Alfred Zong

All Publications

• Room-temperature non-volatile optical manipulation of polar order in a charge density wave NATURE COMMUNICATIONS Liu, Q., Wu, D., Wu, T., Han, S., Peng, Y., Yuan, Z., Cheng, Y., Li, B., Hu, T., Yue, L., Xu, S., Ding, R., Lu, M., Li, R., Zhang, S., Lv, B., Zong, A., Su, Y., Gedik, N., Yin, Z., Dong, T., Wang, N. 2024; 15 (1): 8937

  Abstract

  Utilizing ultrafast light-matter interaction to manipulate electronic states of quantum materials is an emerging area of research in condensed matter physics. It has significant implications for the development of future ultrafast electronic devices. However, the ability to induce long-lasting metastable electronic states in a fully reversible manner is a long-standing challenge. Here, by using ultrafast laser excitations, we demonstrate the capability to manipulate the electronic polar states in the charge-density-wave material EuTe4 in a non-volatile manner. The process is completely reversible and is achieved at room temperature with an all-optical approach. Each induced non-volatile state brings about modifications to the electrical resistance and second harmonic generation intensity. The results point to layer-specific phase inversion dynamics by which photoexcitation mediates the stacking polar order of the system. Our findings extend the scope of non-volatile all-optical control of electronic states to ambient conditions, and highlight a distinct role of layer-dependent phase manipulation in quasi-two-dimensional systems with inherent sublayer stacking orders.

  View details for DOI 10.1038/s41467-024-53323-0

  View details for Web of Science ID 001336262600017

  View details for PubMedID 39414809

  View details for PubMedCentralID PMC11484949

• UV-Induced Reaction Pathways in Bromoform Probed with Ultrafast Electron Diffraction. Journal of the American Chemical Society Hoffmann, L., Toulson, B. W., Yang, J., Saladrigas, C. A., Zong, A., Muvva, S. B., Figueira Nunes, J. P., Reid, A. H., Attar, A. R., Luo, D., Ji, F., Lin, M. F., Fan, Q., Weathersby, S. P., Shen, X., Wang, X., Wolf, T. J., Neumark, D. M., Leone, S. R., Zuerch, M. W., Centurion, M., Gessner, O. 2024

  Abstract

  For many chemical reactions, it remains notoriously difficult to predict and experimentally determine the rates and branching ratios between different reaction channels. This is particularly the case for reactions involving short-lived intermediates, whose observation requires ultrafast methods. The UV photochemistry of bromoform (CHBr3) is among the most intensely studied photoreactions. Yet, a detailed understanding of the chemical pathways leading to the production of atomic Br and molecular Br2 fragments has proven challenging. In particular, the role of isomerization and/or roaming and their competition with direct C-Br bond scission has been a matter of continued debate. Here, gas-phase ultrafast megaelectronvolt electron diffraction (MeV-UED) is used to directly study structural dynamics in bromoform after single 267 nm photon excitation with femtosecond temporal resolution. The results show unambiguously that isomerization contributes significantly to the early stages of the UV photochemistry of bromoform. In addition to direct C-Br bond breaking within <200 fs, formation of iso-CHBr3 (Br-CH-Br-Br) is observed on the same time scale and with an isomer lifetime of >1.1 ps. The branching ratio between direct dissociation and isomerization is determined to be 0.4 ± 0.2:0.6 ± 0.2, i.e., approximately 60% of molecules undergo isomerization within the first few hundred femtoseconds after UV excitation. The structure and time of formation of iso-CHBr3 compare favorably with the results of an ab initio molecular dynamics simulation. The lifetime and interatomic distances of the isomer are consistent with the involvement of a roaming reaction mechanism.

  View details for DOI 10.1021/jacs.4c07165

  View details for PubMedID 39374484

• Coexistence of Interacting Charge Density Waves in a Layered Semiconductor. Physical review letters Lv, B. Q., Zong, A., Wu, D., Nie, Z., Su, Y., Choi, D., Ilyas, B., Fichera, B. T., Li, J., Baldini, E., Mogi, M., Huang, Y. B., Po, H. C., Meng, S., Wang, Y., Wang, N. L., Gedik, N. 2024; 132 (20): 206401

  Abstract

  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 EuTe_{4}, a layered crystal that has drawn considerable attention owing to its anomalous thermal hysteresis and a semiconducting CDW state despite the absence of perfect Fermi surface nesting. By accessing unoccupied conduction bands with time- and angle-resolved photoemission measurements, we find that monolayers and bilayers 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 EuTe_{4} but also provide a general nonequilibrium approach to understand coexisting, layer-dependent orders in a complex system.

