638 research outputs found
The Nematic Energy Scale and the Missing Electron Pocket in FeSe
Superconductivity emerges in proximity to a nematic phase in most iron-based
superconductors. It is therefore important to understand the impact of
nematicity on the electronic structure. Orbital assignment and tracking across
the nematic phase transition prove to be challenging due to the multiband
nature of iron-based superconductors and twinning effects. Here, we report a
detailed study of the electronic structure of fully detwinned FeSe across the
nematic phase transition using angle-resolved photoemission spectroscopy. We
clearly observe a nematicity-driven band reconstruction involving dxz, dyz, and
dxy orbitals. The nematic energy scale between dxz and dyz bands reaches a
maximum of 50 meV at the Brillouin zone corner. We are also able to track the
dxz electron pocket across the nematic transition and explain its absence in
the nematic state. Our comprehensive data of the electronic structure provide
an accurate basis for theoretical models of the superconducting pairing in
FeSe
Observation of Temperature-Induced Crossover to an Orbital-Selective Mott Phase in AFeSe (A=K, Rb) Superconductors
In this work, we study the AFeSe (A=K, Rb) superconductors
using angle-resolved photoemission spectroscopy. In the low temperature state,
we observe an orbital-dependent renormalization for the bands near the Fermi
level in which the dxy bands are heavily renormliazed compared to the dxz/dyz
bands. Upon increasing temperature to above 150K, the system evolves into a
state in which the dxy bands have diminished spectral weight while the dxz/dyz
bands remain metallic. Combined with theoretical calculations, our observations
can be consistently understood as a temperature induced crossover from a
metallic state at low temperature to an orbital-selective Mott phase (OSMP) at
high temperatures. Furthermore, the fact that the superconducting state of
AFeSe is near the boundary of such an OSMP constraints the
system to have sufficiently strong on-site Coulomb interactions and Hund's
coupling, and hence highlight the non-trivial role of electron correlation in
this family of iron superconductors
Spectral Evidence for Emergent Order in BaNaFeAs
We report an angle-resolved photoemission spectroscopy study of the
iron-based superconductor family, BaNaFeAs. This system
harbors the recently discovered double-Q magnetic order appearing in a
reentrant C phase deep within the underdoped regime of the phase diagram
that is otherwise dominated by the coupled nematic phase and collinear
antiferromagnetic order. From a detailed temperature-dependence study, we
identify the electronic response to the nematic phase in an orbital-dependent
band shift that strictly follows the rotational symmetry of the lattice and
disappears when the system restores C symmetry in the low temperature
phase. In addition, we report the observation of a distinct electronic
reconstruction that cannot be explained by the known electronic orders in the
system
Local antiferromagnetic exchange and collaborative Fermi surface as key ingredients of high temperature superconductors
Cuprates, ferropnictides and ferrochalcogenides are three classes of
unconventional high-temperature superconductors, who share similar phase
diagrams in which superconductivity develops after a magnetic order is
suppressed, suggesting a strong interplay between superconductivity and
magnetism, although the exact picture of this interplay remains elusive. Here
we show that there is a direct bridge connecting antiferromagnetic exchange
interactions determined in the parent compounds of these materials to the
superconducting gap functions observed in the corresponding superconducting
materials. High superconducting transition temperature is achieved when the
Fermi surface topology matches the form factor of the pairing symmetry favored
by local magnetic exchange interactions. Our result offers a principle guide to
search for new high temperature superconductors.Comment: 12 pages, 5 figures, 1 table, 1 supplementary materia
Non-Fermi liquid behavior in a correlated flatband pyrochlore lattice
Electronic correlation effects are manifested in quantum materials when
either the onsite Coulomb repulsion is large or the electron kinetic energy is
small. The former is the dominant effect in the cuprate superconductors or
heavy fermion systems while the latter in twisted bilayer graphene or
geometrically frustrated metals. However, the simultaneous cooperation of both
effects in the same quantum material--the design principle to produce a
correlated topological flat bands pinned at the Fermi level--remains rare.
