22 research outputs found
Correlation between the latent heats and cohesive energies of metal clusters
ProducciĂłn CientĂficaDissociation energies have been determined for Al_n^+ clusters (n = 25â83) using a new experimental approach that takes into account the latent heat of melting. According to the arguments presented here, the cohesive energies of the solidlike clusters are made up of contributions from the dissociation energies of the liquidlike clusters and the latent heats for melting. The size-dependent variations in the measured dissociation energies of the liquidlike clusters are small and the variations in the cohesive energies of solidlike clusters result almost entirely from variations in the latent heats for melting. To compare with the measured cohesive energies, density-functional theory has been used to search for the global minimum energy structures. Four groups of low energy structures were found: Distorted decahedral fragments, fcc fragments, fcc fragments with stacking faults, and âdisordered.â For most cluster sizes, the measured and calculated cohesive energies are strongly correlated. The calculations show that the variations in the cohesive energies (and the latent heats) result from a combination of geometric and electronic shell effects. For some clusters an electronic shell closing is responsible for the enhanced cohesive energy and latent heat (e.g., n = 37), while for others (e.g., n = 44) a structural shell closing is the cause
Electronic effects on melting: Comparison of aluminum cluster anions and cations
ProducciĂłn CientĂficaHeat capacities have been measured as a function of temperature for aluminum cluster anions with 35â70 atoms. Melting temperatures and latent heats are determined from peaks in the heat capacities; cohesive energies are obtained for solid clusters from the latent heats and dissociation energies determined for liquid clusters. The melting temperatures, latent heats, and cohesive energies for the aluminum cluster anions are compared to previous measurements for the corresponding cations. Density functional theory calculations have been performed to identify the global minimum energy geometries for the cluster anions. The lowest energy geometries fall into four main families: distorted decahedral fragments, fcc fragments, fcc fragments with stacking faults, and âdisorderedâ roughly spherical structures. The comparison of the cohesive energies for the lowest energy geometries with the measured values allows us to interpret the size variation in the latent heats. Both geometric and electronic shell closings contribute to the variations in the cohesive energies (and latent heats), but structural changes appear to be mainly responsible for the large variations in the melting temperatures with cluster size. The significant charge dependence of the latent heats found for some cluster sizes indicates that the electronic structure can change substantially when the cluster melts
Substituting a copper atom modifies the melting of aluminum clusters
ProducciĂłn CientĂficaHeat capacities have been measured for Al(nâ1)Cuâ clusters (n = 49â62) and compared with results for pure Aln+ clusters. Al(nâ1)Cuâ and Aln+ have the same number of atoms and the same number of valence electrons (excluding the copper d electrons). Both clusters show peaks in their heat capacities that can be attributed to melting transitions; however, substitution of an aluminum atom by a copper atom causes significant changes in the melting behavior. The sharp drop in the melting temperature that occurs between n = 55 and 56 for pure aluminum clusters does not occur for the Al(nâ1)Cuâ analogs. First-principles density-functional theory has been used to locate the global minimum energy structures of the doped clusters. The results show that the copper atom substitutes for an interior aluminum atom, preferably one with a local face-centered-cubic environment. Substitution does not substantially change the electronic or geometric structures of the host cluster unless there are several Aln+ isomers close to the ground state. The main structural effect is a contraction of the bond lengths around the copper impurity, which induces both a contraction of the whole cluster and a stress redistribution between the AlâAl bonds. The size dependence of the substitution energy is correlated with the change in the latent heat of melting on substitution
Quasi-Two-Dimensional Fermi Surface and Heavy Quasiparticles in CeRh2As2
The recent discovery of multiple superconducting phases in CeRh2As2 has
attracted considerable interest. These rich phases are thought to be related to
the locally noncentrosymmetric crystal structure, although the possible role of
a quadrupole density wave preceding the superconductivity remains an open
question. While measurements of physical properties imply that the Ce 4f
electrons could play an essential role, the momentum-resolved electronic
structure remains hitherto unreported, hindering an in-depth understanding of
the underlying physics. Here, we report a high-resolution angle-resolved
photoemission study of CeRh2As2. Our results reveal fine splittings of
conduction bands, which are directly related to the locally noncentrosymmetric
structure, as well as a quasi-two-dimensional Fermi surface, implying weak
interlayer hopping and possible nesting instabilities. Our experiments also
uncover the fine structures and pronounced temperature evolution of the Kondo
peak, demonstrating strong Kondo effect facilitated by excited crystal electric
field states. Our results unveil the salient electronic features arising from
the interplay between the crystal structure and strong electron correlation,
providing spectroscopic insight for understanding the heavy fermion physics and
unconventional quadrupole density wave in this enigmatic compound