  View details for DOI 10.1103/PhysRevLett.132.206401

  View details for PubMedID 38829092

• A solid-state high harmonic generation spectrometer with cryogenic cooling. The Review of scientific instruments Kohrell, F., Nebgen, B. R., Spies, J. A., Hollinger, R., Zong, A., Uzundal, C., Spielmann, C., Zuerch, M. 2024; 95 (2)

  Abstract

  Solid-state high harmonic generation (sHHG) spectroscopy is a promising technique for studying electronic structure, symmetry, and dynamics in condensed matter systems. Here, we report on the implementation of an advanced sHHG spectrometer based on a vacuum chamber and closed-cycle helium cryostat. Using an in situ temperature probe, it is demonstrated that the sample interaction region retains cryogenic temperature during the application of high-intensity femtosecond laser pulses that generate high harmonics. The presented implementation opens the door for temperature-dependent sHHG measurements down to a few Kelvin, which makes sHHG spectroscopy a new tool for studying phases of matter that emerge at low temperatures, which is particularly interesting for highly correlated materials.

  View details for DOI 10.1063/5.0174407

  View details for PubMedID 38416040

• Ultrafast formation of topological defects in a two-dimensional charge density wave NATURE PHYSICS Cheng, Y., Zong, A., Wu, L., Meng, Q., Xia, W., Qi, F., Zhu, P., Zou, X., Jiang, T., Guo, Y., van Wezel, J., Kogar, A., Zuerch, M. W., Zhang, J., Zhu, Y., Xiang, D. 2024; 20 (1)

  View details for DOI 10.1038/s41567-023-02279-x

  View details for Web of Science ID 001137000900003

• Delamination-Assisted Ultrafast Wrinkle Formation in a Freestanding Film. Nano letters Su, Y., Zong, A., Kogar, A., Lu, D., Hong, S. S., Freelon, B., Rohwer, T., Wang, B. Y., Hwang, H. Y., Gedik, N. 2023

  Abstract

  Freestanding films provide a versatile platform for materials engineering thanks to additional structural motifs not found in films with a substrate. A ubiquitous example is wrinkles, yet little is known about how they can develop over as fast as a few picoseconds due to a lack of experimental probes to visualize their dynamics in real time on the nanoscopic scale. Here, we use time-resolved electron diffraction to directly observe light-activated wrinkling formation in freestanding La2/3Ca1/3MnO3 films. Via a "lock-in" analysis of oscillations in the diffraction peak position, intensity, and width, we quantitatively reconstructed how wrinkles develop on the time scale of lattice vibration. Contrary to the common assumption of fixed boundary conditions, we found that wrinkle development is associated with ultrafast delamination at the film boundaries. Our work provides a generic protocol to quantify wrinkling dynamics in freestanding films and highlights the importance of the film-substrate interaction in determining the properties of freestanding structures.

  View details for DOI 10.1021/acs.nanolett.3c02898

  View details for PubMedID 37988604

• Intrinsic 1[Formula: see text] phase induced in atomically thin 2H-MoTe2 by a single terahertz pulse. Nature communications Shi, J., Bie, Y. Q., Zong, A., Fang, S., Chen, W., Han, J., Cao, Z., Zhang, Y., Taniguchi, T., Watanabe, K., Fu, X., Bulović, V., Kaxiras, E., Baldini, E., Jarillo-Herrero, P., Nelson, K. A. 2023; 14 (1): 5905

  Abstract

  The polymorphic transition from 2H to 1[Formula: see text]-MoTe2, which was thought to be induced by high-energy photon irradiation among many other means, has been intensely studied for its technological relevance in nanoscale transistors due to the remarkable improvement in electrical performance. However, it remains controversial whether a crystalline 1[Formula: see text] phase is produced because optical signatures of this putative transition are found to be associated with the formation of tellurium clusters instead. Here we demonstrate the creation of an intrinsic 1[Formula: see text] lattice after irradiating a mono- or few-layer 2H-MoTe2 with a single field-enhanced terahertz pulse. Unlike optical pulses, the low terahertz photon energy limits possible structural damages. We further develop a single-shot terahertz-pump-second-harmonic-probe technique and reveal a transition out of the 2H-phase within 10 ns after photoexcitation. Our results not only provide important insights to resolve the long-standing debate over the light-induced polymorphic transition in MoTe2 but also highlight the unique capability of strong-field terahertz pulses in manipulating quantum materials.

