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Mixed spin states for robust ferromagnetism in strained SrCoO$_3$ thin films
Authors:
Xiquan Zheng,
Nicholas B. Brookes,
Flora Yakhou-Harris,
Yingjie Lyu,
Jianbing Zhang,
Qian Xiao,
Xinyi Jiang,
Qingzheng Qiu,
Qizhi Li,
Shilong Zhang,
Xinqiang Cai,
Pu Yu,
Yi Lu,
Yingying Peng
Abstract:
Epitaxial strain in transition-metal oxides can induce dramatic changes in electronic and magnetic properties. A recent study on the epitaxially strained SrCoO$_3$ thin films revealed persistent ferromagnetism even across a metal-insulator transition. This challenges the current theoretical predictions, and the nature of the local spin state underlying this robustness remains unresolved. Here, we…
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Epitaxial strain in transition-metal oxides can induce dramatic changes in electronic and magnetic properties. A recent study on the epitaxially strained SrCoO$_3$ thin films revealed persistent ferromagnetism even across a metal-insulator transition. This challenges the current theoretical predictions, and the nature of the local spin state underlying this robustness remains unresolved. Here, we employ high-resolution resonant inelastic x-ray scattering (RIXS) at the Co-$L_3$ edge to probe the spin states of strained SrCoO$_3$ thin films. Compared with CoO$_6$ cluster multiplet calculations, we identify a ground state composed of a mixed high- and low-spin configuration, distinct from the previously proposed intermediate-spin state. Our results demonstrate that the robustness of ferromagnetism arises from the interplay between this mixed spin state and the presence of ligand holes associated with negative charge transfer. These findings provide direct experimental evidence for a nontrivial magnetic ground state in SrCoO$_3$ and offer new pathways for designing robust ferromagnetic systems in correlated oxides.
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Submitted 2 August, 2025;
originally announced August 2025.
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Ultrafast Orbital-Selective Photodoping Melts Charge Order in Overdoped Bi-based Cuprates
Authors:
Xinyi Jiang,
Qizhi Li,
Qingzheng Qiu,
Li Yue,
Junhan Huang,
Yiwen Chen,
Byungjune Lee,
Hyeongi Choi,
Xingjiang Zhou,
Tao Dong,
Nanlin Wang,
Hoyoung Jang,
Yingying Peng
Abstract:
High-temperature superconductivity in cuprates remains one of the enduring puzzles of condensed matter physics, with charge order (CO) playing a central yet elusive role, particularly in the overdoped regime. Here, we employ time-resolved X-ray absorption spectroscopy and resonant X-ray scattering at a free-electron laser to probe the transient electronic density of states and ultrafast CO dynamic…
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High-temperature superconductivity in cuprates remains one of the enduring puzzles of condensed matter physics, with charge order (CO) playing a central yet elusive role, particularly in the overdoped regime. Here, we employ time-resolved X-ray absorption spectroscopy and resonant X-ray scattering at a free-electron laser to probe the transient electronic density of states and ultrafast CO dynamics in overdoped (Bi,Pb)$_{2.12}$Sr$_{1.88}$CuO$_{6+δ}$. We reveal a striking pump laser wavelength dependence - the 800 nm light fails to suppress CO, whereas the 400 nm light effectively melts it. This behavior originates from the fact that 400 nm photons can promote electrons from the Zhang-Rice singlet band to the upper Hubbard band or apical oxygen states, while 800 nm photons lack the energy to excite electrons across the charge-transfer gap. The CO recovery time ($\sim$3 ps) matches that of the underdoped cuprates, indicating universal electronic instability in the phase diagram. Additionally, melting overdoped CO requires an order-of-magnitude higher fluence highlighting the role of lattice interactions. Our findings demonstrate orbital-selective photodoping and provide a route to ultrafast control of emergent quantum phases in correlated materials.
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Submitted 5 June, 2025;
originally announced June 2025.
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Photo-generated charge-transfer excitons in NiO revealed by ultrafast time-resolved resonant inelastic x-ray scattering
Authors:
Giacomo Merzoni,
Leonardo Martinelli,
Sergii Parchenko,
Sophia F. R. TenHuisen,
Vasily Lebedev,
Luigi Adriano,
Robert Carley,
Natalia Gerasimova,
Laurent Mercadier,
Martin Teichmann,
Benjamin E. van Kuiken,
Zhong Yin,
Amina Alic,
Denitsa R. Baykusheva,
Sorin G. Chiuzbaian,
Stefano Dal Conte,
Oleg Dogadov,
Alexander Föhlisch,
Maurits W. Haverkort,
Maximilian Kusch,
Tim Laarmann,
Wei Sheng Lee,
Marco Moretti Sala,
Ying Ying Peng,
Qing Zheng Qiu
, et al. (10 additional authors not shown)
Abstract:
Strong electronic correlation can lead to insulating behavior and to the opening of large optical gaps, even in materials with partly filled valence shells. Although the non-equilibrium optical response encodes both local (quasi atomic) and collective (long range) responses, optical spectroscopy is usually more sensitive to the latter. Resonant x-ray techniques are better suited to investigate the…
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Strong electronic correlation can lead to insulating behavior and to the opening of large optical gaps, even in materials with partly filled valence shells. Although the non-equilibrium optical response encodes both local (quasi atomic) and collective (long range) responses, optical spectroscopy is usually more sensitive to the latter. Resonant x-ray techniques are better suited to investigate the quasi-atomic properties of correlated solids. Using time-resolved resonant inelastic x-ray scattering (RIXS), here we study the ultrafast non-equilibrium processes in NiO following photo-excitation by ultraviolet photons with energy exceeding the optical gap. We observe the creation of charge-transfer excitons that decay with a time constant of about 2\,ps, while itinerant photo-doping persists for tens of picoseconds. Following our discovery, which establishes time-resolved high-resolution RIXS as a powerful tool for the study of transient phenomena in condensed matter, the possible presence of charge-transfer excitons will need to be considered when interpreting optical pump-probe experiments on correlated quantum materials.
