Electronic Processes in Quantum Materials
Electronic Processes in Quantum Materials
Research Group Chris Nicholson
Research Group Chris Nicholson

Research

Motivation

Electrons are important in many areas of nature: they are central to biological reactions; they form chemical bonds; and they determine many physical properties. We are interested in how electrons behave in solids, where the interactions between many electrons can produce diverse emergent phases. The goal of our research is to understand the microscopic mechanisms that produce such behaviour, observe how their phases spatially vary at the microscale, and to use external means to control them.

Goals

We aim to enable a combination of advanced lattice modification methods (strain, moiré, and microscale engineering) with ultrafast optical excitation and access to the transient momentum-resolved electronic structure in quasi-2D materials. Access to the microscale will be achieved by exploiting spatially selective apertures in a recently developed “momentum microscope” approach to trARPES. These cutting-edge trARPES capabilities will be extended by coupling to mid-IR excitation, thereby providing a conceptual bridge between static lattice manipulation and transient electronic excitation. Combined uniaxial strain and electronic excitation will be exploited to disentangle microscopic processes in quasi-2D structures, semiconductors and magnets. Moiré engineering and ultrafast mid-IR excitation will be utilized to uncover microscopic mechanisms in model twisted bilayer systems and 2D heterostructures. Further extending the trARPES technique to realise a micro-focussed XUV beam will allow efficient access to the dynamic electronic structure in micro machined samples. Manipulating the electronic behaviour of twisted 2D systems in microscale strain and electric field gradients opens new avenues for probing quantum materials with trARPES. We hope to obtain detailed insight into the microscopic mechanisms underpinning emergent electronic behaviour in 2D materials and inform next-generation material development.

Complementary static and non-equilibrium approaches utilized by our group to control the electronic behaviour of solids.

Methods

We investigate electronic structure and femtosecond dynamics using angle-resolved photoemission spectroscopy (ARPES), time-resolved ARPES, and micro-ARPES both in our home lab and at synchrotrons. We work in close collaboration with the groups of Ralph Ernstorfer (TU Berlin) and Laurenz Rettig (FHI) and have developed a high-harmonic laser system [1] coupled to advanced electron spectrometers allowing state-of-the-art investigation of ultrafast dynamics in quantum materials throughout the full Brillouin zone [2].

[1] M. Puppin, Y. Deng, C. W. Nicholson, J. Feldl, N. Schroeter, H. Vita, P. S. Kirchmann, C. Monney, L. Rettig, M. Wolf, R. Ernstorfer, Time- and angle-resolved photoemission spectroscopy of solids in the extreme ultraviolet at 500 kHz repetition rate, Rev. Sci. Instr. 90, 023104 (2019)
[2] C. W. Nicholson, A. Lücke, W. G. Schmidt, M. Puppin, L. Rettig, R. Ernstorfer, M. Wolf, Beyond the molecular movie: dynamics of bands and bonds during a photo-induced phase transition, Science, 362, 821-825 (2018)

Schematic of the trARPES experiment. (Left) Electrons are excited by a pump pulse (red) into the unoccupied states and then ejected from the solid by the XUV probe pulse (purple). Varrying the delay between the pulses allows us to build a movie of the electronic dynamics on the femtosecond timescale (right).