¹⁷O NMR in YBa₂Cu₃O₇-δ Superconductors.

P. WZIETEK (*), D. KÖNGETER (*), P. AUBAN (*), D. JÉROME (*), J. M. BASSAT (**) J. P. COUTURES (**) , B. DUBOIS (**) and PH. ODIER (**)

(*) Laboratoire de Physique des Solides (associé au CNRS), Université Paris-Sud
F-91405 Orsay Cédex, France

(**) CRPHT-CNRS, 1D Avenue de la Recherche Scientifique
F-45071 Orléans Cédex 2, France

(received 17 March 1988; accepted in final form 12 December 1988)

PACS. 74.30G – Response to electromagnetic fields, nuclear magnetic resonance, ultrasonic attenuation.

PACS. 74.70 – Superconducting materials.

**Abstract.** – We present the first ¹⁷O NMR data obtained in YBa₂Cu₃O₇-δ powder with T_c = 90 K in which oxygen sites on Cu-O chains had been selectively populated by a gas exchange process performed at low temperature. A comparison between fully exchanged and chain only exchanged samples suggests that magnetic-field-swept spectra cannot discriminate between both kinds of samples. Fourier-transformed spectra do however provide a very clear assignment between plane and chain-bridge sites. The spin-lattice relaxation of chain sites measured between 4.2 and 300 K is dramatically nonexponential, as expected for nuclei in the presence of an important quadrupolar interaction. Our data show the existence of a shortening of the chain relaxation in a limited temperature domain slightly below T_c. This hump of relaxation supports the absence of zeros of the superconducting order parameter over the Fermi surface and the s-symmetry for the pair wave function.

***Introduction.*** – So far, superconductivity at high temperatures has only been observed in oxides [1]. This fact suggests that oxygen atoms constitute a major ingredient for high-temperature superconductivity.

It is thus of primary importance to possess a local probe which is able to provide
information about the role played by oxygen atoms in the electronic properties of the
superconducting oxides.

As far as the YBa₂Cu₃O₇-δ series is concerned, it is well established that the oxygen
concentration (0 < δ < 1) determines the nature of the ground state [1].

Neutron diffraction studies [2] of the orthorhombic phase of YBa₂Cu₃O₇-δ (6.5 < 7 - δ < 7) have shown that oxygen atoms can occupy five different crystallographic sites. Sites O(1) and O(5) are related to the (a, b)-plane, but the O(5) are empty at δ≈0 leaving Cu-O chains along the b-direction. O(2) and O(3) sites belong to CuO₂ layers adjacent to Y atoms, while the O(4) sites are located in planes parallel to the c-direction. The oxygen content is related to the temperature at constant partial pressure as shown by thermogravimetric analysis curves under flowing oxygen [3].
Strauven et al. [4] have examined the effect of temperature on the oxygen desorption of a $7-\delta = 6.85$ starting material. Up to $1000$ °C the oxygen evolves in five steps with the first two steps occurring at $650$ °C and $720$ °C. Up to $350$ °C no evolution of oxygen in CuO chains was observed. Around $720$ °C an orthorhombic-tetragonal phase transition occurs (with equal occupancy of O(1) and O(2) sites).

The present letter reports a preliminary attempt to clarify the role of oxygen on the electronic properties of $YBa_2Cu_3O_{7-\delta}$ («1-2-3» compound) through the NMR of $^{17}$O nuclei in selectively gas-exchanged «1-2-3» powders.

The starting «1-2-3» material was prepared by a chemical route using liquid nitrate precursors in order to achieve an excellent chemical homogeneity. After a two hours' treatment at $850$ °C in $O_2$ atmosphere this process leads to a powder exhibiting a monomodal distribution of particles centred at $0.7$ µm which gives the onset of a Meissner signal at 90 K [5]. The isotopic exchange was carried out by a heat treatment of 5 days at $300$ °C in a gaseous mixture enriched up to 51% in $^{17}$O. The $^{17}$O/$^{16}$O ratio in the confined atmosphere was controlled by a quadrupole mass spectrometer during the exchange process. After the 5-day treatment no further change of the ratio $^{17}$O/$^{16}$O could be detected.

