95 research outputs found
Electronic Structure of the YH3 Phase from Angle-Resolved Photoemission Spectroscopy
Yttrium can be loaded with hydrogen up to high concentrations causing
dramatic structural and electronic changes of the host lattice. We report on
angle-resolved photoemission experiments of the Y trihydride phase. Most
importantly, we find the absence of metal d-bands at the Fermi level and a set
of flat, H-induced bands located at much higher binding energy than predicted,
indicating an increased electron affinity at H sites
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Spectromicroscopic measurements of electronic structure variations in atomically thin WSe2
Atomically thin transition metal dichalcogenides (TMDCs) are promising candidates for implementation in next generation semiconducting devices, for which laterally homogeneous behavior is needed. Here, we study the electronic structure of atomically thin exfoliated WSe2, a prototypical TMDC with large spin–orbit coupling, by photoemission electron microscopy, electron energy-loss spectroscopy, and density functional theory. We resolve the inhomogeneities of the doping level by the varying energy positions of the valence band. There appear to be different types of inhomogeneities that respond differently to electron doping, introduced by potassium intercalation. In addition, we find that the doping process itself is more complex than previously anticipated and entails a distinct orbital and thickness dependence that needs to be considered for effective band engineering. In particular, the density of selenium vs tungsten states depends on the doping level, which leads to changes in the optical response beyond increased dielectric screening. Our work gives insight into the inhomogeneity of the electron structure of WSe2 and the effects of electron doping, provides microscopic understanding thereof, and improves the basis for property engineering of 2D materials
Bare electron dispersion from photoemission experiments
Performing an in-depth analysis of the photoemission spectra along the nodal
direction of the high temperature superconductor Bi-2212 we have developed a
procedure to determine the underlying electronic structure and established a
precise relation of the measured quantities to the real and imaginary parts of
the self-energy of electronic excitations. The self-consistency of the
procedure with respect to the Kramers-Kronig transformation allows us to draw
conclusions on the applicability of the spectral function analysis and on the
existence of well defined quasiparticles along the nodal direction even for the
underdoped Bi-2212 in the pseudogap state.Comment: 4 pages 3 figures revtex, corrected misprint
Evolution of Superconductivity in Electron-Doped Cuprates: Magneto-Raman Spectroscopy
The electron-doped cuprates Pr_{2-x}Ce_xCuO_4 and Nd_{2-x}Ce_xCuO_4 have been
studied by electronic Raman spectroscopy across the entire region of the
superconducting (SC) phase diagram. The SC pairing strength is found to be
consistent with a weak-coupling regime except in the under-doped region where
we observe an in-gap collective mode at 4.5 k_{B}T_c while the maximum
amplitude of the SC gap is ~8 k_{B}T_{c}. In the normal state, doped carriers
divide into coherent quasi-particles (QPs) and carriers that remain incoherent.
The coherent QPs mainly reside in the vicinity of (\pi/2, \pi/2) regions of the
Brillouin zone (BZ). We find that only coherent QPs contribute to the
superfluid density in the B_{2g} channel. The persistence of SC coherence peaks
in the B_{2g} channel for all dopings implies that superconductivity is mainly
governed by interactions between the hole-like coherent QPs in the vicinity of
(\pi/2, \pi/2) regions of the BZ. We establish that superconductivity in the
electron-doped cuprates occurs primarily due to pairing and condensation of
hole-like carriers. We have also studied the excitations across the SC gap by
Raman spectroscopy as a function of temperature (T) and magnetic field (H) for
several different cerium dopings (x). Effective upper critical field lines
H*_{c2}(T, x) at which the superfluid stiffness vanishes and
H^{2\Delta}_{c2}(T, x) at which the SC gap amplitude is suppressed by field
have been determined; H^{2\Delta}_{c2}(T, x) is larger than H*_{c2}(T, x) for
all doping concentrations. The difference between the two quantities suggests
the presence of phase fluctuations that increase for x< 0.15. It is found that
the magnetic field suppresses the magnitude of the SC gap linearly at
surprisingly small fields.Comment: 13 pages, 8 figures; submitted to Phys. Rev.
Reply to Comment on:"Nonmonotonic d_{x^2-y^2} Superconducting Order Parameter in Nd_{2-x}Ce_xCuO_4"
We confirm that all the results of scanning SQUID, tunneling, ARPES,
penetration depth and Raman experiments are consistent with a nonmonotonic
d_{x^2-y^2} superconducting order parameter proposed in Phys. Rev. Lett., 88,
107002 (2002).Comment: Reply to Comment by F. Venturini, R. Hackl, and U. Michelucci
cond-mat/020541
Direct evidence for ferroelectric polar distortion in ultrathin lead titanate perovskite films
X-ray photoelectron diffraction is used to directly probe the intra-cell
polar atomic distortion and tetragonality associated with ferroelectricity in
ultrathin epitaxial PbTiO3 films. Our measurements, combined with ab-initio
calculations, unambiguously demonstrate non-centro-symmetry in films a few unit
cells thick, imply that films as thin as 3 unit cells still preserve a
ferroelectric polar distortion, and also show that there is no thick
paraelectric dead layer at the surface
Effect of Zn and Ni impurities on the quasiparticle renormalization in Bi-2212
The Cu substitution by Zn and Ni impurities and its influence on the mass
renormalization effects in angle resolved photoelectron spectra (ARPES) of
Bi-2212 is addressed. We show that the nonmagnetic Zn atoms have much stronger
effect both in nodal and antinodal parts of the Brillouin zone than magnetic
Ni. The observed changes are consistent with the behaviour of the spin
resonance mode as seen by inelastic neutron scattering in YBCO. This strongly
suggests that the "peak-dip-hump" and the "kink" in ARPES on the one side and
neutron resonance on the other are closely related features.Comment: 4 pages, 3 figure
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