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1.
Nuclear quadrupole resonance (NQR)139La and63Cu spin-lattice relaxation rateT 1 t-1 measurements in a La1.94Sr0.06CuO4 single crystal are described. Slowing-down of Cu2+ spin fluctuations is evidenced through a dramatic increase of139 T 1 ?1 on cooling. While the onset of diamagnetism occurs atT c = 8 K,139 T 1 ?1 has a peak atT g ? 5 K, when the characteristic frequency of magnetic fluctuations reaches the NQR frequencyv Q ? 9 MHz. In agreement with a number of previous studies, these results show that the so-called “cluster spin-glass” phase persists in the superconducting regime. Issues concerning the coexistence of the two phases are discussed.  相似文献   

2.
Nuclear quadrupole resonances (NQR) of139La in La2−xMxCuO4 (M=Ba, Sr) and63Cu in YBa2CU3O7 have been investigated with substitution of Cu by magnetic impurities. For La-system, the strong enhancement of the nuclear relaxation rate, 1/T 1 atT c *≈10 K (suggesting the occurrence of magnetic instability with hole-doping), is suppressed by 3D-antiferromagnetism induced by magnetic impurities. In the superconducting region, fluctuations of Cu moments at low temperatures remain extremely fast as at high temperatures. For Y-system, Fe-ions are substituted for both Cu(1) and Cu(2) sites, But Co-ions are preferentially substituted to Cu1 sites. The peaks in the relaxation rate of Cu indicate the appearance of magnetic ordering without destroying superconductivity for the 0.5%-Fe doped sample.  相似文献   

3.
NMR and NQR spectra and spin-lattice relaxation measurements carried out in LASCO and YBCO-type crystals are presented and analyzed in order to derive insights on the correlations and spin-dynamics of the Cu2+ ions and on the microscopic mechanisms of high-T c superconductivity. As an illustrative example on how the magnetic correlation length and spin dynamics properties can be extracted from the relaxation rateW, the35Cl NMR data in the two-dimensional Heisenberg system Sr2CuCl2O2, around the paramagnetic-antiferromagnetic (PA-AF) transition are first considered. Then the139La NQR relaxation measurements in La2?xSrxCuO4 are briefly reviewed and it is shown how a simple picture of localized Cu2+ magnetic moments, whose spin fluctuation times are controlled by the charge defects induced by the doping, leads in a direct way to quantitative estimates for the progressive shift, on cooling, of the spectral density of the low-frequency spin excitations towards the high frequency range. This phenomenon can be described in terms of effective spin at the Cu2+ ions, and its similarities with the analogous effect of progressive delocalization in Heavy Fermions systems are pointed out. Thus, the superconducting transition appears to occur in an unconventional Fermi liquid with AF correlations among itinerant pseudoparticles, possibly involving a mechanism not directly related to the magnetic correlated dynamics. In fact, a universal behavior of the relaxation rates as a function of temperature is observed, regardless of the transition temperatureT c. The independence ofT c from the low frequency static and dynamical spin properties is also indicated by89Y Knight shifts and from63Cu relaxation rates in systems like YBa2Cu4O8 (Y124), whereT c can be changed by atomic substitutions and by controlling the oxygen stoichiometry. The effect of an external magnetic field on the correlated spin dynamics of the AF Fermi liquid is investigated and from a comparison of Cu NQR relaxation and NMR relaxation in oriented powder of YBCO and LASCO it is shown that the external field has the small but unambiguous effect of depressing the relaxation rates aboveT c, besides strongly enhancing them in the superconducting phase. A maximum in the ratio \({{W\left( {NQR} \right)} \mathord{\left/ {\vphantom {{W\left( {NQR} \right)} {W\left( {\vec H\left\| {\vec c} \right.} \right)}}} \right. \kern-0em} {W\left( {\vec H\left\| {\vec c} \right.} \right)}}\) is thus observed around 80 K, either in LASCO or in YBCO, again indicating that the transition could be driven by a mechanism not directly involving the spin dynamic properties. To study the role of the fluxions belowT c 89Y NMR shifts and spectra in oriented powders of YBCO are analyzed. Information on the spin susceptibility and on the structure of the vortex lattice is obtained. In addition, from the temperature behavior of the linewidth a motional narrowing related to flux melting is evidenced. The effective correlation time for the vortex motion is derived and it is discussed why μ+SR cannot detect it in view of the different rigid-lattice line broadening.  相似文献   

