54 research outputs found
Mott transition in Cr-doped V2O3 studied by ultrafast reflectivity: electron correlation effects on the transient response
The ultrafast response of the prototype Mott-Hubbard system (V1-xCrx)2O3 was
systematically studied with fs pump-probe reflectivity, allowing us to clearly
identify the effects of the metal-insulator transition on the transient
response. The isostructural nature of the phase transition in this material
made it possible to follow across the phase diagram the behaviour of the
detected coherent acoustic wave, whose average value and lifetime depend on the
thermodynamic phase and on the correlated electron density of states. It is
also shown how coherent lattice oscillations can play an important role in some
changes affecting the ultrafast electronic peak relaxation at the phase
transition, changes which should not be mistakenly attributed to genuine
electronic effects. These results clearly show that a thorough understanding of
the ultrafast response of the material over several tenths of ps is necessary
to correctly interpret its sub-ps excitation and relaxation regime, and appear
to be of general interest also for other strongly correlated materials.Comment: 6 pages, 3 figures. Europhysics Letters (in press
Coupling of a high-energy excitation to superconducting quasiparticles in a cuprate from Coherent Charge Fluctuation Spectroscopy
Dynamical information on spin degrees of freedom of proteins or solids can be
obtained by Nuclear Magnetic Resonance (NMR) and Electron Spin Resonance (ESR).
A technique with similar versatility for charge degrees of freedom and their
ultrafast correlations could move forward the understanding of systems like
unconventional superconductors. By perturbing the superconducting state in a
high-Tc cuprate using a femtosecond laser pulse, we generate coherent
oscillations of the Cooper pair condensate which can be described by an NMR/ESR
formalism. The oscillations are detected by transient broad-band reflectivity
and found to resonate at the typical scale of Mott physics (2.6 eV), suggesting
the existence of a non-retarded contribution to the pairing interaction, as in
unconventional (non Migdal-Eliashberg) theories.Comment: Accepted for publication in the Proceedings of the National Academy
of Sciences of the U.S.A. (PNAS
Evidence for a Peierls phase-transition in a three-dimensional multiple charge-density waves solid
The effect of dimensionality on materials properties has become strikingly
evident with the recent discovery of graphene. Charge ordering phenomena can be
induced in one dimension by periodic distortions of a material's crystal
structure, termed Peierls ordering transition. Charge-density waves can also be
induced in solids by strong Coulomb repulsion between carriers, and at the
extreme limit, Wigner predicted that crystallization itself can be induced in
an electrons gas in free space close to the absolute zero of temperature.
Similar phenomena are observed also in higher dimensions, but the microscopic
description of the corresponding phase transition is often controversial, and
remains an open field of research for fundamental physics. Here, we photoinduce
the melting of the charge ordering in a complex three-dimensional solid and
monitor the consequent charge redistribution by probing the optical response
over a broad spectral range with ultrashort laser pulses. Although the
photoinduced electronic temperature far exceeds the critical value, the
charge-density wave is preserved until the lattice is sufficiently distorted to
induce the phase transition. Combining this result with it ab initio}
electronic structure calculations, we identified the Peierls origin of multiple
charge-density waves in a three-dimensional system for the first time.Comment: Accepted for publication in Proc. Natl. Acad. Sci. US
Significant reduction of electronic correlations upon isovalent Ru substitution of BaFe2As2
We present a detailed investigation of Ba(Fe0.65Ru0.35)2As2 by transport
measurements and Angle Resolved photoemission spectroscopy. We observe that Fe
and Ru orbitals hybridize to form a coherent electronic structure and that Ru
does not induce doping. The number of holes and electrons, deduced from the
area of the Fermi Surface pockets, are both about twice larger than in
BaFe2As2. The contribution of both carriers to the transport is evidenced by a
change of sign of the Hall coefficient with decreasing temperature. Fermi
velocities increase significantly with respect to BaFe2As2, suggesting a
significant reduction of correlation effects. This may be a key to understand
the appearance of superconductivity at the expense of magnetism in undoped iron
pnictides
Doping-Dependent and Orbital-Dependent Band Renormalization in Ba(Fe_1-xCo_x)_2As_2 Superconductors
Angle resolved photoemission spectroscopy of Ba(Fe1-xCox)2As2 (x = 0.06,
0.14, and 0.24) shows that the width of the Fe 3d yz/zx hole band depends on
the doping level. In contrast, the Fe 3d x^2-y^2 and 3z^2-r^2 bands are rigid
and shifted by the Co doping. The Fe 3d yz/zx hole band is flattened at the
optimal doping level x = 0.06, indicating that the band renormalization of the
Fe 3d yz/zx band correlates with the enhancement of the superconducting
transition temperature. The orbital-dependent and doping-dependent band
renormalization indicates that the fluctuations responsible for the
superconductivity is deeply related to the Fe 3d orbital degeneracy.Comment: 5 pages, 4 figure
Nesting between hole and electron pockets in Ba(Fe1-xCox)2As2 (x=0-0.3) observed with angle-resolved photoemission
