Showing 1–50 of 81 results for author: Weng, Y

Deciphering the lattice vibrational behaviors of CuInP2S6 by angle-resolved polarized Raman scattering

Authors: Yiqi Hu, Jun-Jie Zhang, Zhou Zhou, Shun Wang, Qiankun Li, Yanfei Hou, Tianhao Ying, Lingling Yang, Jingyao Zhang, Shuzhong Yin, Yuyan Weng, Shuai Dong, Jianlin Yao, Liang Fang, Lu You

Abstract: The layered van der Waals (vdW) ferroelectric CuInP2S6 (CIPS) exhibits unique cation hopping-driven phenomena that bring about unconventional properties with intriguing mechanisms and hold promises for advanced applications in nanoelectronics. However, an explicit analysis of its lattice dynamics and vibrational symmetries, pivotal for understanding the material's peculiar ferroelectric and ferroi… ▽ More The layered van der Waals (vdW) ferroelectric CuInP2S6 (CIPS) exhibits unique cation hopping-driven phenomena that bring about unconventional properties with intriguing mechanisms and hold promises for advanced applications in nanoelectronics. However, an explicit analysis of its lattice dynamics and vibrational symmetries, pivotal for understanding the material's peculiar ferroelectric and ferroionic behaviors, remains incomplete. Here, we employ angle-resolved polarized Raman spectroscopy in concert with first-principles calculations to systematically unravel the anisotropic lattice vibrations of CIPS single crystals. By analyzing the polarization-dependent Raman intensities, we determine the symmetry assignments and Raman tensors of all major vibrational modes, revealing good agreement with theoretical predictions. Furthermore, we demonstrate the utility of Raman spectroscopy as a sensitive and non-invasive probe for structural and ferroelectric order evolution, by examining temperature-driven phase transitions and thickness-dependent polarization suppression in CIPS. Our findings establish a foundational framework for correlating lattice dynamics with functional properties in CIPS and provide a methodological blueprint for studying other vdW ferroelectrics. △ Less

Submitted 23 December, 2025; originally announced December 2025.

Comments: Supplementary Table S2 with detialed displacement patterns of major vibrational modes (gif figures) can be found in Figshare (10.6084/m9.figshare.30944357)

Journal ref: Chinese Phys. Lett. 42 120805 (2025)

MatQnA: A Benchmark Dataset for Multi-modal Large Language Models in Materials Characterization and Analysis

Authors: Yonghao Weng, Liqiang Gao, Linwu Zhu, Jian Huang

Abstract: Recently, large language models (LLMs) have achieved remarkable breakthroughs in general domains such as programming and writing, and have demonstrated strong potential in various scientific research scenarios. However, the capabilities of AI models in the highly specialized field of materials characterization and analysis have not yet been systematically or sufficiently validated. To address this… ▽ More Recently, large language models (LLMs) have achieved remarkable breakthroughs in general domains such as programming and writing, and have demonstrated strong potential in various scientific research scenarios. However, the capabilities of AI models in the highly specialized field of materials characterization and analysis have not yet been systematically or sufficiently validated. To address this gap, we present MatQnA, the first multi-modal benchmark dataset specifically designed for material characterization techniques. MatQnA includes ten mainstream characterization methods, such as X-ray Photoelectron Spectroscopy (XPS), X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), etc. We employ a hybrid approach combining LLMs with human-in-the-loop validation to construct high-quality question-answer pairs, integrating both multiple-choice and subjective questions. Our preliminary evaluation results show that the most advanced multi-modal AI models (e.g., GPT-4.1, Claude 4, Gemini 2.5, and Doubao Vision Pro 32K) have already achieved nearly 90% accuracy on objective questions in materials data interpretation and analysis tasks, demonstrating strong potential for applications in materials characterization and analysis. The MatQnA dataset is publicly available at https://huggingface.co/datasets/richardhzgg/matQnA. △ Less

Submitted 14 September, 2025; originally announced September 2025.

Anomalous Landau levels and quantum oscillation in rotation-invariant insulators

Authors: Jianlong Fu, Chun Yu Weng, Hoi Chun Po

Abstract: Landau levels in certain models are known to protrude into the zero-field energy gap. These are known as anomalous Landau levels (ALLs). We study whether ALLs can lead to Fermi-surface like quantum oscillation in the absence of a zero-field Fermi surface. Focusing on two-dimensional multi-band low-energy models of electrons with continuous rotation symmetry, we show that an effective-band descript… ▽ More Landau levels in certain models are known to protrude into the zero-field energy gap. These are known as anomalous Landau levels (ALLs). We study whether ALLs can lead to Fermi-surface like quantum oscillation in the absence of a zero-field Fermi surface. Focusing on two-dimensional multi-band low-energy models of electrons with continuous rotation symmetry, we show that an effective-band description, akin to the semiclassical treatment of Landau level problems in metals, can be used to predict the Landau level spectrum, including possible ALLs. This description then describes ALL induced quantum oscillation for certain insulating models, which we demonstrate through numerical calculations. △ Less

Submitted 14 September, 2025; v1 submitted 28 January, 2025; originally announced January 2025.

Comments: 10 pages, 4 figures

Sliding-mediated ferroelectric phase transition in CuInP2S6 under pressure

Authors: Zhou Zhou, Jun-Jie Zhang, Gemma F. Turner, Stephen A. Moggach, Yulia Lekina, Samuel Morris, Shun Wang, Yiqi Hu, Qiankun Li, Jinshuo Xue, Zhijian Feng, Qingyu Yan, Yuyan Weng, Bin Xu, Yong Fang, Ze Xiang Shen, Liang Fang, Shuai Dong, Lu You

Abstract: Interlayer stacking order has recently emerged as a unique degree of freedom to control crystal symmetry and physical properties in two-dimensional van der Waals (vdW) materials and heterostructures. By tuning the layer stacking pattern, symmetry-breaking and electric polarization can be created in otherwise non-polar crystals, whose polarization reversal depends on the interlayer sliding motion.… ▽ More Interlayer stacking order has recently emerged as a unique degree of freedom to control crystal symmetry and physical properties in two-dimensional van der Waals (vdW) materials and heterostructures. By tuning the layer stacking pattern, symmetry-breaking and electric polarization can be created in otherwise non-polar crystals, whose polarization reversal depends on the interlayer sliding motion. Herein, we demonstrate that in a vdW layered ferroelectric, its existing polarization is closely coupled to the interlayer sliding driven by hydrostatic pressure. Through combined structural, electrical, vibrational characterizations, and theoretical calculations, we clearly map out the structural evolution of CuInP2S6 under pressure. A tendency towards a high polarization state is observed in the low-pressure region, followed by an interlayer-sliding-mediated phase transition from a monoclinic to a trigonal phase. Along the transformation pathway, the displacive-instable Cu ion serves as a pivot point that regulates the interlayer interaction in response to external pressure. The rich phase diagram of CuInP2S6, which is enabled by stacking orders, sheds light on the physics of vdW ferroelectricity and opens an alternative route to tailoring long-range order in vdW layered crystals. △ Less

Submitted 21 February, 2024; originally announced February 2024.

Comments: 22 pages, 4 figures

Journal ref: Applied Physics Reviews 11, 011414 (2024)

Strain tuned magnetotransport of Jeff=1/2 antiferromagnetic Sr2IrO4 thin films

Authors: N. Hu, Y. K. Weng, K. Chen, B. You, Y. Liu, Y. T. Chang, R. Xiong, S. Dong, C. L. Lu

Abstract: In this work, we report observation of strain effect on physical properties of Sr2IrO4 thin films grown on SrTiO3 (001) and LaAlO3 (001) substrates. It is found that the film on LaAlO3 with compressive strain has a lower antiferromagnetic transition temperature (TN~210 K) than the film on SrTiO3 (TN~230 K) with tensile strain, which is probably caused by modified interlayer coupling. Interestingly… ▽ More In this work, we report observation of strain effect on physical properties of Sr2IrO4 thin films grown on SrTiO3 (001) and LaAlO3 (001) substrates. It is found that the film on LaAlO3 with compressive strain has a lower antiferromagnetic transition temperature (TN~210 K) than the film on SrTiO3 (TN~230 K) with tensile strain, which is probably caused by modified interlayer coupling. Interestingly, magnetoresistance due to pseudospin-flip of the film on LaAlO3 is much larger than that of tensile-strained film on SrTiO3, and robust anisotropic magnetoresistance is observed in the former, but H-driven reversal behavior is seen in the latter. By performing first principles calculations, it is revealed that epitaxial strain plays an efficient role in tuning the canting angle of Jeff=1/2 moments and thus net moment at every IrO2 layer, responsible for the difference in magnetoresistance between the films. The reversal of anisotropic magnetoresistance in the thin film on SrTiO3 can be ascribed to stabilization of a metastable stable with smaller bandgap as the Jeff=1/2 moments are aligned along the diagonal of basal plane by H. However, theoretical calculations reveal much higher magnetocrystalline anisotropy energy in the film on LaAlO3. This causes difficulties to drive the Jeff=1/2 moments to reach the diagonal and thereby the metastable state, explaining the distinct anisotropic magnetoresistance between two samples in a qualitative sense. Our findings indicate that strain can be a highly efficient mean to engineer the functionalities of Jeff=1/2 antiferromagnet Sr2IrO4. △ Less

Submitted 16 September, 2023; originally announced September 2023.

Comments: 21 pages and 5 figures

Journal ref: Materials Today Physics 27, 100809 (2022)

Colossal linear magnetoelectricity in polar magnet Fe2Mo3O8

Authors: Yuting Chang, Yakui Weng, Yunlong Xie, Bin You, Junfeng Wang, Liang Li, Jun-Ming Liu, Shuai Dong, Chengliang Lu

Abstract: Linear magnetoelectric effect is an attractive phenomenon in condensed matters and provides indispensable technological functionalities. Here a colossal linear magnetoelectric effect with diagonal component alfa_33 reaching up to ~480 ps/m is reported in a polar magnet Fe2Mo3O8, and this effect can persist in a broad range of magnetic field (~20 T) and is orders of magnitude larger than reported v… ▽ More Linear magnetoelectric effect is an attractive phenomenon in condensed matters and provides indispensable technological functionalities. Here a colossal linear magnetoelectric effect with diagonal component alfa_33 reaching up to ~480 ps/m is reported in a polar magnet Fe2Mo3O8, and this effect can persist in a broad range of magnetic field (~20 T) and is orders of magnitude larger than reported values in literature. Such an exceptional experimental observation can be well reproduced by a theoretical model affirmatively unveiling the vital contributions from the exchange striction, while the sign difference of magnetocrystalline anisotropy can also be reasonably figured out. △ Less

Submitted 16 September, 2023; originally announced September 2023.

