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Atomically Resolved Acoustic Dynamics Coupled with Magnetic Order in a van der Waals Antiferromagnet
Authors:
Faran Zhou,
Kyle Hwangbo,
Sung Soo Ha,
Xiao-Wei Zhang,
Sae Hwan Chun,
Jaeku Park,
Intae Eom,
Qianni Jiang,
Zekai Yang,
Marc Zajac,
Sungwon Kim,
Sungwook Choi,
Zhaodong Chu,
Kyoung Hun Oh,
Yifan Su,
Alfred Zong,
Elton J. G. Santos,
Ting Cao,
Jiun-Haw Chu,
Stephan O. Hruszkewycz,
Nuh Gedik,
Di Xiao,
Hyunjung Kim,
Xiaodong Xu,
Haidan Wen
Abstract:
Magnetoelastic coupling in van der Waals (vdW) magnetic materials enables a unique interplay between the spin and lattice degrees of freedom. Characterizing the elastic responses with atomic and femtosecond resolution across the magnetic transition is essential for guiding the design of magnetically tunable actuators and strain-mediated spintronic devices. Here, ultrafast x-ray diffraction employe…
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Magnetoelastic coupling in van der Waals (vdW) magnetic materials enables a unique interplay between the spin and lattice degrees of freedom. Characterizing the elastic responses with atomic and femtosecond resolution across the magnetic transition is essential for guiding the design of magnetically tunable actuators and strain-mediated spintronic devices. Here, ultrafast x-ray diffraction employed at a free-electron laser reveals that the atomic displacements, wave vectors, and dispersion relations of acoustic phonon modes in a vdW antiferromagnet FePS$_3$ are coupled with the magnetic order, by tracking both in-plane and out-of-plane Bragg peaks upon optical excitation across the Néel temperature (T$_N$). One transverse mode shows that a quasi-out-of-plane atomic displacement undergoes a significant directional change across T$_N$. Its quasi-in-plane wave vector is derived by the comparison between the measured sound velocity and the first-principles calculations. The other transverse mode is an interlayer shear acoustic mode whose amplitude is strongly enhanced in the antiferromagnetic phase, exhibiting eight times stronger amplitude than the longitudinal acoustic mode below T$_N$. The atomically resolved characterization of acoustic phonon dynamics that couple with magnetic ordering opens opportunities for harnessing unique magnetoelastic coupling in vdW magnets on ultrafast timescales.
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Submitted 23 January, 2026;
originally announced January 2026.
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Bidirectional ultrafast control of charge density waves via phase competition
Authors:
Honglie Ning,
Kyoung Hun Oh,
Yifan Su,
Zhengyan Darius Shi,
Dong Wu,
Qiaomei Liu,
B. Q. Lv,
Alfred Zong,
Gyeongbo Kang,
Hyeongi Choi,
Hyun-Woo J. Kim,
Seunghyeok Ha,
Jaehwon Kim,
Suchismita Sarker,
Jacob P. C. Ruff,
B. J. Kim,
N. L. Wang,
Todadri Senthil,
Hoyoung Jang,
Nuh Gedik
Abstract:
The intricate competition between coexisting charge density waves (CDWs) can lead to rich phenomena, offering unique opportunities for phase manipulation through electromagnetic stimuli. Leveraging time-resolved X-ray diffraction, we demonstrate ultrafast control of a CDW in EuTe$_4$ upon optical excitation. At low excitation intensities, the amplitude of one of the coexisting CDW orders increases…
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The intricate competition between coexisting charge density waves (CDWs) can lead to rich phenomena, offering unique opportunities for phase manipulation through electromagnetic stimuli. Leveraging time-resolved X-ray diffraction, we demonstrate ultrafast control of a CDW in EuTe$_4$ upon optical excitation. At low excitation intensities, the amplitude of one of the coexisting CDW orders increases at the expense of the competing CDW, whereas at high intensities, it exhibits a nonmonotonic temporal evolution characterized by both enhancement and reduction. This transient bidirectional controllability, tunable by adjusting photo-excitation intensity, arises from the interplay between optical quenching and phase-competition-induced enhancement. Our findings, supported by phenomenological time-dependent Ginzburg-Landau theory simulations, not only clarify the relationship between the two CDWs in EuTe$_4$, but also highlight the versatility of optical control over order parameters enabled by phase competition.
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Submitted 30 September, 2025;
originally announced October 2025.
