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Research Profile - Department of Materials Science and Metallurgy ...

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James Loudon<br />

Royal Society University <strong>Research</strong> Fellow<br />

MA MSci University <strong>of</strong> Cambridge<br />

PhD University <strong>of</strong> Cambridge<br />

+44 (0) 1223 334564<br />

jcl25@cam.ac.uk<br />

www-hrem.msm.cam.ac.uk/people/loudon<br />

Imaging Flux Vortices in Superconductors<br />

I work in the High-Resolution Electron Microscopy Group<br />

researching the structure <strong>and</strong> dynamics <strong>of</strong> flux vortices in<br />

superconductors using real-time imaging. Previously, I studied<br />

the unconventional phases <strong>of</strong> mixed-valent manganites at<br />

Cambridge <strong>and</strong> Cornell Universities.<br />

Superconductors <strong>and</strong> flux vortices<br />

Superconductors have two remarkable properties: they have no<br />

electrical resistance whatsoever <strong>and</strong> expel magnetic flux from<br />

their interiors. Applying an increasing magnetic field to an ideal<br />

(type-I) superconductor, no flux enters until a critical field strength<br />

at which the material suddenly ceases to be superconducting. In<br />

contrast, in type-II superconductors, the superconducting state<br />

is not wholly destroyed at once but magnetic flux penetrates the<br />

material along narrow channels <strong>of</strong> non-superconducting material<br />

called flux vortices. Each vortex contains one quantum <strong>of</strong> flux:<br />

the smallest amount <strong>of</strong> magnetic flux allowed by the laws <strong>of</strong><br />

quantum mechanics. To emphasize their quantum nature, they<br />

are <strong>of</strong>ten called ‘fluxons’.<br />

The behaviour <strong>of</strong> fluxons is crucial to the performance <strong>of</strong> almost<br />

every superconducting device, including superconducting power<br />

lines, SQUIDs for very sensitive measurements <strong>of</strong> magnetic<br />

fields, superconducting magnets in medical MRI scanners, <strong>and</strong><br />

future technologies such as superconductor-based quantum<br />

computers. To improve performance, it is necessary to observe<br />

fluxons in action to determine their response to pinning sites <strong>and</strong><br />

other constraints <strong>and</strong> stimuli.<br />

Imaging flux vortices<br />

We use transmission electron microscopy to observe individual<br />

vortices in real time. This technique has a higher resolution than<br />

the alternatives <strong>and</strong> can measure the magnetic fields quantitatively.<br />

Worldwide, it had been successfully employed by only two<br />

collaborating groups using specially adapted microscopes.<br />

However, we have recently imaged vortices with a modern<br />

commercial electron microscope (see figure), opening up a world<br />

<strong>of</strong> possibilities. We shall investigate the structure <strong>of</strong> individual<br />

vortices, observe <strong>and</strong> quantify their dynamics <strong>and</strong> investigate their<br />

response to pinning sites <strong>and</strong> constrained geometries.<br />

JC Loudon, LF Kourkoutis, JS Ahn, CL Zhang, SW Cheong & DA Muller,<br />

“Valence changes <strong>and</strong> structural distortions in ‘charge ordered’ manganites<br />

quantified by atomic-scale scanning transmission electron microscopy”<br />

Phys. Rev. Lett. 99, 237205 (2007).<br />

JC Loudon & PA Midgley, “Comparison <strong>of</strong> the ferromagnetic phase<br />

transitions in La 0.7<br />

Ca 0.3<br />

MnO 3<br />

<strong>and</strong> single crystal nickel by micromagnetic<br />

imaging” Philos. Mag., 86, 2941–2956 (2006).<br />

JC Loudon & PA Midgley, “Micromagnetic imaging to determine the nature<br />

<strong>of</strong> the ferromagnetic phase transition in La 0.7<br />

Ca 0.3<br />

MnO 3<br />

” Phys. Rev. Lett. 96,<br />

027214 (2006).<br />

JC Loudon & PA Midgley, “Real-space imaging <strong>of</strong> coexisting chargeordered<br />

<strong>and</strong> monoclinic phases in La 1‐x<br />

Ca x<br />

MnO 3<br />

(x = 0.67 <strong>and</strong> 0.71)” Phys.<br />

Rev. B 71, 220408 (2005).<br />

JC Loudon, S Cox, AJ Williams, JP Attfield, PB Littlewood, PA Midgley<br />

& ND Mathur, “Weak charge-lattice coupling requires reinterpretation <strong>of</strong><br />

stripes <strong>of</strong> charge order in La 1–x<br />

Ca x<br />

MnO 3<br />

” Phys. Rev. Lett. 94, 097202 (2005).<br />

Flux vortices in superconducting Bi 2<br />

Sr 2<br />

CaCu 2<br />

O 8+ō<br />

:<br />

A pseudo-hexagonal array <strong>of</strong> flux vortices in superconducting<br />

Bi 2<br />

Sr 2<br />

CaCu 2<br />

O 8+ō<br />

at 20 K in a field <strong>of</strong> 30 G. Individual vortices<br />

are seen as black-white features<br />

28 <strong>Research</strong> <strong>Pr<strong>of</strong>ile</strong>

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