Quynh Le Nguyen

All Publications

• Observation of time-reversal symmetry breaking in the band structure of altermagnetic RuO2. Science advances Fedchenko, O., Minar, J., Akashdeep, A., D'Souza, S. W., Vasilyev, D., Tkach, O., Odenbreit, L., Nguyen, Q., Kutnyakhov, D., Wind, N., Wenthaus, L., Scholz, M., Rossnagel, K., Hoesch, M., Aeschlimann, M., Stadtmuller, B., Klaui, M., Schonhense, G., Jungwirth, T., Hellenes, A. B., Jakob, G., Smejkal, L., Sinova, J., Elmers, H. 2024; 10 (5): eadj4883

  Abstract

  Altermagnets are an emerging elementary class of collinear magnets. Unlike ferromagnets, their distinct crystal symmetries inhibit magnetization while, unlike antiferromagnets, they promote strong spin polarization in the band structure. The corresponding unconventional mechanism of time-reversal symmetry breaking without magnetization in the electronic spectra has been regarded as a primary signature of altermagnetism but has not been experimentally visualized to date. We directly observe strong time-reversal symmetry breaking in the band structure of altermagnetic RuO2 by detecting magnetic circular dichroism in angle-resolved photoemission spectra. Our experimental results, supported by ab initio calculations, establish the microscopic electronic structure basis for a family of interesting phenomena and functionalities in fields ranging from topological matter to spintronics, which are based on the unconventional time-reversal symmetry breaking in altermagnets.

  View details for DOI 10.1126/sciadv.adj4883

  View details for PubMedID 38295181

• Quenched lattice fluctuations in optically driven SrTiO₃ NATURE MATERIALS Fechner, M., Foerst, M., Orenstein, G., Krapivin, V., Disa, A. S., Buzzi, M., von Hoegen, A., de la Pena, G., Nguyen, Q. L., Mankowsky, R., Sander, M., Lemke, H., Deng, Y., Trigo, M., Cavalleri, A. 2024

  Abstract

  Crystal lattice fluctuations, which are known to influence phase transitions of quantum materials in equilibrium, are also expected to determine the dynamics of light-induced phase changes. However, they have only rarely been explored in these dynamical settings. Here we study the time evolution of lattice fluctuations in the quantum paraelectric SrTiO3, in which mid-infrared drives have been shown to induce a metastable ferroelectric state. Crucial in these physics is the competition between polar instabilities and antiferrodistortive rotations, which in equilibrium frustrate the formation of long-range ferroelectricity. We make use of high-intensity mid-infrared optical pulses to resonantly drive the Ti-O-stretching mode at 17 THz, and we measure the resulting change in lattice fluctuations using time-resolved X-ray diffuse scattering at a free-electron laser. After a prompt increase, we observe a long-lived quench in R-point antiferrodistortive lattice fluctuations. Their enhancement and reduction are theoretically explained by considering the fourth-order nonlinear phononic interactions to the driven optical phonon and third-order coupling to lattice strain, respectively. These observations provide a number of testable hypotheses for the physics of light-induced ferroelectricity.

  View details for DOI 10.1038/s41563-023-01791-y

  View details for Web of Science ID 001155814700001

  View details for PubMedID 38302742

• Ultrafast X-Ray Scattering Reveals Composite Amplitude Collective Mode in the Weyl Charge Density Wave Material (TaSe_{4})_{2}I. Physical review letters Nguyen, Q. L., Duncan, R. A., Orenstein, G., Huang, Y., Krapivin, V., de la Peña, G., Ornelas-Skarin, C., Reis, D. A., Abbamonte, P., Bettler, S., Chollet, M., Hoffmann, M. C., Hurley, M., Kim, S., Kirchmann, P. S., Kubota, Y., Mahmood, F., Miller, A., Osaka, T., Qu, K., Sato, T., Shoemaker, D. P., Sirica, N., Song, S., Stanton, J., Teitelbaum, S. W., Tilton, S. E., Togashi, T., Zhu, D., Trigo, M. 2023; 131 (7): 076901

  Abstract

  We report ultrafast x-ray scattering experiments of the quasi-1D charge density wave (CDW) material (TaSe_{4})_{2}I following ultrafast infrared photoexcitation. From the time-dependent diffraction signal at the CDW sidebands we identify a 0.11 THz amplitude mode derived primarily from a transverse acoustic mode of the high-symmetry structure. From our measurements we determine that this mode interacts with the valence charge indirectly through another collective mode, and that the CDW system in (TaSe_{4})_{2}I has a composite nature supporting multiple dynamically active structural degrees of freedom.

