105 research outputs found
Non-adiabatic Kohn Anomaly in Heavily Boron-doped Diamond
We report evidence of a non-adiabatic Kohn anomaly in boron-doped diamond,
using a joint theoretical and experimental analysis of the phonon dispersion
relations. We demonstrate that standard calculations of phonons using density
functional perturbation theory are unable to reproduce the dispersion relations
of the high-energy phonons measured by high-resolution inelastic x-ray
scattering. On the contrary, by taking into account non-adiabatic effects
within a many-body field-theoretic framework, we obtain excellent agreement
with our experimental data. This result indicates a breakdown of the
Born-Oppenheimer approximation in the phonon dispersion relations of
boron-doped diamond.Comment: 6 pages, 2 figure
Giant Kohn anomaly and the phase transition in charge density wave ZrTe_3
A strong Kohn anomaly in ZrTe_3 is identified in the mostly transverse
acoustic phonon branch along the modulation vector q_P with polarization along
the a* direction. This soft mode freezes to zero frequency at the transition
temperature T_P and the temperature dependence of the frequency is strongly
affected by fluctuation effects. Diffuse x-ray scattering of the incommensurate
superstructure shows a power law scaling of the intensity and the correlation
length that is compatible with an order parameter of dimension n = 2.Comment: 4 pages, 4 figures. accepted at Phys. Rev. Let
Buried double CuO chains in YBaCuO uncovered by nano-ARPES
The electron dynamics in the CuO chains has been elusive in Y-Ba-Cu-O cuprate
systems by means of standard angle-resolved photoemission spectroscopy (ARPES);
cleaved sample exhibits areas terminated by both CuO-chain or BaO layers, and
the size of a typical beam results in ARPES signals that are superposed from
both terminations. Here, we employ spatially-resolved ARPES with submicrometric
beam (nano-ARPES) to reveal the surface-termination-dependent electronic
structures of the double CuO chains in YBaCuO. We present the first
observation of sharp metallic dispersions and Fermi surfaces of the double CuO
chains buried underneath the CuO-plane block on the BaO terminated surface.
While the observed Fermi surfaces of the CuO chains are highly one-dimensional,
the electrons in the CuO-chains do not undergo significant electron
correlations and no signature of a Tomonaga-Luttinger liquid nor a marginal
Fermi liquid is found. Our works represent an important experimental step
toward understanding of the charge dynamics and provides a starting basis for
modelling the high- superconductivity in YBCO cuprate systems.Comment: 10 pages, 5 figures including supplementary material (4 pages, 2
figures
Evolution of the charge density wave superstructure in under pressure
The material ZrTe3 is a well-known example of an incommensurate periodic lattice distortion (PLD) at low temperatures due to a charge density wave (CDW). Previous studies have found a sharp boundary as a function of pressure between CDW below 5 GPa and bulk superconductivity above this value. We present a study of low-temperature-high-pressure single crystal x-ray diffraction along with ab initio density functional theory calculations. The modulation vector qCDW is found to change smoothly with pressure until the PLD is lost. Fermi surface calculations reproduce these changes, but neither these nor the experimental crystal lattice structure show a particular step change at 5 GPa, thus leading to the conclusion that the CDW is lost accidentally by tipping the balance of CDW formation in the Fermi surface nesting that stabilizes it
Anharmonicity due to Electron-Phonon Coupling in Magnetite
We present the results of inelastic x-ray scattering for magnetite and
analyze the energies and spectral widths of the phonon modes with different
symmetries in a broad range of temperature 125<T<293 K. The phonon modes with
X_4 and Delta_5 symmetries broaden in a nonlinear way with decreasing
temperature when the Verwey transition is approached. It is found that the
maxima of phonon widths occur away from high-symmetry points which indicates
the incommensurate character of critical fluctuations. Strong phonon
anharmonicity induced by electron-phonon coupling is discovered within ab
initio calculations which take into account local Coulomb interactions at Fe
ions. It (i) explains observed anomalous phonon broadening, and (ii)
demonstrates that the Verwey transition is a cooperative phenomenon which
involves a wide spectrum of phonons coupled to charge fluctuations condensing
in the low-symmetry phase.Comment: 5 pages, 5 figures, accepted in Physical Review Letter
Phonon anomalies and lattice dynamics in superconducting oxychlorides CaCuOCl
We present a comprehensive study of the phonon dispersion in an underdoped,
superconducting CaCuOCl crystal. We interpret the results using
lattice dynamical calculations based on a shell model, and we compare the
results, to other hole-doped cuprates, in particular to the ones isomorphic to
LaSrCuO (LSCO). We found that an anomalous dip in the Cu-O bond
stretching dispersion develops in oxychlorides with a simultaneous marked
broadening of the mode. The broadening is maximum at that corresponds to the charge-modulations propagation vector. Our analysis
also suggests that screening effects in calculations may cause an apparent
cosine-shaped bending of the Cu-O bond-stretching dispersion along both the
( 0 0) and ( 0) directions, that is not observed on the data close to
optimal doping. This observation suggests that the discrepancy between
experimental data and \textit{ab-initio} calculations on this mode originates
from an overestimation of the doping effects on the mode
Orbital Subband Structures and Chiral Orbital Angular Momentum in the (001) Surface States of SrTiO
We have performed angle resolved photoemission spectroscopy (ARPES)
experiments on the surface states of SrTiO(001) using linearly and
circularly polarized light to investigate the subband structures of
out-of-plane orbitals and chiral orbital angular momentum (OAM).
The data taken in the first Brillouin zone reveal new subbands for
orbitals with Fermi wave vectors of 0.25 and 0.45 in
addition to the previously reported ones. As a result, there are at least two
subbands for all the Ti 3d t orbitals. Our circular dichroism ARPES data
is suggestive of a chiral OAM structure in the surface states and may provide
clues to the origin of the linear Rashba-like surface band splitting.Comment: 7 pages, 3 figures, Journal pape
Short-Range Correlations in Magnetite above the Verwey Temperature
Magnetite, FeO, is the first magnetic material discovered and
utilized by mankind in Ancient Greece, yet it still attracts attention due to
its puzzling properties. This is largely due to the quest for a full and
coherent understanding of the Verwey transition that occurs at K and
is associated with a drop of electric conductivity and a complex structural
phase transition. A recent detailed analysis of the structure, based on single
crystal diffraction, suggests that the electron localization pattern contains
linear three-Fe-site units, the so-called trimerons. Here we show that whatever
the electron localization pattern is, it partially survives up to room
temperature as short-range correlations in the high-temperature cubic phase,
easily discernible by diffuse scattering. Additionally, {\it ab initio}
electronic structure calculations reveal that characteristic features in these
diffuse scattering patterns can be correlated with the Fermi surface topology.Comment: 7 pages, 6 figure
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