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Submitted 23 April, 2025;
originally announced April 2025.
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Photo-induced Dynamics and Momentum Distribution of Chiral Charge Density Waves in 1T-TiSe$_{2}$
Authors:
Qingzheng Qiu,
Sae Hwan Chun,
Jaeku Park,
Dogeun Jang,
Li Yue,
Yeongkwan Kim,
Yeojin Ahn,
Mingi Jho,
Kimoon Han,
Xinyi Jiang,
Qian Xiao,
Tao Dong,
Jia-Yi Ji,
Nanlin Wang,
Jeroen van den Brink,
Jasper van Wezel,
Yingying Peng
Abstract:
Exploring the photoinduced dynamics of chiral states offers promising avenues for advanced control of condensed matter systems. Photoinduced or photoenhanced chirality in 1T-TiSe$_{2}$ has been suggested as a fascinating platform for optical manipulation of chiral states. However, the mechanisms underlying chirality training and its interplay with the charge density wave (CDW) phase remain elusive…
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Exploring the photoinduced dynamics of chiral states offers promising avenues for advanced control of condensed matter systems. Photoinduced or photoenhanced chirality in 1T-TiSe$_{2}$ has been suggested as a fascinating platform for optical manipulation of chiral states. However, the mechanisms underlying chirality training and its interplay with the charge density wave (CDW) phase remain elusive. Here, we use time-resolved X-ray diffraction (tr-XRD) with circularly polarized pump lasers to probe the photoinduced dynamics of chirality in 1T-TiSe$_{2}$. We observe a notable ($\sim$20%) difference in CDW intensity suppression between left- and right-circularly polarized pumps. Additionally, we reveal momentum-resolved circular dichroism arising from domains of different chirality, providing a direct link between CDW and chirality. An immediate increase in CDW correlation length upon laser pumping is detected, suggesting the photoinduced expansion of chiral domains. These results both advance the potential of light-driven chirality by elucidating the mechanism driving chirality manipulation in TiSe$_2$, and they demonstrate that tr-XRD with circularly polarized pumps is an effective tool for chirality detection in condensed matter systems.
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Submitted 4 February, 2025;
originally announced February 2025.
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Spin frustration and unconventional spin twisting state in van der Waals ferromagnet/antiferromagnet heterostructures
Authors:
Tianye Wang,
Qian Li,
Mengmeng Yang,
Yu Sun,
Alpha T. N'Diaye,
Christoph Klewe,
Andreas Scholl,
Xianzhe Chen,
Xiaoxi Huang,
Hongrui Zhang,
Santai Yang,
Xixiang Zhang,
Chanyong Hwang,
Padraic C. Shafer,
Michael F. Crommie,
Ramamoorthy Ramesh,
Zi Q. Qiu
Abstract:
Atomically flat surfaces of van der Waals (vdW) materials pave an avenue for addressing a long-standing fundamental issue of how a perfectly compensated antiferromagnet (AFM) surface frustrates a ferromagnetic (FM) overlayer in FM/AFM heterostructures. By revealing the AFM and FM spin structures separately in vdW Fe5GeTe2/NiPS3 heterostructures, we find that C-type in-plane AFM NiPS3 develops thre…
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Atomically flat surfaces of van der Waals (vdW) materials pave an avenue for addressing a long-standing fundamental issue of how a perfectly compensated antiferromagnet (AFM) surface frustrates a ferromagnetic (FM) overlayer in FM/AFM heterostructures. By revealing the AFM and FM spin structures separately in vdW Fe5GeTe2/NiPS3 heterostructures, we find that C-type in-plane AFM NiPS3 develops three equivalent AFM domains which are robust against external magnetic field and magnetic coupling with Fe5GeTe2. Consequently, spin frustration at the Fe5GeTe2/NiPS3 interface was shown to develop a perpendicular Fe5GeTe2 magnetization in the interfacial region that switches separately from the bulk of the Fe5GeTe2 magnetizations. In particular, we discover an unconventional spin twisting state that the Fe5GeTe2 spins twist from perpendicular direction near the interface to in-plane direction away from the interface in Fe5GeTe2/NiPS3. Our finding of the twisting spin texture is a unique property of spin frustration in van der Waals magnetic heterostructures.
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Submitted 27 January, 2025;
originally announced January 2025.
