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Browsing by Author "Chen, L"

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Antiferromagnetic topological insulating state in Tb0.02Bi1.08Sb0.9Te2S single crystals
(American Physical Society (APS), 2023-03-13) Guo, L; Zhao, WY; Li, Q; Xu, M; Chen, L; Bake, A; Vu, THY; He, YH; Fang, Y; Cortie, DL; Mo, SK; Edmonds, MT; Wang, Xl; Dong, S; Karel, J; Zheng, RK
Topological insulators are emerging materials with insulating bulk and symmetry protected nontrivial surface states. One of the most fascinating transport behaviors in a topological insulator is the quantized anomalous Hall insulator, which has been observed in magnetic-topological-insulator-based devices. In this work, we report a successful doping of rare earth element Tb into Bi1.08Sb0.9Te2S topological insulator single crystals, in which the Tb moments are antiferromagnetically ordered below ∼10 K. Benefiting from the in-bulk-gap Fermi level, transport behavior dominant by the topological surface states is observed below ∼150 K. At low temperatures, strong Shubnikov-de Haas oscillations are observed, which exhibit 2D-like behavior. The topological insulator with long range magnetic ordering in rare earth doped Bi1.08Sb0.9Te2S single crystal provides an ideal platform for quantum transport studies and potential applications. ©2023 American Physical Society.
Early Neolithic diets at Baijia, Wei River valley, China: Stable carbon and nitrogen isotope analysis of human and faunal remains
(Past Global Changes, 2013-02-13) Atahan, P; Dodson, JR; Li, XQ; Zhou, XY; Hu, S; Chen, L; Bertuch, F; Grice, K
The first farmers of the Wei River valley belonged to the Laoguantai period (ca. 8500-7000 yr BP) and lived in small settlements that were sparsely distributed in the landscape. Understanding of Laoguantai farming practices is limited as only a small number of archaeological sites are known. Here we present stable carbon and nitrogen isotope values for faunal and human bone collagen from Baijia, a Laoguantai site in the Wei River valley of Shaanxi Province, China. Five of the collagen samples have been AMS 14C dated and have a calibrated age range of ca. 7659-7339 yr BP. Stable isotope results show millet and aquatic foods, such as fish and shellfish, being included in the human diet. Bovid samples, which are tentatively identified as water buffalo, have stable carbon isotope values reflecting some millet consumption. The question of whether these bovids were grazing on millet growing wild, or had diets directly influenced by humans, remains to be answered. Stable isotope results for a single pig reveal a markedly different diet, one dominated by C3 plants which would have dominated natural vegetation of the region. Overall, stable isotope results conform to the current view of Laoguantai people being millet farmers with subsistence strategies that included hunted wild foods.
Enhanced thermoelectric performance and mechanical strength of n-type BiTeSe materials produced via a composite strategy
(Elsevier, 2022-01) Yang, G; Sang, L; Mitchell, DRG; Yun, FF; See, KW; Ahmed, AJ; Sayyar, S; Bake, A; Liu, P; Chen, L; Yue, ZJ; Cortie, DL; Wang, XL
Zone-melted Bi2Te2.7Se0.3 (ZM BTS) alloys are typical n-type commercial thermoelectric (TE) materials and are utilized for refrigeration and power generation near room temperature. They usually suffer from poor mechanical performance, as well as having a low figure of merit (ZT). In this work, we report an effective composite strategy to improve both the TE and mechanical performance of n-type BTS materials by incorporating carbon microfibers. The introduction of carbon microfibers in BTS effectively reduces the lattice thermal conductivity due to phonon scattering at multi-scale boundaries and due to the large interfacial thermal resistance arising from phonon mismatch between the constituent phases. Simultaneously, it also gives rise to an enhancement of the electrical conductivity, which originates from the increased carrier density without significant limitation on its weighted mobility. Consequently, a high peak ZT of 1.1 at 400 K and an average ZTave value of 0.95 are achieved in the temperature range 300 ~ 550 K, yielding a calculated efficiency of η = 9%. Moreover, the BTS/carbon microfiber composites show superior compressive strength compared to a commercial ZM BTS sample. This improved strength is highly desirable for real-world TE applications. Our results demonstrate a novel way to produce high-performance TE materials, in which interfaces with large thermal resistance are used to achieve low thermal conductivity without significantly degrading the electrical properties of the materials. © 2021 Elsevier B.V.
