Single-enantiomer spin polarisers in superconducting junctions

Abstract

Chiral matter acting as a spin-selective device in biased electron transport is attracting attention for the quantum-technological design of miniaturized electronics. To date, however, experimental reports on spin selectivity are not conclusive. The magnetoresistance in electron transport measurements observed for chiral materials on ferromagnets upon magnetisation reversal is proposed to result from electrostatic rather than from the sought-after chiral effects. Recent break junction studies even question the spin-dependent electron flow across single chiral molecules. Here, we avoid ferromagnetic electrodes and magnetisation reversal to provide unambiguous experimental evidence for the chirality-induced spin selectivity effect of single enantiomers. Functionalising the superconducting tip of a scanning tunnelling microscope with a manganese atom cluster gives rise to Yu-Shiba-Rusinov resonances that serve as spin-sensitive probes of the tunnelling current in junctions of single heptahelicene molecules adsorbed on a crystalline lead surface. Our key finding is the dependence of the signal strength of these states in spectroscopy of the differential conductance on the handedness of the molecule. The experiments unveil the role of the enantiomers as spin polarisers and the irrelevance of electrostatics in the chosen model system.

Figures (4)

• Figure 1: Spin-selective electron transport across enantiomers using superconducting electrodes. (a) Illustration of the tunnelling junction comprising a Mn-terminated Pb tip and a $\Lambda$-[7]H enantiomer on Pb(111). (b) Left: DOS of the tip ($\varrho_\text{t}$) showing the BCS energy gap of width $2\Delta$ and two pairs of spin-polarised YSR states. Right: DOS of the sample ($\varrho_\text{s}$). Quasielectron transport with indicated spin occurs from tip to sample due to positive sample voltage $V=(\mu_\text{t}-\mu_\text{s})/\text{e}>0$. (c) As (b) for $V<0$. (d)--(f) As (a)--(c) for the $\Delta$-[7]H enantiomer. The thickness of the horizontal arrows indicates the quasielectron transmission with the respective spin component.
• Figure 2: Separation of a racemic mixture of $\Lambda$-[7]H and $\Delta$-[7]H on Pb(111). (a) Constant-current STM image of a [7]H island with marked enantiopure domains ($1\,\text{V}$, $50\,\text{pA}$, $30\,\text{nm}\times 20\,\text{nm}$). Inset: atomically resolved Pb(111) lattice ($10\,\text{mV}$, $50\,\text{pA}$, $4\,\text{nm}\times 4\,\text{nm}$). (b) Close-up view of a [7]H island with enantiopure domains indicated by shaded areas and individual $\Lambda$ and $\Delta$ molecules. Numbers $1$--$3$ indicate a submolecular pattern with increasing apparent height (arc with arrow) reflecting the molecular helix with right (top) and left (bottom) handedness ($1\,\text{V}$, $50\,\text{pA}$, $6\,\text{nm}\times 6\,\text{nm}$). (c),(d) STM images of a single (c) $\Delta$-[7]H and (d) $\Lambda$-[7]H molecule embedded in the island ($1\,\text{V}$, $50\,\text{pA}$, $1.4\,\text{nm}\times 1.4\,\text{nm}$).
• Figure 3: Expected effect of spin-selective quasielectron transport across enantiomers probed by a Mn-functionalised Pb tip. (a) Spectrum of $\text{d}I/\text{d}V$ recorded with a Mn-terminated tip atop clean Pb(111) in an external magnetic field of $B=40\,\text{mT}$. The solid line is a fit to the Mn tip spectrum. For comparison, $\text{d}I/\text{d}V$ data obtained with a clean Pb tip at $B=0$ are added as dots. The spectra are normalised to unity at $5\,\text{mV}$. Feedback loop parameters: $1\,\text{V}$, $50\,\text{pA}$. (b) Lorentzian DOS ($\varrho_\text{YSR}$) used for describing the YSR states in the Mn tip spectrum in (a) ($\varepsilon_{i\sigma}$: YSR energy with respect to $E_\text{F}\equiv 0$, $i\in\{0,1\}$, spin $\sigma\in\{\uparrow,\downarrow\}$). (c) Simulated difference spectra $\delta\,\text{d}I/\text{d}V$ of the two enantiomers with indicated polarisation. The dashed lines indicate the position of coherence peaks in the case of a clean Pb--Pb tunnelling junction.
• Figure 4: Experimental evidence for the single-enantiomer CISS effect. (a),(b) Spectra of $\text{d}I/\text{d}V$ obtained at $B=40\,\text{mT}$ with the same Mn-terminated tip as in Fig. \ref{['fig3']}a on the apparently highest part (site $3$) of (a) $\Lambda$-[7]H and (b) $\Delta$-[7]H. Grey dots depict spectroscopic data recorded with a clean Pb tip. The spectra are normalised to unity at $5\,\text{mV}$. Feedback loop parameters: $1\,\text{V}$, $50\,\text{pA}$. (c),(d) Difference spectra of (a) and (b) at (c) $B=40\,\text{mT}$ and (d) $B=0$. (e) Difference spectrum obtained from $\text{d}I/\text{d}V$ data of $\Lambda$-[7]H and $\Delta$-[7]H acquired atop site $2$.