Comments: 14 pages and 4 figures

Journal ref: Phys. Rev. Lett. 131, 136701 (2023)

Optical bulk-boundary dichotomy in a quantum spin Hall insulator

Authors: Junfeng Han, Pengcheng Mao, Hailong Chen, Jia-Xin Yin, Maoyuan Wang, Dongyun Chen, Yongkai Li, Jingchuan Zheng, Xu Zhang, Dashuai Ma, Qiong Ma, Zhi-Ming Yu, Jinjian Zhou, Cheng-Cheng Liu, Yeliang Wang, Shuang Jia, Yuxiang Weng, M. Zahid Hasan, Wende Xiao, Yugui Yao

Abstract: The bulk-boundary correspondence is a key concept in topological quantum materials. For instance, a quantum spin Hall insulator features a bulk insulating gap with gapless helical boundary states protected by the underlying Z2 topology. However, the bulk-boundary dichotomy and distinction are rarely explored in optical experiments, which can provide unique information about topological charge carr… ▽ More The bulk-boundary correspondence is a key concept in topological quantum materials. For instance, a quantum spin Hall insulator features a bulk insulating gap with gapless helical boundary states protected by the underlying Z2 topology. However, the bulk-boundary dichotomy and distinction are rarely explored in optical experiments, which can provide unique information about topological charge carriers beyond transport and electronic spectroscopy techniques. Here, we utilize mid-infrared absorption micro-spectroscopy and pump-probe micro-spectroscopy to elucidate the bulk-boundary optical responses of Bi4Br4, a recently discovered room-temperature quantum spin Hall insulator. Benefiting from the low energy of infrared photons and the high spatial resolution, we unambiguously resolve a strong absorption from the boundary states while the bulk absorption is suppressed by its insulating gap. Moreover, the boundary absorption exhibits a strong polarization anisotropy, consistent with the one-dimensional nature of the topological boundary states. Our infrared pump-probe microscopy further measures a substantially increased carrier lifetime for the boundary states, which reaches one nanosecond scale. The nanosecond lifetime is about one to two orders longer than that of most topological materials and can be attributed to the linear dispersion nature of the helical boundary states. Our findings demonstrate the optical bulk-boundary dichotomy in a topological material and provide a proof-of-principal methodology for studying topological optoelectronics. △ Less

Submitted 13 February, 2023; v1 submitted 7 February, 2023; originally announced February 2023.

Comments: 26 pages, 4 figures

Journal ref: Science Bulletin (2023)

Ferroelectric control of spin-polarized two-dimensional electron gas

Authors: Yakui Weng, Wei Niu, Xin Huang, Ming An, Shuai Dong

Abstract: Spin-polarized two-dimensional electron gas (2DEG) at oxide interfaces is an emerging physical phenomenon, which is technologically important for potential device applications. However, most previous relevant studies only focused on the creation and characterization of the spin-polarized 2DEG. To push forward the device applications, the control of spin-polarized 2DEG by electric field is an impor… ▽ More Spin-polarized two-dimensional electron gas (2DEG) at oxide interfaces is an emerging physical phenomenon, which is technologically important for potential device applications. However, most previous relevant studies only focused on the creation and characterization of the spin-polarized 2DEG. To push forward the device applications, the control of spin-polarized 2DEG by electric field is an important step. Here, a model system based on antiferromagnetic and ferroelectric perovskites, i.e., YTiO3/PbTiO3 superlattice, is designed to manipulate the spin-polarized 2DEG. By switching the direction of polarization, the spin-polarized 2DEG can be effectively tuned for both symmetric interfaces and asymmetric polar interfaces. △ Less

Submitted 27 October, 2021; originally announced October 2021.

Comments: 5 pages, 4 figures

Journal ref: Phys. Rev. B 103, 214101 (2021)

Manipulation of Jeff=3/2 states by tuning the tetragonal distortion

Authors: Yakui Weng, Shuai Dong

Abstract: The spin-orbit entangled quantum states in 4d/5d compounds, e.g., the Jeff=1/2 and Jeff=3/2 states, have attracted great interests for their unique physical roles in unconventional superconductivity and topological states. Here, the key role of tetragonal distortion is clarified, which determines the ground states of 4d1/5d1 systems to be the Jeff=3/2 one (e.g. K2NbCl6) or S=1/2 one (e.g. Rb2NbCl6… ▽ More The spin-orbit entangled quantum states in 4d/5d compounds, e.g., the Jeff=1/2 and Jeff=3/2 states, have attracted great interests for their unique physical roles in unconventional superconductivity and topological states. Here, the key role of tetragonal distortion is clarified, which determines the ground states of 4d1/5d1 systems to be the Jeff=3/2 one (e.g. K2NbCl6) or S=1/2 one (e.g. Rb2NbCl6). By tuning the tetragonal distortion via epitaxial strain, the occupation weights of dxy/dyz/dxz orbitals can be subtly modulated, competing with the spin-orbit coupling. Consequently, quantum phase transitions between S=1/2 state and Jeff=3/2 state, as well as between different Jeff=3/2 states, can be achieved, resulting in significant changes of local magnetic moments. Our prediction points out a reliable route to engineer new functionality of Jeff states in these quantum materials. △ Less

Submitted 27 October, 2021; originally announced October 2021.

Comments: 6 pages, 4 figures

Journal ref: Phys. Rev. B 104, 165150 (2021)

Antinodal kink in the band dispersion of electron-doped cuprate ${\rm La}_{2-x}{\rm Ce}_x{\rm CuO}_{4\pmδ}$

Authors: C. Y. Tang, Z. F. Lin, J. X. Zhang, X. C. Guo, J. Y. Guan, S. Y. Gao, Z. C. Rao, J. Zhao, Y. B. Huang, T. Qian, Z. Y. Weng, K. Jin, Y. J. Sun, H. Ding

Abstract: Angle-resolved photoemission spectroscopy (ARPES) measurements have established the phenomenon of kink in band dispersion of high-$T_{\rm c}$ cuprate superconductors. However, systematic studies of the kink in electron-doped cuprates are still lacking experimentally. We performed $in$-$situ$ ARPES measurements on ${\rm La}_{2-x}{\rm Ce}_x{\rm CuO}_{4\pmδ}$ (LCCO) thin films over a wide electron do… ▽ More Angle-resolved photoemission spectroscopy (ARPES) measurements have established the phenomenon of kink in band dispersion of high-$T_{\rm c}$ cuprate superconductors. However, systematic studies of the kink in electron-doped cuprates are still lacking experimentally. We performed $in$-$situ$ ARPES measurements on ${\rm La}_{2-x}{\rm Ce}_x{\rm CuO}_{4\pmδ}$ (LCCO) thin films over a wide electron doping ($n$) range from 0.05 to 0.23. While the nodal kink is nearly invisible, an antinodal kink around 45 meV, surviving above 200 K, is observed for $n\sim0.05-0.19$, whose position is roughly independent of doping. The fact that the antinodal kink observed at high temperatures and in the highly overdoped region favors the phonon mechanism with contributions from the Cu-O bond-stretching mode and the out-of-plane oxygen buckling mode. Our results also suggest that the antinodal kink of LCCO is only weakly coupled to its superconductivity. △ Less

Submitted 25 October, 2021; originally announced October 2021.

Suppression of Antiferromagnetism in Electron-Doped Cuprate $T'$-${\rm La}_{2-x}{\rm Ce}_x\rm {CuO}_{4\pmδ}$

Authors: C. Y. Tang, Z. F. Lin, J. X. Zhang, X. C. Guo, J. Y. Guan, S. Y. Gao, Z. C. Rao, J. Zhao, Y. B. Huang, T. Qian, Z. Y. Weng, K. Jin, Y. J. Sun, H. Ding

Abstract: We performed systematic angle-resolved photoemission spectroscopy measurements $in$-$situ$ on $T'$-${\rm La}_{2-x}{\rm Ce}_x\rm {CuO}_{4\pmδ}$ (LCCO) thin films over the extended doping range prepared by the refined ozone/vacuum annealing method. Electron doping level ($n$), estimated from the measured Fermi surface volume, varies from 0.05 to 0.23, which covers the whole superconducting dome. We… ▽ More We performed systematic angle-resolved photoemission spectroscopy measurements $in$-$situ$ on $T'$-${\rm La}_{2-x}{\rm Ce}_x\rm {CuO}_{4\pmδ}$ (LCCO) thin films over the extended doping range prepared by the refined ozone/vacuum annealing method. Electron doping level ($n$), estimated from the measured Fermi surface volume, varies from 0.05 to 0.23, which covers the whole superconducting dome. We observed an absence of the insulating behavior around $n \sim$ 0.05 and the Fermi surface reconstruction shifted to $n \sim$ 0.11 in LCCO compared to that of other electron-doped cuprates at around 0.15, suggesting that antiferromagnetism is strongly suppressed in this material. The possible explanation may lie in the enhanced -$t'$ /$t$ in LCCO for the largest $\rm{La^{3+}}$ ionic radius among all the Lanthanide elements. △ Less

Submitted 10 June, 2021; originally announced June 2021.

