanoparticle-Enabled Plasmonic switch with Thermal and Electronic Reconfigurability

Authors: Gregory Tanyi, Daniel Peace, Han-Hao Cheng, Guanghui Ren, Xuan Hiep Dinh, Mohammed Taha, Arnan Mitchell, Karsten Hoehn, Christina Lim, Ranjith Unnithan

Abstract: We present an integrated switch that combines plasmonic and neuromorphic technologies with a single sub-stoichiometric VO2-x nanoparticle. The presented device acts as a versatile plasmonic switch with dual thermal and electrical reconfigurability leveraging the near-room temperature phase transition of the VO2-x nanoparticles combined with the rapid phase recovery to drive the device. The change… ▽ More We present an integrated switch that combines plasmonic and neuromorphic technologies with a single sub-stoichiometric VO2-x nanoparticle. The presented device acts as a versatile plasmonic switch with dual thermal and electrical reconfigurability leveraging the near-room temperature phase transition of the VO2-x nanoparticles combined with the rapid phase recovery to drive the device. The change in both the optical and electrical properties of the VO2-x nanoparticle enables simultaneous optical and electrical readouts making the plasmonic device suitable as a phase change memory cell which is crucial in the convergence of computing and communication technologies. Our demonstration of reversible electrical switching, evidenced by a 6dB modulation depth and concurrent optical and electrical outputs, signifies a major stride in merging electronic and photonic functionalities within phase-change material devices. This novel strategy not only harmonizes optical communication with electronic computing but also advances the development of sophisticated integrated neuromorphic devices. △ Less

Submitted 15 May, 2025; originally announced May 2025.

Strain-induced gyrotropic effects in ferroelectric BaTiS3

Authors: Wei Luo, Asier Zabalo, Guodong Ren, Gwan-Yeong Jung, Massimiliano Stengel, Rohan Mishra, Jayakanth Ravichandran, Laurent Bellaiche

Abstract: Gyrotropic effects, including natural optical activity (NOA) and the nonlinear anomalous Hall effect (NAHE), are crucial for advancing optical and transport devices. We explore these effects in the BaTiS3 system, a quasi-one-dimensional crystal that exhibits giant optical anisotropy. (Niu et al. Nat. Photonics 12, 392 (2018); Zhao et al. Chem. Mater. 34, 5680 (2022)). In the P63cm phase which is s… ▽ More Gyrotropic effects, including natural optical activity (NOA) and the nonlinear anomalous Hall effect (NAHE), are crucial for advancing optical and transport devices. We explore these effects in the BaTiS3 system, a quasi-one-dimensional crystal that exhibits giant optical anisotropy. (Niu et al. Nat. Photonics 12, 392 (2018); Zhao et al. Chem. Mater. 34, 5680 (2022)). In the P63cm phase which is stable under room temperature, we predict two distinct strain-induced phase transitions: a symmetry-lowering transition from the P63cm to P63 phase under tensile strain, which enhances NOA and enables optical rotation; and an isostructural insulator-to-polar Weyl semimetal (WSM) transition under compressive strain, which activates the NAHE and exhibits a strain-induced sign reversal. The low-temperature P21 phase also transforms into a P212121 phase under enough compressive strains with such phase transition exhibiting a large NOA. All these results highlight BaTiS3 as a viable candidate for novel ferroelectrics, optical and transport devices with strain enhanced or activated gyrotropic properties. △ Less

Submitted 14 May, 2025; originally announced May 2025.

Comments: 18 pages, 4 figures

Journal ref: Letter in PRB (2026)

Towards Atomic-Scale Control over Structural Modulations in Quasi-1D Chalcogenides for Colossal Optical Anisotropy

Authors: Guodong Ren, Shantanu Singh, Gwan Yeong Jung, Wooseon Choi, Huandong Chen, Boyang Zhao, Kevin Ye, Andrew R. Lupini, Miaofang Chi, Jordan A. Hachtel, Young-Min Kim, Jayakanth Ravichandran, Rohan Mishra

Abstract: Optically anisotropic materials are sought after for tailoring the polarization of light. Recently, colossal optical anisotropy was reported in a quasi-one-dimensional chalcogenide, Sr1.125TiS3. Compared to SrTiS3, the excess Sr in Sr1.125TiS3 leads to periodic structural modulations and introduces additional electrons that undergo charge ordering on select Ti atoms to form a highly polarizable cl… ▽ More Optically anisotropic materials are sought after for tailoring the polarization of light. Recently, colossal optical anisotropy was reported in a quasi-one-dimensional chalcogenide, Sr1.125TiS3. Compared to SrTiS3, the excess Sr in Sr1.125TiS3 leads to periodic structural modulations and introduces additional electrons that undergo charge ordering on select Ti atoms to form a highly polarizable cloud oriented along the c-axis, hence, resulting in the colossolal optical anisotropy. Here, further enhancement of the colossal optical anisotropy to 2.5 in Sr1.143TiS3 is reported through control over the periodicity of the atomic-scale modulations. The role of structural modulations in tuning the optical properties in a series of SrxTiS3 compounds has been investigated using DFT calculations. The structural modulations arise from various stacking sequences of face-sharing TiS6 octahedra and twist-distorted trigonal prisms, and are found to be thermodynamically stable for x larger than 1 but smaller than 1.5. As x increases, an indirect-to-direct band gap transition is predicted for x equal to and larger than 1.143 along with an increased occupancy of Ti-dz2 states. Together, these two factors result in a theoretically predicted maximum birefriengence of 2.5 for Sr1.143TiS3. Single crystals of Sr1.143TiS3 were grown using a molten-salt flux method. Atomic-scale observations using scanning transmission electron microscopy confirm the feasibility of synthesizing SrxTiS3 with varied modulation periodicities. Overall, these findings demonstrate compositonal tunability of optical properties in SrxTiS3 compounds, and potentially in other hexagonal perovskites having structural modulations. △ Less

Submitted 14 May, 2025; originally announced May 2025.

Strong tunability of epitaxial relationship and reconstruction at improper ferroelectric interface

Authors: Xin Li, Yu Yun, Guodong Ren, Arashdeep Singh Thind, Amit Kumar Shah, Rohan Mishra, Xiaoshan Xu

Abstract: The atomic structures at epitaxial film-substrate interfaces determine scalability of thin films and can result in new phenomena. However, it is challenging to control the interfacial structures since they are decided by the most stable atomic bonding. In this work, we report strong tunability of the epitaxial interface of improper ferroelectric hexagonal ferrites deposited on spinel ferrites. The… ▽ More The atomic structures at epitaxial film-substrate interfaces determine scalability of thin films and can result in new phenomena. However, it is challenging to control the interfacial structures since they are decided by the most stable atomic bonding. In this work, we report strong tunability of the epitaxial interface of improper ferroelectric hexagonal ferrites deposited on spinel ferrites. The selection of two interface types, related by a 90 deg rotation of in-plane epitaxial relations and featured by disordered and hybridized reconstructions respectively, can be achieved by growth conditions, stacking sequences, and spinel compositions. While the disordered type suppresses the primary K3 structure distortion and ferroelectricity in hexagonal ferrites, the hybridized type is more coherent with the distortion with minimal suppression. This tunable interfacial structure provides critical insight on controlling interfacial clamping and may offer a solution for the long-standing problem of practical critical thickness in improper ferroelectrics. △ Less

Submitted 8 May, 2025; originally announced May 2025.

