Showing 1–28 of 28 results for author: Feng, G

Epitaxial Electrodeposition of Fe with Controlled In-Plane Variants for Reversible Metal Anode in Aqueous Electrolyte

Authors: Chenxi Sui, Ching-Tai Fu, Guangxia Feng, Yuqi Li, Junyan Li, Gangbin Yan, Po-Chun Hsu, Steven Chu, Yi Cui

Abstract: The development of reversible metal anodes is a key challenge for advancing aqueous battery technologies, particularly for scalable and safe stationary energy storage applications. Here we demonstrate a strategy to realize epitaxial electrodeposition of iron (Fe) on single-crystal copper (Cu) substrates in aqueous electrolytes. We compare the electrodeposition behavior of Fe on polycrystalline and… ▽ More The development of reversible metal anodes is a key challenge for advancing aqueous battery technologies, particularly for scalable and safe stationary energy storage applications. Here we demonstrate a strategy to realize epitaxial electrodeposition of iron (Fe) on single-crystal copper (Cu) substrates in aqueous electrolytes. We compare the electrodeposition behavior of Fe on polycrystalline and single-crystalline Cu substrates, revealing that the latter enables highly uniform, dense, and crystallographically aligned Fe growth. Comprehensive electron backscatter diffraction (EBSD) and X-ray diffraction (XRD) analysis confirms the formation of Fe with specific out-of-plane and in-plane orientations, including well-defined rotational variants. Our findings highlight that epitaxial electrodeposition of Fe can suppress dendritic growth and significantly enhance Coulombic efficiency during plating/stripping cycles. This approach bridges fundamental crystallography with practical electrochemical performance, providing a pathway toward high-efficiency aqueous batteries utilizing Earth-abundant materials. △ Less

Submitted 12 October, 2025; originally announced October 2025.

Unconventional Superconductivity in $\mathrm{La_{3}Ni_{2}O_{7}}$ from the Perspective of Symmetry

Authors: Guan-Hao Feng, Jun Quan, Yusheng Hou

Abstract: The recently discovered superconductor $\mathrm{La_{3}Ni_{2}O_{7}}$ has attracted significant attention due to its remarkably high transition temperature ($T_{c}$) under high pressure. Shortly after this discovery, thin-film $\mathrm{La_{3}Ni_{2}O_{7}}$ was demonstrated to exhibit ambient-pressure superconductivity; however, the corresponding $T_c$ is only about half that of the pressurized bulk m… ▽ More The recently discovered superconductor $\mathrm{La_{3}Ni_{2}O_{7}}$ has attracted significant attention due to its remarkably high transition temperature ($T_{c}$) under high pressure. Shortly after this discovery, thin-film $\mathrm{La_{3}Ni_{2}O_{7}}$ was demonstrated to exhibit ambient-pressure superconductivity; however, the corresponding $T_c$ is only about half that of the pressurized bulk material. This striking difference raises questions about the underlying mechanisms governing superconductivity in these two structures. To address this issue, we develop a symmetry-based method that investigates superconducting pairings solely based on experimentally determined symmetry and $T_c$, without assuming any specific superconducting parameters. Applying this approach, we find that both pressurized bulk and thin-film $\mathrm{La_{3}Ni_{2}O_{7}}$ exhibit $s_{\pm}$-wave pairing symmetry and two-gap superconductivity, yet their dominant microscopic origins are distinct. In the pressurized bulk, superconductivity is dominated by out-of-plane pairing of the Ni-$d_{z^2}$ orbitals, while in the thin film, in-plane pairing of the Ni-$d_{x^2-y^2}$ orbitals prevails. Furthermore, the observed reduction in $T_c$ can be attributed to this transition of the dominant pairing types, driven by the decreased ratio of inter-layer to intra-layer hoppings in the thin film. Our result sheds lights on the microscopic pairings in $\mathrm{La_{3}Ni_{2}O_{7}}$ and reveal the significance of the symmetry. This method can potentially be generalized to a broader range of unconventional superconductors. △ Less

Submitted 8 October, 2025; v1 submitted 2 June, 2025; originally announced June 2025.

Machine Learning Relationships between Nanoporous Structures and Electrochemical Performance in MOF Supercapacitors

Authors: Zhenxiang Wang, Taizheng Wu, Liang Zeng, Jiaxing Peng, Ding Yu, Ming Gao, Guang Feng

Abstract: The development of supercapacitors is impeded by the unclear relationships between nanoporous electrode structures and electrochemical performance, primarily due to challenges in decoupling the complex interdependencies of various structural descriptors. While machine learning (ML) techniques offer a promising solution, their application is hindered by the lack of large, unified databases. Herein,… ▽ More The development of supercapacitors is impeded by the unclear relationships between nanoporous electrode structures and electrochemical performance, primarily due to challenges in decoupling the complex interdependencies of various structural descriptors. While machine learning (ML) techniques offer a promising solution, their application is hindered by the lack of large, unified databases. Herein, we use constant-potential molecular simulation to construct a unified supercapacitor database with hundreds of metal-organic framework (MOF) electrodes. Leveraging this database, well-trained decision-tree-based ML models achieve fast, accurate, and interpretable predictions of capacitance and charging rate, experimentally validated by a representative case. SHAP analyses reveal that specific surface area (SSA) governs gravimetric capacitance while pore size effects are minimal, attributed to the strong dependence of electrode-ion coordination on SSA rather than pore size. SSA and porosity, respectively, dominate volumetric capacitance in 1D-pore and 3D-pore MOFs, pinnacling the indispensable effects of pore dimensionality. Meanwhile, porosity is found to be the most decisive factor in the charging rate for both 1D-pore and 3D-pore MOFs. Especially for 3D-pore MOFs, an exponential increase with porosity is observed in both ionic conductance and in-pore ion diffusion coefficient, ascribed to loosened ion packing. These findings provide profound insights for the design of high-performance supercapacitor electrodes. △ Less

Submitted 15 January, 2025; originally announced January 2025.