No chemical analyses were undertaken, but our data suggest that at least 20% of the oxygen sites were exchanged by $^{17}$O. The X-ray diffraction of the sample (#1) performed before and after the $^{17}$O exchange provides $a = 3.83$ Å, $b = 3.88$ Å, $c = 11.68$ Å. These X-ray data and the onset of Meissner signal at 90 K are in agreement with a stoichiometry of $7-\delta = 6.85$ [2]. Another sample (#2) has been gas-exchanged in the same $^{17}$O atmosphere for a duration of 3 days at $850$ °C and slowly cooled down to room temperature in order to maintain the high oxygen content. All samples were kept in quartz ampoules filled with one atmosphere of $^{17}$O.

Two kinds of NMR experiments have been performed with a phase-coherent Bruker MSL 400 spectrometer:

i) Field-swept studies between 4.2 and 100 K plotting the amplitude of the spin-echo signal vs. the magnetic field for samples #1 and #2.

ii) Fourier transform of the spin-echo in both samples at fixed field ($v_0 = 34$ MHz) selecting a pulse length of $\pi/6$, in order to preferentially excite the (1/2, -1/2) transition of the quadrupolar spectrum.

The full width at half height of the spin-echo in sample #1 of about $10$ µs is consistent with the width of the central peak of the field-swept spectrum, *vide infra*. The $T_2$ relaxation time amounts to about 0.8 ms at room temperature with only a slight temperature dependence.

Field-swept spectra for samples #1 and #2 at $T = 100$ K are displayed in fig. 1. The shape of the spectrum is characteristic of an NMR powder pattern perturbed by first-order quadrupolar splitting. The central peak is due to the magnetic transition (1/2, -1/2) which is not affected by the first-order quadrupolar coupling, whereas weaker satellites on both sides of the central peak can be attributed to singularities in the powder pattern of the quadrupolar ($\pm 1/2, \pm 3/2$) transitions [6]. The splitting between peaks of satellites amounts to about 760 kHz (insert of fig. 1) for both samples.

The NMR spectrum confirms that oxygen can well be exchanged in the «1-2-3» structure at a temperature as low as $300$ °C. However, we infer from the diffusion properties of oxygen that at such a low temperature O(1) sites which are part of the CuO chains are the only ones to be exchanged easily. Consequently, fig. 1a) should preferentially characterize the resonance pattern of O(1) sites. However, the comparison between fig. 1a) and b) shows that field-swept spectra do not allow to discriminate between different oxygen sites as the
<figure><img src="image_1.png" /><figcaption>Fig. 1. – Field-swept spectra of samples #1, $v_0$=34 MHz (a)) and #2, $v_0$=31.5 MHz (b)). In the insert the vertical-axis scale is increased to show details of the first-order quadrupolar splitting for sample #1.</figcaption></figure>

spectrum of sample #2, for which all oxygen sites of the orthorhombic structure are populated, does not look that different from the spectrum of the selectively exchanged sample #1.