  View details for DOI 10.1038/s41467-023-41291-w

  View details for PubMedID 37737233

  View details for PubMedCentralID PMC10516973

• Spin-mediated shear oscillators in a van der Waals antiferromagnet. Nature Zong, A., Zhang, Q., Zhou, F., Su, Y., Hwangbo, K., Shen, X., Jiang, Q., Liu, H., Gage, T. E., Walko, D. A., Kozina, M. E., Luo, D., Reid, A. H., Yang, J., Park, S., Lapidus, S. H., Chu, J. H., Arslan, I., Wang, X., Xiao, D., Xu, X., Gedik, N., Wen, H. 2023

  Abstract

  Understanding how microscopic spin configuration gives rise to exotic properties at the macroscopic length scale has long been pursued in magnetic materials1-5. One seminal example is the Einstein-de Haas effect in ferromagnets1,6,7, in which angular momentum of spins can be converted into mechanical rotation of an entire object. However, for antiferromagnets without net magnetic moment, how spin ordering couples to macroscopic movement remains elusive. Here we observed a seesaw-like rotation of reciprocal lattice peaks of an antiferromagnetic nanolayer film, whose gigahertz structural resonance exhibits more than an order-of-magnitude amplification after cooling below the Néel temperature. Using a suite of ultrafast diffraction and microscopy techniques, we directly visualize this spin-driven rotation in reciprocal space at the nanoscale. This motion corresponds to interlayer shear in real space, in which individual micro-patches of the film behave as coherent oscillators that are phase-locked and shear along the same in-plane axis. Using time-resolved optical polarimetry, we further show that the enhanced mechanical response strongly correlates with ultrafast demagnetization, which releases elastic energy stored in local strain gradients to drive the oscillators. Our work not only offers the first microscopic view of spin-mediated mechanical motion of an antiferromagnet but it also identifies a new route towards realizing high-frequency resonators8,9 up to the millimetre band, so the capability of controlling magnetic states on the ultrafast timescale10-13 can be readily transferred to engineering the mechanical properties of nanodevices.

  View details for DOI 10.1038/s41586-023-06279-y

  View details for PubMedID 37532936

  View details for PubMedCentralID 10156606

• Probing lithium mobility at a solid electrolyte surface. Nature materials Woodahl, C., Jamnuch, S., Amado, A., Uzundal, C. B., Berger, E., Manset, P., Zhu, Y., Li, Y., Fong, D. D., Connell, J. G., Hirata, Y., Kubota, Y., Owada, S., Tono, K., Yabashi, M., Te Velthuis, S. G., Tepavcevic, S., Matsuda, I., Drisdell, W. S., Schwartz, C. P., Freeland, J. W., Pascal, T. A., Zong, A., Zuerch, M. 2023; 22 (7): 848-852

  Abstract

  Solid-state electrolytes overcome many challenges of present-day lithium ion batteries, such as safety hazards and dendrite formation1,2. However, detailed understanding of the involved lithium dynamics is missing due to a lack of in operando measurements with chemical and interfacial specificity. Here we investigate a prototypical solid-state electrolyte using linear and nonlinear extreme-ultraviolet spectroscopies. Leveraging the surface sensitivity of extreme-ultraviolet-second-harmonic-generation spectroscopy, we obtained a direct spectral signature of surface lithium ions, showing a distinct blueshift relative to bulk absorption spectra. First-principles simulations attributed the shift to transitions from the lithium 1 s state to hybridized Li-s/Ti-d orbitals at the surface. Our calculations further suggest a reduction in lithium interfacial mobility due to suppressed low-frequency rattling modes, which is the fundamental origin of the large interfacial resistance in this material. Our findings pave the way for new optimization strategies to develop these electrochemical devices via interfacial engineering of lithium ions.

  View details for DOI 10.1038/s41563-023-01535-y

  View details for PubMedID 37106132

  View details for PubMedCentralID PMC10313518

• Evidence for Bootstrap Percolation Dynamics in a Photoinduced Phase Transition. Physical review letters Carbin, T., Zhang, X., Culver, A. B., Zhao, H., Zong, A., Acharya, R., Abbamonte, C. J., Roy, R., Cao, G., Kogar, A. 2023; 130 (18): 186902

  Abstract

  Upon intense femtosecond photoexcitation, a many-body system can undergo a phase transition through a nonequilibrium route, but understanding these pathways remains an outstanding challenge. Here, we use time-resolved second harmonic generation to investigate a photoinduced phase transition in Ca_{3}Ru_{2}O_{7} and show that mesoscale inhomogeneity profoundly influences the transition dynamics. We observe a marked slowing down of the characteristic time τ that quantifies the transition between two structures. τ evolves nonmonotonically as a function of photoexcitation fluence, rising from below 200 fs to ∼1.4  ps, then falling again to below 200 fs. To account for the observed behavior, we perform a bootstrap percolation simulation that demonstrates how local structural interactions govern the transition kinetics. Our work highlights the importance of percolating mesoscale inhomogeneity in the dynamics of photoinduced phase transitions and provides a model that may be useful for understanding such transitions more broadly.

