arXiv subjects

Dongsung Choi

Publications and source records attributed to Dongsung Choi.

8 recordsLinked to original sources

arXiv2025

Dynamics of a jointly commensurate moir\'e charge density wave

The advent of two-dimensional moir\'e systems has revolutionized the exploration of phenomena arising from strong correlations and nontrivial band topology. Recently, a moir\'e superstructure formed by two coexisting charge density waves (CDWs) with slightly mismatched wavevectors has been realized. These incommensurate CDWs can collectively exhibit commensurability, resulting in the jointly commensurate CDW (JC-CDW) and establishing a new paradigm for controlling moir\'e potential and periodicity. Achieving such functionality, however, hinges on a key open question: how do the amplitude, phase coherence, and periodicity of this order respond to external perturbations? Here, we address this question using a suite of time- and momentum-resolved diffraction and spectroscopic techniques to probe light-induced CDW dynamics in EuTe$_4$. Our time-resolved diffraction measurements distinguish the instantaneous quenching of the JC-CDW amplitude, as verified by time-resolved photoemission spectroscopy, from the much slower evolution of phase fluctuations. Furthermore, while the JC-CDW wavevector remains locked along the CDW direction upon photoexcitation, indicating a preserved moir\'e periodicity, the correlation length of JC-CDW shows an exclusive reduction perpendicular to its wavevector, unveiling the formation of previously unexplored shear-type defects. Together, this multimodal methodology reconstructs the spatiotemporal evolution of the JC-CDW upon excitation. These findings not only highlight the remarkable robustness of JC-CDWs out of equilibrium, but also provide insight into optical manipulation and engineering of moir\'e quantum materials through defect control.

cond-mat.str-el↗

arXiv2025

Discovery of transient topological crystalline order in optically driven SnSe

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.

cond-mat.mtrl-sci↗

arXiv2025

Time-domain identification of distinct mechanisms for competing charge density waves in a rare-earth tritelluride

Understanding the origin of phase transitions and the interactions between distinct phases remains a central task in condensed matter physics. Charge density wave (CDW) systems provide an ideal platform for investigating these phenomena. While the dominant CDW phases in many materials can be explained through Fermi surface nesting or electron-phonon interactions, certain CDW phase transitions remain poorly understood, challenging conventional paradigms. One notable example is rare-earth tritelluride ErTe3, which hosts two competing CDW orders. While the dominant CDW phase fits within the electron-phonon coupling framework, the formation mechanism of the subdominant CDW remains enigmatic. In this study, we combine time-and-angle-resolved photoemission spectroscopy (trARPES) with time-dependent Ginzburg-Landau (TDGL) theory to establish a time-domain approach for probing phase transitions in solid-state systems. By analyzing the distinct recovery dynamics of the two CDW orders in ErTe3 following light excitation, we reveal a novel nucleation-like growth mechanism that likely drives the secondary CDW phase transition. This work not only uncovers a previously unknown CDW formation mechanism in rare-earth tritellurides but also introduces a non-equilibrium framework for understanding phase transitions and phase competition in quantum materials.

cond-mat.str-el↗

arXiv2024

Direct observation of a photoinduced topological phase transition in Bi-doped (Pb,Sn)Se

Ultrafast photoexcitation offers a novel approach to manipulating quantum materials. One of the long-standing goals in this field is to achieve optical control over topological properties. However, the impact on their electronic structures, which host gapless surface states, has yet to be directly observed. Here, using time- and angle-resolved photoemission spectroscopy, we visualize the photo-induced evolution of the band structure in Biy(Pb1-xSnx)1-ySe(111) films from topological to trivial insulators. Following near-infrared ultrafast laser excitation, we observe that the topological surface state opens a substantial gap of up to 0.1 eV. Considering the topological phase diagram associated with lattice distortion and atomic displacement, we show that a uniaxial strain generated by the ultrafast optical pulse is sufficiently effective and strong for the observed topological phase transition. Our study highlights the potential of optical tuning of materials through laser excitation to control topological properties on ultrafast timescales.

