Showing 1–42 of 42 results for author: Du, K

Cascade of Spin Liquids in a Bilayer Triangular-lattice Antiferromagnet Rb_2Co_2(SeO_3)_3

Authors: Xiaoyu Xu, Yunlong Wang, Xuejuan Gui, Jun Luo, Guijing Duan, Ke Shi, Zhaosheng Wang, Shuo Li, Huifen Ren, Chuanying Xi, Langsheng Ling, Zhanlong Wu, Ying Chen, Xiaohui Bo, Xinyu Shi, Kefan Du, Rui Bian, Jie Yang, Yi Cui, Rui Zhou, Jinchen Wang, Rong Yu, Weiqiang Yu

Abstract: In frustrated Ising magnets, classical spin liquids (CSLs) with macroscopic ground-state degeneracy can survive against conventional magnetic order, as exemplified by systems on triangular, kagome and pyrochlore lattices at zero field. Here we report the discovery of a high-field route toward spin liquids in a bilayer triangular lattice antiferromagnet, Rb$_2$Co$_2$(SeO$_3$)$_3$. We demonstrate th… ▽ More In frustrated Ising magnets, classical spin liquids (CSLs) with macroscopic ground-state degeneracy can survive against conventional magnetic order, as exemplified by systems on triangular, kagome and pyrochlore lattices at zero field. Here we report the discovery of a high-field route toward spin liquids in a bilayer triangular lattice antiferromagnet, Rb$_2$Co$_2$(SeO$_3$)$_3$. We demonstrate that a cascade of CSLs -- characterized by doubly degenerate one-up-one-down local spin configurations and a residual entropy of 1/2(1-M/M_s)Rln2 per mole -- emerges through field-controlled dilution of Ising dimers. Owing to the interplay of intra- and inter-layer interactions, these CSLs are further stabilized by lattice symmetry breaking at fractional magnetization plateaus. Such field-induced spin liquids can be understood as a consequence of generalized ice rules, analogous to those governing in pyrochlore antiferromagnets. In particular, the 5/6-plateau state is a candidate quantum spin liquid. Our results thereby establish a new pathway for exploring diverse spin liquid states across both classical and quantum regimes. △ Less

Submitted 6 April, 2026; v1 submitted 4 April, 2026; originally announced April 2026.

Comments: 8 pages, 4 figures

NMR evidence of spin supersolid and Pomeranchuk effect behaviors in the triangular-lattice antiferromagnet Rb$_2$Ni$_2$(SeO$_3$)$_3$

Authors: Ying Chen, Zhanlong Wu, Xuejuan Gui, Guijing Duan, Shuo Li, Xiaoyu Xu, Kefan Du, Xinyu Shi, Rui Bian, Xiaohui Bo, Guochen Liu, Jun Luo, Jie Yang, Yi Cui, Rui Zhou, Jinchen Wang, Rong Yu, Weiqiang Yu

Abstract: We performed $^{85}$Rb nuclear magnetic resonance (NMR) measurements on the $S$ = 1 bilayer triangular-lattice antiferromagnet Rb$_2$Ni$_2$(SeO$_3$)$_3$ in magnetic fields up to 26 T. In the field range from 3 T to 26 T, the NMR spectral lines split and their respective spectral weight ratios reveal the existence of the magnetic up-up-down (UUD) phase, although the 1/3-plateau phase is only reache… ▽ More We performed $^{85}$Rb nuclear magnetic resonance (NMR) measurements on the $S$ = 1 bilayer triangular-lattice antiferromagnet Rb$_2$Ni$_2$(SeO$_3$)$_3$ in magnetic fields up to 26 T. In the field range from 3 T to 26 T, the NMR spectral lines split and their respective spectral weight ratios reveal the existence of the magnetic up-up-down (UUD) phase, although the 1/3-plateau phase is only reached at fields above 16 T. Two distinct gapless regimes are further identified: one at low fields and low temperatures, and the other at high fields and high temperatures, consistent with the spin supersolid Y and V phases. Notably, the UUD-V phase boundary exhibits a negative slope in $dT/dH$, where the supersolid phase is located at temperatures above the solid phase due to strong low-energy spin fluctuations. △ Less

Submitted 3 March, 2026; originally announced March 2026.

Comments: Accepted in Chinese Physics Letters

Journal ref: Chin. Phys. Lett. 43, 040709 (2026)

Atomically-sharp magnetic soliton in the square-net lattice EuRhAl$_{4}$Si$_{2}$

Authors: Kevin Allen, Juba Bouaziz, Yichen Zhang, Kai Du, Sanu Mishra, Gustav Bihlmayer, Yiqing Hao, Victor Ukleev, Chen Luo, Florin Radu, Yuxiang Gao, Marta Zonno, Sergey Gorovikov, Christopher Lane, Jian-Xin Zhu, Huibo Cao, Sang-Wook Cheong, Ming Yi, Stefan Blügel, Emilia Morosan

Abstract: Topological spin textures are hallmark manifestations of competing interactions in magnetic matter. Their effective description by nonlinear field theories reflects an energetic frustration that destabilizes uniform order while selecting finite-size, topologically nontrivial configurations as stationary states. Among the most extreme realizations are atomically-sharp domain wall excitations, namel… ▽ More Topological spin textures are hallmark manifestations of competing interactions in magnetic matter. Their effective description by nonlinear field theories reflects an energetic frustration that destabilizes uniform order while selecting finite-size, topologically nontrivial configurations as stationary states. Among the most extreme realizations are atomically-sharp domain wall excitations, namely one-dimensional (1D) magnetic solitons, which represent the ultimate scaling limit of magnetic textures. Such solitons may emerge in magnetic systems where effective exchange interactions compete directly with uniaxial magnetic anisotropy. Here we show that the square-net rare earth compound EuRhAl$_{4}$Si$_{2}$ realizes a very susceptible regime where the magnetic anisotropy competes with highly frustrated exchange interactions stabilizing a rare ferrimagnetic $\uparrow\uparrow\downarrow$ state that, under applied magnetic field, supports the formation of atomically-sharp soliton defects. We confirm the bulk response of the 1D magnetic solitons via magnetization and electrical transport measurements. We establish both the zero- and in-field $\uparrow\uparrow\downarrow$ order via neutron diffraction, while magnetic force microscopy visualizes its real-space evolution into a stripe-like array. To elucidate the microscopic origin of the soliton, we relate the Ruderman-Kittel-Kasuya-Yosida (RKKY)-driven exchange interactions and the magnetic anisotropy through density functional theory, and we construct an effective 1D $J_{1}$-$J_{2}$-$K$ model whose atomistic spin dynamics simulations reproduce the observed soliton states as a function of external field. Our results demonstrate that EuRhAl$_{4}$Si$_{2}$ hosts atomically-sharp, field-driven 1D magnetic solitons, providing a new platform for studying 1D topological excitations at the atomic length scale. △ Less

Submitted 10 February, 2026; originally announced February 2026.

Polar enhancement of optical nonlinearities and domain-driven second harmonic contrast in bismuth telluro-halide van der Waals crystals

Authors: Kevin W. C. Kwock, Tenzin Norden, Thomas P. Darlington, Kaiyuan Yao, Kai Du, Ana-Marija Nedić, Thaís V. Trevisan, Ekaterina Dolgopolova, Peter P. Orth, Rohit P. Prasankumar, P. James Schuck, Sang-Wook Cheong, Wilton J. M. Kort-Kamp, Prashant Padmanabhan

Abstract: The BiTeX family of polar van der Waals (vdW) semiconductors offers a unique platform for exploring the interplay between polar crystalline structure and nonlinear optical phenomena. Here, we utilize second harmonic generation (SHG) polarimetry to demonstrate giant anisotropic optical nonlinearities in BiTeBr and BiTeI driven by contributions to the crystals' nonlinear polarizability originating f… ▽ More The BiTeX family of polar van der Waals (vdW) semiconductors offers a unique platform for exploring the interplay between polar crystalline structure and nonlinear optical phenomena. Here, we utilize second harmonic generation (SHG) polarimetry to demonstrate giant anisotropic optical nonlinearities in BiTeBr and BiTeI driven by contributions to the crystals' nonlinear polarizability originating from their permanent dipole moment. In addition, using SHG microscopy, we show that BiTeI displays a distinctive SHG spatial texture consisting of thread-like regions of reduced harmonic intensity. These features demark the boundaries between phase and anti-phase polar domains, confirmed via piezoresponse force microscopy and are attributable to disorder-driven restoration of inversion symmetry and concomitant optical interference effects. Our results unveil the power of polarization-resolved SHG microscopy in elucidating the intricate relationship between structure, symmetry, and nonlinear optical responses in polar vdW materials and highlight the promise of BiTeX as a material platform for domain-engineered nanoscale nonlinear photonics. △ Less

Submitted 31 December, 2025; originally announced January 2026.

