SIMPLE HORMONIC MOTION

http://www.animations.physics.unsw.edu.au/jw/SHM.htm

Fingerprinting magnetic monopoles

Effective Magnetic Monopoles and Universal Conductance Fluctuations

Kjetil M. D. Hals, Anh Kiet Nguyen, Xavier Waintal, and Arne Brataas

Phys. Rev. Lett. 105, 207204 (Published November 11, 2010)

Magnetic monopoles in real space, postulated by Dirac in 1931, have not been seen in nature. However, effective magnetic monopoles in crystal momentum space were observed in the metallic ferromagnet SrRuO₃ a few years ago. Here they arise from energy-band crossings; whenever a charged particle traverses a closed curve in momentum space, its wave function acquires a geometric Berry phase from the monopole fields. The fingerprint of these monopoles is an unconventional behavior in the so-called anomalous Hall effect: the transverse resistivity can show a nonmonotonic temperature dependence and even a sign change.

Writing in Physical Review Letters, Kjetil Hals, Anh Kiet Nguyen, and Arne Brataas from the Norwegian University of Science and Technology, and Xavier Waintal from CEA, Grenoble, France, show that it is possible to manipulate momentum-space magnetic monopoles in ferromagnets with strong spin-orbit coupling by external magnetic fields, and observe this in universal conductance fluctuations (UCF). In general, UCF refers to time-independent fluctuations in the conductance of metals at low temperature that vary between samples but are reproducible for a given sample at a fixed temperature. Hals et al. show that fast conductance oscillations recently observed in experiments on the ferromagnetic semiconductor (Ga,Mn)As are a consequence of the relocation of momentum-space magnetic monopoles. This relocation comes about due to a rotation of the magnetization and leads to a geometric phase change of closed momentum-space curves. This work offers an entirely new probe of magnetic monopoles in momentum space. – Sarma Kancharla

Raising the temperature on density-functional theory

A new analysis clears some of the remaining hurdles to a completely rigorous density-functional theory for calculating the properties of materials at finite temperature.

http://physics.aps.org/articles/v3/99

Thermal and vacuum friction acting on rotating particles

In the Casimir effect, vacuum fluctuations of the electromagnetic field exert a force on closely spaced metal plates, a phenomenon that is well understood theoretically and detectable experimentally. Can a related effect occur for rotating systems, in which vacuum fluctuations alter the spin rate of a particle, resulting in rotational drag? Writing in Physical Review A, Alejandro Manjavacas and Javier García de Abajo of the Instituto de Óptica, Madrid, Spain, show theoretically that this should be an experimentally observable effect.

The phenomenon of vacuum friction for spinning objects is somewhat different than for the static parallel plates: the accelerating charges in a spinning conductive object interact with the vacuum fluctuations and can emit photons. Earlier work by Manjavacas and García de Abajo tackled the problem with a semiclassical model that employed the fluctuation-dissipation theorem to calculate the overall energy transfer between the spinning particle and the vacuum field. In their new calculations, they take a fully quantum mechanical approach, which not only confirms the semiclassical results but extends the results to molecular systems and magnetic interactions. In addition to their intrinsic interest, the findings may be relevant to understanding the dynamical behavior of cosmic nanoparticles such as interstellar dust and the optical spectra of rotating molecules.
Phys. Rev. A 82, 063827 (Published December 23, 2010)