Hole Doping of the CuO2 Chains in (La,Sr,Ca)14Cu24O41

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.

Ca(2)Y(2)Cu(5)O(10): the first frustrated quasi-1D ferromagnet close to criticality

Ca(2)Y(2)Cu(5)O(10) is built up from edge-shared CuO(4) plaquettes forming spin chains. From inelastic neutron scattering data we extract an in-chain nearest-neighbor exchange J(1)≈-170  K and the frustrating next-neighbor J(2)≈32  K interactions, both significantly larger than previous estimates. The ratio α=|J(2)/J(1)|=0.19±0.01 places the system close to the critical point α(c)=0.25 of the J(1)-J(2) chain but in the 1D ferromagnetic regime. We establish that the vicinity to criticality only marginally affects the dispersion and coherence of the spin-wave-like magnetic excitations but instead results in a dramatic T dependence of high-energy Zhang-Rice singlet excitation intensities.

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.

Structural transition, electrical and magnetic properties of the B-site Co doped Sr(14)Cu(24)O(41) compounds

The effect of Co substitution for Cu on the structure and physical properties of Sr(14)(Cu(1-x)Co(x))(24)O(41) compounds was studied by analyzing the selected-area electron diffraction and convergent-beam electron diffraction patterns, and by measuring the magnetic susceptibility, the electrical resistivity and Raman spectra. It is found that the space group of the CuO(2) chain is changed from Amma to Ammm upon Co doping, but the structure of the Cu(2)O(3) ladder remains unchanged. This indicates that the displacement between two neighboring CuO(2) chains has disappeared due to Co doping. Once a small amount of Co ions are doped into the compound, exceptional changes in the Weiss temperature and in the number of dimers occur. The remarkable increase in the absolute value of the Weiss temperature indicates that the antiferromagnetic interaction in CuO(2) chains becomes very strong due to Co doping. The increase in the Curie coefficient and the number of dimers implies that the Co doping causes Zhang-Rice singlets in the chains to be decoupled into free spins Cu(2+) and holes. Then, the free spins Cu(2+) are coupled into dimers, and the holes transfer from chains to ladders, which causes the resistivity to decrease when the Co dopant concentration is low (x<0.10). When the Co dopant concentration is high (x>0.10), some Co ions are directly substituted for the Cu ions in the ladders, which results in an increase in resistivity with increasing Co dopant content.

Interaction between the chain and ladder subsystems in (Ca,Sr,La)(14)Cu(24)O(41) compounds

We investigate the influence of structural modulations on the electronic properties of incommensurate (Ca,Sr,La)(14)Cu(24)O(41) compounds by band structure calculations based on density functional theory and the local density approximation (LDA). Using a supercell approach with ten CuO(2) chain and seven Cu(2)O(3) ladder segments, we take into account the major effects of the structural incommensurability. We find that the LDA electronic states show very little response to these modulations. The coupling between the electronic and structural degrees of freedom, hence, can be well described in terms of two independent subsystems. The incommensurate charge density waves (CDWs) in the chains and the ladders are formed independently of each other.

Magnetic ground state of coupled edge-sharing CuO2 spin chains

By means of density functional theory, we investigate the magnetic ground state of edge-sharing CuO2 spin chains, as found in the (La,Ca,Sr)14Cu24O41 system, for instance. Our data rely on spin-polarized electronic structure calculations including on-site interaction [local density approximation plus the multiorbital mean-field Hubbard model (LDA+U)] and an effective model for the interchain coupling. Strong doping dependence of the magnetic order is characteristic for edge-sharing CuO2 spin chains. We determine the ground state magnetic structure as function of the spin-chain filling and quantify the competing exchange interactions.

One- and two-triplon spectra of a cuprate ladder

We have performed inelastic neutron scattering on the near ideal spin-ladder compound La4Sr10Cu24O41 as a starting point for investigating doped ladders and their tendency toward superconductivity. A key feature was the separation of one-triplon and two-triplon scattering. Two-triplon scattering is observed quantitatively for the first time and so access is realized to the important strong magnetic quantum fluctuations. The spin gap is found to be 26.4+/-0.3 meV. The data are successfully modeled using the continuous unitary transformation method, and the exchange constants are determined by fitting to be Jleg=186 meV and Jrung=124 meV along the leg and rung, respectively; a substantial cyclic exchange of Jcyc=31 meV is confirmed.

Synthesis, Structure, and Magnetic Properties of an Antiferromagnetic Spin-Ladder Complex: Bis(2,3-dimethylpyridinium) Tetrabromocuprate

The complex (2,3-dmpyH)2CuBr4 has been synthesized and its crystal packing determined by single-crystal X-ray diffraction (2,3-dmpyH = 2,3-dimethylpyridinium). The compound crystallizes in the triclinic space group P1. The crystal packing is characterized by the formation of a ladder structure for the CuBr4 anions showing short Br...Br contacts. The rungs of the ladder are formed via a crystallographic inversion center, while the rails are formed via unit cell translations. Variable temperature magnetic susceptibility measurements agree very well with the ladder model [Jrung = -3.10 cm-1 (-4.34 K) and Jrail = -6.02 cm-1 (-8.42 K)]. The assignment as a magnetic ladder is confirmed by first principles bottom-up theoretical calculations which conclude that Jrung = -3.49 cm-1 (-4.89 K) and Jrail = -7.79 cm-1 (-10.9 K), in very good agreement with the experimental values. They also support the absence of additional significant magnetic exchange within the crystals. Thus, (2,3-dmpyH)2CuBr4 represents the second reported example of a weak-exchange limit magnetic ladder (that is, one in which the exchange along the rail is stronger than that across the rung).

