Showing 1–50 of 113 results for author: Hong, X

Strain-tunable charge localization coupled to complex magnetic orders in EuAl$_4$

Authors: M. Baumgartner, Xunyang Hong, Tianren Wang, Fazhi Yang, Yuetong Wu, Junzhang Ma, Tian Shang, J. Oppliger, J. Küspert, M. Hücker, O. Ivashko, F. Igoa Saldaña, M. v. Zimmermann, S. Pyon, K. Kudo, M. Nohara, P. Sačer, A. Akrap, N. Barišić, M. Novak, Qisi Wang, J. Chang

Abstract: Charge localization is particularly interesting when coupled to antiferromagnetic spin structures. Coupled spin-charge orders are well established in elemental chromium and correlated oxide superconductors, yet the interplay between charge order and more complex magnetic textures -- such as skyrmion lattices and chiral spin structures -- remains largely unexplored. Here we report a comprehensive s… ▽ More Charge localization is particularly interesting when coupled to antiferromagnetic spin structures. Coupled spin-charge orders are well established in elemental chromium and correlated oxide superconductors, yet the interplay between charge order and more complex magnetic textures -- such as skyrmion lattices and chiral spin structures -- remains largely unexplored. Here we report a comprehensive study of how charge localization couples to the unusually rich sequence of magnetic phases in EuAl$_4$. Using x-ray diffraction under applied magnetic field and uniaxial pressure, we demonstrate a direct coupling between the charge and spin order parameters. In the absence of external stimuli, charge localization is markedly enhanced upon entering the magnetically ordered phases. Strikingly, this effect is highly susceptible to strain: uniaxial pressure applied along the charge-order propagation direction further enhances localization, whereas pressure applied perpendicular to it weakens it. Application of magnetic field reveals both competitive and possible collaborative interactions between spin and charge ordering.This flexible coupling between spin and charge ordering opens a new route to designing symmetry-breaking states. Chiral charge order may for example be patterned from spin structures with that symmetry. △ Less

Submitted 8 July, 2026; originally announced July 2026.

Direct High-Magnetic-Field Coupling to Stripe Order in a Cuprate Superconductor

Authors: Leonardo Martinelli, Izabela Biało, Jens Oppliger, Julia Küspert, Orion Gerguri, Sandro Brunner, Benedikt Eggert, Mark H. Fischer, Jochen Geck, Marein Rahn, Ellen Fogh, Jaewon Choi, Atsuhiko Miyata, Oleksandr Prokhnenko, Zahirul Islam, Fernando Igoa Saldaña, Martin v. Zimmermann, Rachel Nickel, Kurt Kummer, Nicholas B. Brookes, Adheena Painganoor, Paola Caterina Forino, Rasmus Toft-Petersen, Niels Bech Christensen, Xunyang Hong , et al. (25 additional authors not shown)

Abstract: Superconductivity in cuprates emerges out of a complex normal state that hosts density waves, pseudogap physics, and strange metal properties. Here, we access this normal state by synchronizing free-electron laser x-rays with high-magnetic-field pulses up to 44 T. We observe a linear increase in charge order amplitude and correlation length that persists far above the vortex melting transition. Th… ▽ More Superconductivity in cuprates emerges out of a complex normal state that hosts density waves, pseudogap physics, and strange metal properties. Here, we access this normal state by synchronizing free-electron laser x-rays with high-magnetic-field pulses up to 44 T. We observe a linear increase in charge order amplitude and correlation length that persists far above the vortex melting transition. This behavior is incompatible with standard phase competition between charge order and superconductivity. By means of conventional hard x-ray diffraction and magnetostriction, we show that applied fields also enhance monoclinic lattice distortions. However, this magnetoelastic response is weaker and an epiphenomenon of the stripe order enhancement. Combined with recent observations of field-linear spin freezing, our results point to a direct coupling between magnetic field and the spin component of stripe order in the high-field normal state -- a mechanism independent of superconductivity suppression that has so far remained hidden from scattering probes. △ Less

Submitted 5 June, 2026; originally announced June 2026.

Comments: 3 figures

Nematicity in LaFeAsO single crystals studied by elastoresistance, high-resolution thermal expansion and shear-modulus measurements

Authors: X. C. Hong, S. Sauerland, L. Wang, F. Scaravaggi, A. U. B. Wolter, R. Kappenberger, S. Aswartham, S. Wurmehl, S. Sykora, F. Caglieris, B. Büchner, C. Hess, R. Klingeler

Abstract: Nematicity in LaFeAsO single crystals is studied by means of high-resolution thermal expansion, shear modulus, and elastoresistivity measurements. A softening of the shear modulus $C_{\rm 66}$ towards the structural phase transition at $T_{\rm S}$ is observed. In addition, a similar Curie-Weiss-like divergence of the nematic susceptibilities is found in the temperature dependence of both $χ^{sh}$… ▽ More Nematicity in LaFeAsO single crystals is studied by means of high-resolution thermal expansion, shear modulus, and elastoresistivity measurements. A softening of the shear modulus $C_{\rm 66}$ towards the structural phase transition at $T_{\rm S}$ is observed. In addition, a similar Curie-Weiss-like divergence of the nematic susceptibilities is found in the temperature dependence of both $χ^{sh}$ and $χ^{er}$, which are deduced from the shear modulus (sh) and the elastoresistivity (er) studies, respectively. These observations provide evidence for an electronic origin of nematicity in LaFeAsO. The characteristic energy of the coupling between the lattice and the electronic degrees of freedom is deduced to $\sim$30~K. The comparison to corresponding measurements on BaFe$_2$As$_2$ single crystals reveals a very similar temperature dependence of the shear modulus but yields contrasting results for $χ^{er}$ : In BaFe$_2$As$_2$, $χ^{er}$ diverges similarly as the uncoupled nematicity deduced from the shear modulus data as it is expected from the underlying Landau theory. In contrast, the Weiss temperatures of $χ^{er}$ and $χ^{sh}$ are significantly different in LaFeAsO. This difference is at odds with the commonly anticipated theories of resistivity anisotropy and electronic nematicity in iron pnictides. △ Less

Submitted 12 May, 2026; originally announced May 2026.

Journal ref: Phys. Rev. B 112, 235107 (2025)

Probing GHz Spin Dynamics Across Magnetic Phase Transitions in CrCl3 Nanoflakes Using Nitrogen-Vacancy Microscopy

Authors: Benjamin Hammons, Jitender Kumar, Sehrish Iqbal, Prem Bahadur Karki, Karishma Prasad, Tianlin Li, Aram Pirali, Ayodimeji E. Aregbesola, Rupak Timalsina, Xia Hong, Jian Wang, Kapildeb Ambal, Ilja Fescenko, Abdelghani Laraoui

Abstract: CrCl3, a layered van der Waals (vdW) magnet, exhibits in-plane magnetic anisotropy and enhanced interlayer coupling upon stacking, making it an ideal platform to host exotic nanoscale magnetic phenomena such as magnon hydrodynamics and meron-like topological spin defects. When interfaced with other vdW materials, its antiferromagnetic-to-ferromagnetic and ferromagnetic-to-paramagnetic phase transi… ▽ More CrCl3, a layered van der Waals (vdW) magnet, exhibits in-plane magnetic anisotropy and enhanced interlayer coupling upon stacking, making it an ideal platform to host exotic nanoscale magnetic phenomena such as magnon hydrodynamics and meron-like topological spin defects. When interfaced with other vdW materials, its antiferromagnetic-to-ferromagnetic and ferromagnetic-to-paramagnetic phase transitions and magnetic anisotropy can be tuned by voltage, strain, and layer stacking. Understanding the spin dynamics of CrCl3 at its magnetic phase transitions is crucial to its applications in magnonics. Here, we investigate the spin dynamics of CrCl3 nanoflakes using cryogenic diamond quantum sensing microscopy, based on measuring optically detected magnetic resonance, Rabi oscillations, and spin-lattice relaxation time (T1) of shallow nitrogen vacancy (NV) centers in diamond. In the ferromagnetic regime, we observe a pronounced reduction in the NV spin resonance contrast, a collapse of the Rabi oscillations, and a strong enhancement by two orders of magnitude of the relaxation rate G1 = 1/T1. These observations indicate intensified spin fluctuations in the gigahertz range. Broadband ferromagnetic resonance spectroscopy on CrCl3 crystals reveals resonance frequencies in the 4-15 GHz range together with a linewidth of ~24 mT, further supporting the NV measurements. A phenomenological model of magnetic-noise-induced NV relaxation reproduces the temperature dependence of G1 by combining antiferromagnetic, ferromagnetic, and paramagnetic fluctuation channels, indicating that magnetic noise is strongest in the ferromagnetic regime and evolves markedly across the phase transition. These results are crucial for using CrCl3 in 2D magnonics and hybrid quantum-magnon systems. △ Less

Submitted 20 July, 2026; v1 submitted 27 April, 2026; originally announced April 2026.

Journal ref: ACS nano 20 (22), 16542-16543 (2026)

Collective spin excitations in trilayer nickelate La$_4$Ni$_3$O$_{10}$

Authors: Ying Chan, Yuehong Li, Yujie Yan, Xunyang Hong, Tianren Wang, Marli dos Reis Cantarino, Yinghao Zhu, Enkang Zhang, Lixing Chen, Jun Okamoto, Hsiao-Yu Huang, Di-Jing Huang, N. B. Brookes, Johan Chang, Yao Shen, Jun Zhao, Qisi Wang

Abstract: Ruddlesden-Popper (RP) nickelates have recently emerged as a new family of high-temperature superconductors. In bilayer RP nickelates, magnetic excitations with large exchange couplings have been observed, supporting a spin-mediated pairing mechanism. Whether comparable spin correlations persist in trilayer nickelates, however, remains unknown. Here, we present a Ni $L$-edge resonant inelastic X-r… ▽ More Ruddlesden-Popper (RP) nickelates have recently emerged as a new family of high-temperature superconductors. In bilayer RP nickelates, magnetic excitations with large exchange couplings have been observed, supporting a spin-mediated pairing mechanism. Whether comparable spin correlations persist in trilayer nickelates, however, remains unknown. Here, we present a Ni $L$-edge resonant inelastic X-ray scattering (RIXS) study of La$_4$Ni$_3$O$_{10}$ single crystals. While the orbital excitations remain similar to those of La$_3$Ni$_2$O$_{7}$, the collective spin excitations in La$_4$Ni$_3$O$_{10}$ exhibit a comparable bandwidth of about $60$ meV but substantially suppressed spectral weight, implying a weaker electronic correlation in the trilayer compounds. Our results underscore the three-dimensional and multi-orbital electronic character in La$_4$Ni$_3$O$_{10}$, highlighting important differences from the bilayer nickelates. These findings provide crucial insights into the evolution of magnetism across the RP nickelate family and its connection to superconductivity. △ Less

Submitted 6 April, 2026; originally announced April 2026.

Comments: Supplementary Information available upon request

Magneto-elasto-resistivity in FeSe

Authors: M. Wissmann, L. Fanfarillo, X. -C. Hong, S. Caprara, S. Aswartham, B. Büchner, C. Hess, G. Seibold, F. Caglieris

Abstract: FeSe stands out among iron-based superconductors due to its extended nematic phase without the onset of long-range magnetic order. While strain-dependent electrical resistivity has been extensively explored to probe nematicity, its influence on magneto-transport properties remains less understood. In this work, we present measurements of the magneto-elasto-resistivity in FeSe as a function of temp… ▽ More FeSe stands out among iron-based superconductors due to its extended nematic phase without the onset of long-range magnetic order. While strain-dependent electrical resistivity has been extensively explored to probe nematicity, its influence on magneto-transport properties remains less understood. In this work, we present measurements of the magneto-elasto-resistivity in FeSe as a function of temperature and applied magnetic field. Using a minimal multiband Boltzmann model for transport we derive analytical expressions that capture the magnetic behavior of the whole set of experimental data both in the paramagnetic and in the nematic phase. These findings indicate that a multiband framework can robustly describe the magneto-elasto-transport properties in FeSe and arguably in other iron-based superconductors. △ Less

Submitted 19 December, 2025; v1 submitted 18 December, 2025; originally announced December 2025.

