arXiv subjects

Lars Egil Helseth

Publications and source records attributed to Lars Egil Helseth.

9 recordsLinked to original sources

arXiv2014

Power flow in a small electromagnetic energy harvesting system excited by mechanical motion

In this study the power flow in a coupled mechanical and electromagnetic harvesting system in presence of both positional and electrical fluctuations is analyzed. Explicit expressions for the power into and out of the mechanical and electrical parts of the system are found in the case of weak coupling, and it is shown how the power flows between the domains consistent with energy conservation. The case of thermal fluctuations is considered in particular, and use of the fluctuation-dissipation theorem explicitly demonstrates that the power delivered to the mechanical system from the electrical system is the same as the power delivered to the electrical system from the mechanical system. On the other hand, the power dissipated in the electrical circuit is not the same as the power transferred from the mechanical domain if the electrical circuit contains its own current fluctuations. The electrical noise power dissipated in a load resistor is calculated, and found to consist of a component due to electromagnetic coupling in addition to the well-known Nyquist component. The component due to electromagnetic coupling scales with size, and becomes more important than the Nyquist component only for sufficiently small systems.

physics.class-ph↗

arXiv2010

Under which conditions is quantum brownian motion observable in a microscope?

We investigate under which conditions we can expect to observe quantum brownian motion in a microscope. Using the fluctuation-dissipation theorem, we investigate quantum brownian motion in an ohmic bath, and estimate temporal and spatial accuracy required to observe a crossover from classical to quantum behavior.

cond-mat.stat-mech↗

arXiv2010

General method for focusing of waves using phase and amplitude compensation

A general method for focusing of waves, based on phase and amplitude compensation, is applied to monochromatic, polychromatic and diffusive waves. Monochromatic waves may form spatially localized waves in free space, whereas polychromatic waves form non-decaying traveling evanescent modes conned to subwavelength regions in media where the frequency depends on the wave vector. We suggest an analogy between the phase compensation method and the transformation of frequencies between inertial, relativistic coordinate systems.

physics.optics↗

arXiv2009

Tight focusing of polychromatic waves using angular spectrum compensation in spatially dispersive media

A general method for tight focusing of waves, based on compensation of the angular spectrum, is established. We apply the method to monochromatic, polychromatic and diffusive waves. Diffusive and monochromatic waves may form spatially localized waves in free space, whereas polychromatic waves form light sheets or non-decaying traveling evanescent modes confined to subwavelength regions in media where the frequency depends on the wave vector. We suggest an analogy between our compensation method and the transformation of frequencies between inertial, relativistic coordinate systems.

physics.optics↗

arXiv2004

Anomalous interaction between vortices and nanomagnets

We study a thin film system consisting of a superconducting and a magnetic film, where the superconductor contains a vortex and the magnetic film a nanomagnet. We find that if the magnetic film has planar anisotropy, the vortex induces a magnetization distribution, and its interaction with the nanomagnet crosses over from attractive to repulsive at short distances.

cond-mat.supr-con↗

arXiv2004

Vortex interactions in presence of a soft magnetic film

We study theoretically the behavior of vortices in a thin film superconductor placed close to a soft magnetic film. It is shown that the field from the vortex induces a magnetization distribution in the soft magnetic film, thus modifying the fields and vortex interactions. We suggest that the interaction between two otherwise identical vortices is attractive at short distances, but repulsive at larger distances. This is in contrast to the case without the soft magnetic film, where the force is always repulsive.

cond-mat.supr-con↗

arXiv2003

Interaction between soft magnetic and superconducting films

We study theoretically the interaction between soft magnetic and superconducting films. It is shown that the vortex induces a magnetization distribution in the magnetic film, thus modifying the magnetic field as well as the interactions with e.g. Bloch walls. In particular, the field from the vortex may change sign close to the core as a result of the vortex-induced magnetization, thus resulting in a crossover from attraction to repulsion between vortices and Bloch walls. Moreover, we show that by tuning the anisotropy field of the magnetic film, one may enhance the interaction between Bloch walls and vortices. Finally, we discuss how the structure of a Bloch wall in presence of the thin film superconductor can be revealed by magneto-optic imaging using an additional magneto-optic indicator.

cond-mat.supr-con↗

arXiv2002

Focusing of atoms with strongly confined light potentials

Focusing of atoms with light potentials is studied. In particular, we consider strongly confined, cylindrical symmetric potential, and demonstrate their applications in both red and blue-detuned focusing of atoms. We also study the influence of aberrations, and find that a resolution of 1 nm should in principle be possible.

quant-ph↗

arXiv2002

Interaction between superconductive films and magnetic nanostructures

The interplay between magnetic and superconductive films is studied. The generalized London equation for this system is solved, and the magnetic fields, the energy and the interaction forces are computed. In particular, we focus on how to manipulate vortices using magnetic nanostructures.

cond-mat.supr-con↗