TcSUH
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Special Seminar
Chemical Physics of Strongly Correlated Electron Systems
by: Prof. Frank Steglich
Date: Tuesday June 12, 2001
Time: 11:00 am – 12:00 pm
Location: Houston Science Center – Building 593 — Room 102
Overview
In a number of lanthanide- and actinide-based intermetallic compounds unconventional metallic, superconducting and magnetic states have been observed. These originate in a strong local Coulomb repulsion between the 4f/5f electrons and a weak hybridization of these atomic and the conduction-electron wavefunctions.
I shall discuss (1) the antiferromagnetically ordered heavy-fermion superconductor UPd2Al3 for which recent tunneling and inelastic-neutron-scattering experiments strongly support a magnetic-exciton-mediated pairing mechanism [1], (2) several compounds of the RET2X2 family showing pronounced “non-Fermi-liquid” effects [2-4] which cannot be fully explained by the existing spinfluctuation theories, and (3) the charge-ordered state of Yb4As3 characterized by an extremely low charge-carrier concentration and antiferromagnetic effective S = 1/2 chains. In a transverse magnetic field, the latter give rise to the opening of a spin gap which has the same origin as the pronounced soliton excitations that show up in several physical properties [5, 6]. Finally, I address a recent collaboration between chemists and physicists in our institute devoted to finding new small-gap semiconductors.
References: 1. N. K. Sato et al., Nature 410, 340 (2001); 2. P. Gegenwart et al., Phys. Rev. Lett. 81, 1501 (1998); 3. P. Gegenwart et al., Phys. Rev. Lett. 82, 1293 (1999); 4. O. Trovarelli et al., Phys. Rev. Lett. 85, 626 (2000); 5. M. K?ppen et al., Phys. Rev. Lett. 82, 4548 (1999); 6. F. Steglich et al., Acta Phys. Pol. A 97, 91 (2000).
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Special Seminar
Metal-Metal Bonded Supramolecular Chemistry Assembly, Symmetry, and Molecular Architecture on the Rational Design of Advanced Materials Based on Nanoscale-sized Molecules
by: Dr. Chun Lin
Date: Thursday May 17, 2001
Time: 11:00 am – 12:00 pm
Location: Houston Science Center – Building 593 — Room 102
Overview
This work pioneered the concept of introducing metal-metal bonds into supramolecular chemistry. This cutting-edge project is focused on the design and self-assembly of nanomolecules mediated by pairs of bonded dimetal units. Coupling of such units in pairs can form one-, two-, and three-dimensional materials containing metal-metal bonds wherein cooperative interaction among the dimetal centers may give rise to tunable physical properties in the bulk material.
Metal-metal bonded cationic complexes of the type [M2(DAniF)4-n(MeCN)8-2n]m+, where M = Mo or Rh and DAniF is an N,N'-di-p-anisylformamidinate anion, have been used as precursors for subunit pieces and then linked by various equatorial and axial bridging groups such as polycarboxylate anions, polypyridyls and polynitriles. Characterization of the products by single-crystal X-ray diffraction, CV, DPV, NMR and other spectroscopic techniques have revealed the presence of discrete tetranuclear (chains or loops), hexanuclear (triangles), octanuclear (squares), dodecanuclear (cages) species, and one-, two-, three-dimensional molecular nanotubes. These compounds display a rich electrochemical behavior which is affected by the nature of the linkers.
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Special Seminar
Phase Transitions in Vortex Matter
by: Dr. George Crabtree
Date: Wednesday May 09, 2001
Time: 10:30 am – 11:30 am
Location: Houston Science Center – Building 593 — Room 102
Overview
Magnetic fields penetrate superconductors in the form of vortices, tubes containing one quantum of flux surrounded by circulating supercurrents. Each vortex interacts with its neighbors through the Lorentz force and with the defects in the superconductor through pinning. When placed in a thermal bath, the vortex array forms a rich variety of condensed phases, including lattices, liquids, and glasses. The complexity of these condensed phases and the transitions among them rival those of ordinary atomic matter, hence the name “vortex matter.” An introduction to vortices in superconductors will be followed by a survey of the physics of their condensed phases and how we probe them experimentally.
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Special Seminar
Quantum Critical Behavior in Strongly Correlated Metals
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.
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Special Seminar
Bulk and Thin-Film YBCO High-Temperature Superconductors
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,?)
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