T. M. Riseman

First observation of an antiferromagnetic phase in the Y1Ba2Cu3Ox system

The magnetic properties of four different phases of the YBa2Cu3Ox system, including the famous 90 K superconductor, have been probed by the positive muon spin relaxation method (µ+SR). The oxygen-deficient tetragonal insulating phase (x~6.2) was found to be magnetically ordered, thus most likely antiferromagnetic, near room temperature.

Static magnetic order in La1.875Ba0.125CuO4

Zero field muon spin relaxation measurements of high quality, single phase La1.875Ba0.125CuO4 indicate that this system exhibits static magnetic order below 38 K. At this x = 18 concentration, superconductivity is suppressed. The ordered spins most likely reside on the copper sites.

μ+SR Studies of Magnetic Properties of the YBa2Cu3Ox System

Samples of YBa 2 Cu 3 O x in four different phases have been probed by positive muon spin relaxation (µ + SR) method. Each phase was found to show different magnetic behavior. In the famous orthorhombic 90K superconductor ( x ∼6.9), neither magnetic ordering nor magnetic fluctuations were observed. However, in another orthorhombic superconducting phase ( x ∼6.4, T c ≈60 K) a magnetic fluctuation was observed below 7 K via muon spin-nuclear spin double relaxation. Its magnetic ordering temperature was expected to be below 2.4 K. Similar behavior was observed in an oxygen-rich ( x ∼6.5) tetragonal insulating phase, whereas the oxygen-deficient ( x ∼6.2) tetragonal insulating phase was found to be magnetically ordered, most likely antiferromagnetic, near room temperature. This is the first observation of a magnetically ordered state in the YBa 2 Cu 3 O x system.

Quantum diffusion of muonium in GaAs

The diffusion rate of muonium in the III–V compound semiconductor GaAs has been determined from measurements of muon spinT1 relaxation induced by motion in the presence of nuclear hyperfine interactions. It is shown for the first time in a semiconductor that (a) there is a crossover of the transport mechanism at about 90 K from stochastic to zero-phonon hopping, as evidenced by a steep rise in the hop rate at lower temperatures, and that (b) the muonium diffuses at the hop rate of 1010 s−1 (corresponding diffusion constantD≈10−6 cm2s−1) at lower temperatures as well as at room temperature.

Positive muon Knight shift and spin relaxation in heavy fermion superconductors UPt3 and UBe13

Abstract We report muon spin rotation/relaxation (μSR) measurements of the heavy fermion superconductors UBe 13 and UPt 3 . In both materials we find that the muon Knight shift is unchanged in the superconducting state, consistent with odd-parity pairing (such as p-wave). The magnetic field penetration depths in UPt 3 and UBe 13 are extremely long, greater than 10000 A. We find no evidence of a magnetic transition in UBe 13 below 10 K.

Site of the positive muon in YBa2Cu3O7

A combination of μSR and17O nuclear quadrupolar μLCR has been used to obtain detailed information on the muon site in YBa2Cu3O7. The rms magnitude of the nuclear dipolar fields in YBa2Cu3O7 enriched with 38%17O is about twice that in the natural16O sample. In addition the17O μLCR spectrum shows that the electric field gradient tensor at the oxygen closest to the muon is almost equal to that measured by NMR for the CuO2 plane sites O(2,3). These results suggest that the muon in fully oxygenated YBa2Cu3O7 is located at a single site 1.0 Å from an oxygen ion, probably O(2,3).

Probing magnetism and superconductivity of high-Tc systems with positive muons

Abstract Recent results from our μSR experiments on high- T c systems are presented. We describe studies of static magnetic order in hole-doped and electron-doped systems, correlations between T c and n s / m * (carrier density / effective mass) in hole-doped superconducting cuprates, the temperature dependence of the penetration depth in c-axis oriented sintered ceramic of Y Ba 2 Cu 3 O 7 , as well as the detection of flux pinning phenomena by μSR.

Zero-field muon spin rotation studies in (TMTSF)2PF6

We report zero-field muon spin rotation studies of the organic compound (TMTSF)2PF6 at ambient pressure. An extremely sharp onset of the spin density wave state (SDW) is observed below the transition temperature TM = 12.2 K, which may be first order in nature. The saturation of the magnetic order at low temperatures can be ascribed to the existence of the SDW gap. The broad distribution of local magnetic fields deduced from μSR time spectra is consistent with that expected from an incommensurate SDW.

μ+ SR studies on Sr2CuO2Cl2, La2CuO4 and Nd2CuO4: 2-d magnetism, local fields and muon sitesSR studies on Sr2CuO2Cl2, La2CuO4 and Nd2CuO4: 2-d magnetism, local fields and muon sites

We compare the temperature dependence of the zero-field muon spin precession frequency observed in Sr2CuO2Cl2 and La2CuO4 below the Néel temperature with a 2-d Heisenberg model with an additional small anisotropy. Good agreement of the observed results with this model reflects the highly 2 dimensional magnetic properties of these cuprates. The measured local fields are compared with a simple calculation of dipolar fields in the crystal to search for muon site locations. Our results provide evidence that the muon sites are related to the oxygen atoms in these compounds.

Temperature dependence of the magnetic penetration depth in YBa2Cu3O7 measured by muon spin rotation

We report an improved measurement of the temperature dependence of the magnetic penetration depth in sintered YBa2Cu3O77 using muon spin rotation. The effective isotropic average of the magnetic penetration depth (λ) is estimated from the second moment of the field distribution in the vortex state in an external field of 15kG. The low temperature behaviour of λ is still consistent with an s-wave pairing mechanism although there are deviations from the conventional BCS-like behaviour for T/Tc > 0.2.