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Van-Hove annihilation and nematic instability on a Kagome lattice
Authors:
Yu-Xiao Jiang,
Sen Shao,
Wei Xia,
M. Michael Denner,
Julian Ingham,
Md Shafayat Hossain,
Qingzheng Qiu,
Xiquan Zheng,
Hongyu Chen,
Zi-Jia Cheng,
Xian P. Yang,
Byunghoon Kim,
Jia-Xin Yin,
Songbo Zhang,
Maksim Litskevich,
Qi Zhang,
Tyler A. Cochran,
Yingying Peng,
Guoqing Chang,
Yanfeng Guo,
Ronny Thomale,
Titus Neupert,
M. Zahid Hasan
Abstract:
Novel states of matter arise in quantum materials due to strong interactions among electrons. A nematic phase breaks the point group symmetry of the crystal lattice and is known to emerge in correlated materials. Here we report the observation of an intra-unit-cell nematic order and signatures of Pomeranchuk instability in the Kagome metal ScV6Sn6. Using scanning tunneling microscopy and spectrosc…
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Novel states of matter arise in quantum materials due to strong interactions among electrons. A nematic phase breaks the point group symmetry of the crystal lattice and is known to emerge in correlated materials. Here we report the observation of an intra-unit-cell nematic order and signatures of Pomeranchuk instability in the Kagome metal ScV6Sn6. Using scanning tunneling microscopy and spectroscopy, we reveal a stripe-like nematic order breaking the crystal rotational symmetry within the Kagome lattice itself. Moreover, we identify a set of van Hove singularities adhering to the Kagome layer electrons, which appear along one direction of the Brillouin zone while being annihilated along other high-symmetry directions, revealing a rotational symmetry breaking. Via detailed spectroscopic maps, we further observe an elliptical deformation of Fermi surface, which provides direct evidence for an electronically mediated nematic order. Our work not only bridges the gap between electronic nematicity and Kagome physics, but also sheds light on the potential mechanism for realizing symmetry-broken phases in correlated electron systems.
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Submitted 17 July, 2024; v1 submitted 19 June, 2024;
originally announced June 2024.
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Using magnetic dynamics to measure the spin gap in a candidate Kitaev material
Authors:
Xinyi Jiang,
Qingzheng Qiu,
Cheng Peng,
Hoyoung Jang,
Wenjie Chen,
Xianghong Jin,
Li Yue,
Byungjune Lee,
Sang-Youn Park,
Minseok Kim,
Hyeong-Do Kim,
Xinqiang Cai,
Qizhi Li,
Tao Dong,
Nanlin Wang,
Joshua J. Turner,
Yuan Li,
Yao Wang,
Yingying Peng
Abstract:
Materials potentially hosting Kitaev spin-liquid states are considered crucial for realizing topological quantum computing. However, the intricate nature of spin interactions within these materials complicates the precise measurement of low-energy spin excitations indicative of fractionalized excitations. Using Na$_{2}$Co$_2$TeO$_{6}$ as an example, we study these low-energy spin excitations using…
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Materials potentially hosting Kitaev spin-liquid states are considered crucial for realizing topological quantum computing. However, the intricate nature of spin interactions within these materials complicates the precise measurement of low-energy spin excitations indicative of fractionalized excitations. Using Na$_{2}$Co$_2$TeO$_{6}$ as an example, we study these low-energy spin excitations using the time-resolved resonant elastic x-ray scattering (tr-REXS). Our observations unveil remarkably slow spin dynamics at the magnetic peak, whose recovery timescale is several nanoseconds. This timescale aligns with the extrapolated spin gap of $\sim$ 1 $μ$eV, obtained by density matrix renormalization group (DMRG) simulations in the thermodynamic limit. The consistency demonstrates the efficacy of tr-REXS in discerning low-energy spin gaps inaccessible to conventional spectroscopic techniques.
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Submitted 6 May, 2024;
originally announced May 2024.
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First Principles Studies of Stacking Fault Energies in Ternary Magnesium Alloys
Authors:
Qiwen Qiu,
Stephen Yue,
Jun Song
Abstract:
Magnesium (Mg) alloys have emerged as promising materials due to their low density and high strength-to-weight ratio, offering a wide range of applications across multiple industries. Nevertheless, the inherent brittleness of Mg alloys poses a significant hurdle, necessitating innovative approaches to enhance their mechanical performance. Among the various strategies, manipulating stacking fault e…
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Magnesium (Mg) alloys have emerged as promising materials due to their low density and high strength-to-weight ratio, offering a wide range of applications across multiple industries. Nevertheless, the inherent brittleness of Mg alloys poses a significant hurdle, necessitating innovative approaches to enhance their mechanical performance. Among the various strategies, manipulating stacking fault energy (SFE) has been a key focus, although primarily within the realm of binary alloys. This study investigates SFE in Mg alloys, focusing on ternary compositions. Utilizing first-principles DFT calculations, we analyze solute interactions and their influence on SFE, particularly in Mg-Al-X and Mg-Zn-X configurations. Predictive models are developed for estimating SFE effects, revealing solute pairs that mimic rare earth elements and show potential for improved ductility. The findings contribute to fundamental insights into Mg alloy behavior, offering practical directions for designing advanced materials with superior mechanical properties.
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Submitted 31 March, 2024;
originally announced April 2024.
