Showing 1–50 of 68 results for author: Pi, L

Absence of high-field spin supersolid phase in Rb$_2$Co(SeO$_3$)$_2$ with a triangular lattice

Authors: K. Shi, Y. Q. Han, B. C. Yu, L. S. Ling, W. Tong, C. Y. Xi, T. Shang, Zhaosheng Wang, Li Pi, Long Ma

Abstract: Magnetization, torque magnetometry, specific heat and nuclear magnetic resonance (NMR) are used to study the high field intermediate phase between the 1/3-magnetization plateau and polarized state in the quantum Ising antiferromagnet Rb$_2$Co(SeO$_3$)$_2$ with a triangular lattice. The magnetic phase diagram with the magnetic field up to 30 T is mapped by the comprehensive experimental data. The "… ▽ More Magnetization, torque magnetometry, specific heat and nuclear magnetic resonance (NMR) are used to study the high field intermediate phase between the 1/3-magnetization plateau and polarized state in the quantum Ising antiferromagnet Rb$_2$Co(SeO$_3$)$_2$ with a triangular lattice. The magnetic phase diagram with the magnetic field up to 30 T is mapped by the comprehensive experimental data. The "up-up-down" (UUD) spin configuration of the 1/3-magnetization plateau state is identified by NMR spectral analysis. At higher magnetic fields, this UUD structure persist to the intermediate phase, which is finally destroyed in the polarized state. This observation supplies unambiguous spectroscopic evidence for the absence of proposed high field spin supersolid phase. The high-field phase diagram of this quantum magnet proximate to the Ising-anisotropy limit contradicts with that proposed by theoretical studies. △ Less

Submitted 7 September, 2025; originally announced September 2025.

Comments: 6 pages, 4 figures

Multi-origin driven giant planar Hall effect in topological antiferromagnet EuAl2Si2 with tunable spin texture

Authors: Xiangqi Liu, Ziyi Zhu, Yixuan Luo, Zhengyang Li, Bo Bai, Jingcheng Huang, Xia Wang, Chuanying Xi, Li Pi, Guanxiang Du, Leiming Chen, Wenbo Wang, Wei Xia, Yanfeng Guo

Abstract: In topological materials, the planar Hall effect (PHE) is often regarded as a hallmark of profound quantum phenomena-most notably the Adler-Bell-Jackiw chiral anomaly and Berry curvature-rendering it an indispensable tool for deciphering the topological essence of emergent phases. In this study, we delve into the PHE and anisotropic magnetoresistance in the recently discovered layered topological… ▽ More In topological materials, the planar Hall effect (PHE) is often regarded as a hallmark of profound quantum phenomena-most notably the Adler-Bell-Jackiw chiral anomaly and Berry curvature-rendering it an indispensable tool for deciphering the topological essence of emergent phases. In this study, we delve into the PHE and anisotropic magnetoresistance in the recently discovered layered topological antiferromagnet EuAl2Si2. Our analysis of the robust PHE signal (~3.8 μΩ cm at 2 K and 8 T) unveils a distinct interplay of mechanisms. While Berry curvature plays a minor role, the dominant contributions stem from classical orbital MR in the field-induced ferromagnetic state and field-suppressed spin fluctuations in the paramagnetic regime. These insights not only position EuAl2Si2-with its highly tunable spin texture-as an exemplary system for probing the intricate coupling between spin configurations and band topology in magnetotransport but also pave the way for designing novel materials with tailored PHE responses, highlighting significant application prospects in quantum sensing, spintronic devices, and topologically protected electronic systems. △ Less

Submitted 27 August, 2025; originally announced August 2025.

Comments: 17 pages and 5 figures

Robust field re-entrant superconductivity in ferromagnetic infinite-layer rare-earth nickelates

Authors: Mingwei Yang, Jiayin Tang, Xianfeng Wu, Heng Wang, Wenjing Xu, Haoliang Huang, Zhicheng Pei, Wenjie Meng, Guangli Kuang, Jinfeng Xu, Sixia Hu, Chuanying Xi, Li Pi, Qingyou Lu, Ziqiang Wang, Qikun Xue, Zhuoyu Chen, Danfeng Li

Abstract: Superconductivity and ferromagnetism are naturally competing, while their interplay can give rise to exotic quantum phases, such as triplet pairing, exemplified by heavy-fermion compounds like UTe$_2$, where magnetic fluctuations stabilise multiple superconducting states. However, such phenomena have remained elusive in high-temperature superconductors. Here we report the discovery of robust field… ▽ More Superconductivity and ferromagnetism are naturally competing, while their interplay can give rise to exotic quantum phases, such as triplet pairing, exemplified by heavy-fermion compounds like UTe$_2$, where magnetic fluctuations stabilise multiple superconducting states. However, such phenomena have remained elusive in high-temperature superconductors. Here we report the discovery of robust field-induced re-entrant superconductivity in heavily Eu-doped infinite-layer nickelate Sm$_{0.95-x}$Ca$_{0.05}$Eu$_x$NiO$_2$. In the heavily over-doped regime, we observe a remarkable superconducting state that emerges under high magnetic fields ($>$ 6 Tesla) after the initial suppression of zero-field superconductivity. Both zero-resistance transport and Meissner diamagnetic effect confirm the superconducting nature of this high-field phase, which persists up to at least 45 Tesla. This re-entrant behaviour is featured by the coexistence of ferromagnetism and superconductivity on distinct sublattices -- magnetic Eu$^{2+}$ ions and the Ni-O planes, respectively. Such an exotic state may arise from the compensation between external and internal exchange fields (Jaccarino-Peter effect) combined with magnetic fluctuation-enhanced pairing near quantum criticality. Our findings establish infinite-layer nickelates as a unique platform for high-temperature ferromagnetic superconductivity, opening new avenues for discovering and manipulating unconventional quantum phases in strongly correlated materials. △ Less

Submitted 22 August, 2025; v1 submitted 20 August, 2025; originally announced August 2025.

Comments: Manuscript under review. 26 pages, 11 figures

Enhanced Superconductivity and Mixed-dimensional Behaviour in Infinite-layer Samarium Nickelate Thin Films

Authors: Mingwei Yang, Heng Wang, Jiayin Tang, Junping Luo, Xianfeng Wu, Wenjing Xu, Aile Wang, Yuetong Wu, Ruilin Mao, Ze Wang, Zhicheng Pei, Guangdi Zhou, Zhengang Dong, Bohan Feng, Lingchi Shi, Wenjie Meng, Chuanying Xi, Li Pi, Qingyou Lu, Jun Okamoto, Hsiao-Yu Huang, Di-Jing Huang, Haoliang Huang, Qisi Wang, Peng Gao , et al. (2 additional authors not shown)

Abstract: Rare-earth infinite-layer nickelates represent an emerging class of unconventional superconductors, with materials synthesis largely limited to early lanthanide compounds. Here, we report the synthesis and characterization of phase-pure superconducting samarium-based infinite-layer nickelate thin films, including the first demonstration of Sm$_{1-x}$Sr$_x$NiO$_2$, along with co-doped variants inco… ▽ More Rare-earth infinite-layer nickelates represent an emerging class of unconventional superconductors, with materials synthesis largely limited to early lanthanide compounds. Here, we report the synthesis and characterization of phase-pure superconducting samarium-based infinite-layer nickelate thin films, including the first demonstration of Sm$_{1-x}$Sr$_x$NiO$_2$, along with co-doped variants incorporating europium and calcium. These films, grown on LSAT (001) substrates, exhibit coherent lattice structures up to $\sim$ 9 nm thickness with minimal stacking faults. The co-doped compounds achieve a record-small $c$-axis parameter of 3.26 Å and display remarkable superconducting transition temperatures up to 32.5 K. These results establish a clear correlation between decreasing $c$-axis parameter and increasing critical temperature across different rare-earth systems. In addition, angle-dependent magnetoresistance investigations reveal the existence of a hybrid mixture of 2D and 3D superconductivity in this novel system with enhanced coupling between the rare-earth 5d and Ni 3d orbitals, confirmed by resonant inelastic X-ray scattering experiments. As the concentration of Eu increases, the system exhibits a clear tendency towards 3D superconductivity. Furthermore, we observe distinctive negative magnetoresistance in the europium-containing samples. These findings advocate clear materials design principles for higher transition temperatures and exotic physics in infinite-layer nickelate superconductors through structural engineering of the rare-earth site. △ Less

Submitted 20 August, 2025; v1 submitted 24 March, 2025; originally announced March 2025.

Comments: Main text: 20 pages, 4 figures. Supplementary: 11 pages, 9 figures

Topologically nontrivial $1/3$-magnetization plateau state in a spin-1/2 trimer chain

Authors: Y. Y. Han, B. C. Yu, Z. Du, L. S. Ling, L. Zhang, W. Tong, C. Y. Xi, J. L. Zhang, T. Shang, Li Pi, Long Ma

Abstract: Topologically nontrivial Haldane phase is theoretically proposed to be realized in the 1/3-magnetization ($M$) plateau of spin-1/2 trimer systems. However, the spin excitation gap, typical characteristic of Haldane phase, is not yet experimentally verified. Here, we report the nuclear magnetic resonance investigations into the low-energy spin dynamics in the $S=1/2$ spin-trimer antiferromagnetic c… ▽ More Topologically nontrivial Haldane phase is theoretically proposed to be realized in the 1/3-magnetization ($M$) plateau of spin-1/2 trimer systems. However, the spin excitation gap, typical characteristic of Haldane phase, is not yet experimentally verified. Here, we report the nuclear magnetic resonance investigations into the low-energy spin dynamics in the $S=1/2$ spin-trimer antiferromagnetic chain compound Na$_2$Cu$_3$Ge$_{4-x}$Si$_{x}$O$_{12}$ ($x=0, 0.1\sim1.5$). In the parent compound ($x=0$), the spin-lattice relaxation rate (1/$T_1$) shows significantly different temperature dependence when the external magnetic field is increased above the critical field of $μ_0$$H_{c}$ = 29 T. The spin excitation gap is evidenced from the thermally activated behavior of $1/T_1(T)$ in the 1/3-$M$ plateau state. By substituting Ge$^{4+}$ with Si$^{4+}$, the critical field for the 1/3-$M$ plateau significantly decreases, e.g. $μ_0H_{c}=17$ T in $x=1.0$ samples, which results from the suppressed inter-trimer coupling $J_2$. The gapped spin excitation is confirmed again above 17 T, whose size shows temperature-dependent behavior for $μ_0H\geq25.72$ T. These observations provide further insights into the Haldane physics. △ Less

Submitted 3 July, 2024; originally announced July 2024.

