RAPID COMMUNICATIONS PHYSICAL REVIEW B 88, 241410(R) (2013) Photon energy dependence of circular dichroism in angle-resolved photoemission spectroscopy of Bi 2 Se 3 Dirac states F. Vidal, 1, 2 M. Eddrief, 1, 2 B. Rache Salles, 3, 4 I. Vobornik, 3 E. Velez-Fort, 5, 6 G. Panaccione, 3 and M. Marangolo 1, 2 1 Sorbonne Universit´ es, UPMC Universit´ e Paris 06, UMR 7588, Institut des NanoSciences de Paris, 4 place Jussieu, F-75005 Paris, France 2 CNRS, UMR 7588, Institut des NanoSciences de Paris, 4 place Jussieu, F-75005 Paris, France 3 Istituto Officina dei Materiali (IOM)–CNR, Laboratorio TASC, Area Science Park, S.S.14, Km 163.5, I-34149 Trieste, Italy 4 Instituto de F´ ısica, Universidade Federal do Rio de Janeiro, 21941-972 Rio de Janeiro (RJ), Brazil 5 Laboratoire de Photonique et de Nanostructures (CNRS-LPN), Route de Nozay, 91460 Marcoussis, France 6 IMPMC, Universit´ e Pierre et Marie Curie, CNRS UMR7590, 4 Pl. Jussieu, 75005 Paris, France (Received 4 November 2013; published 26 December 2013) Circular dichroism in angle-resolved photoemission spectroscopy (CD-ARPES) has been recently proposed as a sensitive probe of either the spin texture or the orbital momentum texture of helical Dirac fermions at the surface of three-dimensional topological insulators. Recent results obtained on Bi 2 Te 3 point rather towards a final state effect. In this Rapid Communication, we study the prototypical topological insulator Bi 2 Se 3 by CD-ARPES as a function of the photon energy and as a function of thickness. Analysis of the photon energy dependence of CD-ARPES, as well as the existence of a large dichroic signal from massive Dirac fermion states in ulthathin films, confirm that the dichroism is predominantly a final state effect. DOI: 10.1103/PhysRevB.88.241410 PACS number(s): 73.20.At, 79.60.−i, 71.70.Ej Three-dimensional topological insulators (TIs) are an in- tensively studied class of compounds exhibiting peculiar elec- tronic properties because of band inversion induced by strong spin-orbit coupling. 1– 6 These systems are insulating in the bulk while their surface states are metallic and characterized by a linear dispersion at the center of the Brillouin zone (Ŵ). 7 Furthermore, these Dirac fermions are helical: The spin direction is locked perpendicularly to the momentum and winds by 2π around Ŵ. 8 Such unusual spin texture may be of interest for applications in spintronics and quantum computation and is thus thoroughly investigated. 9 Among three-dimensional TIs, Bi 2 Se 3 and Bi 2 Te 3 are two prototypical compounds and, as such, their band struc- ture has been measured using angle-resolved photoemission spectroscopy recently. 10–13 Understanding the fine details of the spin texture of Dirac fermions as well as tailoring such texture—e.g., by doping with magnetic elements—are currently active fields of research. 14 Experimentally, this issue can be addressed using spin-resolved photoemission spectroscopy. Although providing direct information on the spin polarization, this technique is quite demanding and the signal-to-noise ratio may be such that small details could be missed. In such a context, alternative experimental techniques would be of great interest in order to probe the spin texture around Ŵ in TIs. Recently, circular dichroism in angle-resolved photoemission spectroscopy (CD-ARPES) has been proposed as such alternative tool. 15– 17 CD-ARPES, also known as circular dichroism in the angular dependence of photoemission, 18–20 is a technique that consists in using circularly polarized light (σ + and σ − ) to excite the photoelectrons. The circular dichroism is then obtained by computing the weighted difference of the photoemission intensities, (I σ + − I σ − )/(I σ + + I σ − ). The existence of CD-ARPES has been predicted by Dubs et al. for oriented molecules 21 and was confirmed experimentally by Westphal et al. 22 It has been used to study various systems such as single crystal surfaces, 23, 24 adsorbed atoms, 25, 26 oriented adsorbed molecules, 27 , 28 chiral metal surfaces, 29 adsorbed chiral molecules, 30 , 31 and reconstructed surfaces. 32 In the case of TIs, CD-ARPES was used by Wang et al. to study the helical texture of the Dirac cone of Bi 2 Se 3 , 15 with a photon energy hν = 6.2 eV. A link between the dichroic signal and the spin was established and vectorial mapping of the spin texture was derived from the CD-ARPES data. According to this work, CD-ARPES can be used as a valuable alternative to spin-resolved ARPES for the observation of fine details of the chiral spin geometry in TIs. In particular, small deviations with respect to the simple, commonly accepted, helical spin structure above the Dirac point were evidenced. In another CD-ARPES study of Bi 2 Se 3 with hν = 10 and 13 eV, Park et al. tentatively attributed the dichroic signal to a local orbital-angular momentum locked to the electron momentum in the opposite direction to the spin. 17 The large value of the dichroism and its reversal when crossing the Dirac point were put forward as arguments in favor of a direct link between the CD-ARPES signal and the postulated chiral orbital momentum texture. However, both works were carried out at fixed photon energies. Recent results on the photon energy dependence of CD-ARPES in Bi 2 Te 3 question the simple connection of the dichroic signal with the spin or orbital texture. 33 In this Rapid Communication, we report the study of the dependence of CD-ARPES on the photon energy in the case of Bi 2 Se 3 , on both sides of the Dirac point (E D ). We conclude that there is no simple connection of CD-ARPES and spin/orbital texture in TIs due to final state effects in the photoemission process. The samples were grown using a multichamber molecular beam epitaxy (MBE) setup at Institut des NanoSciences de Paris. High quality GaAs buffer layers were grown homoepi- taxially on GaAs(111) in a Riber compact 21 MBE chamber. The substrates were then transferred under ultrahigh vacuum to a Riber 32 MBE chamber equipped with Bi and Se cells. Growth of Bi 2 Se 3 epilayers was then carried out at T = 300 ◦ C with a Se/Bi beam equivalent pressure ratio higher than 6.5. 1098-0121/2013/88(24)/241410(5) 241410-1 ©2013 American Physical Society