Strong tuning of magnetism and electronic structure by spin orientation

Authors: Yakui Weng, Xing'ao Li, Shuai Dong

Abstract: To efficiently manipulate magnetism is a key physical issue for modern condensed matter physics, which is also crucial for magnetic functional applications. Most previous relevant studies rely on the tuning of spin texture, while the spin orientation is often negligible. As an exception, spin-orbit coupled $J_{\rm eff}$ states of $4d$/$5d$ electrons provide an ideal platform for emergent quantum e… ▽ More To efficiently manipulate magnetism is a key physical issue for modern condensed matter physics, which is also crucial for magnetic functional applications. Most previous relevant studies rely on the tuning of spin texture, while the spin orientation is often negligible. As an exception, spin-orbit coupled $J_{\rm eff}$ states of $4d$/$5d$ electrons provide an ideal platform for emergent quantum effects. However, many expectations have not been realized due to the complexities of real materials. Thus the pursuit for more ideal $J_{\rm eff}$ states remains ongoing. Here a near-ideal $J_{\rm eff}$=$3/2$ Mott insulating phase is predicted in the family of hexachloro niobates, which avoid some common drawbacks of perovskite oxides. The local magnetic moment is nearly compensated between spin and orbital components, rendering exotic recessive magnetism. More interestingly, the electronic structure and magnetism can be strongly tuned by rotating spin axis, which is rare but crucial for spintronic applications. △ Less

Submitted 20 October, 2020; originally announced October 2020.

Comments: 6 pages, 4 figures

Journal ref: Physical Review B 102, 180401(R) (2020)

Dynamical robustness of topological end states in nonreciprocal Su-Schrieffer-Heeger models with open boundary conditions

Authors: Li-Jun Lang, Yijiao Weng, Yunhui Zhang, Enhong Cheng, Qixia Liang

Abstract: For non-Hermitian quantum models, the dynamics is apparently not reflected by the static properties, e.g., the complex energy spectrum, because of the nonorthogonality of the right eigenvectors, the nonunitarity of the time evolution, the breakdown of the adiabatic theory, etc., but in experiments the time evolution of an initial state is commonly used. Here, we pay attention to the dynamics of an… ▽ More For non-Hermitian quantum models, the dynamics is apparently not reflected by the static properties, e.g., the complex energy spectrum, because of the nonorthogonality of the right eigenvectors, the nonunitarity of the time evolution, the breakdown of the adiabatic theory, etc., but in experiments the time evolution of an initial state is commonly used. Here, we pay attention to the dynamics of an initial end state in nonreciprocal Su-Schrieffer-Heeger models under open boundary conditions, and we find that it is dynamically more robust than its Hermitian counterpart, because the non-Hermitian skin effect can suppress the part leaking to the bulk sites. To observe this, we propose a classical electric circuit with only a few passive inductors and capacitors, the mapping of which to the quantum model is established. This work explains how the non-Hermitian skin effect enhances the robustness of the topological end state, and it offers an easy way, via the classical electric circuit, of studying the nonreciprocal quantum dynamics, which may stimulate more dynamical studies of non-Hermitian models in other platforms. △ Less

Submitted 8 January, 2021; v1 submitted 28 August, 2020; originally announced August 2020.

Comments: 8 pages, 4 figures, published version

Journal ref: Phys. Rev. B 103, 014302 (2021)

Ultralong carrier lifetime of topological edge states in a-Bi4Br4

Authors: Pengcheng Mao, Junfeng Han, Jingchuan Zheng, Jinjian Zhou, Zhiming Yu, Wende Xiao, Dongyun Chen, Gang Wang, Jie Ma, Cheng-cheng Liu, Xiang Li, Qinsheng Wang, Junxi Duan, Hailong Chen, Yuxiang Weng, Yugui Yao

Abstract: The rising of quantum spin Hall insulators (QSHI) in two-dimensional (2D) systems has been attracting significant interest in current research, for which the 1D helical edge states, a hallmark of QSHI, are widely expected to be a promising platform for next-generation optoelectronics. However, the dynamics of the 1D edge states has not yet been experimentally addressed. Here, we report the observa… ▽ More The rising of quantum spin Hall insulators (QSHI) in two-dimensional (2D) systems has been attracting significant interest in current research, for which the 1D helical edge states, a hallmark of QSHI, are widely expected to be a promising platform for next-generation optoelectronics. However, the dynamics of the 1D edge states has not yet been experimentally addressed. Here, we report the observation of optical response of the topological helical edge states in a-Bi4Br4, using the infrared-pump infrared-probe microscopic spectroscopy. Remarkably, we observe that the carrier lifetime of the helical edge states reaches nanosecond-scale at room temperature, which is about 2 - 3 orders longer than that of most 2D topological surface states and is even comparable with that of the well developed optoelectronics semiconductors used in modern industry. The ultralong carrier lifetime of the topological edge states may be attributed to their helical and 1D nature. Our findings not only provide an ideal material for further investigations of the carrier dynamics of 1D helical edge states but also pave the way for its application in optoelectronics. △ Less

Submitted 1 July, 2020; originally announced July 2020.

Comments: 18 pages, 4 figures, 57 references

Continuous doping of a cuprate surface: new insights from in-situ ARPES

Authors: Y. G. Zhong, J. Y. Guan, X. Shi, J. Zhao, Z. C. Rao, C. Y. Tang, H. J. Liu, G. D. Gu, Z. Y. Weng, Z. Q. Wang, T. Qian, Y. J. Sun, H. Ding

Abstract: The cuprate superconductors distinguish themselves from the conventional superconductors in that a small variation in the carrier doping can significantly change the superconducting transition temperature (T_c), giving rise to a superconducting dome where a pseudogap (ref. 1,2) emerges in the underdoped region and a Fermi liquid appears in the overdoped region. Thus a systematic study of the prope… ▽ More The cuprate superconductors distinguish themselves from the conventional superconductors in that a small variation in the carrier doping can significantly change the superconducting transition temperature (T_c), giving rise to a superconducting dome where a pseudogap (ref. 1,2) emerges in the underdoped region and a Fermi liquid appears in the overdoped region. Thus a systematic study of the properties over a wide doping range is critical for understanding the superconducting mechanism. Here, we report a new technique to continuously dope the surface of Bi2Sr2CaCu2O8+x through an ozone/vacuum annealing method. Using in-situ ARPES, we obtain precise quantities of energy gaps and the coherent spectral weight over a wide range of doping. We discover that the d-wave component of the quasiparticle gap is linearly proportional to the Nernst temperature that is the onset of superconducting vortices (ref. 3), strongly suggesting that the emergence of superconducting pairing is concomitant with the onset of free vortices, with direct implications for the onset of superconducting phase coherence at T_c and the nature of the pseudogap phenomena. △ Less

Submitted 2 June, 2018; v1 submitted 16 May, 2018; originally announced May 2018.

Comments: 23 pages, 10 figures, 1 table, supplementary materials included

Journal ref: Phys. Rev. B 98, 140507 (2018)

Appearance and disappearance of ferromagnetism in ultra-thin LaMnO$_3$ on SrTiO$_3$ substrate: a viewpoint from first-principles

Authors: Ming An, Yakui Weng, Huimin Zhang, Jun-Jie Zhang, Yang Zhang, Shuai Dong

Abstract: The intrinsic magnetic state (ferromagnetic or antiferromagnetic) of ultra-thin LaMnO$_3$ films on the mostly used SrTiO$_3$ substrate is a long-existing question under debate. Either strain effect or non-stoichiometry was argued to be responsible for the experimental ferromagnetism. In a recent experiment [Science \textbf{349}, 716 (2015)], one more mechanism, namely the self-doping due to polar… ▽ More The intrinsic magnetic state (ferromagnetic or antiferromagnetic) of ultra-thin LaMnO$_3$ films on the mostly used SrTiO$_3$ substrate is a long-existing question under debate. Either strain effect or non-stoichiometry was argued to be responsible for the experimental ferromagnetism. In a recent experiment [Science \textbf{349}, 716 (2015)], one more mechanism, namely the self-doping due to polar discontinuity, was argued to be the driving force of ferromagnetism beyond the critical thickness. Here systematic first-principles calculations have been performed to check these mechanisms in ultra-thin LaMnO$_3$ films as well as superlattices. Starting from the very precise descriptions of both LaMnO$_3$ and SrTiO$_3$, it is found that the compressive strain is the dominant force for the appearance of ferromagnetism, while the open surface with oxygen vacancies leads to the suppression of ferromagnetism. Within LaMnO$_3$ layers, the charge reconstructions involve many competitive factors and certainly go beyond the intuitive polar catastrophe model established for LaAlO$_3$/SrTiO$_3$ heterostructures. Our study not only explains the long-term puzzle regarding the magnetism of ultra-thin LaMnO$_3$ films, but also shed light on how to overcome the notorious magnetic dead layer in ultra-thin manganites. △ Less

Submitted 30 November, 2017; originally announced November 2017.

Comments: 10 pages, 8 figures

Journal ref: Physical Review B 96, 235112 (2017)

(LaTiO$_3$)$_n$/(LaVO$_3$)$_n$ as a model system for unconventional charge transfer and polar metallicity

Authors: Yakui Weng, Jun-Jie Zhang, Bin Gao, Shuai Dong

Abstract: At interfaces between oxide materials, lattice and electronic reconstructions always play important roles in exotic phenomena. In this study, the density functional theory and maximally localized Wannier functions are employed to investigate the (LaTiO$_3$)$_n$/(LaVO$_3$)$_n$ magnetic superlattices. The electron transfer from Ti$^{3+}$ to V$^{3+}$ is predicted, which violates the intuitive band al… ▽ More At interfaces between oxide materials, lattice and electronic reconstructions always play important roles in exotic phenomena. In this study, the density functional theory and maximally localized Wannier functions are employed to investigate the (LaTiO$_3$)$_n$/(LaVO$_3$)$_n$ magnetic superlattices. The electron transfer from Ti$^{3+}$ to V$^{3+}$ is predicted, which violates the intuitive band alignment based on the electronic structures of LaTiO$_3$ and LaVO$_3$. Such unconventional charge transfer quenches the magnetism of LaTiO$_3$ layer mostly and leads to metal-insulator transition in the $n=1$ superlattice when the stacking orientation is altered. In addition, the compatibility among the polar structure, ferrimagnetism, and metallicity is predicted in the $n=2$ superlattice. △ Less

Submitted 30 March, 2017; originally announced March 2017.

