63Cu Nuclear Quadrupole Resonance Research Articles

Impurity-Robust Bulk Gapless Excitation in the Yb-Based Zigzag Chain Compound YbCuS2

We have performed 63Cu-nuclear quadrupole resonance (NQR) measurements using a lump sample of the Yb zigzag-chain compound YbCuS2 with a small surface area to investigate the sample dependence of low-temperature magnetic properties in YbCuS2 by comparing with the previous study with different powdered sample. The line width of NQR signals in the present lump sample is larger than that in the previous powdered sample. In addition, the transition temperature T N ∼ 0.92 K in the present lump sample is lower than that in the previous powdered sample (∼ 0.95 K). These results suggest that the quality of the present lump sample is worse than that of the previous powdered sample. However, the T-linear behavior of the nuclear spin-lattice relaxation rate 1/T1 was observed below 0.5 K and the value of 1/T 1 T in both samples is almost the same even though the sample quality and sample geometry are different. This suggests that T-linear behavior in 1/T 1 arises from the impurity-robust bulk gapless excitation inherent in YbCuS2 rather than from sample issues such as the sample quality or geometry.

Short-range magnetic order and temperature-dependent properties of cupric oxide

The temperature dependence of the optical and magnetic properties of CuO were examinedby means of hybrid density functional theory calculations. Our work shows that thespin exchange interactions in CuO are neither fully one-dimensional nor fullythree-dimensional. The temperature dependence of the optical band gap and the 63Cu nuclear quadrupole resonance frequency of CuO originate from the combined effect of astrong coupling between the spin order and the electronic structure and the progressiveappearance of short-range order with temperature.

Cu nuclear quadrupole resonance study of NdBa 2(Cu,Zn,Ni) 3O 7− δ

We have investigated slightly underdoped NdBa 2(Cu,Zn,Ni) 3O 7− δ with magnetic Ni and non-magnetic Zn impurities by 63Cu nuclear quadrupole resonance (NQR). The larger ionic radius of the Nd compared to Y made it possible to incorporate higher amounts of Zn and Ni impurities (up to 12% in our samples), and thereby superconductivity is completely suppressed. The 63Cu NQR spectra are comparable to similar doped YBCO samples, but are shifted about ≈1.5 MHz to higher frequencies. By doping the samples with Ni the spectra are moderately broadened, whereas the broadening is stronger for Zn doping. The spin lattice relaxation rate, T 1 - 1 , is drastically enhanced by the large Nd moment, and thereby the pseudo gap is not visible in all of these samples.

Valence and magnetic transitions inYbInCu4 probedusing 63Cu nuclear quadrupole resonance under high pressure

Applying pressure to the valence fluctuating compoundYbInCu4 lowers the valence transition temperature and stabilizes the high temperature (HT)localized spin state. For pressure above 2.4 GPa, the valence transition iscompletely suppressed and the magnetically ordered state arises as a newground state below 2.4 K. In order to elucidate microscopically the relationbetween the valence and magnetic transitions in f electron system, we performed63Cu NQRmeasurements of YbInCu4 under pressure up to 2.5 GPa. The experimental results provide clearevidence for the onset of the magnetic ordering in the pressure-stabilizedHT state above 2.3 GPa. The nuclear spin–lattice relaxation rate1/T1 in the HTstate shows 1/T1 = constant behaviour down to 3.0 K and a divergence increase just below 2.4 K.We concluded that the pressure-stabilized localized electron state ofYbInCu4 transforms directly into a magnetically ordered state, without displaying any intermediateheavy Fermi liquid behaviour.

NQR study on Nd doped La 2− xSr xCuO 4

63Cu nuclear quadrupole resonance (NQR) experiments were preformed on six Nd-doped La 2− x Sr x CuO 4 samples. The NQR spectra show a systematic evolution with various Nd and/or Sr doping, except for La 1.48Nd 0.4Sr 0.12CuO 4. A completely different NQR spectrum was observed for the sample, which is similar to that for undoped insulator La 2CuO 4. It is speculated that a static stripe phase on the NQR time scale μs may be responsible for the anomalous spectrum.

