Papers by Pietro Carretta
Il Nuovo Cimento D, 1994
Summary An overview on some works recently carried out by means of pulsed NMR-NQR experiments is... more Summary An overview on some works recently carried out by means of pulsed NMR-NQR experiments is given. The choice of the arguments is aimed at providing a flash on the topics and phenomena most tipically studied with the afore-mentioned approach: from the antiferromagnets (AF) precursors of high-temperature superconductors upon charge and spin doping (such as CuO-doped Li, La2CuO4-doped Sr and Zn)
Magnetization and 63Cu NMR-NQR relaxation measurements are used to study the superconducting fluc... more Magnetization and 63Cu NMR-NQR relaxation measurements are used to study the superconducting fluctuations in YBa2Cu3O6+x (YBCO) oriented powders. In optimally doped YBCO the fluctuating negative magnetization Mfl(H, T) is rather well described by an anisotropic Ginzburg-Landau (GL) functional and the curves Mfl/ √ H cross at Tc. In underdoped YBCO, in- stead, over a wide temperature range an anomalous diamagnetism
Physical Review B, 1998
The spin dynamics of Mn spins in the dodecanuclear manganese cluster of formula ͓Mn 12 O 12 ͑CH 3... more The spin dynamics of Mn spins in the dodecanuclear manganese cluster of formula ͓Mn 12 O 12 ͑CH 3 COO͒ 16 ͑H 2 O͒ 4 ͔•2CH 3-COOH•4H 2 O͑Mn 12) has been investigated by 1 H NMR and muon spinlattice relaxation rate 1/T 1 as a function of temperature ͑10-400 K͒ and external magnetic field ͑0-9.4 T͒. At room temperature, the proton 1/T 1 depends on the measuring frequency. The results can be interpreted in terms of a slow decay of the Mn electronic-spin autocorrelation function, a feature characteristic of the almost zero dimensionality of the system. As the temperature is lowered, 1/T 1 displays a critical enhancement that can be related to the slowing down of the local spin fluctuations as the cluster approaches the condensation into the total spin Sϭ10 ground-state configuration. It is found that the application of an external magnetic field greatly depresses the enhancement of 1/T 1 an effect that could be related to the superparamagnetic behavior of the Mn 12 molecule. ͓S0163-1829͑98͒06801-5͔
International Journal of Modern Physics B, 2000
ABSTRACT The overdoped regime in Y1-xCaxBa2Cu3O7, for x=0.05 and x=0.10 (Tc(0)=82 K and Tc(0) = 7... more ABSTRACT The overdoped regime in Y1-xCaxBa2Cu3O7, for x=0.05 and x=0.10 (Tc(0)=82 K and Tc(0) = 73.8 K respectively) is studied by means of 89Y NMR relaxation and SQUID magnetization measurements. It is shown that for hole concentration close to 0.19, where a critical point is suspected and where the antiferromagnetic correlations should disappear, 1/(T1T) is strictly constant from room temperature down to Tc. This behavior is also observed in the sample at Ca-content 0.10, where the hole concentration is 0.203. The magnetization as a function of temperature and the isothermal magnetization curves, reveal the occurrence of a marked enhancement of the fluctuating diamagnetism in the overdoped regime, with respect to optimally doped YBCO. The magnetization data cannot be justified by anisotropic Ginzburg-Landau functional for the free energy or by scaling arguments. These findings are tentatively related to superconducting "drops" created above Tc or to phase fluctuations in restricted dimensionality.
NMR-MRI, μSR and Mössbauer Spectroscopies in Molecular Magnets
ABSTRACT Phase transitions (PT) are typical collective phenomena occurring in many-body systems w... more ABSTRACT Phase transitions (PT) are typical collective phenomena occurring in many-body systems with interparticle interactions. A rather general description of the PT’s and of the accompanying “critical dynamics” can be given in the framework of Landau-type statistical theory, which includes more specific theories such as the Weiss mean field theory for magnetic systems and the Van der Waals theory for fluids. This approach is suited for second order or slightly first-order PT3.