cond-mat.mtrl-sci↗

arXiv2024

Direct observation of Floquet-Bloch states in monolayer graphene

Floquet engineering is a novel method of manipulating quantum phases of matter via periodic driving [1, 2]. It has successfully been utilized in different platforms ranging from photonic systems [3] to optical lattice of ultracold atoms [4, 5]. In solids, light can be used as the periodic drive via coherent light-matter interaction. This leads to hybridization of Bloch electrons with photons resulting in replica bands known as Floquet-Bloch states. After the direct observation of Floquet-Bloch states in a topological insulator [6], their manifestations have been seen in a number of other experiments [7-14]. By engineering the electronic band structure using Floquet-Bloch states, various exotic phase transitions have been predicted [15-22] to occur. To realize these phases, it is necessary to better understand the nature of Floquet-Bloch states in different materials. However, direct energy and momentum resolved observation of these states is still limited to only few material systems [6, 10, 14, 23, 24]. Here, we report direct observation of Floquet-Bloch states in monolayer epitaxial graphene which was the first proposed material platform [15] for Floquet engineering. By using time- and angle-resolved photoemission spectroscopy (trARPES) with mid-infrared (mid-IR) pump excitation, we detected replicas of the Dirac cone. Pump polarization dependence of these replica bands unequivocally shows that they originate from the scattering between Floquet-Bloch states and photon-dressed free-electron-like photoemission final states, called Volkov states. Beyond graphene, our method can potentially be used to directly observe Floquet-Bloch states in other systems paving the way for Floquet engineering in a wide range of quantum materials.

cond-mat.mes-hall↗

arXiv2024

Coexistence of interacting charge density waves in a layered semiconductor

Coexisting orders are key features of strongly correlated materials and underlie many intriguing phenomena from unconventional superconductivity to topological orders. Here, we report the coexistence of two interacting charge-density-wave (CDW) orders in EuTe4, a layered crystal that has drawn considerable attention owing to its anomalous thermal hysteresis and a semiconducting CDW state despite the absence of perfect FS nesting. By accessing unoccupied conduction bands with time- and angle-resolved photoemission measurements, we find that mono- and bi-layers of Te in the unit cell host different CDWs that are associated with distinct energy gaps. The two gaps display dichotomous evolutions following photoexcitation, where the larger bilayer CDW gap exhibits less renormalization and faster recovery. Surprisingly, the CDW in the Te monolayer displays an additional momentum-dependent gap renormalization that cannot be captured by density-functional theory calculations. This phenomenon is attributed to interlayer interactions between the two CDW orders, which account for the semiconducting nature of the equilibrium state. Our findings not only offer microscopic insights into the correlated ground state of EuTe4 but also provide a general non-equilibrium approach to understand coexisting, layer-dependent orders in a complex system.

cond-mat.str-el↗

arXiv2023

Light-induced insulator-metal transition in Sr$_2$IrO$_4$ reveals the nature of the insulating ground state

Sr$_2$IrO$_4$ has attracted a lot of attention due to its structural and electronic similarities to La$_2$CuO$_4$ which is the parent compound of high-T$_c$ superconducting cuprates. It was proposed to be a strong spin-orbit coupled J$_{eff}$ = 1/2 Mott insulator, but the Mott nature of its insulating ground state and the origin of the gap have not been conclusively established. Here, we use ultrafast laser pulses to realize an insulator-metal transition in Sr$_2$IrO$_4$ and probe the resulting dynamics using time- and angle-resolved photoemission spectroscopy. We observe a closing of the gap and the formation of weakly-renormalized electronic bands in the gap region. Comparing these observations to the expected temperature and doping evolution of Mott gaps and Hubbard bands provides clear evidence that the insulating state does not originate from Mott correlations. We instead propose a correlated band insulator picture, where antiferromagnetic correlations play a key role in the opening of the gap. More broadly, our results demonstrate that energy-momentum resolved nonequilibrium dynamics can be used to clarify the nature of equilibrium states in correlated materials.

cond-mat.str-el↗

arXiv2020

The spontaneous symmetry breaking in Ta$_2$NiSe$_5$ is structural in nature

The excitonic insulator is an electronically-driven phase of matter that emerges upon the spontaneous formation and Bose condensation of excitons. Detecting this exotic order in candidate materials is a subject of paramount importance, as the size of the excitonic gap in the band structure establishes the potential of this collective state for superfluid energy transport. However, the identification of this phase in real solids is hindered by the coexistence of a structural order parameter with the same symmetry as the excitonic order. Only a few materials are currently believed to host a dominant excitonic phase, Ta$_2$NiSe$_5$ being the most promising. Here, we test this scenario by using an ultrashort laser pulse to quench the broken-symmetry phase of this transition metal chalcogenide. Tracking the dynamics of the material's electronic and crystal structure after light excitation reveals surprising spectroscopic fingerprints that are only compatible with a primary order parameter of phononic nature. We rationalize our findings through state-of-the-art calculations, confirming that the structural order accounts for most of the electronic gap opening. Not only do our results uncover the long-sought mechanism driving the phase transition of Ta$_2$NiSe$_5$, but they also conclusively rule out any substantial excitonic character in this instability.

cond-mat.str-el↗