Altermagnetic boosting of chiral phonons

Authors: J. Okamoto, C. Y. Mou, H. Y. Huang, G. Channagowdra, C. Won, K. Du, X. Fang, E. V. Komleva, C. T. Chen, S. V. Streltsov, A. Fujimori, S-W. Cheong, D. J. Huang

Abstract: Chirality characterizes the asymmetry between a structure and its mirror image and underlies a wide range of chiral functionalities. In crystallographically chiral materials, phonons with non-zero linear momentum $\textbf{k}$ can acquire a $k$-induced longitudinal magnetization, giving rise to chiral phonons. Helical spin order, with its proper screw-type configuration, breaks all mirror symmetrie… ▽ More Chirality characterizes the asymmetry between a structure and its mirror image and underlies a wide range of chiral functionalities. In crystallographically chiral materials, phonons with non-zero linear momentum $\textbf{k}$ can acquire a $k$-induced longitudinal magnetization, giving rise to chiral phonons. Helical spin order, with its proper screw-type configuration, breaks all mirror symmetries and therefore carries magnetic chirality. Such helical spins also generate non-relativistic spin splitting for any quasiparticle excitations propagating along the screw axis. To explore the possible connection between chiral phonons and magnetic chirality, we investigated the crystallographically polar and chiral compound (Mn,Ni)$_3$TeO$_6$, which hosts three distinct states: a paramagnetic state, a helical spin state with magnetic chirality, and a collinear spin state without magnetic chirality. We find an approximately tenfold enhancement of chiral-phonon coupling in the helical spin state along the screw axis, compared with both the paramagnetic and collinear spin states. These results identify a new route to amplify chiral phonons through an altermagnetic effect arising from the broken parity-time symmetry in helical spins. %from non-relativistic spin splitting. △ Less

Submitted 29 November, 2025; originally announced December 2025.

Comments: 8 pages, 5 figures. (Supplement: 9 pages, 1 fiture)

Spinons, solitons and random singlets in the spin-chain compound copper benzoate

Authors: Ying Chen, Guijing Duan, Yuejiu Zhao, Ning Xi, Bingying Pan, Xiaoyu Xu, Zhanlong Wu, Kefan Du, Shuo Li, Ze Hu, Rui Bian, Xiaoqun Wang, Wei Li, Long Zhang, Yi Cui, Shiyan Li, Rong Yu, Weiqiang Yu

Abstract: The $S=1/2$ antiferromagnetic Heisenberg chain is a paradigmatic quantum system hosting exotic excitations such as spinons and solitons, and forming random singlet state in the presence of quenched disorder. Realizing and distinguishing these excitations in a single material remains a significant challenge. Using nuclear magnetic resonance (NMR) on a high-quality single crystal of copper benzoate,… ▽ More The $S=1/2$ antiferromagnetic Heisenberg chain is a paradigmatic quantum system hosting exotic excitations such as spinons and solitons, and forming random singlet state in the presence of quenched disorder. Realizing and distinguishing these excitations in a single material remains a significant challenge. Using nuclear magnetic resonance (NMR) on a high-quality single crystal of copper benzoate, we identify and characterize all three excitation types by tuning the magnetic field at ultra-low temperatures. At a low field of 0.2 T, a temperature-independent spin-lattice relaxation rate ($1/T_1$) over more than a decade confirms the presence of spinons. Below 0.4 K, an additional relaxation channel emerges, characterized by $1/T_1 \propto T$ and a spectral weight growing as $-\ln(T/T_0)$, signaling a random-singlet ground state induced by weak quenched disorder. At fields above 0.5 T, a field-induced spin gap $Δ\propto H^{2/3}$ observed in both $1/T_1$ and the Knight shift signifies soliton excitations. Our results establish copper benzoate as a unique experimental platform for studying one-dimensional quantum integrability and the interplay of disorder and correlations. △ Less

Submitted 13 October, 2025; originally announced October 2025.

Evidence for Magnon-Assisted Exciton Radiation in a Layered Antiferromagnetic Semiconductor

Authors: Yingchen Peng, Yanan Ge, Wentao Chen, Zihan Wang, Kang Wang, Kezhao Du, Xingzhi Wang, Ye Yang

Abstract: Layered van der Waals (vdW) magnetic semiconductors open a new avenue for exploring intertwined excitonic and magnetic phenomena. Here, we investigate this interplay in the vdW antiferromagnet MnPS3, uncovering an exceptionally long exciton lifetime (~100 μs) below the Néel temperature (T_N). We demonstrate that the exciton lifetime is governed by phonon-assisted nonradiative recombination and thu… ▽ More Layered van der Waals (vdW) magnetic semiconductors open a new avenue for exploring intertwined excitonic and magnetic phenomena. Here, we investigate this interplay in the vdW antiferromagnet MnPS3, uncovering an exceptionally long exciton lifetime (~100 μs) below the Néel temperature (T_N). We demonstrate that the exciton lifetime is governed by phonon-assisted nonradiative recombination and thus exhibits a strong temperature dependence. In contrast, the exciton radiative recombination shows a distinct temperature dependence that is sensitive to magnetic order. Below T_N, the temperature dependence of the radiative recombination rate is consistent with a magnon-assisted emission pathway, while above T_N it reflects the combined effects of short-range spin correlations and phonons. These findings not only establish MnPS3 as a compelling candidate for excitonic devices due to its long-lifetime and correlation with magnetic orders but also provide crucial insights into the interplay between excitons, spins, and lattice in vdW magnetic semiconductors. △ Less

Submitted 20 July, 2026; v1 submitted 9 October, 2025; originally announced October 2025.

Electrotoroidicity: New Paradigm for Transverse Electromagnetic Responses

Authors: Kai Du, Daegeun Jo, Xianghan Xu, Fei-Ting Huang, Ming-Hao Lee, Ming-Wen Chu, Kefeng Wang, David Vanderbilt, Hyun-Woo Lee, Sang-Wook Cheong

Abstract: The exploration of transverse electromagnetic responses in solids with broken spatial-inversion (I) and/or time-reversal (T) symmetries has unveiled numerous captivating phenomena, including the (anomalous) Hall effect, Faraday rotations, non-reciprocal directional dichroism, and off-diagonal linear magnetoelectricity, all within the framework of magnetotoroidicity. Here, we introduce a novel clas… ▽ More The exploration of transverse electromagnetic responses in solids with broken spatial-inversion (I) and/or time-reversal (T) symmetries has unveiled numerous captivating phenomena, including the (anomalous) Hall effect, Faraday rotations, non-reciprocal directional dichroism, and off-diagonal linear magnetoelectricity, all within the framework of magnetotoroidicity. Here, we introduce a novel class of transverse electromagnetic responses originating from electrotoroidicity in ferro-rotational (FR) systems with preserved I and T symmetries, distinct from magnetotoroidicity. We discover a high-order off-diagonal magnetic susceptibility of FR domains and a reduced linear diagonal magnetic susceptibility at FR domain walls in doped ilmenite FeTiO3. The non-trivial "Hall-like" effect of the former corresponds to an anomalous transverse susceptibility in the presence of spontaneous electrotoroidal moments in FR materials. Our findings unveil an emergent type of transverse electromagnetic responses even in I and T symmetry-conserved conditions and illustrate new functionalities of abundant FR materials. △ Less

Submitted 30 September, 2025; originally announced October 2025.

Comments: Nature Physics (2025)

Spin-supersolidity induced quantum criticality and magnetocaloric effect in the triangular-lattice antiferromagnet Rb$_2$Co(SeO$_3$)$_2$

Authors: Yi Cui, Zhanlong Wu, Zhongcen Sun, Kefan Du, Jun Luo, Shuo Li, Jie Yang, Jinchen Wang, Rui Zhou, Qian Chen, Yoshimitsu Kohama, Atsuhiko Miyata, Zhuo Yang, Rong Yu, Weiqiang Yu

Abstract: We performed high-field magnetization, magnetocaloric effect (MCE), and NMR measurements on the Ising triangular-lattice antiferromagnet Rb$_2$Co(SeO$_3$)$_2$. The observations of the 1/3-magnetization plateau, the split NMR lines, and the thermal activation behaviors of the spin-lattice relaxation rate $1/T_1$ between 2 T and 15.8 T provide unambiguous evidence of a gapped up-up-down (UUD) magnet… ▽ More We performed high-field magnetization, magnetocaloric effect (MCE), and NMR measurements on the Ising triangular-lattice antiferromagnet Rb$_2$Co(SeO$_3$)$_2$. The observations of the 1/3-magnetization plateau, the split NMR lines, and the thermal activation behaviors of the spin-lattice relaxation rate $1/T_1$ between 2 T and 15.8 T provide unambiguous evidence of a gapped up-up-down (UUD) magnetic ordered phase. For fields between 15.8 T and 18.5 T, the anomaly in the magnetic susceptibility, the slow saturation of the NMR line spectral ratio with temperature, and the power-law temperature dependence of $1/T_1$ suggest the ground state to be a spin supersolid with gapless spin excitations. With further increasing the field, the Grüneisen ratio, extracted from the MCE data, reveals a continuous quantum phase transition at $H_{\rm C}\approx$ 19.5 T and a universal quantum critical scaling with the exponents $νz~\approx~$1. Near $H_{\rm C}$, the large high-temperature MCE signal and the broad peaks in the NMR Knight shift and $1/T_1$, manifest the strong spin fluctuations driven by both magnetic frustration and quantum criticality. These results establish Rb$_2$Co(SeO$_3$)$_2$ as a candidate platform for cryogenic magnetocaloric cooling. △ Less

Submitted 30 September, 2025; originally announced September 2025.

