daniele di castro, marcello colapietro and ginestra bianconi- metallic stripes in oxygen doped...

8/3/2019 Daniele Di Castro, Marcello Colapietro and Ginestra Bianconi- Metallic Stripes in Oxygen Doped La2CuO4

1/6

International Journal of Modern Physics B, Vol. 14, Nos. 29, 30 & 31 (2000) 3438-3443 World Scientific Publishing Company

3438

METALLIC STRIPES IN OXYGEN DOPED La2CuO4

DANIELE DI CASTRO

Unit INFM, Dipartimento di Fisica, Universit di Roma La Sapienza, 00185 Roma Italy

MARCELLO COLAPIETRO

Dipartimento di Chimica, Universit di Roma La Sapienza, 00185 Roma Italy

GINESTRA BIANCONI

Department of Physics, Notre Dame University, 46556 Indiana

We have investigated the charge ordering in an oxygen doped La2CuO4 crystal by diffuse high-

resolution x-ray diffraction using synchrotron radiation. The micro-strain in the CuO2 plane is

close to the micro-strain 0 for the formation of static charge ordered phases. The crystal shows theco-existence of a first phase with an insulating crystal of charge strings and a secondsuperconducting phase of "supestripes". The first insulating phase is due to charges at the criticaldoping 1/8 self trapped into a crystal of ordered strings of finite length (~ 60 ) and acommensurate period of 4 lattice units. The superconducting phase shows diffuse diffraction peakswith short coherence length due to fluctuations of stripes with an incommensurate period of about4.8 unit cells and length longer than 500 .

1. Introduction

The superconducting phase in cuprate superconductors occurs in a superlattice ofmetallic quantum stripes, called "superstripes" [1-5], where the hopping between the

stripes for a single particle (t) is smaller that for an electron pair (t2N0, where N0 is thedensity of states at the Fermi level). The fact that the stripes show up clearly in somehigh-Tc compounds and not in others has been explained recently by reveling thatdifferent compounds show very slow or very fast stripe fluctuations and therefore theirdetection depends on the experimental measuring time. The stripes formation anddynamics (and hence their fluctuations) has been discovered experimentally to becontrolled by the elastic strain field [6-11],due to the lattice mismatch of bcc CuO2layer with neighbor rocksalt fcc layers. The "superstripes" appear for an elastic strainlarger than a critical value c. High-Tc superconductivity shows up around this criticalpoint in a region of micro strain c/c0.

As it has been shown in a recent paper [11] the La2CuO4+ compounds, have a highvalue of micro-strain, therefore they show superconducting superstripes at (~0.16,

~0.072) and static charge ordering at (=1/8, ~0.072). By choosing a sample with aneffective hole density of~0.15 we have found here the presence of domains made ofcrystals of strings at hole doping 1/8 that co-exist with macroscopic domains where asuperlattice of superstripes appears. Hence we were able to investigate thetransformation of an insulating electronic crystal of ordered localized charges into ametallic striped phase. This allowed us to study the distribution of the lengths of theshort static strings, very picked at m = 11a (where a is the axis of the unit cell), and ofthe metallic superstripes, more broadened, with lengths going from about m = 20a to"infinite" (a*~0) with an average length of m = 77a (more than 400).


2/6


3439

2. Experimental

A flux grown single crystal of La2CuO4+y was doped by electrochemical oxidation. Thecrystal shows a spinodal macroscopic decomposition into two domains with about equalprobability: phase (1) made of macroscopic domains with static antiferromagnetic orderbelow 40 K [12] and a second phase (2) made of superconducting domains with a singlesuperconducting transition Tc=40 K, where the hole density is the same as in theoptimum Sr doped La2CuO4 superconductors with =0.16 holes per Cu site [13, 14].

The charge ordering has been studied by temperature dependent diffraction datacollected on the crystallography beam-line at the synchrotron radiation facility Elettra atTrieste [15-17]. The X-ray beam emitted by the wiggler source at the 2 GeV electronstorage ring was monochromatized by a Si(111) double crystal monochromator. The

temperature of the crystal was monitored with an accuracy of 1 K. The data wascollected with a photon energy of 12.4 KeV (=1 ), using an imaging plate as a 2Ddetector. The sample oscillation around the b axis was in a range 0


3/6


3440

component) from the peaks due to the charge ordering (diffuse and with almost zero a*component). The metallic superconducting phase shows a pattern of diffuse spots (Fig.2a and Fig.3a) due to charge ordering with a coherence length of about 350 and with awavevector:q2=0.013 (0.003)a*, 0.208 (0.003)b*, 0.290 (0.005)c*and a pattern of narrower, resolution limited, diffraction spots, with wavevector:q2oxy= 0.037 (0.001) a*, 0.198 (0.002) b*, 0.290(0.005) c*.

