TcSUH
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Special Seminar
Impurities and Inhomogeneities in the High [Tc] Superconductor Bi2Sr2CaCu2O8+δ
by: Dr. Shuheng H. Pan
Date: Wednesday February 07, 2001
Time: 3:00 pm – 4:00 pm
Location: Houston Science Center – Building 593 — Room 102
Overview
I will show the manifestations of a microscopic electronic “inhomogeneity” in the optimally doped high-[Tc] superconductor Bi2Sr2CaCu2O8+x using scanning tunnelling microscopy/spectroscopy (STM/S). We observe strong spatial variations in the local density of states (LDOS) spectrum independent of the sample quality and fabrication method. We find that both the integrated LDOS and the superconducting energy gap exhibit Gaussian-like distributions. They are strongly crosscorrelated with each other in space and vary on surprisingly short length scales characterized by auto-correlation decay lengths of ~14. More remarkably, these amplitude correlation decay lengths are smaller than the measured in-plane superconducting coherence length [chi]ab, but coincide with the average spacing of the oxygen dopants. Our observations suggest that these local electronic variations are directly associated with the spatial distribution of the individual oxygen dopant atoms. This microscopic picture emerging from our findings may lead to a deeper understanding of high-[Tc] superconductivity.
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Special Seminar
Experimental Constraints on the Physics of Cuprates
by: Dr. Guo-Meng Zhao
Date: Tuesday January 23, 2001
Time: 11:00 am – 12:00 pm
Location: Houston Science Center – Building 593 — Room 102
Overview
Many anomalous physical properties have been observed in the high-[Tc] cuprate superconductors over the last decade. These include: (1) The unconventional isotope effects on the supercarrier mass, on the charge-stripe formation temperature, on the spin-glass freezing temperature, and on the antiferromagnetic ordering temperature; (2) The exotic pairing symmetry; (3) The magnetic resonance peak in the superconducting state revealed by inelastic neutron scattering; (4) The peak/dip/hump features seen in angle-resolved photoemission spectroscopy and in tunneling spectra; (5) The strong-coupling features in optical and tunneling data; (6) The pseudogap in the normal-state of underdoped cuprates; (7) The unusually large supercarrier mass anisotropy and its novel doping dependence.
I will talk about these important experimental results which place crucial constraints on the physics of cuprates. The conclusion is that high-[Tc] superconductivity in cuprates is a cooperative phenomenon between the strong electron-electron correlation and strong electron-phonon coupling.
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Special Seminar
X-Ray Study of Thin Ferroelectric Films and Their Interfaces
by: Prof. Markus Aspelmeyer
Date: Thursday December 14, 2000
Time: 3:30 pm – 4:30 pm
Location: Houston Science Center – Building 593 — Room 102
Overview
Thin ferroelectric films are important in high-technology applications as high-[epsilon] dielectrics, transducers, actuators and future low-power non-volatile memories (Fe-NVRAMs). The growing need for still smaller devices raises the question of the principal limits of ferroelectricity. It is well known that the properties of ferroelectric thin films are largely determined by surface effects and the atomic and electronic structure of the interfaces between the films and their metal electrodes. In addition, constrained lattice dynamics due to finite size effects can even lead to a suppression of the ferroelectric phase transition. However, the underlying mechanisms are not fully understood.
The use of highly brilliant synchrotron x-ray sources together with modern x-ray techniques provide a unique non-destructive tool for the investigation of such complex multi-scale phenomena, allowing us to characterize such thin film systems on a nanometer-scale. Measurements of the electron density profile, the structure and correlations of (buried) interfaces and of the elastic deformation fields in the films can thereby be accomplished. The present study is focused on
- nanoscale morphological effects at the interfaces due to domain growth and associated defect properties;
- the influence of the interface structure on the functionality of ferroelectric systems (i.e. permeability, polarizability, fatigue, etc.);
- anharmonic lattice dynamical effects of the ferroelectric phase transition such as thermal expansion.
To cover both fundamental aspects concerning the theory of phase transitions as well as technological relevance, two different ferrolectric systems will be presented: Single crystalline BaxSr1-xTiO3 (BST)-thin films were studied as a model system for the investigation of a ferroelectric transformation of purely displacive character. BST is also of special interest for capacitor applications in high-density dynamic random access memories (DRAMs). In a collaboration with Infineon Technologies, polycrystalline SrBi2Ta2O9 (SBT)-thin films on layered electrode systems were also investigated. These are very promising for future low-power Fe-NVRAM applications due to their nearly fatigue-free cycling behavior.
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Special Seminar
Electronic Structure of High-Temperature Superconductors: Reports from Einstein's Electrons
by: Prof. Zhi-Xun Shen
Date: Tuesday December 05, 2000
Time: 11:00 am – 12:00 pm
Location: Houston Science Center – Building 593 — Room 102
Overview
Almost a century ago, Einstein advanced the revolutionary idea that treated electric-magnetic waves as quantized particles (photons) and thus resolved the long-standing puzzle raised by the famous discharge experiments of Hertz. The photoelectric effect, as we know it today, is one of the few experiments that laid the foundation for the birth of quantum mechanics. Half a century ago, a spectroscopy technique (photoemission spectroscopy) based on this phenomenon was developed to study atoms, molecules and solids, but its impact has not been felt in low-temperature physics because of the limited resolution. The last decade saw a significant improvement in both energy and momentum resolution of angle-resolved photoemission spectroscopy, extending its unique power to address the fundamental questions raised by the extraordinary discovery of high-temperature superconductors by Bednorz and Muller, and by Chu and his colleagues. This talk showcases the progress we have made to understand the physics of high-temperature superconductivity via Einstein's electrons.
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Special Seminar
Coupling of Two Superconductors Through a Ferromagnet- Sign Reversal Superconducting Order Parameter in a Ferromagnet
by: Dr. V. V. Ryazanov
Date: Tuesday November 28, 2000
Time: 11:00 am – 12:00 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 the point of 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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