273 research outputs found
The direct Cu NQR Study of the Stripe Phase in the Lanthanum Cuprates
Using Cu NQR in Eu-doped La_(2-x)Sr_xCuO_4 we find the evidence of the pinned
stripe phase at 1.3K for 0.08<x<0.18. The pinned fraction increases by one
order of magnitude near hole doping x=1/8. The NQR lineshape reveals three
inequivalent Cu positions. A dramatic change of the NQR signal for x > 0.18
correlating with the onset of bulk superconductivity corresponds to the
depinning of the stripe phase.Comment: 4 pages, 3 figures, to appear in Physica C, Proceedings of the 6th
International Conference on Materials and Mechanisms of Superconductivity,
Houston, February 200
Mapping of the energetically lowest exciton in bulk -HfS
By combining electron energy-loss spectroscopy and state-of-the-art
computational methods, we were able to provide an extensive picture of the
excitonic processes in -HfS. The results differ significantly from the
properties of the more scrutinized group VI semiconducting transition metal
dichalcogenides such as MoS and WSe. The measurements revealed a
parabolic exciton dispersion for finite momentum parallel to the
K direction which allowed the determination of the effective exciton
mass. The dispersion decreases monotonically for momentum exchanges parallel to
the M high symmetry line. To gain further insight into the excitation
mechanisms, we solved the ab-initio Bethe-Salpeter equation for the system. The
results matched the experimental loss spectra closely, thereby confirming the
excitonic nature of the observed transitions, and produced the
momentumdependent binding energies. The simulations also demonstrated that the
excitonic transitions for || M occur exactly along that
particular high symmetry line. For || K on the other hand,
the excitations traverse the Brillouin zone crossing various high symmetry
lines. A particular interesting aspect of our findings was that the calculation
of the electron probability density revealed that the exciton assumes a
six-pointed star-like shape along the real space crystal planes indicating a
mixed Frenkel-Wannier character.Comment: 12 pages, 10 figure
Electron Energy-Loss Spectroscopy: A versatile tool for the investigations of plasmonic excitations
The inelastic scattering of electrons is one route to study the vibrational
and electronic properties of materials. Such experiments, also called electron
energy-loss spectroscopy, are particularly useful for the investigation of the
collective excitations in metals, the charge carrier plasmons. These plasmons
are characterized by a specific dispersion (energy-momentum relationship),
which contains information on the sometimes complex nature of the conduction
electrons in topical materials. In this review we highlight the improvements of
the electron energy-loss spectrometer in the last years, summarize current
possibilities with this technique, and give examples where the investigation of
the plasmon dispersion allows insight into the interplay of the conduction
electrons with other degrees of freedom
Recommended from our members
Charge transfer characteristics of F6TCNNQ–gold interface
The metal–organic interface between polycrystalline gold and hexafluorotetracyanonaphthoquinodimethane (F6TCNNQ) was investigated by photoelectron spectroscopy with the focus on the charge transfer characteristics from the metal to the molecule. The valence levels, as well as the core levels of the heterojunction, indicate a full electron transfer and a change in the chemical environment. The changes are observed in the first F6TCNNQ layers, whereas for further film growth, only neutral F6TCNNQ molecules could be detected. New occupied states below the Fermi level were observed in the valence levels, indicating a lowest unoccupied molecular orbital (LUMO) occupation due to the charge transfer. A fitting of the spectra reveals the presence of a neutral and a charged F6TCNNQ molecules, but no further species were present
Amorphous ferromagnetism and re-entrant magnetic glassiness in SmMoO: new insights into the electronic phase diagram of pyrochlore molybdates
We discuss the magnetic properties of a SmMoO single
crystal as investigated by means of different experimental techniques. In the
literature, a conventional itinerant ferromagnetic state is reported for the
Mo sublattice below K. However, our results of dc
magnetometry, muon spin spectroscopy (SR) and high-harmonics magnetic
ac susceptibility unambiguously evidence highly disordered conditions in this
phase, in spite of the crystalline and chemical order. This disordered magnetic
state shares several common features with amorphous ferromagnetic alloys. This
scenario for SmMoO is supported by the anomalously high
values of the critical exponents, as mainly deduced by a scaling analysis of
our dc magnetization data and confirmed by the other techniques. Moreover,
SR detects a significant static magnetic disorder at the microscopic
scale. At the same time, the critical divergence of the third-harmonic
component of the ac magnetic susceptibility around K leads to
additional evidence towards the glassy nature of this magnetic phase. Finally,
the longitudinal relaxation of spin polarization (also supported by
results of ac susceptibility) evidences re-entrant glassy features similar to
amorphous ferromagnets.Comment: 15 pages, 13 figure
Plasmons and Interband Transitions of CaSrCuO investigated by Electron Energy-Loss Spectroscopy
Electron energy-loss spectroscopy studies have been performed in order to get
a deeper insight into the electronic structure and elementary excitations of
the two-leg ladder system CaSrCuO. We find a strong
anisotropy of the loss function for momentum transfers along the a and
c-crystallographic axis, and a remarkable linear plasmon dispersion for a
momentum transfer parallel to the legs of the ladders. The investigated
spectral features are attributed to localized and delocalized charge-transfer
excitations and the charge carrier plasmon. The charge carrier plasmon position
and dispersion in the long wave-length limit agree well with expectations based
upon the band structure of the two-leg ladder, while the observed quasi-linear
plasmon dispersion might be related to the peculiar properties of underdoped
cuprates in general.Comment: 16 pages, 8 figure
Signatures of electronic polarons in LaSrMnO observed by electron energy-loss spectroscopy
The dielectric properties of LaSrMnO single crystals with
x = 0, 0.125, 0.25, and 0.5 were studied by means of electron energy-loss
spectroscopy as a function of temperature and momentum transfer. A clear
signature of the doped holes is observed around 1.65 eV energy loss, where
spectral weight emerges with increasing x. For all , this
doping-induced excitation can propagate within the ab-plane, as revealed by a
clear upward dispersion of the corresponding loss peak with increasing momentum
transfer. The hole-induced excitation also shifts to higher energies with the
onset of magnetic correlations for x = 0.5, implying a strong coupling of
charge and spin dynamics. We conclude that (i) the loss feature at 1.65 eV is a
signature of electronic polarons, which are created around doped holes, and
that (ii) this low-energy excitation involves the charge transfer between
manganese and oxygen. The finite dispersion of these excitations further
indicates significant polaron-polaron interactions.Comment: 7 pages, 4 figure
Exciton dispersion in para-quaterphenyl: Significant molecular interactions beyond Coulomb coupling
We have experimentally determined the momentum dependence of the electronic excitation spectra of para-quaterphenyl single crystals. The parallel arrangement of para-quaterphenyl molecules results in a strong Coulomb coupling of the molecular excitons. Such crystals have been considered to be a very good realization of the Frenkel exciton model, including the formation of H-type aggregates. Our data reveal an unexpected exciton dispersion of the upper Davydov component, which cannot be rationalized in terms of inter-molecular Coulomb coupling of the excitons. A significant reduction of the nearest neighbor coupling due to additional charge-transfer processes is able to provide an explanation of the data. Furthermore, the spectral onset of the excitation spectrum, which represents a heavy exciton resulting from exciton-phonon coupling, also shows a clear dispersion, which had been unknown so far. Finally, an optically forbidden excitation about 1 eV above the excitation onset is observed. © 2021 Author(s)
- …