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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.

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.

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.

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.

Special Seminar

Measurements of Electromagnetic Properties of Materials at Microwave Frequencies

by: Prof. Jerry Krupka

Date: Friday September 15, 2000

Time: 11:00 am – 12:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

  • Transmission line measurements.
  • General concepts of measurements by resonance methods.
  • Measurements of dielectric and magnetic materials in resonant cavities.
  • Dielectric resonator techniques.
  • Mode dielectric resonators.
  • Split cavities and split post dielectric resonators.
  • Measurements of ultra-low loss materials using whispering gallery mode resonators.
  • Measurements of anisotropic materials.
  • Permeability measurements of gyromagnetic and paramagnetic materials.
  • Surface resistance measurements of superconductors

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