Seminar "Ultrafast phonon and spin dynamics in chalcogenide materials" by Dr. Richarj Mondal, Tsukuba Univ.

Speaker: Dr. Richarj Mondal, Division of Applied Physics, University of Tsukuba

Title : Ultrafast phonon and spin dynamics in chalcogenide materials

Abstract: Topological insulators (TIs) (Bi₂Se₃, Bi₂Te₃ and Sb₂Te₃) have a great interest in materials science for application in spintronic and optoelectronic devices, since spin is locked orthogonally to momentum [1]. While, chalcogenide superlattice (CSL), GeTe/Sb₂Te₃, which is formed with an alternate stacking of the ferroelectric normal insulator (NI) GeTe with the TI Sb₂Te₃, is a promising candidate for the next generation non-volatile electronic memory. This newly formed CSL, also referred to as interfacial phase change memory (iPCM) [2], is crystalline in both the RESET (low-conductivity) and SET (high conductivity) phases.  Theoretical calculations based on ab initio, it has been predicted that GeTe/Sb₂Te₃ CSLs becomes TIs, Dirac semimetals, or NIs depending upon the thickness of the individual blocks of GeTe and Sb₂Te₃ [3].

In first part of my talk, I will discuss the coherent phonon dynamics in TI Sb₂Te₃ for several quintuple layers thick. The transient coherent phonon spectra at different time delay exhibit a Fano-like asymmetric line shape of the A²_1g phonon mode, which is attributed to quantum interference between continuum like coherent Dirac plasmons and phonons. The inverse of the Fano asymmetry parameter decays within 1 ps, and its time dependence can be well fit to an electron temperature calculated by the two-temperature model (TTM), whereas the TTM cannot well explain the Gaussian decay profile observed up to ≈200 fs after photoexcitation. The observed Gaussian decay is a consequence of the coherence time of the Dirac plasmon (continuum state), which couples with a discrete phonon state [4].

In second part of my talk, I will discuss the time resolve Kerr measurement, which was used to discern the topological nature in chalcogenide materials [5, 6]. We have investigated ultrafast time-resolved magneto-optical Kerr spectroscopy in TIs and CSLs. We unraveled the topological states of TIs and CSLs using helicity dependent magneto-optical Kerr effect. The amplitude of the helicity dependent Kerr signal shows a four cyclic oscillation with π/2 periodicity, which is interpreted based on the cascading nonlinear magneto-optical effect induced via strong spin-orbit coupling, suggesting the existence of a Dirac like cone in TI and CSLs.

References:

1. H. Zhang et al., Nature Physics, Vol. 5, 438 (2009).
2. R. E. Simpson et al., Nature Nanotech. Vol. 6, 501 (2011).
3. J. Tominaga, A. V. Kolobov, P. Fons, T. Nakano, and S. Murakami, Adv. Mater. Interfaces, Vol. 1, 1300027 (2014).
4. R. Mondal, A. Arai, Y. Saito, P. Fons, A. V. Kolobov, J. Tominaga, and M. Hase, Phys. Rev. B, Vol. 97, 144306 (2018)
5. R. Mondal, Y. Saito, Y. Aihara, P. Fons, A. V. Kolobov, J. Tominaga, S. Murakami, and M. Hase, Sci. Rep. Vol. 8, 3908 (2018).
6. R. Mondal, Y. Aihara , Y. Saito, P. Fons, A. V. Kolobov, J. Tominaga, and M. Hase, ACS Appl. Mater. Interfaces, Vol. 10, 26781 (2018).