Mesoscale and Nanoscale Physics (cond-mat.mes-hall)

Abstract: The shape of crystalline nanoparticles (NP) can often be described by polyhedra with flat facet surfaces. Thus, structural studies of polyhedral bodies can help to describe geometric details of NPs. Here we consider compact polyhedra of cubic point symmetry Oh.as simple models. Their surfaces are described by facets with normal vectors along selected directions (a, b, c) together with their symmetry equivalents forming a direction family {abc}. For given {abc} this yields generic polyhedra with up to 48 facets where we focus on polyhedra with facets of {abc} = {100}, {110}, and {111}, suggested for metal NPs with cubic lattices. The resulting generic polyhedra, cubic, rhombohedral, and octahedral, can serve for the description of non-generic polyhedra as intersections of corresponding generic species. Their structural properties are shown to be fully determined by only three structure parameters, facet distances R100, R110, and R111 of three types of facets. This provides a phase diagram to completely classify the corresponding Oh symmetry polyhedra. Structural properties of all polyhedra, such as shape, size, and facet geometries, are discussed in analytical and numerical detail with visualization of characteristic examples. The results may be used for respective nanoparticle simulations but also as a repository assisting the interpretation of structures of real compact nanoparticles observed by experiment.

Abstract: In superconductors (SC), the Higgs amplitude mode is a coherent oscillation of the order parameter typically generated by THz laser irradiation. In this paper we propose to probe the Higgs mode using electronic transport in ballistic superconducting hybrid devices. We first confirm the existence of a non-zero amplitude mode in the clean case using the Keldysh-Eilenberger formalism. We then investigate two different geometries, respectively a normal-insulating-superconductor (NIS) tunnel junction and a NSN junction with transparent interfaces, the superconductor being irradiated in both situations. In the NIS case, the Higgs manifests itself in the second-order AC current response which is resonant at the Higgs frequency. In the NSN case, the DC differential conductance allows to probe the gaps generated by the Higgs mode in the Floquet spectrum.

Abstract: A magnonic spin current crossing a ferromagnet-metal interface is accompanied by spin current shot noise arising from the discrete quanta of spin carried by magnons. In thin films, e.g., the spin of so-called squeezed magnons have been shown to deviate from the common value $\hbar$, with corresponding changes in the spin noise. In experiments, spin currents are typically converted to charge currents via the inverse spin Hall effect. We here analyze the magnitude of the spin current shot noise in the charge channel for a typical electrically detected spin pumping experiment, and find that the voltage noise originating from the spin current shot noise is much smaller than the inevitable Johnson-Nyquist noise. Furthermore, due to the local nature of the spin-charge conversion, the ratio between spin current shot noise and Johnson-Nyquist noise does not scale with sample geometry and sensitively depends on material-specific transport properties. Our analysis thus provides guidance for the experimental detection of squeezed magnons through spin pumping shot noise.

Abstract: Chirality-induced spin selectivity has been reported in many experiments, but a generally accepted theoretical explanation has not yet been proposed. Here, we introduce a simple model system of a straight cylindrical free-electron wire, containing a helical string of atomic scattering centers, with spin-orbit interaction. The advantage of this simple model is that it allows deriving analytical expressions for the spin scattering rates, such that the origin of the effect can be easily followed. We find that spin-selective scattering can be viewed as resulting from constructive interference of partial waves scattered by the spin-orbit terms. We demonstrate that forward scattering rates are independent of spin, while back scattering is spin dependent over wide windows of energy. Although the model does not represent the full details of electron transmission through chiral molecules, it clearly reveals a mechanism that could operate in chiral systems.

Abstract: We present a theoretical analysis of heat transport through a single-molecule junction with two possible transport channels for electrons where interactions between electrons on the molecule and phonons in the nuclear environment is strong and Marcus-type processes predominate in the electron transport. We show that within the steady state regime the competition between transport channels may result in negative differential heat conductance and cooling of the molecule environment. Also, we analyze the effect of a slowly driven molecule level (provided that another level is fixed) on the heat transport and power generated in the system.

Abstract: Spintronics has become a broad and important research field that intersects with magnetism, nano-electronics, and materials science. Its overarching aim is to provide a fundamental understanding of spin-dependent phenomena in solid-state systems that can enable a new generation of spin-based logic devices. Over the past decade, graphene and related 2D van der Waals crystals have taken center stage in expanding the scope and potential of spintronic materials. Their distinctive electronic properties and atomically thin nature have opened new opportunities to probe and manipulate internal electronic degrees of freedom. Purely electrical control over conduction-electron spins can be attained in graphene-transition metal dichalcogenide heterostructures, due to proximity effects combined with graphene's high electronic mobility. Specifically, graphene experiences a proximity-induced spin-orbit coupling that enables efficient spin-charge interconversion processes; the two most well-known and at the forefront of current research are the spin Hall and inverse spin galvanic effects, wherein an electrical current yields a spin current and non-equilibrium spin polarization, respectively. This article provides an overview of the basic principles, theory, and experimental methods underpinning the nascent field of 2D material-based spintronics.