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Search Results: 1 - 10 of 461784 matches for " A. Varykhalov "
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Massless Dirac fermions in epitaxial graphene on Fe(110)
A. Varykhalov,J. Sánchez-Barriga,P. Hlawenka,O. Rader
Physics , 2012,
Abstract: Graphene grown on Fe(110)by chemical vapor deposition using propylene is investigated by means of angle-resolved photoemission. The presence of massless Dirac fermions is clearly evidenced by the observation of a fully intact Dirac cone. Unlike Ni(111) and Co(0001), the Fe(110) imposes a strongly anisotropic quasi-one-dimensional structure on the graphene. Certain signatures of a superlattice effect appear in the dispersion of its \sigma-bands but the Dirac cone does not reveal any detectable superlattice or quantum-size effects although the graphene corrugation is twice as large as in the established two-dimensional graphene superlattice on Ir(111).
Rashba splitting of 100 meV in Au-intercalated graphene on SiC
D. Marchenko,A. Varykhalov,J. Sánchez-Barriga,Th. Seyller,O. Rader
Physics , 2015,
Abstract: Intercalation of Au can produce giant Rashba-type spin-orbit splittings in graphene but this has not yet been achieved on a semiconductor substrate. For graphene/SiC(0001), Au intercalation yields two phases with different doping. Here, we report the preparation of an almost pure p-type graphene phase after Au intercalation. We observe a 100 meV Rashba-type spin-orbit splitting at 0.9 eV binding energy. We show that this giant splitting is due to hybridization and much more limited in energy and momentum space than for Au-intercalated graphene on Ni.
Topological surface state under graphene for two-dimensional spintronics in air
A. Varykhalov,D. Marchenko,M. R. Scholz,E. Rienks,T. K. Kim,O. Rader
Physics , 2011, DOI: 10.1103/PhysRevLett.108.066804
Abstract: Spin currents which allow for a dissipationless transport of information can be generated by electric fields in semiconductor heterostructures in the presence of a Rashba-type spin-orbit coupling. The largest Rashba effects occur for electronic surface states of metals but these cannot exist but under ultrahigh vacuum conditions. Here, we reveal a giant Rashba effect ({\alpha}_R ~ 1.5E-10 eVm) on a surface state of Ir(111). We demonstrate that its spin splitting and spin polarization remain unaffected when Ir is covered with graphene. The graphene protection is, in turn, sufficient for the spin-split surface state to survive in ambient atmosphere. We discuss this result along with evidences for a topological protection of the surface state.
Graphene for spintronics: giant Rashba splitting due to hybridization with Au
D. Marchenko,A. Varykhalov,M. R. Scholz,G. Bihlmayer,E. I. Rashba,A. Rybkin,A. M. Shikin,O. Rader
Physics , 2012, DOI: 10.1038/ncomms2227
Abstract: Graphene in spintronics has so far primarily meant spin current leads of high performance because the intrinsic spin-orbit coupling of its pi-electrons is very weak. If a large spin-orbit coupling could be created by a proximity effect, the material could also form active elements of a spintronic device such as the Das-Datta spin field-effect transistor, however, metal interfaces often compromise the band dispersion of massless Dirac fermions. Our measurements show that Au intercalation at the graphene-Ni interface creates a giant spin-orbit splitting (~100 meV) in the graphene Dirac cone up to the Fermi energy. Photoelectron spectroscopy reveals hybridization with Au-5d states as the source for the giant spin-orbit splitting. An ab initio model of the system shows a Rashba-split dispersion with the analytically predicted gapless band topology around the Dirac point of graphene and indicates that a sharp graphene-Au interface at equilibrium distance will account for only ~10 meV spin-orbit splitting. The ab initio calculations suggest an enhancement due to Au atoms that get closer to the graphene and do not violate the sublattice symmetry.
High spin polarization and circular dichroism of topological surface states on Bi2Te3
M. R. Scholz,J. Sánchez-Barriga,D. Marchenko,A. Varykhalov,A. Volykhov,L. V. Yashina,O. Rader
Physics , 2011,
Abstract: Topological insulators have been successfully identified by spin-resolved photoemission but the spin polarization remained low (~20%). We show for Bi2Te3 that the in-gap surface state is much closer to full spin polarization with measured values reaching 80% at the Fermi level. When hybridizing with the bulk it remains highly spin polarized which may explain recent unusual quantum interference results on Bi2Se3. The topological surface state shows a large circular dichroism in the photoelectron angle distribution with an asymmetry of ~20% the sign of which corresponds to that of the measured spin.
Tolerance of topological surface states towards magnetic moments: Fe on Bi2Te3 and Bi2Se3
M. R. Scholz,J. Sánchez-Barriga,D. Marchenko,A. Varykhalov,A. Volykhov,L. V. Yashina,O. Rader
Physics , 2011, DOI: 10.1103/PhysRevLett.108.256810
Abstract: Topological insulators(1-8) are a novel form of matter which features metallic surface states with quasirelativistic dispersion similar to graphene(9). Unlike graphene, the locking of spin and momentum and the protection by time-reversal symmetry(1-8) open up tremendous additional possibilities for external control of transport properties(10-18). Here we show by angle-resolved photoelectron spectroscopy that the topological sur-face states of Bi2Te3 and Bi2Se3 are stable against the deposition of Fe without opening a band gap. This stability extends to low submonolayer coverages meaning that the band gap reported recently(19) for Fe on Bi2Se3 is incorrect as well as to complete monolayers meaning that topological surface states can very well exist at interfaces with ferromagnets in future devices.