Magnetic properties of molecular beam epitaxy-grown ultrathin Cr2Ge2Te6 films down to monolayer limit on Si substrates

Authors: Pengfei Ji, Ruixuan Liu, Tianchen Zhu, Jinxuan Liang, Yang Chen, Yitian Tong, Yunhe Bai, Zuhan Geng, Fangting Chen, Yunyi Zang, Xiyu Hong, Jiatong Zhang, Luyi Yang, Qi-Kun Xue, Ke He, Xiao Feng

Abstract: Cr2Ge2Te6, a prototypical van der Waals ferromagnetic semiconductor, have attracted significant interest for its potential applications in high-performance spintronics. However, the magnetic ground state of monolayer Cr2Ge2Te6 remains elusive due to fragile and irregular-shaped thin flake samples with weak magnetic signals. Here, we successfully grow uniform ferromagnetic Cr2Ge2Te6 films down to m… ▽ More Cr2Ge2Te6, a prototypical van der Waals ferromagnetic semiconductor, have attracted significant interest for its potential applications in high-performance spintronics. However, the magnetic ground state of monolayer Cr2Ge2Te6 remains elusive due to fragile and irregular-shaped thin flake samples with weak magnetic signals. Here, we successfully grow uniform ferromagnetic Cr2Ge2Te6 films down to monolayer by molecular beam epitaxy. By exploiting a self-limiting growth mode, we achieve synthesis of uniform monolayer Cr2Ge2Te6 films across entire millimeter-scale Si substrates. Through a combination of superconducting quantum interference device magnetometry and anomalous Hall effect measurements, we establish that monolayer Cr2Ge2Te6 exhibits intrinsic ferromagnetism with perpendicular magnetic anisotropy below ~10 K, albeit with strong magnetic fluctuations characteristic of its two-dimensional nature. Furthermore, a systematic thickness-dependent study reveals a crossover from this fluctuation-dominated two-dimensional magnetism turns into conventional long-range ferromagnetic order as the film thickness increases. Our work not only definitively establishes the intrinsic ferromagnetic ground state of monolayer Cr2Ge2Te6, but also provides a scalable, silicon-compatible route for preparing the two-dimensional magnet for future spintronic or quantum devices. △ Less

Submitted 10 December, 2025; originally announced December 2025.

Impact of Electron Correlations on Infinite-Layer Cuprates and Nickelates

Authors: Xunyang Hong, Yuetong Wu, Ying Chan, Sze Tung Li, I. Biało, L. Martinelli, A. Drewanowski, Qiang Gao, Xiaolin Ren, Xingjiang Zhou, Zhihai Zhu, A. Galdi, D. G. Schlom, K. M. Shen, J. Choi, M. Garcia Fernandez, Ke-Jin Zhou, N. B. Brookes, H. M. Rønnow, Qisi Wang, J. Chang

Abstract: Optimization of unconventional superconductivity involves a balance of interaction strengths. Precise determination of correlation strength across different material families is therefore important. Here, we present a combined X-ray absorption spectroscopy (XAS) and resonant inelastic X-ray scattering (RIXS) study of infinite-layer PrNiO$_2$ and SrCuO$_2$ that enables fair comparison of their inte… ▽ More Optimization of unconventional superconductivity involves a balance of interaction strengths. Precise determination of correlation strength across different material families is therefore important. Here, we present a combined X-ray absorption spectroscopy (XAS) and resonant inelastic X-ray scattering (RIXS) study of infinite-layer PrNiO$_2$ and SrCuO$_2$ that enables fair comparison of their interaction strengths. For both compounds, we study the orbital and magnetic excitations and extract their dispersions along high-symmetry directions. Using a single-band Hubbard model and including higher-order exchange interactions, we derive the correlation factor $U/t$ for both compounds. A key finding is that despite a smaller Coulomb repulsion $U$, PrNiO$_2$ exhibits a correlation strength that is 20% stronger than that of its isostructural cuprate counterpart SrCuO$_2$. This indicates that a moderation of the correlation strength may further optimize superconductivity in nickelates. △ Less

Submitted 12 November, 2025; originally announced November 2025.

Comments: Supplementary information available upon request

Journal ref: Communications Materials 7, 182 (2026)

Nematic Fluctuations and Electronic Correlations in Heavily Hole-Doped Ba$_{1-x}$K$_x$Fe$_2$As$_2$ Probed by Elastoresistance

Authors: Franz Eckelt, Steffen Sykora, Xiaochen Hong, Vilmos Koscis, Vadim Grinenko, Bernd Büchner, Kunihiro Kihou, Chu-Ho Lee, Christian Hess

Abstract: This work investigates nematic fluctuations and electronic correlations in the hole-doped iron pnictide superconductor Ba$_{1-x}$K$_x$Fe$_2$As$_2$ by means of longitudinal and transverse elastoresistance measurements over a wide doping range ($0.63 < x < 0.98$). For this purpose, the orbital character of the electronic response was revealed by decomposition of the elastoresistance into the… ▽ More This work investigates nematic fluctuations and electronic correlations in the hole-doped iron pnictide superconductor Ba$_{1-x}$K$_x$Fe$_2$As$_2$ by means of longitudinal and transverse elastoresistance measurements over a wide doping range ($0.63 < x < 0.98$). For this purpose, the orbital character of the electronic response was revealed by decomposition of the elastoresistance into the $A_{1g}$ and $B_{2g}$ symmetry channels. It was shown that at lower doping levels nematic fluctuations in the $B_{2g}$ channel dominate, while for $x > 0.68$ the $A_{1g}$ channel becomes dominant and reaches a pronounced maximum at $x \approx 0.8$ which indicates strong orbital-selective electronic correlations. Despite the dominance of the $A_{1g}$ signal at high doping, a weak contribution in the $B_{2g}$ channel persists, which can be interpreted as a remnant of nematic fluctuations. Model calculations based on a five-orbital tight-binding Hamiltonian with interactions attribute the observed enhancement in the $A_{1g}$ channel to an orbital-selective Kondo-like resonance, predominantly involving the $d_{xy}$ orbital. We discuss our results in relation to the evolution of the Sommerfeld coefficient reported in the literature and a reported change of the superconducting order parameter. All this indicates that for $x > 0.68$ qualitatively new physics emerges. Our findings suggest that electronic correlations in the strongly hole-doped regime play an important role in superconductivity, while the detectable weak nematic fluctuations may also be of relevance. △ Less

Submitted 10 October, 2025; originally announced October 2025.

Thickness-Induced Topological Phase Transition Investigated by Helicity Dependent Photocurrent in $α$-Sn/CdTe(110)

Authors: Tengfei Liu, Xiyu Hong, Zhe Li, Shenzhong Chen, Leyi Li, Xin-Yi Tang, Shuying Cheng, Yunfeng Lai, Yonghai Chen, Zhu Diao, Ke He, Qi-kun Xue, Jinling Yu

Abstract: $α$-Sn exhibits a rich topological phase diagram, yet experimental methods to tune and distinguish these phases remain limited. Here, we investigated the helicity-dependent photocurrent (HDPC) in $α$-Sn films of varying thickness grown on CdTe(110) by molecular beam epitaxy. The HDPC of the 5 nm $α… ▽ More $α$-Sn exhibits a rich topological phase diagram, yet experimental methods to tune and distinguish these phases remain limited. Here, we investigated the helicity-dependent photocurrent (HDPC) in $α$-Sn films of varying thickness grown on CdTe(110) by molecular beam epitaxy. The HDPC of the 5 nm $α$-Sn film shows an odd-function dependence on incident angle, whereas that of the 10 and 30 nm films exhibit an even-function dependence. Combined with high-resolution transmission electron microscopy (HR-TEM), point-group symmetry analysis, and first-principles calculations, it is revealed that a thickness-driven topological phase transition from a two dimensional (2D) to a three dimensional (3D) topological insulator occurs between 5 and 10 nm. These results demonstrate that HDPC serves as a sensitive diagnostic tool for topological phase transitions. The tunable electronic properties of $α$-Sn(110) films enable thickness- and strain-mediated control of topological states, establishing a versatile platform for exploring emerging topological phenomena and developing spin-based devices. △ Less

Submitted 4 September, 2025; originally announced September 2025.

Itinerant and topological excitations in a honeycomb spiral spin liquid candidate

Authors: Yuqian Zhao, Xuping Yao, Xun Chen, Zongtang Wan, Zhaohua Ma, Xiaochen Hong, Yuesheng Li

Abstract: The frustrated insulating magnet can stabilize a spiral spin liquid, arising from cooperative fluctuations among a subextensively degenerate manifold of spiral configurations, with ground-state wave vectors forming a continuous contour or surface in reciprocal space. The atomic-mixing-free honeycomb antiferromagnet GdZnPO has recently emerged as a promising spiral spin-liquid candidate, hosting no… ▽ More The frustrated insulating magnet can stabilize a spiral spin liquid, arising from cooperative fluctuations among a subextensively degenerate manifold of spiral configurations, with ground-state wave vectors forming a continuous contour or surface in reciprocal space. The atomic-mixing-free honeycomb antiferromagnet GdZnPO has recently emerged as a promising spiral spin-liquid candidate, hosting nontrivial topological excitations. Despite growing interest, the transport and topological properties of spiral spin liquids remain largely unexplored experimentally. Here, we report transport measurements on high-quality, electrically insulating GdZnPO single crystals. We observe a giant low-temperature magnetic thermal conductivity down to $\sim$50 mK, described by $κ_{xx}^\mathrm{m}$ $\sim$ $κ_0+κ_1T$, where both $κ_0$ and $κ_1$ are positive constants associated with excitations along and off the spiral contour in reciprocal space, respectively. This behavior parallels the magnetic specific heat, underscoring the presence of mobile low-energy excitations intrinsic to the putative spiral spin liquid. Furthermore, the observed positive thermal Hall effect confirms the topological nature of at least some of these excitations. Our findings provide key insights into the itinerant and topological properties of low-lying spin excitations in the spiral spin-liquid candidate. △ Less

Submitted 26 August, 2025; originally announced August 2025.

Comments: accepted in Nature Communications. Supporting Information is available from the authors

Report number: Nat. Commun. 16, 8410 (2025)

Journal ref: Nat. Commun. 16, 8410 (2025)

Magnetic Excitations of a Half-Filled Tl-based Cuprate

Authors: I. Biało, Q. Wang, J. Küspert, X. Hong, L. Martinelli, O. Gerguri, Y. Chan, K. von Arx, O. K. Forslund, W. R. Pudełko, C. Lin, N. C. Plumb, Y. Sassa, D. Betto, N. B. Brookes, M. Rosmus, N. Olszowska, M. D. Watson, T. K. Kim, C. Cacho, M. Horio, M. Ishikado, H. M. Rønnow, J. Chang

Abstract: Strong electron correlations drive Mott insulator transitions. Yet, there exists no framework to classify Mott insulators by their degree of correlation. Cuprate superconductors, with their tunable doping and rich phase diagrams, offer a unique platform to investigate the evolution of these interactions. However, spectroscopic access to a clean half-filled Mott-insulating state is lacking in compo… ▽ More Strong electron correlations drive Mott insulator transitions. Yet, there exists no framework to classify Mott insulators by their degree of correlation. Cuprate superconductors, with their tunable doping and rich phase diagrams, offer a unique platform to investigate the evolution of these interactions. However, spectroscopic access to a clean half-filled Mott-insulating state is lacking in compounds with the highest superconducting onset temperature. To fill this gap, we introduce a pristine, half-filled thallium-based cuprate system, Tl$_2$Ba$_5$Cu$_4$O$_{x}$. Using high-resolution resonant inelastic x-ray scattering, we probe long-lived magnon excitations and uncover a pronounced kink in the magnon dispersion, marked by a simultaneous change in group velocity and lifetime broadening. Modeling the dispersion within a Hubbard-Heisenberg approach, we extract the interaction strength and compare it with other cuprate systems. Our results establish a cuprate universal relation between electron-electron interaction and magnon zone-boundary dispersion. Superconductivity seems to be optimal at intermediate correlation strength, suggesting an optimal balance between localization and itinerancy. △ Less

Submitted 16 January, 2026; v1 submitted 29 July, 2025; originally announced July 2025.

