arXiv subjects

Omar Abdul-Aziz

Publications and source records attributed to Omar Abdul-Aziz.

5 recordsLinked to original sources

arXiv2025

Tracking the Catastrophic Collapse of Hybrid Exciton-Phonon Order in a Quantum Material

Revealing the interactions binding electronic and lattice components of cooperative quantum order is central to sculpting new states of matter. This challenge is epitomized by the charge density wave material 1T-TiSe$_2$, where photoexcitation disrupts its presumed hybrid exciton-phonon order. This exposes a paradox: the electronic component collapses within femtoseconds while the periodic lattice distortion persists. If the lattice distortion outlives the excitonic condensate, were they truly intertwined? Here we resolve this by uncovering a low-frequency mode (approx. 0.13 THz) emerging only in the ordered state, signaling exciton-phonon coupling. This mode is consistent with a locked phason -- a collective excitation arising if coupling between the excitonic condensate and lattice reduces continuous phase symmetry to a discrete one, giving the excitonic Goldstone mode finite mass. This is captured by an effective theory describing a shared potential landscape. At a critical threshold, the collapse of excitonic order flattens the potential, triggering an exciton-phonon catastrophe: selective overheating of the charge density wave phonon, disappearance of the locked phason, and sudden loss of electronic coherence. Remarkably, the lattice distortion survives as a dynamically trapped, non-thermal remnant, confirmed by the anomalous temperature dependence of the phononic response. These findings demonstrate that coupled potential energy landscapes can be manipulated to selectively dismantle complex quantum orders, advancing material control through dynamical design.

physics.app-ph↗

arXiv2025

Photoinduced Electronic Band Dynamics and Defect-mediated Surface Potential Evolution in PdSe$_2$

We use time- and angle-resolved photoemission spectroscopy (TR-ARPES) combined with density functional theory to investigate ultrafast carrier dynamics in low-symmetry layered semiconducting PdSe$_2$. The indirect bandgap is determined to be 0.55~eV. Following photoexcitation above this gap, we resolve a valence band shift and broadening, both lasting less than a picosecond, consistent with bandgap renormalization and carrier scattering, indicative of strong many-body interactions. Subsequently, hot carriers populate the conduction band minimum and are captured by defect states. A surface photovoltage (SPV) of $\sim$ 67~meV emerges, persisting for over 50~ps, driven by defect-assisted charge separation. The formation of native vacancies, promoted by the low-symmetry lattice, likely gives rise to the mid-gap states responsible for this long-lived SPV response. Detailed analysis of TR-ARPES spectra disentangles the contributions of bandgap renormalization, carrier scattering, defect states, and SPV. These findings establish PdSe$_2$ as a prototypical layered quantum material exhibiting exotic photoresponses on ultrafast timescales.

cond-mat.mes-hall↗

arXiv2024

Unveiling Ultrafast Spin-Valley Dynamics and Phonon-Mediated Charge Transfer in MoSe$_{2}$/WSe$_{2}$ Heterostructures

We use helicity-resolved ultrafast transient absorption spectroscopy to study spin-valley polarization dynamics in a vertically stacked MoSe$_{2}$/WSe$_{2}$ heterostructure. The experimental findings reveal details of interlayer charge transfer on ultrafast timescales, showing that the spin-valley polarized state of photoexcited carriers is conserved during the charge transfer and formation of interlayer excitons. Our results confirm that phonon scattering mediates the interlayer charge transfer process, while a high phonon population at elevated temperatures causes a significant decrease in spin-valley selective charge transfer. Moreover, the experimental findings demonstrate the possibility that interlayer excitons and their spin-valley polarization can be probed in the optical response of intralayer excitons. These findings pave the way for ultrafast detection, control, and manipulation of spin-valley polarized excitons in transition metal dichalcogenide-based 2D heterostructures.

physics.app-ph↗

arXiv2024

Resonance-Induced Anomalies in Temperature-Dependent Raman Scattering of PdSe$_{2}$

We report a comprehensive Raman study of the phonon behaviour in PdSe$_2$ in the temperature range of 5 K to 300 K. A remarkable change in the Raman spectrum is observed at 120 K: a significant enhancement of the out-of-plane phonon A$^{1}_{g}$ mode, accompanied by a suppression of the in-plane A$^{2}_{g}$ and B$^{2}_{1g}$ modes. This intriguing behavior is attributed to a temperature-dependent resonant excitation effect. The results are supported by density functional theory (DFT) calculations, which demonstrate that the electron-phonon coupling for the phonon modes varies and is strongly associated with the relevant electronic states. Furthermore, nonlinear frequency shifts are identified in all modes, indicating the decay of an optical phonon into multiple acoustic phonons. The study of Raman emission reported here, complemented by linear optical spectroscopy, reveals an unexpected scenario for the vibrational properties of PdSe$_2$ that holds substantial promise for future applications in PdSe$_2$-based optoelectronics.

cond-mat.str-el↗

arXiv2024

Ultrafast Optical Control of Exciton Diffusion in WSe$_2$/Graphene Heterostructures Revealed by Heterodyne Transient Grating Spectroscopy

Using heterodyne transient grating spectroscopy, we observe a significant enhancement of exciton diffusion within a monolayer WSe$_2$ stacked on top of graphene. We further demonstrate that the diffusion dynamics can be optically tuned on the ultrafast time scale (i.e. a few picoseconds) by altering the photoexcited charge carrier density in graphene. The results reveal that, on a time scale of a few picoseconds, the effective diffusion constant in the WSe$_2$/graphene heterostructure is approximately 40 cm$^2 $/s, representing a substantial improvement over the 2 cm$^2 $/s typical for an isolated monolayer of WSe$_2$. The enhanced diffusion can be understood in terms of a transient screening of impurities, charge traps, and defect states in WSe$_2$ by photoexcited charge carriers in graphene. Furthermore, we observe that the diffusion within WSe$_2$ is affected by interlayer interactions, like charge transfer, exhibiting different dynamic states depending on the incident excitation fluence. These findings underscore the dynamic nature of screening and diffusion processes in heterostructures of 2D semiconductors and graphene and provide valuable insights for future applications of these systems in ultrafast optoelectronic devices.

cond-mat.mes-hall↗