Non-monotonic pseudo-gap in high-Tc cuprates
A. A. Kordyuk,S. V. Borisenko,V. B. Zabolotnyy,R. Schuster,D. S. Inosov,R. Follath,A. Varykhalov,L. Patthey,H. Berger
Physics , 2008, DOI: 10.1103/PhysRevB.79.020504
Abstract: The mechanism of high temperature superconductivity is not resolved for so long because the normal state of cuprates is not yet understood. Here we show that the normal state pseudo-gap exhibits an unexpected non-monotonic temperature dependence, which rules out the possibility to describe it by a single mechanism such as superconducting phase fluctuations. Moreover, this behaviour, being remarkably similar to the behaviour of the charge ordering gap in the transition-metal dichalcogenides, completes the correspondence between these two classes of compounds: the cuprates in the PG state and the dichalcogenides in the incommensurate charge ordering state reveal virtually identical spectra of one-particle excitations as function of energy, momentum and temperature. These results suggest that the normal state pseudo-gap, which was considered to be very peculiar to cuprates, seems to be a general complex phenomenon for 2D metals. This may not only help to clarify the normal state electronic structure of 2D metals but also provide new insight into electronic properties of 2D solids where the metal-insulator and metal-superconductor transitions are considered on similar basis as instabilities of particle-hole and particle-particle interaction, respectively.
Anisotropic effect of warping on the lifetime broadening of topological surface states in angle-resolved photoemission from Bi$_2$Te$_3$
J. Sánchez-Barriga,M. R. Scholz,E. Golias,E. Rienks,D. Marchenko,A. Varykhalov,L. V. Yashina,O. Rader
Physics , 2015, DOI: 10.1103/PhysRevB.90.195413
Abstract: We analyze the strong hexagonal warping of the Dirac cone of Bi$_2$Te$_3$ by angle-resolved photoemission. Along $\overline{\Gamma}$$\overline{\rm M}$, the dispersion deviates from a linear behavior meaning that the Dirac cone is warped outwards and not inwards. We show that this introduces an anisotropy in the lifetime broadening of the topological surface state which is larger along $\overline{\Gamma}$$\overline{\rm K}$. The result is not consistent with nesting. Based on the theoretically predicted behavior of the ground-state spin texture of a strongly warped Dirac cone, we propose spin-dependent scattering processes as explanation for the anisotropic scattering rates. These results could help paving the way for optimizing future spintronic devices using topological insulators and controlling surface-scattering processes via external gate voltages.
Angle-resolved and core-level photoemission study of interfacing the topological insulator Bi1.5Sb0.5Te1.7Se1.3 with Ag, Nb and Fe
N. de Jong,E. Frantzeskakis,B. Zwartsenberg,Y. K. Huang,D. Wu,P. Hlawenka,J. Sánchez-Barriga,A. Varykhalov,E. van Heumen,M. S. Golden
Physics , 2015, DOI: 10.1103/PhysRevB.92.075127
Abstract: Interfaces between a bulk-insulating topological insulator (TI) and metallic adatoms have been studied using high-resolution, angle-resolved and core-level photoemission. Fe, Nb and Ag were evaporated onto Bi1.5Sb0.5Te1.7Se1.3 (BSTS) surfaces both at room temperature and 38K. The coverage- and temperature-dependence of the adsorption and interfacial formation process have been investigated, highlighting the effects of the overlayer growth on the occupied electronic structure of the TI. For all coverages at room temperature and for those equivalent to less than 0.1 monolayer at low temperature all three metals lead to a downward shift of the TI's bands with respect to the Fermi level. At room temperature Ag appears to intercalate efficiently into the van der Waals gap of BSTS, accompanied by low-level substitution of the Te/Se atoms of the termination layer of the crystal. This Te/Se substitution with silver increases significantly for low temperature adsorption, and can even dominate the electrostatic environment of the Bi/Sb atoms in the BSTS near-surface region. On the other hand, Fe and Nb evaporants remain close to the termination layer of the crystal. On room temperature deposition, they initially substitute isoelectronically for Bi as a function of coverage, before substituting for Te/Se atoms. For low temperature deposition, Fe and Nb are too immobile for substitution processes and show a behaviour consistent with clustering on the surface. For both Ag and Fe/Nb, these differing adsorption pathways leads to the qualitatively similar and remarkable behavior for low temperature deposition that the chemical potential first moves upward (n-type dopant behavior) and then downward (p-type behavior) on increasing coverage.
Multiband quasiparticle interference in the topological insulator Cu_(x)Bi_(2)Te_(3)
E. van Heumen,S. Johnston,J. Kaas,N. de Jong,F. Massee,J. Oen,E. Rienks,A. Varykhalov,J. B. Goedkoop,Y. Huang,M. S. Golden
Physics , 2011,
Abstract: We present angle resolved photoemission experiments and scanning tunneling spectroscopy results on the doped topological insulator Cu_(0.2)Bi_(2)Te_(3). Quasi particle interference (QPI) measurements, based on high resolution conductance maps of the local density of states show that there are three distinct energy windows for quasi particle scattering. Using a model Hamiltonian for this system two new scattering channels are identified: the first between the surface states and the conduction band and the second between conduction band states. The comparison of the QPI data with self- energies extracted from photoemission indicates that spin-orbit terms are relevant for surface state to conduction band scattering
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