  View details for DOI 10.1103/PhysRevLett.130.186902

  View details for PubMedID 37204876

• The spontaneous symmetry breaking in Ta2NiSe5 is structural in nature. Proceedings of the National Academy of Sciences of the United States of America Baldini, E., Zong, A., Choi, D., Lee, C., Michael, M. H., Windgaetter, L., Mazin, I. I., Latini, S., Azoury, D., Lv, B., Kogar, A., Su, Y., Wang, Y., Lu, Y., Takayama, T., Takagi, H., Millis, A. J., Rubio, A., Demler, E., Gedik, N. 2023; 120 (17): e2221688120

  Abstract

  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, Ta2NiSe5 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 spectroscopic fingerprints that are compatible only 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 gap opening. Our results suggest that the spontaneous symmetry breaking in Ta2NiSe5 is mostly of structural character, hampering the possibility to realize quasi-dissipationless energy transport.

  View details for DOI 10.1073/pnas.2221688120

  View details for PubMedID 37071679

  View details for PubMedCentralID PMC10151608

• Emerging ultrafast techniques for studying quantum materials NATURE REVIEWS MATERIALS Zong, A., Nebgen, B. R., Lin, S., Spies, J. A., Zuerch, M. 2023; 8 (4): 224-240

  View details for DOI 10.1038/s41578-022-00530-0

  View details for Web of Science ID 000936714900001

• Dynamical criticality of spin-shear coupling in van der Waals antiferromagnets NATURE COMMUNICATIONS Zhou, F., Hwangbo, K., Zhang, Q., Wang, C., Shen, L., Zhang, J., Jiang, Q., Zong, A., Su, Y., Zajac, M., Ahn, Y., Walko, D. A., Schaller, R. D., Chu, J., Gedik, N., Xu, X., Xiao, D., Wen, H. 2022; 13 (1): 6598

  Abstract

  The interplay between a multitude of electronic, spin, and lattice degrees of freedom underlies the complex phase diagrams of quantum materials. Layer stacking in van der Waals (vdW) heterostructures is responsible for exotic electronic and magnetic properties, which inspires stacking control of two-dimensional magnetism. Beyond the interplay between stacking order and interlayer magnetism, we discover a spin-shear coupling mechanism in which a subtle shear of the atomic layers can have a profound effect on the intralayer magnetic order in a family of vdW antiferromagnets. Using time-resolved X-ray diffraction and optical linear dichroism measurements, interlayer shear is identified as the primary structural degree of freedom that couples with magnetic order. The recovery times of both shear and magnetic order upon optical excitation diverge at the magnetic ordering temperature with the same critical exponent. The time-dependent Ginzburg-Landau theory shows that this concurrent critical slowing down arises from a linear coupling of the interlayer shear to the magnetic order, which is dictated by the broken mirror symmetry intrinsic to the monoclinic stacking. Our results highlight the importance of interlayer shear in ultrafast control of magnetic order via spin-mechanical coupling.

  View details for DOI 10.1038/s41467-022-34376-5

  View details for Web of Science ID 000878823900035

  View details for PubMedID 36329063

  View details for PubMedCentralID PMC9633802

• Periodic dynamics in superconductors induced by an impulsive optical quench COMMUNICATIONS PHYSICS Dolgirev, P. E., Zong, A., Michael, M. H., Curtis, J. B., Podolsky, D., Cavalleri, A., Demler, E. 2022; 5 (1)

  View details for DOI 10.1038/s42005-022-01007-w

  View details for Web of Science ID 000856981800003

• Light-induced dimension crossover dictated by excitonic correlations. Nature communications Cheng, Y., Zong, A., Li, J., Xia, W., Duan, S., Zhao, W., Li, Y., Qi, F., Wu, J., Zhao, L., Zhu, P., Zou, X., Jiang, T., Guo, Y., Yang, L., Qian, D., Zhang, W., Kogar, A., Zuerch, M. W., Xiang, D., Zhang, J. 2022; 13 (1): 963

  Abstract

  In low-dimensional systems with strong electronic correlations, the application of an ultrashort laser pulse often yields novel phases that are otherwise inaccessible. The central challenge in understanding such phenomena is to determine how dimensionality and many-body correlations together govern the pathway of a non-adiabatic transition. To this end, we examine a layered compound, 1T-TiSe2, whose three-dimensional charge-density-wave (3D CDW) state also features exciton condensation due to strong electron-hole interactions. We find that photoexcitation suppresses the equilibrium 3D CDW while creating a nonequilibrium 2D CDW. Remarkably, the dimension reduction does not occur unless bound electron-hole pairs are broken. This relation suggests that excitonic correlations maintain the out-of-plane CDW coherence, settling a long-standing debate over their role in the CDW transition. Our findings demonstrate how optical manipulation of electronic interaction enables one to control the dimensionality of a broken-symmetry order, paving the way for realizing other emergent states in strongly correlated systems.