Comments: 6 pages, 4 figures

Observation of quantum oscillations near the Mott-Ioffe-Regel limit in CaAs3

Authors: Yuxiang Wang, Minhao Zhao, Jinglei Zhang, Wenbin Wu, Shichao Li, Yong Zhang, Wenxiang Jiang, Nesta Benno Joseph, Liangcai Xu, Yicheng Mou, Yunkun Yang, Pengliang Leng, Yong Zhang, Li Pi, Alexey Suslov, Mykhaylo Ozerov, Jan Wyzula, Milan Orlita, Fengfeng Zhu, Yi Zhang, Xufeng Kou, Zengwei Zhu, Awadhesh Narayan, Dong Qian, Jinsheng Wen , et al. (3 additional authors not shown)

Abstract: The Mott-Ioffe-Regel limit sets the lower bound of carrier mean free path for coherent quasiparticle transport. Metallicity beyond this limit is of great interest because it is often closely related to quantum criticality and unconventional superconductivity. Progress along this direction mainly focuses on the strange-metal behaviors originating from the evolution of quasiparticle scattering rate… ▽ More The Mott-Ioffe-Regel limit sets the lower bound of carrier mean free path for coherent quasiparticle transport. Metallicity beyond this limit is of great interest because it is often closely related to quantum criticality and unconventional superconductivity. Progress along this direction mainly focuses on the strange-metal behaviors originating from the evolution of quasiparticle scattering rate such as linear-in-temperature resistivity, while the quasiparticle coherence phenomena in this regime are much less explored due to the short mean free path at the diffusive bound. Here we report the observation of quantum oscillations from Landau quantization near the Mott-Ioffe-Regel limit in CaAs3. Despite the insulator-like temperature dependence of resistivity, CaAs3 presents giant magnetoresistance and prominent Shubnikov-de Haas oscillations from Fermi surfaces, indicating highly coherent band transport. In contrast, the quantum oscillation is absent in the magnetic torque. The quasiparticle effective mass increases systematically with magnetic fields, manifesting a much larger value than the expectation given by magneto-infrared spectroscopy. It suggests a strong many-body renormalization effect near Fermi surface. We find that these unconventional behaviors may be explained by the interplay between the mobility edge and the van Hove singularity, which results in the formation of coherent cyclotron orbits emerging at the diffusive bound. Our results call for further study on the electron correlation effect of the van Hove singularity. △ Less

Submitted 14 March, 2024; originally announced March 2024.

Comments: 18 pages, 5 figures

Journal ref: National Science Review, nwae127 (2024)

High magnetic field phase diagram and weak FM breaking in (Ni0.93Co0.07)3V2O8

Authors: Jiating Wu, Minjie Zhang, Ke Shi, Huxin Yin, Yuyan Han, Lansheng Ling, Wei Tong, Chuanying Xi, Li Pi, Zhaosheng Wang

Abstract: We present magnetostriction and thermal expansion measurements on multiferroic (Ni0.93Co0.07)3V2O8. The high field phase diagrams up to 33 T along the a, b and c directions are built. For H//a, as the magnetic field increases, two intermediate phases appear between the incommensurate phase and the paramagnetic phase at about 7 K, and then a magnetically induced phase appears above the paramagnetic… ▽ More We present magnetostriction and thermal expansion measurements on multiferroic (Ni0.93Co0.07)3V2O8. The high field phase diagrams up to 33 T along the a, b and c directions are built. For H//a, as the magnetic field increases, two intermediate phases appear between the incommensurate phase and the paramagnetic phase at about 7 K, and then a magnetically induced phase appears above the paramagnetic phase. For H//b,thermal expansion measurement indicates a mutation in the spin lattice coupling of the high field phases. The interlaced phase boundary suggests a mixed state in the optical high field phase. For H//c, an intermediate phase between the commensurate phase and the incommensurate phase is detected. A nonlinear boundary between the intermediate phase and the low temperature incommensurate phase, and a clear boundary between the commensurate phase and the paramagnetic phase are found. These results indicate that doping Co2+ breaks the weak ferromagnetic moment of the commensurate phase, which exists in the parent compound Ni3V2O8 and (Ni0.9Co0.1)3V2O8. This nonlinear influence reflects complicated spin modulation in Ni3V2O8 by doping Co2+. △ Less

Submitted 31 December, 2023; originally announced January 2024.

Comments: 7 pages, 4 figures

Journal ref: Phys. Rev. B 108, 214108(2023)

Giant domain wall anomalous Hall effect in an antiferromagnet

Authors: Wei Xia, Bo Bai, Xuejiao Chen, Yichen Yang, Yang Zhang, Jian Yuan, Qiang Li, Kunya Yang, Xiangqi Liu, Yang Shi, Haiyang Ma, Huali Yang, Mingquan He, Lei Li, Chuanying Xi, Li Pi, Xiaodong Lv, Xia Wang, Xuerong Liu, Shiyan Li, Xiaodong Zhou, Jianpeng Liu, Yulin Chen, Jian Shen, Dawei Shen , et al. (3 additional authors not shown)

Abstract: Generally, the dissipationless Hall effect in solids requires time-reversal symmetry breaking (TRSB), where TRSB induced by external magnetic field results in ordinary Hall effect, while TRSB caused by spontaneous magnetization gives rise to anomalous Hall effect (AHE) which scales with the net magnetization. The AHE is therefore not expected in antiferromagnets with vanishing small magnetization.… ▽ More Generally, the dissipationless Hall effect in solids requires time-reversal symmetry breaking (TRSB), where TRSB induced by external magnetic field results in ordinary Hall effect, while TRSB caused by spontaneous magnetization gives rise to anomalous Hall effect (AHE) which scales with the net magnetization. The AHE is therefore not expected in antiferromagnets with vanishing small magnetization. However, large AHE was recently observed in certain antiferromagnets with noncolinear spin structure and nonvanishing Berry curvature. Here, we report another origin of AHE in a layered antiferromagnet EuAl2Si2, namely the domain wall (DW) skew scattering with Weyl points near the Fermi level, in experiments for the first time. Interestingly, the DWs form a unique periodic stripe structure with controllable periodicity by external magnetic field, which decreases nearly monotonically from 975 nm at 0 T to 232 nm at 4 T. Electrons incident on DW with topological bound states experience strong asymmetric scattering, leading to a giant AHE, with the DW Hall conductivity (DWHC) at 2 K and 1.2 T reaching a record value of ~ 1,5100 S cm-1 among bulk systems and being two orders of magnitude larger than the intrinsic anomalous Hall conductivity. The observation not only sets a new paradigm for exploration of large anomalous Hall effect, but also provides potential applications in spintronic devices. △ Less

Submitted 18 October, 2024; v1 submitted 12 December, 2023; originally announced December 2023.

Comments: 20 pages Main Text+ SI 33 pages, 5 main figures

Journal ref: Physical Review Letters 2024, 133: 216602

Magnetic-field-induced nonlinear transport in HfTe5

Authors: Cheng Zhang, Jinshan Yang, Zhongbo Yan, Xiang Yuan, Yanwen Liu, Minhao Zhao, Alexey Suslov, Jinglei Zhang, Li Pi, Zhong Wang, Faxian Xiu

Abstract: The interplay of electron correlations and topological phases gives rise to various exotic phenomena including fractionalization, excitonic instability, and axionic excitation. Recently-discovered transition-metal pentatellurides can reach the ultra-quantum limit in low magnetic fields and serve as good candidates for achieving such a combination. Here, we report evidences of density wave and meta… ▽ More The interplay of electron correlations and topological phases gives rise to various exotic phenomena including fractionalization, excitonic instability, and axionic excitation. Recently-discovered transition-metal pentatellurides can reach the ultra-quantum limit in low magnetic fields and serve as good candidates for achieving such a combination. Here, we report evidences of density wave and metal-insulator transition in HfTe5 induced by intense magnetic fields. Using the nonlinear transport technique, we detect a distinct nonlinear conduction behavior in the longitudinal resistivity within the a-c plane, corresponding to the formation of a density wave induced by magnetic fields. In high fields, the onset of the nonlinear conduction in the Hall resistivity indicates an impurity-pinned magnetic freeze-out as the possible origin of the insulating behavior. These frozen electrons can be gradually re-activated into mobile states above a threshold electric field. These experimental evidences call for further investigations into the underlying mechanism for the bulk quantum Hall effect and field-induced phase transtions in pentatellurides. △ Less

Submitted 19 November, 2023; originally announced November 2023.

Comments: 13 pages, 4 figures

Journal ref: National Science Review, 9(10), nwab208 (2022)

The Ground State Lattice Distortion of CsV$_{3}$Sb$_{5}$ Revealed by de Haas-van Alphen Oscillations

Authors: Senyang Pan, Feng Du, Yong Zhang, Zheng Chen, Qi Li, Yingcai Qian, Xiangde Zhu, Chuanying Xi, Li Pi, Wing Chi Yu, Qun Niu, Jinglei Zhang, Mingliang Tian

Abstract: The recently discovered AV$_{3}$Sb$_{5}$ (A = K, Rb, Cs) compounds have garnered intense attention in the scientific community due to their unique characteristics as kagome superconductors coexisting with a charge density wave (CDW) order. To comprehend the electronic properties of this system, it is essential to understand the lattice distortions associated with the CDW order and the ground state… ▽ More The recently discovered AV$_{3}$Sb$_{5}$ (A = K, Rb, Cs) compounds have garnered intense attention in the scientific community due to their unique characteristics as kagome superconductors coexisting with a charge density wave (CDW) order. To comprehend the electronic properties of this system, it is essential to understand the lattice distortions associated with the CDW order and the ground state electronic behavior. Here, we comprehensively examine the Fermi surface by a combination of angle-dependent torque magnetometry and density functional theory calculations. We observe magnetic breakdown in the de Haas-van Alphen oscillations under high magnetic fields. Additionally, by examining the angular and temperature variations in quantum oscillation frequencies, we gain insight into the evolution of the three-dimensional like Fermi surfaces and the cyclotron masses of the orbits, which are consistent with weak electron-phonon coupling. Notably, further comparisons indicate that the 2$\times$2$\times$2 Star-of-David (SoD) distortion is more compatible with both high frequency data above 1000\,T and low frequency data below 500\,T, while the 2$\times$2$\times$2 Tri-Hexagonal (TrH) distortion aligns well with experimental data at mid frequencies. This finding implies the inherent coexistence of both TrH and SoD 2$\times$2$\times$2 patterns within the CDW order. These observations provide key insights into the interplay among effective electronic dimensionality, CDW state, and superconductivity. △ Less

Submitted 9 November, 2023; originally announced November 2023.

Huge magnetostriction in superconducting single-crystalline BaFe$_{1.908}$Ni$_{0.092}$As$_{2}$

Authors: Minjie Zhang, Jiating Wu, Ke Shi, Langsheng Ling, Wei Tong, Chuanying Xi, Li Pi, J. Wosnitza, Huiqian Luo, Zhaosheng Wang

Abstract: The performance of iron-based superconductors in high magnetic fields plays an important role for their practical application. In this work, we measured the magnetostriction and magnetization of BaFe$_{1.908}$Ni$_{0.092}$As$_{2}$ single crystals using pulsed magnetic fields up to 60 T and static magnetic fields up to 33 T, respectively. A huge longitudinal magnetostriction (of the order of 10… ▽ More The performance of iron-based superconductors in high magnetic fields plays an important role for their practical application. In this work, we measured the magnetostriction and magnetization of BaFe$_{1.908}$Ni$_{0.092}$As$_{2}$ single crystals using pulsed magnetic fields up to 60 T and static magnetic fields up to 33 T, respectively. A huge longitudinal magnetostriction (of the order of 10$ ^{-4} $) was observed in the direction of the twin boundaries. The magnetization measurements evidence a high critical-current density due to strong bulk pinning. By using magnetization data with an exponential flux-pinning model, we can reproduce the magnetostriction curves qualitatively. This result shows that the magnetostriction of BaFe$_{1.908}$Ni$_{0.092}$As$_{2}$ can be well explained by a flux-pinning-induced mechanism. △ Less

Submitted 21 August, 2023; originally announced August 2023.

