DISORDER TO ORDER-LIKE TRANSITION IN La 2 CuO 4.1 SUPERCONDUCTOR INDUCED BY HIGH INTENSITY X-RAYS

Transcription

1 International Journal of Modern Physics B, Vol. 16, Nos. 11 & 12 (2002) c World Scientific Publishing Company DISORDER TO ORDER-LIKE TRANSITION IN La 2 CuO 4.1 SUPERCONDUCTOR INDUCED BY HIGH INTENSITY X-RAYS D. DI CASTRO and G. CAMPI Unità INFM and Dipartimento di Fisica, Università di Roma La Sapienza, P.le Aldo Moro 2, Roma, Italy G. BIANCONI Physics Department, Notre Dame Univeristy, IN-46566, Indiana, USA Received 10 December 2001 Revised 20 March 2002 We have studied effect of X-ray illumination on charge ordering superstructures in oxygen doped La 2 CuO 4+δ superconducting system. Formation of long range crystalline phase of polarons is favored in this system by mobile counter-ions (interstitial oxygen ions in the La 2 O 2 layer) in the temperature range K. We find a clear signature of disorder to order like X-ray induced phase transition. This transition appears to show up beyond a threshold dose of about photons/cm 2, the value which increases with decreasing temperature. Keywords: Synchrotron X-ray diffraction; micro-strain and charge ordering; X-ray photoinduced phase transitions; cuprate perovskites; nanostructure manipulation. 1. Introduction The physics of cuprates appears to be confined in the microscopically inhomogeneous CuO 4 square plane, involving various degrees of freedom, charge, lattice and spin excitations. 1 The anomalous superconducting phase, that appears at high metallic density (beyond 0.06 holes per Cu site), survives in a perovskite lattice containing metallic units (bcc CuO 2 sublayers) intercalated by ionic block layers (fcc Ba, Sr, La, Y, Ca oxides). Recent experiments reveal that the strength of the electron-lattice interaction in the metallic CuO 2 plane depends on the Cu-O microstrain due to lattice mismatch between the bcc CuO 2 plane and the intercalated fcc layers. 2 Indeed, the micro-strain of the metallic Cu-O units increases by decreasing the atomic radius of the intercalated ion. Furthermore, high resolution angular resolved photoemission experiments 3 have shown an increasing renormalization of the electronic band dispersion with the micro-strain in the energy range of about 80 mev, extending from the van Hove singularity to the Fermi level. The oxygen doped La 2 CuO 4+δ superconducting system, having a high value of micro-strain, is the simplest system to study the ordering of dopants and the charge 1627

2 1628 D. D. Castro, G. Campi & G. Bianconi ordering in the CuO 2 plane among the high-t c superconductors. In this material the dopant oxygen ions in the LaO layers, that are the negative counterions, are free to move in at a temperature T>180 K. 4 7 X-ray diffraction measurements have revealed co-existence of different superstructures in this system 7 which are classified through the staging of oxygen along the c-axis, 4 i.e. the modulation wave-vectors along the c axis. First two superstructures are characterized by 3-dimensional (3D) ordering and narrow, resolution limited, diffraction peaks with the wavevectors (stage 2): q 1 =0.089 (±0.003)a (±0.002)b (±0.005)c ; and q 2 = (±0.003)a (±0.002)b (±0.005)c. These superstructures coexist with other diffuse spots with the wavevector (stage 3.5): q 3 = (±0.002)a (±0.003)b (±0.005)c ; and q 4 = (±0.001)a (±0.002)b (±0.005)c. In this work we have studied effect of temperature and X-ray photon illumination on different superstructures to explore various kind of phase transitions in the oxygen doped La 2 CuO 4. While photo-induced phase transitions (PIPT) 8 have been widely studied by laser photon pumping, the present work exploits possibility of X-ray photon pumping using high brilliance synchrotron radiation sources. We find a clear signature of X-ray photon induced phase transition like behavior as a function of temperature and X-ray photon dose. The X-ray irradiation induced phase transition like behavior has been studied at 300 and 220 K. Here we will focus on the evolution of the stage 2 superstructures while the characteristic features of the stage 3.5 superstructures are discussed elsewhere Experimental A single crystal grown by flux method of La 2 CuO 4.1, doped by electrochemical oxidation, that shows a sharp superconducting transition at 38 K was used for temperature dependent diffraction measurements. The diffraction data were collected on the crystallography beam-line at the Elettra storage ring at Trieste. The X-ray beam emitted by a wiggler source at the 2 GeV Elettra storage ring was monochromatized by a Si(111) double crystal and focused on the La 2 CuO 4.1 single crystal. We have collected the data in the K geometry, with photon energy of 12.4 KeV, using a 2-dimensional (2D) X-ray CCD detector. The sample oscillation around the b axis was in a range 0 <θ<30,whereθ is the angle between the direction of the photon beam and the a-axis. Thanks to the high brilliance source, it was possible to record a large number of weak superstructure spots due to charge ordering around the main peaks of the average structure. Twinning of the single crystal was accounted to index the diffraction images containing the normal peaks characterizing the average structure and the superstructure diffraction spots. The lattice parameters of single crystal were determined to be a =5.351 Å, b =5.418 Å, c = Åatroomtemperature.