139La and 63Cu NQR in Zn-doped lanthanum cuprate

139La and 63Cu nuclear quadrupole resonance (NQR) relaxation measurements have been performed in a series of La 1.87Sr 0.13Cu 1− y Zn y O 4 (0 ≤ y ≤ 0.03) samples which show a dramatic suppression of the superconducting transition with Zn doping. 139La 139 T 1 −1 exhibits an enhancement at low T similar to the spin glass behavior in other lanthanum cuprates, but the peak indicating spin freezing is not observed down to 4.2 K. The opening of spin pseudogap is found to be suppressed as T c decreases. These results appear to be associated with a disordered state of pinned charge stripes.

63Cu NQR evidence of dimensional crossover to anisotropic 2D regime in S = 1 / 2 three-Leg ladder Sr2Cu3O5

We probed spin-spin correlations up to 725 K with 63Cu nuclear quadrupole resonance in the S = 1 / 2 three-leg ladder Sr2Cu3O5. We present experimental evidence that below 300 K weak interladder coupling causes dimensional crossover of the spin-spin correlation length xi from the quasi-1D (xi approximately 1 / T) to the anisotropic 2D regime ( xi approximately exp2pirho(s) / T, where 2pirho(s) = 290+/-30 K is the effective spin stiffness).

Charge states of atoms in the lattices of the high-temperature superconductors Tl2Ba2Ca n−1CunO2n+4 and Bi2Sr2Ca n−1CunO2n+4

To determine the charges of atoms in the lattices of the compounds Tl2Ba2Can−1CunO2n+4 and Bi2Sr2Can−1CunO2n+4 (n=1,2,3), the parameters of the electric field gradient tensor at the copper sites of the indicated lattices were found by emission Mossbauer spectroscopy on the isotopes 61Cu(61Ni) and 67Cu(67Zn), and a calculation of these parameters was performed in the point-charge approximation. A comparison of the resulting values and the published data on 63Cu nuclear quadrupole resonance showed that agreement between the experimental and computed values of the parameters obtains for models in which the holes resulting from a reduction in the valence of some of the thallium (bismuth) atoms are localized predominantly at oxygen sites located in the same plane as the copper atoms (for the compounds Tl2Ba2Ca2Cu3O10 and Bi1.6Pb0.4Sr2Ca2Cu3O10—at oxygen sites in the same plane as the Cu(2) atoms).

Atomic Charge States in Tl2Ba2Can?1CunO2n+4 as Determined by the61Cu(61Ni),67Cu(67Zn) and133Ba(133Cs) Emission M�ssbauer Spectroscopy

Parameters of the electric field gradient tensors have been found for the Cu and Ba sites of the Tl2Ba2Can—1CunO2n+4 lattices by the 61Cu(61Ni), 67Cu(67Zn) and 133Ba(133Cs) emission Mössbauer spectroscopy. The same parameters have been calculated using the point charge approximation. An analysis of the obtained quantities has been carried out taking into consideration literature data on the 63Cu nuclear quadrupole resonance. Agreement of the measured and calculated parameters could be reached assuming the holes resulting from lowering the valence of part of the Tl atoms to be placed mainly at oxygen sites of the Cu–O planes (especially, at oxygen sites of the Cu(2)–O planes for the Tl2Ba2Ca2Cu3O10 compound).

67Cu(67Zn) Mössbauer emission spectroscopy of Tl2Ba2CuO6 and Tl2Ba2CaCu2O8

The parameters of the crystal electric-field gradient at the copper sites of Tl2Ba2CuO6 and Tl2Ba2CaCu2O8 have been determined by Mossbauer emission spectroscopy using the 67Cu(67Zn) isotope, and calculated in the point-charge approximation. The results obtained are analyzed using the available 63Cu nuclear quadrupole resonance data. The experimental and calculated data can be brought in agreement if one assumes that the holes appearing after part of the thallium atoms lower their valency become localized primarily at the oxygen sites lying in the plane of the copper ions.

