11B and 195Pt NMR study of the superconductors Li2(Pd1−xPtx)3B without inversion symmetry

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Abstract

We report the 11B and 195Pt NMR measurements in non-centrosymmetric superconductors Li2(Pd1−xPtx)3B (x = 0.0, 0.2, 0.5, 1.0). From the measurements of spin–lattice relaxation time (T1), we found that there was a coherence peak (CP) just below superconducting transition temperature (Tc) for x = 0–0.5 but no CP in x = 1. We demonstrated that the system for x = 0–0.5 were BCS superconductors but there existed line node in the superconducting gap for x = 1.0. The 195Pt Knight Shift in x = 0.2 decreased below Tc, indicating spin-singlet state. The results showed that BCS superconducting state evolves into an exotic state with line-nodes in the gap function when x is increased, as the spin–orbit coupling is enhanced.

Introduction

In the superconducting materials with an inversion center in the crystal structure, the Cooper pairs are either in the spin-singlet state or in the spin-triplet state because of parity conservation. If the structure lacks the inversion symmetry, a mixing of spin-singlet and spin-triplet states becomes possible [1], [2]. The extent of the mixing depends on the strength of the spin–orbit coupling [2].

After the discovery of the non-centrosymmetric compound CePt3Si [3], many superconductors without inversion symmetry have been found. Among them, Li2M3B (M = Pd, Pt) [4], [5] are weakly electron-correlated and are particularly suitable for studying the effects of inversion symmetry breaking. Here we report the properties of these compounds studied by NMR.

The crystal structure of Li2(Pd1−xPtx)3B (cubic, P4332) is shown in Fig. 1. It consists of distorted octahedral units which are formed from BM6 (M = Pd, Pt). It was found that Tc decreases smoothly from ∼8 K to ∼2.8 K with increasing x and the crystal structure is almost unchanged except slight decrease in lattice constant [5]. 11B spin–lattice relaxation rate (1/T1) in Li2Pd3B shows a coherence peak just below Tc and decreases exponentially with further decreasing temperature, which is a characteristic of BCS superconductors. Also, the Knight Shift changes below Tc [6]. In the case of Li2Pt3B, 1/T1 shows no coherence peak and decreases with T3 which is a characteristic of superconductors with line-nodes in the gap function. Furthermore the Knight Shift does not change across Tc. From these experiments, it was concluded that the spin-singlet state is dominant in Li2Pd3B but the spin-triplet state is dominant in Li2Pt3B [7]. In this paper we study the evolution of the superconducting gap in Li2(Pd1−xPtx)3B by the measurements of T1 and also Knight Shift.

Section snippets

Experimental result

Polycrystal samples of Li2(Pd1−xPtx)3B (x = 0.2, 0.5, 1.0) were prepared by the two-step arc melting method [4], [5]. For NMR measurements, the polycrystalline samples were crushed into powder. Tc at zero magnetic field was obtained from the temperature dependence of the self-inductance of the NMR coil. The NMR measurements were conducted at low magnetic fields to prevent the reduction in Tc by the magnetic field. The NMR spectrum were obtained by the fast Fourier transform (FFT) of the spin echo.

Conclusion

In conclusion, we have presented the 11B and 195Pt NMR results in the alloyed Li2(Pd1−xPtx)3B system. For 1/T1, it decreases exponentially below Tc with a coherence peak for x = 0.0–0.5, which indicates isotropic gap. In contrast, 1/T1 decreases in proportion to T3 below Tc with no coherence peak in x = 1.0, indicating anisotropic gap with line-nodes in the gap function. The Knight Shift measured by 195Pt NMR, it deceases below Tc for x = 0.2, which indicates that the Cooper pairs are in the

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