Design Of The Magnetic Penetration Depth Measurement System And Its Applications In Probing Superconducting Order Parameters

Penetration depth is a parameter which can connect the electromagnetic phenomena and the micro-mechanism of superconductivity. Measurements of the temperature dependence of penetration depth and/or superfluid density may provide insights to the superconducting order parameters and, therefore, are important for revealing the superconducting pairing mechanisms.In this dissertation, we have designed and constructed a high-resolution device for measuring the magnetic penetration depth down to40mK by using a technique based on the tunnel-diode-oscillator (TDO). An LC circuit is powered by a tunnel diode, which operates at a frequency of7MHz with a precision of2ppb. The ultra-low noise of the TDO cuircuit allows measurement of changes in the penetration depth with a precision of0.3A.By using this technique, together with some other measurements, we have studied the superconducting order parameters of several newly discovered materials, namely the filled skutterudites superconductors (SCs) PrPt4Ge12and LaPt4Ge12, the non-centrosymmetry SC BiPd and the intermetallic SC LaNiGa2. The main findings include:1. By means of measuring the London penetration depth and heat capacity, together with the μSR experiments, we have systematically investigated the superconducting order parameters of the filled skutterudite SCs PrPt4Ge1and LaPt4Ge12. The superfluid density ρs(T) and the electronic specific heat Ce (T)/T of PrPt4Ge12can be consistently described in terms of a two-gap BCS model. For LaPt4Ge12, the superfluid density ρsTDO derived from the London penetration depth λ(T), shows a tail near Tc and can be only fitted by a two-band y-model. On the other hand, the superfluid density ρsμSR from the TF-μSR measurements, performed at a field of75mT, can be fitted by using a single-band BCS model with a gap size close to the large gap derived from our penetration depth measurements. We argue that such a difference of the superfluid density might be a consequence of multiband superconductivity with weak interband coupling; the small gap is likely destroyed by a small field in the μSR measurements. These findings suggest that multiband superconductivity might generally exist in the filled skutterudite superconductors and the4f-electrons may increase the interband coupling. 2. BiPd is a newly discovered noncentrosymmetric superconductor with a monoclinic crystal structure at low temperatures. Our measurements of magnetic penetration depth shows pronounced anisotropic behavior. The in-plane penetration depth△λac (T) follows an exponential decrease, but the interplane penetration depth△λb(T) shows power-law-type behavior. The superfluid density ρs (T) is best fitted by an anisotropic two-band model. We argue that such a complex order parameter is attributed to the admixture of spin-singlet and spin-triplet pairing states as a result of antisymmetric spin-orbit coupling in BiPd.3. The recent μSR experiments showed evidence of time reversal symmetry (TRS) breaking in LaNiGa2while entering the superconducting state. Our results of magnetic penetration depth below0.3Tc can be nicely described by the BCS model, suggesting nodeless superconductivity for LaNiGa2. Detailed analysis shows that temperature dependence of the superfluid density ρs(T), electronic specific heat Ce/T and the upper critical field μ0Hc2(T) could be consistently fitted by a two band γ-model. These results provide strong evidence of multiband superconductivity for LaNiGa2. The multiband character provides a key to reconcile the nonunitary triplet pairing with a fully-gapped scenario which is needed for understanding the TRS breaking in this compound.