Comments: 7 pages, 5 figures

Journal ref: npjQM 11, 52 (2026)

Correlation between Exciton Dynamics and Spin Structure in van der Waals Antiferromagnet NiPS3

Authors: Kang Wang, Yingchen Peng, Boying Huang, Chun Zhou, Qianlu Sun, Fujie Tang, Zhenglu Li, Weigao Xu, Kezhao Du, Xingzhi Wang, Ye Yang

Abstract: The emerging magnetic van der Waals (vdW) materials provide a platform for exploring novel physics regarding magnetism in low dimensions and developing ultrathin spintronic applications. Here, we investigate the ultrafast dynamics of excitons in a vdW NiPS3 crystal. The temporal evolution of the transient reflection spectra indicates that the spin-correlated exciton is formed through photocarrier… ▽ More The emerging magnetic van der Waals (vdW) materials provide a platform for exploring novel physics regarding magnetism in low dimensions and developing ultrathin spintronic applications. Here, we investigate the ultrafast dynamics of excitons in a vdW NiPS3 crystal. The temporal evolution of the transient reflection spectra indicates that the spin-correlated exciton is formed through photocarrier localization, the rate of which is independent of the magnetic degrees of freedom. However, the recombination rate of these excitons is connected with the long-range magnetic order, and this connection probably arise from a spin-flip rooted in the underlying antiferromagnetic background during the recombination. Our findings uncover intertwined coupling between carrier, lattice and spin degrees of freedom in NiPS3, which may pave the path toward ultrafast optical manipulation of spin-related quantum states in vdW antiferromagnets. △ Less

Submitted 8 August, 2025; originally announced August 2025.

Large magnon dichroism and other optical properties of hexagonal ferrite h-Lu0.6Sc0.4FeO3 with altermagnetic A2 spin ordering

Authors: V. A. Martinez, A. A. Sirenko, L. Bugnon, P. Marsik, C. Bernhard, Qing Zhang, G. L. Pascut, F. Lyzwa, Z. Liu, K. Du, S. -W. Cheong

Abstract: Multiferroic hexagonal h-Lu0.6Sc0.4FeO3 single crystals with non-collinear spins were studied using the THz and Raman scattering spectroscopies and ellipsometry. Antiferromagnetic resonances, or magnons, were found at about 0.85 THz and 1.2 THz. These magnons harden as temperature increases and disappear above 130 K. This behavior is consistent with the magnetic susceptibility and a phase transiti… ▽ More Multiferroic hexagonal h-Lu0.6Sc0.4FeO3 single crystals with non-collinear spins were studied using the THz and Raman scattering spectroscopies and ellipsometry. Antiferromagnetic resonances, or magnons, were found at about 0.85 THz and 1.2 THz. These magnons harden as temperature increases and disappear above 130 K. This behavior is consistent with the magnetic susceptibility and a phase transition to a previously reported weak ferromagnetic state. A strong dichroism at the resonance with the AFM doublet has been observed at zero external magnetic field using both conventional circular polarization and THz vector vortex beams. This observation is attributed to the strong altermagnetic properties of h-Lu0.6Sc0.4FeO3 with a broken PT symmetry. The splitting of the magnon doublet in an external magnetic field applied long the c axis yields a g-factor of 3.0 for the Fe3+ ions. Raman spectra of the optical phonons revealed a Fano-type asymmetry due to their interaction with a continuum of polar excitations. Electronic transitions were studied with ellipsometry and the results were compared with the modelled using DFT+eDMFT. △ Less

Submitted 29 July, 2025; originally announced July 2025.

Topological Magneto-optical Kerr Effect without Spin-orbit Coupling in Spin-compensated Antiferromagnet

Authors: Camron Farhang, Weihang Lu, Kai Du, Yunpeng Gao, Junjie Yang, Sang-Wook Cheong, Jing Xia

Abstract: The magneto-optical Kerr effect (MOKE), the differential reflection of oppositely circularly polarized light, has traditionally been associated with relativistic spin-orbit coupling (SOC), which links a particle's spin with its orbital motion. In ferromagnets, large MOKE signals arise from the combination of magnetization and SOC, while in certain coplanar antiferromagnets, SOC-induced Berry curva… ▽ More The magneto-optical Kerr effect (MOKE), the differential reflection of oppositely circularly polarized light, has traditionally been associated with relativistic spin-orbit coupling (SOC), which links a particle's spin with its orbital motion. In ferromagnets, large MOKE signals arise from the combination of magnetization and SOC, while in certain coplanar antiferromagnets, SOC-induced Berry curvature enables MOKE despite zero net magnetization. Theoretically, large MOKE can also arise in a broader class of magnetic materials with compensated spins, without relying on SOC - for example, in systems exhibiting real-space scalar spin chirality. The experimental verification has remained elusive. Here, we demonstrate such a SOC- and magnetization-free MOKE in the noncoplanar antiferromagnet Co1/3TaS2. Using a Sagnac interferometer microscope, we image domains of scalar spin chirality and their reversal. Our findings establish experimentally a new mechanism for generating large MOKE signals and position chiral spin textures in compensated magnets as a compelling platform for ultrafast, stray-field-immune opto-spintronic applications. △ Less

Submitted 19 February, 2026; v1 submitted 13 July, 2025; originally announced July 2025.

Journal ref: Nature Communications, 17, 3386 (2026)

NMR study of supersolid phases in the triangular-lattice antiferromagnet Na2BaCo(PO4)2

Authors: Xiaoyu Xu, Zhanlong Wu, Ying Chen, Qing Huang, Ze Hu, Xinyu Shi, Kefan Du, Shuo Li, Rui Bian, Rong Yu, Yi Cui, Haidong Zhou, Weiqiang Yu

Abstract: We report ultra-low-temperature $^{23}$Na NMR measurements on the Ising triangular lattice antiferromagnet Na$_2$BaCo(PO$_4$)$_2$, which precisely resolve the phase diagram under magnetic field applied along the crystalline $c$ axis. With increasing field, the NMR spectra resolve three ordered phases with distinct spin configurations: the Y, up-up-down (UUD), and V phases. The spin-lattice relaxat… ▽ More We report ultra-low-temperature $^{23}$Na NMR measurements on the Ising triangular lattice antiferromagnet Na$_2$BaCo(PO$_4$)$_2$, which precisely resolve the phase diagram under magnetic field applied along the crystalline $c$ axis. With increasing field, the NMR spectra resolve three ordered phases with distinct spin configurations: the Y, up-up-down (UUD), and V phases. The spin-lattice relaxation rate $1/T_1$ data demonstrate gapless excitations in the Y and V phases, strongly supporting their supersolid nature. However, the phase transitions from the UUD phase to the two supersolid phases exhibit dramatically different behaviors upon cooling. Prior to entering the Y phase, $1/T_1$ identifies a gapless regime within the UUD phase, suggesting a Berezinskii-Kosterlitz-Thouless phase above a second-order phase transition. In contrast, the coexistence of the UUD and V phases observed in our experiments provides direct evidence of a first-order phase transition between these phases. △ Less

Submitted 11 April, 2025; originally announced April 2025.

Comments: 5 pages, 4 figures

Journal ref: Phys. Rev. B 112, 125163 (2025)

Universal Superconductivity in FeTe and All-Iron-Based Ferromagnetic Superconductor Heterostructures

Authors: Hee Taek Yi, Xiong Yao, Deepti Jain, Ying-Ting Chan, An-Hsi Chen, Matthew Brahlek, Kim Kisslinger, Kai Du, Myung-Geun Han, Yimei Zhu, Weida Wu, Sang-Wook Cheong, Seongshik Oh

Abstract: Ferromagnetism (FM) and superconductivity (SC) are two of the most famous macroscopic quantum phenomena. However, nature normally does not allow SC and FM to coexist without significant degradation. Here, we introduce the first fully iron-based SC/FM heterostructures, composed of Fe(Te,Se) and Fe3GeTe2, and show that in this platform strong FM and high-temperature SC robustly coexist. We subsequen… ▽ More Ferromagnetism (FM) and superconductivity (SC) are two of the most famous macroscopic quantum phenomena. However, nature normally does not allow SC and FM to coexist without significant degradation. Here, we introduce the first fully iron-based SC/FM heterostructures, composed of Fe(Te,Se) and Fe3GeTe2, and show that in this platform strong FM and high-temperature SC robustly coexist. We subsequently discover that chemical proximity effect from neighboring layers can universally drive the otherwise non-superconducting FeTe films into a SC state. This suggests that the ground state of FeTe is so close to the SC state that it could be driven in and out of the SC state with various other perturbations. Altogether, this shows that Fe-Te-based heterostructures provide a unique opportunity to manipulate magnetism, superconductivity and topological physics, paving the way toward new superconducting technologies. △ Less

Submitted 3 February, 2025; originally announced February 2025.

Journal ref: Adv. Funct. Mater. 2025, 2418259

Haldane phase, field-induced magnetic ordering and Tomonaga-Luttinger liquid behavior in a spin-one chain compound NiC$_2$O$_4$$\cdot$2NH$_3$

Authors: Shuo Li, Zhanlong Wu, Yanhong Wang, Jun Luo, Kefan Du, Xiaoyu Xu, Ze Hu, Ying Chen, Jie Yang, Zhengxin Liu, Rong Yu, Yi Cui, Rui Zhou, Hongcheng Lu, Weiqiang Yu

Abstract: We performed single-crystal magnetic susceptibility and $^1$H NMR measurements on a quasi-1D, spin-1 antiferromagnet NiC$_2$O$_4$$\cdot$2NH$_3$, with temperature down to 100 mK and with field up to 26 T. With field applied along the chain direction (crystalline $b$ direction), a spin gap is determined at low fields. Our susceptibility and spin-lattice relaxation measurements reveal a Haldane phase… ▽ More We performed single-crystal magnetic susceptibility and $^1$H NMR measurements on a quasi-1D, spin-1 antiferromagnet NiC$_2$O$_4$$\cdot$2NH$_3$, with temperature down to 100 mK and with field up to 26 T. With field applied along the chain direction (crystalline $b$ direction), a spin gap is determined at low fields. Our susceptibility and spin-lattice relaxation measurements reveal a Haldane phase at low field, with an intrachain exchange coupling $J$ $\approx$ 35 K and an easy-plane single-ion anisotropy of 17 K. A field-induced antiferromagnetic (AFM) ordering emerges at fields of 2.1 T, which sets a three-dimensional (3D) quantum critical point (QCP). The high-temperature spin-lattice relaxation rates $1/T_1$ resolves an onset of Tomonaga-Luttinger liquid behavior at field above $3.5$ T, which characterizes a hidden 1D QCP. △ Less

Submitted 29 November, 2024; originally announced November 2024.