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Observation of giant circular dichroism induced by electronic chirality
Authors:
Qian Xiao,
Oleg Janson,
Sonia Francoual,
Qingzheng Qiu,
Qizhi Li,
Shilong Zhang,
Wu Xie,
Pablo Bereciartua,
Jeroen van den Brink,
Jasper van Wezel,
Yingying Peng
Abstract:
Chiral phases of matter, characterized by a definite handedness, abound in nature, ranging from the crystal structure of quartz to spiraling spin states in helical magnets. In $1T$-TiSe$_2$ a source of chirality has been proposed that stands apart from these classical examples as it arises from combined electronic charge and quantum orbital fluctuations. This may allow its chirality to be accessed…
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Chiral phases of matter, characterized by a definite handedness, abound in nature, ranging from the crystal structure of quartz to spiraling spin states in helical magnets. In $1T$-TiSe$_2$ a source of chirality has been proposed that stands apart from these classical examples as it arises from combined electronic charge and quantum orbital fluctuations. This may allow its chirality to be accessed and manipulated without imposing either structural or magnetic handedness. However, direct bulk evidence that broken inversion symmetry and chirality are intrinsic to TiSe$_2$ remains elusive. Here, employing resonant elastic scattering of x-rays, we reveal the presence of giant circular dichroism up to $\sim$ 40$\%$ at forbidden Bragg peaks that emerge at the charge and orbital ordering transition. The dichroism varies dramatically with incident energy and azimuthal angle. Comparison to calculated scattering intensities unambiguously traces its origin to bulk chiral electronic order in ${\mathrm{TiSe}}_2$ and establishes resonant elastic x-ray scattering as a sensitive probe to electronic chirality.
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Submitted 19 December, 2023;
originally announced December 2023.
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Charge redistribution, charge order and plasmon in La$_{2-x}$Sr$_{x}$CuO$_{4}$/La$_{2}$CuO$_{4}$ superlattices
Authors:
Qizhi Li,
Lele Ju,
Hsiaoyu Huang,
Yuxuan Zhang,
Changwei Zou,
Tianshuang Ren,
A. Singh,
Shilong Zhang,
Qingzheng Qiu,
Qian Xiao,
Di-Jing Huang,
Yanwu Xie,
Zhen Chen,
Yingying Peng
Abstract:
Interfacial superconductors have the potential to revolutionize electronics, quantum computing, and fundamental physics due to their enhanced superconducting properties and ability to create new types of superconductors. The emergence of superconductivity at the interface of La$_{2-x}$Sr$_{x}$CuO$_{4}$/La$_{2}$CuO$_{4}$ (LSCO/LCO), with a T$_c$ enhancement of $\sim$ 10 K compared to the La…
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Interfacial superconductors have the potential to revolutionize electronics, quantum computing, and fundamental physics due to their enhanced superconducting properties and ability to create new types of superconductors. The emergence of superconductivity at the interface of La$_{2-x}$Sr$_{x}$CuO$_{4}$/La$_{2}$CuO$_{4}$ (LSCO/LCO), with a T$_c$ enhancement of $\sim$ 10 K compared to the La$_{2-x}$Sr$_{x}$CuO$_{4}$ bulk single crystals, provides an exciting opportunity to study quantum phenomena in reduced dimensions. To investigate the carrier distribution and excitations in interfacial superconductors, we combine O K-edge resonant inelastic X-ray scattering and atomic-resolved scanning transmission electron microscopy measurements to study La$_{2-x}$Sr$_{x}$CuO$_{4}$/La$_{2}$CuO$_{4}$ superlattices (x=0.15, 0.45) and bulk La$_{1.55}$Sr$_{0.45}$CuO$_{4}$ films. We find direct evidence of charge redistribution, charge order and plasmon in LSCO/LCO superlattices. Notably, the observed behaviors of charge order and plasmon deviate from the anticipated properties of individual constituents or the average doping level of the superlattice. Instead, they conform harmoniously to the effective doping, a critical parameter governed by the T$_c$ of interfacial superconductors.
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Submitted 4 September, 2023;
originally announced September 2023.
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Pressure-induced color change in the lutetium dihydride LuH2
Authors:
Pengfei Shan,
Ningning Wang,
Xiquan Zheng,
Qingzheng Qiu,
Yingying Peng,
Jinguang Cheng
Abstract:
The lutetium dihydride LuH2 is stable at ambient conditions. Here we show that its color undergoes sequential changes from dark blue at ambient pressure to pink at ~2.2 GPa and then to bright red at ~4 GPa upon compression in a diamond anvil cell. Such a pressure-induced color change in LuH2 is reversible and it is very similar to that recently reported in the N-doped lutetium hydride. However, ou…
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The lutetium dihydride LuH2 is stable at ambient conditions. Here we show that its color undergoes sequential changes from dark blue at ambient pressure to pink at ~2.2 GPa and then to bright red at ~4 GPa upon compression in a diamond anvil cell. Such a pressure-induced color change in LuH2 is reversible and it is very similar to that recently reported in the N-doped lutetium hydride. However, our preliminary resistance measurements on LuH2 under pressures up to 7.7 GPa evidenced no superconductivity down to 1.5 K.
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Submitted 12 March, 2023;
originally announced March 2023.
