Seminars 2006

To gain an initial understanding of the copper-based catalysts in commercially important chemical reactions such as the oxygen-assisted water-gas shift reaction (OWGS), we performed density-functional theory calculations, investigating the interaction of oxygen and copper and focusing on the relative stability of surface oxides and oxide surfaces of the O/Cu system.

By employing the technique of "ab initio atomistic thermodynamics", we show that surface oxides are only metastable at pressures and temperatures relevant to technical catalysis, with no stable chemisorption phase observed even at very low coverage.

Having identified the bulk oxide as the most stable phase under realistic catalytic conditions, we highlight the importance of defects in the bulk and at the surface. Two (defected) copper oxide surfaces are shown to have a lower surface free energy compared to the stoichiometric surface for the considered range of oxygen chemical potential and could be catalytically relevant. Reaction pathways for the elementary processes of the OWGS reaction over these surfaces are being investigated and will be reported.

In recent years, femtosecond real-time spectroscopy has been shown to present an excellent experimental alternative for studying temperature-dependent changes of the low-lying electronic structure of superconductors and other strongly correlated electron systems like charge density waves, manganites, or heavy fermions. In these experiments, a femtosecond laser pump pulse excites electron-hole pairs. These high-energy quasiparticles rapidly, within 100s of femtoseconds, thermalize via electron-electron and electron-phonon collisions, reaching states near the Fermi energy. Further relaxation dynamics are strongly affected by the low-energy electronic structure in these materials. The dynamics are extracted either by time-resolved measurements of the photoinduced changes in reflectivity or transmission at optical frequencies, or by directly measuring conductivity dynamics, with the probe wavelength in the terahertz (far-infrared) range.

In my work in Los Alamos I have extended the utility of ultrafast spectroscopy to another class of correlated electron materials, namely spin-density-wave (SDW) compounds, whether in its pure state or when it coexists with another order such as superconductivity. My talk will present time-resolved photoinduced reflectivity data of itinerant antiferromagnets UMGa5 (M=Ni, Pt), and their parent compound UGa3 down to 10K. For UNiGa5 (TN = 85 K) the relaxation time τ shows a sharp increase at TN consistent with the opening of a SDW gap, which is the charge gap that opens up along the nested regions of the Fermi surface. In addition, the temperature dependence of τ below TN is consistent with the opening of a SDW gap leading to a quasiparticle recombination bottleneck as revealed by the Rothwarf-Taylor model. For UPtGa5 (TN = 26 K) however, no change in τ was observed across TN, suggesting that no SDW gap opens up in the antiferromagnetic phase. We attribute this to UPtGa5 having a different type of magnetic order compared to UNiGa5, leading to a gapless quasiparticle spectrum. Our results thus challenge the conventional wisdom that a SDW phase necessarily implies a SDW gap at the Fermi level.

Finally, I will present data on the high-temperature superconductor Tl2Ba2Ca2Cu3Oy (Tl-2223). Without applying any external magnetic field, we see a qualitative change in the relaxation dynamics below ~40 K, which is suggestive of an entry into the coexistence phase where superconductivity and antiferromagnetism coexist. To quantitatively explain our data, we combined a coupled model describing the time-evolution of quasiparticles and high-frequency phonons in the presence of a gap in the density of states, and a mean field model that gives rise to a decrease in the superconducting gap as one enters the coexistence state. Our study once again points to a magnetic origin in the mechanism of high-temperature superconductivity

In their seminal paper [Nature 409, 46 (2001) ], Knill, Laflamme and Milburn demonstrated that computationally efficient quantum information processing employing linear optics components should in principle be feasible. In our talk, we elaborate on the concatenation of increasingly nontrivial ideas that finally leads to this unexpected and conceptually important insight.

Modern Science-Fiction often assumes future “discoveries” in theoretical physics. Most of the stories would be impossible if our present understanding of the world of particles and forces stands firm. Since big modifications of what we found cannot be expected, one may wonder what humanity’s future might look like. Can outer space be colonized? Can intelligent robots be built? Can we travel to the stars? Can we control the Earth’s climate? Lecturer speculates on what can be done, not on what will be done.

The chromosome ends in eukaryotic cells are called Telomeres. Although Telomeres are essential for genome integrity and play an important role in cellular aging and cancer, knowledge about their physical structures is still limited. Telomeric DNA consists of tandem repeats of cytosine-rich sequences on one strand and guanine-rich sequences on the other strand. Both strands are prone to formation of structures that are different from the canonical Watson-Crick double helix. In this seminar, studies of the so-called i-motif and G-quadruplex structures adopted by DNA Telomeric sequences will be presented and their implications for anticancer drug design will be discussed.

