SPring-8, the large synchrotron radiation facility

Speaker : Dr. Vladmir N. Strocov

Language : English

Affiliate : Swiss Light Source Paul Scherrer Institute

Title : Soft-X-Ray ARPES Facility at SLS: Instrumentation and First Results

Abstract :
ARPES experiments in the soft-X-ray energy range bring virtues of free-electron final states, simplified matrix elements and increasing photoelectron escape depth λ. The latter, along with enhancement of the bulk sensitivity, implies improvement of the intrinsic resolution in surface-perpendicular momentum Δk_z =λ^-1 which allows reliable ARPES studies of 3-dimensional electronic structure.

The ADRESS beamline (V.N. Strocov et al, J. Synchrotron Rad. 17 (2010) 631) delivers soft-X-ray radiation with variable polarizations in the hv range from 300 to 1600 eV. The photon flux tops up 10¹³ photons/s/0.01%BW at 1 keV which has allowed us to break through the notorious problem of small valence band crossection in the soft-X-ray range. The ARPES endstation uses grazing light incidence to increase the photoyield. The analyser is rotatable to permit the slit orientation along the beam to exploit symmetry of the valence states. Operation with a combined energy resolution of 110 meV delivers spectra of publication quality within 5 min, and with 60 meV within 30 min.

We illustrate the achieved performance with first results on 3-dimensional electronic structure of VSe₂. Variation of hv has allowed us to slice its FS in k_z. Each 2-dimensional slice was acquired in ~4 hours. A perpendicular cut of the FS demonstrated its 3-dimensional warping connected with formation of CDWs. The experimental FS shows a textbook clarity by virtue of free-electron final states, their good definition with small Δk_z and smooth matrix elements achieved with soft-X-ray energies. Furthermore, I will present a few most recent results on excitonic mediated CDWs in TiSe₂ and 3 dimensional effects in pnictides

Organizer : Toyohiko Kinoshita
toyohiko@spring8.or.jp