The Apparatus

The measurements were performed on the M15 surface $\mu ^{+}$ beamline at TRIUMF. The beam consisted of low momentum (28.6MeV/c), $100\%$backward spin polarized positive muons. The short range of these muons makes this beamline ideal for the study of thin samples. A crossed-field separator was used to rotate the muon spins perpendicular to their momentum direction. After passing through a 1cm diameter collimator and a thin muon defining counter, a small fraction of the incoming beam came to rest in the crystals ( $30,000 \mu^{+} \mbox{s}^{-1}$ stops out of $200,000 \mu^{+} \mbox{s}^{-1}$ incoming).

Great effort was devoted to reducing the background signal in this experiment, which superimposes itself on the measured field distribution originating from the sample. The background signal originates from muons stopping in the sample holder or in the cryostat walls and windows, which then precess in the externally applied field. Previous $\mu ^{+}$SR experiments on single crystals have been seriously plagued by the inseparable nature of the often large background signal [77]. A novel experimental setup was used to eliminate most of the background signal produced by those muons which missed the sample [86]. The three crystals were mounted on a thin layer of aluminized mylar using a minute portion of Apiezon N grease, with their c-axes parallel to the applied field. Recall with this orientation of the field it is $\lambda _{ab} (T)$which is measured. The thin aluminized mylar provides no appreciable signal. The three crystals together provided a total area of $36 \mbox{mm}^{2}$ for the incoming muon beam. Data taken with only one of the three single crystals of $\mbox{YBa$_{2}$ Cu$_{3}$ O$_{6.95}$ }$, showed no further appreciable reduction in the background signal and gave the same foreground signal as the 3-crystal sample. Consequently, a mosaic consisting of all three single crystals was used to maximize the counting rates for muons striking the sample.

 

The mylar with the mosaic of crystals was stretched over the end of a hollow 4.45cm-diameter, cylindrical, aluminum sample holder, as indicated in Fig. 4.1. A horizontal ⁴He gas-flow cryostat with an internal diameter of 4.92cm, allowed cooling of the crystalline $\mbox{YBa$_{2}$ Cu$_{3}$ O$_{6.95}$ }$ down to $\sim 2.6$K. A 7T warm-bore superconducting magnet called Helios was used to produce magnetic fields transverse to the initial muon spin direction.

The positrons emitted from the muon decay were readily detected by a cylindrical arrangement of positron counters, coaxial with the magnet axis. Overlapping forward (F) and backward (B) counters were used to define a solid angle for muon decay events originating from the sample. A veto (V) counter in the form of a cylindrical scintillator cup, was employed to discriminate against those muons which missed the sample. A ``good'' muon stop was defined as $M \cdot \bar{V}$ ($M \equiv$ muon counter). A ``good'' positron stop (i.e. a positron originating from a muon that stopped in the sample) was defined as $F \cdot B \cdot \bar{V}$.