1. Superconductivity in CuxBi2Se3 and its Implications for the Undoped Topological Insulator
Garrett Vanacore, Sean Vig, Xiaoxiao Wang, Jiang Wang, University of Illinois at Urbana-Champaign
Structural Analysis Results
Overview
Single crystal is chemically single phase.
X-ray diffraction indicates excellent long-range crystal quality. HREM shows no signs of stacking faults, intergrowths, or amorphous regions, indicating good quality on the nanoscale, though there is no ordering of Cu in intersitial sites
It has been theorized that topological insulators (TI) could host exotic quasiparticles (anyons) that may significantly advance experimental quantum computation.
To create these anyons, superconductivity must be induced in the surface (conducting) states of the TI, though this feat has yet to be experimentally realized.
Hor et al. have observed superconductivity in copper-doped Bi2Se3, and hope this may lead to induced superconductivity in undoped Bi2Se3.
Superconducting Characterization Results
Magnetic characterization showed only CuxBi2Se3 becomes superconducting above 1.8 K, no other Cu-Bi-Se system shows superconductivity. Superconductivity is achieved in the doping range 0.10<x<0.30, with optimal single crystals between x=0.12 and x=0.15 (see FIG 2).
Field cooled and zero field cooled magnetization measurements in Cu0.12Bi2Se3 show an onset of superconductivity at 3.8 K with the zero field cooled measurement reaching about 20% of full diamagnetism at 1.8 K (see FIG 3).
Given the magnetization results, resistivity measurements are performed with CuxBi2Se3 with doping parameter x=0.12 and show a superconducting transition at 3.8 K (see FIG 4)
FIG 2:Magnetization of various Cu-Bi-Se systems as a function of temperature. Note only CuxBi2Se3 systems show a drop in magnetization indicating a superconducting transition.
Crystal Growth and Structural Analysis Methods
CuxBi2Se3 is grown by melting Cu, Bi, and Se at 850°C, slow cooling to 650°C and quenching in cold water.
Structural properties are determined through an ensemble of X-ray powder diffraction, high resolution electron microscopy (HREM), and ultra-high vacuum low-temperature scanning tunneling microscopy.
Structural tests are pivotal for differentiating between CuxBi2Se3 (see FIG 1) and an alternate structure created by doping, Bi2-xCuxSe3.
FIG 1: The crystal structure of CuxBi2Se3. Bi2Se3 is formed from double-layers of Bi2Se6 octahedra. Cu may either intercalate between Se layers - giving CuxBi2Se3 (shown below) - or substitute for Bi atoms - giving Cu-intercalated Bi2-xCuxSe3.
Conclusions and Future Work
This is the first observation of superconductivity in a material that is chemically similar to a TI, and the close chemical similarity may allow it to be used to induce superconductivity in an undoped TI.
The complexity of particle states in CuxBi2Se3 is not well understood; e.g. it is not clear where Cooper pairing arises in the material. This will be a focus of future research.
Superconducting Characterization
Characterization is performed on Cu-intercalcated CuxBi2Se3 for various values of the doping parameter, x, in addition to Cu-substituted Bi2-xCuxSe3 and other Cu-Bi-Se systems.
Both AC and DC magnetization is measured, the AC measurements with a proprietary Physical Property Measurement System (PPMS), the DC measurements with a superconducting quantum interference device. Resistivity is measured using a standard four-probe technique, with currents applied in the basal plane.
FIG 4. Resistivity of Cu0.12Bi2Se3 measured parallel to the basal plane, showing a superconducting transition at 3.8 K.
FIG 3: Zero field cooled and field cooled magnetization measurements on Cu0.12Bi2Se3 as a function of temperature. This shows a superconducting transition at 3.8 K.
Acknowledgments
Professor Lance Cooper, Celia Elliott, and Y.S. Hor et al.
Source: Superconductivity in CuxBi2Se3 and its Implications for Pairing in the Undoped Topological Insulator, Y.S. Hor et al., Phys. Rev. Lett. 104, (2010)