  View details for DOI 10.1103/PhysRevLett.131.076901

  View details for PubMedID 37656841

• Circular dichroism in hard X-ray photoelectron diffraction observed by time-of-flight momentum microscopy. Ultramicroscopy Tkach, O., Vo, T., Fedchenko, O., Medjanik, K., Lytvynenko, Y., Babenkov, S., Vasilyev, D., Nguyen, Q. L., Peixoto, T. R., Gloskowskii, A., Schlueter, C., Chernov, S., Hoesch, M., Kutnyakhov, D., Scholz, M., Wenthaus, L., Wind, N., Marotzke, S., Winkelmann, A., Rossnagel, K., Minar, J., Elmers, H., Schonhense, G. 2023; 250: 113750

  Abstract

  X-ray photoelectron diffraction (XPD) is a powerful technique that yields detailed structural information of solids and thin films that complements electronic structure measurements. Among the strongholds of XPD we can identify dopant sites, track structural phase transitions, and perform holographic reconstruction. High-resolution imaging of kll-distributions (momentum microscopy) presents a new approach to core-level photoemission. It yields full-field kx-ky XPD patterns with unprecedented acquisition speed and richness in details. Here, we show that beyond the pure diffraction information, XPD patterns exhibit pronounced circular dichroism in the angular distribution (CDAD) with asymmetries up to 80%, alongside with rapid variations on a small kll-scale (0.1 A-1). Measurements with circularly-polarized hard X-rays (hnu=6keV) for a number of core levels, including Si, Ge, Mo and W, prove that core-level CDAD is a general phenomenon that is independent of atomic number. The fine structure in CDAD is more pronounced compared to the corresponding intensity patterns. Additionally, they obey the same symmetry rules as found for atomic and molecular species, and valence bands. The CD is antisymmetric with respect to the mirror planes of the crystal, whose signatures are sharp zero lines. Calculations using both the Bloch-wave approach and one-step photoemission reveal the origin of the fine structure that represents the signature of Kikuchi diffraction. To disentangle the roles of photoexcitation and diffraction, XPD has been implemented into the Munich SPRKKR package to unify the one-step model of photoemission and multiple scattering theory.

  View details for DOI 10.1016/j.ultramic.2023.113750

  View details for PubMedID 37178606

• Observation of a massive phason in a charge-density-wave insulator. Nature materials Kim, S., Lv, Y., Sun, X., Zhao, C., Bielinski, N., Murzabekova, A., Qu, K., Duncan, R. A., Nguyen, Q. L., Trigo, M., Shoemaker, D. P., Bradlyn, B., Mahmood, F. 2023

  Abstract

  The lowest-lying fundamental excitation of an incommensurate charge-density-wave material is believed to be a massless phason-a collective modulation of the phase of the charge-density-wave order parameter. However, long-range Coulomb interactions should push the phason energy up to the plasma energy of the charge-density-wave condensate, resulting in a massive phason and fully gapped spectrum1. Using time-domain terahertz emission spectroscopy, we investigate this issue in (TaSe4)2I, a quasi-one-dimensional charge-density-wave insulator. On transient photoexcitation at low temperatures, we find the material strikingly emits coherent, narrowband terahertz radiation. The frequency, polarization and temperature dependences of the emitted radiation imply the existence of a phason that acquires mass by coupling to long-range Coulomb interactions. Our observations underscore the role of long-range interactions in determining the nature of collective excitations in materials with modulated charge or spin order.

  View details for DOI 10.1038/s41563-023-01504-5

  View details for PubMedID 36894771

• Direct observation of enhanced electron-phonon coupling in copper nanoparticles in the warm-dense matter regime Direct observation of enhanced electron-phonon coupling in copper nanoparticles in the warm-dense matter regime Nguyen, Q. L. 2023; 8 (131): 085101

  View details for DOI 10.1103/PhysRevLett.131.085101

• Necklace-structured high-harmonic generation for low-divergence, soft x-ray harmonic combs with tunable line spacing. Science advances Rego, L., Brooks, N. J., Nguyen, Q. L., Roman, J. S., Binnie, I., Plaja, L., Kapteyn, H. C., Murnane, M. M., Hernandez-Garcia, C. 2022; 8 (5): eabj7380

  Abstract

  The extreme nonlinear optical process of high-harmonic generation (HHG) makes it possible to map the properties of a laser beam onto a radiating electron wave function and, in turn, onto the emitted x-ray light. Bright HHG beams typically emerge from a longitudinal phased distribution of atomic-scale quantum antennae. Here, we form a transverse necklace-shaped phased array of linearly polarized HHG emitters, where orbital angular momentum conservation allows us to tune the line spacing and divergence properties of extreme ultraviolet and soft x-ray high-harmonic combs. The on-axis HHG emission has extremely low divergence, well below that obtained when using Gaussian driving beams, which further decreases with harmonic order. This work provides a new degree of freedom for the design of harmonic combs-particularly in the soft x-ray regime, where very limited options are available. Such harmonic beams can enable more sensitive probes of the fastest correlated charge and spin dynamics in molecules, nanoparticles, and materials.

