Detection of a Chirality-Induced Spin Selective Quantum Capacitance in α-Helical Peptides

Abstract

Advanced Kelvin probe force microscopy simultaneously detects the quantum capacitance and surface potential of an α-helical peptide monolayer. These indicators shift when either the magnetic polarization or the enantiomer is toggled. A model based on a triangular quantum well in thermal and chemical equilibrium and electron–electron interactions allows for calculating the electrical potential profile from the measured data. The combination of the model and the measurements shows that no global charge transport is required to produce effects attributed to the chirality-induced spin selectivity effect. These experimental findings support the theoretical model of Fransson et al. Nano Letters 2021, 21 (7), 3026–3032. Measurements of the quantum capacitance represent a new way to test and refine theoretical models used to explain strong spin polarization due to chirality-induced spin selectivity.

Chiral Induced Spin Selectivity

“…DNA (see refs , , , , , − , , − ) and α-helical oligopeptides (see refs , , − , , , , , , − , , , , , , , , , − ) have been widely used to explore the CISS effect and its connection with molecular properties. Having been investigated by spin-dependent photoemission, transport, electrochemical, and spin-dependent polarization experiments, they comprise testbed systems for comparisons between methods.…”

Mechanism for Electrostatically Generated Magnetoresistance in Chiral Systems without Spin-Dependent Transport

“…Recent experimental results report a modification of the (electrostatic) equilibrium properties of a chiral system coupled to a ferromagnet in the absence of a bias current. − It was found that the contact potential of chiral molecules on a ferromagnet is modified either by reversal of the magnetization direction or by interchanging the chirality of the system. Theiler et al recently reported a shift of the contact potential measured by Kelvin probe force microscopy (KPFM) of 50 mV when the direction of the magnetization is reversed, which is similar in magnitude to the values previously reported by KPFM measurements and photoemission experiments . We note that no electron transport is involved in these measurements of the contact potential shifts.…”

“…To check the feasibility of this mechanism, we calculate the areal electron density, N , that is required to build up a potential difference of Δψ ⇆ = 50 mV over a typical length scale of a molecule d = 1 nm, using a parallel plate capacitor approximation, N = ϵ s normalΔ ψ false⇆ e d where ϵ s = ϵ r ϵ 0 is the permittivity and e is the electron charge. With ϵ r = 5, we find that N = 10 –2 nm –2 , which corresponds to 2.5 × 10 –3 electron charge per molecule for a molecular packing density of 4 nm –2 .…”

“…The magnitude of the contact potential shift of ∼50 mV is large compared to k B T , where k B is the Boltzmann constant and T is the (room) temperature, which makes the change in contact potential relevant for the (low bias) electron transport and the MR. An electrostatic contact potential modification by reversal of the magnetization direction or by changing the chirality of the system is not expected based on time reversal symmetry and parity inversion considerations . However, it has been clearly observed in a number of experiments. − …”

“…Then, we provide an alternative mechanism for the origin of the MR of chiral systems coupled to a ferromagnet. We argue that experiments showed a persistent/equilibrium electrostatic contact potential/work function of the system that is altered by either reversal of the magnetization direction or chirality. − We investigate the implications of this for the modification of the potential profiles of a Schottky barrier-like junction where the chiral system is in direct contact with a ferromagnetic metal as well as the case where they are separated by a tunnel barrier. We find that the barrier height and the energy bands within the screening length are adjusted and thus can modify the charge transport.…”