69 research outputs found
Ba nuclear resonance in YBa2Cu2Oy
The first observations of NMR and NQR of both isotopes135Ba and134Ba in isotopically enriched samples of YBa2Cu3Oy with oxygen concentrations y=6.0, 6.2, and 7.0 are described. The pure NQR frequencies and asymmetry parameter are in good agreement with theoretical predictions. The temperature dependence of the NQR frequency of Ba for y=7 is qualitatively similar to that for Cu(2) but much stronger. The temperature dependence of the longitudinal and transverse relaxation times opens new questions. © 1992 Springer
Decrease of the homogeneous width of the Tm3+ electron energy levels in TmBa2CCu3 O6+x at the superconducting transition
Enhanced NMR of 169Tm (I= 1 2) enhanced NMR has been studied in TmBa2Cu3O6+x, x=0, 0.2, 0.4, at temperatures 1.2-40K. The nuclear magnetic relaxation rates at T>20K increase exponentially as 1 T1(2) = R1(2)exp(-Δ/kT), with Δ = 176 K being the energy of the first excited state of the Tm3+ ion, while the R1(2) pre-exponential factors are inversely proportional to the homogenous width Γo of this doublet state. The pre-exponential factors measured at the frequencies of 48 and 69 MHz show homogeneous broadening of the 4f-levels due to the decreasing dipole-dipole transfer of the electron excitations from ion to ion at increasing because of crystal structure disordering. A sharp decrease of Γo (by about 0.9K) is observed in the superconducting compound with x=0.4 (Tc onset at 20K) at low temperatures and is attributed to the weakening of the 4f-electron-phonon interaction. At the temperatures around 6K the same sample displays enhancement of the transverse relaxation due to the relatively slow (W ≈ 109s-1) fluctuations of the internal magnetic fields caused by Cu2+ ions in CuO2 planes. © 1992
NMR studies of singlet-ground-state rare-earth ions in high-Tc superconductors
Many of presently known high-Tc superconductors contain rare-earth (RE) ions with an even number of electrons in an unfilled 4f-shell (Pr3+, Tb3+, Ho3+, Tm3+). If the ground state of 4f-electrons is non-degenerate and separated from excited states by high enough energy intervals, one can observe the so-called "enhanced NMR" of RE nuclei at low temperatures. In the present paper some aspects of the enhanced NMR are analyzed in applications to the crystal and electron structure of high-Tc superconductors. © 1992 Springer
Anomalously large oxygen-ordering contribution to the thermal expansion of untwinned YBa2Cu3O6.95 single crystals: a glass-like transition near room temperature
We present high-resolution capacitance dilatometry studies from 5 - 500 K of
untwinned YBa2Cu3Ox (Y123) single crystals for x ~ 6.95 and x = 7.0. Large
contributions to the thermal expansivities due to O-ordering are found for x ~
6.95, which disappear below a kinetic glass-like transition near room
temperature. The kinetics at this glass transition is governed by an energy
barrier of 0.98 +- 0.07 eV, in very good agreement with other O-ordering
studies. Using thermodynamic arguments, we show that O-ordering in the Y123
system is particularly sensitive to uniaxial pressure (stress) along the chain
axis and that the lack of well-ordered chains in Nd123 and La123 is most likely
a consequence of a chemical-pressure effect.Comment: 4 pages, 3 figures, submitted to PR
Planar 17O NMR study of Pr_yY_{1-y}Ba_2Cu_3O_{6+x}
We report the planar ^{17}O NMR shift in Pr substituted YBa_{2}Cu_{3}O_{6+x},
which at x=1 exhibits a characteristic pseudogap temperature dependence,
confirming that Pr reduces the concentration of mobile holes in the CuO_{2}
planes. Our estimate of the rate of this counterdoping effect, obtained by
comparison with the shift in pure samples with reduced oxygen content, is found
insufficient to explain the observed reduction of T_c. From the temperature
dependent magnetic broadening of the ^{17}O NMR we conclude that the Pr moment
and the local magnetic defect induced in the CuO_2 planes produce a long range
spin polarization in the planes, which is likely associated with the extra
reduction of T_c. We find a qualitatively different behaviour in the oxygen
depleted Pr_yY_{1-y}Ba_2Cu_3O_{6.6}, i.e. the suppression of T is nearly
the same, but the magnetic broadening of the ^{17}O NMR appears weaker. This
difference may signal a weaker coupling of the Pr to the planes in the
underdoped compound, which might be linked with the larger Pr to CuO_2 plane
distance, and correspondingly weaker hybridization.Comment: 8 pages, 9 figures, accepted in Phys Rev
On the stability of 2 \sqrt{2} x 2 \sqrt{2} oxygen ordered superstructures in YBa2Cu3O6+x
We have compared the ground-state energy of several observed or proposed " 2
\sqrt{2} x 2 \sqrt{2} oxygen (O) ordered superstructures " (from now on HS),
with those of "chain superstructures" (CS) (in which the O atoms of the basal
plane are ordered in chains), for different compositions x in YBa2Cu3O6+x. The
model Hamiltonian contains i) the Madelung energy, ii) a term linear in the
difference between Cu and O hole occupancies which controls charge transfer,
and iii) covalency effects based on known results for models in one and
two dimensions. The optimum distribution of charge is determined minimizing the
total energy, and depends on two parameters which are determined from known
results for x=1 and x=0.5. We obtain that on the O lean side, only CS are
stable, while for x=7/8, a HS with regularly spaced O vacancies added to the
x=1 structure is more stable than the corresponding CS for the same x. We find
that the detailed positions of the atoms in the structure, and long-range
Coulomb interactions, are crucial for the electronic structure, the mechanism
of charge transfer, the stability of the different phases, and the possibility
of phase separation.Comment: 24 text pages, Latex, one fig. included as ps file, to be publisheb
in Phys. Rev.
Hydrogen Diffusion and Electronic Structure Parameters in Tantalum Hydrides Determined by Pulsed NMR Measurements
OBSERVATION OF ANTIFERROMAGNETIC ORDER IN YBa2Cu3O6.15 by CU NQR
The NQR spectra of YBa2Cu3Oy (6.0 < y < 6.25) show two different antiferromagnetic structures with a different stacking sequence. In both structures the magnetic moment is located at the Cu-2 sites perpendicular to the c-axis. The structure with antiferromagnetic coupling between all Cu-2 planes reveals strong temperature dependent magnetic fluctuations with correlation times of about 10-6 s at 15 K. No fluctuations are found in the second structure which reveals a constant internal field of about 0.2 T
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