Effects of spin vacancies on the correlated spin dynamics in La2Cu1−xZnxO4 from 63Cu nuclear quadrupole resonance relaxation

63Cu nuclear quadrupole resonance (NQR) relaxation measurements in La2CuO4 doped Zn are used in order to investigate the temperature dependence of the in-plane magnetic correlation length ξ2D and the effects associated to spin vacancies in two dimensional quantum Heisenberg antiferromagnets (QHAF). The relaxation rates T1−1 and T2−1 have been related to the static generalized susceptibility χ(q,0) and to the decay rate Γq of the normal excitations. By using scaling arguments for χ(q,0) and Γq, the relaxation rates have been expressed in close form in terms of ξ2D(x,T) and its dependence on temperature and spin doping x thus extracted. The experimental findings are analyzed in light of the renormalized classical (RC) and quantum critical (QC) behaviors predicted for ξ2D by recent theories for S=1/2 HAF in square lattices. It is first shown that in pure La2CuO4, ξ2D is consistent with a RC regime up to about 900 K, with tendency toward the QC regime above. The spin vacancies reduce the Néel temperature according to the law TN(x)≈TN(0)(1–3.5x). From the temperature dependence of 63Cu NQR relaxation rate T1−1, T2−1 and from the composition dependence of TN it is consistently proved that the effect on ξ2D can be accounted for by the modification of the spin stiffness in a simple dilutionlike model, the system still remaining in the RC regime, at least for T⩽900 K.

Cuprous oxide manometer for high-pressure magnetic resonance experiments

We present a manometer designed to measure pressures of 1–20 kbar in temperatures between 4–300 K in cylinder-piston type chambers, with an accuracy of ∼100 bar. The manometer is based on pressure-dependent zero-field 63Cu nuclear quadrupole resonance frequency corresponding to ‖±3/2〉↔‖±1/2〉 transition in Cu2O. The nuclear quadrupole resonance frequency νQ varies linearly with pressure and its temperature dependence is adequately explained by a model of lattice vibrational modes in O—Cu—O bonds. This manometer is particularly convenient for zero or high-field magnetic resonance experiments.

63Cu NQR Nutation Spectroscopy and Spin Counting in Copper Oxides

AbstractNutation spectroscopy of 63Cu nuclei at zero static magnetic field is explored as a method for the determination of the quadrupole asymmetry parameters η of Cu sites in copper oxides, with special emphasis on high‐Tc superconductors. Since the nutation spectra reveal the actual radio frequency (rf) intensities experienced by the 63Cu nuclei, the method also provides a verification of a quantitative Cu spin counting procedure based on integration of nuclear quadrupole resonance (NQR) echo‐signal intensities over wide frequency ranges. The paper presents numerical simulations of spectra and signal intensities that were obtained with a density‐matrix formalism whereby the spin dynamics is described as precessions in a 3D operator space. The quantitative effects of the echo refocusing pulse, rf amplitude fluctuations, and variations in η are examined. Examples of 63Cu NQR nutation spectra are shown for randomly oriented powders of Cu2O and YBa2Cu3O7.

NQR investigation of high T c superconductors under pressure

The pressure dependence of the 63Cu nuclear quadrupole resonance (NQR) frequency, ν Q, and T c have been measured for La 1.85Sr 0.15CuO 4 and YBa 2Cu 4O 8 under high pressure up to 1.3GPa. ν Q was found to decrease with increasing pressure in La 1.85Sr 0.15CuO 4, while ν Q in the CuO 2 plane increases with pressure in YBa 2Cu 4O 8. It is evidenced that charge transfer takes place under pressure. Based on the analysis that ν Q arises mainly from the on-site holes, a schematic model relating T c and the ratio of the in-plane hole concentration in the Cu3d and O2p orbitals, n(Cu)/n(O), is constructed. It is proposed that n(Cu)/n(O) is a primarily important factor governing T c and that the enhancement of T c under pressure is due to the change of the ratio approaching the most favorable value (∼1).

