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Register a new userLattice and Magnetic Effects on a d-d Excitation in NiO Using a 25 meV Resolution X-ray Spectrometer
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We investigate the behavior of a $d$-$d$ transition in NiO using a new x-ray spectrometer with 0.025 eV resolution at 15816 eV, and via ab-initio ligand field theory calculations. The transition at ~1.7 eV energy transfer is measured at temperatures between 20 and 800 K, at a momentum transfer |$bf{Q}$| = 6.52 AA$^{-1}$. Fine structure is clearly observed at 20 K. As temperature is increased, the excitation shifts to lower energy and broadens. We explain the energy shift as being related to thermal expansion and to magnetism. The broadening is well fit considering thermal fluctuations of the Ni-O bond length, with a scale factor found to be in reasonable agreement with calculation.
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Luminescence spectra of NiO have been investigated under vacuum ultraviolet (VUV) and soft X-ray (XUV) excitation. Photoluminescence (PL) spectra show broad emission bands centered at about 2.3 and 3.2 eV. The PL excitation (PLE) spectral evolution and lifetime measurements reveal that two mechanisms with short and long decay times, attributed to the d($e_g$)-d($e_g$) and p($pi$)-d charge transfer (CT) transitions in the range 4-6,eV, respectively, are responsible for the observed emissions, while the most intensive p($sigma$)-d CT transition at 7,eV appears to be a weak if any PL excitation mechanism. The PLE spectra recorded in the 4-7,eV range agree with the RIXS and reflectance data. Making use of the XUV excitation allows us to avoid the predominant role of the surface effects in luminescence and reveal bulk luminescence with puzzling well isolated doublet of very narrow lines with close energies near 3.3,eV characteristic for recombination transitions in self-trapped emph{d}-emph{d} CT excitons formed by coupled Jahn-Teller Ni$^+$ and Ni$^{3+}$ centers. This conclusion is supported both by a comparative analysis of the luminescence spectra for NiO and solid solutions Ni$_{x}$Zn$_{1-x}$O, and by a comprehensive cluster model assignement of different emph{p}-emph{d} and emph{d}-emph{d} CT transitions, their relaxation channels. To the best of our knowledge it is the first observation of the self-trapping for emph{d}-emph{d} CT excitons. Our paper shows the time resolved luminescence measurements provide an instructive tool for elucidation of the emph{p}-emph{d} and emph{d}-emph{d} CT excitations and their relaxation in 3d oxides.
We investigate effects of optical lattice potential in one- and two-dimensional two-component trapped Fermi gases with population imbalances. Using the exact diagonalization and the density matrix renormalization group methods complementarily, we calculate the atom density profile from the ground state many-body wavefunction as a function of attractive interaction strength for various population imbalances. The numerical results reveal that although a phase separation between the superfluid core and the shell cloud of excess atoms occurs as observed in experiments without the optical lattice, the population imbalance generally remains in the core region in contrast to the non-lattice cases. The essence of the numerical results in a strong attractive regime can be explained by an effective model composed of Cooper pairs and excess major fermions.
We present a study of the resonant inelastic scattering response of ybin excited at the tender Yb $M_5$ X-ray edge. In the high-temperature, paramagnetic phase, we observe a multiplet structure which can be understood at an ionic level. Upon cooling through the valence transition at $T_vsim$ 40$K$, we observe a strong renormalization of the low-energy spectra, indicating a sensitivity to the formation of an intermediate valence phase at low temperatures. Similar spectrum renormalization has been observed in the optical conductivity, which suggests that the low-energy electronic structure possesses both mixed conduction and localized character.
A quantum spin liquid state has long been predicted to arise in spin-1/2 Heisenberg square-lattice antiferromagnets at the boundary region between Neel (nearest-neighbor interaction dominates) and columnar (next-nearest-neighbor dominates) antiferromagnetic order. However, there are no known compounds in this region. Here we use $d^{10}$-$d^0$ cation mixing to tune the magnetic interactions on the square lattice while simultaneously introducing disorder. We find spin-liquid-like behavior in the double perovskite Sr$_2$Cu(Te$_{0.5}$W$_{0.5}$)O$_6$, where the isostructural end phases Sr$_2$CuTeO$_6$ and Sr$_2$CuWO$_6$ are Neel and columnar type antiferromagnets, respectively. We show that magnetism in Sr$_2$Cu(Te$_{0.5}$W$_{0.5}$)O$_6$ is entirely dynamic down to 19 mK. Additionally, we observe at low temperatures for Sr$_2$Cu(Te$_{0.5}$W$_{0.5}$)O$_6$, similar to several spin liquid candidates, a plateau in muon spin relaxation rate and a strong $T$-linear dependence in specific heat. Our observations for Sr$_2$Cu(Te$_{0.5}$W$_{0.5}$)O$_6$ highlight the role of disorder in addition to magnetic frustration in spin liquid physics.
We show how strongly correlated materials could be described within the framework of an excitonic insulator formalism, and delineate the relationship between inter- and intra-band ordering phenomena. Our microscopic model of excitons clarifies the fundamental role of Van-Hove-singularity-nesting in driving both inter- and intra-band ordering transitions, and uncovers an interesting connection with resonating-valence-bond physics.
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