Journal ref: Nano Lett. 2025, 25,12509-12515

Antiferroelectricity with metastable polar state from Kittel model

Authors: Amit Kumar Shah, Xin Li, Guodong Ren, Yu Yun, Rohan Mishra, Xiaoshan Xu

Abstract: We have revisited the Kittel model that describes antiferroelectricity (AFE) in terms of two sublattices of spontaneous polarization with antiparallel couplings. By constructing the comprehensive phase diagram including the antiferroelectric, ferroelectric, and paraelectric phases in the parameter space, we identified an AFE phase with stable antipolar states and metastable polar state (SAMP) due… ▽ More We have revisited the Kittel model that describes antiferroelectricity (AFE) in terms of two sublattices of spontaneous polarization with antiparallel couplings. By constructing the comprehensive phase diagram including the antiferroelectric, ferroelectric, and paraelectric phases in the parameter space, we identified an AFE phase with stable antipolar states and metastable polar state (SAMP) due to the weak coupling between sublattices. We found that the metastability of the polar state in SAMP phase could lead to apparent remanent polarization, depending on the measurement timescale. This explains the observed ferroelectric behavior of orthorhombic hafnia, which is predicted to be antipolar by density functional theories. △ Less

Submitted 6 May, 2025; v1 submitted 5 April, 2025; originally announced April 2025.

Journal ref: Appl. Phys. Lett. 127, 152902 (2025)

Giant coercivity and enhanced intrinsic anomalous Hall effect at vanishing magnetization in a compensated kagome ferrimagnet

Authors: Jonathan M. DeStefano, Elliott Rosenberg, Guodong Ren, Yongbin Lee, Zhenhua Ning, Olivia Peek, Kamal Harrison, Saiful I. Khondaker, Liqin Ke, Igor I. Mazin, Juan Carlos Idrobo, Jiun-Haw Chu

Abstract: Ferrimagnets that can be driven to magnetic compensation show promise for use in spintronics as they exhibit a finite anomalous Hall effect at zero magnetic field without having a significant magnetic moment. Compensated ferrimagnet spintronic devices with both a large anomalous Hall effect and a high coercivity would be simultaneously easy to read and difficult to erase. The kagome ferrimagnet Tb… ▽ More Ferrimagnets that can be driven to magnetic compensation show promise for use in spintronics as they exhibit a finite anomalous Hall effect at zero magnetic field without having a significant magnetic moment. Compensated ferrimagnet spintronic devices with both a large anomalous Hall effect and a high coercivity would be simultaneously easy to read and difficult to erase. The kagome ferrimagnet TbMn$_6$Sn$_6$ has been reported to host a large intrinsic anomalous Hall effect. Here, we demonstrate that doping the Mn sites with Cr drives the system towards magnetic compensation. For nearly compensated compositions at low temperatures, giant coercive fields exceeding 14 T are observed. Additionally, Cr doping significantly enhances the intrinsic anomalous Hall effect, which can be attributed to a shift in the Fermi level. Our results extend the range of unique magnetic states observed in kagome materials, demonstrating that chemical doping is an effective strategy to tune and realize these states. △ Less

Submitted 6 February, 2025; originally announced February 2025.

Improper flexoelectricity in hexagonal rare-earth ferrites

Authors: Xin Li, Guodong Ren, Yu Yun, Arashdeep Singh Thind, Amit Kumar Shah, Abbey Bowers, Rohan Mishra, Xiaoshan Xu

Abstract: Flexoelectricity is a universal effect that generates electric polarization due to broken inversion symmetry caused by local strain gradient. The large strain gradient at nanoscale makes flexo-electric effects, especially in nanoscopic ferroelectric materials, promising in sensors, actuator, energy harvesting, and memory applications. In this work, we studied flexoelectricity in hexagonal ferrites… ▽ More Flexoelectricity is a universal effect that generates electric polarization due to broken inversion symmetry caused by local strain gradient. The large strain gradient at nanoscale makes flexo-electric effects, especially in nanoscopic ferroelectric materials, promising in sensors, actuator, energy harvesting, and memory applications. In this work, we studied flexoelectricity in hexagonal ferrites h-YbFeO3, an improper ferroelectric expected to have weak piezoelectricity and low sensitivity to depolarization field, which are advantageous for studying flexoelectric effects. We show that in h-YbFeO3 epitaxial thin films, strain gradient on the order of 10^6 m-1 occurs near grain boundaries and edge dislocation, which has a significant impact on the non-polar K3 structural distortion that induces spontaneous polarization. The phenomenological model based on the Landau theory of improper ferroelectricity suggests an indirect flexoelectric effect on the order of 10 nC/m in h-YbFeO3, which is substantially larger than the expectation from Kogan mechanism. These results reveal a novel microscopic mechanism of coupling between strain gradient and polarization mediated by structural distortion, which we call improper flexoelectricity. △ Less

Submitted 28 May, 2025; v1 submitted 25 September, 2024; originally announced September 2024.

Polarization Pinning at Antiphase Boundaries in Multiferroic YbFeO$_3$

Authors: Guodong Ren, Pravan Omprakash, Xin Li, Yu Yun, Arashdeep S. Thind, Xiaoshan Xu, Rohan Mishra

Abstract: The switching characteristics of ferroelectrics and multiferroics are influenced by the interaction of topological defects with domain-walls. We report on the pinning of polarization due to antiphase boundaries in thin films of the multiferroic hexagonal YbFeO$_3$. We have directly resolved the atomic structure of a sharp antiphase boundary (APB) in YbFeO$_3$ thin films using a combination of aber… ▽ More The switching characteristics of ferroelectrics and multiferroics are influenced by the interaction of topological defects with domain-walls. We report on the pinning of polarization due to antiphase boundaries in thin films of the multiferroic hexagonal YbFeO$_3$. We have directly resolved the atomic structure of a sharp antiphase boundary (APB) in YbFeO$_3$ thin films using a combination of aberration-corrected scanning transmission electron microscopy (STEM) and total energy calculations based on density-functional theory (DFT). We find the presence of a layer of FeO$_6$ octahedra at the APB that bridge the adjacent domains. STEM imaging shows a reversal in the direction of polarization on moving across the APB, which DFT calculations confirm is structural in nature as the polarization reversal reduces the distortion of the FeO$_6$ octahedral layer at the APB. Such APBs in hexagonal perovskites are expected to serve as domain-wall pinning sites and hinder ferroelectric switching of the domains. △ Less

Submitted 11 October, 2024; v1 submitted 13 September, 2024; originally announced September 2024.

