Sparkling STARS for CLF collaboration with Diamond
27 Mar 2015



A CLF-Diamond collaboration has gained insights that will aid development of new materials for opto-electronic and spintronic devices using a new technique called STARS


​​​​The laser excited electrons can be visualised as a sea of spin up and spin down electrons that can relax back to the (lower energy) ground state in a similar way to the water in the waterfalls in this image. One waterfall (representing one spin channel) is much quicker at moving its water than the other which has a bottleneck restricting the flow analogous to the surface resonance state.

'Pigeon River High Falls' bySharon Mollerus via Flickr under the CC BY 2.0 licence

Scientists have gained insights that will aid development of new materials for opto-electronic and spintronic devices by understanding spin relaxation in topological insulators.

A European collaboration, including UK scientists from the Central Laser Facility and Diamond Light Source, has successfully gained new insights into the excited state spin dynamics of topological insulators. Using a new technique known as Spin, Time and Angle Resolved Spectroscopies (STARS), the team used the Artemis spin-resolved electron time-of-flight end-station and a high repetition rate ultrafast laser on Diamond’s Nanoscience beamline (I06). They measured spin-dependent scattering between bulk and surface states in the archetypal topological insulator Bi2Se3. The results were published on the 4th March 2015 in the journal Physical Review Letters.​​

The experiment was the first demonstration of STARS – adding spin-resolution to the time-and angle-resolved photoemission technique available on Artemis. The 250 kHz repetition rate of the system makes it possible to acquire add spin-detection, discriminating between electrons with spin-up and spin-down. However, the low pulse energy at this repetition rate means that only a fourth harmonic (6 eV) probe can be used, rather than the 20-40 eV XUV probing available on Artemis.

“There is currently wide interest in the idea of being able to optically induce and control a spin-polarised current in spintronic devices” explains lead author Dr Cephise Cacho from Artemis, adding that “the out-of-equilibrium electronic and spin structure of topological insulators has revealed some novel dynamics at work.” 

The collaboration involved researchers from academic institutes and facilities across 8 European countries, who spent 6 months preparing for these experiments at Diamond and then over a month taking measurements in the Nanoscience beamline’s laser cabin. 

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Contact: Springate, Emma (STFC,RAL,CLF)