Singlet-triplet superconducting quantum magnetometer

Motivated by the recent experimental realization of a quantum interference transistor based on the superconducting proximity effect, we here demonstrate that the inclusion of a textured ferromagnet both strongly enhances the flux sensitivity of such a device and additionally allows for singlet-triplet switching by tuning a bias voltage. This functionality makes explicit use of the induced spin-triplet correlations due to the magnetic texture. Whereas the existence of such triplet correlations is well known, our finding demonstrates how spin-triplet superconductivity may be utilized for concrete technology, namely, to improve the functionality of ultrasensitive magnetometers.

Figure 1

Schematic setup of the proposed SQM. An inhomogeneous FM and a normal-metal (N) nanowire each of length $d$ are connected in parallel with an insulating layer separating them. This structure is sandwiched between two bulk $s$-wave superconductors along the $x$ axis. A third S electrode of width $W$ is deposited on top of the nanowires along the $y$ direction. By applying a voltage $V$, a quasiparticle current flows between the electrode and nano-wires (we refer to the third superconducting electrode as the “collector electrode”). The bulk S are assumed to be part of a closed loop circuit threaded by an external flux, so that any supercurrent flowing into the collector electrode can be neglected. The external flux induces a macroscopic phase difference $\varphi$ between the bulk S.

Figure 2

Top row (a)–(c): DOS vs quasiparticle energy $\epsilon$ for several phase differences $\varphi$. The columns correspond to (a) a S/N/S junction, (b) a S/uniform FM/S junction, and (c) a S/conical FM/S junction. The DOS has been averaged over the collector electrode width $W=d/2$ and $d=0.5{\xi }_S$. Bottom row (a)–(c): the device sensitivity $d{I}_{qp}/d\varphi$ vs phase difference $\varphi$ for different choices of bias voltage $eV/{\Delta }_0=\{1.4,1.5,1.6\}$ with $d=0.3{\xi }_S$. In the column (d), we show the sensitivity for our proposed SQM device which incorporates both an N and conical FM nanowire (see Fig. 1).

Figure 3

Imaginary and real parts of spin-triplet superconducting correlations averaged over the junction length plotted vs $\varphi$ in the ballistic regime where a conical FM is sandwiched between two SC banks. (a) $d$$=$$0.3{\xi }_S$ and (b) $d$$=$$0.5{\xi }_S$.