Pouvoir thermoélectronique sous pression. C.Pasquier, Laboratoire de Physique des Solides ORSAY

Transcription

1 Pouvoir thermoélectronique sous pression C.Pasquier, Laboratoire de Physique des Solides ORSAY

2 Outline Experimental setup TEP at the Mott Insulator-Metal transition in V 2 O 3 Metal+hidden density wave state= colossal Power factor in TTF[Ni(dmit) 2 ] 2 TEP in Dirac cone systems : not graphene but α-(bedt-ttf) 2 I 3 Conclusions GDR Thermoélectricité Orsay 11/07/2011 2

3 Outline Experimental setup TEP at the Mott Insulator-Metal transition in V 2 O 3 Metal+hidden density wave state= colossal Power factor in TTF[Ni(dmit) 2 ] 2 TEP in Dirac cone systems : not graphene but α-(bedt-ttf) 2 I 3 Conclusions GDR Thermoélectricité Orsay 11/07/2011 3

4 Experimental set-up In a clamp pressure cell not a lot of place but An excellent thermal screen heater V T S = V / T l = 62 mm work = 4 or 5 mm MAXI T 1 mm Φ ext = 35 mm T mk, 1K for T>400K Pressure = Gpa. 2.8GPa GDR Thermoélectricité Orsay 11/07/2011 4

5 Outline Experimental setup TEP at the Mott Insulator-Metal transition in V 2 O 3 Metal+hidden density wave state= colossal Power factor in TTF[Ni(dmit) 2 ] 2 TEP in Dirac cone systems : not graphene but α-(bedt-ttf) 2 I 3 Conclusions GDR Thermoélectricité Orsay 11/07/2011 5

6 Mott Insulator-metal transition Coefficient 2 S ZT = T κρ Power Factor: 2 S P F = ρ ZT or P F large needs S large (insulator) and ρ small (metal) What is the situation at the metal-insulator transition? Exemple : Cr doped V 2 O 3 temperature (K) crossover metal transition metal transition insulator crossover insulator Insulator zone of coexistance pressure (bar) critical point 3300 bar and 460 K Metal GDR Thermoélectricité Orsay 11/07/2011 6

7 Mott Insulator-metal transition In a Mott Insulator (1/2 band filling) and a one band model : S=0! S(µV/K) S (µv/k) (b) temperature (K) (a) Insulator Metal crossover metal transition metal transition insulator crossover insulator Insulator 3333 bar 3400 bar 3600 bar 3800 bar 4800 bar 5740 bar Temperature (K) zone of coexistance pressure (bar) critical point 3300 bar and 460 K Metal 500 bar 1000 bar 1720 bar 2260 bar 3333 bar S (µv/k) bar 3213 bar 460 Tc 3290 bar Power factor (µw. K 2.cm -1 ) Metal Temperature (K) P=3333 bar P=4050 bar pc Pressure (bar) S.Populoh, P.Auban-Senzier, P.Wzietek, C.Pasquier, submitted to APL GDR Thermoélectricité Orsay 11/07/ Temperature (K) INSULATOR Temperature (K) METAL Power factor is maximal far from the MI transition!

8 Outline Experimental setup TEP at the Mott Insulator-Metal transition in V 2 O 3 Metal+hidden density wave state= colossal Power factor in TTF[Ni(dmit) 2 ] 2 TEP in Dirac cone systems : not graphene but α-(bedt-ttf) 2 I 3 Conclusions GDR Thermoélectricité Orsay 11/07/2011 8

9 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 NbSe 2 (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) GDR Thermoélectricité Orsay 11/07/2011 9

10 TTF[Ni(dmit) 2 ] 2 : 1D multi-bandes Chaînes TTF TTF CDW at 1 bar visible in X-ray at 40 K S. Ravy, E. Canadell and J.P. Pouget; Proceedings of the ISSP inetrnational Symposium, Tokyo, Japan, August 28-30, 1989 CDW transition not visible at 1 bar in resistivity measurements Superconductor at high Pressure, T c =1.6K Chaînes Ni(dmit) 2 Thermoelectric properties???? GDR Thermoélectricité Orsay 11/07/

11 TTF[Ni(dmit) 2 ] 2 : resistivity Band structure : hole and electron bands A. Kobayashi, H. Kim, Y. Saaki, R. Kato and H. Kobayashi; SSC 62, 57 (1987) CDW transition non visible whatever the pressure W.Kaddour, P.Auban-Senzier, C.P unpublished GDR Thermoélectricité Orsay 11/07/

