arXiv subjects

Qun Yang

Publications and source records attributed to Qun Yang.

30 records · Page 2Linked to original sources

arXiv2022

Bonding and Electronic Nature of the Anionic Framework in LaPd$_3$S$_4$

Double Dirac materials are a topological phase of matter in which a non-symmorphic symmetry enforces greater electronic degeneracy than normally expected up to eightfold. The cubic palladium bronzes NaPd$_3$O$_4$ and LaPd$_3$S$_4$ are built of Pd$_3$X$_4$ (X = O, S) anionic frameworks that are ionically bonded to A cations (A = Na, La). These materials were recently identified computationally as harboring eightfold fermions. Here we report the preparation of single crystals and electronic properties of LaPd$_3$S$_4$. Measurements down to T = 0.45 K and in magnetic fields up to $μ$0H = 65 T are consistent with normal Fermi liquid physics of a Dirac metal in the presence of dilute magnetic impurities. This interpretation is further confirmed by analysis of specific heat, magnetization measurements and comparison to density functional theory (DFT) calculations. Through a bonding analysis of the DFT electronic structure of NaPd$_3$O$_4$ and LaPd$_3$S$_4$, we identify the origin of the stability of the anionic Pd$_3$X$_4$ framework at higher electron counts for X = S than X = O, and propose chemical tuning strategies to enable shifting the 8-fold fermion points to the Fermi level.

cond-mat.mtrl-sci↗

arXiv2021

Three-Dimensional Real Space Invariants, Obstructed Atomic Insulators and A New Principle for Active Catalytic Sites

Topologically trivial insulators come in two kinds: atomic, where the Wannier charge centers (WCCs) are localized on the atoms, and obstructed atomic, where the WCCs are located away from the atoms. The latter, which can exhibit interesting surface states and possibly have much larger band gaps than the topological insulators, have so far not been classified in three-dimensional (3D) crystalline materials. In this paper, we developed the 3D real space invariants (RSIs) for the 1651 Shubnikov space groups with the spin-orbit coupling and provide the full classification of 3D obstructed atomic insulators (OAIs) by the RSIs. We then apply the theory to the entire database of materials on the Topological Quantum Chemistry website and Topological Magnetic Materials website, obtaining all the OAIs so far existing in nature. We find that, out of the 34013 paramagnetic and 296 magnetic topologically trivial insulators, there are 3383 paramagnetic and 30 magnetic OAIs. All of them present a filling anomaly under certain open-boundary conditions and exhibit obstructed surface states (OSSs). We further refine the atomic insulator concept to obtain the orbital-selected OAIs (OOAIs), where the WCC of the system is located at a Wyckoff position occupied by an atom but forms a symmetric representation that does not belong to the outer-shell electrons of the given atom. In such a way, we obtain a further 121 OOAIs. Furthermore, we analyze the catalytic properties of one of the OAIs in a "proof of principle" experiment. By using the single crystal of 2H-MoS2 as a hydrogen evolution catalyst, we directly prove that the catalytic activities arise from the surfaces with OSSs. Additional potential applications of the 3D RSIs and OAIs in, for example, electrochemistry, asymmetric catalysis, superconductivity, and Josephson diode will be discussed.

cond-mat.mtrl-sci↗

arXiv2021

Obstructed surface states as the origin of catalytic activity in inorganic heterogeneous catalysts

The discovery of new catalysts that are efficient, sustainable, and low-cost is a major research endeavor for many industrial chemical processes. This requires an understanding and determination of the catalytic origins for the given catalysts, which still remains a challenge. Here we describe a novel method to identify new catalysts based on searching for crystalline symmetry-protected obstructed atomic insulators (OAIs) that have metallic surface states on otherwise semiconducting or insulating compounds. The Wannier charge centers in OAIs are pinned by symmetries at some empty Wyckoff positions so that surfaces that accommodate these sites are guaranteed to have metallic obstructed surface states (OSSs). Beyond the well-studied 2H-MoS2, we further verified our theory on the catalysts, 2H-MoTe2, and 1T'-MoTe2, whose catalytic active sites are consistent with our calculations of obstructed Wannier charge centers (OWCCs) and OSSs. In addition, we have predicted the location of catalytic active sites and confirmed these predictions by exploring the hydrogen evolution reaction on NiPS3 bulk single crystals, which we find to be one of the most promising new catalysts with high activity and, moreover, of low cost. Most importantly, we successfully identified several high-efficient catalysts just by considering the number of OWCCs and the crystal symmetry of the OAIs. Using the real space invariant theory and high-throughput computational methods applied to a database of 34013 topologically trivial insulators, we have identified 1788 unique OAIs (3383 ICSDs), of which 465 are potential high-quality catalysts for heterogeneous reactions. The Miller indices of the active surfaces are also obtained. Our new methodology will facilitate and accelerate the discovery of new catalysts for a wide range of heterogeneous redox reactions, where sustainability, toxicity, and cost must be considered.

