Atomic relaxations at the (0001) surface of Bi2Se3 single crystals and ultrathin films

Under the terms of the Creative Commons Attribution License 3.0 (CC-BY).-- et al.We present a surface x-ray analysis of the atomic structure of the (0001) surface of the topological insulator Bi2Se3, which was grown as a single crystal and as an ultrathin film on Si(111) using molecular beam epitaxy (MBE). In general we find that the top Se-Bi layer spacing is expanded between 2% and 17% relative to the bulk, while deeper layers and the first van der Waals gap are unrelaxed. The top layer expansion is directly related to the amount of surface contamination by carbon and oxygen. The near-surface structures of the single crystal and the MBE-grown thin film differ in the degree of (static) disorder: for the former an overall Debye parameter (B) per quintuple layer (QL) of 5Å2 is found to decrease slowly with depth. MBE-grown Bi2Se3 films exhibit the opposite scenario, characterized by an increase in B from about 10Å2 for the topmost QL to values of B=20-40 Å2 for the fourth QL. This is attributed to the lattice misfit to the Si(111) surface. Ab initio calculations reveal carbon to act as an n-dopant, while the first interlayer spacing expansion induces a shift of the Dirac point towards the Bi2Se3 bulk conduction band minimum.We acknowledge the financial support from the DFG through priority program SPP1666 (Topological Insulators).Peer Reviewe

Atomic and electronic structure of bismuth-bilayer-terminated Bi2Se3(0001) prepared by atomic hydrogen etching

Under the terms of the Creative Commons Attribution License 3.0 (CC-BY).A bilayer of bismuth is recognized as a prototype two-dimensional topological insulator. Here we present a simple and well reproducible top-down approach to prepare a flat and well ordered bismuth bilayer with a lateral size of several hundred nanometers on Bi2Se3(0001). Using scanning tunneling microscopy, surface x-ray diffraction, and Auger electron spectroscopy we show that exposure of Bi2Se3(0001) to atomic hydrogen completely removes selenium from the top quintuple layer. The band structure of the system, calculated from first principles for the experimentally derived atomic structure, is in excellent agreement with recent photoemission data. Our results open interesting perspectives for the study of topological insulators in general.This work is supported by the Deutsche Forschungsgemeinschaft through priority program SPP 1666 (Topological Insulators). M.M.O. and E.V.C. thank the Tomsk State University Academic D.I. Mendeleev Fund Program (Research Grant No. 8.1.05.2015).Peer Reviewe

Atomic and electronic structure of bismuth-bilayer-terminated Bi2Se3(0001) prepared by atomic hydrogen etching

A bilayer of bismuth is recognized as a prototype two-dimensional topological insulator. Here we present a simple and well reproducible top-down approach to prepare a flat and well ordered bismuth bilayer with a lateral size of several hundred nanometers on Bi2Se3(0001). Using scanning tunneling microscopy, surface x-ray diffraction, and Auger electron spectroscopy we show that exposure of Bi2Se3(0001) to atomic hydrogen completely removes selenium from the top quintuple layer. The band structure of the system, calculated from first principles for the experimentally derived atomic structure, is in excellent agreement with recent photoemission data. Our results open interesting perspectives for the study of topological insulators in general

Surface alloying and iron selenide formation in Fe/Bi2Se3(0001) observed by x-ray absorption fine structure experiments

Under the terms of the Creative Commons Attribution License 3.0 (CC-BY).-- et al.The atomic structure of ultrathin iron films deposited on the (0001) surface of the topological insulator Bi2Se3 is analyzed by surface x-ray absorption spectroscopy. Iron atoms deposited on a Bi2Se3 (0001) surface kept at 160 K substitute bismuth atoms within the first quintuple layer. Iron atoms are neighbored by six selenium atoms at a distance in the 2.4 Å range indicating substantial atomic relaxations. Mild annealing up to 520 K leads to the formation of α-FeSe, characterized by a local order extending up to the sixth shell (5.80 Å). Ab initio calculations predict a noncollinear magnetic ordering with a transition temperature of 3.5-10 K depending on the iron concentration and the number of the layers in which Fe is located.We acknowledge financial support from DFG through priority program SPP1666 (Topological Insulators). M.M.O. and E.V.C. thank the Tomsk State University Academic D. I. Mendeleev Fund Program (Research Grant No. 8.1.05.2015). Partial support by the Saint Petersburg State University Project No. 11.50.202.2015 is also acknowledged.Peer Reviewe