Comments: 9 pages, 8 figures

Journal ref: Phys. Rev. B 111, 195164 (2025)

Two Plaquette-Singlet Phases and Emergent SO(5) Deconfined Quantum Criticality in SrCu2(BO3)2

Authors: Yi Cui, Kefan Du, Zhanlong Wu, Shuo Li, Pengtao Yang, Ying Chen, Xiaoyu Xu, Hongyu Chen, Chengchen Li, Juanjuan Liu, Bosen Wang, Wenshan Hong, Shiliang Li, Zhiyuan Xie, Jinguang Cheng, Bruce Normand, Rong Yu, Weiqiang Yu

Abstract: The deconfined quantum critical point (DQCP) has become a central open concept in the physics of quantum matter, and its proposed presence in the Shastry-Sutherland model was followed by the experimental observation of at least a minimal DQC scenario induced by an applied magnetic field in SrCu$_2$(BO$_3$)$_2$. However, the nature of the plaquette-singlet phase in SrCu$_2$(BO$_3$)$_2$ remains unre… ▽ More The deconfined quantum critical point (DQCP) has become a central open concept in the physics of quantum matter, and its proposed presence in the Shastry-Sutherland model was followed by the experimental observation of at least a minimal DQC scenario induced by an applied magnetic field in SrCu$_2$(BO$_3$)$_2$. However, the nature of the plaquette-singlet phase in SrCu$_2$(BO$_3$)$_2$ remains unresolved, and with it the identification of the DQCP symmetry from among several theoretical scenarios. Here we perform detailed high-pressure $^{11}$B NMR studies to reveal the presence of both the full-plaquette (FP) and empty-plaquette (EP) phases in SrCu$_2$(BO$_3$)$_2$, phase-separated at a first-order, pressure-driven transition with a volume-fraction effect. The field-driven transition from the EP to the antiferromagnetic (AFM) phase complements our previous observations of the FP--AFM transition, with both showing deconfined quantum criticality, while the scaling of the spin-lattice relaxation rate near the EP--AFM transition, $1/T_1 \propto T^{0.6}$, suggests a DQCP governed by a different universality class. We discuss possible extensions to the Shastry-Sutherland model that account for these pressure and field effects. The expanded phase space we discover mandates an SO(5) DQCP symmetry, and hence our results take an important step towards a complete understanding of deconfined quantum criticality in SrCu$_2$(BO$_3$)$_2$. △ Less

Submitted 8 June, 2025; v1 submitted 31 October, 2024; originally announced November 2024.

Comments: 7 pages, 5 figures

Directionally asymmetric nonlinear optics in planar chiral MnTiO$_3$

Authors: Xinshu Zhang, Tyler Carbin, Kai Du, Bingqing Li, Kefeng Wang, Casey Li, Tiema Qian, Ni Ni, Sang-Wook Cheong, Anshul Kogar

Abstract: Planar chiral structures possess a two dimensional handedness that is associated with broken mirror symmetry. Such motifs span vast length scales; examples include certain pinwheel molecules, nautilus shells, cyclone wind patterns and spiral galaxies. Although pervasive in nature, it has only recently been found that condensed matter systems can exhibit a form of planar chirality through toroidal… ▽ More Planar chiral structures possess a two dimensional handedness that is associated with broken mirror symmetry. Such motifs span vast length scales; examples include certain pinwheel molecules, nautilus shells, cyclone wind patterns and spiral galaxies. Although pervasive in nature, it has only recently been found that condensed matter systems can exhibit a form of planar chirality through toroidal arrangements of electric dipoles, known as ferro-rotational (FR) order. A key characteristic of such order is that enantiomorph conversion occurs when the solid is flipped by 180 degrees about an in-plane axis. Consequently, ferro-rotationally ordered materials may exhibit directionally asymmetric response functions, even while preserving inversion and time-reversal symmetry. Such an effect, however, has yet to be observed. Using second harmonic interferometry, we show here that when circularly polarized light is incident on MnTiO$_3$, the generated nonlinear signal exhibits directional asymmetry. Depending on whether the incident light is parallel or anti-parallel to the FR axis, we observe a different conversion efficiency of two right (left) circularly polarized photons into a frequency-doubled left (right) circularly polarized photon. Our work uncovers a fundamentally new optical effect in ordered solids and opens up the possibility for developing novel nonlinear and directionally asymmetric optical devices. △ Less

Submitted 7 October, 2024; originally announced October 2024.

Ferroaxial phonons in chiral and polar NiCo2TeO6

Authors: V. A. Martinez, Y. Gao, J. Yang, F. Lyzwa, Z. Liu, C. J. Won, K. Du, V. Kiryukhin, S-W. Cheong, A. A. Sirenko

Abstract: Perfect circular dichroism has been observed in the Raman scattering by the optical phonons in single chiral domain NiCo2TeO6 crystals. The selection rules for the optical phonons are determined by the combination of the chiral structure C and the electric polarization P along the c-axis. These two symmetry operations are equivalent to the ferroaxial order (C dot P) = A, so the observed optical ph… ▽ More Perfect circular dichroism has been observed in the Raman scattering by the optical phonons in single chiral domain NiCo2TeO6 crystals. The selection rules for the optical phonons are determined by the combination of the chiral structure C and the electric polarization P along the c-axis. These two symmetry operations are equivalent to the ferroaxial order (C dot P) = A, so the observed optical phonons are referred to as "ferroaxial". For a given Raman scattering geometry the observed effect may also be described as a complete non-reciprocal propagation of the optical phonons, whose preferable vector direction is determined by the sign of C dot P. The combination of Raman scattering and polarization plane rotation of the transmitted white light allows for identification of the direction of electric polarization P in mono domain chiral crystals. △ Less

Submitted 14 October, 2024; originally announced October 2024.

Surface Magnetism in Fe$_3$GeTe$_2$ Crystals

Authors: T. A. Tyson, S. Amarasinghe, AM M. Abeykoon, R. Lalancette, S. K. Du, X. Fang, S. -W. Cheong, A. Al-Mahboob, J. T. Sadowski

Abstract: The surface magnetization of Fe$_3$GeTe$_2$ was examined by low-energy electron microscopy (LEEM) using an off-normal incidence electron beam. We found that the 180$^o$ domain walls are of Bloch type. Temperature-dependent LEEM measurements yield a surface magnetization with a surface critical exponent $β$1 = 0.79 +/- 0.02. This result is consistent with surface magnetism in the 3D semi-infinite H… ▽ More The surface magnetization of Fe$_3$GeTe$_2$ was examined by low-energy electron microscopy (LEEM) using an off-normal incidence electron beam. We found that the 180$^o$ domain walls are of Bloch type. Temperature-dependent LEEM measurements yield a surface magnetization with a surface critical exponent $β$1 = 0.79 +/- 0.02. This result is consistent with surface magnetism in the 3D semi-infinite Heisenberg ($β$1 = 0.84 +/- 0.01) or Ising ($β$1 = 0.78 +/- 0.02) models, which is distinctly different from the bulk exponent ($β$ = 0.34 +/- 0.07). The measurements reveal the power of LEEM with a tilted beam to determine magnetic domain structure in quantum materials. Single crystal diffraction measurements reveal inversion symmetry-breaking weak peaks and yield space group P-6m2. This Fe site defect-derived loss of inversion symmetry enables the formation of skyrmions in this Fe$_3$GeTe$_2$ crystal. △ Less

Submitted 5 September, 2024; originally announced September 2024.

Low-energy spin dynamics in a Kitaev material Na3Ni2BiO6 investigated by NMR

Authors: Xinyu Shi, Yi Cui, Yanyan Shangguan, Xiaoyu Xu, Zhanlong Wu, Ze Hu, Shuo Li, Kefan Du, Ying Chen, Long Ma, Zhengxin Liu, Jinsheng Wen, Jinshan Zhang, Weiqiang Yu

Abstract: We performed 23Na NMR and magnetization measurements on an S = 1, quasi-2D honeycomb lattice antiferromagnet Na3Ni2BiO6. A large positive Curie-Weiss constant of 22.9 K is observed. The NMR spectra at low fields are consistent with a "zigzag" magnetic order, indicating a large easy-axis anisotropy. With field applied along the c* axis, the NMR spectra confirm the existence of a 1/3-magnetization p… ▽ More We performed 23Na NMR and magnetization measurements on an S = 1, quasi-2D honeycomb lattice antiferromagnet Na3Ni2BiO6. A large positive Curie-Weiss constant of 22.9 K is observed. The NMR spectra at low fields are consistent with a "zigzag" magnetic order, indicating a large easy-axis anisotropy. With field applied along the c* axis, the NMR spectra confirm the existence of a 1/3-magnetization plateau phase between 5.1 T and 7.1 T. The transition from the zigzag order to the 1/3-magnetization plateau phase is also found to be a first-order type. A monotonic decrease of the spin gap is revealed in the 1/3-magnetization plateau phase, which reaches zero at a quantum critical field Hc = 8.35 T before entering the fully polarized phase. These data suggest the existence of exchange frustration in the system along with strong ferromagnetic interactions, hosting the possibility for Kitaev physics. Besides, well below the ordered phase, the 1/T1 at high fields shows either a level off or an enhancement upon cooling below 3 K, which suggests the existence of low-energy fluctuations. △ Less

Submitted 11 April, 2024; originally announced April 2024.

