Signatures of Andreev Blockade in a Double Quantum Dot Coupled to a Superconductor

We investigate an electron transport blockade regime in which a spin triplet localized in the path of current is forbidden from entering a spin-singlet superconductor. To stabilize the triplet, a double quantum dot is created electrostatically near a superconducting Al lead in an InAs nanowire. The quantum dot closest to the normal lead exhibits Coulomb diamonds, and the dot closest to the superconducting lead exhibits Andreev bound states and an induced gap. The experimental observations compare favorably to a theoretical model of Andreev blockade, named so because the triplet double dot configuration suppresses Andreev reflections. Observed leakage currents can be accounted for by finite temperature. We observe the predicted quadruple level degeneracy points of high current and a periodic conductance pattern controlled by the occupation of the normal dot. Even-odd transport asymmetry is lifted with increased temperature and magnetic field. This blockade phenomenon can be used to study spin structure of superconductors. It may also find utility in quantum computing devices that use Andreev or Majorana states.

Figure 1

(a) Schematic of AB. The blockaded configuration is indicated with a red cross showing how a spin triplet in the DQD is prevented from forming a spin-singlet Cooper pair. (b) SEM image of a device similar to the one studied in the main text. Section marked “$\mathrm{Al}/\mathrm{InAs}$” is an InAs nanowire covered by an Al shell. A section where the shell is etched is marked “InAs.” Vertical lines mark gate electrodes used in creating the DQD; other visible gates are floating.

Figure 2

Differential conductance spectra for (a),(b) ${\mathrm{QD}}_S$ and (c),(d) ${\mathrm{QD}}_N$. Spectra are taken by fixing one dot at a degenerate state while tuning the other dot with the ($V_S$, $V_N$) combination. Voltage combinations are indicated in Fig. 3. Large arrows and small arrows indicate resonance peaks with different amplitudes.

Figure 3

(a),(b) Experimental and (c),(d) simulated results of a “unit” stability diagram with $2\times 2$ quadruple DPs. Parities in ${\mathrm{QD}}_N$ and ${\mathrm{QD}}_S$ are labeled on top and right axes in panel (a) (E—even, O—odd). Dashed lines indicate traces along which spectra in Fig. 2 are taken. The bias is indicated in white. Parameters for simulation (in a.u., which match system energies in meV for convenience): source-drain bias ${\mu }_S-{\mu }_N=-0.1$ in (c), 0.1 in (d), charging energy $U_N=4$, $U_S=0.7$, interdot charging energy $U_{NS}=0.01$, induced gap ${\mathrm{\Delta }}_S=0.2$, temperature $T=0.02$.

Figure 4

Stability diagrams in a larger $(V_S,V_N)$ parameter space. The dashed rectangle in panel (a) encloses the region studied in Figs. 2, 3, and 5. Parities in ${\mathrm{QD}}_N$ (${\mathrm{QD}}_S$) are labeled on top (right) axis. Blue (white) arrows indicate columns of conductance triangles with low (high) current. Bias is indicated in white.

Figure 5

Stability diagrams at different biases and fields (a)–(d) $B=0$ and (e)–(h) 0.6 T. The bias is indicated in black. The magnetic field is in the sample plane at a 24° angle with the nanowire.