Comments: 8 pages, 5 figures

Journal ref: Physical Review B 95, 155117 (2017)

Magnetic and electronic properties of La$_3M$O$_7$ and possible polaron formation in hole-doped La$_3M$O$_7$ ($M$=Ru and Os)

Authors: Bin Gao, Yakui Weng, Jun-Jie Zhang, Huimin Zhang, Yang Zhang, Shuai Dong

Abstract: Oxides with $4d$/$5d$ transition metal ions are physically interesting for their particular crystalline structures as well as the spin-orbit coupled electronic structures. Recent experiments revealed a series of $4d$/$5d$ transition metal oxides $R_3M$O$_7$ ($R$: rare earth; $M$: $4d$/$5d$ transition metal) with unique quasi-one-dimensional $M$ chains. Here first-principles calculations have been… ▽ More Oxides with $4d$/$5d$ transition metal ions are physically interesting for their particular crystalline structures as well as the spin-orbit coupled electronic structures. Recent experiments revealed a series of $4d$/$5d$ transition metal oxides $R_3M$O$_7$ ($R$: rare earth; $M$: $4d$/$5d$ transition metal) with unique quasi-one-dimensional $M$ chains. Here first-principles calculations have been performed to study the electronic structures of La$_3$OsO$_7$ and La$_3$RuO$_7$. Our study confirm both of them to be Mott insulating antiferromagnets with identical magnetic order. The reduced magnetic moments, which are much smaller than the expected value for ideal high-spin state ($3$ $t_{2g}$ orbitals occupied), are attributed to the strong $p-d$ hybridization with oxygen ions, instead of the spin-orbit coupling. The Ca-doping to La$_3$OsO$_7$ and La$_3$RuO$_7$ can not only modulate the nominal carrier density but also affect the orbital order as well as the local distortions. The Coulombic attraction and particular orbital order would prefer to form polarons, which might explain the puzzling insulating behavior of doped $5d$ transition metal oxides. In addition, our calculation predict that the Ca-doping can trigger ferromagnetism in La$_3$RuO$_7$ but not in La$_3$OsO$_7$. △ Less

Submitted 26 January, 2017; originally announced January 2017.

Comments: 12 pages, 4 figures

Journal ref: J. Phys.: Condens Matter 29, 095803 (2017)

Nanostructure analysis of InGaN/GaN quantum wells based on semi-polar-faced GaN nanorods

Authors: Yu-Sheng Huang, Yu-Hsin Weng, Yung-Sheng Chen, Shih-Wei Feng, Chie-Tong Kuo, Ming-Yen Lu, Yung-Chen Cheng, Ya-Ping Hsieh, Hsiang-Chen Wang

Abstract: We demonstrate a series of InGaN/GaN double quantum well nanostructure elements. We grow a layer of 2 μm undoped GaN template on top of a (0001)-direction sapphire substrate. A 100 nm SiO2 thin film is deposited on top as a masking pattern layer. This layer is then covered with a 300 nm aluminum layer as the anodic aluminum oxide (AAO) hole pattern layer. After oxalic acid etching, we transfer the… ▽ More We demonstrate a series of InGaN/GaN double quantum well nanostructure elements. We grow a layer of 2 μm undoped GaN template on top of a (0001)-direction sapphire substrate. A 100 nm SiO2 thin film is deposited on top as a masking pattern layer. This layer is then covered with a 300 nm aluminum layer as the anodic aluminum oxide (AAO) hole pattern layer. After oxalic acid etching, we transfer the hole pattern from the AAO layer to the SiO2 layer by reactive ion etching. Lastly, we utilize metal-organic chemical vapor deposition to grow GaN nanorods approximately 1.5 μm in size. We then grow two layers of InGaN/GaN double quantum wells on the semi-polar face of the GaN nanorod substrate under different temperatures. We then study the characteristics of the InGaN/GaN quantum wells formed on the semi-polar faces of GaN nanorods. We report the following findings from our study: first, using SiO2 with repeating hole pattern, we are able to grow high-quality GaN nanorods with diameters of approximately 80-120 nm; second, photoluminescence (PL) measurements enable us to identify Fabry-Perot effect from InGaN/GaN quantum wells on the semi-polar face. We calculate the quantum wells' cavity thickness with obtained PL measurements. Lastly, high resolution TEM images allow us to study the lattice structure characteristics of InGaN/GaN quantum wells on GaN nanorod and identify the existence of threading dislocations in the lattice structure that affects the GaN nanorod's growth mechanism. △ Less

Submitted 13 December, 2016; originally announced December 2016.

Comments: 8 pages, 6 figures

Synthesis, Optical, and Magnetic Properties of Ba$_2$Ni$_3$F$_{10}$ Nanowires

Authors: Shuang Zhou, Ji Wang, Yakui Weng, Zhangting Wu, Zhenhua Ni, Qingyu Xu, Jun Du, Shuai Dong

Abstract: A low temperature hydrothermal route has been developed, and pure phase Ba$_2$Ni$_3$F$_{10}$ nanowires have been successfully prepared under the optimized conditions. Under the 325 nm excitation, the Ba$_2$Ni$_3$F$_{10}$ nanowires exhibit three emission bands with peak positions locating at 360 nm, 530 nm, and 700 nm, respectively. Combined with the first-principles calculations, the photoluminesc… ▽ More A low temperature hydrothermal route has been developed, and pure phase Ba$_2$Ni$_3$F$_{10}$ nanowires have been successfully prepared under the optimized conditions. Under the 325 nm excitation, the Ba$_2$Ni$_3$F$_{10}$ nanowires exhibit three emission bands with peak positions locating at 360 nm, 530 nm, and 700 nm, respectively. Combined with the first-principles calculations, the photoluminescence property can be explained by the electron transitions between the t2g and eg orbitals. Clear hysteresis loops observed below the temperature of 60 K demonstrates the weak ferromagnetism in Ba$_2$Ni$_3$F$_{10}$ nanowires, which has been attributed to the surface strain of nanowires. Exchange bias with blocking temperature of 55 K has been observed, which originates from the magnetization pinning under the cooling field due to antiferromagnetic core/weak ferromagnetic shell structure of Ba2Ni3F10 nanowires. △ Less

Submitted 15 November, 2016; originally announced November 2016.

Journal ref: ACS Applied Materials & Interfaces 8, 26213-26219 (2016)

Block antiferromagnetism and possible ferroelectricity in KFe$_2$Se$_2$

Authors: Yang Zhang, Huimin Zhang, Yakui Weng, Lingfang Lin, Xiaoyan Yao, Shuai Dong

Abstract: Superconductors and multiferroics are two of the hottest branches in condensed matter physics. The connections between those two fields are fundamentally meaningful to unify the physical rules of correlated electrons. Recently, BaFe$_2$Se$_3$, was predicted to be multiferroic [Phys. Rev. Lett. 113, 187204 (2014)] due to its unique one-dimensional block-type antiferromagnetism. Here, another iron-s… ▽ More Superconductors and multiferroics are two of the hottest branches in condensed matter physics. The connections between those two fields are fundamentally meaningful to unify the physical rules of correlated electrons. Recently, BaFe$_2$Se$_3$, was predicted to be multiferroic [Phys. Rev. Lett. 113, 187204 (2014)] due to its unique one-dimensional block-type antiferromagnetism. Here, another iron-selenide KFe$_2$Se$_2$, a parent state of iron-based superconductor, is predicted to be multiferroic. Its two-dimensional block-type antiferromagnetism can generate a moderate electric dipole for each Fe-Se layer via the Fe-Se-Fe exchange striction. Different stacking configurations of these magnetic blocks give closely proximate energies and thus the ground state of KFe$_2$Se$_2$ may be switchable between antiferroelectric and ferroelectric phases. △ Less

Submitted 22 October, 2016; v1 submitted 8 September, 2016; originally announced September 2016.

Comments: 5 pages, 4 figures

Journal ref: Physica Status Solidi-Rapid Research Letters 10, 757-761 (2016)

Inversion of ferrimagnetic magnetization by ferroelectric switching via a novel magnetoelectric coupling

Authors: Yakui Weng, Lingfang Lin, Elbio Dagotto, Shuai Dong

Abstract: Although several multiferroic materials/heterostructures have been extensively studied, finding strong magnetoelectric couplings for the electric field control of the magnetization remains challenging. Here, a novel interfacial magnetoelectric coupling based on three components (ferroelectric dipole, magnetic moment, and antiferromagnetic order) is analytically formulated. As an extension of carri… ▽ More Although several multiferroic materials/heterostructures have been extensively studied, finding strong magnetoelectric couplings for the electric field control of the magnetization remains challenging. Here, a novel interfacial magnetoelectric coupling based on three components (ferroelectric dipole, magnetic moment, and antiferromagnetic order) is analytically formulated. As an extension of carrier-mediated magnetoelectricity, the new coupling is shown to induce an electric-magnetic hysteresis loop. Realizations employing BiFeO$_3$ bilayers grown along the [$111$] axis are proposed. Without involving magnetic phase transitions, the magnetization orientation can be switched by the carrier modulation driven by the field effect, as confirmed using first-principles calculations. △ Less

Submitted 23 June, 2016; originally announced June 2016.

Comments: 10 pages, 8 figures

Journal ref: Phys. Rev. Lett. 117, 037601 (2016)

Superconductivity in HfTe5 Induced via Pressures

Authors: Y. Liu, Y. J. Long, L. X. Zhao, S. M. Nie, S. J. Zhang, Y. X. Weng, M. L. Jin, W. M. Li, Q. Q. Liu, Y. W. Long, R. C. Yu, X. L. Fen, Q. Li, H. M. Weng, X. Dai, Z. Fang, G. F. Chen, C. Q. Jin

Abstract: Recently, ZrTe5 and HfTe5 are theoretically studied to be the most promising layered topological insulators since they are both interlayer weakly bonded materials and also with a large bulk gap in the single layer. It paves a new way for the study of novel topological quantum phenomenon tuned via external parameters. Here, we report the discovery of superconductivity and properties evolution in Hf… ▽ More Recently, ZrTe5 and HfTe5 are theoretically studied to be the most promising layered topological insulators since they are both interlayer weakly bonded materials and also with a large bulk gap in the single layer. It paves a new way for the study of novel topological quantum phenomenon tuned via external parameters. Here, we report the discovery of superconductivity and properties evolution in HfTe5 single crystal induced via pressures. Our experiments indicated that anomaly resistance peak moves to low temperature first before reverses to high temperature followed by disappearance which is opposite to the low pressure effect on ZrTe5. HfTe5 became superconductive above ~5.5 GPa up to at least 35 GPa in the measured range. The highest superconducting transition temperature (Tc) around 5 K was achieved at 20 GPa. High pressure Raman revealed that new modes appeared around pressure where superconductivity occurs. Crystal structure studies shown that the superconductivity is related to the phase transition from Cmcm structure to monoclinic C2/m structure. The second phase transition from C2/m to P-1 structure occurs at 12 GPa. The combination of transport, structure measurement and theoretical calculations enable a completely phase diagram of HfTe5 at high pressures. △ Less

Submitted 1 March, 2016; originally announced March 2016.

