Metal-Insulator Transitions in Complex Oxides probed by

Compton Scattering

B. Barbiellini, Northeastern University, Boston, USA

 

Theory Group, Northeastern [NU]

 

Arun Bansil [NU] Robert S. Markiewicz [NU] Bernardo Barbiellini [NU] Peter Mijnarends [NU & Delft, Holland] Stanislas Kaprzyk [NU & AGH, Poland] Matti Lindroos [NU & TTKK, Finland] Jouko Nieminen [  “              “     ] Seppo Sahrakorpi [NU] Dan Nissenbaum [NU]* Ville Arpiainen [TTKK, Finland] Hsin Lin [NU]* Tanmoy Das [NU] Wael Al-Sawai [NU] Susmita Basak [NU] Ray Wang [NU] Cristian Kusko [Romania] Millard Baublitz [NU and Boston U.] Many Experimental Collaborations

DFT

Interactions

Spectroscopies

ARPES RIXS

STM

X-rayCompton

PositronAnnihilation

Matrix Elements

QMC

Many-bodyOne-body

Motivation & Outline

• Coulomb repulsion tends to localize individual d electrons on atoms.

• Hybridization with the oxygen p electron states tends to de-localize the d electrons.

• These competing effects explain the MIT.

• Compton scattering is a unique MIT probe in Y1-xPrxCu3O7, La2-2xSr1+2xMn2O7, Fe3O4.

The Compton cross section

2/ ( )ed d d d const p

The cross section is related to the Electron Momentum Density (EMD)

G. Kaplan, B. Barbiellini and A. Bansil, Phys. Rev. B 68, 235104 (2003).

Electron Momentum Density

2( ) 2 i in p p

The EMD is given by the formula:

Important information on the wave functions and the occupation numbers values can be extracted from the EMD.

The occupation ni can be calculated within AGP or RVB.

Barbiellini & Bansil, J. Phys Chem. Solids 62, 2181 (2001).

Occupation number sum rule

1

0 1

ˆ( ) 2

i

ii

n

Tr n N

The occupation numbers are the eigenvalues of the one-particle density and they satisfy the relations:

For independent particles the orbitals are either occupied or empty.Virtual orbitals appear because of electronic correlation.

MIT in RBa2Cu3O7 (R=Y or Pr)

A. Shukla, B. Barbiellini, A. Erb, A.A. Manuel, T. Buslaps V. Honkimaki and P. Suortti,Phys. Rev. B 59,12127 (1999); ESRF Highlights (1997)

The [100]-[110] Compton profile anisotropy displays a remarkable difference between insulating PBCO and metallic YBCO.

The theory gives good agreement for the shape in the metallic YBCObut it must be rescaled.

MIT in La2-2xSr1+2xMn2O7

• The Colossal Magneto-Resistance (CMR) phenomenon is a huge decrease of resistance induced by application of a magnetic field.

• We have considered x=0.35 under 3 conditions: (1) FM metallic T<Tc, (2) PM insulating, (3) CMR.

B. Barbiellini, A. Koizumi, P. E. Mijnarends, W. Al-Sawai, Hsin Lin, T. Nagao, K. Hirota, M. Itou, Y. Sakurai and A. Bansil, Phys. Rev. Lett. 102, 206402 (2009)

eg

t2g

2d-EMD in the FM phase

KKR LDA calculation in good agreement with Magnetic Compton scattering.

In the C4v anisotropyFS & p and d wave function effects become visible.

The amplitude of the t2g and eg peaks is about 9 % while the amplitude of the p related peaks is less than 4 %.

Yinwan Li, P. A. Montano, J.F. Mitchell, B. Barbiellini, P. E. Mijnarends, S. Kaprzyk, and A. Bansil, Phys. Rev. Lett. 93, 207206 (2004).

[100]-[110] CP anisotropy

Oxygen p electronsplay a major role at low momentum range.

FT reveals electron localization

Coherence peaks have shorter range in the PM phase.

Number of electrons involved in MIT

In the present case we do need to rescale the theoryCP anisotropy.

We can therefore estimate the number of electron displaced in the MIT:

0.71 FM-PM0.67 CMR-PM

The role of p electrons is important.

Fe3O4 studied by Magnetic Compton Scattering

H. Kobayashi, M. Itou, S. Todo, B.Barbiellini, P.E. Mijnarends and A. Bansil,Manuscript submitted in Phys. Rev. B (2009)

Magnetite is ferrimagnetic with a Curie temperature > 800 K.

Below TV= 122 K a MIT occurs (Verwey)

MCS allows us to study the magnetic electrons behavior at the MIT.

Cubic spinel structure above Tv

O2- (Fe2+, Fe3+) octa Fe3+ tetra

Tetra (A) siteocta (B) site

Magnetic electron contributions

This contribution is cancelledby anti-ferromagnetic A B sitesContribution from ferromagnetic

B sites

Magnetite Spin MD

KKR LDA calculation.

FS breaks, p and d wave function effects are visible.

3d-Spin MD in the basal plane.

Magnetic Compton Profile

Good agreement between the calculation and the experiment.

MCP anisotropy

The MCP anisotropy decreases below the Verwey temperature. This was also observed by

Y. Li, P.A. Montano, B. Barbiellini, P.E. Mijnarends, S. Kaprzyk, and A. Bansil, J. Phys. Chem. Solids 68, 1556 (2007).

2d-reconstruction (expt.)

The reconstructed 2d-EMD changes from being negative at 300 K to positive at 12 K in the low momentum region.

The number of electrons per cell (with two Fe B sites) whose wave functions localize ne=0.59 should be compared to the charge ordering on the iron B-sites of about 0.40 electrons per unit cell (from RXS).

The higher value of ne indicates that the iron-oxygen hybridization plays also a role in the MIT.

T=300 K

T=12 K

Conclusion

• Compton scattering in transition-metal oxides shows clearly that and valence electrons are shared in states that cannot be assigned solely to metal or oxygen.

• Models in which oxides are simulated in terms of cation charge fluctuations with an on-site Coulomb interaction, ignoring oxygen states are too schematic to describe correctly the MIT.

Japan Synchrotron Radiation Research Institute, Press Release May 19, 2009

LaOFeAs is a member of the Fe-based superconductors with simple

tetragonal crystal structure stacking with FeAs layers and LaO layers.

Calculated Lock-Crisp-West(LCW) folding in the

paramagnetic tetragonal Brillouin zone of LaOFeAs.

FS topology and Fe-based superconductivity

Y. J. Wang, Hsin Lin, B. Barbiellini, P.E. Mijnarends,R.S. Markiewicz, S. Kaprzyk and A. Bansil

Please see poster by Y.J. Wang