arXiv subjects

Vikas Arora

Publications and source records attributed to Vikas Arora.

4 recordsLinked to original sources

arXiv2024

Ultrafast Dynamics of Coherent Phonon Modes in Excitonic Insulator Ta$_2$NiSe$_5$

The spontaneous condensation of excitons in the excitonic insulating phase has been reported in Ta$_2$NiSe$_5$ below 325 K. In this context, we present the temperature-dependent optical pump optical probe spectroscopy of Ta$_2$NiSe$_5$, with a focus on coherent phonon dynamics. In addition to the fast relaxation process involving excitonic recombination, we observe a systematic behavior for the slow relaxation process associated with the relaxation of hot phonons. The asymmetry parameter and cubic anharmonicity of the 3 THz mode demonstrate the structural transition across T$_C$=325 K, whereas the order parameter nature and asymmetry of 2 THz modes reveal its coupling with the excitonic phase of Ta$_2$NiSe$_5$. Coherent phonon modes display less anharmonicity compared to the corresponding Raman modes. Continuous Wavelet Transform (CWT) reveals that the peak time t$_{peak}$ of phonons is similar for all modes except the 3 THz mode. The temperature dependence of t$_{peak}$ for the M3 mode exhibits a possible role of excitonic condensate below T$_c$ in the formation of quasiparticle (phonon). CWT analysis supports the time-dependent asymmetry of the M3 mode caused by photoexcited carriers. This study illustrates the role of photoexcited carriers in depicting a structural transition and dressing of coherent phonons and, hence, demonstrating many-body effects.

cond-mat.mtrl-sci↗

arXiv2024

Pressure Dependence of Ultrafast Carrier Dynamics in Excitonic Insulator Ta$_2$NiSe$_5$

An excitonic insulator (EI) phase is a consequence of collective many-body effects where an optical band gap is formed by the condensation of electron-hole pairs or excitons. We report pressure-dependent optical pump optical probe spectroscopy of EI Ta$_2$NiSe$_5$ in an on-site in situ geometry. The fast relaxation process depicts the transition across P$_{C_1}$ $\sim$1 GPa from EI phase to a semiconductor and P$_{C_2}$ $\sim$3 GPa from a semiconductor to a semimetallic phase. The instability of the EI phase beyond P$_{C_1}$ is captured by the Rothwarf-Taylor model by incorporating the decrease of the bandgap under pressure. The pressure coefficient of the bandgap decreases, 65 meV/GPa closely agrees with the first principle calculations.

cond-mat.mtrl-sci↗

arXiv2024

Twist Angle Dependent Ultrafast Transient Dynamics of MoSe$_2$/WSe$_2$ van der Waals Heterostructures beyond the Exciton Mott Transition

Two-dimensional van der Waals heterostructures (HS) exhibit twist-angle ($\theta$) dependent interlayer charge transfer, driven by moir\'e potential that tunes the electronic band structure with varying $\theta$. Apart from the magic angles of $\sim$3$^\circ$ and $\sim$57.5$^\circ$ that show flat valence bands (twisted WSe$_2$ bilayer), the commensurate angles of 21.8$^\circ$ and 38.2$^\circ$ reveal the Umklapp light coupling of interlayer excitons. We report our results on non-degenerate optical pump-optical probe spectroscopy of MoSe$_2$/WSe$_2$ HS at large twist angles under high photoexcitation densities above the Mott transition threshold, generating interlayer localized charge carriers. We show that the recombination time of electrons and holes is minimum at the commensurate angles. The strength of non-radiative interlayer Auger recombination also shows a minimum at the commensurate angles. The fluence dependence of interlayer carrier recombination time suggests additional relaxation channels near the commensurate angles. This study emphasizes the significance of the large twist angle of HS in developing transition metal dichalcogenides-based optoelectronic devices.

cond-mat.mes-hall↗

arXiv2024

Ultrafast Spectroscopy of Dirac Semimetal Cd3As2 under Pressure

Topological properties of a three-dimensional Dirac semimetal Cd3As2, protected by crystal rotation and time-reversal symmetry, can be tuned with the application of pressure. Ultrafast spectroscopy is a unique tool to investigate the character and time evolution of electronic states, emphasizing the signatures of transition. We designed an experimental setup for in-situ pressure-dependent ultrafast optical pump optical probe spectroscopy of Cd3As2 using a symmetric diamond anvil cell. The fast relaxation processes show significant changes across pressure-induced phase transitions at PC1, approximately 3 GPa, and PC2, approximately 9 GPa. A new sub-picosecond time scale relaxation dynamics emerges beyond PC2. Theoretical calculations of differential reflectivity for both interband and intraband processes indicate that the negative (positive) differential reflectivity (Delta R/R) results from the interband (intraband) processes. The pressure-dependent behavior of relaxation dynamics amplitudes beyond PC1 emphasized the necessity of incorporating quadratic band opening in the calculations, explaining the transition of Cd3As2 from a Dirac semimetal to a semiconducting phase. The time evolution of differential reflectivity is calculated using the electronic temperature as a function of time, as provided by the two-temperature model, which fits the experimental data.

cond-mat.mtrl-sci↗