Outline
q Experimental setup
qTEP at the Mott Insulator-Metal transition in V2O3
qMetal+hidden density wave state= ‘colossal’ Power factor in TTF[Ni(dmit)2]2
q TEP in Dirac cone systems : not graphene but α-(BEDT-TTF)2I3
q Conclusions
GDR Thermoélectricité Orsay 11/07/2011 2
Outline
q Experimental setup
qTEP at the Mott Insulator-Metal transition in V2O3
qMetal+hidden density wave state= ‘colossal’ Power factor in TTF[Ni(dmit)2]2
q TEP in Dirac cone systems : not graphene but α-(BEDT-TTF)2I3
q Conclusions
GDR Thermoélectricité Orsay 11/07/2011 3
Experimental set-up
GDR Thermoélectricité Orsay 11/07/2011 4
In a clamp pressure cell… not a lot of place Lbut … An excellent thermal screen ☺
l = 62 mm
Φext = 35 mm
∅work = 4 or 5 mm MAXI
S = ∆V / ∆T
∆V
∆T
T
heater
1 mm
∆T∼ 100-400 mK , 1K for T>400KPressure = 0-1.1 Gpa …. 2.8GPa
Outline
q Experimental setup
qTEP at the Mott Insulator-Metal transition in V2O3
qMetal+hidden density wave state= ‘colossal’ Power factor in TTF[Ni(dmit)2]2
q TEP in Dirac cone systems : not graphene but α-(BEDT-TTF)2I3
q Conclusions
GDR Thermoélectricité Orsay 11/07/2011 5
Mott Insulator-metal transition
GDR Thermoélectricité Orsay 11/07/2011 6
TSZTκρ
2
= ρ
2SPF =Coefficient Power Factor:
ZT or PF large needs S large (insulator) and ρ small (metal)
What is the situation at the metal-insulator transition ?
3000 3100 3200 3300 3400 3500 3600 3700425
430
435
440
445
450
455
460
465
470
475
480
485
490
495
tem
pera
ture
(K)
pressure (bar)
crossovermetal
transitionmetal
transitioninsulator
crossoverinsulator
critical point3300 bar and 460 K
zone of coexistance
InsulatorMetal
Exemple :Cr doped V2O3
Mott Insulator-metal transition
GDR Thermoélectricité Orsay 11/07/2011 7
3000 3100 3200 3300 3400 3500 3600 3700425
430
435
440
445
450
455
460
465
470
475
480
485
490
495
tem
pera
ture
(K)
pressure (bar)
crossovermetal
transitionmetal
transitioninsulator
crossoverinsulator
critical point3300 bar and 460 K
zone of coexistance
Insulator
Metal
In a Mott Insulator (1/2 band filling) and a one band model : S=0 !
300 350 400 450 50016
18
20
22
24 (b)
S (µ
V/K
)
Temperature (K)
3333 bar 3400 bar 3600 bar 3800 bar 4800 bar 5740 bar
0
50
100
150
200
250 500 bar 1000 bar 1720 bar 2260 bar 3333 bar
S(µV
/K)
(a)Insulator
Metal420 430 440 450 460 470 480
162024283236404448525660
3290 bar
3213 bar
S (µ
V/K
)
Temperature (K)
3093 bar
3000 3200 3400 3600420
440
460
480
Tc
INSU
LATO
R
Tem
pera
ture
(K)
Pressure (bar)
METAL
pc
Power factor is maximal far from the MI transition !S.Populoh, P.Auban-Senzier, P.Wzietek, C.Pasquier, submitted to APL
300 350 400 450 5000.0
0.2
0.4
0.6
0.8
P=4050 bar
Pow
er fa
ctor
(µW
. K2 .c
m-1
)
Temperature (K)
P=3333 bar
Metal
Outline
q Experimental setup
q TEP at the Mott Insulator-Metal transition in V2O3
qMetal+hidden density wave state= ‘colossal’ Power factor in TTF[Ni(dmit)2]2
q TEP in Dirac cone systems : not graphene but α-(BEDT-TTF)2I3
q Conclusions
GDR Thermoélectricité Orsay 11/07/2011 8
Quasi-1D materials : CDW occurrence
GDR Thermoélectricité Orsay 11/07/2011 9
1D system: Peierls transition at low temperatures due to nesting of the Fermi surface => Metal-Insulator transition
TRUE if nesting is perfect for all bands…
In case of many bands at the Fermi level, the situation is more complex !