Influence of magnetic fields on NQR line shape parameters in oriented YBa 2Cu 3O 7−δ powder samples

63Cu nuclear quadrupole resonance (NQR) experiments on the Cu(1) site of oriented YBa2Cu3O7−δ powder samples are performed at 4.2 K. The application of magnetic fields during the cooling procedure or after zero-field cooling influences the NQR lineshape parameters, i.e. linewidth, intensity, and amplitude. The NQR measurements for different magnetic states of the samples are presented together with corresponding measurements of the remanent magnetic field.

Copper NQR and NMR in the superconductor YBa 2Cu 4O 8+x

The frequencies ν Q, the linewidth and the spin-lattice and spin-spin relaxation times T 1 and T 2, respectively, of the 63Cu nuclear quadrupole resonance (NQR) signals for chain and planar copper sites in YBa 2Cu 4O 8 were measured between 6 and 750 K; in addition nuclear magnetic resonance (NMR) spectra were taken at 150 and 300 K. The results are compared with data known for the related structure YBa 2Cu 3O 7. Above T c the tensors of the electric field gradient and the anisotropic Knight shift at the copper sites differ in a subtle way from those of YBa 2Cu 3O 7. At the Cu2 site, the derivative of ν Q with respect to temperature changes its sign around 200 K. No indication for magnetic ordering at the Cu sites was found. Below T c there is no evidence for BCS type relaxation and the Cu2 T c rates vary like exp( T T 0 ) and the Cu1 rates like T n with n≈1.35. Above T c the Cu1 rates suggest the presence of quasiparticles (and phonon interactions) while the Cu2 rates can perhaps be explained by a model of Monien et al. postulating the coupling of Cu nuclei to itinerant interacting quasiparticles.

NMR and NQR studies in YBa 2Cu 3O 7−δ

This paper reviews the nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) experiments done in this laboratory on the superconductor YBa 2Cu 3O 7−δ. Measurements involve the isotopes 63Cu and 65Cu (above and below T c) and 89Y (above T c). The following topics will be discussed: the assignment of the Cu NQR frequencies to the atomic sites based on intensity measurements, the determination of the electric field gradients and the Knight shifts at the Cu sites (at 300 K), the calculation of the field gradients, the temperature dependence of the 63Cu NQR frequencies and relaxation times for both Cu sites; some 89Y relaxation and Knight shift data. The results will be compared with those of other groups where corresponding data are available.

Unusual behavior of nuclear relaxation in CeCu 2Si 2 “possible evidence for triplet superconductivity”

Nuclear relaxation of 63Cu in the superconducting state of the Kondo-lattice system CeCu 2Si 2 has been studied with the use of the 63Cu nuclear quadrupole resonance technique under zero field and down to 65mK. The nuclear spin-lattice relaxation rate (1/T 1) decreases drastically just below T c =0.67 K down to 0.5T c without the apparent enchanced behavior and then is found to be almost temperature independent below 0.3T c. These results suggest that the superconductivity in CeCu 2Si 2 is not in the usual BCS regime. The analysis based upon the existing triplet pairing model with an anisotropic energy gap describes well the behavior from T c down to 0.5T c, while the temperature independence below 0.3T c remains unexplained.

Coordination bonds in KCu(CN) 2 as studied by 14N and 63Cu NQR

The 63Cu nuclear quadrupole spin-lattice relaxation time, T 1, was measured as a function of temperature between 77 and 260 K for potassium dicyanocuprate(I), KCu(CN) 2 and found to depend on the temperature in T −2. This shows that the quadrupole relaxation is governed by the vibration of the anion and that possibilities of reorientation on self-diffusion of ionic species are excluded below 260 K. Our proposed bond model was thus indirectly reinforced. The Zeeman effect experiment on a single crystal for the 63Cu nuclear quadrupole resonance line at 200 K was not conclusive as to the temperature dependence of the asymmetry parameter.