12 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 GDR Thermoélectricité Orsay 11/07/

13 TTF[Ni(dmit) 2 ] 2 : Thermoelectric properties 2000 S (µv/k) TTF[Ni(dmit)2]2 T=19K Pressure (kbar) S increases when P increases and σ increases when P increases Power factor (µw. K 2.cm -1 ) TTF[Ni(dmit)2]2 T=19K Pressure (kbar) 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)? GDR Thermoélectricité Orsay 11/07/2011 W.Kaddour, P.Auban-Senzier, C.P unpublished 13

14 Outline Experimental setup TEP at the Mott Insulator-Metal transition in V 2 O 3 Metal+hidden density wave state= colossal Power factor in TTF[Ni(dmit) 2 ] 2 TEP in Dirac cone systems : not graphene but α-(bedt-ttf) 2 I 3 Conclusions GDR Thermoélectricité Orsay 11/07/

15 Dirac cones Graphene α-(bedt-ttf) 2 I 3 Topological insulators 2DEG systems: m e = 0 Graphene m e > 0 Bilayer Graphene 2D system 3D system better screening Volume = Insulator Surface: Dirac cone Conventional 2DEG GDR Thermoélectricité Orsay 11/07/

16 Dirac cones (II) Theoretica l predictions GDR Thermoélectricité Orsay 11/07/

17 Dirac cones in α-(bedt-ttf) 2 I 3 A complex phase diagram. Charge Order 200 M.Monteverde, P.Auban-Senzier, C.P unpublished α-(bedt-ttf)2i3 150 Metal Métal Temperature (K) Insulator Pressure (kbar) Existence of Dirac cones proved by Quantum Hall Effect At which pressure does Dirac cones appear? Resistivity GDR is not Thermoélectricité enough to Orsay answer 11/07/

18 Dirac cones in α-(bedt-ttf) 2 I 3 Thermopower measurements may help Dirac cones may imply S=0 (electron-hole symmetry) M.Monteverde, P.Auban-Senzier, C.P unpublished S (µv/k) kb 7kb 6kb5kb4kb 2kb 1 bar S (µv/k) kb 7kb 6kb 5kb 4kb 3kb 1 bar Temperature (K) Temperature (K) 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 GDR Thermoélectricité Orsay 11/07/

19 Dirac cones in α-(bedt-ttf) 2 I 3 New phase diagram α-(bedt-ttf)2i3 Metal E What is the nature of the intermediate phase? Temperature (K) Charge order? Insulator k Pressure (kbar) E k M.Monteverde, P.Auban-Senzier, C.P unpublished GDR Thermoélectricité Orsay 11/07/2011 F. Piéchon, G. Montambaux 19

20 Outline Experimental setup TEP at the Mott Insulator-Metal transition in V 2 O 3 Metal+hidden density wave state= colossal Power factor in TTF[Ni(dmit) 2 ] 2 TEP in Dirac cone systems : not graphene but α-(bedt-ttf) 2 I 3 Conclusions GDR Thermoélectricité Orsay 11/07/

21 Conclusions TEP under pressure allows to obtain a new image of the phase diagram of many materials and is complementary to many other experimental techniques. In 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 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) 2 I 3 or may be in topological insulators GDR Thermoélectricité Orsay 11/07/

22 Remerciements S. Populoh, W. Kaddour Expériences au LPS P.Auban-Senzier, M. Monteverde C.Mézière, P.Batail MOLTECH Angers L. Valade LCC Toulouse GDR Thermoélectricité Orsay 11/07/

23 GDR Thermoélectricité Orsay 11/07/

24 Pnictides SmFeAsO 0.85 Thermoelectric power (TEP) is a thermodynamic measurement which reveals features non visible in resistivity measurements such as GDR Thermoélectricité Orsay 11/07/

25 Cônes de Dirac (V) Comparaison graphène - α(bedt-ttf) 2 I 3 Géométrie graphène sur SiO 2 α(bedt-ttf) 2 I 3 2D Empilement de couches 2D Vitesse de Fermi 10 6 m/s 10 5 m/s Mobilité (4K) 10 4 cm 2 /Vs 10 5 cm 2 /Vs Densité minimum de porteurs de charges Largeur naturelle niveau de Landau cm cm K 1K On voit tout l intérêt pour la physique des systèmes à cônes de Dirac de l étude de α-(bedt-ttf) 2 I 3 GDR Thermoélectricité Orsay 11/07/