Superconductivity in ${\mathrm{Cu}}_x{\mathrm{Bi}}_2{\mathrm{Se}}_3$ and its Implications for Pairing in the Undoped Topological Insulator

${\mathrm{Bi}}_2{\mathrm{Se}}_3$ is one of a handful of known topological insulators. Here we show that copper intercalation in the van der Waals gaps between the ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ layers, yielding an electron concentration of $\sim 2\times {10}^{20}{\mathrm{cm}}^{-3}$, results in superconductivity at 3.8 K in ${\mathrm{Cu}}_x{\mathrm{Bi}}_2{\mathrm{Se}}_3$ for $0.12\le x\le 0.15$. This demonstrates that Cooper pairing is possible in ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ at accessible temperatures, with implications for studying the physics of topological insulators and potential devices.

Figure 1

(a) The crystal structure of Cu intercalated ${\mathrm{Bi}}_2{\mathrm{Se}}_3$. (b) X-ray diffraction scan showing the $00L$ reflections from the basal plane of a cleaved ${\mathrm{Cu}}_{0.12}{\mathrm{Bi}}_2{\mathrm{Se}}_3$ single crystal with Si powder diffraction as a calibration. (c) The $⟨110⟩$ zone axis electron diffraction pattern for ${\mathrm{Cu}}_{0.12}{\mathrm{Bi}}_2{\mathrm{Se}}_3$. (d) High resolution electron microscope image of a representative area of ${\mathrm{Cu}}_{0.12}{\mathrm{Bi}}_2{\mathrm{Se}}_3$, showing the regular array of layers (labeled by atom type) and the absence of stacking defects on the nanoscale.

Figure 2

The temperature dependent magnetization of a single crystal of ${\mathrm{Cu}}_{0.12}{\mathrm{Bi}}_2{\mathrm{Se}}_3$ shows a superconducting transition $T_c\sim 3.8\mathrm{K}$ in zero field cooled (ZFC) and field cooled (FC) measurements. Upper inset: superconductivity occurs only in a narrow window of $x$ in ${\mathrm{Cu}}_x{\mathrm{Bi}}_2{\mathrm{Se}}_3$. Superconductivity is not found for $x<0.1$ and $x>0.3$, or in ${\mathrm{Bi}}_{2-x}{\mathrm{Cu}}_x{\mathrm{Se}}_3$, ${\mathrm{Cu}}_2\mathrm{Se}$, ${\mathrm{CuBi}}_3{\mathrm{Se}}_5$, ${\mathrm{BiCuSe}}_2$, and ${\mathrm{BiCu}}_3{\mathrm{Se}}_3$ (data labeled in inset as “${\mathrm{Bi}}_{1.9}{\mathrm{Cu}}_{0.1}{\mathrm{Se}}_3$ and others”). Lower inset: temperature dependence of the superconducting upper critical field, $H_{c2}(T)$ of ${\mathrm{Cu}}_{0.12}{\mathrm{Bi}}_2{\mathrm{Se}}_3$ for the magnetic field applied parallel to the $c$ axis and parallel to the $ab$ plane.

Figure 3

The resistivity of a ${\mathrm{Cu}}_{0.12}{\mathrm{Bi}}_2{\mathrm{Se}}_3$ crystal with applied current in the $ab$ plane. The lower inset shows that the superconducting transition occurs at $\sim 3.8\mathrm{K}$. The upper inset shows the magnetoresistance plot at $T=1.8\mathrm{K}$ for the field applied parallel to the $c$ axis. An upper critical field of ${\mu }_o{H}_C\sim 1\mathrm{T}$ is observed. The third inset shows the comparison of the Seebeck coefficients of ${\mathrm{Cu}}_x{\mathrm{Bi}}_2{\mathrm{Se}}_3$ and ${\mathrm{Bi}}_{2-x}{\mathrm{Cu}}_x{\mathrm{Se}}_3$.

Figure 4

STM topographic images over a 250 Å by 250 Å area showing the filled states at bias voltages of (a) $-1.0\mathrm{V}$ and (b) $-0.5\mathrm{V}$; and the unoccupied states at bias voltages of (c) $+0.5\mathrm{V}$ and (d) $+1.0\mathrm{V}$. Three types of topological features are identified, denoted as 1, 2, and 3 in the legend. Type 1 and type 2 features are identified as Cu on the cleaved surface and intercalated in van der Waals layers beneath the surface, respectively.