We present a comprehensive angle-resolved photoemission study of the
three-dimensional electronic structure of Ba(Fe1-xCox)2As2. The wide range of
dopings covered by this study, x=0 to x=0.3, allows to extract systematic
features of the electronic structure. We show that there are three different
hole pockets around the G point, the two inner ones being nearly degenerate and
rather two dimensional, the outer one presenting a strong three dimensional
character. The structure of the electron pockets is clarified by studying high
doping contents, where they are enlarged. They are found to be essentially
circular and two dimensional. From the size of the pockets, we deduce the
number of holes and electrons present at the various dopings. We find that the
net number of carriers is in good agreement with the bulk stoichiometry, but
that the number of each species (holes and electrons) is smaller than predicted
by theory. Finally, we discuss the quality of nesting in the different regions
of the phase diagram. The presence of the third hole pocket significantly
weakens the nesting at x=0, so that it may not be a crucial ingredient in the
formation of the Spin Density Wave. On the other hand, superconductivity seems
to be favored by the coexistence of two-dimensional hole and electron pockets
of similar sizes
Orbitally resolved lifetimes in Ba(Fe0.92Co0.08)2As2 measured by ARPES
Despite many ARPES investigations of iron pnictides, the structure of the
electron pockets is still poorly understood. By combining ARPES measurements in
different experimental configurations, we clearly resolve their elliptic shape.
Comparison with band calculation identify a deep electron band with the dxy
orbital and a shallow electron band along the perpendicular ellipse axis with
the dxz/dyz orbitals. We find that, for both electron and hole bands, the
lifetimes associated with dxy are longer than for dxz/dyz. This suggests that
the two types of orbitals play different roles in the electronic properties and
that their relative weight is a key parameter to determine the ground state
Ultrafast evolution and transient phases of a prototype out-of-equilibrium Mott-Hubbard material
The study of photoexcited strongly correlated materials is attracting growing interest since their rich phase diagram often translates into an equally rich out-of-equilibrium behaviour. With femtosecond optical pulses, electronic and lattice degrees of freedom can be transiently decoupled, giving the opportunity of stabilizing new states inaccessible by quasi-adiabatic pathways. Here we show that the prototype Mott-Hubbard material V2O3 presents a transient non-thermal phase developing immediately after ultrafast photoexcitation and lasting few picoseconds. For both the insulating and the metallic phase, the formation of the transient configuration is triggered by the excitation of electrons into the bonding a1g orbital, and is then stabilized by a lattice distortion characterized by a hardening of the A1g coherent phonon, in stark contrast with the softening observed upon heating. Our results show the importance of selective electron-lattice interplay for the ultrafast control of material parameters, and are relevant for the optical manipulation of strongly correlated systems. \ua9 The Author(s) 2017
Satellites and large doping- and temperature-dependence of electronic properties in hole-doped BaFe2As2
Over the last years, superconductivity has been discovered in several
families of iron-based compounds. Despite intense research, even basic
electronic properties of these materials, such as Fermi surfaces, effective
electron masses, or orbital characters are still subject to debate. Here, we
address an issue that has not been considered before, namely the consequences
of dynamical screening of the Coulomb interactions among Fe-d electrons. We
demonstrate its importance not only for correlation satellites seen in
photoemission spectroscopy, but also for the low-energy electronic structure.
From our analysis of the normal phase of BaFe2As2 emerges the picture of a
strongly correlated compound with strongly doping- and temperature-dependent
properties. In the hole overdoped regime, an incoherent metal is found, while
Fermi-liquid behavior is recovered in the undoped compound. At optimal doping,
the self-energy exhibits an unusual square-root energy dependence which leads
to strong band renormalizations near the Fermi level
Temperature-dependent electron-phonon coupling in LaSrCuO probed by femtosecond X-ray diffraction
The strength of the electron-phonon coupling parameter and its evolution
throughout a solid's phase diagram often determines phenomena such as
superconductivity, charge- and spin-density waves. Its experimental
determination relies on the ability to distinguish thermally activated phonons
from those emitted by conduction band electrons, which can be achieved in an
elegant way by ultrafast techniques. Separating the electronic from the
out-of-equilibrium lattice subsystems, we probed their re-equilibration by
monitoring the transient lattice temperature through femtosecond X-ray
diffraction in LaSrCuO single crystals with =0.1 and 0.21.
The temperature dependence of the electron-phonon coupling is obtained
experimentally and shows similar trends to what is expected from the
\textit{ab-initio} calculated shape of the electronic density-of-states near
the Fermi energy. This study evidences the important role of band effects in
the electron-lattice interaction in solids, in particular in superconductors
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