cond-mat.mtrl-sci↗

arXiv2021

Data-Driven Electron Microscopy: Electron Diffraction Imaging of Materials Structural Properties

Transmission electron diffraction is a powerful and versatile structural probe for the characterization of a broad range of materials, from nanocrystalline thin films to single crystals. With recent developments in fast electron detectors and efficient computer algorithms, it now becomes possible to collect unprecedently large datasets of diffraction patterns (DPs) and process DPs to extract crystallographic information to form images or tomograms based on crystal structural properties, giving rise to data-driven electron microscopy. Critical to this kind of imaging is the type of crystallographic information being collected, which can be achieved with a judicious choice of electron diffraction techniques, and the efficiency and accuracy of DP processing, which requires the development of new algorithms. Here, we review recent progress made in data collection, new algorithms, and automated electron DP analysis. These progresses will be highlighted using application examples in materials research. Future opportunities based on smart sampling and machine learning are also discussed.

cond-mat.mtrl-sci↗

arXiv2021

Cepstral Scanning Transmission Electron Microscopy Imaging of Severe Lattice Distortions

The development of four-dimensional (4D) scanning transmission electron microscopy (STEM) using fast detectors has opened-up new avenues for addressing some of long-standing challenges in electron imaging. One of these challenges is how to image severely distorted crystal lattices, such as at a dislocation core. Here we introduce a new 4D-STEM technique, called Cepstral STEM, for imaging disordered crystals using electron diffuse scattering. Local fluctuations of diffuse scattering are captured by scanning electron nanodiffraction (SEND) using a coherent probe. The harmonic signals in electron diffuse scattering are detected through Cepstral analysis and used for imaging. By integrating Cepstral analysis with 4D-STEM, we demonstrate that information about the distortive part of electron scattering potential can be separated and imaged at nm spatial resolution. We apply our technique to the analysis of a dislocation core in SiGe and lattice distortions in high entropy alloy.

cond-mat.mtrl-sci↗

arXiv2020

In situ modification of delafossite type PdCoO2 bulk single crystal for reversible hydrogen sorption and fast hydrogen evolution

The observation of extraordinarily high conductivity in delafossite-type PdCoO2 is of great current interest, and there is some evidence that electrons behave like a fluid when flowing in bulk crystals of PdCoO2. Thus, this material is an ideal platform for the study of the electron transfer processes in heterogeneous reactions. Here, we report the use of bulk single crystal PdCoO2 as a promising electrocatalyst for hydrogen evolution reactions (HERs). An overpotential of only 31 mV results in a current density of 10 mA cm^(-2), accompanied by high long-term stability. We have precisely determined that the crystal surface structure is modified after electrochemical activation with the formation of strained Pd nanoclusters in the surface layer. These nanoclusters exhibit reversible hydrogen sorption and desorption, creating more active sites for hydrogen access. The bulk PdCoO2 single crystal with ultra-high conductivity, which acts as a natural substrate for the Pd nanoclusters, provides a high-speed channel for electron transfer

cond-mat.mtrl-sci↗

arXiv2020

Observation of giant spin-split Fermi-arc with maximal Chern number in the chiral topological semimetal PtGa

Non-symmorphic chiral topological crystals host exotic multifold fermions, and their associated Fermi arcs helically wrap around and expand throughout the Brillouin zone between the high-symmetry center and surface-corner momenta. However, Fermi-arc splitting and realization of the theoretically proposed maximal Chern number rely heavily on the spin-orbit coupling (SOC) strength. In the present work, we investigate the topological states of a new chiral crystal, PtGa, which has the strongest SOC among all chiral crystals reported to date. With a comprehensive investigation using high-resolution angle-resolved photoemission spectroscopy, quantum-oscillation measurements, and state-of-the-art ab initio calculations, we report a giant SOC-induced splitting of both Fermi arcs and bulk states. Consequently, this study experimentally confirms the realization of a maximal Chern number equal to |4| for the first time in multifold fermionic systems, thereby providing a platform to observe large-quantized photogalvanic currents in optical experiments.