Comments: 12 pages, 8 figures, 2 tables

Persistent paramagnons in high-temperature infinite-layer nickelate superconductors

Authors: Yujie Yan, Ying Chan, Xunyang Hong, S. Lin Er Chow, Zhaoyang Luo, Yuehong Li, Tianren Wang, Yuetong Wu, Izabela Biało, Nurul Fitriyah, Saurav Prakash, Xing Gao, King Yau Yip, Qiang Gao, Xiaolin Ren, Jaewon Choi, Ganesha Channagowdra, Jun Okamoto, Xingjiang Zhou, Zhihai Zhu, Liang Si, Mirian Garcia-Fernandez, Ke-Jin Zhou, Hsiao-Yu Huang, Di-Jing Huang , et al. (3 additional authors not shown)

Abstract: The recent discovery of high-temperature superconductivity in hole-doped SmNiO$_2$, exhibiting the record-high transition temperature $T_c$ among infinite-layer (IL) nickelates, has opened a new avenue for exploring design principles of superconductivity. Experimentally determining the electronic structure and magnetic interactions in this new system is crucial to elucidating the mechanism behind… ▽ More The recent discovery of high-temperature superconductivity in hole-doped SmNiO$_2$, exhibiting the record-high transition temperature $T_c$ among infinite-layer (IL) nickelates, has opened a new avenue for exploring design principles of superconductivity. Experimentally determining the electronic structure and magnetic interactions in this new system is crucial to elucidating the mechanism behind the enhanced superconductivity. Here, we report a Ni $L$-edge resonant inelastic x-ray scattering (RIXS) study of superconducting Sm-based IL nickelate thin films Sm$_{1-x-y-z}$Eu$_x$Ca$_y$Sr$_z$NiO$_2$ (SECS). Dispersive paramagnonic excitations are observed in both optimally and overdoped SECS samples, supporting a spin-fluctuation-mediated pairing scenario. However, despite the two-fold enhancement of $T_c$ in the Sm-based nickelates compared to their Pr-based counterparts, the effective exchange coupling strength is reduced by approximately $20\%$. This behavior contrasts with hole-doped cuprates, where magnetic interactions correlate positively with $T_c$, highlighting essential differences in their superconducting mechanisms. △ Less

Submitted 24 July, 2025; originally announced July 2025.

Comments: Supplementary Information available upon request

Journal ref: Nature Communications 17, 6645 (2026)

Direct probe of magnetic field effects on phonons by ultrasound propagation in a quasi-two-dimensional honeycomb magnet Na$_2$Co$_2$TeO$_6$

Authors: Xiaochen Hong, Maximilian Schiffer, Beat Valentin Schwarze, Marc Uhlarz, Xianghong Jin, Weiliang Yao, Lukas Janssen, Sergei Zherlitsyn, Bernd Büchner, Yuan Li, Young Sun, Christian Hess

Abstract: We study the phonon behavior of a Co-based honeycomb frustrated magnet Na$_2$Co$_2$TeO$_6$ under magnetic field applied perpendicular to the honeycomb plane. The temperature and field dependence of the sound velocity and sound attenuation unveil prominent spin-lattice coupling in this material, promoting ultrasound as a sensitive probe for magnetic properties. An out-of-plane ferrimagnetic order i… ▽ More We study the phonon behavior of a Co-based honeycomb frustrated magnet Na$_2$Co$_2$TeO$_6$ under magnetic field applied perpendicular to the honeycomb plane. The temperature and field dependence of the sound velocity and sound attenuation unveil prominent spin-lattice coupling in this material, promoting ultrasound as a sensitive probe for magnetic properties. An out-of-plane ferrimagnetic order is determined below the Néel temperature $T_N=27$~K. A comprehensive analysis of our data further supports a triple-Q ground state of Na$_2$Co$_2$TeO$_6$. Furthermore, the ultrasound data were systematically compared to the thermal transport results from literature, to unveil the importance of phononic contribution to the observed transport behaviors. △ Less

Submitted 25 August, 2025; v1 submitted 1 June, 2025; originally announced June 2025.

Comments: to appear in Phys. Rev. B

Journal ref: Phys. Rev. B 112, 094403 (2025)

Nitrogen-Vacancy Magnetometry of Edge Magnetism in WS2 Flakes

Authors: Ilja Fescenko, Raman Kumar, Thitinun Gas-Osoth, Yifei Wang, Suvechhya Lamichhane, Tianlin Li, Adam Erickson, Nina Raghavan, Tom Delord, Cory D. Cress, Nicholas Proscia, Samuel W. LaGasse, Sy-Hwang Liou, Xia Hong, Jose J. Fonseca, Toshu An, Carlos A. Meriles, Abdelghani Laraoui

Abstract: Two-dimensional (2D) magnets are of significant interest both as a platform for exploring novel fundamental physics and for their potential in spintronic and optoelectronic devices. Recent bulk magnetometry studies have indicated a weak ferromagnetic response in WS2, and theoretical predictions suggest edge-localized magnetization in flakes with partial hydrogenation. Here, we use room-temperature… ▽ More Two-dimensional (2D) magnets are of significant interest both as a platform for exploring novel fundamental physics and for their potential in spintronic and optoelectronic devices. Recent bulk magnetometry studies have indicated a weak ferromagnetic response in WS2, and theoretical predictions suggest edge-localized magnetization in flakes with partial hydrogenation. Here, we use room-temperature wide-field quantum diamond magnetometry to image pristine and Fe-implanted WS2 flakes of varying thicknesses (45-160 nm), exfoliated from bulk crystals and transferred to NV-doped diamond substrates. We observe direct evidence of edge-localized stray magnetic fields, which scale linearly with applied external magnetic field (4.4-220 mT), reaching up to 4.7 uT. The edge signal shows a limited dependence on the flake thickness, consistent with dipolar field decay and sensing geometry. Magnetic simulations using five alternative models favor the presence of edge magnetization aligned along an axis slightly tilted from the normal to the WS2 flake plane, consistent with spin canting in antiferromagnetically coupled edge states. Our findings establish WS2 as a promising platform for edge-controlled 2D spintronics. △ Less

Submitted 27 July, 2025; v1 submitted 16 May, 2025; originally announced May 2025.

Journal ref: Advanced Functional Materials e12391 (2025)

Ultrafast dynamics of ferroelectric polarization of NbOI$_{2}$ captured with femtosecond electron diffraction

Authors: Yibo Wang, Md Sazzad Hossain, Tianlin Li, Yanwei Xiong, Cuong Le, Jesse Kuebler, Nina Raghavan, Lucia Fernandez-Ballester, Xia Hong, Alexander Sinitskii, Martin Centurion

Abstract: Two-dimensional (2D) ferroelectric materials like NbOI$_{2}$ have garnered significant interest, yet their temporal response and synergetic interaction with light remain underexplored. Previous studies on the polarization of oxide ferroelectrics have relied on time-resolved optical second harmonic generation or ultrafast X-ray scattering. Here, we probe the laser-induced polarization dynamics of 2… ▽ More Two-dimensional (2D) ferroelectric materials like NbOI$_{2}$ have garnered significant interest, yet their temporal response and synergetic interaction with light remain underexplored. Previous studies on the polarization of oxide ferroelectrics have relied on time-resolved optical second harmonic generation or ultrafast X-ray scattering. Here, we probe the laser-induced polarization dynamics of 2D NbOI$_{2}$ nanocrystals using ultrafast transmission electron diffraction and deflectometry. The deflection of the electron pulses is directly sensitive to the changes in the polarization, while the diffraction signal captures the structural evolution. Excited with a UV laser pulse, the polarization of NbOI$_{2}$ is initially suppressed for two picoseconds, then it recovers and overshoots, leading to a transiently enhanced polarization persisting for over 200 ps. This recovery coincides with coherent acoustic phonon generation, triggering a piezoresponse in the NbOI$_{2}$ nanocrystals. Our results offer a new method for sensing the ferroelectric order parameter in femtosecond time scales. △ Less

Submitted 10 April, 2025; originally announced April 2025.

Comments: 26 pages, 9 figures

Observation of collective charge excitations in a cuprate superconductor

Authors: Xunyang Hong, Yujie Yan, L. Martinelli, I. Biało, K. von Arx, J. Choi, Y. Sassa, S. Pyon, T. Takayama, H. Takagi, Zhenglu Li, M. Garcia-Fernandez, Ke-Jin Zhou, J. Chang, Qisi Wang

Abstract: Emergent symmetry breakings in condensed matter systems are often intimately linked to collective excitations. For example, the intertwined spin-charge stripe order in cuprate superconductors is associated with spin and charge excitations. While the collective behavior of spin excitations is well established, the nature of charge excitations remains to be understood. Here we present a high-resolut… ▽ More Emergent symmetry breakings in condensed matter systems are often intimately linked to collective excitations. For example, the intertwined spin-charge stripe order in cuprate superconductors is associated with spin and charge excitations. While the collective behavior of spin excitations is well established, the nature of charge excitations remains to be understood. Here we present a high-resolution resonant inelastic x-ray scattering (RIXS) study of charge excitations in the stripe-ordered cuprate La$_{1.675}$Eu$_{0.2}$Sr$_{0.125}$CuO$_4$. The RIXS spectra consist of both charge and phonon excitations around the charge ordering wave vector. By modeling the momentum-dependent phonon intensity, the charge-excitation spectral weight is extracted for a wide range of energy. As such, we reveal the highly dispersive nature of the charge excitations, with an energy scale comparable to the spin excitations. Since charge order and superconductivity in cuprates are possibly driven by the same electronic correlations, determining the interaction strength underlying charge order is essential to establishing a comprehensive microscopic model of high-temperature superconductivity. △ Less

Submitted 6 March, 2025; originally announced March 2025.

Comments: Supplementary Information available upon request

Designing Guidance for Multiple Valley-based Topological States Driven by Magnetic Substrates: Potential Applications at High Temperatures

Authors: Xiyu Hong, Zhe Li

Abstract: Valley-based topological phases offer a wealth of exotic quantum states with tunable functionalities, driven by the valley degree of freedom. In this work, by constructing heterostructures of germanene (silicene, stanene) on various magnetic substrates, we address key tuning factors such as the spin-orbit coupling (SOC) strength of the substrate, magnetic orientations, and stacking orders, all of… ▽ More Valley-based topological phases offer a wealth of exotic quantum states with tunable functionalities, driven by the valley degree of freedom. In this work, by constructing heterostructures of germanene (silicene, stanene) on various magnetic substrates, we address key tuning factors such as the spin-orbit coupling (SOC) strength of the substrate, magnetic orientations, and stacking orders, all of which govern multiple valley-based topological features. We present a comprehensive guiding principle for the efficient manipulation of these features, achieved simply by designing and modulating the magnetic properties of the underlying substrates. Specifically, increasing the SOC strength of the magnetic substrate acilitates a range of topological phase transitions characterized by different Chern numbers, with many systems exhibiting a transition from quantum valley Hall to quantum anomalous Hall (QAH) states. Additionally, rotating the in-plane magnetic orientation of the substrate enables tunability of the Chern number and chirality, within a moderate range of SOC strength. Furthermore, the antiferromagnetic coupling of the magnetic substrate can induce valley-based QAH states with substantial valley gaps, leveraging its high Curie temperature (TC) to enable the realization of multiple tunable magnetic topologies at elevated temperatures. Our findings provide a straightforward strategy for the design and manipulation of spintronic and valleytronic devices that can potentially operate under high-temperature conditions. △ Less

Submitted 8 January, 2025; originally announced January 2025.

