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Special Seminar

Collective Modes in Unconventional Superconductors

by: Prof. Peter Brusov

Date: Friday September 24, 2004

Time: 12:00 pm – 1:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

After the recent discovery of collective modes in unconventional superconductors (USC), their study becomes very important. Collective modes (CM) in HTSC exhibit themselves in many experiments: ultrasound attenuation (UA) and microwave absorption (MWA), neutron scattering, photoemission and Raman scattering, etc. The large peak in the dynamical spin susceptibility in HTSC arises from a weakly damped spindensity- wave CM. This gives rise to a dip between the sharp low energy peak and the higher binding energy hump in the ARPES spectrum. Also, the CM of amplitude fluctuation of the d-wave gap yields a broad peak above the pairbreaking threshold in the B1g Raman spectrum. The contribution of collective modes to UA and MWA may be substantial. We consider two-dimensional and three-dimensional models of p- and d-pairing for superconductors built by the path integration technique. Within these models we calculate the collective excitations in different unconventional superconductors [high temperature superconductors (HTSC), heavy fermion superconductors (HFSC), etc.] under p- and d-pairing. We consider both bulk and 2D systems. Some recent ideas concerning realization in HTSC of the mixtures of different states are investigated. In particular, we consider the mixture of two d-wave states in HTSC (of dx2-y2 and dXY states). Obtained results could be used for interpretation of the sound attenuation and microwave absorption data as well as for identi?cation of the type of pairing and order parameter in unconventional superconductors. They allow us to distinguish pure d-wave state from the mixture of two d-wave states in HTSC.

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Special Seminar

Prospects of Dilute Nitrides III-V Heterostructures for IR Photovoltaics and Recent Developments at TCSAM

by: Alex Freundlich

Date: Friday August 06, 2004

Time: 12:00 pm – 1:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

The unusual bandgap shrinkage that accompanies the incorporation of small amounts of nitrogen (< few percent) in III-V semiconductors has sparked both theoretical and experimental research in the arena of dilute nitrogen containing III-V alloys. While considerable progress has been made, the encountered low solubility of N in these alloys and their poor optical properties have thus far hindered the expected rapid proliferation of the technology for IR and optoelectronic applications. The presentation highlights results of recent investigations at University of Houston on development of Ga(In)AsN based alloys and heterostructures (by RF- chemical beam epitaxy). Some key conditions favoring incorporation of large amounts of N (up to 7%) in epilayers are deciphered. In particular, the criticality of N-RF-plasma conditions (real time plasma spectroscopy) upon the incorporation of nitrogen in the solid and in controlling the optical properties of grown epilayers will be addressed. Based on experimental data presented here, a new dilute nitride-superlattice material design (lattice matched to InP) is devised and its potential for mid-IR applications (0.4-0.6 eV) will be discussed.

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Special Seminar

The Promise of MgB2 Superconductors in Electric Power Energy

by: Prof. Kamel Salama

Date: Friday May 07, 2004

Time: 12:00 pm – 1:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

Since the discovery of its superconductivity in early 2001, Magnesium Diboride (MgB 2) has drawn a great deal of worldwide research interest. This new high temperature superconductor has a critical temperature of 39.4 K, an upper critical ?eld of 29 Tesla, and a relatively long coherence length of about 5 nm. In addition, MgB 2 exhibits no intrinsic current blockage by grain boundaries and comparatively weak anisotropy and thermal ?uctuations. Research on MgB2 at TCSAM demonstrates promising results of high current carrying capability sustained to applied magnetic ?eld in the temperature range of liquid hydrogen and liquid neon. Thirty-meter long Fe-sheathed MgB 2 wires and tapes have been fabricated using the powder-in-tube method with ultra-?ne starting precursors. High critical current density of 3 x 105 A/cm2 at 20 K and self-?eld has been obtained. Coils wound from MgB 2 wires also possess superior superconducting properties. These results reveal the promise of MgB for electric power applications at 20-5 K.

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Special Seminar

Theory of High-Temperature Superconductivity - A New Perspective

Prof. W. P. Su

by: Prof. W. P. Su

Date: Friday January 16, 2004

Time: 12:00 pm – 1:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

Theoretical analysis of an effective model of d-wave superconductivity in two dimensions reveals a subtle interplay between phase separation, superconductivity and antiferromagnetism. The main theoretical result can be described in terms of elementary phase separation concepts in a binary alloy. This provides a deep understanding of the phase diagram of a hole-doped cuprate superconductor. A microscopic model of d-wave superconductivity will also be discussed.

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Special Seminar

Nanoscale Magnetic Recording

by: Dr. Dimitri Litvinov

Date: Friday September 05, 2003

Time: 12:00 pm – 1:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

Magnetic recording is rapidly shifting into the realm of nanoscale technologies. At 1 Terabit/in2, the size of a recording bit is 50x13nm2 (assuming 4:1 bit aspect ratio.) To store such small magnetic features, the characteristic dimensions of all supporting recording system components have to be shrunk correspondingly. The read/write transducers are being scaled down into 40nm range. The characteristic size of the smallest magnetic feature in a recording medium is being refined to hit the 5nm mark. The flying height of the recording heads is targeted to be a few nanometers at the most. The servoing capabilities of the tracking system are being optimized for the capability to position a recording transducer on a 3.5” diameter media disk with a precision of 10nm in a fraction of a millisecond. This presentation will review the major critical issues related to the recording physics, device fabrication, and system integration at nanoscale. The prospects of extending magnetic recording technologies above Terabit/in2 range as well as alternative approaches to data storage and retrieval will be discussed. Among the open issues critical to the development of nanoscale probe recording is the ability to fabricate magnetic transducers with the dimensions in the nanometer range as well as the detailed understanding of the recording physics of such nanomagnetic devices. Magnetic probe heads with a cross-section of 60x60nm2 were successfully fabricated using focused ion-beam (FIB) processing. The ability of such probe heads to controllably conduct magnetic flux is critical to the performance of a recording system. The results of both experimental and theoretical study of micro/nano-magnetic behavior of fabricated nanoscale probe heads will be reported. The recording demonstration using 60nm wide magnetic nanowriters will be presented. The key advantages of FIB technology with respect to magnetic materials processing will be reviewed. The resolution limits and Ga+ ions interaction with magnetic materials will be discussed.

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