132 research outputs found
Electronic properties across metal-insulator transition in \beta-pyrochlore-type CsW2O6 epitaxial films
In CsW2O6, which undergoes a metal-insulator transition (MIT) at 213 K, the
emergence of exotic properties associated with rattling motion of Cs is
expected owing to its characteristic \beta-pyrochlore-type structure. However,
a hurdle for crystal growth hampers elucidation of detailed properties and
mechanisms of the MIT. Here we report on the epitaxial growth of
\beta-pyrochlore-type CsW2O6 films and their electronic properties across the
MIT. Using pulsed-laser deposi-tion technique, we grew single-crystalline
CsW2O6 films exhibiting remarkably lower resistivity compared with a
poly-crystalline bulk and sharp MIT around 200 K. Negative magnetoresistance
and positive Hall coefficient were found, which became pronounced below 200 K.
The valence-band and core-levels photoemission spectra indicated the drastic
changes across the MIT. In the valence band photoemission spectrum, the finite
density of states was observed at the Fermi level in the metallic phase. In
contrast, an energy gap appeared in the insulating phase. The split of W 4f
core-level spectrum suggested the charge disproportionation of W5+ and W6+ in
the insulating phase. The change of spectral shape in the Cs 4d core levels
reflected the rattling motion of Cs+ cations. These results strongly suggest
that CsW2O6 is a novel material, in which MIT is driven by the charge
disproportionation associated with the rattling motion.Comment: 8 pages, 6 figure
Origin of the anomalous mass renormalization in metallic quantum well states of correlated oxide SrVO
angle-resolved photoemission spectroscopy (ARPES) has been
performed on SrVO ultrathin films, which show metallic quantum well (QW)
states, to unveil the origin of the anomalous mass enhancement in the QW
subbands. The line-shape analysis of the ARPES spectra reveals that the
strength of the electron correlation increases as the subband bottom energy
approaches the Fermi level. These results indicate that the anomalous
subband-dependent mass enhancement mainly arises from the quasi-one-dimensional
character of confined V states as a result of their orbital-selective
quantization.Comment: 6 pages, 3 figure
Emergence of quantum critical behavior in metallic quantum-well states of strongly correlated oxides
Controlling quantum critical phenomena in strongly correlated electron
systems, which emerge in the neighborhood of a quantum phase transition, is a
major challenge in modern condensed matter physics. Quantum critical phenomena
are generated from the delicate balance between long-range order and its
quantum fluctuation. So far, the nature of quantum phase transitions has been
investigated by changing a limited number of external parameters such as
pressure and magnetic field. We propose a new approach for investigating
quantum criticality by changing the strength of quantum fluctuation that is
controlled by the dimensional crossover in metallic quantum well (QW)
structures of strongly correlated oxides. With reducing layer thickness to the
critical thickness of metal-insulator transition, crossover from a Fermi liquid
to a non-Fermi liquid has clearly been observed in the metallic QW of SrVO
by \textit{in situ} angle-resolved photoemission spectroscopy. Non-Fermi liquid
behavior with the critical exponent is found to emerge in the
two-dimensional limit of the metallic QW states, indicating that a quantum
critical point exists in the neighborhood of the thickness-dependent Mott
transition. These results suggest that artificial QW structures provide a
unique platform for investigating novel quantum phenomena in strongly
correlated oxides in a controllable fashion.Comment: 6 pages, 3 figure
Natural van der Waals heterostructural single crystals with both magnetic and topological properties
Heterostructures having both magnetism and topology are promising materials
for the realization of exotic topological quantum states while challenging in
synthesis and engineering. Here, we report natural magnetic van der Waals
heterostructures of (MnBi2Te4)m(Bi2Te3)n that exhibit controllable magnetic
properties while maintaining their topological surface states. The interlayer
antiferromagnetic exchange coupling is gradually weakened as the separation of
magnetic layers increases, and an anomalous Hall effect that is well coupled
with magnetization and shows ferromagnetic hysteresis was observed below 5 K.
The obtained homogeneous heterostructure with atomically sharp interface and
intrinsic magnetic properties will be an ideal platform for studying the
quantum anomalous Hall effect, axion insulator states, and the topological
magnetoelectric effect.Comment: 40 pages, 15 figure
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