4.
Two peaks are observed at T=35 and 47 K in the transverse relaxation rate for Cu(2) nuclei in YBa2Cu3O7?y . A comparison of the relaxation rates for isotopes 63Cu(2) and 65Cu(2) at T=47 K indicates the magnetic nature of relaxation. The enhancement of local magnetic field fluctuations perpendicular to CuO2 planes at T=47 K is associated with critical fluctuations of orbital currents. The peak at T=35 K is attributed to the emergence of an inhomogeneous superconducting phase. The obtained experimental results and the available data from the literature made it possible to propose a qualitatively new phase diagram of the superconducting state.  相似文献   

5.
We report detailed 17O, 139La, and 63,65Cu Nuclear Magnetic Resonance (NMR) and Nuclear Quadrupole Resonance (NQR) measurements in a stripe ordered La1.875Ba0.125CuO4 single crystal and in oriented powder samples of La1.8−x Eu0.2Sr x CuO4. We observe a partial wipeout of the 17O NMR intensity and a simultaneous drop of the 17O electric field gradient (EFG) at low temperatures where the spin stripe order sets in. In contrast, the 63,65Cu intensity is completely wiped out at the same temperature. The drop of the 17O quadrupole frequency is compatible with a charge stripe order. The 17O spin lattice relaxation rate shows a peak similar to that of the 139La, which is of magnetic origin. This peak is doping dependent and is maximal at x ≈ 1/8.  相似文献   

6.
The nuclear quadrupole resonance (NQR) has been investigated for Cu in La2–x A x CuO y (A=Sr and Ba). Three Cu NQR lines, A, B and C, have been observed, which correspond to three different Cu sites. From the analysis of these NQR intensities, site assignments are possibly as follows. These lines A, B and C are attributed to CuO6 octahedral, CuO5 pyramidal and CuO4 planar sites, respectively. The apical oxygen O(2) is induced to transfer to the interstitial O(3) site between LaO planes when two or more Sr2+ ions are located in its neighboring La sites. The excess holes doped into the CuO2 plane are provided mainly by O(3) as well as the unpaired Sr2+ ion.  相似文献   

7.
We have performed57Fe Mössbauer measurements on aligned iron-doped crystallites of La1.85Sr0.15CuO4?y and Pb2Sr2R0.50Cu3O8?y where R=Eu or Ho. From the intensity ratio of the two lines of the quadrupole doublets we deduce that the main axis of the electric field gradient does not coincide with thec axis of the lattice. This surprising result indicates that at the iron probe, substituted in the Cu site of the CuO2 plane, the local electric charge symmetry is different from the lattice symmetry. We discuss this result in relation with the electronic configuration nearby this Cu site. The Pb based samples without Ca are magnetic at room temperature. Therefore in these samples Ca plays the same role as Sr in (La, Sr)2CuO4?y .  相似文献   

8.
We report experimental results of nuclear magnetic resonance (NMR) at the La site and nuclear quadrupole resonance (NQR) at the As site in the normal state of the superconducting compound LaOs4As12. Measurements have been performed on powder sample obtained from high quality single crystals. The temperature dependences of the nuclear spin-lattice relaxation rates, 1/T1, of 75As and 139La nuclei were measured. No scaling between them was found indicating a local character of relaxation processes. The relaxation of 75As nuclei can consistently be understood in terms of antiferromagnetic spin fluctuations, as deduced from the T-dependence of (1/T1T)=C/(Tθ)1/2.  相似文献   

9.
Corti  M.  Faffa  F.  Rigamonti  A.  Tabak  F.  Carretta  P.  Licci  F.  Raffo  L. 《Il Nuovo Cimento D》1994,16(10):1793-1797
Il Nuovo Cimento D - 139La NQR spectra, μSR precessional frequencies and NQR relaxation rates W driven by the Cu2+ spin fluctuations are used to study the static magnetic properties and the...  相似文献   