Giant linear magnetoresistance in half-metallic Sr2CrMoO6 thin films
(Springer Nature, 2021-12-01) Wang, ZC; Chen, L; Li, SS; Ying, JS; Tang, F; Gao, GY; Fang, Y; Zhao, WY; Cortie, DL; Wang, XL; Zheng, RK
Linear magnetoresistance (LMR) is a special case of a magnetic-field induced resistivity response, which has been reported in highly disordered semiconductor systems and in topological materials. In this work, we observe LMR effect in half-metallic perovskite Sr2CrMoO6 thin films, of which the maximum MR value exceeds +1600% at 2 K and 14 T. It is an unusual behavior in ferrimagnetic double perovskite material like Sr2CrMoO6, which are known for intrinsic tunneling-type negative magnetoresistance. In the thin films, the high carriers’ density (~1022 cm−3) and ultrahigh mobility (~104 cm2 V−1 s−1) provide a low-resistivity (~10 nΩ·cm) platform for spin-polarized current. Our DFT calculations and magnetic measurements further support the half-metal band structure. The LMR effect in Sr2CrMoO6 could possibly originate from transport behavior that is governed by the guiding center motion of cyclotron orbitals, where the magnetic domain structure possibly provides disordered potential. The ultrahigh mobility and LMR in this system could broaden the applications of perovskites, and introduce more research on metallic oxide ferri-/ferro-magnetic materials. © The Author(s) 2021 - Open Access CC BY licence.
Lead-free (Ag,K)NbO3materials for high-performance explosive energy conversion
(Australian Institute of Nuclear Science and Engineering (AINSE), 2020-11-11) Liu, Z; Lu, T; Xue, F; Withers, RL; Studer, AJ; Narayanan, N; Dong, XL; Yu, D; Chen, L; Wang, G; Liu, Y
Explosive energy conversion materials with extremely rapid response times have a diverse and growing range of applications in energy, medical, and mining areas. Research into the underlying mechanisms and the search for new candidate materials is so limited that Pb0.99(Zr0.95Ti0.05)0.98Nb0.02O3 is still the dominant material after half a century. In this work, we report the discovery of a new, lead-free ferroelectric material, (Ag0.935K0.065)NbO3 for explosive energy conversion applications. This material not only possesses a record-high energy storage density of 5.401 J/g, but also exhibits excellent temperature stability (up to a disruptive ferroelectric to ferroelectric phase transition at 150oC) by comparison with Pb0.99(Zr0.95Ti0.05)0.98Nb0.02O3 (which exhibits the ferroelectric to ferroelectric phase transition but at the much lower temperature of 41~70oC). (Ag0.935K0.065)NbO3 enables extremely high power, energy conversion within 1.8 microseconds, generating a pulse with e.g. a current ~ 22 A. Furthermore, pressure-dependent physical characterization, together with transmission electron microscopy, in-situ neutron diffraction analysis and theoretical modelling, reveals the mechanism underlying the observed explosive energy conversion behavior. It is found that the fast release of the stored energy can be attributed to a pressure-induced octahedral tilt change from a-a-c+ to AgNbO3-type a-a-c-/a-a-c+, in accordance with an irreversible pressure driven FE-AFE phase transition. This work provides not only an alternative (with significantly better performance) to the current commercially-employed lead-containing materials, but also provides guidance for the further development of new materials and devices for explosive energy conversion applications. Copyright © 2020 The Authors.
Machine learning prediction of activation energy in cubic Li-argyrodites with hierarchically encoding crystal structure-based (HECS) descriptors
(Elsevier, 2021-07-30) Zhao, Q; Avdeev, M; Chen, L; Shi, S
Rational design of solid-state electrolytes (SSEs) with high ionic conductivity and low activation energy (Ea) is vital for all solid-state batteries. Machine learning (ML) techniques have recently been successful in predicting Li+ conduction property in SSEs with various descriptors and accelerating the development of SSEs. In this work, we extend the previous efforts and introduce a framework of ML prediction for Ea in SSEs with hierarchically encoding crystal structure-based (HECS) descriptors. Taking cubic Li-argyrodites as an example, an Ea prediction model is developed to the coefficient of determination (R2) and root-mean-square error (RMSE) values of 0.887 and 0.02 eV for training dataset, and 0.820 and 0.02 eV for test dataset, respectively by partial least squares (PLS) analysis, proving the prediction power of HECS-descriptors. The variable importance in projection (VIP) scores demonstrate the combined effects of the global and local Li+ conduction environments, especially the anion size and the resultant structural changes associated with anion site disorder. The developed Ea prediction model directs us to optimize and design new Li-argyrodites with lower Ea, such as Li6–xPS5–xCl1+x (<0.322 eV), Li6+xPS5+xBr1–x (<0.273 eV), Li6+xPS5+xBr0.25I0.75–x (<0.352 eV), Li6+(5–n)yP1–yNyS5I (<0.420 eV), Li6+(5–n)yAs1–yNyS5I (<0.371 eV), Li6+(5–n)yAs1–yNySe5I (<0.450 eV), by broadening bottleneck size, invoking site disorder and activating concerted Li+ conduction. This analysis shows great potential in promoting rational design of advanced SSEs and the same approach can be applied to other types of materials.© 2021 Published by Elsevier B.V. on behalf of Science China Press.