Desorption of an active Brownian polymer from a homogeneous attractive surface

Authors: Guo-qiang Feng, Wen-de Tian

Abstract: The interfacial behavior of active Brownian polymer (ABPO) is studied by Langevin dynamics simulations. On the dependence of adsorption strength and activity characterized by Peclet number (Pe), the polymer displays two typical states on the surface: adsorption and desorption states. We find the diffusion behavior of ABPO parallel to the surface yields the "active Rouse model" and activity causes… ▽ More The interfacial behavior of active Brownian polymer (ABPO) is studied by Langevin dynamics simulations. On the dependence of adsorption strength and activity characterized by Peclet number (Pe), the polymer displays two typical states on the surface: adsorption and desorption states. We find the diffusion behavior of ABPO parallel to the surface yields the "active Rouse model" and activity causes the adsorption-desorption transition at a certain adsorption strength. Particular attention is paid to how the desorption time changes with the activity. At intermediate activity, desorption time displays an exponential decay with the inverse of effective temperature. Further, we observed a non-monotonic dependence of desorption time on the rotation diffusion coefficient of the monomer and found it exists a scaling relation with chain length N. Our results highlight the activity can be used to regulate the polymer adsorption and desorption behavior. △ Less

Submitted 25 August, 2024; originally announced August 2024.

Nodal higher-order topological superconductivity from C6-symmetric Dirac semimetals

Authors: Guan-Hao Feng

Abstract: Three-dimensional Dirac semimetals (DSMs) have been shown to exhibit one-dimensional hinge modes which are termed the higher-order hinge Fermi-arc (HOFA) states. They are the topological consequences of Dirac points. Superconducting states from Dirac semimetals can inherit the Dirac points to form nodal Dirac superconducting states, raising a question of whether there exists a topological supercon… ▽ More Three-dimensional Dirac semimetals (DSMs) have been shown to exhibit one-dimensional hinge modes which are termed the higher-order hinge Fermi-arc (HOFA) states. They are the topological consequences of Dirac points. Superconducting states from Dirac semimetals can inherit the Dirac points to form nodal Dirac superconducting states, raising a question of whether there exists a topological superconducting bulk-hinge correspondence similar to DSMs. In this work, we discuss the nodal superconducting states from half-filled DSMs respecting non-magnetic (Type-II) Shubnikov space group (SSG) $P6/mmm1'$. We find that the BdG Dirac points can lead to higher-order topological Dirac superconducting (HOTDSC) states instead of the expected higher-order Majorana-arc (HOMA) states. The HOTDSC states can be regarded as a crossing between the HOFAs in normal states and the BdG shadow states. We demonstrate that HOTDSC states can be indicated by relative topologies of BdG Dirac points by utilizing the theory of magnetic topological quantum chemistry (MTQC). △ Less

Submitted 8 November, 2024; v1 submitted 18 August, 2024; originally announced August 2024.

Organic solvent boosts charge storage and charging dynamics of conductive MOF supercapacitors

Authors: Ming Chen, Taizheng Wu, Liang Niu, Ting Ye, Wenlei Dai, Liang Zeng, Alexei A. Kornyshev, Zhenxiang Wang, Zhou Liu, Guang Feng

Abstract: Conductive metal-organic frameworks (c-MOFs) and ionic liquids (ILs) have emerged as auspicious combinations for high-performance supercapacitors. However, the nanoconfinement from c-MOFs and high viscosity of ILs slow down the charging process. This hindrance can, however, be resolved by adding solvent. Here, we performed constant-potential molecular simulations to scrutinize the solvent impact o… ▽ More Conductive metal-organic frameworks (c-MOFs) and ionic liquids (ILs) have emerged as auspicious combinations for high-performance supercapacitors. However, the nanoconfinement from c-MOFs and high viscosity of ILs slow down the charging process. This hindrance can, however, be resolved by adding solvent. Here, we performed constant-potential molecular simulations to scrutinize the solvent impact on charge storage and charging dynamics of MOF-IL-based supercapacitors. We find conditions for >100% enhancement in capacity and ~6 times increase in charging speed. These improvements were confirmed by synthesizing near-ideal c-MOFs and developing multiscale models linking molecular simulations to electrochemical measurements. Fundamentally, our findings elucidate that the solvent acts as an ionophobic agent to induce a substantial enhancement in charge storage, and as an ion traffic police to eliminate convoluted counterion and co-ion motion paths and create two distinct ion transport highways to accelerate charging dynamics. This work paves the way for the optimal design of MOF supercapacitors. △ Less

Submitted 2 March, 2024; originally announced March 2024.

Moment-Tensor-Based Constant-Potential Modeling of Electrical Double Layers

Authors: Zhenxiang Wang, Ming Chen, Jiedu Wu, Xiangyu Ji, Liang Zeng, Jiaxing Peng, Jiawei Yan, Alexei A. Kornyshev, Bingwei Mao, Guang Feng

Abstract: Constant-potential molecular dynamics (MD) simulations are indispensable for understanding the capacitance, structure, and dynamics of electrical double layers (EDLs) at the atomistic level. However, the classical constant-potential method, relying on the so-called 'floating charges' to keep electrode equipotential, overlooks quantum effects on the electrode and always underestimates EDL capacitan… ▽ More Constant-potential molecular dynamics (MD) simulations are indispensable for understanding the capacitance, structure, and dynamics of electrical double layers (EDLs) at the atomistic level. However, the classical constant-potential method, relying on the so-called 'floating charges' to keep electrode equipotential, overlooks quantum effects on the electrode and always underestimates EDL capacitance for typical electrochemical systems featuring metal electrodes in aqueous electrolytes. Here, we propose a universal theoretical framework as moment-tensor-based constant potential method (mCPM) to capture electronic structure variations with electric moments. For EDLs at Au(111) electrodes, mCPM-based MD reveals bell-shaped capacitance curves in magnitude and shape both quantitatively consistent with experiments. It further unveils the potential-dependent local electric fields, agreeing with experimental observations of redshift vibration of interfacial water under negative polarization and predicting a blueshift under positive polarization, and identifies geometry dependence of two time scales during EDL formation. △ Less

Submitted 30 January, 2024; originally announced January 2024.

