We observe a giant spin-orbit splitting in bulk and surface states of the non-centrosymmetric semiconductor BiTeI. We show that the Fermi level can be placed in the valence or in the conduction band by controlling the surface termination. In both cases it intersects spin-polarized bands, in the corresponding surface depletion and accumulation layers. The momentum splitting of these bands is not affected by adsorbate-induced changes in the surface potential. These findings demonstrate that two properties crucial for enabling semiconductor-based spin electronics -a large, robust spin splitting and ambipolar conduction -are present in this material.

We report a comprehensive study of the paradigmatic quasi-1D compound (TaSe(4))(2)I performed by means of angle-resolved photoemission spectroscopy (ARPES) and first-principles electronic structure calculations. We find it to be a zero-gap semiconductor in the nondistorted structure, with non-negligible interchain coupling. Theory and experiment support a Peierls-like scenario for the charge-density wave formation below T(CDW)=263  K, where the incommensurability is a direct consequence of the finite interchain coupling. The formation of small polarons, strongly suggested by the ARPES data, explains the puzzling semiconductor-to-semiconductor transition observed in transport at T(CDW).

We present a scanning tunneling microscopy investigation of the dynamics of hot electrons and holes in Ag pyramidal nanostructures. The geometry of the nanostructure leads to a strong reduction of the decay mechanism into the bulk states and then to a large reflection coefficient of the surface electronic waves. Therefore, in contrast to quantum corrals and adatom islands which show a dominant lossy scattering contribution to the linewidth, the narrow observed structures in the differential conductivity spectra reveal the expected linewidth minimum at the Fermi energy. The electron-phonon contribution to the lifetime is shown to be dominant, in agreement with previous photoemission measurements.

The non-centro-symmetric semiconductor BiTeI exhibits two distinct surface terminations that support spinsplit Rashba surface states. Their ambipolarity can be exploited for creating spin-polarized p-n junctions at the boundaries between domains with different surface terminations. We use scanning tunneling microscopy (STM) and spectroscopy (STS) to locate such junctions and investigate their atomic and electronic properties. The Te-and I-terminated surfaces are identified owing to their distinct chemical reactivity and an apparent height mismatch of electronic origin. The Rashba surface states are revealed in the STS spectra by the onset of a van Hove singularity at the band edge. Eventually, an electronic depletion is found on interfacial Te atoms, consistent with the formation of a space-charge area in typical p-n junctions. In inversion-asymmetric systems, the spin-orbit interaction lifts the spin degeneracy. This effect can occur either at surfaces (the Rashba-Bychkov effect [1]) or in the bulk of non-centro-symmetric crystals (the Rashba-Dresselhaus effect [2,3]). Such Rashba systems are promising candidates for manipulating electron spin by means of electric field in the context of emerging spintronic devices [4]. Significant research efforts are currently directed toward the search of materials exhibiting a "giant" Rashba effect that would enable nanometer-scale spintronic devices operating at room temperature. To date, the largest spin splitting, measured by angle-resolved photoemission spectroscopy (ARPES), has been reported in the BiAg 2 /Ag(111) surface alloy [5] and both the surface and bulk states of non-centro-symmetric semiconductor BiTeI [6][7][8][9].In BiTeI, photoemission data show two distinct types of surface domains with different surface terminations (Te and I) and opposite surface band bendings that support p-or n-type surface states [8]. We anticipate that Rashba p-n junctions can be observed at the boundaries between such domains. This can be used for fulfilling another requirement for fabricating logic devices-ambipolarity, which is the possibility to control carriers' nature (electrons or holes). Moreover, a number of novel transport phenomena have been predicted for p-n junctions in systems with strong spin-orbit coupling [10][11][12][13] that can be further exploited in practical applications. In view of applications, the questions of current dissipation to the bulk or controlled growth of the junction by epitaxy are important, but well beyond the scope of this paper. Here, cleaved BiTeI is rather considered as a toy-system providing a quite unique opportunity to study a p-n junction in two dimensions.In this work, we employ scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) to locate and investigate p-n junctions naturally occurring at a cleaved BiTeI surface. The surface domains with Te and I terminations are identified owing to their distinct chemical reactivity, apparent height, and electronic structure. The onset of a van * cedric.tournier@epfl.ch Hov...

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 ...