Tunnel spectroscopy of a proximity Josephson junction

We present tunnel spectroscopy experiments on the proximity effect in lateral superconductor–normal-metal–superconductor Josephson junctions. Our weak link is embedded into a superconducting ring allowing phase biasing of the Josephson junction by an external magnetic field. We explore the temperature and phase dependence of both the induced minigap and the modification of the density of states in the normal metal. Our results agree with a model based on the quasiclassical theory in the diffusive limit. The device presents an advanced version of the superconducting quantum interference proximity transistor, now reaching flux sensitivities of 3 nA$/{\Phi }_0$, where ${\Phi }_0$ is the flux quantum.

Figure 1

Schematic view of the experimental setup: a superconducting Al loop with an area of $100\mu$m${}^2$ (blue) is interrupted using a weak link made out of copper (magenta). One superconducting tunnel probe is attached to the middle of the weak link. The scaleup image on the right depicts a scanning electron micrograph of the sample core showing the Al electrode (of width $\sim 100$ nm) connected via a tunnel junction to the Cu wire as well as the Al/Cu/Al superconductor–normal-metal–superconductor proximity junction with a length $L=300$ nm. $\Phi$ symbolizes the applied magnetic flux threading the loop. Our basic electrical setup consists of a voltage bias and current measurement using a preamplifier at room temperature.

Figure 2

(a) Sketch of the density of states of a superconductor (Al, left of the tunnel junction) and proximimized Cu island (right of the tunnel junction) at zero bias (left) and at a bias voltage of $({\Delta }_1-{\Delta }_2)/e$ and the occupation at elevated temperature of$T\approx 1/3T_{\mathrm{C}}$ of Al. Blue areas symbolize occupied states. (b) Measured differential conductance vs voltage curves at three different bath temperatures. Arrows indicate the position of ${\Delta }_1-{\Delta }_2$. (c) Measured aluminum gap (blue stars) and induced minigap (orange triangles) as a function of bath temperature $T$. The BCS gap temperature dependence on aluminum is shown by the full line calculated using the measured gap value. The dashed line follows the same result scaled down by the ratio of the two gaps (${\Delta }_2/{\Delta }_1$).

Figure 3

(a),(c),(e) Differential conductance and (b),(d),(f) current as a function of applied bias voltage. Measurements (c),(d) are compared to the theoretical models (a),(b) and (e),(f). The electron micrograph depicts the investigated superconductor–normal-metal–superconductor junction with the attached tunnel probe. Top panels: calculated current vs voltage curve (b) assuming ${\Delta }_1\left(0\right)=220\mu \mathrm{eV}$ and a weak link length $L=2{\xi }_0$ and the corresponding conductance curve (a) at two extreme flux values where the minigap is maximized ($\Phi =0$, green) and fully closed ($\Phi ={\Phi }_0/2$, red). Middle panels: measured currents at a bath temperature of 585 mK (d) and conductances (c) calculated as a numerical derivative of the current as a function of applied voltage for the same flux values as above. Bottom panels: Current vs voltage curve (f) and corresponding conductance (e) calculated including the influence of an electromagnetic environment at $\Phi =0$ with $T_{\text{env}}=4.2$ K (dashed blue line) and $T_{\text{env}}=T=585$ mK (green line).

Figure 4

SQUIPT. (a) Current through the device at two different voltage bias ($V_{\mathrm{BIAS}}$) points as a function of magnetic flux through the ring. Curves corresponding to different temperatures are vertically offset for clarity by current values indicated with the arrows. (b) Contour plot of $\partial I/\partial \Phi$ measured with a lockin technique using a field modulation as a function of voltage bias and external flux. Color scale is from $-3\mathrm{nA}/{\Phi }_0$ (blue) to 3 $\mathrm{nA}/{\Phi }_0$ (red) with a step of 0.3 $\mathrm{nA}/{\Phi }_0$. (c) Vertical line in (b) corresponds to $V_{\mathrm{BIAS}}=0.32$ mV plotted here and shows the SQUIPT sensitivity $\partial I/\partial \Phi$ at this biasing point as a function of $\Phi$.