Observation of Vortices in Superconductors
- PDF / 1,865,922 Bytes
- 4 Pages / 576 x 792 pts Page_size
- 45 Downloads / 220 Views
MRS BULLETIN/JUNE 1994
through thin-film superconductors in a 350 kV cold-tip field-emission TEM.8"10 Both electron holography and Lorentz microscopy were used to image vortices. The specimens chosen for these experiments were type II superconductors, Nb and Bi2Sr18CaCu2Ox (BSCCO-2212), because these materials allow magnetic fields to penetrate them in the form of extremely thin filaments called f luxons. Each f luxon is quantized, containing a flux of exactly h\2e, and the fluxons arrange themselves within the superconductor to form a lattice, the vortex lattice. Because these vortices play a significant role in superconductivity, it is important to investigate their interactions with specimen defects to elucidate various fundamental properties, such as the flux pinning mechanisms. Vortex lattice behavior has been a source of much controversy lately. For example, an interesting feature in high Tc materials is the "irreversibility line" in the H-T phase diagram. Below this line vortices can be effectively pinned, but above it pinning becomes so weak that the magnetization is reversible and the critical current vanishes.11 Various theories, such as flux creep12 or lattice melting,13 have been proposed to explain the phenomenon. The ability to directly observe the dynamical behavior of vortices can provide meaningful insights into the exact nature of the mechanism and is useful in evaluating the theoretical predictions. This article presents recent results on the observation of vortices in both conventional and high Tc superconductors. Experimental Method Observation of the vortex lattice requires a highly coherent and penetrating beam of illuminating electrons and the
holography electron microscope was specially developed for this purpose.14 The TEM is equipped with a liquid He stage that cools specimens down to 4.5 K and magnetic fields up to 150 G can be applied (See Figure 1). An attached TV camera system allows the dynamical behavior of fluxons to be viewed in real time and recorded on videotape (30 frames/s). We have observed the dynamics of vortices under changing temperature and magnetic fields. Niobium (99%) specimens were prepared for transmission observation by chemically polishing sections of coldrolled foils that had been annealed to ~2000°C in a vacuum of 10"6Pa. The TEM specimens were about 100 run in thickness and had electron transparent regions about 100 /um in diameter. BSCCO specimens were prepared by cleaving singlecrystal sections so that a thin region could be obtained. The Nb and BSCCO thin foils were subsequently sandwiched between copper grids and then mounted in the microscope. To observe the vortex lattice within a superconductor, the specimen must be tilted with respect to the electron beam. The reason for this tilt is simple: Only those components of the magnetic field perpendicular to the electron beam will produce a phase difference between two coherent electron waves due to the AB effect;4 a flux of h/2e causes a phase difference of exactly IT. While the value of the phase differenc
Data Loading...