Comments: 7 pages, 7 figures

Journal ref: Chinese Phys. B 33 067601 (2024)

Molecular intercalation in the van der Waals antiferromagnets FePS3 and NiPS3

Authors: Cong Li, Ze Hu, Xiaofei Hou, Sheng Xu, Zhanlong Wu, Kefan Du, Shuo Li, Xiaoyu Xu, Ying Chen, Zeyu Wang, Tiancheng Mu, Tian-Long Xia, Yanfeng Guo, B. Normand, Weiqiang Yu, Yi Cui

Abstract: We have performed electrochemical treatment of the van der Waals antiferromagnetic materials FePS$_3$ and NiPS$_3$ with the ionic liquid EMIM-BF$_4$, achieving significant molecular intercalation. Mass analysis of the intercalated compounds, EMIM$_x$-FePS$_3$ and EMIM$_x$-NiPS$_3$, indicated respective intercalation levels, $x$, of approximately 27\% and 37\%, and X-ray diffraction measurements de… ▽ More We have performed electrochemical treatment of the van der Waals antiferromagnetic materials FePS$_3$ and NiPS$_3$ with the ionic liquid EMIM-BF$_4$, achieving significant molecular intercalation. Mass analysis of the intercalated compounds, EMIM$_x$-FePS$_3$ and EMIM$_x$-NiPS$_3$, indicated respective intercalation levels, $x$, of approximately 27\% and 37\%, and X-ray diffraction measurements demonstrated a massive (over 50\%) enhancement of the $c$-axis lattice parameters. To investigate the consequences of these changes for the magnetic properties, we performed magnetic susceptibility and $^{31}$P nuclear magnetic resonance (NMR) studies of both systems. For EMIM$_x$-FePS$_3$, intercalation reduces the magnetic ordering temperature from $T_N = 120$~K to 78~K, and we find a spin gap in the antiferromagnetic phase that drops from 45~K to 30~K. For EMIM$_x$-NiPS$_3$, the ordering temperature is almost unaffected (changing from 148~K to 145~K), but a change towards nearly isotropic spin fluctuations suggests an alteration of the magnetic Hamiltonian. Such relatively modest changes, given that the huge extension of the $c$ axes is expected to cause a very strong suppression any interlayer interactions, point unequivocally to the conclusion that the magnetic properties of both parent compounds are determined solely by two-dimensional (2D), intralayer physics. The changes in transition temperatures and low-temperature spin dynamics in both compounds therefore indicate that intercalation also results in a significant modulation of the intralayer magnetic interactions, which we propose is due to charge doping and localization on the P sites. Our study offers chemical intercalation with ionic liquids as an effective method to control not only the interlayer but also the intralayer interactions in quasi-2D magnetic materials. △ Less

Submitted 3 March, 2024; originally announced March 2024.

Journal ref: Physical Review B 109, 184407(2024)

Giant X-ray circular dichroism in a time-reversal invariant altermagnet

Authors: Jun Okamoto, Ru-Pan Wang, Yen-Yi Chu, Hung-Wei Shiu, Amol Singh, Hsiao-Yu Huang, Chung-Yu Mou, Sucitto Teh, Horng-Tay Jeng, Kai Du, Xianghan Xu, Sang-Wook Cheong, Chao-Hung Du, Chien-Te Chen, Atsushi Fujimori, Di-Jing Huang

Abstract: X-ray circular dichroism, arising from the contrast in X-ray absorption between opposite photon helicities, serves as a spectroscopic tool to measure the magnetization of ferromagnetic materials and identify the handedness of chiral crystals. Antiferromagnets with crystallographic chirality typically lack X-ray magnetic circular dichroism because of time-reversal symmetry, yet exhibit weak X-ray n… ▽ More X-ray circular dichroism, arising from the contrast in X-ray absorption between opposite photon helicities, serves as a spectroscopic tool to measure the magnetization of ferromagnetic materials and identify the handedness of chiral crystals. Antiferromagnets with crystallographic chirality typically lack X-ray magnetic circular dichroism because of time-reversal symmetry, yet exhibit weak X-ray natural circular dichroism. Here, we report the observation of giant natural circular dichroism in the Ni $L_3$-edge X-ray absorption of Ni$_3$TeO$_6$, a polar and chiral antiferromagnet with effective time-reversal symmetry. To unravel this intriguing phenomenon, we propose a phenomenological model that classifies the movement of photons in a chiral crystal within the same symmetry class as that of a magnetic field. The coupling of X-ray polarization with the induced magnetization yields giant X-ray natural circular dichroism, revealing the altermagnetism of Ni$_3$TeO$_6$. Our findings provide evidence for the interplay between magnetism and crystal chirality in natural optical activity. Additionally, we establish the first example of a new class of magnetic materials exhibiting circular dichroism with time-reversal symmetry. △ Less

Submitted 23 February, 2024; v1 submitted 12 December, 2023; originally announced December 2023.

Comments: Accepted by Advanced Materials (2024.2.16) Revised title: Giant X-ray circular dichroism in a time-reversal invariant altermagnet Revised drafts: Main 14 pages, 4 figures, and SI 20 pages, 8 figures

Journal ref: adv. mater. 36, 2309172 (2024)

Hydrogen-induced tunable remanent polarization in a perovskite nickelate

Authors: Yifan Yuan, Michele Kotiuga, Tae Joon Park, Yuanyuan Ni, Arnob Saha, Hua Zhou, Jerzy T. Sadowski, Abdullah Al-Mahboob, Haoming Yu, Kai Du, Minning Zhu, Sunbin Deng, Ravindra S. Bisht, Xiao Lyu, Chung-Tse Michael Wu, Peide D. Ye, Abhronil Sengupta, Sang-Wook Cheong, Xiaoshan Xu, Karin M. Rabe, Shriram Ramanathan

Abstract: Materials with field-tunable polarization are of broad interest to condensed matter sciences and solid-state device technologies. Here, using hydrogen (H) donor doping, we modify the room temperature metallic phase of a perovskite nickelate NdNiO3 into an insulating phase with both metastable dipolar polarization and space-charge polarization. We then demonstrate transient negative differential ca… ▽ More Materials with field-tunable polarization are of broad interest to condensed matter sciences and solid-state device technologies. Here, using hydrogen (H) donor doping, we modify the room temperature metallic phase of a perovskite nickelate NdNiO3 into an insulating phase with both metastable dipolar polarization and space-charge polarization. We then demonstrate transient negative differential capacitance in thin film capacitors. The space-charge polarization caused by long-range movement and trapping of protons dominates when the electric field exceeds the threshold value. First-principles calculations suggest the polarization originates from the polar structure created by H doping. We find that polarization decays within ~1 second which is an interesting temporal regime for neuromorphic computing hardware design, and we implement the transient characteristics in a neural network to demonstrate unsupervised learning. These discoveries open new avenues for designing novel ferroelectric materials and electrets using light-ion doping. △ Less

Submitted 20 November, 2023; originally announced November 2023.

Comments: 13 pages, 5 figures

Spin-Mediated Direct Photon Scattering by Plasmons in BiTeI

Authors: A. C. Lee, S. Sarkar, K. Du, H. -H. Kung, C. J. Won, K. Wang, S. -W. Cheong, S. Maiti, G. Blumberg

Abstract: We use polarization resolved Raman spectroscopy to demonstrate that for a 3D giant Rashba system the bulk plasmon collective mode can directly couple to the Raman response even in the long wavelength $\mathbf q \rightarrow 0$ limit. Although conventional theory predicts the plasmon spectral weight to be suppressed as the square of its quasi-momentum and thus negligibly weak in the Raman spectra, w… ▽ More We use polarization resolved Raman spectroscopy to demonstrate that for a 3D giant Rashba system the bulk plasmon collective mode can directly couple to the Raman response even in the long wavelength $\mathbf q \rightarrow 0$ limit. Although conventional theory predicts the plasmon spectral weight to be suppressed as the square of its quasi-momentum and thus negligibly weak in the Raman spectra, we observe a sharp in-gap plasmon mode in the Raman spectrum of BiTeI below the Rashba continuum. This coupling, in a polar system with spin-orbit coupling, occurs without assistance from phonons when the incoming photon excitation is resonant with Rashba-split intermediate states. We discuss the distinctive features of BiTeI's giant Rashba system band structure that enable the direct observation of plasmon in Raman scattering. △ Less

Submitted 18 February, 2024; v1 submitted 6 October, 2023; originally announced October 2023.

Comments: Editors' Suggestion

Journal ref: Phys. Rev. B 109, L041111 (2024)

Light-induced electronic polarization in antiferromagnetic Cr2O3

Authors: Xinshu Zhang, Tyler Carbin, Adrian B. Culver, Kai Du, Kefeng Wang, Sang-Wook Cheong, Rahul Roy, Anshul Kogar

Abstract: In a solid, the electronic subsystem can exhibit incipient order with lower point group symmetry than the crystal lattice. External fields that couple to electronic order parameters have rarely been investigated, however, despite their potential importance to inducing exotic effects. Here, we show that when inversion symmetry is broken by the antiferromagnetic (AFM) order in Cr2O3, transmitting a… ▽ More In a solid, the electronic subsystem can exhibit incipient order with lower point group symmetry than the crystal lattice. External fields that couple to electronic order parameters have rarely been investigated, however, despite their potential importance to inducing exotic effects. Here, we show that when inversion symmetry is broken by the antiferromagnetic (AFM) order in Cr2O3, transmitting a linearly polarized light pulse through the crystal gives rise to an in-plane rotational symmetry breaking (from C3 to C1) via optical rectification. Using interferometric time-resolved second harmonic generation, we show that the ultrafast timescale of the symmetry reduction is indicative of a purely electronic response; the underlying spin and crystal structures remain unaffected. The symmetry-broken state exhibits a dipole moment, and its polar axis can be controlled with the incident light. Our results establish a coherent nonlinear optical protocol by which to break electronic symmetries and produce unconventional electronic effects in solids. △ Less

Submitted 9 January, 2024; v1 submitted 21 July, 2023; originally announced July 2023.