  View details for DOI 10.1038/s41467-022-28309-5

  View details for PubMedID 35181649

  View details for PubMedCentralID PMC8857203

• Unconventional Hysteretic Transition in a Charge Density Wave. Physical review letters Lv, B. Q., Zong, A., Wu, D., Rozhkov, A. V., Fine, B. V., Chen, S. D., Hashimoto, M., Lu, D. H., Li, M., Huang, Y. B., Ruff, J. P., Walko, D. A., Chen, Z. H., Hwang, I., Su, Y., Shen, X., Wang, X., Han, F., Po, H. C., Wang, Y., Jarillo-Herrero, P., Wang, X., Zhou, H., Sun, C. J., Wen, H., Shen, Z. X., Wang, N. L., Gedik, N. 2022; 128 (3): 036401

  Abstract

  Hysteresis underlies a large number of phase transitions in solids, giving rise to exotic metastable states that are otherwise inaccessible. Here, we report an unconventional hysteretic transition in a quasi-2D material, EuTe_{4}. By combining transport, photoemission, diffraction, and x-ray absorption measurements, we observe that the hysteresis loop has a temperature width of more than 400 K, setting a record among crystalline solids. The transition has an origin distinct from known mechanisms, lying entirely within the incommensurate charge density wave (CDW) phase of EuTe_{4} with no change in the CDW modulation periodicity. We interpret the hysteresis as an unusual switching of the relative CDW phases in different layers, a phenomenon unique to quasi-2D compounds that is not present in either purely 2D or strongly coupled 3D systems. Our findings challenge the established theories on metastable states in density wave systems, pushing the boundary of understanding hysteretic transitions in a broken-symmetry state.

  View details for DOI 10.1103/PhysRevLett.128.036401

  View details for PubMedID 35119886

• Polarization-Resolved Extreme-Ultraviolet Second-Harmonic Generation from LiNbO_{3}. Physical review letters Uzundal, C. B., Jamnuch, S., Berger, E., Woodahl, C., Manset, P., Hirata, Y., Sumi, T., Amado, A., Akai, H., Kubota, Y., Owada, S., Tono, K., Yabashi, M., Freeland, J. W., Schwartz, C. P., Drisdell, W. S., Matsuda, I., Pascal, T. A., Zong, A., Zuerch, M. 2021; 127 (23): 237402

  Abstract

  Second harmonic generation (SHG) spectroscopy ubiquitously enables the investigation of surface chemistry, interfacial chemistry, as well as symmetry properties in solids. Polarization-resolved SHG spectroscopy in the visible to infrared regime is regularly used to investigate electronic and magnetic order through their angular anisotropies within the crystal structure. However, the increasing complexity of novel materials and emerging phenomena hampers the interpretation of experiments solely based on the investigation of hybridized valence states. Here, polarization-resolved SHG in the extreme ultraviolet (XUV-SHG) is demonstrated for the first time, enabling element-resolved angular anisotropy investigations. In noncentrosymmetric LiNbO_{3}, elemental contributions by lithium and niobium are clearly distinguished by energy dependent XUV-SHG measurements. This element-resolved and symmetry-sensitive experiment suggests that the displacement of Li ions in LiNbO_{3}, which is known to lead to ferroelectricity, is accompanied by distortions to the Nb ion environment that breaks the inversion symmetry of the NbO_{6} octahedron as well. Our simulations show that the measured second harmonic spectrum is consistent with Li ion displacements from the centrosymmetric position while the Nb─O bonds are elongated and contracted by displacements of the O atoms. In addition, the polarization-resolved measurement of XUV-SHG shows excellent agreement with numerical predictions based on dipole-induced SHG commonly used in the optical wavelengths. Our result constitutes the first verification of the dipole-based SHG model in the XUV regime. The findings of this work pave the way for future angle and time-resolved XUV-SHG studies with elemental specificity in condensed matter systems.

  View details for DOI 10.1103/PhysRevLett.127.237402

  View details for PubMedID 34936786

• Role of Equilibrium Fluctuations in Light-Induced Order PHYSICAL REVIEW LETTERS Zong, A., Dolgirev, P. E., Kogar, A., Su, Y., Shen, X., Straquadine, J. W., Wang, X., Luo, D., Kozina, M. E., Reid, A. H., Li, R., Yang, J., Weathersby, S. P., Park, S., Sie, E. J., Jarillo-Herrero, P., Fisher, I. R., Wang, X., Demler, E., Gedik, N. 2021; 127 (22)

  View details for DOI 10.1103/PhysRevLett.127.227401

  View details for Web of Science ID 000723128100004

• Role of Equilibrium Fluctuations in Light-Induced Order. Physical review letters Zong, A., Dolgirev, P. E., Kogar, A., Su, Y., Shen, X., Straquadine, J. A., Wang, X., Luo, D., Kozina, M. E., Reid, A. H., Li, R., Yang, J., Weathersby, S. P., Park, S., Sie, E. J., Jarillo-Herrero, P., Fisher, I. R., Wang, X., Demler, E., Gedik, N. 2021; 127 (22): 227401