Comments: 16 pages, 4 figures

Journal ref: Chinese Phys. B 33 118502 (2024)

Strain-driven stabilization of a room-temperature chiral multiferroic with coupled ferroaxial and ferroelectric order

Authors: Guodong Ren, Gwan Yeong Jung, Huandong Chen, Chong Wang, Boyang Zhao, Rama K. Vasudevan, Jordan A. Hachtel, Andrew R. Lupini, Miaofang Chi, Di Xiao, Jayakanth Ravichandran, Rohan Mishra

Abstract: Noncollinear ferroic materials are sought after as testbeds to explore the intimate connections between topology and symmetry, which result in electronic, optical and magnetic functionalities not observed in collinear ferroic materials. For example, ferroaxial materials have ordered rotational structural distortions that break mirror symmetry and induce chirality. When ferroaxial order is coupled… ▽ More Noncollinear ferroic materials are sought after as testbeds to explore the intimate connections between topology and symmetry, which result in electronic, optical and magnetic functionalities not observed in collinear ferroic materials. For example, ferroaxial materials have ordered rotational structural distortions that break mirror symmetry and induce chirality. When ferroaxial order is coupled with ferroelectricity arising from a broken inversion symmetry, it offers the prospect of electric-field-control of the ferroaxial distortions and opens up new tunable functionalities. However, chiral multiferroics, especially ones stable at room temperature, are rare. We report the discovery of a strain-stabilized, room-temperature chiral multiferroic phase in single crystals of BaTiS$_3$, a quasi-one-dimensional (1D) hexagonal chalcogenide. Using first-principles calculations, we predict the stabilization of this multiferroic phase having $P6_3$ space group for biaxial tensile strains exceeding 1.5% applied on the basal ab-plane of the room temperature $P6_3cm$ phase of BaTiS$_3$. The chiral multiferroic phase is characterized by rotational distortions of select TiS$_6$ octahedra around the long $c$-axis and polar displacement of Ti atoms along the $c$-axis. We used an innovative approach using focused ion beam milling to make appropriately strained samples of BaTiS$_3$. The ferroaxial and ferroelectric distortions, and their domains in $P6_3$-BaTiS$_3$ were directly resolved using atomic resolution scanning transmission electron microscopy. Landau-based phenomenological modeling predicts a strong coupling between the ferroelectric and the ferroaxial order making $P6_3$-BaTiS$_3$ an attractive test bed for achieving electric-field control of chirality-related phenomena such as circular photo-galvanic current and the Rashba effect. △ Less

Submitted 2 December, 2024; v1 submitted 7 August, 2024; originally announced August 2024.

Comments: 20 pages, 4 figures

Antiferroelectric Hafnia Down to the 2D Limit

Authors: Xin Li, Guodong Ren, Haidong Lu, Kartik Samanta, Amit Kumar Shah, Kai Huang, Pravan Omprakash, Yu Yun, Pratyush Buragohain, Huibo Cao, Yan Wu, Jordan A. Hachtel, Andrew R. Lupini, Miaofang Chi, Juan Carlos Idrobo, Evgeny Y. Tsymbal, Alexei Gruverman, Rohan Mishra, Xiaoshan Xu

Abstract: Antiferroelectricity is a material property characterized by alternating electric dipoles spontaneously ordered in antiparallel directions. Antiferroelectrics are promising for energy storage, solid-state cooling, and memory technologies; however, these materials are scarce, and their scalability remains largely unexplored. In this work, we demonstrate that single-crystalline hafnia, a lead-free C… ▽ More Antiferroelectricity is a material property characterized by alternating electric dipoles spontaneously ordered in antiparallel directions. Antiferroelectrics are promising for energy storage, solid-state cooling, and memory technologies; however, these materials are scarce, and their scalability remains largely unexplored. In this work, we demonstrate that single-crystalline hafnia, a lead-free CMOS-compatible material, exhibits antiferroelectricity under compressive-strain conditions. We observe antiparallel sublattice polarization and stable double-hysteresis in single-crystalline (111)-oriented epitaxial La-doped hafnia films grown on yttrium-stabilized zirconia and show that the antipolar orthorhombic phase of hafnia adheres to the Kittel model of antiferroelectricity. Notably, compressive strain strengthens the antiferroelectric order in thinner La-doped hafnia films, achieving an unprecedented 850 C ordering temperature in the two-dimensional limit, highlighting hafnia's potential for advanced antiferroelectric devices. △ Less

Submitted 2 August, 2025; v1 submitted 3 August, 2024; originally announced August 2024.

Emergent Atomic Scale Polarisation Vortices in BaTiS3

Authors: Boyang Zhao, Gwan Yeong Jung, Shantanu Singh, Robert B. Smith, Huandong Chen, Guodong Ren, Chuangtang Wang, Sara Termos, Sean T. Holmes, Frederic Mentink-Vigier, Weizhe Zhang, Zhengyu Du, Claire Wu, M. J. Swamynadhan, Qinai Zhao, Kevin Ye, Donald A. Walko, Nicholas S. Settineri, Simon J. Teat, Liuyan Zhao, Robert W. Schurko, Haidan Wen, Rohan Mishra, Jayakanth Ravichandran

Abstract: Topological defects, such as vortices and skyrmions in magnetic and dipolar systems, can give rise to properties that are not observed in typical magnets and dielectrics. Here, we report the discovery of long-range ordered periodic dipole arrays of atomic-scale vortices and antivortices in the unconventional charge-density-wave (CDW) phase of BaTiS3, a quasi-1D chalcogenide. Synchrotron X-ray diff… ▽ More Topological defects, such as vortices and skyrmions in magnetic and dipolar systems, can give rise to properties that are not observed in typical magnets and dielectrics. Here, we report the discovery of long-range ordered periodic dipole arrays of atomic-scale vortices and antivortices in the unconventional charge-density-wave (CDW) phase of BaTiS3, a quasi-1D chalcogenide. Synchrotron X-ray diffraction (XRD) reveals the presence of a multi-q ordering in BaTiS3 that confines vortex-vortex-antivortex polarisation triplets to the a-b plane with alternating handedness along the c-axis. The multi-q displacive distortions are characterised by three distinctive off-centre TiS6 configurations, whose ratios are independently confirmed by 47/49Ti solid-state nuclear magnetic resonance (SSNMR). Using first-principles calculations and phenomenological modelling, we show that the dipolar vortex unit cell in BaTiS3 arises from the coupling between multiple lattice instabilities arising from flat, soft phonon bands. This mechanism contrasts with classical dipolar textures in ferroelectric heterostructures that emerge from the competition between electrostatic and strain energies. The observation of dipolar vortices in BaTiS3 brings the ultimate scaling limit for real-space dipolar topological structures down to about a nanometre and unveils the intimate connection between crystal symmetry and real-space topology. Our work sets up zero-filling semiconducting materials with competing structural instabilities as a playground for realising and understanding quantum polarisation topologies. △ Less

Submitted 21 October, 2025; v1 submitted 13 June, 2024; originally announced June 2024.