Tuning the dirac point position in Bi2Se3(0001) via surface carbon doping

Under the terms of the Creative Commons Attribution License 3.0 (CC-BY).-- et al.Angular resolved photoemission spectroscopy in combination with ab initio calculations show that trace amounts of carbon doping of the Bi2Se3 surface allows the controlled shift of the Dirac point within the bulk band gap. In contrast to expectation, no Rashba-split two-dimensional electron gas states appear. This unique electronic modification is related to surface structural modification characterized by an expansion of the top Se-Bi spacing of approximate to 11% as evidenced by surface x-ray diffraction. Our results provide new ways to tune the surface band structure of topological insulators.This work is supported by the DFG through Priority Program “Topological Insulators (SPP 1666)” and by Science Development Foundation under the President of the Republic of Azerbaijan [Grant No. EIF-2011-1(3)-82/69/4-M-50].Peer Reviewe

Atomic structure and properties of magnetic adsorbates on the topological insulator Bi2Se

Resumen del trabajo presentado al Moscow International Symposium on Magnetism (MISM), celebrado en Moscú (Rusia) del 29 de junio al 3 de julio de 2014.Support by DFG through SPP1666 is acknowledged.Peer reviewe

Ascertaining the nanocluster formation within an ion-irradiated Pt/Ni/C multi-trilayer with X-ray absorption spectroscopy

In this work nanoclusters formed in a Pt/Ni/C multi-trilayer by the ion-irradiated method of synthesis are characterized. In particular, an attempt to understand the role of interfaces in the synthesis is made. With this objective, ion-irradiation-induced structural changes in a Pt/Ni/C multi-trilayer using X-ray absorption spectroscopy (at the Ni K-edge) in conjunction with the X-ray standing-wave technique are investigated. The XANES analysis identifies chemical binding at pristine Ni/C and Ni/Pt interfaces, in contrast with physical adsorption at the Pt/C interface. The chemical nature of the interfaces determines their relative stability with respect to irradiation and controls the extent of metallic diffusion. The most interesting structural change, upon irradiation, is the disruption of the Pt/C interface and subsequent migration of Pt atoms towards pre-diffused Ni atoms within the C layer, leading to the formation of Ni-centered Ni-Pt bimetallic nanoclusters (with Ni:Pt = 60:40). These clusters are highly disordered beyond their nearest neighbor and find wide-scale applications as, for example, magnetic devices etc. The implications of these findings on the design goals are discussed

Atomic and electronic structure of bismuth-bilayer-terminated Bi2Se3(0001) prepared by atomic hydrogen etching

A bilayer of bismuth is recognized as a prototype two-dimensional topological insulator. Here we present a simple and well reproducible top-down approach to prepare a flat and well ordered bismuth bilayer with a lateral size of several hundred nanometers on Bi2Se3(0001). Using scanning tunneling microscopy, surface x-ray diffraction, and Auger electron spectroscopy we show that exposure of Bi2Se3(0001) to atomic hydrogen completely removes selenium from the top quintuple layer. The band structure of the system, calculated from first principles for the experimentally derived atomic structure, is in excellent agreement with recent photoemission data. Our results open interesting perspectives for the study of topological insulators in general

Polymer dynamics of well-defined, chain-end-functionalized polystyrenes by dielectric spectroscopy

A novel strategy is described to study polymer dynamics by using a combination of dielectric spectroscopy and functionalized polymers. The first results are presented using various well-defined, chain-end-functionalized polystyrenes (PS) synthesized using a combination of modern anionic polymerization techniques and hydrosilylation chemistry. The end-functionalized polystyrenes investigated contain the cyano (−CN), hydroxyl (−OH), acetyl (−OCOCH3, −Ac), or ethyl ether (−OCH2CH3, −OEt) groups. By applying broadband dielectric spectroscopy (BDS) over an extensive temperature range (approximately 50−413 K), it was possible to fully characterize the polymer dynamics associated with the segmental α-relaxation as well as the local secondary process related to the specific movement of the functional groups themselves. Combining these data with the results from differential scanning calorimetry (DSC), it is shown that for rather large functional groups the overall polymer matrix properties are altered, giving rise to a decrease in the glass transition temperature. The trend can be rationalized in terms of free volume effects caused by the bulky functional groups and points toward matrix plasticization effects. However, for cyano-functionalized PS the inclusion of this group does not significantly affect the matrix properties. By taking advantage of the strong dipole moment of the CN group, a clear dielectric signal can be obtained that can be used to selectively study the specific dynamics where the group is located. In other words, by appropriately attaching cyano groups at different parts of the chains, these can be exploited as in situ dielectric probes that allow determination of specific contributions to dynamical processes in polymers.The University of Akron researchers gratefully acknowledge Repsol YPF for partial support of this research and also FMC, Lithium Division, for providing samples of sec-butyllithium. The Basque Country Government (IT-436-07) and the Spanish Ministry of Education and Science (MAT 2007-63681), Consolider (Spanish Ministry of Science), are gratefully acknowledged.Peer reviewe