Comments: 4 pages, 3 figures

Journal ref: Appl. Phys. Lett. 123, 072602 (2023)

High-field NMR study of the spin correlations in the spin-cluster mineral Na$_2$Cu$_3$O(SO$_4$)$_3$

Authors: Long Ma, J. X. Li, L. S. Ling, Y. Y. Han, L. Zhang, L. Hu, W. Tong, C. Y. Xi, Li Pi

Abstract: We report NMR study on the spin correlations in the spin-cluster based mineral Na$_2$Cu$_3$O(SO$_4$)$_3$ with magnetic fields ranged from 1 T to 33 T. The long-range magnetic order is observed from both the sudden spectral broadening at $T_N$ and critical slowing down behavior in the temperature dependence of spin-lattice relaxation rates ($1/T_1(T)$). The hump behavior of $1/T_1(T)$ persists to… ▽ More We report NMR study on the spin correlations in the spin-cluster based mineral Na$_2$Cu$_3$O(SO$_4$)$_3$ with magnetic fields ranged from 1 T to 33 T. The long-range magnetic order is observed from both the sudden spectral broadening at $T_N$ and critical slowing down behavior in the temperature dependence of spin-lattice relaxation rates ($1/T_1(T)$). The hump behavior of $1/T_1(T)$ persists to $μ_0H=7.25$ T, above which a spin excitation gap is observed from the thermally activated temperature dependence of $1/T_1$. The gap size shows a linear field dependence, whose slope and intercept respectively yield an effective magnetic moment of 2.54 $μ_B$ and a 0.94 meV spin excitation gap under zero magnetic field. These results indicate the existence of short-range order and prominent easy-plane spin anisotropy, which are important for understanding the spin excitation spectrum in A$_2$Cu$_3$O(SO$_4$)$_3$. △ Less

Submitted 24 June, 2023; originally announced June 2023.

Comments: 5 pages, 4 figures

Journal ref: Phys. Rev. B 107, 245134 (2023)

A disorder-sensitive emergent vortex phase identified in high-Tc superconductor (Li,Fe)OHFeSe

Authors: Dong Li, Peipei Shen, Jinpeng Tian, Ge He, Shunli Ni, Zhaosheng Wang, Chuanying Xi, Li Pi, Hua Zhang, Jie Yuan, Kui Jin, Evgeny F. Talantsev, Li Yu, Fang Zhou, Jens Hänisch, Xiaoli Dong, Zhongxian Zhao

Abstract: The magneto-transport properties are systematically measured under c-direction fields up to 33 T for a series of single-crystal films of intercalated iron-selenide superconductor (Li,Fe)OHFeSe. The film samples with varying degree of disorder are grown hydrothermally. We observe a magnetic-field-enhanced shoulder-like feature in the mixed state of the high-Tc (Li,Fe)OHFeSe films with weak disorder… ▽ More The magneto-transport properties are systematically measured under c-direction fields up to 33 T for a series of single-crystal films of intercalated iron-selenide superconductor (Li,Fe)OHFeSe. The film samples with varying degree of disorder are grown hydrothermally. We observe a magnetic-field-enhanced shoulder-like feature in the mixed state of the high-Tc (Li,Fe)OHFeSe films with weak disorder, while the feature fades away in the films with enhanced disorder. The irreversibility field is significantly suppressed to lower temperatures with the appearance of the shoulder feature. Based on the experiment and model analysis, we establish a new vortex phase diagram for the weakly disordered high-Tc (Li,Fe)OHFeSe, which features an emergent dissipative vortex phase intermediate between the common vortex glass and liquid phases. The reason for the emergence of this intermediate vortex state is further discussed based on related experiments and models. △ Less

Submitted 6 May, 2022; originally announced May 2022.

Comments: 15 pages, 4 figures

Journal ref: Supercond. Sci. Technol. 35 (2022) 064007

Incommensurate magnetic order in Sm$_3$BWO$_9$ with the distorted kagome lattice

Authors: K. Y. Zeng, F. Y. Song, L. S. Ling, W. Tong, Shiliang Li, Z. M. Tian, Long Ma, Li Pi

Abstract: We investigate the magnetic ground state of Sm$_3$BWO$_9$ with the distorted kagome lattice. A magnetic phase transition is identified at $T_N=0.75$ K from the temperature dependence of specific heat. From $^{11}$B nuclear magnetic resonance (NMR) measurements, an incommensurate magnetic order is shown by the double-horn type spectra under a $c$-axis magnetic field. While, absence of line splittin… ▽ More We investigate the magnetic ground state of Sm$_3$BWO$_9$ with the distorted kagome lattice. A magnetic phase transition is identified at $T_N=0.75$ K from the temperature dependence of specific heat. From $^{11}$B nuclear magnetic resonance (NMR) measurements, an incommensurate magnetic order is shown by the double-horn type spectra under a $c$-axis magnetic field. While, absence of line splitting is observed for field oriented within the $ab$-plane, indicating the incommensurate modulation of the internal field strictly along $c$-axis. From the spin dynamics, the critical slowing down behavior is observed in the temperature dependence of $1/T_1$ with $μ_0H\perp c$-axis, which is completely absent in that with $μ_0H||c$-axis. Based on the local symmetry of $^{11}$B sites, we analyze the hyperfine coupling tensors and propose two constraints on the possible magnetic structure. The single ion anisotropy should play an important role in the determination of the contrasting ground states of Sm$_3$BWO$_9$ and Pr$_3$BWO$_9$. △ Less

Submitted 28 September, 2022; v1 submitted 5 May, 2022; originally announced May 2022.

Comments: 7 pages, 5 figures

Journal ref: Chin. Phys. Lett. 39, 107501 (2022)

Enhanced optoelectronic performance and photogating effect in quasi-one-dimensional BiSeI wires

Authors: H. J. Hu, W. L. Zhen, S. R. Weng, Y. D. Li, R. Niu, Z. L. Yue, F. Xu, L. Pi, C. J. Zhang, W. K. Zhu

Abstract: Quasi-one-dimensional (quasi-1D) materials are a newly arising topic in low-dimensional researches. As a result of reduced dimensionality and enhanced anisotropy, the quasi-1D structure gives rise to novel properties and promising applications such as photodetectors. However, it remains an open question whether performance crossover will occur when the channel material is downsized. Here we report… ▽ More Quasi-one-dimensional (quasi-1D) materials are a newly arising topic in low-dimensional researches. As a result of reduced dimensionality and enhanced anisotropy, the quasi-1D structure gives rise to novel properties and promising applications such as photodetectors. However, it remains an open question whether performance crossover will occur when the channel material is downsized. Here we report on the fabrication and testing of photodetectors based on exfoliated quasi-1D BiSeI thin wires. Compared with the device on bulk crystal, a significantly enhanced photoresponse is observed, which is manifested by a series of performance parameters, including ultrahigh responsivity (7 x 10$^4$ A W$^{-1}$), specific detectivity (2.5 x 10$^{14}$ Jones) and external quantum efficiency (1.8 x 10$^7$%) when $V_{\textrm {ds}}$ = 3 V, $λ$ = 515 nm and $P$ = 0.01 mW cm$^{-2}$. The conventional photoconductive effect is unlikely to account for such a superior photoresponse, which is ultimately understood in terms of the increased specific surface area and the photogating effect caused by trapping states. This work provides a perspective for the modulation of optoelectronic properties and performance in quasi-1D materials. △ Less

Submitted 4 May, 2022; originally announced May 2022.

Comments: 23 pages, 4 figures and SI

Journal ref: Appl. Phys. Lett. 120, 201101 (2022)

Large Magnetoresistance and Weak Antilocalization in V1-delta Sb2 Single Crystal

Authors: Yong Zhang, Xinliang Huang, Wenshuai Gao, Xiangde Zhu, Li Pi

Abstract: The binary pnictide semimetals have attracted considerable attention due to their fantastic physical properties that include topological effects, negative magnetoresistance, Weyl fermions and large non-saturation magnetoresistance. In this paper, we have successfully grown the high-quality V1-deltaSb2 single crystals by Sb flux method and investigated their electronic transport properties. A large… ▽ More The binary pnictide semimetals have attracted considerable attention due to their fantastic physical properties that include topological effects, negative magnetoresistance, Weyl fermions and large non-saturation magnetoresistance. In this paper, we have successfully grown the high-quality V1-deltaSb2 single crystals by Sb flux method and investigated their electronic transport properties. A large positive magnetoresistance that reaches 477% under a magnetic field of 12 T at T = 1.8 K was observed. Notably, the magnetoresistance showed a cusp-like feature at the low magnetic fields and such feature weakened gradually as the temperature increased, which indicated the presence of weak antilocalization effect (WAL). The angle-dependent magnetoconductance and the ultra-large prefactor alpha extracted from the Hikami-Larkin-Nagaoka equation revealed that the WAL effect is a 3D bulk effect originated from the three-dimensional bulk spin-orbital coupling. △ Less

Submitted 16 September, 2021; originally announced September 2021.

Comments: 18 pages,5 figures, 1 table

Journal ref: Chinese Phys. B 31, 037102 (2022)

Local evidence for collective spin excitations in the distorted kagome antiferromagnet Pr$_3$BWO$_9$

Authors: K. Y. Zeng, F. Y. Song, Z. M. Tian, Qiao Chen, Shun Wang, Bo Liu, Shiliang Li, L. S. Ling, W. Tong, Long Ma, Li Pi

Abstract: We report the local probe investigation of a frustrated antiferromagnet Pr$_3$BWO$_9$ with the distorted kagome lattice. Absence of magnetic order or spin freezing is indicated by the spectral analysis down to 0.3 K and specific heat measurements down to 0.09 K. The Knight shifts show an upturn behavior with the sample cooling down, which is further suppressed by external field. For the spin dynam… ▽ More We report the local probe investigation of a frustrated antiferromagnet Pr$_3$BWO$_9$ with the distorted kagome lattice. Absence of magnetic order or spin freezing is indicated by the spectral analysis down to 0.3 K and specific heat measurements down to 0.09 K. The Knight shifts show an upturn behavior with the sample cooling down, which is further suppressed by external field. For the spin dynamics, gapped spin excitation is observed from the temperature dependence of spin-lattice relaxation rates, with the gap size proportional to the applied magnetic field intensity. Comparatively, an unexpected sharp peak is observed in the nuclear spin-spin relaxation rate data at $T^*\sim 4-5$ K. These results indicate an unconventional persistent fluctuating paramagnetic ground state with antiferromagnetic collective spin excitations in the strongly frustrated spin system. △ Less

Submitted 1 November, 2021; v1 submitted 15 July, 2021; originally announced July 2021.

Comments: 6 pages, 4 figures

Journal ref: Phys. Rev. B 104, 155150 (2021)

Unusual normal and superconducting state properties observed in hydrothermal Fe1-xSe flakes

Authors: Shaobo Liu, Sheng Ma, Zhaosheng Wang, Wei Hu, Zian Li, Qimei Liang, Hong Wang, Yuhang Zhang, Zouyouwei Lu, Jie Yuan, Kui Jin, Jian-Qi Li, Li Pi, Li Yu, Fang Zhou, Xiaoli Dong, Zhongxian Zhao

Abstract: The electronic and superconducting properties of Fe1-xSe single-crystal flakes grown hydrothermally are studied by the transport measurements under zero and high magnetic fields up to 38.5 T. The results contrast sharply with those previously reported for nematically ordered FeSe by chemical-vapor-transport (CVT) growth. No signature of the electronic nematicity, but an evident metal-to-nonmetal c… ▽ More The electronic and superconducting properties of Fe1-xSe single-crystal flakes grown hydrothermally are studied by the transport measurements under zero and high magnetic fields up to 38.5 T. The results contrast sharply with those previously reported for nematically ordered FeSe by chemical-vapor-transport (CVT) growth. No signature of the electronic nematicity, but an evident metal-to-nonmetal crossover with increasing temperature, is detected in the normal state of the present hydrothermal samples. Interestingly, a higher superconducting critical temperature Tc of 13.2 K is observed compared to a suppressed Tc of 9 K in the presence of the nematicity in the CVT FeSe. Moreover, the upper critical field in the zero-temperature limit is found to be isotropic with respect to the field direction and to reach a higher value of ~42 T, which breaks the Pauli limit by a factor of 1.8. △ Less

Submitted 25 April, 2021; originally announced April 2021.