In NbSe2 (2D), nesting maintain free electron pockets: low resitivity (superconductor at low T) and S is not very large (few µV/K) (in agreement with a metallic behavior)
TTF[Ni(dmit)2]2 : 1D multi-bandes
GDR Thermoélectricité Orsay 11/07/2011 10
ßChaînes TTF à
ßChaînes Ni(dmit)2à
CDW at 1 bar visible in X-ray at ∼40 KS. Ravy, E. Canadell and J.P. Pouget; Proceedings of the ISSP inetrnational
Symposium, Tokyo, Japan, August 28-30, 1989
TTF
CDW transition not visible at 1 bar in resistivity measurementsSuperconductor at high Pressure, Tc=1.6K
Thermoelectric properties ????
TTF[Ni(dmit)2]2 : resistivity
GDR Thermoélectricité Orsay 11/07/2011 11
A. Kobayashi, H. Kim, Y. Saaki, R. Kato and H. Kobayashi; SSC 62, 57 (1987)
Band structure : hole and electron
bands
CDW transition non visible whatever the pressureW.Kaddour, P.Auban-Senzier, C.P unpublished
TTF[Ni(dmit)2]2 : TEP
GDR Thermoélectricité Orsay 11/07/2011 12
T>30K : TEP compatible with a metallic behavior even if it is quite complex
? W.Kaddour, P.Auban-Senzier, C.P unpublished
TTF[Ni(dmit)2]2: Thermoelectric properties
GDR Thermoélectricité Orsay 11/07/2011 13
0 2 4 6 8 10 12-300
-250
-200
-150
-100
-50
0
TTF[Ni(dmit)2]2 T=19K
S (µ
V/K
)
Pressure (kbar)
0 2 4 6 8 10 120
500
1000
1500
2000
Pow
er fa
ctor
(µW
. K2 .c
m-1
)
Pressure (kbar)
TTF[Ni(dmit)2]2 T=19K
|S| increases when P increasesand
σ increases when P increases
At low temperature, ‘colossal’ power factor.
Preliminary thermal conductivity measurements show that ZT(single crystal)>1 at 19K and 10kbar.
Physical origin ? Unknown but certainly related to e-ph coupling
Towards new types of thermoelectric materials (1D+multiband)? W.Kaddour, P.Auban-Senzier, C.P unpublished
Outline
q Experimental setup
qTEP at the Mott Insulator-Metal transition in V2O3
qMetal+hidden density wave state= ‘colossal’ Power factor in TTF[Ni(dmit)2]2
q TEP in Dirac cone systems : not graphene but α-(BEDT-TTF)2I3
q Conclusions
GDR Thermoélectricité Orsay 11/07/2011 14
Dirac cones
15GDR Thermoélectricité Orsay 11/07/2011
2DEG systems:
Conventional 2DEG
Bilayer Graphene
me > 0
Grapheneme = 0Graphene Topological insulators α-(BEDT-TTF)2I3
2D system 3D system èbetter screening
Volume = InsulatorSurface: Dirac cone
Dirac cones in α-(BEDT-TTF)2I3
17GDR Thermoélectricité Orsay 11/07/2011
Métal
Charge Order
A complex phase diagram….
