TcSUH
Special Seminar
Quantum Fluctuations and Phase Transition in SrTiO3 Thin Films
by: Prof. Xiaoxing Xi
Date: Friday May 15, 1998
Time: 2:00 pm – 3:00 pm
Location: Houston Science Center – Building 593 — Room 102
Overview
Perovskite oxide SrTiO3 is an exceptional material in which quantum fluctuations play a central role. As established by Moller and Burkard, quantum fluctuations suppress the ferroelectric ordering in SrTiO3, leading to a quantum paraelectric ground state at low temperature. The possibility of a coherent quantum state proposed by Moller et al. has prompted numerous theoretical and experimental studies on quantum phase transition in this system. In this talk I will present our measurement of the complex dielectric permittivity as a function of temperature in high-quality SrTiO3 thin films deposited by pulsed laser deposition. We found that a peak in the real part of the dielectric constant and a low temperature loss peak, both previously suggested as indications of a quantum phase transition, showed markedly distinct thickness and electric field dependence. This behavior is qualitatively different from that in the SrTiO3 single crystals, and is consistent with the 2D Ising model in transverse field if one assumes a stronger ferroelectric coupling with respect to the quantum fluctuations in the thin films.
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Special Seminar
Optical Response of Thin Metallic and HTSC Films
by: Prof. Josef Humlicek
Date: Monday May 04, 1998
Time: 11:00 am – 12:00 pm
Location: Houston Science Center – Building 593 — Room 102
Overview
We discuss the optical (mainly infrared) response of conducting thin films on suitable substrates, such as TiN on silicon and YBa2Cu3O7 on MgO and SrTiO3, focusing on the unique features of the film samples compared to bulk materials. Special emphasis is placed on possible inhomogeneities of the free-carrier polarizability in the growth direction. We also discuss the peculiarities of the highly anisotropic spectra of HTSC films in the TO and LO phonon range of the c-axis response. The emphasized experimental technique is Fourier-transform ellipsometry and polarized reflectance at oblique incidence.
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Special Seminar
Progress in HTS SQUID Magnetometers and Instrumentation for Magnetocardiography
by: Dr. Alex Braginski
Date: Friday May 01, 1998
Time: 11:00 am – 12:00 pm
Location: Houston Science Center – Building 593 — Room 102
Overview
The sensitivity (magnetic field resolution) of high-temperature (HTS) SQUID sensors under development is approaching a level of 10 to 30 fT/Hz1/2 in the required bandwidth. This makes it possible to develop practical multichannel HTS SQUID systems for biomagnetic applications, and especially for magnetocardiography (MCG). With further sensitivity improvements, HTS systems for magnetoencephalography (MEG), where ultimate sensitivities are required, might eventually be possible. The purpose of this talk is twofold: (1) to review the status of HTS SQUID sensors and system-oriented demonstrations of multichannel operation and to identify the remaining technological obstacles; and (2) to analyze system requirements for MCG, the features of low-temperature multichannel (LTS) systems, and conditions which could make HTS systems operationally and economically preferable. The sensitivity of future HTS systems can, at best, only approach that of their LTS counterparts. New solutions for reducing magnetic shielding and attaining shielding-free operation can equally benefit HTS and LTS and are easier to implement in the latter. Hence, for large systems with over 50 sensing channels, and a total of about 100 SQUIDs, the margin for HTS market success is very narrow and questionable. For small, relatively inexpensive systems with a limited number of sensing channels, operational advantages of HTS appear greater, and a large market might develop for these. However, a key unresolved issue is the acceptance of MCG in medical diagnostics. Statistical validation of present diagnostic methods and the development of new clinical methods requires a collaboration between cardiologists, electrophysiologists, and SQUID developers. The cost of MCG systems may be higher than that of most established electrocardiographic instrumentation. Hence, one must unambiguously determine whether MCG offers only operational convenience or if it has, as claimed, much higher diagnostic information content and better localization potential. Only large clinical studies can answer this question and possibly lead to the success of MCG in the market.
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