Airborne microparticles such as bacterial spores are hard to detect from distance. Fast and sensitive detection technique which carries the spectroscopic fingerprint of characteristic chemical compounds would be a very useful tool when incorporated into a LIDAR system. This requires real time detection of backscattered signal which is too weak with existing techniques such as laser induced fluorescence and Raman spectroscopy. Coherent anti-Stokes Raman spectroscopy (CARS) is probably one of the nonlinear techniques that comes closest for the purpose. I will show that the CARS signal can be enhanced via quantum coherence using carefully shaped and delayed pulses. The theory goes beyond the usual perturbative regime and takes into account the focusing effect of the microparticle. A large backscattered signal shows that this technique could be promising for LIDAR system. Extension of the theory to particles composed of complex molecules is possible using transform theory.

Surface emitting lasers and short wavelength optoelectronic devices are two important separate topics of considerable recent research and commercial interest. However, surface emitting lasers in the wavelength from blue green to UV are still under developed. Taking advantage of the unique material properties inherent to ZnO, especially high excitonic oscillator strength and high exciton binding energy, it is expected to directly generate blue-green to UV vertical cavity lasers with high power and high beam quality for a wide range of applications. This talk will first review the recent advance in surface emitting lasers and ZnOrelated materials, and then discuss the impact of introducing ZnO-related materials into vertical cavity lasers on both research community and industrial applications.

In the last decade, dilute nitride III–V alloy semiconductors, in particular GaInNAs quaternary compounds, have received intensive attention for promising active materials in optical fiber communication. In this presentation, I will review the basic physical properties of this novel material system and demonstrate how it could be applied to a range of surface emitting devices in the 1.3–1.6 μm wavelength region

Hybrid nanocomposite coatings through multiple processes is offering a significant impact on surface technology because of their unique capabilities and novel functionalities through nano-structuring and nano-processing. Such coatings containing multiphases in composition and multiple or multi-layer in structure render substantially improved mechanical, chemical and tribological properties as well as additional functions such as anti-sticking, self-lubricating, selfcleaning, anti-fogging, thermal insulating abilities and environment- and bio-compatibility. In this presentation, nano-phased hybrid multi-functional coating systems will be reviewed. Case studies in coating design and processes will be shared and discussed for various applications using vapour deposition, sol-gel and electro-chemical processes.

Si-based epitaxy has found more and more applications in advanced Si technologies. While some epitaxial processes have been mature enough for high volume manufacturing, there are a lot of unknowns in other new epitaxial processes as well as their integration into Si technologies. This talk will give a brief summary of our work in this area. We will present our recent results on growth kinetics, integration, as well as engineered substrates.

GaN is said to be the next semiconductor after silicon. In year 2005, the market for GaN devices reached about US$ 5.2 billions. Major applications of GaN include solid-state lighting using semiconductor light emitting diodes (LEDs), backlighting of LCD TVs, mobile phones, automotives, latest-generation DVD players (blu-ray/HD-DVD players, up to 200 GB per disk), etc.

In this talk, the speaker will give an overview of GaN and related materials, including material growth, properties, processing, and devices. He will focus on the following topics:

1. Applications of GaN devices
2. GaN and related materials
3. Processing of GaN and related materials
4. GaN devices
5. Power GaN devices made at Tinggi Technologies

A brief insight in the field of ultracold atom physics. Under unimaginably cold conditions, the elusive quantum world — usually hidden at higher temperatures — comes into focus.

Superfluidity of liquid helium is one of the earliest and most tangible examples. This phenomenon occurs at just 2 K, modern laser-cooling technology can attain orders of magnitude lower temperatures.

So what happens at almost absolute zero and how can we reach it?

The Sun is a very common star, one of the billions populating our Galaxy. The Sun is of course very special to us for it is the reason for habitability on Earth. It faithfully rises and sets each day except for the occasional startle of a solar eclipse. The Sun is a much more exciting object to the astrophysicists and space scientists. Space Age began in the late fifties when Russian and American satellites went into space for the first time. Eventually astronomers got their turn to put instruments into space in order to observe the Sun above our blanketing atmosphere. Observations from space together with the more traditional kinds from the ground have revealed a dynamic Sun, at times explosive and evolving on all observable time scales down to less than a second. The Sun has a global magnetic field that reverses direction faithfully once every 11 years. A diversity of solar processes originates from this remarkable phenomenon to influence our climate and control our terrestrial as well as our near-space environments. This talk is about how the Sun is interesting and how lucky we do get in understanding the physics of this natural system.