Domain walls at the SDW endpoint of (TMTSF) 2 PF 6 under pressure C.Pasquier, Laboratoire de Physique des Solides, Orsay S. Brazovskii LPTMS, Orsay Acknowledgments: P. Grigoriev SDW M(SC) N. Kang, B.Salameh, P. Auban-Senzier, D.Jérome M(SC)

SC SDW Outline Superconductivity at the border of density wave states The case of (TMTSF) 2 ReO 4 Phase separation in (TMTSF) 2 PF 6

SC SDW SC/CDW proximity Superconductivity at the end point of a charge density wave state in organic and inorganic systems SC CDW L.Brossard et al, PRB (1990) A. F. Kusmartseva et al., PRL 103, (2009) TiSe 2 Per 2 [Au(mnt) 2 ] D. Graf et al, EPL, (2009) 1T- TaS 2 T c,max  6-8K TTF [Ni(dmit) 2 ] 2

SC SDW SC/(SDW or AF) proximity S. Nandi et al., PRL 104, (2010). Superconductivity at the end point of a spin density wave (or AF) state in organic and inorganic systems  -(BEDT-TTF) 2 X (TMTTF) 2 X & (TMTSF) 2 X

SC SDW SC/DW proximity Superconductivity at the end point of density wave is therefore a common feature in unconventional superconductivity. How does SC emerge from a density wave state ? We will focus on a 1D organic systems, essentially (TMTSF) 2 PF 6 It appears that there is a phase coexistence with the formation of domains and not ‘stripes’. We have to be careful and check that such phase coexistence is not due to structural transition like in (TMTSF) 2 ReO 4 : what happens in this case ?

SC SDW Phase coexistence in (TMTSF) 2 ReO 4 Moret R., Pouget J.-P., Comes R. and Bechgaard K., Phys.Rev.Lett., 49 (1982) 1008 Parkin S.S.P. Jérome D. and Bechgaard K., Mol.Cryst.Liq.Cryst., 79 (1981) 213 SC at low Temperature above 8kbar a b c Insulator Metal

SC SDW Phase coexistence in (TMTSF) 2 ReO 4 Self- organisation along a C.Colin et al., EPL, 75, 301 (2006) (log scale)

SC SDW Phase coexistence in (TMTSF) 2 ReO 4 (2a,2c) (a,2c) Metal Semiconductor 2 possible orientations for each anion Simple model : anisotropic Ising model Pseudospin : |+> if lattice parameter = 2a |-> if lattice parameter = a anisotropic interactions between spins  anisotropic interactions between chains Filaments or anisotropic bubbles oriented along a Onsager (1941) Pouget, Ravy,… a

SC SDW Phase coexistence in (TMTSF) 2 PF 6 c-axis a-axis b-axis

SC SDW Phase coexistence in (TMTSF) 2 PF 6 SC along c PHASE A : SC visible along c* only!

SC SDW Phase coexistence in (TMTSF) 2 PF 6  c =0 at low T Double transition in  b which disappears when P increases. Clear non-linearities as a function of current Some features are field independent PHASE B : SC visible along c* and b’!

SC SDW Phase coexistence in (TMTSF) 2 PF 6 Non linearities at zero bias persist up to high fields. They appear with SC at low pressure and disappear for P  P c0 PHASE A: 7.5kbar PHASE B: 8kbar H

SC SDW Phase coexistence in (TMTSF) 2 PF 6 Double transition in  a which disappears when P increases. PHASE C : SC visible along c*, b’ and a!

SC SDW Phase coexistence in (TMTSF) 2 PF 6 From bubbles to slabs by adjusting hydrostatic pressure Josephson junctions Tunnel junctions a c b SD W SC SD W SC SDW SC SDW SC

SDW Phase coexistence in (TMTSF) 2 PF 6 How to understand this texture evolution ? Why SC does appear first along c (the worst conducting direction!!!!) ? Many theories have been developed for cuprates… …..but only one theory seems to fit our data Soliton model : Existence of soliton domain walls (metallic) perpendicular to a- axis and expected peak of the anisotropy  b,c /  a at the DW / Metal transition S. Brazovskii, L.P. Gorkov and A.G. Lebed, JETP 56 (1982) 683 L.P.Gorkov, P.D.Grigoriev, EPL 71,425 (2005); PRB, 75, R20507 (2007) a c b SD W SC SD W SC SDW SC SDW SC See also experiments by Lee et al (PRL 2002,PRL 2005)

SC SDW Phase coexistence in (TMTSF) 2 PF 6 An image with the hands of the soliton model : how do metal (SC) emerge from a DW E creation of a soliton < SDW gap N. Kang et al. PRB (2010) Journées labo, 7 Octobre 2010  SDW Low pressure: Homogeneous SDW  SDW Phases B and C: Bands in the SDW gap ‘soliton phase’  SC  SDW Phase A: Midgap state in SDW gap High pressure : SC homogeneous phase  SC

SC SDW Phase coexistence in (TMTSF) 2 PF 6 We believe that the deep in dV/dI characteristics is related to this particular band structure (as we are doing tunneling experiments!)  SDW Low pressure: Homogeneous SDW  SDW Phases B and C: Bands in the SDW gap ‘soliton phase’  SC  SDW Phase A: Midgap state in SDW gap High pressure : SC homogeneous phase  SC PHASE B: 8kbar

SC SDW Phase coexistence in (TMTSF) 2 PF 6 a c b SD W SC SD W SC SDW SC SDW SC ? Why c first ??? J.P.Pouget, S.Ravy, Synth. Metals 85,1523 (1997) T.Takahashi et al, JPSJ 55,1364 (1986) Experiments :

SC SDW Phase coexistence in (TMTSF) 2 PF 6 a c b SD W SC SD W SC SDW SC SDW SC Why c first ??? governs the evolution from SDW to metal = deviation from nesting As q b  ¼, the term in k b is small, the term in k c is dominant. So ‘’’’’everything’’’’’ is fixed along k a and k b but not k c.

SC SDW Conclusion We have followed experimentally the evolution of the Metal (SC) concentration in the SDW matrix in (TMTSF) 2 PF 6 : bubbles - filaments - slabs evolution This evolution is understandable within a ‘soliton model’ Future : Is this evolution observable in other 1D systems or other materials with SDW/SC competition at the mesoscopic scale? Is it related to the particular Fermi surface of (TMTSF) 2 PF 6 where electrons for SC and SDW come from the same band. Same features for CDW/SC competition ?

SC SDW

SC SDW Cargese August 18, 2011 The ‘green flash’ spot ?