Physical 08B ( 1981 ) 1181-1 182 North.Holland Publishing Company
LOW TEMPERATURE PROPERTIES
OF AN ORGANIC SUPERCONDUCTOR
R. L. Greene, P. Haen and E. M. Engler IBM Research Laboratory, San Jose, California 95193 and J. F. Kwak and J. E. Schirber Sandia Laboratories, Albuquerque, New Mexico 87185 A review of some properties of (TMTSF)2PF 6 is given. We argue that increased interchain coupling is the major cause of the pressure induced transition from a quasi ID SDW insulator to an anisotropic 2D superconductor. A narrow region of SDW and superconductivity coexistence is observed. The synthesis [I] of the class of quasi onedimensional (ID) metals, (TMTSF)2X[X=PF6, C104, AsFS, etc.], has led to several important dlscoveries: i) the first observation of superconductivity (SC) in an organic solid , and 2) the first example of a spin density wave (SDW) ground state in an anlsotroplc metal . In thls paper we briefly review some experiments designed to understand the origin of these novel phenomena. At ambient pressure (TMTSF)2PF 6 exhibits quasi ID metallic conductivity along the a crystal axis with a metal-lnsulator transition to a SDW state at TMI~I2K. In Figure i we show the longitudinal (a axis) resistance of (TMTSF)2PF 6 below 4K at two higher pressures. At 10.5 kbar the SC state is observed alone with Tc-0.8K. The data at 6.5 kbar suggests the coexistence of the SC and SDW states, as was earlier proposed by two of us . This suggestion was confirmed by the observation of the Meissner effect at 6.5 kbar . In Figure 2 we show the pres-
sure variation of both TMI and T c up to 14 kbar. The region of coexistence (dashed) is not well defined by our present data. However, at 5 kbar we find no SC down to 20mK, and above 8 kbar we see no evidence (from resistivity) of the SDW state. Experiments to look for SC in the regions between 5 and 6.5 kbar and the SDW state above 7 kbar are in progress. These results suggest the transverse (interchain) tunnelling integral, tA, plays the crucial role in determining the relative stability of the SC and SDW states. The insert of Figure 2 schematically shows the theoretical  variation of TMI(Tp) and T c as t I is increased. Application of pressure to (TMTSF)2PF 6 increases t I (since the transverse conductivity is increased ) which drives TMI to zero, produces the SC state and then decreases T c. Since TMI never increases with pressure, the ambient pressure state must already have a substantial t± and cannot be in the ID fluctuation region (indicated by the
6.5 k bar
s ~ ~ t
o s •
k.\\\~ b~\\\N 0.2 P(k bar)
Figure i : Resistance below 4K for (TMTSF)2PF 6 at two pressures. Data normalized t o unity at 4K.
Figure 2 : Schematic phase diagram for (TMTSF)2PF 6. Dashed lines roughly indicate region of SDW and SC coexlstance. Insert dlscussed in text.
I 18 i