Structural Phase Transition and Dynamic Processes in Metastable Cu2S
This report delves into probing structural phase transitions and dynamic processes in metastable Cu2S, conducted by Junjie Li, Jun Li, and Jing Tao from the Condensed Matter Physics & Materials Science Division at Brookhaven National Laboratory. The research aims to shed light on the unique properties of Cu2S and its behavior during phase transitions, offering valuable insights into its dynamic behavior.
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UE80 experiment report Probing Structural Phase Transition and Separating Dynamic Processes in Metastable Cu2S Junjie Li, Jun Li, Jing Tao Condensed Matter Physics & Materials Science Division Brookhaven National Laboratory Research sponsored by the U. S. DOE, BES, Division of Materials Sciences and Engineering and Early Career Award
Camera speed: 1000 frame/s = 10-3 s Ultrafast: 100 fs = 10-13 s Optical pump electrons Pump Detector ~1ps ~10fs Sample e e e - spins - phonons - e e - e - ~100ps - e - Electron probe Probe Time Delay Pump: initiate the process and set the time zero Probe: take snapshot at different delay times Delay time: optical length difference, 1um ~3.3fs
Laser Pulse Variable Time Delay Polarized Laser Pulse Electron Pulse Crystal Specimen CCD Camera BNL-type RF e-Gun SPECS Temporal resolution ~ 200 fs 106 electrons per pulse Acceleration voltage = 2.8 MeV
Motivation Cu2S single-crystals demonstrate a surprisingly high susceptibility of their structures and properties to external perturbations such as electron and photon irradiation. T = 20 C T = 90 C L-phase, low symmetry H-phase, high symmetry H-phase L-phase J. Tao et al., PNAS 114, 9832 (2017) UED results would be critical to reveal the structural dynamics and understand the nature of the structural phase transition in Cu2S.
Preliminary UED results Sample: Cu2S/SiN 100 m 1 m (110) (120) (200) (332)(420)
UED pump-probe patterns Two characteristics were measured: After laser pump 1. Peak Center position 2. Peak width 6 ps 2.5 ps (120) reflection profile Before laser pump p
Here structural phase transition is characterized by two measurements: UED pump-probe patterns After laser pump 1. Peak Center position ---- lattice constants 2. Peak width ---- lattice symmetry (equivalent wave numbers at high-symmetry phase result in sharp reflection rings) 6 ps 2.5 ps Before laser pump p Symmetry breaking and lattice expansion take place with distinct dynamics during the transition in Cu2S crystals.
Unveiling the pathway of structural phase transition Phase 1 Phase 2 Ongoing work Phase 2 Energy associated with the structures at the locations in the phase diagram needs to be calculated to confirm the energetical path way of the transition revealed by UED in Cu2S. Lattice Constant Pathway from phase 1 to phase 2? Pathway from phase 1 to phase 2? Energy Scale Phase 1 Or Symmetry Phase 1 Phase 2 Identifying the role of specific phonon modes in structural phase transition Ongoing work 1. Calculation of phonon modes in Cu2S using frozen-phonon method and perturbation theory. Heisenberg uncertainty principle: ? ? 2 It takes different time for energy distributing to the phonon modes at different energy levels. Optical phonons seem to be responsible for the symmetry change while acoustic phonons are related to lattice expansion in the transition. 2. Correlation between the calculation and experimental UED results.
Junjie Li Jun Li Acknowledgement Brookhaven National Laboratory (CMPMS & CFN) J. Li, J. Li, L. Wu, Y. Zhu M. Liu, D. Lu, M. Lu, Y. Cao Brookhaven National Laboratory (ATF) M. Palmer, M. Fedurin, M. Babzien, C. Swinson, K. Kusche Princeton University R. J. Cava University of Arkansas L. Mathurin, J-Y. Chen American Physical Society Y. Li