Comments: 11 pages, 3 figures

Journal ref: CHIN. PHYS. LETT. 38 (2021) 057401 (Exp.)

Multiple Weyl fermions in the noncentrosymmetric semimetal LaAlSi

Authors: Hao Su, Xianbiao Shi, Jian Yuan, Yimin Wan, Erjian Cheng, Chuanying Xi, Li Pi, Xia Wang, Zhiqiang Zou, Na Yu, Weiwei Zhao, Shiyan Li, Yanfeng Guo

Abstract: The noncentrosymmetric RAlPn (R = rare earth, Pn = Si, Ge) family, predicted to host nonmagnetic and magnetic Weyl states, provide an excellent platform for investigating the relation between magnetism and Weyl physics. By using high field magnetotransport measurements and first principles calculations, we have unveiled herein both type-I and type-II Weyl states in the nonmagnetic LaAlSi. By a car… ▽ More The noncentrosymmetric RAlPn (R = rare earth, Pn = Si, Ge) family, predicted to host nonmagnetic and magnetic Weyl states, provide an excellent platform for investigating the relation between magnetism and Weyl physics. By using high field magnetotransport measurements and first principles calculations, we have unveiled herein both type-I and type-II Weyl states in the nonmagnetic LaAlSi. By a careful comparison between experimental results and theoretical calculations, nontrivial Berry phases associated with the Shubnikov-de Haas oscillations are ascribed to the electron Fermi pockets related to both types of Weyl points located ~ 0.1 eV above and exactly on the Fermi level, respectively. Under high magnetic field, signatures of Zeeman splitting are also observed. These results indicate that, in addition to the importance for exploring intriguing physics of multiple Weyl fermions, LaAlSi as a comparison with magnetic Weyl semimetals in the RAlPn family would also yield valuable insights into the relation between magnetism and Weyl physics. △ Less

Submitted 10 February, 2021; originally announced February 2021.

Comments: 18 pages, 5 figures, 1 table

Journal ref: Phys. Rev. B 103, 165128 (2021)

Disorder-robust high-field superconducting phase of FeSe single crystals

Authors: Nan Zhou, Yue Sun, C. Y. Xi, Z. S. Wang, J. L. Zhang, Y. Zhang, Y. F. Zhang, C. Q. Xu, Y. Q. Pan, J. J. Feng, Y. Meng, X. L. Yi, L. Pi, T. Tamegai, Xiangzhuo Xing, Zhixiang Shi

Abstract: When exposed to high magnetic fields, certain materials manifest an exotic superconducting (SC) phase that has attracted considerable attention. A proposed explanation for the origin of the high-field SC phase is the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state. This state is characterized by inhomogeneous superconductivity, where the Cooper pairs have finite center-of-mass momenta. Recently, the… ▽ More When exposed to high magnetic fields, certain materials manifest an exotic superconducting (SC) phase that has attracted considerable attention. A proposed explanation for the origin of the high-field SC phase is the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state. This state is characterized by inhomogeneous superconductivity, where the Cooper pairs have finite center-of-mass momenta. Recently, the high-field SC phase was observed in FeSe, and it was deemed to originate from the FFLO state. Here, we synthesize FeSe single crystals with different levels of disorder. The level of disorder is expressed by the ratio of the mean free path to the coherence length and ranges between 35 and 1.2. The upper critical field \textit{B}$_{\rm{c}2}$ was obtained by both resistivity and magnetic torque measurements over a wide range of temperatures, which went as low as $\sim$0.5 K, and magnetic fields, which went up to $\sim$38 T along the \textit{c} axis and in the \textit{ab} plane. In the high-field region parallel to the \textit{ab} plane, an unusual SC phase was confirmed in all the crystals, and the phase was found to be robust against disorder. This result suggests that the high-field SC phase in FeSe is not a conventional FFLO state. △ Less

Submitted 19 November, 2021; v1 submitted 3 February, 2021; originally announced February 2021.

Comments: 11 pages, 11 figures

NMR study of the spin correlations in the $S=1$ armchair chain Ni$_2$NbBO$_6$

Authors: K. Y. Zeng, Long Ma, L. M. Xu, Z. M. Tian, L. S. Ling, Li Pi

Abstract: We report our nuclear magnetic resonance (NMR) study on the structurally spin chain compound Ni$_2$NbBO$_6$ with complex magnetic coupling. The antiferromagnetic transition is monitored by the line splitting resulting from the staggered internal hyperfine field. The magnetic coupling configuration proposed by the first-principle density functional theory (DFT) is supported by our NMR spectral anal… ▽ More We report our nuclear magnetic resonance (NMR) study on the structurally spin chain compound Ni$_2$NbBO$_6$ with complex magnetic coupling. The antiferromagnetic transition is monitored by the line splitting resulting from the staggered internal hyperfine field. The magnetic coupling configuration proposed by the first-principle density functional theory (DFT) is supported by our NMR spectral analysis. For the spin dynamics, a prominent peak at $T\sim35$ K well above the Néel temperature ($T_N\sim20$ K at $μ_0H=10$ T) is observed from the spin-lattice relaxation data. As compared with the dc-susceptibility, this behavior indicates a antiferromagnetic coupling with the typical energy scale of $\sim3$ meV. Thus, the Ni$_2$NbBO$_6$ compound can be viewed as strongly ferromagnetically coupled armchair spin chains along the crystalline $b$-axis. These facts place strong constraints to the theoretical model for this compound. △ Less

Submitted 5 August, 2020; originally announced August 2020.

Comments: 6 pages, 4 figures

Journal ref: Chin. Phys. B, 30, 047503 (2021)

Magnetism-induced topological transition in EuAs3

Authors: Erjian Cheng, Wei Xia, Xianbiao Shi, Chengwei Wang, Chuanying Xi, Shaowen Xu, Darren C. Peets, Linshu Wang, Hao Su, Li Pi, Wei Ren, Xia Wang, Na Yu, Yulin Chen, Weiwei Zhao, Zhongkai Liu, Yanfeng Guo, Shiyan Li

Abstract: The nature of the interaction between magnetism and topology in magnetic topological semimetals remains mysterious, but may be expected to lead to a variety of novel physics. We present $ab$ $initio$ band calculations, electrical transport and angle-resolved photoemission spectroscopy (ARPES) measurements on the magnetic semimetal EuAs$_3$, demonstrating a magnetism-induced topological transition… ▽ More The nature of the interaction between magnetism and topology in magnetic topological semimetals remains mysterious, but may be expected to lead to a variety of novel physics. We present $ab$ $initio$ band calculations, electrical transport and angle-resolved photoemission spectroscopy (ARPES) measurements on the magnetic semimetal EuAs$_3$, demonstrating a magnetism-induced topological transition from a topological nodal-line semimetal in the paramagnetic or the spin-polarized state to a topological massive Dirac metal in the antiferromagnetic (AFM) ground state at low temperature, featuring a pair of massive Dirac points, inverted bands and topological surface states on the (010) surface. Shubnikov-de Haas (SdH) oscillations in the AFM state identify nonzero Berry phase and a negative longitudinal magnetoresistance ($n$-LMR) induced by the chiral anomaly, confirming the topological nature predicted by band calculations. When magnetic moments are fully polarized by an external magnetic field, an unsaturated and extremely large magnetoresistance (XMR) of $\sim$ 2$\times10^5$ % at 1.8 K and 28.3 T is observed, likely arising from topological protection. Consistent with band calculations for the spin-polarized state, four new bands in quantum oscillations different from those in the AFM state are discerned, of which two are topologically protected. Nodal-line structures at the $Y$ point in the Brillouin zone (BZ) are proposed in both the spin-polarized and paramagnetic states, and the latter is proven by ARPES. Moreover, a temperature-induced Lifshitz transition accompanied by the emergence of a new band below 3 K is revealed. These results indicate that magnetic EuAs$_3$ provides a rich platform to explore exotic physics arising from the interaction of magnetism with topology. △ Less

Submitted 29 June, 2020; originally announced June 2020.

Comments: 15 pages, with METHODS AND EXTENDED DATA

Journal ref: Nature Communications 12, 6970 (2021)

Emergence of superconductivity in strongly correlated hole-dominated Fe1-xSe

Authors: S. L. Ni, J. P. Sun, S. B. Liu, J. Yuan, Li Yu, M. W. Ma, L. Zhang, L. Pi, P. Zheng, P. P. Shen, D. Li, D. E. Shi, G. B. Li, J. L. Sun, G. M. Zhang, K. Jin, J. -G. Cheng, F. Zhou, X. L. Dong, Z. X. Zhao

Abstract: Here we establish a more complete phase diagram for FeSe system, based on experimental results of nonstoichiometric Fe1-xSe single crystals that we have developed recently, as well as nearly stoichiometric FeSe single crystals. The electronic correlation is found to be strongly enhanced in hole-dominated Fe1-xSe, as compared with electron-dominated FeSe, from the magnetic susceptibility and electr… ▽ More Here we establish a more complete phase diagram for FeSe system, based on experimental results of nonstoichiometric Fe1-xSe single crystals that we have developed recently, as well as nearly stoichiometric FeSe single crystals. The electronic correlation is found to be strongly enhanced in hole-dominated Fe1-xSe, as compared with electron-dominated FeSe, from the magnetic susceptibility and electrical transport measurements in the normal state. A superconducting dome is found to emerge starting from the strongly correlated hole-dominated regime with electron doping, while the tetragonal-orthorhombic phase transition at ~90 K is observed only at higher electron-doping levels in the electron-dominated regime. △ Less

Submitted 29 December, 2019; originally announced December 2019.

Comments: 16 pages, 4 figures. First submitted on May 24, 2019

Giant enhancement of critical current density at high field in superconducting (Li,Fe)OHFeSe films by Mn doping

Authors: Dong Li, Jie Yuan, Peipei Shen, Chuanying Xi, Jinpeng Tian, Shunli Ni, Jingsong Zhang, Zhongxu Wei, Wei Hu, Zian Li, Li Yu, Jun Miao, Fang Zhou, Li Pi, Kui jin, Xiaoli Dong, Zhongxian Zhao

Abstract: Critical current density (Jc) is one of the major limiting factors for high field applications of iron-based superconductors. Here, we report that Mn-ions are successfully incorporated into nontoxic superconducting (Li,Fe)OHFeSe films. Remarkably, the Jc is significantly enhanced from 0.03 to 0.32 MA/cm^2 under 33 T, and the vortex pinning force density monotonically increases up to 106 GN/m^3, wh… ▽ More Critical current density (Jc) is one of the major limiting factors for high field applications of iron-based superconductors. Here, we report that Mn-ions are successfully incorporated into nontoxic superconducting (Li,Fe)OHFeSe films. Remarkably, the Jc is significantly enhanced from 0.03 to 0.32 MA/cm^2 under 33 T, and the vortex pinning force density monotonically increases up to 106 GN/m^3, which is the highest record so far among all iron-based superconductors. Our results demonstrate that Mn incorporation is an effective method to optimize the performance of (Li,Fe)OHFeSe films, offering a promising candidate for high-field applications. △ Less

Submitted 18 December, 2019; originally announced December 2019.