We have not attempted to fit the powder spectra of fig. 1 with a model involving the quadrupolar coupling constant, the asymmetry parameter $\eta$ of the electric-field gradient tensor and possible anisotropies in chemical and paramagnetic shifts. However, assuming $\eta=0$, the peak-to-peak separation $v_Q$ = 760 kHz leads to a quadrupolar coupling constant $e^2qQ/h$ = 5.06 MHz; such a coupling gives rise to a second-order broadening of the central transition of 25 kHz (or 43 Oe) which is much smaller than the observed full width at half height of 160 G. The magnitude of the quadrupolar coupling constant is in fairly good agreement with the empirical law between $e^2qQ/h$ and the ionic character of oxygen which has been established in various oxides [7]. Since dipolar couplings with neighbouring Cu atoms are too small to account for the line broadening, we are forced to admit that the line broadening of the (1/2, -1/2) field-swept spectrum is inhomogeneous. This is confirmed by the lineshape of the (1/2, -1/2) transition obtained after Fourier transformation of the spin-echo in samples #1 and #2, fig. 2. Sample #2 reveals the existence of two lines separated by about 2000 p.p.m. at 273 K. The relatively narrow weakly shifted peak (380 p.p.m. down-field shift from a H$_2$$^{17}$O reference signal at room temperature) is assigned to the O(1) sites [8]. The more intense and down-field shifted signal can be attributed to the resonance of O(2, 3) plane sites [8]. However, following intensity considerations for the two resonance lines of sample #2 (fig. 2), we suggest that the weakly shifted peak should also contain the resonance coming from $^{17}$O nuclei on O(4) sites. We notice the existence of a very weak signal in sample #1, fig. 2, occurring at the position of the plane signal in sample #2. Consequently, we may say that even for an exchange temperature as low as 300 °C O(4) sites can be easily populated and even a finite fraction of the plane sites can be exchanged with $^{17}$O.
<figure><img src="image_2.png" /><figcaption>Fig. 2. – Fourier-transformed spectra of samples #1 (lower trace) and #2 (upper trace) at T = 270 K with corresponding magnetization recoveries (T = 196 K): single line of sample #1 and plane sites in sample #2 (H₂¹⁷O line is taken as spectrum reference).</figcaption></figure>

The determination of the spin-lattice relaxation rate has been an important aspect of this work since in metals and in particular in superconductors it provides useful information about the gap which opens in the single-particle density of states at the Fermi level and the symmetry of the pair wave function [9].

The measurements have been carried out on samples #1 and #2 at various temperatures between 4.2 and 200 K by plotting either the regrowth of the integrated echo intensity (sample #1) or the growth of the FT spectra (#2) vs. the delay τ between a short comb saturation (40 pulses of 6 µs for comb length of 1.4 ms) and the observation sequence. Figure 2 shows the time dependence of the recovery for the single line (O(1) peak) of sample #1 and the plane sites of sample #2 at T = 190 K. The recovery of sample #1 is clearly nonexponential since an exponential growth is attained approximately only after the signal has reached about 80% of the equilibrium value. As shown in fig. 2, the recovery of plane sites signals is also nonexponential although time constants involved are about ten times smaller than those of the chain sites. The nonexponential growth requests a determination of the signal over two decades. This implies a large enough signal-to-noise ratio and therefore long accumulation times (about 1 day per data point) are needed.

A nonexponential growth of the magnetization is indeed expected for nuclei ($I > 1/2$) located in lattice positions exhibiting large electric-field gradients [10, 11]. Furthermore the inequality of level spacings between the six Zeeman levels prevents the spectral diffusion which would maintain a Boltzman distribution. Following a procedure already used in the case of nuclei in noncubic crystals [10, 11] we have performed an analysis of the raw data such as those of fig. 2 with a summation of three exponential growths characterized by three respective time constants and amplitudes. We have checked that the time dependence of the recovery agrees fairly well with the theoretical law for a nuclear relaxation induced by the fluctuations of the magnetic (hyperfine) field [11] (details will be published elsewhere [12]).
<figure><img src="image_3.png" /><figcaption>Fig. 3. – Temperature dependence of the long-time tail $T_1$, sample #1.</figcaption></figure>

The long-tail time constant of the fit is similar to the relaxation time which can be derived from the semilog plot of $M(\tau)$ vs. $\tau$ when $M(\tau)$ exceeds 80% of its thermal equilibrium value.

