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Near surface vortex lattice in NbSe₂ studies with low energy beta - NMR Wang, Doug

Abstract

In this thesis, the magnetic field distribution in the vortex state of NbSe₂ is investigated with the technique of depth resolved β-detected NMR using a beam of highly polarized ⁸Li produced at the TRIUMF ISAC facility. The β-NMR lineshape is a direct measure of the local magnetic field distribution weighted according to the implantation profile of the ⁸Li beam. By varying the implantation energy between 1~ 30keV, one can control the average implantation depth corresponds in a range between 5 ~ 136nm. Above Tc = 7.OK a relatively narrow resonance is observed whose width is attributed to magnetic dipolar broadening from the ⁹³Nb nuclear moments. Below Tc a much broader asymmetric lineshape is observed, which is characteristic of a triangular magnetic vortex lattice. Modeling the magnetic field distribution allows one to determine both the effective in plane penetration depth λıı and the effective in plane coherence length ξıı or vortex core radius. In a magnetic field of 302mT, we obtain λıı = 279(30)nm and ξıı = 12(1)nm. In a smaller magnetic field of 10.84mT, the effective coherence length increases dramatically to a value of 77(10)nm. This is more than an order of magnitude larger than the expected from the coherence length in NbSe₂. The origin of the giant vortices is discussed. We propose an explanation involving the multiband nature of NbSe₂.

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