Kheong Sann Chan

The influence of preparation conditions on the surface area of zirconia

Abstract The conditions for preparation of high surface area zirconia were studied. Samples were prepared by precipitation from aqueous solutions of zirconium chloride with ammonium hydroxide. The order of addition of the reactants was found to affect the surface area. Digestion of the hydrous zirconia is the key to high surface area zirconia without the necessity of adding other oxides or dopants. Both the temperature and the time of digestion are important parameters. Zirconia with surface area in excess of 220 m 2 /g after calcination at 500°C have been obtained. The materials maintained a surface area of >90 m 2 /g even after heat treatment at 900°C for 12 h. In addition, digestion led to the formation of the tetragonal allotrope of zirconia. Samples which had been digested for long times at 100°C are tetragonal and maintain this phase up to 1000°C. The effects of digestion seems to be related to a phase transformation of the hydrous precursor at around 80°C. A mechanism based on defect density is postulated to explain the phase stability.

Catalytic carbon monoxide oxidation over strontium, cerium and copper-substituted lanthanum manganates and cobaltates

Abstract The influence of either A or B-site substitution in perovskite-type mixed oxides on the catalytic oxidation of carbon monoxide has been studied. The following systems were investigated: (La,Sr) MnO 3 , La(Mn,Cu)O 3 , (La,Sr)CoO 3 and (La,Ce)CoO 3 . Cobaltates are generally more active than the manganates. Substitution in the A or B-site improved the catalytic activity with oxidation starting from 75 °C. A volcano plot of activity versus composition was obtained for each series with up to a 10-fold increase in catalytic activity for the substituted compounds. Lattice oxygen participates in the reaction even under stoichiometric conditions. The catalysts show a positive rate dependence on the carbon monoxide partial pressure so that under reducing conditions, the reaction is not inhibited. A bistability in the rate of catalytic oxidation at high carbon monoxide concentration was observed over La 1− x Sr x MnO 3 and LaMn 1− x Cu x O 3 (0⩽x⩽0.2). This bistability has been attributed to a carbon monoxide-driven reconstruction of the reduced surface, leading to pairs of Mn 2 ions with a Mn-Mn distance comparable to the spacing in the metal. These pairs provide reactive sites for carbon monoxide oxidation and oxygen chemisorption. Such metal-metal pairs are not found in the perovskite lattice but are a structural feature of the closely related hexagonal 4-layered packing which is the normal crystal structure of SrMnO 3 . The change back to the less active state is due to reoxidation of the surface. It was confirmed that a low mobility of lattice oxygen is a necessary condition for hysteresis in these oxides.

Channel Models and Detectors for Two-Dimensional Magnetic Recording

Two-dimensional magnetic recording (TDMR) is a novel recording architecture intended to support densities beyond those of conventional recording systems. The gains from TDMR come primarily from more powerful coding and signal processing algorithms that allow the bits to be packed more tightly on the disk, and yet be retrieved with acceptable error rates. In this paper, we present some preliminary results for an advanced channel model based on micromagnetic simulations, coined the Grain Flipping Probability model. This model requires a one-time computationally complex characterization phase, but subsequently provides fast and accurate two-dimensional (2-D) readback waveforms that include effects captured from micromagnetic simulations and the statistical effects derived from the granularity of the recording medium. We also show the performance of several detectors over a pre-existing TDMR channel model directly as a function of channel density rather than the signal-to-noise ratio (SNR).

TDMR Platform Simulations and Experiments

Two-dimensional magnetic recording (TDMR) is a novel architecture for magnetic recording systems proposed to achieve densities towards 10 Tb/in2. TDMR differs from other solutions in that it does not require a complete redesign of the head or medium for its gains, relying instead on powerful 2-D codes and signal processing to reliably store and retrieve the information. To explore the viability of TDMR via simulation, models for the 2-D medium, writing, and readback are needed. In this paper, we propose models for these processes and discuss ongoing progress in 2-D experimental waveform retrieval that will be used to validate the models. We show some initial results achieved using these models which demonstrate the viability of TDMR and that will provide an indication of what densities may be possible with TDMR, when the 2-D codes and detectors are completed.

Trapping of ultra-cold atoms with the magnetic field of vortices in a thin-film superconducting micro-structure

We store and control ultra-cold atoms in a new type of trap using the magnetic fields of vortices in a high-temperature superconducting micro-structure. We generate the attractive trapping potential for the atoms by combining the magnetic field of a superconductor in the remanent state with external homogeneous magnetic fields. We show the control of crucial atom trap characteristics such as an efficient intrinsic loading mechanism, spatial positioning of the trapped atoms and the vortex density in the superconductor. The measured trap characteristics are in good agreement with our numerical simulations.

