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Bi-Weekly Seminar

Does DNA act as it's own sunscreen?

Prof. Eric  Bittner

by: Prof. Eric Bittner

Date: Friday July 07, 2006

Time: 12:00 pm – 1:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

DNA is a surprisingly robust molecular system in spite of its rather large UV absorption cross-section in the 310-290 nm range. Part of this robustness comes from DNA's ability to rapidly dissipate the electronic photoexcitation energy into heat, thus preventing to some extent photochemical processes that can lead to mutation. One of the key questions is whether or not this dissipation is due to base-pairing and hydrogen transfer in localized excited states or if it is due to base-stacking effects. In this talk, I shall give an overview of our two-band lattice model for the excited states of DNA double helices. Our theoretical calculations corroborates recent ultrafast experimental results that indicate that base-stacking dictates the fate of an excitation in A-T DNA. Moreover, our work suggest that in AT DNA, excitonic dynamics along the A chain is dramatically different than along the T chain. Finally, we speculate that these processes may have played a crucial role in the evolutionary selection of DNA.

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Special Seminar

Midgap states as a powerful tool to investigate unconventional superconductivity - An overview

by: Prof. Chia-Ren Hu

Date: Wednesday June 28, 2006

Time: 4:00 pm – 5:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

In the summer of 1993, the first phase-sensitive test of high Tc superconductors had just appeared in preprint form [1], which was designed to show that the pairing order parameter or gap function of these superconductors has a cos(2θ)-like sign variation on an essentially cylindrical Fermi surface. The speaker proposed [2] then at TcSUH that as a topological consequence of this sign variation alone, at any non-(100) surface of such a superconductor there must exist a sizable number of quasi-particle excitations with energy essentially at the center of the superconducting gap, i.e., at the Fermi energy. These “midgap states” are nearly dispersionless in that they have momentum along the surface ranging from -kF to + kF but essentially no kinetic energy associated with them. These states, called “zero-energy Andreev bound states” by some researchers, are a direct signature of unconventional (i.e. non-s-wave) pairing. Many strong experimental evidences on the existence of such states in high Tc superconductors have since been obtained, and these states have since become a powerful tool to test whether many kinds of more recently discovered superconductors have unconventional pairing. Very recently, the speaker and his collaborators at TcSUH have shown that a variation of these states can also provide a clear signature for the so-called FFLO (Fulde-Ferrell-Larkin-Ovchinnikov) state for pairing of fermions with mismatched Fermi surfaces [3,4]. The FFLO state can also occur in trapped atomic mixtures, and in a quark-gluon plasma, and is therefore also of strong interest to atomic and nuclear/particle physicists.

[1] D. A. Wollman et al., Phys. Rev. Lett. 71, 2134 (1993).[2] C.-R. Hu, Phys. Rev. Lett. 72, 1526 (1994); J. Yang and C.-R. Hu, Phys. Rev. B, 50, 16766 (1994).[3] Q. Wang, H.-Y. Chen, C.-R. Hu, and C. S. Ting, Phys. Rev. Lett.96, 117006 (2006), and Q. Wang, C.-R. Hu, and C. S. Ting, arXiv cond-mat/0605417, to be published.[4] Q. Cui, C.-R. Hu, J. Y. T. Wei, and K. Yang, to appear in the proceedings of the 24th International conference on Low Temperature Physics; and arXiv cond-mat/0510717, to appear inPhys. Rev. B.

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Special Seminar

Magnetic Glasses in Colossal Magnetoresistive Manganites

by: Dr. Weida Wu

Date: Thursday May 18, 2006

Time: 12:00 pm – 1:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

Spin glasses are founded in the frustration and randomness of microscopic magnetic interactions. They are non-ergodic systems, not described by thermodynamics. Magnetic glassy behaviour has been observed in many colossal magnetoresistive manganites, yet there is no consensus that they are spin glasses. Here, an intriguing glass transition in (La,Pr,Ca)MnO3 is imaged using a variable-temperature magnetic force microscope. In contrast to the speculated spin glass picture, our results show that the observed static magnetic configuration seen below the glass temperature arises from the cooperative freezing of the first order antiferromagnetic (charge ordered) to ferromagnetic transition, leading to a non-ergodic state. Our data also suggest that accommodation strain plays an important role in the kinetics of the phase transition. This cooperative freezing idea has been applied to conventional glass systems including window glasses and supercooled liquids, and may be applicable across many systems to any first-order phase transition occurring on a complex free energy landscape.

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Student Symposium

31st Semiannual TcSUH Student Symposium and BBQ Picnic

by: TcSUH Administration

Date: Tuesday May 16, 2006

Time: 1:00 pm – 5:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

All members of TCSUH are invited to attend the 31st Semiannual TCSUH Student Symposium. Students will present original research results to TCSUH members, guests and a panel of judges.

The Symposium will begin at 1:00 p.m. with a brief review of TcSUH’s progress. The afternoon will then continue with a series of 15-minute presentations representing novel work by undergraduate and graduate students from each of TcSUH’s laboratories. A Symposium Program, including the schedule and abstracts for each presentation, will be provided at the door.

BBQ, soft drinks and beer will be provided at the picnic following the symposium. Prize winners will be announced at the picnic.

Picnic RSVP: by Friday, May 12, TcSUH Office of Public Affairs, x38210

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Special Seminar

Nesting Phenomena in High Temperature Superconductors

by: Prof. John Ruvalds

Date: Thursday May 11, 2006

Time: 4:00 pm – 5:00 pm

Location: Science & Research Building 1 – Building 550 — Room 634

Overview

The anomalous quasiparticle damping and high temperature superconductivity in cuprates is explained by Coulomb interactions among electrons [ or holes ] on a nested Fermi surface. In YBCO and other copper oxides, a nearly half filled tight binding energy band naturally produces nesting in the form of parallel segments of the square Fermi surface. Our Nested Fermi Liquid theory derives the anomalous quasiparticle damping and provides a mechanism for d-wave superconductivity at room temperature. Neutron , photoemission, and light scattering experiments confirm various predictions of the nesting theory. Our analysis predicts new materials, such as sulfides, that may become superconducting when a competing spin density wave is suppressed.

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