The authors report on NMR and susceptibility measurements in Sr14−xNaxCu24O41 for 0⩽x⩽2. The analysis of the susceptibility evidences that only a small fraction of Cu2+ spins are involved in the dimerization of Cu(1)O2 chains. The introduction of extra holes by Na+ doping is found to decrease the Cu(1)2+ uniform static susceptibility without affecting the amplitude of the dimerization gap. On the other hand, the amplitude of the spin gap in the Cu(2)2O3 two-leg-ladders and the resistivity are reduced by doping. The origin of the discrepancy in the amplitude of the gap estimated by means of nuclear spin-lattice relaxation and susceptibility measurements is briefly discussed.
The authors report on NMR and susceptibility measurements in Sr14−xNaxCu24O41 for 0⩽x⩽2. The analysis of the susceptibility evidences that only a small fraction of Cu2+ spins are involved in the dimerization of Cu(1)O2 chains. The introduction of extra holes by Na+ doping is found to decrease the Cu(1)2+ uniform static susceptibility without affecting the amplitude of the dimerization gap. On the other hand, the amplitude of the spin gap in the Cu(2)2O3 two-leg-ladders and the resistivity are reduced by doping. The origin of the discrepancy in the amplitude of the gap estimated by means of nuclear spin-lattice relaxation and susceptibility measurements is briefly discussed.
We report comprehensive Cu NMR studies on single crystals of ${\mathrm{Sr}}_{14\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{Cu}}_{24}{\mathrm{O}}_{41},$ which contain simple CuO2 chains and two-leg Cu2O3 ladders. From measurements of the 63Cu NMR shift, it is clear that the spin gap in the ladders decreases with isovalent Ca substitution from $\ensuremath{\Delta}=550\ifmmode\pm\else\textpm\fi{}30$ K for Sr14Cu24O41 (Sr14) to $350\ifmmode\pm\else\textpm\fi{}30$ K, $280\ifmmode\pm\else\textpm\fi{}30$ K, and $270\ifmmode\pm\else\textpm\fi{}30$ K for x=6 (Ca6), x=9 (Ca9), and x=11.5 (Ca11.5), respectively. The exponential decrease of the nuclear spin-lattice relaxation rate 1/T1 below \ensuremath{\sim}130 K is consistent with the presence of the spin gap in the spin excitation spectrum. In the T range higher than $\ensuremath{\sim}200$ K, we observed the following dependences: 1/T1=const and the square of Gaussian spin-echo decay time, ${T}_{2G}^{2}\ensuremath{\propto}T$ which are consistent with the scaling theory for the S=1/2 one-dimensional (1D) Heisenberg model. The value of T2G/T1T is compatible with the theoretical prediction of an exchange constant along the leg ${J}_{\ensuremath{\parallel}}\ensuremath{\sim}1800$ K for Ca6 and ${J}_{\ensuremath{\parallel}}\ensuremath{\sim}1500$ K for Ca9 and Ca11.5. A notable finding is that the magnitude of the spin gap remains nearly constant and characteristics of novel 1D-like spin dynamics are maintained in the content varying from Ca9 to Ca11.5. On the other hand, the charge transport changes with increasing Ca content so that the more conductive Ca11.5 exhibits pressure-induced superconductivity exceeding 3.5 GPa. We have found that T2G2, which is proportional to the inverse spin correlation length ${\ensuremath{\xi}}^{\ensuremath{-}1}$, deviates from a linear T dependence upon cooling and is described by $A+BT\mathrm{exp}(\ensuremath{-}\ensuremath{\Delta}/T)$, regardless of the Ca substitution. We point out that the value of T2G2(T=0)=A is proportional to the finite value of ${\ensuremath{\xi}}_{0}^{\ensuremath{-}1}=\ensuremath{\Delta}{/c}_{1D},$ where ${c}_{1D}=(\ensuremath{\pi}{/2)J}_{\ensuremath{\parallel}}$ is the spin-wave velocity. From the result that the values of ${A}^{\ensuremath{-}1}\ensuremath{\sim}{\ensuremath{\xi}}_{\mathrm{eff}}$ for Ca6, Ca9, and Ca11.5 are significantly reduced compared to that for Sr14, it is suggested that ${\ensuremath{\xi}}_{\mathrm{eff}}$ is dominated at low T by an average distance d among mobile holes obeying the relation ${\ensuremath{\xi}}_{\mathrm{eff}}\ensuremath{\sim}d={\ensuremath{\xi}}_{h}.