Universally optimizable strategy for magnetic gaps towards high-temperature quantum anomalous Hall states via magnetic-insulator/topological-insulator building-blocks

Authors: Zhe Li, Feng Xue, Xin-Yi Tang, Xiyu Hong, Yang Chen, Xiao Feng, Ke He

Abstract: Optimizing the magnetic Zeeman-splitting term, specifically the magnetic gap of the topological surface states (TSSs), is a crucial issue and central challenge in advancing higher-temperature quantum anomalous Hall (QAH) states. In this work, we demonstrate a counterintuitive, nonmonotonic relationship between the magnetic gap and the hybridization strength in ferromagnetic-insulator (FMI)/topolog… ▽ More Optimizing the magnetic Zeeman-splitting term, specifically the magnetic gap of the topological surface states (TSSs), is a crucial issue and central challenge in advancing higher-temperature quantum anomalous Hall (QAH) states. In this work, we demonstrate a counterintuitive, nonmonotonic relationship between the magnetic gap and the hybridization strength in ferromagnetic-insulator (FMI)/topological-insulator (TI) sandwich structures. Concretely, insufficient hybridization strength fails to induce a substantial magnetic gap; while excessive hybridization incandesces the competition between kinetic and Coulomb exchange interactions, thereby reducing the gap. Strong hybridization strength also spatially delocalizes the TSSs, diminishing the effective orbital coupling between TSS-based p and magnetic d orbitals, which further weakens kinetic and Coulomb exchange interaction strength. Moreover, modifying the stacking order offers an experimentally viable approach to optimizing magnetic gaps, enabling the tunability of Chern numbers, chirality and maximizing global gaps. These findings unveil a universal guiding principle for optimizing magnetic gaps in FMI-TI proximity-based QAH systems, offering valuable insights for advancing experimental implementations in this field. △ Less

Submitted 3 February, 2025; v1 submitted 11 November, 2024; originally announced November 2024.

Comments: 37 pages, 31 figures

Journal ref: Phys. Rev. B 111, 075106 (2025)

Ultrafast dynamics of a spin-polarized electron plasma with magnetic ions

Authors: Benjamin Bakri, Nicolas Crouseilles, Paul-Antoine Hervieux, Xue Hong, Giovanni Manfredi

Abstract: We construct a mean-field model that describes the nonlinear dynamics of a spin-polarized electron gas interacting with fixed, positively-charged ions possessing a magnetic moment that evolves in time. The mobile electrons are modeled by a four-component distribution function in the two-dimensional phase space $(x,v)$, obeying a Vlasov-Poisson set of equations. The ions are modeled by a Landau-Lif… ▽ More We construct a mean-field model that describes the nonlinear dynamics of a spin-polarized electron gas interacting with fixed, positively-charged ions possessing a magnetic moment that evolves in time. The mobile electrons are modeled by a four-component distribution function in the two-dimensional phase space $(x,v)$, obeying a Vlasov-Poisson set of equations. The ions are modeled by a Landau-Lifshitz equation for their spin density, which contains ion-ion and electron-ion magnetic exchange terms. We perform a linear response study of the coupled Vlasov-Poisson-Landau-Lifshitz (VPLL) equations for the case of a Maxwell-Boltzmann equilibrium, focussing in particular on the spin dispersion relation. Condition of stability or instability for the spin modes are identified, which depend essentially on the electron spin polarization rate $η$ and the electron-ion magnetic coupling constant $K$. We also develop an Eulerian grid-based computational code for the fully nonlinear VPLL equations, based on the geometric Hamiltonian method first developed in [N. Crouseilles et al. Journal of Plasma Physics, 89(2):905890215, 2023]. This technique allows us to achieve great accuracy for the conserved quantities, such as the modulus of the ion spin vector and the total energy. Numerical tests in the linear regime are in accordance with the estimations of the linear response theory. For two-stream equilibria, we study the interplay of instabilities occurring in both the charge and the spin sectors. The set of parameters used in the simulations, with densities close to those of solids ($\approx 10^{29} \rm m^{-3}$) and temperatures of the order of 10 eV, may be relevant to the warm dense matter regime appearing in some inertial fusion experiments. △ Less

Submitted 24 October, 2024; originally announced October 2024.

Journal ref: J. Plasma Phys. 91 (2025) E9

Decoupling of Static and Dynamic Charge Correlations revealed by Uniaxial Strain in a Cuprate Superconductor

Authors: L. Martinelli, I. Biało, X. Hong, J. Oppliger, C. Lin, T. Schaller, J. Küspert, M. H. Fischer, T. Kurosawa, N. Momono, M. Oda, D. V. Novikov, A. Khadiev, E. Weschke, J. Choi, S. Agrestini, M. Garcia-Fernandez, Ke-Jin Zhou, Q. Wang, J. Chang

Abstract: We use uniaxial strain in combination with ultra-high-resolution Resonant Inelastic X-ray Scattering (RIXS) at the oxygen K- and copper L3-edges to study the excitations stemming from the charge ordering wave vector in La1.875Sr0.125CuO4. By detwinning stripe ordering, we demonstrate that the optical phonon anomalies do not show any stripe anisotropy. The low-energy charge excitations also retain… ▽ More We use uniaxial strain in combination with ultra-high-resolution Resonant Inelastic X-ray Scattering (RIXS) at the oxygen K- and copper L3-edges to study the excitations stemming from the charge ordering wave vector in La1.875Sr0.125CuO4. By detwinning stripe ordering, we demonstrate that the optical phonon anomalies do not show any stripe anisotropy. The low-energy charge excitations also retain an in-plane four-fold symmetry. As such, we find that both phonon and charge excitations are decoupled entirely from the strength of static charge ordering. The almost isotropic character of charge excitations is indicative of a quantum critical behaviour and remains a possible source for the strange metal properties found in the normal state of cuprate superconductors. △ Less

Submitted 24 February, 2025; v1 submitted 21 June, 2024; originally announced June 2024.

Comments: 9 pages, 6 figures

Multimechanism quantum anomalous Hall and Chern number tunable states in germanene (silicene, stanene)/$M$Bi$_2$Te$_4$ heterostructures

Authors: Zhe Li, Jiatong Zhang, Xiyu Hong, Xiao Feng, Ke He

Abstract: By constructing germanene (silicene, stanene)/$M$Bi$_2$Te$_4$ ($M$ = 3d-transition elements) heterostructures, we discovered and designed multimechanism quantum-anomalous-Hall (QAH) systems, including $Γ$-based QAH, $K$-$K'$-connected QAH, and valley-polarized $K$- or $K'$-based QAH states via first-principle computations. The unique systems possess a global gap and tunable Chern number. The coexi… ▽ More By constructing germanene (silicene, stanene)/$M$Bi$_2$Te$_4$ ($M$ = 3d-transition elements) heterostructures, we discovered and designed multimechanism quantum-anomalous-Hall (QAH) systems, including $Γ$-based QAH, $K$-$K'$-connected QAH, and valley-polarized $K$- or $K'$-based QAH states via first-principle computations. The unique systems possess a global gap and tunable Chern number. The coexisting conventional $Γ$-based QAH state of $M$Bi$_2$Te$_4$ and valley-polarized $K$($K'$)-based QAH state of germanene (silicene, stanene), with opposite chirality, can interact with each other. Adjusting magnetic configurations of $M$Bi$_2$Te$_4$-layers not only switch on (off) the QAH conductance, but also modulate Chern numbers exactly. For example, the germanene/bilayer-NiBi$_2$Te$_4$ possesses the Chern number $C = +1$ in ferromagnetic couplings and $C = +2$ in antiferromagnetic couplings. The novel multimechanism QAH insulators, which are achievable in experiments, provide a new approach to spintronics and valleytronics based on topological states of matter. △ Less

Submitted 17 June, 2024; v1 submitted 16 January, 2024; originally announced January 2024.

Comments: 9 pages, 4 figures

Journal ref: Phys. Rev. B 109, 235132(2024)

Spinon heat transport in the three-dimensional quantum magnet PbCuTe$_2$O$_6$

Authors: Xiaochen Hong, Matthias Gillig, Abanoub R. N. Hanna, Shravani Chillal, A. T. M. Nazmul Islam, Bella Lake, Bernd Büchner, Christian Hess

Abstract: Quantum spin liquids (QSL) are novel phases of matter which remain quantum disordered even at the lowest temperature. They are characterized by emergent gauge fields and fractionalized quasiparticles. Here we show that the sub-Kelvin thermal transport of the three-dimensional $S=1/2$ hyper-hyperkagome quantum magnet PbCuTe$_2$O$_6$ is governed by a sizeable charge-neutral fermionic contribution wh… ▽ More Quantum spin liquids (QSL) are novel phases of matter which remain quantum disordered even at the lowest temperature. They are characterized by emergent gauge fields and fractionalized quasiparticles. Here we show that the sub-Kelvin thermal transport of the three-dimensional $S=1/2$ hyper-hyperkagome quantum magnet PbCuTe$_2$O$_6$ is governed by a sizeable charge-neutral fermionic contribution which is compatible with the itinerant fractionalized excitations of a spinon Fermi surface. We demonstrate that this hallmark feature of the QSL state is remarkably robust against sample crystallinity, large magnetic field, and field-induced magnetic order, ruling out the imitation of QSL features by extrinsic effects. Our findings thus reveal the characteristic low-energy features of PbCuTe$_2$O$_6$ which qualify this compound as a true QSL material. △ Less

Submitted 20 November, 2023; originally announced November 2023.

Transport Anisotropy in One-dimensional Graphene Superlattice in the High Kronig-Penney Potential Limit

Authors: Tianlin Li, Hanying Chen, Kun Wang, Yifei Hao, Le Zhang, Kenji Watanabe, Takashi Taniguchi, Xia Hong

Abstract: One-dimensional graphene superlattice subjected to strong Kronig-Penney (KP) potential is promising for achieving electron lensing effect, while previous studies utilizing the modulated dielectric gates can only yield a moderate, spatially dispersed potential profile. Here, we realize high KP potential modulation of graphene via nanoscale ferroelectric domain gating. Graphene transistors are fabri… ▽ More One-dimensional graphene superlattice subjected to strong Kronig-Penney (KP) potential is promising for achieving electron lensing effect, while previous studies utilizing the modulated dielectric gates can only yield a moderate, spatially dispersed potential profile. Here, we realize high KP potential modulation of graphene via nanoscale ferroelectric domain gating. Graphene transistors are fabricated on PbZr$_{0.2}$Ti$_{0.8}$O$_{3}$ back-gates patterned with periodic, 100-200 nm wide stripe domains. Due to band reconstruction, the h-BN top-gating induces satellite Dirac points in samples with current along the superlattice vector $\hat{s}$, a feature absent in samples with current perpendicular to $\hat{s}$. The satellite Dirac point position scales with the superlattice period ($L$) as $\propto L^β$, with $β= -1.18 \pm 0.06$. These results can be well explained by the high KP potential scenario, with the Fermi velocity perpendicular to $\hat{s}$ quenched to about 1% of that for pristine graphene. Our study presents a promising material platform for realizing electron supercollimation and investigating flat band phenomena. △ Less

Submitted 9 September, 2023; originally announced September 2023.

Comments: 12 pages, 5 figures, and Supplemental Material

Journal ref: Phys. Rev. Lett. 132, 056204 (2024)

Phonon thermal transport shaped by strong spin-phonon scattering in a Kitaev material Na$_2$Co$_2$TeO$_6$

Authors: Xiaochen Hong, Matthias Gillig, Weiliang Yao, Lukas Janssen, Vilmos Kocsis, Sebastian Gass, Yuan Li, Anja U. B. Wolter, Bernd Büchner, Christian Hess

Abstract: The recent report of a half-quantized thermal Hall effect in the Kitaev material $α$-RuCl$_3$ has sparked a strong debate on whether it is generated by Majorana fermion edge currents or whether other more conventional mechanisms involving magnons or phonons are at its origin. A more direct evidence for Majorana fermions which could be expected to arise from a contribution to the longitudinal heat… ▽ More The recent report of a half-quantized thermal Hall effect in the Kitaev material $α$-RuCl$_3$ has sparked a strong debate on whether it is generated by Majorana fermion edge currents or whether other more conventional mechanisms involving magnons or phonons are at its origin. A more direct evidence for Majorana fermions which could be expected to arise from a contribution to the longitudinal heat conductivity $κ_{xx}$ at $T\rightarrow0$ is elusive due to a very complex magnetic field dependence of $κ_{xx}$. Here, we report very low temperature (below 1~K) thermal conductivity ($κ$) of another candidate Kitaev material, Na$_2$Co$_2$TeO$_6$. The application of a magnetic field along different principal axes of the crystal reveals a strong directional-dependent magnetic-field ($\bf B$) impact on $κ$. We show that no evidence for mobile quasiparticles except phonons can be concluded at any field from 0~T to the field polarized state. In particular, severely scattered phonon transport is observed across the $B-T$ phase diagram, which is attributed to prominent magnetic fluctuations. Cascades of phase transitions are uncovered for all $\bf B$ directions by probing the strength of magnetic fluctuations via a precise record of $κ$($B$). Our results thus rule out recent proposals for itinerant magnetic excitations in Na$_2$Co$_2$TeO$_6$, and emphasise the importance of discriminating true spin liquid transport properties from scattered phonons in candidate materials. △ Less

Submitted 29 June, 2023; originally announced June 2023.

