As a new PhD student, the opportunity to undertake a short exploratory training project (ETP) is one of the main appeals of a Centre for Doctoral Training. Getting a sneak preview of the experiments, research groups and supervisors that will be available to us before choosing the PhD project is a great opportunity to help make up our minds. The projects are also a chance for us to try out some of the techniques that may be helpful in our future work. It was with some excitement then that I embarked on an ETP with Professor Simon Bending at the University of Bath with the intriguing title ‘Vortex Melting in a High Tc Superconductor’. The aim of this project is to study the structure of lines of magnetic flux (known as vortices) which penetrate into type II superconductors. In this case the target is BSCCO, a material in the cuprate family of superconductors. The eventual aim of the project is to measure the ‘melting’ of the vortex structure as a function of the angle of the applied magnetic field. The measurements we’ve taken for this project have been with the help of a phenomenon called the Hall Effect.
What is the Hall effect?
The Hall effect is observed in charge carriers flowing in a conductor: on the application of a magnetic field, the charge carriers experience a transverse force perpendicular to both the current direction and the applied magnetic field. The charge carriers are pushed towards the edge of the conductor resulting in a voltage build-up, which can be measured with carefully positioned contacts. This effect is used in a Hall Effect sensor for sensitive measurements of the magnetic field. In condensed matter physics, the Hall Effect is used as a probe for the measurement of a range of materials including semiconductors, magnetic materials and superconductors.
Using the Hall Effect in my ETP
The crystal we’ve been studying, BSCCO, is a member of the cuprate family of superconductors. These materials are characterised by high superconducting transition temperatures and copper oxide layers, which are responsible for the superconducting phase. When a magnetic field is applied to BSCCO, vortices enter and form a known structure. Beyond a certain applied field the structure ‘melts’, producing a peak in the magnetic field inside the superconductor. Measurements of the Hall voltage as a function of applied magnetic field are able to reveal the location of this melting line. So far we’ve been introduced to the struggles of experimental physics as we’ve experienced several difficulties with the resolution of our probe! But we’ve finally managed to track the melting line over a range of temperatures. The next step is to observe melting as a function of applied magnetic field angle. These measurements are important in BSCCO because of its high anisotropy, therefore the measurements will help us understand how the vortices act along the different directions of the crystal.
Advanced Hall Effect Measurement Probes
Measurements using the Hall effect have been advanced in recent years thanks to a new technique known as Scanning Hall probe microscopy (SHPM) developed by Simon and his group in Bath. The technique makes use of small GaAs/AlGaAs Hall probes with extremely high sensitivity in combination with a scanning tunnelling microscope. Scanning Hall microscopes can therefore spatially map the magnetic state of a material making them a useful tool in many areas of condensed matter physics. Figure 2 (a) shows a SHPM image of a superconductor-ferrimagnet bilayer. The black and white regions in the SHPM image show magnetisation in and out of the plane, clearly revealing the labyrinthine structure of the material. The authors of this paper went further by performing an SHPM scan on the material with an applied current and were able to observe how the current flows through the complex domain structure. SHPM has so far been used for many interesting studies including observations of data bits on hard disks and other magnetic structures. SHPM makes use of the Hall effect in a new way and brings many possible new applications.
The first short project of my PhD has been an interesting (if sometimes a little frustrating!) introduction into the world of superconductivity, Hall effect measurements and experimental physics in general. I’ve discovered that the Hall effect can be used in several different ways and I’m sure this introduction will help me in my future work.
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Scanning Hall probe microscopy image of a superconductor-ferrimagnet bilayer reproduced from M Marchevsky et al 2011 Supercond. Sci. Technol. 24 024006. Copyright IOP Publishing Ltd 2011.