Orbital-selective Mass Enhancements in Multi-band Ca2−x_{2-x}Srx_{x}RuO4_{4} Systems Analyzed by the Extended Drude Model

We investigated optical spectra of quasi-two-dimensional multi-band Ca$_{2-x}$Sr$_{x}$RuO$_{4}$ systems. The extended Drude model analysis on the ab-plane optical conductivity spectra indicates that the effective mass should be enhanced near $x=0.5$. Based on the sum rule argument, we showed that the orbital-selective Mott-gap opening for the $d_{yz/zx}$ bands, the widely investigated picture, could not be the origin of the mass enhancement. We exploited the multi-band effects in the extended Drude model analysis, and demonstrated that the intriguing heavy mass state near $x=0.5$ should come from the renormalization of the $d_{xy}$ band.Comment: 4 figure

Epitaxial growth and the magnetic properties of orthorhombic YTiO3 thin films

High-quality YTiO3 thin films were grown on LaAlO3 (110) substrates at low oxygen pressures (<10-8 Torr) using pulsed laser deposition. The in-plane asymmetric atomic arrangements at the substrate surface allowed us to grow epitaxial YTiO3 thin films, which have an orthorhombic crystal structure with quite different a- and b-axes lattice constants. The YTiO3 film exhibited a clear ferromagnetic transition at 30 K with a saturation magnetization of about 0.7 uB/Ti. The magnetic easy axis was found to be along the [1-10] direction of the substrate, which differs from the single crystal easy axis direction, i.e., [001].Comment: 14 pages, 4 figure

Predictability of reset switching voltages in unipolar resistance switching

In unipolar resistance switching of NiO capacitors, Joule heating in the conducting channels should cause a strong nonlinearity in the low resistance state current-voltage (I-V) curves. Due to the percolating nature of the conducting channels, the reset current IR, can be scaled to the nonlinear coefficient Bo of the I-V curves. This scaling relationship can be used to predict reset voltages, independent of NiO capacitor size; it can also be applied to TiO2 and FeOy capacitors. Using this relation, we developed an error correction scheme to provide a clear window for separating reset and set voltages in memory operations

Observation of inhomogeneous domain nucleation in epitaxial Pb(Zr,Ti)O3 capacitors

We investigated domain nucleation process in epitaxial Pb(Zr,Ti)O3 capacitors under a modified piezoresponse force microscope. We obtained domain evolution images during polarization switching process and observed that domain nucleation occurs at particular sites. This inhomogeneous nucleation process should play an important role in an early stage of switching and under a high electric field. We found that the number of nuclei is linearly proportional to log(switching time), suggesting a broad distribution of activation energies for nucleation. The nucleation sites for a positive bias differ from those for a negative bias, indicating that most nucleation sites are located at ferroelectric/electrode interfaces

Multiple conducting carriers generated in LaAlO3/SrTiO3 heterostructures

We have found that there is more than one type of conducting carriers generated in LaAlO3/SrTiO3 heterostructures by comparing the sheet carrier density and mobility from optical transmission spectroscopy with those from dc-transport measurements. When multiple types of carriers exist, optical characterization dominantly reflects the contribution from the high-density carriers whereas dc-transport measurements may exaggerate the contribution of the high-mobility carriers even though they are present at low-density. Since the low-temperature mobilities determined by dc-transport in the LaAlO3/SrTiO3 heterostructures are much higher than those extracted by optical method, we attribute the origin of high-mobility transport to the low-density conducting carriers.Comment: 3 figures, supplemental materia

Dielectric constants of Ir, Ru, Pt, and IrO2: Contributions from bound charges

We investigated the dielectric functions $\epsilon$($\omega$) of Ir, Ru, Pt, and IrO$_2$, which are commonly used as electrodes in ferroelectric thin film applications. In particular, we investigated the contributions from bound charges $\epsilon^{b}$($\omega$), since these are important scientifically as well as technologically: the $\epsilon_1^{b}$(0) of a metal electrode is one of the major factors determining the depolarization field inside a ferroelectric capacitor. To obtain $\epsilon_1^{b}$(0), we measured reflectivity spectra of sputtered Pt, Ir, Ru, and IrO2 films in a wide photon energy range between 3.7 meV and 20 eV. We used a Kramers-Kronig transformation to obtain real and imaginary dielectric functions, and then used Drude-Lorentz oscillator fittings to extract $\epsilon_1^{b}$(0) values. Ir, Ru, Pt, and IrO$_2$ produced experimental $\epsilon_1^{b}$(0) values of 48$\pm$10, 82$\pm$10, 58$\pm$10, and 29$\pm$5, respectively, which are in good agreement with values obtained using first-principles calculations. These values are much higher than those for noble metals such as Cu, Ag, and Au because transition metals and IrO$_2$ have such strong d-d transitions below 2.0 eV. High $\epsilon_1^{b}$(0) values will reduce the depolarization field in ferroelectric capacitors, making these materials good candidates for use as electrodes in ferroelectric applications.Comment: 26 pages, 6 figures, 2 table