Journal ref: npj Quantum Mater. 9, 18 (2024)

Phononic-magnetic dichotomy of the thermal Hall effect in the Kitaev-Heisenberg candidate material Na$_2$Co$_2$TeO$_6$

Authors: Matthias Gillig, Xiaochen Hong, Christoph Wellm, Vladislav Kataev, Weiliang Yao, Yuan Li, Bernd Büchner, Christian Hess

Abstract: Majorana fermions as emergent excitations of the Kitaev quantum spin liquid ground state constitute a promising concept in fault tolerant quantum computation. Experimentally, the recently reported topological half-quantized thermal Hall effect in the Kitaev material $α$-RuCl$_3$ seems to confirm the Majorana nature of the material's magnetic excitations. It has been argued, however, that the therm… ▽ More Majorana fermions as emergent excitations of the Kitaev quantum spin liquid ground state constitute a promising concept in fault tolerant quantum computation. Experimentally, the recently reported topological half-quantized thermal Hall effect in the Kitaev material $α$-RuCl$_3$ seems to confirm the Majorana nature of the material's magnetic excitations. It has been argued, however, that the thermal Hall signal in $α$-RuCl$_3$ rather stems from phonons or topological magnons than from Majorana fermions. Here we investigate the thermal Hall effect of the closely related Kitaev quantum material Na$_2$Co$_2$TeO$_6$, and we show that the thermal Hall signal emerges from at least two components, phonons and magnetic excitations. This dichotomy results from our discovery that the transversal heat conductivity $κ_{xy}$ carries clear signatures of the phononic $κ_{xx}$, but changes sign upon entering the low-temperature, magnetically ordered phase. We systematically resolve the two components by considering the detailed temperature and field dependence of both $κ_{xy}$ and $κ_{xx}$. Our results demonstrate that uncovering a genuinely quantized magnetic thermal Hall effect in a Kitaev topological quantum spin liquid requires to disentangle phonon vs. magnetic contributions where the latter include potentially fractionalized excitations such as the expected Majorana fermions. △ Less

Submitted 6 March, 2023; originally announced March 2023.

Interface-tuning of ferroelectricity and quadruple-well state in CuInP$_2$S$_6$ via ferroelectric oxide

Authors: Kun Wang, Du Li, Jia Wang, Yifei Hao, Hailey Anderson, Li Yang, Xia Hong

Abstract: Ferroelectric van der Waals CuInP$_2$S$_6$ possesses intriguing quadruple-well states and negative piezoelectricity. Its technological implementation has been impeded by the relatively low Curie temperature (bulk $T_C$ ~42 °C) and the lack of precise domain control. Here we show that CuInP$_2$S$_6$ can be immune to the finite size effect and exhibits enhanced ferroelectricity, piezoelectricity, an… ▽ More Ferroelectric van der Waals CuInP$_2$S$_6$ possesses intriguing quadruple-well states and negative piezoelectricity. Its technological implementation has been impeded by the relatively low Curie temperature (bulk $T_C$ ~42 °C) and the lack of precise domain control. Here we show that CuInP$_2$S$_6$ can be immune to the finite size effect and exhibits enhanced ferroelectricity, piezoelectricity, and polar alignment in the ultrathin limit when interfaced with ferroelectric oxide PbZr$_{0.2}$Ti$_{0.8}$O$_3$ films. Piezoresponse force microscopy studies reveal that the polar domains in thin CuInP$_2$S$_6$ fully conform to those of underlying PbZr$_{0.2}$Ti$_{0.8}$O$_3$, where the piezoelectric coefficient changes sign and increases sharply with reducing thickness. High temperature $in$ $situ$ domain imaging points to a significantly enhanced $T_C$ exceeding 200 °C for 13 nm CuInP$_2$S$_6$ on PbZr$_{0.2}$Ti$_{0.8}$O$_3$. Density functional theory modeling and Monte Carlo simulations show that the enhanced polar alignment and $T_C$ can be attributed to interface-mediated structure distortion in CuInP$_2$S$_6$. Our study provides an effective material strategy to engineer the polar properties of CuInP$_2$S$_6$ for flexible nanoelectronic, optoelectronic, and mechanical applications. △ Less

Submitted 15 August, 2023; v1 submitted 2 February, 2023; originally announced February 2023.

Comments: 21 pages, 5 figures, and Supporting Information

Journal ref: ACS Nano 17 (16), 15787 (2023)

Remote Surface Optical Phonon Scattering in Ferroelectric Ba$_{0.6}$Sr$_{0.4}$TiO$_{3}$ Gated Graphene

Authors: Hanying Chen, Tianlin Li, Yifei Hao, Anil Rajapitamahuni, Zhiyong Xiao, Stefan Schoeche, Mathias Schubert, Xia Hong

Abstract: We report the effect of remote surface optical (RSO) phonon scattering on carrier mobility in monolayer graphene gated by ferroelectric oxide. We fabricate monolayer graphene transistors back-gated by epitaxial (001) Ba$_{0.6}$Sr$_{0.4}$TiO$_{3}$ films, with field effect mobility up to 23,000 cm$^{2}$V$^{-1}$s$^{-1}$ achieved. Switching the ferroelectric polarization induces nonvolatile modulation… ▽ More We report the effect of remote surface optical (RSO) phonon scattering on carrier mobility in monolayer graphene gated by ferroelectric oxide. We fabricate monolayer graphene transistors back-gated by epitaxial (001) Ba$_{0.6}$Sr$_{0.4}$TiO$_{3}$ films, with field effect mobility up to 23,000 cm$^{2}$V$^{-1}$s$^{-1}$ achieved. Switching the ferroelectric polarization induces nonvolatile modulation of resistance and quantum Hall effect in graphene at low temperatures. Ellipsometry spectroscopy studies reveal four pairs of optical phonon modes in Ba$_{0.6}$Sr$_{0.4}$TiO$_{3}$, from which we extract the RSO phonon frequencies. The temperature dependence of resistivity in graphene can be well accounted for by considering the scattering from the intrinsic longitudinal acoustic phonon and the RSO phonon, with the latter dominated by the mode at 35.8 meV. Our study reveals the room temperature mobility limit of ferroelectric-gated graphene transistors imposed by RSO phonon scattering. △ Less

Submitted 30 September, 2022; originally announced September 2022.

Comments: 15 pages, 6 figures

Physical Vapor Transport Growth of Antiferromagnetic CrCl$_3$ Flakes Down to Monolayer Thickness

Authors: Jia Wang, Zahra Ahmadi, David Lujan, Jeongheon Choe, Takashi Taniguchi, Kenji Watanabe, Xiaoqin Li, Jeffrey E. Shield, Xia Hong

Abstract: The van der Waals magnets CrX$_3$ (X = I, Br, and Cl) exhibit highly tunable magnetic properties and are promising candidates for developing novel two-dimensional (2D) magnetic devices such as magnetic tunnel junctions and spin tunneling transistors. Previous studies of CrCl$_3$ have mainly focused on mechanically exfoliated samples. Controlled synthesis of high quality atomically thin flakes is c… ▽ More The van der Waals magnets CrX$_3$ (X = I, Br, and Cl) exhibit highly tunable magnetic properties and are promising candidates for developing novel two-dimensional (2D) magnetic devices such as magnetic tunnel junctions and spin tunneling transistors. Previous studies of CrCl$_3$ have mainly focused on mechanically exfoliated samples. Controlled synthesis of high quality atomically thin flakes is critical for their technological implementation but has not been achieved to date. Here, we report the growth of large CrCl$_3$ flakes with well-defined facets down to monolayer thickness (~0.6 nm) via the physical vapor transport technique. Both isolated flakes with well-defined facets and long stripe samples with the trilayer portion exceeding 60 $μ$m have been obtained. High-resolution transmission electron microscopy studies show that the CrCl$_3$ flakes are single crystalline in the monoclinic structure, consistent with the Raman results. The room temperature stability of the CrCl$_3$ flakes decreases with decreasing thickness. The tunneling magnetoresistance of graphite/CrCl$_3$/graphite tunnel junctions confirms that few-layer CrCl$_3$ possesses in-plane magnetic anisotropy and Néel temperature of 17 K. Our study paves the path for developing CrCl$_3$-based scalable 2D spintronic applications. △ Less

Submitted 4 December, 2022; v1 submitted 12 August, 2022; originally announced August 2022.

Comments: 14 pages, 5 figures, with Supporting Information

Journal ref: Advanced Science, 2203548 (2022)

Novel Valence Transition in Elemental Metal Europium around 80 GPa

Authors: Bijuan Chen, Mingfeng Tian, Jurong Zhang, Bing Li, Yuming Xiao, Paul Chow, Curtis Kenney-Benson, Hongshan Deng, Jianbo Zhang, Raimundas Sereika, Xia Yin, Dong Wang, Xinguo Hong, Changqing Jin, Yan Bi, Hanyu Liu, Haifeng Liu, Jun Li, Ke Jin, Qiang Wu, Jun Chang, Yang Ding, Ho-kwang Mao

Abstract: Valence transition could induce structural, insulator-metal, nonmagnetic-magnetic and superconducting transitions in rare-earth metals and compounds, while the underlying physics remains unclear due to the complex interaction of localized 4f electrons as well as their coupling with itinerant electrons. The valence transition in the elemental metal europium (Eu) still has remained as a matter of de… ▽ More Valence transition could induce structural, insulator-metal, nonmagnetic-magnetic and superconducting transitions in rare-earth metals and compounds, while the underlying physics remains unclear due to the complex interaction of localized 4f electrons as well as their coupling with itinerant electrons. The valence transition in the elemental metal europium (Eu) still has remained as a matter of debate. Using resonant x-ray emission scattering and x-ray diffraction, we pressurize the states of 4f electrons in Eu and study its valence and structure transitions up to 160 GPa. We provide compelling evidence for a valence transition around 80 GPa, which coincides with a structural transition from a monoclinic (C2/c) to an orthorhombic phase (Pnma). We show that the valence transition occurs when the pressure-dependent energy gap between 4f and 5d electrons approaches the Coulomb interaction. Our discovery is critical for understanding the electrodynamics of Eu, including magnetism and high-pressure superconductivity. △ Less

Submitted 28 June, 2022; v1 submitted 18 June, 2022; originally announced June 2022.

Journal ref: Phys. Rev. Lett. 129, 016401 (2022)

Perspective on Epitaxial NiCo2O4 Film as an Emergent Spintronic Material: Magnetism and Transport Properties

Authors: Xiaoshan Xu, Corbyn Mellinger, Zhi Gang Cheng, Xuegang Chen, Xia Hong

Abstract: The ferrimagnetic inverse spinel NiCo2O4 has attracted extensive research interests for its versatile electrochemical properties, robust magnetic order, high conductivity, and fast spin dynamics, as well as its highly tunable nature due to the closely coupled charge, spin, orbital, lattice, and defect effects. Single-crystalline epitaxial thin films of NiCo2O4 present a model system for elucidatin… ▽ More The ferrimagnetic inverse spinel NiCo2O4 has attracted extensive research interests for its versatile electrochemical properties, robust magnetic order, high conductivity, and fast spin dynamics, as well as its highly tunable nature due to the closely coupled charge, spin, orbital, lattice, and defect effects. Single-crystalline epitaxial thin films of NiCo2O4 present a model system for elucidating the intrinsic physical properties and strong tunability, which are not viable in bulk single crystals. In this perspective, we discuss the recent advances in epitaxial NiCo2O4 thin films, focusing on understanding its unusual magnetic and transport properties in light of crystal structure and electronic structure. The perpendicular magnetic anisotropy in compressively strained NiCo2O4 films is explained by considering the strong spin-lattice coupling, particularly on Co ions. The prominent effect of growth conditions reveals the complex interplay between the crystal structure, cation stoichiometry, valence state, and site occupancy. NiCo2O4 thin films also exhibit various magnetotransport anomalies, including linear magnetoresistance and sign change in anomalous Hall effect, which illustrate the competing effects of band intrinsic Berry phase and impurity scattering. The fundamental understanding of these phenomena will facilitate the functional design of NiCo2O4 thin films for nanoscale spintronic applications. △ Less

Submitted 4 June, 2022; originally announced June 2022.

