Transverse-field muon spin rotation $(\text{TF-}\ensuremath{\mu}\text{SR})$ experiments in the heavy-fermion superconductor ${\text{PrOs}}_{4}{\text{Sb}}_{12}({T}_{c}=1.85\text{ }\text{K})$ suggest that the superconducting penetration depth $\ensuremath{\lambda}(T)$ is temperature independent at low temperatures, consistent with a gapped quasiparticle excitation spectrum. In contrast, radio frequency inductive measurements yield a stronger temperature dependence of $\ensuremath{\lambda}(T)$, indicative of point nodes in the gap. Muon Knight-shift measurements in the normal state of ${\text{PrOs}}_{4}{\text{Sb}}_{12}$ suggest that the perturbing effect of the muon charge on the neighboring ${\text{Pr}}^{3+}$ crystalline electric field is negligibly small and therefore is unlikely to cause the difference between the $\text{TF-}\ensuremath{\mu}\text{SR}$ and rf results. The discrepancy appears to be related to multiband superconductivity in ${\text{PrOs}}_{4}{\text{Sb}}_{12}$.

The effective superconducting penetration depth measured in the vortex state of PrOs4Sb12 using transverse-field muon spin rotation (TF-μSR) exhibits an activated temperature dependence at low temperatures, consistent with a nonzero gap for quasiparticle excitations. In contrast, Meissner-state radiofrequency (rf) inductive measurements of the penetration depth yield a T2 temperature dependence, suggestive of point nodes in the gap. A scenario based on the recent discovery of extreme two-band superconductivity in PrOs4Sb12 is proposed to resolve this difference. In this picture a large difference between large- and small-gap coherence lengths renders the field distribution in the vortex state controlled mainly by supercurrents from a fully gapped large-gap band. In zero field all bands contribute, yielding a stronger temperature dependence to the rf inductive measurements.

Highly disordered magnetism confined to individual weakly interacting vortices is detected by muon spin rotation in two different families of high-transition-temperature superconductors, but only in samples on the low-doping side of the low-temperature normal state metal-to-insulator crossover (MIC). The results support an extended quantum phase transition (QPT) theory of competing magnetic and superconducting orders that incorporates the coupling between CuO2 planes. Contrary to what has been inferred from previous experiments, the static magnetism that coexists with superconductivity near the field-induced QPT is not ordered. Our findings unravel the mystery of the MIC and establish that the normal state of high-temperature superconductors is ubiquitously governed by a magnetic quantum critical point in the superconducting phase.

We report on muon spin rotation measurements of the internal magnetic field distribution n(B) in the vortex solid phase of YBa2Cu3Oy (YBCO) single crystals, from which we have simultaneously determined the hole-doping dependences of the in-plane Ginzburg-Landau (GL) length scales in the underdoped regime. We find that T-c has a sublinear dependence on 1/lambda(2)(ab), where lambda(ab) is the in-plane magnetic penetration depth in the extrapolated limits T -> 0 and H -> 0. The power coefficient of the sublinear dependence is close to that determined in severely underdoped YBCO thin films, indicating that the same relationship between T-c and the superfluid density is maintained throughout the underdoped regime. The GL coherence length xi(ab) (vortex core size) is found to increase with decreasing hole-doping concentration and to exhibit a field dependence that is explained by proximity-induced superconductivity on the CuO chains. Both lambda(ab) and xi(ab) are enhanced near 1/8 hole doping, supporting the belief by some that stripe correlations are a universal property of high-T-c cuprates.

Watery bridges play the key role in a new basic structural motif exhibited by M(μ‐OH 2 ) 2 [Au(CN) 2 ] 2 (M=Cu, Ni) materials. This motif is unique for cyanometallate‐based polymers and rare in aqueous coordination chemistry: a 1D chain propagated by double aqua‐bridges. These metal–water chains, shown here cascading down a rock‐studded waterfall, aggregate through hydrogen bonds to form stacked ribbons in 3D. The magnetic properties of both compounds were studied by using short‐lived subatomic muons as highly sensitive local probes in zero‐magnetic field, as shown here. D. B. Leznoff, J. E. Sonier et al. describe these Cu II and Ni II cyanoaurate coordination polymers on page 6748 ff.

We present μSR measurements of the low temperature magnetic field dependence of the vortex core size in the multiband superconductor NbSe2. The spatially extended bound core states associated with the smaller energy gap rapidly delocalize with increasing magnetic field, resulting in a shrinking of the vortex core size. The more tightly bound core states associated with the larger energy gap, on the other hand, lead to a field-independent core size for fields greater than 4kOe. The field dependence of the extracted magnetic penetration depth is explained in terms of the effect of delocalized quasiparticles on the spatial field variation around the vortex cores.

