137 research outputs found
PHOENEXS: System for Angle- and Spin-Resolved Photoemission at BESSY II
Article addresses overall performance, technical features and sample preparation facilities of movable endstation PHOENEXS at BESSY II which is used for spin- and angle-resolved photoemission
Evidence for topological band inversion of the phase change material Ge2Sb2Te5
We present an angle-resolved photoemission study of a ternary phase change
material, namely Ge2Sb2Te5, epitaxially grown on Si(111) in the metastable
cubic phase. The observed upper bulk valence band shows a minimum at Gamma-bar
being 0.3 eV below the Fermi level E_F and a circular Fermi contour around
Gamma-bar with a dispersing diameter of 0.27-0.36 Anstroms^-1. This is in
agreement with density functional theory calculations of the Petrov stacking
sequence in the cubic phase which exhibits a topological surface state. The
topologically trivial cubic KH stacking shows a valence band maximum at Gamma
in line with all previous calculations of the hexagonal stable phase exhibiting
the valence band maximum at Gamma for a trivial Z_2 topological invariant nu_0
and away from Gamma for non-trivial nu_0. Scanning tunneling spectroscopy
exhibits a band gap of 0.4 eV around E_F
One-dimensional electronic structure of phosphorene chains
Phosphorene, a 2D allotrope of phosphorus, is technologically very appealing
because of its semiconducting properties and narrow band gap. Further reduction
of the phosphorene dimensionality may spawn exotic properties of its electronic
structure, including lateral quantum confinement and topological edge states.
Phosphorene atomic chains self-assembled on Ag(111) have recently been
characterized structurally but were found by angle-resolved photoemission
(ARPES) to be electronically 2D. We show that these chains, although aligned
equiprobably to three directions of the Ag(111) surface, can be
characterized by ARPES because the three rotational variants are separated in
the angular domain. The dispersion of the phosphorus band measured along and
perpendicular to the chains reveals pronounced electronic confinement resulting
in a 1D band, flat and dispersionless perpendicular to the chain direction in
momentum space. Our density functional theory calculations reproduce the 1D
band for the experimentally determined structure of P/Ag(111). We predict a
semiconductor-to-metal phase transition upon increasing the density of the
chain array so that a 2D structure would be metallic
2D layered transport properties from topological insulator BiSe single crystals and micro flakes
Low-field magnetotransport measurements of topological insulators such as
BiSe are important for revealing the nature of topological surface
states by quantum corrections to the conductivity, such as
weak-antilocalization. Recently, a rich variety of high-field magnetotransport
properties in the regime of high electron densities ( cm)
were reported, which can be related to additional two-dimensional layered
conductivity, hampering the identification of the topological surface states.
Here, we report that quantum corrections to the electronic conduction are
dominated by the surface states for a semiconducting case, which can be
analyzed by the Hikami-Larkin-Nagaoka model for two coupled surfaces in the
case of strong spin-orbit interaction. However, in the metallic-like case this
analysis fails and additional two-dimensional contributions need to be
accounted for. Shubnikov-de Haas oscillations and quantized Hall resistance
prove as strong indications for the two-dimensional layered metallic behavior.
Temperature-dependent magnetotransport properties of high-quality BiSe
single crystalline exfoliated macro and micro flakes are combined with high
resolution transmission electron microscopy and energy-dispersive x-ray
spectroscopy, confirming the structure and stoichiometry. Angle-resolved
photoemission spectroscopy proves a single-Dirac-cone surface state and a
well-defined bulk band gap in topological insulating state. Spatially resolved
core-level photoelectron microscopy demonstrates the surface stability.Comment: Sci. Rep. (2016
Suppression of nematicity by tensile strain in multilayer FeSe/SrTiO films
The nematicity in multilayer FeSe/SrTiO films has been previously
suggested to be enhanced with decreasing film thickness. Motivated by this,
there have been many discussions about the competing relation between
nematicity and superconductivity. However, the criterion for determining the
nematicity strength in FeSe remains highly debated. The understanding of
nematicity and its relation to superconductivity in FeSe films is therefore
still controversial. Here, we fabricate multilayer FeSe/SrTiO films using
molecular beam epitaxy and study the nematic properties by combining
angle-resolved photoemission spectroscopy, nuclear magnetic resonance, and
scanning tunneling microscopy experiments. We unambiguously demonstrate that,
near the interface, the nematicity is suppressed by the SrTiO-induced
tensile strain; in the bulk region further away from the interface, the
strength of nematicity recovers to the bulk value. Our results not only solve
the controversy about the nematicity in multilayer FeSe films, but also offer
valuable insights into the relationship between nematicity and
