38 research outputs found
Prediction of perovskite-related structures in ACuO (A Ca, Sr, Ba, Sc, Y, La) using density functional theory and Bayesian optimization
Oxygen vacancy ordering in perovskite-type transition-metal oxides plays an
important role in the emergence of exotic electronic properties, as typified by
superconducting cuprates. In this study, we predict the stability of
oxygen-deficient perovskite structures in ACuO (A Ca, Sr, Ba, Sc,
Y, La) by density functional theory calculation. We introduce a combination of
the cluster expansion method, Gaussian process, and Bayesian optimization to
find stable oxygen-deficient structures among a considerable number of
candidates. Our calculations not only reproduce the reported structures but
suggest the presence of several unknown oxygen-deficient perovskite structures,
some of which are stabilized at high pressures. This work demonstrates the
great applicability of the present computational procedure for the elucidation
of the structural stability of strongly correlated oxides with a large
tolerance to oxygen deficiency.Comment: 11 pages, 9 figure
Rich structural phase diagram and thermoelectric properties of layered tellurides Mo1-xNbxTe2
MoTe2 is a rare transition-metal ditelluride having two kinds of layered
polytypes, hexagonal structure with trigonal prismatic Mo coordination and
monoclinic structure with octahedral Mo coordination. The monoclinic distortion
in the latter is caused by anisotropic metal-metal bonding. In this work, we
have examined the Nb doping effect on both polytypes of MoTe2 and clarified a
structural phase diagram for Mo1-xNbxTe2 containing four kinds of polytypes. A
rhombohedral polytype crystallizing in polar space group has been newly
identified as a high-temperature metastable phase at slightly Nb-rich
composition. Considering the results of thermoelectric measurements and the
first principles calculations, the Nb ion seemingly acts as a hole dopant in
the rigid band scheme. On the other hand, the significant interlayer
contraction upon the Nb doping, associated with the Te p-p hybridization, is
confirmed especially for the monoclinic phase, which implies a shift of the
p-band energy level. The origin of the metal-metal bonding in the monoclinic
structure is discussed in terms of the d electron counting and the Te p-p
hybridization.Comment: 16 pages, 6 figures, 1 table, to be published in APL Material
Giant enhancement of cryogenic thermopower by polar structural instability in the pressurized semimetal MoTe2
We found that a high mobility semimetal 1T'-MoTe2 shows a significant
pressure-dependent change in the cryogenic thermopower in the vicinity of the
critical pressure, where the polar structural transition disappears. With the
application of a high pressure of 0.75 GPa, while the resistivity becomes as
low as 10 {\mu}{\Omega}cm, thermopower reached the maximum value of 60
{\mu}VK-1 at 25 K, leading to a giant thermoelectric power factor of 300
{\mu}WK-2cm-1. Based on semiquantitative analyses, the origin of this behavior
is discussed in terms of inelastic electron-phonon scattering enhanced by the
softening of zone center phonon modes associated with the polar structural
instability.Comment: 13 pages, 4 figures Physical review B (accepted
Real-Space Observation of Ligand Hole State in Cubic Perovskite SrFeO
An anomalously high valence state sometimes shows up in transition-metal
oxide compounds. In such systems, holes tend to occupy mainly the ligand
orbitals, giving rise to interesting physical properties such as
superconductivity in cuprates and rich magnetic phases in ferrates. However, no
one has ever observed the distribution of ligand holes in real space. Here, we
report a successful observation of the spatial distribution of valence
electrons in cubic perovskite SrFeO by high-energy X-ray diffraction
experiments and precise electron density analysis using a core differential
Fourier synthesis method. A real-space picture of ligand holes formed by the
orbital hybridization of Fe 3 and O 2 is revealed. The anomalous valence
state in Fe is attributed to the considerable contribution of the ligand hole,
which is related to the metallic nature and the absence of Jahn-Teller
distortions in this system.Comment: 14 pages, 4 figure
Possible helimagnetic order in Co4+-containing perovskites Sr1-xCaxCoO3
We systematically synthesized perovskite-type oxides Sr1-xCaxCoO3 containing
unusually high valence Co4+ ions by a high pressure technique, and investigated
the effect of systematic lattice change on the magnetic and electronic
properties. As the Ca content x exceeds about 0.6, the structure changes from
cubic to orthorhombic, which is supported by the first-principles calculations
of enthalpy. Upon the orthorhombic distortion, the ground state remains to be
apparently ferromagnetic with a slight drop of the Curie temperature.
Importantly, the compounds with x larger than 0.8 show antiferromagnetic
behavior with positive Weiss temperatures and nonlinear magnetization curves at
lowest temperature, implying that the ground state is noncollinear
antiferromagnetic or helimagnetic. Considering the incoherent metallic behavior
and the suppression of the electronic specific heat at high x region, the
possible emergence of a helimagnetic state in Sr1-xCaxCoO3 is discussed in
terms of the band-width narrowing and the double-exchange mechanism with the
negative charge transfer energy as well as the spin frustration owing to the
next-nearest neighbor interaction.Comment: 13 pages, 4 figure
Superconductivity in a ferroelectric-like topological semimetal SrAuBi
Given the rarity of metallic systems that exhibit ferroelectric-like
transitions, it is apparently challenging to find a system that simultaneously
possesses superconductivity and ferroelectric-like structural instability.
Here, we report the observation of superconductivity at 2.4 K in a layered
semimetal SrAuBi characterized by strong spin-orbit coupling (SOC) and
ferroelectric-like lattice distortion. Single crystals of SrAuBi have been
successfully synthesized and found to show a polar-nonpolar structure
transition at 214 K, which is associated with the buckling of Au-Bi honeycomb
lattice. On the basis of the band calculations considering SOC, we found
significant Rashba-type spin splitting and symmetry-protected multiple Dirac
points near the Fermi level. We believe that this discovery opens up new
possibilities of pursuing exotic superconducting states associated with the
semimetallic band structure without space inversion symmetry and the
topological surface state with the strong SOC.Comment: 17 pages, 6 figures (npj Quantum Materials in press
Unveiling the orbital-selective electronic band reconstruction through the structural phase transition in TaTe
Tantalum ditelluride TaTe belongs to the family of layered transition
metal dichalcogenides but exhibits a unique structural phase transition at
around 170 K that accompanies the rearrangement of the Ta atomic network from a
"ribbon chain" to a "butterfly-like" pattern. While multiple mechanisms
including Fermi surface nesting and chemical bonding instabilities have been
intensively discussed, the origin of this transition remains elusive. Here we
investigate the electronic structure of single-crystalline TaTe with a
particular focus on its modifications through the phase transition, by
employing core-level and angle-resolved photoemission spectroscopy combined
with first-principles calculations. Temperature-dependent core-level
spectroscopy demonstrates a splitting of the Ta core-level spectra through
the phase transition indicative of the Ta-dominated electronic state
reconstruction. Low-energy electronic state measurements further reveal an
unusual kink-like band reconstruction occurring at the Brillouin zone boundary,
which cannot be explained by Fermi surface nesting or band folding effects. On
the basis of the orbital-projected band calculations, this band reconstruction
is mainly attributed to the modifications of specific Ta states, namely
the orbitals (the ones elongating along the ribbon chains) at the
center Ta sites of the ribbon chains. The present results highlight the strong
orbital-dependent electronic state reconstruction through the phase transition
in this system and provide fundamental insights towards understanding complex
electron-lattice-bond coupled phenomena.Comment: 21 pages, 5 figure