Structural and electronic inhomogeneity of superconducting Nb-doped Bi2Se3

Abstract

The crystal structure, electronic structure, and transport properties of crystals with the nominal composition Nb 0.25 Bi 2 Se 3 are investigated. X-ray diffraction reveals that the as-grown crystals display phase segregation and contain major contributions of BiSe and the superconducting misfit layer compound (BiSe) 1.1 NbSe 2 . The inhomogeneous character of the samples is also reflected in the electronic structure and transport properties of different single crystals. Angle-resolved photoemission spectroscopy (ARPES) reveals an electronic structure that resembles poor-quality Bi 2 Se 3 with an ill-defined topological surface state. High-quality topological surface states are instead observed when using a highly focused beam size, i.e., nanoARPES. While the superconducting transition temperature is found to vary between 2.5 and 3.5 K, the majority of the bulk single crystals does not exhibit a zero-resistance state suggesting filamentary superconductivity in the materials. Susceptibility measurements of the system together with the temperature dependence of the coherence length extracted from the upper critical field are consistent with conventional BCS superconductivity of a type II superconductor.

Local structure of Nb in superconducting Nb-doped Bi2Se3

“…Rather, it is found to primarily be located in the misfit phase. Our findings are in good agreement with recent reports linking the origin of superconductivity to the presence of the misfit phase [30,31].…”

“…Despite the lack of clarity on the origin of the reduced amplitude of the EXAFS oscillations, the sum of the evidence suggests a strong tendency toward the formation of the misfit phase rather than incorporation of Nb dopants into the Bi 2 Se 3 host structure. In a recent study [30], we estimated that the total phase content across a crystal boule (from which S1 originates) fits approximately with the assumption that all or most of the Nb in the synthesis appear in the misfit phase. We also found that all superconducting samples contained this phase.…”

“…Three crystal boules of Nb-doped Bi 2 Se 3 were grown from the pure elements Bi (99.999%), Se (99.999%), and Nb (99.99%) through a melt-growth method described in detail elsewhere [30]. Two of these (C1, C3) were synthesized with nominal stoichiometry Nb x Bi 2 Se 3 and one (C2) with Nb x Bi 2−x Se 3 , where x = 0.25 for all samples.…”

“…We recently showed that the resulting crystal boules have inhomogeneous composition with multiple phases present: Bi 2 Se 3 , trigonal BiSe, and the misfit phase. Furthermore, the phase content and physical properties vary greatly across the crystal boule [30]. The significant phase segregation present in Nb x Bi 2 Se 3 crystal boules imposes a challenge for identifying the structural origin of the superconductivity.…”

Relevance of sample geometry on the in-plane anisotropy of SrxBi2Se3 superconductor

“…However, to which extent the observed two-fold symmetric superconductivity is originated from an intrinsic topological nature still re- * mingquan.he@cqu.edu.cn mains elusive. Other factors, such as built-in strain or distortion [17,18], structural inhomogeneity and impurity phases [19][20][21][22], inhomogeneously distributed superconducting regions [4,16,[23][24][25], cannot be completely ruled out at this stage. Exclusion of possible extrinsic origins is of great importance to clarify the putative topological nature of the nematic superconductivity found in doped Bi 2 Se 3 systems.…”