170 research outputs found
Electrochemically driven efficient enzymatic conversion of CO2 to formic acid with artificial cofactors
Enzymatic reduction of CO2 to formic acid with the enzyme formate dehydrogenase (FDH) and a cofactor is a promising method for CO2 conversion and utilization. However, the natural cofactor nicotinamide adenine dinucleotide (NADH) shows some drawbacks such as a low reduction efficiency and forms isomers or dimers (1,6 - NADH or NAD dimer) in the regeneration reaction. To overcome them and to improve the production of formic acid, in this work, the artificial cofactors, i.e., the bipyridinium-based salts of methyl viologen (MV2+), 1,1’-dicarboxymethyl-4,4’-bipyridinium bromine (DC2+), and 1,1’-diaminoethyl-4,4’-bipyridinium bromine (DA2+), were used to replace NADH, and the effect of different functional groups on the electrochemical regeneration and catalytic performance in the enzymatic reaction was studied systematically. Also, studies using the natural cofactor NADH were carried out for comparison. It was found that the cofactor with amino groups showed the highest catalytic efficiency (kcat/Km) of 0.161 mM-1min-1, which is 536 times higher than that of the natural cofactor NADH. Molecular Dynamics simulations were conducted to give further molecular insight into the behavior of the cofactors. Analyzing the free energy profiles of the complexes between CO2 in the FDH active site with different artificial cofactors indicated that the artificial cofactor with the amino groups had the highest affinity for CO2, being consistent with the experimental observations
Structure and Spin Dynamics of LaSrMnO
Neutron scattering has been used to study the structure and spin dynamics of
LaSrMnO. The magnetic structure of this system is
ferromagnetic below T_C = 235 K. We see anomalies in the Bragg peak intensities
and new superlattice peaks consistent with the onset of a spin-canted phase
below T_{CA} = 205 K, which appears to be associated with a gap at q = (0, 0,
0.5) in the spin-wave spectrum. Anomalies in the lattice parameters indicate a
concomitant lattice distortion. The long-wavelength magnetic excitations are
found to be conventional spin waves, with a gapless (< 0.02 meV) isotropic
dispersion relation . The spin stiffness constant D has a
dependence at low T, and the damping at small q follows . An
anomalously strong quasielastic component, however, develops at small wave
vector above 200 K and dominates the fluctuation spectrum as T -> T_C. At
larger q, on the other hand, the magnetic excitations become heavily damped at
low temperatures, indicating that spin waves in this regime are not eigenstates
of the system, while raising the temperature dramatically increases the
damping. The strength of the spin-wave damping also depends strongly on the
symmetry direction in the crystal. These anomalous damping effects are likely
due to the itinerant character of the electrons.Comment: 8 pages (RevTex), 9 figures (encapsulated postscript
The Structure of Nanoscale Polaron Correlations in La1.2Sr1.8Mn2O7
A system of strongly-interacting electron-lattice polarons can exhibit charge
and orbital order at sufficiently high polaron concentrations. In this study,
the structure of short-range polaron correlations in the layered colossal
magnetoresistive perovskite manganite, La1.2Sr1.8Mn2O7, has been determined by
a crystallographic analysis of broad satellite maxima observed in diffuse X-ray
and neutron scattering data. The resulting q=(0.3,0,1) modulation is a
longitudinal octahedral-stretch mode, consistent with an incommensurate
Jahn-Teller-coupled charge-density-wave fluctuations, that implies an unusual
orbital-stripe pattern parallel to the directions.Comment: Reformatted with RevTe
Novel stripe-type charge ordering in the metallic A-type antiferromagnet Pr{0.5}Sr{0.5}MnO{3}
We demonstrate that an A-type antiferromagnetic (AFM) state of
Pr{0.5}Sr{0.5}MnO{3} exhibits a novel charge ordering which governs the
transport property. This charge ordering is stripe-like, being characterized by
a wave vector q ~ (0,0,0.3) with very anisotropic correlation parallel and
perpendicular to the stripe direction. This charge ordering is specific to the
manganites with relatively wide one-electron band width (W) which often exhibit
a metallic A-type AFM state, and should be strictly distinguished from the
CE-type checkerboard-like charge ordering which is commonly observed in
manganites with narrower W such as La{1-x}Ca{x}MnO{3} and Pr{1-x}Ca{x}MnO{3}.Comment: REVTeX4, 5 pages, 4 figure
Stripes Induced by Orbital Ordering in Layered Manganites
Spin-charge-orbital ordered structures in doped layered manganites are
investigated using an orbital-degenerate double-exchange model tightly coupled
to Jahn-Teller distortions. In the ferromagnetic phase, unexpected diagonal
stripes at = (=integer) are observed, as in recent experiments.