The Stokes-Einstein-Debye relation in ortho-terphenyl liquid

Authors: Gan Ren

Abstract: The Stokes-Einstein-Debye (SED) relation is proposed to be breakdown in supercooled liquids by many studies. However, the conclusions are usually drawn by testing some variants of the SED relation rather than its original form. In this work, the rationality of the SED relation and its variants is examined by performing molecular dynamics simulations with the Lewis-Wahnstrom model of ortho-terpheny… ▽ More The Stokes-Einstein-Debye (SED) relation is proposed to be breakdown in supercooled liquids by many studies. However, the conclusions are usually drawn by testing some variants of the SED relation rather than its original form. In this work, the rationality of the SED relation and its variants is examined by performing molecular dynamics simulations with the Lewis-Wahnstrom model of ortho-terphenyl (OTP). The results indicate the original SED relation is valid for OTP but the three variants are all breakdown. The inconsistency between SED relation and its variants is attributed to the adopted assumptions and approximations, heterogeneous dynamics and the interactions among molecules. Therefore, care should be taken when employing its variants to judge the validity of the SED relation in supercooled liquids. △ Less

Submitted 20 April, 2024; originally announced April 2024.

Molten flux growth of single crystals of quasi-1D hexagonal chalcogenide BaTiS3

Authors: Huandong Chen, Shantanu Singh, Hongyan Mei, Guodong Ren, Boyang Zhao, Mythilli Surendran, Yan-Ting Wang, Rohan Mishra, Mikhail A. Kats, Jayakanth Ravichandran

Abstract: BaTiS3, a quasi-1D complex chalcogenide, has gathered considerable scientific and technological interest due to its giant optical anisotropy and electronic phase transitions. However, the synthesis of high-quality BaTiS3 crystals, particularly those featuring crystal sizes of millimeters or larger, remains a challenge. Here, we investigate the growth of BaTiS3 crystals utilizing a molten salt flux… ▽ More BaTiS3, a quasi-1D complex chalcogenide, has gathered considerable scientific and technological interest due to its giant optical anisotropy and electronic phase transitions. However, the synthesis of high-quality BaTiS3 crystals, particularly those featuring crystal sizes of millimeters or larger, remains a challenge. Here, we investigate the growth of BaTiS3 crystals utilizing a molten salt flux of either potassium iodide, or a mixture of barium chloride and barium iodide. The crystals obtained through this method exhibit a substantial increase in volume compared to those synthesized via the chemical vapor transport method, while preserving their intrinsic optical and electronic properties. Our flux growth method provides a promising route towards the production of high-quality, large-scale single crystals of BaTiS3, which will greatly facilitate advanced characterizations of BaTiS3 and its practical applications that require large crystal dimensions. Additionally, our approach offers an alternative synthetic route for other emerging complex chalcogenides. △ Less

Submitted 9 March, 2024; originally announced March 2024.

Giant Modulation of Refractive Index from Picoscale Atomic Displacements

Authors: Boyang Zhao, Guodong Ren, Hongyan Mei, Vincent C. Wu, Shantanu Singh, Gwan-Yeong Jung, Huandong Chen, Raynald Giovine, Shanyuan Niu, Arashdeep S. Thind, Jad Salman, Nick S. Settineri, Bryan C. Chakoumakos, Michael E. Manley, Raphael P. Hermann, Andrew R. Lupini, Miaofang Chi, Jordan A. Hachtel, Arkadiy Simonov, Simon J. Teat, Raphaële J. Clément, Mikhail A. Kats, J. Ravichandran, Rohan Mishra

Abstract: Structural disorder has been shown to enhance and modulate magnetic, electrical, dipolar, electrochemical, and mechanical properties of materials. However, the possibility of obtaining novel optical and optoelectronic properties from structural disorder remains an open question. Here, we show unambiguous evidence of disorder in the form of anisotropic, picoscale atomic displacements modulating the… ▽ More Structural disorder has been shown to enhance and modulate magnetic, electrical, dipolar, electrochemical, and mechanical properties of materials. However, the possibility of obtaining novel optical and optoelectronic properties from structural disorder remains an open question. Here, we show unambiguous evidence of disorder in the form of anisotropic, picoscale atomic displacements modulating the refractive index tensor and resulting in the giant optical anisotropy observed in BaTiS$_3$, a quasi-one-dimensional hexagonal chalcogenide. Single crystal X-ray diffraction studies reveal the presence of antipolar displacements of Ti atoms within adjacent TiS$_6$ chains along the c-axis, and three-fold degenerate Ti displacements in the a-b plane. $^{47/49}$Ti solid-state NMR provides additional evidence for those Ti displacements in the form of a three-horned NMR lineshape resulting from a low symmetry local environment around Ti atoms. We used scanning transmission electron microscopy to directly observe the globally disordered Ti a-b plane displacements and find them to be ordered locally over a few unit cells. First-principles calculations show that the Ti a-b plane displacements selectively reduce the refractive index along the ab-plane, while having minimal impact on the refractive index along the chain direction, thus resulting in a giant enhancement in the optical anisotropy. By showing a strong connection between structural disorder with picoscale displacements and the optical response in BaTiS$_3$, this study opens a pathway for designing optical materials with high refractive index and functionalities such as large optical anisotropy and nonlinearity. △ Less

Submitted 19 March, 2024; v1 submitted 6 October, 2023; originally announced October 2023.

