We report on 6,7 Li nuclear magnetic resonance measurements of the spin-chain compound LiCu2O2 in the paramagnetic and magnetically ordered states. Below T ≈24 K the NMR lineshape presents a clear signature of incommensurate (IC) static modulation of the local magnetic field consistent with an IC spiral modulation of the magnetic moments.7 Li NMR reveals strong phason-like dynamical fluctuations extending well below 24 K. We hypothesize that a series of phase transitions at 24.2, 22.5, and 9 K reflects a "Devil's staircase" type behavior generic for IC systems. LDA based calculations of exchange integrals reveal a large in-chain frustration leading to a magnetical spiral.Despite extensive efforts during the last decades spin ordering in frustrated S=1/2 quantum spin chains still remains a matter of broad activities. [1,2,3,4,5, 6] Rich phase diagrams with commensurate (C) and incommensurate (IC) phases, with spin-and charge ordering, dimerization, or superconductivity have been predicted. Most studies have been focused on various cuprates with corner-or edge-shared CuO 4 plaquettes. Edge-sharing of CuO 4 plaquettes leads to CuO 2 chains with a nearly 90• Cu-O-Cu bond angle causing a reduced nearest neighbor (nn) transfer and a next-nearest neighbor (nnn) transfer of similar size allowing frustration effects. IC spiral states driven by ferromagnetic (FM) nn exchange and in-chain frustration have been predicted theoretically for CuO 2 chain compounds such as Ca 2 Y 2 Cu 5 O 10 but discarded experimentally. [5, 6] The observation of in-chain IC effects in undoped quasi-1D cuprates is so far restricted by a sharp magnetic field driven C-IC transition observed in the spin-Peierls system CuGeO 3 at high magnetic fields. [7] Here we report on 6,7 Li nuclear magnetic resonance measurements (NMR) and local density (LDA) based analysis of the electronic and magnetic structure of the chain compound LiCu 2 O 2 . We show that the observed spontaneous magnetic order can be described by a spiral modulation of the magnetic moments. Independently, LDA calculations and a subsequent Heisenberganalysis reveal strong in-chain frustration driving spiral ordering in accord with the NMR data.LiCu 2 O 2 is an insulating orthorhombic compound [1,8,9, 10] with bilayers of edge-shared Cu 2+ -O chains running parallel to the b-axis separated by Cu 1+ planes. It exhibits a high-temperature antiferromagnetic(AFM)-like Curie-Weiss susceptibility χ(T ). Low-temperature χ(T ) and specific heat stufies [1,11] point to a series of intrinsic phase transitions at T ≈24.2 K, T ≈22.5 K, and T ≈9 K pointing to a complex multi-stage rearrangement of the spin structure. Magnetization studies performed in external fields up to 5 T did not reveal any signatures of field-induced transitions. µSR data[11] point to a broad distribution of magnetic fields at the muon stopping sites. LSDA(+U) calculations point to an FM in-chain ordering.[10] However, a simple FM ordering is in conflict with the µSR data[11] and the AFM dimer liquid picture. [1,12] Thus, to...

The optical response of the technologically interesting multiferroic BiFeO3 and related complex iron oxides, having high Néel or Curie temperature, is studied in the wide spectral range from 0.6 up to 5.8 eV by means of spectroscopic ellipsometry. The investigated iron oxides have different crystal symmetry with FeO6 octa hedral and FeO4 tetrahedral centers distorted to a certain degree. One of the two groups of materials includes BiFeO3, ErFeO3, Y.95Bi.05FeO3, oe-Fe2O3, Fe2-xGaxO3, and Fe3BO6 in which iron Fe3+ ions occupy only octahedral centrosymmetric or noncentrosymmetric positions and distortions range of 1-20 %. The second group includes LiFe5O8, BaFe12Ow, Sm3Fe5O12, and Ca2Fe2O5 in which Fe3+ ions occupy both octahedral and tetrahedral positions with a rising tetra/ortho ratio. We show that in the spectral range up to ~3.7 eV, the optical response is dominated by p-d charge transfer (CT) transitions, while at E > 3.7 eV both p-d and d-d CT transitions are revealed. At variance with several previous investigations, we present a correct and unified assignment of different dipole-allowed and dipole-forbidden CT transitions. All the ferrites investigated are CT insulators with the band gap determined by a dipole-forbidden p-d CT transition t1g ^ t2g, forming a ~2.5 eV band on the tail of a strong 3.0 eV band assigned to dipole-allowed p-d CT transitions t2u(w) ^ t2g in octahe dral FeOg centers. A noticeable enhancement of the optical response in BiFeO3 at ~4 eV as compared with other related iron oxides is attributed to CT transitions within the Bi-O bonds. We report an observation of unexpected midinfrared CT bands in calcium ferrite Ca2Fe2O5 and an enhanced structureless spectral weight in a wide range below the main CT bands in BiFeO3 with a remarkable smearing of the fundamental absorption edge. All these anomalies are assigned to CT instabilities accompanied by a self-trapping of p-d CT excitons and nucleation of electron-hole droplets. The optical detection of this CT instability agrees with the observation of a metal-insulator transition in bismuth ferrite.

