Zeroand longitudinal-field muon-spin relaxation (it+ SR) measurements have been performed on La2Sr„Cu04 alloys in both single-crystal and sintered powder forms above and below their magnetic transition temperatures, Tf. The p+ precession frequency v depends only weakly on x and Tf, an observation which together with resistivity data implies classical freezing of magnetic moments in the regime where the carriers are localized. For x 0.05, critical dynamics are observed near Tf. The p SR technique is shown to be very sensitive to ferromagnetically aligned pairs of Cu + moments; the population of such pairs increases greatly with x.

We present a study of the vortex lattice in untwinned YBa2Cu3O7−x crystals, using a combination of muon spin rotation and neutron small angle scattering measurements. Both methods show a very sharp melting temperature consistent with a first order transition. The dependence of the melting temperature on the angle of the field with respect to the crystallographic c-axis is studied. The results are compared to thermal measurements.

We present a study of the vortex lattice in untwinned YBa2Cu3O7−x crystals, using a combination of muon spin rotation and neutron small angle scattering measurements. Both methods show a very sharp melting temperature consistent with a first order transition. The dependence of the melting temperature on the angle of the field with respect to the crystallographic c-axis is studied. The results are compared to thermal measurements.

We investigate the superconducting behaviour of Bi doped Pb. Pure lead shows type-I behaviour entering an intermediate state in a magnetic field. High dopings of Bi (>3%) lead to type-II behaviour showing a mixed state, where the magnetic field penetrates the superconductor in the form of a flux lattice. At intermediate doping, the sample shows both type-I and type-II behaviour depending on the temperature. This arises because the Ginzburg-Landau parameter κ passes through its critical value of 1/ √

The asymmetry signal of a transverse field μSR time histogram is primarily the Fourier transform of the local field distribution P(b) for samples with static local fields b. The maximum entropy technique is used to produce a μSR lineshape which automatically deconvolutes the pulse structure of the beam (for pulsed facilities) and the time window. In this paper, we describe how to combine several component kernels to construct the transformation matrix for converting the lineshape into a time signal. We discuss the implications for improving the μSR maximum entropy (ME) algorithm under certain circumstances, and how dipolar broadening might be deconvoluted safely. This transformation may also be used to create accurate theoretical time signals for conventional fitting.

The maximum entropy (ME) analysis of μSR time spectra is used to produce the local field distribution P(b). The ME technique has user-defined constants in addition to the usual data manipulation parameters (such as the time range and binning) used in traditional fitting or Fourier transforms. The ME constants are the default level (used to suppress signals below the noise level) and a looseness factor (used to smooth the line shape). In this paper, we discuss how to automatically select values for these constants to ensure that the resulting P(b) is minimally distorted from the ideal and that the calculated errors are sensible.

In muon spin rotation (μSR) measurements, the details of the local field distribution P(b) are of interest, so either a fast Fourier transform (FFT) or the maximum entropy method (MEM) is applied to extract P(b) from the μSR time spectra. Ideally, one would like to analyze the resulting P(b) as if it were a data set in its own right: fitting P(b) to appropriate theoretical functions and calculating its mean, second moment and third moment. This is only possible if there are rigorous errors associated with P(b). In this paper, we show that the maximum entropy method finds its maximum in a way analogous to χ2 fitting. Calculation of a covariance matrix naturally follows. The covariance matrix demonstrates that the maximum entropy (ME) result P(b) has intrinsically correlated, not independent, errors. With proper care, P(b) can be fit and have its moments calculated. This method is not limited to μSR applications, it can be applied to other cases involving Fourier transforms of data, such as NMR.

We present a study of the vortex lattice in untwinned YBa$_2$Cu$_3$O$_{7-x}$ crystals, using a combination of muon spin rotation and neutron small angle scattering measurements. Both methods show a very sharp melting temperature consistent with a first order transition. The dependence of the melting temperature on the angle of the field with respect to the crystallographic c-axis is studied. The results are compared to thermal measurements.

We investigate the superconducting behaviour of Bi doped Pb. Pure lead shows type-I behaviour entering an intermediate state in a magnetic field. High dopings of Bi (>3 the magnetic field penetrates the superconductor in the form of a flux lattice. At intermediate doping, the sample shows both type-I and type-II behaviour depending on the temperature. This arises because the Ginzburg-Landau parameter κ passes through its critical value of 1/√(2) with temperature.

We have used a variety of microscopic techniques to reveal the structure and motion of flux line arrangements, when the flux lines in low Tc type II superconductors are caused to move by a transport current. Using small-angle neutron scattering by the flux line lattice (FLL), we are able to demonstrate directly the alignment by motion of the nearest-neighbour FLL direction. This tends to be parallel to the direction of flux line motion, as had been suspected from two-dimensional simulations. We also see the destruction of the ordered FLL by plastic flow and the bending of flux lines. Another technique that our collaboration has employed is the direct measurement of flux line motion, using the ultra-high-resolution spectroscopy of the neutron spin-echo technique to observe the energy change of neutrons diffracted by moving flux lines. The μSR technique gives the distribution of values of magnetic field within the FLL. We have recently shown that one can perform μSR measurements while the FLL is moving. Such measurements give complementary information about the local speed and orientation of the FLL motion. We conclude by discussing the possible application of this technique to thin film superconductors.