  View details for DOI 10.1126/sciadv.abj7380

  View details for PubMedID 35119926

• Bright, single helicity, high harmonics driven by mid-infrared bicircular laser fields OPTICS EXPRESS Dorney, K. M., Fan, T., Nguyen, Q. D., Ellis, J. L., Hickstein, D. D., Brooks, N., Zusin, D., Gentry, C., Hernandez-Garcia, C., Kapteyn, H. C., Murnane, M. M. 2021; 29 (23): 38119-38128

  Abstract

  High-harmonic generation (HHG) is a unique tabletop light source with femtosecond-to-attosecond pulse duration and tailorable polarization and beam shape. Here, we use counter-rotating femtosecond laser pulses of 0.8 µm and 2.0 μm to extend the photon energy range of circularly polarized high-harmonics and also generate single-helicity HHG spectra. By driving HHG in helium, we produce circularly polarized soft x-ray harmonics beyond 170 eV-the highest photon energy of circularly polarized HHG achieved to date. In an Ar medium, dense spectra at photon energies well beyond the Cooper minimum are generated, with regions composed of a single helicity-consistent with the generation of a train of circularly polarized attosecond pulses. Finally, we show theoretically that circularly polarized HHG photon energies can extend beyond the carbon K edge, extending the range of molecular and materials systems that can be accessed using dynamic HHG chiral spectro-microscopies.

  View details for DOI 10.1364/OE.440813

  View details for Web of Science ID 000716468800091

  View details for PubMedID 34808871

• Diol isomer revealed as a source of methyl ketene from propionic acid unimolecular decomposition INTERNATIONAL JOURNAL OF CHEMICAL KINETICS Rogers, C. O., Lockwood, K. S., Nguyen, Q. D., Labbe, N. J. 2021

  View details for DOI 10.1002/kin.21532

  View details for Web of Science ID 000697229600001

• Detection of the keto-enol tautomerization in acetaldehyde, acetone, cyclohexanone, and methyl vinyl ketone with a novel VUV light source PROCEEDINGS OF THE COMBUSTION INSTITUTE Couch, D. E., Nguyen, Q. D., Liu, A., Hickstein, D. D., Kapteyn, H. C., Murnane, M. M., Labbe, N. J. 2021; 38 (1): 1737-1744

  View details for DOI 10.1016/j.proci.2020.06.139

  View details for Web of Science ID 000668887400020

• Highly-excited state properties of cumulenone chlorides in the vacuum-ultraviolet PHYSICAL CHEMISTRY CHEMICAL PHYSICS Nguyen, Q. D., Peters, W. K., Fortenberry, R. C. 2020; 22 (21): 11838-11849

  Abstract

  Recent observations of chloromethane in interstellar environments suggest that other organohalogens, which are known to be critically important in Earth's atmosphere, may also be of significance beyond our own terrestrial veil. This raises the question of how such molecules behave under extreme conditions such as when exposed to vacuum ultraviolet (VUV) radiation. VUV photons promote molecules to highly excited states that fragment in non-statistical patterns controlled by the initial femtosecond dynamics. A detailed understanding of VUV-driven photochemistry in complex organic molecules that consist of more than one functional group is a particularly challenging task. This quantum chemical analysis reports the electronic states and ionization potentials up to the VUV range (6-11 eV) of the chlorine-substituted cumulenone series molecules. The valence and Rydberg properties of lone-pair terminated, π-conjugated systems are explored for their potential resonance with lone pairs from elsewhere in the system. The carbon chain elongation within the family ClHCnO, where n = 1-4, influences the electronic excitations, associated wavefunctions, and ionization potentials of the molecules. The predicted geometries and ionization potentials are in good agreement with the available experimental photoelectron spectra for formyl chloride and chloroketene, n = 1-2. Furthermore, comparison between the regular cumulenone species and the corresponding chlorinated derivatives exhibit similar behaviors especially for n = 3, where the allene backbone in propadienone chloride is severely bent. Most notably for the excited states is that the Rydberg character becomes more dominant as the energy increases, with some retaining valence characters.

