92 research outputs found
Anomalous momentum dependence of the multiband electronic structure of FeSe_1-xTe_x superconductors induced by atomic disorder
When periodicity of crystal is disturbed by atomic disorder, its electronic
state becomes inhomogeneous and band dispersion is obscured. In case of
Fe-based superconductors, disorder of chalcogen/pnictogen height causes
disorder of Fe 3d level splitting. Here, we report an angle-resolved
photoemission spectroscopy study on FeSe_1-xTe_x with the chalcogen height
disorder, showing that the disorder affects the Fe 3d band dispersions in an
orbital-selective way instead of simple obscuring effect. The reverse of the Fe
3d level splitting due to the chalcogen height difference causes the splitting
of the hole band with Fe 3d x^2-y^2 character around the Gamma point.Comment: 5 pages, 4 figure
Prediction of Orbital Ordering in Single-Layered Ruthenates
The key role of the orbital degree of freedom to understand the magnetic
properties of layered ruthenates is here discussed. In the G-type
antiferromagnetic phase of CaRuO, recent X-ray experiments reported the
presence of 0.5 hole per site in the orbital, while the
and orbitals contain 1.5 holes. This unexpected hole
distribution is explained by a novel state with orbital ordering (OO),
stabilized by a combination of Coulomb interactions and lattice distortions. In
addition, the rich phase diagram presented here suggests the possibility of
large magnetoresistance effects, and predicts a new ferromagnetic OO phase in
ruthenates.Comment: 4 pages, Revtex, with 2 figures embedded in the text. Submitted to
Phys. Rev. Let
Inhibition of the photoinduced structural phase transition in the excitonic insulator TaNiSe
Femtosecond time-resolved mid-infrared reflectivity is used to investigate
the electron and phonon dynamics occurring at the direct band gap of the
excitonic insulator TaNiSe below the critical temperature of its
structural phase transition. We find that the phonon dynamics show a strong
coupling to the excitation of free carriers at the \Gamma\ point of the
Brillouin zone. The optical response saturates at a critical excitation fluence
~mJ/cm due to optical absorption saturation. This
limits the optical excitation density in TaNiSe so that the system
cannot be pumped sufficiently strongly to undergo the structural change to the
high-temperature phase. We thereby demonstrate that TaNiSe exhibits a
blocking mechanism when pumped in the near-infrared regime, preventing a
nonthermal structural phase transition
Role of the Ce valence in the coexistence of superconductivity and ferromagnetism of CeOFBiS revealed by Ce -edge x-ray absorption spectroscopy
We have performed Ce -edge x-ray absorption spectroscopy (XAS)
measurements on CeOFBiS, in which the superconductivity of the
BiS layer and the ferromagnetism of the CeOF layer are induced
by the F-doping, in order to investigate the impact of the F-doping on the
local electronic and lattice structures. The Ce -edge XAS spectrum of
CeOBiS exhibits coexistence of (Ce) and (Ce)
state transitions revealing Ce mixed valency in this system. The spectral
weight of the state decreases with the F-doping and completely
disappears for where the system shows the superconductivity and the
ferromagnetism. The results suggest that suppression of Ce-S-Bi coupling
channel by the F-doping appears to drive the system from the valence
fluctuation regime to the Kondo-like regime, leading to the coexistence of the
superconducting BiS layer and the ferromagnetic CeOF layer.Comment: 5 pages, 5 figure
Ultrafast Electronic Band Gap Control in an Excitonic Insulator
We report on the nonequilibrium dynamics of the electronic structure of the
layered semiconductor TaNiSe investigated by time- and angle-resolved
photoelectron spectroscopy. We show that below the critical excitation density
of mJ cm, the band gap transiently, while it is
above . Hartree-Fock calculations reveal that this effect can
be explained by the presence of the low-temperature excitonic insulator phase
of TaNiSe, whose order parameter is connected to the gap size. This
work demonstrates the ability to manipulate the band gap of TaNiSe with
light on the femtosecond time scale
Inhibition of the photoinduced structural phase transition in the excitonic insulator
Femtosecond time-resolved midinfrared reflectivity is used to investigate the electron and phonon dynamics occurring at the direct band gap of the excitonic insulator Ta2NiSe5 below the critical temperature of its structural phase transition. We find that the phonon dynamics show a strong coupling to the excitation of free carriers at the Γ point of the Brillouin zone. The optical response saturates at a critical excitation fluence FC=0.30±0.08 mJ/cm2 due to optical absorption saturation. This limits the optical excitation density in Ta2NiSe5 so that the system cannot be pumped sufficiently strongly to undergo the structural change to the high-temperature phase. We thereby demonstrate that Ta2NiSe5 exhibits a blocking mechanism when pumped in the near-infrared regime, preventing a nonthermal structural phase transition
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