RESUMEN
Time- and angle-resolved photoelectron spectroscopy (trARPES) is a powerful method to track the ultrafast dynamics of quasiparticles and electronic bands in energy and momentum space. We present a setup for trARPES with 22.3 eV extreme-ultraviolet (XUV) femtosecond pulses at 50-kHz repetition rate, which enables fast data acquisition and access to dynamics across momentum space with high sensitivity. The design and operation of the XUV beamline, pump-probe setup, and ultra-high vacuum endstation are described in detail. By characterizing the effect of space-charge broadening, we determine an ultimate source-limited energy resolution of 60 meV, with typically 80-100 meV obtained at 1-2 × 1010 photons/s probe flux on the sample. The instrument capabilities are demonstrated via both equilibrium and time-resolved ARPES studies of transition-metal dichalcogenides. The 50-kHz repetition rate enables sensitive measurements of quasiparticles at low excitation fluences in semiconducting MoSe2, with an instrumental time resolution of 65 fs. Moreover, photo-induced phase transitions can be driven with the available pump fluence, as shown by charge density wave melting in 1T-TiSe2. The high repetition-rate setup thus provides a versatile platform for sensitive XUV trARPES, from quenching of electronic phases down to the perturbative limit.
RESUMEN
Regular tricobalt tetraoxide (Co(3)O(4)) nanocubes with tunable sizes have been synthesized by a simple magnetic field assisted hydrothermal reaction. In contrast to other traditional methods, no surfactant is added to the reaction system, the morphology of the product is controlled by the application of an external magnetic field and the size distribution of the product is tuned by simply modifying the ratio of distilled water to ethanol in the solvent. The growth process of Co(3)O(4) nanocubes is investigated and discussed in detail. It is found that the differences in polarity and dielectric constant between distilled water and ethanol and thus the difference of cobalt coordination ions concentration in the different solvents are the major factors that determine the final size distribution of Co(3)O(4) nanocubes. Magnetic properties of Co(3)O(4) nanocubes synthesized under (MF) and not under (ZF) an external magnetic field are then investigated. It is believed that during their growth, the alignment of spins in the Co(3)O(4) particles and thus the magnetic and crystal lattices of Co(3)O(4) are influenced by the external magnetic field. Spins in MF arrange in a less-ordered manner and cannot be totally compensated by each other, therefore makes them have a stronger tendency to align into an ordered figuration, which leads to a relatively larger magnetization and higher Néel temperature (T(N)) of MF comparing to sample ZF.
