COPPER NUCLEAR-QUADRUPOLE RESONANCE IN GDBA2CU3O7 - DETERMINATION OF SITE ASSIGNMENT

VOLUME 38, NUMBER 4 PHYSICAL REVIEW B Copper nuclear quadrupole AUGUST 1988 of site assignment resonance in GdBa2Cu307. Determination P. C. Hammel, M. Takigawa, R. H. Heffner, and Z. Fisk Los A/amos National Laboratory, Los Alamos, Ne~ Mexico 87545 (Received 18 April 1988) We have measured the copper nuclear relaxation rate in GdBa2Cuq07 in zero applied field us- ing nuclear quadrupole resonance. Fluctuations in the 4f moment associated with the gadolinium contribute strongly to the copper relaxation rate, and this contribution will depend strongly on the This separation differs considerably for the two copper sites. copper-gadolinium separation. Comparison of the relaxation rates shows that the copper signal at the higher frequency (32 MHz) originates from the Cu(2) site, which is located closer to the gadolinium than is the Cu(1) site. The discovery of superconductivity at temperatures above 90 K in the yttrium-based copper oxides' has stimulated much study of the properties of YBa2Cu307 — s (Y-Ba-Cu-0). Copper nuclear quadrupole resonance (NQR) has been a very eff'ective tool in exploring the electronic properties of Y-Ba-Cu-0 both above and below the superconducting transition temperature T, . Copper occupies two distinct sites in the Y-Ba-Cu-0 crystal, the so-called chain or Cu(1) site and the plane or Cu(2) site (see Fig. 1). 63Cu nuclear quadrupole resonances have been observed at two distinct frequencies, 22. 0 and 31. 5 MHz, corresponding to these two Cu sites. A major prob- is the identification of which NQR resonances correspond to the chain and plane copper sites, respectively. This will allow the study of the behavior at the two sites separately. The spin-lattice relaxation rates 1/Ti for the two resonances 21') 3 4 show completely different temperature dependences both above and below the superconducting transition temperature T, (see Fig. lem to be addressed GdBa2Cug0 7 f=32. 3 MHz ii+ ~i~ li GdBa2Cug07 f=22. 5 MHz I V) Ba Cu(2) YBapCug07 f=22. 1 MHz Y, Gd sooo— k k 0(S YBa2Cug07 g k k f=31.5 MHz TEMPERATURE ( K ) FIG. 2. Spin-lattice relaxation rates (1/Ti) for GdBa2Cup07 O(1) FIG. 1. The crystal structure of RBa2Cu&O& — b (R Y, Gd). Note the large difference in the Cu(1)-Gd and the Cu(2)-Gd separation. The YBa2Cu&07 — & crystal structure shown is due to Ref. 17. (this work) and YBa2Cuq07 — s (Ref. 2) as a function of temper- ature. Note the break in the vertical axis. The temperature- independent contribution to the relaxation rate due to the gado- linium 4f electronic moment is evident. In GdBa2Cuq07 — q, this contribution is 18 times larger for the 32 MHz signal than for the 22 MHz signal, demonstrating that the 32 MHz signal origi- inates from the Cu(2) site which is much closer to the gadolini- um. In Gd-Ba-Cu-O, 1/Ti at 32. 3 MHz is shown by open cir- cles and at 22. 5 MHz by closed circles, in Y-Ba-Cu-0 at 31. 5 MHz by open triangles and at 21. 5 MHz by closed triangles. The American Physical Society