  Abstract

  Engineering novel states of matter with light is at the forefront of materials research. An intensely studied direction is to realize broken-symmetry phases that are "hidden" under equilibrium conditions but can be unleashed by an ultrashort laser pulse. Despite a plethora of experimental discoveries, the nature of these orders and how they transiently appear remain unclear. To this end, we investigate a nonequilibrium charge density wave (CDW) in rare-earth tritellurides, which is suppressed in equilibrium but emerges after photoexcitation. Using a pump-pump-probe protocol implemented in ultrafast electron diffraction, we demonstrate that the light-induced CDW consists solely of order parameter fluctuations, which bear striking similarities to critical fluctuations in equilibrium despite differences in the length scale. By calculating the dynamics of CDW fluctuations in a nonperturbative model, we further show that the strength of the light-induced order is governed by the amplitude of equilibrium fluctuations. These findings highlight photoinduced fluctuations as an important ingredient for the emergence of transient orders out of equilibrium. Our results further suggest that materials with strong fluctuations in equilibrium are promising platforms to host hidden orders after laser excitation.

  View details for DOI 10.1103/PhysRevLett.127.227401

  View details for PubMedID 34889631

• A versatile sample fabrication method for ultrafast electron diffraction. Ultramicroscopy Bie, Y. Q., Zong, A., Wang, X., Jarillo-Herrero, P., Gedik, N. 2021; 230: 113389

  Abstract

  Integral to the exploration of nonequilibrium phenomena in solid-state systems is the study of lattice motion after photoexcitation by a femtosecond laser pulse. For the past two decades, ultrafast electron diffraction (UED) has played a critical role in this regard. Despite remarkable progress in instrumental development, this technique is still bottlenecked by a demanding sample preparation process, where ultrathin single crystals of large lateral size are typically required. In this work, we describe an efficient, versatile method that yields high-quality, laterally extended (≥ 100 µm), and thin (≤ 50 nm) single crystals on amorphous films of Si3N4 windows. It applies to most exfoliable materials, including those reactive in ambient conditions, and promises clean, flat surfaces. Besides the natural extension to fabricating van der Waals heterostructures, our method can also be applied to future-generation UED that enables additional control of sample parameters, such as electrostatic gating and excitation by a locally enhanced terahertz field. Our work significantly expands the type of samples for UED studies and also finds application in other time-resolved techniques such as attosecond extreme-ultraviolet absorption spectroscopy. This method hence provides further opportunities to explore photoinduced transitions and to discover novel states of matter out of equilibrium.

  View details for DOI 10.1016/j.ultramic.2021.113389

  View details for PubMedID 34530284

• Unconventional light-induced states visualized by ultrafast electron diffraction and microscopy MRS BULLETIN Zong, A., Kogar, A., Gedik, N. 2021; 46 (8): 720-730

  View details for DOI 10.1557/s43577-021-00163-8

  View details for Web of Science ID 000688399300001

• Second harmonic generation as a probe of broken mirror symmetry PHYSICAL REVIEW B Fichera, B. T., Kogar, A., Ye, L., Gokce, B., Zong, A., Checkelsky, J. G., Gedik, N. 2020; 101 (24)

  View details for DOI 10.1103/PhysRevB.101.241106

  View details for Web of Science ID 000538715500001

• Self-similar dynamics of order parameter fluctuations in pump-probe experiments PHYSICAL REVIEW B Dolgirev, P. E., Michael, M. H., Zong, A., Gedik, N., Demler, E. 2020; 101 (17)

  View details for DOI 10.1103/PhysRevB.101.174306

  View details for Web of Science ID 000531182800002

• High resolution time- and angle-resolved photoemission spectroscopy with 11 eV laser pulses REVIEW OF SCIENTIFIC INSTRUMENTS Lee, C., Rohwer, T., Sie, E. J., Zong, A., Baldini, E., Straquadine, J., Walmsley, P., Gardner, D., Lee, Y. S., Fisher, I. R., Gedik, N. 2020; 91 (4)

  View details for DOI 10.1063/1.5139556

  View details for Web of Science ID 000526759500002

• High resolution time- and angle-resolved photoemission spectroscopy with 11 eV laser pulses. The Review of scientific instruments Lee, C., Rohwer, T., Sie, E. J., Zong, A., Baldini, E., Straquadine, J., Walmsley, P., Gardner, D., Lee, Y. S., Fisher, I. R., Gedik, N. 2020; 91 (4): 043102

