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.

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We report on muon spin rotation (µSR) 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 Tc has a sublinear dependence on 1/λ 2 ab , where λ 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 Tc and the superfluid density is maintained throughout the underdoped regime. The GL coherence length ξ ab (vortex core size) is found to increase with decreasing hole doping concentration, and exhibit a field dependence that is explained by proximity-induced superconductivity on the CuO chains. Both λ ab and ξ ab are enhanced near 1/8 hole doping, supporting the belief by some that stripe correlations are a universal property of high-Tc cuprates.

We report muon spin rotation measurements on the conventional type-II superconductor V3Si that provide clear evidence for changes to the inner structure of a vortex due to the delocalization of bound quasiparticle core states. The experimental findings described here confirm a key prediction of recent microscopic theories describing interacting vortices. The effects of vortex-vortex interactions on the magnetic and electronic structure of the vortex state are of crucial importance to the interpretation of experiments on both conventional and exotic superconductors in an applied magnetic field.PACS numbers: 74.20.Fg, 74.25.Qt, 74.70.Ad, 76.75.+i In 1964, a breakthrough paper by Caroli, de Gennes and Matricon [1] showed that in the framework of the microscopic theory, quasiparticles (QPs) bound to an isolated vortex of a conventional s-wave type-II superconductor occupy discrete energy levels. Twenty-five years later, localized vortex core states were observed for the first time in NbSe 2 by scanning tunneling microscopy (STM) [2]. Our understanding of the vortex state in type-II superconductors has accordingly progressed from Abrikosov's initial prediction [3] based on the macroscopic Ginzburg-Landau (GL) theory [4], to current theories describing the electronic structure of magnetic vortices on a microscopic level. However, it is only in recent years that predictions have emerged from the microscopic theory on the effects of vortex-vortex interactions. In analogy with bringing atoms close together to form a conducting solid, increasing the vortex density by applying a stronger magnetic field H enhances the overlap of bound state wave functions of neighboring vortices, resulting in the formation of energy bands that allow the intervortex transfer of QPs [5,6,7,8]. This is expected to strongly influence experiments on conventional superconductors that are sensitive to quasiparticle excitations, such as specific heat and thermal transport, and to have profound effects on the magnetic structure of the vortex state. However, understanding the potential interplay between vortices and quasiparticles is also of crucial importance in the study of high-temperature superconductors. In these and other exotic superconductors, comparatively little is known about the structure of the vortex state and its effect on experiments in large magnetic fields. It is therefore essential to have a solid understanding of the behavior of interacting vortices in conventional superconductors, and to establish the connections with quasiparticle properties.The effect of delocalized QP core states on the spatial variation of the pair potential ∆(r) at a vortex site has been considered in the framework of the quasiclassical Eilenberger theory [7,8,9]. These calculations show that the effect of the intervortex transfer of QPs on ∆(r) leads to a reduction of the size of the vortex cores with increasing H. Such shrinking of the vortex cores has in fact been observed by muon spin rotation (µSR) [10,11,12,13,14,15,16] and STM [17], and proposed as...

The structures of two cyanoaurate-based coordination polymers, M(mu-OH(2))(2)[Au(CN)(2)](2) (M=Cu, Ni), were determined by using a combination of powder and single-crystal X-ray diffraction techniques. The basic structural motif for both polymers contains rarely observed M(mu-OH(2))(2)M double aqua-bridges, which generate an infinite chain; two trans [Au(CN)(2)](-) units also dangle from each metal center. The chains form ribbons that interact three dimensionally through CNH hydrogen bonding. The magnetic properties of both compounds and of the dehydrated analogue Cu[Au(CN)(2)](2) were investigated by direct current (dc) and alternating current (ac) magnetometry; muon spin-relaxation data was also obtained to probe their magnetic properties in zero-field. In M(mu-OH(2))(2)[Au(CN)(2)](2), ferromagnetic chains of M(mu-OH(2))(2)M are present below 20 K. Interchain magnetic interactions mediated through hydrogen bonding, involving water and cyanoaurate units, yield a long-range magnetically ordered system in Cu(mu-OH(2))(2)[Au(CN)(2)](2) below 0.20 K, as indicated by precession in the muon spin polarization decay. Ni(mu-OH(2))(2)[Au(CN)(2)](2) undergoes a transition to a spin-glass state in zero-field at 3.6 K, as indicated by a combination of muon spin-relaxation and ac-susceptibility data. This transition is probably due to competing interactions that lead to spin frustration. A phase transition to a paramagnetic state is possible for Ni(mu-OH(2))(2)[Au(CN)(2)](2) upon application of an external field; the critical field was determined to be 700 Oe at 1.8 K. The dehydrated compound Cu[Au(CN)(2)](2) shows weak antiferromagnetic interactions at low temperatures.