Comments: 19 pages+8 Figures with supplementary materials

Strong room-temperature blue-violet photoluminescence of multiferroic BaMnF$_4$

Authors: Shuang Zhou, Yakui Weng, Zhangting Wu, Jinlong Wang, Lingzhi Wu, Zhenhua Ni, Qingyu Xu, Shuai Dong

Abstract: BaMnF$_4$ microsheets have been prepared by hydrothermal method. Strong room-temperature blue-violet photoluminescence has been observed (absolute luminescence quantum yield 67%), with two peaks located at 385 nm and 410 nm, respectively. More interestingly, photon self-absorption phenomenon has been observed, leading to unusual abrupt drop of luminescence intensity at wavelength of 400 nm. To und… ▽ More BaMnF$_4$ microsheets have been prepared by hydrothermal method. Strong room-temperature blue-violet photoluminescence has been observed (absolute luminescence quantum yield 67%), with two peaks located at 385 nm and 410 nm, respectively. More interestingly, photon self-absorption phenomenon has been observed, leading to unusual abrupt drop of luminescence intensity at wavelength of 400 nm. To understand the underlying mechanism of such emitting, the electronic structure of BaMnF$_4$ has been studied by first principles calculations. The observed two peaks are attributed to electrons' transitions between the upper-Hubbard bands of Mn's $t_{2g}$ orbitals and the lower-Hubbard bands of Mn's $e_g$ orbitals. Those Mott gap mediated d-d orbital transitions may provide additional degrees of freedom to tune the photon generation and absorption in ferroelectrics. △ Less

Submitted 18 December, 2015; originally announced December 2015.

Comments: 13 pages, 4 figures, 1 table

Journal ref: Physical Chemistry Chemical Physics 18, 2054-2058 (2016)

Possible ferrimagnetism and ferroelectricity of half-substituted rare-earth titanate: a first-principles study on Y$_{0.5}$La$_{0.5}$TiO$_3$

Authors: Ming An, Huimin Zhang, Yakui Weng, Yang Zhang, Shuai Dong

Abstract: Titanates with the perovskite structure, including ferroelectrics (e.g., BaTiO$_3$) and ferromagnetic ones (e.g., YTiO$_3$), are important functional materials. Recent theoretical studies predicted multiferroic states in strained EuTiO$_3$ and titanate superlattices, the former of which has already been experimental confirmed. Here, a first-principles calculation is performed to investigate the st… ▽ More Titanates with the perovskite structure, including ferroelectrics (e.g., BaTiO$_3$) and ferromagnetic ones (e.g., YTiO$_3$), are important functional materials. Recent theoretical studies predicted multiferroic states in strained EuTiO$_3$ and titanate superlattices, the former of which has already been experimental confirmed. Here, a first-principles calculation is performed to investigate the structural, magnetic, and electronic properties of Y half-substituted LaTiO3. Our results reveal that the magnetism of Y$_{0.5}$La$_{0.5}$TiO$_3$ sensitively depends on its structural details because of the inherent phase competition. The lowest energy state is the ferromagnetic state, resulting in 0.25 $μ_{\rm B}$/Ti. Furthermore, some configurations of Y$_{0.5}$La$_{0.5}$TiO$_3$ exhibit hybrid improper polarizations, which can be significantly affected by magnetism, resulting in the multiferroic properties. Because of the quenching disorder of substitution, the real Y$_{0.5}$La$_{0.5}$TiO3 material with random A-site ions may exhibit interesting relaxor behaviors. △ Less

Submitted 11 December, 2015; originally announced December 2015.

Comments: 13 pages, 4 figures, 2 tables

Journal ref: Frontiers of Physics 11, 117501 (2016)

Topological magnetic phase in LaMnO$_3$ (111) bilayer

Authors: Yakui Weng, Xin Huang, Yugui Yao, Shuai Dong

Abstract: Candidates for correlated topological insulators, originated from the spin-orbit coupling as well as Hubbard type correlation, are expected in the ($111$) bilayer of perovskite-structural transition-metal oxides. Based on the first-principles calculation and tight-binding model, the electronic structure of a LaMnO$_3$ ($111$) bilayer sandwiched in LaScO$_3$ barriers has been investigated. For the… ▽ More Candidates for correlated topological insulators, originated from the spin-orbit coupling as well as Hubbard type correlation, are expected in the ($111$) bilayer of perovskite-structural transition-metal oxides. Based on the first-principles calculation and tight-binding model, the electronic structure of a LaMnO$_3$ ($111$) bilayer sandwiched in LaScO$_3$ barriers has been investigated. For the ideal undistorted perovskite structure, the Fermi energy of LaMnO$_3$ ($111$) bilayer just stays at the Dirac point, rendering a semi-metal (graphene-like) which is also a half-metal (different from graphene nor previous studied LaNiO$_3$ ($111$) bilayer). The Dirac cone can be opened by the spin-orbit coupling, giving rise to nontrivial topological bands corresponding to the (quantized) anomalous Hall effect. For the realistic orthorhombic distorted lattice, the Dirac point moves with increasing Hubbard repulsion (or equivalent Jahn-Teller distortion). Finally, a Mott gap opens, establishing a phase boundary between the Mott insulator and topological magnetic insulator. Our calculation finds that the gap opened by spin-orbit coupling is much smaller in the orthorhombic distorted lattice ($\sim$$1.7$ meV) than the undistorted one ($\sim$$11$ meV). Therefore, to suppress the lattice distortion can be helpful to enhance the robustness of topological phase in perovskite ($111$) bilayers. △ Less

Submitted 9 November, 2015; originally announced November 2015.

Comments: 8 pages, 8 figures

Journal ref: Physical Review B 92, 195114 (2015)

Charge transfer and hybrid ferroelectricity in (YFeO$_{3}$)$_{n}$/(YTiO$_{3}$)$_{n}$ magnetic superlattices

Authors: Huimin Zhang, Yakui Weng, Xiaoyan Yao, Shuai Dong

Abstract: Interfaces in oxide heterostructures always provide a fertile ground for emergent properties. Charge transfer from a high energy band to a low energy opponent is naturally expected, as occurring in semiconductor $p$-$n$ junctions. In this study, several exceptional physical phenomena have been predicted in (YFeO$_3$)$_n$/(YTiO$_3$)$_n$ superlattices. First, the charge transfer between these Mott i… ▽ More Interfaces in oxide heterostructures always provide a fertile ground for emergent properties. Charge transfer from a high energy band to a low energy opponent is naturally expected, as occurring in semiconductor $p$-$n$ junctions. In this study, several exceptional physical phenomena have been predicted in (YFeO$_3$)$_n$/(YTiO$_3$)$_n$ superlattices. First, the charge transfer between these Mott insulators is in opposite to the intuitive band alignment scenario. Second, hybrid ferroelectricity with a moderate polarization is generated in the $n=2$ magnetic superlattice. Furthermore, the ferroelectric-type distortion can persist even if the ($AB$O$_3$)$_2$/($AB$'O$_3$)$_2$ system turns to be metallic, rending possible metallic ferroelectricity. △ Less

Submitted 22 May, 2015; originally announced May 2015.

Comments: 6 pages, 4 figures

Journal ref: Physical Review B 91, 195145 (2015)

Magnetism and electronic structure of ($001$)- and ($111$)-oriented LaTiO$_3$ bilayers sandwiched in LaScO$_3$ barriers

Authors: Yakui Weng, Shuai Dong

Abstract: In this study, the magnetism and electronic structure of LaTiO$_3$ bilayers along both the ($001$) and ($111$) orientations are calculated using the density functional theory. The band insulator LaScO$_3$ is chosen as the barrier layer and substrate to obtain the isolating LaTiO$_3$ bilayer. For both the ($001$)- and ($111$)-oriented cases, LaTiO$_3$ demonstrates the G-type antiferromagnetism as t… ▽ More In this study, the magnetism and electronic structure of LaTiO$_3$ bilayers along both the ($001$) and ($111$) orientations are calculated using the density functional theory. The band insulator LaScO$_3$ is chosen as the barrier layer and substrate to obtain the isolating LaTiO$_3$ bilayer. For both the ($001$)- and ($111$)-oriented cases, LaTiO$_3$ demonstrates the G-type antiferromagnetism as the ground state, similar to the bulk material. However, the electronic structure is significantly changed. The occupied bands of Ti are much narrower in the ($111$) case, giving a nearly flat band. As a result, the exchange coupling between nearest-neighbor Ti ions are reformed in these superlattices, which will affect the Néel temperature significantly. △ Less

Submitted 27 October, 2014; originally announced October 2014.

Comments: 4 pages, 4 figures. The 59th Annual Magnetism and Magnetic Materials (MMM) Conference

Journal ref: Journal of Applied Physics 117, 17C716 (2015)

Strain driven sequential magnetic transitions in strained GdTiO3 on compressive substrates: a first-principles study

Authors: Li-Juan Yang, Ya-Kui Weng, Hui-Min Zhang, Shuai Dong

Abstract: The compressive strain effects on the magnetic ground state and electronic structure of strained GdTiO 3 have been studied by the first-principles method. Different from the congeneric YTiO3 and LaTiO3 cases both of which becomes the A-type antiferromagnetism on the (001) LaAlO3 substrate despite their contrastive magnetism, the ground state of strained GdTiO3 on the LaAlO3 substrate changes from… ▽ More The compressive strain effects on the magnetic ground state and electronic structure of strained GdTiO 3 have been studied by the first-principles method. Different from the congeneric YTiO3 and LaTiO3 cases both of which becomes the A-type antiferromagnetism on the (001) LaAlO3 substrate despite their contrastive magnetism, the ground state of strained GdTiO3 on the LaAlO3 substrate changes from the original ferromagnetism to G-type antiferromagnetim, instead of the A-type one although Gd 3+ is between Y3+ and La3+. Only when the in-plane compressive strain is large enough, e.g. on the (001) YAlO3 substrate, the ground state finally becomes the A-type one. The band structure calculation shows that these compressive strained GdTiO3 remain insulating, although the band gap changes a little in these strained GdTiO3. △ Less

Submitted 5 October, 2014; originally announced October 2014.