Machine learning of (1+1)-dimensional directed percolation based on raw and shuffled configurations

Authors: Shen Jianmin, Wang Shanshan, Li Wei, Xu Dian, Yang Yuxiang, Wang Yanyang, Gao Feng, Zhu Yueying, Tuo Kui

Abstract: Machine learning (ML) can process large sets of data generated from complex systems, which is ideal for classification tasks as often appeared in critical phenomena. Meanwhile ML techniques have been found effective in detecting critical points, or in a broader sense phase separation, and extracting critical exponents. But there are still many unsolved issues with the ML, one of which is the meani… ▽ More Machine learning (ML) can process large sets of data generated from complex systems, which is ideal for classification tasks as often appeared in critical phenomena. Meanwhile ML techniques have been found effective in detecting critical points, or in a broader sense phase separation, and extracting critical exponents. But there are still many unsolved issues with the ML, one of which is the meaning of hidden variables of unsupervised learning. Some say that the hidden variables and the principal component may contain basic information regarding the order parameter of the system of interest, which sounds plausible but lacks evidence. This study aims at searching for evidence supporting the conjecture that the autoencoder's (AE) single latent variable and PCA's first principal component can only serve as signals related to particle density, which happens to be the order parameter of the non-equilibrium DP model. Indeed, in some phase transition (PT) models the order parameter is the particle density, whereas in some PT models it is not. Having conducted a certain degree of random shuffling on the DP configurations, which are then fed to the neural networks as input, we find that AE's single latent variable and PCA's first principal component can indeed represent particle density. It is found that shuffling does affect the size of maximum cluster in the system, which suggests that the second principal component of the PCA is related to the maximal cluster. This has been supported by changes in the correlation length of the transition system with variations in the shuffle ratio. △ Less

Submitted 6 May, 2024; v1 submitted 20 November, 2023; originally announced November 2023.

Comments: 18 pages, 15 figures

Theory and Experiments of Pressure-Tunable Broadband Light Emission from Self-Trapped Excitons in Metal Halide Crystals

Authors: Shenyu Dai, Xinxin Xing, Viktor G. Hadjiev, Zhaojun Qin, Tian Tong, Guang Yang, Chong Wang, Lijuan Hou, Liangzi Deng, Zhiming Wang, Guoying Feng, Jiming Bao

Abstract: Hydrostatic pressure has been commonly applied to tune broadband light emissions from self-trapped excitons (STE) in perovskites for producing white light and study of basic electron-phonon interactions. However, a general theory is still lacking to understand pressure-driven evolution of STE emissions. In this work we first identify a theoretical model that predicts the effect of hydrostatic pres… ▽ More Hydrostatic pressure has been commonly applied to tune broadband light emissions from self-trapped excitons (STE) in perovskites for producing white light and study of basic electron-phonon interactions. However, a general theory is still lacking to understand pressure-driven evolution of STE emissions. In this work we first identify a theoretical model that predicts the effect of hydrostatic pressure on STE emission spectrum, we then report the observation of extremely broadband photoluminescence emission and its wide pressure spectral tuning in 2D indirect bandgap CsPb2Br5 crystals. An excellent agreement is found between the theory and experiment on the peculiar experimental observation of STE emission with a nearly constant spectral bandwidth but linearly increasing energy with pressure below 2 GPa. Further analysis by the theory and experiment under higher pressure reveals that two types of STE are involved and respond differently to external pressure. We subsequently survey published STE emissions and discovered that most of them show a spectral blue-shift under pressure, as predicted by the theory. The identification of an appropriate theoretical model and its application to STE emission through the coordinate configuration diagram paves the way for engineering the STE emission and basic understanding of electron-phonon interaction. △ Less

Submitted 23 September, 2022; originally announced September 2022.

Journal ref: Materials Today Physics 30 (2023): 100926

Probing Robust Majorana Signatures by Crossed Andreev Reflection with a Quantum Dot

Authors: Guan-Hao Feng, Hong-Hao Zhang

Abstract: We propose a three-terminal structure to probe robust signatures of Majorana zero modes. This structure consists of a quantum dot coupled to the normal metal, s-wave superconducting and Majorana Y-junction leads. The zero-bias differential conductance at zero temperature of the normal-metal lead peaks at $2e^{2}/h$, which will be deflected after Majorana braiding. This quantized conductance can en… ▽ More We propose a three-terminal structure to probe robust signatures of Majorana zero modes. This structure consists of a quantum dot coupled to the normal metal, s-wave superconducting and Majorana Y-junction leads. The zero-bias differential conductance at zero temperature of the normal-metal lead peaks at $2e^{2}/h$, which will be deflected after Majorana braiding. This quantized conductance can entirely arise from the Majorana-induced crossed Andreev reflection, protected by the energy gap of the superconducting lead. We find that the effect of thermal broadening is significantly suppressed when the dot is on resonance. In the case that the energy level of the quantum dot is much larger than the superconducting gap, tunneling processes are dominated by Majorana-induced crossed Andreev reflection. Particularly, a novel kind of crossed Andreev reflection equivalent to the splitting of charge quanta $3e$ occurs after Majorana braiding. △ Less

Submitted 26 January, 2022; v1 submitted 6 May, 2021; originally announced May 2021.