Physical review. B, Condensed matter, 1993
... In this way Jeff has been related to a correlation time of the form (h cc(COeXchnh )1 cxeE(x)... more ... In this way Jeff has been related to a correlation time of the form (h cc(COeXchnh )1 cxeE(x)/TE(x) being the energy gap from localized to itinerant states. ... Mag. Reson. 12, 39 (1990). 18R. G. Graham, PC Riedi, and BM Wanklyn, J. Phys.: Condens. Matter 3, 135 (1991). 19G. ...
Physical review. B, Condensed matter, 1996
Evidence is presented of superconducting fluctuations in the 63Cu NQR-NMR relaxation rate in YBa2... more Evidence is presented of superconducting fluctuations in the 63Cu NQR-NMR relaxation rate in YBa2Cu3O7-delta, as obtained from a careful comparison of measurements carried out in the absence and in the presence of a field parallel to the c axis. It is shown that the field causes a reduction of the relaxation rate W in a range of about 10 K
Physical Review Letters, 1994
We describe muon spin relaxation measurements of the magnetic heavy fermion compound CeCu22Si2. S... more We describe muon spin relaxation measurements of the magnetic heavy fermion compound CeCu22Si2. Static magnetic order appears below about 1 K, most likely of spin glass nature. We find that the superconducting transition onsetting at 0.6 K partially destroys the magnetic state and that the magnetic volume fraction decreases within the superconducting state. This observation supports the idea that in CeCu22Si2, magnetic order and superconductivity compete with each other rather than coexist.
Physical Review B, 2003
63,65 Cu NQR-NMR relaxation measurements in CeCu 6Ϫx Au x for xϭ0, xϭ0.1, and xϭ0.8 are used to d... more 63,65 Cu NQR-NMR relaxation measurements in CeCu 6Ϫx Au x for xϭ0, xϭ0.1, and xϭ0.8 are used to derive insights on temperature ͑T͒ and magnetic-field ͑H͒ dependences of the spin dynamics around the quantum critical point ͑QCP͒. The relaxation rate 2W is related to the k integrated, low energy, generalized susceptibility (k,,H,T→0). For xϭ0 a Fermi-liquid behavior is detected, while for xϭ0.8 the temperature dependence of 2W is the one expected for a nearly antiferromagnetic metal ordering at T N ϭ2.2 K. Instead, for xϭ0.1, around the QCP, a response function of the form suggested by neutron scattering, namely of twodimensional character with anomalous exponent and (/T) scaling, is found to explain the main experimental findings. An effect is observed in the low-temperature range for Hտ1 T, with a crossover to a gapped phase for the spin excitations at a field-dependent temperature.