Comments: 6 pages, 5 figures

Journal ref: Supercond. Sci. Tech. 32, 12LT01 (2019)

NMR study of the spin excitations in the frustrated antiferromagnet Yb(BaBO$_3$)$_3$ with a triangular lattice

Authors: K. Y. Zeng, Long Ma, Y. X. Gao, Z. M. Tian, L. S. Ling, Li Pi

Abstract: In this paper, we study the spin excitation properties of the frustrated triangular-lattice antiferromagnet Yb(BaBO$_3$)$_3$ with nuclear magnetic resonance. From the spectral analysis, neither magnetic ordering nor spin freezing is observed with temperature down to $T=0.26$ K, far below its Curie-Weiss temperature $|θ_w|\sim2.3$ K. From the nuclear relaxation measurement, precise temperature-inde… ▽ More In this paper, we study the spin excitation properties of the frustrated triangular-lattice antiferromagnet Yb(BaBO$_3$)$_3$ with nuclear magnetic resonance. From the spectral analysis, neither magnetic ordering nor spin freezing is observed with temperature down to $T=0.26$ K, far below its Curie-Weiss temperature $|θ_w|\sim2.3$ K. From the nuclear relaxation measurement, precise temperature-independent spin-lattice relaxation rates are observed at low temperatures under a weak magnetic field, indicating the gapless spin excitations. Further increasing the field intensity, we observe a spin excitation gap with the gap size proportional to the field intensity. These phenomena suggest a very unusual strongly correlated quantum disordered phase, and the implications for the quantum spin liquid state are further discussed. △ Less

Submitted 30 July, 2020; v1 submitted 8 December, 2019; originally announced December 2019.

Comments: 6 pages, 6 figures. Supplemental materials available upon request

Journal ref: Phys. Rev. B 102, 045149 (2020)

The upper critical field and its anisotropy in RbCr$_{3}$As$_{3}$

Authors: Qimei Liang, Tong Liu, Chuanying Xi, Yuyan Han, Gang Mu, Li Pi, Zhi-An Ren, Zhaosheng Wang

Abstract: The temperature dependence of the upper critical field ($H_{c2}$) in RbCr$% _{3}$As$_{3}$ single crystals ($T_{c}\approx $ 7.3 K) has been determined by means of magnetoresistance measurements with temperature down to 0.35 K in static magnetic fields up to 38 T. The magnetic field was applied both for directions parallel ($H\parallel c $, $H_{c2}^{\parallel c}$) and perpendicular ($H\perp c$,… ▽ More The temperature dependence of the upper critical field ($H_{c2}$) in RbCr$% _{3}$As$_{3}$ single crystals ($T_{c}\approx $ 7.3 K) has been determined by means of magnetoresistance measurements with temperature down to 0.35 K in static magnetic fields up to 38 T. The magnetic field was applied both for directions parallel ($H\parallel c $, $H_{c2}^{\parallel c}$) and perpendicular ($H\perp c$, $H_{c2}^{\perp c}$) to the Cr chains. The curves $H_{c2}^{\parallel c}(T)$ and $H_{c2}^{\perp c}(T)$ cross at $\sim $ 5.5 K. As a result, the anisotropy parameter $γ(T)=H_{c2}^{\perp c}/H_{c2}^{\parallel c}(T)$ increases from 0.5 near $T_{c}$ to 1.6 at low temperature. Fitting with the Werthamer-Helfand-Hohenberg (WHH) model yields zero-temperature critical fields of $μ_0H_{c2}^{\parallel c}(0)\approx $ 27.2 T and $μ_0H_{c2}^{\perp c}(0)\approx $ 43.4 T, both exceeding the BCS weak-coupling Pauli limit $μ_0H_{p}=1.84T_{c}=13.4$ T. The results indicate that the paramagnetic pair breaking effect is strong for $H \parallel c$ but absent for $H \perp c$, which was further confirmed by the angle dependent magnetoresistance and $H_{c2}$ measurements. △ Less

Submitted 8 December, 2019; originally announced December 2019.

Comments: 6 pages, 5 figures

Journal ref: Physical Review B 100, 214512 (2019)

Bulk Fermi surface of the layered superconductor TaSe3 with three-dimensional strong topological insulator state

Authors: Wei Xia, Xianbiao Shi, Yong Zhang, Hao Su, Qin Wang, Linchao Ding, Leiming Chen, Xia Wang, Zhiqiang Zou, Na Yu, Li Pi, Yufeng Hao, Bin Li, Zengwei Zhu, Weiwei Zhao, Xufeng Kou, Yanfeng Guo

Abstract: High magnetic field transport measurements and ab initio calculations on the layered superconductor TaSe3 have provided compelling evidences for the existence of a three-dimensional strong topological insulator state. Longitudinal magnetotransport measurements up to ~ 33 T unveiled striking Shubnikov-de Hass oscillations with two fundamental frequencies at 100 T and 175 T corresponding to a nontri… ▽ More High magnetic field transport measurements and ab initio calculations on the layered superconductor TaSe3 have provided compelling evidences for the existence of a three-dimensional strong topological insulator state. Longitudinal magnetotransport measurements up to ~ 33 T unveiled striking Shubnikov-de Hass oscillations with two fundamental frequencies at 100 T and 175 T corresponding to a nontrivial electron Fermi pocket at the B point and a nontrivial hole Fermi pocket at the Γ point respectively in the Brillouin zone. However, calculations revealed one more electron pocket at the B point, which was not detected by the magnetotransport measurements, presumably due to the limited carrier momentum relaxation time. Angle dependent quantum oscillations by rotating the sample with respect to the magnetic field revealed clear changes in the two fundamental frequencies, indicating anisotropic electronic Fermi pockets. The ab initio calculations gave the topological Z2 invariants of (1; 100) and revealed a single Dirac cone on the (1 0 -1) surface at the X point with helical spin texture at a constant-energy contour, suggesting a strong topological insulator state. The results demonstrate TaSe3 an excellent platform to study the interplay between topological phase and superconductivity and a promising system for the exploration of topological superconductivity. △ Less

Submitted 1 December, 2019; originally announced December 2019.

Comments: 16 pages, 4 figures, 1 table. arXiv admin note: text overlap with arXiv:1911.03844

Journal ref: Phys. Rev. B 101, 155117 (2020)

Experimental evidence of crystal symmetry protection for the topological nodal line semimetal state in ZrSiS

Authors: C. C. Gu, J. Hu, X. L. Chen, Z. P. Guo, B. T. Fu, Y. H. Zhou, C. An, Y. Zhou, R. R. Zhang, C. Y. Xi, Q. Y. Gu, C. Park, H. Y. Shu, W. G. Yang, L. Pi, Y. H. Zhang, Y. G. Yao, Z. R. Yang, J. H. Zhou, J. Sun, Z. Q. Mao, M. L. Tian

Abstract: Tunable symmetry breaking plays a crucial role for the manipulation of topological phases of quantum matter. Here, through combined high-pressure magneto-transport measurements, Raman spectroscopy, and X-ray diffraction, we demonstrate a pressure-induced topological phase transition in nodal-line semimetal ZrSiS. Symmetry analysis and first-principles calculations suggest that this pressure-induce… ▽ More Tunable symmetry breaking plays a crucial role for the manipulation of topological phases of quantum matter. Here, through combined high-pressure magneto-transport measurements, Raman spectroscopy, and X-ray diffraction, we demonstrate a pressure-induced topological phase transition in nodal-line semimetal ZrSiS. Symmetry analysis and first-principles calculations suggest that this pressure-induced topological phase transition may be attributed to weak lattice distortions by non-hydrostatic compression, which breaks some crystal symmetries, such as the mirror and inversion symmetries. This finding provides some experimental evidence for crystal symmetry protection for the topological semimetal state, which is at the heart of topological relativistic fermion physics. △ Less

Submitted 18 November, 2019; originally announced November 2019.

Comments: 27 pages, 17 figures

Journal ref: Physical Review B 100, 205124 (2019)

Observation of charge density wave transition in TaSe3 mesowires

Authors: J. Yang, Y. Q. Wang, R. R. Zhang, L. Ma, W. Liu, Z. Qu, L. Zhang, S. L. Zhang, W. Tong, L. Pi, W. K. Zhu, C. J. Zhang

Abstract: The quasi-one-dimensional (quasi-1D) TaSe3 attracts considerable attention for its intriguing superconductivity and possible interplay with nontrivial topology and charge density wave (CDW) state. However, unlike the isostructural analogues, CDW has not been observed for TaSe3 despite its quasi-1D character that is supposed to promote Peierls instabilities and CDW. Here we synthesize TaSe3 mesowir… ▽ More The quasi-one-dimensional (quasi-1D) TaSe3 attracts considerable attention for its intriguing superconductivity and possible interplay with nontrivial topology and charge density wave (CDW) state. However, unlike the isostructural analogues, CDW has not been observed for TaSe3 despite its quasi-1D character that is supposed to promote Peierls instabilities and CDW. Here we synthesize TaSe3 mesowires (MWs) using a one-step approach. For the MW of ~300 nm thick, a distinct CDW transition occurs at 65 K in the resistivity measurement, which has not been reported before and is further evidenced by the Raman characterization and susceptibility measurement. For comparison, we have also prepared bulk single crystal TaSe3. Although no anomaly appears in the resistivity and magnetoresistance measurements, the carrier type detected by Hall effect varies from n-type to p-type below 50 K, suggesting a reconstruction of Fermi surface that could be associated with CDW. The enhancement of CDW in the MWs is attributed to the reduced dimensionality. TaSe3 is demonstrated to be a promising platform to study the correlation and competition of CDW and superconductivity in the quasi-1D systems. △ Less

Submitted 22 June, 2019; originally announced June 2019.

Comments: 21 pages, 5 figures, Applied Physics Letters

Journal ref: Appl. Phys. Lett. 115, 033102 (2019)

Magnetotransport in Al6Re

Authors: Erjian Cheng, Darren C. Peets, Chuanying Xi, Yeyu Huang, Li Pi, Shiyan Li

Abstract: Since very few Type-I superconductors are known and most are elemental superconductors, there are very few experimental platforms where the interaction between Type-I superconductivity and topologically nontrivial band structure can be probed. The rhenium aluminide Al$_6$Re has recently been identified as a Type-I superconductor with a transition of 0.74\,K and a critical field of $\sim$50\,Oe. He… ▽ More Since very few Type-I superconductors are known and most are elemental superconductors, there are very few experimental platforms where the interaction between Type-I superconductivity and topologically nontrivial band structure can be probed. The rhenium aluminide Al$_6$Re has recently been identified as a Type-I superconductor with a transition of 0.74\,K and a critical field of $\sim$50\,Oe. Here, we report its magnetotransport behavior including de Haas-van Alphen (dHvA) and Shubnikov-de Haas (SdH) oscillations. Angular dependence of the magnetoresistance reveals a highly anisotropic Fermi surface with dominant hole character. From the strong oscillatory component $ΔR_{xx}$ in high magnetic fields up to 33\,T, the Landau index infinite-field intercept in the case of a single oscillation frequency, and the phase factor $\varphi$ where multiple frequencies coexist, are both $\sim$1/4. This intermediate value is suggestive of possible nontrivial band topology but does not allow strong conclusions. △ Less

Submitted 12 August, 2019; v1 submitted 22 April, 2019; originally announced April 2019.

