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    On the "spin-freezing" mechanism in underdoped superconducting cuprates

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    The letter deals with the spin-freezing process observed by means of NMR-NQR relaxation or by muon spin rotation in underdoped cuprate superconductors. This phenomenon, sometimes referred as coexistence of antiferromagnetic and superconducting order parameters, is generally thought to result from randomly distributed magnetic moments related to charge inhomogeneities (possibly stripes) which exhibit slowing down of their fluctuations on cooling below Tc_c . Instead, we describe the experimental findings as due to fluctuating, vortex-antivortex, orbital currents state coexisting with d-wave superconducting state. A direct explanation of the experimental results, in underdoped Y1x_{1-x}Cax_xBa2_2Cu3_3O6.1_{6.1} and La2x_{2-x}Sr%_xCuO4_4, is thus given in terms of freezing of orbital current fluctuations

    Network patterns and strength of orbital currents in layered cuprates

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    In a frame of the tJGt-J-G model we derive the microscopical expression for the circulating orbital currents in layered cuprates using the anomalous correlation functions. In agreement with μ\mu-on spin relaxation (μ\muSR), nuclear quadrupolar resonance (NQR) and inelastic neutron scattering(INS) experiments in YBa2_2Cu3_3O6+x_{6+x} we successfully explain the order of magnitude and the monotonous increase of the {\it internal} magnetic fields resulting from these currents upon cooling. However, the jump in the intensity of the magnetic fields at Tc_c reported recently seems to indicate a non-mean-field feature in the coexistence of current and superconducting states and the deviation of the extended charge density wave vector instability from its commensurate value {\bf Q}(π,π\approx(\pi,\pi) in accordance with the reported topology of the Fermi surface

    Evidence for the formation of magnetic moments in the cuprate superconductor Hg0.8_{0.8}Cu0.2_{0.2}Ba2_2Ca2_2Cu3_3O8+δ_{8+\delta} below TcT_c seen by NQR

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    We report pure zero field nuclear magnetic resonance (NQR) measurements on the optimally doped three layer high-Tc T_{c} -compounds HgBaCaCuO and HgBaCaCuO(F) with TcT_c 134 K. Above Tc T_{c} two Cu NQR line pairs are observed in the spectra corresponding to the two inequivalent Cu lattice sites. Below Tc T_{c} the Cu NQR spectra show additional lines leading to the extreme broadened Cu NQR spectra at 4.2 K well known for the HgBaCaCuO compounds. The spin-lattice relaxation curves follow a triple exponential function with coefficients depend onto the saturation time (number of saturation pulses), whereas the spin-spin relaxation curve is described by a single exponential function. From the spin-lattice relaxation we deduced a complete removal of the Kramers degeneracy of the Cu quadrupole indicating that the additional lines are due to a Zeemann splitting of the 63/65^{63/65}Cu lines due to the spontaneous formation of magnetic moments within the CuO layers. Below 140 K, the spectra are well fitted by a number of 6 63/65^{63/65}Cu line pairs. From the number of the Cu lines, the position of the lines relative to each other and the complete removal of the Kramers degeneracy we deduced an orientation of the magnetic moments parallel to the symmetry axis of the electric field gradient tensor with magnitudes of the order of 1000 G. We also discuss the possible microscopic origin of the observed internal magnetic fields.Comment: 11 pages, 12 figure
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