engleski

Collective Electronic States of the New Quasi-One-Dimensional Materials

The spin/charge density-wave (SDW/CDW) phases in relation with super- conductivity (SC) were studied by linear and non-linear dc resistivity and low-frequency dielectric spectroscopy. Quasi-one-dimensional (q1D) mate- rials from two di&reg ; ; ; erent families were studied: single chain (TMTSF)2PF6, an organic Bechgaard-Fabre charge-transfer salt and (La, Sr, Ca)14Cu24O41, composite chain/ladder cuprates. Using an improved control of the applied pressure, in the phase diagram of (TMTSF)2PF6 a 0.8 kbar wide pressure range, featuring an SDW/SC coexistence, was found. The inhomogeneous character is supported by the analysis of the resistivity between TSDW and TSC and the superconducting critical current. TSC = 1:2K is practically con- stant here and only the SC domain proportion is increasing under pressure. Experimental results are correlated with a model comparing the free energy of a SDW/SC segregated phase and of homogeneous phases. Nonisovalently substituted q1D cuprates LaySr14&iexcl ; ; ; y&iexcl ; ; ; xCaxCu24O41 are insulators with hop- ping transport along the chains, a 1D disordered system. Disorder is due to random structural distortions of chains induced by irregular coordination of the La/Sr/Ca ions. The transport switches from the chains to ladders in iso- valently substituted Sr14&iexcl ; ; ; xCaxCu24O41, due to mobile holes transferred into the ladders. A phase transition is identi&macr ; ; ; ed below which a 2D long-range CDW order develops in the ladders. That is, in addition to the mode along the ladders, the collective CDW screened mode is observed in the direction of the ladder rungs. The transport perpendicular to the ladder planes is dominated by hopping. Ca-substitution suppresses CDW phase with TCDW decreasing from 210 K (x=0) to 7 K (x=9), and the CDW gap from 1300 K to 30 K, respectively. The suppression is due to the changes in coupling parameters in this strongly coupled system, as well as to disorder introduced at the Sr sites. This ground state falls between the CDW order of itinerant charges and the charge order of localized charges. Its non-standard nature might also be characterized by non-s-wave like pairing. SC is reported to be established in x > 10 when CDW is completely removed under pressure. CDW properties speak in favor of also non-s-wave 2D nature of SC.

spin-density wave; charge-density wave; superconductivity; organic superconductors; quasi-one-dimensional cuprates

nije evidentirano