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Student Eklund Award Lectures

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Yu Pan, and Lucas F. Hackl; The Pennsylvania State University
02 February 2017 from 4:00 PM to 5:00 PM
117 Osmond Laboratory
Contact Name
Jun Zhu
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Yu Pan, Advisor: Nitin Samarth

Origin of the helicity dependent photocurrent in 3D topological insulators

Narrow band gap semiconductors such as the Bi-chalcogenides have attracted much contemporary interest as three-dimensional (3D) "topological insulators" (TIs) that host gapless spin- textured surface states which reside within the bulk band gap. The spin-momentum locking in these helical Dirac states provides a unique opportunity for “topological spintronics" devices that function at technologically relevant temperatures (300 K and above). Optical methods have also been adopted to control electron spin and charge currents in 3D TIs. In particular, experiments have shown that circularly polarized light induces a directional helicity-dependent photocurrent (HDPC) in 3D TIs. Surprisingly, the phenomenon is readily observed at photon energies that excite electrons to states far above the spin-momentum locked Dirac cone and the underlying mechanism for the helicity- dependent photocurrent is still not understood. We resolve the puzzle through a comprehensive study of the helicity-dependent photocurrent in (Bi1−xSbx)2Te3 thin films as a function of the incidence angle of the optical excitation, its wavelength and the gate-tuned chemical potential. Our observations allow us to unambiguously identify the circular photo-galvanic effect as the dominant mechanism for the helicity-dependent photocurrent. Additionally, we use a first principles calculation to relate the directional nature of the photocurrent to asymmetric optical transitions between the topological surface states and bulk bands. The insights we obtain are important for engineering opto-spintronic devices that rely on optical steering of spin and charge currents.


Lucas F. Hackl, Advisor: Eugenio Bianchi

Entanglement production through parametric resonance

Entanglement plays a crucial role for studying quantum correlations in physical systems. In order to predict the time evolution of correlations, it is important to identify the mechanism that produces entanglement. In my talk, I will explain how parametric resonance leads to entanglement production with linear growth of the entanglement entropy. I will then discuss the relevance of this result for reheating in cosmology and periodically driven Bose condensates.