Comments: 24 pages, 3 figures

Direct in-situ measurement of electrical properties of solid electrolyte interphase on lithium metal anode

Authors: Yaobin Xu, Hao Jia, Peiyuan Gao, Diego E. Galvez-Aranda, Saul Perez Beltran, Xia Cao, Phung M. L. Le, Jianfang Liu, Mark H Engelhard, Shuang Li, Gang Ren, Jorge M. Seminario, Perla B. Balbuena, Ji-Guang Zhang, Wu Xu, Chongmin Wang

Abstract: Solid electrolyte interphase (SEI), a thin layer that dynamically forms between active electrode and electrolyte during battery operation, critically governs the performance of rechargeable batteries1-5. An ideal SEI is expected to be electrically insulative to prevent persistently parasitic reactions between the electrode and the electrolyte, while ionically conductive to facilitate Faradaic reac… ▽ More Solid electrolyte interphase (SEI), a thin layer that dynamically forms between active electrode and electrolyte during battery operation, critically governs the performance of rechargeable batteries1-5. An ideal SEI is expected to be electrically insulative to prevent persistently parasitic reactions between the electrode and the electrolyte, while ionically conductive to facilitate Faradaic reactions of the electrode1,2,6. However, the true nature of the electrical properties of an SEI layer remains hitherto unclear due to the lack of a direct characterization method, leaving a range of behaviors of rechargeable batteries unelucidated. Here, we use in-situ bias transmission electron microscopy, for the first time, to directly measure the electrical properties of SEIs formed on copper (Cu) and lithium (Li) substrates. Surprisingly, we discover that, in terms of electrical behavior, SEI is distinctively different from a typical electrical insulator as what has been widely, and up to date, assumed ever since the discovery of SEI; rather, SEI shows voltage-dependent differential conductance. △ Less

Submitted 22 April, 2023; originally announced April 2023.

Testing the variants of the Stokes-Einstein relation in the framework of self-consistent generalized Langevin equation theory

Authors: Wanmei Zhang, Wenhan Zhang, Gan Ren

Abstract: The two functional forms, D~1/tau and D~T/tau, are usually adopted as the variants of the Stokes-Einstein relation; where D is the diffusion constant, tau the relaxation time and T the temperature. The self-consistent generalized Langevin equation (SCGLE) theory is presented as an analytical tool to predict the long time dynamics of colloids and molecular liquids. In this work, taking truncated Le… ▽ More The two functional forms, D~1/tau and D~T/tau, are usually adopted as the variants of the Stokes-Einstein relation; where D is the diffusion constant, tau the relaxation time and T the temperature. The self-consistent generalized Langevin equation (SCGLE) theory is presented as an analytical tool to predict the long time dynamics of colloids and molecular liquids. In this work, taking truncated Lennard-Jones-like liquids as an example, the rationality of the two variants were tested in the framework of the SCGLE theory. Our results indicate that D~1/tau is a good variant of the Stokes-Einstein relation in the framework of SCGLE theory; however, D~T/tau is not a good one but taking a fractional from as D~(T/tau)^zeta with an exponent zeta is not equal to 1.0 even the Stokes-Einstein relation is established in SCGLE theory. △ Less

Submitted 7 March, 2023; originally announced March 2023.

Colossal optical anisotropy from atomic-scale modulations

Authors: Hongyan Mei, Guodong Ren, Boyang Zhao, Jad Salman, Gwan Yeong Jung, Huandong Chen, Shantanu Singh, Arashdeep S. Thind, John Cavin, Jordan A. Hachtel, Miaofang Chi, Shanyuan Niu, Graham Joe, Chenghao Wan, Nick Settineri, Simon J. Teat, Bryan C. Chakoumakos, Jayakanth Ravichandran, Rohan Mishra, Mikhail A. Kats

Abstract: In modern optics, materials with large birefringence (Δn, where n is the refractive index) are sought after for polarization control (e.g. in wave plates, polarizing beam splitters, etc.), nonlinear optics and quantum optics (e.g. for phase matching and production of entangled photons), micromanipulation, and as a platform for unconventional light-matter coupling, such as Dyakonov-like surface pol… ▽ More In modern optics, materials with large birefringence (Δn, where n is the refractive index) are sought after for polarization control (e.g. in wave plates, polarizing beam splitters, etc.), nonlinear optics and quantum optics (e.g. for phase matching and production of entangled photons), micromanipulation, and as a platform for unconventional light-matter coupling, such as Dyakonov-like surface polaritons and hyperbolic phonon polaritons. Layered "van der Waals" materials, with strong intra-layer bonding and weak inter-layer bonding, can feature some of the largest optical anisotropy; however, their use in most optical systems is limited because their optic axis is out of the plane of the layers and the layers are weakly attached, making the anisotropy hard to access. Here, we demonstrate that a bulk crystal with subtle periodic modulations in its structure -- Sr9/8TiS3 -- is transparent and positive-uniaxial, with extraordinary index n_e = 4.5 and ordinary index n_o = 2.4 in the mid- to far-infrared. The excess Sr, compared to stoichiometric SrTiS3, results in the formation of TiS6 trigonal-prismatic units that break the infinite chains of face-shared TiS6 octahedra in SrTiS3 into periodic blocks of five TiS6 octahedral units. The additional electrons introduced by the excess Sr subsequently occupy the TiS6 octahedral blocks to form highly oriented and polarizable electron clouds, which selectively boost the extraordinary index n_e and result in record birefringence (Δn > 2.1 with low loss). The connection between subtle structural modulations and large changes in refractive index suggests new categories of anisotropic materials and also tunable optical materials with large refractive-index modulation and low optical losses. △ Less

Submitted 21 July, 2023; v1 submitted 28 February, 2023; originally announced March 2023.

Comments: Main text + supplementary

Stabilizing polar phases in binary metal oxides by hole doping

Authors: Tengfei Cao, Guodong Ren, Ding-Fu Shao, Evgeny Y. Tsymbal, Rohan Mishra

Abstract: The recent observation of ferroelectricity in the metastable phases of binary metal oxides, such as HfO2, ZrO2, Hf0.5Zr0.5O2, and Ga2O3, has garnered a lot of attention. These metastable ferroelectric phases are typically stabilized through epitaxial growth, alloying, or defect engineering. Here, we propose hole doping plays a key role in stabilizing the polar phases in binary metal oxides. Using… ▽ More The recent observation of ferroelectricity in the metastable phases of binary metal oxides, such as HfO2, ZrO2, Hf0.5Zr0.5O2, and Ga2O3, has garnered a lot of attention. These metastable ferroelectric phases are typically stabilized through epitaxial growth, alloying, or defect engineering. Here, we propose hole doping plays a key role in stabilizing the polar phases in binary metal oxides. Using first-principles density-functional-theory calculations, we show that holes in these oxides mainly occupy one of the two oxygen sublattices. This hole localization, which is more pronounced in the polar phase than in the nonpolar phase, lowers the electrostatic energy of the system, and makes the polar phase more stable at sufficiently large concentrations. We demonstrate that this electrostatic mechanism is responsible for stabilization of the ferroelectric phase of HfO2 aliovalently doped with elements that introduce holes to the system, such as La and N. Finally, we show that the spontaneous polarization in HfO2 is robust to hole doping, and a large polarization persists even under a high concentration of holes. △ Less

Submitted 1 March, 2023; v1 submitted 19 September, 2022; originally announced September 2022.

