Muons and magnets

Abstract:

The Hofstadter butterfly is the energy spectrum of an infinite square lattice, plotted as a function of the magnetic field. We illustrate a method of calculating similar spectra for finite lattices in a magnetic field, using methods that consider the appropriate molecular orbitals, and find that the spectra resemble the Hofstadter butterfly. We relate the bonding and antibonding orbitals used to describe small systems to the Landau levels of the infinite system. This approach provides an unusual, but instructive, method of introducing the physics of Landau levels from the basic quantum mechanics of small systems. © 2004 American Association of Physics Teachers.

Abstract:

The spin-crossover phenomenon is a cooperative low-spin (LS) to high-spin (HS) transition which can be initiated using temperature or light irradiation. We have used muon-spin relaxation (muSR) to study this transition in two salts which show this effect. muSR provides local magnetic information and hence a means of examining this transition from a local perspective. For both salts, the LS phase gives rise to root-exponential relaxation which we associate with a dilute distribution of fluctuating moments resulting from incomplete spin crossover. The low temperature HS fraction which remains is small but can be altered by rapid cooling. We relate the observed muon relaxation to the underlying fluctuating moment distribution and compare our results to Monte-Carlo simulations of these distributions.

Abstract:

The response of the superconducting charge carriers to an applied magnetic field has been probed in a number of organic superconductors using muSR. Systematic studies have been made of the in-plane superconducting penetration depth lambda in superconductors based on the ET and BETS molecules. These studies have revealed a strong correlation between lambda and T-c. This type of correlation was first noted in high T-c cuprates by Uemura et al who found a linear relation between T-c and lambda(-2). In the case of organic superconductors, however, we find that the overall con-elation is better described as T-c proportional to lambda(-3). Understanding this characteristic trend should provide some important clues about the nature of the superconductivity in the organic superconductors; several possible models are considered here for describing this behaviour. It is concluded that models in which the superconducting carrier concentration is tied to the nominal band filling are inconsistent with the observed scaling behaviour.

Abstract:

We present the results of numerical calculations of the energy levels and eigenfunctions of finite sections of a tight-binding square lattice in the presence of a perpendicular magnetic field. The energy spectrum of such a system, plotted as a function of magnetic field, resembles the Hofstadter butterfly found for the infinite system. In high magnetic fields, each eigenstate carries a persistent current which has a chirality associated with whether the eigenstate exists in the bulk or the edge of the system. We present simulations of electric dipole transitions between the chirally distinguished states for an isotropic lattice. These transitions correspond to harmonics in the cyclotron resonance.

Abstract:

Radical ion salts of the neutral organic radicals, 2-imidazolyl nitronyl nitroxide (2-IMNN) and 2-benz-imidazolyl nitronyl nitroxide (2-BIMNN), and a diradical cation salt of pyridine-2,6-diylbis(nitronyl nitroxide) (2,6-PYBNN), were prepared. The radical ion salts of 2-BIMNN, Li+(2-BIMNN)(.-) and (2-BIMNN).-BF4-, exhibit strong antiferromagnetic interactions with exchange coupling constants J/k = -42 K and -510 K, respectively, while the 2-IMNN salts show Curie-Weiss behaviour with Weiss constants theta = -9.0 K for Li+(2-IMNN)(.-) and theta = -1.8 K for (2-IMNN)Cl-.-(-), indicating weak interactions. The diradical cation salt (2,6-PYBNN)Cl-..+(-) 14 shows intramolecular ferromagnetic interaction with J/k = +4.7 K and intermolecular antiferromagnetic interaction with theta = -0.7 K, which is remarkably reduced from J/k = -57 K observed in the diradical 2,6-PYBNN.