Momentum and photon energy dependence of the circular dichroic photoemission in the bulk Rashba semiconductorsBiTeX(X=I, Br, Cl)

Abstract

Bulk Rashba systems BiTeX (X = I, Br, Cl) are emerging as important candidates for developing spintronics devices because of the coexistence of spin-split bulk and surface states, along with the ambipolar character of the surface charge carriers. The need to study the spin texture of strongly spin-orbit-coupled materials has recently promoted circular dichroic angular resolved photoelectron spectroscopy (CD-ARPES) as an indirect tool to measure the spin and the angular degrees of freedom. Here we report a detailed photon-energy-dependent study of the CD-ARPES spectra in BiTeX (X = I, Br, Cl). Our work reveals a large variation in the magnitude and sign of the dichroism. Interestingly, we find that the dichroic signal modulates differently for the three compounds and for the different spin-split states. These findings show a momentum and photon-energy dependence for the CD-ARPES signals in the bulk Rashba semiconductor BiTeX (X = I, Br, Cl). Finally, the outcome of our experiment indicates the important relation between the modulation of the dichroism and the phase differences between the wave functions involved in the photoemission process. This phase difference can be due to initialor final-state effects. In the former case the phase difference results in possible interference effects among the photoelectrons emitted from different atomic layers and characterized by entangled spin-orbital polarized bands. In the latter case the phase difference results from the relative phases of the expansion of the final state in different outgoing partial waves. The need for novel and advanced spintronics devices has stimulated the quest for materials hosting metallic spinpolarized bands embedded in a semiconducting bulk. Starting from the present knowledge on topological insulators (TIs) [1][2][3][4][5], the design of materials with spin-polarized bands requires the tailoring of the spin texture at the Fermi level (E F ), hence the synthesis of systems such as ternary TIs [6,7] or the bulk Rashba semiconductors BiTeX (X = I, Br, Cl) characterized by ambipolar surface states [8][9][10][11][12]. Nowadays one of the major challenges is to study the fully threedimensional spin properties of ternary TIs, and the bulk Rashba semiconductors, as is done for magnetic doped TIs [13].Spin-resolved angular resolved photoelectron spectroscopy (SR-ARPES) offers the unique possibility to directly address the spin polarization. Unfortunately, SR-ARPES, based on high-energy spin-dependent Mott scattering, is characterized by a low efficiency (1 × 10 −3 -1 × 10 −4 ) [14]. This limitation has recently renewed the interest for alternative spin detection devices based on higher-efficiency low-energy electron diffraction (IV-LEED with 1 × 10 −1 -1 × 10 −2 ) [15]. This context well explains why the possibility of indirectly studying the spin polarization via circular dichroic ARPES (CD-ARPES) was regarded as a major breakthrough [16]. CD-ARPES measures the difference between the photoemission intensities obtained with the two opposite helicities ...

Termination-dependent surface properties in the giant-Rashba semiconductorsBiTeX(X=Cl, Br, I)

“…This could point to a different n-type doping at the surface or in the bulk. Another possible explanation are strong cross section effects with excitation energy which were reported recently [25]. For the Br/Cl-terminated surface conduction band states do not appear at the Fermi level due to p-type band bending as well as no surface states emerge near the valence band in agreement with earlier ARPES measurements on BiTeCl and in contradiction with a theoretical prediction [6].…”

“…On the Teterminated surface we observe a Rashba-split band close to the Fermi level that derives from the conduction band bottom and the onset of valence band states at a binding energy of approximately 1 eV. We note that only one set of parabolic bands is visible in our data whereas previous studies observed two to three sets of bands [7,25]. In Refs.…”

Bulk and surface band structure of the new family of semiconductors BiTeX (X=I, Br, Cl)

“…The same SO interaction behind the Rashba effect is indeed also responsible for the band inversion in topological insulators, all of which contain one or more heavy elements. A topological state has also been reported at ambient pressure for BiTeCl [15], an observation which is however in contrast with all other experimental studies that have appeared to this point [16][17][18].…”

“…Their intensity is weak at the low (<50 eV) photon energy typically used in ARPES beamlines [26] or in laboratory sources [11], but it is already detectable at ∼100 eV [25] and clearly visible in the soft x-ray range [26,27]. Moreover, in the sister compounds BiTeBr and BiTeCl an additional pair of bands was identified closer to the Fermi level, both by laser-based photoemission [16] and by circular dichroism [18].…”

Direct measurement of the bulk spin structure of noncentrosymmetric BiTeCl

“…It has been shown that circular dichroism (CD) measurements of the surface states of BiTeCl and related compounds are strongly photon energy dependent and thus CD is not a suitable probe for the spin polarization [30,31]. It is under debate how and whether the photoelectron spin polarization measured in SARPES portrays the spin structure of the initial state.…”