Comments: 15 pages, 8 figures, accepted for publication in Physical Review B

Journal ref: Phys. Rev. B 105, 035148 (2022)

Adding salt to expand voltage window of humid ionic liquids

Authors: Ming Chen, Jiedu Wu, Ting Ye, Jinyu Ye, Sheng Bi, Jiawei Yan, Bingwei Mao, Guang Feng

Abstract: Humid hydrophobic ionic liquids, widely used as electrolytes, have narrowed electrochemical windows, because their water, absorbed on the electrode surface, gets involved in electrolysis. In this work, we performed molecular dynamics simulations to explore effects of adding Li-salt in humid ionic liquids on the water adsorbed on the electrode surface. Results reveal that most of water molecules ar… ▽ More Humid hydrophobic ionic liquids, widely used as electrolytes, have narrowed electrochemical windows, because their water, absorbed on the electrode surface, gets involved in electrolysis. In this work, we performed molecular dynamics simulations to explore effects of adding Li-salt in humid ionic liquids on the water adsorbed on the electrode surface. Results reveal that most of water molecules are pushed away from both cathode and anode, by adding salt. The water remained on the electrode is almost bound with Li+, which has significantly lowered activity. The Li+-bonding and re-arrangement of the surface-adsorbed water both facilitate the inhibition of water electrolysis, and thus prevent the reduction of electrochemical windows of humid hydrophobic ionic liquids. This finding is testified by cyclic voltammetry measurements where salt-in-humid ionic liquids exhibit enhanced electrochemical windows. Our work provides the underlying mechanism and a simple but practical approach for protection of humid ionic liquids from performance degradation. △ Less

Submitted 24 February, 2020; originally announced February 2020.

Comments: 20 pages, 5 figures

Self-formed 2D/3D Heterostructure on the Edge of 2D Ruddlesden-Popper Hybrid Perovskites Responsible for Intriguing Optoelectronic Properties and Higher Cell Efficiency

Authors: Zhaojun Qin, Shenyu Dai, Chalapathi Charan Gajjala, Chong Wang, Viktor G. Hadjiev, Guang Yang, Jiabing Li, Xin Zhong, Zhongjia Tang, Yan Yao, Arnold M. Guloy, Rohith Reddy, David Mayerich, Liangzi Deng, Qingkai Yu, Guoying Feng, Zhiming Wang, Jiming Bao

Abstract: The observation of low energy edge photoluminescence and its beneficial effect on the solar cell efficiency of Ruddlesden-Popper perovskites has unleashed an intensive research effort to reveal its origin. This effort, however, has been met with more challenges as the underlying material structure has still not been identified; new modellings and observations also do not seem to converge. Using 2D… ▽ More The observation of low energy edge photoluminescence and its beneficial effect on the solar cell efficiency of Ruddlesden-Popper perovskites has unleashed an intensive research effort to reveal its origin. This effort, however, has been met with more challenges as the underlying material structure has still not been identified; new modellings and observations also do not seem to converge. Using 2D (BA)2(MA)2Pb3Br10 as an example, we show that 3D MAPbBr3 is formed due to the loss of BA on the edge. This self-formed MAPbBr3 can explain the reported edge emission under various conditions, while the reported intriguing optoelectronic properties such as fast exciton trapping from the interior 2D perovskite, rapid exciton dissociation and long carrier lifetime can be understood via the self-formed 2D/3D lateral perovskite heterostructure. The 3D perovskite is identified by submicron infrared spectroscopy, the emergence of XRD signature from freezer-milled nanometer-sized 2D perovskite and its photoluminescence response to external hydrostatic pressure. The revelation of this edge emission mystery and the identification of a self-formed 2D/3D heterostructure provide a new approach to engineering 2D perovskites for high-performance optoelectronic devices. △ Less

Submitted 26 January, 2020; originally announced January 2020.

Monte Carlo studies of modified scalable designs for quantum computation

Authors: Guan-Hao Feng, Jingwei Chen, Hong-Hao Zhang

Abstract: As the building blocks of topological quantum computation, Majorana zero modes (MZMs) have attracted tremendous attention in recent years. Scalable mesoscopic island designs with MZMs show great potential in quantum information processing. However, these systems are susceptible to quasi-particle poisoning which would induce various parity-breaking errors. To solve this problem, we modify the mesos… ▽ More As the building blocks of topological quantum computation, Majorana zero modes (MZMs) have attracted tremendous attention in recent years. Scalable mesoscopic island designs with MZMs show great potential in quantum information processing. However, these systems are susceptible to quasi-particle poisoning which would induce various parity-breaking errors. To solve this problem, we modify the mesoscopic islands with gate-tunable valves and non-topological backbones. We study the lifetime of the Majorana qubits on these modified islands which are coupled to local bosonic and fermionic thermal baths. We consider both the parity-breaking and parity-preserving errors, and propose a parity correction scheme. By using Jordan-Wigner transformation, we analyze the probability of logical X and Y errors. The open quantum system is described by the Pauli master equation, and standard Monte Carlo simulations are applied to observe the behavior of the system when the parity correction proposal is implemented. The results demonstrate that (1) our parity correction proposal is effective to most of the parity-breaking errors; (2) the lifetime of the qubit benefits from larger island size before it meets the threshold; (3) small chemical potential $ μ$ on the non-topological backbones and fine tuned paring potential $ Δ$ of the topological bulk segment are required for high probability of correctness. Our results provide an effective error correction scheme for the parity-breaking errors. △ Less

Submitted 15 November, 2019; originally announced November 2019.