The recent development at the Paul Scherrer Institute of a beam of low energy muons allows depth dependent muon spin rotation and relaxation investigations in thin samples, multilayers and near surface regions (low energy μSR, LE-μSR). After a brief overview of the LE-μSR method, some representative experiments performed with this technique will be presented. The first direct determination of the field profile just below the surface of a high-temperature superconductor in the Meissner phase illustrates the power and sensitivity of low energy muons as near-surface probe and is an example of general application to depth profiling of magnetic fields. The evolution of the flux line lattice distribution across the surface of a YBa2Cu3O7 film in the vortex phase has been investigated by implanting muons on both sides of a normal-superconducting boundary. A determination of the relaxation time and energy barrier to thermal activation in iron nanoclusters, embedded in a silver thin film matrix (500nm), demonstrates the use of slow muons to measure the properties of samples that cannot be made thick enough for the use of conventional μSR. Other experiments investigated the magnetic properties of thin Cr(001) layers at thicknesses above and below the collapse of the spin density wave.

This paper describes a study of superparamagnetism in iron nanoclusters using low energy μSR, including for the first time zero field measurements, which yield an intrinsic nanocluster superparamagnetic relaxation rate of ν0=67±15 MHz and an energy barrier of ΔE=37±4 K. TEM micrographs of the embedded clusters show there to be a range of shapes and also sizes, which is bourne out by SQUID magnetometry measurements.

The range of low-energy μ+ in a thin Al film deposited on a quartz glass substrate has been investigated as a function of the μ+ implantation energy by measuring the amplitude of the diamagnetic signal in a transverse field experiment. The μ+ implantation energy was varied between 3.4 and 29.4 keV. The μ+ asymmetry, which reflects the fraction of μ+ stopped in Al, is found to decrease with increasing energy. The results are compared and found in good agreement with predictions of a Monte Carlo program, which simulates the implantation profiles of muons in matter.

The phenomenon of superconductivity continues to be of considerable scientific and practical interest. Underlying this phenomenon is the formation of electron pairs, which in conventional superconductors do not rotate about their centre of mass (‘ s -wave’ pairing; refs 1 , 2 ). This contrasts with the situation in high-temperature superconductors, where the electrons in a pair are believed to have two units of relative angular momentum (‘ d -wave’ pairing; ref. 3 and references therein). Here we report small-angle neutron-scattering measurements of magnetic flux lines in the perovskite superconductor Sr 2 RuO 4 ( ref. 4 ), which is a candidate for another unconventional paired electron state—‘ p -wave’ pairing, which has one unit of angular momentum 5 , 6 , 7 . We find that the magnetic flux lines form a square lattice over a wide range of fields and temperatures, which is the result predicted by a recent theory 8 , 9 of p -wave superconductivity in Sr 2 RuO 4 . This theory also indicates that only a fraction of the electrons are strongly paired and that the orientation of the square flux lattice relative to the crystal lattice will determine which parts of the three-sheet Fermi surface of this material are responsible for superconductivity. Our results suggest that superconductivity resides mainly on the ‘γ’ sheet 9 .

The asymmetry signal of a mu SR time histogram is theoretically the Fourier transform of the local field distribution P(b), for samples with static local fields. Commonly, P(b) is extracted from the time histograms by either a fast Fourier transform (FFT) or maximum entropy (ME). In this paper, we discuss how ME suppresses noise, and directly compare ME line shapes with FFTs. (C) 2000 Elsevier Science B.V. All rights reserved.

We have measured the diffracted neutron scattering intensities from the square magnetic flux lattice in the perovskite superconductor Sr2RuO4, which is thought to exhibit p-wave pairing with a two-component order parameter. The relative intensities of different flux lattice Bragg reflections over a wide range of field and temperature have been shown to be inconsistent with a single component Ginzburg-Landau theory but qualitatively agree with a two-component p-wave Ginzburg-Landau theory.

This round table discussion considered coordinating improvements in the portability and quality of μSR analysis software. Panel members were Tanya Riseman, Ivan D. Reid (PSI), Steve P. Cottrell (ISIS), Jess H. Brewer (TRIUMF), and Mark Koennecke (PSI); Mark is a developer of the new neutron and X-ray scattering data format NeXus.

We report on low energy muon spin rotation (LE-μSR) measurements of the magnetic penetration depth λab of a 700 nm thick YBa2Cu3O7−δ film. The temperature dependence of the penetration depth is in good agreement with the results from conventional μSR measurements on single crystals. This demonstrates the potential of the new technique for thin film studies.

The phenomenon of superconductivity continues to be of considerable scientific and practical interest. Underlying this phenomenon is the formation of electron pairs, which in conventional superconductors do not rotate about their centre of mass ('s-wave' pairing; refs 1, 2). This contrasts with the situation in high-temperature superconductors, where the electrons in a pair are believed to have two units of relative angular momentum ('d-wave' pairing; ref. 3 and references therein). Here we report small-angle neutron-scattering measurements of magnetic flux lines in the perovskite superconductor Sr2RuO4 (ref. 4), which is a candidate for another unconventional paired electron state-'p-wave' pairing, which has one unit of angular momentum(5-7). We find that the magnetic flux lines form a square lattice over a wide range of fields and temperatures, which is the result predicted by a recent theory(8,9) of p-wave superconductivity in Sr2RuO4. This theory also indicates that only a fraction of the electrons are strongly paired and that the orientation of the square flux lattice relative to the crystal lattice will determine which parts of the three-sheet Fermi surface of this material are responsible for superconductivity. Our results suggest that superconductivity resides mainly on the 'gamma' sheet(9).