Back home

Back home

UH logoHouston Science CenterBuilding 593 – (713) 743-8200

TcSUH Events

Home » Events » Bi-Weekly Seminar

Bi-Weekly Seminar

The Unified Electronic Phase Diagram of High Tc

Prof. Pei  Hor

by: Prof. Pei Hor

Date: Friday February 01, 2008

Time: 12:00 pm – 1:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

I will discuss the construction of a unified electronic phase diagram (UEPD) by analyzing various characteristic temperatures and energies of high-Tc cuprates using a dimensionless universal hole-doping concentration (pu). There are three converging characteristic temperatures (T*'s) and their corresponding energies (E*'s) as pu increases in the underdoped regime. T*'s and E*'s merge together with the Tc (pu) and 3.5kB Tc (pu) curves at pu ~ 1.1 in the overdoped regime, respectively. They finally go to zero at pu ~ 1.3. The UEPD follows an asymmetric half-bell-shaped Tc-curve in which Tc appears at pu ~ 0.4, reaches a maximum at pu~ 1, and rapidly goes to zero at pu ~1.3. The asymmetric UEPD curve is at odds with the well-known symmetric superconducting dome for La2-xSrxCuO4 in which two characteristic temperatures and energies that converge as pu increases and merge together when Tc goes to zero at pu ~ 1.6,. The unified phase diagram clearly shows that pseudogap is necessary for high temperature superconductivity. I will discuss some universal intrinsic properties of high- Tc that can be easily understood in terms of the UEPD.

Download: Event PDF

Bi-Weekly Seminar

Progress Report on Two Nano-Material Studies

Prof. Wei-Kan  Chu

by: Prof. Wei-Kan Chu

Date: Friday November 09, 2007

Time: 12:00 pm – 1:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

Ion-Beam Assisted Fabrication of GaN nanorod and Applications Abstract: GaN is a direct Band Gap Semiconductor. Its ternary compound with In or with Al could cover a very broad band region with potential applications in Laser source, Light Emitting Diode, high efficiency solar cell and optoelectronics. We have studied GaN nanorod formation during MBE growth of GaN film on Si[111] substrate, and its fabrication assisted by ion implantation on Si before the the MBE growth. In this talk, I will give a progress report on our nanorod growth studies, Ion Beam Assisted growth, Nanorod characterization, and its potential applications. (Collaboration with Q. Y. Chen, L. W. Tu, and H.W. Seo). Field Ionization of Carbon Nano Tubes and Applications Abstract: The removal of electrons from any species by interaction with a high electrical field is called Field Ionization. The most notable work on field ionization conducted by Mueller?s team at Penn State [Phys. Rev 102, 624 (1956)] is a perfect example, which later developed into the famous Field Ion Microscope (FIM). Focus Ion Beam (FIB) is another example, where a sharp tip can emit focused liquid metal ions such as Ga ions when positively biased. We have studied field ionization of Carbon Nano Tubes under residual hydrogen gas, and produced huge proton current. In this talk, I will discuss the implication of our experiment and its potential applications. (Collaboration with Jiarui Liu).

Download: Event PDF

Bi-Weekly Seminar

Nanostructured Pt alloy Core-Shell Fuel Cell Electrocatalysts - Synthesis, Structure, and Performance

by: Dr. Peter Strasser

Date: Thursday October 25, 2007

Time: 12:00 pm – 1:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

The identification of more active, more cost-effective and more stable electrocatalysts for the oxygen reduction reaction (ORR) continues to bea scientific priority in low-temperature Fuel Cell catalysis research.Among all currently known electrocatalyst materials, Pt alloys have remained one of the most attractive catalyst concepts, in particularfrom a power density perspective.

We have recently discovered a new class of Pt core shell nanoparticle electrocatalysts which exhibit ORR activities exceeding those of conventional uniformly alloyed Pt-rich catalysts. We also have put forward a hypothesis for the enhancement mechanism which focused on lattice strain in the Pt rich Shell of the nanoparticles resulting from the electrochemical de alloying synthesis. Our experiments have been corroborated by DFT computational modeling.

We also report on recent strategies to experimentally realize the high electrocatalytic RDE activities of our new catalysts in realistic single PEM fuel cell devices.

Download: Event PDF

Bi-Weekly Seminar

Oxygen Diffusion and Surface Exchange in Mixed Conducting Metal Oxides

Dr. Allan J. Jacobson

by: Dr. Allan J. Jacobson

Date: Friday September 28, 2007

Time: 12:00 pm – 1:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

The performance of many energy conversion and storage devices depend on the properties of mixed electronic-ionic conducting materials. Mixed or ambipolar conductors simultaneously transport ions and electrons and provide the critical interface between chemical and electrical energy in devices such as fuel cells and batteries. Enhancements in storage capacity, reversibility, power density and life all require new materials and a better understanding of the fundamentals of ambipolar conductivity. In this presentation, I will describe some recent results for a remarkable new class of oxygen ion mixed conductors with potential applications in fuel cells and ion transport membranes.

We have shown that mixed-conducting non-stoichiometric perovskite oxides with ordered A site cations have remarkably high oxygen ion conductivity and surface reaction rates for oxygen exchange relative to conventional materials. Subsequent to our own studies, two other groups have demonstrated comparably high oxygen diffusion in similar compounds confirming that this class of compounds represents a significant enhancement in the achievable rates of oxygen diffusion in mixed conducting oxides.

In PrBaCo2O5+x (PBCO), a representative example of this class of compounds, the barium and praseodymium cations are located in planes that alternate along the c axis; oxygen vacancies occur only in the ab plane containing the Pr3+ cations. The oxygen diffusion coefficient measured in PrBaCo2O5+x as a function of temperature surpasses the diffusion coefficients of the compounds La0.5Sr0.5CoO3-x and La2NiO4+x which are among the highest of the known mixed conducting oxides.

The surface exchange coefficient for oxygen exchange has been measured on thin films of PrBaCo2O5+x by electrical conductivity relaxation and by oxygen-isotope exchange and depth profiling. Microstructural studies indicate that the PBCO films, prepared by pulsed laser deposition, have excellent single-crystal quality and epitaxial nature. The measurements reveal that the PBCO films have high electronic conductivity and more rapid surface exchange kinetics than those of other perovskites.

Reasons for the high oxide ion diffusion and surface exchange coefficients and the relation to the high electronic conductivity and diffusion pathways will be discussed together with the potential use of the compounds as electrodes for oxygen reduction in fuel cells and as membranes for oxygen separation.

Download: Event PDF

Bi-Weekly Seminar

Finding the Key to the High Tc Puzzle

by: Prof. Young Kim

Date: Friday September 14, 2007

Time: 11:00 am – 12:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

The high-Tc puzzle remains unsolved despite extensive experimental collection of the puzzle pieces over the past two decades. Granted, this could be due to the complexity of the problem, but it could also be very likely that some key building blocks might have been overlooked as they were hidden behind various experiments on different high temperature superconducting materials and, therefore, too subtle to be recognized. In order to solve this puzzle, we have re-searched the key pieces in hand and put them together to bring about a coherent picture that captures the essential physics of high Tc superconductivity.

Back to the top of the page

Copyright © 2009 Texas Center for Superconductivity (TcSUH) – 3201 Cullen Suite 202, Houston, Texas 77004 – (713) 743-8200 – Houston Science Center – Buillding 593 – Mail Code: TCSUH 5002

Problems or feedback? Email: