Superconductivity in non-centrosymmetric ThNiSi
Introduction
Since the discovery of unconventional superconductivity in the heavy-fermion compound CePt3Si [1], superconductivity in intermetallics lacking an inversion symmetry in their crystal structure has continuously attracted considerable attention. The absence of inversion center results in the presence of electric field parallel to the unique crystal axis, which gives rise to antisymmetric spin orbit coupling (ASOC) that splits the Fermi surface and removes the spin degeneracy of electrons. Another inherent property of non-centrosymmetric (NCS) superconductors is mixing of spin-singlet and spin-triplet Cooper-pairing channels [2], as experimentally proved for CePt3Si [1] and a few other NCS materials, such as CeRhSi3 [3], CeIrSi3 [4] or UIr [5]. Remarkably, all these compounds exhibit strong electronic correlations, which hampers an extraction of features arising purely due to the NCS nature. To address the problem, a variety of weakly correlated materials has been investigated in recent years. While almost all of them have been characterized as conventional BCS superconductors with s-wave energy gaps [6], a line nodal gap structure has been inferred for LaNiC2 [7] and Li2Pt3B [8]. Most interestingly, for the former compound the effect of time reversal symmetry (TRS) breaking has been reported [9]. One should note however that a TRS breaking has been found also in Re6Zr [10], SrPtAs [11] and Lu5Rh6Sn18 [12], which are s-wave BCS superconductors.
In this paper we report on our study on a nonmagnetic superconductor ThNiSi. According to Zhong et al. [13], the compound crystallizes with the tetragonal LaPtSi-type structure (space group I41md), which lacks inversion symmetry along the fourfold axis, and exhibits superconductivity below 2 K. The presence of thorium is expected to enhance ASOC and hence rise a possibility to observe some unconventional character of the superconducting condensate. Thus, we decided to investigate the electronic ground state in ThNiSi in a systematic manner by means of magnetization, electrical resistivity and heat capacity measurements.
Section snippets
Experimental details
Polycrystalline sample of ThNiSi was synthesized by arc-melting stoichiometric amounts of the elemental constituents (purity: Th - 98 wt.%, Ni – 99.99 wt.%, Si – 99.999 wt.%) on a water cooled cooper hearth under titanium-gettered argon atmosphere. The button was turned over and remelted several times to promote homogeneity. Subsequently, the ingot was wrapped in molybdenum foil, sealed in evacuated quartz ampoule, and annealed at 1173 K for 14 days.
The crystal structure of the alloy was
Results and discussion
The powder XRD pattern of ThNiSi is presented in Fig. 1. It was indexed within a tetragonal unit cell of the LaPtSi-type (space group I41md) with the lattice parameters: a = 4.0697 (1) Å and c = 14.0891 (7) Å, in good agreement with the literature data [13]. The EDS analysis yielded the chemical composition Th34(2)Ni31(2)Si35(2) (at.%), which corresponds to a nearly stoichiometric equiatomic phase. No foreign phases were detected in the EDS data, however the XRD data revealed tiny amount (0.9%)
Conclusions
The silicide ThNiSi crystallizes with the noncentrosymetric tetragonal LaPtSi-type structure. Below Tc = 0.84 K, the compound exhibits weak-coupling type-II superconductivity with somewhat abnormal temperature dependence of the upper critical field. Though the normal state properties of ThNiSi are very similar to those of the isostructural material ThCoSi [18], the superconducting characteristics of these two ternaries are considerably different. Contrary to the Co-bearing phase, the
Acknowledgement
The authors thank Professor Piotr Wiśniewski for his kind assistance in our 3He magnetic susceptibility measurements.
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