Magnetic Reversal Time in Open Long Range Systems

Topological phase space disconnection has been recently found to be a general phenomenon in isolated anisotropic spin systems. It sets a general framework to understand the emergence of ferromagnetism in finite magnetic systems starting from microscopic models without phenomenological on-site barriers. Here we study its relevance for finite systems with long range interacting potential in contact with a thermal bath. We show that, even in this case, the induced magnetic reversal time is exponentially large in the number of spins, thus determining {\it stable} (to any experimental observation time) ferromagnetic behavior. Moreover, the explicit temperature dependence of the magnetic reversal time obtained from the microcanonical results, is found to be in good agreement with numerical simulations. Also, a simple and suggestive expression, indicating the Topological Energy Threshold at which the disconnection occurs, as a real energy barrier for many body systems, is obtained analytically for low temperature

Robust nonfullerene solar cells approaching unity external quantum efficiency enabled by suppression of geminate recombination

Nonfullerene solar cells have increased their efficiencies up to 13%, yet quantum efficiencies are still limited to 80%. Here we report efficient nonfullerene solar cells with quantum efficiencies approaching unity. This is achieved with overlapping absorption bands of donor and acceptor that increases the photon absorption strength in the range from about 570 to 700 nm, thus, almost all incident photons are absorbed in the active layer. The charges generated are found to dissociate with negligible geminate recombination losses resulting in a short-circuit current density of 20 mA cm−2 along with open-circuit voltages >1 V, which is remarkable for a 1.6 eV bandgap system. Most importantly, the unique nano-morphology of the donor:acceptor blend results in a substantially improved stability under illumination. Understanding the efficient charge separation in nonfullerene acceptors can pave the way to robust and recombination-free organic solar cells

On the free energy of a ferromagnet : anisotropy and rotation

Using the property of rotational invariance of the free energy, F can be expanded as the sum of Fa the magnetocrystalline energy, F me its derivative with respect to the symmetrical strains and F r its derivative with respect to the rotations. This latter term, always ignored, provides a simple analytical expression of the magnetic torque for any given direction of the magnetization.Mettant à profit la propriété d'invariance rotationnelle de l'énergie libre, on peut développer celle-ci, F, comme la somme de l'énergie magnétocristalline Fa, de sa dérivée par rapport aux déformations symétriques F me, et de sa dérivée par rapport aux rotations, Fr. Ce dernier terme, toujours négligé, fournit une expression analytique simple du couple magnétique pour n'importe quelle direction de l'aimantation