Low-energy excitations in electron-doped metal phthalocyanine from NMR in Li0.5_{0.5}MnPc

$^7$Li and $^1$H NMR and magnetization measurements in \lpc (Pc$\equiv$C$_{32}$H$_{16}$N$_8$), recently proposed as a strongly correlated metal, are presented. Two different low-frequency dynamics are evidenced. The first one, probed by $^1$H nuclei gives rise to a slowly relaxing magnetization at low temperature and is associated with the freezing of MnPc $S=3/2$ spins. This dynamic is similar to the one observed in pristine $\beta$-MnPc and originates from Li depleted chain segments. The second one, evidenced by $^7$Li spin-lattice relaxation rate, is associated with the hopping of the electrons along Li-rich chains. The characteristic correlation times for the two dynamics are derived and the role of disorder is briefly discussed.Comment: 7 two-columns pages, 11 figure

Detection of entanglement between collective spins

Entanglement between individual spins can be detected by using thermodynamics quantities as entanglement witnesses. This applies to collective spins also, provided that their internal degrees of freedom are frozen, as in the limit of weakly-coupled nanomagnets. Here, we extend such approach to the detection of entanglement between subsystems of a spin cluster, beyond such weak-coupling limit. The resulting inequalities are violated in spin clusters with different geometries, thus allowing the detection of zero- and finite-temperature entanglement. Under relevant and experimentally verifiable conditions, all the required expectation values can be traced back to correlation functions of individual spins, that are now made selectively available by four-dimensional inelastic neutron scattering

Magnetic field induced non-Fermi liquid to Fermi liquid crossover at the quantum critical point of YbCu5−x_{5-x}Aux_{x}

The temperature (T) dependence of the muon and $^{63}$Cu nuclear spin-lattice relaxation rates $1/T_1$ in YbCu4.4Au0.6 is reported over nearly four decades. It is shown that for $T\to 0$ $1/T_1$ diverges following the behaviour predicted by the self-consistent renormalization (SCR) theory developed by Moriya for a ferromagnetic quantum critical point. On the other hand, the static uniform susceptibility $\chi_s$ is observed to diverge as $T^{-2/3}$ and $1/T_1T\propto \chi_s^2$, a behaviour which is not accounted for by SCR theory. The application of a magnetic field $H$ is observed to induce a crossover to a Fermi liquid behaviour and for $T\to 0$ $1/T_1$ is found to obey the scaling law $1/T_1(H)= 1/T_1(0)[1+(\mu_BH/k_BT)^2]^{-1}$.Comment: 4 pages, 4 figure

Vortex Lattice Melting of a NbSe2 single grain probed by Ultrasensitive Cantilever Magnetometry

Using dynamic cantilever magnetometry, we study the vortex lattice and its corresponding melting transition in a micrometer-size crystallite of superconducting NbSe2. Measurements of the cantilever resonance frequency as a function of magnetic field and temperature respond to the magnetization of the vortex-lattice. The cantilever dissipation depends on thermally activated vortex creep motion, whose pinning energy barrier is found to be in good agreement with transport measurements on bulk samples. This approach reveals the phase diagram of the crystallite, and is applicable to other micro- or nanometer-scale superconducting samples.Comment: 5 pages, 4 figure

A view from inside iron-based superconductors

Muon spin spectroscopy is one of the most powerful tools to investigate the microscopic properties of superconductors. In this manuscript, an overview on some of the main achievements obtained by this technique in the iron-based superconductors (IBS) are presented. It is shown how the muons allow to probe the whole phase diagram of IBS, from the magnetic to the superconducting phase, and their sensitivity to unravel the modifications of the magnetic and the superconducting order parameters, as the phase diagram is spanned either by charge doping, by an external pressure or by introducing magnetic and non-magnetic impurities. Moreover, it is highlighted that the muons are unique probes for the study of the nanoscopic coexistence between magnetism and superconductivity taking place at the crossover between the two ground-states.Comment: 28 pages, 18 figure

NMR as a probe of the relaxation of the magnetization in magnetic molecules

We investigate the time autocorrelation of the molecular magnetization $M(t)$ for three classes of magnetic molecules (antiferromagnetic rings, grids and nanomagnets), in contact with the phonon heat bath. For all three classes, we find that the exponential decay of the fluctuations of $M(t)$, associated with the irreversible exchange of energy with the heat bath, is characterized by a single characteristic time $\tau (T,B)$ for not too high temperature $T$ and field $B$. This is reflected in a nearly single-lorentzian shape of the spectral density of the fluctuations. We show that such fluctuations are effectively probed by NMR, and that our theory explains the recent phenomenological observation by Baek et al. (PRB70, 134434) that the Larmor-frequency dependence of $1/T_1$ data in a large number of AFM rings fits to a single-lorentzian form.Comment: Published as Phys. Rev. Letters 94, 077203 (2005) in slightly reduced for

Quantum-gate implementation in permanently coupled AF spin rings without need of local fields

We propose a scheme for the implementation of quantum gates which is based on the qubit encoding in antiferromagnetic molecular rings. We show that a proper engineering of the intercluster link would result in an effective coupling that vanishes as far as the system is kept in the computational space, while it is turned on by a selective excitation of specific auxiliary states. These are also shown to allow the performing of single- and two-qubit gates without an individual addressing of the rings by means of local magnetic fields.Comment: To appear in Physical Review Letter

Many-body models for molecular nanomagnets

We present a flexible and effective ab-initio scheme to build many-body models for molecular nanomagnets, and to calculate magnetic exchange couplings and zero-field splittings. It is based on using localized Foster-Boys orbitals as one-electron basis. We apply this scheme to three paradigmatic systems, the antiferromagnetic rings Cr8 and Cr7Ni and the single molecule magnet Fe4. In all cases we identify the essential magnetic interactions and find excellent agreement with experiments.Comment: 5 pages, 3 figure

Superconductivity emerging from an electronic phase separation in the charge ordered phase of RbFe2_2As2_2

$^{75}$As, $^{87}$Rb and $^{85}$Rb nuclear quadrupole resonance (NQR) and $^{87}$Rb nuclear magnetic resonance (NMR) measurements in RbFe$_2$As$_2$ iron-based superconductor are presented. We observe a marked broadening of $^{75}$As NQR spectrum below $T_0\simeq 140$ K which is associated with the onset of a charge order in the FeAs planes. Below $T_0$ we observe a power-law decrease in $^{75}$As nuclear spin-lattice relaxation rate down to $T^*\simeq 20$ K. Below that temperature the nuclei start to probe different dynamics owing to the different local electronic configurations induced by the charge order. A fraction of the nuclei probes spin dynamics associated with electrons approaching a localization while another fraction probes activated dynamics possibly associated with a pseudogap. These different trends are discussed in the light of an orbital selective behaviour expected for the electronic correlations.Comment: 5 pages, 3 figures and 4 pages of supplemental materia