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Absence of localized $5d^1$ electrons in KTaO$_3$ interface superconductors
Authors:
Xinqiang Cai,
Jungho Kim,
Leonardo Martinelli,
Piero Florio,
Matteo Corti,
Weiliang Qiao,
Yanqiu Sun,
Jiasen Niu,
Quentin Faure,
Christoph Sahle,
Qingzheng Qiu,
Qian Xiao,
Xiquan Zheng,
Qizhi Li,
Changwei Zou,
Xinyi Jiang,
Giacomo Ghiringhelli,
Wei Han,
Yanwu Xie,
Yi Lu,
Marco Moretti Sala,
Yingying Peng
Abstract:
Recently, an exciting discovery of orientation-dependent superconductivity was made in two-dimensional electron gas (2DEG) at the interfaces of LaAlO$_3$/KTaO$_3$ (LAO/KTO) or EuO/KTaO$_3$ (EuO/KTO). The superconducting transition temperature can reach a $T_c$ of up to $\sim$ 2.2 K, which is significantly higher than its 3$d$ counterpart LaAlO$_3$/SrTiO$_3$ (LAO/STO) with a $T_c$ of $\sim$ 0.2 K.…
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Recently, an exciting discovery of orientation-dependent superconductivity was made in two-dimensional electron gas (2DEG) at the interfaces of LaAlO$_3$/KTaO$_3$ (LAO/KTO) or EuO/KTaO$_3$ (EuO/KTO). The superconducting transition temperature can reach a $T_c$ of up to $\sim$ 2.2 K, which is significantly higher than its 3$d$ counterpart LaAlO$_3$/SrTiO$_3$ (LAO/STO) with a $T_c$ of $\sim$ 0.2 K. However, the underlying origin remains to be understood. To uncover the nature of electrons in KTO-based interfaces, we employ x-ray absorption spectroscopy (XAS) and resonant inelastic x-ray spectroscopy (RIXS) to study LAO/KTO and EuO/KTO with different orientations. We reveal the absence of $dd$ orbital excitations in all the measured samples. Our RIXS results are well reproduced by calculations that considered itinerant $5d$ electrons hybridized with O $2p$ electrons. This suggests that there is a lack of localized Ta $5d^1$ electrons in KTO interface superconductors, which is consistent with the absence of magnetic hysteresis observed in magneto-resistance (MR) measurements. These findings offer new insights into our understanding of superconductivity in Ta $5d$ interface superconductors and their potential applications.
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Submitted 3 March, 2023;
originally announced March 2023.
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Prevailing charge order in overdoped cuprates beyond the superconducting dome
Authors:
Qizhi Li,
Hsiao-Yu Huang,
Tianshuang Ren,
Eugen Weschke,
Lele Ju,
Changwei Zou,
Shilong Zhang,
Qingzheng Qiu,
Jiarui Liu,
Shuhan Ding,
Amol Singh,
Oleksandr Prokhnenko,
Di-Jing Huang,
Ilya Esterlis,
Yao Wang,
Yanwu Xie,
Yingying Peng
Abstract:
The extremely overdoped cuprates are generally considered to be Fermi liquid metals without exotic orders, whereas the underdoped cuprates harbor intertwined states. Contrary to this conventional wisdom, using Cu $L_3$ edge and O $K$ edge resonant x-ray scattering, we reveal a charge order (CO) in overdoped La$_{2-x}$Sr$_x$CuO$_4$ (0.35 $\leq$ x $\leq$ 0.6) beyond the superconducting dome. This CO…
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The extremely overdoped cuprates are generally considered to be Fermi liquid metals without exotic orders, whereas the underdoped cuprates harbor intertwined states. Contrary to this conventional wisdom, using Cu $L_3$ edge and O $K$ edge resonant x-ray scattering, we reveal a charge order (CO) in overdoped La$_{2-x}$Sr$_x$CuO$_4$ (0.35 $\leq$ x $\leq$ 0.6) beyond the superconducting dome. This CO has a periodicity of $\sim$ 6 lattice units with correlation lengths of $\sim 3 - 20$ lattice units. It shows similar in-plane momentum and polarization dependence and dispersive excitations as the CO of underdoped cuprates, but its maximum intensity differs along the c-direction and persists up to 300 K. This CO cannot be explained by either the Fermi surface instability or the doped Hubbard model and its origin remains to be understood. Our results suggest that CO is prevailing in the overdoped metallic regime and superconductivity emerges out of the CO phase upon decreasing hole carriers.
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Submitted 17 October, 2022; v1 submitted 18 August, 2022;
originally announced August 2022.
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Coexistence of Multiple Stacking Charge Density Waves in Kagome Superconductor ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$
Authors:
Qian Xiao,
Yihao Lin,
Qizhi Li,
Xiquan Zheng,
Sonia Francoual,
Christian Plueckthun,
Wei Xia,
Qingzheng Qiu,
Shilong Zhang,
Yanfeng Guo,
Ji Feng,
Yingying Peng
Abstract:
The recently discovered Kagome family ${\mathrm{AV}}_3{\mathrm{Sb}}_5$ (A = K, Rb, Cs) exhibits rich physical phenomena, including non-trivial topological electronic structure, giant anomalous Hall effect, charge density waves (CDW) and superconductivity. Notably, CDW in ${\mathrm{AV}}_3{\mathrm{Sb}}_5$ is evidenced to intertwine with its superconductivity and topology, but its nature remains elus…
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The recently discovered Kagome family ${\mathrm{AV}}_3{\mathrm{Sb}}_5$ (A = K, Rb, Cs) exhibits rich physical phenomena, including non-trivial topological electronic structure, giant anomalous Hall effect, charge density waves (CDW) and superconductivity. Notably, CDW in ${\mathrm{AV}}_3{\mathrm{Sb}}_5$ is evidenced to intertwine with its superconductivity and topology, but its nature remains elusive. Here, we combine x-ray scattering experiments and density-functional theory calculations to investigate the CDWs in ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$ and demonstrate the coexistence of 2 $\times$ 2 $\times$ 2 and 2 $\times$ 2 $\times$ 4 CDW stacking phases. Competition between these CDW phases is revealed by tracking the temperature evolution of CDW intensities, which also manifests in different transition temperatures during warming- and cooling measurements. We also identify a meta-stable quenched state of ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$ after fast-cooling process. Our study demonstrates the coexistence of competing CDW stacking in ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$, offering new insights in understanding the novel properties of this system.