Quasi-epitaxial growth of BaTiS$_3$ films

Authors: M. Surendran, B. Zhao, G. Ren, S. Singh, A. Avishai, H. Chen, J. Han, M. Kawasaki, R. Mishra, J. Ravichandran

Abstract: Perovskite chalcogenides have emerged as a new class of semiconductors with tunable band gap in the visible-infrared region. High quality thin films are critical to understand the fundamental properties and realize the potential applications based on these materials. We report growth of quasi-epitaxial thin films of quasi one-dimensional (quasi-1D) hexagonal chalcogenide BaTiS$_3$ by pulsed laser… ▽ More Perovskite chalcogenides have emerged as a new class of semiconductors with tunable band gap in the visible-infrared region. High quality thin films are critical to understand the fundamental properties and realize the potential applications based on these materials. We report growth of quasi-epitaxial thin films of quasi one-dimensional (quasi-1D) hexagonal chalcogenide BaTiS$_3$ by pulsed laser deposition. We identified the optimal growth conditions by varying the growth parameters such as the substrate temperature and H2S partial pressure and examined their effects on the thin film structure. High resolution thin film X-Ray diffraction shows strong texture in the out-of-plane direction, whereas no evidence of in-plane relationship between the film and the substrate is observed. Grazing incidence wide-angle X-ray scattering and scanning transmission electron microscopy studies reveal the presence of weak epitaxial relationships of the film and the substrate, despite a defective interface. Our study opens up a pathway to realize quasi-1D hexagonal chalcogenide thin films and their heterostructures with perovskite chalcogenides. △ Less

Submitted 18 August, 2022; originally announced August 2022.

Comments: 25 pages, 6 figures

Unconventional Charge-density-wave Order in a Dilute d-band Semiconductor

Authors: Huandong Chen, Batyr Ilyas, Boyang Zhao, Emre Ergecen, Josh Mutch, Gwan Yeong Jung, Qian Song, Connor A. Occhialini, Guodong Ren, Sara Shabani, Eric Seewald, Shanyuan Niu, Jiangbin Wu, Nan Wang, Mythili Surendran, Shantanu Singh, Jiang Luo, Sanae Ohtomo, Gemma Goh, Bryan C. Chakoumakos, Simon J. Teat, Brent Melot, Han Wang, Di Xiao, Abhay N. Pasupathy , et al. (5 additional authors not shown)

Abstract: Electron-lattice coupling effects in low dimensional materials give rise to charge density wave (CDW) order and phase transitions. These phenomena are critical ingredients for superconductivity and predominantly occur in metallic model systems such as doped cuprates, transition metal dichalcogenides, and more recently, in Kagome lattice materials. However, CDW in semiconducting systems, specifical… ▽ More Electron-lattice coupling effects in low dimensional materials give rise to charge density wave (CDW) order and phase transitions. These phenomena are critical ingredients for superconductivity and predominantly occur in metallic model systems such as doped cuprates, transition metal dichalcogenides, and more recently, in Kagome lattice materials. However, CDW in semiconducting systems, specifically at the limit of low carrier concentration region, is uncommon. Here, we combine electrical transport, synchrotron X-ray diffraction and optical spectroscopy to discover CDW order in a quasi-one-dimensional (1D), dilute d-band semiconductor, BaTiS3, which suggests the existence of strong electron-phonon coupling. The CDW state further undergoes an unusual transition featuring a sharp increase in carrier mobility. Our work establishes BaTiS3 as a unique platform to study the CDW physics in the dilute filling limit to explore novel electronic phases. △ Less

Submitted 23 July, 2022; originally announced July 2022.

Journal ref: Adv. Mater. 2023, 2303283

Orientation Controlled Anisotropy in Single Crystals of Quasi-1D BaTiS3

Authors: Boyang Zhao, Md Shafkat Bin Hoque, Gwan Yeong Jung, Hongyan Mei, Shantanu Singh, Guodong Ren, Milena Milich, Qinai Zhao, Nan Wang, Huandong Chen, Shanyuan Niu, Sang-Jun Lee, Cheng-Tai Kuo, Jun-Sik Lee, John A. Tomko, Han Wang, Mikhail Kats, Rohan Mishra, Patrick E Hopkins, J. Ravichandran

Abstract: Low-dimensional materials with chain-like (one-dimensional) or layered (twodimensional) structures are of significant interest due to their anisotropic electrical, optical, thermal properties. One material with chain-like structure, BaTiS3 (BTS), was recently shown to possess giant in-plane optical anisotropy and glass-like thermal conductivity. To understand the origin of these effects, it is nec… ▽ More Low-dimensional materials with chain-like (one-dimensional) or layered (twodimensional) structures are of significant interest due to their anisotropic electrical, optical, thermal properties. One material with chain-like structure, BaTiS3 (BTS), was recently shown to possess giant in-plane optical anisotropy and glass-like thermal conductivity. To understand the origin of these effects, it is necessary to fully characterize the optical, thermal, and electronic anisotropy of BTS. To this end, BTS crystals with different orientations (aand c-axis orientations) were grown by chemical vapor transport. X-ray absorption spectroscopy (XAS) was used to characterize the local structure and electronic anisotropy of BTS. Fourier transform infrared (FTIR) reflection/transmission spectra show a large inplane optical anisotropy in the a-oriented crystals, while the c-axis oriented crystals were nearly isotropic in-plane. BTS platelet crystals are promising uniaxial materials for IR optics with their optic axis parallel to the c-axis. The thermal conductivity measurements revealed a thermal anisotropy of ~4.5 between the c- and a-axis. Time-domain Brillouin scattering showed that the longitudinal sound speed along the two axes is nearly the same suggesting that the thermal anisotropy is a result of different phonon scattering rates. △ Less

Submitted 7 April, 2022; originally announced April 2022.

The Properties of doping ZrCo

Authors: Gan Ren

Abstract: ZrCo alloy is promising to substitute uranium for handling hydrogen isotope storage in thermonuclear reactor. The alloying substitution of Zr in ZrCo with Hf or Ti can enhance the ability of anti-disproportionation. In this work, Zr$_{0.75}$Hf$_{0.25}$Co and Zr$_{0.75}$Ti$_{0.25}$Co were considered in the framework of density functional theory, aimed to investigate the properties of the alloying s… ▽ More ZrCo alloy is promising to substitute uranium for handling hydrogen isotope storage in thermonuclear reactor. The alloying substitution of Zr in ZrCo with Hf or Ti can enhance the ability of anti-disproportionation. In this work, Zr$_{0.75}$Hf$_{0.25}$Co and Zr$_{0.75}$Ti$_{0.25}$Co were considered in the framework of density functional theory, aimed to investigate the properties of the alloying substitution of Zr with Hf or Ti in ZrCo. Our results found that the optimized lattice constants of the alloying substitutions, Zr$_{0.75}$Hf$_{0.25}$Co and Zr$_{0.75}$Ti$_{0.25}$Co, are smaller than ZrCo. The thermodynamic stability reduces in the order Zr$_{0.75}$Hf$_{0.25}$Co > ZrCo > Zr$_{0.75}$Ti$_{0.25}$Co, as demonstrated by the enthalpy of formation. The valence electrons are mainly localized at the ion core and the chemical bonds are polarized in Zr$_{0.75}$Hf$_{0.25}$Co and Zr$_{0.75}$Ti$_{0.25}$Co analogous to ZrCo. △ Less

Submitted 15 January, 2022; originally announced January 2022.