Comments: 28 pages, 13 figures

Photoluminescence mapping and time-domain thermo-photoluminescence for rapid imaging and measurement of thermal conductivity of boron arsenide

Authors: Shuai Yue, Geethal Amila Gamage, Mohammadjavad Mohebinia, David Mayerich, Vishal Talari, Yu Deng, Fei Tian, Shenyu Dai, Haoran Sun, Viktor G. Hadjiev, Wei Zhang, Guoying Feng, Jonathan Hu, Dong Liu, Zhiming Wang, Zhifeng Ren, Jiming Bao

Abstract: Cubic boron arsenide (BAs) is attracting greater attention due to the recent experimental demonstration of ultrahigh thermal conductivity \k{appa} above 1000 W/mK. However, its bandgap has not been settled and a simple yet effective method to probe its crystal quality is missing. Furthermore, traditional \k{appa} measurement methods are destructive and time consuming, thus they cannot meet the urg… ▽ More Cubic boron arsenide (BAs) is attracting greater attention due to the recent experimental demonstration of ultrahigh thermal conductivity \k{appa} above 1000 W/mK. However, its bandgap has not been settled and a simple yet effective method to probe its crystal quality is missing. Furthermore, traditional \k{appa} measurement methods are destructive and time consuming, thus they cannot meet the urgent demand for fast screening of high \k{appa} materials. After we experimentally established 1.82 eV as the indirect bandgap of BAs and observed room-temperature band-edge photoluminescence, we developed two new optical techniques that can provide rapid and non-destructive characterization of \k{appa} with little sample preparation: photoluminescence mapping (PL-mapping) and time-domain thermo-photoluminescence (TDTP). PL-mapping provides nearly real-time image of crystal quality and \k{appa} over mm-sized crystal surfaces; while TDTP allows us to pick up any spot on the sample surface and measure its \k{appa} using nanosecond laser pulses. These new techniques reveal that the apparent single crystals are not only non-uniform in \k{appa}, but also are made of domains of very distinct \k{appa}. Because PL-mapping and TDTP are based on the band-edge PL and its dependence on temperature, they can be applied to other semiconductors, thus paving the way for rapid identification and development of high-\k{appa} semiconducting materials. △ Less

Submitted 15 October, 2019; originally announced October 2019.

Revealing the Origin of Luminescence Center in 0D Cs4PbBr6 Perovskite

Authors: Zhaojun Qin, Shenyu Dai, Viktor G. Hadjiev, Chong Wang, Lingxi Ouyang, Lixin Xie, Yizhou Ni, Chunzheng Wu, Guang Yang, Shuo Chen, Liangzi Deng, Qingkai Yu, Ching-Wu Chu, Guoying Feng, Zhiming Wang, Jiming Bao

Abstract: Zero dimensional perovskite Cs4PbBr6 has attracted considerable attention recently not only because of its highly efficient green photoluminescence (PL), but also its two highly debated opposing mechanisms of the luminescence: embedded CsPbBr3 nanocrystals versus intrinsic Br vacancy states. After a brief discussion on the root cause of the controversy, we provide sensitive but non-invasive method… ▽ More Zero dimensional perovskite Cs4PbBr6 has attracted considerable attention recently not only because of its highly efficient green photoluminescence (PL), but also its two highly debated opposing mechanisms of the luminescence: embedded CsPbBr3 nanocrystals versus intrinsic Br vacancy states. After a brief discussion on the root cause of the controversy, we provide sensitive but non-invasive methods that can not only directly correlate luminescence with the underlying structure, but also distinguish point defects from embedded nanostructures. We first synthesized both emissive and non-emissive Cs4PbBr6 crystals, obtained the complete Raman spectrum of Cs4PbBr6 and assigned all Raman bands based on density functional theory simulations. We then used correlated Raman-PL as a passive structure-property method to identify the difference between emissive and non-emissive Cs4PbBr6 crystals and revealed the existence of CsPbBr3 nanocrystals in emissive Cs4PbBr6. We finally employed a diamond anvil cell to probe the response of luminescence centers to hydrostatic pressure. The observations of fast red-shifting, diminishing and eventual disappearance of both green emission and Raman below Cs4PbBr6 phase transition pressure of ~3 GPa is compatible with CsPbBr3 nanocrystal inclusions as green PL emitters and cannot be explained by Br vacancies. The resolution of this long-lasting controversy paves the way for further device applications of low dimensional perovskites, and our comprehensive optical technique integrating structure-property with dynamic pressure response is generic and can be applied to other emerging optical materials to understand the nature of their luminescent centers. △ Less

Submitted 10 April, 2019; v1 submitted 9 April, 2019; originally announced April 2019.

Journal ref: Chem. Mater. 2019, 31, 21, 9098-9104

Molecular understanding of charge storage and charging dynamics in supercapacitors with MOF electrodes and ionic liquid electrolytes

Authors: Sheng Bi, Ming Chen, Runxi Wang, Jiamao Feng, Mircea Dinca, Alexei A. Kornyshev, Guang Feng

Abstract: We present a computational microscopy analysis (targeted molecular dynamics simulations) of the structure and performance of conductive metal organic framework (MOF) electrodes in supercapacitors with room temperature ionic liquids. The molecular modeling predicts the characteristic shapes of the potential dependence of electrode capacitance, relying on the structure of MOF electrodes and particul… ▽ More We present a computational microscopy analysis (targeted molecular dynamics simulations) of the structure and performance of conductive metal organic framework (MOF) electrodes in supercapacitors with room temperature ionic liquids. The molecular modeling predicts the characteristic shapes of the potential dependence of electrode capacitance, relying on the structure of MOF electrodes and particularly how ions transport and reside in MOFs under polarization. Transmission line model was adopted to characterize the charging dynamics process and build up a bridge to evaluate the capacitive performance of practical supercapacitor devices at macroscale from the simulation-obtained data at nanoscale. Such nanoscale-to-macroscale analysis demonstrates the potential of MOF supercapacitors for achieving unprecedentedly high volumetric energy and power densities. The investigation gives molecular insights into the preferred structures of MOF for achieving these results, which could provide a blueprint for future experimental characterization of these new systems. △ Less

Submitted 1 March, 2019; originally announced March 2019.

