Weyl fermions as emergent quasiparticles can arise in Weyl semimetals (WSMs) in which the energy bands are nondegenerate, resulting from inversion or time-reversal symmetry breaking. Nevertheless, experimental evidence for magnetically induced WSMs is scarce. Here, using photoemission spectroscopy, we observe that the degeneracy of Bloch bands is already lifted in the paramagnetic phase of EuCd2As2. We attribute this effect to the itinerant electrons experiencing quasi-static and quasi–long-range ferromagnetic fluctuations. Moreover, the spin-nondegenerate band structure harbors a pair of ideal Weyl nodes near the Fermi level. Hence, we show that long-range magnetic order and the spontaneous breaking of time-reversal symmetry are not essential requirements for WSM states in centrosymmetric systems and that WSM states can emerge in a wider range of condensed matter systems than previously thought.
Plasmon induced water splitting is a promising research area with the potential for efficient conversion of solar to chemical energy, yet its atomic mechanism is not well understood. Here, ultrafast electron-nuclear dynamics of water splitting on gold nanoparticles upon exposure to femtosecond laser pulses was directly simulated using real time time-dependent density functional theory (TDDFT). Strong correlation between laser intensity, hot electron transfer, and reaction rates has been identified. The rate of water splitting is dependent not only on respective optical absorption strength, but also on the quantum oscillation mode of plasmonic excitation. Odd modes are more efficient than even modes, owing to faster decaying into hot electrons whose energy matches well the antibonding orbital of water. This finding suggests photocatalytic activity can be manipulated by adjusting the energy level of plasmon-induced hot carriers, through altering the cluster size and laser parameter, to better overlap adsorbate unoccupied level in plasmon-assisted photochemistry.
Half-Heusler (HH) compounds are important high temperature thermoelectric (TE) materials and have gained ever-increasing popularity. In recent years, p-type FeNbSb-based heavy-band HH compounds have attracted considerable attention with the record-high zT value of 1.5. Here, we use first-principles based methods to predict a very high zT value of 1.54 at 1200 K in p-type RuTaSb alloys. The high band degeneracy and low band effective mass contribute to a high power factor. Although the electrical thermal conductivity is high due to the high carrier mobility and hence electrical conductivity, the total thermal conductivity is moderate because of the low lattice thermal conductivity. The predicted high zT demonstrates that the p-type RuTaSb HH alloys are promising as TE materials for high temperature power generation.
Hydrogen gas production from solar water splitting provides a renewable energy cycle to address the grand global energy challenge; however, its dynamics and fundamental mechanism remain elusive. We directly explore by first-principles the ultrafast electron-nuclear quantum dynamics on the time scale of ∼100 fs during water photosplitting on a plasmonic cluster embedded in liquid water. Water molecule splitting is assisted by rapid proton transport in liquid water in a Grotthuss-like mechanism. We identify that a plasmon-induced field enhancement effect dominates water splitting, while charge transfer from gold to the antibonding orbital of a water molecule also plays an important role. "Chain-reaction" like rapid H production is observed via the combination of two hydrogen atoms from different water molecules. These results provide a route toward a complete understanding of water photosplitting in the ultimate time and spatial limit.
As ynergistic catalytic method combining photoredox catalysis,hydrogen-atom transfer,and proton-reduction catalysis for the dehydrogenative silylation of alkenes was developed. With this approach,ahighly concise route to substituted allylsilanes has been achieved under very mild reaction conditions without using oxidants.T his transformation features good to excellent yields,o perational simplicity, and high atom economy.B ased on control experiments, apossible reaction mechanism is proposed. Scheme 1. Strategies for the synthesis of substituted allylsilanes.
Localized surface plasmon resonances (LSPR) supported in metal nanostructures can be efficiently harnessed to drive photocatalytic reactions, whose atomic scale mechanism remains a challenge. Here, real-time dynamics of H2 photosplitting on a linear silver atomic chain, upon exposure to femtosecond laser pulses, has been investigated using time-dependent density functional theory. The wavelength dependent H2 splitting process is strongly coupled to LSPR excitation in silver chain. We identify that hot electrons produced in the silver chain by plasmon excitation are transferred to the antibonding state of the adsorbed H2 and trigger H2 dissociation, consistent with experimental observations. Increasing illumination intensity and the length of atomic chain promote H2 splitting, thanks to stronger LSPR. Dynamic electronic response can be quantitatively described within the present approach, providing insights towards a complete fundamental understanding on plasmon-induced chemical reactions at the microscopic scale.
Revealing the nature of a hydrogen-bond network in water structures is one of the imperative objectives of science. With the use of a low-temperature scanning tunneling microscope, water clusters on a Au(111) surface were directly imaged with molecular resolution by a functionalized tip. The internal structures of the water clusters as well as the geometry variations with the increase of size were identified. In contrast to a buckled water hexamer predicted by previous theoretical calculations, our results present deterministic evidence for a flat configuration of water hexamers on Au(111), corroborated by density functional theory calculations with properly implemented van der Waals corrections. The consistency between the experimental observations and improved theoretical calculations not only renders the internal structures of absorbed water clusters unambiguously, but also directly manifests the crucial role of van der Waals interactions in constructing water-solid interfaces.
Electrochemistry and Raman spectroscopy have shown that aquocob(III)alamin (Cbl(III)) can be reduced by nitric oxide (NO) to form Cbl(II) on an electrode surface. The Cbl(II) formed in this way can bind NO to form nitrosyl-cobalamin, Cbl(II)-NO, which is reduced to form Cbl(I) at about -1.0 V vs a KCl saturated Ag/AgCl reference electrode. In addition, nitrite was found to bind both Cbl(III) and Cbl(II) and a binding constant of 3.5 x 10(2) M(-1) was measured for (NO(2)-Cbl(II))(1-). UV-vis spectrophotometry and mass spectroscopy were used to show that Cbl(I) reduces NO to form Cbl(II)-NO and N(2)O and N(2), and this reaction is involved in the cyclic voltammetry of cobalamin in the presence of excess NO where a catalytic reduction of NO occurs involving the cycling of Cbl(II)-NO/Cbl(I). This redox couple is also involved in the electrochemical catalytic reduction of nitrite. These results can be used to explain a number of physiological effects involving NO interaction in biological systems with added cobalamin or with cobalamin in the methionine synthase enzyme.
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