TcSUH
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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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Special Seminar
Supersolid Phase in Spin Dimer XXZ Systems under Magnetic Field
by: Dr. T. K. Lee
Date: Wednesday May 10, 2006
Time: 4:00 pm – 5:00 pm
Location: Science & Research Building 1 – Building 550 — Room 634
Overview
Using quantum Monte Carlo method, we study, under external magnetic fields, the phase diagram of the two-dimensional spin S=1/2 dimer model with an anisotropic intra-plane antiferromagnetic coupling. With a reaonable size of anisotropy, a supersolid phase with both finite checkerboard structure factor and superfluid density is found. We demonstrate that the supersolid phase is characterized by a non-uniform bose condensate density that breaks translational symmetry. The rich phase diagram also contains a checkerboard solid and two different types of superfluid phase formed by S_z=+1 and S_z=0 spin triplets, with finite staggered magnetization in z-axis and in-plane direction, respectively. As we show, the model can be realized as a consequence of including the next nearest neighbor coupling among dimers and our results suggest that spin dimer systems may be a more natural place to look for supersolids.
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Special Seminar
Development of MgB2 conductors towards industrial applications
by: Dr. Giovanni Grasso
Date: Monday May 08, 2006
Time: 12:00 pm – 1:00 pm
Location: Houston Science Center – Building 593 — Room 102
Overview
Superconductivity at 40 K in MgB2 was unexpectedly announced by Prof. Akimitsu in January 2001. This stunning news attracted the interest of the entire superconductivity community for many months, leading to the submission of hundreds if not thousands of relevant scientific papers. Their experimental outcomes were very promising in view of an industrial application of MgB2 in the near future. Reports by several groups indeed confirmed upper critical fields in thin films often larger than 60 Tesla at low temperatures. Several worldwide superconductor wire manufacturers soon started a survey of the possibility of manufacturing superconducting wires based on such a simple and inexpensive binary compound. In this talk, the progresses achieved so far in this respect will be reported, including an overview of the potential markets for such an innovative type of superconductor just after a few years of development.
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Special Seminar
Probing interfacial defects in YBa2Cu3O7-δ films and their effects on transport properties
by: Haiyan Wang
Date: Thursday May 04, 2006
Time: 12:00 pm – 1:00 pm
Location: Houston Science Center – Building 593 — Room 102
Overview
Wires that carry electrical current without resistance are fabricated by coating metal substrates with the high-temperature superconductor YBa2Cu3O7-δ (YBCO). One problem with this material is that its current-carrying ability (Jc) is decreasing as film thickness increases, namely YBCO thickness dependence. Another problem is that its Jc is dramatically reduced by the presence of a magnetic field, and many envisioned applications, such as magnets, motors, generators and transformers, require operation in a moderate-to-strong field. Recently, significant progress has been made to overcome these two challenges using multilayer architecture and nano-particle doping approaches. These additional interfaces and grain boundaries create extra interfacial defects in the YBCO that will pin magnetic flux lines and prevent the dissipative motion that degrades performance. In this talk, several successful examples for improving both YBCO Jc self-field and Jc in-field will be introduced and correlated with the microstructure characteristics of these additional defects.
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Special Seminar
Coexistence of Superconductivity and Ferromagnetism in Dilute Co-doped La1.89Ce0.11CuO4+δ
by: Bai-ru Zhao
Date: Friday April 28, 2006
Time: 12:00 pm – 1:00 pm
Location: Houston Science Center – Building 593 — Room 102
Overview
The thin films of the optimally electron doped T'-phase superconductor La1.89Ce0.11CuO4±δ (LCCO) are investigated under the dilute Co doping, which are formulated as the La1.89Ce0.11(Cu1-xCox)O4±δ (LCCCO) with x = 0.01 - 0.05. In this whole Co concentration range, the ferromagnetic ordering is observed which is suggested to be formed through the RKKY interaction. In the case of very low Co concentration, x = 0.01 and 0.02, the coexistence of superconductivity and ferromagnetism is obviously detected. For such formulated LCCCO (substitution of Co for Cu), all these facts should take place in the CuO2 plane. This let us believe that there may be two kinds of charge carriers (electrons and holes) in the LCCO system, as has been suggested for other electron-doped high-Tc superconductors. In this talk, the anisotropic transport property of (La,Ce)2CuO4 will also be reported.
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