$ From an estimate of ${\ensuremath{\xi}}_{0}/a\ensuremath{\sim}$ 5.2 for Sr14, where a is the Cu-Cu distance along the leg, ${\ensuremath{\xi}}_{h}/a$ is obtained as $\ensuremath{\sim}3.5$, 2.3, and 2.0, and hole content x as $\ensuremath{\sim}0.14$, 0.22, and 0.25 per Cu2O3 ladder for Ca6, Ca9, and Ca11.5, respectively. These values were consistent with x= 0.14, 0.2, and 0.22 for Ca6, Ca9, and Ca11 estimated from the optical conductivity experiment by Osafune et al. [Phys. Rev. Lett. 78, 1980 (1997)]. The ${\mathrm{Sr}}_{14\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{Cu}}_{24}{\mathrm{O}}_{41}$ compounds are thus hole-doped two-leg spin-ladder systems which reveal the metallic behavior dominated by the 1D-like spin dynamics at high T and accompanied by the spin gap formation at low T. For Ca11.5, as the spin gap is formed upon cooling below $\ensuremath{\sim}180$ K, the resistivity increases in the direction perpendicular to the ladder, whereas the conductivity along the ladder remains metallic, followed by the localization of mobile holes in both directions below ${T}_{L}\ensuremath{\sim}60$ K. We point out that preformed pairs are confined in each ladder and localized below $\ensuremath{\sim}60$ K at an ambient pressure.
We have performed inelastic-neutron-scattering and dc susceptibility measurements on a polycrystalline sample of (Sr0.8Ca0.2)14Cu24O41, which has a complex crystal structure comprising alternating layers containing isolated CuO2 chains and Cu2O3 two-legged ladders. The observed excitation spectrum is consistent with a spin gap for the ladder of 35 meV. At lower energy transfers we have observed a broad, nondispersive, feature centered on 10 meV that corresponds to singlet-triplet excitations in the CuO2 chains. We see some evidence in our susceptibility and neutron-scattering measurements to suggest that portions of the CuO2 chains are dimerized, or broken up into larger finite chains by nonmagnetic CuO2 units.
Magnetic properties of SrCu2O3 and Sr2Cu3O5, containing two-leg and three-leg S=21 ladders made of antiferromagnetic Cu-O-Cu linear bonds, were investigated. The susceptibility of the two-leg ladder material, SrCu2O3, is characteristic of thermal excitation from a nonmagnetic ground state with a spin gap of about 420 K, while the susceptibility of Sr2Cu3O5, containing three-leg ladders, reflects a gapless spin excitation spectrum. The temperature dependence of the nuclear-spin lattice relaxation rate, T11, of 63Cu NMR also indicates the existence of a spin gap only for SrCu2O3. The spin gap we observed for SrCu2O3 confirms a recent theoretical prediction for the magnetic behavior of spin = \textonehalf{} antiferromagnetically coupled Heisenberg chains.
We have performed magnetic susceptibility and electron spin resonance measurements on the quasi-one-dimensional magnet Sr14Cu24O41, which has simple chains and two-leg ladder chains of copper ions. The magnetic signals which we observed below room temperature mainly come from the simple chains. The experimental results show that the simple chain unexpectedly has a spin gap in the excitation spectrum, which originates from a dimerized state. \textcopyright{} 1996 The American Physical Society.
Inelastic neutron scattering measurements have been performed on the S=1/2 quasi-one-dimensional system ${\mathrm{Sr}}_{14\mathrm{\ensuremath{-}}\mathit{x}}$YxCu24O41, which has both simple chains and two-leg ladders of copper ions. Strong magnetic inelastic peaks, which originate from the simple chains, have been observed at 9--14 meV in Sr14Cu24O41. These spin gaps have been confirmed to originate from a dimerized state of the chain. The dimers are formed between spins which are separated by 2 and 4 times the distance between the nearest-neighbor copper ions in the simple chain. In addition, a weaker magnetic inelastic peak around 11 meV, which originates from a dimerized state in the ladder, has also been observed. These dimers are formed between the nearest-neighbor copper ions which are connected by the interladder coupling. We have also studied the effect of Y3+ substitution for Sr2+ site on the dimerized states. It was found that the yttrium substitution suppresses the gap energies drastically. Possible origins of the dimerized ground state and the excitations are discussed. \textcopyright{} 1996 The American Physical Society.