Comments: 6 pages, 4 figures

Journal ref: Phys. Rev. B 100, 054509 (2019)

Anomalous thermoelectric effects of ZrTe$_{5}$ in and beyond the quantum limit

Authors: J. L. Zhang, C. M. Wang, C. Y. Guo, X. D. Zhu, J. Y. Yang, Y. Q. Wang, Z. Qu, L. Pi, H. Z. Lu, M. L. Tian

Abstract: Thermoelectric effects are more sensitive and promising probes to topological properties of emergent materials, but much less addressed compared to other physical properties. Zirconium pentatelluride (ZrTe$_{5}$) has inspired active investigations recently because of its multiple topological nature. We study the thermoelectric effects of ZrTe$_{5}$ in a magnetic field and find several anomalous be… ▽ More Thermoelectric effects are more sensitive and promising probes to topological properties of emergent materials, but much less addressed compared to other physical properties. Zirconium pentatelluride (ZrTe$_{5}$) has inspired active investigations recently because of its multiple topological nature. We study the thermoelectric effects of ZrTe$_{5}$ in a magnetic field and find several anomalous behaviors. The Nernst response has a steplike profile near zero field when the charge carriers are electrons only, suggesting the anomalous Nernst effect arising from a nontrivial profile of Berry curvature. Both the thermopower and Nernst signal exhibit exotic peaks in the strong-field quantum limit. At higher magnetic fields, the Nernst signal has a sign reversal at a critical field where the thermopower approaches to zero. We propose that these anomalous behaviors can be attributed to the Landau index inversion, which is resulted from the competition of the $\sqrt{B}$ dependence of the Dirac-type Landau bands and linear-$B$ dependence of the Zeeman energy ($B$ is the magnetic field). Our understanding to the anomalous thermoelectric properties in ZrTe$_{5}$ opens a new avenue for exploring Dirac physics in topological materials. △ Less

Submitted 16 April, 2019; v1 submitted 31 March, 2019; originally announced April 2019.

Comments: 6 pages, 4 figures

Journal ref: Phys. Rev. Lett. 123, 196602 (2019)

Anomalies of upper critical field in the spinel superconductor LiTi$_2$O$_{4-δ}$

Authors: Zhongxu Wei, Ge He, Wei Hu, Zhongpei Feng, Jie Yuan, Chuanying Xi, Qian Li, Beiyi Zhu, Fang Zhou, Xiaoli Dong, Li Pi, Fedor V. Kusmartsev, Zhongxian Zhao, Kui Jin

Abstract: High-field electrical transport and point-contact tunneling spectroscopy were used to investigate superconducting properties of the unique spinel oxide, LiTi$_2$O$_{4-δ}$ films with various oxygen content. We find that the upper critical field $B_\mathrm{c2}$ gradually increases as more oxygen impurities are brought into the samples by carefully tuning the deposition atmosphere. It is striking tha… ▽ More High-field electrical transport and point-contact tunneling spectroscopy were used to investigate superconducting properties of the unique spinel oxide, LiTi$_2$O$_{4-δ}$ films with various oxygen content. We find that the upper critical field $B_\mathrm{c2}$ gradually increases as more oxygen impurities are brought into the samples by carefully tuning the deposition atmosphere. It is striking that although the superconducting transition temperature and energy gap are almost unchanged, an astonishing isotropic $B_\mathrm{c2}$ up to $\sim$ 26 Tesla is observed in oxygen-rich sample, which is doubled compared to the anoxic sample and breaks the Pauli limit. Such anomalies of $B_\mathrm{c2}$ were rarely reported in other three dimensional superconductors. Combined with all the anomalies, three dimensional spin-orbit interaction induced by tiny oxygen impurities is naturally proposed to account for the remarkable enhancement of $B_\mathrm{c2}$ in oxygen-rich LiTi$_2$O$_{4-δ}$ films. Such mechanism could be general and therefore provides ideas for optimizing practical superconductors with higher $B_\mathrm{c2}$. △ Less

Submitted 21 January, 2019; v1 submitted 28 December, 2018; originally announced December 2018.

Journal ref: Phys. Rev. B 100, 184509 (2019)

Novel magnetic field tuning of quantum spin excitations in a weakly coupled $S=$1/2 Heisenberg spin chain as seen from NMR

Authors: Long Ma, Z. Wang, L. Hu, Z. Qu, N. Hao, Li Pi

Abstract: We report our NMR study of the spin excitations in the quasi-one dimensional (1D) $S=1/2$ quantum magnet CH$_3$NH$_3$Cu(HCOO)$_3$ lying in the 1D-3D dimensional crossover regime with Dzyaloshinskii-Moriya(DM) interactions. Above $T_N$, the spinon excitation is observed from the constant $1/T_1$ at low temperatures contributed from the staggered spin susceptibility. At low temperatures well below… ▽ More We report our NMR study of the spin excitations in the quasi-one dimensional (1D) $S=1/2$ quantum magnet CH$_3$NH$_3$Cu(HCOO)$_3$ lying in the 1D-3D dimensional crossover regime with Dzyaloshinskii-Moriya(DM) interactions. Above $T_N$, the spinon excitation is observed from the constant $1/T_1$ at low temperatures contributed from the staggered spin susceptibility. At low temperatures well below $T_N$, the $1/T_1$ begins to flatten out under weak magnetic fields. With the increasing field intensity, $1/T_1$ tends to show a power-law temperature dependence gradually, with the index increasing from zero to $\sim3$, and finally $\sim5$, which are the respective typical characteristic for the dominating two-magnon Raman process and three-magnon scattering contributions to the nuclear relaxation in a conventional 3D magnet. A possible physical mechanism for this novel magnetic field tuning of quantum spin excitations related with the enhanced effective staggered field created by the DM interactions under magnetic field is discussed. △ Less

Submitted 17 September, 2019; v1 submitted 15 December, 2018; originally announced December 2018.

Comments: 8 pages, 5 figures

Journal ref: Phys. Rev. B 100, 125126 (2019)

Phase transition and anomalous scaling in the quantum Hall transport of topological insulator Sn-Bi1.1Sb0.9Te2S devices

Authors: Faji Xie, Shuai Zhang, Qianqian Liu, Chuanying Xi, Ting-Ting Kang, Rui Wang, Boyuan Wei, Xing-Chen Pan, Minhao Zhang, Fucong Fei, Xuefeng Wang, Li Pi, Geliang L. Yu, Baigeng Wang, Fengqi Song

Abstract: The scaling physics of quantum Hall transport in optimized topological insulators with a plateau precision of ~1/1000 e2/h is considered. Two exponential scaling regimes are observed in temperature-dependent transport dissipation, one of which accords with thermal activation behavior with a gap of 2.8 meV (> 20 K), the other being attributed to variable range hopping (1-20 K). Magnetic field-drive… ▽ More The scaling physics of quantum Hall transport in optimized topological insulators with a plateau precision of ~1/1000 e2/h is considered. Two exponential scaling regimes are observed in temperature-dependent transport dissipation, one of which accords with thermal activation behavior with a gap of 2.8 meV (> 20 K), the other being attributed to variable range hopping (1-20 K). Magnetic field-driven plateau-to-plateau transition gives scaling relations of (dR$_{xy}$/dB)$^{max}$ \propto T$^{-κ}$ and \DeltaB$^{-1}$ \propto T$^{-κ}$ with a consistent exponent of κ~ 0.2, which is half the universal value for a conventional two-dimensional electron gas. This is evidence of percolation assisted by quantum tunneling, and reveals the dominance of electron-electron interaction of the topological surface states. △ Less

Submitted 12 December, 2018; originally announced December 2018.

Journal ref: Phys. Rev. B 99, 081113 (2019)

Origin of planar Hall effect in type-II Weyl semimetal MoTe2

Authors: D. D. Liang, Y. J. Wang, W. L. Zhen, J. Yang, S. R. Weng, X. Yan, Y. Y. Han, W. Tong, L. Pi, W. K. Zhu, C. J. Zhang

Abstract: Besides the negative longitudinal magnetoresistance (MR), planar Hall effect (PHE) is a newly emerging experimental tool to test the chiral anomaly or nontrivial Berry curvature in Weyl semimetals (WSMs). However, the origins of PHE in various systems are not fully distinguished and understood. Here we perform a systematic study on the PHE and anisotropic MR (AMR) of Td-MoTe2, a type-II WSM. Altho… ▽ More Besides the negative longitudinal magnetoresistance (MR), planar Hall effect (PHE) is a newly emerging experimental tool to test the chiral anomaly or nontrivial Berry curvature in Weyl semimetals (WSMs). However, the origins of PHE in various systems are not fully distinguished and understood. Here we perform a systematic study on the PHE and anisotropic MR (AMR) of Td-MoTe2, a type-II WSM. Although the PHE and AMR curves can be well fitted by the theoretical formulas, we demonstrate that the anisotropic resistivity arises from the orbital MR (OMR), instead of the negative MR as expected in the chiral anomaly effect. In contrast, the absence of negative MR indicates that the large OMR dominates over the chiral anomaly effect. This explains why it is difficult to measure negative MR in type-II WSMs. We argue that the measured PHE can be related with the chiral anomaly only when the negative MR is simultaneously observed. △ Less

Submitted 4 September, 2018; originally announced September 2018.

Comments: 14 pages, 4 figures

Journal ref: AIP Advances 9, 055015 (2019)

Current jetting distorted planar Hall effect in a Weyl semimetal with ultrahigh mobility

Authors: J. Yang, W. L. Zhen, D. D. Liang, Y. J. Wang, X. Yan, S. R. Weng, J. R. Wang, W. Tong, L. Pi, W. K. Zhu, C. J. Zhang

Abstract: A giant planar Hall effect (PHE) and anisotropic magnetoresistance (AMR) is observed in TaP, a nonmagnetic Weyl semimetal with ultrahigh mobility. The perpendicular resistivity (i.e., the planar magnetic field applied normal to the current) far exceeds the zero-field resistivity, which thus rules out the possible origin of negative longitudinal magnetoresistance. The giant PHE/AMR is finally attri… ▽ More A giant planar Hall effect (PHE) and anisotropic magnetoresistance (AMR) is observed in TaP, a nonmagnetic Weyl semimetal with ultrahigh mobility. The perpendicular resistivity (i.e., the planar magnetic field applied normal to the current) far exceeds the zero-field resistivity, which thus rules out the possible origin of negative longitudinal magnetoresistance. The giant PHE/AMR is finally attributed to the large anisotropic orbital magnetoresistance that stems from the ultrahigh mobility. Furthermore, the mobility-enhanced current jetting effects are found to strongly deform the line shape of the curves, and their evolution with the changing magnetic field and temperature is also studied. Although the giant PHE/AMR suggests promising applications in spintronics, the enhanced current jetting shows the other side of the coin, which needs to be considered in the future device design. △ Less

Submitted 14 January, 2019; v1 submitted 17 July, 2018; originally announced July 2018.

Comments: Physical Review Materials

Journal ref: Phys. Rev. Materials 3, 014201 (2019)

Extreme magnetoresistance and Shubnikov-de Haas oscillations in ferromagnetic DySb

Authors: D. D. Liang, Y. J. Wang, C. Y. Xi, W. L. Zhen, J. Yang, L. Pi, W. K. Zhu, C. J. Zhang

Abstract: The electronic structures of a representative rare earth monopnictide (i.e., DySb) under high magnetic field (i.e., in the ferromagnetic state) are studied from both experimental and theoretical aspects. A non-saturated extremely large positive magnetoresistance (XMR) is observed (as large as 3.7*10^4% at 1.8 K and 38.7 T), along with the Shubnikov-de Haas oscillations that are well reproduced by… ▽ More The electronic structures of a representative rare earth monopnictide (i.e., DySb) under high magnetic field (i.e., in the ferromagnetic state) are studied from both experimental and theoretical aspects. A non-saturated extremely large positive magnetoresistance (XMR) is observed (as large as 3.7*10^4% at 1.8 K and 38.7 T), along with the Shubnikov-de Haas oscillations that are well reproduced by our first principles calculations. Three possible origins of XMR are examined. Although a band inversion is found theoretically, suggesting that DySb might be topologically nontrivial, it is deeply underneath the Fermi level, which rules out a topological nature of the XMR. The total densities of electron-like and hole-like carriers are not fully compensated, showing that compensation is unlikely to account for the XMR. The XMR is eventually understood in terms of high mobility that is associated with the steep linear bands. This discovery is important to the intensive studies on the XMR of rare earth monopnictides. △ Less

Submitted 26 August, 2018; v1 submitted 25 May, 2018; originally announced May 2018.

