Department of Physics & Astronomy

Spring 2009 Colloquia

 

Ophir Auslaender

Ophir Auslaender

Geballe Laboratory for Advanced Materials
Stanford University
Probing microscopic and dynamical properties of superconducting vortices by vortex dragging
March 2, 2009, 4PM, Thornton 411

Superconductors often contain vortices, quantized microscopic whirlpools of electrons. Apart from their technological significance (it is vortex motion that limits the utility of the high temperature superconductors), vortices are a rich playground for condensed matter physics because of the interplay between thermal fluctuations, vortex-vortex interactions, vortex structure, and the interaction for a vortex with the defects in a real material. We use a home built, low temperature magnetic force microscope (MFM) to both image and manipulate individual vortices. This allows us to go beyond more conventional methods that cannot probe the behavior of a specific, individual, vortex. I will mainly present data from YBCO, a high temperature superconductor. In a YBCO thin film we find that if the force exerted by the magnetic tip of the MFM is strong enough to overcome the pinning potential created by defects which trap a vortex, it jumps as a whole to a new pinning site. This behavior is strikingly different from the behavior in a high quality, slightly overdoped, YBCO single crystal, where vortices behave like one-dimensional elastic objects - they stretch rather than jump when we drag them. In this sample we find an anisotropy in the mechanical properties of a vortex. We attribute this anisotropy to a combination of microscopic parameters and spatial correlations in the defect structure. Surprisingly, we find that when we wiggle the top of a vortex we can drag it significantly further than when we do not, giving rise to a dramatic dynamic anisotropy between the fast and the slow directions in a scan. In an underdoped YBCO single crystal, a material where superconductivity is so anisotropic that a vortex should be viewed as a stack of two dimensional pancakes rather then an elastic string, we show that vortices kink rather then tilt when we pull on them. These results demonstrate the power afforded by direct single-vortex manipulation and imaging for exploring the interesting behavior of these extended objects.

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