Michela FratiniDipartimento di Fisica Università degli studi di Roma “La Sapienza”

6th INTERNATIONAL CONFERENCE OF THE STRIPES SERIES

STRIPES 08

Quantum Phenomena in Complex Matter

CRITICAL OPALESCENCE IN SUPEROXYGENATED

La2CuO4+y

Erice, July 26-August 1, 2008

OUTLINE

We have investigated the ordering of interstitial oxygen (iO) in superoxigenated La2CuO4

•Method: synchrotron x-ray micro-diffraction as a probe to detect microscopic phase separation giving self-organized electronic textures

•Evidence for :1. The coexistence of ordered and disordered domains2. The order to disorder phase transition at 330K3. The statistical distribution of the size of ordered domains shows a power law distribution indicating critical opalescence near a critical point

•These results open the way to control and manipulate the phase separation for new functional devices

HIGH Tc SUPERCONDUCTORS:SHOW A COMMON STRUCTURE

MgB2Cuprates ROFeAs

•The nanoscale architecture: The lamellar Structure

PHASE DIAGRAM OF CUPRATES:

1 2 3

Critical point for structural phase transition

from LTT to LTO

STRUCTURE OF La2CuO4+y

a = 5.3462±0.0006 alfa = 89.969± 0.026

b = 5.3866±0.0004beta = 90.014 ± 0.020

c = 13.2039 ± 0.0026gamma = 89.979 ± 0.010

• Space group=Fmmm

• The cuprate perovskite La2CuO4 is formed by bcc CuO2 monolayers

intercalated by fcc rocksalt LaO bilayers with a periodicity of 1.3 nanometers.

•The oxygen dopants in the LaO planes are mobile above 200 K

Interstitial oxygen

PHASE COEXISTING OF SUPERCONDUCTIVITY AND MAGNETIC ORDER (Savici et al. Phys. Lett. 95 157001 (2005))

•High Tc superconductor

Lee et al. Phys. Rev. B 60 3643 (1999)

The system shows a coexistence between superconducting and magnetic domains below 41K

Tc=41K

0 0.06 0.12 0.180

1

2

3

4

Nor

mal

ized

Inte

nsity

h (r.l.u.)

q2(a*)

q2(a*)(II)

0.1 0.2 0.3 0.4 0.5k (r.l.u.)

q2(b*)

q2(b*)(II)

0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2l (r.l.u.)

q2(c*)

q2(c*)(II)

The existence of an ordered phase of interstitial oxygen has been detected at room temperature

OXYGEN ORDERED PHASE

q2=0.09 a* +0.25 b*+0.50 c*

X-ray diffraction pattern with:higher harmonics and narrow lines

indicatesDomains of 3D commensurate ordered

oxygen dopants

Interstitial oxygen ordering T=300-350K

a) Hysteresis cycle of Q2 phase using a photon flux of

1.5*1014 photons/cm2/s in the range from 300K to 350K.

b) Hysteresis cycle of Q2 phase using a photon flux of 10*1014photons/cm2/s

The result of the x.ray illumination is a reduction of the Hysteresis loop

CONTINUOUS FIRST ORDER PHASE TRANSITION

PHOTO-INDUCED ORDERING PROCESS AT DIFFERENT TEMPERATURES.

•Under x-ray illumination the size of the ordered domains increases in the temperature range between 250 K and 300 K.

•The photo-induced ordering process shows a threshold characteristic of cooperative phenomena.

INHOMOGENEOUS phase with the COEXISTENCE of

domains of ordered and disordered interstitial oxygen ions

Below 350K

•bubbles of 3D ordered iO’s in a disordered medium

What is the spatial distribution

of the ordered domains?

Space-resolved X-ray diffraction based upon a beam size of ~ 1 µmMETHOD

specialises in the delivery of the microfocused X-ray beam

•Photon sources in the range 12/13

Kev

•The focussed beam is defined by

a pinhole of 5 micron diameter

OXYGEN ORDERING PHASE

The large orthorhombicity makes easy identify the twinning of the crystal domains and to index the

superstructure peaks.

q2= (0.09; 0; 0.50)

q2 (0;0.25; 0.50)

006 peak in the diffraction pattern

We have done a mesh of the whole sample, with a beam size of about 1 micron, after locating q2, present in this oxide cuprate at room temperature around the bragg peak

(0,0,6).

MICROMAPPING SUPEROXYGENATED SAMPLE

Tc=41 K

Mean Intensity =1.7233

•The colours dots show the intensity of the q2

superstructure due to interstitial oxygen

ordering

•The figure shows the intensity on 5000

diffraction superstructure spots due to charge ordered domains

STATISTICAL DISTRIBUTION OF THE Q2 PHASE

)/exp()( xxr

Mean Intensity =1.7233

in SUPEROXYGENATED SAMPLE

γ = 1.7

ξ = 2.6

MICROMAPPING SAMPLE withINTERMEDIATE OXYGEN CONTENT

Y (

1 pi

xel=

4.9

mic

ron)

X (1 pixel=20.3 micron)

Tc= 32 K and 41 K

Mean Intensity =1.5752

STATISTICAL DISTRIBUTION OF THE Q2 PHASE

in SAMPLE with INTERMEDIATE OXYGEN CONTENT.

)/exp()( xxx

γ = 1.7

ξ = 1.5

Mean Intensity = 1.5752

RANK

MICROMAPPING IN A SAMPLE withLOW OXYGEN CONTENT

Y (

1 pi

xel=

4.9

mic

ron)

X (1 pixel=20.3 micron)

Tc=12K and 36K

Mean Intensity =1.0954

STATISTICAL DISTRIBUTION OF THE Q2 PHASE

IN SAMPLE with LOW OXYGEN CONCENTRATION DOPING.

γ = 1.7

ξ = 1.1

Mean Intensity =1.0954

RANK

CONCLUSIONS

•The proximity to a quantum critical point determines the critical opalescence with a power law distribution of the size of

oxygen ordered domains

•This is supported by the scale invariance

•This result is assigned to the criticality of the system being close to the tricritical point identified in the phase diagram

(Tc, doping and chemical pressure)

•This conclusion is supported by the photoinduced phase transition near the critical point.

Finally I would like to thank my collaborators

• Alessandra Vittorini-Orgeas• Nicola Poccia

• Ginestra Bianconi • Luisa Barba (Elettra)

• Manfred Burghammer (ESRF)• Gaetano Campi