Journal ref: APL Mater. 6, 086105 (2018)

Frustration induced non-Curie-Weiss paramagnetism in La3Ir3O11: a fractional-valence-state iridate

Authors: J. Yang, J. R. Wang, W. L. Zhen, L. Ma, L. S. Ling, W. Tong, C. J. Zhang, L. Pi, W. K. Zhu

Abstract: Experimental and theoretical studies are performed on La3Ir3O11, an iridate hosting a +4.33 fractional valence state for Ir ions and a three-dimensional frustrated structure composed of edge-shared Ir2O10 dimers. These features are expected to enhance inter-site hoppings and reduce magnetic moments of Ir ions. However, a spin-orbit driven Mott insulating transport is observed, which is supported b… ▽ More Experimental and theoretical studies are performed on La3Ir3O11, an iridate hosting a +4.33 fractional valence state for Ir ions and a three-dimensional frustrated structure composed of edge-shared Ir2O10 dimers. These features are expected to enhance inter-site hoppings and reduce magnetic moments of Ir ions. However, a spin-orbit driven Mott insulating transport is observed, which is supported by our first principles calculations. Most importantly, geometrical frustration and competing interactions result in a non-Curie-Weiss paramagnetic ground state, revealing no magnetic order down to 2 K. This unusual state is further demonstrated by a theoretical modeling process, suggesting a possible candidate for the spin liquid state. △ Less

Submitted 25 October, 2019; v1 submitted 20 March, 2018; originally announced March 2018.

Comments: Physical Review B

Journal ref: Physical Review B 100, 205107 (2019)

Field Driven Quantum Criticality in the Spinel Magnet ZnCr$_2$Se$_4$

Authors: C. C. Gu, Z. Y. Zhao, X. L. Chen, M. Lee, E. S. Choi, Y. Y. Han, L. S. Ling, L. Pi, Y. H. Zhang, G. Chen, Z. R. Yang, H. D. Zhou, X. F. Sun

Abstract: We report detailed dc and ac magnetic susceptibilities, specific heat, and thermal conductivity measurements on the frustrated magnet ZnCr$_2$Se$_4$. At low temperatures, with increasing magnetic field, this spinel material goes through a series of spin state transitions from the helix spin state to the spiral spin state and then to the fully polarized state. Our results indicate a direct quantum… ▽ More We report detailed dc and ac magnetic susceptibilities, specific heat, and thermal conductivity measurements on the frustrated magnet ZnCr$_2$Se$_4$. At low temperatures, with increasing magnetic field, this spinel material goes through a series of spin state transitions from the helix spin state to the spiral spin state and then to the fully polarized state. Our results indicate a direct quantum phase transition from the spiral spin state to the fully polarized state. As the system approaches the quantum criticality, we find strong quantum fluctuations of the spins with the behaviors such as an unconventional $T^2$-dependent specific heat and temperature independent mean free path for the thermal transport. We complete the full phase diagram of ZnCr$_2$Se$_4$ under the external magnetic field and propose the possibility of frustrated quantum criticality with extended densities of critical modes to account for the unusual low-energy excitations in the vicinity of the criticality. Our results reveal that ZnCr$_2$Se$_4$ is a rare example of 3D magnet exhibiting a field-driven quantum criticality with unconventional properties. △ Less

Submitted 14 March, 2018; originally announced March 2018.

Comments: 6 pages, 4 figures + supplementary: 2 pages, 1 figure; accepted for publication in Phys. Rev. Lett

Journal ref: Phys. Rev. Lett. 120, 147204 (2018)

Non-saturating Quantum Magnetization in Weyl semimetal TaAs

Authors: Cheng-Long Zhang, C. M. Wang, Zhujun Yuan, Chi-Cheng Lee, Li Pi, Changying Xi, Hsin Lin, Neil Harrison, Hai-Zhou Lu, Jinglei Zhang, Shuang Jia

Abstract: Detecting the spectroscopic signatures of Dirac-like quasiparticles in emergent topological materials is crucial for searching their potential applications. Magnetometry is a powerful tool for fathoming electrons in solids, yet its ability for discerning Dirac-like quasiparticles has not been recognized. Adopting the probes of magnetic torque and parallel magnetization for the archetype Weyl semim… ▽ More Detecting the spectroscopic signatures of Dirac-like quasiparticles in emergent topological materials is crucial for searching their potential applications. Magnetometry is a powerful tool for fathoming electrons in solids, yet its ability for discerning Dirac-like quasiparticles has not been recognized. Adopting the probes of magnetic torque and parallel magnetization for the archetype Weyl semimetal TaAs in strong magnetic field, we observed a quasi-linear field dependent effective transverse magnetization and a strongly enhanced parallel magnetization when the system is in the quantum limit. Distinct from the saturating magnetic responses for massive carriers, the non-saturating signals of TaAs in strong field is consistent with our newly developed magnetization calculation for a Weyl fermion system in an arbitrary angle. Our results for the first time establish a thermodynamic criterion for detecting the unique magnetic response of 3D massless Weyl fermions in the quantum limit. △ Less

Submitted 24 February, 2018; originally announced February 2018.

Comments: Submitted on 14th November 17. Version of original submission. SI is not included

Journal ref: Nature communications, 2019, 10(1): 1028

Non-stoichiometry effects on the extreme magnetoresistance in Weyl semimetal WTe2

Authors: J. X. Gong, J. Yang, M. Ge, Y. J. Wang, D. D. Liang, L. Luo, X. Yan, W. L. Zhen, S. R. Weng, L. Pi, C. J. Zhang, W. K. Zhu

Abstract: Non-stoichiometry effect on the extreme magnetoresistance is systematically investigated for the Weyl semimetal WTe2. Magnetoresistance and Hall resistivity are measured for the as-grown samples with a slight difference in Te vacancies and the annealed samples with increased Te vacancies. The fittings to a two-carrier model show that the magnetoresistance is strongly dependent on the residual resi… ▽ More Non-stoichiometry effect on the extreme magnetoresistance is systematically investigated for the Weyl semimetal WTe2. Magnetoresistance and Hall resistivity are measured for the as-grown samples with a slight difference in Te vacancies and the annealed samples with increased Te vacancies. The fittings to a two-carrier model show that the magnetoresistance is strongly dependent on the residual resistivity ratio (i.e., the degree of non-stoichiometry), which is eventually understood in terms of electron doping which not only breaks the balance between electron-type and hole-type carrier densities but also reduces the average carrier mobility. Thus, compensation effect and ultrahigh mobility are probably the main driving force of the extreme magnetoresistance in WTe2. △ Less

Submitted 29 April, 2018; v1 submitted 29 December, 2017; originally announced December 2017.

Journal ref: Chin. Phys. Lett. 35, 097101 (2018)

Tricritical point and phase diagram based on critical scaling in monoaxial chiral helimagnet Cr1/3NbS2

Authors: Hui Han, Lei Zhang, Deepak Sapkota, Ningning Hao, Langsheng Ling, Haifeng Du, Li Pi, Changjin Zhang, David G. Mandrus, Yuheng Zhang

Abstract: In this work, the magnetism of the single crystal Cr$_{1/3}$NbS$_{2}$, which exhibits chiral magnetic soliton lattice (CSL) state, is investigated. The magnetization displays strong magnetic anisotropy when the field is applied perpendicularly and parallel to the $c$-axis in low field region ($H<H_{S}$, $H_{S}$ is the saturation field). The critical exponents of Cr$_{1/3}$NbS$_{2}$ are obtained as… ▽ More In this work, the magnetism of the single crystal Cr$_{1/3}$NbS$_{2}$, which exhibits chiral magnetic soliton lattice (CSL) state, is investigated. The magnetization displays strong magnetic anisotropy when the field is applied perpendicularly and parallel to the $c$-axis in low field region ($H<H_{S}$, $H_{S}$ is the saturation field). The critical exponents of Cr$_{1/3}$NbS$_{2}$ are obtained as $β=$ 0.370(4), $γ=$ 1.380(2), and $δ=$ 4.853(6), which are close to the theoretical prediction of three-dimensional Heisenberg model. Based on the scaling equation and the critical exponents, the $H-T$ phase diagram in the vicinity of the phase transition is constructed, where two critical points are determined. One is a tricritical point which locates at the intersection between the CSL, forced ferromagnetic (FFM), and paramagnetic (PM) states. The other one is a critical point situated at the boundaries between CSL, helimagnetic (HM), and PM states. △ Less

Submitted 20 September, 2017; originally announced September 2017.

Comments: Published in Physical Review B

Manipulating Multiple Order Parameters via Oxygen Vacancies: The case of Eu0.5Ba0.5TiO3-δ

Authors: Weiwei Li, Qian He, Le Wang, Huizhong Zeng, John Bowlan, Langsheng Ling, Dmitry A. Yarotski, Wenrui Zhang, Run Zhao, Jiahong Dai, Junxing Gu, Shipeng Shen, Haizhong Guo, Li Pi, Haiyan Wang, Yongqiang Wang, Ivan A. Velasco-Davalos, Yangjiang Wu, Zhijun Hu, Bin Chen, Run-Wei Li, Young Sun, Kuijuan Jin, Yuheng Zhang, Hou-Tong Chen , et al. (5 additional authors not shown)

Abstract: Controlling functionalities, such as magnetism or ferroelectricity, by means of oxygen vacancies (VO) is a key issue for the future development of transition metal oxides. Progress in this field is currently addressed through VO variations and their impact on mainly one order parameter. Here we reveal a new mechanism for tuning both magnetism and ferroelectricity simultaneously by using VO. Combin… ▽ More Controlling functionalities, such as magnetism or ferroelectricity, by means of oxygen vacancies (VO) is a key issue for the future development of transition metal oxides. Progress in this field is currently addressed through VO variations and their impact on mainly one order parameter. Here we reveal a new mechanism for tuning both magnetism and ferroelectricity simultaneously by using VO. Combined experimental and density-functional theory studies of Eu0.5Ba0.5TiO3-δ, we demonstrate that oxygen vacancies create Ti3+ 3d1 defect states, mediating the ferromagnetic coupling between the localized Eu 4f7 spins, and increase an off-center displacement of Ti ions, enhancing the ferroelectric Curie temperature. The dual function of Ti sites also promises a magnetoelectric coupling in the Eu0.5Ba0.5TiO3-δ. △ Less

Submitted 28 August, 2017; originally announced August 2017.

Comments: Accepted by Physical Review B, 2017

Topological semimetal state and field-induced Fermi surface reconstruction in antiferromagnetic monopnictide NdSb

Authors: Yongjian Wang, J. H. Yu, Y. Q. Wang, C. Y. Xi, L. S. Ling, S. L. Zhang, J. R. Wang, Y. M. Xiong, Tao Han, Hui Han, Jun Yang, Jixiang Gong, Lei Luo, W. Tong, Lei Zhang, Zhe Qu, Y. Y. Han, W. K. Zhu, Li Pi, X. G. Wan, Changjin Zhang, Yuheng Zhang

Abstract: We report the experimental realization of Dirac semimetal state in NdSb, a material with antiferromagnetic ground state. The occurrence of topological semimetal state has been well supported by our band structure calculations and the experimental observation of chiral anomaly induced negative magnetoresistance. A field-induced Fermi surface reconstruction is observed, in response to the change of… ▽ More We report the experimental realization of Dirac semimetal state in NdSb, a material with antiferromagnetic ground state. The occurrence of topological semimetal state has been well supported by our band structure calculations and the experimental observation of chiral anomaly induced negative magnetoresistance. A field-induced Fermi surface reconstruction is observed, in response to the change of spin polarization. The observation of topological semimetal state in a magnetic material provides an opportunity to investigate the magneto-topological phenomena. △ Less

Submitted 15 March, 2018; v1 submitted 26 February, 2017; originally announced February 2017.