0 2 4 6 8 10 12 14 16 18 200
50
100
150
200
Metal
Tem
pera
ture
(K)
Pressure (kbar)
Insulator
α-(BEDT-TTF)2I3
Existence of Dirac cones proved by Quantum Hall Effect
At which pressure does Dirac cones appear ? Resistivity is not enough to answer
M.Monteverde, P.Auban-Senzier, C.P unpublished
Dirac cones in α-(BEDT-TTF)2I3
18GDR Thermoélectricité Orsay 11/07/2011
Thermopower measurements may help …Dirac cones may imply S=0 (electron-hole symmetry)
0 50 100 150 200 250 300-10
0
10
20
30
7kb6kb5kb4kb3kb
2kb
S (µ
V/K
)
Temperature (K)
1 bar
0 50 100-0.30
-0.25
-0.20
-0.15
-0.10
-0.05
0.00
0.05
0.10
8kb 7kb 6kb 5kb 4kb 3kb
S (µ
V/K
)
Temperature (K)
1 bar
Existence of Dirac cones ???
TEP reveals 2 different insulating phases : -) a low pressure phase associated to charge order proved by NMR for instance-) a higher pressure phase where Dirac cone physics seems already there
M.Monteverde, P.Auban-Senzier, C.P unpublished
Dirac cones in α-(BEDT-TTF)2I3
19GDR Thermoélectricité Orsay 11/07/2011
New phase diagram
0 2 4 6 8 10 12 14 16 18 200
50
100
150
200
Metal
Te
mpe
ratu
re (K
)
Pressure (kbar)
Insulator
α-(BEDT-TTF)2I3
M.Monteverde, P.Auban-Senzier, C.P unpublished
Char
ge
orde
r
?
What is the nature of the
intermediate phase ?E
k
E
k
F. Piéchon, G. Montambaux
Outline
q Experimental setup
qTEP at the Mott Insulator-Metal transition in V2O3
qMetal+hidden density wave state= ‘colossal’ Power factor in TTF[Ni(dmit)2]2
q TEP in Dirac cone systems : not graphene but α-(BEDT-TTF)2I3
q Conclusions
GDR Thermoélectricité Orsay 11/07/2011 20
Conclusions
q TEP under pressure allows to obtain a new image of the phase diagram of many
materials and is complementary to many other experimental techniques.
qIn pure 1D multiband material TTF[Ni(dmit)2]2, a ‘colossal’ power factor has been
shown at low temperatures : the first molecular thermoelectric material. This may open
new way of chemical synthesis in the oxides for large ZT @RT.
q TEP under pressure allows to study deeply the physics of Dirac cones in 3D materials
(similar to graphene) that is following the evolution of the Fermi surface : the birth, life
and death of Dirac cones in α-(BEDT-TTF)2I3 or may be in topological insulators
GDR Thermoélectricité Orsay 11/07/2011 21
Remerciements
GDR Thermoélectricité Orsay 11/07/2011 22
S. Populoh, W. Kaddour Expériences au LPSP.Auban-Senzier, M. Monteverde
C.Mézière, P.Batail MOLTECH AngersL. Valade LCC Toulouse
Pnictides
GDR Thermoélectricité Orsay 11/07/2011 24
SmFeAsO0.85
Thermoelectric power (TEP) is a thermodynamic measurement which reveals features non visible in resistivity measurements such as
Cônes de Dirac (V)
25GDR Thermoélectricité Orsay 11/07/2011
Comparaison graphène - α(BEDT-TTF)2I3
graphène sur SiO2 α(BEDT-TTF)2I3
Géométrie 2D Empilement de couches 2D
Vitesse de Fermi 106 m/s 105 m/s
Mobilité (4K) 104 cm2/Vs 105 cm2/Vs
Densité minimum de porteurs de charges
1011 cm-2 108 cm-2
Largeur naturelle niveau de Landau 100K 1K
On voit tout l’intérêt pour la physique des systèmes à cônes de Dirac de l’étude de α-(BEDT-TTF)2I3