superconductivity.Comment: 23 pages, 4 figure
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Suppression of nematicity by tensile strain in multilayer FeSe/SrTiO3 films
The nematicity in multilayer FeSe/SrTiO3 films has been previously suggested to be enhanced with decreasing film thickness. Motivated by this, there have been many discussions about the competing relation between nematicity and superconductivity. However, the criterion for determining the nematicity strength in FeSe remains highly debated. The understanding of nematicity as well as its relation to superconductivity in FeSe films is therefore still controversial. Here, we fabricate multilayer FeSe/SrTiO3 films using molecular beam epitaxy and study the nematic properties by combining angle-resolved photoemission spectroscopy, Se77 nuclear magnetic resonance, and scanning tunneling microscopy experiments. We unambiguously demonstrate that, near the interface, the nematic order is suppressed by the SrTiO3-induced tensile strain; in the bulk region further away from the interface, the strength of nematicity recovers to the bulk value. Our results not only solve the recent controversy about the nematicity in multilayer FeSe films, but also offer valuable insights into the relationship between nematicity and superconductivity
Electrical Transport Properties of VanadiumâDoped Bi2Te2.4Se0.6
Vanadiumâdoped Bi2âxTe2.4Se0.6 single crystals, with xâ=â0.015 and 0.03, are grown by the Bridgman method. Bandstructure characterization by angleâresolved photoemission spectroscopy (ARPES) measurements shows gapless topological surface states for both vanadium concentrations. The VanâderâPauw resistivity, the Hall charge carrier density, and the mobility in the temperature range from 0.3 to 300âK are strongly dependent on vanadium concentration, with carrier densities as low as 1.5âĂâ1016âcmâ3 and mobilities as high as 570âcm2âVâ1sâ1. As expected for transport in gapless topological surface states, the resistivity, carrier density, and mobility are constant below 10âK. The magnetoresistance shows weak antilocalization for both vanadium concentrations in the same temperature range. The weak antilocalization is analyzed with the HikamiâLarkinâNagaoka model, which yields phaseâcoherence lengths of up to 250ânm for xâ=â0.015.Deutsche Forschungsgemeinschaft
http://dx.doi.org/10.13039/501100001659Helmholtz-Gemeinschaft
http://dx.doi.org/10.13039/501100001656Peer Reviewe
Observation of Quantum-Tunneling Modulated Spin Texture in Ultrathin Topological Insulator Bi2Se3 Films
Understanding the spin-texture behavior of boundary modes in ultrathin
topological insulator films is critically essential for the design and
fabrication of functional nano-devices. Here by using spin-resolved
photoemission spectroscopy with p-polarized light in topological insulator
Bi2Se3 thin films, we report tunneling-dependent evolution of spin
configuration in topological insulator thin films across the metal-to-insulator
transition. We observe strongly binding energy- and wavevector-dependent spin
polarization for the topological surface electrons in the ultra-thin
gapped-Dirac-cone limit. The polarization decreases significantly with enhanced
tunneling realized systematically in thin insulating films, whereas magnitude
of the polarization saturates to the bulk limit faster at larger wavevectors in
thicker metallic films. We present a theoretical model which captures this
delicate relationship between quantum tunneling and Fermi surface spin
polarization. Our high-resolution spin-based spectroscopic results suggest that
the polarization current can be tuned to zero in thin insulating films forming
the basis for a future spin-switch nano-device.Comment: To appear in Nature Communications (2014); Expanded version of
http://arxiv.org/abs/1307.548
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Potential landscape-scale pollinator networks across Great Britain: structure, stability and influence of agricultural land cover
Understanding spatial variation in the structure and stability of plant-pollinator networks, and their relationship with anthropogenic drivers, is key to maintaining pollination services and mitigating declines. Constructing sufficient networks to examine patterns over large spatial scales remains challenging. Using biological records (citizen science), we constructed potential plant-pollinator networks at 10km resolution across Great Britain, comprising all potential interactions inferred from recorded floral visitation and species co-occurrence. We calculated network metrics (species richness, connectance, pollinator and plant generality) and adapted existing methods to assess robustness to sequences of simulated plant extinctions across multiple networks. We found positive relationships between agricultural land cover and both pollinator generality and robustness to extinctions under several extinction scenarios. Increased robustness was attributable to changes in plant community composition (fewer extinction-prone species) and network structure (increased pollinator generality). Thus, traits enabling persistence in highly agricultural landscapes can confer robustness to potential future perturbations on plant-pollinator networks
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