Professor Stephen Blundell

Abstract:

A muon is an unstable spin-1/2 particle with a lifetime of 2.2 μs. Beams of spinpolarized positive muons can be prepared at accelerator facilities and then subsequently implanted in various types of condensed matter. Both the time and direction dependence of the subsequent positron emission can be monitored. This allows the precession and relaxation of the average muon-spin polarization to be measured and the local magnetic field in the sample to be directly inferred. The muon thus behaves essentially as a 'microscopic magnetometer' and is used to follow the magnetic order at a local level and to investigate both static and dynamic effects. This article outlines the principles of various experimental techniques that involve implanted muons, and reviews some recent experimental data on organic and molecular magnets.

Abstract:

Using a variant of the Corbino geometry in pulsed magnetic fields of up to 60 T, we have made direct measurements of the Hall potential in (Formula presented) within the quantum Hall regime. This method enables the in-plane components of the resistivity tensor, which are normally very difficult to measure, to be investigated and the nonlinear behavior of the sample’s (Formula presented) characteristics to be studied. It is found that an increasing probability of magnetic breakdown at higher fields leads to a degradation of the quantum Hall effect. © 1999 The American Physical Society.

Abstract:

We describe a quantum-mechanical model of Fermi-surface traversal resonance (FTR), a magneto-optical resonance that occurs in quasi-one-dimensional metals. We show that the predictions of this model are in quantitative agreement with earlier semiclassical models of FTR. The agreement between the two approaches, whose starting assumptions are very different, demonstrates that it is a fundamental property of quasi-one-dimensional systems. © 1999 The American Physical Society.

Abstract:

A positive muon is a spin-1/2 particle. Beams of muons with all their spins polarized can be prepared and subsequently implanted in various types of condensed matter. The subsequent precession and relaxation of their spins can then be used to investigate a variety of static and dynamic effects in a sample and hence to deduce properties concerning magnetism, superconductivity and molecular dynamics. Though strictly a lepton, and behaving essentially like a heavy electron, it is convenient to think of a muon as a light proton, and it is often found with a captured electron in a hydrogen-like atom known as muonium. This article outlines the principles of various experimental techniques which involve implanted muons and describes some recent applications. The use of muons in condensed matter physics has shed new light on subjects as diverse as passivation in semiconductors, frustrated spin systems, vortex lattice melting, and quantum diffusion of light particles.

Abstract:

We report muon-spin-rotation (Formula presented)SR) measurements on a series of compounds with composition (Formula presented) with strontium doping level x between 0 and 1. A magnetic transition is found in all the samples studied, which occurs at a composition-dependent temperature, (Formula presented) Below (Formula presented) clear precession signals are observed in zero applied magnetic field for all (Formula presented), indicating the existence of at least short-range magnetic order on a time scale longer than (Formula presented) s. Above (Formula presented) the magnetic correlation times decrease by several orders of magnitude. At (Formula presented), we find peaks in both (Formula presented) and the zero-temperature staggered magnetization, an observation which we attribute to the higher degree of localization of the holes at this doping level. The measurements of (Formula presented) as a function of x extend the determination of Néel temperatures by previous neutron diffraction and (Formula presented)SR measurements. © 1999 The American Physical Society.