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Special Seminar

Cluster-based Model, a New Route Towards Description of Complex Atomic Structures

by: Prof. Chuang Dong

Date: Friday September 26, 2008

Time: 12:00 pm – 1:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

Atomic structures are generally described by crystallography, which is entirely based on planar periodicity. However such a method largely fails in describing complex atomic structures such as complex metallic alloys and disordered systems. Alternative to planar periodicity is spherical periodicity arising from resonance between electron wave and static atomic strictures, which has been proven widely present in disordered systems. One unique advantage is that only local 1st-neighbor cluster structure information is needed to depict the global structure. It implies that a structure, no matter how complex it can be from crystallographic viewpoint, consists simply of a cluster plus a few glue atoms, or in cluster formula (cluster)1(glue atom)x. Such a cluster-plus-glue-atom model is applied to give the composition formulas of a variety of complex structures such as crystalline intermetallic compounds, bulk metallic glasses, quasicrystals, and even multi-element solid solution limit alloys.

Special Seminar

Current Carrying State and Its Implication in a D-Wave Superconductor

by: Dr. Jian-Xin Zhu

Date: Tuesday July 08, 2008

Time: 4:00 pm – 5:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

A fundamental property of all known superconductors is the formation of Cooper pairs in the superconducting state. A far-reaching implication of this fact is the quantization of magnetic flux in multiply connected superconducting geometries. In this talk, I will discuss the magnetic flux dependence of order parameter and supercurrent in a hollow $d$-wave superconducting cylinder. It is shown that the existence of line nodal quasiparticles in a pure $d_{x^2-y^2}$ pairing state gives rise to an $hc/e$ periodicity in the order parameter and a first-order quantum phase transition for a large system size. We demonstrate that the flux periodicity in the supercurrent is sensitive to the detailed electronic band structure and electron filling factor. In particular, we find that, in cooperation with the increase of the cylinder circumference, the $hc/2e$ periodicity can be restored significantly in the supercurrent by avoiding the particle-hole symmetry point. A similar study of a $d_{x^2-y^2}+id_{xy}$ pairing state verifies the peculiarity of unconventional superconductors with nodal structure. I will also discuss the possibility of an impurity quantum phase transition as driven by the supercurrent.

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Special Seminar

Supercurrents Through a Ferromagnet, Josephson π- Junctions as Superconducting Phase Inverters

by: Dr. V. V. Ryazanov

Date: Thursday May 22, 2008

Time: 12:00 pm – 1:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

It was predicted by Larkin and Ovchinnikov and by Fulde and Ferrel that superconducting pairing can occur when the electron momenta at the Fermi energy are different for the two electron spin directions, for instance as the result of an exchange field in magnetic superconductors. The resulting 'LOFF'-state is qualitatively different from the zero-momentum state: it is spatially inhomogeneous and the order parameter contains nodes where the phase changes by π. The LOFF state was never observed in bulk material, but we present experimental evidence that it can be induced in a weak ferromagnet (F) sandwiched between two superconductors (S). Such an SFS junction can yield a phase shift of π between the superconducting banks. The phase change of the superconducting order parameter in the ferromagnet arises as a response of the Cooper pair, which consists of two electrons of opposite spin and momentum, to the energy difference between two spin directions in the ferromagnet. This shift manifests itself in reentrant superconducting behavior of the critical supercurrent temperature dependence, Ic(T), of the Josephson SFS junction as well as in half-period shift of Ic(H)-dependence of a triangular SFS junction array at point of a transition of the junctions from a "0-" to a "π"- state. The π-state offers fundamentally new ways for studying the coexistence of superconductivity and magnetism and may also be important for superconducting electronics, in particular in quantum computing: several schemes for the realization of the necessary qubits (quantum two level systems) rely on the use of phase shifts of π in a superconducting network.

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Special Seminar

Chemistry and Physics of Intermetallic Clathrates and Skutterudite-like Compounds

by: Dr. Yuri Grin

Date: Tuesday February 26, 2008

Time: 11:00 am – 12:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

Crystal structures of intermetallic clathrates and skutterudite derivatives are formed by the 3D host frameworks with the differently sized filled or non-filled cages. Beside a variety of physical behaviours these compounds attracted the worldwide attention, e.g., as promising thermoelectric materials. Chemical bonding in clathrates and skutterudites is one of the key questions for the creation of the new materials of these classes. While the bonding in intermetallic clathrates may be sufficiently described by the Zintl concept with some modifications, the presence of transition metals in the filled skutterudites does not allow the straight forward description with simple electron counting. A more general description is possible with the new quantum chemical toolbox for bonding analysis in real space - electron localizability indicator [1]. Understanding of the chemical bonding allows to concept new preparation routes for synthesis of new representatives of this materials family. Especially, the (partial) control of the cage filling is achievable on this way [2]. This allows to prepare ‘empty’ clathrates, e.g., new modification of germanium [3]. Two new groups of filled skutterudites were successfully prepared. The iron-antimonides of the alkali metals NaFe4Sb12, KFe4Sb12 [4] and TlFe4Sb12 [5] represent the electron-poorest members of this family and reveal a wide spectrum of electronic properties. The novel family of REPt4Sb12 compounds shows i.e., superconductivity at relatively high temperatures [6]. [1] A. Leithe-Jasper et al. Phys. Rev. B. 2004 70 214418. [2] B. Bo?hme et al. J. Am. CHem. Soc. 2007 129 5348. [3] A. M. Guloy et al. Nature 2006 443 320-323. [4] W. Schnelle et al. 2008 submitted. [5] A. Leithe-Jasper et al. Phys. Rev. B. 2008 in press. [6] R. Gumeniuk et al. Phys. Rev. Lett. 2008 100 017002.

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Special Seminar

Magnetic Resonance Imaging: an Evolving Clinical Imaging Tool

by: Prof. Raja Muthupillai

Date: Friday February 22, 2008

Time: 12:00 pm – 1:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

This lecture will focus on the various mechanisms through which contrast could be generated in Magnetic Resonance Images (MRI), and the clinical applications of MR. Among diagnostic imaging modalities, MRI is arguably the most versatile. Magnetic resonance imaging allows non-invasive evaluation of an array of tissue properties in vivo, e.g., tissue magnetic-resonance relaxation (anatomic imaging), metabolite distribution within tissue (spectroscopy), tissue micro-structure (diffusion), tissue micro-vascular flow (perfusion), tissue velocity, etc. This versatility has resulted in widespread use of MRI as a diagnostic imaging tool to assess pathology in patients. Recent technological advances have made it possible to use MR not just as diagnostic imaging modality, but also to use in conjunction with interventional, therapeutic procedures, e.g., MR guided focused ultrasound, and other MR guided interventions.

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