3 Disorder to Order-Like Transition Results and Discussion The two superstructures q 1 and q 2 (stage 2) are characterized by narrow resolution limited peaks and associated to two different oxygen ordered phases. The first ordered phase, called O1, is characterized by strings of finite length, of about 11a, separated by d = 4b with doubling of the unit cell along the c- axis (q 1 =0.089 (±0.003)a (±0.002)b (±0.005)c ). The second one, called O2, is made of longer strings, forming an incommensurate superlattice with a period of 3.75 lattice units in the b direction (q 2 =0.049 (±0.003)a (±0.002)b (±0.005)c ). 7,10 While the superstructure q 1, characterizing the phase O1, appears below the temperature T c1 = 356 K, the q 2 shows-up (phase O2) below T c2 380 K. 11 The profiles of the superstructures peaks in the h direction are shown in Fig. 1(a). The peaks correspond to the ordered phases (stage 2), namely O1 and O2. Fig. 1. (a) Profiles of the q 1 and q 2 diffraction peaks in the Q =(0,h,6+0.5) direction at T = 300 K. (b) Profiles of the q 1 and q 2 diffraction peaks in the Q =(0,h,6+0.5) direction for three different dose values (1, 2 and 3 in the circles) at T = 300 K.

4 1630 D. D. Castro, G. Campi & G. Bianconi As evident from the figure [Fig. 1(a)], the O2 has larger probability (60%) than that of the phase O1 (40%). The relative weight of the two ordered phases changes under X-ray illumination, as evident from Fig. 1(b), where we have shown the profiles of the two superstructures for different X-ray dose values. Starting from the initial state [denoted by 1 in Fig. 1(b)], the weight of O1 (q 1 ) is pumped up to more than 4 times its initial value, while the weight of the O2 (q 2 ) gets decreased [see e.g. the profile denoted by 3 in Fig. 1(b)]. Figure 2 shows evolution of the relative weight of q 1 and q 2 as a function of X-ray dose. Here the relative weight corresponds to the integrated intensity I(q 1 ) on /I(q 1 ) off,wherei(q 1 ) on is the intensity of the peak under illumination while I(q 1 ) off without illumination. It is interesting to note that the relative weight shows a threshold photon flux of Φ th photons/cm 2 above which the q 1 increases sharply with the X-ray photon dose. On the other hand, the relative weight of q 2 has a higher threshold value Φ th photons/cm 2,above which it starts decreasing. The sum of the intensities of the two superstructures shows an increase. In order to obtain a disordered phase the sample was heated up to 375 K to suppress the ordered phases. The heating was followed by a rapid quenching of the sample down to 220 K. At this temperature, the same experiment was repeated to investigate effect of the X-ray illumination. Figure 3 shows evolution of the relative weight of the superstructure q 1 as a function of X-ray doses. We observe that weight Fig. 2. Effect of the X-ray illumination dose on the relative integrated intensity I(q 1,2 ) on/i(q 1,2 ) off of the charge ordering modulations q 1 and q 2 at T = 300 K. The sum of the intensities of the two superstructures is also shown (diamonds). It appears that the two photon flux thresholds are Φ th photons/cm 2 for the phase O1 and Φ th photons/cm 2 for the phase O2 respectively. The number 1, 2 and 3 indicate the dose values corresponding to the three profiles of q 1 and q 2 shown in the Fig. 1(b). The lines are guides to the eyes.

