We report a Rashba spin splitting of a two-dimensional electron gas in the topological insulator Bi(2)Se(3) from angle-resolved photoemission spectroscopy. We further demonstrate its electrostatic control, and show that spin splittings can be achieved which are at least an order-of-magnitude larger than in other semiconductors. Together these results show promise for the miniaturization of spintronic devices to the nanoscale and their operation at room temperature.

High spin (S = 1) organic diradicals may offer enhanced properties with respect to several emerging technologies, but typically exhibit low singlet triplet energy gaps and possess limited thermal stability. We report triplet ground state diradical 2 with a large singlet-triplet energy gap, ΔE ST ≥ 1.7 kcal mol −1 , leading to nearly exclusive population of triplet ground state at room temperature, and good thermal stability with onset of decomposition at ~160 °C under inert atmosphere. Magnetic properties of 2 and the previously prepared diradical 1 are characterized by SQUID magnetometry of polycrystalline powders, in polystyrene glass, and in other matrices. Polycrystalline diradical 2 forms a novel one-dimensional (1D) spin-1 (S = 1) chain of organic radicals with intrachain antiferromagnetic coupling of J′/k = −14 K, which is associated with the N•••N and N•••O intermolecular contacts. The intrachain antiferromagnetic coupling in 2 is by far strongest among all studied 1D S = 1 chains of organic radicals, which also makes 1D S = 1 chains *

The fundamental limits of inorganic semiconductors for light emitting applications, such as holographic displays, biomedical imaging and ultrafast data processing and communication, might be overcome by hybridization with their organic counterparts, which feature enhanced frequency response and colour range. Innovative hybrid inorganic/organic structures exploit efficient electrical injection and high excitation density of inorganic semiconductors and subsequent energy transfer to the organic semiconductor, provided that the radiative emission yield is high. An inherent obstacle to that end is the unfavourable energy level offset at hybrid inorganic/organic structures, which rather facilitates charge transfer that quenches light emission. Here, we introduce a technologically relevant method to optimize the hybrid structure's energy levels, here comprising ZnO and a tailored ladder-type oligophenylene. The ZnO work function is substantially lowered with an organometallic donor monolayer, aligning the frontier levels of the inorganic and organic semiconductors. This increases the hybrid structure's radiative emission yield sevenfold, validating the relevance of our approach.

From a combination of high resolution angle-resolved photoemission spectroscopy and density functional calculations, we show that BaFe2As2 possesses essentially two-dimensional electronic states, with a strong change of orbital character of two of the Γ-centered Fermi surfaces as a function of kz. Upon Co doping, the electronic states in the vicinity of the Fermi level take on increasingly three-dimensional character. Both the orbital variation with kz and the more three-dimensional nature of the doped compounds have important consequences for the nesting conditions and thus possibly also for the appearance of antiferromagnetic and superconducting phases. 74.25.Jb, Since the discovery of high T c superconductivity in Fepnictides [1], many experiments have been carried out to reveal the physical and electronic properties of these materials [2,3,4,5]. The parent compounds of Fepnictide superconductors are antiferromagnetic (AFM) metals. Both electron and hole doping suppresses the AFM order and leads to a superconducting phase. The AFM ordering is supposed to occur by nesting of hole pockets at the center of the Brillouin zone (BZ) and electron pockets at the zone corner. Nesting may be also important for the pairing mechanism in these compounds [6] although there are alternative scenarios based on the high polarizability of the As ions [7]. The nesting scenario could explain why in the SmFeAsO-based superconductors [8], predicted to have an almost twodimensional electronic structure [9, 10], higher superconducting transition temperatures T c are observed than in BaFe 2 As 2 -based systems [2] which are predicted to have a more three-dimensional electronic structure [11]. In general, reduction of the dimensionality increases the number of states that could be considered to be well nested. Furthermore, we point out that the orbital character of the states at the Fermi level E F is very important for the nesting conditions as the interband transitions which determine the electronic susceptibility, as calculated by the Lindhard function, are (in weak coupling scenarios) by far strongest when the two Fermi surfaces have the same orbital character [12]. The admixture of threedimensionality, arising from interlayer coupling, makes the materials potentially more useful in devices and other applications. Thus the dimensionality of the electronic structure, i.e., the k z dispersion of the electronic states is of great importance for the understanding and application of these new superconductors.Although angle-resolved photoemission spectroscopy (ARPES) is an ideal tool to study the dispersion of bands parallel and perpendicular to the FeAs layers there exist only a few experimental studies of these issues [13,14,15]. In this letter, we report a systematic study of the dimensionality of the electronic structure of BaFe 2−x Co x As 2 (x= 0 to 0.4) using polarization dependent ARPES, uncovering two new factors which are of great signi cance for the nesting of the Fermi surfaces of these systems. Firstly we show that the Co d...