  Abstract

  Performing time- and angle-resolved photoemission (tr-ARPES) spectroscopy at high momenta necessitates extreme ultraviolet laser pulses, which are typically produced via high harmonic generation (HHG). Despite recent advances, HHG-based setups still require large pulse energies (from hundreds of μJ to mJ) and their energy resolution is limited to tens of meV. Here, we present a novel 11 eV tr-ARPES setup that generates a flux of 5 × 1010 photons/s and achieves an unprecedented energy resolution of 16 meV. It can be operated at high repetition rates (up to 250 kHz) while using input pulse energies down to 3 µJ. We demonstrate these unique capabilities by simultaneously capturing the energy and momentum resolved dynamics in two well-separated momentum space regions of a charge density wave material ErTe3. This novel setup offers the opportunity to study the non-equilibrium band structure of solids with exceptional energy and time resolutions at high repetition rates.

  View details for DOI 10.1063/1.5139556

  View details for PubMedID 32357712

• Amplitude dynamics of the charge density wave in LaTe₃: Theoretical description of pump-probe experiments PHYSICAL REVIEW B Dolgirev, P. E., Rozhkov, A., Zong, A., Kogar, A., Gedik, N., Fine, B. 2020; 101 (5)

  View details for DOI 10.1103/PhysRevB.101.054203

  View details for Web of Science ID 000513181300003

• Light-induced charge density wave in LaTe3 NATURE PHYSICS Kogar, A., Zong, A., Dolgirev, P. E., Shen, X., Straquadine, J., Bie, Y., Wang, X., Rohwer, T., Tung, I., Yang, Y., Li, R., Yang, J., Weathersby, S., Park, S., Kozina, M. E., Sie, E. J., Wen, H., Jarillo-Herrero, P., Fisher, I. R., Wang, X., Gedik, N. 2020; 16 (2): 159-+

  View details for DOI 10.1038/s41567-019-0705-3

  View details for Web of Science ID 000511518200012

• Spontaneous gyrotropic electronic order in a transition-metal dichalcogenide. Nature Xu, S. Y., Ma, Q., Gao, Y., Kogar, A., Zong, A., Mier Valdivia, A. M., Dinh, T. H., Huang, S. M., Singh, B., Hsu, C. H., Chang, T. R., Ruff, J. P., Watanabe, K., Taniguchi, T., Lin, H., Karapetrov, G., Xiao, D., Jarillo-Herrero, P., Gedik, N. 2020; 578 (7796): 545-549

  Abstract

  Chirality is ubiquitous in nature, and populations of opposite chiralities are surprisingly asymmetric at fundamental levels1,2. Examples range from parity violation in the subatomic weak force to homochirality in biomolecules. The ability to achieve chirality-selective synthesis (chiral induction) is of great importance in stereochemistry, molecular biology and pharmacology2. In condensed matter physics, a crystalline electronic system is geometrically chiral when it lacks mirror planes, space-inversion centres or rotoinversion axes1. Typically, geometrical chirality is predefined by the chiral lattice structure of a material, which is fixed on formation of the crystal. By contrast, in materials with gyrotropic order3-6, electrons spontaneously organize themselves to exhibit macroscopic chirality in an originally achiral lattice. Although such order-which has been proposed as the quantum analogue of cholesteric liquid crystals-has attracted considerable interest3-15, no clear observation or manipulation of gyrotropic order has been achieved so far. Here we report the realization of optical chiral induction and the observation of a gyrotropically ordered phase in the transition-metal dichalcogenide semimetal 1T-TiSe2. We show that shining mid-infrared circularly polarized light on 1T-TiSe2 while cooling it below the critical temperature leads to the preferential formation of one chiral domain. The chirality of this state is confirmed by the measurement of an out-of-plane circular photogalvanic current, the direction of which depends on the optical induction. Although the role of domain walls requires further investigation with local probes, the methodology demonstrated here can be applied to realize and control chiral electronic phases in other quantum materials4,16.

  View details for DOI 10.1038/s41586-020-2011-8

  View details for PubMedID 32103195

  View details for PubMedCentralID 5504287

• Dynamical Slowing-Down in an Ultrafast Photoinduced Phase Transition. Physical review letters Zong, A., Dolgirev, P. E., Kogar, A., Ergeçen, E., Yilmaz, M. B., Bie, Y. Q., Rohwer, T., Tung, I. C., Straquadine, J., Wang, X., Yang, Y., Shen, X., Li, R., Yang, J., Park, S., Hoffmann, M. C., Ofori-Okai, B. K., Kozina, M. E., Wen, H., Wang, X., Fisher, I. R., Jarillo-Herrero, P., Gedik, N. 2019; 123 (9): 097601

  Abstract

  Complex systems, which consist of a large number of interacting constituents, often exhibit universal behavior near a phase transition. A slowdown of certain dynamical observables is one such recurring feature found in a vast array of contexts. This phenomenon, known as critical slowing-down, is well studied mostly in thermodynamic phase transitions. However, it is less understood in highly nonequilibrium settings, where the time it takes to traverse the phase boundary becomes comparable to the timescale of dynamical fluctuations. Using transient optical spectroscopy and femtosecond electron diffraction, we studied a photoinduced transition of a model charge-density-wave (CDW) compound LaTe_{3}. We observed that it takes the longest time to suppress the order parameter at the threshold photoexcitation density, where the CDW transiently vanishes. This finding can be captured by generalizing the time-dependent Landau theory to a system far from equilibrium. The experimental observation and theoretical understanding of dynamical slowing-down may offer insight into other general principles behind nonequilibrium phase transitions in many-body systems.

