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Very low-frequency excitations in frustrated two-dimensional S=1/2 Heisenberg antiferromagnets
No full textMagnetic field effect at the quantum critical point from NQR-NMR relaxation
No full text63Cu NQR–NMR relaxation measurements in CeCu5.9Au0.1 are used to achieve insights on the k-integrated, lowenergy,
generalized susceptibility chi(k,w,H,T->0) around the quantum critical point. The response function indicated by neutron scattering, namely of 2D character with anomalous exponent and with (w/T) scaling, appears to justify the main experimental findings. A breakdown of that form of w is found to occur below a field-dependent temperature T*(H); where a behavior characteristic of a gapped system sets in
Correlated spin dynamics and phase transitions in pure and disordered 2D S=1/2 antiferromagnets: Insights from NMR-NQR
No full textSpin dilution in frustrated two-dimensional S= 1/2 antiferromagnets on a square lattice
No full text7Li and 29Si nuclear magnetic resonance, muon spin relaxation (muSR), and magnetization measurements in Li2V1−xOTixSiO4, for 0<x<0.2, are presented. The x=0 compound is a prototype of frustrated twodimensional Heisenberg antiferromagnet on a square-lattice with competing nearest- (J1) and next-nearest- (J2)
neighbor exchange interactions. Ti4+(S=0) for V4+(S=1/2) substitution yields the spin dilution of the antiferromagnetic
layers. The analysis of the magnetization and of the nuclear spin-lattice relaxation rate shows that spin dilution not only reduces the spin stiffness by a factor .around (1−x)^2, but also causes the decrease of the effective ratio J2(x) /J1(x). Moreover, the sublattice magnetization curves derived from zero-field muSR measurements in the collinear phase point out that, at variance with nonfrustrated two-dimensional Heisenberg
antiferromagnets, spin dilution affects the low-temperature staggered magnetization only to a minor extent. This observation is supported also by the x dependence of the collinear ordering temperature. The results obtained for the Ti doped samples are discussed in the light of the results previously obtained in the pure x=0 compound and in nonfrustrated two-dimensional Heisenberg antiferromagnets with spin dilution
Frustration driven lattice distortions in Li2VOSiO4 and VOMoO4
No full textNMR and EPR measurements in the two-dimensional frustrated anti-ferromagnets on a square lattice Li2VOSiO4 and VOMoO4 are presented. It is found that a structural distortion, possibly driven by the frustration, takes place in both compounds. The modifications induced by the distortion in the NMR and EPR spectra on cooling can be accounted for by a progressive growth of the size of the domains where the lattice is distorted
From weakly antiferromagnetic metal to quantum criticality: NQR and mu SR results for CeCu6-xAux with 0<=x<=0.8
No full textIn this report, we present and discuss new results for NQR and muSR experiments in CeCu6-xAux, for x=0.3 (Neel temperature TN about 0.5 K) and x = 0.8 (TN about 1.8 K), carried out as part of our study of the spin dynamics, the magnetic ground-state and precursor effects on approaching TN. In these compounds the NQR relaxation rate for T>>TK (the Kondo temperature) is consistent with a relaxation process driven by screened local Ce moments with a weak temperature and x dependence on cooling. The low temperature effective screening increases with decreasing x. The low-temperature zero-field muSR reveals an enhanced relaxation rate that does not appear to follow a critical law, as expected on approaching the ordering temperature
Magnetic properties of frustrated two-dimensional S=1/2 antiferromagnets on a square lattice
No full textAn overview on the basic magnetic properties of vanadates which
represent prototypes of frustrated two-dimensional S=1/2 antiferromagnets on a square lattice is presented. It will be shown how information on the ground state sublattice magnetization on the static uniform susceptibility and on the frustration driven lattice distortions can be achieved by means of NMR spectroscopy and magnetization measurements. The low-energy spin excitations investigated by means of NMR and muSR relaxation measurements will be analyzed and the anomalous
very-low-frequency dynamics originating from the degeneracy of the ground state discussed. Finally the effects of an hydrostatic pressure on the degree of frustration of the vanadates will be addressed
On the magnetic field dependence of the susceptibility at the quantum critical point in CeCu5.9Au0.1
No full textMagnetization measurements and the field dependence of 63Cu NMR relaxation rates are used to analyse the effect of an external
magnetic field H at the quantum critical point of CeCu5.9Au0.1. It is argued that the scaling argument in which H is included through an effective temperature Teff holds only in weak field, while in strong fields the data are rather accounted for by direct scaling
of the field dependence of magnetization
Dynamical susceptibility and magnetic field effect at the quantum criticality in CeCu6-xAux from Cu NQR-NMR relaxation
No full textCu-63,Cu-65 NQR-NMR relaxation measurements in CeCu6-xAux for x=0, x=0.1, and x=0.8 are used to derive insights on temperature (T) and magnetic-field (H) dependences of the spin dynamics around the quantum critical point (QCP). The relaxation rate 2W is related to the k integrated, low energy, generalized susceptibility chi(k,omega,H,T-->0). For x=0 a Fermi-liquid behavior is detected, while for x=0.8 the temperature dependence of 2W is the one expected for a nearly antiferromagnetic metal ordering at T-N=2.2 K. Instead, for x=0.1, around the QCP, a response function of the form suggested by neutron scattering, namely of two-dimensional character with anomalous exponent and (omega/T) scaling, is found to explain the main experimental findings. An effect is observed in the low-temperature range for Hgreater than or similar to1 T, with a crossover to a gapped phase for the spin excitations at a field-dependent temperature
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