Superconductivity in the Topological Nodal-line Semimetal NaAlSi

Abstract

NaAlSi is an sp electron superconductor crystallizing in a layered structure of the anti-PbFCl type with a relatively high transition temperature Tc of ~7 K. Recent electronic state calculations revealed the presence of topological nodal lines in the semimetallic band structure, which attracted much attention owing to the superconductivity. However, experimental investigation remained limited because of the lack of single crystals. Here, we successfully prepared single crystals of NaAlSi by a Na-Ga flux method and characterized their superconducting and normal-state properties through electrical resistivity, magnetization, and heat capacity measurements. A sharp superconducting transition with a Tc of 6.8 K is clearly observed, and heat capacity data suggest an anisotropic superconducting gap. Surprisingly, despite the sp electron system, the normal state is governed by the electron correlations, which is indicated by a T 2 resistivity and a Wilson ratio of 2.0. The origin of the electron correlation may be related to the orthogonal saddleshaped Fermi surfaces derived from the Si px and py states, which intersect with the light Al s bands to form the nodal lines near the Fermi level. These results strongly suggest that the superconductivity of NaAlSi is not caused by a simple phonon mechanism but involves a certain unconventional aspect, although its relevance to the nodal lines is unclear.

Pseudogap Formation in the Nodal-Line Semimetal NaAlGe

“…Single crystals of NaAlGe were grown using the Na-Ga flux method, as previously described. 20,22) A mixture of Na : Al : Ge : Ga = 3 : 1 : 1 : 0.5 was placed in a boron nitride crucible under an argon atmosphere and sealed in a stainless-steel reaction container. The container was heated to 1073 K in an electric furnace and then slowly cooled to 823 K for 80-100 hours.…”

“…4. NaAlSi exhibited a high degree of anisotropy in its : 20) c (B || c) was significantly less than a (B || a) due to the large contribution of Landau orbital diamagnetism, which was estimated to be -2.81 × 10 -5 cm 3 mol -1 at 300 K as the difference between c and a. Thus, in Fig.…”

“…As with NaAlSi, 20) the Hall resistivity xy is proportional to the applied magnetic field along the c axis over the entire temperature range. This suggests that light electrons dominate the transport, while heavy holes play a secondary role.…”

“…The average chemical composition of five crystals, as determined by the WDX spectroscopy, was Na1.13Al0.97Ga0.01Ge0.89, indicating that approximately 10% excess Na was substituted for Ge and ~1% Ga remained at the Al site from the flux; for NaAlSi and Si-rich solid solutions, obtained crystals were nearly stoichiometric with 1-2% Ga contamination. 20,22) However, an XRD structural refinement based on the assumption of a mixture of Na and Ge atoms at the Ge site revealed that there was no such substitution of Na for Ge: the occupancy of the Ge atom was 1.002 (3). This indicates that a nearly stoichiometric sample was obtained.…”

“…11,12) The two bands cross to generate complex nodal-line structures near the Fermi level in the absence of spin-orbit interactions (SOI). [12][13][14][15] NaAlSi exhibits superconductivity below Tc = 7 K. 16) The superconductivity is most likely of the conventional s-wave type based on phonon-mediated Cooper pairing; 14,[17][18][19][20] However, recent study utilizing single crystals demonstrated that fractional superconductivity could exist at magnetic fields greater than the bulk upper critical field, which was suggested to originate from surface states. 21) Despite their similar electronic structures, NaAlGe was reported to be a nonsuperconductor above 2 K, 16) which was recently confirmed by resistivity and magnetic susceptibility measurements.…”