Diapositiva 1

Elena Cannuccia

Multiferroicity and para/ferroelectric transition in GeV4S8

04/04/2017

Lab. Physique des Interactions Ioniques et Moleculaires Campus Saint Jerome, Marseille

Dipartimento di Fisica, Universit de Rome Tor Vergata

Institut Laue Langevin, Grenoble

Ferromagnetism and Ferroelectricity

spin

dipoles

Hz

Ez

Multiferroic Materials

Hi, I have strong polarization and weak magneto-electric coupling

Hi, I have weak polarization and large magneto-electric coupling

Type I

Type II

Watch out for these two guys (2 types of Multiferroic Materials)

Towards a new kind of type I multiferroic compounds

We are an assorted group: charge and orbital order MF compunds are my friends

I do not know them !

Type I

Type II

GeV4S8 is a new type I multiferroic compounds

Ge

S

V

Formation of molecular-like electronic states within the clusters.

Special class of Mott insulators because ofthe possibility to tune the insulator-to-metal transitions

T [K]

32 K

Metal, P=0

Insulating, P=?

Phase transitions in GeV4S8

space group compatible with ferroelectricity

300

cubic

Assigned to F-43m SG

paraelectric

not centrosymm. but not polar

insulating

32

17

orthorhombic

Assigned to Imm2 SG

ferroelectric

Not centrosym, polar

insulating

orthorhombic

Assigned to Pmn21 SG

antiferromagnetic

Polar, insulating

K.Singh, et al. Phys. Rev. Lett. 113, 137602 (2014)

TN

TS

The search of ferroelectricity was then engaged

K.Singh, et al. Phys. Rev. Lett. 113, 137602 (2014)

Presence of finite polarization

The polarization increases discontinuously: paraelectric/ferroelectric phase transition

The polarization decreases abruptly: Magneto-electric coupling

The magneto-electric coupling in GeV4S8

K.Singh, et al. Phys. Rev. Lett. 113, 137602 (2014)

The external magnetic fieldaffects the P magnitude and the T dependence of P below TN

A deeper examination of the GeV4S8 structure below TS is instructive in order to address the origin of the ferroelectricity.

GeV4S8 is a very promising multiferroic compound.

Orbital ordering

Purely electricpolarization

Enhancement of P

Structural Transition

The origin of ferroelectricityin GeV4S8

V1-V1

V2-V2

K.Singh, et al. Phys. Rev. Lett. 113, 137602 (2014)

Lattice dynamics study to deepunderstand the para/ferro transition

T [K]

32

orthorhombic

Assigned to Imm2 SG

ferroelectric

polar

Raman and Infra-red measurement

8-300 K 22 300 K

Lattice dynamics study to deepunderstand the para/ferro transition

Low Temperature phase

High Temperature phase

cubic

Assigned to F-43m SG

paraelectric

not centrosymm. but not polar

insulating

Phonons in the ferroelectric (LT) phase

IR

4 main modes: 309, 322, 370 and 445 cm-1

50 K

zoom

445

370

309

322

2 softened modes (445 and 322 cm-1)below 50K

Theoretical phonon modes and predicted IR intensities

E. Cannuccia, et al. JPC 121, 3522 (2017)

2 new modes (333 and 342 cm-1)visible at TS

Theoretical phonon modes and predicted Raman intensities

0 new modes visible at TS

300 K on a single crystal

Detailed view of low intensity region

50 K

zoom

E

B1

B2

E

E

E

B2

B2

A1

419

377

287

260

Phonons in the ferroelectric (LT) phase

E. Cannuccia, et al. JPC 121, 3522 (2017)

Phonons in the paraelectric (HT)phase F43m

We predicted a metallic ground state: WRONG

DOS

Experimental and optimized geometry significantly differ each other

Global assignment of intense exper. modes:NOT POSSIBLE

the B3LYP functional, we tried B1WC and PBE0 functionals

We questioned the spin-unpolarized calculation

the quality of the basis set, we repeated with a plane-wave code

Lets tackle the problem from a more physical point of view

Having equivalent extinction rules, a small symm. breaking is difficult to detect in X-ray

F43m

I4m2

DOS

E. Cannuccia, et al. JPC 121, 3522 (2017)

V1-V1

V2-V2

Ge

S

Irrep E: -200 cm-1

Physical interpretation of some phonon mode

Irrep B2: 322 cm-1

Softening of the mode below Ts

Bond/stretching elongation V4S4 cluster

Any softening of the mode below Ts

Irrep E: 305 cm-1

Any intracluster movements

E. Cannuccia, et al. JPC 121, 3522 (2017)

Conclusions

We propose the assignement of the HT phase phonon spectrum in the I4m2 group

Several phonon modes are softened below Ts,and drive the intracluster atomic movementsobserved in the ferroelectric transition

The simultaneous weakening/strengthening of V-V bonds is a direct consequence of the orbital order instability seen as driving force of the ferroelectric transition

Marie Bernadette Lepetit

Laurent Cario

Etienne Janod

Vinh Ta Phuoc

And Kiran Singh, Charles Simon, Benoit Corraze, Benjamin Brire

Acknowledgements

Questions?