Vikas Arora

working with
Prof. Ajay K Sood
and Dr Victor S Muthu

Temperature and Pressure tuning of carrier relaxation dynamics
in Excitonic Insulator Ta2 NiSe5


The development in short duration processes capturing techniques has lead us to understand the carriers relaxation dynamics of materials. Ta2 NiSe5 (TNSE) is a narrow bandgap material, where binding energy of excitons overcomes the bandgap, and it behaves as Excitonic Insulator (EI) in the ground state under ambient conditions. We present the optical pump optical probe studies on TNSE with varying Temperature and Pressure. We measure the ultrafast recombination of photoexcited quasiparticles. Temperature studies: The relaxation dynamics support the evidence of material falling into BEC-BCS crossover regime. We witness a significant change in relaxation time behavior across T ̃ 320 K manifesting the transition from EI to semiconducting state. The coherent optical phonons, observed in photoexcited carrier relaxation dynamics, reveals a structure phase transition at T∼320 K by disappearance of B 1g Raman mode at 122 cm−1 . This transition is supported by Raman studies on TNSE from our group. Using Wavelet transform, we quantify the evolution of coherent phonon modes with delay time as a function of temperature and fluence. Pressure studies: The relaxation time of these quasiparticles reveals a linear decrease with increase in pressure. It reveals three different slopes for pressures 0 < P < 1.3 GPa and 1.3 < P < 3.2GPa and P>3.2GPa. This corresponds to two transitions P1 and P2 at P=1.3 GPa and P=3.2 GPa respectively. The P1 transition is a transformation of TNSE from EI to semiconductor and the P2 transition is another transformation of TNSE from semiconductor to semimetallic phase, similar to x-ray diffraction studies [1]. Pressure tunes the bandgap of TNSE and reveals its different phases.



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