Anisotropic charge modulation in ladder planes ofSr14−xCaxCu24O41

Abstract

The charge response of the ladders in Sr 14−x Ca x Cu 24 O 41 is characterized by dc resistivity, low frequency dielectric, and optical spectroscopy in all three crystallographic directions. The collective charge-density wave screened mode is observed in the direction of the rungs for x = 0, 3, and 6, in addition to the mode along the legs. For x = 8 and 9, the charge-density-wave response along the rungs fully vanishes, while the one along the legs persists. The transport perpendicular to the planes is always dominated by hopping.The physics of doped Mott-Hubbard insulators challenges conventional theories of metals and insulators. 1 The effect of strong Coulomb interactions produces a rich variety of exotic ordering phenomena, which have been the focus of intense scientific activity in recent years. The spin-chain and ladder self-doped compound Sr 14−x Ca x Cu 24 O 41 has attracted much attention since it is the first superconducting copper oxide with a nonsquare lattice. 2 Theoretically, in doped two-leg Cuu O ladders, superconductivity ͑SC͒ is tightly associated with the spin gap and in competition with charge-density wave ͑CDW͒. 3 While both the spin gap and CDW were established in the ladders of Sr 14−x Ca x Cu 24 O 41 , 4-6 the relevance of these objects to electronic properties and superconductivity is still subject of intensive discussion. Recently, it was shown, on the basis of dielectric response data, that substitution of Sr 2+ by Ca 2+ gradually suppresses the insulating CDW phase, which eventually vanishes for x Ͼ 9, Ref. 7. In contrast to these results, dynamical Raman response observed above RT for x = 0 was assigned to CDW fluctuations and found to persist in the metallic phase of x = 12, a system which becomes SC under pressure. 8 It is of particular interest to learn more about the nature of CDW order in the spin ladders, which presents a nice experimental system of strongly interacting electrons. Although the ground state for 0 ഛ x ഛ 9 reveals a number of well-known fingerprints of the conventional CDW, such as the pinned phason 9 and the broad dispersion at radio frequencies due to screening of the CDW by free carriers, 5-7 its origin is certainly more complicated, since the system does not undergo a metal-to-insulator but an insulator-to-insulator transition. The role of Ca substitution is another open issue. Suppression of the CDW phase was ascribed 7 to worsened nesting conditions 10 implying that the system becomes more 2D already at ambient pressure for large x. On the other hand, it was suggested that at ambient pressure ͑for all x͒ the charge dynamics is essentially one-dimensional 2,11 ͑1D͒ and that only the application of pressure induces the dimensional crossover from 1 to 2. 2 In this Report, we address these important questions concerning the charge-ordered ground state in the ladders of Sr 14−x Ca x Cu 24 O 41 . Our results give evidence that the CDW is two dimensional with an anisotropic dispersion: the longrange charge order develops only in ladder planes, leading to a screen...

Cooperative dynamics in charge-ordered state ofα-(BEDT-TTF)2I3

“…[7][8][9] At much higher fields the giant nonlinear conductivity and negative differential resistance (NDR) were reported 10-14 which were very different from those known in conventional CDWs. Distinct mechanisms to explain these remarkable nonlinear effects have been suggested ranging from melting of charge order in θ-(BEDT-TTF) 2 CsM ′ (SCN) 4 (M ′ = Zn, Co) compounds 11 to dielectric breakdown and depinning and sliding of holes within charge stripes in La 2−x Sr x NiO 4+δ .…”

Temperature and composition phase diagram in the iron-based ladder compoundsBa1−xCsxFe2Se3

“…The spatial dimensionality should have effects on magnetic interaction, orbital selectivity and topology of Fermi-surface of Fe atoms. In this regard, it is noteworthy that one-dimensional spin-ladder systems (La, Y, Sr, Ca) 14 Cu 24 O 41 provide great insight into the understanding of two-dimensional copper-oxide superconductors 16 .…”

Spin-gap evolution upon Ca doping in the spin-ladder series Sr14−xCaxCu24

“…When gradually substituting Ca for Sr, the resistivity of Sr 14−x Ca x Cu 24 O 41 decreases mainly due to the redistribution of holes from chains to ladders. 7 It is believed that the holes are localized on chains and become mobile when transferring to ladders. In the compounds with x > 10, a superconducting phase was observed under hydrostatic pressure ranging from 3 to 8 GPa.…”

“…Interestingly, this system is an inherently self-hole-doped system, containing six holes in one formula unit. 7 The parent compound is easy to synthesize and has almost all six holes on chain sites. Its physics was widely investigated and is well understood.…”