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"Emergence of spin singlets with inhomogeneous gaps in the kagome lattice Heisenberg antiferromagnets Zn-barlowite and herbertsmithite" — Jiaming Wang: Weishi Yuan, Philip M. Singer, Rebecca W. Smaha, Wei He, Jiajia Wen, Young S. Lee & Takashi Imai; Nature Physics, 08/05/21.
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Weishi Yuan, Philip M. Singer, Rebecca W. Smaha, Wei He, Jiajia Wen, Young S. Lee & Takashi Imai
Abstract
The kagome Heisenberg antiferromagnet formed by frustrated spins arranged in a lattice of corner-sharing triangles is a prime candidate for hosting a quantum spin liquid (QSL) ground state consisting of entangled spin singlets1. However, the existence of various competing states makes a convincing theoretical prediction of the QSL ground state difficult2, calling for experimental clues from model materials. The kagome lattice materials Zn-barlowite (ZnCu3(OD)6FBr)3,4,5 and herbertsmithite (ZnCu3(OD)6Cl2)6,7,8,9,10 do not exhibit long-range order and are considered the best realizations of the kagome Heisenberg antiferromagnet known so far. Here we use 63Cu nuclear quadrupole resonance combined with the inverse Laplace transform11,12,13 to locally probe the inhomogeneity of delicate quantum ground states affected by disorder14,15,16,17. We present direct evidence for the gradual emergence of spin singlets with spatially varying excitation gaps, but even at temperatures far below the super-exchange energy scale their fraction is limited to ~60% of the total spins. Theoretical models18,19 need to incorporate the role of disorder to account for the observed inhomogeneously gapped behaviour.
"Magnetic excitations in infinite-layer nickelates" — H. Lu: M. Rossi, A. Nag, M. Osada, D. F. Li, K. Lee, B. Y. Wang, M. Garcia-Fernandez, S. Agrestini, Z. X. Shen, E. M. Been, B. Moritz, T. P. Devereaux, J. Zaanen, H. Y. Hwang, Ke-Jin Zhou, W. S. Lee; Science, 07/09/21.
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M. Rossi, A. Nag, M. Osada, D. F. Li, K. Lee, B. Y. Wang, M. Garcia-Fernandez, S. Agrestini, Z. X. Shen, E. M. Been, B. Moritz, T. P. Devereaux, J. Zaanen, H. Y. Hwang, Ke-Jin Zhou, W. S. Lee
Abstract
The discovery of superconductivity in infinite-layer nickelates brings us tantalizingly close to a material class that mirrors the cuprate superconductors. We measured the magnetic excitations in these nickelates using resonant inelastic x-ray scattering at the Ni L3-edge. Undoped NdNiO2 possesses a branch of dispersive excitations with a bandwidth of approximately 200 milli–electron volts, which is reminiscent of the spin wave of strongly coupled, antiferromagnetically aligned spins on a square lattice. The substantial damping of these modes indicates the importance of coupling to rare-earth itinerant electrons. Upon doping, the spectral weight and energy decrease slightly, whereas the modes become overdamped. Our results highlight the role of Mottness in infinite-layer nickelates.
"Bifunctional Asymmetric Fabric with Tailored Thermal Conduction and Radiation for Personal Cooling and Warming" — Yucan Peng : Hiang Kwee Lee, David S Wu, Yi Cui; Engineering, 05/28/21.
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Hiang Kwee Lee, David S Wu, Yi Cui
Abstract
Personal thermal management is emerging as a promising strategy to provide thermal comfort for the human body while conserving energy. By improving control over the heat dissipating from the human body, personal thermal management can provide effective personal cooling and warming. Here, we propose a facile surface modification approach to tailor the thermal conduction and radiation properties based on commercially available fabric, to realize better management of the whole heat transport pathway from the human body to the ambient. A bifunctional asymmetric fabric (BAF) offering both a cooling and a warming effect is demonstrated. Due to the advantages of roughness asymmetry and surface modification, the BAF demonstrates an effective cooling effect through enhanced heat conduction and radiation in the cooling mode; in the warming mode, heat dissipation along both routes is reduced for personal warming. As a result, a 4.6 °C skin temperature difference is measured between the cooling and warming BAF modes, indicating that the thermal comfort zone of the human body can be enlarged with one piece of BAF clothing. We expect this work to present new insights for the design of personal thermal management textiles as well as a novel solution for the facile modification of available fabrics for both personal cooling and warming.