Magnon-hole scattering and charge order in Sr14-xCaxCu24O41

The magnon thermal conductivity kappa(mag) of the hole-doped spin ladders in Sr14-xCaxCu24O41 has been investigated at low doping levels x. The analysis of kappa(mag) reveals a strong doping and temperature dependence of the magnon mean free path l(mag), which is a local probe for the interaction of magnons with the doped holes in the ladders. In particular, this novel approach to studying charge degrees of freedom via spin excitations shows that charge ordering of the holes in the ladders leads to a freezing out of magnon-hole scattering processes.

Influence of the incommensurability in Sr14-xCaxCu24O41 Family Compounds

We study the influence of the structural modulation on the low energy physics of the Sr14-xCaxCu24O41 oxides, using ab initio determination of the on-site and nearest-neighbor effective parameters. The structural modulations appear to be the key degree of freedom responsible for the low energy properties, such as the electron localization, the formation of dimers in the x=0 compound, or the antiferromagnetic order in the x=13.6 compound.

Charge-order-induced sharp Raman peak in Sr14Cu24O41

In the two-leg S=1/2 ladders of Sr14Cu24O41, a modulation of the exchange coupling arises from the charge order within the other structural element, the CuO2 chains. In general, breaking transla-tional invariance by modulation causes gaps within the dispersion of elementary excitations. We show that the gap induced by the charge order can drastically change the magnetic Raman spectrum, leading to the sharp peak observed in Sr14Cu24O41. This sharp Raman line gives insight into the charge-order periodicity and hence into the distribution of carriers. The much broader spectrum of La6Ca8Cu24O41 reflects the response of an undoped ladder in the absence of charge order.

Observation of two-magnon bound states in the two-leg ladders of (Ca,La)(14)Cu(24)O(41)

Phonon-assisted two-magnon absorption is studied in the spin- 1/2 two-leg ladders of (Ca,La)(14)Cu(24)O(41) for E parallel c (legs) and E parallel a (rungs). We verify the theoretically predicted existence of two-magnon singlet bound states, which give rise to peaks at approximately equal to 2140 and 2800 cm(-1). The two-magnon continuum is observed at approximately equal to 4000 cm(-1). Two different theoretical approaches (Jordan-Wigner fermions and perturbation theory) describe the data very well for J parallel approximately equal to 1020-1100 cm(-1), J parallel/J perpendicular approximately equal to 1-1.2. At high energies, the magnetic contribution to sigma(omega) is strikingly similar in the ladders and in the undoped high-T(c) cuprates, which emphasizes the importance of strong quantum fluctuations in the latter.

Strong anisotropy of superexchange in the copper-oxygen chains of La(14)-xCaxCu24O41

Electron spin resonance data of Cu2+ ions in La(14)-xCaxCu24O41 single crystals (x = 9,11,12) reveal a very large width of the resonance line in the paramagnetic state. This signals an unusually strong anisotropy of approximately 10% of the isotropic Heisenberg superexchange in the Cu-O chains of this compound. The strong anisotropy can be explained by the specific geometry of two symmetrical 90 degrees Cu-O-Cu bonds, which boosts the importance of orbital degrees of freedom. Our data show the apparent limitations of the applicability of an isotropic Heisenberg model to the low-dimensional cuprates.

Evidence for Successive Doping-Induced Dimensionality Transitions in the Spin-Chain Compound Ca2+xY2-xCu5O10

We report the results of heat capacity, SQUID magnetometry, and x-ray diffraction studies of polycrystalline and magnetically aligned Ca 2+xY 2-xCu 5O (10), a quasi-1D compound. As holes are doped into the chains from x = 0-2, the data indicate a change from 3D long-range order to 1D chain behavior to cluster behavior. We quantitatively model the magnetic and thermal properties of heavily doped Ca 4Cu 5O (10) in terms of spin clusters. Anisotropic susceptibility data indicate a doping-independent easy axis of magnetization perpendicular to the planes containing the chains; also, a spin-flop transition is observed, providing a measure of the anisotropy field.

Structural refinement of crystals with m = n = 1 in the series of [M2Cu2O3] m [CuO2] n -type cuprates

The series of cuprates [M2CU2O3] m [CuO2] n (where M are alkaline earth or trivalent metals) consisting of two sublattices was studied. The first members [M2Cu2O3][Cu1+δ O2+γ ] of the series were synthesized. The structural investigations of the obtained single crystals have shown that the latter structure can be interpreted as the commensurate version of two fragments due to the reconstruction of the Cu–O ribbons. Depending on the chemical composition, the crystals studied were either superconducting (T c = 85 K) or non-superconducting.