Journal ref: Physical Review B 97, 115133 (2018)

Stepwise quantized surface states and delayed Landau level hybridization in Co cluster-decorated BiSbTeSe2 topological insulator devices

Authors: Shuai Zhang, Li Pi, Rui Wang, Geliang Yu, Xing-Chen Pan, Zhongxia Wei, Jinglei Zhang, Chuanying Xi, Zhanbin Bai, Fucong Fei, Mingyu Wang, Jian Liao, Yongqing Li, Xuefeng Wang, Fengqi Song, Yuheng Zhang, Baigeng Wang, Dingyu Xing, Guanghou Wang

Abstract: In three-dimensional topological insulators (TIs), the nontrivial topology in their electronic bands casts a gapless state on their solid surfaces, using which dissipationless TI edge devices based on the quantum anomalous Hall (QAH) effect and quantum Hall (QH) effect have been demonstrated. Practical TI devices present a pair of parallel-transport topological surface states (TSSs) on their top a… ▽ More In three-dimensional topological insulators (TIs), the nontrivial topology in their electronic bands casts a gapless state on their solid surfaces, using which dissipationless TI edge devices based on the quantum anomalous Hall (QAH) effect and quantum Hall (QH) effect have been demonstrated. Practical TI devices present a pair of parallel-transport topological surface states (TSSs) on their top and bottom surfaces. However, due to the no-go theorem, the two TSSs always appear as a pair and are expected to quantize synchronously. Quantized transport of a separate Dirac channel is still desirable, but has never been observed in graphene even after intense investigation over a period of 13 years, with the potential aim of half-QHE. By depositing Co atomic clusters, we achieved stepwise quantization of the top and bottom surfaces in BiSbTeSe2 (BSTS) TI devices. Renormalization group flow diagrams13, 22 (RGFDs) reveal two sets of converging points (CVPs) in the (Gxy, Gxx) space, where the top surface travels along an anomalous quantization trajectory while the bottom surface retains 1/2 e2/h. This results from delayed Landau-level (LL) hybridization (DLLH) due to coupling between Co clusters and TSS Fermions. △ Less

Submitted 10 February, 2017; originally announced February 2017.

Journal ref: Nature Communications 8,977(2017)

Disruption of the accidental Dirac semimetal state in ZrTe$_{5}$ under hydrostatic pressure

Authors: J. L. Zhang, C. Y. Guo, X. D. Zhu, L. Ma, G. L. Zheng, Y. Q. Wang, L. Pi, Y. Chen, H. Q. Yuan, M. L. Tian

Abstract: We study the effect of hydrostatic pressure on the magnetotransport properties of the zirconium pentatelluride. The magnitude of resistivity anomaly gets enhanced with increasing pressure, but the transition temperature $T^{\ast}$ is almost independent of it. In the case of H $\parallel$ $b$, the quasi-linear magnetoresistance decreases drastically from 3300$\%$ (9 T) at ambient pressure to 400… ▽ More We study the effect of hydrostatic pressure on the magnetotransport properties of the zirconium pentatelluride. The magnitude of resistivity anomaly gets enhanced with increasing pressure, but the transition temperature $T^{\ast}$ is almost independent of it. In the case of H $\parallel$ $b$, the quasi-linear magnetoresistance decreases drastically from 3300$\%$ (9 T) at ambient pressure to 400$\%$ (14 T) at 2.5 GPa. Besides, the change of the quantum oscillation phase from topological nontrivial to trivial is revealed around 2 GPa. Both demonstrate that the pressure breaks the accidental Dirac node in ZrTe$_{5}$. For H $\parallel$ $c$, in contrast, subtle changes can be seen in the magnetoresistance and quantum oscillations. In the presence of pressure, ZrTe$_{5}$ evolves from a highly anisotropic to a nearly isotropic electronic system, which accompanies with the disruption of the accidental Dirac semimetal state. It supports the assumption that ZrTe$_{5}$ is a semi-3D Dirac system with linear dispersion along two directions and a quadratic one along the third. △ Less

Submitted 12 January, 2017; originally announced January 2017.

Journal ref: Phys. Rev. Lett. 118, 206601 (2017)

Evolution of Weyl orbit and quantum Hall effect in Dirac semimetal Cd3As2

Authors: Cheng Zhang, Awadhesh Narayan, Shiheng Lu, Jinglei Zhang, Huiqin Zhang, Zhuoliang Ni, Xiang Yuan, Yanwen Liu, Ju-Hyun Park, Enze Zhang, Weiyi Wang, Shanshan Liu, Long Cheng, Li Pi, Zhigao Sheng, Stefano Sanvito, Faxian Xiu

Abstract: Owing to the coupling between open Fermi arcs on opposite surfaces, topological Dirac semimetals exhibit a new type of cyclotron orbit in the surface states known as Weyl orbit. Here, by lowering the carrier density in Cd3As2 nanoplates, we observe a crossover from multiple- to single-frequency Shubnikov-de Haas (SdH) oscillations when subjected to out-of-plane magnetic field, indicating the domin… ▽ More Owing to the coupling between open Fermi arcs on opposite surfaces, topological Dirac semimetals exhibit a new type of cyclotron orbit in the surface states known as Weyl orbit. Here, by lowering the carrier density in Cd3As2 nanoplates, we observe a crossover from multiple- to single-frequency Shubnikov-de Haas (SdH) oscillations when subjected to out-of-plane magnetic field, indicating the dominant role of surface transport. With the increase of magnetic field, the SdH oscillations further develop into quantum Hall state with non-vanishing longitudinal resistance. By tracking the oscillation frequency and Hall plateau, we observe a Zeeman-related splitting and extract the Landau level index as well as sub-band number. Different from conventional two-dimensional systems, this unique quantum Hall effect may be related to the quantized version of Weyl orbits. Our results call for further investigations into the exotic quantum Hall states in the low-dimensional structure of topological semimetals. △ Less

Submitted 20 September, 2017; v1 submitted 18 December, 2016; originally announced December 2016.

Comments: Accepted by Nature Communications

Superconductivity and Charge Density Wave in ZrTe$_{3-x}$Se$_{x}$

Authors: Xiangde Zhu, Wei Ning, Lijun Li, Langsheng Ling, Ranran Zhang, Jinglei Zhang, Kefeng Wang, Yu Liu, Li Pi, Yongchang Ma, Haifeng Du, Minglian Tian, Yuping Sun, Cedomir Petrovic, Yuheng Zhang

Abstract: Charge density wave (CDW), the periodic modulation of the electronic charge density, will open a gap on the Fermi surface that commonly leads to decreased or vanishing conductivity. On the other hand superconductivity, a commonly believed competing order, features a Fermi surface gap that results in infinite conductivity. Here we report that superconductivity emerges upon Se doping in CDW conducto… ▽ More Charge density wave (CDW), the periodic modulation of the electronic charge density, will open a gap on the Fermi surface that commonly leads to decreased or vanishing conductivity. On the other hand superconductivity, a commonly believed competing order, features a Fermi surface gap that results in infinite conductivity. Here we report that superconductivity emerges upon Se doping in CDW conductor ZrTe$_{3}$ when the long range CDW order is gradually suppressed. Superconducting critical temperature $T_c(x)$ in ZrTe$_{3-x}$Se$_x$ (${0\leq}x\leq0.1$) increases up to 4 K plateau for $0.04$$\leq$$x$$\leq$$0.07$. Further increase in Se content results in diminishing $T_{c}$ and filametary superconductivity. The CDW modes from Raman spectra are observed in $x$ = 0.04 and 0.1 crystals, where signature of ZrTe$_{3}$ CDW order in resistivity vanishes. The electronic-scattering for high $T_{c}$ crystals is dominated by local CDW fluctuations at high temperures, the resistivity is linear up to highest measured $T=300K$ and contributes to substantial in-plane anisotropy. △ Less

Submitted 7 June, 2016; originally announced June 2016.

Comments: 15 pages, 4 figures

Journal ref: Scientific Reports 6, 26974 (2016)

Pressure induced re-emergence of superconductivity in superconducting topological insulator Sr0.065Bi2Se3

Authors: Yonghui Zhou, Xuliang Chen, Ranran Zhang, Jifeng Shao, Xuefei Wang, Chao An, Ying Zhou, Changyong Park, Wei Tong, Li Pi, Zhaorong Yang, Changjin Zhang, Yuheng Zhang

Abstract: The recent-discovered Sr$_x$Bi$_2$Se$_3$ superconductor provides an alternative and ideal material base for investigating possible topological superconductivity. Here, we report that in Sr$_{0.065}$Bi$_{2}$Se$_3$, the ambient superconducting phase is gradually depressed upon the application of external pressure. At high pressure, a second superconducting phase emerges at above 6 GPa, with a maximu… ▽ More The recent-discovered Sr$_x$Bi$_2$Se$_3$ superconductor provides an alternative and ideal material base for investigating possible topological superconductivity. Here, we report that in Sr$_{0.065}$Bi$_{2}$Se$_3$, the ambient superconducting phase is gradually depressed upon the application of external pressure. At high pressure, a second superconducting phase emerges at above 6 GPa, with a maximum $T_c$ value of $\sim$8.3 K. The joint investigations of the high-pressure synchrotron x-ray diffraction and electrical transport properties reveal that the re-emergence of superconductivity in Sr$_{0.065}$Bi$_{2}$Se$_3$ is closely related to the structural phase transition from ambient rhombohedral phase to high-pressure monoclinic phase around 6 GPa, and further to another high-pressure tetragonal phase above 25 GPa. △ Less

Submitted 27 April, 2016; originally announced April 2016.

Comments: 13 pages, 4 figures

Journal ref: Phys. Rev. B 93, 144514 (2016)

Single Crystal Growth of the New Pressure-induced-superconductor CrAs via Chemical Vapor Transport

Authors: Xiangde Zhu, Langsheng Ling, Yuyan Han, Junmin Xu, Yongjian Wang, Hongwei Zhang, Changjin Zhang, Li Pi, Yuheng Zhang

Abstract: Mono-arsenide CrAs, endures a helical anti-ferromagnetic order transition at~ 265 K under ambient pressure. Recently, pressure-induced-superconductivity was discovered vicinity to the helical anti-ferromagnetic order in CrAs[Wei Wu et al., Nature Communications 5, 5508 (2014).]. However, the size of crystal grown via tin flux method is as small as 1 mm in longest dimension. In this work, we report… ▽ More Mono-arsenide CrAs, endures a helical anti-ferromagnetic order transition at~ 265 K under ambient pressure. Recently, pressure-induced-superconductivity was discovered vicinity to the helical anti-ferromagnetic order in CrAs[Wei Wu et al., Nature Communications 5, 5508 (2014).]. However, the size of crystal grown via tin flux method is as small as 1 mm in longest dimension. In this work, we report the single crystal growth of CrAs with size of 1 * 5 * 1 mm3 via chemical vapor transport method and its physical properties. △ Less

Submitted 16 March, 2016; originally announced March 2016.

Comments: 9 pages, 3 figures

Journal ref: Journal of Alloys and Compounds 677, 57 (2016)