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 report the determination of the Dzyaloshinsky-Moriya interaction, the dominant magnetic anisotropy term in the kagome spin-1/2 compound ZnCu3(OH)6Cl2. Based on the analysis of the high-temperature electron spin resonance (ESR) spectra, we find its main component |Dz|=15(1) K to be perpendicular to the kagome planes. Through the temperature dependent ESR linewidth, we observe a building up of nearest-neighbor spin-spin correlations below approximately 150 K.

We report, through 17O NMR, an unambiguous local determination of the intrinsic kagome lattice spin susceptibility as well as that created around nonmagnetic defects arising from natural Zn/Cu exchange in the S=1/2 (Cu2+) herbertsmithite ZnCu3(OH)6Cl2 compound. The issue of a singlet-triplet gap is addressed. The magnetic response around a defect is found to markedly differ from that observed in nonfrustrated antiferromagnets. Finally, we discuss our relaxation measurements in the light of Cu and Cl NMR data and suggest a flat q dependence of the excitations.

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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.

Shubnikov-de Haas (SdH) oscillations and upper critical magnetic field (Hc2) of the iron-based superconductor FeSe (Tc = 8.6 K) have been studied by tunnel diode oscillatorbased measurements in magnetic fields of up to 55 T and temperatures down to 1.6 K. Several Fourier components enter the SdH oscillations spectrum with frequencies definitely smaller than predicted by band structure calculations indicating band renormalization and reconstruction of the Fermi surface at low temperature, in line with previous ARPES data. The Werthamer-Helfand-Hohenberg model accounts for the temperature dependence of Hc2 for magnetic field applied both parallel (H ab) and perpendicular (H c) to the iron conducting plane, suggesting that one band mainly controls the superconducting properties in magnetic fields despite the multiband nature of the Fermi surface. Whereas Pauli pair breaking is negligible for H c, a Pauli paramagnetic contribution is evidenced for H ab with Maki parameter α = 2.1, corresponding to Pauli field HP = 36.5 T.

We report17 O NMR measurements in the S = 1/2 (Cu 2+ ) kagomé antiferromagnet Herbertsmithite ZnCu3(OH)6Cl2 down to 45 mK in magnetic fields ranging from 2 T to 12 T. While Herbertsmithite displays a gapless spin-liquid behavior in zero field, we uncover an instability toward a spin-solid phase at sub-kelvin temperature induced by an applied magnetic field. The latter phase shows largely suppressed moments < ∼ 0.1µB and gapped excitations. The H − T phase diagram suggests the existence of a quantum critical point at the small but finite magnetic field µ0Hc = 1.55(25) T. We discuss this finding in light of the perturbative Dzyaloshinskii-Moriya interaction which was theoretically proposed to sustain a quantum critical regime for the quantum kagomé Heisenberg antiferromagnet model.Quantum spin liquids (QSL) are appealing states of matter which do not break any symmetry of the spin Hamiltonian. While QSL behaviors are well established for one-dimensional spin systems, their existence in higher dimensions remains questionable. The quantum kagomé Heisenberg antiferromagnet (QKHA) is considered as the best candidate to stabilize such a QSL phase in two dimensions [1][2][3]. The specificity of this model relies on the unique combination of strong quantum fluctuations enhanced by low spins S = 1/2 and high geometrical frustration of the lattice of corner-sharing triangles. Early numerical studies have shown, despite finite size limitations, that the ground state of the QKHA model does not support any simple order parameter [4][5][6] and have evidenced an original excitation spectrum with dense sets of low energy excitations in all spin sectors [5,6]. A recent state-of-the-art calculation [7] favors a QSL ground state with a small gap much like the resonating-valence-bond state [8,9]. Other recent proposals with similar ground-state energies encompass large unit-cell valence-bond-crystals [10] as well as gapless 'critical' spin liquids with algebraic spin correlations [11,12]. Clearly, the exact ground state of QKHA remains a challenging and highly debated issue [13,14]. In this respect, studying the effect of perturbation, such as disorder, anisotropies or external field, is not only necessary to compare to real materials but also proves to be efficient to discriminate between the competing ground states -critical QSL are expected to be easily destabilized [15,16] while gapped ones should be more robust.To confront theories, a significant step was achieved with the synthesis [17] of the first "structurally perfect" QKHA, ZnCu 3 (OH) 6 Cl 2 , called Herbertsmithite. The electron spin moments of Cu 2+ ions (S=1/2) form undistorted kagomé planes, well separated from each other by diamagnetic Zn 2+ triangular planes which ensure the quasi-two-dimensionality of the magnetic net ( Fig. 1). Despite a sizable antiferromagnetic superexchange J = 180(10) K, Herbertsmithite develops no onsite magnetization [18,19] and remains in an unfrozen, fluctuating state under zero field with short-ranged spin correlations down to at least 50...

Analytical formulae for de Haas-van Alphen (dHvA) oscillations in linear chain of coupled two-dimensional (2D) orbits (Pippard's model) are derived systematically taking into account the chemical potential oscillations in magnetic field. Although corrective terms are observed, basic (α) and magnetic-breakdown-induced (β and 2β − α) orbits can be accounted for by the Lifshits-Kosevich (LK) and Falicov-Stachowiak semiclassical models in the explored field and temperature ranges. In contrast, the "forbidden orbit" β − α amplitude is described by a non-LK equation involving a product of two classical orbit amplitudes. Furthermore, strongly non-monotonic field and temperature dependence may be observed for the second harmonics of basic frequencies such as 2α and the magnetic breakdown orbit β + α, depending on the value of the spin damping factors. These features are in agreement with the dHvA oscillation spectra of the strongly 2D organic metal θ-(ET)4CoBr4(C6H4Cl2).