High Fermi-level spin polarization in the $({\mathrm{Bi}}_{1-x}{\mathrm{Sb}}_x){}_2{\mathrm{Te}}_3$ family of topological insulators: A point contact Andreev reflection study

Point contact Andreev reflection spectroscopy is employed to extract the effective Fermi-level spin polarization of three distinct compositions from the (${\mathrm{Bi}}_{1-x}{\mathrm{Sb}}_x$)${}_2{\mathrm{Te}}_3$ topological insulator family. The end members, ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ and ${\mathrm{Sb}}_2{\mathrm{Te}}_3$, exhibit high polarization of 70(4)% and 57(3)%, respectively. High-field $({\mu }_{0}H=14\mathrm{T})$ point contact spectroscopy shows carrier depletion close to the Fermi level for these two compositions with small activation gaps of 0.40(4) and 0.28(2) meV, respectively. The almost fully suppressed bulk conductivity in the (${\mathrm{Bi}}_{0.18}{\mathrm{Sb}}_{0.82}$)${}_2{\mathrm{Te}}_3$ results in an even higher spin polarization of 83(9)%. Further, it is demonstrated that magnetic doping with Cr and V tends to reduce the spin-polarization values with respect to the ones of the pure compositions. ${\mathrm{Bi}}_{1.97}{\mathrm{Cr}}_{0.03}{\mathrm{Te}}_3, {\mathrm{Sb}}_{1.975}{\mathrm{Cr}}_{0.025}{\mathrm{Te}}_3, {\mathrm{Bi}}_{1.975}{\mathrm{V}}_{0.025}{\mathrm{Te}}_3$, and ${\mathrm{Sb}}_{1.97}{\mathrm{V}}_{0.03}{\mathrm{Te}}_3$ exhibit spin polarization of 52%, 52%, 58%, and 50%, respectively. In view of the rather high effective polarization, nonmagnetic topological insulators close to (${\mathrm{Bi}}_{0.18}{\mathrm{Sb}}_{0.82}$)${}_2{\mathrm{Te}}_3$ may provide a path towards the characterization of pair-breaking mechanisms in spin-triplet superconductors.

Figure 1

PCAR spectra, fits, and extracted parameters for the three different compositions. (a) ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ with $P=70\left(4\right)\%$, (b) ${\mathrm{Sb}}_2{\mathrm{Te}}_3$ with $P=57\left(3\right)\%$, and (c) (${\mathrm{Bi}}_{0.18}{\mathrm{S}}_{0.82}$)${}_2{\mathrm{Te}}_3$ with $P=83\left(9\right)\%$. The model-extracted parameters are indicated with asterisks. (d) The electron injection process in PCAR. The electron spins of the TI surface states are in plane locked perpendicular to the momentum. Cooper pairs (CP) with out-of-plane spin components are not allowed to be injected elastically in the TI surface states due to momentum conservation.

Figure 2

Temperature scans of ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ and ${\mathrm{Sb}}_2{\mathrm{Te}}_3$. (a) PCAR spectra of ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ between 2.0 and 10.0 K, (b) PCAR spectra of ${\mathrm{Sb}}_2{\mathrm{Te}}_3$ between 2.0 and 10.0 K, (c) PCS of ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ in magnetic field of ${\mu }_{0}H=14\mathrm{T}$, and (d) PCS of ${\mathrm{Sb}}_2{\mathrm{Te}}_3$ in a magnetic field of ${\mu }_{0}H=14\mathrm{T}$, where ${\mathrm{\Delta }}_2=1.5\mathrm{meV}$ is the bulk superconducting gap of Nb and $q$ is the elementary charge.

Figure 3

Temperature evolution of the ZBA of PCAR (top panels) and PCS (bottom panels) along with extracted critical exponents $\gamma$ of the superconducting transition and the bulk band gap $E_{\mathrm{g}}$. (a) ZBA of the PCAR temperature scan in zero magnetic field of ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ with $\gamma =0.91\left(2\right)$, (b) ZBA of the PCAR temperature scan of ${\mathrm{Sb}}_2{\mathrm{Te}}_3$ in zero magnetic field with $\gamma =0.71\left(2\right)$, (c) ZBA of the PCS temperature scan of ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ in ${\mu }_{0}H=14\mathrm{T}$ with $E_{\mathrm{g}}=0.40\left(4\right)\mathrm{meV}$, and (d) ZBA of the PCS temperature scan of ${\mathrm{Sb}}_2{\mathrm{Te}}_3$ in ${\mu }_{0}H=14\mathrm{T}$ with $E_{\mathrm{g}}=0.28\left(2\right)\mathrm{meV}$. The presented ZBA plots are extracted from the ZBA temperature dependence in Fig. 2.

Figure 4

Fitted PCAR data along with the extracted parameters of four magnetically doped TI compositions. (a) ${\mathrm{Bi}}_{1.97}{\mathrm{Cr}}_{0.03}{\mathrm{Te}}_3$, (b) ${\mathrm{Sb}}_{1.975}{\mathrm{Cr}}_{0.025}{\mathrm{Te}}_3$, (c) ${\mathrm{Bi}}_{1.975}{\mathrm{V}}_{0.025}{\mathrm{Te}}_3$, (d) ${\mathrm{Sb}}_{1.97}{\mathrm{V}}_{0.03}{\mathrm{Te}}_3$, and (e) AHE temperature scan of ${\mathrm{Sb}}_{1.97}{\mathrm{V}}_{0.03}{\mathrm{Te}}_3$ with the extracted Curie temperature of $T_{\mathrm{C}}=11.6\left(6\right)\mathrm{K}$. (f) Example of a covariant matrix on the fit in (a) and the vector of the parameters errors.

Figure 5

Magnetic field scan of a ${\mathrm{Bi}}_2{\mathrm{Te}}_3$/Nb point contact at $2.0\mathrm{K}$. (a) The low-field scan up to ${\mu }_{0}H=1.0\mathrm{T}$ and (b) the high-field scan from $1\mathrm{T}$ up to ${\mu }_{0}H=14.0\mathrm{T}$. ${\mathrm{\Delta }}_2=1.5\mathrm{meV}$ is the bulk superconducting gap of Nb, and $q$ is the elementary charge.