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Special Seminar

Supersolid Phase in Spin Dimer XXZ Systems under Magnetic Field

Dr. T. K. Lee

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

 Haiyan  Wang

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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Special Seminar

Approaching a quantitative understanding of dendritic flux avalanches

by: Prof. Tom H. Johansen

Date: Tuesday April 25, 2006

Time: 12:00 pm – 1:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

Magneto-optical imaging (MOI) has revealed that metastable distributions of pinned vortices often break down by abrupt avalanches creating dendritic flux patterns in the superconductor. This intermittent dynamics, the distinct fingering character of the patterns, and that it all occurs only in thin films, has for more than a decade been a puzzle. We present here MOI results allowing for the first time a direct quantitative comparison between experiment and a model based on a thermo-magnetic feedback mechanism (flux jump) and the non-local electrodynamics typical of film superconductors. Measurements on MgB2 and Nb films are shown to agree with the model on key features like the magnitude of the instability onset magnetic field, and how it varies with both temperature and sample size. Another observation that only 5% anisotropy in Jc can lead to a strongly anisotropic avalanche activity also finds an easy explanation within the model.

The talk reports also a recent breakthrough in using low-temperature fluorescent thermal imaging (FTI) to study dissipative processes in superconductors. The method is based on the temperature dependent fluorescence in a thin rare-earth chelate film deposited directly on the superconductor. It will be shown that FTI can detect distributions of local heating with a temperature resolution of 0.05 K and can locate hot spots down to a few microns in size, demonstrating that the method is very promising for diagnostic testing of powered superconducting devices operating at 77 K. If we succeed to improve the method at even lower temperatures it will open a new window also for studies of thermo-magnetic avalanches in vortex matter.

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