  View details for DOI 10.1039/d0cp01835j

  View details for Web of Science ID 000540513500004

  View details for PubMedID 32426777

• Generation of extreme-ultraviolet beams with time-varying orbital angular momentum SCIENCE Rego, L., Dorney, K. M., Brooks, N. J., Nguyen, Q. L., Liao, C., San Roman, J., Couch, D. E., Liu, A., Pisanty, E., Lewenstein, M., Plaja, L., Kapteyn, H. C., Murnane, M. M., Hernandez-Garcia, C. 2019; 364 (6447): 1253-+

  Abstract

  Light fields carrying orbital angular momentum (OAM) provide powerful capabilities for applications in optical communications, microscopy, quantum optics, and microparticle manipulation. We introduce a property of light beams, manifested as a temporal OAM variation along a pulse: the self-torque of light. Although self-torque is found in diverse physical systems (i.e., electrodynamics and general relativity), it was not realized that light could possess such a property. We demonstrate that extreme-ultraviolet self-torqued beams arise in high-harmonic generation driven by time-delayed pulses with different OAM. We monitor the self-torque of extreme-ultraviolet beams through their azimuthal frequency chirp. This class of dynamic-OAM beams provides the ability for controlling magnetic, topological, and quantum excitations and for manipulating molecules and nanostructures on their natural time and length scales.

  View details for DOI 10.1126/science.aaw9486

  View details for Web of Science ID 000473271300039

  View details for PubMedID 31249031

• Controlling the polarization and vortex charge of attosecond high-harmonic beams via simultaneous spin-orbit momentum conservation NATURE PHOTONICS Dorney, K. M., Rego, L., Brooks, N. J., San Roman, J., Liao, C., Ellis, J. L., Zusin, D., Gentry, C., Nguyen, Q. L., Shaw, J. M., Picon, A., Plaja, L., Kapteyn, H. C., Murnane, M. M., Hernandez-Garcia, C. 2019; 13 (2): 123-+

  View details for DOI 10.1038/s41566-018-0304-3

  View details for Web of Science ID 000456652900018

• Strong field ionization with orbital angular momentum light beams measured with three-dimensional velocity imaging technique Quynh Nguyen, Dorney, K., Peters, W., Couch, D., Wooldridge, L., Li, W., Kapteyn, H., Murnane, M. AMER CHEMICAL SOC. 2018

  View details for Web of Science ID 000447609103645

• High harmonics with spatially varying ellipticity OPTICA Ellis, J. L., Dorney, K. M., Hickstein, D. D., Brooks, N. J., Gentry, C., Hernandez-Garcia, C., Zusin, D., Shaw, J. M., Nguyen, Q. L., Mancuso, C. A., Jansen, G., Witte, S., Kapteyn, H. C., Murnane, M. M. 2018; 5 (4): 479-485

  View details for DOI 10.1364/OPTICA.5.000479

  View details for Web of Science ID 000430601100024

• Ultrafast 25-fs relaxation in highly excited states of methyl azide mediated by strong nonadiabatic coupling PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Peters, W. K., Couch, D. E., Mignolet, B., Shi, X., Nguyen, Q. L., Fortenberry, R. C., Schlegel, H., Remacle, F., Kapteyn, H. C., Murnane, M. M., Li, W. 2017; 114 (52): E11072-E11081

  Abstract

  Highly excited electronic states are challenging to explore experimentally and theoretically-due to the large density of states and the fact that small structural changes lead to large changes in electronic character with associated strong nonadiabatic dynamics. They can play a key role in astrophysical and ionospheric chemistry, as well as the detonation chemistry of high-energy density materials. Here, we implement ultrafast vacuum-UV (VUV)-driven electron-ion coincidence imaging spectroscopy to directly probe the reaction pathways of highly excited states of energetic molecules-in this case, methyl azide. Our data, combined with advanced theoretical simulations, show that photoexcitation of methyl azide by a 10-fs UV pulse at 8 eV drives fast structural changes and strong nonadiabatic coupling that leads to relaxation to other excited states on a surprisingly fast timescale of 25 fs. This ultrafast relaxation differs from dynamics occurring on lower excited states, where the timescale required for the wavepacket to reach a region of strong nonadiabatic coupling is typically much longer. Moreover, our theoretical calculations show that ultrafast relaxation of the wavepacket to a lower excited state occurs along one of the conical intersection seams before reaching the minimum energy conical intersection. These findings are important for understanding the unique strongly coupled non-Born-Oppenheimer molecular dynamics of VUV-excited energetic molecules. Although such observations have been predicted for many years, this study represents one of the few where such strongly coupled non-Born-Oppenheimer molecular dynamics of VUV-excited energetic molecules have been conclusively observed directly, making it possible to identify the ultrafast reaction pathways.

