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Measurement of the London penetration depth in the Meissner state of NbSe₂ using low energy polarized ⁸Li Hossain, Md Masrur

Abstract

In this thesis, the Meissner state of NbSe₂ was investigated using low energy beam of spin polarized ⁸Li. The ⁸Li nuclear spin relaxation rate 1/T₁ was measured as a function of temperature and magnetic field. The spin relaxation rate is sensitive to low frequency nuclear spin dynamics of the host Nb spins and is strongly field dependent. This is used to determine the reduction in the magnetic field upon cooling into the Meissner state. Using a calculated implantation profile and a model field distribution, one can extract a measure of the absolute value of the London penetration depth λ in Meissner state. In addition, a model field distribution, assuming a suppression of order parameter near surface, was developed. In this case, we can extract another length scale which is related to the "coherence" length ξ. The value of λ depends on the model field distribution but is significantly longer than that obtained previously in the vortex state using μSR. From the measured internal magnetic field distribution, London penetration λL is extracted as a function of temperature. There is also evidence of the coherence peak in 1/T₁ of host nuclear spins. λL( T ) follows the two-fluid model of superconductivity. Depending on the model for internal field distribution, λL(0) varies in the range (1795-2434) °A.

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