Low-Complexity Iterative Row-Column Soft Decision Feedback Algorithm for 2-D Inter-Symbol Interference Channel Detection With Gaussian Approximation

In this paper, we study the complexity reduction problem of the iterative row-column soft decision feedback algorithm (IRCSDFA) for 2-D inter-symbol interference (ISI) detection. Specifically, Gaussian approximation (GA) is employed in both the component row and column detectors of the IRCSDFA in order to reduce its computational complexity. With the employment of GA, the state space dimension of the ISI trellis of either component detector can be reduced enormously (i.e., the number of branches in one ISI trellis section decreases). Bahl-Cocke-Jelinek-Raviv (BCJR) algorithm is then employed to perform detection over the GA-simplified ISI trellis. For brevity, we refer to the IRCSDFA with BCJR detection over the GA-simplified ISI trellis as “IRCSDFA-GA-BCJR”. Next, the iteration scheduling of component detectors and decoder in coded 2-D ISI channels with low density parity check (LDPC) coding and IRCSDFA-GA-BCJR detection is studied. Specifically, three iteration schemes: single detector (row or column) scheme, alternate detector scheme, and combined detector scheme, are considered, with the last scheme showing the best coded performance. Finally, the computational complexity of the proposed IRCSDFA-GA-BCJR is analyzed, and shown to have significant reduction with a cost of only about 0.3 and 0.35 dB in coded BER/FER performance loss compared to the conventional IRCSDFA without GA and the optimal symbol-based BCJR algorithm, respectively.

Modeling of 2-D Magnetic Recording and a Comparison of Data Detection Schemes

Two-dimensional magnetic recording (TDMR) together with shingled magnetic recording (SMR) are technologies proposed to extend the life of conventional granular magnetic recording. The grain flipping probability (GFP) model has been proposed to mimic the performance of micromagnetic ( μ-mag) simulations for the purpose of signal reproduction. Other work in TDMR includes the proposal of a Gaussian mixture model (GMM) that produces improved likelihood information at the output of the detector, combined with low density parity check (LDPC) codes. The contribution of this paper is threefold. First, we aim to simulate a TDMR/SMR recording system with the GFP model, both with and without the GMM detector, and with various random and structured LDPC codes, of both 4 k and 16 k block lengths, to determine areal densities that might be achieved. Second, we perform a comparison of the various model order reduced (MOR) GFP implementations to compare the effect of writing with various factors taken out of the picture. Third, we perform a validation of the GFP model and the setup as a whole, by running the system with a parameter set close to that of conventional recording. The results of these experiments give an assurance of the validity of our model, give an indication of the expected density that might be achieved in a TDMR/SMR system, and give a direction for which parameter(s) in magnetic recording systems might be optimized to yield the most gain.

A Study of SMR/TDMR With a Double/Triple Reader Head Array and Conventional Read Channels

Two-dimensional magnetic recording is a novel technology proposed to extend the life of conventional granular magnetic recording (CGMR). It is the readback counterpart technology to shingled magnetic recording (SMR) that is being pursued by the industry today. In SMR, a 2-D block of bits is written as a single unit: modifying any bit within the block requires a modification to the entire 2-D block. In TDMR, by comparison, a 2-D block is read and processed as a single unit. Latency during writing of a 2-D block is not a serious problem as the data is buffered in the RAM. Latency during readback, however, results in a performance degradation to the user who has to wait before getting his file. If there is a single reader head, 2-D readback necessitates multiple revolutions of the disk leading to a significant latency, which may not be acceptable to the user and is one of the reasons the industry is hesitating for 2-D magnetic recording (TDMR). Recent events have seen companies working on the development of multiple (two or three) reader heads that read multiple tracks simultaneously with a single pass of the head. Such double/triple reader head arrays could be used to support TDMR, obviating the need for multiple revolutions of the media and reducing the read latency. An alternative use for the multireader heads would be in an SMR scenario, where only a single track is readback at a time. In this case, the extra readers could be used to estimate and mitigate the intertrack interference coming from the adjacent tracks. The difference between these two modes is the number of tracks detected with each head pass. For TDMR there would be two or three tracks detected with a single pass while for SMR there would be just one. In this paper, we study and optimize the impact of parameters of the double/triple reader head array being used in the SMR and TDMR readback scenarios. The key difference between these scenarios is in the design of the GPR equalizer and we compare and contrast the recording system varying several key design parameters.

Analysis of the partitioned frequency-domain block LMS (PFBLMS) algorithm

In this paper, we present a new analysis of the partitioned frequency-domain block least-mean-square (PFBLMS) algorithm. We analyze the matrices that control the convergence rates of the various forms of the PFBLMS algorithm and evaluate their eigenvalues for both white and colored input processes. Because of the complexity of the problem, the detailed analyses are only given for the case where the filter input is a first-order autoregressive process (AR-1). However, the results are then generalized to arbitrary processes in a heuristic way by looking into a set of numerical examples. An interesting finding (that is consistent with earlier publications) is that the unconstrained PFBLMS algorithm suffers from slow modes of convergence, which the FBLMS algorithm does not. Fortunately, however, these modes are not present in the constrained PFBLMS algorithm, A simplified version of the constrained PFBLMS algorithm, which is known as the schedule-constrained PFBLMS algorithm, is also discussed, and the reason for its similar behavior to that of its fully constrained version is explained.

Programmable trap geometries with superconducting atom chips

We employ the hysteretic behavior of a superconducting thin film in the remanent state to generate different traps and flexible magnetic potentials for ultracold atoms. The trap geometry can be programed by externally applied fields. This approach for atom optics is demonstrated by three different trap types realized on a single microstructure: a Z-type trap, a double trap, and a bias-field-free trap. Our studies show that superconductors in the remanent state provide a versatile platform for atom optics and applications in ultracold quantum gases.