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Joint commensuration in moiré charge-order superlattices drives shear topological defects
Authors:
Kyoung Hun Oh,
Yifan Su,
Honglie Ning,
B. Q. Lv,
Alfred Zong,
Dong Wu,
Qiaomei Liu,
Gyeongbo Kang,
Hyeongi Choi,
Hyun-Woo J. Kim,
Seunghyeok Ha,
Jaehwon Kim,
Suchismita Sarker,
Jacob P. C. Ruff,
Xiaozhe Shen,
Duan Luo,
Stephen Weathersby,
Patrick Kramer,
Xinxin Cheng,
Dongsung Choi,
Doron Azoury,
Masataka Mogi,
B. J. Kim,
N. L. Wang,
Hoyoung Jang
, et al. (1 additional authors not shown)
Abstract:
The advent of two-dimensional moiré systems has revolutionized the exploration of phenomena arising from strong correlations and nontrivial band topology. Recently, a moiré superstructure formed by two coexisting charge density wave (CDW) orders with slightly mismatched wavevectors has been realized. These incommensurate CDWs can collectively exhibit commensurability, resulting in the jointly comm…
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The advent of two-dimensional moiré systems has revolutionized the exploration of phenomena arising from strong correlations and nontrivial band topology. Recently, a moiré superstructure formed by two coexisting charge density wave (CDW) orders with slightly mismatched wavevectors has been realized. These incommensurate CDWs can collectively exhibit commensurability, resulting in the jointly commensurate CDW (JC-CDW). This JC-CDW hosts phenomena including electronic anisotropy and phase-modulated hysteresis, and holds promise for non-volatile optoelectronic memory devices. Realizing such functionality requires understanding how the spatial periodicity, coherence, and amplitude of this order evolve under perturbations. Here, we address these questions using time- and momentum-resolved techniques to probe light-induced dynamics in EuTe$_4$. Our time-resolved diffraction results show that under intense photoexcitation, the JC-CDW wavevector and coherence length remain locked along the CDW direction, indicating preserved moiré periodicity while the moiré potential depth is suppressed. This robustness governs the configuration of the photoexcited JC-CDW and leads to the formation of previously underexplored shear-type topological defects. Furthermore, we developed an approach to simultaneously track the temporal evolution of the amplitude and phase of a CDW by following two diffraction peaks corresponding to one order, with findings verified by time-resolved photoemission and electron diffraction. This methodology enables reconstruction of the momentum- and time-resolved evolution of the JC-CDW and direct visualization of shear-type topological defect formation. These findings not only highlight the unique robustness of JC-CDWs out of equilibrium, but also establish a platform for optical moiré engineering and manipulation of quantum materials through topological defect control.
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Submitted 19 September, 2025;
originally announced September 2025.
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Discovery of transient topological crystalline order in optically driven SnSe
Authors:
Masataka Mogi,
Dongsung Choi,
Kyoung Hun Oh,
Diana Golovanova,
Yufei Zhao,
Yifan Su,
Zongqi Shen,
Doron Azoury,
Haoyu Xia,
Batyr Ilyas,
Tianchuang Luo,
Noriaki Kida,
Taito Osaka,
Tadashi Togashi,
Binghai Yan,
Nuh Gedik
Abstract:
Ultrafast optical excitation provides a powerful route for accessing emergent quantum phases far from equilibrium, enabling transient light-induced phenomena such as magnetism, ferroelectricity, and superconductivity. However, extending this approach to induce topological phases, especially in conventional semiconductors, remains challenging. Here, we report the observation of a thermally inaccess…
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Ultrafast optical excitation provides a powerful route for accessing emergent quantum phases far from equilibrium, enabling transient light-induced phenomena such as magnetism, ferroelectricity, and superconductivity. However, extending this approach to induce topological phases, especially in conventional semiconductors, remains challenging. Here, we report the observation of a thermally inaccessible, transient topological crystalline order in the layered semiconductor SnSe, a trivial insulator with a sizable (~ 0.8 eV) band gap, induced by femtosecond above-gap excitation. Time- and angle-resolved photoemission spectroscopy directly reveals the sub-picosecond emergence of Dirac-like linear dispersions within the band gap. Their features, including a high Fermi velocity (~ 2.5x10^5 m/s), multiple Dirac points away from high-symmetry momenta, and independence from probe photon energy, are consistent with mirror-symmetry-protected surface states of a topological crystalline insulator. The observed spectral dynamics, combined with density functional theory calculations, indicate that the femtosecond excitation transiently increases lattice symmetry, enabling topological crystalline order to emerge. Our discovery opens new avenues for ultrafast optical control of topological quantum phases in semiconductors, with potential applications in quantum and spintronic devices.
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Submitted 16 May, 2025; v1 submitted 20 February, 2025;
originally announced February 2025.