"High-temperature superconductivity" — Xingjiang Zhou: Wei-Sheng Lee, Masatoshi Imada, Nandini Trivedi, Philip Phillips, Hae-Young Kee, Päivi Törmä and Mikhail Eremets; Nature Reviews Physics, 05/28/21.
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Wei-Sheng Lee, Masatoshi Imada, Nandini Trivedi, Philip Phillips, Hae-Young Kee, Päivi Törmä and Mikhail Eremets
Abstract
Despite decades of intense theoretical, experimental and computational effort, a microscopic theory of high-temperature superconductivity is not yet established. Eight researchers share their contributions to the search for a better understanding of unconventional superconductivity and their hopes for the future of the field.
"Sensitive, portable heavy-metal-ion detection by the sulfidation method on a superhydrophobic concentrator (SPOT)" — Hiang Kwee Lee: Wenxiao Huang, Yusheng Ye, Jinwei Xu, Yucan Peng, Tong Wu, Ankun Yang, Lien-Yang Chou, Xin Xiao, Xin Gao, Fang Liu, Hansen Wang, Bofei Liu, Jiangyan Wang, and Yi Cui; One Earth, 05/21/21.
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Wenxiao Huang, Yusheng Ye, Jinwei Xu, Yucan Peng, Tong Wu, Ankun Yang, Lien-Yang Chou, Xin Xiao, Xin Gao, Fang Liu, Hansen Wang, Bofei Liu, Jiangyan Wang, and Yi Cui
Abstract
One in three people worldwide does not have access to safe drinking water. Notably, heavy-metal ions (HMIs) are major water pollutants threatening human health because of their severe toxicity, even at trace levels. Efficient HMI detection thus plays a major defense against metal poisoning by enabling early pollution warning and efficient regulatory enforcement. However, it remains a formidable challenge to accurately detect these pollutants on site at ultratrace levels in a cost- or time-effective manner. Here, we introduce an efficient, portable sensor to concurrently quantify five different HMIs down to the sub-nanomolar level by sulfiding them on a superhydrophobic surface. Sulfidation serves as a colorimetric reaction while the superhydrophobic surface concentrates analytes for sensitive visual detection. Our superhydrophobic concentrator (SPOT) sensor can be made portable by being integrated with a smartphone application to quantify HMIs in <8 min and at $0.02 per analysis. Decentralizing water monitoring by using our SPOT design is crucial to ensuring that clean water is accessible to everyone.

"Alloying a single and a double perovskite: a Cu+/2+ mixed-valence layered halide perovskite with strong optical absorption" — Bridget A. Connor: Rebecca W. Smaha, Jiayi Li, Aryeh Gold-Parker, Alexander J. Heyer, Michael F. Toney, Young S. Lee and Hemamala I. Karunadasa; Chemical Science, 05/14/21.
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Rebecca W. Smaha, Jiayi Li, Aryeh Gold-Parker, Alexander J. Heyer, Michael F. Toney, Young S. Lee and Hemamala I. Karunadasa
Abstract
Introducing heterovalent cations at the octahedral sites of halide perovskites can substantially change their optoelectronic properties. Yet, in most cases, only small amounts of such metals can be incorporated as impurities into the three-dimensional lattice. Here, we exploit the greater structural flexibility of the two-dimensional (2D) perovskite framework to place three distinct stoichiometric cations in the octahedral sites. The new layered perovskites AI4[CuII(CuIInIII)0.5Cl8] (1, A = organic cation) may be derived from a CuI–InIII double perovskite by replacing half of the octahedral metal sites with Cu2+. Electron paramagnetic resonance and X-ray absorption spectroscopy confirm the presence of Cu2+ in 1. Crystallographic studies demonstrate that 1 represents an averaging of the CuI–InIII double perovskite and CuII single perovskite structures. However, whereas the highly insulating CuI–InIII and CuII perovskites are colorless and yellow, respectively, 1 is black, with substantially higher electronic conductivity than that of either endmember. We trace these emergent properties in 1 to intervalence charge transfer between the mixed-valence Cu centers. We further propose a tiling model to describe how the Cu+, Cu2+, and In3+ coordination spheres can pack most favorably into a 2D perovskite lattice, which explains the unusual 1 : 2 : 1 ratio of these cations found in 1. Magnetic susceptibility data of 1 further corroborate this packing model. The emergence of enhanced visible light absorption and electronic conductivity in 1 demonstrates the importance of devising strategies for increasing the compositional complexity of halide perovskites.