6

6.2

6.4

6.6

6.8

7

7.2

-0.5 0 0.5

l(r.l.u.)

k (r.l.u.)Fig.1: X-ray diffraction pattern of the La2Cu O4.1 in the (c*,b*) plane by an image plate detector. The

diffraction peaks along the a* direction due to the twinning are also observable (enclosed rectangles).

The third harmonic of the q2 modulation is also observable (Fig.2a) showing anadded evidence of the anharmonicity. This second wavevector q2is associated with inplane superconducting stripe ordering. The length of the stripes in the a* directionbecomes longer than 77a (~400 ), of the same order of magnitude of the coherencelength of the modulation. The transverse modulation of the superlattice of stripes alongthe b-axis is the same as the one found in superconducting Bi2212, and characterized bydiagonal stripes with wavevector of 0.208b* indicating a separation between the stripesof R~4.8b. The period along the c axis direction is found to be 3.5 c.

The profiles of the first harmonics of this domain along the b* and a* directions areshown (denoted by (2)) in Fig.4a and Fig.4b respectively. In Fig.5 we show the secondharmonics where the oxygen modulation, with long coherence length 1/=101a and thecharge modulation, with short coherence length 1/

=44.5a could be separated.


4/6


3441

In summary this work provides an experimental evidence for the coexistence of twodifferent phases with different hole densities in the same sample (La2CuO4.1) showing thecoexistence in the same sample of a crystal of strings at doping 1/8 and a superlattice ofsuperconducting stripes in the high Tc superconducting phase at higher doping,according with the new phase diagram of the doped perovskites[11]. Thesuperconducting phase shows diffuse diffraction peaks with an incommensurate period

of about 4.8b and stripes with a distribution of lengths broadened around the averagevalue of about 400 . On the other hand, the doped charges at the critical 1/8 doping areself trapped into a crystal of ordered strings of finite length of about 60 and acommensurate period of 4 lattice units and characterized by sharp

0.18 0.2 0.22 0.24 0.26 0.280

4

8

12

16

k scan

l=0.5l=0.29

(1)(2)

(a)

No

rmalizedintensity

0 0.05 0.1 0.150

4

8

12

16

h scan

l=0.5

l=0.29

(1)(2)

(b)

No

rmalizedintensity

FIGURE 4. Scans along the Q= (0,k,6+l) (panel a) and along the Q= (h,0,6+l) (panel b) due to the diffusescattering peaks of stage 3.5 superstructure (2), (open circles), and the resolution limited peaks due to the stage2 superstructure (1) open squares).

6

7

l

(r.

l.u.

)

a)

6

7

-0.5 0 0.5k (r.l.u.)

l

(r.

l.u.

)

b)

Fig.2: modulation peaks along the b*direction of the two domains: insulating(panel b) and superconducting (panel a).

6

7

l

(r.l.u.)

a)

6

7

-0.5 0 0.5h (r.l.u.)

l

(r.

l.u.

)

b)

Fig.3: modulation peaks along the a*direction of the two domains: insulating(panel b) and superconducting (panel a).


5/6


3442

0.36 0.4 0.44 0.48 0.52 0.560

1

2

3

4

k scan

l=1l=0.58

(1)

(2)

(a)

Normalizedintensity

0 0.05 0.1 0.15 0.2 0.250

1

2

3

h scan

Normalizedintensity

l=1

l=0.58

(1)(2)

(b)

FIGURE 5. Scans along the second harmonic Q = (0,k,6+l) (panel a) and along the Q = (h,0,6+l) (panel b)due to the diffuse scattering peaks of stage 3.5 superstructure (2) (open circles), and the resolution limitedpeaks due to the stage 2 superstructure (1) (open squares).

diffraction peaks. In conclusion the present results show coexistence of strings andstripes in the oxygen doped La2CuO4 system.