Journal ref: J. Appl. Phys. 132, 020901 (2022)

Heat transport of the kagomé Heisenberg quantum spin liquid candidate YCu$_3$(OH)$_{6.5}$Br$_{2.5}$: localized magnetic excitations and spin gap

Authors: Xiaochen Hong, Mahdi Behnami, Long Yuan, Boqiang Li, Wolfram Brenig, Bernd Büchner, Yuesheng Li, Christian Hess

Abstract: The spin-1/2 kagomé Heisenberg antiferromagnet is generally accepted as one of the most promising two-dimensional models to realize a quantum spin liquid state. Previous experimental efforts were almost exclusively on only one archetypal material, the herbertsmithite ZnCu$_3$(OH)$_6$Cl$_2$, which unfortunately suffers from the notorious orphan spins problem caused by magnetic disorders. Here we tu… ▽ More The spin-1/2 kagomé Heisenberg antiferromagnet is generally accepted as one of the most promising two-dimensional models to realize a quantum spin liquid state. Previous experimental efforts were almost exclusively on only one archetypal material, the herbertsmithite ZnCu$_3$(OH)$_6$Cl$_2$, which unfortunately suffers from the notorious orphan spins problem caused by magnetic disorders. Here we turn to YCu$_3$(OH)$_{6.5}$Br$_{2.5}$, recently recognized as another host of a globally undistorted kagomé Cu$^{2+}$ lattice free from the orphan spins, thus a more feasible system for studying the intrinsic kagomé quantum spin liquid physics. Our high-resolution low-temperature thermal conductivity measurements yield a vanishing small residual linear term of $κ/T$ ($T\rightarrow 0$), and thus clearly rule out itinerant gapless fermionic excitations. Unusual scattering of phonons grows exponentially with temperature, suggesting thermally activated phonon-spin scattering and hence a gapped magnetic excitation, consistent with a $\mathbb{Z}_2$ quantum spin liquid ground state. Additionally, the analysis of magnetic field impact on the thermal conductivity reveals a field closing of the spin gap, while the excitations remain localized. △ Less

Submitted 16 June, 2022; v1 submitted 26 May, 2022; originally announced May 2022.

Comments: 4 figures

Elastoresistivity of heavily hole doped 122 iron pnictides superconductors

Authors: Xiaochen Hong, Steffen Sykora, Federico Caglieris, Mahdi Behnami, Igor Morozov, Saicharan Aswartham, Vadim Grinenko, Kunihiro Kihou, Chul-Ho Lee, Bernd Büchner, Christian Hess

Abstract: Nematicity in the heavily hole-doped iron pnictide superconductors remains controversial. Sizeable nematic fluctuations and even nematic orders far from a magnetic instability were declared in RbFe$_2$As$_2$ and its sister compounds. Here we report a systematic elastoresistance study of series of isovalent- and electron-doped KFe$_2$As$_2$ crystals. We found divergent elastoresistance upon cooling… ▽ More Nematicity in the heavily hole-doped iron pnictide superconductors remains controversial. Sizeable nematic fluctuations and even nematic orders far from a magnetic instability were declared in RbFe$_2$As$_2$ and its sister compounds. Here we report a systematic elastoresistance study of series of isovalent- and electron-doped KFe$_2$As$_2$ crystals. We found divergent elastoresistance upon cooling for all the crystals along their [110] direction. The amplitude of elastoresistivity diverges if K is substituted with larger ions or if the system is driven towards a Lifshitz transition. However, we conclude none of them necessarily indicates an independent nematic critical point. Instead, the increased nematicity can be associated with another electronic criticality. In particular, we propose a mechanism how elastoresistivity is enhanced at a Lifshitz transition. △ Less

Submitted 26 May, 2022; originally announced May 2022.

Journal ref: Front. Phys. {\bf 10} 853717 (2022)

Anomalous Hall Effect and Perpendicular Magnetic Anisotropy in Ultrathin Ferrimagnetic NiCo$_2$O$_4$ Films

Authors: Xuegang Chen, Qiuchen Wu, Le Zhang, Yifei Hao, Myung-Geun Han, Yimei Zhu, Xia Hong

Abstract: The inverse spinel ferrimagnetic NiCo$_2$O$_4$ possesses high magnetic Curie temperature $T_C$, high spin polarization, and strain-tunable magnetic anisotropy. Understanding the thickness scaling limit of these intriguing magnetic properties in NiCo$_2$O$_4$ thin films is critical for their implementation in nanoscale spintronic applications. In this work, we report the unconventional magnetotrans… ▽ More The inverse spinel ferrimagnetic NiCo$_2$O$_4$ possesses high magnetic Curie temperature $T_C$, high spin polarization, and strain-tunable magnetic anisotropy. Understanding the thickness scaling limit of these intriguing magnetic properties in NiCo$_2$O$_4$ thin films is critical for their implementation in nanoscale spintronic applications. In this work, we report the unconventional magnetotransport properties of epitaxial (001) NiCo$_2$O$_4$ films on MgAl$_2$O$_4$ substrates in the ultrathin limit. Anomalous Hall effect measurements reveal strong perpendicular magnetic anisotropy for films down to 1.5 unit cell (1.2 nm), while $T_C$ for 3 unit cell and thicker films remains above 300 K. The sign change in the anomalous Hall conductivity ($σ_{xy}$) and its scaling relation with the longitudinal conductivity ($σ_{xx}$) can be attributed to the competing effects between impurity scattering and band intrinsic Berry curvature, with the latter vanishing upon the thickness driven metal-insulator transition. Our study reveals the critical role of film thickness in tuning the relative strength of charge correlation, Berry phase effect, spin orbit interaction, and impurity scattering, providing important material information for designing scalable epitaxial magnetic tunnel junctions and sensing devices using NiCo$_2$O$_4$. △ Less

Submitted 13 June, 2022; v1 submitted 31 March, 2022; originally announced April 2022.

Comments: 12 pages, 4 figures

Journal ref: Applied Physics Letters 120, 242401 (2022)

Ubiquitous enhancement of nematic fluctuations across the phase diagram of iron based superconductors probed by the Nernst effect

Authors: Christoph Wuttke, Federico Caglieris, Steffen Sykora, Frank Steckel, Xiaochen Hong, Sheng Ran, Seunghyun Khim, Rhea Kappenberger, Sergey L. Bud'ko, Paul C. Canfield, Sabine Wurmehl, Saicharan Aswartham, Bernd Büchner, Christian Hess

Abstract: The role of nematic fluctuations for unconventional superconductivity has been subject of intense discussions for many years. In iron-based superconductors, the most established probe for electronic-nematic fluctuations, i.e. the elastoresistivity seems to imply that superconductivity is reinforced by electronic-nematic fluctuations, since the elastoresistivity amplitude peaks at or close to optim… ▽ More The role of nematic fluctuations for unconventional superconductivity has been subject of intense discussions for many years. In iron-based superconductors, the most established probe for electronic-nematic fluctuations, i.e. the elastoresistivity seems to imply that superconductivity is reinforced by electronic-nematic fluctuations, since the elastoresistivity amplitude peaks at or close to optimal $T_C$. However, on the over-doped side of the superconducting dome, the diminishing elastoresistivity suggests a negligible importance in the mechanism of superconductivity. Here we introduce the Nernst coefficient as a genuine probe for electronic nematic fluctuations, and we show that the amplitude of the Nernst coefficient tracks the superconducting dome of two prototype families of iron-based superconductors, namely Rh-doped $BaFe_{2}As_{2}$ and Co-doped $LaFeAsO$. Our data thus provide fresh evidence that in these systems nematic fluctuations foster the superconductivity throughout the phase diagram. △ Less

Submitted 1 February, 2022; originally announced February 2022.

Comments: Main article 15 pages, 3 figures. Supplemental Material 15 pages, 7 figures

Absence of nematic instability in LiFeAs

Authors: Michael Wissmann, Federico Caglieris, Xiaochen Hong, Saicharan Aswartham, Anna Vorobyova, Igor Morozov, Bernd Büchner, Christian Hess

Abstract: The relationship between unconventional superconductivity, antiferromagnetism and nematic order in iron-based superconductors (FeSCs) is still highly debated. In many FeSCs superconductivity is in proximity of a nematically and magnetically ordered state. LiFeAs is an exceptional stoichiometric FeSC becoming superconducting below 18 K, without undergoing a structural or magnetic transition. Howeve… ▽ More The relationship between unconventional superconductivity, antiferromagnetism and nematic order in iron-based superconductors (FeSCs) is still highly debated. In many FeSCs superconductivity is in proximity of a nematically and magnetically ordered state. LiFeAs is an exceptional stoichiometric FeSC becoming superconducting below 18 K, without undergoing a structural or magnetic transition. However, some recent experimental studies suggested the existence of finite nematic fluctuations and even a nematic superconducting state. In this study, we employ elastoresistance as a measure of nematic fluctuations in pristine LiFeAs and compare the findings with the elastoresistance of LiFeAs at low Co and V doping levels as well with that of magnetically and nematically ordering NaFeAs. We find LiFeAs and cobalt-doped LiFeAs far away from a nematic instability. △ Less

Submitted 31 January, 2022; originally announced February 2022.

Quantum Critical Magnetic Excitations in Spin-1/2 and Spin-1 Chain Systems

Authors: Y. Xu, L. S. Wang, Y. Y. Huang, J. M. Ni, C. C. Zhao, Y. F. Dai, B. Y. Pan, X. C. Hong, P. Chauhan, S. M. Koohpayeh, N. P. Armitage, S. Y. Li

Abstract: The study of CoNb$_2$O$_6$ sits at the confluence of simplicity and complexity: on one hand, the model for Ising chains -- the building blocks of CoNb$_2$O$_6$ -- in a transverse field, can be exactly solved and, thus, serves as an archetype of quantum criticality; on the other hand, the weak but nonzero interchain coupling adds geometric frustration to the stage, substantially complicating the ph… ▽ More The study of CoNb$_2$O$_6$ sits at the confluence of simplicity and complexity: on one hand, the model for Ising chains -- the building blocks of CoNb$_2$O$_6$ -- in a transverse field, can be exactly solved and, thus, serves as an archetype of quantum criticality; on the other hand, the weak but nonzero interchain coupling adds geometric frustration to the stage, substantially complicating the phase diagram. Here we utilize low-temperature specific heat and thermal conductivity measurements to study the low-lying magnetic excitations in CoNb$_2$O$_6$ and its spin-1 analogue NiNb$_2$O$_6$. The thermal conductivity is found to be suppressed around the quantum critical point, where the specific heat is enhanced due to gapless magnetic excitations, pointing to the localized nature of the latter. These results highlight the predominant role of frustration in determining the quantum critical magnetic excitations of spin chains, which may furthermore underlie the remarkable similarities between the phenomenology of these spin-1/2 and spin-1 systems. △ Less

Submitted 27 May, 2022; v1 submitted 3 December, 2021; originally announced December 2021.

