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Seminar | X-Ray Science Division

Superconductors and Semiconductors Physics in Extreme Conditions

XSD/TRR Special Presentation

Abstract: I will present the response of materials to extreme conditions as measured with in-house developed instrumentation.  First, I will describe magnetization measurements of high-temperature superconductor (HTS) coated conductors at very high magnetic field and temperatures down to 4.2 K. Progress in manufacturing HTS tapes has made possible coated conductors carrying critical currents (?c) of the order of ~2 kAmp at liquid helium temperatures. Accurate ?c evaluation over a broad range of applied field (?), field angle (?),and temperature (?) is essential for high-field magnet technology development. Critical currents are measured in high fields by four-probe transport methods, which are difficult in the small bore of high-field magnets due to limited space, cooling problems arising from liquid helium levitation (“He-bubble problem”), and large forces and torques on the sample that challenge material strength. During measurement, many samples are destroyed, resulting in a very limited set of data in very high field and beyond B/c-axis configuration. To avoid such issues, I developed a vibrating coil magnetometer (VCM) that provides a contact-free method to indirectly evaluate strongly anisotropic high critical currents from magnetization measurements (?c~M). The VCM relays on vibrating sensing coils, thus allowing in situ sample rotation in a high magnetic field and better temperature control. Magnetization measurements of several conductors show that ?c(?,?,?) is reliably extracted from ?(?,?,?) by rescaling on one critical current value obtained from transport measurements.

Second, I will describe the response of a silicon lattice to a dense optical excitation. Photogenerated carrier-phonon interaction influence the transient optical and electrical properties of silicon (i.e., the performance and potential for downsizing of small silicon-based electronic devices. By employing TR pump-and-probe spectroscopy, we studied the response of zone-center LO phonon of Si(001) to nonequilibrium photo-excited carriers as observed through its frequency and decay time change (i.e., the complex self-energy associated with dressing the phonon by electronic excitations). The photo-induced changes give rise to a transient lattice softening manifested through a corresponding change in reflectivity. The LO phonon frequency and decay rate is extracted from transient reflectivity measured over the first ~6 ps after excitation and analyzed in the context of the many body interactions occurring during and after excitation. The results for Si(001) display an intricate time-dependent lattice softening based on the interplay between hot carriers and lattice temperatures contributions, initial doping concentration and type, and photo-excitation intensity.

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