arXiv subjects

A. H. Zewail

Publications and source records attributed to A. H. Zewail.

4 recordsLinked to original sources

arXiv2020

Observation of a phonon avalanche in highly photoexcited hybrid perovskite single crystals

In hybrid lead halide perovskites, the coupling between photogenerated charges and the ionic degrees of freedom plays a crucial role in defining the intrinsic limit of carrier mobility and lifetime. However, direct investigation of this fundamental interaction remains challenging because its relevant dynamics occur on ultrashort spatial and ultrafast temporal scales. Here, we unveil the coupled electron-lattice dynamics of a CH3NH3PbI3 single crystal upon intense photoexcitation through a unique combination of ultrafast electron diffraction, time-resolved photoelectron spectroscopy, and time-dependent ab initio calculations. We observe the structural signature of a hot-phonon bottleneck effect that prevents rapid carrier relaxation, and we uncover a phonon avalanche mechanism responsible for breaking the bottleneck. The avalanche involves a collective emission of low-energy phonons - mainly associated with the organic sub-lattice - that proceeds in a regenerative manner and correlates with the accumulation and confinement of photocarriers at the crystal surface. Our results indicate that in hybrid perovskites carrier transport and spatial confinement are key to controlling the electron-phonon interaction and their rational engineering is relevant for future applications in optoelectronic devices.

cond-mat.mtrl-sci↗

arXiv2017

High-temporal-resolution electron microscopy for imaging ultrafast electron dynamics

Ultrafast Electron Microscopy (UEM) has been demonstrated to be an effective table-top technique for imaging the temporally-evolving dynamics of matter with subparticle spatial resolution on the time scale of atomic motion. However, imaging the faster motion of electron dynamics in real time has remained beyond reach. Here, we demonstrate more than an order of magnitude (16 times) enhancement in the typical temporal resolution of UEM by generating isolated 30 fs electron pulses, accelerated at 200 keV, via the optical-gating approach, with sufficient intensity for efficiently probing the electronic dynamics of matter. Moreover, we investigate the feasibility of attosecond optical gating to generate isolated subfemtosecond electron pulses, attaining the desired temporal resolution in electron microscopy for establishing the Attomicroscopy to allow the imaging of electron motion in the act.

physics.optics↗

arXiv2010

Real time observation of cuprates structural dynamics by Ultrafast Electron Crystallography

The phonon-mediated attractive interaction between carriers leads to the Cooper pair formation in conventional superconductors. Despite decades of research, the glue holding Cooper pairs in high-temperature superconducting cuprates is still controversial, and the same is true as for the relative involvement of structural and electronic degrees of freedom. Ultrafast electron crystallography (UEC) offers, through observation of spatio-temporally resolved diffraction, the means for determining structural dynamics and the possible role of electron-lattice interaction. A polarized femtosecond (fs) laser pulse excites the charge carriers, which relax through electron-electron and electron-phonon coupling, and the consequential structural distortion is followed diffracting fs electron pulses. In this review, the recent findings obtained on cuprates are summarized. In particular, we discuss the strength and symmetry of the directional electron-phonon coupling in Bi2Sr2CaCu2O8+δ(BSCCO), as well as the c-axis structural instability induced by near-infrared pulses in La2CuO4 (LCO). The theoretical implications of these results are discussed with focus on the possibility of charge stripes being significant in accounting for the polarization anisotropy of BSCCO, and cohesion energy (Madelung) calculations being descriptive of the c-axis instability in LCO.

cond-mat.supr-con↗

arXiv2009

Direct Role of Structural Dynamics in Electron-Lattice Coupling of Superconducting Cuprates

The mechanism of electron pairing in high-temperature superconductors is still the subject of intense debate. Here, we provide direct evidence of the role of structural dynamics, with selective atomic motions (buckling of copper-oxygen planes), in the anisotropic electronlattice coupling. The transient structures were determined using time-resolved electron diffraction, following carrier excitation with polarized femtosecond heating pulses, and examined for different dopings and temperatures. The deformation amplitude reaches 0.5 % of the c-axis value of 30 A when the light polarization is in the direction of the copper-oxygen bond, but its decay slows down at 45 degrees. These findings suggest a selective dynamical lattice involvement with the anisotropic electron-phonon coupling being on a time scale (1 to 3.5 ps depending on direction) of the same order of magnitude as that of the spin exchange of electron pairing in the high-temperature superconducting phase.

cond-mat.supr-con↗