Magnetic exchange field signature in ultrafast electron dynamics in monolayer WSe2 by ultrashort optical pulse

Abstract

We theoretically investigate the ultrafast electron dynamics in monolayer tungsten diselenide interacting with an irradiated ultrashort strong optical pulse in the presence of a magnetic exchange field. The dynamics of Dirac fermions in monolayer WSe2 is affected by spin and valley splitting, of course, enhanced by applied magnetic exchange field. There exists remarkable interaction between the exchange field and valley index because of huge spin–orbit coupling at the valence band. Regarding the optical pulse duration is smaller than the electron scattering time, the electron dynamics remains coherent and is described by the time-dependent Schrodinger equation. The strong electric field of the pulse, interacted with monolayer WSe2 causes appearance of a dipole moment connecting two valence and conduction bands. Because of the K and K′ valleys behave differently by the magnetic exchange field, the perfect valley-spin polarization effect is achieved. We explore the irreversible conduction band electron transition affected by a magnetic exchange field. The signature of the exchange field on such transitions is quite obvious. The conduction band electron redistribution around the Dirac points is represented, when the pulse ends. Asymmetric hot spots pattern in two different K and K′ valleys depends more or less on the magnitude of the magnetic exchange field. By reversing the direction of the exchange field, the process of spin polarization is completely reversed and it results in the electron distribution pattern. The high spin-valley resolved splitting in monolayer WSe2 provides a unique platform to investigate the magnetic exchange field driven phenomena, leading to valley polarization effect.