Horacio Miguel Pastawski

Chiral Induced Spin Selectivity (CISS) is an intriguing phenomenon in chiral molecules, in which spin polarization emerges at room temperature in two-terminal junctions without requiring ferromagnetic contacts or strong intrinsic spin–orbit coupling. This work develops a unified tight-binding framework that reproduces charge and spin transport in single- and double-helical molecules, including ordered oligopeptides and DNA. We first reproduce the spin-independent conductance–distance behavior observed by Giese, Curr. Opin. Chem. Biol. 6, 612 (2002), and Lindsay, Life 10, 72 (2020), by including electron–phonon interactions through Einstein phonon reservoirs at temperature T. Upon introducing spin–orbit coupling under the tunneling barrier for single-stranded DNA, we obtain a clear spin-conductance asymmetry, leading to strong spin polarization (20%–40%) that increases with molecular length and reverses sign with molecular chirality. The temperature dependence of the polarization exhibits a linear increase near room temperature, consistent with experimental trends. Double-stranded configurations yield similar spin-selective behavior within the experimental setup. We argue that this class of models provides the closest microscopic correspondence to current CISS measurements and that further refinements can be achieved by identifying specific decoherence processes and barrier parameters associated with each molecular system.

Static disorder in a three-dimensional crystal degrades the ideal ballistic dynamics until it produces a localized regime. This metal-insulator transition is often preceded by coherent diffusion. By studying three paradigmatic one-dimensional models, namely, the Harper-Hofstadter-Aubry-André and Fibonacci tight-binding chains, along with the power-banded random matrix model, we show that whenever coherent diffusion is present, transport is exceptionally stable against decoherent noise. This is completely at odds with what happens for coherently ballistic and localized dynamics, where the diffusion coefficient strongly depends on the environmental decoherence. A universal dependence of the diffusion coefficient on the decoherence strength is analytically derived: The diffusion coefficient remains almost decoherence independent until the coherence time becomes comparable to the mean elastic scattering time. Thus, systems with a quantum diffusive regime could be used to design robust quantum wires. Moreover, our results might shed light on the functionality of many biological systems, which often operate at the border between the ballistic and localized regimes.

Through an appropriate election of the molecular orbital basis, we show analytically that the molecular dissociation occurring in a Heyrovsky reaction can be interpreted as a quantum dynamical phase transition, i.e., an analytical discontinuity in the molecular energy spectrum induced by the catalyst. The metallic substrate plays the role of an environment that produces an energy uncertainty on the adatom. This broadening induces a critical behavior not possible in a quantum closed system. We use suitable approximations on symmetry, together with both Lanczos and canonical transformations, to give analytical estimates for the critical parameters of molecular dissociation. This occurs when the bonding to the surface is 2 times the molecular bonding. This value is slightly weakened for less symmetric situations. However simple, this conclusion involves a high order perturbative solution of the molecule-catalyst system. This model is further simplified to discuss how an environment-induced critical phenomenon can be evaluated through an idealized perturbative tunneling microscopy set-up. In this case, the energy uncertainties in one or both atoms are either Lorentzian or Gaussian. The former results from the Fermi golden rule, i.e., a Markovian approximation. The Gaussian uncertainty, associated with non-Markovian decoherent processes, requires the introduction of a particular model of a spin bath. The partially coherent tunneling current is obtained from the generalized Landauer–Büttiker equations. The resonances observed in these transport parameters reflect, in many cases, the critical properties of the resonances in the molecular spectrum.

In recent years, there has been an increasing interest in nanoelectromechanical devices, current-driven quantum machines, and the mechanical effects of electric currents on nanoscale conductors. Here, we carry out a thorough study of the current-induced forces and the electronic friction of systems whose electronic effective Hamiltonian can be described by an archetypal model, a single energy level coupled to two reservoirs. Our results can help better understand the general conditions that maximize the performance of different devices modeled as a quantum dot coupled to two electronic reservoirs. Additionally, they can be useful to rationalize the role of current-induced forces in the mechanical deformation of one-dimensional conductors.

A study of long-range interactions in disordered systems yields a surprising result: Transport can increase with disorder.