Magnetotransport and Berry phase tuning in Gd-doped Bi2Se3 topological insulator single crystals
(American Physical Society, 2022-05-01) Chen, L; Li, SS; Zhao, W; Bake, A; Cortie, DL; Wang, XL; Karel, J; Li, H; Zheng, RK
The Berry phase is an important concept in solids, correlated to the band topology, axion electrodynamics, and potential applications of topological materials. Here, we investigate the magnetotransport and Berry phase of rare earth element Gd-doped Bi2Se3 (Gd:Bi2Se3) topological insulators (TIs) at low temperatures and high magnetic fields. Gd:Bi2Se3 single crystals show Shubnikov-de Haas (SdH) oscillations with nontrivial Berry phase, while Bi2Se3 single crystals show zero Berry phase in SdH oscillations. A fitting of the temperature-dependent magnetization curves using the Curie-Weiss law reveals that the Gd dopants in the crystals show paramagnetism in the 3-300 K region, indicating that the origin of the Berry phase is not long-range magnetic ordering. Moreover, Gd doping has limited influence on the quantum oscillation parameters (e.g., frequency of oscillation, area of Fermi surface, effective electron mass, and Fermi wave vectors) but has a significant impact on the Hall mobility, carrier density, and band topology. Our results demonstrate that Gd doping can tune the Berry phase of TIs effectively, which may pave the way for the future realization of many predicted exotic transport phenomena of topological origin. ©2022 American Physical Society
Millet agriculture in north-central China: evidence from human remains
(Australasian Quaternary Association, 2010-07-16) Atahan, P; Dodson, JR; Li, XQ; Zhou, XY; Chen, L; Grice, K
Not available - Powerpoint slides only.
A novel graphene oxide wrapped Na2Fe2(SO4)3/C cathode composite for long life and high energy density sodium‐ion batteries
(Wiley, 2018-08-06) Chen, MZ; Cortie, DL; Hu, Z; Jin, H; Wang, S; Gu, QF; Hua, WB; Wang, E; Lai, WH; Chen, L; Chou, SL; Wang, XL; Dou, SX
The cathode materials in the Na‐ion battery system are always the key issue obstructing wider application because of their relatively low specific capacity and low energy density. A graphene oxide (GO) wrapped composite, Na2Fe2(SO4)3@C@GO, is fabricated via a simple freeze‐drying method. The as‐prepared material can deliver a 3.8 V platform with discharge capacity of 107.9 mAh g−1 at 0.1 C (1 C = 120 mA g−1) as well as offering capacity retention above 90% at a discharge rate of 0.2 C after 300 cycles. The well‐constructed carbon network provides fast electron transfer rates, and thus, higher power density also can be achieved (75.1 mAh g−1 at 10 C). The interface contribution of GO and Na2Fe2(SO4)3 is recognized and studied via density function theory calculation. The Na storage mechanism is also investigated through in situ synchrotron X‐ray diffraction, and pseudocapacitance contributions are also demonstrated. The diffusion coefficient of Na+ ions is around 10−12–10−10.8 cm2 s−1 during cycling. The higher working voltage of this composite is mainly ascribed to the larger electronegativity of the element S. The research indicates that this well‐constructed composite would be a competitive candidate as a cathode material for Na‐ion batteries. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA.
