Search

The isotope $^{98}\mathrm{Tc}$ decays to $^{98}\mathrm{Ru}$ with a half-life of $4.2\ifmmode\times\else\texttimes\fi{}{10}^{6}\text{ }\text{ }\text{ }\mathrm{yr}$ and could have been present in the early Solar System. In this Letter, we report on the first calculations of the production of $^{98}\mathrm{Tc}$ by neutrino-induced reactions in core-collapse supernovae (the $\ensuremath{\nu}$ process). Our predicted $^{98}\mathrm{Tc}$ abundance at the time of solar system formation is not much lower than the current measured upper limit raising the possibility for its detection in the not too distant future. We show that, if the initial abundance were to be precisely measured, the $^{98}\mathrm{Tc}$ nuclear cosmochronometer could be used to evaluate a much more precise value of the duration time from the last core-collapse supernova to the formation of the solar system. Moreover, a unique and novel feature of the $^{98}\mathrm{Tc}$ $\ensuremath{\nu}$-process nucleosynthesis is the large contribution ($\ensuremath{\sim}20%$) from charged current reactions with electron antineutrinos. This means that $^{98}\mathrm{Tc}$ becomes a unique new $\ensuremath{\nu}$-process probe of the temperature of the electron antineutrinos.

${\text{Ca}}_{1\ensuremath{-}x}{\text{Sr}}_{x}{\text{VO}}_{3}$ is a Mott-Hubbard-type correlated electron system whose bandwidth can be varied by the V-O-V bond angle but the actual effect of bandwidth control on the electronic structure has been controversial in previous photoemission experiments. In this work, band dispersions and Fermi surfaces of ${\text{SrVO}}_{3}$ and ${\text{CaVO}}_{3}$ are studied by angle-resolved photoemission spectroscopy. Near the Fermi level $({E}_{F})$, three bands forming cylindrical Fermi surfaces derived from the three $\text{V}\text{ }3d$ ${t}_{2g}$ orbitals have been observed. The observed bandwidths for both compounds are almost half of those predicted by local-density approximation band-structure calculation, confirming mass renormalization caused by electron correlation. It has been clearly demonstrated that the width of the $d$ band in ${\text{CaVO}}_{3}$ is narrower than that in ${\text{SrVO}}_{3}$, qualitatively consistent with the result of band-structure calculation. Roles of the orthorhombic lattice distortion and electron correlation in the observed band narrowing are discussed.

We report the first observation of the multilayer band splitting in the optimally doped trilayer cuprate ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{Ca}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{10+\ensuremath{\delta}}$ (Bi2223) by angle-resolved photoemission spectroscopy. The observed energy bands and Fermi surfaces are originated from the outer and inner ${\mathrm{CuO}}_{2}$ planes (OP and IP). The OP band is overdoped with a large $d$-wave gap around the node of ${\ensuremath{\Delta}}_{0}\ensuremath{\sim}43\text{ }\text{ }\mathrm{meV}$ while the IP is underdoped with an even large gap of ${\ensuremath{\Delta}}_{0}\ensuremath{\sim}60\text{ }\text{ }\mathrm{meV}$. These energy gaps are much larger than those for the same doping level of the double-layer cuprates, which leads to the large ${T}_{c}$ in Bi2223. We propose possible origins of the large superconducting gaps for the OP and IP: (1) minimal influence of out-of-plane disorder and a proximity effect and (2) interlayer tunneling of Cooper pairs between the OP and IP.

How out-of-plane disorder affects the electronic structure has been investigated for the single-layer cuprates Bi{sub 2}Sr{sub 1.6}Ln{sub 0.4}CuO{sub 6+{delta}} (Ln = La, Nd, Gd) by angle-resolved photoemission spectroscopy. We have observed that, with increasing disorder, while the Fermi surface shape and band dispersions are not affected, the quasi-particle width increases, the anti-nodal gap is enhanced and the superconducting gap in the nodal region is depressed. The results indicate that the superconductivity is significantly depressed by out-of-plane disorder through the enhancement of the anti-nodal gap and the depression of the superconducting gap in the nodal region.

We have performed angle-resolved photoemission and core-level x-ray photoemission studies of the single-layer cuprate Bi{sub 2}Sr{sub 2-x}La{sub x}CuO{sub 6+{delta}} (Bi2201) and revealed the doping evolution of the electronic structure from the lightly-doped to optimally-doped regions. We have observed the formation of the dispersive quasi-particle band, evolution of the Fermi 'arc' into the Fermi surface and the shift of the chemical potential with hole doping as in other cuprates. The doping evolution in Bi2201 is similar to that in Ca{sub 2-x}Na{sub x}CuO{sub 2}Cl{sub 2} (Na-CCOC), where a rapid chemical potential shift toward the lower Hubbard band of the parent insulator has been observed, but is quite different from that in La{sub 2-x}Sr{sub x}CuO{sub 4} (LSCO), where the chemical potential does not shift, yet the dispersive band and the Fermi arc/surface are formed around the Fermi level already in the lightly-doped region. The (underlying) Fermi surface shape and band dispersions are quantitatively analyzed using tightbinding fit, and the deduced next-nearest-neighbor hopping integral t also confirm the similarity to Na-CCOC and the difference from LSCO.

We have measured photoemission spectra of ${\mathrm{SrTiO}}_{3}/{\mathrm{LaTiO}}_{3}$ superlattices with a topmost ${\mathrm{SrTiO}}_{3}$ layer of variable thickness. A finite coherent spectral weight with a clear Fermi cutoff was observed at chemically abrupt ${\mathrm{SrTiO}}_{3}/{\mathrm{LaTiO}}_{3}$ interfaces, indicating that an ``electronic reconstruction'' occurs at the interface between the Mott insulator ${\mathrm{LaTiO}}_{3}$ and the band insulator ${\mathrm{SrTiO}}_{3}$. For ${\mathrm{SrTiO}}_{3}/{\mathrm{LaTiO}}_{3}$ interfaces annealed at high temperatures ($\ensuremath{\sim}1000\text{ }\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$), which leads to $\mathrm{Sr}/\mathrm{La}$ atomic interdiffusion and hence to the formation of ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{TiO}}_{3}$-like material, the intensity of the incoherent part was found to be dramatically reduced whereas the coherent part with a sharp Fermi cutoff was enhanced due to the spread of charge. These important experimental features are well reproduced by layer dynamical-mean-field-theory calculation.