4. Acknowledgements

Thanks are due the A. Valletta and P. Radaelli for help in the early stage of this work, toC. Chaillout for neutron diffraction sample characterization. This research has beensupported by Istituto Nazionale di Fisica della Materia (INFM) in the frame of the

progetto PAIS Stripes, the Ministero dellUniversit e della Ricerca Scientifica(MURST) Programmi di Ricerca Scientifica di Rilevante Interesse Nazionalecoordinated by R. Ferro, e Progetto 5% Superconduttvit del Consiglio Nazionale delleRicerche (CNR).

References

1. A. Bianconi, M. Missori J. Phys. I (France) 4, 361 (1994); A. Bianconi S. dellaLonga M. Missori, I. Pettiti, M. Pompa, and A. Soldatov Jpn. J. Appl. Phys. 32Suppl. 32-2, 578 (1993) 2. A. Bianconi Sol. State Commun. 91, 1 (1994); and Physica C235-240, 269 (1994). 3. A. Bianconi N. L. Saini, A. Lanzara, M. Missori, T. Rossetti, H. Oyanagi, H.Yamaguchi, K. Oka, T. Ito Phys. Rev. Lett76, 3412-3415 (1996).

4. A. Bianconi, A. Valletta, A. Perali, and N. L. Saini Physica C296, 269-280 (1998). 5. A. Bianconi, Int. J. Modern Physics B,to be Published(2000). 6. A. Bianconi S. Agrestini, G. Bianconi, D. Di Castro and N. L. Saini, in "Stripes and Related Phenomena", Edited by A. Bianconi and N. L. Saini, KluwerAcademic/Plenum Publishers, New York, pag. 9 (2000) 7. N. L. Saini, A. Bianconi and Y. Oyanagi J. of Phys. Soc. Jpn., to be published(2000); N. L. Saini, A. Bianconi A. Lanzara, S. Agrestini D. Di Castro and Y.OyanagiInt. J. Modern Physics B,to be published(2000).


6/6


3443

8. A. Bianconi, D. Di Castro, N. L. Saini and G. Bianconi, in Phase transitions andself-organization in electronic and molecular networks, edited by M. F. Thorpe andJ. C. Phillips, Klwer Academic/Plenum Publisher, Fundamental Materials ResearchSeries (Proc. of the meeting at Cambridge 11-14 July, 2000) 9. A. Bianconi, S. Agrestini, G. Bianconi, D. Di Castro and N. L. Saini, J. of Alloysand Compounds (2000) in press;10. D. Di Castro, G. Bianconi, M. Colapietro, A. Pifferi, N. L. Saini, S. Agrestini and A.Bianconi,Eur. Phys. Journ B, to be published (2000).11. A. Bianconi, N. L. Saini, S. Agrestini, D. Di Castro and G. Bianconi, Int. J. ModernPhysics B,to be Published(2000)12. Y. S. Lee, R. J. Birgeneau, M. A. Kastner, Y. Endoh, S. Wakimoto, K. Yamada, R.W. Erwin, S. -H. Lee and G. Shirane Phys. Rev. B 60, 3643-3654 (1999).

13. P. C. Hammel, A. P. Reyes, S. -W Cheong, Z. Fisk and J. E. Schriber Phys. Rev.Letters 71, 440 (1993); P. C. Hammel, A. P. Reyes, Z. Fisk, M. Takigawa, J. D.Thompson, R. H. Heffner, S. Cheong, and J. E. Schirber Phys. Rev. B 42, 6781(1990); E. T. Ahrens, A. P. Reyes, P. C. Hammel, J. D. Thompson, P. C. Canfield,Z. Fisk, J. E. Schirber, Physica C212, 317 (1993)14. P. C. Hammel, B. W. Statt, R. L. Martin, F, C. Chou, D. C. Johnston, and S. -W.Cheong Phys. Rev. B 57, R712 (1998)15. F. V. Kusmartsev, D. Di Castro, G. Bianconi & A. Bianconi, Physics Letters A 275,117 (2000)16. A. Bianconi, D. Di Castro, G. Bianconi, A. Pifferi, N. L. Saini, F. C. Chou, D. C.Johnston and M. Colapietro, Physica C, 341-348, 1719 (2000).17. G. Bianconi D. Di Castro, N.L. Saini, A. Bianconi, M. Colapietro, and A. Pifferi in

X-rays and Inner Shell processes ed. by R.W. Dunford et al., American Institute of

Physics, Conference Proc., N.Y. 2000 pag. 358.

daniele di castro, marcello colapietro and ginestra bianconi- metallic stripes in oxygen doped...

Documents