856 research outputs found
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
A family-oriented therapy program for youths with substance abuse: long-term outcomes related to relapse and academic or social status
A family-oriented treatment program for youths with ketamine abuse and their caregivers: a pilot study in Taiwan
Uniaxial ferromagnetism in the kagome metal TbVSn
The synthesis and characterization of the vanadium-based kagome metal
TbVSn is presented. X-ray measurements confirm this material forms
with the same crystal structure type as the recently investigated kagome metals
GdVSn and YVSn, with space group symmetry P6/mmm. A signature
of a phase transition at 4.1K is observed in heat capacity, resistivity, and
magnetic susceptibility measurements, and both resistivity and magnetization
measurements exhibit hysteresis in magnetic field. Furthermore, a strikingly
large anisotropy in the magnetic susceptibility was observed, with the c-axis
susceptibility nearly 100 times the ab plane susceptibility at 2K. This is
highly suggestive of uniaxial ferromagnetism, and the large size of
9.4/f.u. indicates the Tb electronic moments cooperatively
align perpendicular to the V kagome lattice plane. The entropy at the phase
transition is nearly Rln(2), indicating that the CEF ground state of the
Tb ion is a doublet, and therefore the sublattice of electrons in
this material can be shown to map at low temperatures to the Ising model in a
D symmetry environment. Hall measurements at temperatures from 300K to
1.7K can be described by two-band carrier transport at temperatures below
around 150K, with a large increase in both hole and electron mobilities,
similar to YVSn, and an anomalous Hall effect is seen below the
ordering temperature. Angle-resolved photoemission measurements above the
magnetic ordering temperature reveal typical kagome dispersions. Our study
presents TbVSn as an ideal system to study the interplay between
Ising ferromagnetism and non-trivial electronic states emerging from a kagome
lattice
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
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
Intertwined magnetism and charge density wave order in kagome FeGe
Electron correlations often lead to emergent orders in quantum materials.
Kagome lattice materials are emerging as an exciting platform for realizing
quantum topology in the presence of electron correlations. This proposal stems
from the key signatures of electronic structures associated with its lattice
geometry: flat band induced by destructive interference of the electronic
wavefunctions, topological Dirac crossing, and a pair of van Hove singularities
(vHSs). A plethora of correlated electronic phases have been discovered amongst
kagome lattice materials, including magnetism, charge density wave (CDW),
nematicity, and superconductivity. These materials can be largely organized
into two types: those that host magnetism and those that host CDW order.
Recently, a CDW order has been discovered in the magnetic kagome FeGe,
providing a new platform for understanding the interplay between CDW and
magnetism. Here, utilizing angle-resolved photoemission spectroscopy, we
observe all three types of electronic signatures of the kagome lattice: flat
bands, Dirac crossings, and vHSs. From both the observation of a
temperature-dependent shift of the vHSs towards the Fermi level as well as
guidance via first-principle calculations, we identify the presence of the vHSs
near the Fermi level (EF) to be driven by the development of underlying
magnetic exchange splitting. Furthermore, we show spectral evidence for the CDW
order as gaps that open on the near-EF vHS bands, as well as evidence of
electron-phonon coupling from a kink on the vHS band together with phonon
hardening observed by inelastic neutron scattering. Our observation points to
the magnetic interaction-driven band modification resulting in the formation of
the CDW order, indicating an intertwined connection between the emergent
magnetism and vHS charge order in this moderately-correlated kagome metal.Comment: submitted on April 22, 202
Discovery of charge density wave in a correlated kagome lattice antiferromagnet
A hallmark of strongly correlated quantum materials is the rich phase diagram
resulting from competing and intertwined phases with nearly degenerate ground
state energies. A well-known example is the copper oxides, where a charge
density wave (CDW) is ordered well above and strongly coupled to the magnetic
order to form spin-charge separated stripes that compete with
superconductivity. Recently, such rich phase diagrams have also been revealed
in correlated topological materials. In two-dimensional kagome lattice metals
consisting of corner-sharing triangles, the geometry of the lattice can produce
flat bands with localized electrons, non-trivial topology, chiral magnetic
order, superconductivity and CDW order. While CDW has been found in weakly
electron correlated nonmagnetic AV3Sb5 (A = K, Rb, Cs), it has not yet been
observed in correlated magnetic ordered kagome lattice metals. Here we report
the discovery of CDW within the antiferromagnetic (AFM) ordered phase of kagome
lattice FeGe. The CDW in FeGe occurs at wavevectors identical to that of
AV3Sb5, enhances the AFM ordered moment, and induces an emergent anomalous Hall
effect. Our findings suggest that CDW in FeGe arises from the combination of
electron correlations-driven AFM order and van Hove singularities-driven
instability possibly associated with a chiral flux phase, in stark contrast to
strongly correlated copper oxides and nickelates, where the CDW precedes or
accompanies the magnetic order.Comment: 36 pages, 4 figures in main tex
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