The classification of superconductors using muon spin rotation

In this paper we explore the role of muon spin rotation (μSR) techniques in the characterization and classification of superconducting materials. In particular we focus upon the Uemura classification scheme which considers the correlation between the superconducting transition temperature,T _c, and the effective Fermi temperature,T _F, determined from μSR measurements of the penetration depth. Within this scheme strongly correlated “exotic” superconductors, i.e, hightT _C cuprates, heavy fermions, Chevrel phases and the organic superconductors, form a common but distinct group, characterized by a universal scaling ofT _C withT _F such that 1/100 <T _C/T _F<1/10. For conventional BCS superconductorsT _C/T _F<1/1000. The results of new μSR measurements of the penetration depth in superconducting Y(Ni_1?x Co _x )₂B₂C and YB₆ are also presented. In Y(Ni_1?x Co _x )₂B₂C the decrease ofT _C with increasing Co concentration is linked to a marked decrease in the carrier density from 2.9·10²⁸ m^?3 atx=0 to 0.6·10²⁸ m^?3, atx=0.1, while the carrier mass enhancement remains almost constant at approximately 10. For YB₆ we find evidence of a modest enhancement of the carrier mass (m ^*/m=3), and a relatively low carrier density of 0.24·10²⁸ m^?3. These results are discussed within the Uemura classification scheme. It is found that neither Y(Ni_1?x Co _x )₂B₂C withT _c/T _F>>1/250 nor YB₆ withT _C/T _F>>1/340 can be definitively classified as either “exttic” or “conventional”, but instead the compounds display behavior which interpolates between the two regimes.     

Ferromagnetic spin alignment in head-to-tail coupled oligo(1, 4-phenyleneethynylene)s and Oligo(1,4-phenylenevinylene)s bearing pendant p-phenylenediamine radical cations

The intramolecular and long-range ferromagnetic coupling between p-phenylenediamine radical cations in head-to-tail coupled oligo(1, 4-phenyleneethynylene)s and oligo(1,4-phenylenvinylene)s between neighbors and next-nearest neighbors is described. UV/vis/near-IR experiments show that the radical cations are localized in the pendant p-phenylenediamine units of the conjugated oligomers. The ESR spectra of these oligo(1,4-phenyleneethynylene) and oligo(1, 4-phenylenvinylene) di(radical cation)s are consistent with those of a triplet state. A linear behavior is observed for the doubly integrated ESR intensity of the DeltaM(s) = +/-1 and DeltaM(s) = +/-2 signals with the inverse temperature (I approximately 1/T), consistent with Curie's law. This behavior indicates a triplet ground-state diradical with a large triplet-singlet energy gap or possibly a degeneracy of singlet and triplet states.     

Spin correlations in amorphous Bi2Sr2CaCu2Ox

Zero and longitudinal field SR has been used to probe the magnetic state of amorphous Bi₂Sr₂CaCu₂O_x prepared by rapid quenching from the melt. The results suggest quasi-static spin correlations between extremely dilute Cu²⁺ ions.Work performed while at the J J Thomson Physical Laboratory, University of Reading.     

Spin dynamics in the spin glass phase of Y(Mn1−xAlx)2

Zero field SR has been used to probe spin dynamics in the concentrated spin glass Y(Mn_0.9Al_0.1)₂. The spectra follow a stretched exponential form, G_z(t)=exp(–(t)), with reaching 1/3 as the freezing temperature, T_g=60K, is approached. The evolution of G_z(t) with temperature is suggestive of the behaviour of the spin correlation function found by Ogielski in Monte Carlo simulations of the dynamics of the Ising spin glass model. The SR results are correlated with results previously obtained by inelastic neutron scattering.     

Supramolecular phthalocyanine dimers based on the secondary dialkylammonium cation/dibenzo-24-crown-8 recognition motif

[formula: see text] New unsymmetrically substituted DB24C8-phthalocyanines, which are able to form complexes with suitable dialkylammonium cations, have been prepared. These complexes most probably have a pseudorotaxane geometry.