Spin-glass magnetism confined to individual weakly interacting vortices is detected in two different families of high-transition-temperature (T_c) superconductors, but only in samples on the low-doping side of the low-temperature normal state metal-to-insulator crossover (MIC). Our findings unravel the mystery of the MIC, but more importantly identify the true location of the field-induced quantum phase transition (QPT) in the superconducting state. The non-uniform appearance of magnetism in the vortex state favours a surprisingly exotic phase diagram, in which spatially inhomogeneous competing order is stabilized at the QPT, and an `avoided' quantum critical point (QCP) is realized at zero magnetic field.

Muon spin rotation measurements in ultra-clean single crystals of heavily underdoped superconducting YBa2Cu3O6+x (x≈0.365) are presented. The material shows a sharp superconducting transition below T=15K. By field cooling and shifting the applied field, we show that the superconducting state pins magnetic flux and develops a flux lattice below Tc, indicating that the superconducting state exists throughout the sample on a microscopic scale. At the same temperature, a disordered magnetic state appears on a nanoscale with at least one well-defined internal field probed by the muon. These two states coexist on a nanometer lengthscale and over a narrow region of oxygen doping.

We report muon spin rotation measurements on a single crystal of the marginal type-II superconductor V. The measured internal magnetic field distributions are modeled assuming (i) solutions of the Ginzburg-Landau (GL) equations for an ideal vortex lattice obtained using an iterative procedure developed by Brandt [Phys. Rev. Lett. 78, 2208 (1997)], (ii) a variational GL method, and (iii) a modified London model. Remarkably the models yield qualitatively similar results. The magnetic penetration depth lambda and the coherence length xi determined from the data analysis exhibit strong field dependences, which are attributed to changes in the electronic structure of the vortex lattice. We find that the zero-field extrapolated value of lambda is essentially independent of the assumed model and agrees well with the value obtained by experimental techniques that probe the Meissner state. On the other hand, only fits to either of the GL models yield reliable values of xi.

We report muon spin rotation measurements on a 〈111〉 single crystal of the marginal type-II superconductor vanadium. The temperature dependence of the magnetic penetration depth is extracted from the data using an iterative solution of the Ginzburg–Landau (GL) equations. This model is valid for low- and high-κ superconductors. The results are compared with the modified London and analytical GL models.

Muon spin rotation and magnetization measurements in ultraclean single crystals of superconducting YBa2Cu3O6+x (x=0.375) reveal transitions at T=19 K to a bulk superconducting state and below T=15 K to a disordered antiferromagnetic state. The superconducting state pins magnetic flux. At the lowest temperatures an internal magnetic field with a magnitude close to that seen in the antiferromagnetic parent compound is present throughout the bulk of the sample. The two states coexist on a nanometer length scale, and over a narrow region of oxygen doping.

The magnetic field dependence of the vortex core size in the multiband superconductor NbSe2 has been determined from muon spin rotation measurements. The spatially extended nature of the quasiparticle core states associated with the smaller gap leads to a rapid field-induced shrinkage of the core size at low fields, while the more tightly bound nature of the states associated with the larger gap leads to a field-independent core size for fields greater than 4 kOe. A simple model is proposed for the density of delocalized core states that establishes a direct relationship between the field-induced reduction of the vortex core size and the corresponding enhancement of the electronic thermal conductivity. We show that this model accurately describes both NbSe2 and the single-band superconductor V3Si.

This paper has been withdrawn by the authors, because of results obtained from a more exhaustive study of both YBa_2Cu_3O_y and La_{2-x}Sr_xCuO_4 (see cond-mat/0610051).

Transverse-field muon spin relaxation rates in single crystals of the heavy-fermion superconductor PrOs4Sb12 (Tc = 1.85 K) are nearly constant in the vortex state for temperatures below ~0.5Tc. This suggests that the superconducting penetration depth lambda(T) is temperature-independent at low temperatures, consistent with a gapped quasiparticle excitation spectrum. In contrast, radiofrequency measurements yield a stronger temperature dependence of lambda(T), indicative of point nodes in the gap. A similar discrepancy exists in superconducting Sr2RuO4 which, like PrOs4Sb12, breaks time-reversal symmetry (TRS) below Tc, but not in a number of non-TRS-breaking superconductors.

We have performed magnetic force microscopy at various temperatures utilizing piezoelectric quartz tuning forks as probes. Due to their large force constants (∼104N∕m), quartz tuning forks are intrinsically less sensitive to force gradients than conventional cantilevers. However, we demonstrate that the technique of Q-control can be used to increase their sensitivity, making their use as probes for variable temperature magnetic force microscopy a viable option.