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Distinguished Lecture Series

The Role of Vortices in Limiting Tc in Cuprate Superconductors

Prof. N. P. Ong

by: Prof. N. P. Ong

Date: Friday April 18, 2008

Time: 4:00 pm – 6:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

Superconductivity in the copper oxides occurs at temperatures much higher than in all other metals. There is growing evidence that the Cooper pairs actually survive to even higher temperatures. I will discuss Nernst and torque magnetometry experiments which suggest the scenario that, above Tc in the cuprates, long-range phase stiffness is destroyed, rather than the gap order parameter. In the Nernst experiment, the vortex current produced by a temperature gradient generates a Josephson E-field perpendicular to the applied field H. A large Nernst signal eN persisting to a high onset temperature ~130 K is observed in nearly all cuprate families. Extensive Nernst experiments in the cuprates LSCO, Bi 2201, and 2212 yield a 3D phase diagram (x, T, H) in fields up to 45 T. This picture has been confirmed by recent torque magnetometry experiments. In a tilted H, local planar supercurrents associated with vortices above Tc produce a torque that deflects a cantilever. The inferred diamagnetism provides thermodynamic evidence for the vortex liquid picture suggested by the Nernst effect. Recent high-temperature STM experiments providing direct test of these ideas will also be described.

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Bi-Weekly Seminar

The Newly-Discovered Fe-Based Layered High Temperature Superconductors

Dr. Ching Wu Chu

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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Joint Seminar

Substitution Effects in Intermetallic Compounds

by: Prof. Thomas F. Faessler

Date: Friday April 04, 2008

Time: 11:00 am – 12:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

Intermetallic compounds are fascinating with respect to their atomic structures and their electronic band structures. On the one hand atomic structures can be as simple as close packed arrays of atoms, and they can also reach a complexity with up to thousand atoms per unit cell (complex metallic alloys). On the other hand the electronic band structure in many cases offers particularities such as pseudo-gaps or high density of states at the Fermi level, which permits their applications in various fields of material sciences. As a borderline case semi-conducting Zintl phases gain increasingly influence in technological relevant materials. Fine tuning of atomic and electronic structures can be achieved by partial substitution of specific components of an intermetallic compound. In this context the effect on structural and electronic properties of atom by atom substitution is demonstrated by means of the systems KPb2-KBi2, BaSn3-BaBi3, mercury substitution in clathrate-I A8Ge44 and A8Sn44 (A = alkali metal), and zinc substitution in binary compound of the system Na-Sn. As a result the formation of complex structures containing atom clusters or long-range orders up to 100Ã… reveal as basic elements, as well as qualitative changes in the electronic properties.

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Bi-Weekly Seminar

Electronic Mechanism of Two-Dimensional D-Wave Superconductivity

Prof. W. P. Su

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

 Milinda  Abeykoon

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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