Understanding how the electronic and magnetic properties of materials are modified by the injection of charge carriers is of central importance across a range of fundamental and applied research. For example, the selective doping of impurity atoms into semiconductors such as silicon provides electrons or holes thus allowing the carrier concentration to be modified. With careful engineering, nanoscale transistors can be constructed which are at the core of smartphones, computers, and almost every electronic appliance we use today. However, charge injection through chemical doping has a number of limitations, including difficulties associated with integration of the dopant atom in the host lattice, and control of the precise number and distribution of dopant atoms. Furthermore, once the device has been fabricated, that's it: there is no possibility of varying the doping level, and hence the electrical properties remain fixed.
By contrast, doping by exciting electrons into the conduction band with light – photo doping – provides a way of controlling and varying the number of charge carriers at will. The use of pulsed lasers to achieve photo doping opens up the possibility of investigating the dynamics of charge carrier creation and recombination, as well as the opportunity to study the creation of exotic, non-equilibrium electronic and magnetic states.
An international collaboration of scientists from the LCN, United States, Japan, Germany, Switzerland, Spain and the Netherlands have recently reported in Nature Materials the development of novel X-ray scattering techniques for studying the electronic and magnetic correlations of photo induced transient states. The lifetime of such transient states is typically very short – on the order of 10-12 seconds – so the experimenters used state of the art X-ray free-electron laser (XFEL) facilities in California and Japan to generate intense, very short X-ray pulses in order to probe the dynamics of the excited state in a Sr2IrO4 thin film. This included the first ever time-resolved resonant inelastic X-ray scattering (t-RIXS) measurements performed at a XFEL in the hard X-ray regime. With careful selection of the infra-red laser pump energy, electrons are excited across the optical gap in Sr2IrO4, transforming it from an insulator to a metal.
The authors discovered that the magnetic correlations were radically influenced by the metal-insulator transition induced by photo doping. Sr2IrO4 is normally a magnetic insulator below 240 K. The magnetic correlations were shown to decay concurrently with the onset of the metallic phase, only to recover with the reformation of the insulating ground state as carriers recombined across the gap. Through the use of the newly developed t-RIXS technique, it was possible to separate the observed behaviour into in-plane and out-of-plane components, which can be directly related to the intrinsic magnetism of the insulating ground state.
Figure: (a) The scattering setup. The IR pump pulse (shown in red) is incident on the surface of Sr2IrO4. XFEL pulses (shown in purple) probe the resulting transient state. X-rays that are scattered close to 90 degrees are either directly measured, to access the magnetic Bragg peak that probes the presence or absence of 3D magnetic order, or energy analyzed to access the inelastic spectrum that is particularly sensitive to the 2D magnetic correlations; (b) Schematic of the excitation processes. The IR pump beam photo-dopes the sample by exciting an electron across the optical gap, denoted as the difference in energy between the occupied lower Hubbard band (LHB) and unoccupied upper Hubbard band (UHB). XFEL pulses tuned to the Ir L3 absorption edge (shown in purple) probe the resulting transient state. The resulting emitted photon encodes the magnetic and orbital configuration of the transient state; (c) Illustration of how the pump pulse damps the magnetic correlations at high energies, leading to an overall broadening of the spin wave dispersion.
James Vale, a PhD student at the LCN and co-investigator for this work, commented: “We were able to show that resonant inelastic X-ray scattering, which is uniquely sensitive to spin, orbital and charge dynamics, can be extended into the time-resolved domain. This opens up a number of potential future experiments, including studies into the transient dynamics of superconductors and materials exhibiting exotic quantum states. Furthermore, imagine being able to engineer a device which responds differently depending on the wavelength and intensity of light you shine on it, and on ultrafast timescales. Such a device may revolutionise memory storage for example, and is merely one potential future application of photo-doping.”