Comments: 19 pages, 6 figures

Journal ref: Nature Materials, 2020

Kinetics of Ion Transport in Ionic Liquids: Two Dynamical Diffusion States

Authors: Guang Feng, Ming Chen, Sheng Bi, Zachary A. H. Goodwin, Eugene B. Postnikov, Michael Urbakh, Alexei A. Kornyshev

Abstract: Using classical molecular dynamics simulations, we investigate the mobility of ions in [Bmim][TFSI], a typical room temperature ionic liquid. Analyzing the trajectories of individual cations and anions, we estimate the time that ions spend in bound, clustered states, and when the ions move quasi-freely. Using this information, we evaluate the average portion of free ions that dominate conductivity… ▽ More Using classical molecular dynamics simulations, we investigate the mobility of ions in [Bmim][TFSI], a typical room temperature ionic liquid. Analyzing the trajectories of individual cations and anions, we estimate the time that ions spend in bound, clustered states, and when the ions move quasi-freely. Using this information, we evaluate the average portion of free ions that dominate conductivity. The amount of thus defined free ions comprises 15-25%, monotonically increasing with temperature in the range of 300-600 K, with the rest of the ions being temporarily bound, moving rather in local potentials. The conductivities as a function of temperature, calculated from electric current autocorrelation functions, reproduce reported experimental data well. Interestingly, for free ions the Nernst-Einstein relationship between the mobility and diffusion coefficient holds fairly well. In analogy with electronic semiconductors, one can speak about an ionic semiconductor model for ionic liquids with valence (or excitonic) and conduction band states for ions, separated by an energy gap. The obtained band gap for the ionic liquid is, however, very small, about 0.026 eV, allowing for easy interchanges between the two dynamic states. △ Less

Submitted 2 May, 2018; originally announced May 2018.

Journal ref: Physical Review X, 2019

Enhanced Raman sideband cooling of caesium atoms in a vapour-loaded magneto-optical trap

Authors: Y. Li, J. Wu, G. Feng, J. Nute, S. Piano, L. Hackermuller, J. Ma, L. Xiao, S. Jia

Abstract: We report enhanced three-dimensional degenerated Raman sideband cooling (3D DRSC) of caesium (Cs) atoms in a standard single-cell vapour-loading magneto-optical trap. Our improved scheme involves using a separate repumping laser and optimized lattice detuning. We load $1.5 \times 10^7$ atoms into the Raman lattice with a detuning of -15.5 GHz (to the ground F = 3 state). Enhanced 3D DRSC is used t… ▽ More We report enhanced three-dimensional degenerated Raman sideband cooling (3D DRSC) of caesium (Cs) atoms in a standard single-cell vapour-loading magneto-optical trap. Our improved scheme involves using a separate repumping laser and optimized lattice detuning. We load $1.5 \times 10^7$ atoms into the Raman lattice with a detuning of -15.5 GHz (to the ground F = 3 state). Enhanced 3D DRSC is used to cool them from 60 $μ$K to 1.7 $μ$K within 12 ms and the number of obtained atoms is about $1.2 \times 10^7$. A theoretical model is proposed to simulate the measured number of trapped atoms. The result shows good agreement with the experimental data. The technique paves the way for loading a large number of ultracold Cs atoms into a crossed dipole trap and efficient evaporative cooling in a single-cell system. △ Less

Submitted 19 March, 2015; originally announced March 2015.

Comments: 9 pages, 4 figures. Accepted in Laser Physics Letters

Interfacial Ionic Liquids: Connecting Static and Dynamic Structures

Authors: Ahmet Uysal, Hua Zhou, Guang Feng, Sang Soo Lee, Song Li, Peter T. Cummings, Pasquale F. Fulvio, Sheng Dai, John K. McDonough, Yury Gogotsi, Paul Fenter

Abstract: It is well-known that room temperature ionic liquids (RTILs) often adopt a charge-separated layered structure, i.e., with alternating cation- and anion-rich layers, at electrified interfaces. However, the dynamic response of the layered structure to temporal variations in applied potential is not well understood. We used in situ, real-time X-ray reflectivity (XR) to study the potential-dependent e… ▽ More It is well-known that room temperature ionic liquids (RTILs) often adopt a charge-separated layered structure, i.e., with alternating cation- and anion-rich layers, at electrified interfaces. However, the dynamic response of the layered structure to temporal variations in applied potential is not well understood. We used in situ, real-time X-ray reflectivity (XR) to study the potential-dependent electric double layer (EDL) structure of an imidazolium-based RTIL on charged epitaxial graphene during potential cycling as a function of temperature. The results suggest that the graphene-RTIL interfacial structure is bistable in which the EDL structure at any intermediate potential can be described by the combination of two extreme-potential structures whose proportions vary depending on the polarity and magnitude of the applied potential. This picture is supported by the EDL structures obtained by fully atomistic molecular dynamics (MD) simulations at various static potentials. The potential-driven transition between the two structures is characterized by an increasing width but with an approximately fixed hysteresis magnitude as a function of temperature. The results are consistent with the coexistence of distinct anion and cation adsorbed structures separated by an energy barrier (~0.15 eV). △ Less

Submitted 6 December, 2014; v1 submitted 21 November, 2014; originally announced November 2014.

