Pressure effects of NQR parameters in YBa2Cu3Oy

Physica C 162-164 (1989) 173-174 North-Holland

PRESSURE E F F E C T S OF NQR PARAMETERS IN YBa2CusOv Katharina MI~LLER, M. MALI, J. ROOS and D. BRINKMANN Physik-Institut der Universitlit Zfirich, Sch6nberggasse 9, CH-8001 Zfirich, Switzerland

We have studied the effect of hydrostatic pressure (up to 0.6 GPa) on the spin-lattice rdaxation' time T1 and on the resonance frequency vQ of Cu NQR signals in YBa~CusO v for y ~ 7 and ~ 6. For y ~ 7 and above To, T1 is independent of pressure. For y ~ 7, vQ of Cul (Cu2) decreases (increases) linearly with pressure while for y ~ 6, uQ of Cul increases, vq is calculated in terms of the point-charge model to yield the ion valencies.

We have studied the effect of hydrostatic pressure (up to 0.6 GPa) on the resonance frequency v~ of the Cu NQR (nuclear quadrupole resonance) signals for the Cu(1) site in YBazCu30s and on vq and the spin lattice relaxation time 2"1 of Cu NQR signals for both Cu sites in YBa2Cu30~. In YBa2CuaOs an antiferromagnet below Tt¢ = 418 K oniy ~/~ of the non-magnetic Cul+(1) could be measured by pure NQR. However, from the quadrupolar split NMR (nuclear magnetic resonance) in the internal magnetic fidd 1 the vQ of the magnetic CuZ+(2) is also known. At normal pressure I/Q of Cu(1) stays constant up to 120 K where by abruptly changing the slope it starts to decrease linearly by -0.97 ± 0.03 kHz/K with increasing temperature (Fig. 1). Applying pressure shifts the z/Q(T) curve linearly by +540 ± 50 kHz/ G P a to higher values. In the past a similar temperature and pressure dependence of z/Q was observed 2 for the monovalent Cu in Cu~O. To determine vQ theoreticaUy we evaluated by means of a point charge model the electric field gradient (EFG) at the Cu sites. The EFG is usually composed of an intra-atomic or valence contribution if the quadrupolar nucleus is located in an ion having incomplete electronic shall and a lattice contribution which is due to charges on neighboring ions. In case of Cul+(1) the valence part is zero. The lattice contribution of the EFG evaluated with an experimental Cu ~+ Sternheimer antishielding factor 700 = - 5 . 5 from Cu20 together with the ~Cu dectric quadrupole moment Q = -0.211 • 10 -zs m 2 yields for Cu(1) Z/Q = 31.0 M H z which is in excellent 0921-4534/89/$03.50 © Elsevier Science Publishers B.V. (North-HoUand)

agreement with the 29.9 MHz experimental value. It seems that the point charge model is adequate for the EFG calculation at the Cul+(1) site. In case of Cu2+(2) having an incomplete electronic shell a valence EFG contribution has to be added to the lattice part. The other way around, from the experimental 1 Cu(2) v~ and calculated lattice EFG the va. lence EFG can be extracted. This way we get 80 MHz for the valence quadrupole coupling a value appreciably smaller than the 130 MHz observed in distorted octahedral divalent copper complexess. i

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FIGURE 1 Temperature dependence of t he a~Ca (1) N QR frequency vQ in YBa~CusOs at normal pressure and at 0.6 GPa. Insert: NQR signal.

K. Mailer et al. I Pressure effects of NQR parameters in YBa2CusOy

174

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FIGURE 2 Temperature dependence of the °sCu NQR frequencies vQ for both Cu sites in YBa2CusOr at normal pressure and at 0.6 GPa. In the superconductor YBa2CusOr and above T¢, Tx for Cu(1) and Cu(2) is independent of pressure up to 0.6 GPa, i.e. the high frequency fluctuations giving rise to relaxation are not affected. However, the EFG and through it ~'0 are influenced by pressure. At normal pressure the vq of Cu(1) [Cu(2)] increase~ by 0.8 =k 0.1 kHz/K [decreases by 1.6 ± 0.1 kHz/K] with increasing temperature (Fig. 2). By applying pressure v~ changes linearly with pressure, however, with different slopes for Cu(1) and Cu(2) (Fig. 3). The VQ(T) curve is shifted for Cu(1) by 480 ± 40 kHz/GPa to lower and for Cu(2) by 150 5=70 k H z / G P a to higher values, thus pressure quasi enhances the effect of temperature on vQ (Fig. 2). A comparison with YBa2CusOs clearly shows beside the big difference in vQ's a completely different temperature and pressure dependence of the Cu(1) vQ in both compounds. To explain this different behavior and the extreme asymmetry (77 ~ 1) of the EFG at the Cu(1) site various point charge models 4 were invoked. To restrict the possibilities of the charge distribution we allowed variation of the oxygen's and Cu(1) charge but kept ys+, BaZ+ and Cu2+(2) constant. Further

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FIGURE 3 Pressure dependence of the °SCu NQR frequencies i.,Q in YBa2CuaOs and YBa2CuaOT. we used 7o0 = - 5 . 5 and in case of Cu 2+ valence contributions that did not deviate more than 10 % from the value obtained in YBa2CusOo. A good overall agreement with the experimental data has been achieved for Cu~+(1), chain oxygen O l's- and the rest oxygens 01"96- . Of course, using the point charge model one has to be aware of its weaknesses and should not be deceived by excellent fits. REFERENCES 1. H. Yasuoka, T. Shimizu, Y. Ueda and K. Kosuge, J. Phys. Soc. Jpn. 57 (1988) 2659. 2. T. Kushida, G.B. Benedek and N. Bloembergen, Phys. Rev. 104 (1956) 1364. 3. E. K6nig~ Magnetische Eigenschaften der Koordinations- und metallorganischen Verbindungen der Uebergangselemente (Springer Verlag, Berlin, 1966). 4. C.H. Pennington, D.J. Durand, C.P. Slichter, J.P. Rice, E.D. Bukowski and D.M. Ginsberg, Phys. Rev. B39 (1989) 2902. M Mali, J. Roos and D. Brinkmann, Physica 153-155 (1988) 737.