125 research outputs found
Paramagnon dispersion in -FeSe observed by Fe -edge resonant inelastic x-ray scattering
We report an Fe -edge resonant inelastic x-ray scattering (RIXS) study of
the unusual superconductor -FeSe. The high energy resolution of this
RIXS experiment (55meV FWHM) made it possible to resolve
low-energy excitations of the Fe manifold. These include a broad peak
which shows dispersive trends between 100-200meV along the and
directions of the one-Fe square reciprocal lattice, and which can
be attributed to paramagnon excitations. The multi-band valence state of FeSe
is among the most metallic in which such excitations have been discerned by
soft x-ray RIXS
Possible strong electron-lattice interaction and giant magneto-elastic effects in Fe-pnictides
The possibility for an appreciable many-body contribution to the
electron-phonon interaction (EPI) in Fe-pnictides is discussed in the model
where EPI is due to the electronic polarization of As- ions. The EPI-pol
coupling ismuch larger than the one obtained in the LDA band structure
calculations. It contributes significantly to the intra-band s-wave pairing and
an appreciable positive As-isotope effect in the superconducting critical
temperature is expected. In the Fe-breathing mode the linear (in the
Fe-displacements) EPI-pol coupling vanishes, while the non-linear (quadratic)
one is very strong. The part of the EPI-pol coupling, which is due to the
"potential" energy (the Hubbard U) changes, is responsible for the giant
magneto-elastic effects in MFe_{2}As_{2}, M=Ca, Sr, Ba since it gives much
larger contribution to the magnetic pressure than the band structure effects
do. This mechanism is contrary to the LDA prediction where the magneto-elastic
effects are due to the "kinetic" energy effects, i.e. the changes in the
density of states by the magneto-elastic effects. The proposed $EPI-pol is
expected to be operative (and strong) in other Fe-based superconductors with
electronically polarizable ions such as Se, Te, S etc., and in high-temperature
superconductors due to the polarizability of the O-ions.Comment: 6 pages, 2 figures; new References are added, text improved, typos
correcte
Emerging symmetric strain response and weakening nematic fluctuations in strongly hole- doped iron-based superconductors
Electronic nematicity is often found in unconventional superconductors, suggesting its rele-
vance for electronic pairing. In the strongly hole-doped iron-based superconductors, the
symmetry channel and strength of the nematic fluctuations, as well as the possible presence
of long-range nematic order, remain controversial. Here, we address these questions using
transport measurements under elastic strain. By decomposing the strain response into the
appropriate symmetry channels, we demonstrate the emergence of a giant in-plane sym-
metric contribution, associated with the growth of both strong electronic correlations and the
sensitivity of these correlations to strain. We find weakened remnants of the nematic fluc-
tuations that are present at optimal doping, but no change in the symmetry channel of
nematic fluctuations with hole doping. Furthermore, we find no indication of a nematic-
ordered state in the AFe2As2 (A =K, Rb, Cs) superconductors. These results revise the
current understanding of nematicity in hole-doped iron-based superconductors
Emergence of the nematic electronic state in FeSe
We present a comprehensive study of the evolution of the nematic electronic
structure of FeSe using high resolution angle-resolved photoemission
spectroscopy (ARPES), quantum oscillations in the normal state and
elastoresistance measurements. Our high resolution ARPES allows us to track the
Fermi surface deformation from four-fold to two-fold symmetry across the
structural transition at ~87 K which is stabilized as a result of the dramatic
splitting of bands associated with dxz and dyz character. The low temperature
Fermi surface is that a compensated metal consisting of one hole and two
electron bands and is fully determined by combining the knowledge from ARPES
and quantum oscillations. A manifestation of the nematic state is the
significant increase in the nematic susceptibility as approaching the
structural transition that we detect from our elastoresistance measurements on
FeSe. The dramatic changes in electronic structure cannot be explained by the
small lattice effects and, in the absence of magnetic fluctuations above the
structural transition, points clearly towards an electronically driven
transition in FeSe stabilized by orbital-charge ordering.Comment: Latex, 8 pages, 4 figure
The monoclinic crystal structure of -RuCl and the zigzag antiferromagnetic ground state
The layered honeycomb magnet alpha-RuCl3 has been proposed as a candidate to
realize a Kitaev spin model with strongly frustrated, bond-dependent,
anisotropic interactions between spin-orbit entangled jeff=1/2 Ru4+ magnetic
moments. Here we report a detailed study of the three-dimensional crystal
structure using x-ray diffraction on untwinned crystals combined with
structural relaxation calculations. We consider several models for the stacking
of honeycomb layers and find evidence for a crystal structure with a monoclinic
unit cell corresponding to a stacking of layers with a unidirectional in-plane
offset, with occasional in-plane sliding stacking faults, in contrast with the
currently-assumed trigonal 3-layer stacking periodicity. We report electronic
band structure calculations for the monoclinic structure, which find support
for the applicability of the jeff=1/2 picture once spin orbit coupling and
electron correlations are included. We propose that differences in the
magnitude of anisotropic exchange along symmetry inequivalent bonds in the
monoclinic cell could provide a natural mechanism to explain the spin gap
observed in powder inelastic neutron scattering, in contrast to spin models
based on the three-fold symmetric trigonal structure, which predict a gapless
spectrum within linear spin wave theory. Our susceptibility measurements on
both powders and stacked crystals, as well as neutron powder diffraction show a
single magnetic transition at TN ~ 13K. The analysis of the neutron data
provides evidence for zigzag magnetic order in the honeycomb layers with an
antiferromagnetic stacking between layers. Magnetization measurements on
stacked single crystals in pulsed field up to 60T show a single transition
around 8T for in-plane fields followed by a gradual, asymptotic approach to
magnetization saturation, as characteristic of strongly anisotropic exchange
interactions.Comment: 13 pages, 9 figures, published in Physical Review
Quenched nematic criticality separating two superconducting domes in an iron-based superconductor under pressure
The nematic electronic state and its associated nematic critical fluctuations
have emerged as potential candidates for superconducting pairing in various
unconventional superconductors. However, in most materials their coexistence
with other magnetically-ordered phases poses significant challenges in
establishing their importance. Here, by combining chemical and hydrostatic
physical pressure in FeSeS, we provide a unique access to a
clean nematic quantum phase transition in the absence of a long-range magnetic
order. We find that in the proximity of the nematic phase transition, there is
an unusual non-Fermi liquid behavior in resistivity at high temperatures that
evolves into a Fermi liquid behaviour at the lowest temperatures. From quantum
oscillations in high magnetic fields, we trace the evolution of the Fermi
surface and electronic correlations as a function of applied pressure. We
detect experimentally a Lifshitz transition that separates two distinct
superconducting regions: one emerging from the nematic electronic phase with a
small Fermi surface and strong electronic correlations and the other one with a
large Fermi surface and weak correlations that promotes nesting and
stabilization of a magnetically-ordered phase at high pressures. The lack of
mass divergence suggests that the nematic critical fluctuations are quenched by
the strong coupling to the lattice. This establishes that superconductivity is
not enhanced at the nematic quantum phase transition in the absence of magnetic
order.Comment: 4 figures, 9 page
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