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Dynamical decoding of the competition between charge density waves in a kagome superconductor
Authors:
Honglie Ning,
Kyoung Hun Oh,
Yifan Su,
Alexander von Hoegen,
Zach Porter,
Andrea Capa Salinas,
Quynh L Nguyen,
Matthieu Chollet,
Takahiro Sato,
Vincent Esposito,
Matthias C Hoffmann,
Adam White,
Cynthia Melendrez,
Diling Zhu,
Stephen D Wilson,
Nuh Gedik
Abstract:
The kagome superconductor CsV$_3$Sb$_5$ hosts a variety of charge density wave (CDW) phases, which play a fundamental role in the formation of other exotic electronic instabilities. However, identifying the precise structure of these CDW phases and their intricate relationships remain the subject of intense debate, due to the lack of static probes that can distinguish the CDW phases with identical…
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The kagome superconductor CsV$_3$Sb$_5$ hosts a variety of charge density wave (CDW) phases, which play a fundamental role in the formation of other exotic electronic instabilities. However, identifying the precise structure of these CDW phases and their intricate relationships remain the subject of intense debate, due to the lack of static probes that can distinguish the CDW phases with identical spatial periodicity. Here, we unveil the competition between two coexisting $2\times2\times2$ CDWs in CsV$_3$Sb$_5$ harnessing time-resolved X-ray diffraction. By analyzing the light-induced changes in the intensity of CDW superlattice peaks, we demonstrate the presence of both phases, each displaying a significantly different amount of melting upon excitation. The anomalous light-induced sharpening of peak width further shows that the phase that is more resistant to photo-excitation exhibits an increase in domain size at the expense of the other, thereby showcasing a hallmark of phase competition. Our results not only shed light on the interplay between the multiple CDW phases in CsV$_3$Sb$_5$, but also establish a non-equilibrium framework for comprehending complex phase relationships that are challenging to disentangle using static techniques.
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Submitted 5 March, 2024;
originally announced March 2024.
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Direct Observation of Collective Modes of the Charge Density Wave in the Kagome Metal CsV$_3$Sb$_5$
Authors:
Doron Azoury,
Alexander von Hoegen,
Yifan Su,
Kyoung Hun Oh,
Tobias Holder,
Hengxin Tan,
Brenden R. Ortiz,
Andrea Capa Salinas,
Stephen D. Wilson,
Binghai Yan,
Nuh Gedik
Abstract:
A new group of kagome metals AV$_3$Sb$_5$ (A = K, Rb, Cs) exhibit a variety of intertwined unconventional electronic phases, which emerge from a puzzling charge density wave phase. Understanding of this parent charge order phase is crucial for deciphering the entire phase diagram. However, the mechanism of the charge density wave is still controversial, and its primary source of fluctuations - the…
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A new group of kagome metals AV$_3$Sb$_5$ (A = K, Rb, Cs) exhibit a variety of intertwined unconventional electronic phases, which emerge from a puzzling charge density wave phase. Understanding of this parent charge order phase is crucial for deciphering the entire phase diagram. However, the mechanism of the charge density wave is still controversial, and its primary source of fluctuations - the collective modes - have not been experimentally observed. Here, we use ultrashort laser pulses to melt the charge order in CsV$_3$Sb$_5$ and record the resulting dynamics using femtosecond angle-resolved photoemission. We resolve the melting time of the charge order and directly observe its amplitude mode, imposing a fundamental limit for the fastest possible lattice rearrangement time. These observations together with ab-initio calculations provide clear evidence for a structural rather than electronic mechanism of the charge density wave. Our findings pave the way for better understanding of the unconventional phases hosted on the kagome lattice.
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Submitted 24 January, 2023;
originally announced January 2023.
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Molecular Dynamics Simulation of Electron Trapping in the Sapphire Lattice
Authors:
C. Rambaut,
K. H. Oh,
H. Jaffrezic,
J. Kohanoff,
S. Fayeulle
Abstract:
Energy storage and release in dielectric materials can be described on the basis of the charge trapping mechanism. Most phenomenological aspects have been recently rationalized in terms of the space charge model~\cite{blaise,blaise1}. Dynamical aspects are studied here by performing Molecular Dynamics simulations. We show that an excess electron introduced into the sapphire lattice (\alumina) ca…
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Energy storage and release in dielectric materials can be described on the basis of the charge trapping mechanism. Most phenomenological aspects have been recently rationalized in terms of the space charge model~\cite{blaise,blaise1}. Dynamical aspects are studied here by performing Molecular Dynamics simulations. We show that an excess electron introduced into the sapphire lattice (\alumina) can be trapped only at a limited number of sites. The energy gained by allowing the electron to localize in these sites is of the order of 4-5 eV, in good agreement with the results of the space charge model. Displacements of the neighboring ions due to the implanted charge are shown to be localized in a small region of about 5~Å. Detrapping is observed at 250 $K$. The ionic displacements turn out to play an important role in modifying the potential landscape by lowering, in a dynamical way, the barriers that cause localization at low temperature.
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Submitted 16 October, 1995;
originally announced October 1995.