3,033 research outputs found
Post density functional theoretical studies of highly polar semiconductive Pb(TiNi)O solid solutions: The effects of cation arrangement on band gap
We use a combination of conventional density functional theory (DFT) and
post-DFT methods, including the local density approximation plus Hubbard
(LDA+), PBE0, and self-consistent to study the electronic properties of
Ni-substituted PbTiO (Ni-PTO) solid solutions. We find that LDA
calculations yield unreasonable band structures, especially for Ni-PTO solid
solutions that contain an uninterrupted NiO layer. Accurate treatment of
localized states in transition-metal oxides like Ni-PTO requires post-DFT
methods. -site Ni/Ti cation ordering is also investigated. The -site
cation arrangement alters the bonding between Ni and O, and therefore strongly
affects the band gap () of Ni-PTO. We predict that Ni-PTO solid
solutions should have a direct band gap in the visible light energy range, with
polarization similar to the parent PbTiO. This combination of properties
make Ni-PTO solid solutions promising candidate materials for solar energy
conversion devices.Comment: 19 pages, 6 figure
Positron potential and wavefunction in LaFeAsO
We report calculations of the positron potential and wavefunction in LaFeAsO.
These calculations show that the positron wavefunction does sample the entire
unit cell although it is largest in the interstices of the La layer adjacent to
As atoms. The implication is that angular correlation of annihilation radiation
(ACAR) is a viable probe of the Fermi surfaces in this material. The results
also apply to positive muons, and indicate that these will be localized in the
La layer adjacent to As
PERFORMANCE AND FLOW CHARACTERISTICS OF NOZZLES FOR INITIALLY SUBCOOLED HOT WATER (INFLUENCE OF TURBULENCE AND DECOMPRESSION RATE)
The purpose of this study was to improve the performance characteristics of a convergent-divergent nozzle for flashing expansion of initially subcooled hot water. The slip between the vapor and liquid along the divergent passage and the maximum nonequilibrium pressure drop at the nozzle throat(this is related to the thermal nonequilibrium) cause a decline in the nozzle efficiency. To decrease the maximum pressure drop,a nozzle with thin wires just upstream of the throat and a nozzle with a smaller convergent angle were manufactured and tested. Initially subcooled hot water at an inlet pressure of 0.47MPa and inlet temperatures of 409.9 to 421.8K (inlet subcoolings of 0.9-12.9K) were used with the nozzle back pressures ranging from 5 to 101.3kPa. The following results were attained:(1)the maximum nonequilibrium pressure drop at the throat decreased and the thrust coefficient increased by 10%; (2)the thrust coefficient was found to be independent of the inlet subcooling and exit stream-expansion ratio; (3)a correlation between the ratio of the exit pressure to the inlet pressure and the appropriate exit stream-expansion ratio was obtained empirically;and (4)the critical flow rate was found to be lower than that for an ordinary convergent-divergent nozzle
Full-gap superconductivity robust against disorder in heavy-fermion CeCu2Si2
A key aspect of unconventional pairing by the antiferromagnetic
spin-fluctuation mechanism is that the superconducting energy gap must have
opposite sign on different parts of the Fermi surface. Recent observations of
non-nodal gap structure in the heavy-fermion superconductor CeCuSi were
then very surprising, given that this material has long been considered a
prototypical example of a superconductor where the Cooper pairing is
magnetically mediated. Here we present a study of the effect of controlled
point defects, introduced by electron irradiation, on the temperature-dependent
magnetic penetration depth in CeCuSi. We find that the
fully-gapped state is robust against disorder, demonstrating that low-energy
bound states, expected for sign-changing gap structures, are not induced by
nonmagnetic impurities. This provides bulk evidence for -wave
superconductivity without sign reversal.Comment: 5 pages, 4 figures + Supplemental Material (1 page, 1 figure). Will
appear in Phys. Rev. Let
One-Center Charge Transfer Transitions in Manganites
In frames of a rather conventional cluster approach, which combines the
crystal field and the ligand field models we have considered different charge
transfer (CT) states and O 2p-Mn 3d CT transitions in MnO octahedra.