A large body of experimental data point towards a charge transfer instability of parent insulating cuprates to be their unique property. We argue that the true charge transfer gap in these compounds is as small as 0.4-0.5 eV rather than 1.5-2.0 eV as usually derived from the optical gap measurements. In fact we deal with a competition of the conventional (3d 9 ) ground state and a charge transfer (CT) state with formation of electron-hole dimers which evolves under doping to an unconventional bosonic system. Our conjecture does provide an unified standpoint on the main experimental findings for parent cuprates including linear and nonlinear optical, Raman, photoemission, photoabsorption, and transport properties anyhow related with the CT excitations. In addition we suggest a scenario for the evolution of the CuO2 planes in the CT unstable cuprates under a nonisovalent doping. II. ELECTRON-LATTICE RELAXATION AND CT INSTABILITY OF PARENT CUPRATESMinimal energy cost of the optically excited disproportionation or electron-hole formation due to a direct Franck-Condon (FC) CT transition in insulating cuprates is E opt gap ≈1.5-2 eV. This relatively small value of the optical gap is addressed to be an argument against the "negative-U " disproportionation reaction 2Cu(II) = Cu(III) + Cu(I) 9 , or more correctlyHowever, the question arises, what is the energy cost for the thermal excitation of such a local disproportionation?The answer implies first of all the knowledge of relaxation energy, or the energy gain due to the lattice polarization by the localized charges. The full polarization energy R includes the cumulative effect of electronic and ionic terms, related with the displacement of electron shells and ionic cores, respectively. The former term R opt is due

We report on susceptibility, magnetization, 23 Na NMR, and specific heat data of the spin-chain material NaCu2O2 in the paramagnetic and ordered phases. Below 13 K, where a sharp field-dependent specific heat peak appears, the NMR lineshape points to an incommensurate static modulation of the local magnetic field consistent with a spiral state of the Cu magnetic moments. At 2 K weak ferromagnetism with an ordered moment of about 4•10 −3 µB has been observed. LDA-based estimates of exchange integrals reveal a large inchain frustration leading to a magnetic spiral.

In frames of a rather conventional cluster approach, which combines the crystal field and the ligand field models we have considered different charge transfer (CT) states and O 2p-Mn 3d CT transitions in MnO 9− 6 octahedra. The many-electron dipole transition matrix elements were calculated using the Racah algebra for the cubic point group. Simple "local" approximation allowed to calculate the relative intensity for all dipole-allowed π − π and σ − σ CT transitions. We present a self-consistent description of the CT bands in insulating stoichiometric LaMn 3+ O3 compound with the only Mn 3+ valent state and idealized octahedral MnO 9− 6 centers which allows to substantially correct the current interpretation of the optical spectra. Our analysis shows the multi-band structure of the CT optical response with the weak low-energy edge at 1.7 eV, associated with forbidden t1g(π) − eg transition and a series of the weak and strong dipole-allowed high-energy transitions starting from 2.5 and 4.5 eV, respectively, and extending up to nearly 11 eV. The most intensive features are associated with two strong composite bands near 4.6 ÷ 4.7 eV and 8 ÷ 9 eV, respectively, resulting from the superposition of the dipole-allowed σ − σ and π − π CT transitions. These predictions are in good agreement with experimental spectra. The experimental data point to a strong overscreening of the crystal field parameter Dq in the CT states of MnO 9− 6 centers.