Quantum Brownian motion in ratchet potentials: duality relation and its consequences

Quantum Brownian motion in ratchet potentials is investigated by means of an approach based on a duality relation. This relation links the long-time dynamics in a tilted ratchet potential in the presence of dissipation with the one in a driven dissipative tight-binding model. The application to quantum ratchet yields a simple expression for the ratchet current in terms of the transition rates in the tight-binding system.Comment: Chemical Physics (in press

Duality Relation for Quantum Ratchets

A duality relation between the long-time dynamics of a quantum Brownian particle in a tilted ratchet potential and a driven dissipative tight-binding model is reported. It relates a situation of weak dissipation in one model to strong dissipation in the other one, and vice versa. We apply this duality relation to investigate transport and rectification in ratchet potentials: From the linear mobility we infer ground-state delocalization for weak dissipation. We report reversals induced by adiabatic driving and temperature in the ratchet current and its dependence on the potential shape.Comment: Modified content, corrected typo

Spin-transfer torque in magnetic multilayer nanopillars

We consider a quasi one-dimensional configuration consisting of two small pieces of ferromagnetic material separated by a metallic one and contacted by two metallic leads. A spin-polarized current is injected from one lead. Our goal is to investigate the correlation induced between the magnetizations of the two ferromagnets by spin-transfer torque. This torque results from the interaction between the magnetizations and the spin polarization of the current. We discuss the dynamics of a single ferromagnet, the extension to the case of two ferromagnets, and give some estimates for the parameters based on experiments.Comment: To appear in the Journal of Physics: Conference Series (Proceedings of the International Conference on Nanoscience and Technology, Basel, 2006

Multi-band quantum ratchets

We investigate directed motion in non-adiabatically rocked ratchet systems sustaining few bands below the barrier. Upon restricting the dynamics to the lowest M bands, the total system-plus-bath Hamiltonian is mapped onto a discrete tight-binding model containing all the information both on the intra- and inter-well tunneling motion. A closed form for the current in the incoherent tunneling regime is obtained. In effective single-band ratchets, no current rectification occurs. We apply our theory to describe rectification effects in vortex quantum ratchets devices. Current reversals upon variation of the ac-field amplitude or frequency are predicted.Comment: Accepted for publication in Physical Review Letter

Ageing of a Microscopic Sliding Gold Contact at Low Temperatures

Nanometer-scale friction measurements on a Au(111) surface have been performed at temperatures between 30 and 300 K by means of atomic force microscopy. Stable stick slip with atomic periodicity is observed at all temperatures, showing only weak dependence on temperature between 300 and 170 K. Below 170 K, friction increases with time and a distortion of the stick-slip characteristic is observed. Low friction and periodic stick slip can be reestablished by pulling the tip out of contact and subsequently restoring the contact. A comparison with molecular dynamics simulations indicates that plastic deformation within a growing gold junction leads to the observed frictional behavior at low temperatures. The regular stick slip with atomic periodicity observed at room temperature is the result of a dynamic equilibrium shape of the contact, as microscopic wear damage is observed to heal in the sliding contact

Quantum Ratchets

In this thesis, ratchet systems operating in the quantum regime are investigated. Ratchet systems, also known as Brownian motors, are periodic systems presenting an intrinsic asymmetry which can be exploited to extract work out of unbiased forces. As a model for ratchet systems, we consider the motion of a particle in a one-dimensional periodic and asymmetric potential, interacting with a thermal environment, and subject to an unbiased driving force. In quantum ratchets, intrinsic quantum fluctuations as well as the tunnel effect enrich the transport mechanisms. The investigation of quantum ratchets allows one to gain fundamental understanding on the dynamics of quantum dissipative systems. Starting from a continuous ratchet potential and applying a path integral formalism, we develop two approaches, beyond a semiclassical description, where the dynamics can be mapped onto that of an effective tight-binding model. In the first approach, a parameter regime is chosen such that only few low energy quantum states in each well of the periodic potential are involved in the dynamics of the particle. The second approach leads to a duality relation between the original system and a single-band tight-binding model. The two methods are valid in different regimes of parameters, the second one including the classical limit. We get an analytical expression for the stationary velocity of the particle, which shows reversals as a function of the driving force and of the temperature. The second method allows us to extract the explicit dependence of the velocity on the parameters of the potential, and thus to characterize the potentials which lead to the highest rectification efficiency. We also discuss the connection between this theoretical model and measurements of the dynamics of vortices in quasi one-dimensional Josephson junction arrays.Applied Science

