Electron transport under an ultrafast laser pulse: Implication for spin transport*

In the electric-field induced transport, a voltage bias is applied

across a device. The chemical potential difference between the drain

and source drives the charge carrier across the device.  So the moving

direction of charge carriers is along the electric field. But in

laser-induced electron transport, both the electric and magnetic

fields are transversal to the ligt propagation, so how the electrons

move along the longitudinal direction is not obvious, but is very

critical to our current understanding of spin transport. In this talk,

we show that a general mechanism is working. Although the magnetic

field B is weak, it is this field that steers the electron

moving along the light propagation direction, besides its strong

transverse motion.  We employ the formalism put forth by Varga and

Toroke and confirm that if we only include E, the electron

only moves transversely with a large velocity, but once including both

B and  E, and using real experimental laser parameters, we

are able to demonstrate that a laser pulse can drive the electron

along the axial direction by 40 to 400 Å, consistent with the

experiments.  The key insight is that  B changes the direction of

the electron and allows the electron to move along the Poynting vector

of light. Our finding has an important consequence.  Because a nonzero

B means a spatially dependent vector potential A(r, t), B =▽×A(r, t), this points out that

the Coulomb gauge, that is, replacing  A(r, t) by a spatially independent A(r) is unable to describe electron and spin

transport under laser excitation.  Our finding is expected to have a

potential impact the investigation of laser-driven spin transport.