Special issue on μSR: muon spin rotation, relaxation or resonance

Abstract

To a particle physicist a muon is a member of the lepton family, a heavy electron possessing a mass of about 1/9 that of a proton and a spin of 1/2, which interacts with surrounding atoms and molecules electromagnetically. Since its discovery in 1937, the muon has been put to many uses, from tests of special relativity to deep inelastic scattering, from studies of nuclei to tests of weak interactions and quantum electrodynamics, and most recently, as a radiographic tool to see inside heavy objects and volcanoes. In 1957 Richard Garwin and collaborators, while conducting experiments at the Columbia University cyclotron to search for parity violation, discovered that spin-polarized muons injected into materials might be useful to probe internal magnetic fields. This eventually gave birth to the modern field of μSR, which stands for muon spin rotation, relaxation or resonance, and is the subject of this special issue of Journal of Physics: Condensed Matter. Muons are produced in accelerators when high energy protons (generally >500 MeV) strike a target like graphite, producing pions which subsequently decay into muons. Most experiments carried out today use relatively low-energy (~4 MeV), positively-charged muons coming from pions decaying at rest in the skin of the production target. These muons have 100% spin polarization, a range in typical materials of about 180 mg cm-2, and are ideal for experiments in condensed matter physics and chemistry. Negatively-charged muons are also occasionally used to study such things as muonic atoms and muon-catalysed fusion. The μSR technique provides a local probe of internal magnetic fields and is highly complementary to inelastic neutron scattering and nuclear magnetic resonance, for example. There are four primary μSR facilities in the world today: ISIS (Didcot, UK), KEK (Tsukuba, Japan), PSI (Villigen, Switzerland) and TRIUMF (Vancouver, Canada), serving about 500 researchers world-wide. A new facility, JPARC (Tokai, Japan), is currently being built to replace the current Japanese μSR capability at KEK. These μSR institutions provide scientists a variety of sample environments, including a range of temperatures, magnetic fields and applied pressure. In addition, very low-energy muon beams (< 1 keV) have been developed for studies of thin films and nano-materials. In 2002 this world-wide community founded the International Society of μSR Spectroscopy (http://musr.org/~isms/) in order to promote the health of this growing field of research.The 20 papers presented in this volume are intended to highlight some of the current μSR research activities of interest to condensed matter physicists. It is not an exhaustive review. In particular, the active and exciting area of muonium chemistry is left to a future volume. The group of papers in section I addresses the physics of strongly correlated electrons in solids, one of the most active fields of condensed matter research today. Strong electron correlations arise from (Coulomb) interactions which render Landau's theory of electron transport for weakly interacting systems invalid. Included in this category are unconventional heavy-fermion superconductors, high-temperature copper-oxide superconductors, non-Fermi liquid (NFL) systems and systems with strong electron-lattice-spin coupling, such as the colossal magnetoresistance manganites. Two key properties often make the muon a unique probe of these materials: (1) the muon's large magnetic moment (~3 μp) renders it extremely sensitive to the tiny magnetic fields (~1 Gauss) found, for example, in many NFL systems and in superconductors possessing time-reversal-violating order parameters, and (2) the muon's spin 1/2 creates a simple μSR lineshape (no quadrupolar coupling), ideal for measuring spin-lattice-relaxation, local susceptibilities and magnetic-field distributions in ordered magnets and superconductors. Section II contains studies which exploit the unique sensitivities of μSR just noted to elucidate new and hidden properties of novel magnetic materials, including the use of very low energy muons to study thin films. Sections I and II are concerned with the bare positive muon as a probe of internal magnetic fields in metals. The papers in section III describe studies which exploit the fact that in semiconductors the muon appears as a light isotope of the hydrogen atom, called muonium. These studies provide important new information regarding the electronic structure and motion of light, dilute hydrogen-like impurities in semiconductors, which is useful for both semiconductor fundamentals and applications. Finally, in section IV experiments which probe electron transfer in large molecular systems are presented, including future prospects for investigating some materials of biological interest. These latter experiments exploit the sensitivity of muonium to the motion of electrons inside molecular systems, the so-called `labeled-electron' method.It is our hope that even this limited perspective shows the extraordinary degree to which the μSR technique is contributing deeply to our understanding of condensed matter physics. Richard Garwin's early experiments have indeed borne unexpected fruit!The editor is very grateful to all the invited authors for their timely contributions to this special section of Journal of Physics: Condensed Matter.