Stuff in this page about my research work:

My path through Physics

List of publications

A pretty neutron picture

Superconductivity web sites

My path through Physics
(or a sort of very extended Curiculum Vitae)

The phenomenon of high temperature superconductivity (and its own phenomenon as a research field) took off in 1987, the same year I entered Manchester University as an undergraduate; from the introductory week lecture the subject has shaped my interests ever since.

A final year project in electron tunnelling in low-Tc superconductors (enthusiastically supervised by Peter Lucas) was enough to convince of the fun of real research, and the experience, that is hit and miss frustration, of laying down thin films to form junctions between different superconductors for the electrons to tunnel through was a pointer to the importance of materials properties in superconductivity.

I pursued this theme in my PhD, for which I joined Peter Hatton's group at the Univesity of Edinburgh in 1991. Using x-ray scattering techniques to characterise the structural subtleties of the high-Tc superconductor BiSrCaCuO, drawing correlations between structure and growth parameters (succesfully [p1]), and with superconductivity (less successfully [p2]). Although I had little comprehension of the word 'Incommensurate' at the outset, it was eventually (once I'd looked it up in the dictionary) to become the main concept of interest during my thesis work, partly because in a way it breaks the rules of well-established Bragg diffraction.

I was fortunate to be employed by Ted Forgan in 1996, at the University of Birmingham, and have had the privilege of working with his research group at various times since then. This broadened my interests to include the magnetic properties of superconductors, and I have been involved with experiments probing magnetic flux-lines using neutrons and muons; the experiments being performed at the Institut Laue Langevin (Grenoble, France), Paul Scherrer Institut (Zurich, Switzerland), and Rutherford Appleton Laboratory (Oxfordshire, England).

In 1997 I moved on to the Laboratoire de Physique des Solides at the Univerisite de Paris-Sud in Orsay, France. The position was made possible by a Marie Curie Fellowship from the European Commission. A memorable time was had in the group of Ian Campbell, learning the micro-craft of torque magnetometers, and investigating the dynamics of flux-lines in the presence of columnar defects.


neutron scattering pattern
Raw small angle neutron scattering data from a YBCO crystal -
of interest is the weak ring (just 1% of the total scattering) from the flux-lines.

Publications (selected principle ones)

  • Observation of transverse field components in the flux line lattice in the anisotropic superconductor YBa2Cu3O7
    • P G Kealey, D Charalambous, E M Forgan, S L Lee, S T Johnson, P Schleger, R Cubitt, D McK Paul, C M Aegerter, S Tajima, A Rykov
    Phys. Rev. B 64, 174501 (2001)

  • Measurement of Vortex Motion in a Type-II Superconductor: A Novel Use of the Neutron Spin-Echo Technique
    • E M Forgan, P G Kealey, S T Johnson, A Pautrat, Ch. Simon, S L Lee, C M Aegerter, R Cubitt, B Farago and P Schleger
    Physical Review Letters 85, 3488 (2000).

  • Small angle neutron scattering and vortex lattice dynamical phase diagram
    • C. Goupil, A. Pautrat, Ch. Simon, P. G. Kealey, E. M. Forgan, S. L. Lee, S. T. Johnson, G. Lazard, B. Placais, Y. Simon, P. Mathieu, R. Cubitt, Ch. Dewhurst
    Physica C (Proceedings of M2SHTSC VI, pt. 2, Houston, Texas, USA, Feb. 20-25, 2000) 341-348,999 (2000)

  • Angular-dependent muon-spin rotation and torque magnetometry on the mixed state of the high-temperature superconductor YBa2Cu3O7
    • C Ager, F Y Ogrin, S L Lee, C M Aegerter, S Romer, H Keller, I M Savic, S H Lloyd, S T Johnson, E M Forgan, T Riseman, P G Kealey, S Tajima, A Rykov
    Physical Review B 62, 3528 (2000).

  • Evidence for an angular dependent contribution from columnar defects to the equilibrium magnetization of YBa_2Cu_3O_(7-d)
    • B Hayani, S T Johnson, L Fruchter,and C J van der Beek.
    Physical Review B 61, 717 (2000).
    Download the article including figures as either postscript(103K) or a latex document(61K).
    Or from the cond-mat preprint archive #9903407.

  • Flux-line lattice structures in untwinned YBa_2Cu_3O_(7-d)
    • S T Johnson, E M Forgan, S H Lloyd, C M Aegerter, S L Lee, R Cubitt, P G Kealey C Ager, S Tajima, A Rykov, D McK Paul.
    Physical Review Letters 82, 2792 (1999).
    Download the article including figures as either postscript(103K) or a latex document(61K).
    Or from the cond-mat preprint archive #9804159.

  • Observation of vortex lattice melting in twinned YBa_2Cu_3O_(7-d) using neutron small-angle scattering
    • C M Aegerter, S T Johnson, W J Nuttall, S H Lloyd, E M Forgan, R Cubitt, S L Lee, D McK Paul, M Yethiraj, and H A Mook
    Physical Review B Vol. 57, 14 511 (1998).

  • A study of the effects of annealing upon the high-Tc superconductor Bi_2Sr_2CaCu_2O_8
    • S T Johnson, P D Hatton, A J S Chowdhury, J Gardner, G Balakrishnan, D McK Paul, and J Hodby. Physica C (1998).

  • An in situ x-ray scattering study of high-temperature structural changes in Bi_2Sr_2CaCu_2O_8
    • S T Johnson and P D Hatton Physica C (1996).

  • X-ray scattering studies of the incommensurate high-Tc superconductor BiSrCaCuO
    • S T Johnson University of Edinburgh Thesis (1995).

  • In search of low temperature structural changes in BiSrCaCuO.
    • S T Johnson, P D Hatton, A J S Chowdury, and B M Wanklyn Solid State Communications, Vol. 94, pp. 261-265 (1995).

  • X-ray scattering studies of a variety of BiSrCaCuO single crystals.
    • S T Johnson, P D Hatton, A J S Chowdury, B M Wanklyn, Y F Yan, Z X Zhao, and A Marshall Physica C 219, pp. 61-66 (1994)

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