Superconductivity in YbxMyHfNCl (M=NH3and

Abstract

We report the observation of superconductivity in rare-earth metal cointercalated compounds Yb x (M) y HfNCl with M = NH 3 and tetrahydrofuran (THF). The superconducting transition temperature is about 23 and 24.6 K for Yb 0.2 (NH 3 ) y HfNCl and Yb 0.3 (NH 3 ) y HfNCl, respectively. Replacing the NH 3 with a larger molecule THF, the superconducting transition temperature increases to 25.2 K in Yb 0.2 (THF) y HfNCl, which is almost the same as the highest T c reported in the alkali-metal intercalated HfNCl superconductors. The T c of Yb 0.2 (THF) y HfNCl is apparently suppressed by pressure up to 0.5 GPa, while the pressure effect on T c becomes very small above 0.5 GPa. Our results suggest that for the most part, the superconductivity in these layered intercalated superconductors does not rely on intercalated metal ions, even magnetic ions. PACS number(s): 74.62.Fj High-T c superconductivity has been observed in layered cuprates and recently discovered iron-based superconductors. 1-3 The proximity to magnetically ordered states for these systems suggests that the magnetic interactions are the crucial force for Cooper pairing in such high-T c superconductivity, which gives rise to the unconventional nature of the superconductivity in these systems. Superconductivity in various other exotic materials, such as Na x CoO 2 , Sr 2 RuO 4 , and heavy-fermion systems, was also found to be closely connected to magnetism. 4-6 However, for another type of superconducting material of the layered metallonitride halides (MNX, M = Ti, Zr, Hf; X = Cl, Br, I) with the maximum T c as high as 25.5 K, 7 their parent compounds are band insulators, and superconductivity seems to have no correlation with magnetism. 8 There exist two types of layered nitride compounds: one is the (FeOCl)-type structure (so-called α structure) with a two-dimensional (2D) metal-nitrogen (MN) layer of a rectangular lattice; the other is the (SmSI)-type structure (so-called β structure) with a 2D MN layer of a honeycomb lattice. 9 For the former, K x TiNCl was reported to display superconductivity with T c = 16 K. 10 For the latter, usually with M = Hf,Zr and X = Cl, a maximum of T c = 25.5 K has been achieved in Li x (THF) y HfNCl. 7 The parent compounds of the latter, so-called β-MNCl, consist of an alternative stacking of honeycomb MN bilayers sandwiched by Cl bilayers. 11 Superconductivity is usually induced through doping charge carriers by means of alkali-metal intercalation or producing the Cl deficiency. 12,13 Unlike the large pressure effect on T c observed in cuprates or iron-based superconductors, the T c in this type of superconductors decreases slightly as the pressure increases. 14,15 However, for cointercalated β-ZrNCl and β-HfNCl, the interlayer spacing would strongly affect the superconducting transition temperature. An increase of the basal spacing would lead to a reduction of the negligible warping along the K z direction, and thus to an increase in the nesting of the Fermi surface. 16 It is assumed that a modification of the Fermi su...

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