Researchers: James Loudon and Charlotte Bowell
Flux Vortices in Superconductors
Superconductors have two remarkable properties: they have no electrical resistance whatsoever and they expel magnetic field from their interiors. If you apply a magnetic
field to an ideal (type I) superconductor, no field enters unless the magnetic field exceeds a critical strength at which the material suddenly ceases to superconduct.
However, there is another type of superconductor (called type II) where the whole superconducting state is not destroyed at once but above a critical field, magnetic flux
penetrates the material by flowing along narrow channels called flux vortices where superconductivity is suppressed. Each flux vortex contains one quantum of flux: the
smallest amount of magnetic flux allowed in a superconductor by the laws of quantum mechanics. To emphasise their quantum nature, they are often called fluxons.
Flux Vortices in BSCCO: (a) Experimental Arrangment for imaging vortices. (b) A vortex lattice. (c) Image of a single
vortex found by averaging those in (b).
At first sight, superconductors look ideal for use in power lines: they have no resistance, so electricity flows without any power loss. The trouble is that when an
electrical current is passed along a wire, it generates a magnetic field and if this field becomes larger than the critical field, the wire suddenly becomes resistive and
there is catastrophic heating. The critical fields of type I superconductors are too low to make a viable power line and they only superconduct at very low temperatures
requiring expensive cooling with liquid helium. However, so-called high temperature superconductors have the advantages that the field needed to destroy superconductivity
completely is much higher and they superconduct at much higher temperatures so that they can be cooled by inexpensive liquid nitrogen. The disadvantage is that high
temperature superconductors are type II and when they conduct electricity, the magnetic fields associated with the electrical current generate fluxons. As the current
flows, it induces a force on the fluxons and they move in response. This movement uses up energy and so type II superconductors appear to have electrical resistance even
though they are superconducting. The only way to retain the zero-resistance state is to prevent the fluxons from moving which is usually done by introducing defects into
the crystal structure of the superconductor which pin the vortices. However, determining how the vortices respond to these pinning sites and what type of defect best pins
the vortices is no easy matter.
There are several ways to observe fluxons but the method we use, developed by the Tonomura research group at the Hitachi Advanced Research Laboratories in Japan, employs
transmission electron microscopy, has a higher resolution than the alternatives, can measure the magnetic fields quantitatively and observe the fluxon motion in real time.
Despite these advantages, this technique has only been successfully employed by two research groups using dedicated microscopes. We have recently imaged
vortices with a modern commercial electron microscope, opening up a world of possibilities.
We shall use this technique to examine how fluxons respond to naturally occurring defects as well as introducing our own artificial pinning sites. We shall investigate the
pinning by different patterns of defects created by ion beam irradiation and also by arrays of dots of magnetic material deposited on the surface of the superconductor
which pin vortices due to their magnetic interaction. In some superconductors, under the right temperature and magnetic field, the pinning forces on the fluxons abruptly
disappear in a process termed flux lattice melting. We plan to characterise this transition by analysing videos taken of the fluxon motion as the transition is
Loudon and P.A. Midgley, Ultramicroscopy, 109, 6, 700, 2009 for more information.)
Imaging Phase Transitions in Action
A phase transition is an abrupt change in the properties of a system in response to changes in the external conditions. Examples in solid state physics include the onset of
superconductivity as a sample is cooled, the change in magnetic properties as a ferromagnet is heated above its Curie point and the changes in crystal structure materials
exhibit in response to changes in temperature and pressure. Phase transitions are important throughout science and occur in fields ranging from the study of the early
universe to the study of banking crises.
Phase transitions are frequently classified as first order, involving the coexistence of two separate phases, or second order, where one phase changes uniformly and
continuously into another. However, classifying the phase transition only gives a hint as to the underlying physics which drives it. Using the electron microscope, we aim
to see the details of the phase transition as it occurs in real time to go beyond this classification
scheme and instead elucidate the mechanism by which one phase transforms into another.