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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.

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

Special Seminar

Discovery of Superconductivity in Quarternary Borocarbides and Related Studies at TIFR, Bombay

by: Prof. R. Nagarajan

Date: Wednesday August 16, 2000

Time: 4:00 pm – 5:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

After the revolutionary discovery of high Tc oxide superconductors, discovery of superconductivity in quaternary borocarbides in 1993 was a landmark in superconductivity. The talk will describe the discovery made at the Tata Institute of Fundamental Research (TIFR), Bombay; and discuss some of the studies conducted before and after the discovery. A summary of the studies on the rare earth series RNi4B (R = Y, rare earth) which preceded the discovery will be given. In this, mention will be made about the narrow domain wall, as narrow as an atomic layer, behaviour in SmNi4B. After describing the discovery of superconductivity in Y-Ni-B-C which led to the subject of quaternary borocarbides, some of the studies on quaternary borocarbides carried out at TIFR and in collaboration will be given. The discussion will include suppression of superconductivity by non magnetic impurities in the magnetic superconductor DyNi2B2C ([Tc] < TN), suppression of superconductivity in the moderate heavy fermion YbNi2B2C, which was synthesized at TIFR. The talk will conclude with the recent work on magnetic properties of Sm-Ru-B-C system in which a magnetic phase exhibits a very large coercive field of about 15T at 5K.

Special Seminar

What Problems of Physics and Astrophysics Seem Now to be Especially Important and Interesting?

by: Prof. Vitaly L. Ginzburg

Date: Thursday November 18, 1999

Time: 12:00 pm – 1:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

The author proposes an educational program aimed primarily at widening the scientific horizons of the young generation of physicists. For this purpose, a list of the top problems of the day are given and addressed, although the list is admittedly subjective and unavoidably inexhaustive and limited (one cannot do the impossible).

Special Seminar

Spin Dephasing vs Optical Dephasing

by: Dr. Ming-Wei Wu

Date: Tuesday November 02, 1999

Time: 11:00 am – 12:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

We study the kinetics of spin coherence of optically excited electrons in an undoped insulating ZnSe/Zn1-xCdxSe quantum well under moderate magnetic fields in the Voigt configuration. After clarifying the optical coherence and the spin coherence, we build the kinetic Bloch equations and calculate dephasing and relaxation kinetics of laser-pulse-excited plasma due to statically screened Coulomb scattering and electron hole spin exchange. We find that the Coulomb scattering, which is the main mechanism for the optical dephasing, cannot cause the spin dephasing, and that the electron-hole spin exchange is the main mechanism of the spin decoherence. Moreover, the beat frequency in the Faraday rotation angle is determined mainly by the Zeeman splitting, red shifted by the Coulomb scattering and the electron hole spin exchange. Our numerical results are in agreement with experimental findings. A possible scenario for the contribution of electron-hole spin exchange to the spin dephasing of the n-doped material is also discussed.

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