arXiv subjects

M. Jonson

Publications and source records attributed to M. Jonson.

At least 37 records · Page 2Linked to original sources

arXiv2014

Electronic spin working mechanically

A single-electron tunneling (SET) device with a nanoscale central island that can move with respect to the bulk source- and drain electrodes allows for a nanoelectromechanical (NEM) coupling between the electrical current through the device and mechanical vibrations of the island. Although an electromechanical "shuttle" instability and the associated phenomenon of single-electron shuttling were predicted more than 15 years ago, both theoretical and experimental studies of NEM-SET structures are still carried out. New functionalities based on quantum coherence, Coulomb correlations and coherent electron-spin dynamics are of particular current interest. In this article we present a short review of recent activities in this area.

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arXiv2013

Nanoelectromechanics of shuttle devices

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arXiv2012

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.

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arXiv2012

Dynamics of Current Induced Magnetic Superstructures in Exchange-Spring Devices

Thermoelectric manipulation of the magnetization of a magnetic layered stack in which a low-Curie temperature magnet is sandwiched between two strong magnets (exchange spring device) is considered. Controllable Joule heating produced by a current flowing in the plane of the magnetic stack (CIP configuration) induces a spatial magnetic and thermal structure along the current flow --- a magneto-thermal-electric domain (soliton). We show that such a structure can experience oscillatory in time dynamics if the magnetic stack is incorporated into an electric circuit in series with an inductor. The excitation of these magneto-thermionic oscillations follow the scenario either of "soft" of "hard" instability: in the latter case oscillations arise if the initial perturbation is large enough. The frequency of the temporal oscillations is of the order of $10^5 ÷10^7 s^{-1}$ for current densities $j\sim 10^6 ÷10^7 A/cm^3$.

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arXiv2012

Current-induced magnetic superstructures in exchange-spring devices

We investigate the potential to use a magneto-thermo-electric instability that may be induced in a mesoscopic magnetic multi-layer (F/f/F) to create and control magnetic superstructures. In the studied multilayer two strongly ferromagnetic layers (F) are coupled through a weakly ferromagnetic spacer (f) by an "exchange spring" with a temperature dependent "spring constant" that can be varied by Joule heating caused by an electrical dc current. We show that in the current-in-plane (CIP) configuration a distribution of the magnetization, which is homogeneous in the direction of the current flow, is unstable in the presence of an external magnetic field if the length L of the sample in this direction exceeds some critical value Lc ~ 10 μm. This spatial instability results in the spontaneous formation of a moving domain of magnetization directions, the length of which can be controlled by the bias voltage in the limit L >> Lc. Furthermore, we show that in such a situation the current-voltage characteristics has a plateau with hysteresis loops at its ends and demonstrate that if biased in the plateau region the studied device functions as an exponentially precise current stabilizer.

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arXiv2011

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.

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arXiv2011

Suppression of stochastic fluctuations of suspended nanowires by temperature-induced single-electron tunnelling

We investigate theoretically the electromechanical properties of freely suspended nanowires that are in tunnelling contact with the tip of a scanning tunnelling microscope (STM) and two supporting metallic leads. The aim of our analysis is to characterize the fluctuations of the dynamical variables of the nanowire when a temperature drop is mantained between the STM tip and the leads, which are all assumed to be electrically grounded. By solving a quantum master equation that describes the coupled dynamics of electronic and mechanical degrees of freedom we find that the stationary state of the mechanical oscillator has a Gaussian character, but that the amplitude of its root-mean square center-of-mass fluctuations is smaller than would be expected if the system were coupled only to the leads at thermal equilibrium.

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arXiv2011

Electron-hole asymmetry in two-terminal graphene devices

A theoretical model is proposed to describe asymmetric gate-voltage dependence of conductance and noise in two-terminal ballistic graphene devices. The model is analyzed independently within the self-consistent Hartree and Thomas-Fermi approximations. Our results justify the prominent role of metal contacts in recent experiments with suspended graphene flakes. The contact-induced electrostatic potentials in graphene demonstrate a power-law decay with the exponent varying from -1 to -0.5. Within our model we explain electron-hole asymmetry and strong Fabri-Perot oscillations of the conductance and noise at positive doping, which were observed in many experiments with submicrometer samples. Limitations of the Thomas-Fermi approximation in a vicinity of the Dirac point are discussed.

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arXiv2011

Sub-Wavelength Terahertz Spin-Flip Laser Based on a Magnetic Point-Contact Array

We present a novel design for a single-mode, truly sub-wavelength THz disk laser based on a nano-composite gain medium comprising an array of metal/ferromagnetic point contacts embedded in a thin dielectric layer. Stimulated emission of light occurs in the point contacts as a result of spin-flip relaxation of spin-polarized electrons that are injected from the ferromagnetic side of the contacts. Ultra-high electrical current densities in the contacts and a dielectric material with a large refractive index, neither condition being achievable in conventional semiconductor media, allows the thresholds of lasing to be overcome for the lowest-order modes of the disk, hence making single-mode operation possible.

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arXiv2011

Stimulated emission and absorption of photons in magnetic point contacts: toward metal-based spin-lasers

Point contacts between high anisotropy ferromagnetic SmCo5 and normal metal Cu are used to achieve a strong spin-population inversion in the contact core. Subjected to microwave irradiation in resonance with the Zeeman splitting in Cu, the inverted spin-population relaxes through stimulated spin-flip photon emission, detected as peaks in the point contact resistance. Resonant spin-flip photon absorption is detected as resistance minima, corresponding to sourcing the photon field energy into the electrical circuit. These results demonstrate fundamental mechanisms that are potentially useful for designing metallic spin-based lasers.