Physical Review B, 1993
Zero-field copper NMR measurements on YBa2Cu3O6 up to 380 K are reported. The temperature depende... more Zero-field copper NMR measurements on YBa2Cu3O6 up to 380 K are reported. The temperature dependence of the local magnetic moment at the Cu(2) plane site is compared with the predictions of spin-wave theory, based on the exchange coupling constants obtained from inelastic neutron scattering. The agreement is satisfactory, within the limits of the theoretical approximations, but discrepancies with elastic neutron-scattering
Physical Review B, 1995
La NQR and zero-field @+SR in antiferromagnetic (AF) LazCu, "Zn"O~, for x up to 0.13 and in the t... more La NQR and zero-field @+SR in antiferromagnetic (AF) LazCu, "Zn"O~, for x up to 0.13 and in the temperature range 1.6-350 K, are used to study the effects related to the substitution of magnetic Cu + 5 =-' with homovalent diamagnetic S =0 Zn in La&Cu04. We report measurements both of static magnetic properties, such as Neel temperatures T~, sublattice magnetization and field~h~at the La nucleus or at the p site, as well as of NQR relaxation rates lV. These quantities are used to study the effects of Zn doping on the low-energy Cu + spin excitations. It is found that T& decreases with x in a way close to the one expected by diluting quasi-two-dimensional Heisenberg magnets on square lattice, while the sublattice magnetization is slightly affected by Zn doping. Mean-field arguments based on the dilution model for the interplanar interactions allow one to conclude that the in-plane magnetic correlation length is little sensitive to the Zn presence. Up to x =-0.08 the temperature dependence of the AF field~h~is close to the one in pure LaiCuO&, with a sharp decrease for T~T~indicative of a continuous transition with a small critical exponent P. For strong doping the low-temperature dependence of~hã ppears to depart from the one in pure LazCu04. For x~0.05 both NQR spectra and pSR reveal the presence of regions where the long-range AF order is suppressed. For temperature above 100 K up to Tz the " La relaxation rate 8' due to the Cu + spin Auctuations shows only slight corrections with respect to pure La2Cu04 which are possibly related to the disorder in the AF interactions or to finite-size effects. A novel and remarkable effect of Zn doping is the appearance in 8; for T~100 K, of large and marked maxima, which are x dependent. This phenomenon is attributed to the cooperative freezing of local magnetic moments induced by Zn on Cu orbitals, interacting via the underlying AF matrix. The maxima in 8'occur when the Auctuation frequencies of the anomalous spins become of the order of the NQR frequency, thus driving the system to a spin-glass state superimposed to the AF matrix.
Superconductor Science and Technology, 2012
We investigate the effect of external pressure on magnetic order in undoped LnFeAsO (Ln = La, Ce,... more We investigate the effect of external pressure on magnetic order in undoped LnFeAsO (Ln = La, Ce, Pr, La) by using muon-spin relaxation measurements and ab-initio calculations. Both magnetic transition temperature T m and Fe magnetic moment decrease with external pressure. The effect is observed to be lanthanide dependent with the strongest response for Ln = La and the weakest for Ln = Sm. The trend is qualitatively in agreement with our DFT calculations. The same calculations allow us to assign a value of 0.68(2) µ B to the Fe moment, obtained from an accurate determination of the muon sites. Our data further show that the magnetic lanthanide order transitions do not follow the simple trend of Fe, possibly as a consequence of the different f-electron overlap.
Physical Review Letters, 2005
We investigate the time autocorrelation of the molecular magnetization M (t) for three classes of... more 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 τ (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/T1 data in a large number of AFM rings fits to a single-lorentzian form.
Physical Review Letters, 2000
Physical Review Letters, 1995
We present and analyze the normal phase temperature dependence of resistivity and nuclear spinlat... more We present and analyze the normal phase temperature dependence of resistivity and nuclear spinlattice relaxation of (TMTSF)2C104 under strong transverse magnetic fields. From a field-dependent scaling theory we show that the singular enhancement of the one-dimensional electron-electron umklapp scattering and antiferromagnetic spin correlations under field play a fundamental role in the large enhancement of magnetoresistance and nuclear relaxation of Bechgaard salts.
Physical Review Letters, 2000
Physical Review Letters, 1998
Measurements of the spin-lattice relaxation rate are reported for muons and protons as a function... more Measurements of the spin-lattice relaxation rate are reported for muons and protons as a function of temperature for different values of the applied magnetic field in the Mn 12 O 12 molecular cluster. Strongly field dependent maxima in the relaxation rate versus temperature are observed below 50 K. The results are explained in terms of thermal fluctuations of the total magnetization of the cluster among the different orientations with respect to the anisotropy axis. The lifetimes of the different m components of the total spin, S T 10, of the molecule are obtained from the experiment and shown to be consistent with the ones expected from a spin-phonon coupling mechanism. No clear evidence for macroscopic quantum tunneling was observed in the field dependence of the proton relaxation rate at low T. [S0031-9007(98)07391-8]
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Papers by Pietro Carretta