These stripes are induced by the orbital degree of freedom, which forms a
staggered pattern in the background. A -shift in the orbital order across
stripes is identified, analogous to the -shift in spin order across
stripes in cuprates. At =1/4 and 1/3, another non-magnetic phase with
diagonal static charge stripes is stabilized at intermediate values of the
-spins exchange coupling.Comment: reordering of figure
Optical Studies of a Layered Manganite La_{1.2}Sr_{1.8}Mn_2O_7 : Polaron Correlation Effect
Optical conductivity spectra of a cleaved ab-plane of a
La_{1.2}Sr_{1.8}Mn_2O_7 single crystal exhibit a small polaron absorption band
in the mid-infrared region at overall temperatures. With decreasing temperature
(T) to Curie temperature (T_C), the center frequency of the small polaron band
moves to a higher frequency, resulting in a gap-like feature, and that it
collapses to a lower frequency below T_C. Interestingly, with decreasing T, the
stretching phonon mode hardens above T_C and softens below T_C. These
concurring changes of lattice and electronic structure indicate that short
range polaron correlation exist above T_C but disappear with a magnetic
ordering.Comment: 4 pages including 5 figures. submitted to Phys. Rev.
Magnon Broadening Effect by Magnon-Phonon Interaction in Colossal Magnetoresistance Manganites
In order to study the magnetic excitation behaviors in colossal
magnetoresistance manganites, a magnon-phonon interacting system is
investigated. Sudden broadening of magnon linewidth is obtained when a magnon
branch crosses over an optical phonon branch. Onset of the broadening is
approximately determined by the magnon density of states. Anomalous magnon
damping at the brillouine zone boundary observed in low Curie temperature
manganites is explained.Comment: 4 pages incl. 4 figs. New e-mail: [email protected]
Low-temperature electrical transport in bilayer manganite LaSrMnO
The temperature and magnetic field dependence of anisotropic in-plane
and out-of-plane resistivities have been investigated in
single crystals of the bilayer manganite LaSrMnO.
Below the Curie transition temperature 125 K, and
display almost the same temperature dependence with an up-turn around 50 K. In
the metallic regime (50 K 110 K), both and
follow a dependence, consistent with the two-magnon
scattering. We found that the value of the proportionality coefficient
and the ratio of the exchange interaction obtained
by fitting the data are in excellent agreement with the calculated
based on the two-magnon model and deduced from neutron scattering,
respectively. This provides further support for this scattering mechanism. At
even lower , in the non-metallic regime ( 50 K), {\it both} the in-plane
and out-of-plane conductivities obey a
dependence, consistent with weak localization effects. Hence, this demonstrates
the three-dimensional metallic nature of the bilayer manganite
LaSrMnO at .Comment: 7 pages and 5 figures, accepted for publication in Phys. Rev.
The phase-separated states in antiferromagnetic semiconductors with polarizable lattice
The possibility of the slab or stripe phase separation (alternating
ferromagnetic highly- conductive and insulating antiferromagnetic layers) is
proved for isotropic degenerate antiferromagnetic semiconductors. This type of
phase separation competes with the droplet phase separation (ferromagnetic
droplets in the antiferromagnetic host or vice versa). The interaction of
electrons with optical phonons alone cannot cause phase-separated state with
alternating highly-conductive and insulating regions but it stabilizes the
magnetic phase separation. The magnetostriction deformation of the lattice in
the phase-separated state is investigated.Comment: 17 Pages, 1 EPS Figur
Orbital Structure and Magnetic Ordering in Layered Manganites: Universal Correlation and Its Mechanism
Correlation between orbital structure and magnetic ordering in bilayered
manganites is examined. A level separation between the and
orbitals in a Mn ion is calculated in the ionic model for a
large number of the compounds. It is found that the relative stability of the
orbitals dominates the magnetic transition temperatures as well as the magnetic
structures. A mechanism of the correlation between orbital and magnetism is
investigated based on the theoretical model with the two orbitals under
strong electron correlation.Comment: 4 pages, 4 figure
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