An O3‐type oxide with low sodium content as the phase‐transition‐free anode for sodium‐ion batteries
(Wiley, 2018-06-11) Zhao, C; Avdeev, M; Chen, L; Hu, YS
Layered transition metal oxides NaxMO2 (M=transition metal) with P2 or O3 structure have attracted attention in sodium‐ion batteries (NIBs). A universal law is found to distinguish structural competition between P2 and O3 types based on the ratio of interlayer distances of the alkali metal layer d(O‐Na‐O) and transition‐metal layer d(O‐M‐O). The ratio of about 1.62 can be used as an indicator. O3‐type Na0.66Mg0.34Ti0.66O2 oxide is prepared as a stable anode for NIBs, in which the low Na‐content (ca. 0.66) usually undergoes a P2‐type structure with respect to NaxMO2. This material delivers an available capacity of about 98 mAh g−1 within a voltage range of 0.4–2.0 V and exhibits a better cycling stability (ca. 94.2 % of capacity retention after 128 cycles). In situ X‐ray diffraction reveals a single‐phase reaction in the discharge–charge process, which is different from the common phase transitions reported in O3‐type electrodes, ensuring long‐term cycling stability. © 2018 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim
On the microstructure and high-temperature stability of nano-grained Zircaloy-4
(Elsevier, 2022-03-15) Chen, L; Wang, ZY; Zhu, HL; Burr, PA; Qu, JT; Huang, Y; Balogh, L; Preuss, M; Muránsky, O
A nano-grained microstructure of an α-Zr alloy (Zircaloy-4) was produced by high-pressure torsion, which shows evidence of a metastable ω-Zr phase, rather than β-Zr, determined by combining synchrotron X-ray diffraction and detailed electron microscopy observations. The ω-Zr phase is retained at ambient conditions and shows a new orientation relationship of [1011]α // [1100]ω and (1011)α // (1120)ω with the α-Zr matrix but is thermally unstable, fully reverting back to α-Zr phase upon heating above 350 °C. © 2021 Acta Materialia Inc. Published by Elsevier Ltd.
P2-Na0.6[Cr0.6Ti0.4]O2 cation-disordered electrode for high-rate symmetric rechargeable sodium-ion batteries
(Springer Nature, 2015-04-24) Wang, Y; Xiao, R; Hu, YS; Avdeev, M; Chen, L
Most P2-type layered oxides exhibit Na+/vacancy-ordered superstructures because of strong Na+–Na+ interaction in the alkali metal layer and charge ordering in the transition metal layer. These superstructures evidenced by voltage plateaus in the electrochemical curves limit the Na+ ion transport kinetics and cycle performance in rechargeable batteries. Here we show that such Na+/vacancy ordering can be avoided by choosing the transition metal ions with similar ionic radii and different redox potentials, for example, Cr3+ and Ti4+. The designed P2-Na0.6[Cr0.6Ti0.4]O2 is completely Na+/vacancy-disordered at any sodium content and displays excellent rate capability and long cycle life. A symmetric sodium-ion battery using the same P2-Na0.6[Cr0.6Ti0.4]O2 electrode delivers 75% of the initial capacity at 12C rate. Our contribution demonstrates that the approach of preventing Na+/vacancy ordering by breaking charge ordering in the transition metal layer opens a simple way to design disordered electrode materials with high power density and long cycle life. Copyright © 2015, The Author(s)
Phase transition enhanced thermoelectric performance in Cu2Se
(Australian Institute of Physics, 2014-02-06) Liu, H; Shi, X; Zhang, W; Chen, L; Danilkin, SA
Worldwide efforts to searching for good thermoelectric materials are frequently focusing on normal phases in crystalline semiconductors. The material’s thermoelectric performance is described the parameter of figure of merit, zT, which is around unity around room temperature and above 1.5 at high temperatures. In the Cu2Se with anti-fluorite structure above 400K, Se atoms form a rigid face-centred cubic lattice, while the copper ions are highly disordered or moving around the tetrahedral voids with liquid-like mobility, resulting in an extraordinarily low lattice thermal conductivity, which enables zT up to 1.5 at 1,000K. Here, we report significantly enhanced thermoelectric performance during the phase transitions in Cu2Se and iodine doped Cu2Se. It is showed that the critical electron and phonon scattering greatly improve the thermopower and strongly reduce the thermal coductivity, leading to the improvement in the figure of merit more than 3-7 times compared to the normal phases, and achieving zT value of 2.3 at 400K. This mechanism pave a new way to increase the figure of merit of thermoelectric materials, and expend the utility of thermoelectrics in electronic cooling industry.z
Spin reorientation transition and negative magnetoresistance in ferromagnetic NdCrSb3 single crystals