Metal halide perovskites have emerged as materials of high interest for solar energy-to-electricity conversion, and in particular, the use of mixed-ion structures has led to high power conversion efficiencies and improved stability. For this reason, it is important to develop means to obtain atomic level understanding of the photoinduced behavior of these materials including processes such as photoinduced phase separation and ion migration. In this paper, we implement a new methodology combining visible laser illumination of a mixed-ion perovskite ((FAPbI3)0.85(MAPbBr3)0.15) with the element specificity and chemical sensitivity of core-level photoelectron spectroscopy. By carrying out measurements at a synchrotron beamline optimized for low X-ray fluxes, we are able to avoid sample changes due to X-ray illumination and are therefore able to monitor what sample changes are induced by visible illumination only. We find that laser illumination causes partially reversible chemistry in the surface region, including enrichment of bromide at the surface, which could be related to a phase separation into bromide- and iodide-rich phases. We also observe a partially reversible formation of metallic lead in the perovskite structure. These processes occur on the time scale of minutes during illumination. The presented methodology has a large potential for understanding light-induced chemistry in photoactive materials and could specifically be extended to systematically study the impact of morphology and composition on the photostability of metal halide perovskites.

We report high resolution angle-resolved photoemission spectroscopy ͑ARPES͒ studies of the electronic structure of BaFe 2 As 2 , which is one of the parent compounds of the Fe-pnictide superconductors. ARPES measurements have been performed at 20 and 300 K, corresponding to the orthorhombic antiferromagnetic phase and the tetragonal paramagnetic phase, respectively. Photon energies between 30 and 175 eV and polarizations parallel and perpendicular to the scattering plane have been used. Measurements of the Fermi surface yield two hole pockets at the ⌫ point and an electron pocket at each of the X points. The topology of the pockets has been concluded from the dispersion of the spectral weight as a function of binding energy. Changes in the spectral weight at the Fermi level upon variation in the polarization of the incident photons yield important information on the orbital character of the states near the Fermi level. No differences in the electronic structure between 20 and 300 K could be resolved. The results are compared with density functional theory band structure calculations for the tetragonal paramagnetic phase.

We have studied the electronic structure of EuFe2As2−xPx using high resolution angle-resolved photoemission spectroscopy. Upon substituting As with the isovalent P, which leads to a chemical pressure and to superconductivity, we observe a non-rigid-band like change of the electronic structure along the center of the Brillouin zone (BZ): an orbital and kz dependent increase or decrease in the size of the hole pockets near the Γ − Z line. On the other hand, the diameter of the Fermi surface cylinders at the BZ corner forming electron pockets, hardly changes. This is in stark contrast to p and n-type doped iron pnictides where, on the basis of ARPES experiments, a more rigid-band like behavior has been proposed. These findings indicate that there are different ways in which the nesting conditions can be reduced causing the destabilization of the antiferromagnetic order and the appearance of the superconducting dome.

Using angle resolved photoemission it is shown that the low lying electronic states of the iron pnictide parent compound EuFe2As2 are strongly modified in the magnetically ordered, low temperature, orthorhombic state compared to the tetragonal, paramagnetic case above the spin density wave transition temperature. Back-folded bands, reflected in the orthorhombic/ anti-ferromagnetic Brillouin zone boundary hybridize strongly with the non-folded states, leading to the opening of energy gaps. As a direct consequence, the large Fermi surfaces of the tetragonal phase fragment, the low temperature Fermi surface being comprised of small droplets, built up of electron and hole-like sections. These high resolution ARPES data are therefore in keeping with quantum oscillation and optical data from other undoped pnictide parent compounds. The electronic structure and properties of the newly discovered iron-pnictide superconductors [1] are a focus of much research. A central theme is the understanding of the origins of the magnetic ordering and its possible interplay with superconductivity. The characteristics of the low-temperature, orthorhombic, antiferromagnetic (AFM) phase have been experimentally determined, for example for the undoped 122 -parent compounds, M Fe 2 As 2 (with M = Ca, Sr, Ba, Eu. . . ). A common proposition is that the magnetism is of an itinerant, spin density wave (SDW) type, connected to nesting of the warped cylindrical Fermi surfaces (FS's) centered at the (0, 0) (Γ) and (π, π) points (X) of the tetragonal Brillouin zone (BZ) [2]. Such an SDW would have dramatic consequences for the band structure and Fermi surface, leading to reconstruction, the opening of gaps and major Fermi surface depletion. Recently, quantum oscillation (QO) experiments have presented evidence for Fermi surfaces comprising small pocketsdue to the effects of the SDW order -in SrFe 2 As 2 [3] and BaFe 2 As 2 [4]. Both studies find the existence of three distinct orbits, with FS areas of only 0.3%, 0.6% and 1.5% of the tetragonal BZ (compared to a total FS area larger than 20% in the tetragonal phase). In addition, the opening of gaps as well as a dramatic reduction of the free charge carrier density upon entering the orthorhombic AFM state has been concluded from optical conductivity measurements [5,6]. Given that a FS-nesting-driven SDW is rooted in k-space, it is of great importance whether the SDW 'fingerprints' of reconstruction, gapping and FS depletion really take place in the E(k)-space hosting the electronic states of these materials. Angle-resolved photoemission (ARPES) is a powerful probe of such issues, and consequently there have been numerous studies of the parent compounds of the pnictide superconductors using ARPES. Early studies were unable to detect significant changes below the magnetic ordering temperature [7,8]. More recently, ARPES studies have shown the existence of small, additional FS pockets (either hole or electron-like) around the (0, 0) or (π, π) point [9,10,11,12], and a detailed temperature-dependent...