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Submitted 10 March, 2023; v1 submitted 13 January, 2022;
originally announced January 2022.
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Establishing the carrier scattering phase diagram for ZrNiSn-based half-Heusler thermoelectric materials
Authors:
Qingyong Ren,
Chenguang Fu,
Qinyi Qiu,
Shengnan Dai,
Zheyuan Liu,
Takatsugu Masuda,
Shinichiro Asai,
Masato Hagihala,
Sanghyun Lee,
Shuki Torri,
Takashi Kamiyama,
Lunhua He,
Xin Tong,
Claudia Felser,
David J. Singh,
Tiejun Zhu,
Jiong Yang,
Jie Ma
Abstract:
Chemical doping is one of the most important strategies for tuning electrical properties of semiconductors, particularly thermoelectric materials. Generally, the main role of chemical doping lies in optimizing the carrier concentration, but there can potentially be other important effects. Here, we show that chemical doping plays multiple roles for both electron and phonon transport properties in…
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Chemical doping is one of the most important strategies for tuning electrical properties of semiconductors, particularly thermoelectric materials. Generally, the main role of chemical doping lies in optimizing the carrier concentration, but there can potentially be other important effects. Here, we show that chemical doping plays multiple roles for both electron and phonon transport properties in half-Heusler thermoelectric materials. With ZrNiSn-based half-Heusler materials as an example, we use high-quality single and polycrystalline crystals, various probes, including electrical transport measurements, inelastic neutron scattering measurement, and first-principles calculations, to investigate the underlying electron-phonon interaction. We find that chemical doping brings strong screening effects to ionized impurities, grain boundary, and polar optical phonon scattering, but has negligible influence on lattice thermal conductivity. Furthermore, it is possible to establish a carrier scattering phase diagram, which can be used to select reasonable strategies for optimization of the thermoelectric performance.
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Submitted 22 June, 2020; v1 submitted 25 March, 2020;
originally announced March 2020.
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Coherent transfer of spin angular momentum by evanescent spin waves within antiferromagnetic NiO
Authors:
Maciej Dabrowski,
Takafumi Nakano,
David M. Burn,
Andreas Frisk,
David G. Newman,
Christoph Klewe,
Qian Li,
Mengmeng Yang,
Padraic Shafer,
Elke Arenholz,
Thorsten Hesjedal,
Gerrit van der Laan,
Zi Q. Qiu,
Robert J. Hicken
Abstract:
Insulating antiferromagnets are efficient and robust conductors of spin current. To realise the full potential of these materials within spintronics, the outstanding challenges are to demonstrate scalability down to nanometric lengthscales and the transmission of coherent spin currents. Here, we report the coherent transfer of spin angular momentum by excitation of evanescent spin waves of GHz fre…
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Insulating antiferromagnets are efficient and robust conductors of spin current. To realise the full potential of these materials within spintronics, the outstanding challenges are to demonstrate scalability down to nanometric lengthscales and the transmission of coherent spin currents. Here, we report the coherent transfer of spin angular momentum by excitation of evanescent spin waves of GHz frequency within antiferromagnetic NiO at room temperature. Using element-specific and phase-resolved x-ray ferromagnetic resonance, we probe the injection and transmission of ac spin current, and demonstrate that insertion of a few nanometre thick epitaxial NiO(001) layer between a ferromagnet and non-magnet can even enhance the flow of spin current. Our results pave the way towards coherent control of the phase and amplitude of spin currents at the nanoscale, and enable the realization of spin-logic devices and spin current amplifiers that operate at GHz and THz frequencies.
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Submitted 11 December, 2019;
originally announced December 2019.