"Alloying a single and a double perovskite: a Cu+/2+ mixed-valence layered halide perovskite with strong optical absorption" — Bridget A. Connor,: Rebecca W. Smaha, Jiayi Li, Aryeh Gold-Parker, Alexander J. Heyer, Michael F. Toney, Young S. Lee and Hemamala I. Karunadasa; Chemical Science, 05/14/21.
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Rebecca W. Smaha, Jiayi Li, Aryeh Gold-Parker, Alexander J. Heyer, Michael F. Toney, Young S. Lee and Hemamala I. Karunadasa
Abstract
Introducing heterovalent cations at the octahedral sites of halide perovskites can substantially change their optoelectronic properties. Yet, in most cases, only small amounts of such metals can be incorporated as impurities into the three-dimensional lattice. Here, we exploit the greater structural flexibility of the two-dimensional (2D) perovskite framework to place three distinct stoichiometric cations in the octahedral sites. The new layered perovskites AI4[CuII(CuIInIII)0.5Cl8] (1, A = organic cation) may be derived from a CuI–InIII double perovskite by replacing half of the octahedral metal sites with Cu2+. Electron paramagnetic resonance and X-ray absorption spectroscopy confirm the presence of Cu2+ in 1. Crystallographic studies demonstrate that 1 represents an averaging of the CuI–InIII double perovskite and CuII single perovskite structures. However, whereas the highly insulating CuI–InIII and CuII perovskites are colorless and yellow, respectively, 1 is black, with substantially higher electronic conductivity than that of either endmember. We trace these emergent properties in 1 to intervalence charge transfer between the mixed-valence Cu centers. We further propose a tiling model to describe how the Cu+, Cu2+, and In3+ coordination spheres can pack most favorably into a 2D perovskite lattice, which explains the unusual 1 : 2 : 1 ratio of these cations found in 1. Magnetic susceptibility data of 1 further corroborate this packing model. The emergence of enhanced visible light absorption and electronic conductivity in 1 demonstrates the importance of devising strategies for increasing the compositional complexity of halide perovskites.
"Carbon nanotube thermoelectric devices by direct printing: Toward wearable energy converters" — Hye Ryoung Lee: Naoki Furukawa, Antonio J. Ricco, Eric Pop, Yi Cui, and Yoshio Nishi; Applied Physics Letters, 04/27/21.
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Naoki Furukawa, Antonio J. Ricco, Eric Pop, Yi Cui, and Yoshio Nishi
Abstract
Thermoelectric devices convert thermal energy to electrical energy and are particularly well-suited for energy harvesting from waste heat. Even as the number of electronic devices used in daily life proliferates, technical advances diminish the average power such devices require to perform a given function. Localized thermal gradients that abound in our living environments, despite having modest energy densities, are therefore becoming increasingly viable and attractive to power such devices. With this motivation, we report the design, fabrication, and characterization of single-wall carbon nanotube thermoelectric devices (CNT-TDs) on flexible polyimide substrates as a basis for wearable energy converters. Our aqueous-solution-based film fabrication process could enable readily scalable, low-cost TDs; here, we demonstrate CNT-hydroxypropyl cellulose (HPC) composite thermoelectric films by aerosol jet printing. The electrical conductivity of the composite films is controlled through the number of CNT/HPC layers printed in combination with control of the annealing conditions. The HPC initially disperses the CNTs in deionized water, the greenest of solvents, and is subsequently partially eliminated from the film by annealing, with concomitant morphological changes that we characterized by TEM. HPC removal is key to obtaining good electrical conductivity (0.94 to 1.10 × 105 S/m) and Seebeck coefficients (36 to 43 μV/K). We also report a power factor of 208 μW m−1 K−2 for a CNT-TD composed of 15 layers of CNT/HPC, promising performance for CNT-based flexible TDs that are deposited from aqueous solution, stable in air, and require no additional doping or sorting processes.
