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Special Seminar

New Properties in Old Materials: Doped Layered Dichalcogenides

by: Prof. Emilia Morosan

Date: Monday October 22, 2007

Time: 4:00 pm – 5:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

Transition metal dichalcogenides MX2 (M is transition metal, X = S, Se, or Te) have long been known and explored. Due to their reduced dimensionality, such compounds sometimes display charge density wave (CDW) transitions, which are periodic modulations of the conduction electron density. In addition, the CDW state is believed to compete with a superconducting state (SC), both the CDW and the SC representing collective electron states induced by Fermi surface instabilities. Upon doping with various complexes, the transition metal dichalcogenides often reveal dramatic changes of their physical properties. I will discuss the effects of transition metal intercalation on the properties of two layered chalcogenide materials, TiSe2 and TaS2. Although TiSe2 is one of the first known CDW-bearing materials, the nature of its CDW transition remains controversial. Recently the interest in TiSe2 has been renewed by our discovery of the new superconducting state SC that emerges upon Cu doping. Thus CuxTiSe2 provides the first example of a system in which controlled chemical doping can be used to study the competition between the CDW and SC. I will also discuss experiments on FexTaS2 aimed at studying the sharp switching of the magnetization that we recently observed in this compound for x = 1/4. For this particular Fe content, FexTaS2 orders ferromagnetically below 160 K and displays very sharp hysteresis loops in the ordered state for H||c. This is indicative of a very rapid switch of the magnetization direction, and the time dependence of this magnetization switch reveals unexpected time dependence.

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Special Seminar

Current State of the MgB2 Superconductor and Devices Development at Columbus Superconductors

by: Dr. Giovanni Grasso

Date: Tuesday May 29, 2007

Time: 11:00 am – 12:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

Six years is a limited amount of time to bring a new material to a successful level of development. However, this is indeed happening for MgB2, as recently proven by a number of impressive news items coming from industries and institutions working on it. The capability of MgB2 wires and tapes to carry large currents at intermediate temperatures between those of liquid helium and liquid nitrogen has been recently demonstrated by the realization of a full scale prototype of an open MRI fully working system. Such a system contains many innovative features, such as the cryogenic-free operation and the innovative magnet design that reduces claustrophobia issues, as well as the use of as much as 18 Km of multifilamentary conductor. In this talk, the further progress of MgB2 wire development will be discussed, with the aim of drawing a realistic picture of the future impact of MgB2 on many superconducting devices.

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Special Seminar

Theoretical Understanding of Rotational Symmetry Breaking in Sodium-Doped Cuprate Superconductors

by: Dr. Y. Chen

Date: Tuesday May 22, 2007

Time: 4:00 pm – 5:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

The recent atomically resolved STM studies on strongly underdoped NaCCOC revealed a surprisingly complex pattern with the square symmetry of thelattice broken on a local scale. This has raised an interesting question about the origin of the broken local square symmetry. Our theoretical investigation shows that for reasonable parameters the single hole impurity state of t-t'-J model has a doubly degenerate ground state whose components can be represented as states with even (odd) reflection symmetry around the x(y)-axes. The conductance pattern for one state is anisotropic as the STM tip scans above the Cu-O-Cu bonds along the x(y)-axes. This strong anisotropy appears at lower voltages while much weaker anisotropy shows up at higher voltages. Our results agree qualitatively with recent experiments. In addition, we demonstrate that the satisfaction of low-energy sum rules shown in STM measurements strengthens the validity of the description of the low-energy physics by an effective single-band model instead of the three-band model. Some preliminary results will be discussed.

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Special Seminar

Bond Bending Enhanced Ferroelectricity in Multiferroic Materials

by: Prof. C. D. Hu

Date: Monday May 21, 2007

Time: 4:00 pm – 5:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

We studied transition metal oxides with spiral spin distribution. The bond bending at oxygen sites enables the hybridization of the t¬2g orbitalsof transition elements and the p-orbitals of oxygen atoms. Bloch functions are coupled due to the spiral spin distribution. The resulting wave functiongives rise to electronic distribution which has net electric polarization if spin-orbit interaction is considered.

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Special Seminar

The 1-D BOSE GAS

Prof. Norm  Frankel

by: Prof. Norm Frankel

Date: Wednesday April 11, 2007

Time: 11:00 am – 12:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

The one dimensional delta function Bose gas is a classic system in the quantum many body problem. It has excited much study, with seminal contributions from theoretical physics and mathematics, over the past five decades.

The wonderful advent of exceedingly low temperature cyrogenics has brought the Bose gas into prominence. Very recently, with the application of sophisticated anisotropic traps, the Bose gas can be fashioned into a genuinely 1-D configuration, thus bringing this 1-D Bose gas into the special spotlight experimentally.

In the light of our recent works, we will focus on the density matrix and concomitantly the occupation numbers [distribution function] along with the pair correlation function and concomitantly the structure factor. These are the signature quantities of the Bose gas.

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