Comments: 18 pages, 5 figures, 3 tables

Journal ref: Journal of Physics: Condensed Matter 26, 476001 (2014)

A Natural Topological Insulator

Authors: Pascal Gehring, Hadj M. Benia, Ye Weng, Robert Dinnebier, Christian R. Ast, Marko Burghard, Klaus Kern

Abstract: The earth's crust and outer space are rich sources of technologically relevant materials which have found application in a wide range of fields. Well-established examples are diamond, one of the hardest known materials, or graphite as a suitable precursor of graphene. The ongoing drive to discover novel materials useful for (opto)electronic applications has recently drawn strong attention to topol… ▽ More The earth's crust and outer space are rich sources of technologically relevant materials which have found application in a wide range of fields. Well-established examples are diamond, one of the hardest known materials, or graphite as a suitable precursor of graphene. The ongoing drive to discover novel materials useful for (opto)electronic applications has recently drawn strong attention to topological insulators. Here, we report that Kawazulite, a mineral with the approximate composition Bi$_2$(Te,Se)$_2$(Se,S), represents a naturally occurring topological insulator whose electronic properties compete well with those of its synthetic counterparts. Kawazulite flakes with a thickness of a few tens of nanometers were prepared by mechanical exfoliation. They exhibit a low intrinsic bulk doping level and correspondingly a sizable mobility of surface state carriers of more than $1000 \text{cm}^2/(\text{V s})$ at low temperature. Based on these findings, further minerals which due to their minimized defect densities display even better electronic characteristics may be identified in the future. △ Less

Submitted 26 November, 2013; originally announced November 2013.

Journal ref: Nano Letters, 13, 1179 (2013)

Magnetic orders of LaTiO$_3$ under epitaxial strain: a first-principles study

Authors: Yakui Weng, Xin Huang, Yankun Tang, Shuai Dong

Abstract: Perovskite LaTiO$_3$ bulk is a typical Mott-insulator with G-type antiferromagnetic order. In this work, the biaxial strain effects on the ground magnetic order of LaTiO$_3$ films grown on various substrates have been studied. For the compressive strain, LaTiO$_3$ films grown on LaAlO$_3$, LaGaO$_3$, and SrTiO$_3$ substrates undergo a phase transition from the original G-type antiferromagnet to A-… ▽ More Perovskite LaTiO$_3$ bulk is a typical Mott-insulator with G-type antiferromagnetic order. In this work, the biaxial strain effects on the ground magnetic order of LaTiO$_3$ films grown on various substrates have been studied. For the compressive strain, LaTiO$_3$ films grown on LaAlO$_3$, LaGaO$_3$, and SrTiO$_3$ substrates undergo a phase transition from the original G-type antiferromagnet to A-type antiferromagnet. The underlying physical mechanisms are the lattice distortions tunned by strain. While for the tensile strain, the BaTiO$_3$ and LaScO$_3$ substrates have been tested, which show a tendency to transit the LaTiO$_3$ to the C-type antiferromagnet. Furthermore, our calculations find that the magnetic transitions under epitaxial strain do not change the insulating fact of LaTiO$_3$. △ Less

Submitted 20 October, 2013; originally announced October 2013.

Comments: 3 pages, 3 figures. Proceeding of the 58th MMM conference, accepted

Journal ref: Journal of Applied Physics 115, 17E108 (2014)

Spin-roton excitations in the cuprate superconductors

Authors: J. W. Mei, Z. Y. Weng

Abstract: We identify a new kind of elementary excitations, spin-rotons, in the doped Mott insulator. They play a central role in deciding the superconducting transition temperature Tc, resulting in a simple Tc formula,Tc=Eg/6, with Eg as the characteristic energy scale of the spin rotons. We show that the degenerate S=1 and S=0 rotons can be probed by neutron scattering and Raman scattering measurements,… ▽ More We identify a new kind of elementary excitations, spin-rotons, in the doped Mott insulator. They play a central role in deciding the superconducting transition temperature Tc, resulting in a simple Tc formula,Tc=Eg/6, with Eg as the characteristic energy scale of the spin rotons. We show that the degenerate S=1 and S=0 rotons can be probed by neutron scattering and Raman scattering measurements, respectively, in good agreement with the magnetic resonancelike mode and the Raman A1g mode observed in the high-Tc cuprates. △ Less

Submitted 30 September, 2009; originally announced September 2009.

Comments: 10 pages, 9 figures

Journal ref: Phys. Rev. B 81, 014507 (2010)

Phase diagram of the frustrated, spatially anisotropic S=1 antiferromagnet on a square lattice

Authors: H. C. Jiang, F. Krüger, J. E. Moore, D. N. Sheng, J. Zaanen, Z. Y. Weng

Abstract: We study the S=1 square lattice Heisenberg antiferromagnet with spatially anisotropic nearest neighbor couplings $J_{1x}$, $J_{1y}$ frustrated by a next-nearest neighbor coupling $J_{2}$ numerically using the density-matrix renormalization group (DMRG) method and analytically employing the Schwinger-Boson mean-field theory (SBMFT). Up to relatively strong values of the anisotropy, within both me… ▽ More We study the S=1 square lattice Heisenberg antiferromagnet with spatially anisotropic nearest neighbor couplings $J_{1x}$, $J_{1y}$ frustrated by a next-nearest neighbor coupling $J_{2}$ numerically using the density-matrix renormalization group (DMRG) method and analytically employing the Schwinger-Boson mean-field theory (SBMFT). Up to relatively strong values of the anisotropy, within both methods we find quantum fluctuations to stabilize the Néel ordered state above the classically stable region. Whereas SBMFT suggests a fluctuation-induced first order transition between the Néel state and a stripe antiferromagnet for $1/3\leq J_{1x}/J_{1y}\leq 1$ and an intermediate paramagnetic region opening only for very strong anisotropy, the DMRG results clearly demonstrate that the two magnetically ordered phases are separated by a quantum disordered region for all values of the anisotropy with the remarkable implication that the quantum paramagnetic phase of the spatially isotropic $J_{1}$-$J_{2}$ model is continuously connected to the limit of decoupled Haldane spin chains. Our findings indicate that for S=1 quantum fluctuations in strongly frustrated antiferromagnets are crucial and not correctly treated on the semiclassical level. △ Less

Submitted 20 January, 2009; originally announced January 2009.

Comments: 10 pages, 10 figures

Journal ref: Phys. Rev. B 79, 174409 (2009)

Supersolid Order of Frustrated Hard-Core Bosons in a Triangular Lattice System

Authors: H. C. Jiang, M. Q. Weng, Z. Y. Weng, D. N. Sheng, L. Balents

Abstract: We numerically demonstrate that a supersolid phase exists in a frustrated hard-core boson system on a triangular lattice over a wide range of interaction strength. In the infinite repulsion (Ising) limit, we establish a mapping to the same problem with unfrustrated hopping, which connects the supersolid to the known results in that case. The weak superfluidity can be destroyed or strongly enhanc… ▽ More We numerically demonstrate that a supersolid phase exists in a frustrated hard-core boson system on a triangular lattice over a wide range of interaction strength. In the infinite repulsion (Ising) limit, we establish a mapping to the same problem with unfrustrated hopping, which connects the supersolid to the known results in that case. The weak superfluidity can be destroyed or strongly enhanced by a next nearest neighbor hopping term, which provides valuable information for experimental realization of a supersolid phase on optical lattice. △ Less

Submitted 6 October, 2008; originally announced October 2008.

Comments: 4 pages, 4 figures

Journal ref: Phys. Rev. B 79, 020409(R) (2009)

Second Renormalization of Tensor-Network States

Authors: Z. Y. Xie, H. C. Jiang, Q. N. Chen, Z. Y. Weng, T. Xiang

Abstract: We propose a second renormalization group method to handle the tensor-network states or models. This method reduces dramatically the truncation error of the tensor renormalization group. It allows physical quantities of classical tensor-network models or tensor-network ground states of quantum systems to be accurately and efficiently determined. We propose a second renormalization group method to handle the tensor-network states or models. This method reduces dramatically the truncation error of the tensor renormalization group. It allows physical quantities of classical tensor-network models or tensor-network ground states of quantum systems to be accurately and efficiently determined. △ Less

Submitted 14 October, 2009; v1 submitted 1 September, 2008; originally announced September 2008.

Comments: 5 figures, 4 pages

Journal ref: Phys.Rev.Lett.103, 160601 (2009)

Accurate determination of tensor network state of quantum lattice models in two dimensions

Authors: H. C. Jiang, Z. Y. Weng, T. Xiang

Abstract: We have proposed a novel numerical method to calculate accurately the physical quantities of the ground state with the tensor-network wave function in two dimensions. We determine the tensor network wavefunction by a projection approach which applies iteratively the Trotter-Suzuki decomposition of the projection operator and the singular value decomposition of matrix. The norm of the wavefunctio… ▽ More We have proposed a novel numerical method to calculate accurately the physical quantities of the ground state with the tensor-network wave function in two dimensions. We determine the tensor network wavefunction by a projection approach which applies iteratively the Trotter-Suzuki decomposition of the projection operator and the singular value decomposition of matrix. The norm of the wavefunction and the expectation value of a physical observable are evaluated by a coarse grain renormalization group approach. Our method allows a tensor-network wavefunction with a high bond degree of freedom (such as D=8) to be handled accurately and efficiently in the thermodynamic limit. For the Heisenberg model on a honeycomb lattice, our results for the ground state energy and the staggered magnetization agree well with those obtained by the quantum Monte Carlo and other approaches. △ Less

Submitted 23 June, 2008; originally announced June 2008.

