Figure 1

(Color online) Device A. (a) Bias spectroscopy at $T=0.3\text{K}$ of a SWCNT coupled to Ti/Nb/Ti leads. (b) Conductance within the dashed rectangle in (a). The onset of quasiparticle cotunneling is clearly seen at $e{V}_{\text{sd}}=\pm 2\Delta$ (horizontal green arrows). (c) Schematic energy diagram for a viable cotunneling process in the center of a diamond. (d) Bias cuts at positions (I,II) in (b) showing conductance peaks due to QP cotunneling, and sequential tunneling into GS and ES, respectively.Reuse & Permissions

Figure 2

(Color online) Device B. (a) Bias spectroscopy at $T=6.5\text{K}$, i.e., above $T_c\approx 1.7\text{K}$. Fourfold shell structure is observed with occupation numbers written in the large Coulomb diamonds corresponding to a full shell (zero occupation does not correspond to half filling). (b) Same gate region restricted to $\left|V_{\text{sd}}\right|\le 3\text{mV}$ at $T=0.3\text{K}$ showing inelastic cotunneling features (blue arrows) as well as the onset of elastic quasiparticle cotunneling (horizontal green arrows). Every fourth diamond shows pronounced gate-dependent conductance peaks near $\left|V_{\text{sd}}\right|=2\Delta /e$ (vertical green arrow).Reuse & Permissions

Figure 3

(Color online) Device B. (a) Detailed bias spectroscopy at $T=0.3\text{K}$ of one four-electron shell [dashed blue rectangle in Fig. 2b]. [(b)–(e)] Schematic energy diagrams illustrating inelastic cotunneling processes relevant for fillings of 1–4 electrons in a shell, e.g., corresponding to charge states 81–84. The thick blue (thin red) level represents the strongly (weakly) coupled orbital, which both are coupled more weakly to the right electrode (drain) shown by a thicker barrier.Reuse & Permissions

Figure 4

(Color online) Comparison of (a) measured and (b) calculated conductance versus $V_{\text{sd}}$ in the center of diamonds 81–84 in Fig. 3a. In the calculation $\delta =3\Delta$, $k_{B}T=0.03\text{meV}$ and addition energies (12.6, 12.6, 10.8, and 20.7) meV extracted from the width of the diamonds 81–84 have been used.Reuse & Permissions

Figure 5

(Color online) Device B. (a) Bias spectroscopy at $T=0.3\text{K}$ for a strongly coupled gate region with pronounced change in the subgap structure in every fourth diamond. Numbers indicate the additional electron number in correspondence with Fig. 2, while the large/small diamonds are assigned even (E)/odd (O) electron filling. The horizontal green arrows indicate the elastic quasiparticle cotunneling lines at $e{V}_{\text{sd}}=\pm 2\Delta \approx \pm 0.5\text{meV}$ together with the lower-lying lines corresponding to a single Andreev reflection. (b) Linear conductance (zero bias) versus gate voltage hinting that well and poorly coupled orbitals are filled consecutively. [(c) and (d)] Zoom at the gap structure around two Coulomb blockade resonances marked by arrows in (a) with the gate voltage in units of $\Delta$.Reuse & Permissions