The effective hydrodynamic radius is not a constant

Authors: Gan Ren

Abstract: The effective hydrodynamic radius is usually assumed to be a constant in testing the Stokes-Einstein relation by its variants. We have performed molecular dynamics simulations with ortho-terphenyl and Kob-Andersen model to examine the assumption and found the effective hydrodynamic radius is not a constant but decreases with decreasing temperature. The variant of Stokes-Einstein relation is not es… ▽ More The effective hydrodynamic radius is usually assumed to be a constant in testing the Stokes-Einstein relation by its variants. We have performed molecular dynamics simulations with ortho-terphenyl and Kob-Andersen model to examine the assumption and found the effective hydrodynamic radius is not a constant but decreases with decreasing temperature. The variant of Stokes-Einstein relation is not established but Stokes-Einstein relation is valid by considering the changes of the effective hydrodynamic radius. We propose the assumption should be seriously evaluated when using variants to test Stokes-Einstein relation. △ Less

Submitted 15 January, 2022; originally announced January 2022.

Modeling Temperature, Frequency, and Strain Effects on the Linear Electro-Optic Coefficients of Ferroelectric Oxides

Authors: Yang Liu, Guodong Ren, Tengfei Cao, Rohan Mishra, Jayakanth Ravichandran

Abstract: An electro-optic modulator offers the function of modulating the propagation of light in a material with electric field and enables seamless connection between electronics-based computing and photonics-based communication. The search for materials with large electro-optic coefficients and low optical loss is critical to increase the efficiency and minimize the size of electro-optic devices. We pre… ▽ More An electro-optic modulator offers the function of modulating the propagation of light in a material with electric field and enables seamless connection between electronics-based computing and photonics-based communication. The search for materials with large electro-optic coefficients and low optical loss is critical to increase the efficiency and minimize the size of electro-optic devices. We present a semi-empirical method to compute the electro-optic coefficients of ferroelectric materials by combining first-principles density-functional theory calculations with Landau-Devonshire phenomenological modeling. We apply the method to study the electro-optic constants, also called Pockels coefficients, of three paradigmatic ferroelectric oxides: BaTiO3, LiNbO3, and LiTaO3. We present their temperature-, frequency- and strain-dependent electro-optic tensors calculated using our method. The predicted electro-optic constants agree with the experimental results, where available, and provide benchmarks for experimental verification. △ Less

Submitted 18 November, 2021; v1 submitted 5 June, 2021; originally announced June 2021.

Comments: 30 pages, 6 figures, 2 tables and 2 boxes

Conservation of the Stokes-Einstein Relation in Supercooled Water

Authors: Gan Ren, Yanting Wang

Abstract: The Stokes-Einstein (SE) relation is commonly regarded as being breakdown in supercooled water. However, this conclusion is drawn upon testing the validities of some variants of the SE relation rather than its original form, and it appears conflicting with the fact that supercooled water is in its local equilibrium. In this work, we show by molecular dynamics simulation that both the Einstein and… ▽ More The Stokes-Einstein (SE) relation is commonly regarded as being breakdown in supercooled water. However, this conclusion is drawn upon testing the validities of some variants of the SE relation rather than its original form, and it appears conflicting with the fact that supercooled water is in its local equilibrium. In this work, we show by molecular dynamics simulation that both the Einstein and Stokes relations are indeed conserved in supercooled water. The inconsistency between the original SE relation and its variants comes from two facts: (1) the substitutes of the shear viscosity in the SE variants are wavevector-dependent, so it is only a cursory approximation; (2) the effective hydrodynamic radius actually decreases with decreasing temperature, instead of being a constant as assumed in the SE variants. Besides supercooled water, this inconsistency may also exist in other supercooled liquids. △ Less

Submitted 20 August, 2021; v1 submitted 1 March, 2021; originally announced March 2021.

Electron and hole trapping in the Ce3+ and Pr3+ doped lutetium pyrosilicate scintillator crystals studied by electron paramagnetic resonance

Authors: V. Laguta, M. Buryi, Y. Wu, G. Ren, M. Nik

Abstract: Electron and hole trapping was studied in the Ce3+ and Pr3+ doped Lu2Si2O7 scintillation single crystals (LPS:Ce and LPS:Pr) by Electron Paramagnetic Resonance (EPR). Detailed EPR measurements of the X-ray irradiated LPS crystals revealed that holes generated by irradiation are predominantly trapped at oxygen lattice ions creating O- centers. The same X-ray irradiation creates also electron type c… ▽ More Electron and hole trapping was studied in the Ce3+ and Pr3+ doped Lu2Si2O7 scintillation single crystals (LPS:Ce and LPS:Pr) by Electron Paramagnetic Resonance (EPR). Detailed EPR measurements of the X-ray irradiated LPS crystals revealed that holes generated by irradiation are predominantly trapped at oxygen lattice ions creating O- centers. The same X-ray irradiation creates also electron type centers which were attributed to Lu2+ ions, where the trapped electron at Lu lattice ion is stabilized by a defect nearby, such as oxygen vacancy and Ir3+ impurity ion. Both the hole and electron centers can be thus considered as a bound small polarons, which makes the charge trapping in scintillation mechanism quite competitive. The hole O- and electron Lu2+ centers show thermal stability well above room temperature. The thermal decays of their concentrations correlate well with the appearance of the thermally stimulated luminescence glow peaks at 470-550 K. The presence of the same intrinsic traps in the Ce and Pr doped LPS crystals suggests that difference in the light yield of these crystals is an intrinsic property of the Ce3+ and Pr3+ activator centers in LPS lattice. Charge traps origin in this pyrosilicate structure and their role in scintillation mechanism is compared with the results previously described in literature in orthosilicates. △ Less

Submitted 18 January, 2020; originally announced January 2020.