Comments: 9 pages, 3 figures

Journal ref: Physical Review X 12, 021020 (2022)

Thermal transport of the frustrated spin-chain mineral linarite: Magnetic heat transport and strong spin-phonon scattering

Authors: Matthias Gillig, Xiaochen Hong, Piyush Sakrikar, Gaël Bastien, A. U. B. Wolter, Leonie Heinze, Satoshi Nishimoto, Bernd Büchner, Christian Hess

Abstract: The mineral linarite (PbCuSO$_4$(OH)$_2$) forms a monoclinic structure where a sequence of Cu(OH)$_2$ units forms a spin-$\frac{1}{2}$ chain. Competing ferromagnetic nearest-neighbor ($J_1$) and antiferromagnetic next-nearest-neighbor interactions ($J_2$) in this quasi-one-dimensional spin structure imply magnetic frustration and lead to magnetic ordering below $T_N =$2.8 K in a mutliferroic ellip… ▽ More The mineral linarite (PbCuSO$_4$(OH)$_2$) forms a monoclinic structure where a sequence of Cu(OH)$_2$ units forms a spin-$\frac{1}{2}$ chain. Competing ferromagnetic nearest-neighbor ($J_1$) and antiferromagnetic next-nearest-neighbor interactions ($J_2$) in this quasi-one-dimensional spin structure imply magnetic frustration and lead to magnetic ordering below $T_N =$2.8 K in a mutliferroic elliptical spin-spiral ground state. Upon the application of a magnetic field along the spin-chain direction, distinct magnetically ordered phases can be induced. We studied the thermal conductivity $κ$ in this material across the magnetic phase diagram as well as in the paramagnetic regime in the temperature ranges 0.07-1 K and 9-300 K. We found that in linarite the heat is carried mainly by phonons but shows a peculiar non-monotonic behavior in field. In particular, $κ$ is highly suppressed at the magnetic phase boundaries, indicative of strong scattering of the phonons off critical magnetic fluctuations. Even at temperatures far above the magnetically ordered phases, the phononic thermal conductivity is reduced due to scattering off magnetic fluctuations. The mean free path due to spin-phonon scattering ($l_{\text{spin-phonon}}$) was determined as function of temperature. A power law behavior was observed mainly above 0.5 K indicating the thermal activation of spin fluctuations. In the critical regime close to the saturation field, $l_{\text{spin-phonon}}$ shows a $1/T$ dependence. Furthermore, a magnon thermal transport channel was verified in the helical magnetic phase. We estimate a magnon mean free path which corresponds to about 1000 lattice spacings. △ Less

Submitted 5 October, 2021; originally announced October 2021.

A Unified Theoretical Treatment on Statistical Properties of the Semi-batch Self-condensing Vinyl Polymerization System

Authors: Fang Gu, Jiangtao Li, Xiaozhong Hong, Haijun Wang

Abstract: We present a novel generating function (GF) method for the self-condensing vinyl polymerization (SCVP) system with any initial distribution of preexisted polymers. Such a method was proven to be especially useful to investigate the semi-batch SCVP system allowing a sequence of feeding operations during the polymerization. Consequently, the number-, weight-, and z-average molecular weights as well… ▽ More We present a novel generating function (GF) method for the self-condensing vinyl polymerization (SCVP) system with any initial distribution of preexisted polymers. Such a method was proven to be especially useful to investigate the semi-batch SCVP system allowing a sequence of feeding operations during the polymerization. Consequently, the number-, weight-, and z-average molecular weights as well as polydispersity index of hyperbranched polymers can be explicitly given, which are determined by predetermined feeding details and conversions in each polymerization step. These analytical results are further confirmed by the corresponding Monte Carlo simulation. Therefore the present GF method has provided a unified treatment on the semi-batch SCVP system. Accordingly, hyperbranched polymers with desired properties can be prepared by designing feeding details and presetting conversions at each step based on the present GF method. △ Less

Submitted 5 March, 2021; originally announced March 2021.

Journal ref: Chin J Polym Sci (2021)

Unusual heat transport of the Kitaev material Na$_2$Co$_2$TeO$_6$: putative quantum spin liquid and low-energy spin excitations

Authors: Xiaochen Hong, Matthias Gillig, Richard Hentrich, Weiliang Yao, Vilmos Kocsis, Arthur R. Witte, Tino Schreiner, Danny Baumann, Nicolás Pérez, Anja U. B. Wolter, Yuan Li, Bernd Büchner, Christian Hess

Abstract: We studied the field dependent thermal conductivity ($κ$) of Na$_2$Co$_2$TeO$_6$, a compound considered as the manifestation of the Kitaev model based on the high-spin $d^7$ Co$^{2+}$ ions. We found that in-plane magnetic fields beyond a critical value $B_c \approx$~10 T are able to drastically enhance $κ$ at low temperatures, resulting in a double-peak structure of $κ(T)$ that closely resembles t… ▽ More We studied the field dependent thermal conductivity ($κ$) of Na$_2$Co$_2$TeO$_6$, a compound considered as the manifestation of the Kitaev model based on the high-spin $d^7$ Co$^{2+}$ ions. We found that in-plane magnetic fields beyond a critical value $B_c \approx$~10 T are able to drastically enhance $κ$ at low temperatures, resulting in a double-peak structure of $κ(T)$ that closely resembles the behavior of $α$-RuCl$_3$. This result suggests that heat transport in Na$_2$Co$_2$TeO$_6$ is primarily phononic, and it is strongly affected by scattering from magnetic excitations that are highly tunable by external fields. Interestingly, for magnetic fields $B // a$ (i.e., along the zigzag direction of the Co-Co bonds), there is an extended field range which separates the long-range magnetic order for $B\leq B_c\approx10$ T and the partially spin-polarized gapped high-field phase for $B\gtrsim 12$ T. The low-energy phonon scattering is particularly strong in this field range, consistent with the notion that the system becomes a quantum spin liquid with prominent spin fluctuations down to energies of no more than 2 meV. △ Less

Submitted 28 January, 2021; originally announced January 2021.

Journal ref: Phys. Rev. B 104, 144426 (2021)

Giant Transport Anisotropy in ReS$_2$ Revealed via Nanoscale Conducting Path Control

Authors: Dawei Li, Shuo Sun, Zhiyong Xiao, Jingfeng Song, Ding-Fu Shao, Evgeny Y. Tsymbal, Stephen Ducharme, Xia Hong

Abstract: The low in-plane symmetry in layered 1T'-ReS$_2$ results in strong band anisotropy, while its manifestation in the electronic properties is challenging to resolve due to the lack of effective approaches for controlling the local current path. In this work, we reveal the giant transport anisotropy in monolayer to four-layer ReS$_2$ by creating directional conducting paths via nanoscale ferroelectri… ▽ More The low in-plane symmetry in layered 1T'-ReS$_2$ results in strong band anisotropy, while its manifestation in the electronic properties is challenging to resolve due to the lack of effective approaches for controlling the local current path. In this work, we reveal the giant transport anisotropy in monolayer to four-layer ReS$_2$ by creating directional conducting paths via nanoscale ferroelectric control. By reversing the polarization of a ferroelectric polymer top layer, we induce conductivity switching ratio of >1.5x10$^8$ in the ReS$_2$ channel at 300 K. Characterizing the domain-defined conducting nanowires in an insulating background shows that the conductivity ratio between the directions along and perpendicular to the Re-chain can exceed 5.5x10$^4$. Theoretical modeling points to the band origin of the transport anomaly, and further reveals the emergence of a flat band in few-layer ReS$_2$. Our work paves the path for implementing the highly anisotropic 2D materials for designing novel collective phenomena and electron lensing applications. △ Less

Submitted 27 September, 2021; v1 submitted 18 January, 2021; originally announced January 2021.

Comments: 13 pages, 4 figures, w. Supplemental Material

Journal ref: Phys. Rev. Lett. 127, 136803 (2021)

Revisiting the phase diagram of LaFe$_{1-x}$Co$_x$AsO on single crystals by thermodynamic methods

Authors: Francesco Scaravaggi, Sven Sauerland, Liran Wang, Rhea Kappenberger, Piotr Lepucki, Adam P. Dioguardi, Xiaochen Hong, Federico Caglieris, Christoph Wuttke, Christian Hess, Hans-Joachim Grafe, Saicharan Aswartham, Sabine Wurmehl, Rüdiger Klingeler, Anja U. B. Wolter, Bernd Büchner

Abstract: In this work we revisit the phase diagram of Co-doped LaFeAsO using single crystals and thermodynamic methods. From magnetic susceptibility studies we track the doping evolution of the antiferromagnetic phase, revealing a continuous suppression of $T_\mathrm{N}$ up to 5$\%$ Co doping. In order to study the evolution of the so-called nematic phase, the temperature dependence of the lengths changes… ▽ More In this work we revisit the phase diagram of Co-doped LaFeAsO using single crystals and thermodynamic methods. From magnetic susceptibility studies we track the doping evolution of the antiferromagnetic phase, revealing a continuous suppression of $T_\mathrm{N}$ up to 5$\%$ Co doping. In order to study the evolution of the so-called nematic phase, the temperature dependence of the lengths changes along the $a$ and $b$ orthorhombic directions, $ΔL/L_0$, was determined by high-resolution capacitance dilatometry. The results clearly show a gradual reduction of the orthorhombic distortion $δ$ and of $T_\mathrm{S}$ with increasing Co content up to 4.5$\%$, while it is completely suppressed for 7.5$\%$ Co. Bulk superconductivity was found in a small doping region around 6$\%$ Co content, while both $T_\mathrm{c}$ and the superconducting volume fraction rapidly drop in the neighbouring doping regime. Ultimately, no microscopic coexistence between the superconducting and magnetic phases can be assessed within our resolution limit, in sharp contrast with other iron-pnictide families, e.g., electron- and hole-doped BaFe$_2$As$_2$. △ Less

Submitted 14 January, 2021; originally announced January 2021.

Journal ref: Phys. Rev. B 103, 174506 (2021)

High-field thermal transport properties of the Kitaev quantum magnet alpha-RuCl3: evidence for low-energy excitations beyond the critical field

Authors: Richard Hentrich, Xiaochen Hong, Matthias Gillig, Federico Caglieris, Matija Culo, Maryam Shahrokhvand, Uli Zeitler, Maria Roslova, Anna Isaeva, Thomas Doert, Lukas Janssen, Matthias Vojta, Bernd Büchner, Christian Hess

Abstract: We investigate the phononic in-plane longitudinal low-temperature thermal conductivity kappa_ab of the Kitaev quantum magnet alpha-RuCl3 for large in-plane magnetic fields up to 33 T. Our data reveal for fields larger than the critical field Bc ~ 8 T, at which the magnetic order is suppressed, a dramatic increase of kappa_ab at all temperatures investigated. The analysis of our data shows that the… ▽ More We investigate the phononic in-plane longitudinal low-temperature thermal conductivity kappa_ab of the Kitaev quantum magnet alpha-RuCl3 for large in-plane magnetic fields up to 33 T. Our data reveal for fields larger than the critical field Bc ~ 8 T, at which the magnetic order is suppressed, a dramatic increase of kappa_ab at all temperatures investigated. The analysis of our data shows that the phonons are not only strongly scattered by a magnetic mode at relatively large energy which scales roughly linearly with the magnetic field, but also by a small-energy mode which emerges near Bc with a square-root-like field dependence. While the former is in striking agreement with recent spin wave theory (SWT) results of the magnetic excitation spectrum at the Gamma point, the energy of the latter is too small to be compatible with the SWT-expected magnon gap at the M point, despite the matching field dependence. Therefore, an alternative scenario based on phonon scattering off the thermal excitation of random-singlet states is proposed. △ Less

Submitted 30 October, 2020; originally announced October 2020.

