Synthesis and crystal growth

Abstract:

We report on the interplay between a van Hove singularity and a charge density wave state in 2⁢𝐻−TaSe2. We use angle-resolved photoemission spectroscopy to investigate changes in the Fermi surface of this material under surface doping with potassium. At high doping, we observe modifications which imply the disappearance of the (3×3) charge density wave and formation of a different correlated state. Using a tight-binding-based approach as well as an effective model, we explain our observations as a consequence of coupling between the single-particle Lifshitz transition during which the Fermi level passes a van Hove singularity and the charge density order. In this scenario, the high electronic density of states associated with the van Hove singularity induces a change in the periodicity of the charge density wave from the known (3×3) to a new (2×2) superlattice.

Abstract:

In the rapidly expanding field of topological materials there is growing interest in systems whose topological electronic band features can be induced or controlled by magnetism. Magnetic Weyl semimetals, which contain linear band crossings near the Fermi level, are of particular interest owing to their exotic charge and spin transport properties. Up to now, the majority of magnetic Weyl semimetals have been realized in ferro- or ferrimagnetically ordered compounds, but a disadvantage of these materials for practical use is their stray magnetic field which limits the minimum size of devices. Here we show that Weyl nodes can be induced by a helical spin configuration, in which the magnetization is fully compensated. Using a combination of neutron diffraction and resonant elastic x-ray scattering, we find that below TN = 14.5 K the Eu spins in EuCuAs develop a planar helical structure which induces two quadratic Weyl nodes with Chern numbers C = ±2 at the A point in the Brillouin zone.

Abstract:

Photocatalytic splitting of seawater for hydrogen evolution has attracted a great deal of attention in recent years. However, the poor energy conversion efficiency and stability of photocatalysts in a salty environment have greatly hindered further applications of this technology. Moreover, the effects of electrolytes in seawater remain controversial. Here we present electrolyte-assisted charge polarization over an N-doped TiO2 photocatalyst, which demonstrates the stoichiometric evolution of H2 and O2 from the thermo-assisted photocatalytic splitting of seawater. Our extensive characterizations and computational studies show that ionic species in seawater can selectively adsorb on photo-polarized facets of the opposite charge, which can prolong the charge-carrier lifetime by a factor of five, leading to an overall energy conversion efficiency of 15.9 ± 0.4% at 270 °C. Using a light-concentrated furnace, a steady hydrogen evolution rate of 40 mmol g−1 h−1 is demonstrated, which is of the same order of magnitude as laboratory-scale electrolysers.