Kibble-Zurek mechanism of Ising domains

Authors: Kai Du, Xiaochen Fang, Choongjae Won, Chandan De, Fei-ting Huang, Fernando J. Gomez-Ruiz, Adolfo Del Campo, Sang-Wook Cheong

Abstract: The formation of topological defects after a symmetry-breaking phase transition is an overarching phenomenon that encodes rich information about the underlying dynamics. Kibble-Zurek mechanism (KZM), which describes these nonequilibrium dynamics, predicts defect densities of these second-order phase transitions driven by thermal fluctuations. It has been verified as a successful model in a wide va… ▽ More The formation of topological defects after a symmetry-breaking phase transition is an overarching phenomenon that encodes rich information about the underlying dynamics. Kibble-Zurek mechanism (KZM), which describes these nonequilibrium dynamics, predicts defect densities of these second-order phase transitions driven by thermal fluctuations. It has been verified as a successful model in a wide variety of physical systems, finding applications from structure formation in the early universe to condensed matter systems. However, whether topologically-trivial Ising domains, one of the most common and fundamental types of domains in condensed matter systems, also obey the KZM has never been investigated in the laboratory. We examined two different kinds of three-dimensional (3D) structural Ising domains: clockwise (CW)/counter-clockwise (CCW) ferro-rotation domains in NiTiO3 and up/down polar domains in BiTeI. While the KZM slope of ferro-rotation domains in NiTiO3 agrees well with the prediction of the 3D Ising model, the KZM slope of polar domains in BiTeI surprisingly far exceeds the theoretical limit, setting an exotic example where possible weak long-range dipolar interactions play a critical role in steepening the KZM slope of non-topological quantities. Our results demonstrate the validity of KZM for Ising domains and reveal an enhancement of the power-law exponent and a possible reduction of the dynamic critical exponent z for transitions with long-range interactions. △ Less

Submitted 15 April, 2024; v1 submitted 27 June, 2023; originally announced June 2023.

Journal ref: Nature Physics 19, 1495 (2023)

Topological Surface Magnetism and Neel Vector Control in a Magnetoelectric Antiferromagnet

Authors: Kai Du, Xianghan Xu, Choongjae Won, Kefeng Wang, Scott A. Crooker, Sylvie Rangan, Robert Bartynski, Sang-Wook Cheong

Abstract: Antiferromagnetic states with no stray magnetic fields can enable high-density ultra-fast spintronic technologies. However, the detection and control of antiferromagnetic Neel vectors remain challenging. Linear magnetoelectric antiferromagnets (LMAs) may provide new pathways, but applying simultaneous electric and magnetic fields, necessary to control Neel vectors in LMAs, is cumbersome and imprac… ▽ More Antiferromagnetic states with no stray magnetic fields can enable high-density ultra-fast spintronic technologies. However, the detection and control of antiferromagnetic Neel vectors remain challenging. Linear magnetoelectric antiferromagnets (LMAs) may provide new pathways, but applying simultaneous electric and magnetic fields, necessary to control Neel vectors in LMAs, is cumbersome and impractical for most applications. Herein, we show that Cr2O3, a prototypical room-temperature LMA, carries a topologically-protected surface magnetism in all surfaces, which stems from intrinsic surface electric fields due to band bending, combined with the bulk linear magnetoelectricity. Consequently, bulk Neel vectors with zero bulk magnetization can be simply tuned by magnetic fields through controlling the magnetizations associated with the surface magnetism. Our results imply that the surface magnetizations discovered in Cr2O3 should be also present in all LMAs. △ Less

Submitted 3 April, 2023; originally announced April 2023.

Journal ref: npj Quantum Materials volume 8, Article number: 17 (2023)

Effects of Shape on Interaction Dynamics of Tetrahedral Nanoplastics and the Cell Membrane

Authors: Xin Yong, Ke Du

Abstract: Cellular uptake of nanoplastics is instrumental in their environmental accumulation and transfer to humans through the food chain. Despite extensive studies using spherical plastic nanoparticles, the influence of the morphological characteristics of environmentally released nanoplastics is understudied. Using dissipative particle dynamics simulations, we modeled the interactions between a cell mem… ▽ More Cellular uptake of nanoplastics is instrumental in their environmental accumulation and transfer to humans through the food chain. Despite extensive studies using spherical plastic nanoparticles, the influence of the morphological characteristics of environmentally released nanoplastics is understudied. Using dissipative particle dynamics simulations, we modeled the interactions between a cell membrane and hydrophobic nanotetrahedra, which feature high shape anisotropy and large surface curvature seen for environmental nanoplastics. We observe robust uptake of nanotetrahedra with sharp vertices and edges by the lipid membrane. Two local energy minimum configurations of nanotetrahedra embedded in the membrane bilayer were identified for particles of large sizes. Further analysis of particle dynamics within the membrane shows that the two interaction states exhibit distinct translational and rotational dynamics in the directions normal and parallel to the plane of the membrane. The membrane confinement significantly arrests the out-of-plane motion, resulting in caged translation and subdiffusive rotation. While the in-plane diffusion remains Brownian, we find that the translational and rotational modes decouple from each other as the particle size increases. The rotational diffusion decreases by a greater extent compared to the translational diffusion, deviating from the continuum theory predictions. These results provide fundamental insights into the shape effect on the nanoparticle dynamics in crowded lipid membranes. △ Less

Submitted 27 August, 2023; v1 submitted 20 October, 2022; originally announced October 2022.

Comments: 25 pages, 10 figures, 1 table

Journal ref: J. Phys. Chem. B 2023, 127, 1652-1663

Preferred corrosion pathways for oxygen in Al2Ca-twin boundaries and dislocations

Authors: Nicolas J. Peter, Daniela Zander, Xumeng Cao, Chunhua Tian, Siyuan Zhang, Kui Du, Christina Scheu, Gerhard Dehm

Abstract: With an ongoing discussion on the oxygen diffusion along crystal defects remaining, it is difficult to study this phenomenon in Al containing intermetallic materials due to its rapid and passivating oxide formation. We report here the observation of enhanced oxygen diffusion along crystal defects, i.e. dislocations and twin boundaries, in the C15 Al 2 Ca Laves phase and how the presence of oxygen… ▽ More With an ongoing discussion on the oxygen diffusion along crystal defects remaining, it is difficult to study this phenomenon in Al containing intermetallic materials due to its rapid and passivating oxide formation. We report here the observation of enhanced oxygen diffusion along crystal defects, i.e. dislocations and twin boundaries, in the C15 Al 2 Ca Laves phase and how the presence of oxygen induces structural changes at these defects. Three main phases were identified and characterized structurally by aberration-corrected, atomic resolution scanning transmission electron microscopy, analytically by energy dispersive X-ray spectroscopy and electron energy loss spectroscopy. Unlike the C15 bulk phase, the twin boundary and dislocation transformed into a few nanometer wide amorphous phase, which depletes in Al and Ca but is highly enriched in oxygen. The dislocation even shows coexistence of the amorphous phase with a simple Al-rich A1 fcc phase. This A1 phase only depletes in Ca, not in Al (Al remains at bulk concentration), and is also enriched in oxygen. The Al-rich A1 phase is coherent with the C15 matrix. Electron energy loss spectroscopy revealed the amorphous phase to be Al 2 O 3 . We thereby show as one of the first studies that oxygen diffusion along crystal defects, especially also at the twin boundary can induce the formation of an amorphous oxide along themselves. The identification of oxygen-induced transformation at strained defects has to be considered when the material is exposed to air during plastic deformation at elevated temperatures. △ Less

Submitted 9 August, 2022; v1 submitted 1 August, 2022; originally announced August 2022.

Comments: 6 pages, 3 figures

Chiral Electronic Excitations in a Quasi-2D Rashba System BiTeI

Authors: A. C. Lee, B. Peng, K. Du, H. -H. Kung, B. Monserrat, S. -W. Cheong, C. J. Won, G. Blumberg

Abstract: The optical transitions between spin-polarized bands of the quasi-two dimensional Rashba system BiTeI are investigated using polarization resolved resonant Raman spectroscopy. We detect chiral excitations between states with opposite helicity and compare spectra to calculations within a three-band model. Using the resonant Raman excitation profile, we deduce the Rashba parameters and band gaps of… ▽ More The optical transitions between spin-polarized bands of the quasi-two dimensional Rashba system BiTeI are investigated using polarization resolved resonant Raman spectroscopy. We detect chiral excitations between states with opposite helicity and compare spectra to calculations within a three-band model. Using the resonant Raman excitation profile, we deduce the Rashba parameters and band gaps of the higher conduction bands near the Fermi level, and compare the parameters to values obtained by ab initio density function theory (DFT). △ Less

Submitted 25 April, 2022; v1 submitted 7 February, 2022; originally announced February 2022.