Figure 3b) displays the time dependence of the $T_1$ derived by the fit. Due to the nonexponentiality, the uncertainty of the data is admittedly rather large. However, we feel confident that the existence of the hump of $1/T_1$ is real. The hump of $1/T_1$ can be understood as the signature of concomitant effects due to a superconducting gap opening over the entire Fermi surface and coherence factors which are typical for s-wave pairing [13]. A somewhat similar hump has been observed for the relaxation of ¹³⁹La ($I = 7/2$) below the transition temperature of La₁.₈Sr₀.₂CuO₄₋δ [14].

However, as far as the «1-2-3» superconductor is concerned, neither Cu(1) nor Cu(2) relaxation rates show any hump below $T_c$ [15]. This feature has been understood for Cu(2) sites by the existence of localized moments on Cu atoms providing the dominant relaxation channel [16] and consequently masking possible relaxation through itinerant carriers. A recent relaxation study of ¹⁷O in «1-2-3» materials which has appeared as the present letter was under revision [17] has concluded to the existence of a 1/T₁ hump for the oxygen plane sites.

To summarize, the ¹⁷O relaxation study on selectively exchanged superconducting YBa₂Cu₃O₇₋δ powder indicates that oxygen nuclei pertaining to CuO chains and possibly O(4) sites show a severely nonexponential relaxation via the fluctuations of hyperfine field. The slight increase of 1/T₁ which is observed just below $T_c$ has been attributed to a Hebel-Slichter anomaly. However, we have observed that $T_1$'s of chain sites for the totally exchanged sample #2 are about a factor 2 shorter than those of sample #1 [12] (shown in fig. 3). This effect could be attributed to a spin-diffusion-assisted relaxation from rapidly relaxing plane sites. Consequently, we cannot rule out as the origin of the 1/T₁ hump in sample #1 a possible effect of the residual population of plane sites (vide supra) on the chain
relaxation via spin diffusion between planes and chains. Anyway, this does not affect the salient conclusion in favour of the absence of zeros for the superconducting gap over the 3D or 2D Fermi surface.

We wish to acknowledge a fruitful cooperation with P. BERNIER and the support of a DRET contract 86/057. D. KÖNGETER has benefited from an EEC contract ST2J0315-C.

## REFERENCES

[1] A recent review can be found in MULLER J. and OLSEN J. L., *High Temperature Superconductors and Materials and Mechanisms of Superconductivity*, Physica C, 153 (North Holland) 1988.

[2] JORGENSEN J. D. et al., *Phys. Rev. B*, 36 (1987) 3608.

[3] DUBOIS B., DONY A., GERVAIS M., CABANNES F. and PH. ODIER, in ref. [1].

[4] STRAUVEN H. et al., *Solid State Commun.*, 65 (1988) 293.

[5] MONOD P., DUBOIS B. and PH. ODIER, in ref. [1].

[6] ABRAGAM A., *Principles of Nuclear Magnetism* (Oxford University Press, Oxford) 1961.

[7] SCHRAMM S. and OLDFIELD E., *J. Am. Chem. Soc.*, 106 (1984) 2502.

[8] BLEIER H. et al., *J. Phys. (Paris)*, 49 (1988) 1825.

[9] MAC LAUGHLIN D., *Solid State Phys.*, 31 (1976) 1.

[10] ANDREW E. R. and TUNSTALL D. P., *Proc. Phys. Soc.*, 78 (1961) 1.

[11] NARATH A., *Phys. Rev.*, 162 (1967) 320.

[12] WZIETEK P. et al., to be published (and Ph.D. Thesis).

[13] HEBEL L. C. and SLICHTER C. P., *Phys. Rev.*, 113 (1957) 1504.

[14] SEIDEL H. et al., *Europhys. Lett.*, 5 (1988) 647.

[15] YASUOKA H. et al., preprint (August 1988).

[16] HORVATIC M. et al., preprint (July 1988).

[17] KITAOKA Y. et al., preprint (July 1988).