X-ray Temperature Difference Absorption Spectra (XTDAS): Y/NdBa2Cu3O7-d Phase Transitions
L. Nguyen, C. M. Burch, T. H. Vu, Q. v. Le, K.K. Singh, C.T. Lin and J.V. Acrivos
Department of Chemistry San José State University
Abstract
XAS measurements taken as Y/NdBa2Cu3O7-d undergo superconducting phase transitions can detect the increase in fluctuation of the inter-atomic potential near Tc. These must be differentiated from for instrumental glitches. We have developed a technique that analyzes the difference between the absorbance at a given temperature T relative to that at Tc. The temperature is ramped, at a constant rate of 5 to 10 K per XAS spectrum, by using the remote control Oxford ITC4 program and a PC to produce a continuous temperature cycle from 4 to 100 K - 100 K to 4 K, etc. The changes in the atomic absorbance and in the bond distance that have been detected by conventional XAFS analysis are confirmed in this manner. The fine structure (~1 Å) in the Ba XTDAFS indicate changes in atomic xafs due to electron density changes near Tc.
Introduction
The Y/NdBa2Cu3O7-d were prepared at IRC for Superconductivity, Cambridge University (Professor W.Y. Liang's Research Group) and Morris Laboratories, Berkeley.
Discussion of Results: SSRL 2425V
Near the superconducting transition temperature Tc in cuprate oxides (figures 1a, 2a) an increased transparency is observed in the post-edge region of Ba and Nd L2,3 and Cu K edges. The explanation for this effect is that the relaxation time t if for the final states in the post-edge region decreases near Tc due to fluctuations in the inter-atomic potential. The normalized atomic absorption coefficient K versus energy (E = hv) in the neighborhood of an x-ray edge (E0 = hv0) for the absorber A, is given by the sum of Lorentzian curves1:
K(t , (v-v0)) = S aA/(1+ (2p t AE)2(vAE - v)2) ™ (p /2 + ArcTan(2p t if (v - v0))/p .
The magnitude of the slope:
m = (K(E) - K(E0))/(E - E0)/K(E0)
versus E - E0 = h(v - v0) was used to calibrate the range of 2p t if /h = 1 to 0. 4 eV-1 for m = 0 to -1E-5 eV-1 in fig. 1b. The observations near Tc are reproducible in the increasing and decreasing parts of temperature cycle for all samples with different Tc and some with two Tc. These phenomena occur only near Tc. The results in figure 2a show that the electron density changes near Tc involve the so called non-conducting Ba-O layers. The changes in the different Ba-O bond distances in the cuprate superconductors are in agreement with the changes in the Raman modes involving the Ba atoms; the most important effect uncovered by the XTDAS is the changes in the Ba atomic exafs at the phase transition (fig. 2b,c).
Conclusion
We have shown the power of XAS to measure phase transitions in these solid state materials. A Ba-O bond distance change near Tc indicates a change in electron density along the Ba-O bond.
Acknowledgements
The research, and publications2-6 were supported by NSF DMR9612873.
References
1. F.K. Richtmyer et al., Phys. Rev. 46, 843 (1934)
2. T. Norman, A. T. Nguyen, L. Nguyen and J. Acrivos, Bull. APS 43, 571 (1998)
3. C. Burch, L. Nguyen and J. Acrivos, Bull. APS 43, 584 (1998)
4. Q. v. Le, T. H. Vu, L. Nguyen, A. Alvarado and J.V. Acrivos, ACS Meeting Las Vegas, September 1997
5. J.V Acrivos, ACS Meeting Las Vegas, 1997
6. L. Nguyen, ACS Meeting Las Vegas, 1997
Figure 1: YBa2Cu3O7-d Cu-K Edge XAS in transmission at the orientation: q = (c^ Ex-ray field) = p /2, f = (a^ Ex-ray field) = p /6, and T (increasing) showing two phase transitions: (a) Absorbance versus energy at the differentT indicated. (b) Calculated K according to reference 1, versus 2 p t if /h and E - E0. (c) XTDAS for figure 1a. The absence of x-ray fine structure in the XTDAS indicates negligible changes in bond distance at Tc.
Figure 2: Nd1.1Ba1.95Cu3O7-d : (a) Absorbance near the Ba-L3 edge. (b) Ba-Oi bond distances vs T. (c,d) XTDAS for figure 2a. The presence of x-ray fine structure indicates the Ba atomic XAFS change near 1 Å, near Tc. This indicates that a change in the electron density occurs in the BaO layer near Tc.