Here, using angle-resolved photoemission spectroscopy, we report the
observation of a flat band at the Fermi level in a 3 pyrochlore metal
CuVS. From a combination of first-principles calculations and
slave-spin calculations, we understand the origin of this band to be a
destructive quantum-interference effect associated with the V pyrochlore
sublattice and further renormalization to the Fermi level by electron
interactions in the partially filled V orbitals. As a result, we find
transport behavior that indicates a deviation from Fermi-liquid behavior as
well as a large Sommerfeld coefficient. Our work demonstrates the pathway into
correlated topology by constructing and pinning correlated flat bands near the
Fermi level out of a pure -electron system by the combined cooperation of
local Coulomb interactions and geometric frustration in a pyrochlore lattice
system.Comment: 23 pages, 4 figures, to appear in Nature Physic
Reduction in Visceral Adiposity is Highly Related to Improvement in Vascular Endothelial Dysfunction among Obese Women: An Assessment of Endothelial Function by Radial Artery Pulse Wave Analysis
Because obesity is frequently complicated by other cardiovascular risk factors, the impact of a reduction in visceral adiposity on vascular endothelial dysfunction (VED) in obese patients is difficult to determine. In the present study, we evaluated the impact of a reduction in visceral adiposity on VED in obese women. Thirty-six premenopausal obese women (BMI ≥ 25 kg/m2) without complications were enrolled in the study. VED was evaluated by determining the augmentation index (AIx) from radial artery pulse waves obtained by applanation tonometry. Changes in AIx in response to nitroglycerin-induced endothelium-independent vasodilatation (ΔAIx-NTG) and in response to salbutamol administration (ΔAIx-Salb) were determined before and after weight reduction. After a 12-week weight reduction program, the average weight loss was 7.96±3.47 kg, with losses of 21.88±20.39 cm2 in visceral fat areas (p < 0.001). Pulse wave analysis combined with provocative pharmacological testing demonstrated preserved endothelium-independent vasodilation in healthy premenopausal obese women (ΔAIx-NTG: 31.36±9.80% before weight reduction vs. 28.25 ± 11.21% after weight reduction, p > 0.1) and an improvement in endothelial-dependent vasodilation following weight reduction (ΔAIx-Salb: 10.03±6.49% before weight reduction vs. 19.33 ± 9.28% after reduction, p < 0.001). A reduction in visceral adipose tissue was found to be most significantly related to an increase in ΔAIx-Salb (β=-0.57, p < 0.001). A reduction in visceral adiposity was significantly related to an improvement in VED. This finding suggests that reduction of visceral adiposity may be as important as the control of other major risk factors in the prevention of atherosclerosis in obese women
Non-Thermal Emergence of an Orbital-Selective Mott Phase in FeTeSe
Electronic correlation is of fundamental importance to high temperature
superconductivity. Iron-based superconductors are believed to possess moderate
correlation strength, which combined with their multi-orbital nature makes them
a fascinating platform for the emergence of exotic phenomena. A particularly
striking form is the emergence of an orbital selective Mott phase, where the
localization of a subset of orbitals leads to a drastically reconstructed Fermi
surface. Here, we report spectroscopic evidence of the reorganization of the
Fermi surface from FeSe to FeTe as Se is substituted by Te. We uncover a
particularly transparent way to visualize the localization of the
electron orbital through the suppression of its hybridization with the more
coherent electron orbitals, which leads to a redistribution of the
orbital-dependent spectral weight near the Fermi level. These noteworthy
features of the Fermi surface are accompanied by a divergent behavior of a band
renormalization in the orbital. All of our observations are further
supported by our theoretical calculations to be salient spectroscopic
signatures of such a non-thermal evolution from a strongly correlated metallic
phase towards an orbital-selective Mott phase in FeTeSe as Se
concentration is reduced.Comment: 11 pages, 5 figure
A Unified Description of Cuprate and Iron Arsenide Superconductors
We propose a unified description of cuprate and iron-based superconductivity.
Consistency with magnetic structure inferred from neutron scattering implies
significant constraints on the symmetry of the pairing gap for the iron-based
superconductors. We find that this unification requires the orbital pairing
formfactors for the iron arsenides to differ fundamentally from those for
cuprates at the microscopic level.Comment: 12 pages, 10 figures, 2 table
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