(MDPI, 2023-02-20) Chen, L; Zhao, WY; Wang, ZC; Tang, F; Fang, Y; Zeng, Z; Xia, ZC; Cheng, ZX; Cortie, DL; Rule, KC; Wang, XL; Zheng, RK
High-quality NdCrSb3 single crystals are grown using a Sn-flux method, for electronic transport and magnetic structure study. Ferromagnetic ordering of the Nd3+ and Cr3+ magnetic sublattices are observed at different temperatures and along different crystallographic axes. Due to the Dzyaloshinskii–Moriya interaction between the two magnetic sublattices, the Cr moments rotate from the b axis to the a axis upon cooling, resulting in a spin reorientation (SR) transition. The SR transition is reflected by the temperature-dependent magnetization curves, e.g., the Cr moments rotate from the b axis to the a axis with cooling from 20 to 9 K, leading to a decrease in the b-axis magnetization f and an increase in the a-axis magnetization. Our elastic neutron scattering along the a axis shows decreasing intensity of magnetic (300) peak upon cooling from 20 K, supporting the SR transition. Although the magnetization of two magnetic sublattices favours different crystallographic axes and shows significant anisotropy in magnetic and transport behaviours, their moments are all aligned to the field direction at sufficiently large fields (30 T). Moreover, the magnetic structure within the SR transition region is relatively fragile, which results in negative magnetoresistance by applying magnetic fields along either a or b axis. The metallic NdCrSb3 single crystal with two ferromagnetic sublattices is an ideal system to study the magnetic interactions, as well as their influences on the electronic transport properties. © 2023 The Authors, Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license.
Temporal trends in millet consumption in northern China
(Elsevier, 2014-10-01) Atahan, P; Dodson, JR; Li, XQ; Zhou, XY; Chen, L; Barry, LA; Bertuch, F
Temporal trends in prehistoric millet consumption are investigated in two regions of northern China, in the Wei River valley and a northern zone that encompasses north-eastern Shaanxi, western Shanxi and south-central Inner Mongolia. By directly radiocarbon dating each sample investigated, inferences about the timing of dietary shifts inferred from stable carbon and nitrogen isotope compositions can be made with a high degree of precision. Evidence presented here indicates that humans living around 4000 years ago in both the Wei River valley and the northern zone were heavily dependent on millet for their subsistence. By ca. 2500 cal. yr BP, a major diversification of diet had occurred in the Wei River valley, with some consuming much larger proportions of C3 foods than previously. These C3 foods may have included the western-derived cereals – wheat, barley and oats – and also rice. © 2014, Elsevier Ltd.
Topological insulator VxBi1.08-x Sn0.02Sb0.9Te2S as a promising n-type thermoelectric material
(Elsevier, 2022-10) Chen, L; Zhao, WY; Li, M; Yang, G; Guo, L; Bake, A; Liu, P; Cortie, DL; Zheng, RK; Cheng, ZX; Wang, XL
As one of the most important n-type thermoelectric (TE) materials, Bi2Te3 has been studied for decades, with efforts to enhance the thermoelectric performance based on element doping, band engineering, etc. In this study, we report a novel bulk-insulating topological material system as a replacement for n-type Bi2Te3 materials: V doped Bi1.08Sn0.02Sb0.9Te2S (V:BSSTS). The V:BSSTS is a bulk insulator with robust metallic topological surface states. Furthermore, the bulk band gap can be tuned by the doping level of V, which is verified by magnetotransport measurements. Large linear magnetoresistance is observed in all samples. Excellent thermoelectric performance is obtained in the V:BSSTS samples, e.g., the highest figure of merit ZT of ~ 0.8 is achieved in the 2% V doped sample (denoted as V0.02) at 530 K. The high thermoelectric performance of V:BSSTS can be attributed to two synergistic effects: (1) the low conductive secondary phases Sb2S3, and V2S3 are believed to be important scattering centers for phonons, leading to lower lattice thermal conductivity; and (2) the electrical conductivity is increased due to the high-mobility topological surface states at the boundaries. In addition, by replacing one third of costly tellurium with abundant, low-cost, and less-toxic sulfur element, the newly produced BSSTS material is inexpensive but still has comparable TE performance to the traditional Bi2Te3-based materials, which offers a cheaper plan for the electronics and thermoelectric industries. Our results demonstrate that topological materials with unique band structures can provide a new platform in the search for new high performance TE materials. © 2022 Elsevier B.V.

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