Improved Single-Crystal Growth of Sr2RuO4

Abstract

High-quality single crystals are essentially needed for the investigation of the novel bulk properties of unconventional superconductors. The availability of such crystals grown by the floating-zone method has helped to unveil the unconventional superconductivity of the layered perovskite Sr 2 RuO 4 , which is considered as a strong candidate of a topological spin-triplet superconductor. Yet, recent progress of investigations urges further efforts to obtain ultimately high-quality crystalline samples. In this paper, we focus on the method of preparation of feed rods for the floating-zone melting and report on the improvements of the crystal growth. We present details of the improved methods used to obtain crystals with superconducting transition temperatures T c that are consistently as high as 1.4 K, as well as the properties of these crystals.Despite the key experimental results supporting spin-triplet pairing [19][20][21][22][23], such a first-order transition is difficult to explain within the context of spin-triplet superconductivity. This first-order transition becomes second order when the T c is suppressed below 1.45 K, which corresponds to an impurity level of~50 ppm [10]. Thus, pristine samples with impurity levels less than 10 ppm are required in order to deepen our knowledge of the superconducting state of Sr 2 RuO 4 .Recent innovations in the design of uniaxial-stress cells enables the T c of Sr 2 RuO 4 to be enhanced up to 3.5 K when the stress is along the crystalline [100] direction [24][25][26][27]. The origin of this enhancement of T c is attributed to the Fermi-level crossing of the van-Hove singularity in one of the three quasi-two-dimensional Fermi surfaces. It is hoped that detailed investigations of this phenomenon will lead to the clarification of the superconducting symmetry and mechanism in Sr 2 RuO 4 . The T c enhancement has actually been known for many years in the eutectic crystals of Sr 2 RuO 4 with micron-size metallic Ru platelets, which introduce strong strains in Sr 2 RuO 4 near the interfaces [28][29][30]. In order to investigate the strain-induced superconducting phase, high-quality Ru-inclusion free single crystals of Sr 2 RuO 4 with a specific in-plane crystalline direction are in demand.In this study, we examine how we can further improve the quality of crystals of Sr 2 RuO 4 grown by the floating-zone method with an infrared image furnace. The floating-zone technique has been used to produce high-quality crystals of relatively large size useful for most experimental purposes including inelastic neutron scattering. Because the technique is essentially crucible-free, it can be used to achieve the minimum possible impurity levels. The floating-zone technique has been used for the successful crystal growth of Sr 2 RuO 4 [31], as well as other ruthenates [32][33][34][35][36][37]. Previous reports mainly describe optimization of the atomic compositions of the feed rod and various parameters of the final growth process. On the other hand, there are a number of processes (from pow...

Split superconducting and time-reversal symmetry-breaking transitions in Sr2RuO4 under stress

“…Samples and sample holder for uniaxial stress. Single crystals of Sr 2 RuO 4 were grown by a floating zone method 58 . Data from six samples, labelled A-F, are reported; samples E and F were cut from sample C after the measurement of sample C. Sample A approximately grew along the 〈110〉 lattice direction, and the remaining samples along the 〈100〉 direction; it was necessary to select samples that approximately grew along the 〈100〉 direction to obtain samples of sufficient length along this direction for mounting in the uniaxial stress apparatus.…”

High-sensitivity heat-capacity measurements on Sr 2 RuO 4 under uniaxial pressure

“…High-quality single-crystal Sr 2 RuO 4 samples were grown in a floating zone furnace (Canon Machinery) using techniques refined over many years to those described recently in ref. 41. They were aligned using a bespoke Laue X-ray camera and cut using a wire saw into thin bars with whose long axis aligned with the [100] direction of the crystal.…”

A Review of Electronic Transport in Superconducting Sr2RuO4 Junctions

“…It is challenging to obtain pure SRO214 single crystals as impurity phases form as embedded inclusions during growth, including ferromagnetic SrRuO 3 (SRO113, n = ∞; T curie of 160 K), metamagnetic Sr 3 Ru 2 O 7 (SRO327, n = 2; T curie ≈ 100 K under 1 T magnetic field) and pure Ru metal. The floating zone technique results in high-quality SRO214 single crystals [49,50].…”