Volborthite compound is one of the very few realizations of S=1/2 quantum spins on a highly frustrated kagomé-like lattice. Low-T SQUID measurements reveal a broad magnetic transition below 2 K which is further confirmed by a peak in the 51 V nuclear spin relaxation rate (1/T1) at 1.4 K±0.2 K. Through 51 V NMR, the ground state (GS) appears to be a mixture of different spin configurations, among which 20% correspond to a well defined short range order, possibly of the √ 3 × √ 3 type. While the freezing involve all the Cu 2+ spins, only 40% of the copper moment is actually frozen which suggests that quantum fluctuations strongly renormalize the GS.

Using muon spin rotation, well-defined bulk approximately 100% magnetic phases in NaxCoO2 are revealed. A novel magnetic phase is detected for x=0.85 with the highest transition temperature ever observed for x>or=0.75. This stresses the diversity of x>or=0.75 magnetic phases and the link between magnetic and structural degrees of freedom. For the charge-ordered x=0.50 compound, a cascade of transitions is observed below 85 K. From a detailed analysis of our data, we conclude that the ordered moment varies continuously with temperature and suggest that the two secondary transitions at 48 and 29 K correspond to a moderate reorientation of antiferromagnetically coupled moments.

We present µSR experiments in the S = 3 2 kagomé bilayer compound Ba2Sn2ZnGa10−7pCr7pO22 (BSZCGO(p)) and compare it to the isostructural SrCr9pGa12−9pO19 (SCGO(p)), including for the latter new results for p ≥ 0.89. Quantum-dynamical low energy magnetic excitations are evidenced in this novel compound. We study the evolution of the muon relaxation rate with p, T and field. A phenomenological model for the muon relaxation based on sporadic dynamics due to spin excitations in a singlet sea proposed by Uemura et al. [Phys. Rev. Lett. 73, 3306 (1994)]. is extended to all fields and T -range. Its connexion to the RVB picture is discussed, and we argue that such coherent states might mediate the interactions between "impurities" which induce the spin glass freezing.PACS numbers: 75.40. Gb, 75.50.Lk, 76.75.+i The stabilization of a spin liquid state, in which all kinds of magnetic correlation functions are short ranged, seems now granted theoretically in the S = Among all corner sharing highly frustrated magnets, only a few are good candidates fulfilling the important conditions of the pure Heisenberg lattice with NN couplings. The combination of the weakness of the singleion anisotropy and of a direct overlap exchange are certainly the major advantages of the recently discovered chromate S = 3 2 kagomé bilayer BSZCGO(p) [5,6] and the long studied SCGO(p) [7], where p is the Cr 3+ network covering rate. Beyond the absence of ordering well below the Curie-Weiss temperature, the unusual large value of the specific heat unveils a high density of low lying excitations [5,8] and its field independence suggests that the excited states are mostly singlets [9]. Moreover, their GS is found essentially fluctuating as proven by neutron experiments [10], and µSR for SCGO(p) [11,12], although an intrinsic spin glass (SG) signature is observed in susceptibility measurements [7,13]. The origin of such a SG state in a disorder-free system still awaits for a complete understanding although recent theoretical approaches catch some of the facets of this original GS [14].In this context BSZCGO(p) appears as a unique compound : it has a more pronounced 2D character than SCGO(p), with a larger interbilayer distance [5] and all the magnetic sites lying on a corner sharing lattice [22]. In addition, a twice lower SG temperature T g has been reported [5] and a new type of defects other than nonmagnetic substitutions (likely bond defects), dominates the low-T susceptibility [6]. This offers the possibility of probing the influence of T g and the relevance of dilution on the evolution of the T → 0 dynamics, by a simple comparison of two quasi-identical kagomé bilayers.In this Letter, we present the first Muon Spin Relaxation (µSR) study of the spin dynamics in BSZCGO(p) and revisit the case of SCGO(p) for low dilutions now available. This technique has proven to be a front tool for the study of quantum dynamical states in a vast family of singlet GS systems. Whereas specific heat is sensitive to all kinds of excitations, including likely domi...

We present microscopic and macroscopic magnetic properties of the highly frustrated antiferromagnet Ba(2)Sn(2)ZnCr(7p)Ga(10-7p)O22, respectively, probed with NMR and SQUID experiments. The T variation of the intrinsic susceptibility of the Cr3+ frustrated Kagomé bilayer, chi(Kag), displays a maximum around 45 K. The dilution of the magnetic lattice has been studied in detail for 0.29

Compelling evidence for band-type conductivity and even bulk superconductivity below Tc approximately 8 K has been found in (69,71)Ga NMR experiments in crystalline ordered, giant Ga84 cluster compounds. This material appears to represent the first realization of a theoretical model proposed by Friedel in 1992 for superconductivity in ordered arrays of weakly coupled, identical metal nanoparticles.

compounds are two highly-frustrated magnets possessing a quasi-two-dimensional Kagomé bilayer of spin 3 2 / chromium ions with antiferromagnetic interactions. Their magnetic susceptibility was measured by local nuclear magnetic resonance and nonlocal (SQUID) techniques, and their low-temperature spin dynamics by muon spin resonance. Consistent with the theoretical picture drawn for geometrically frustrated systems, the Kagomé bilayer is shown here to exhibit: (i) short range spin-spin correlations down to a temperature much lower than the Curie-Weiss temperature, no conventional long-range transition occurring; (ii) a Curie contribution to the susceptibility from paramagnetic defects generated by spin vacancies; (iii) low-temperature spin fluctuations, at least down to 30 mK, which are a trademark of a dynamical ground state. These properties point to a spin-liquid ground state, possibly built on resonating valence bonds with unconfined spinons as the magnetic excitations.

Volborthite, Cu 3 V 2 O 7 (OH) 2 •2H 2 O, is a natural frustrated antiferromagnet (θ CW 130 K) with S = 1/2 spins (Cu 2+) sitting at the vertices of a Kagomélike lattice built on isosceles triangles. We report on the static (SQUID, 51 V NMR) magnetic properties of the pure and 5% Zn/Cu substituted compounds and on an extensive µSR study of the dilution effect (up to 15% Zn substitutions) on the spin dynamics. Although volborthite shares most of the unusual features already exhibited in Kagomé bilayer compounds, namely a dynamical state as T → 0 and a low temperature maximum in the local susceptibility, we found some surprising specificities. The T → 0 dynamical state is less robust against dilution and the low temperature local susceptibility studied by means of NMR also strongly depends on dilution. Such a sensitivity to dilution questions the role of the asymmetry of the exchange constants.

A neutron spin-echo investigation of the low temperature spin dynamics in two well-characterized kagomé bilayer compounds SrCr9xGa12-9xO19 (x=0.95, SCGO) and Ba2Sn2ZnCr7xGa10-7xO22 (x=0.97, BSZCGO) reveals two novel features. One is the slowing down of the relaxation rate without critical behavior at Tg, where a macroscopic spin-glass-like freezing occurs. The second is, in SCGO at 4 K (approximately Tg)