We report muon spin rotation measurements on the S=1/2 (Cu2+) paratacamite ZnxCu4-x(OH)6Cl2 family. Despite a Weiss temperature of approximately -300 K, the x=1 compound is found to have no transition to a magnetic frozen state down to 50 mK as theoretically expected for the kagomé Heisenberg antiferromagnet. We find that the limit between a dynamical and a partly frozen ground state occurs around x=0.5. For x=1, we discuss the relevance to a singlet picture.

We confirm the recent prediction that interstitial protium may act as a shallow donor in zinc oxide, by direct spectroscopic observation of its muonium counterpart. On implantation into ZnO, positive muons--chemically analogous to protons in this context--form paramagnetic centers below about 40 K. The muon-electron contact hyperfine interaction, as well as the temperature and activation energy for ionization, imply a shallow level. Similar results for the cadmium chalcogenides suggest that such shallow donor states are generic to the II-VI compounds. The donor level depths should serve as a guide for the electrical activity of interstitial hydrogen.

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We have performed extensive zero field µSR experiments on pure YBa2Cu3O6+y and diluted Yrare-earth substituted Y0.92Eu0.08Ba2Cu3O6+y and Y0.925Nd0.075Ba2Cu3O6+y at light hole-doping. A common magnetic behavior is detected for all the three families, demonstrating negligible effects of the isovalent Y-substituent disorder. Two distinct regimes are identified, separated by a crossover, whose origin is attributed to the concurrent thermal activation of spin and charge degrees of freedom: a thermally activated and a re-entrant antiferromagnetic regime. The peculiar temperature and hole density dependence of the magnetic moment m(h, T ) fit a model with a (spin) activation energy for the crossover between the two regimes throughout the entire investigated range. The magnetic moment is suppressed by a simple dilution mechanism both in the re-entrant regime (0 ≤ h ≤ 0.056) and in the so-called Cluster Spin Glass state coexisting with superconductivity (0.056 < h 0.08). We argue a common magnetic ground state for these two doping regions and dub it frozen antiferromagnet. Conversely either frustration or finite-size effects prevail in the thermally activated antiferromagnetic state, that vanishes at the same concentration where superconductivity emerges, suggesting the presence of a quantum critical point at hc = 0.056(2).

We present extensive magnetic measurements of the (CaxLa1−x)(Ba1.75−xLa0.25+x)Cu3Oy system with its four different families (x) having a T max c (x) variation of 28% and minimal structural changes. For each family, we measured the Néel temperature, the anisotropies of the magnetic interactions, and the spin glass temperature. Our results exhibit a universal relation Tc = cJns for all families, where c ∼ 1, J is the in plane Heisenberg exchange, and ns is the carrier density. This relates cuprate superconductivity to magnetism in the same sense that phonon mediated superconductivity is related to atomic mass.The critical temperature for superconductivity T c in metallic superconductors varies with isotope substitution [1]. This observation, known as the isotope effect, played a key role in exposing their mechanism for superconductivity. In contrast, the mechanism for superconductivity in the cuprate is still elusive, but is believed to be of magnetic origin [2]. Verifying this belief would require an experiment similar to the isotope effect, namely, a measurement of T c versus the magnetic interaction strength J, with no other structural changes in the compounds under investigation. Here we present such an experiment using the (Ca x La 1−x )(Ba 1.75−x La 0.25+x )Cu 3 O y (CLBLCO) system with its four different superconducting families, for which maximum T c (T max c ) varies by 28%. This is a large change compared to Sn, which has the strongest isotope effect in nature where T c varies only by 4%. For each family, we measured the Néel Temperature T N and the anisotropies of the magnetic interactions. This allows us to obtain the Heisenberg coupling J. In addition, we determine the spin glass temperature T g of underdoped samples. J, T g and T c allow us to generate a unified phase diagram for magnetism and superconductivity from no doping to over doping. We combine this result with a previous determination of the superconducting carrier density n s [3], and demonstrate experimentally a magnetic analog of the isotope effect.