Microscopic origin of dimerization in the CuO2 chains in Sr14Cu24O41

Terahertz spectroscopic signature of the charge density wave in the spin-ladder compound, Sr14Cu24O41

We provide spectroscopic evidence for the charge density wave (CDW) phason mode at ≈0.93 THz in the two-leg, spin-1/2 ladders of Sr₁₄Cu₂₄O₄₁ using terahertz time-domain spectroscopy. We find that annealing in an oxygen atmosphere or doping with a low concentration of Co (≾1%) does not affect the CDW phason mode. However, Co doping at higher concentrations (10%), wherein the Co enters the ladder layers, destabilizes the CDW. We believe that the suppression of the CDW phase is due to an increase in intraladder overlap integrals through the shrinkage of interplane distance upon Co doping.

Effect of impurities on the long-distance and Zhang-Rice dimers in the quantum magnet Sr14Cu24O41

We study the effect of impurities on the two types of spin-dimers in the hybrid chain-ladder spin 1/2 quantum magnet SrCuO. Four different impurities were used, namely, the non-magnetic Zn (0.0025 and 0.01 per Cu) and Al (0.0025 and 0.01 per Cu), and magnetic Ni (0.0025 and 0.01 per Cu) and Co (0.01, 0.03, 0.05 and 0.1 per Cu). These impurities were doped in high-quality single-crystals synthesized by the floating-zone method. The magnetic susceptibility of pristine SrCuOis analyzed rigorously to confirm that at low temperatures (K), the 'free' spins in the chains undergo a long-distance dimerization as proposed in a recent study (Sahlinget al2015Nat. Phys.11255). The effect of impurity on these dimers is analyzed by measuring the specific heat down toK. We found that even at the lower impurity concentration, the long-distance dimers are significantly severed but the quantum entangled spin dimerized state of the chains persists. On the other hand, the other type of spin dimers that form at relatively higher temperatures via an intervening Zhang-Rice singlet are found to be practically unaffected at the lower impurity concentration; but at 1% doping these dimers are considerably severed. The effect of Co impurity turned out to be most unusual displaying a strongly anisotropic response, and with a dimerization gap that suppresses faster along the chain/ladder direction than perpendicular to it as a function of increasing Co concentration.

Structural modulation and hole distribution in Sr(14-x)Ca(x)Cu(24)O(41)

Structural properties and hole distribution in the spin-ladder compound Sr(14-x)Ca(x)Cu(24)O(41) have been extensively investigated by transmission electron microscopy (TEM). The complex electron diffraction patterns and high-resolution TEM observations reveal a clear incommensurate structural modulation along the c-axis direction attributable to two mismatched sublattices; this modulation strongly depends on hole distribution in the compounds. The fine structures of the O K and Cu L(2,3) ionization edges for the Sr(14-x)Ca(x)Cu(24)O(41) compounds recorded under different conditions indicate that more doped holes reside in the chains than the ladders, and that substituting Ca for Sr atoms results in a charge redistribution between the chains and ladders. Based on the experimental findings, the theoretical results, including the partial density of states and optical conductivity spectra calculated by the density functional theory, are also discussed.

Strain amplification of the 4k_(F) chain instability in Sr14Cu24O41

We used resonant soft x-ray scattering to study the chain ordering in Sr14Cu24O41 (SCO). We observed, for the first time, both the chain and ladder orders in SCO with the same probe. We found that the chain modulation in SCO is incommensurate with wave vector L_(c)=0.318, is strongly temperature (T) dependent, and is accompanied by a substantial hole modulation. By contrast, the chain modulation in a hole-depleted control sample La6Ca8Cu24O41 was commensurate (L_(c)=0.3), T independent, and purely structural. We conclude that the chain charge order in SCO is a 4k_(F) charge density wave stabilized by the misfit strain between the ladder and chain substructures.

Quantum melting of the hole crystal in the spin ladder of Sr(14-x)CaxCu24O41

We have used resonant soft x-ray scattering to study the effects of discommensuration on the hole Wigner crystal (HC) in the spin ladder Sr(14-x)CaxCu24O41 (SCCO). As the hole density is varied the HC forms only with the commensurate wave vectors L(L) = 1/5 and L(L) = 1/3; for incommensurate values it "melts." A simple scaling between L(L) and temperature is observed, tau1/3/tau1/5 = 5/3, indicating an inverse relationship between the interaction strength and wavelength. Our results suggest that SCCO contains hole pairs that are crystallized through an interplay between lattice commensuration and Coulomb repulsion, reminiscent of the "pair density wave" scenario.

Crystallization of charge holes in the spin ladder of Sr14Cu24O41

Determining the nature of the electronic phases that compete with superconductivity in high-transition-temperature (high-T(c)) superconductors is one of the deepest problems in condensed matter physics. One candidate is the 'stripe' phase, in which the charge carriers (holes) condense into rivers of charge that separate regions of antiferromagnetism. A related but lesser known system is the 'spin ladder', which consists of two coupled chains of magnetic ions forming an array of rungs. A doped ladder can be thought of as a high-T(c) material with lower dimensionality, and has been predicted to exhibit both superconductivity and an insulating 'hole crystal' phase in which the carriers are localized through many-body interactions. The competition between the two resembles that believed to operate between stripes and superconductivity in high-T(c) materials. Here we report the existence of a hole crystal in the doped spin ladder of Sr14Cu24O41 using a resonant X-ray scattering technique. This phase exists without a detectable distortion in the structural lattice, indicating that it arises from many-body electronic effects. Our measurements confirm theoretical predictions, and support the picture that proximity to charge ordered states is a general property of superconductivity in copper oxides.