TcSUH
Bi-Weekly Seminar
The Newly-Discovered Fe-Based Layered High Temperature Superconductors
by: Dr. Ching Wu Chu
Date: Friday April 11, 2008
Time: 12:00 pm – 1:00 pm
Location: Houston Science Center – Building 593 — Room 102
Overview
Built upon insight gained from cuprate high temperature superconductors and work on the equiatomic quarternary rare-earth transition metal oxypnictide superconductors, ROTP with R = rare-earth, O = oxygen, T = transition metal, and P = pnictogen over the last few years, Hideo Hosona's group in the Tokyo Institute of Technology reported in late February of this year that they achieved superconductivity with a Tc of 26 K in F-doped LaOFeAs. The report has generated great excitement. Last week, three groups in China achieved a Tc = 41 K in F-doped CeOFeAs, a Tc = 43K in F-doped SmOFeAs, and a Tc = 50 K in F-doped PrOFeAs, respectively. Tens of papers flooded the scientific community in a very short period of time, due to the possible scientific implications on high temperature superconductivity and the intriguing physics involved. In this talk, I will present experimental results from our group on some of these compounds at TcSUH and discuss the similarities and differences between the cuprate and pnictide superconductors, after briefly presenting the history on and summarizing the present status of the study on superconducting ROTP.
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Bi-Weekly Seminar
Electronic Mechanism of Two-Dimensional D-Wave Superconductivity
by: Prof. W. P. Su
Date: Friday March 28, 2008
Time: 12:00 pm – 1:00 pm
Location: Houston Science Center – Building 593 — Room 102
Overview
Motivated by high-temperature superconductors, it is useful to discuss a BCS-like effective Hamiltonian for a two-dimensional d-wave superconductor independent of the origin of the pairing potential and to explore the consequences of such a model. A particular aspect of this model which has so far not received adequate attention is the intrinsic instability of the charge carriers towards phase separation. Such an instability can lead to phenomenologies strikingly similar to those observed in the cuprates including inhomogeneous superconductivity and the ‘pseudogap’ phase diagram. This raises the issue of how much of the cuprate phenomenology is independent of the origin of the pairing potential. In the case of the s-wave superconductors, it is well-known that most of the properties of the superconductors can be explained by an effective Hamiltonian which contains an attractive pairing potential which might or might not be mediated by phonons.
As an example of electronic mechanism for d-wave pairing, we have studied a model inspired by an idea proposed by W. A. Little in the sixties. In this model, a charge moving in a conduction plane can induce charge polarization in polarizable molecules surrounding the conduction plane. This polarization can in turn attract a second charge carrier thus establishing an effective attractive interaction between two carriers. For certain choice of parameters, the model does seem to exhibit d-wave superconductivity. Interestingly, the model also seem to exhibit phase separation.
For the cuprates, a more likely origin of the pairing force is probably interlayer polarization. Theoretical as well as experimental arguments will be presented to support this speculation.
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Bi-Weekly Seminar
X-ray Scattering Studies of Semiconductor Nanoclusters in Zeolites
by: Milinda Abeykoon
Date: Friday February 29, 2008
Time: 12:00 pm – 1:00 pm
Location: Houston Science Center – Building 593 — Room 102
Overview
When electrons and holes in a semiconductor are confined to ultra-small regions of space (typically 1-25 nm), the optical and electronic properties of the semiconductor become strongly size-dependent. Such structures are called quantum dots, nanowires or nanoclusters, depending upon their shape and dimensionality. These nanostructures are of great interest for a variety of potential electronic, photochemical and nonlinear optical applications and are necessary for an analysis of the transition from molecular to bulk semiconductor properties.
This talk will discuss the structure of HgSe and Se semiconductor nanoclusters synthesized in both Nd-Y (spherical pore) and LTL (tubular pore) zeolites. The molecular structures of these systems were modeled by performing the Rietveld refinement on X-ray Bragg data. A remarkable feature in our X-ray diffraction patterns, continuous diffuse scattering under the Bragg peaks, will also be discussed along with our PDF (Pair Distribution Function) data. We use the results of optical studies to complement our X-ray structural work.
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Bi-Weekly Seminar
The Ultrasensitive SQUID-Based Sensing Applications for Biomedical Imaging and Diagnostics
Date: Friday February 15, 2008
Time: 12:00 pm – 1:00 pm
Location: Houston Science Center – Building 593 — Room 102
Overview
Superconducting QUantum Interference Device (SQUID) is the most sensitive and stable detector of magnetic flux available. The SQUID sensing provides the unmatched sensitivity and temporal resolution used for detection of the electromagnetic field perturbation associated with the neuronal currents in the brain, fetal cardiac activity, and the nuclear spin magnetization in ultra-low field NMR/MRI. In this presentation, I will summarize the current status of SQUID biomedical applications relevant to their present scientific and technological challenges, focusing on those applications that convey fundamental technological breakthroughs in corresponding biomedical fields. I will then describe our activities in the area of fetal cardiac monitoring, vulnerable plaque detection, cancer diagnostics and ultra-low field MR imaging. I will also present new research highlights from our recent clinical study in London aimed at developing a new ultrasensitive magnetic probe for detecting the spread of breast cancer.
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Bi-Weekly Seminar
The Unified Electronic Phase Diagram of High Tc
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.
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