543 research outputs found
Persistence of singlet fluctuations in the coupled spin tetrahedra system Cu2Te2O5Br2 revealed by high-field magnetization and 79Br NQR - 125Te NMR
We present high-field magnetization and Br nuclear quadrupole
resonance (NQR) and Te nuclear magnetic resonance (NMR) studies in the
weakly coupled Cu () tetrahedral system CuTeOBr.
The field-induced level crossing effects were observed by the magnetization
measurements in a long-ranged magnetically ordered state which was confirmed by
a strong divergence of the spin-lattice relaxation rate 1/T1 at T0=13.5 K. In
the paramagnetic state, 1/T1 reveals an effective singlet-triplet spin gap much
larger than that observed by static bulk measurements. Our results imply that
the inter- and the intra-tetrahedral interactions compete, but at the same time
they cooperate strengthening effectively the local intratetrahedral exchange
couplings. We discuss that the unusual feature originates from the frustrated
intertetrahedral interactions.Comment: 5 pages, 4 figures, accepted in Phys. Rev. B as a Rapid
Communication
La NMR investigation in underdoped LaSrCuO
We report La and Cu nuclear magnetic and quadrupole resonance
(NMR/NQR) studies in an underdoped LaSrCuO single
crystal, focusing on the La NMR in the normal state. We demonstrate
that the local structural distortions in the low-temperature orthorhombic
structure cause the tilting of the direction of the electric field gradient
(EFG) at the nuclei from the c axis, resulting in two NMR central transition
spectra at both the La and Cu nuclei in an external field.
Taking into account the tilt angle of the EFG, the temperature dependence of
the La spectra allowed us to determine the La Knight shift and
the structural order parameter. The angle and temperature dependence of the
La spectrum is in perfect agreement with the macroscopic average
structure and proves a displacive transition. The La nuclear
spin-lattice relaxation rates, , suggest that
LaSrCuO undergoes a gradual change to a
temperature-independent paramagnetic regime in the high temperature region.
Both the spectra and of the La as a function of temperature
reveal a sharp anomaly around K, implying a first-order-like
structural transition, and a dramatic change below ~70 K arising from
collective glassy spin freezing.Comment: 7 pages, 7 figures, published in PR
NMR Evidence for Charge Inhomogeneity in Stripe Ordered La_{1.8-x}Eu_{0.2}Sr_{x}CuO_4
We report ^{17}O Nuclear Magnetic Resonance (NMR) results in the stripe
ordered La_{1.8-x}Eu_{0.2}Sr_{x}CuO_4 system. Below a temperature T_q ~ 80K,
the local electric field gradient (EFG) and the absolute intensity of the NMR
signal of the planar O site exhibit a dramatic decrease. We interpret these
results as microscopic evidence for a spatially inhomogeneous charge
distribution, where the NMR signal from O sites in the domain walls of the spin
density modulation are wiped out due to large hyperfine fields, and the
remaining signal arises from the intervening Mott insulating regions.Comment: 4 pages, to appear in Phys. Rev. Let
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