  View details for DOI 10.1103/PhysRevLett.123.097601

  View details for PubMedID 31524450

• Dynamical Slowing-Down in an Ultrafast Photoinduced Phase Transition PHYSICAL REVIEW LETTERS Zong, A., Dolgirev, P. E., Kogar, A., Ergecen, E., Yilmaz, M. B., Bie, Y., Rohwer, T., Tung, I., Straquadine, J., Wang, X., Yang, Y., Shen, X., Li, R., Yang, J., Park, S., Hoffmann, M. C., Ofori-Okai, B. K., Kozina, M. E., Wen, H., Wang, X., Fisher, I. R., Jarillo-Herrero, P., Gedik, N. 2019; 123 (9)

  View details for DOI 10.1103/PhysRevLett.123.097601

  View details for Web of Science ID 000483048500015

• Observation of multiple types of topological fermions in PdBiSe PHYSICAL REVIEW B Lv, B. Q., Feng, Z., Zhao, J., Yuan, N. Q., Zong, A., Luo, K. E., Yu, R., Huang, Y., Strocov, V. N., Chikina, A., Soluyanov, A. A., Gedik, N., Shi, Y., Qian, T., Ding, H. 2019; 99 (24)

  View details for DOI 10.1103/PhysRevB.99.241104

  View details for Web of Science ID 000470840400001

• Evidence for topological defects in a photoinduced phase transition NATURE PHYSICS Zong, A., Kogar, A., Bie, Y., Rohwer, T., Lee, C., Baldini, E., Ergecen, E., Yilmaz, M. B., Freelon, B., Sie, E. J., Zhou, H., Straquadine, J., Walmsley, P., Dolgirev, P. E., Rozhkov, A. V., Fisher, I. R., Jarillo-Herrero, P., Fine, B. V., Gedik, N. 2019; 15 (1): 27-+

  View details for DOI 10.1038/s41567-018-0311-9

  View details for Web of Science ID 000454733100015

• Combining time-resolved optical (TOS), electronic (trARPES) and structural (UED) probes on the class of rare earth tritellurides RTe3 Rohwero, T., Zong, A., Kogar, A., Bie, Y., Lee, C., Baldini, E., Ergecen, E., Yilmaz, M. B., Freelon, B., Sie, E. J., Zhou, H., Straquadine, J., Walmsley, P., Dolgirev, P. E., Rozhkov, A., Fisher, I. R., Jarillo-Herrero, P., Fine, B., Gedik, N., Cerullo, G., Ogilvie, J., Kartner, F., Khalil, M., Li, R. E D P SCIENCES. 2019

  View details for DOI 10.1051/epjconf/201920504009

  View details for Web of Science ID 000570451400078

• Ultrafast manipulation of mirror domain walls in a charge density wave SCIENCE ADVANCES Zong, A., Shen, X., Kogar, A., Ye, L., Marks, C., Chowdhury, D., Rohwer, T., Freelon, B., Weathersby, S., Li, R., Yang, J., Checkelsky, J., Wang, X., Gedik, N. 2018; 4 (10): eaau5501

  Abstract

  Domain walls (DWs) are singularities in an ordered medium that often host exotic phenomena such as charge ordering, insulator-metal transition, or superconductivity. The ability to locally write and erase DWs is highly desirable, as it allows one to design material functionality by patterning DWs in specific configurations. We demonstrate such capability at room temperature in a charge density wave (CDW), a macroscopic condensate of electrons and phonons, in ultrathin 1T-TaS2. A single femtosecond light pulse is shown to locally inject or remove mirror DWs in the CDW condensate, with probabilities tunable by pulse energy and temperature. Using time-resolved electron diffraction, we are able to simultaneously track anti-synchronized CDW amplitude oscillations from both the lattice and the condensate, where photoinjected DWs lead to a red-shifted frequency. Our demonstration of reversible DW manipulation may pave new ways for engineering correlated material systems with light.

  View details for DOI 10.1126/sciadv.aau5501

  View details for Web of Science ID 000449221200072

  View details for PubMedID 30345365

  View details for PubMedCentralID PMC6195337