Bulletin of the American Physical Society
APS March Meeting 2023
Volume 68, Number 3
Las Vegas, Nevada (March 5-10)
Virtual (March 20-22); Time Zone: Pacific Time
Session B25: Ultrafast Dynamics in Cooperative SystemsInvited
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Sponsoring Units: DLS Chair: Susan Dexheimer, Washington State University Room: Room 217/218 |
Monday, March 6, 2023 11:30AM - 12:06PM |
B25.00001: Nonequilibrium quantum phenomena: A gilded age of driving fields and monitoring capabilities Invited Speaker: Keith A Nelson Light at THz frequencies is extremely well matched to the excitations and dynamics characteristic of many cooperative systems. Strong THz fields can be used for control as well as nonlinear spectroscopy of the collective responses of primary interest in quantum materials. Recent results involving collective electronic, vibrational, and spin degrees of freedom will be presented. A THz-induced transition into a topological insulator phase of the transition metal dichalcogenide MoTe2 will be discussed. A single THz pulse drives the phase transition electronically, after which the new phase persists indefinitely [1]. A persistent optically-induced change in TaS2 will also be discussed briefly. The transition is monitored on a single-shot basis with both optical and THz probe light [2]. Strong THz fields drive a “soft” lattice vibrational mode to induce a transition from the quantum paraelectric phase of SrTiO3 to a transient ferroelectric phase [3]. Recent ultrafast x-ray diffraction measurements provide information beyond what could be determined through optical probes. Finally, THz-induced nonlinear responses of collective spin waves (magnons) have revealed coupling that is inherent in canted antiferromagnetic materials [4]. Two-dimensional THz spectroscopy, conducted using single-shot readout of the time-dependent signal field, reveals the coupled spin responses. |
Monday, March 6, 2023 12:06PM - 12:42PM |
B25.00002: Mapping Atomic Motions with Ultrabright Electrons: Fundamental Space-Time Limits to Imaging Molecular Dynamics Invited Speaker: R J Dwayne Miller One of the long sought objectives in science has been to watch atomic motions on the primary timescales governing structural transitions. From a chemistry perspective, this capability would give a direct observation of reaction forces and probe the central unifying concept of transition states that links chemistry to biology. To achieve this objective, there are not only extraordinary requirements for simultaneous spatial-temporal resolution but equally important, due to sample limitations, also one on source brightness. With the development of ultrabright electrons capable of literally lighting up atomic motions, this experiment has been realized (Siwick et al Science 2003) and efforts accelerated with the onset of XFELs (Miller, Science 2014). A number of different chemical reactions will be discussed from electrocyclization with conserved stereochemistry, intermolecular electron transfer for organic systems, metal to metal electron transfer, to the direct observation of a bimolecular collision and bond formation in condensed phase for the classic I3- system, in a process analogous to a molecular Newton's cradle. These studies have discovered that these high dimensional problems, order 3N, distilled down to atomic projections along a few principle reaction coordinates. The specific details depend on the spatial resolution to these motions, for which <.01 Å changes in atomic position (less than the background thermal motion) has now been achieved on the 100 fs timescale. Without any detailed analysis, the key large-amplitude modes can be identified by eye from the molecular movies. This reduction in dimensionality appears to be general, arising from the very strong anharmonicity of the many body potential in the barrier crossing region. We now are beginning to see the underlying physics for the generalized reaction mechanisms that have been empirically discovered over time. The "magic of chemistry" is this enormous reduction in dimensionality in the barrier crossing region that ultimately makes chemical concepts transferrable. How far can this reductionist view be extended with respect to complexity? The spatial-temporal correlations discoverd in this work provide new insight into how chemistry scaled in complexity up to biological systems – leading to living systems. |
Monday, March 6, 2023 12:42PM - 1:18PM |
B25.00003: Nonequilibrium lattice dynamics measurements with ultrafast x-ray pulses Invited Speaker: David A Reis There is growing interest in the use of ultrafast light pulses to drive nonequilibrium states of materials with novel properties. Our understanding of how to generate and control these states requires time-resolved atomic-scale probes of structure and dynamics. In this talk, we report results of femtosecond x-ray free-electron laser-based scattering experiments which help elucidate the excited-state dynamics of photo-excited materials. In the case of SnSe, we find that above-gap photoexcitation drives the material towards a higher-symmetry structure that does not exist thermal equilibrium. First principle calculations help us identify how photoexcitation from localized valence bands drives changes in the orbital hybridization leading to this instability. From the measured excited-state phonon dispersion we further identify changes in the interlayer bonding that are responsible for driving this novel instability, consistent with the changes in the orbital character of the bands. Our work suggest the importance of pump-wavelength for control of structural distortions through orbitally-selective above-gap excitation. |
Monday, March 6, 2023 1:18PM - 1:54PM |
B25.00004: From Excitons to topological excitonic states and their fingerprints on the electronic bandstructure Invited Speaker: Alessandra Lanzara Coulomb-bound electrons and holes forming excitonic states have attracted significant interest given their critical roles in both fundamental science and applications. Whether these excitonic state can be driven in the presence of topological invariants, what properties of the topological state persists and what are their fingerprints in the material’s band structure are all open questions. Here I will discuss some recent work where, by using the ultrafast angle resolved photoemission spectroscopy we study and reveal under which conditions excitonic states can be driven in a topological insulators. The differences with excitonic states in bulk semiconnductors and their fingerprints on the electronic structure are discussed. |
Monday, March 6, 2023 1:54PM - 2:30PM |
B25.00005: Insignts into non-equilibrium spectroscopy from the theoretical perspective Invited Speaker: Alexander F Kemper Time-resolved angle-resolved photoemission spectroscopy is one of the most powerful pump–probe measurements of materials driven far from equilibrium. Unlike the linear-response regime, where the frequency-dependent response function is independent of time, in a far-from-equilibrium experiment, responses functions depend on two times in the time domain. We describe how one can transition from the two independent times to time-dependent frequency response functions and how they involve contributions from times that are near to each other. However, they should not be thought of as a frequency-dependent response at a single definite time. Instead, the Fourier uncertainty relations show that they involve contributions from ranges of times and must be interpreted in this light. |
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