Comments: 8 pages, 4 figures

Journal ref: Phys. Rev. B 102, 235155 (2020)

Multibeam Electron Diffraction

Authors: Xuhao Hong, Steven E Zeltmann, Benjamin H Savitzky, Luis Rangel DaCosta, Alexander Mueller, Andrew M Minor, Karen Bustillo, Colin Ophus

Abstract: One of the primary uses for transmission electron microscopy (TEM) is to measure diffraction pattern images in order to determine a crystal structure and orientation. In nanobeam electron diffraction (NBED) we scan a moderately converged electron probe over the sample to acquire thousands or even millions of sequential diffraction images, a technique that is especially appropriate for polycrystall… ▽ More One of the primary uses for transmission electron microscopy (TEM) is to measure diffraction pattern images in order to determine a crystal structure and orientation. In nanobeam electron diffraction (NBED) we scan a moderately converged electron probe over the sample to acquire thousands or even millions of sequential diffraction images, a technique that is especially appropriate for polycrystalline samples. However, due to the large Ewald sphere of TEM, excitation of Bragg peaks can be extremely sensitive to sample tilt, varying strongly for even a few degrees of sample tilt for crystalline samples. In this paper, we present multibeam electron diffraction (MBED), where multiple probe forming apertures are used to create mutiple STEM probes, all of which interact with the sample simultaneously. We detail designs for MBED experiments, and a method for using a focused ion beam (FIB) to produce MBED apertures. We show the efficacy of the MBED technique for crystalline orientation mapping using both simulations and proof-of-principle experiments. We also show how the angular information in MBED can be used to perform 3D tomographic reconstruction of samples without needing to tilt or scan the sample multiple times. Finally, we also discuss future opportunities for the MBED method. △ Less

Submitted 18 September, 2020; originally announced September 2020.

Comments: 14 pages, 6 figures

Journal ref: Microsc Microanal 27 (2021) 129-139

The incommensurate magnet iron monophosphide FeP: Crystal growth and characterization

Authors: I. O. Chernyavskii, S. E. Nikitin, Y. A. Onykiienko, D. S. Inosov, Q. Stahl, J. Geck, X. C. Hong, C. Hess, S. Gaß, A. U. B. Wolter, D. Wolf, A. Lubk, D. V. Efremov, F. Yokaichiya, S. Aswartham, B. Büchner, I. V. Morozov

Abstract: We report an optimized chemical vapor transport method, which allows growing FeP single crystals up to 500 mg in mass and 80 $mm^{3}$ in volume. The high quality of the crystals obtained by this method was confirmed by means of EDX, high-resolution TEM, low-temperature single crystal XRD and neutron diffraction experiments. We investigated the transport and magnetic properties of the single crysta… ▽ More We report an optimized chemical vapor transport method, which allows growing FeP single crystals up to 500 mg in mass and 80 $mm^{3}$ in volume. The high quality of the crystals obtained by this method was confirmed by means of EDX, high-resolution TEM, low-temperature single crystal XRD and neutron diffraction experiments. We investigated the transport and magnetic properties of the single crystals and calculated the electronic band structure of FeP. We show both theoretically and experimentally, that the ground state of FeP is metallic. The examination of the magnetic data reveals antiferromagnetic order below T$_{N}$ =119 K while transport remains metallic in both the paramagnetic and the antiferromagnetic phase. The analysis of the neutron diffraction data shows an incommensurate magnetic structure with the propagation vector Q=(0, 0, $\pmδ$), where $δ$ $\sim$ 0.2. For the full understanding of the magnetic state, further experiments are needed. The successful growth of large high-quality single crystals opens the opportunity for further investigations of itinerant magnets with incommensurate spin structures using a wide range of experimental tools. △ Less

Submitted 31 March, 2020; v1 submitted 24 March, 2020; originally announced March 2020.

Report number: 4, 083403, (2020)

Journal ref: Phys. Rev. Materials 4, 083403 (2020)

Spontaneous emission of a quantum emitter near a Chern insulator: interplay of time reversal symmetry breaking and van Hove singularity

Authors: Bing-Sui Lu, Khatee Zathul Arifa, Xing Ru Hong

Abstract: We consider the generic problem of a two-level quantum emitter near a two-dimensional Chern insulator in the dipole approximation, and study how the frequency-dependent response and electronic density of states of the insulator modifies the transition rate of the emitter between the ground and excited levels. To this end, we obtain the full real-frequency behavior of the conductivity tensor by per… ▽ More We consider the generic problem of a two-level quantum emitter near a two-dimensional Chern insulator in the dipole approximation, and study how the frequency-dependent response and electronic density of states of the insulator modifies the transition rate of the emitter between the ground and excited levels. To this end, we obtain the full real-frequency behavior of the conductivity tensor by performing a tight-binding calculation based on the Qi-Wu-Zhang model and using a Kubo formula, and derive the full electromagnetic Green tensor of the system, which breaks Onsager reciprocity. This enables us to find that for frequencies smaller than the maximum band gap, the system is sensitive to time reversal symmetry-breaking, whereas for much larger frequencies the system becomes insensitive, with implications for the discrimination of the state of a circularly polarised dipole emitter. We also study the impact of a van Hove singularity on the surface-induced correction to the transition rate, finding that it can enhance its amplitude by a few orders of magnitude compared to the case where the conductivity is set to its static value. By considering configurations in which the dipole is circularly polarised or parallel with the surface of the Chern insulator, we find that the surface correction to the transition rate can exhibit a novel decay with sine integral-like oscillations. △ Less

Submitted 11 May, 2020; v1 submitted 10 March, 2020; originally announced March 2020.

Comments: 23 pages, 11 figures

Journal ref: Phys. Rev. B 101, 205410 (2020)

Domain Wall Enabled Steep Slope Switching in MoS$_2$ Transistors Towards Hysteresis-Free Operation

Authors: Jingfeng Song, Yubo Qi, Zhiyong Xiao, Kun Wang, Dawei Li, Seung-Hyun Kim, Angus I. Kingon, Andrew M. Rappe, Xia Hong

Abstract: The device concept of ferroelectric-based negative capacitance (NC) transistors offers a promising route for achieving energy-efficient logic applications that can outperform the conventional semiconductor technology, while viable operation mechanisms remain a central topic of debate. In this work, we report steep slope switching in MoS$_2$ transistors back-gated by single-layer polycrystalline Pb… ▽ More The device concept of ferroelectric-based negative capacitance (NC) transistors offers a promising route for achieving energy-efficient logic applications that can outperform the conventional semiconductor technology, while viable operation mechanisms remain a central topic of debate. In this work, we report steep slope switching in MoS$_2$ transistors back-gated by single-layer polycrystalline PbZr$_{0.35}$Ti$_{0.65}$O$_3$. The devices exhibit current on/off ratios up to 8$\times$10$^6$ within an ultra-low gate voltage window of $V_g$ = $\pm$0.5 V and subthreshold swing (SS) as low as 9.7 mV decade$^{-1}$ at room temperature, transcending the 60 mV decade$^{-1}$ Boltzmann limit without involving additional dielectric layers. Theoretical modeling reveals the dominant role of the metastable polar states within domain walls in enabling the NC mode, which is corroborated by the relation between SS and domain wall density. Our findings shed light on a hysteresis-free mechanism for NC operation, providing a simple yet effective material strategy for developing low-power 2D nanoelectronics. △ Less

Submitted 24 December, 2022; v1 submitted 30 August, 2019; originally announced September 2019.

Comments: 26 pages, 5 figures, and Supplementary Information

Journal ref: npj 2D Materials and Applications 6, 77 (2022)

Evolution of the nematic susceptibility in LaFe$_{1-x}$Co$_x$AsO

Authors: Xiaochen Hong, Federico Caglieris, Rhea Kappenberger, Sabine Wurmehl, Saicharan Aswartham, Francesco Scaravaggi, Piotr Lepucki, Anja U. B. Wolter, Hans-Joachim Grafe, Bernd Büchner, Christian Hess

Abstract: The identification of electronic nematicity across series of iron-based superconductors raises the question of its relationship with superconductivity and other ordered states. Here, we report a systematic elastoresistivity study on LaFe$_{1-x}$Co$_x$AsO single crystals, which have well separated structural and magnetic transition lines. All crystals show Curie-Weiss-like nematic susceptibility in… ▽ More The identification of electronic nematicity across series of iron-based superconductors raises the question of its relationship with superconductivity and other ordered states. Here, we report a systematic elastoresistivity study on LaFe$_{1-x}$Co$_x$AsO single crystals, which have well separated structural and magnetic transition lines. All crystals show Curie-Weiss-like nematic susceptibility in the tetragonal phase. The extracted nematic temperature is monotonically suppressed upon cobalt doping, and changes sign around the optimal doping level, indicating a possible nematic quantum critical point beneath the superconducting dome. The amplitude of nematic susceptibility shows a peculiar double-peak feature. This could be explained by a combined effect of different contributions to the nematic susceptibility, which are amplified at separated doping levels of LaFe$_{1-x}$Co$_x$AsO. △ Less

Submitted 3 August, 2020; v1 submitted 1 August, 2019; originally announced August 2019.

Comments: 4 figures

Journal ref: Phys. Rev. Lett. 125, 067001 (2020)

Abrupt Enhancement of Spin-Orbit Scattering Time in Ultrathin Semimetallic SrIrO$_3$ Close to the Metal-Insulator Transition

Authors: L. Zhang, X. Jiang, X. Xu, X. Hong

Abstract: We report a magnetotransport study of spin relaxation in 1.4-21.2 nm epitaxial SrIrO$_3$ thin films coherently strained on SrTiO$_3$ substrates. Fully charge compensated semimetallic transport has been observed in SrIrO$_3$ films thicker than 1.6 nm, where the charge mobility at 10 K increases from 45 cm$^2$/Vs to 150 cm$^2$/Vs with decreasing film thickness. In the two-dimensional regime, the cha… ▽ More We report a magnetotransport study of spin relaxation in 1.4-21.2 nm epitaxial SrIrO$_3$ thin films coherently strained on SrTiO$_3$ substrates. Fully charge compensated semimetallic transport has been observed in SrIrO$_3$ films thicker than 1.6 nm, where the charge mobility at 10 K increases from 45 cm$^2$/Vs to 150 cm$^2$/Vs with decreasing film thickness. In the two-dimensional regime, the charge dephasing and spin-orbit scattering lengths extracted from the weak localization/anti-localization effects show power-law dependence on temperature, pointing to the important role of electron-electron interaction. The spin-orbit scattering time $τ_\text{so}$ exhibits an Elliott-Yafet mechanism dominated quasi-linear dependence on the momentum relaxation time $τ_\text{p}$. Ultrathin films approaching the critical thickness of metal-insulator transition show an abrupt enhancement in $τ_\text{so}$, with the corresponding $τ_\text{so}/τ_\text{p}$ about 7.6 times of the value for thicker films. A likely origin for such unusual enhancement is the onset of strong electron correlation, which leads to charge gap formation and suppresses spin scattering. △ Less

Submitted 28 April, 2020; v1 submitted 26 July, 2019; originally announced July 2019.

Comments: 23 pages, 4 figures

Journal ref: APL Materials 8, 051108 (2020)

Strain-derivative of thermoelectric properties: a sensitive probe for nematicity

Authors: Federico Caglieris, Christoph Wuttke, Xiaochen Hong, Steffen Sykora, Rhea Kappenberger, Saicharan Aswartham, Sabine Wurmehl, Bernd Büchner, Christian Hess

Abstract: The nematic instability of iron-based superconductors is an undebatable ingredient of the physics of iron-based superconductors. Yet, its origin remains enigmatic as it involves a fermiology with an intricate interplay of lattice-, orbital- and spin- degrees of freedom. It is well known that thermoelectric transport is an excellent probe for revealing even subtle signatures of instabilities and pe… ▽ More The nematic instability of iron-based superconductors is an undebatable ingredient of the physics of iron-based superconductors. Yet, its origin remains enigmatic as it involves a fermiology with an intricate interplay of lattice-, orbital- and spin- degrees of freedom. It is well known that thermoelectric transport is an excellent probe for revealing even subtle signatures of instabilities and pertinent fluctuations. In this paper, we report a strong response of the thermoelectric transport properties of two underdoped 1111 iron-based superconductors to a vanishingly small strain. By introducing the strain-derivative of the Seebeck and the Nernst coefficients, we provide a novel description of the nematic order parameter, proving the existence of an anisotropic Peltier-tensor beside an anisotropic conductivity-tensor. Our measurements reveal that the transport nematic phenomenology is the result of the combined effect of both an anisotropic scattering time and Fermi surface distortions, pointing out that in a realistic description, abreast of the spin-fluctuations also the orbital character is a fundamental ingredient. In addition, we show that nematic fluctuations universally relax in a Curie-Weiss fashion above T_S in all the elasto-transport measurements and we provide evidences that nematicity must be band-selective. △ Less

Submitted 10 August, 2020; v1 submitted 28 May, 2019; originally announced May 2019.