Local shape of the vapor-liquid critical point on the thermodynamic surface and the van der Waals equation of state

Authors: J. S. Yu, X. Zhou, J. F. Chen, W. K. Du, X. Wang, Q. H. Liu

Abstract: Differential geometry is powerful tool to analyze the vapor-liquid critical point on the surface of the thermodynamic equation of state. The existence of usual condition of the critical point $\left( \partial p/\partial V\right) _{T}=0$ requires the isothermal process, but the universality of the critical point is its independence of whatever process is taken, and so we can assume… ▽ More Differential geometry is powerful tool to analyze the vapor-liquid critical point on the surface of the thermodynamic equation of state. The existence of usual condition of the critical point $\left( \partial p/\partial V\right) _{T}=0$ requires the isothermal process, but the universality of the critical point is its independence of whatever process is taken, and so we can assume $\left( \partial p/\partial T\right) _{V}=0$. The distinction between the critical point and other points on the surface leads us to further assume that the critical point is geometrically represented by zero Gaussian curvature. A slight extension of the van der Waals equation of state is to letting two parameters $a$ and $b$ in it vary with temperature, which then satisfies both assumptions and reproduces its usual form when the temperature is approximately the critical one. △ Less

Submitted 29 May, 2021; v1 submitted 26 January, 2021; originally announced January 2021.

Comments: 6 pages. substantially revised

Journal ref: Front Phys (2021) 9:679083

Permutable SOS (Symmetry Operational Similarity)

Authors: Sang-Wook Cheong, Seongjoon Lim, Kai Du, Fei-Ting Huang

Abstract: Based on symmetry consideration, quasi-one-dimensional (1D) objects, relevant to numerous observables or phenomena, can be classified into eight different types. We provide various examples of each 1D type, and discuss their Symmetry Operational Similarity (SOS) relationships, which are often permutable. A number of recent experimental observations, including current-induced magnetization in polar… ▽ More Based on symmetry consideration, quasi-one-dimensional (1D) objects, relevant to numerous observables or phenomena, can be classified into eight different types. We provide various examples of each 1D type, and discuss their Symmetry Operational Similarity (SOS) relationships, which are often permutable. A number of recent experimental observations, including current-induced magnetization in polar or chiral conductors, non-linear Hall effect in polar conductors, spin-polarization of tunneling current to chiral conductors, and ferro-rotational domain imaging with linear gyration are discussed in terms of (permutable) SOS. In addition, based on (permutable) SOS, we predict a large number of new phenomena in low symmetry materials that can be experimentally verified in the future. △ Less

Submitted 8 January, 2021; originally announced January 2021.

Comments: 28 pages, 6 figures

Journal ref: npj Quantum Mater. 6, 58 (2021)

Highly tunable ferroelectricity in hybrid improper ferroelectric Sr3Sn2O7

Authors: Xianghan Xu, Yazhong Wang, Fei-Ting Huang, Kai Du, Elizabeth A. Nowadnick, Sang-Wook Cheong

Abstract: The successful theoretical prediction and experimental demonstration of hybrid improper ferroelectricity (HIF) provides a new pathway to couple octahedral rotations, ferroelectricity, and magnetism in complex materials. To enable technological applications, a HIF with a small coercive field is desirable. We successfully grow Sr3Sn2O7 single crystals, and discover that they exhibit the smallest ele… ▽ More The successful theoretical prediction and experimental demonstration of hybrid improper ferroelectricity (HIF) provides a new pathway to couple octahedral rotations, ferroelectricity, and magnetism in complex materials. To enable technological applications, a HIF with a small coercive field is desirable. We successfully grow Sr3Sn2O7 single crystals, and discover that they exhibit the smallest electric coercive field at room temperature among all known HIFs. Furthermore, we demonstate that a small external stress can repeatedly erase and re-generate ferroelastic domains. In addition, using in-plane piezo-response force microscopy, we characterize abundant charged and neutral domain walls. The observed small electrical and mechanical coercive field values are in accordance with the results of our first-principles calculations on Sr3Sn2O7, which show low energy barriers for both 90° and 180° polarization switching compared to those in other experimentally demonstrated HIFs. Our findings represent an advance towards the possible technological implemetation of functional HIFs. △ Less

Submitted 3 August, 2020; originally announced August 2020.

Comments: Accepted by Advanced Functional Materials

Direct Visualization of Irreducible Ferrielectricity in Crystals

Authors: Kai Du, Lei Guo, Jin Peng, Xing Chen, Zheng-Nan Zhou, Yang Zhang, Ting Zheng, Yan-Ping Liang, Jun-Peng Lu, Zhen-Hua Ni, Shan-Shan Wang, Gustaaf Van Tendeloo, Ze Zhang, Shuai Dong, He Tian

Abstract: In solids, charge polarity can one-to-one correspond to spin polarity phenomenologically, e.g. ferroelectricity/ferromagnetism, antiferroelectricity/antiferromagnetism, and even dipole-vortex/magnetic-vortex, but ferrielectricity/ferrimagnetism kept telling a disparate story in microscopic level. Since the definition of a charge dipole involves more than one ion, there may be multiple choices for… ▽ More In solids, charge polarity can one-to-one correspond to spin polarity phenomenologically, e.g. ferroelectricity/ferromagnetism, antiferroelectricity/antiferromagnetism, and even dipole-vortex/magnetic-vortex, but ferrielectricity/ferrimagnetism kept telling a disparate story in microscopic level. Since the definition of a charge dipole involves more than one ion, there may be multiple choices for a dipole unit, which makes most ferrielectric orders equivalent to ferroelectric ones, i.e. this ferrielectricity is not necessary to be a real independent branch of polarity. In this work, by using the spherical aberration-corrected scanning transmission electron microscope, we visualize a nontrivial ferrielectric structural evolution in BaFe2Se3, in which the development of two polar sub-lattices is out-of-sync, for which we term it as irreducible ferrielectricity. Such irreducible ferrielectricity leads to a non-monotonic behavior for the temperature-dependent polarization, and even a compensation point in the ordered state. Our finding unambiguously distinguishes ferrielectrics from ferroelectrics in solids. △ Less

Submitted 24 July, 2020; originally announced July 2020.

Comments: 15 figures

Journal ref: NPJ Quantum Materials 5, 49 (2020)

Efficient light-emitting diodes based on oriented perovskite nanoplatelets

Authors: Jieyuan Cui, Yang Liu, Yunzhou Deng, Chen Lin, Zhishan Fang, Chensheng Xiang, Peng Bai, Kai Du, Xiaobing Zuo, Kaichuan Wen, Shaolong Gong, Haiping He, Zhizhen Ye, Yunan Gao, He Tian, Baodan Zhao, Jianpu Wang, Yizheng Jin

Abstract: Solution-processed planar perovskite light-emitting diodes (LEDs) promise high-performance and cost-effective electroluminescent (EL) devices ideal for large-area display and lighting applications. Exploiting emission layers with high ratios of horizontal transition dipole moments (TDMs) is expected to boost photon outcoupling of planar LEDs. However, LEDs based on anisotropic perovskite nanoemitt… ▽ More Solution-processed planar perovskite light-emitting diodes (LEDs) promise high-performance and cost-effective electroluminescent (EL) devices ideal for large-area display and lighting applications. Exploiting emission layers with high ratios of horizontal transition dipole moments (TDMs) is expected to boost photon outcoupling of planar LEDs. However, LEDs based on anisotropic perovskite nanoemitters remains to be inefficient (external quantum efficiency, EQE <5%), due to the difficulties of simultaneously controlling the orientations of TDMs, achieving high photoluminescence quantum yields (PLQYs) and realizing charge balance in the films of the assembled nanostructures. Here we demonstrate efficient EL from an in-situ grown continuous perovskite film comprising of a monolayer of face-on oriented nanoplatelets. The ratio of horizontal TDMs of the perovskite nanoplatelet films is ~84%, substantially higher than that of isotropic emitters (67%). The nanoplatelet film shows a high PLQY of ~75%. These merits enable LEDs with a peak EQE of 23.6%, representing the most efficient perovskite LEDs. △ Less

Submitted 22 September, 2021; v1 submitted 13 June, 2020; originally announced June 2020.

Comments: 52 pages, 23 figures

No existence of the geometric potential for a Dirac fermion on two-dimensional curved surfaces of revolution

Authors: Z. Q. Yang, X. Y. Zhou, Z. Li, W. K. Du, Q. H. Liu

Abstract: For a free particle that non-relativistically moves on a curved surface, there are curvature-induced quantum potentials that significantly influence the surface quantum states, but the experimental results in topological insulators, whenever curved or not, indicate no evidence of such a potential, implying that there does not exist such a quantum potential for the relativistic particles, constrain… ▽ More For a free particle that non-relativistically moves on a curved surface, there are curvature-induced quantum potentials that significantly influence the surface quantum states, but the experimental results in topological insulators, whenever curved or not, indicate no evidence of such a potential, implying that there does not exist such a quantum potential for the relativistic particles, constrained on the surface or not. Within the framework of Dirac quantization scheme, we demonstrate a general result that for a Dirac fermion on a two-dimensional curved surface of revolution, no curvature-induced quantum potential is permissible. △ Less

Submitted 22 December, 2019; v1 submitted 5 December, 2019; originally announced December 2019.

