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Special Seminar

Quantum Critical Behavior in Strongly Correlated Metals

Dr. Qimiao  Si

by: Dr. Qimiao Si

Date: Thursday May 03, 2001

Time: 4:00 pm – 5:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

Studies in the high [Tc] cuprates have raised a number of fundamental questions on quantum critical phenomena in metallic systems. One basic question concerns how strong correlations affect the quantum critical fluctuations and the associated non-Fermi liquid behavior. We find that the effect can be dramatic, with new critical but spatially local modes emerging near the critical point. These local modes co-exist with the usual long-wavelength modes. We argue that such a local criticality has already been observed in heavy fermion metals. It also sheds new light on the physics of the cuprates, especially on the striking puzzle raised by photoemission experiments namely, why there are no cold quasiparticles anywhere on the Fermi surface.

Special Seminar

Bulk and Thin-Film YBCO High-Temperature Superconductors

Prof. Herbert C. Freyhardt

by: Prof. Herbert C. Freyhardt

Date: Wednesday May 02, 2001

Time: 10:30 am – 11:30 am

Location: Houston Science Center – Building 593 — Room 102

Overview

Since its discovery the high-temperature superconductor (HTS) Y-Ba-Cu-O developed into one of the most attractive candidates for applications. However, because of its complex physical behaviour this became only possible on the basis of an understanding of the underlying fundamental principles. This will be demonstrated by considering on the one hand the sophisticated growth as well as the growth induces microstructure of bulk monolithic YBCO and YBCO-coated conductors, i.e. conductors of the second generation, and connecting both on the other hand to the critical current limiting mechanisms, particularly determined by the presence of grain boundaries. As a consequence, for bulk HTS and coated conductors extremely high trapped magnetic fields and current-carrying capabilities could be achieved, which opens attractive prospects for applications in, e.g. electrical and power engineering (flywheels, motors, transformers, fault-current-limiting devices,?)

Special Seminar

Synthesis and Consolidation of Nanomaterials

by: Dr. E. G. Baburaj

Date: Monday April 30, 2001

Time: 10:30 am – 11:30 am

Location: Houston Science Center – Building 593 — Room 102

Overview

Nanomaterials display novel and often enhanced properties compared to traditional materials. While considerable progress in basic understanding of this class of materials has been made, the shift from basic science to technological applications has been slow because of the absence of commercially viable processes to produce nanomaterials. This presentation is a review of the work on scalable processes for the synthesis and consolidation of nanocrystalline materials pursued by the speaker. Formation of solid solutions and intermetallic compounds either by mechanical alloying (MA) or by MA followed by reaction hot isostatic pressing (RHIP), synthesis of metals, alloys and carbides by mechanochemical processing (MCP), auto ignition synthesis of oxides, and the consolidation of single and multiphase nanomaterials will be discussed in detail.

Special Seminar

A Toy Model for the Pseudogap Phase in Cuprate Superconductors

by: Dr. Kun Yang

Date: Thursday February 15, 2001

Time: 4:00 pm – 5:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

In this talk I discuss a simple model we introduced[1] that have a number of features qualitatively similar to the phenomenology of Cuprate superconductors, especially the existence of a pseudogap region in the phase diagram. In this model, Cooper pairing is induced by an attractive interaction with a finite range. While returning to the usual BCS limit when the range of the pairing interaction [lambda] is shorter than the mean-field coherence length [xi], the physics becomes qualitatively different from that of BCS when [lambda] is much larger than [xi]. In this regime the system support a new class of low-energy collective modes absent in the BCS theory, whose fluctuations greatly depress the superconducting transition temperature ([Tc]) from its mean-field level, and lead to pseudogap behavior above [Tc]. These fluctuations are not the usual phase fluctuations of the standard superconducting order parameter, and lead to a highly unusual Ginzburg-Landau description in which the center of mass and relative degrees of freedom of the order parameter must be included on equal footing. We will discuss how such a long-range pairing interaction may arise in Cuprate superconductors, especially in the underdoped regime.

[1] Kun Yang and S. L. Sondhi, Phys. Rev. B 62, 11778 (2000)

Special Seminar

Impurities and Inhomogeneities in the High [Tc] Superconductor Bi2Sr2CaCu2O8+δ

Dr. Shuheng H. Pan

by: Dr. Shuheng H. Pan

Date: Wednesday February 07, 2001

Time: 3:00 pm – 4:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

I will show the manifestations of a microscopic electronic “inhomogeneity” in the optimally doped high-[Tc] superconductor Bi2Sr2CaCu2O8+x using scanning tunnelling microscopy/spectroscopy (STM/S). We observe strong spatial variations in the local density of states (LDOS) spectrum independent of the sample quality and fabrication method. We find that both the integrated LDOS and the superconducting energy gap exhibit Gaussian-like distributions. They are strongly crosscorrelated with each other in space and vary on surprisingly short length scales characterized by auto-correlation decay lengths of ~14. More remarkably, these amplitude correlation decay lengths are smaller than the measured in-plane superconducting coherence length [chi]ab, but coincide with the average spacing of the oxygen dopants. Our observations suggest that these local electronic variations are directly associated with the spatial distribution of the individual oxygen dopant atoms. This microscopic picture emerging from our findings may lead to a deeper understanding of high-[Tc] superconductivity.

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