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Elena Cannuccia
Multiferroicity and
para/ferroelectric
transition in GeV4S8
04/04/2017
Lab. “Physique des Interactions Ion...
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-
electri...
Towards a new kind of
type I multiferroic
compounds
We are an assorted
group: charge and
orbital order MF
compunds are my
...
GeV4S8 is a new type
I multiferroic
compounds
Ge
S
< 3 ˚A
≈ 4 ˚A
V
Formation of molecular-like electronic
states within th...
T [K]T [K]
Phase transitions in GeV4S8
300
●
cubic
●
Assigned to F-43m SG
●
paraelectric
●
not centrosymm. but
not polar
●...
The search of ferroelectricity
was then engaged
K.Singh, et al. Phys. Rev. Lett. 113, 137602 (2014)
Presence of finite
pol...
The magneto-electric coupling
in GeV4S8
K.Singh, et al. Phys. Rev. Lett. 113, 137602 (2014)
The external magnetic field
af...
Orbital ordering Purely electric
polarization
Enhancement
of P
Structural
Transition
The origin of ferroelectricity
in GeV...
Lattice dynamics study to deep
understand the para/ferro transitio
T [K]T [K]32
●
orthorhombic
●
Assigned to Imm2 SG
●
ferroelectric
●
polar
Raman and Infra-red
measurement
8-300 K 22 – 300...
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 m...
Theoretical phonon modes
and predicted Raman intensities
0 new modes
visible at TS
300 K on a
single crystal
Detailed view...
Phonons in the paraelectric (HT)
phase F43m
✗ We predicted a metallic ground state:
WRONG
DOS
✗ Experimental and optimized...
Let’s tackle the problem
from a more physical
point of view
Title:/srv/www/bcs_branch_www//bcs/www
Creator:dvips(k) 5.98 C...
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 mod...
Conclusions
We propose the assignement of the HT phase
phonon spectrum in the I4m2 group
Several phonon modes are softened...
Marie Bernadette Lepetit
Laurent Cario Etienne Janod
Vinh Ta
Phuoc
And Kiran Singh, Charles Simon, Benoit Corraze, Benjami...
Questions?
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Multiferroicity and para/ferroelectric transition in GeV4S8

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Multiferroicity and para/ferroelectric transition in GeV4S8

  1. 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, Université de Rome “Tor Vergata” Institut Laue Langevin, Grenoble
  2. 2. Ferromagnetism and Ferroelectricity spin dipoles Hz Ez
  3. 3. Multiferroic Materials
  4. 4. 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) P∼(1−100)μC /cm 2 P∼10−2 μC/cm2
  5. 5. 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 P=0.75μC /cm2
  6. 6. GeV4S8 is a new type I multiferroic compounds Ge S < 3 ˚A ≈ 4 ˚A V Formation of molecular-like electronic states within the clusters. Special class of Mott insulators because of the possibility to tune the insulator-to-metal transitions
  7. 7. T [K]T [K] Phase transitions in GeV4S8 300 ● cubic ● Assigned to F-43m SG ● paraelectric ● not centrosymm. but not polar ● insulating 3217 ● 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
  8. 8. 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
  9. 9. The magneto-electric coupling in GeV4S8 K.Singh, et al. Phys. Rev. Lett. 113, 137602 (2014) The external magnetic field affects the P magnitude and the T dependence of P below TN A deeper examination of the GeV4 S8 structure below TS is instructive in order to address the origin of the ferroelectricity. GeV4 S8 is a very promising multiferroic compound.
  10. 10. Orbital ordering Purely electric polarization Enhancement of P Structural Transition The origin of ferroelectricity in GeV4S8 V1 -V1 V2 -V2 K.Singh, et al. Phys. Rev. Lett. 113, 137602 (2014)
  11. 11. Lattice dynamics study to deep understand the para/ferro transitio
  12. 12. T [K]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 deep understand the para/ferro transitio Low Temperature phase High Temperature phase ● cubic ● Assigned to F-43m SG ● paraelectric ● not centrosymm. but not polar ● insulating
  13. 13. 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
  14. 14. 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)
  15. 15. 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
  16. 16. Let’s tackle the problem from a more physical point of view Title:/srv/www/bcs_branch_www//bcs/www Creator:dvips(k) 5.98 Copyright 2009 Rad CreationDate:Mon Apr 3 11:27:25 2017Having 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)
  17. 17. 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)
  18. 18. 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 movements observed 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
  19. 19. Marie Bernadette Lepetit Laurent Cario Etienne Janod Vinh Ta Phuoc And Kiran Singh, Charles Simon, Benoit Corraze, Benjamin Brière Acknowledgements
  20. 20. Questions?

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