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Coherent ac spin current transmission across an antiferromagnetic CoO insulator
Authors:
Q. Li,
M. Yang,
C. Klewe,
P. Shafer,
A. T. N'Diaye,
D. Hou,
T. Y. Wang,
N. Gao,
E. Saitoh,
C. Hwang,
R. J. Hicken,
J. Li,
E. Arenholz,
Z. Q. Qiu
Abstract:
The recent discovery of spin-current transmission through antiferromagnetic (AFM) insulating materials opens up unprecedented opportunities for fundamental physics and spintronics applications. The great mystery currently surrounding this topic is: how could THz AFM magnons mediate a GHz spin current? This mis-match of frequencies becomes particularly critical for the case of coherent ac spin-curr…
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The recent discovery of spin-current transmission through antiferromagnetic (AFM) insulating materials opens up unprecedented opportunities for fundamental physics and spintronics applications. The great mystery currently surrounding this topic is: how could THz AFM magnons mediate a GHz spin current? This mis-match of frequencies becomes particularly critical for the case of coherent ac spin-current, raising the fundamental question of whether a GHz ac spin-current can ever keep its coherence inside an AFM insulator and so drive the spin precession of another FM layer coherently? Utilizing element- and time-resolved x-ray pump-probe measurements on Py/Ag/CoO/Ag/Fe75Co25/MgO(001) heterostructures, we demonstrate that a coherent GHz ac spin current pumped by the permalloy (Py) ferromagnetic resonance (FMR) can transmit coherently across an antiferromagnetic CoO insulating layer to drive a coherent spin precession of the FM Fe75Co25 layer. Further measurement results favor thermal magnons rather than evanescent spin waves as the mediator of the coherent ac spin current in CoO.
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Submitted 1 June, 2019;
originally announced June 2019.
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Discovery of intrinsic ferromagnetism in 2D van der Waals crystals
Authors:
Cheng Gong,
Lin Li,
Zhenglu Li,
Huiwen Ji,
Alex Stern,
Yang Xia,
Ting Cao,
Wei Bao,
Chenzhe Wang,
Yuan Wang,
Z. Q. Qiu,
R. J. Cava,
Steven G. Louie,
Jing Xia,
Xiang Zhang
Abstract:
It has been long hoped that the realization of long-range ferromagnetic order in two-dimensional (2D) van der Waals (vdW) crystals, combined with their rich electronic and optical properties, would open up new possibilities for magnetic, magnetoelectric and magneto-optic applications. However, in 2D systems, the long-range magnetic order is strongly hampered by thermal fluctuations which may be co…
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It has been long hoped that the realization of long-range ferromagnetic order in two-dimensional (2D) van der Waals (vdW) crystals, combined with their rich electronic and optical properties, would open up new possibilities for magnetic, magnetoelectric and magneto-optic applications. However, in 2D systems, the long-range magnetic order is strongly hampered by thermal fluctuations which may be counteracted by magnetic anisotropy, according to the Mermin-Wagner theorem. Prior efforts via defect and composition engineering, and proximity effect only locally or extrinsically introduce magnetic responses. Here we report the first experimental discovery of intrinsic long-range ferromagnetic order in pristine Cr2Ge2Te6 atomic layers by scanning magneto-optic Kerr microscopy. In such a 2D vdW soft ferromagnet, for the first time, an unprecedented control of transition temperature of ~ 35% - 57% enhancement is realized via surprisingly small fields (<= 0.3 Tesla in this work), in stark contrast to the stiffness of the transition temperature to magnetic fields in the three-dimensional regime. We found that the small applied field enables an effective anisotropy far surpassing the tiny magnetocrystalline anisotropy, opening up a sizable spin wave excitation gap. Confirmed by renormalized spin wave theory, we explain the phenomenon and conclude that the unusual field dependence of transition temperature constitutes a hallmark of 2D soft ferromagnetic vdW crystals. Our discovery of 2D soft ferromagnetic Cr2Ge2Te6 presents a close-to-ideal 2D Heisenberg ferromagnet for studying fundamental spin behaviors, and opens the door for exploring new applications such as ultra-compact spintronics.
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Submitted 16 March, 2017;
originally announced March 2017.
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Direct detection of pure spin-current by x-ray pump-probe measurements
Authors:
J. Li,
L. R. Shelford,
P. Shafer,
A. Tan,
J. X. Deng,
P. S. Keatley,
C. Hwang,
E. Arenholz,
G. van der Laan,
R. J. Hicken,
Z. Q. Qiu
Abstract:
By synchronizing a microwave waveform with the synchrotron x-ray pulses, we use the ferromagnetic resonance (FMR) of the Py (Ni81Fe19) layer in a Py/Cu/Cu75Mn25/Cu/Co multilayer to pump a pure spin current into the Cu75Mn25 spacer layer, and then directly probe the spin current in the Cu75Mn25 layer by a time-resolved x-ray magnetic circular dichroism (XMCD). This element-specific pump-probe measu…
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By synchronizing a microwave waveform with the synchrotron x-ray pulses, we use the ferromagnetic resonance (FMR) of the Py (Ni81Fe19) layer in a Py/Cu/Cu75Mn25/Cu/Co multilayer to pump a pure spin current into the Cu75Mn25 spacer layer, and then directly probe the spin current in the Cu75Mn25 layer by a time-resolved x-ray magnetic circular dichroism (XMCD). This element-specific pump-probe measurement unambiguously identifies the AC spin current in the Cu75Mn25 layer. In addition, phase resolved x-ray measurements reveal a characteristic bipolar phase behavior of the Co spins that is a fingerprint of spin-current driven spin precession.
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Submitted 16 July, 2016; v1 submitted 15 May, 2015;
originally announced May 2015.