"Subterahertz collective dynamics of polar vortices" — Qian Li: Vladimir A. Stoica, Marek Paściak, Yi Zhu, Yakun Yuan, Tiannan Yang, Margaret R. McCarter, Sujit Das, Ajay K. Yadav, Suji Park, Cheng Dai, Hyeon Jun Lee, Youngjun Ahn, Samuel D. Marks, Shukai Yu, Christelle Kadlec, Takahiro Sato, Matthias C. Hoffmann, Matthieu Chollet, Michael E. Kozina, Silke Nelson, Diling Zhu, Donald A. Walko, Aaron M. Lindenberg, Paul G. Evans, Long-Qing Chen, Ramamoorthy Ramesh, Lane W. Martin, Venkatraman Gopalan, John W. Freeland, Jirka Hlinka & Haidan Wen; Nature, 04/14/21.
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Vladimir A. Stoica, Marek Paściak, Yi Zhu, Yakun Yuan, Tiannan Yang, Margaret R. McCarter, Sujit Das, Ajay K. Yadav, Suji Park, Cheng Dai, Hyeon Jun Lee, Youngjun Ahn, Samuel D. Marks, Shukai Yu, Christelle Kadlec, Takahiro Sato, Matthias C. Hoffmann, Matthieu Chollet, Michael E. Kozina, Silke Nelson, Diling Zhu, Donald A. Walko, Aaron M. Lindenberg, Paul G. Evans, Long-Qing Chen, Ramamoorthy Ramesh, Lane W. Martin, Venkatraman Gopalan, John W. Freeland, Jirka Hlinka & Haidan Wen
Abstract
The collective dynamics of topological structures1,2,3,4,5,6 are of interest from both fundamental and applied perspectives. For example, studies of dynamical properties of magnetic vortices and skyrmions3,4 have not only deepened our understanding of many-body physics but also offered potential applications in data processing and storage7. Topological structures constructed from electrical polarization, rather than electron spin, have recently been realized in ferroelectric superlattices5,6, and these are promising for ultrafast electric-field control of topological orders. However, little is known about the dynamics underlying the functionality of such complex extended nanostructures. Here, using terahertz-field excitation and femtosecond X-ray diffraction measurements, we observe ultrafast collective polarization dynamics that are unique to polar vortices, with orders-of-magnitude higher frequencies and smaller lateral size than those of experimentally realized magnetic vortices3. A previously unseen tunable mode, hereafter referred to as a vortexon, emerges in the form of transient arrays of nanoscale circular patterns of atomic displacements, which reverse their vorticity on picosecond timescales. Its frequency is considerably reduced (softened) at a critical strain, indicating a condensation (freezing) of structural dynamics. We use first-principles-based atomistic calculations and phase-field modelling to reveal the microscopic atomic arrangements and corroborate the frequencies of the vortex modes. The discovery of subterahertz collective dynamics in polar vortices opens opportunities for electric-field-driven data processing in topological structures with ultrahigh speed and density.
"Aspects of the synthesis of thin film superconducting infinite-layer nickelates" — Kyuho Lee: Berit H. Goodge, Danfeng Li, Motoki Osada, Bai Yang Wang, Yi Cui, Lena F. Kourkoutis, Harold Y. Hwang; APL Materials, 04/09/21.
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Berit H. Goodge, Danfeng Li, Motoki Osada, Bai Yang Wang, Yi Cui, Lena F. Kourkoutis, Harold Y. Hwang
Abstract
The recent observation of superconductivity in Nd0.8Sr0.2NiO2 calls for further investigation and optimization of the synthesis of this infinite-layer nickelate structure. Here, we present our current understanding of important aspects of the growth of the parent perovskite compound via pulsed laser deposition on SrTiO3 (001) substrates and the subsequent topotactic reduction. We find that to achieve single-crystalline, single-phase superconducting Nd0.8Sr0.2NiO2, it is essential that the precursor perovskite Nd0.8Sr0.2NiO3 thin film is stabilized with no visible impurity phases; in particular, a Ruddlesden–Popper-type secondary phase is often observed. We have further investigated the evolution of the soft-chemistry topotactic reduction conditions to realize full transformation to the infinite-layer structure with no film decomposition or formation of other phases. We find that capping the nickelate film with a subsequent SrTiO3 layer provides an epitaxial template to the top region of the nickelate film, much like the substrate. Thus, for currently optimized growth conditions, we can stabilize superconducting single-phase Nd0.8Sr0.2NiO2 (001) epitaxial thin films up to ∼10 nm.