Comments: 6 pages, no figures. Typos corrected

General covariant geometric momentum, gauge potential and a Dirac fermion on a two-dimensional sphere

Authors: Q. H. Liu, Z. Li, X. Y. Zhou, Z. Q. Yang, W. K. Du

Abstract: For a particle that is constrained on an ($N-1$)-dimensional ($N\geq2$) curved surface, the Cartesian components of its momentum in $N$-dimensional flat space is believed to offer a proper form of momentum for the particle on the surface, which is called the geometric momentum as it depends on the mean curvature. Once the momentum is made general covariance, the spin connection part can be interpr… ▽ More For a particle that is constrained on an ($N-1$)-dimensional ($N\geq2$) curved surface, the Cartesian components of its momentum in $N$-dimensional flat space is believed to offer a proper form of momentum for the particle on the surface, which is called the geometric momentum as it depends on the mean curvature. Once the momentum is made general covariance, the spin connection part can be interpreted as a gauge potential. The present study consists in two parts, the first is a discussion of the general framework for the general covariant geometric momentum. The second is devoted to a study of a Dirac fermion on a two-dimensional sphere and we show that there is the generalized total angular momentum whose three cartesian components form the $su(2)$ algebra, obtained before by consideration of dynamics of the particle, and we demonstrate that there is no curvature-induced geometric potential for the fermion. △ Less

Submitted 2 July, 2019; v1 submitted 19 May, 2019; originally announced May 2019.

Comments: 8 pages, no figure. Presentation improved

Microwave Conductivity of Ferroelectric Domains and Domain Walls in Hexagonal Rare-earth Ferrite

Authors: Xiaoyu Wu, Kai Du, Lu Zheng, Di Wu, Sang-Wook Cheong, Keji Lai

Abstract: We report the nanoscale electrical imaging results in hexagonal $Lu_{0.6}Sc_{0.4}FeO_3$ single crystals using conductive atomic force microscopy (C-AFM) and scanning microwave impedance microscopy (MIM). While the dc and ac response of the ferroelectric domains can be explained by the surface band bending, the drastic enhancement of domain wall (DW) ac conductivity is clearly dominated by the diel… ▽ More We report the nanoscale electrical imaging results in hexagonal $Lu_{0.6}Sc_{0.4}FeO_3$ single crystals using conductive atomic force microscopy (C-AFM) and scanning microwave impedance microscopy (MIM). While the dc and ac response of the ferroelectric domains can be explained by the surface band bending, the drastic enhancement of domain wall (DW) ac conductivity is clearly dominated by the dielectric loss due to DW vibration rather than mobile-carrier conduction. Our work provides a unified physical picture to describe the local conductivity of ferroelectric domains and domain walls, which will be important for future incorporation of electrical conduction, structural dynamics, and multiferroicity into high-frequency nano-devices. △ Less

Submitted 21 August, 2018; originally announced August 2018.

Comments: 4 figures

Journal ref: Phys. Rev. B 98 (2018) 081409(R)

Vortex Ferroelectric Domains, Large-loop Weak Ferromagnetic Domains, and Their Decoupling in Hexagonal (Lu, Sc)FeO3

Authors: Kai Du, Bin Gao, Yazhong Wang, Xianghan Xu, Jaewook Kim, Rongwei Hu, Fei-Ting Huang, Sang-Wook Cheong

Abstract: The direct domain coupling of spontaneous ferroelectric polarization and net magnetic moment can result in giant magnetoelectric (ME) coupling, which is essential to achieve mutual control and practical applications of multiferroics. Recently, the possible bulk domain coupling, the mutual control of ferroelectricity (FE) and weak ferromagnetism (WFM) have been theoretically predicted in hexagonal… ▽ More The direct domain coupling of spontaneous ferroelectric polarization and net magnetic moment can result in giant magnetoelectric (ME) coupling, which is essential to achieve mutual control and practical applications of multiferroics. Recently, the possible bulk domain coupling, the mutual control of ferroelectricity (FE) and weak ferromagnetism (WFM) have been theoretically predicted in hexagonal LuFeO3. Here, we report the first successful growth of highly-cleavable Sc-stabilized hexagonal Lu0.6Sc0.4FeO3 (h-LSFO) single crystals, as well as the first visualization of their intrinsic cloverleaf pattern of vortex FE domains and large-loop WFM domains. The vortex FE domains are on the order of 0.1-1 μm in size. On the other hand, the loop WFM domains are ~100 μm in size, and there exists no interlocking of FE and WFM domain walls. These strongly manifest the decoupling between FE and WFM in h-LSFO. The domain decoupling can be explained as the consequence of the structure-mediated coupling between polarization and dominant in-plane antiferromagnetic spins according to the theoretical prediction, which reveals intriguing interplays between FE, WFM, and antiferromagnetic orders in h-LSFO. Our results also indicate that the magnetic topological charge tends to be identical to the structural topological charge. This could provide new insights into the induction of direct coupling between magnetism and ferroelectricity mediated by structural distortions, which will be useful for the future applications of multiferroics. △ Less

Submitted 5 July, 2018; v1 submitted 26 June, 2018; originally announced June 2018.

Comments: 32 pages including supplementary information

Journal ref: npj Quantum Materials,volume 3, Article number: 33 (2018)

Tunable Semiconductors: Control over Carrier States and Excitations in Layered Hybrid Organic-Inorganic Perovskites

Authors: Chi Liu, William Huhn, Ke-Zhao Du, Alvaro Vazquez-Mayagoitia, David Dirkes, Wei You, Yosuke Kanai, David B. Mitzi, Volker Blum

Abstract: For a class of 2D hybrid organic-inorganic perovskite semiconductors based on $π$-conjugated organic cations, we predict quantitatively how varying the organic and inorganic component allows control over the nature, energy and localization of carrier states in a quantum-well-like fashion. Our first-principles predictions, based on large-scale hybrid density-functional theory with spin-orbit coupli… ▽ More For a class of 2D hybrid organic-inorganic perovskite semiconductors based on $π$-conjugated organic cations, we predict quantitatively how varying the organic and inorganic component allows control over the nature, energy and localization of carrier states in a quantum-well-like fashion. Our first-principles predictions, based on large-scale hybrid density-functional theory with spin-orbit coupling, show that the interface between the organic and inorganic parts within a single hybrid can be modulated systematically, enabling us to select between different type-I and type-II energy level alignments. Energy levels, recombination properties and transport behavior of electrons and holes thus become tunable by choosing specific organic functionalizations and juxtaposing them with suitable inorganic components. △ Less

Submitted 26 September, 2018; v1 submitted 19 March, 2018; originally announced March 2018.

Journal ref: Phys. Rev. Lett. 121, 146401 (2018)

Optimal Orientations of Quartz Crystals for Bulk Acoustic Wave Resonators with the Consideration of Thermal Properties

Authors: J. Wang, L. M. Zhang, S. Y. Wang, L. T. Xie, B. Huang, T. F. Ma, J. K. Du, M. C. Chao, S. Shen, R. X. Wu, H. F. Zhang

Abstract: Piezoelectric crystals are widely used for acoustic wave resonators of different functioning modes and types including BAW and SAW. It is well-known that only some special orientations of crystals will exhibit desirable properties such as mode couplings, thermal sensitivity, acceleration sensitivity, and others that are important in design and applications of resonators. With extensive studies on… ▽ More Piezoelectric crystals are widely used for acoustic wave resonators of different functioning modes and types including BAW and SAW. It is well-known that only some special orientations of crystals will exhibit desirable properties such as mode couplings, thermal sensitivity, acceleration sensitivity, and others that are important in design and applications of resonators. With extensive studies on physical properties in last decades and increasing industrial needs of novel products, it is necessary to comb the known knowledge of quartz crystal material for novel orientations and better products as agendas in the industry. With known material properties like elastic, piezoelectric, dielectric, and thermal constants, we can establish the relationships between vibrations and bias fields such as temperature to ensure a resonator immunizing from excessive response to changes causing significant degradation of resonator properties and performances. Since the theoretical framework of wave propagation in piezoelectric solids is known, we need to use the existing data and results for the validation of current orientations in actual products. The agreement will give us needed confidence of the theory and analytical procedures. Through rotations, we calculated physical properties as functions of angles and bias fields, enabling the calculation of resonator properties for the identification of optimal cuts. Such a procedure can also be applied to similar crystals for a careful examination of possible orientations to maximize the potential use of materials in acoustic wave resonators. △ Less

Submitted 26 December, 2017; originally announced December 2017.

Comments: 5 pages, 8 figures

Interstitial-Boron Solution Strengthened WB$_{3+x}$

Authors: Xiyue Cheng, Wei Zhang, Xing-Qiu Chen, Haiyang Niu, Peitao Liu, Kui Du, Gang Liu, Dianzhong Li, Hui-Ming Cheng, Hengqiang Ye, Yiyi Li

Abstract: By means of variable-composition evolutionary algorithm coupled with density functional theory and in combination with aberration-corrected high-resolution transmission electron microscopy experiments, we have studied and characterized the composition, structure and hardness properties of WB$_{3+x}$ ($x$ $<$ 0.5). We provide robust evidence for the occurrence of stoichiometric WB$_3$ and non-stoic… ▽ More By means of variable-composition evolutionary algorithm coupled with density functional theory and in combination with aberration-corrected high-resolution transmission electron microscopy experiments, we have studied and characterized the composition, structure and hardness properties of WB$_{3+x}$ ($x$ $<$ 0.5). We provide robust evidence for the occurrence of stoichiometric WB$_3$ and non-stoichiometric WB$_{3+x}$ both crystallizing in the metastable $hP$16 ($P6_3/mmc$) structure. No signs for the formation of the highly debated WB$_4$ (both $hP$20 and $hP$10) phases were found. Our results rationalize the seemingly contradictory high-pressure experimental findings and suggest that the interstitial boron atom is located in the tungsten layer and vertically interconnect with four boron atoms, thus forming a typical three-center boron net with the upper and lower boron layers in a three-dimensional covalent network, which thereby strengthen the hardness. △ Less

Submitted 12 September, 2013; v1 submitted 10 September, 2013; originally announced September 2013.

Comments: 5 pages, 5 figures