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Spin-current probe for phase transition in an insulator
Authors:
Zhiyong Qiu,
Jia Li,
Dazhi Hou,
Elke Arenholz,
Alpha T. NDiaye,
Ali Tan,
Ken-ichi Uchida,
K. Sato,
Satoshi Okamoto,
Yaroslav Tserkovnyak,
Z. Q. Qiu,
Eiji Saitoh
Abstract:
Spin fluctuation and transition have always been one of central topics of magnetism and condensed matter science. Experimentally, the spin fluctuation is found transcribed onto scattering intensity in the neutron scattering process, which is represented by dynamical magnetic susceptibility and maximized at phase transitions. Importantly, a neutron carries spin without electric charge, and it can b…
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Spin fluctuation and transition have always been one of central topics of magnetism and condensed matter science. Experimentally, the spin fluctuation is found transcribed onto scattering intensity in the neutron scattering process, which is represented by dynamical magnetic susceptibility and maximized at phase transitions. Importantly, a neutron carries spin without electric charge, and it can bring spin into a sample without being disturbed by electric energy, although large facilities such as a nuclear reactor is necessary. Here we show that spin pumping, frequently used in nanoscale spintronic devices, provides a desktop micro probe for spin transition; spin current is a flux of spin without an electric charge and its transport reflects spin excitation. We demonstrate detection of antiferromagnetic transition in ultra-thin CoO films via frequency dependent spin-current transmission measurements, which provides a versatile probe for phase transition in an electric manner in minute devices.
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Submitted 30 August, 2016; v1 submitted 14 May, 2015;
originally announced May 2015.
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Skyrmion Motion Driven by Oscillating Magnetic Field
Authors:
Kyoung-Woong Moon,
Duck-Ho Kim,
Soong-Geun Je,
Byong Sun Chun,
Wondong Kim,
Z. Q. Qiu,
Sug-Bong Choe,
Chanyong Hwang
Abstract:
Magnetic skyrmion motion induced by an electric current has drawn much interest because of its application potential in next-generation magnetic memory devices. Recently, unidirectional skyrmion motion driven by an oscillating magnetic field was also demonstrated on large (20 micrometer) bubble domains with skyrmion topology. At smaller length scale which is more relevant to high-density memory de…
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Magnetic skyrmion motion induced by an electric current has drawn much interest because of its application potential in next-generation magnetic memory devices. Recently, unidirectional skyrmion motion driven by an oscillating magnetic field was also demonstrated on large (20 micrometer) bubble domains with skyrmion topology. At smaller length scale which is more relevant to high-density memory devices, we here show by numerical simulation that a skyrmion of a few tens of nanometers could also be driven by high-frequency field oscillations but with the motion direction different from the tilted oscillating field direction. We found that high-frequency field for small size skyrmions could excite skyrmion resonant modes and that a combination of different modes would result in the final skyrmion motion with a helical trajectory. Because this helical motion depends on the frequency of the field, we can control both the speed and the direction of the skyrmion motion, which is a distinguishable characteristic compared with other methods.
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Submitted 23 April, 2015;
originally announced April 2015.
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Measurement of the spectral line shapes for orbital excitations in the Mott insulator CoO using high-resolution resonant inelastic x-ray scattering
Authors:
L. Andrew Wray,
J. Li,
Z. Q. Qiu,
Jinsheng Wen,
Zhijun Xu,
Genda Gu,
Shih-Wen Huang,
Elke Arenholz,
Wanli Yang,
Zahid Hussain,
Yi-De Chuang
Abstract:
We establish the spectral line shape of orbital excitations created by resonant inelastic X-ray scattering for the model Mott insulator CoO. Improved experimental energy resolution reveals that the line shapes are strikingly different from expectations in a first principles-based atomic multiplet model. Extended theoretical simulations are performed to identify the underlying physical origins, whi…
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We establish the spectral line shape of orbital excitations created by resonant inelastic X-ray scattering for the model Mott insulator CoO. Improved experimental energy resolution reveals that the line shapes are strikingly different from expectations in a first principles-based atomic multiplet model. Extended theoretical simulations are performed to identify the underlying physical origins, which include a pronounced thermal tail reminiscent of anti-Stokes scattering on the energy gain side of excitations, and an essential contribution from interatomic many-body dynamics on the energy loss side of excitations.
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Submitted 11 January, 2014;
originally announced January 2014.
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Reduction factors for the icosahedral T_1u x h_g Jahn-Teller system
Authors:
Q. C. Qiu,
J. L. Dunn,
C. A. Bates,
M. Abou-Ghantous,
V. Z. Polinger
Abstract:
Reduction factors (RF), that are needed when modeling vibronic systems by an effective Hamiltonian in an electronic basis, are calculated for the T_1u x h_g Jahn-Teller (JT) system. The results obtained will be useful when modeling the fullerene anion C_60-, which is believed to exhibit a T_1u x h_g JT effect in its ground state. First-order RF's are calculated using symmetry-adapted vibronic gr…
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Reduction factors (RF), that are needed when modeling vibronic systems by an effective Hamiltonian in an electronic basis, are calculated for the T_1u x h_g Jahn-Teller (JT) system. The results obtained will be useful when modeling the fullerene anion C_60-, which is believed to exhibit a T_1u x h_g JT effect in its ground state. First-order RF's are calculated using symmetry-adapted vibronic ground and tunneling states in which the system is allowed to tunnel between equivalent minima in the potential energy surface. The effect of anisotropy in the minima is also considered. Second-order RF's are calculated incorporating coupling to excited harmonic-oscillator states associated with the minima.
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Submitted 31 July, 2002;
originally announced July 2002.