I.M. Gat

μSR studies of intercalated HfNCl superconductor

Abstract Muon spin relaxation measurements of the magnetic field penetration depth λ have been performed in a quasi two-dimensional (2-d) superconductor based on HfNCl intercalated with Li and organic molecules. The relaxation rate σ(T→0)∼0.5 μs −1 was obtained with transverse fields applied perpendicular to the aligned HfN planes. Using the relation σ∝λ −2 ∝n s2d /m ∗ (2-d superconducting carrier density/effective mass), we deduced the 2-d Fermi temperature TF2d. The ratio Tc/TF2d for the HfNCl-(Li) system is very close to those in two other quasi-2-d superconductors, YBa2Cu3O7 and (BEDT-TTF)2-X. Compared to the Kosterlitz–Thouless (KT) transition temperature TKT=TF2d/8 for the limit of strong coupling, Tcs of these systems are reduced by a factor of 3–4, indicating the importance of carrier overlapping and/or 3-d coupling.

Muon spin relaxation and susceptibility studies of the pure and diluted spin 1/2 kagomé-like lattice system (CuxZn(1-x))3V2O7(OH2) 2H2O.

Muon spin relaxation and magnetic susceptibility measurements have been performed on the pure and diluted spin 1/2 kagomé system (CuxZn(1-x))3V2O7(OH)2 2H2O. In the pure x=1 system we found a slowing down of Cu spin fluctuations with decreasing temperature towards T approximately 1 K, followed by slow and nearly temperature-independent spin fluctuations persisting down to T=50 mK, indicative of quantum fluctuations. No indication of static spin freezing was detected in either of the pure (x=1.0) or diluted samples. The observed magnitude of fluctuating fields indicates that the slow spin fluctuations represent an intrinsic property of kagomé network rather than impurity spins.

Superfluid density and volume fraction of static magnetism in stripe-stabilized La1.85−yEuySr0.15CuO4

Abstract Muon spin relaxation measurements have been performed in the optimally doped La1.85−yEuySr0.15CuO4, where the static spin stripes are stabilized by the rare-earth doping. Zero-field measurements indicate that the volume fraction VCu of static magnetism increases with increasing Eu concentration, while the ordered moment size remains constant. Transverse-field measurements show that n s /m ∗ (superconducting carrier density/effective mass) at T→0 is proportional to the non-magnetic volume fraction (1−VCu). These results indicate that superconductivity and static stripe magnetism occur in different spatial regions, exclusively, suggestive of microscopic phase separation.

Static magnetism in superconducting stage-4 La2CuO4+y(y=0.12)

Abstract Zero-field (ZF) and transverse-field (TF) μSR measurements have been performed on a superconducting La2CuO4+y crystal with y=0.12 where excess oxygen is intercalated to form a stage-4 structure. Measurements are made on the same single-crystal sample studied in the neutron scattering measurements of Lee et al. [Phys. Rev. B 60 (1999) 3643] who find that both superconductivity and static incommensurate magnetic modulation develop below T c ∼T N ∼42 K . The amplitude of the muon precession signal observed in ZF-μSR increases progressively below TN and saturates at T→0 to the value which indicates that a static magnetic field from Cu moments exists in only a part of muon sites corresponding to a magnetic site fraction of VM=41%. The observed frequency ∼3.5 MHz of the precession signal suggests the development of a large (at least 0.4 μ B ) static Cu moment near the “magnetic” muon sites. Comparing the TF-μSR results with computer simulations, we obtained VM(T→0)=42%.

Muon spin relaxation measurements of magnetic-field penetration depth in Ba8Si46

Abstract Muon spin relaxation (μSR) and high-field magnetization measurements have been performed in Ba8Si46, a superconductor with T c =8 K having a silicon clathrate three-dimensional cage structure. The upper critical field H c2 =6.5 T , determined from the magnetization, indicates a short coherence length ξ=72 A . The magnetic field penetration depth λ(T→0)=4000 A was obtained from the muon spin relaxation rate σ(T→0)∼0.46 μs −1 observed in transverse fields of 0.1–1 T. These values of ξ and λ indicate that this system is a strongly type-II superconductor with a high value of κ=56. We deduce the effective Fermi temperature TF and compare the present system with other superconducting systems in a plot of Tc versus TF.

Unconventional superconductivity in (TMTSF)2ClO4

Abstract We have performed detailed measurements to characterize the superconducting (T c ≈1.1 K ) state of (TMTSF)2ClO4 using both normal and deuterated samples. Using ZF-μSR measurements to search for spontaneous magnetic fields below Tc, we see no evidence for such fields within our resolution, thus we conclude the time reversal symmetry is likely not broken. We have also performed precession measurements in the vortex state of (TMTSF)2ClO4. The superconducting relaxation rate decreases strongly with increasing field, which may indicate a dimensional crossover in the nature of the vortex lattice. We estimate that the penetration depth is roughly 535 nm .

Exponential field distribution in Sr(Cu1−xZnx)2O3

Abstract We discuss the use of the Kubo Golden rule (KGR) in the derivation of new muon polarization functions for arbitrary static field distributions in both zero (ZF) and longitudinal (LF) applied fields. ZF/LF muon spin relaxation measurements in spin-ladder system Sr(Cu1−xZnx)2O3 are reported, with observations of dilute moment freezing for x≲0.008 and dense moment freezing for x≳0.008. In neither the dilute nor the dense case do the data conform to classic muon polarization functions. Using the KGR we derive new muon polarization functions to deal with both of these cases. The dilute case assumes dilute moments with a dense moment background, while the dense case assumes an exponential distribution of fields. Simulation of this system is consistent with an exponential distribution of fields.

Spin dynamics in the two-dimensional spin system SrCu2(BO3)2

Abstract Muon spin relaxation measurements have been performed in the two dimensional spin system SrCu2(BO3)2. As the temperature decreases, the relaxation rate increases below T=10 K , and shows a saturation below T=3 K . The line shape at low temperatures is nearly Gaussian, yet it cannot be easily decoupled by an application of longitudinal field. These results indicate the existence of magnetically active spins that show slowing down of fluctuations with decreasing temperatures, yet remain dynamic even at 0.1 K . We have also examined a sample doped with small amount of non-magnetic Zn2+ impurity, Sr(Cu0.99Zn0.01)2(BO3)2. The result was quite similar to the pure sample. Thus, anomalous behavior of the pure sample observed in low temperatures is not due to magnetic moments induced by impurities or a lattice defect.

Magnetic order of the disturbed spin-Peierls compound (Cu,Mg)GeO3

Abstract We have measured muon spin relaxation (μSR) in (Cu1−xMgx)GeO3, a spin-Peierls compound disturbed by the Mg (S=0) doping of the Cu ( S= 1 2 ) spin chain. Static magnetic order was observed in the zero-field μSR measurements below the Neel temperatures determined by the susceptibility measurements. The zero-field muon relaxation exhibits a characteristic line shape in disturbed spin-Peierls compounds; muon spin precession together with exponential static relaxation. The amplitude of the precession, which is a measure of spacial homogeneity of the ordered moment size, takes the maximum at Mg concentration x∼0.03, which is just above the phase boundary in the temperature–Mg concentration (T–x) phase diagram.

μSR studies of two-dimensional antiferromagnets CaV3O7 and SrV3O7

Abstract A discontinuous change of muon-spin precession frequencies in CaV3O7 is observed at T∼0.2 K , indicating a possible spin reorientation. Our results suggest that CaV3O7 possesses the identical spin direction as isostructural SrV3O7 below ∼0.2 K , although the spin directions in the two compounds are different at higher temperatures as determined by elastic neutron scattering.