5 Disorder to Order-Like Transition 1631 Fig. 3. Effect of the X-ray illumination dose on the relative integrated intensity of the charge ordering modulation q 1 at T = 220 K. The number 1, 2 and 3 indicate the dose values corresponding to the three profiles of q 1 and q 2 shown as an inset to the figure. The lines are guides to the eyes. The threshold flux for the phase O1 appears to be of Φ th photons/cm 2. of the peak q 1 is increased by seven times its initial value. In addition, the flux threshold Φ th photons/cm 2 is greater than one found at 300 K (Fig. 2). It should be mentioned that the q 2 appears with a small weight once the sample was quenched to 220 K, after the heating up to 375 K, and almost disappears as a result of the follow-up X-ray photon illumination (see e.g. inset of Fig. 3). The results presented in the Figs. 2 and 3 provide a clear indication of photo induced charge ordering. It appears that the O1 is formed mainly from a disordered phase D with a minor contribution due to the transformation from the weak O2 phase. In fact, the process of this ordering under a photon flux φ can be separated in two processes: (a) from the disordered phase D to the O1 ordered phase and b) from the incommensurate phase O2 to the commensurate phase O1. In summary we have studied effect of X-ray illumination and temperature on the superstructures measured on the oxygen doped La 2 CuO 4 single crystal. The results provide a clear signature of an X-ray-induced phase transition-like behavior from a disordered phase to a commensurate ordered phase with period of 4 unit cells in the b direction. The ordering phase transition shows a temperature dependent threshold value for the X-ray dose illumination. The data suggests nucleation of a commensurate polaron crystal accompanied by the ordering of mobile interstitial oxygen ions at δ =1/8. 7,10 In conclusion, the present experiment provides a possibility to manipulate charge and dopant ordering with temperature and X-ray illumination, the approach that has direct implication on future perspectives to create nanostructures.

6 1632 D. D. Castro, G. Campi & G. Bianconi Acknowledgments This work is supported by progetto cofinanziamento Leghe e composti intermetallici: stabilità termodinamica, proprietà fisiche e reattività of MURST, by Istituto Nazionale Fisica della Materia (INFM), and by Progetto 5% Superconduttività of Consiglio Nazionale delle Ricerche (CNR). References 1. A. Bianconi and N. L. Saini, eds. Stripes and Related Phenomena (Kluwer/Plenum Publishers, New York, 2000). 2. A. Bianconi, G. Bianconi, S. Caprara, D. Di Castro, H. Oyanagi and N. L. Saini, J. Phys. Cond. Matt. 12, (2000). 3. A. Lanzara, P. V. Bogdanov, X. J. Zhou, S. A. Kellar, D. L. Feng, E. D. Lu, T. Yoshida, H. Eisaki, A. Fujimori, K. Kishio, J.-I. Shimoyama, T. Noda, S. Uchida, Z. Hussain and Z.-X. Shen, Nature 412, 510 (2001). 4. B. O. Wells, Y. S. Lee, M. A. Kastner, R. J. Christianson, R. J. Birgeneau, K. Yamada, Y. Endoh and G. Shirane, Science 277, 1067 (1997). 5. J. C. Grenier, N. Lagueyte, A. Wattiaux, J. P. Doumerc, P. Dordor, J. Etourneau, M. Pouchard, J. B. Goodenough and J. S. Zhou, Physica C202, 209 (1992). 6. C. Chaillout, J. Chenavas, S. W. Cheong, Z. Fisk, M. Marezio, B. Morosin and J. E. Schirber, Physica C170, 87 (1990). 7. A. Bianconi, D. Di Castro, G. Bianconi, A. Pifferi, N. L. Saini, F. C. Chou, D. C. Johnston and M. Colapietro, Physica C , 1719 (2000); D. Di Castro, M. Colapietro and G. Bianconi, Int. J. Mod. Phys. B14, 3438 (2000). 8. K. Nasu, Phase Transitions, in press (2002). 9. G. Campi et al., to be published (2002). 10. F. V. Kusmartsev, D. Di Castro, G. Bianconi and A. Bianconi, Phys. Lett. A275 (Nos. 1 2), 118 (2000). 11. D. Di Castro et al., to be published (2002).