Journal ref: J. Phys.: Condens. Matter 27 (2015) 032101

High quality superconducting tunnel junction barriers using atomic layer deposition

Authors: Stephanie M. Moyerman, Guangyuan Feng, Lisa Krayer, Nathan Stebor, Brian G. Keating

Abstract: We demonstrate a technique for creating high quality, large area tunnel junction barriers for normal-insulating- superconducting or superconducting-insulating-superconducting tunnel junctions. We use atomic layer depo- sition and an aluminum wetting layer to form a nanometer scale insulating barrier on gold films. Electronic transport measurements confirm that single-particle electron tunneling is… ▽ More We demonstrate a technique for creating high quality, large area tunnel junction barriers for normal-insulating- superconducting or superconducting-insulating-superconducting tunnel junctions. We use atomic layer depo- sition and an aluminum wetting layer to form a nanometer scale insulating barrier on gold films. Electronic transport measurements confirm that single-particle electron tunneling is the dominant transport mechanism, and the measured current-voltage curves demonstrate the viability of using these devices as self-calibrated, low temperature thermometers with a wide range of tunable parameters. The potential for fabricating high performance junction refrigerators is also highlighted. △ Less

Submitted 26 March, 2013; v1 submitted 26 March, 2013; originally announced March 2013.

Comments: 4 pages, 5 figures - This paper has been withdrawn by the author for edits

Multiscale Algorithms for Eigenvalue Problems

Authors: Nimal Wijesekera, Guogang Feng, Thomas L. Beck

Abstract: Iterative multiscale methods for electronic structure calculations offer several advantages for large-scale problems. Here we examine a nonlinear full approximation scheme (FAS) multigrid method for solving fixed potential and self-consistent eigenvalue problems. In principle, the expensive orthogonalization and Ritz projection operations can be moved to coarse levels, thus substantially reducin… ▽ More Iterative multiscale methods for electronic structure calculations offer several advantages for large-scale problems. Here we examine a nonlinear full approximation scheme (FAS) multigrid method for solving fixed potential and self-consistent eigenvalue problems. In principle, the expensive orthogonalization and Ritz projection operations can be moved to coarse levels, thus substantially reducing the overall computational expense. Results of the nonlinear multiscale approach are presented for simple fixed potential problems and for self-consistent pseudopotential calculations on large molecules. It is shown that, while excellent efficiencies can be obtained for problems with small numbers of states or well-defined eigenvalue cluster structure, the algorithm in its original form stalls for large-molecule problems with tens of occupied levels. Work is in progress to attempt to alleviate those difficulties. △ Less

Submitted 16 April, 2003; originally announced April 2003.

Effects of a Parallel Magnetic Field on the Metal-Insulator Transition in a Dilute Two-Dimensional Electron System

Authors: Kevin Eng, X. G. Feng, Dragana Popovic, S. Washburn

Abstract: The temperature dependence of conductivity $σ(T)$ of a two-dimensional electron system in silicon has been studied in parallel magnetic fields B. At B=0, the system displays a metal-insulator transition at a critical electron density $n_c(0)$, and $dσ/dT >0$ in the metallic phase. At low fields ($B\lesssim 2$ T), $n_c$ increases as $n_c(B) - n_c(0) \propto B^β$ ($β\sim 1$), and the zero-temperat… ▽ More The temperature dependence of conductivity $σ(T)$ of a two-dimensional electron system in silicon has been studied in parallel magnetic fields B. At B=0, the system displays a metal-insulator transition at a critical electron density $n_c(0)$, and $dσ/dT >0$ in the metallic phase. At low fields ($B\lesssim 2$ T), $n_c$ increases as $n_c(B) - n_c(0) \propto B^β$ ($β\sim 1$), and the zero-temperature conductivity scales as $σ(n_s,B,T=0)/σ(n_s,0,0)=f(B^β/δ_n)$ (where $δ_n=(n_s-n_c(0))/n_c(0)$, and $n_s$ is electron density) as expected for a quantum phase transition. The metallic phase persists in fields of up to 18 T, consistent with the saturation of $n_c$ at high fields. △ Less

Submitted 23 April, 2002; v1 submitted 18 December, 2001; originally announced December 2001.

Comments: published version

Journal ref: Phys. Rev. Letters 88, 136402 (2002)

Effects of a parallel magnetic field on the novel metallic behavior in two dimensions

Authors: K. Eng, X. G. Feng, Dragana Popovic, S. Washburn

Abstract: Magnetoconductance (MC) in a parallel magnetic field B has been measured in a two-dimensional electron system in Si, in the regime where the conductivity decreases as σ(n_s,T,B=0)=σ(n_s,T=0) + A(n_s)T^2 (n_s -- carrier density) to a non-zero value as temperature T->0. Very near the B=0 metal-insulator transition, there is a large initial drop in σwith increasing B, followed by a much weaker σ(B)… ▽ More Magnetoconductance (MC) in a parallel magnetic field B has been measured in a two-dimensional electron system in Si, in the regime where the conductivity decreases as σ(n_s,T,B=0)=σ(n_s,T=0) + A(n_s)T^2 (n_s -- carrier density) to a non-zero value as temperature T->0. Very near the B=0 metal-insulator transition, there is a large initial drop in σwith increasing B, followed by a much weaker σ(B). At higher n_s, the initial drop of MC is less pronounced. △ Less

Submitted 5 December, 2000; originally announced December 2000.