The many-electron dipole transition matrix elements were calculated using the
Racah algebra for the cubic point group. Simple "local" approximation allowed
to calculate the relative intensity for all dipole-allowed and
CT transitions. We present a self-consistent description of
the CT bands in insulating stoichiometric LaMnO compound with the
only Mn valent state and idealized octahedral MnO centers
which allows to substantially correct the current interpretation of the optical
spectra. Our analysis shows the multi-band structure of the CT optical response
with the weak low-energy edge at 1.7 eV, associated with forbidden
transition and a series of the weak and strong
dipole-allowed high-energy transitions starting from 2.5 and 4.5 eV,
respectively, and extending up to nearly 11 eV. The most intensive features are
associated with two strong composite bands near eV and
eV, respectively, resulting from the superposition of the dipole-allowed
and CT transitions. These predictions are in good
agreement with experimental spectra. The experimental data point to a strong
overscreening of the crystal field parameter in the CT states of
MnO centers.Comment: 10 pages, 3 figure
Magnetoresistance in Heavily Underdoped YBa_2Cu_3O_{6+x}: Antiferromagnetic Correlations and Normal-State Transport
We report on a contrasting behavior of the in-plane and out-of-plane
magnetoresistance (MR) in heavily underdoped antiferromagnetic (AF)
YBa_2Cu_3O_{6+x} (x<0.37). The out-of-plane MR (I//c) is positive over most of
the temperature range and shows a sharp increase, by about two orders of
magnitude, upon cooling through the Neel temperature T_N. A contribution
associated with the AF correlations is found to dominate the out-of-plane MR
behavior for H//c from far above T_N, pointing to the key role of spin
fluctuations in the out-of-plane transport. In contrast, the transverse
in-plane MR (I//a(b);H//c) appears to be small and smooth through T_N, implying
that the development of the AF order has little effect on the in-plane
resistivity.Comment: 4 pages, 5 figures, accepted for publication in Phys.Rev.Let
Transport Anomalies and the Role of Pseudogap in the "60-K Phase" of YBa_{2}Cu_{3}O_{7-\delta}
We report the result of our accurate measurements of the a- and b-axis
resistivity, Hall coefficient, and the a-axis thermopower in untwinned
YBa_{2}Cu_{3}O_{y} single crystals in a wide range of doping. It is found that
both the a-axis resistivity and the Hall conductivity show anomalous
dependences on the oxygen content y in the "60-K phase" below the pseudogap
temperature T^*. The complete data set enables us to narrow down the possible
pictures of the 60-K phase, with which we discuss a peculiar role of the
pseudogap in the charge transport.Comment: 4 pages, 4 figures, accepted for publication in PR
DYW domain structures imply an unusual regulation principle in plant organellar RNA editing catalysis
RNA上の遺伝情報を書き換える酵素であるDYWドメインの構造を解明 --植物オルガネラRNA編集のユニークな活性制御--. 京都大学プレスリリース. 2021-06-23.RNA editosomes selectively deaminate cytidines to uridines in plant organellar transcripts—mostly to restore protein functionality and consequently facilitate mitochondrial and chloroplast function. The RNA editosomal pentatricopeptide repeat proteins serve target RNA recognition, whereas the intensively studied DYW domain elicits catalysis. Here we present structures and functional data of a DYW domain in an inactive ground state and activated. DYW domains harbour a cytidine deaminase fold and a C-terminal DYW motif, with catalytic and structural zinc atoms, respectively. A conserved gating domain within the deaminase fold regulates the active site sterically and mechanistically in a process that we termed gated zinc shutter. Based on the structures, an autoinhibited ground state and its activation are cross-validated by RNA editing assays and differential scanning fluorimetry. We anticipate that, in vivo, the framework of an active plant RNA editosome triggers the release of DYW autoinhibition to ensure a controlled and coordinated cytidine deamination playing a key role in mitochondrial and chloroplast homeostasis
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