136 research outputs found
BiOCuS: A new superconducting compound with oxypnictide - related structure
The discovery of about 50 K superconductivity in the tetragonal Fe-based
pnictides has stimulated the search for superconductivity in a wide class of
materials with similar structure. Copper forms compounds isostructural to
LaOFeAs. Single phase BiOCuS can be prepared by a solid state reaction at
temperature lower than 500 C from a mixture of Bi2O3, Bi2S3 and Cu2S. The
samples have been characterized by means of EDX analysis, X-ray diffraction,
magnetic and electrical measurements. The cell parameters are a = 3.8708 A, c =
8.565 A. Charge carrier doping can be realized either by F substitutions for O,
or by Cu off-stoichiometry. The latter doping route leads to the occurrence of
superconductivity below Tc = 5.8 K
Doping nature of native defects in 1T-TiSe2
The transition metal dichalcogenide 1T-TiSe2 is a quasi two-dimensional
layered material with a charge density wave (CDW) transition temperature of
TCDW 200 K. Self-doping effects for crystals grown at different temperatures
introduce structural defects, modify the temperature dependent resistivity and
strongly perturbate the CDW phase. Here we study the structural and doping
nature of such native defects combining scanning tunneling
microscopy/spectroscopy and ab initio calculations. The dominant native single
atom dopants we identify in our single crystals are intercalated Ti atoms, Se
vacancies and Se substitutions by residual iodine and oxygen.Comment: 5 pages, 3 figure
STM microscopy of the CDW in 1T-TiSe2 in the presence of single atom defects
We present a detailed low temperature scanning tunneling microscopy study of
the commensurate charge density wave (CDW) in 1-TiSe in the presence of
single atom defects. We find no significant modification of the CDW lattice in
single crystals with native defects concentrations where some bulk probes
already measure substantial reductions in the CDW phase transition signature.
Systematic analysis of STM micrographs combined with density functional theory
modelling of atomic defect patterns indicate that the observed CDW modulation
lies in the Se surface layer. The defect patterns clearly show there are no
2-polytype inclusions in the CDW phase, as previously found at room
temperature [Titov A.N. et al, Phys. Sol. State 53, 1073 (2011). They further
provide an alternative explanation for the chiral Friedel oscillations recently
reported in this compound [J. Ishioka et al., Phys. Rev. B 84, 245125, (2011)].Comment: 5 pages, 4 figure
Rashba spin-splitting control at the surface of the topological insulator Bi2Se3
The electronic structure of Bi2Se3 is studied by angle-resolved photoemission
and density functional theory. We show that the instability of the surface
electronic properties, observed even in ultra-high-vacuum conditions, can be
overcome via in-situ potassium deposition. In addition to accurately setting
the carrier concentration, new Rashba-like spin-polarized states are induced,
with a tunable, reversible, and highly stable spin splitting. Ab-initio slab
calculations reveal that these Rashba state are derived from the 5QL
quantum-well states. While the K-induced potential gradient enhances the spin
splitting, this might be already present for pristine surfaces due to the
symmetry breaking of the vacuum-solid interface.Comment: A high-resolution version can be found at
http://www.physics.ubc.ca/~quantmat/ARPES/PUBLICATIONS/Articles/BiSe_K.pd
Doping nature of native defects in 1T−TiSe₂
The transition-metal dichalcogenide 1T−TiSe₂is a quasi-two-dimensional layered material with a charge density wave (CDW) transition temperature of TCDW≈200  K. Self-doping effects for crystals grown at different temperatures introduce structural defects, modify the temperature-dependent resistivity, and strongly perturbate the CDW phase. Here, we study the structural and doping nature of such native defects combining scanning tunneling microscopy or spectroscopy and ab initio calculations. The dominant native single atom dopants we identify in our single crystals are intercalated Ti atoms, Se vacancies, and Se substitutions by residual iodine and oxygen
Infrared and Raman spectroscopy measurements of a transition in the crystal structure and a closing of the energy gap of BiTeI under pressure
BiTeI is a giant Rashba spin splitting system, in which a non-centro
symmetric topological phase has recently been suggested to appear under high
pressure. We investigated the optical properties of this compound, reflectivity
and transmission, under pressures up to GPa. The gap feature in the
optical conductivity vanishes above GPa and does not reappear up to
at least GPa. The plasma edge, associated with intrinsically doped charge
carriers, is smeared out through a phase transition at GPa. Using high
pressure Raman spectroscopy, we follow the vibrational modes of BiTeI,
providing additional clear evidence that the transition at 9 GPa involves a
change of crystal structure. This change of crystal structure possibly inhibits
the high-pressure topological phase from occurring.Comment: 11 pages, 7 figures, with Supplemental Material, Accepted by PRL Dec
17, 201
Short-range phase coherence and origin of the charge density wave
The impact of variable Ti self-doping on the 1T−TiSe2 charge density wave (CDW) is studied by scanning tunneling microscopy. Supported by density functional theory, we show that agglomeration of intercalated-Ti atoms acts as preferential nucleation centers for the CDW that breaks up in phase-shifted CDW domains whose size directly depends on the intercalated-Ti concentration and which are separated by atomically sharp phase boundaries. The close relationship between the diminution of the CDW domain size and the disappearance of the anomalous peak in the temperature-dependent resistivity allows to draw a coherent picture of the 1T−TiSe2 CDW phase transition and its relation to excitons
Local resilience of the 1T\text{\ensuremath{-}}{\mathrm{TiSe}}_{2} charge density wave to Ti self-doping
In Ti-intercalated self-doped 1T−TiSe2 crystals, the charge density wave (CDW) superstructure induces two nonequivalent sites for Ti dopants. Recently, it has been shown that increasing Ti doping dramatically influences the CDW by breaking it into phase-shifted domains. Here, we report scanning tunneling microscopy and spectroscopy experiments that reveal a dopant-site dependence of the CDW gap. Supported by density functional theory, we demonstrate that the loss of the long-range phase coherence introduces an imbalance in the intercalated-Ti site distribution and restrains the CDW gap closure. This local resilient behavior of the 1T−TiSe2 CDW reveals an entangled mechanism between CDW, periodic lattice distortion, and induced nonequivalent defects
Scanning tunneling microscopy of the charge density wave in in the presence of single atom defects
We present a detailed low-temperature scanning tunneling microscopy (STM) study of the commensurate charge density wave (CDW) in in the presence of single atom defects. We find no significant modification of the CDW lattice in single crystals with native defect concentrations where some bulk probes already measure substantial reductions in the CDW phase transition signature. A systematic analysis of STM micrographs combined with density functional theory modeling of atomic defect patterns indicate that the observed CDW modulation lies in the Se surface layer. The defect patterns clearly show there are no 2H-polytype inclusions in the CDW phase, as previously found at room temperature [A. N. Titov et al., Phys. Solid State 53, 1073 (2011)]. They further provide an alternative explanation for the chiral Friedel oscillations recently reported in this compound [J. Ishioka et al., Phys. Rev. B 84, 245125 (2011)]
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