Ground-State Spin Blockade in a Single-Molecule Junction.

It is known that the quantum mechanical ground state of a nanoscale junction has a significant impact on its electrical transport properties. This becomes particularly important in transistors consisting of a single molecule. Because of strong electron-electron interactions and the possibility of accessing ground states with high spins, these systems are eligible hosts of a current-blockade phenomenon called a ground-state spin blockade. This effect arises from the inability of a charge carrier to account for the spin difference required to enter the junction, as that process would violate the spin selection rules. Here, we present a direct experimental demonstration of a ground-state spin blockade in a high-spin single-molecule transistor. The measured transport characteristics of this device exhibit a complete suppression of resonant transport due to a ground-state spin difference of 3/2 between subsequent charge states. Strikingly, the blockade can be reversibly lifted by driving the system through a magnetic ground-state transition in one charge state, using the tunability offered by both magnetic and electric fields.

Quantification of Functional Dynamics of Membrane Proteins Reconstituted in Nanodiscs Membranes by Single Turnover Functional Readout.

Single-molecule measurements are emerging as a powerful tool to study the individual behavior of biomolecules, revolutionizing our understanding of biological processes. Their ability to measure the distribution of behaviors, instead of the average behavior, allows the direct observation and quantification of the activity, abundance, and lifetime of multiple states and transient intermediates in the energy landscape that are typically averaged out in nonsynchronized ensemble measurements. Studying the function of membrane proteins at the single-molecule level remains a formidable challenge, and to date there is limited number of available functional assays. In this chapter, we describe in detail our recently developed methodology to reconstitute membrane proteins such as the integral membrane protein cytochrome P450 oxidoreductase on membrane systems such as Nanodiscs and study their functional dynamics by recordings at the fundamental resolution of individual catalytic turnovers using prefluorescent substrate analogues. We initially describe the methodology for reconstitution, surface immobilization, and data acquisition of individual enzyme catalytic turnovers. We then explain in detail the statistical analysis, with an emphasis on the model development, the potential pitfalls for correctly identifying the abundance, lifetime, and likelihood of sampling protein functional states. This methodology may enable studies of functional dynamics and their role in biology for a spectrum of membrane proteins.

Interaction-induced negative differential resistance in asymmetric molecular junctions.

Combining insights from quantum chemistry calculations with master equations, we discuss a mechanism for negative differential resistance (NDR) in molecular junctions, operated in the regime of weak tunnel coupling. The NDR originates from an interplay of orbital spatial asymmetry and strong electron-electron interaction, which causes the molecule to become trapped in a nonconducting state above a voltage threshold. We show how the desired asymmetry can be selectively introduced in individual orbitals in, e.g., oligo(phenyleneethynylene)-type molecules by functionalization with a suitable side group, which is in linear conjugation to one end of the molecule and cross-conjugated to the other end.

Interdependence of magnetism and superconductivity in the borocarbide TmNi2B2C

We have discovered a new antiferromagnetic phase in TmNi2B2C by neutron diffraction. The ordering vector is Q(A) = (0.48,0,0) and the phase appears above a critical in-plane magnetic field of 0.9 T. The field was applied in order to test the assumption that the zero-field magnetic structure at Q(F) = (0.094,0.094,0) would change into a c-axis ferromagnet if superconductivity were destroyed. We present theoretical calculations which show that two effects are important: a suppression of the ferromagnetic component of the RKKY exchange interaction in the superconducting phase and a reduction of the superconducting condensation energy due to the periodic modulation of the moments at Q(A).