| Summary: | In this thesis, muon spin rotation (μSR) measurements of the internal magnetic field
distribution in the vortex state of the high-T[sub c] superconductor YBa₂Cu₃O₇_[sub ⍴] and the
conventional type-II superconductor NbSe₂ are presented. From the measured field
distributions, the "characteristic length scales of superconductivity" are extracted. It
is found that both the a-b plane magnetic penetration depth λ[sub ab] and the vortex-core
radius r[sub o] (which is closely related to the coherence length Ɛ[sub ab] vary as functions of
temperature and magnetic field in both materials.
The behaviour of λ[sub ab](H, T) and r[sub 0](H, T) at low temperatures is found to be substantially
different in YBa₂Cu₃0₇_⍴ from what is observed in NbSe₂. This reflects the
unconventional nature of the pairing mechanism in this compound. The temperature
dependence of λ[sub ab] in the vortex state of YBa₂Cu₃0₇_⍴ agrees well with microwave
cavity measurements in the Meissner state. The magnetic field dependence of λ[sub ab], in
YBa₂Cu₃0₇_⍴ is found to be considerably stronger than in NbSe₂. This is likely due
to both the nonlinear and the nonlocal effects associated with nodes in the superconducting
energy gap. However, in NbSe₂ (where nonlocal effects are negligible), it is
not clear whether the field dependence of can be explained solely in terms of the
nonlinear effects associated with an isotropic s-wave energy gap.
The vortex-core radius r[sub 0] is found to decrease with increasing magnetic field in
both superconductors. The reduction in the vortex-core size appears to be due to
the increased strength of the vortex-vortex interactions. An important consequence of
this variation with field is that Ɛ[sub ab] in the vortex state, which is generally regarded to
be extremely small i n the high-T[sub c] compounds, is comparatively large at low magnetic fields.
The vortex-core radius is also found to increase with increasing temperature. The
strength of this variation is considerably weaker in YBa₂Cu₃0₇_⍴ than in NbSe₂. One
possible interpretation is that the quantum limit is realized at much higher temperatures
in the high-T[sub c] compound. The measured temperature dependence of r[sub 0] in both
superconductors is weaker than current theoretical predictions for an isolated vortex.
Finally, the effects of vortex pinning and thermal fluctuations of the vortex lines are
considered. It is found that the vortex lattice is strongly pinned in YBa₂Cu₃0₇_⍴. The
vortex lattice in the underdoped compound YBa₂Cu₃O₆.₆₀ is found to exhibit quasi-2D
behaviour. In particular, a field-induced transition of the 3D-vortex lattice to a 2Dvortex
lattice is observed—which appears to be due to the small c-axis coherence length
and vortex pinning in the Cu0₂ layers of this material. Also, the 3D-solid vortex lattice
in YBa₂Cu₃O₆.₆₀ at low temperatures is found to melt and/or undergo a transition to
a 2D-vortex lattice as the temperature is increased. === Science, Faculty of === Physics and Astronomy, Department of === Graduate
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