Dipôle magnétique | EPFL Graph Search

vignette|Dipôle magnétique de la Terre Un dipôle magnétique est l'équivalent pour le champ magnétique de ce qu'est un dipôle électrostatique pour le champ électrique. Il est entièrement caractérisé par le vecteur moment magnétique (ou moment dipolaire magnétique), l'équivalent pour le magnétisme de ce qu'est le moment dipolaire pour l'électrostatique. Boucle de courant La représentation matérielle la plus simple d'un dipôle magnétique est une boucle de courant, c'est-à-dire un courant électrique circulaire. Le moment magnétique de ce dipôle élémentaire est le vecteur \vec\mu=I,\vec S, où I est l'intensité du courant et \vec S le vecteur surface (vecteur de module égal à l'aire S du cercle, d'origine O au centre du cercle, dirigé suivant l'axe du cercle, et orienté en fonction du sens du courant selon la règle du tire-bouchon). Au sens strict, un dipôle magnétique est la limite d'une boucle de courant quand on fait tendr

Shape staggering of midshell mercury isotopes from in-source laser spectroscopy compared with density-functional-theory and Monte Carlo shell-model calculations

Neutron-deficient Hg177-185 isotopes were studied using in-source laser resonance-ionization spectroscopy at the CERN-ISOLDE radioactive ion-beam facility in an experiment combining different detection methods tailored to the studied isotopes. These include either alpha-decay tagging or multireflection time-of-flight gating for isotope identification. The endpoint of the odd-even nuclear shape staggering in mercury was observed directly by measuring for the first time the isotope shifts and hyperfine structures of Hg177-185. Changes in the meansquare charge radii for all mentioned isotopes, magnetic dipole, and electric quadrupole moments of the odd-A isotopes and arguments in favor of I = 7/2 spin assignment for Hg-177,Hg-179 were deduced. Experimental results are compared with density functional theory (DFT) and Monte Carlo shell model (MCSM) calculations. DFT calculations using Skyrme parametrizations predict a jump in the charge radius around the neutron N = 104 midshell, with an odd-even staggering pattern related to the coexistence of nearly degenerate oblate and prolate minima This near-degeneracy is highly sensitive to many aspects of the effective interaction, a fact that renders perfect agreement with experiments out of reach for current functionals. Despite this inherent difficulty, the SLy5s1 and a modified UNEDF1(SO) parametrization predict a qualitatively correct staggering that is off by two neutron numbers. MCSM calculations of states with the experimental spins and parities show good agreement for both electromagnetic moments and the observed charge radii. A clear mechanism for the origin of shape staggering within this context is identified: a substantial change in occupancy of the proton pi h(9/2) and neutron nu i(13/2) orbitals.

AMER PHYSICAL SOC2019

Nonreciprocal surface acoustic wave propagation via magneto-rotation coupling

Dirk Grundler

A fundamental form of magnon-phonon interaction is an intrinsic property of magnetic materials, the “magnetoelastic coupling.” This form of interaction has been the basis for describing magnetostrictive materials and their applications, where strain induces changes of internal magnetic fields. Different from the magnetoelastic coupling, more than 40 years ago, it was proposed that surface acoustic waves may induce surface magnons via rotational motion of the lattice in anisotropic magnets. However, a signature of this magnon-phonon coupling mechanism, termed magneto-rotation coupling, has been elusive. Here, we report the first observation and theoretical framework of the magneto-rotation coupling in a perpendicularly anisotropic film Ta/CoFeB(1.6 nanometers)/MgO, which consequently induces nonreciprocal acoustic wave attenuation with an unprecedented ratio of up to 100% rectification at a theoretically predicted optimized condition. Our work not only experimentally demonstrates a fundamentally new path for investigating magnon-phonon coupling but also justifies the feasibility of the magneto-rotation coupling application.

2020

A quantum magnetic analogue to the critical point of water

Kazimierz Conder, Ellen Fogh, Julio Antonio Larrea Jiménez, Frédéric Mila, Bruce Normand, Henrik Moodysson Rønnow, Ludger Weber, Mohamed Zayed

At the liquid-gas phase transition in water, the density has a discontinuity at atmospheric pressure; however, the line of these first-order transitions defined by increasing the applied pressure terminates at the critical point(1), a concept ubiquitous in statistical thermodynamics(2). In correlated quantum materials, it was predicted(3) and then confirmed experimentally(4,5) that a critical point terminates the line of Mott metal-insulator transitions, which are also first-order with a discontinuous charge carrier density. In quantum spin systems, continuous quantum phase transitions(6) have been controlled by pressure(7,8), applied magnetic field(9,10) and disorder(11), but discontinuous quantum phase transitions have received less attention. The geometrically frustrated quantum antiferromagnet SrCu2(BO3)(2) constitutes a near-exact realization of the paradigmatic Shastry-Sutherland model(12-14) and displays exotic phenomena including magnetization plateaus(15), low-lying bound-state excitations(16), anomalous thermodynamics(17) and discontinuous quantum phase transitions(18,19). Here we control both the pressure and the magnetic field applied to SrCu2(BO3)(2) to provide evidence of critical-point physics in a pure spin system. We use high-precision specific-heat measurements to demonstrate that, as in water, the pressure-temperature phase diagram has a first-order transition line that separates phases with different local magnetic energy densities, and that terminates at an Ising critical point. We provide a quantitative explanation of our data using recently developed finite-temperature tensor-network methods(17,20-22). These results further our understanding of first-order quantum phase transitions in quantum magnetism, with potential applications in materials where anisotropic spin interactions produce the topological properties(23,24) that are useful for spintronic applications.

2021

A quantum magnetic analogue to the critical point of water

Kazimierz Conder, Ellen Fogh, Julio Antonio Larrea Jiménez, Frédéric Mila, Bruce Normand, Henrik Moodysson Rønnow, Ludger Weber, Mohamed Zayed

2021