Comments: 2 pages, 3 figures; to be publ. in the Proceedings of the 25th Int. Conf. on the Physics of Semiconductors (Osaka, Sept. 2000)

Suppression of metallic behavior in two dimensions by spin flip scattering

Authors: X. G. Feng, Dragana Popovic, S. Washburn

Abstract: We study the effect of the disorder on the metallic behavior of a two-dimensional electron system in silicon. The temperature dependence of conductivity $σ(T)$ was measured for different values of substrate bias, which changes both potential scattering and the concentration of disorder-induced local magnetic moments. We find that the latter has a much more profound effect on $dσ/dT$. In fact, th… ▽ More We study the effect of the disorder on the metallic behavior of a two-dimensional electron system in silicon. The temperature dependence of conductivity $σ(T)$ was measured for different values of substrate bias, which changes both potential scattering and the concentration of disorder-induced local magnetic moments. We find that the latter has a much more profound effect on $dσ/dT$. In fact, the data suggest that in the limit of $T\to 0$ the metallic behavior, as characterized by $dσ/dT < 0$, is suppressed by an arbitrarily small amount of spin flip scattering by local magnetic moments. △ Less

Submitted 11 May, 1999; originally announced May 1999.

Comments: 5 pages, latex, (usues moriond.sty, included) plus three postscript figures, Proceedings of the Rencontres de Moriond 1999

Novel metallic behavior in two dimensions

Authors: X. G. Feng, Dragana Popovic, S. Washburn, V. Dobrosavljevic

Abstract: Experiments on a sufficiently disordered two-dimensional (2D) electron system in silicon reveal a new and unexpected kind of metallic behavior, where the conductivity decreases as σ(n_s,T)=σ(n_s,T=0)+A(n_s)T^2 (n_s-carrier density) to a non-zero value as temperature T->0. In 2D, the existence of a metal with dσ/dT>0 is very surprising. In addition, a novel type of a metal-insulator transition ob… ▽ More Experiments on a sufficiently disordered two-dimensional (2D) electron system in silicon reveal a new and unexpected kind of metallic behavior, where the conductivity decreases as σ(n_s,T)=σ(n_s,T=0)+A(n_s)T^2 (n_s-carrier density) to a non-zero value as temperature T->0. In 2D, the existence of a metal with dσ/dT>0 is very surprising. In addition, a novel type of a metal-insulator transition obtains, which is unlike any known quantum phase transition in 2D. △ Less

Submitted 8 December, 2000; v1 submitted 15 March, 1999; originally announced March 1999.

Comments: 4 pages, 4 figures, revtex; includes many more details

Journal ref: Phys. Rev. Lett. 86, 2625 (2001)

Effect of Local Magnetic Moments on the Metallic Behavior in Two Dimensions

Authors: X. G. Feng, Dragana Popovic, S. Washburn

Abstract: The temperature dependence of conductivity $σ(T)$ in the metallic phase of a two-dimensional electron system in silicon has been studied for different concentrations of local magnetic moments. The local moments have been induced by disorder, and their number was varied using substrate bias. The data suggest that in the limit of $T\to 0$ the metallic behavior, as characterized by $dσ/dT < 0$, is… ▽ More The temperature dependence of conductivity $σ(T)$ in the metallic phase of a two-dimensional electron system in silicon has been studied for different concentrations of local magnetic moments. The local moments have been induced by disorder, and their number was varied using substrate bias. The data suggest that in the limit of $T\to 0$ the metallic behavior, as characterized by $dσ/dT < 0$, is suppressed by an arbitrarily small amount of scattering by local magnetic moments. △ Less

Submitted 30 March, 1999; v1 submitted 3 February, 1999; originally announced February 1999.

Comments: 4 pages, revtex, plus four encapsulated postscript figures

Journal ref: Phys. Rev. Lett. 83, 368 (1999)

Many-body correlations probed by plasmon-enhanced drag measurements in double quantum well structures

Authors: H. Noh, S. Zelakiewicz, X. G. Feng, T. J. Gramila, L. N. Pfeiffer, K. W. West

Abstract: Electron drag measurements of electron-electron scattering rates performed close to the Fermi temperature are reported. While evidence of an enhancement due to plasmons, as was recently predicted [K. Flensberg and B. Y.-K. Hu, Phys. Rev. Lett. 73, 3572 (1994)], is found, important differences with the random-phase approximation based calculations are observed. Although static correlation effects… ▽ More Electron drag measurements of electron-electron scattering rates performed close to the Fermi temperature are reported. While evidence of an enhancement due to plasmons, as was recently predicted [K. Flensberg and B. Y.-K. Hu, Phys. Rev. Lett. 73, 3572 (1994)], is found, important differences with the random-phase approximation based calculations are observed. Although static correlation effects likely account for part of this difference, it is argued that correlation-induced multiparticle excitations must be included to account for the magnitude of the rates and observed density dependences. △ Less

Submitted 2 September, 1998; originally announced September 1998.

Comments: 4 pages, 3 figures, revtex Accepted in Phys. Rev. B

Negative Electron Drag and Hole-Like behavior in the Integer Quantum Hall Regime

Authors: X. G. Feng, S. Zelakiewicz, H. Noh, T. J. Ragucci, T. J. Gramila, L. N. Pfeiffer, K. W. West

Abstract: Electron drag between two two-dimensional electron gases in magnetic fields has been observed with a polarity opposite that for zero field. This negative drag requires that the electrons have a hole-like dispersion. Density dependence measurements in the integer quantum Hall regime show that drag is negative only when the upper Landau level of one layer is more than half filled while the other i… ▽ More Electron drag between two two-dimensional electron gases in magnetic fields has been observed with a polarity opposite that for zero field. This negative drag requires that the electrons have a hole-like dispersion. Density dependence measurements in the integer quantum Hall regime show that drag is negative only when the upper Landau level of one layer is more than half filled while the other is less than half filled, indicating that hole-like dispersion is present in a half of each Landau level. Negative drag is argued to be a consequence of disorder. △ Less

Submitted 2 September, 1998; originally announced September 1998.

Comments: 4 pages, 4 figures included, revtex, Accepted for publication in Physical Review Letters