10.
This presentation gives a personal review of nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) spin-lattice relaxation studies in cuprate superconductors mainly dealing with the YBa2Cu4O8 compound with many examples from the Zürich laboratory. The studies were performed in both the normal and the superconducting state with various NMR isotopes (e.g.,17O,63,65Cu,135,137Ba). The relatively broad signals were mostly obtained by a phase-alternating add-subtract spin-echo technique. We will discuss the general behavior of spin-lattice relaxation in the normal state and the calculation of the dynamic spin including an approach (on the basis of thet-J model) to calculate the relaxation for plane copper, oxygen, and yttrium. An application of the Luttingerliquid model to the relaxation of chain copper in YBa2Cu3O7 and YBa2Cu4O8 is also given. We then will deal with characteristic features of the YBa2Cu4O8 structure: the spin gap, an electronic crossover in the normal state, the single-spin fluid model, and the d-wave pairing.  相似文献   

11.
Mössbauer emission spectroscopy on the 61Cu(61Ni) isotope has been used to determine the quadrupole coupling constant C(Ni) and magnetic induction B(Ni) for the 61Ni2+ probe at copper sites in Cu2O, CuO, La2 ?xBaxCuO4, Nd2?xCexCuO4, RBa2Cu3O6, and RBa2Cu3O7 (R=Y, Nd, Gd, Yb). The compounds containing divalent copper were found to exhibit linear C(Ni) vs. C(Cu) and B(Ni) vs. B(Cu) relations [C(Cu) and B(Cu) are the quadrupole coupling constant and magnetic induction for the 63Cu probe, respectively, found by NMR], which is interpreted as an argument for the copper being in divalent state. The deviation of the data points corresponding to the Cu(1) sites in RBa2Cu3O6 and RBa2Cu3O7 from the C(Ni) vs. C(Cu) straight line may be due either to the copper valence being other than 2+ (in the RBa2Cu3O6 compounds) or to the principal axes of the total and valence electric field gradient being differently oriented (in the RBa2Cu3O7 compounds).  相似文献   

12.
The magnetic structure of the Sr2Cu3O4Cl2 two-subsystem antiferromagnet is studied by the nuclear quadrupole resonance (NQR) method on the 63, 65Cu and 35Cl nuclei. The resonance spectrum above T N2 = 40 K is determined by the Zeeman splitting of the levels of the 63, 65Cu nuclei of the copper atoms at the Cu1 site with the first-order quadrupole perturbation. The magnetic field on the copper nuclei is equal to 93 kOe. The spectrum below n is significantly different: it includes a low-frequency part, which is associated with the ordering of the second magnetic subsystem Cu2. The splitting of the NQR lines of 35Cl is observed above and below T N2. This fact indicates the ferromagnetic ordering of the moments of the Cu1 subsystem, which are located along the c axis of the crystal, and makes it possible to determine the direction of the magnetic field on Cu1 copper as (110).  相似文献   

13.
Anelastic relaxation and 139LaNQR relaxation rates in La2–xSrxCuO4 for Sr content around 2 and 3 percent are discussed in terms of spin and lattice excitations and of the related ordering processes. It is argued how the phase diagram of La2–xSrxCuO4 at the boundary between the antiferromagnetic (AF) and the spin-glass phase (x = 0.02) could be more complicate than previously thought, with a transition to a quasi-long range ordered state at K, as indicated by neutron scattering data. On the other hand, the 139LaNQR spectra are compatible with a transition to an AF phase around K, in agreement with the phase diagram commonly accepted in literature. In this case the peaks in NQR and anelastic relaxation rates around 150 K and 80 K respectively in La1.98Sr0.02CuO4, yield the first evidence of freezing process involving simultaneously lattice and spin excitations, possibly corresponding to motion of charged stripes. Received 18 May 2000 and Received in final form 11 July 2000  相似文献   

14.
The magnetic and superconducting properties in the high-T c cuprates have been investigated over a wide hole doping range by63Cu,17O and205Tl NMR and NQR in the lightly-doped La2?xSrxCuO4 (LSCO), the heavily-doped Tl2Ba2CuO6+y (TBCO) and the Zn-doped YBa2Cu3O7 (YBCO7). In low doping region, the large antiferromagnetic (AF) spin correlation around the zone boundary (q=Q) causes the Curie-Weiss behavior of63(1/T 1 T) associated with that of the staggered susceptibility χO(T) in LSCO. In the vicinity of the hole content whereT c has a peak, the AF spin correlation still survives, although the magnetic coherence length ξM is considerably short being presumably (ξM/a) ~ 1. The further doping destroys progressively the AF spin correlation, which is no longer present is non-superconducting TBCO compounds. These NMR evidences signify that there is an intimate relation between the presence of the AF spin correlation and the onset of the superconductivity. The local collapse of AF spin correlation is a primary cause for the unexpected strong reduction ofT c in case of the substitution of Zn impurities into the CuO2 plane. The superconducting properties clarified by NMR experiments cannot be accounted for by the conventional BCS model or other isotropic s-wave models. A d-wave model is applicable in interpreting consistently most of the NMR results, if the finite density of states at the Fermi level is taken into consideration and is associated with the pair breaking effect. There are increasing evidences that the magnetic mechanism for the superconductivity is promising in high-T c cuprates.  相似文献   

