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作者 Ling, Dah-Chin
書名 An investigation of electrodynamic properties of high transition temperature superconducting yttrium barium(2) copper(3) oxygen(7-delta) single crystals
說明 88 p
附註 Source: Dissertation Abstracts International, Volume: 56-05, Section: B, page: 2700
Adviser: Juei-Teng Chen
Thesis (Ph.D.)--Wayne State University, 1995
Microwave induced dc voltages along the c-axis with or without a dc bias current and magnetic interference effects in YBCO single crystals have been studied. To study the microwave induced dc voltages, the sample was mounted near the end of an X-band waveguide such that the microwave H-field is in the ab-planes and the E-field parallel to the c-axis of the single crystal sample. Since the maxima of E- and H-fields of the microwaves are separated by one quarter of wavelength near the end of the waveguide, the single crystal sample can be coupled to the E- or the H-fields of the microwaves selectively by moving the adjustable end near the end of the waveguide. Microwave induced dc voltages have been studied under a variety of conditions including various sample positions, frequencies, bias current levels and media such as liquid oxygen, liquid nitrogen, oxygen gas and nitrogen gas. With a dc bias current, the effect is larger when the single crystal sample is coupled to the maximum of the microwave H-field, suggesting that the induced dc voltages are due to vortex flow. Without a dc bias current, the induced dc voltages resemble the inverse ac Josephson effect and require the presence of both the E- and H-fields of the microwave. The observed phenomena indicate the existence of multiple Josephson junctions inside a single crystal sample
For the magnetic interference measurements, the sample was biased at a constant dc current in the c-axis direction such that a small dc voltage developed and then a dc magnetic field parallel to the ab-planes of the single crystal was applied to search for an effect. Since the change in the voltage signal is comparable to the noise level of the nanovoltmeter, a more sensitive technique was employed. In a addition to a dc magnetic field, a low-frequency magnetic field perpendicular to the dc field was applied to modulate the critical current of the sample. Using this technique, the small voltage changes as a function of dc magnetic field can be detected at modulation frequency by a lock-in amplifier. We have observed periodic voltage oscillations as a function of the applied dc magnetic field parallel the ab-planes of a $\rm YBa\sb2Cu\sb3O\sb{7-\delta}$ single crystal with a dc bias current along the c-axis. The modulated voltage versus magnetic field patterns are similar to the magnetic-field interference patterns associated with a Josephson junction or a dc SQUID, strongly suggesting that Josephson junctions may occur naturally in $\rm YBa\sb2Cu\sb3O\sb{7-\delta}$ single crystals. The observed field periods indicate that the Josephson junctions are probably at the stacking faults along the c-axis within a single crystal sample or in a large unit cell of superlattice structures, such as $\rm Y\sb2Ba\sb4Cu\sb7O\sb{15-\delta}$
School code: 0254
DDC
Host Item Dissertation Abstracts International 56-05B
主題 Physics, Condensed Matter
0611
Alt Author Wayne State University
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