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Special Seminar
Midgap states as a powerful tool to investigate unconventional superconductivity - An overview
by: Prof. Chia-Ren Hu
Date: Wednesday June 28, 2006
Time: 4:00 pm – 5:00 pm
Location: Houston Science Center – Building 593 — Room 102
Overview
In the summer of 1993, the first phase-sensitive test of high Tc superconductors had just appeared in preprint form [1], which was designed to show that the pairing order parameter or gap function of these superconductors has a cos(2θ)-like sign variation on an essentially cylindrical Fermi surface. The speaker proposed [2] then at TcSUH that as a topological consequence of this sign variation alone, at any non-(100) surface of such a superconductor there must exist a sizable number of quasi-particle excitations with energy essentially at the center of the superconducting gap, i.e., at the Fermi energy. These “midgap states” are nearly dispersionless in that they have momentum along the surface ranging from -kF to + kF but essentially no kinetic energy associated with them. These states, called “zero-energy Andreev bound states” by some researchers, are a direct signature of unconventional (i.e. non-s-wave) pairing. Many strong experimental evidences on the existence of such states in high Tc superconductors have since been obtained, and these states have since become a powerful tool to test whether many kinds of more recently discovered superconductors have unconventional pairing. Very recently, the speaker and his collaborators at TcSUH have shown that a variation of these states can also provide a clear signature for the so-called FFLO (Fulde-Ferrell-Larkin-Ovchinnikov) state for pairing of fermions with mismatched Fermi surfaces [3,4]. The FFLO state can also occur in trapped atomic mixtures, and in a quark-gluon plasma, and is therefore also of strong interest to atomic and nuclear/particle physicists.
[1] D. A. Wollman et al., Phys. Rev. Lett. 71, 2134 (1993).[2] C.-R. Hu, Phys. Rev. Lett. 72, 1526 (1994); J. Yang and C.-R. Hu, Phys. Rev. B, 50, 16766 (1994).[3] Q. Wang, H.-Y. Chen, C.-R. Hu, and C. S. Ting, Phys. Rev. Lett.96, 117006 (2006), and Q. Wang, C.-R. Hu, and C. S. Ting, arXiv cond-mat/0605417, to be published.[4] Q. Cui, C.-R. Hu, J. Y. T. Wei, and K. Yang, to appear in the proceedings of the 24th International conference on Low Temperature Physics; and arXiv cond-mat/0510717, to appear inPhys. Rev. B.
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Special Seminar
Magnetic Glasses in Colossal Magnetoresistive Manganites
by: Dr. Weida Wu
Date: Thursday May 18, 2006
Time: 12:00 pm – 1:00 pm
Location: Houston Science Center – Building 593 — Room 102
Overview
Spin glasses are founded in the frustration and randomness of microscopic magnetic interactions. They are non-ergodic systems, not described by thermodynamics. Magnetic glassy behaviour has been observed in many colossal magnetoresistive manganites, yet there is no consensus that they are spin glasses. Here, an intriguing glass transition in (La,Pr,Ca)MnO3 is imaged using a variable-temperature magnetic force microscope. In contrast to the speculated spin glass picture, our results show that the observed static magnetic configuration seen below the glass temperature arises from the cooperative freezing of the first order antiferromagnetic (charge ordered) to ferromagnetic transition, leading to a non-ergodic state. Our data also suggest that accommodation strain plays an important role in the kinetics of the phase transition. This cooperative freezing idea has been applied to conventional glass systems including window glasses and supercooled liquids, and may be applicable across many systems to any first-order phase transition occurring on a complex free energy landscape.
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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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