Journal ref: Phys. Rev. Applied 13, 044060 (2020)

Multiple-stage structure transformation of organic-inorganic hybrid perovskite CH3NH3PbI3

Authors: Qiong Chen, Henan Liu, Hui-Seon Kim, Yucheng Liu, Mengjin Yang, Naili Yue, Gang Ren, Kai Zhu, Shengzhong Liu, Nam-Gyu Park, Yong Zhang

Abstract: By performing spatially resolved Raman and photoluminescence spectroscopy with varying excitation wavelength, density, and data acquisition parameters, we have achieved a unified understanding towards the spectroscopy signatures of the organic-inorganic hybrid perovskite, transforming from the pristine state (CH3NH3PbI3) to fully degraded state (i.e., PbI2) for samples with varying crystalline dom… ▽ More By performing spatially resolved Raman and photoluminescence spectroscopy with varying excitation wavelength, density, and data acquisition parameters, we have achieved a unified understanding towards the spectroscopy signatures of the organic-inorganic hybrid perovskite, transforming from the pristine state (CH3NH3PbI3) to fully degraded state (i.e., PbI2) for samples with varying crystalline domain size from mesoscopic scale (approximately 100 nm) to macroscopic size (cm), synthesized by three different techniques. We show that the hybrid perovskite exhibits multiple stages of structure transformation occurring either spontaneously or under light illumination, with exceptionally high sensitivity to the illumination conditions (e.g., power, illumination time and interruption pattern). We highlight four transformation stages (Stage 1 - 4, with Stage 1 being the pristine state) along a primary structure degradation path exhibiting distinctly different Raman spectroscopy features at each stage, and point out that previously reported Raman spectra in the literature reflect degraded structures of either Stage 3 or 4. Additional characteristic optical features of partially degraded materials under the joint action of spontaneous and photo degradation are given. This study offers reliable benchmark results for understanding the intrinsic material properties and structure transformation of this unique category of hybrid materials, and a straightforward method to monitor the structure degradation after the material is used in a device or characterized by other techniques. The findings are pertinently important to a wide range of potential applications where the hybrid material is expected to function in greatly different environment and light-matter interaction conditions. △ Less

Submitted 10 April, 2016; originally announced April 2016.

Comments: 38 pages (double spaced), 8 figures

Journal ref: Phys. Rev. X 6, 031042 (2016)

Statistical Theory of Initiation of Explosives by Impact

Authors: Gan Ren, Yingzhe Liu, Weipeng Lai, Tao Yu

Abstract: When a given weight dropped onto an explosive charge, explosion or not is probabilistic for certain impact energy and the frequency of explosion is always increase with increasing impact energy. Based on experimental results and recently theoretical work, we propose that the hot spot formation is attributed to the activated molecules decomposition and the number of molecules initiation is proporti… ▽ More When a given weight dropped onto an explosive charge, explosion or not is probabilistic for certain impact energy and the frequency of explosion is always increase with increasing impact energy. Based on experimental results and recently theoretical work, we propose that the hot spot formation is attributed to the activated molecules decomposition and the number of molecules initiation is proportional to the impact energy but not the dropped weight heating as the previous hot spot theory. A theoretical model based on two states model has been put forward for this phenomena. It is shown that the activated molecules to form a hot spot determine the probabilistic nature of initiation by impact. It is shown a good agreement tested with Hexogen (RDX) experimental impact data. △ Less

Submitted 11 January, 2016; originally announced January 2016.

Saturated Sodium Chloride Solution under an External Static Electric Field: a Molecular Dynamics Study

Authors: Gan Ren, Yanting Wang

Abstract: The behavior of saturated aqueous sodium chloride solutions under a constant external electric field (E) was studied by molecular dynamics (MD) simulation. Our dynamic MD simulations have indicated that the irreversible nucleation process towards crystallization is accelerated by a moderate E, but retarded or even prohibited under a stronger E, which can be understood by the competition between se… ▽ More The behavior of saturated aqueous sodium chloride solutions under a constant external electric field (E) was studied by molecular dynamics (MD) simulation. Our dynamic MD simulations have indicated that the irreversible nucleation process towards crystallization is accelerated by a moderate E, but retarded or even prohibited under a stronger E, which can be understood by the competition between self-diffusion and drift motion. The former increases with E resulting in the acceleration of the nucleation process, and the latter tears oppositely charged ions more apart under a stronger E leading to the deceleration of nucleation. Moreover, our steady-state MD simulations have indicated that a first-order phase transition happens in saturated solutions only when the applied E is below a certain threshold Ec, and the ratio of crystallized ions does not change with the electric field. The magnitude of Ec increases with concentration, because larger clusters are easy to form in a more concentrated solution and require a stronger E to dissociate them. △ Less

Submitted 2 May, 2015; originally announced May 2015.

Comments: 19 pages, 4 tables, 10 figures, and 41 references

Journal ref: Chinese Physics B, Vol. 24, No. 12, 126402 (2015)

Reversible Transient Nucleation in Ionic Solutions as the Precursor of Ion Crystallization

Authors: Gan Ren, Yanting Wang

Abstract: Molecular dynamics simulations for aqueous sodium chloride solutions were carried out at various concentrations. Supplementary to the Debye-Hückel theory, reversible transient nucleation of ions was observed even in dilute solutions. The average size of formed ion clusters and the lifetime of ion pairs increase with concentration until the saturation point, when ion clusters become stable and indi… ▽ More Molecular dynamics simulations for aqueous sodium chloride solutions were carried out at various concentrations. Supplementary to the Debye-Hückel theory, reversible transient nucleation of ions was observed even in dilute solutions. The average size of formed ion clusters and the lifetime of ion pairs increase with concentration until the saturation point, when ion clusters become stable and individual ions adjust their positions to form ordered lattice structures, leading to irreversible ion crystallization, which is beyond the description of the classical nucleation theory. △ Less

Submitted 9 April, 2014; v1 submitted 10 December, 2013; originally announced December 2013.

Comments: 4 pages, 5 figures, and 46 references

Journal ref: EPL 107,30005(2014)

Trans-phonon effects in ultrafast nano-devices

Authors: Zhiping Xu, Quanshui Zheng, Qing Jiang, Chi-Chiu Ma, Yang Zhao, Guanhua Chen, Huajian Gao, Gexue Ren

Abstract: We report a novel phenomenon in carbon nanotube (CNT) based devices, the transphonon effects, which resemble the transonic effects in aerodynamics. It is caused by dissipative resonance of nanotube phonons similar to the radial breathing mode, and subsequent drastic surge of the dragging force on the sliding tube, and multiple phonon barriers are encountered as the intertube sliding velocity rea… ▽ More We report a novel phenomenon in carbon nanotube (CNT) based devices, the transphonon effects, which resemble the transonic effects in aerodynamics. It is caused by dissipative resonance of nanotube phonons similar to the radial breathing mode, and subsequent drastic surge of the dragging force on the sliding tube, and multiple phonon barriers are encountered as the intertube sliding velocity reaches critical values. It is found that the transphonon effects can be tuned by applying geometric constraints or varying chirality combinations of the nanotubes. △ Less

Submitted 6 September, 2007; originally announced September 2007.

Journal ref: Nanotechnology 19, 255705 (2008)