Session A2: Condensed Matter I

Friday, October 21, 2005

The study of phenomena applicable to the development of nonvolatile memory technology has been ongoing for nearly four decades. One such research route lies in the reversible pulse-induced resistive switches reported recently across various metal-perovskite oxide-metal configurations. Several models have been proposed to attribute the observations to bulk phase-separation, changes in the interfacial defects, Schottky-like barriers, or hopping between specially arranged non-percolating metallic domains imbedded in an insulating matrix. Such different mechanisms predicate rather different optimal configurations, switch speed, retention, and device potentials. Impedance spectroscopy, I-V characterization, and the temperature dependency of the resistance, therefore, are used to compare with various models and to deduce key parameters. The data confirm that the switches mainly occur in an active interfacial layer no thicker than a few nm with a defect density that changes significantly during the switching.   

Friday, October 21, 2005

The magnetic phase transitions in the phase diagrams of RMnO$_{3}$ (R = Er, Tm, Ho) are correlated to changes in their dielectric properties proving the existence of strong spin-lattice coupling in these compounds. For the case when R = Tm and Er, their N\'{e}el phase transition line decreases slightly with increasing external magnetic fields up to 7 Tesla and at low temperatures a phase transition line is detected at higher external fields which possibly merges with the N\'{e}el phase transition line at fields above 7 Tesla. For the case when R = Ho, a far more complex phase diagram was discovered including spin reorientation phase transitions and additional previously unknown phases with magnetic structures of which have yet to be explored.   

Friday, October 21, 2005

The magnetic structure of the Ce(Mn$_{1-x}$Cu$_{x})_{2}$Si$_{2}$ (0 $\le $ x $\le $ 1) series, which evolves from a non-magnetic Kondo-heavy-fermion system at $x$=1 to a 3d-antiferromagnetically ordered mixed valence system at $x$=0, has been studied by both temperature and field dependent magnetization ($M)$ measurements using a SQUID magnetometer. Data were taken in both zero-field-cooled (ZFC) and field cooled (FC) processes. It is found that for 0 $\le x <$ 0.4, Neel temperature decreases rapidly with the increase of the Cu concentration $x$. In the range of $x$ from 0.4 $\le $ x $<$ 0.8, ferromagnetic phase is observed below 150 K, and both the ordering temperature $T_{c}$ and Qurie-Weiss $\theta $ decrease with the increase of $x$ . For $x \ge $ 0.8, the $M$ (T) curves are reversible and display paramagnetic behavior. Thus, the system is non-magnetic as it approaches heavy-fermion compound CeCu$_{2}$Si$_{2}$. A magnetic phase diagram is proposed for this compound series.   

Friday, October 21, 2005

The mechanism of cuprate high-temperature superconductivity is still controversial. It is widely accepted that HTSC occurs primarily within CuO planes of cuprates, yet there is contention concerning the in-plane low-energy physics. Using 2$\times $2 high-Tc superlattices (SL) (Ba$_{0.9}$Nd$_{0.1}$CuO$_{2+x})_{2}$/(CaCuO$_{2})_{2}$, we probe the CuO$_{2}$ planes by applying x-ray emission/absorption spectroscopy. The pulsed-laser deposited SL consist of two separately insulating layers and exhibit a T$c$ of 80K. Superconductivity occurs exclusively within the infinite layer (IL) and not the charge reservoir (CR) of the SL. We demonstrate resonant x-ray emission and absorption to be insightful tools for studying the IL, CR and superlattice structures. We measure the O 1s density of state to be insulating for the component layers and metallic for the superlattice. Using resonant inelastic scattering (RIXS) we make the first direct observation of Zhang-Rice singlets in artificial high-Tc superconducting heteroepitaxial structures. A comparison of the x-ray emission spectra of the SL and its component layers gives evidence of charge transport from the so-called charge reservoir layer to the infinite layer.   

Friday, October 21, 2005

Nd$_{1/3}$Eu$_{1/3}$Gd$_{1/3}$Ba$_{2}$Cu$_{3}$O$_{x}$ (NEG)/YBCO multi-layer up to 6 layer films are made and enhanced $J_{c }$is achieved compared to pure YBCO film. The enhancement is observed over a wide range of temperatures. Possible reason for this enhancement is investigated. Magneto-optical imaging shows that these films have excellent macroscopic magnetic uniformity.