Page 36 - ILLS Annual Report 2018

MAGNETISM
Mark Laver. British University of Birmingham, UK www.birmingham.ac.uk/laver
‘I did my PhD at the ILL many years ago. I am now a lecturer at the University of Birmingham. My research interests include superconductors and functional materials, e.g. for nuclear applications.’
Vortex disordering and the peak effect in
superconductors
Small-angle diffractometer D22 SANS-II instrument at PSI NG7 instrument at NCNR
Underlying disorder pins vortices in type-II superconductors, giving zero resistivity below a critical current
jc. Peaks are often observed in the temperature and field dependences of jc.
This ‘peak effect’ is commonly believed to stem from an order-disorder transition of the vortex ensemble [1]. Using SANS, we find an order-disorder transition from a quasi-long-range- ordered phase (the ‘Bragg glass’) to
a vortex glass in a vanadium crystal. The peak effect, however, lies at higher fields and temperatures, challenging the common dogma of its origin.
Figure 1
Experimental geometry used on the D22 SANS instrument. The magnetic field profile presented by superconducting vortices diffracts neutrons. Rocking curves are collected by rotating the sample, field and vortices together through the Bragg condition. A typical image of the 2D SANS multidetector at the peak of the rocking curve of the right Bragg spot is shown.
AUTHORS
R. Toft-Petersen and A.B. Abrahamsen (Technical University of Denmark, Denmark)
S. Balog (University of Fribourg, Switzerland)
L. Porcar (ILL)
M. Laver (University of Birmingham, UK) ARTICLE FROM
Nat. Commun. 9 (2018) 901—All figures adapted from article
REFERENCES
[1] See, e.g. T. Matsushita, Flux Pinning in Superconductors (Springer, 2007) [2] A.I. Larkin, Sov. Phys. JETP. 31 (1970) 784
[3] T. Giamarchi and P. Le Doussal, Phys. Rev. B 52 (1995) 1242
[4] G.P. Mikitik and E.H. Brandt, Phys. Rev. B 64 (2001) 184514
[5] M. Laver et al., Phys. Rev. Lett. 100 (2008) 107001
It has taken many decades to unravel the effects of weak disorder on the ensemble of vortices that form in type-II superconductors. In the 1970s it was initially thought that any random disorder, no matter how weak, would destroy long-range order in the vortex ensemble [2].
However, it turns out that lattice periodicity becomes crucial at large scales, where vortex displacements grow to be
of the order of inter-vortex spacing—a distance set by
flux quantisation. At these asymptotic scales the growth of displacements slows, giving rise to quasi-long-range order with algebraically diverging Bragg peaks. The resulting phase is accordingly known as the ‘Bragg glass’ [3].
The Bragg glass picture, which holds when underlying disorder is weak, breaks down when dislocations become important. Upon increasing field or disorder strength, a transition to a short-range ordered vortex glass phase is expected. Then, at temperatures close to the upper critical field Bc2(T) where bulk superconductivity disappears, thermal fluctuations subsequently drive a proliferation of dislocations and a thermodynamic melting of the vortex lattice.
ANNUAL REPORT 2018
Applied field and vortices 0.01 roughly parallel to beam
Pixel number
40 50 60 70 80 90
90 80 70 60
10 50
40 0 30
20
incoming beam
ILL cold neutron source
0
-0.01
-0.01
aperture
0 0.01 qx (Å-1)
unscattered beam
velocity selector
aperture
collimated incoming beam
SANS multidetector
Pixel number
Scattered neutron intensity (counts / 90s)
qy (Å-1)
scattered neutrons