Friday 06 Dec 2013Colloquium: Vortices in Unconventional Superconductors

Prof. Simon Bending - University of Bath

Newman Red 12:00-13:00

The mixed state of type II superconductors contains topological excitations called vortices that usually carry a single quantum of magnetic flux. These flux lines typically form a triangular lattice under the influence of weak mutual repulsion. Since the characteristic elastic moduli of the lattice are small, the mixed state represents a form of soft (vortex) matter that exhibits thermodynamic phases and true phase transitions (e.g., melting) in which thermal fluctuations and disorder play key roles.

Not only is a better understanding of vortices crucial to the exploitation of superconducting materials, it can also yield key information about the superconducting pairing state and Fermi surface and effective mass anisotropies. Direct imaging using scanning probes has proved to be a powerful tool for studying vortex matter and is also capable of providing limited dynamic information. Recent case studies will be presented illustrating the use of scanning Hall probe microscopy (SHPM) to explore the physics of two very different superconducting systems.

Bi2Sr2CaCu2O8+? exhibits the highest crystalline anisotropy of any known cuprate superconductor. As a consequence it undergoes a first order vortex lattice melting transition far from the phase boundary to the normal state that can be imaged yielding direct information about vortex fluctuation amplitudes [1]. In addition it shows a rich family of broken symmetry vortex structures in tilted magnetic fields that can be understood in terms of interacting Josephson and pancake vortices [2]. It is even possible to exploit this coupled vortex system to realise vortex pumps and lenses in ratchet-like devices [3].

Sr2RuO4 is widely believed to be a p-wave spin triplet superconductor with a two-component chiral order parameter [4]. Since the Cooper pairs have angular momentum, spontaneous supercurrents are predicted anywhere that translational symmetry is broken, e.g., at sample edges and chiral domain walls. Although spontaneous magnetic fields have been observed in SR experiments below the critical temperature [4] they have never been resolved using local probes. Our scanning Hall probe images also fail to reveal evidence for spontaneous currents at H=0. However, interesting structural transitions of the vortex lattice are observed at low fields that lead to the formation of a quite well ordered square lattice for H>10Oe [6]. Furthermore, in mesostructured samples we also find an abrupt change in the vortex pinning behaviour for H?25Oe which might be the signature of a field-driven change of the order parameter. These results will be discussed in the light of relevant theories of field-dependent transitions [7].

[1] A.Oral et al., Phys. Rev. Lett. 80, 3610 (1998).
[2] A.N.Grigorenko et al., Nature 414, 728 (2001).
[3] D.Cole et al., Nature Materials 5, 305 (2006).
[4] A.P. Mackenzie and Y. Maeno, Rev. Mod. Phys. 75, 657 (2003).
[5] G.M. Luke et al., Nature 394, 558 (1998).
[6] P.J.Curran et al., Phys. Rev. B 84, 104507 (2011).
[7] R.Heeb and D.F.Agterberg, Phys. Rev. B 59, 7076 (1999); J.F.Annett et al, Phys. Rev. B 78, 054511 (2008).

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