Welcome

Our research is focused on the understanding of complex materials both in and out of equilibrium.

Non-equilibrium Physics Explained: 5 Questions with Lex Kemper

Our research was featured in the NC State news! Untangling Complex Interactions – With Lasers

Prof. Kemper's group studies the emergent properties of complex materials such as superconductivity and topological phases using theoretical and computational approaches. They are particularly interested in how these materials behave when driven out of equilibrium by strong ultrafast laser pulses, either in the optical/IR or X-ray regime. In the same sense that you have to hit a tuning fork to hear its sound, hitting materials with light pulses allows us to examine the properties of the electrons and atoms that make up the materials. We build up a stroboscopic movie of the material constituents, and by analyzing the movie on femtosecond time scales we can glean information about the interactions that lead to the emergent phenomena.

 

Turning this idea around, we are aiming at controlling materials using the same laser pulses. By picking specific polarizations, we can drive the material from one phase into another, creating the possibility of making ultrafast switches or ultrafast control of material topology, to design them to suit our purposes.

 

Non-equilibrium spectroscopy
arpes_hight_strongUsing numerical simulations, we are developing a fundamental understanding of pump-probe spectroscopy and how it can be used to understand the underlying physics of emergent phenomena in complex quantum materials. The figure shows a system of electrons and phonons under the influence of a very strong field -- one can observe the breakdown of the band structure in favor of Wannier-Stark states.

 

Controlling Material Topology with light
On the other side of the coin, rather than probing the underlying material with laser pulses, we can use the same pulses to change the material's properties. This was demonstrated by using circularly polarized light to change the topological state of graphene. With a continuous driving field, one can produce so-called Floquet states, which can have a different topological character from the base material.  These investigations have continued into  3D Dirac materials such as Na3Bi.