Comments: 4 pages 5 figures 2 tables

Journal ref: Phys. Rev. Lett. 101, 090603 (2008)

DMRG Numerical Study of the Kagomé Antiferromagnet

Authors: H. C. Jiang, Z. Y. Weng, D. N. Sheng

Abstract: We numerically study the spin-1/2 antiferromagnetic Heisenberg model on the kagomé lattice using the density-matrix renormalization group (DMRG) method. We find that the ground state is a magnetically disordered spin liquid, characterized by an exponential decay of spin-spin correlation function in real space and a magnetic structure factor showing system-size independent peaks at commersurate a… ▽ More We numerically study the spin-1/2 antiferromagnetic Heisenberg model on the kagomé lattice using the density-matrix renormalization group (DMRG) method. We find that the ground state is a magnetically disordered spin liquid, characterized by an exponential decay of spin-spin correlation function in real space and a magnetic structure factor showing system-size independent peaks at commersurate antiferromangetic wavevectors. We obtain a spin triplet excitation gap $ΔE(S=1)=0.055\pm 0.005$ by extrapolation based on the large size results, and confirm the presence of gapless singlet excitations. The physical nature of such an exotic spin liquid is also discussed. △ Less

Submitted 10 April, 2008; originally announced April 2008.

Comments: 5 pages, 5 figures and 1 table

Journal ref: Phys. Rev. Lett. 101, 117203 (2008)

On the sign structure of doped Mott insulators

Authors: K. Wu, Z. Y. Weng, J. Zaanen

Abstract: We demonstrate that the sign structure of the t-J model on a hypercubic lattice is entirely different from that of a Fermi gas, by inspecting the high temperature expansion of the partition function up to all orders, as well as the multi-hole propagator of the half-filled state and the perturbative expansion of the ground state energy. We show that while the fermion signs can be completely gauge… ▽ More We demonstrate that the sign structure of the t-J model on a hypercubic lattice is entirely different from that of a Fermi gas, by inspecting the high temperature expansion of the partition function up to all orders, as well as the multi-hole propagator of the half-filled state and the perturbative expansion of the ground state energy. We show that while the fermion signs can be completely gauged away by a Marshall sign transformation at half-filling, the bulk of the signs can be also gauged away in a doped case, leaving behind a rarified "irreducible" sign structure that can be enumerated easily by counting exchanges of holes with themselves and spins on their real space paths. Such a sparse sign structure implies a mutual statistics for the quantum states of the doped Mott insulator. △ Less

Submitted 3 February, 2008; originally announced February 2008.

Comments: 4 pages, 1 figure

Journal ref: Phys. Rev. B77, 155102 (2008)

Self-Retracting Motion of Graphite Microflakes

Authors: Quanshui Zheng, Bo Jiang, Shoupeng Liu, Jing Zhu, Qing Jiang, Yuxiang Weng, Li Lu, Sheng Wang, Qikun Xue, Lianmao Peng

Abstract: We report the observation of a novel phenomenon, the self-retracting motion of graphite, in which tiny flakes of graphite, after being displaced to various suspended positions from islands of highly orientated pyrolytic graphite, retract back onto the islands under no external influences. Our repeated probing and observing such flakes of various sizes indicate the existence of a critical size of… ▽ More We report the observation of a novel phenomenon, the self-retracting motion of graphite, in which tiny flakes of graphite, after being displaced to various suspended positions from islands of highly orientated pyrolytic graphite, retract back onto the islands under no external influences. Our repeated probing and observing such flakes of various sizes indicate the existence of a critical size of flakes, approximately 35 micrometer, above which the self-retracting motion does not occur under the operation. This helps to explain the fact that the self-retracting motion of graphite has not been reported, because samples of natural graphite are typical larger than this critical size. In fact, reports of this phenomenon have not been found in the literature for single crystals of any kinds. A model that includes the static and dynamic shear strengths, the van der Waals interaction force, and the edge dangling bond interaction effect, was used to explain the observed phenomenon. These findings may conduce to create nano-electromechanical systems with a wide range of mechanical operating frequency from mega to giga hertzs. △ Less

Submitted 25 September, 2007; originally announced September 2007.

Superfluid-Mott-Insulator Transition in a One-Dimensional Optical Lattice with Double-Well Potentials

Authors: H. C. Jiang, Z. Y. Weng, T. Xiang

Abstract: We study the superfluid-Mott-insulator transition of ultracold bosonic atoms in a one-dimensional optical lattice with a double-well confining trap using the density-matrix renormalization group. At low density, the system behaves similarly as two separated ones inside harmonic traps. At high density, however, interesting features appear as the consequence of the quantum tunneling between the tw… ▽ More We study the superfluid-Mott-insulator transition of ultracold bosonic atoms in a one-dimensional optical lattice with a double-well confining trap using the density-matrix renormalization group. At low density, the system behaves similarly as two separated ones inside harmonic traps. At high density, however, interesting features appear as the consequence of the quantum tunneling between the two wells and the competition between the "superfluid" and Mott regions. They are characterized by a rich step-plateau structure in the visibility and the satellite peaks in the momentum distribution function as a function of the on-site repulsion. These novel properties shed light on the understanding of the phase coherence between two coupled condensates and the off-diagonal correlations between the two wells. △ Less

Submitted 12 January, 2007; originally announced January 2007.

Comments: 5 pages, 7 figures

Journal ref: Phys. Rev. B 76, 224515 (2007)

Quantum Spin Hall Effect and Topologically Invariant Chern Numbers

Authors: D. N. Sheng, Z. Y. Weng, L. Sheng, F. D. M. Haldane

Abstract: We present a topological description of quantum spin Hall effect (QSHE) in a two-dimensional electron system on honeycomb lattice with both intrinsic and Rashba spin-orbit couplings. We show that the topology of the band insulator can be characterized by a $2\times 2$ traceless matrix of first Chern integers. The nontrivial QSHE phase is identified by the nonzero diagonal matrix elements of the… ▽ More We present a topological description of quantum spin Hall effect (QSHE) in a two-dimensional electron system on honeycomb lattice with both intrinsic and Rashba spin-orbit couplings. We show that the topology of the band insulator can be characterized by a $2\times 2$ traceless matrix of first Chern integers. The nontrivial QSHE phase is identified by the nonzero diagonal matrix elements of the Chern number matrix (CNM). A spin Chern number is derived from the CNM, which is conserved in the presence of finite disorder scattering and spin nonconserving Rashba coupling. By using the Laughlin's gedanken experiment, we numerically calculate the spin polarization and spin transfer rate of the conducting edge states, and determine a phase diagram for the QSHE. △ Less

Submitted 2 March, 2006; originally announced March 2006.

Comments: 4 pages and 4 figures

Journal ref: Phys. Rev. Lett. 97, 036808 (2006)

Quantum Hall Effect of Dirac Fermions in Graphene: Disorder Effect and Phase Diagram

Authors: D. N. Sheng, L. Sheng, Z. Y. Weng

Abstract: We numerically study the interplay of band structure, topological invariant and disorder effect in two-dimensional electron system of graphene in a magnetic field. Two \emph{distinct} quantum Hall effect (QHE) regimes exist in the energy band with the unconventional "half-integer" QHE appearing near the band center, consistent with the experimental observation. The latter is more robust against… ▽ More We numerically study the interplay of band structure, topological invariant and disorder effect in two-dimensional electron system of graphene in a magnetic field. Two \emph{distinct} quantum Hall effect (QHE) regimes exist in the energy band with the unconventional "half-integer" QHE appearing near the band center, consistent with the experimental observation. The latter is more robust against disorder scattering than the conventional QHE states near the band edges. The phase diagram for the unconventional QHE is obtained where the destruction of the Hall plateaus at strong disorder is through the float-up of extended levels toward band center and higher plateaus always disappear first. We further predict a new insulating phase between $ν=\pm 2$ QHE states at the band center, which may explain the experimentally observed resistance discontinuity near zero gate voltage. △ Less

Submitted 7 February, 2006; v1 submitted 7 February, 2006; originally announced February 2006.

Comments: 4 pages, 4 figures

Journal ref: Phys. Rev. B 73, 233406 (2006).

Spin Liquid Phase in Anisotropic Triangular Lattice Heisenberg Model: Exact diagonalization and density-matrix renormalization group calculations

Authors: M. Q. Weng, D. N. Sheng, Z. Y. Weng, Robert J. Bursill

Abstract: Based on exact diagonalization and density matrix renormalization group (DMRG) method, we show that an anisotropic triangle lattice Heisenberg spin model has three distinct quantum phases. In particular, a spin-liquid phase is present in the weak interchain coupling regime, which is characterized by an exponentially-decaying spin correlator along the weaker coupling direction and a quasi-one-dim… ▽ More Based on exact diagonalization and density matrix renormalization group (DMRG) method, we show that an anisotropic triangle lattice Heisenberg spin model has three distinct quantum phases. In particular, a spin-liquid phase is present in the weak interchain coupling regime, which is characterized by an exponentially-decaying spin correlator along the weaker coupling direction and a quasi-one-dimensional spin structure factor along the chain direction, in agreement with experiments on the Cs2CuCl4 compounds. In the obtained phase diagram, the spin liquid phase is found to persist up to a relatively large critical anisotropic coupling ratio J'/J=0.78, which is stabilized by strong quantum fluctuations, with a parity symmetry distinct from two magnetic ordered states in the stronger coupling regime. △ Less

Submitted 1 August, 2006; v1 submitted 7 August, 2005; originally announced August 2005.