  View details for DOI 10.1073/pnas.1712566114

  View details for Web of Science ID 000418722400003

  View details for PubMedID 29109279

  View details for PubMedCentralID PMC5748188

• A new electron-ion coincidence 3D momentum-imaging method and its application in probing strong field dynamics of 2-phenylethyl-N, N-dimethylamine JOURNAL OF CHEMICAL PHYSICS Fan, L., Lee, S., Tu, Y., Mignolet, B., Couch, D., Dorney, K., Quynh Nguyen, Wooldridge, L., Murnane, M., Remacle, F., Schlegel, H., Li, W. 2017; 147 (1): 013920

  Abstract

  We report the development of a new three-dimensional (3D) momentum-imaging setup based on conventional velocity map imaging to achieve the coincidence measurement of photoelectrons and photo-ions. This setup uses only one imaging detector (microchannel plates (MCP)/phosphor screen) but the voltages on electrodes are pulsed to push both electrons and ions toward the same detector. The ion-electron coincidence is achieved using two cameras to capture images of ions and electrons separately. The time-of-flight of ions and electrons are read out from MCP using a digitizer. We demonstrate this new system by studying the dissociative single and double ionization of PENNA (2-phenylethyl-N,N-dimethylamine). We further show that the camera-based 3D imaging system can operate at 10 kHz repetition rate.

  View details for DOI 10.1063/1.4981526

  View details for Web of Science ID 000405089400021

  View details for PubMedID 28688410

• Photophysical properties of pyrrolocytosine, a cytosine fluorescent base analogue PHYSICAL CHEMISTRY CHEMICAL PHYSICS Nguyen, Q. L., Spata, V. A., Matsika, S. 2016; 18 (30): 20189-20198

  Abstract

  The photophysical behavior of pyrrolocytosine (PC), a fluorescent base analogue of cytosine, has been investigated using theoretical approaches. The similarities between the PC and cytosine structures allow PC to maintain the pseudo-Watson-Crick base-pairing arrangement with guanine. Cytosine, similar to the other natural nucleobases, is practically non-fluorescent, because of ultrafast radiationless decay occurring through conical intersections. PC displays a much higher fluorescence quantum yield than cytosine, making it an effective fluorescent marker to study the structure, function, and dynamics of DNA/RNA complexes. Similar to 2-aminopurine, a constitutional isomer of adenine that base-pairs with thymine, PC's fluorescence is quenched when it is incorporated into a dinucleotide or a trinucleotide. In this work we examine the photophysical properties of isolated PC, microhydrated PC, as well as, complexes where PC is either base-stacked or hydrogen-bonded with guanine. Our results indicate that hydration affects the radiationless decay pathways in PC by destabilizing conical intersections. The calculations of dimers and trimers show that the radiative decay is affected by π stacking, while the presence of charge transfer states between PC and guanine may contribute to radiationless decay.

  View details for DOI 10.1039/c6cp01559j

  View details for Web of Science ID 000381428600020

  View details for PubMedID 27251599

  View details for PubMedCentralID PMC5030112

• Fluorescent properties of DNA/RNA base analog pyrrolocytosine Nguyen, Q., Matsika, S. AMER CHEMICAL SOC. 2014

  View details for Web of Science ID 000348457603783

• Exciplexes and conical intersections lead to fluorescence quenching in pi-stacked dimers of 2-aminopurine with natural purine nucleobases PHOTOCHEMICAL & PHOTOBIOLOGICAL SCIENCES Liang, J., Nguyen, Q. L., Matsika, S. 2013; 12 (8): 1387-1400

  Abstract

  Fluorescent analogues of the natural DNA bases are useful in the study of nucleic acids' structure and dynamics. 2-Aminopurine (2AP) is a widely used analogue with environmentally sensitive fluorescence behavior. The quantum yield of 2AP has been found to be significantly decreased when engaged in π-stacking interactions with the native bases. We present a theoretical study on fluorescence quenching mechanisms in dimers of 2AP π-stacked with adenine or guanine as in natural DNA. Relaxation pathways on the potential energy surfaces of the first excited states have been computed and reveal the importance of exciplexes and conical intersections in the fluorescence quenching process.

  View details for DOI 10.1039/c3pp25449f

  View details for Web of Science ID 000322448600013

  View details for PubMedID 23625036

  View details for PubMedCentralID PMC5006741