cond-mat.mtrl-sci↗

arXiv2019

Fast Safety Assessment and Correction Framework for Maintenance Work Zones

A framework is proposed to assess the safety of maintenance work zones in a timely manner, show whether there are safety hazards, whether adjustments need to be made and how to adjust it. By means of advanced data acquisition technologies such as multi video detection and portable device based naturalistic driving, the microscopic vehicle behaviour data can be collected. Based on this data, a method for expressing and displaying the distribution of unsafe vehicle behaviour is used to show whether safety hazards exist. Using Vissim, the impacts of the length and speed limit of the warning area, the length and type of the upstream transition area and the length of the work area of the maintenance work zone on the distribution of unsafe vehicle behaviour are simulated to establish the safety correction matrix, which can tell maintenance departments the direction of adjustment when safety hazards exist in maintenance work zones.

cs.OH↗

arXiv2019

Predicting Complex Relaxation Processes in Metallic Glass

Relaxation processes significantly influence the properties of glass materials. However, understanding their specific origins is difficult, even more challenging is to forecast them theoretically. In this study, using microseconds molecular dynamics simulations together with an accurate many-body interaction potential, we predict that an Al$_{\text{90}}$Sm$_{\text{10}}$ metallic glass would have complex relaxation behaviors: In addition to the main ($α$) relaxation, the glass (i) shows a pronounced secondary ($β$) relaxation at cryogenic temperatures and (ii) exhibits an anomalous relaxation process ($α_2$) accompanying $α$ relaxation. Both of the predictions are verified by experiments. Computational simulations reveal the microscopic origins of relaxation processes: while the pronounced $β$ relaxation is attributed to the abundance of string-like cooperative atomic rearrangements, the anomalous $α_2$ process is found to correlate with the decoupling of the faster motions of Al with slower Sm atoms. The combination of simulations and experiments represents a first glimpse of what may become a predictive routine and integral step for glass physics.

cond-mat.mtrl-sci↗

arXiv2019

Road Maintenance Operation Start Time Optimization Based on Real-time Traffic Map Data

Optimizing the maintenance operation start time can greatly reduce the delays caused by the maintenance operations. A real-time traffic status data acquisition method based on real-time traffic map was first proposed, and then a method that can convert real-time traffic status into real-time traffic volume was put forward. Based on this real-time traffic volume data and the classic delay calculation method based on queuing theory, the delays caused by maintenance operations at different start time can be calculated and compared, and therefore the optimal maintenance operation start time can be obtained. The feasibility of the real-time traffic status data to real-time traffic volume data conversion method and the feasibility of optimizing the maintenance operation start time based on real-time traffic map data are verified by actual cases.

stat.AP↗

arXiv2019

A Method for Expressing and Displaying the Vehicle Behavior Distribution in Maintenance Work Zones

Maintenance work zones on the road network have impacts on the normal travelling of vehicles, which increase the risk of traffic accidents. The traffic characteristic analysis in maintenance work zones is a basis for maintenance work zone related research such as layout design, traffic control and safety assessment. Due to the difficulty in vehicle microscopic behaviour data acquisition, traditional traffic characteristic analysis mainly focuses on macroscopic characteristics. With the development of data acquisition technology, it becomes much easier to obtain a large amount of microscopic behaviour data nowadays, which lays a good foundation for analysing the traffic characteristics from a new point of view. This paper puts forward a method for expressing and displaying the vehicle behaviour distribution in maintenance work zones. Using portable vehicle microscopic behaviour data acquisition devices, lots of data can be obtained. Based on this data, an endpoint detection technology is used to automatically extract the segments in behaviour data with violent fluctuations, which are segments where vehicles take behaviours such as acceleration or turning. Using the support vector machine classification method, the specific types of behaviours of the segments extracted can be identified, and together with a data combination method, a total of ten types of behaviours can be identified. Then the kernel density analysis is used to cluster different types of behaviours of all passing vehicles to show the distribution on maps. By this method, how vehicles travel through maintenance work zones, and how different vehicle behaviours distribute in maintenance work zones can be displayed intuitively on maps, which is a novel traffic characteristic and can shed light to maintenance work zone related researches such as safety assessment and design method.

cs.OH↗

arXiv2019

The Paracrystalline Nature of Lattice Distortion in a High Entropy Alloy

Severe lattice distortion is suggested for high entropy alloys (HEAs), however, evidence for such effect so far is lacking, and the nature of distortion is yet to be understood. Here, we reveal the distortion in an fcc HEA, Al0.1CrFeCoNi, by direct imaging using electron nanodiffraction. Information about crystal symmetry, lattice strain and atomic distortion are data-mined and mapped from many (~10^4) diffraction patterns. Application to the HEA reveals two embodiments of distortion, nm-sized mosaic blocks of paracrystals and strained nano-clusters. Their interaction gives rise to fractal strain field across nanoscopic to mesoscopic scales. As lattice distortion impedes dislocation motion and contributes to strengthening, results here thus provide critical insights about the complex nature of distortion in a HEA.

cond-mat.mtrl-sci↗