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arXiv2011

Spin-thermo-electronic oscillator based on inverse giant magnetoresistance

A spin-thermo-electronic valve with the free layer of exchange-spring type and inverse magnetoresistance is investigated. The structure has S-shaped current-voltage characteristics and can exhibit spontaneous oscillations when integrated with a conventional capacitor within a resonator circuit. The frequency of the oscillations can be controlled from essentially dc to the GHz range by the circuit capacitance.

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arXiv2011

Hot electrons in magnetic point contacts as a photon source

We propose to use a point contact between a ferromagnetic and a normal metal in the presence of a magnetic field for creating a large inverted spin-population of hot electrons in the contact core. The key point of the proposal is that when these hot electrons relax by flipping their spin, microwave photons are emitted, with a frequency tunable by the applied magnetic field. While point contacts is an established technology their use as a photon source is a new and potentially very useful application. We show that this photon emission process can be detected by means of transport spectroscopy and demonstrate stimulated emission of radiation in the 10-100 GHz range for a model point contact system using a minority-spin ferromagnetic injector. These results can potentially lead to new types of lasers based on spin injection in metals.

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arXiv2010

Cooling of nanomechanical resonator by thermally activated single-electron transport

We show that the vibrations of a nanomechanical resonator can be cooled to near its quantum ground state by tunnelling injection of electrons from an STM tip. The interplay between two mechanisms for coupling the electronic and mechanical degrees of freedom results in a bias-voltage dependent difference between the probability amplitudes for vibron emission and absorption during tunneling. For a bias voltage just below the Coulomb blockade threshold we find that absorption dominates, which leads to cooling corresponding to an average vibron population of the fundamental bending mode of 0.2.

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arXiv2010

Quantum entanglement of nanocantilevers

We propose a scheme to entangle two mechanical nanocantilevers through indirect interactions mediated by a gas of ultra cold atoms. We envisage a system of nanocantilevers magnetically coupled to a Bose-Einstein condensate of atoms and focus on studying the dark states of the system. These dark states are entangled states of the two nanocantilevers, with no coupling to the atomic condensate. In the absence of dissipation, the degree of entanglement is found to oscillate with time, while if dissipation is included, the system is found to relax to a statistical mixture of dark states which remains time independent until the inevitable thermal dephasing destroys the nanocantilever coherence. This opens up the possibility of achieving long-lived entangled nanocantilever states.

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arXiv2010

Superconductive pumping of nanomechanical vibrations

We demonstrate that a supercurrent can pump energy from a battery that provides a voltage bias into nanomechanical vibrations. Using a device containing a nanowire Josephson weak link as an example we show that a nonlinear coupling between the supercurrent and a static external magnetic field leads to a Lorentz force that excites bending vibrations of the wire at resonance conditions. We also demonstrate the possibility to achieve more than one regime of stationary nonlinear vibrations and how to detect them via the associated dc Josephson currents and we discuss possible applications of such a multistable nanoelectromechanical dynamics.

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arXiv2010

Thermoelectrical manipulation of nanomagnets

We investigate the interplay between the thermodynamic properties and spin-dependent transport in a mesoscopic device based on a magnetic multilayer (F/f/F), in which two strongly ferromagnetic layers (F) are exchange-coupled through a weakly ferromagnetic spacer (f) with the Curie temperature in the vicinity of room temperature. We show theoretically that the Joule heating produced by the spin-dependent current allows a spin-thermo-electronic control of the ferromagnetic-to-paramagnetic (f/N) transition in the spacer and, thereby, of the relative orientation of the outer F-layers in the device (spin-thermo-electric manipulation of nanomagnets). Supporting experimental evidence of such thermally controlled switching from parallel to antiparallel magnetization orientations in F/f(N)/F sandwiches is presented. Furthermore, we show theoretically that local Joule heating due to a high concentration of current in a magnetic point contact or a nanopillar can be used to reversibly drive the weakly ferromagnetic spacer through its Curie point and thereby exchange couple and decouple the two strongly ferromagnetic F-layers. For the devices designed to have an antiparallel ground state above the Curie point of the spacer, the associated spin-thermionic parallel-to-antiparallel switching causes magneto-resistance oscillations whose frequency can be controlled by proper biasing from essentially DC to GHz. We discuss in detail an experimental realization of a device that can operate as a thermo-magneto-resistive switch or oscillator.

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arXiv2008

The Influence of Electro-Mechanical Effects on Resonant Electron Tunneling Through Small Carbon Nano-Peapods

The influence of a fullerene molecule trapped inside a single-wall carbon nanotube on resonant electron transport at low temperatures and strong polaronic coupling is theoretically discussed. Strong peak to peak fluctuations and anomalous temperature behavior of conductance amplitudes are predicted and investigated. The influence of the chiral properties of carbon nanotubes on transport is also studied.

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arXiv2008

Self-organization of irregular NEM vibrations in multi-mode shuttle structures

We investigate theoretically multi-mode electromechanical "shuttle" instabilities in DC voltage-biased nanoelectromechanical single-electron tunneling (NEM-SET) devices. We show that initially irregular (quasi-periodic) oscillations, that occur as a result of the simultaneous self-excitation of several mechanical modes with incommensurable frequencies, self-organize into periodic oscillations with a frequency corresponding to the eigenfrequency of one of the unstable modes. This effect demonstrates that a local probe can selectively excite global vibrations of extended objects.

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