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$VO_2$ undergoes the insulator-metal transition (IMT) and monoclinic-rutile structural phase transition (SPT) near $67^oC$. The IMT switching has many applications. However, there is an unresolved issue whether the IMT is a Mott transition or a Peierls transition. This complication is caused by metal and insulator coexistence, which is an inherent property of the IMT region. Thus, the acquired data in the IMT region are averaged over the two phases in many experiments. We overcome the issue by probing the electronic state of the monoclinic structure and by introducing a model that accounts for the coexisting phases. We reveal the Mott IMT in the non-distorted monoclinic nanodomain between $55-63^oC$, and the distortion-assisted SPT above $60^oC$., Comment: 18 pages, 4 main figures, 5 supporting figures

GISAXS has been used to study morphology change of $\alpha$-Fe$_2$O$_3$ nanocubes after annealing processes. A submonolayer of the nanocubes was deposited on a Si(100) substrate. While an annealing at 400 $^\circ$C in a vacuum does not change a GISAXS pattern from as-prepared nanocubes submonolayer, annealing in the air at the same temperature altered the GISAXS pattern significantly. SEM image showed that the air-annealed nanocubes were coated with thin layers which were identified as amorphous carbon layers based on Raman measurements. GISAXS simulations from morphologies of nanocube with 38 nm side-length and core-shell (nanocube-core and 7 nm thick carbon-shell) nicely reproduced measured patterns from the vacuum annealed and the air-annealed nanocubes, respectively. The current study provides new approach for in-situ characterization of carbon deposition on a uniform shape nanoparticle through monitoring of deposited carbon thickness., Comment: 15 pages, 6 figures

Atomic structure of a mono-dispersive hematite ($\alpha$-Fe$_2$O$_3$) nanocube $(0 1\bar1 2)$ surface was determined with synchrotron X-ray diffraction. The $\alpha$-Fe$_2$O$_3$ nanocubes were prepared through a hydrothermal process and a single layer of nanocubes was deposited on a silicon substrate with a drop cast method. The alignment of nanocubes with their $(0 1\bar1 2)$ surfaces parallel to the substrate is confirmed with grazing incidence X-ray diffraction. Specular crystal truncation rods (CTR's) from as-prepared and vacuum annealed nanocube surfaces have been measured and they are drastically different from previously reported CTR's from macroscopic single crystal $(0 1\bar1 2)$ surfaces. The measured CTR's from nanocube surfaces are explained well with the atomic structure models of half of atoms in top Fe layer being missing while extra oxygen-layers cover the half-missing Fe layer. An acidic environment during hydrothermal nanocube synthesis process is proposed as the main cause of the difference., Comment: 18 pages, 5 figures

A structural transition in an ABO$_{3}$ perovskite thin film involving the change of the BO$_{6}$ octahedral rotation pattern can be hidden under the global lattice symmetry imposed by the substrate and often easily overlooked. We carried out high-resolution x-ray diffraction experiments to investigate the structures of epitaxial Ca$_{0.5}$Sr$_{0.5}$IrO$_{3}$ (CSIO) perovskite iridate films grown on the SrTiO$_{3}$ (STO) and GdScO$_{3}$ (GSO) substrates in detail. Although the CSIO/STO film layer displays a global tetragonal lattice symmetry evidenced by the reciprocal space mapping, synchrotron x-ray data indicates that its room temperature structure is monoclinic due to Glazer's a$^{+}$a$^{-}$c$^{-}$-type rotation of the IrO$_{6}$ octahedra. In order to accommodate the lower-symmetry structure under the global tetragonal symmetry, the film breaks into four twinned domains, resulting in the splitting of the (half-integer, 0, integer) superlattice reflections. Surprisingly, the splitting of these superlattice reflections decrease with increasing temperature, eventually disappearing at T$_{S}$ = 510(5) K, which signals a structural transition to an orthorhombic phase with a$^{+}$a$^{-}$c$^{0}$ octahedral rotation. In contrast, the CSIO/GSO film displays a stable monoclinic symmetry with a$^{+}$b$^{-}$c$^{-}$ octahedral rotation, showing no structural instability caused by the substrate up to 520 K. Our study illustrates the importance of the symmetry in addition to the lattice mismatch of the substrate in determining the structure of epitaxial thin films., Comment: 5 figures

We report Resonant Inelastic X-ray Scattering (RIXS) study of the magnetic excitation spectrum in a highly insulating Eu$_{2}$Ir$_{2}$O$_{7}$ single crystal that exhibits a metal-insulator transition at $T_{MI}$ = 111(7) K. A propagating magnon mode with 20 meV bandwidth and 28 meV magnon gap is found in the excitation spectrum at 7 K, which is expected in the all-in-all-out (AIAO) magnetically ordered state. This magnetic excitation exhibits substantial softening as temperature is raised towards $T_{MI}$, and turns into highly damped excitation in the paramagnetic phase. Remarkably, the softening occurs throughout the whole Brillouin zone including the zone boundary. This observation is inconsistent with magnon renormalization expected in a local moment system, and indicates that the strength of electron correlation in Eu$_{2}$Ir$_{2}$O$_{7}$ is only moderate, so that electron itinerancy should be taken into account in describing its magnetism., Comment: 5 pages, 3 figures