Temperature dependence of coulomb drag between finite-length quantum wires

We evaluate the Coulomb drag current in two finite-length Tomonaga-Luttinger-liquid wires coupled by an electrostatic backscattering interaction. The drag current in one wire shows oscillations as a function of the bias voltage applied to the other wire, reflecting interferences of the plasmon standing waves in the interacting wires. In agreement with this picture, the amplitude of the current oscillations is reduced with increasing temperature. This is a clear signature of non-Fermi-liquid physics because for coupled Fermi liquids the drag resistance is always expected to increase as the temperature is raised

Molecular dynamics simulation of gold solid film lubrication

The lubrication mechanisms in ultrathin solid gold films confined between two rough nickel surfaces have been investigated employing classical molecular dynamics with a second moment tight-binding potential. Three types of nickel surfaces are considered: Ni(111), Ni(001) single- and an Ni(001) - (111) bicrystal. In all three systems, gold layers that have been quenched from the melt organise in (111) layers parallel to the nickel interfaces. The relative sliding of the two single crystal nickel tribopartners requires a shear stress of around 170 MPa - a value that is almost one order of magnitude lower than the ideal plastic shear stress of single crystal bulk gold. This reduced stress can be explained by a misfit dislocation mechanism in a single plane close to the Ni/Au interface. In the case of the Ni(001) - (111) bicrystal, the nickel grain boundaries induce grain boundaries in the quenched gold film which vanish during sliding. During subsequent sliding the nickel grain boundaries act as nucleation centres for dislocation loops leading to an increased shear stress of 490 MPa. The same is observed for an embedded hydrocarbon impurity. Also here dislocation loops are emitted on (111) planes that are tilted with respect to the sliding plane

Partikelbasierte Simulation magnetorheologischer Flüssigkeiten für die Anwendung in Kupplungen

Magnetorheologische Flüssigkeiten (MRF) bestehen aus magnetisierbaren Feststoffpartikeln (oftmals Eisen) in einem Trägeröl. Beim Anlegen eines externen magnetischen Feldes bilden sich Ketten aus Eisenpartikeln entlang der Feldlinien aus. Die MRF geht dadurch innerhalb von Millisekunden von einem flüssigen in einen festen Zustand über. Dies macht sie interessant für zahlreiche industrielle Anwendungen. MRF finden unter anderem in Kupplungen und Dämpfern Einsatz. Zu den Vorteilen der MRF zählen: Schnelle Reaktionszeit, geringer Verschleiß, geringe Empfindlichkeit gegenüber Verunreinigungen, sowie die exakte Steuerbarkeit der Viskosität und damit der Kraftübertragung durch das externe Magnetfeld. Wir nutzen die Diskrete-Elemente-Methode, um MRF für die Anwendung in Kupplungen auf Partikelebene zu modellieren. Die Feststoffpartikel in der MRF wurden als kugelförmige Eisenpartikel modelliert. Um die magnetische Dipol-Wechselwirkung zwischen den Partikeln zu beschreiben, wurden Magnetisierungsmodelle aus der Literatur in den Code implementiert und miteinander verglichen. Die Modelle beschreiben die Magnetisierung jedes einzelnen Partikels in Abhängigkeit der lokalen Flussdichte. Die Modellparameter wurden an die Magnetisierungskurve einer gebräuchlichen MRF-Zusammensetzung angepasst. Mit dem Ziel eine maximale Drehmomentübertragung innerhalb der Kupplung zu erreichen, wurden gezielt die relevanten Parameter (z.B. Füllgrad, Bewandung, externes magnetisches Feld) variiert, um optimale Material- und Betriebsparameter zu ermitteln