Comments: 4 pages, 3 figures; revised version

Journal ref: Phys. Rev. B 74, 012407 (2006)

Numerical Study of Spin Hall Transport in a Two Dimensional Hole Gas System

Authors: W. Q. Chen, Z. Y. Weng, D. N. Sheng

Abstract: We present a numerical study of the spin Hall effect in a two-dimensional hole gas (2DHG) system in the presence of disorder. We find that the spin Hall conductance (SHC), extrapolated to the thermodynamic limit, remains finite in a wide range of disorder strengths for a closed system on torus. But there is no intrinsic spin Hall accumulation as induced by an external electric field once the dis… ▽ More We present a numerical study of the spin Hall effect in a two-dimensional hole gas (2DHG) system in the presence of disorder. We find that the spin Hall conductance (SHC), extrapolated to the thermodynamic limit, remains finite in a wide range of disorder strengths for a closed system on torus. But there is no intrinsic spin Hall accumulation as induced by an external electric field once the disorder is turned on. The latter is examined by performing a Laughlin's Gedanken gauge experiment numerically with the adiabatical insertion of a flux quantum in a belt-shaped sample, in which the absence of level crossing is found under the disorder effect. Without disorder, on the other hand, energy levels do cross each other, which results in an oscillating spin-density-modulation at the sample boundary after the insertion of one flux quantum in the belt-shaped system. But the corresponding net spin transfer is only about one order of magnitude smaller than what is expected from the bulk SHC. These apparently contradictory results can be attributed to the violation of the spin conservation law in such a system. We also briefly address the dissipative Fermi surface contribution to spin polarization, which may be relevant to experimental measurements. △ Less

Submitted 26 October, 2005; v1 submitted 3 August, 2005; originally announced August 2005.

Comments: 9 pages, 11 figures; final version to appear in Phys. Rev. B

Journal ref: Physical Review B 72, 235315 (2005)

Spin Hall Effect and Spin Transfer in Disordered Rashba Model

Authors: D. N. Sheng, L. Sheng, Z. Y. Weng, F. D. M. Haldane

Abstract: Based on numerical study of the Rashba model, we show that the spin Hall conductance remains finite in the presence of disorder up to a characteristic length scale, beyond which it vanishes exponentially with the system size. We further perform a Laughlin's gauge experiment numerically and find that all energy levels cannot cross each other during an adiabatic insertion of the flux in accordance… ▽ More Based on numerical study of the Rashba model, we show that the spin Hall conductance remains finite in the presence of disorder up to a characteristic length scale, beyond which it vanishes exponentially with the system size. We further perform a Laughlin's gauge experiment numerically and find that all energy levels cannot cross each other during an adiabatic insertion of the flux in accordance with the general level-repulsion rule. It results in zero spin transfer between two edges of the sample as each state always evolves back after the insertion of one flux quantum, in contrast to the quantum Hall effect. It implies that the topological spin Hall effect vanishes with the turn-on of disorder. △ Less

Submitted 19 October, 2005; v1 submitted 8 April, 2005; originally announced April 2005.

Comments: 4 pages, 4 figures final version

Journal ref: Phys. Rev. B 72, 153307 (2005)

Bosonic resonating valence bond wave function for doped Mott insulators

Authors: Z. Y. Weng, Y. Zhou, V. N. Muthukumar

Abstract: We propose a new class of ground states for doped Mott insulators in the electron second-quantization representation. They are obtained from a bosonic resonating valence bond (RVB) theory of the t-J model. At half filling, the ground state describes spin correlations of the S=1/2 Heisenberg model very accurately. Its spin degrees of freedom are characterized by RVB pairing of spins, the size of… ▽ More We propose a new class of ground states for doped Mott insulators in the electron second-quantization representation. They are obtained from a bosonic resonating valence bond (RVB) theory of the t-J model. At half filling, the ground state describes spin correlations of the S=1/2 Heisenberg model very accurately. Its spin degrees of freedom are characterized by RVB pairing of spins, the size of which decreases continuously as holes are doped into the system. Charge degrees of freedom emerge upon doping and are described by twisted holes in the RVB background. We show that the twisted holes exhibit an off diagonal long range order (ODLRO) in the pseudogap ground state, which has a finite pairing amplitude, but is short of phase coherence. Unpaired spins in such a pseudogap ground state behave as free vortices, preventing superconducting phase coherence. The existence of nodal quasiparticles is also ensured by such a hidden ODLRO in the ground state, which is non-Fermi-liquid-like in the absence of superconducting phase coherence. Two distinct types of spin excitations can also be constructed. The superconducting instability of the pseudogap ground state is discussed and a d-wave superconducting ground state is obtained. This class of pseudogap and superconducting ground states unifies antiferromagnetism, pseudogap, superconductivity, and Mott physics into a new state of matter. △ Less

Submitted 7 June, 2005; v1 submitted 24 February, 2005; originally announced February 2005.

Comments: 28 pages, 5 figures, final version to appear in Phys. Rev. B

Journal ref: Physical Review B 72, 014503 (2005)

Numerical Study of the Spin Hall Conductance in the Luttinger Model

Authors: W. Q. Chen, Z. Y. Weng, D. N. Sheng

Abstract: We present first numerical studies of the disorder effect on the recently proposed intrinsic spin Hall conductance in a three dimensional (3D) lattice Luttinger model. The results show that the spin Hall conductance remains finite in a wide range of disorder strength, with large fluctuations. The disorder-configuration-averaged spin Hall conductance monotonically decreases with the increase of d… ▽ More We present first numerical studies of the disorder effect on the recently proposed intrinsic spin Hall conductance in a three dimensional (3D) lattice Luttinger model. The results show that the spin Hall conductance remains finite in a wide range of disorder strength, with large fluctuations. The disorder-configuration-averaged spin Hall conductance monotonically decreases with the increase of disorder strength and vanishes before the Anderson localization takes place. The finite-size effect is also discussed. △ Less

Submitted 27 July, 2005; v1 submitted 24 February, 2005; originally announced February 2005.

Comments: 4 pages, 4 figures; the final version appearing in PRL

Journal ref: Physical Review Letters 95, 086605 (2005)

Spin dynamics in a doped-Mott-insulator superconductor

Authors: W. Q. Chen, Z. Y. Weng

Abstract: We present a systematic study of spin dynamics in a superconducting ground state, which itself is a doped-Mott-insulator and can correctly reduce to an antiferromagnetic (AF) state at half-filling with an AF long-range order (AFLRO). Such a doped Mott insulator is described by a mean-field theory based on the phase string formulation of the t-J model. We show that the spin wave excitation in the… ▽ More We present a systematic study of spin dynamics in a superconducting ground state, which itself is a doped-Mott-insulator and can correctly reduce to an antiferromagnetic (AF) state at half-filling with an AF long-range order (AFLRO). Such a doped Mott insulator is described by a mean-field theory based on the phase string formulation of the t-J model. We show that the spin wave excitation in the AFLRO state at half-filling evolves into a resonancelike peak at a finite energy in the superconducting state, which is located around the AF wave vectors. The width of such a resonancelike peak in momentum space decides a spin correlation length scale which is inversely proportional to the square root of doping concentration, while the energy of the resonancelike peak scales linearly with the doping concentration at low doping. An important prediction of the theory is that, while the total spin sum rule is satisfied at different doping concentrations, the weight of the resonancelike peak does not vanish, but is continuously saturated to the weight of the AFLRO at zero-doping limit. Besides the low-energy resonancelike peak, we also show that the high-energy excitations still track the spin wave dispersion in momentum space, contributing to a significant portion of the total spin sum rule. The fluctuational effect beyond the mean-field theory is also examined, which is related to the broadening of the resonancelike peak in energy space. In particular, we discuss the incommensurability of the spin dynamics by pointing out that its visibility is strongly tied to the low-energy fluctuations below the resonancelike peak. We finally investigate the interlayer coupling effect on the spin dynamics as a function of doping, by considering a bilayer system. △ Less

Submitted 21 February, 2005; v1 submitted 10 June, 2004; originally announced June 2004.

Comments: 24 pages, 17 figures, revised version to appear in Phys. Rev. B

Journal ref: Physical Review B 71, 134516 (2005)

Microscopic origin of local moments in a zinc-doped high-$T_{c}$ superconductor

Authors: X. L. Qi, Z. Y. Weng

Abstract: The formation of a local moment around a zinc impurity in the high-$T_{c}$ cuprate superconductors is studied within the framework of the bosonic resonating-valence-bond (RVB) description of the $t-J$ model. A topological origin of the local moment has been shown based on the phase string effect in the bosonic RVB theory. It is found that such an $S=1/2$ moment distributes near the zinc in a for… ▽ More The formation of a local moment around a zinc impurity in the high-$T_{c}$ cuprate superconductors is studied within the framework of the bosonic resonating-valence-bond (RVB) description of the $t-J$ model. A topological origin of the local moment has been shown based on the phase string effect in the bosonic RVB theory. It is found that such an $S=1/2$ moment distributes near the zinc in a form of staggered magnetic moments at the copper sites. The corresponding magnetic properties, including NMR spin relaxation rate, uniform spin susceptibility, and dynamic spin susceptibility, etc., calculated based on the theory, are consistent with the experimental measurements. Our work suggests that the zinc substitution in the cuprates provide an important experimental evidence for the RVB nature of local physics in the original (zinc free) state. △ Less

Submitted 29 December, 2004; v1 submitted 28 May, 2004; originally announced May 2004.

Comments: The topological reason of local moment formation is given. One figure is added

Journal ref: Phys. Rev. B 71, 184507 (2005)

Self-localization of holes in a lightly doped Mott insulator

Authors: Su-Peng Kou, Z. Y. Weng

Abstract: We show that lightly doped holes will be self-trapped in an antiferromagnetic spin background at low-temperatures, resulting in a spontaneous translational symmetry breaking. The underlying Mott physics is responsible for such novel self-localization of charge carriers. Interesting transport and dielectric properties are found as the consequences, including large doping-dependent thermopower and… ▽ More We show that lightly doped holes will be self-trapped in an antiferromagnetic spin background at low-temperatures, resulting in a spontaneous translational symmetry breaking. The underlying Mott physics is responsible for such novel self-localization of charge carriers. Interesting transport and dielectric properties are found as the consequences, including large doping-dependent thermopower and dielectric constant, low-temperature variable-range-hopping resistivity, as well as high-temperature strange-metal-like resistivity, which are consistent with experimental measurements in the high-T$_c$ cuprates. Disorder and impurities only play a minor and assistant role here. △ Less

Submitted 5 March, 2004; v1 submitted 12 February, 2004; originally announced February 2004.

Comments: 13 pages, 4 figures; additional experimental data are included in Fig. 3 and some comments are added in the text

Journal ref: Eur. Phys. J. B 47, 37-46 (2005)