Modèle de Bohr | EPFL Graph Search

Le modèle de Bohr est une théorie obsolète dans le domaine de la physique/chimie, cherchant à comprendre la constitution d'un atome, et plus particulièrement celui de l'hydrogène et des ions hydrogénoïdes (ions ne possédant qu'un seul électron). Élaborée par Niels Bohr en 1913, cette théorie établie sur le modèle planétaire de Rutherford rencontra un succès immédiat car elle expliquait de manière simple les raies spectrales des éléments hydrogénés tout en effectuant un rapprochement entre les premiers modèles de l'atome et la théorie des quanta. Ce modèle sera généralisé au cas des électrons relativistes par Arnold Sommerfeld afin d'écrire de façon quantitative la structure fine des lignes spectrales de l'hydrogène. Cependant, cette théorie ne peut expliquer le spectre d'éléments à plusieurs électrons (comme celui de l'hélium), ni la nature des liaisons chimiques, et elle est totalement abandonnée au profit de la mécanique quantique à partir de 1925. Histoire Dével

Atomistic simulation of ½ screw dislocations in BCC tungsten

Atomistic simulations are used to describe the ½ screw dislocation in tungsten. Two different embedded atom model (EAM) potentials and one bond-order potential (BOP) are compared. A new analytical approach for constructing asymmetrical screw dislocations is presented.

Trans Tech Publications Ltd, Laublsrutistr 24, Ch-8717 Stafa-Zurich, Switzerland2009

Magnetoresistance from Fermi surface topology

Yi Liu, Quansheng Wu, Oleg Yazyev, Shengnan Zhang

Extremely large nonsaturating magnetoresistance has recently been reported for a large number of both topologically trivial and nontrivial materials. Different mechanisms have been proposed to explain the observed magnetotransport properties, yet without arriving to definitive conclusions or portraying a global picture. In this work, we investigate the transverse magnetoresistance of materials by combining the Fermi surfaces calculated from first principles with the Boltzmann transport theory approach relying on the semiclassical model and the relaxation time approximation. We first consider a series of simple model Fermi surfaces to provide a didactic introduction into the charge-carrier compensation and open-orbit mechanisms leading to nonsaturating magnetoresistance. We then address in detail magnetotransport in three representative materials: (i) copper, a prototypical nearly free-electron metal characterized by the open Fermi surface that results in an intricate angular magnetoresistance, (ii) bismuth, a topologically trivial semimetal in which very large magnetoresistance is known to result from charge-carrier compensation, and (iii) tungsten diphosphide WP2, a recently discovered type-II Weyl semimetal that holds the record of magnetoresistance in compounds. In all three cases our calculations show excellent agreement with both the field dependence of magnetoresistance and its anisotropy measured at low temperatures. Furthermore, the calculations allow for a full interpretation of the observed features in terms of the Fermi surface topology. Our study thus establishes guidelines to clarifying the physical mechanisms underlying the magnetotransport properties in a broad range of materials. These results will help addressing a number of outstanding questions, such as the role of the topological phase in the pronounced large nonsaturating magnetoresistance observed in topological materials.

2019

Highly anisotropic interlayer magnetoresitance in ZrSiS nodal-line Dirac semimetal

Xiaoqiang Wang, Quansheng Wu, Oleg Yazyev, Shengnan Zhang

We instigate the angle-dependent magnetoresistance (AMR) of the layered nodal-line Dirac semimetal ZrSiS for the in-plane and out-of-plane current directions. This material has recently revealed an intriguing butterfly-shaped in-plane AMR whose origin is not well understood. Our aim was to understand the mechanism behind this peculiar shape of AMR and also to probe AMR in the out-of-plane current direction. In contrast to the in-plane AMR, the polar out-of-plane AMR shows a surprisingly different response with a pronounced cusplike feature. The maximum of the cusplike anisotropy is reached when the magnetic field is oriented in the a-b plane. Moreover, the AMR for the azimuthal out-of-plane current direction exhibits a very strong fourfold a-b plane anisotropy. Combining the Fermi surfaces calculated from first principles with the Boltzmann's semiclassical transport theory, we reproduce all the prominent features of the unusual behavior of the in-plane and out-of-plane AMR. We can conclude that the dominant contribution the cusplike AMR lies in open orbits of the hole pocket and, in general, AMR is strongly influenced by charge compensation effect and the off-diagonal conductivity tensor elements, which give rise to peculiar butterfly-shaped AMR. Finally, the semiclassical model was also able to clarify the origin of strong nonsaturating (subquadratic) transverse magnetoresistance observed in this material, as an effect of imperfect charge-carrier compensation and open orbits.

AMER PHYSICAL SOC2019

Magnetoresistance from Fermi surface topology

Yi Liu, Quansheng Wu, Oleg Yazyev, Shengnan Zhang

2019