Shuttling of Spin Polarized Electrons in Molecular Transistors

Shuttling of electrons in single-molecule transistors with magnetic leads in the presence of an external magnetic field is considered theoretically. For a current of partially spin-polarized electrons a shuttle instability is predicted to occur for a finite interval of external magnetic field strengths. The lower critical magnetic field is determined by the degree of spin polarization and it vanishes as the spin polarization approaches 100%. The feasibility of detecting magnetic shuttling in a $C_{60}$-based molecular transistor with magnetic (Ni) electrodes is discussed [A.~N.~Pasupathy et al., Science 306, 86 (2004)].Comment: Submitted to a special issue of "Synthetic Metals" to appear in March 201

The icephobic performance of alkyl-grafted aluminum surfaces

This work analyzes the anti-icing performance of flat aluminum surfaces coated with widely used alkyl-group based layers of octadecyltrimethoxysilane, fluorinated alkylsilane and stearic acid as they are subjected to repeated icing/deicing cycles. The wetting properties of the samples upon long-term immersion in water are also evaluated. The results demonstrate that smooth aluminum surfaces grafted with alkyl groups are prone to gradual degradation of their hydrophobic and icephobic properties, which is caused by interactions and reactions with both ice and liquid water. This implies that alkyl-group based monolayers on aluminum surfaces are not likely to be durable icephobic coatings unless their durability in contact with ice and/or water is significantly improved

Mechanically Induced Thermal Breakdown in Magnetic Shuttle Structures

A theory of a thermally induced single-electron "shuttling" instability in a magnetic nanomechanical device subject to an external magnetic field is presented in the Coulomb blockade regime of electron transport. The model magnetic shuttle device considered comprises a movable metallic grain suspended between two magnetic leads, which are kept at different temperatures and assumed to be fully spin polarized with antiparallel magnetizations. For a given temperature difference shuttling is found to occur for a region of external magnetic fields between a lower and an upper critical field strength, which separate the shuttling regime from normal small-amplitude "vibronic" regimes. We find that (i) the upper critical magnetic field saturates to a constant value in the high temperature limit and that the shuttle instability domain expands with a decrease of the temperature, (ii) the lower critical magnetic field depends not only on the temperature independent phenomenological friction coefficient used in the model but also on intrinsic friction (which vanishes in the high temperature limit) caused by magnetic exchange forces and electron tunneling between the quantum dot and the leads. The feasibility of using thermally driven magnetic shuttle systems to harvest thermal breakdown phenomena is discussed.Comment: 9 pages, 2 figure

Magnetopolaronic effects in electron transport through a single-level vibrating quantum dot

Magneto-polaronic effects are considered in electron transport through a single-level vibrating quantum dot subjected to a transverse (to the current flow) magnetic field. It is shown that the effects are most pronounced in the regime of sequential electron tunneling, where a polaronic blockade of the current at low temperatures and an anomalous temperature dependence of the magnetoconductance are predicted. In contrast, for resonant tunneling of polarons the peak conductance is not affected by the magnetic field.Comment: 7 pages, 2 figure

Umklapp-Assisted Electron Transport Oscillations in Metal Superlattices

We consider a superlattice of parallel metal tunnel junctions with a spatially non-homogeneous probability for electrons to tunnel. In such structures tunneling can be accompanied by electron scattering that conserves energy but not momentum. In the special case of a tunneling probability that varies periodically with period $a$ in the longitudinal direction, i.e., perpendicular to the junctions, electron tunneling is accompanied by "umklapp" scattering, where the longitudinal momentum changes by a multiple of $h/a$. We predict that as a result a sequence of metal-insulator transitions can be induced by an external electric- or magnetic field as the field strength is increased.Comment: 5 pages, 3 figure

Homotopy types of stabilizers and orbits of Morse functions on surfaces

Let $M$ be a smooth compact surface, orientable or not, with boundary or without it, $P$ either the real line $R^1$ or the circle $S^1$, and $Diff(M)$ the group of diffeomorphisms of $M$ acting on $C^{\infty}(M,P)$ by the rule $h\cdot f\mapsto f \circ h^{-1}$, where $h\in Diff(M)$ and $f \in C^{\infty}(M,P)$. Let $f:M \to P$ be a Morse function and $O(f)$ be the orbit of $f$ under this action. We prove that $\pi_k O(f)=\pi_k M$ for $k\geq 3$, and $\pi_2 O(f)=0$ except for few cases. In particular, $O(f)$ is aspherical, provided so is $M$. Moreover, $\pi_1 O(f)$ is an extension of a finitely generated free abelian group with a (finite) subgroup of the group of automorphisms of the Reeb graph of $f$. We also give a complete proof of the fact that the orbit $O(f)$ is tame Frechet submanifold of $C^{\infty}(M,P)$ of finite codimension, and that the projection $Diff(M) \to O(f)$ is a principal locally trivial $S(f)$-fibration.Comment: 49 pages, 8 figures. This version includes the proof of the fact that the orbits of a finite codimension of tame action of tame Lie group on tame Frechet manifold is a tame Frechet manifold itsel

Joule Heating and Current-Induced Instabilities in Magnetic Nanocontacts

We consider the electrical current through a magnetic point contact in the limit of a strong inelastic scattering of electrons. In this limit local Joule heating of the contact region plays a decisive role in determining the transport properties of the point contact. We show that if an applied constant bias voltage exceeds a critical value, the stationary state of the system is unstable, and that periodic, non-harmonic oscillations in time of both the electrical current through the contact and the local temperature in the contact region develop spontaneously. Our estimations show that the necessary experimental conditions for observing such oscillations with characteristic frequencies in the range $10^8 \div 10^9$ Hz can easily be met. We also show a possibility to manipulate upon the magnetization direction of a magnetic grain coupled through a point contact to a bulk ferromagnetic by exciting the above-mentioned thermal-electric oscillations.Comment: 9 pages, 6 figures, submitted to Physical Review