42 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
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
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
Erratum: A microscopic view on the Mott transition in chromium-doped V 2 O 3
Nature Communications 1, Article number: 105 (2010); published: 02 November 2010; updated: 17 January 2012. In Figure 2 of this Article, panel labels c and d were inadvertently switched. A typographical error was also introduced in the last sentence of the legend, which should have read 'The scale bar in panel c represents 10 μm'
Gap-dependent quasiparticle dynamics and coherent acoustic phonons in parent iron pnictide CaFe2As2 across the spin density wave phase transition
We report ultrafast quasiparticle (QP) dynamics and coherent acoustic phonons
in undoped CaFe_2As_2 iron pnictide single crystals exhibiting spin-density
wave (SDW) and concurrent structural phase transition at temperature TSDW ~ 165
K using femtosecond time-resolved pump-probe spectroscopy. The contributions in
transient differential reflectivity arising from exponentially decaying QP
relaxation and oscillatory coherent acoustic phonon mode show large variations
in the vicinity of T_SDW. From the temperature-dependence of the QP
recombination dynamics in the SDW phase, we evaluate a BCS-like temperature
dependent charge gap with its zero-temperature value of ~(1.6+/-0.2)k_BT_SDW,
whereas, much above T_SDW, an electron-phonon coupling constant of ~0.13 has
been estimated from the linear temperature-dependence of the QP relaxation
time. The long-wavelength coherent acoustic phonons with typical time-period of
~100 ps have been analyzed in the light of propagating strain pulse model
providing important results for the optical constants, sounds velocity and the
elastic modulus of the crystal in the whole temperature range of 3 K to 300 K.Comment: Revised version (to appear as Full Paper in Journal of Physical
Society of Japan (2013)); http://jpsj.ipap.jp/link?JPSJ/82/044715
Ultrafast transient generation of spin-densitywave order in the normal state of BaFe2As2 driven by coherent lattice vibrations
The interplay among charge, spin and lattice degrees of freedom in solids
gives rise to intriguing macroscopic quantum phenomena such as colossal
magnetoresistance, multiferroicity and high-temperature superconductivity.
Strong coupling or competition between various orders in these systems presents
the key to manipulate their functional properties by means of external
perturbations such as electric and magnetic fields or pressure. Ultrashort and
intense optical pulses have emerged as an interesting tool to investigate
elementary dynamics and control material properties by melting an existing
order. Here, we employ few-cycle multi-terahertz pulses to resonantly probe the
evolution of the spin-density-wave (SDW) gap of the pnictide compound BaFe2As2
following excitation with a femtosecond optical pulse. When starting in the
low-temperature ground state, optical excitation results in a melting of the
SDW order, followed by ultrafast recovery. In contrast, the SDW gap is induced
when we excite the normal state above the transition temperature. Very
surprisingly, the transient ordering quasi-adiabatically follows a coherent
lattice oscillation at a frequency as high as 5.5 THz. Our results attest to a
pronounced spin-phonon coupling in pnictides that supports rapid development of
a macroscopic order on small vibrational displacement even without breaking the
symmetry of the crystal