15.
Powder samples of YBa2Cu3O6 were magnetically aligned and the anisotropies in the systems were studied by means of Cu(1) nuclear quadrupole resonance (NQR) in the absence of external magnetic fields. Our room temperature measurements of the NQR lineshapes and the spin–lattice and spin–spin relaxation times as a function of the aligning magnetic field indicate that full microscopic alignment can be achieved by using a magnetic field of about 4.7 T, for which doublet line patterns arising from a hyperfine splitting were observed.  相似文献   

16.
Nuclear quadrupole resonance spectroscopy (around 30 MHz) on the chain site Cu(1) nuclei in oxygen deficient YBa2(Cu1?x Fe x )3O6 doped with different amounts of57Fe (x≤0.01) reveal an onset of magnetic order at low temperatures represented by a symmetrical doublet contribution to the nuclear quadrupole resonance (NQR) spectrum. The onset temperatureT N2 depends on the concentration of Fe reaching 130 K forx=0.01. The splitting forx=0.01 at 100 K corresponds to a net internal field of 0.09 T with a distribution of ≈0.08 T representing about 70 percent of the Cu(1) nuclei.57Fe and57Co Mössbauer spectroscopy at 4.2 K with and without an external magnetic field of 5 T revealed that belowx=0.00015 Fe spins are decoupled from the Cu(2) moments in the antiferromagnetic state. Results are interpreted in terms of the magnetic model structure suggested by Kadowaki et al. [1].  相似文献   

17.
We present a study of shape of the Cu(2) NQR spectra in YBa2Cu3O7, TmBa2Cum3O7, and TmBa2Cu4O8 compounds at temperatures of 4.2–300 K. The results of the quantitative analysis lead us to conclude that the shape of the Cu(2) NQR spectra in all the samples studied can be described in the framework of the “motional narrowing” model, which implies that the Cu(2) nucleus possesses two different NQR frequencies between which it can rapidly jump. The difference in frequencies seems to be related to the charge-stripe correlations in CuO2 planes resulting in a dynamical modulation of the electric field gradients at the Cu(2) nuclei. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 9, 594–598 (10 November 1997)  相似文献   

18.
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.  相似文献   

19.
Quasi-one-dimensional (1-D) cupric oxide Ca1?xCuO2 (x = 0.164) is the system with 25–40% hole-doped edge-sharing CuO2 chains. However, the holes are almost localized in Ca1?xCuO2 and its magnetic susceptibility with a peak at 30 K was explained by the model considering both 1-D antiferromagnetic chains and spin dimers (Z. Hiroi, M. Okumura, Y. Nabeshima, T. Yamada, M. Takano: J. Phys. Soc. Jpn.69, 1824, 2000). To clarify the magnetic nature of Ca1?xCuO2, we performed submillimeter-wave electron spin resonance (ESR) measurements on a powder sample of Ca0.83?6CuO2. The resonance above 12 K showed typical powder ESR of Cu2+ and theg-values were determined to be g∥= 2.33 and g = 2.06 from the analysis. The resonance below 12 K changed completely from ESR. The frequency-field relation of ESR at 1.8 K clearly showed the easy-axis type antiferromagnetic resonance.  相似文献   

20.
Results of TDPAC studies of hyperfine interaction in high temperature superconducting ceramics are reported. The γ-ray cascade of 329–487 keV in140Ce excited in the decay of140La in La2-xSrxCuO4 samples (x=0 and 0.15), and 133–482 keV cascade in181Ta excited in the decay of181Hf in YBa2Cu3O7-δ samples were used. The procedure of introducing radioactive181Hf into the ceramics is described and indirect evidence for the occupation of Cu sites by the181Hf-181Ta probe is presented.  相似文献   

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