113 research outputs found
Solving spin quantum-master equations with matrix continued-fraction methods: application to superparamagnets
We implement continued-fraction techniques to solve exactly quantum master
equations for a spin with arbitrary S coupled to a (bosonic) thermal bath. The
full spin density matrix is obtained, so that along with relaxation and
thermoactivation, coherent dynamics is included (precession, tunnel, etc.). The
method is applied to study isotropic spins and spins in a bistable anisotropy
potential (superparamagnets). We present examples of static response, the
dynamical susceptibility including the contribution of the different relaxation
modes, and of spin resonance in transverse fields.Comment: Resubmitted to J. Phys. A: Math. Gen. Some rewriting here and there.
Discussion on positivity in App.D3 at request of one refere
13C NMR study of superconductivity near charge instability realized in beta"-(BEDT-TTF)4[(H3O)Ga(C2O4)3]C6H5NO2
To investigate the superconducting (SC) state near a charge instability, we
performed ^{13}C NMR experiments on the molecular superconductor
beta"-(BEDT-TTF)_{4}[(H_{3}O)Ga(C_{2}O_{4})_{3}]C_{6}H_{5}NO_{2}, which
exhibits a charge anomaly at 100 K. The Knight shift which we measured in the
SC state down to 1.5 K demonstrates that Cooper pairs are in spin-singlet
state. Measurements of the nuclear spin-lattice relaxation time reveal strong
electron-electron correlations in the normal state. The resistivity increase
observed below 10 K indicates that the enhanced fluctuation has an electric
origin. We discuss the possibility of charge-fluctuation-induced
superconductivity.Comment: 5 pages, 4 figure
Equilibrium susceptibilities of superparamagnets: longitudinal & transverse, quantum & classical
The equilibrium susceptibility of uniaxial paramagnets is studied in a
unified framework which permits to connect traditional results of the theory of
quantum paramagnets, \Sm=1/2, 1, 3/2, ..., with molecular magnetic clusters,
\Sm\sim5, 10, 20, all the way up, \Sm=30, 50, 100,... to the theory of
classical superparamagnets. This is done using standard tools of quantum
statistical mechanics and linear response theory (the Kubo correlator
formalism). Several features of the temperature dependence of the
susceptibility curves (crossovers, peaks, deviations from Curie law) are
studied and their scalings with \Sm identified and characterized. Both the
longitudinal and transverse susceptibilities are discussed, as well as the
response of the ensemble with anisotropy axes oriented at random. For the
latter case a simple approximate formula is derived too, and its range of
validity assessed, so it could be used in modelization of experiments.Comment: 32 pages, 5 figures. Submitted to J.Phys.Condens.Matte
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