multiferroicity and para/ferroelectric transition in gev4s8
TRANSCRIPT
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?