arXiv subjects

Rudolf Smorka

Publications and source records attributed to Rudolf Smorka.

5 recordsLinked to original sources

arXiv2025

The Role of Quantum Vibronic Effects in the Spin Polarization of Charge Transport through Molecular Junctions

The connection between molecular vibrations and spin polarization in charge transport through molecular junctions is currently a topic of high interest, with important consequences for a variety of phenomena, such as chirality-induced spin selectivity (CISS). In this work, we follow this theme by exploring the relationship between vibronic dynamics and the corresponding spin polarization of the nonequilibrium charge current in a molecular junction. We employ the hierarchical equations of motion (HEOM) approach, which, since it is numerically exact and treats the vibrational degrees of freedom quantum mechanically, extends previous analyses of similar models that relied on approximate transport methods. We find significant spin polarization of the charge current in the off-resonant, low-voltage regime, where the vibrations must be treated quantum mechanically. Furthermore, we are able to connect the spin polarization in the charge transport to a corresponding polarization of the vibrational dynamics, which manifests itself in the vibrational angular momentum and excitation. Our analysis covers multiple molecule-lead couplings, temperatures, orbital energies, and spin-orbit couplings, demonstrating that the vibrationally assisted spin polarization is robust across a broad range of parameters.

cond-mat.mes-hall↗

arXiv2024

Influence of Nonequilibrium Vibrational Dynamics on Spin Selectivity in Chiral Molecular Junctions

We explore the role of molecular vibrations in the chirality-induced spin selectivity (CISS) effect in the context of charge transport through a molecular nanojunction. We employ a mixed quantum-classical approach that combines Ehrenfest dynamics for molecular vibrations with the hierarchical equations of motion method for the electronic degrees of freedom. This approach treats the molecular vibrations in a nonequilibrium manner, which is crucial for the dynamics of molecular nanojunctions. To explore the effect of vibrational dynamics on spin selectivity, we also introduce a new figure of merit, the displacement polarization, which quantifies the difference in vibrational displacements for opposing lead magnetizations. We analyze the dynamics of single trajectories, investigating how the spin selectivity depends on voltage and electronic-vibrational coupling. Furthermore, we investigate the dynamics and temperature dependence of ensemble-averaged observables. We demonstrate that spin selectivity is correlated in time with the vibrational polarization, indicating that dynamics of the molecular vibrations is the driving force of CISS in this model within the Ehrenfest approach.

cond-mat.mes-hall↗

arXiv2024

Dynamics of spin relaxation in nonequilibrium magnetic nanojunctions

We investigate nonequilibrium phenomena in magnetic nano-junctions using a numerical approach that combines classical spin dynamics with the hierarchical equations of motion technique for quantum dynamics of conduction electrons. Our focus lies on the spin dynamics, where we observe non-monotonic behavior in the spin relaxation rates as a function of the coupling strength between the localized spin and conduction electrons. Notably, we identify a distinct maximum at intermediate coupling strength, which we attribute to a competition that involves the increasing influence of the coupling between the classical spin and electrons, as well as the influence of decreasing local density of states at the Fermi level. Furthermore, we demonstrate that the spin dynamics of a large open system can be accurately simulated by a short chain coupled to semi-infinite metallic leads. In the case of a magnetic junction subjected to an external DC voltage, we observe resonant features in the spin relaxation, reflecting the electronic spectrum of the system. The precession of classical spin gives rise to additional side energies in the electronic spectrum, which in turn leads to a broadened range of enhanced damping in the voltage.

cond-mat.mes-hall↗

arXiv2022

Nonequilibrium dynamics in a spin valve with noncollinear magnetization

We utilize a hybrid quantum-classical equation of motion approach to investigate the spin dynamics and spin-transfer torque in a spin valve under bias voltage. We show that the interplay between localized classical magnetic moments and conduction electrons induces a complex effective exchange coupling between the magnetic layers. This leads to a declination of magnetizations from layers anisotropy axes even in equilibrium. Introducing a finite bias voltage triggers spin currents and related spin-transfer torques which further tilt the magnetizations and govern the relaxation processes of the spin dynamics. Analyzing different scenarios of the applied bias voltage, we show that symmetric and asymmetric voltage drops can lead to relaxation times of the spin dynamics that differ by several orders of magnitude at comparable charge currents. In both cases we observe resonant features, where the relaxation is boosted whenever the chemical potential of the leads matches the maxima in the density of the states of the spin-valve electrons.

cond-mat.mes-hall↗

arXiv2020

Electronic transport through correlated electron systems with nonhomogeneous charge orderings

The spinless Falicov-Kimball model exhibits outside the particle-hole symmetric point different stable nonhomogeneous charge orderings. These include the well known charge stripes and a variety of orderings with phase separated domains, which can significantly influence the charge transport through the correlated electron system. We show this by investigating a heterostructure, in which the Falicov-Kimball model on a finite two-dimensional lattice is located between two noninteracting semi-infinite leads. We use a combination of nonequilibrium Green's functions techniques with a sign-problem-free Monte Carlo method for finite temperatures or a simulated annealing technique for the ground state to address steady-state transport through the system. We show that different ground-state phases of the central system can lead to simple metallic-like or insulating charge transport characteristics, but also to more complicated current-voltage dependencies reflecting a multi-band character of the transmission function. Interestingly, with increasing temperature, the orderings tend to form transient phases before the system reaches the disordered phase. This leads to nontrivial temperature dependencies of the transmission function and charge current.

cond-mat.str-el↗