102 research outputs found
Visualizing the Zhang-Rice singlet, molecular orbitals and pair formation in cuprate
The parent compound of cuprates is a charge-transfer-type Mott insulator with
strong hybridization between the Cu and O orbitals.
A key question concerning the pairing mechanism is the behavior of doped holes
in the antiferromagnetic (AF) Mott insulator background, which is a
prototypical quantum many-body problem. It was proposed that doped hole on the
O site tends to form a singlet, known as Zhang-Rice singlet (ZRS), with the
unpaired Cu spin. But experimentally little is known about the properties of a
single hole and the interplay between them that leads to superconductivity.
Here we use scanning tunneling microscopy to visualize the electronic states in
hole-doped , aiming to establish the atomic-scale local
basis for pair formation. A single doped hole is shown to have an in-gap state
and a clover-shaped spatial distribution that can be attributed to a localized
ZRS. When the dopants are close enough, they develop delocalized molecular
orbitals with characteristic stripe- and ladder-shaped patterns, accompanied by
the opening of a small gap around the Fermi level (). With
increasing doping, the molecular orbitals proliferate in space and gradually
form densely packed plaquettes, but the stripe and ladder patterns remain
nearly the same. The low-energy electronic states of the molecular orbitals are
intimately related to the local pairing properties, thus play a vitally
important role in the emergence of superconductivity. We propose that the
Cooper pair is formed by two holes occupying the stripe-like molecular orbital,
while the attractive interaction is mediated by the AF spin background
Momentum-Resolved Visualization of Electronic Evolution in Doping a Mott Insulator
High temperature superconductivity in cuprates arises from doping a parent
Mott insulator by electrons or holes. A central issue is how the Mott gap
evolves and the low-energy states emerge with doping. Here we report
angle-resolved photoemission spectroscopy measurements on a cuprate parent
compound by sequential in situ electron doping. The chemical potential jumps to
the bottom of the upper Hubbard band upon a slight electron doping, making it
possible to directly visualize the charge transfer band and the full Mott gap
region. With increasing doping, the Mott gap rapidly collapses due to the
spectral weight transfer from the charge transfer band to the gapped region and
the induced low-energy states emerge in a wide energy range inside the Mott
gap. These results provide key information on the electronic evolution in
doping a Mott insulator and establish a basis for developing microscopic
theories for cuprate superconductivity.Comment: 23 pages, 5 figure
Observation of Flat Band and Van Hove Singularity in Non-superconducting Nitrogen-doped Lutetium Hydride
Hydrogen-rich materials offer a compelling avenue towards room temperature
superconductivity, albeit under ultra-high pressure. However, the experimental
investigation of the electronic band structure remains elusive, due to the
inherent instability of most of the hydrogen-rich materials upon pressure
release. Very recently, nitrogen-doped lutetium hydride was claimed to host
room temperature superconductivity under near ambient pressure but was
disproven by following works. Upon decompression, nitrogen doped lutetium
hydride manifests a stable metallic phase with dark blue color. Moreover, high
temperature superconductivity has been reported in lutetium hydrides Lu4H23
(~71 K) under around 200 GPa. These properties engender an unprecedented
opportunity, allowing for the experimental investigation of the electronic band
structure intrinsic to hydrogen-rich material. In this work, using angle
resolved photoemission spectroscopy to investigate the non-superconducting
nitrogen doped lutetium hydride, we observed significant flat band and Van Hove
singularity marginally below the Fermi level. These salient features,
identified as critical elements, proffer potential amplifiers for the
realization of heightened superconductivity, as evidenced by prior research.
Our results not only unveil a confluence of potent strong correlation effects
and anisotropy within the Lu-H-N compound, but also provide a prospect for
engineering high temperature superconductivity through the strategic
manipulation of flat band and the VHS, effectively tailoring their alignment
with the Fermi energy.Comment: 26 pages, 9 figure
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