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C.Pasquier,Laboratoire de Physique des Solides

ORSAY

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 Conclusions


Experimental set-up


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 Conclusions


Mott Insulator-metal transition


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


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 TEP at the Mott Insulator-Metal transition in V2O3



q Conclusions


Quasi-1D materials : CDW occurrence


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


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


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


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


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 Conclusions


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 (II)


Theoretical predictions

Dirac cones in α-(BEDT-TTF)2I3


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



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




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


Char

ge

orde

r

?

What is the nature of the

intermediate phase ?E

k

E

k

F. Piéchon, G. Montambaux

Outline





q Conclusions


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


Remerciements


S. Populoh, W. Kaddour Expériences au LPSP.Auban-Senzier, M. Monteverde

C.Mézière, P.Batail MOLTECH AngersL. Valade LCC Toulouse

Pnictides


SmFeAsO0.85

Thermoelectric power (TEP) is a thermodynamic measurement which reveals features non visible in resistivity measurements such as

Cônes de Dirac (V)


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

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